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4. COAL FIRING

4.1 The Combustion Process

4.2 Design Procedures for Coal-fired Combustors

4.2.1 Scope of Section 44.2.2 Choice of coal feeding method

4.3 In-bed Feeding of Crushed Coal

4.3.1 Calculation of combustion in the bed and freeboardwithout fines recycle

4.3.1.1 Bed combustion4.3.1.2 Freeboard combustion4.3.1.3 Allowance for coals of different reactivity4.3.1.4 Combustion of low grade, high ash coals

4.3.2 Freeboard design4.3.2.1 Freeboard treated as a unit4.3.2.2 Stepwise calculation of freeboard combustion

4.4 Overbed Feeding

4.4.1 General considerations4.4.2 Overbed feeding techniques4.4.3 Estimation of combustion efficiency

4.4.3.1 Combustion efficiency correlation 14.4.3.2 Combustion efficiency correlation 24.4.3.3 Choice of correlation

4.5 Methods of Reducing Unburnt Carbon Loss

4.5.1 Freeboard baffles4.5.2 Combustion of elutriated carbon in a separate bed

4.5.2.1 Carbon burn-up cell design with in-bed feeding

4.5.3 Refiring of fines to the main bed4.5.3.1 Combustion loss estimation for fines recycle with

in-bed feeding

4.5.4 Reduction in fines content of feed4.5.5 Operation at higher temperature or higher excess air

levels

4.6 Assessment of Coals for Suitability in FluidisedCombustors

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4.7 Effect of Coal Characteristics on Combustor Design andOperation

4.7.1 General effects4.7.2 Moisture content4.7.3 Swelling number4.7.4 Coal rank4.7.5 Ash content

4.7.5.1 Effect of ash content on combustion4.7.5.2 Effect of ash content on distributor design4.7.5.3 Coal ash as a bed material

4.8 Estimation of the Initial Ash Size Distribution Producedby the Combustion of Coal

4.9 Conversion of Existing Equipment to Coal Firing

4.10 Turndown

4.11 Firing of Coal/Liquid Mixtures

4.12 Example Calculation

4.12.1 Calculation basis4.12.2 Design and Operating Data4.12.3 Calculation 1 - no recycle4.12.4 Calculation 2 - recycle of 80% of the primary.cyclone

catch

4.13 References

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4. COAL FIRING

4.1 The Combustion Process

A coal-fired fluidised bed combustor contains burning coal

particles Chat are dispersed uniformly throughout a fluidised bed of hot

inert particles. The coal is fed at one or more points and the action of

the fluidising process disperses it throughout the bed. Two feeding

methods can be used; they are termed in-bed feeding and over bed feeding.

The in-bed feeding method consists of the injection of the coal

into the bed at points situated near the base of the bed and arranged

evenly over the bed cross-section. This method is suitable for feeding

crushed coal with a top size of up to 6 mm (0.25 in.). Usually the coal is

conveyed pneumatically, using air, through the injection tubes which pass

into the bed either vertically upwards through the air distribution plate

or horizontally through the combustion side walls. Screw feeders can also

be used if the injection tubes are horizontal.

The above bed feeding method consists in feeding the coal to the

surface of the bed. This method is more suitable for use with uncrushed

coals. The coal can be simply dropped onto the bed surface through one or

more steeply inclined chutes passing through the freeboard side walls or it

can be thrown onto the bed surface by a device such as a spreader stoker.

The bed solids may be an inert material like sand,j orfalumina, or---

limestone if sulphur retention is required. It is also possible that part

of the coal ash formed on combustion can provide a suitable bed material.

The mineral content of cbal is present in two forms known as "inherent" ash

and "discrete" ash. Inherent ash takes the form of widely-dispersed small

particles imbedded in the combustible part of the coal substance. Discrete

ash is lumps of mainly mineral matter of a size comparable with that of the

rest of the coal particles. On combustion, the inherent ash usually

produces particles that are so small that they are rapidly elutriated ftom

the bed. On the other hand, the discrete ash after combustion may leave

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particles that are of a suitable size and strength to form the bed solids.

The coal fed to the bed is heated rapidly by radiation and

conduction, and moisture in the coal is evaporated. At a particle

temperature of between 230' and 350°C, (440'- 660'F), decomposition of the

coal substance starts, with evolution of carbon monoxide, hydrogen, gaseous

hydrocarbons and tars. This stage is referred to as devolatilisation, and

the substances evolved are known as coal volatiles. The devolatilisation

stage is longer than the initial stage (4.1) and when it is complete, the

particle is at a temperature approaching the bed temperature. There then

follows a further rapid rise to the particle equilibrium combustion

temperature which may be as much as 215°C (390'F) above the mean bed

temperature (4.1, 4.2, 4.38).

The time required for the initial rapid heating stage is a

function of the particle diameter and is quite short. A duration of less

than 3 seconds for particles less than 8 mm (0.3 in.) in diameter has been

quoted (4.1). When considering the spacing of coal feed points, the

thermal load resulting locally in the bed from this rapid heating stage may

be important, especially when, firing high-moisture, low calorific value,

fine coals. It should be borne in mind that solids mixing times are of the

order of 5 - 10 seconds, and it is desirable to maintain as uniform a bed

temperature as possible.

Devolatilisation times are typically ten or twenty times as long

as the initial heating times, but vary greatly with coal type, as does the

quantity of volatiles involved. The devolatilised, or "char", particles

remain burning in the bed until they have become so reduced in size that

they are elutriated. The time needed for complete char burn-out is longer

still than that for devolatilisation and may be up to 2000 seconds for

large particles.

It is important for both economic and environmental reasons that'

the combustion efficiency of a coal-fired combustor should be high. For

maximum efficiency, both the volatiles and the fixed carbon (char)

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components of the coal must be burned as completely as possible. The rate

of combustion of the volatiles is controlled mainly by the rate at which

they mix with oxygen, but rapid chilling may quench the reaction even if

oxygen is present. For example, when crushed coal (3 mm (0.12 in.) top

size) is being fired, the time needed for devolatilisation is only 10 to 30

seconds at a bed temperature of 875°C (1610°F), and much of this occurs

within the first five seconds (from the finest fraction of the coal) (4.3).

In these circumstances, therefore, the combustion of the volatiles makes

heavy demands on the supply of oxygen in the vicinity of the coal feed

points, and if insufficient oxygen is available for their complete

combustion within the bed, combustion will continue in the gas mixing zone

above the bed giving rise to a luminous flame which may be smoky and

contribute to a low combustion efficiency. Similar considerations would

apply to the combustion of the fines content of uncrushed coals. To ensure

that sufficient oxygen is available it is suggested, therefore, that the

bed area served by a single feed point should not exceed 0.6 m2 (6 ft ) for

in-bed feeding and 4.0 m (43 ft ) for above bed feeding using a drop tube

feeder or twice that area using a spreader type feeder.

The char remains in, the bed for long enough to be transported

well away from the feed points, and burns away eventually to a size at

which it can be elutriated from the bed. The elutriation of solid

carbonaceous particles is usually the biggest cause of combustion

inefficiency in coal fired fluidised bed combustors. When firing high ash

coals, or when using high sorbent feed rates (see Section 11), significant

carbon losses can also occur in the bed withdrawal stream. The carbon loss

resulting from elutriation of unburnt carbon depends on many factors: the

proportion of fine particles in the coal feed; the susceptibility of the

coal to breakage due to heating and attrition (4.4, 4.5); the coal

reactivity, excess air and bed temperature (which jointly determine the

concentration of carbon in the bed and hence the elutriation rate - (see

Section 9)); and the fluidising velocity.

Further combustion of elutriated carbon occurs in the freeboard

and factors controlling this include excess air, freeboard gas mixing,

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freeboard dimensions, and gas residence time and temperature in the

freeboard. The region of the freeboard immediately above the bed surface

is termed the "splash zone" because both inert bed particles and burning

char particles are continuously.being ejected into it and falling back to

the bed. The splash zone has no sharp upper boundary; with increasing

distance from the bed surface the solids concentration falls and eventually

reaches a value corresponding to steady state elutriation conditions. The

combustion contribution of the freeboard, and especially of the splash

zone, is important and to allow for it a freeboard height of at least 1 m

(3.3 ft) is recommended.

The influence of different variables on combustion rates depends

on the combustion regime, i.e. whether the rate-limiting mechanism is the

diffusion of oxygen to the particle surface, or the chemical reaction

between oxygen and carbon at the external and internal surfaces of the

particle (4.6). Diffusion control, which is more likely to occur when

temperatures are high and particle sizes large, shows only a small

dependence of rate upon temperature and does not vary with coal type.

Chemical rate control, however, is strongly temperature dependent and

varies markedly with coal type, low rank (high-volatile) coals being much

more reactive than high rank (low-volatile) coals. The combustion of coal

particles in fluidised beds at atmospheric pressure has been shown to have

a component of chemical rate control, even at temperatures as high as

1000°C (1830°F) (4.3, 4.38) and the same probably applies in the freeboard

zone (4.7).

Mineral matter closely associated with a burning particle of

coal, such as inherent ash, can acquire a temperature roughly equal to that

of the burning particle. Since burning particle temperatures may exceed

significantly the surrounding bed temperature, and as it is imperative that

the ash sintering temperature is not exceeded either in the bed or in the

freeboard, the mean bed and freeboard temperatures should therefore not

exceed a temperature '220°C (400°F) below the ash sintering temperatures.

For most coals this imposes an upper temperature of about 900 - 950°C

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(1650 - 1750°F). A test procedure for the determination of ash sintering

temperature is described in Section 17.

4.2 Design Procedures for Coal-Fired Combustors

4.2.1 Scope of Section 4

The general procedure for the design of a fluidised bed combustor

is outlined in Section 3 and the main purpose of Section 4 is to describe

methods for predicting the combustion efficiency during the coal firing of

fluidised combustors operating at atmospheric pressure, based on experience

gained on experimental and commercial combustors. Knowledge of behaviour

during combustion at elevated pressures is considered in Section 8. The

variations in both the applications of coal firing and the properties of

the coals burnt are considerable, however, so some discussion of the

efforts of such variations on the combustor design is included.

The methods recommended for estimating the combustion efficiency

of a coal-fired fluidised bed combustor will depend on the coal feeding

method adopted. For in-bed feeding a stepwise mathematical procedure is

described in Section 4.3. The calculation of the proportions of the heat

of combustion released in the bed and in the freeboard is described, and as

part of the heat released in the freeboard is transferred back to the bed,

instructions are given for calculating this transfer and for locating any

freeboard cooling surfaces. A worked example using the procedure is given

in Section 4.12. For above bed feeding an empirically based correlation

for the combustion efficiency of the bed and freeboard together is

described in Section 4.4 along with a summary of current experience of over

bed firing.

For both feeding methods Section 4.5 lists ways of reducing the

unburnt carbon loss, should this be unacceptably high. The recycle of

fines is one commonlyiused method. For in-bed feeding the modifications to

the calculation procedures of Section 4.3 necessary to take account of

fines recycle are given and, also for in-bed feeding, a calculation

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procedure for the design of a separate carbon burn-up bed is described.

Section 4.7 includes notes on the probable effects of coal

characteristics on combustor design and operation and also a discussion of

ash behaviour in the bed and ways of predicting the resistance of coal ash

to abrasion and the estimation of the ash size distribution formed on

combustion of coal.

Advice is given in Section 4.9 regarding the conversion of

existing equipment to coal-fired fluidised bed combustion. A brief

discussion of the considerations relating to the combustion of coal/liquid

mixtures is included in Section 4;11.

A number of computational aids is available. A microcomputer

program "FBC Design" has been written in BASIC for the preliminary

estimation of combustor size for given operating conditions and fuel

properties (4.39). The in-bed feeding calculation procedures have been

programmed for a main frame computer using the PL1 language; the program

is described in reference (4.8). A suite of programs has been written for

a microcomputer for estimating the performance of an industrial boiler

under given operating conditions when using the above bed feeding method

and the bed expansion method for turndown (4.40, 4.41, 4.42).

It should be noted that the calculation procedures given in this

section have been derived mainly from work on the combustion of a number of

low rank bituminous coals, with some limited experience of high rank and

sub-bituminous coals. Because of this, and because coals of similar

composition may vary in their resistance to thermal shock and mechanical

attrition in the bed, the accuracy of desigh predictions is likely to be

improved by conducting experimental tests on the fuel to be used. A

description of test facilities available for this purpose is given in

Section 17 of this Manual.

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4.2.2 Choice of coal feeding method

The choice of a coal feeding method will depend on the

requirements of the application and on the coal properties. It is not

possible to suggest hard and fast rules but some indications as to the

suitability of the two feeding methods are given in Table 4.1.

The in-bed feeding method was developed originally for the

combustion of unwashed coals in power generation applications. Since much

of the mineral matter in unwashed coals is present as discrete ash of a

size similar to that of the combustible coal it was necessary to crush the

coal so that the ash particles formed on combustion would be sized

correctly to act as the bed solids. The overbed feeding method was

developed as a simpler alternative for use in industrial boiler

applications. It is the cheaper method as fewer feed points are needed and

no crushing is required and has been used successfully for feeding washed

graded coals. In such coals most of the mineral matter is present as

inherent ash. As a result almost all the ash formed during combustion is

elutriated and a refractory material such as sand has to be provided and

added periodically to maintain the bed. Even when firing washed coals

there is a gradual accumulation of oversize ash in the bed, and most

fluidised bed fired industrial boilers and furnaces incorporate

bed-cleaning devices, in which a small stream of bed material is removed,

classified and the material freed from large particles is returned to the

bed. See Section 15.

The flat plate type of air distributor with stand pipe air

nozzles (see Section 15) is normally used with both coal feeding methods.

However, it becomes increasingly difficult with the flat plate type to

clean the bed effectively as the ash content of coals fed by the overbed

method is increased. Recent experience (4.43) has shown that the use of a

flat distributor plate with directed air nozzles to sweep oversize

particles to ash removal ports will enable coals with ash contents up to

25% to be handled. The overbed firing of coals with ash contents over 25%

will probably require the use of the more specialised types of distributors

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Table 4.1

Indications for Choice of Coal Feeding Method

Air Distributor: Flat plate type

with stand pipe nozzles and

few ash ports in plate

In-bed feeding indicated if - Above bed feeding indicated if -

Scale moderate to large, e.g. Scale medium to small e.g.power generation, bulk steam, industrial boilers <50 MW, firewater tube boilers tube boilers, steam or hot water

Combustion efficiency required Combustion efficiency: 94-97%high. >97% acceptable

Fuel: crushed, unwashed coals. Fuel: washed, graded coal,Coals with high fines content. singles or smalls. Top size 25 orTop size 6 mm (114 in.) 50 mm (1 or 2 in.)

Cost of crushing economic Cost of crushing uneconomic

Fuel ash content <15%. Higherash contents can be handled ifcoal top size is limited. SeeFigure 4.12.

High sulphur retention needed, Sulphur retention not needed or90% + moderate retention

Fan power costs less critical Fan power costs critical toapplication economics

Deep beds 0.5-1 m (20-40 in.) Shallow beds 0.15-0.5 mstatic (6-20 in.) static

Bed material: ash (or sorbent for Bed material: sand (or sorbentsulphur retention) for sulphur retention)

Bed solids removal required Ash mostly elutriated;intermittent bed cleaningrequired

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Table 4.1 (continued)

Indications for Choice of Coal Feeding Method

Novel designs of distributor

available for handling high ash coals

In-bed feeding indicated if - Above bed feeding indicated if -

Air distributor: Flat plate typewith directed stand pipe nozzles.Ash ports in plate

Fuel ash 15-25%.Bed cleaning continuous

Air distributor: Recirculating bed typeor spouted bed type with central ashofftake system

Fuel ash up to 80%Ash offtake continuous

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that are available for the combustion of low grade fuels. See Sections 7

and 15.

The ash content of the coal may affect the design of other

combustor components besides its effects on distributor and coal feeder

design. It is recommended that Section 4.7, which contains a discussion of

the effects of various coal properties on combustion efficiency and

combustor design, should be consulted at the outset of any design. It

should also be noted that while a fluidised bed combustor can be designed

to burn almost any fuel efficiently, any individual combustor may not

attain its rated output if fed with a fuel that differs significantly from

that for which it was designed.

4.3 In-bed Feeding of Crushed Coal

4.3.1 Calculation of combustion in the bed and freeboard without fines

recycle

The calculation described here is based on the combustion of low

rank, bituminous coals which contained fines, in a 0.92 m (3 ft) square

atmospheric pressure combustor. (4.7, 4.9, 4.10). Freeboard temperatures

were not allowed to rise above 900°C (1650°F). Temperature control was by

injection of saturated steam into the freeboard. Applied to combustors of

a similar configuration and firing a similar type of coal, the predictions

are more reliable than for a radically different type of combustor, or a

coal differing widely from those in the original work. Those coal

properties which are expected to have most effect on combustion are rank,

and inherent ash content. The calculation procedure (equation 4.1) does

not apply to any systems which may be developed for assisting coal mixing

by inducing circulating currents in the bed or by other means, e.g. by

directional coal injection nozzles to induce "gulf-streaming", or by use of

inclined baffles above the bed.

The first step of the calculation gives the combustible loss from

the bed surface, C , by consideration of bed temperature, fluidising

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velocity, excess air, and a coal mixing parameter, J, representing the

uniformity of distribution of coal in the bed. The arrangement of bed

cooling tubes affects the mixing of coal through the bed, and is therefore

important.

The second step calculates the combustion occurring above the bed

as a function of C , excess air, fluidising velocity, freeboard temperature

and freeboard residence time, and estimates the proportion of the heat

release which is transferred back to the bed. If the maximum freeboard

temperature is a design constraint the freeboard cooling needed to prevent

this temperature being exceeded is calculated by a heat balance on the

freeboard gases. If it is desired to predict the temperature profile in

the freeboard for a given cooling surface, an alternative stepwise

calculation of freeboard combustion may be used in which the freeboard is

divided into a number of slices. The whole calculation has been programmed

for computer operation. (4.8).

4.3.1.1 Bed combustion

The amount of combustion occurring in the bed is calculated by

the following procedure. Throughout the calculations the combustible

losses are expressed as fractions of the potential heat input.

1. Calculate a coal mixing parameter: t

_______Ab _ _P_ _... ... ... 4.1

N b [1 - Bh(1.5 - 0.3ph/DO)]

Note:

a. The parameter, 3, takes the value 0.01 for SI units and 0.305

for British Units when P is in atmospheres.

t Symbols are defined in Section 1 of this Manual.

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b. When only part of the bed cross-section is occupied by

tubes, J is calculated for the two exteme cases: i). All

the bed area occupied, and ii). no tubes in the bed. A

weighted average value is then taken. In the case of

combustors where the bed containment is formed by sides

which are not vertical A.b, the bed area, should be

calculated by dividing the bed volume by the bed depth.

Example

It is required to calculate the coal mixing parameter for an

atmospheric pressure combustor with a rectangular bed, 1.5 m

square, containing an array of cooling tubes in the form of

a tube bank 1.0 x 1.4 m in size, and formed of 50 mm outside

diameter tubes with 100 mm horizontal pitch. The bed depth

(fluidised) is 1.2 m, the height from the bottom of the

tubes in the first bank to the top of the tubes in the top

bank is 0.8 m, and there is a single coal feed point.

Then B = 0.8/1.2 = 0.667h

Ph/DO = 100/50 = 2.0

and

0.01 X (1.5)2 X 100J,=.- = 4.688

1 x 1.2 [1 - 0.667 (1.5 - 0.3 x 2.0)]

where J1is the value of J if the whole bed area were occupied

by tube banks, and

0.01 X (1.5)2 X 100J = = 1.875° 1 x 1.2 [1 - 0]

where J is the value of J if there were no tubes in the bed,

i.e. B = 0

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The required value of J is then,

1.4 x 4.688 + (2.25 - 1.4) x 1.875

2.25

3.60

2. Estimate Cd, the loss of combustibles from the bed surface at

datum conditions, for the selected fluidising velocity and bed

temperature. This can be done by reading Cd from Figure 4.1 or

by using the following equation on which the lines drawn in

Figure 4.1 are based.

-(ln Cd) 5.709 x 105 r 13552 21.768Cd u -e exp - + U r ] ... 4.2

Cd Uf Tb/ f J b/ 1

The parameters, 0 and Oil are unity for SI Units with Uf in m/s

and T' in 0K, and are 0.305 and 1.8 respectively for Britishb

Units with UJ in ft/s and Tb in OR.I b

3. Calculate, C%, the loss from the bed surface corrected for the

effects of the coal mixing parameter J, from the empirical

equation,

Cj Cd (0.7 + 0.185 J) ... ... ... ... ... 4.3

This correction is to account for the non-uniformity of coal

concentration in the bed, resulting from the combined processes

of mixing and combustion, and it applies for the combustion of

crushed coal of all ranks in beds at least 0.6 m in depth.

Experience in shallower beds, when feeding a graded or smalls

coal overbed, has shown that it is more convenient, with this

mode of firing, to apply a correction instead to the excess air

level. See equation 4.18 in Section 4.4.3. The correction is

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1.0Datum Conditions

Area per coal feed a 0.81 m2

Bed height Lb 0.6 mPressure P 100 kNlm2J

0.9 Size of tube-bank Bh 0.67Tube diameter DO 57 mmTube pitch Ph 230 mmMixing parameter J 1.63 atm mExcess air level X 0 % -

0.8

0.64,......

CBd m r ....... 0.5

HFra8cttionrpF of ob/sHeat Input

0.4

0.3 ..

0,2

0.1

D - ~ ~ 0.5 I 1.5 2 2.5 3Fluidising Velocity ms

Figure 4.1 (SI Units)

Variation of Cd with Fluidising Velocity and Bed Temperatureat Datum Conditions

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1.0

0.9 Bed height L 2 ft

Pressure P 1 atmSize of tube-bank 1d i2te2Tube diameter 2.25 in. tTube pitch Ph 9.05 in. : .. iMixing parameter J 1.63 atm m0.8Excess air level X 0 %

Datum n: ::Combustibles

Loss 0.7 2From Bed W11,10 i!i:P

Figure 4.1(British1 Unit)Cd

Fraction of wn l d nHeat input

0.1

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quite large for low rank coals but is only small for anthracite.

It is believed that it corrects for the effects of the rapid

evolution of volatile matter close to the coal feed point, which

results in an area of the bed being starved of oxygen. This

phenomenon is less usually noticed with crushed coal feeding for

two reasons: the air used to transport the coal to the feed

point tends to counteract the deficiency of oxygen in this

region, and the use of deeper beds encourages better lateral

mixing of gases in (and just above) the bed.

4. Calculate the combustible loss from the bed surface at the design

operating conditions, C . C is a function of Cj and of the

excess air, X. The relationship is shown in Figure 4.2 and can

be represented by the following equation (4.11).

1 - CO = (1 + 0.01 X) £ 1 - (Cj)A ... ... ... 4.4

where

C1

[C (1 + 0.01 X) ]

Equation 4.4 satisfies the following limiting conditions: as X

approaches - 100, CO approaches 1; as X approaches infinity, C

approaches a constant value of

C /(Cj -ln Cj)

The solution of equation 4.4 is iterative. If a computer program

is not used C may be estimated either from Figure 4.2 or from

Table 4.2. Linear interpolation is permissible on both Figure

4.2 and Table 4.2.

If the fuel is not a low rank bituminous coal the value of C0

obtained above should be corrected to allow for the different

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zHiiJC M X X w t M t g

t t iLM ig M1 fit -. lll'

C-)~mmh~l n 4, w

Figure 4.2

Effect of Excess Air and Cj on Combustible Loss from the Bed

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Copy NoAWTable 4.2

Effect of Excess Air on Co

Exce J 0.05 0.10 0.15 0.20 0.25 0.30 0.40 0.50ExcessAir,X,%

Values of C0

200.0 0.020 0.050 0.086 0.128 0.174 0.224 0.331 0.444190.0 0.020 0.050 0.087 0.129 0.175 0.225 0.332 0.445180.0 0.020 0.050 0.087 0.130 0.176 0.226 0.333 0.446170.0 0.020 0.051 0.088 0.130 0.177 0.227 0.334 0.447160.0 0.021 0.051 0.089 0.131 0.178 0.228 0.335 0.448150.0 0.021 0.052 0.090 0.132 0.179 0.230 0.337 0.449140.0 0.021 0.052 0.090 0.134 0.181 0.231 0.338 0.450130.0 0.021 0.053 0.091 0.135 0.182 0.233 0.340 0.452120.0 0.022 0.054 0.093 0.136 0.184 0.234 0.342 0.453110.0 0.022 0.055 0.094 0.138 0.186 0.236 0.344 0.455100.0 0.022 0.056 0.095 0.140 0.188 0.239 0.346 0.457

95.0 0.023 0.056 0.096 0.141 0.189 0.240 0.347 0.45890.0 0.023 0.056 0.096 0.141 0.189 0.240 0.347 0.45885.0 0.023 0.057 0.098 0.143 0.192 0.243 0.350 0.46080.0 0.023 0.058 0.099 0.144 0.193 0.244 0.351 0.46275.0 0.024 0.059 0.100 0.146 0.195 0.246 0.353 0.46370.0 0.024 0.059 0.101 0.147 0.196 0.248 0.354 0.46465.0 0.025 0.060 :0.102 0.149 0.198 0.249 0.356 0.46660.0 0.025 0.061 0.104 0.151 0.200 0.251 0.358 0.46750.0 0.026 0.062 0.105 0.152 0.202 0.254 0.360 0.45950.0 0.026 0.064 0.107 0.155 0.204 0.256 0.362 0.471

48.0 0.026 0.064 0.108 0.155 0.205 0.257 0.363 0.47246.0 0.027 0.065 0.109 0.156 0.206 0.258 0.364 0.47244.0 0.027 0.065 0.110 0.157 0.208 0.259 0.365 0.47342.0 0.027 0.066 0.111 0.158 0.209 0.260 0.366 0.47440.0 0.028 0.067 0.111 0.160 0.210 0.262 0.367 0.47538.0 0.028 0.067 0.112 0.161 0.211 0.263 0.369 0.47636.0 0.028 0.068 0.113 0.162 0.212 0.264 0.370 0.47734.0 0.029 0.069 0.115 0.163 0.214 0.265 0.371 0.47832.0 0.029 0.070 0.116 0.164 0.215 0.267 0.372 0.47930.0 0.030 0.071 0.117 0.166 0.216 0.268 0.373 0.48028.0 0.030 0.072 0.118 0.167 0.218 0.270 0.375 0.48126.0 0.031 0.073 0.120 0.169 0.220 0.271 0.376 0.48224.0 0.031 0.074 0.121 0.170 0.221 0.273 0.377 0.48322.0 0.032 0.075 0.123 0.172 0.223 0.275 0.379 0.48420.0 0.033 0.076 0.124 0.174 0.225 0.276 0.380 0.485

0~~~~~~~~~~~

Issue 002 Section 4

Page 19 of 96

Copy No.t))iTable 4.2 (continued)

Effect of Excess air on C0

\ CJ 0.05 0.10 0.15 0.20 0.25 0.30 0.40 0.50ExcessAir,X,Z

Values of C0

18.0 0.034 0.078 0.126 0.176 0.227 0.278 0.382 0.48616.0 0.035 0.079 0.128 0.178 0.229 0.280 0.384 0.48814.0 0.036 0.081 0.130 0.180 0.231 0.282 0.385 0.48912.0 0.037 0.083 0.132 0.182 0.233 0.284 0.387 0.49010.0 0.038 0.085 0.134 0.185 0.236 0.287 0.389 0.4928.0 0.040 0.087 0.137 0.187 0.238 0.289 0.391 0.4936.0 0.044 0.093 0.143 0.193 0.244 0.294 0.393 0.4974.0 0.044 0.093 0.143 0.193 0.244 0.294 0.395 0.4972.0 0.046 0.096 0.146 0.196 0.247 0.297 0.398 0.498

0.0 0.050 0.100 0.150 0.200 0.250 0.300 0.400 0.500

-2.0 0.055 0.104 0.154 0.204 0.253 0.303 0.402 0.502-4.0 0.061 0.109 0.159 0.208 0.257 0.307 0.405 0.504-6.0 0.071 0.116 0.164 0.212 0.261 0.310 0.408 0.506-8.0 0.084 0.123 0.169 0.217 0.266 0.314 0.411 0.508

-10.0 0.101 0.131 0.176 0.223 0.270 0.318 0.414 0.510-12.0 0.120 0.142 0.183 0.229 0.275 0.322 0.417 0.513-14.0 0.140 0.154 O0.191 0.235 0.281 0.327 0.421 0.515-16.0 0.160 0.168 '0.201 0.242 0.287 0.337 0.424 0.518-18.0 0.180 0.185 0.211 0.250 0.293 0.337 0.428 0.520-20.0 0.200 0.202 0.223 0.259 0.300 0.343 0.432 0.523-22.0 0.220 0.221 0.237 0.269 0.308 0.349 0.436 0.526-24.0 0.240 0.241 0.252 0.279 0.316 0.356 0.441 0.529-26.0 0.260 0.260 0.268 0.291 0.325 0.363 0.446 0.533-28.0 0.280 0.280 0.285 0.304 0.335 0.371 0.451 0.536-30.0 0.300 0.300 0.303 0.318 0.345 0.379 0.457 0.540

0~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Issue 002 Section 4

Page 20 of 96Copy No.<k.

reactivity of the coal. Section 4.3.1.3 describes the method of

estimating the relevant correction factors.

5. Calculate the amount of combustion occurring in the bed (as a

fraction of potential heat input), Cb, as

Cb 1 - C ... ... ... ... ... ... ... ... 4.5Cb = ~ 0

The combustible loss from the bed surface, C , comprises heat in

unburnt carbon, carbon monoxide and volatiles. In terms of

potential heat release, the relative amounts of these in typical

combustors are approximately: carbon - 86%, CO - 7%, volatiles -

7%. To give a typical example, for a value of C of 0.1, the

heat loss in elutriated unburnt carbon would be about 8.6% of the

heat in the coal feed, and CO and volatiles concentrations in the

flue gas, % vlv, (for an excess air level about 20%) would be

about 0.17% and 0.04% respectively.

4.3.1.2 Freeboard combustion

The carbon elutriated from the bed continues to burn in the

freeboard. There is also combustion of particles ejected temporarily in

the splashing which accompanies the arrival of bubbles at the bed surface.

A considerable amount of heat is returned to the bed with the burning

particles as they fall back. The rest is available to heat up the gas and

solids passing forward to the top of the freeboard.

The heating of the freeboard gases is a phenomenon of

considerable importance because it affects the ratio of bed cooling to

freeboard cooling required. Also, in order to avoid the fouling of

convective heat transfer surfaces in the freeboard it is necessary to

ensure that the temperature of the gases impinging on them does not exceed

a critical temperature which is related to the ash fusion characteristics.

(See Section 4.1). When the freeboard has uncooled refractory walls it is

advisable to avoid temperatures in excess of 950°C (1740°F) anywhere in the

Issue 002 Section 4

Page 21 of 96Copy No.g

freeboard unless the coal used has an ash with a sintering temperature in

excess of 12000C (2190°F).

The procedure for calculating the amount of combustion occurring

in the freeboard (or in any specified part of it) under specified

conditions of fluidising velocity, excess air and freeboard temperature is

as follows:

1. Find the combustion rate constant, K, from

K 10 (0.472 6 Uf) + 0.93] exp (-12 620 1/Ta) ... 4.6

where the freeboard temperature, T', is in degrees absolute. Ifa

Uf, and T' have units of m/s and °K respectively then

e = 01 = 1.0. If U and T' have units of ft/s and °a0 1 ~ ~~~f arespectively then O = 0.305; °1 = 1.8.

The values of K correlated by equation 4.6 have been derived

assuming perfect mixing of the freeboard gases over a

cross-section and no slip between gas and solid particles.

2. Estimate the mean residence time, tf, of the off-gas in the

freeboard, in seconds from

t = Volume of freeboardf Gas volumetric flowrate.4-7

3. Calculate the combustibles remaining in the combustor off-gas,

ct from the following equation

ct (C + 0.Oi X) [ 01XlnC (C + 0.01 x) | +0.01 xJ K tf ... 4.8

S~~~~~~~~~ Ct+00 ) 1+00

Issue 002 Section 4

Page 22 elf 96Copy No _

or, for X =0 from

K= Rtf 4.9.t o

(Note that gas residence time, not particle residence time is

used.)

4. Calculate the amount of combustion occurring in the freeboard,

Cf, as

cf = c C. ... ... ... *- ... ... 4.10

4.3.1.3 Allowance for coals of different reactivity

The calculations given in sections 4.2.1.1 and 4.2.1.2 are

suitable as they stand for use when low rank bituminous coals are to be

fired. If the fuel to be used is of a different type, e.g. an anthracite,

semi-anthracite, or a sub-bituminous coal such as lignite, some adjustments

must be made to the calculations to take account of the different

reactivities of these fuels, and of their different volatile contents.

Anthracite-derived chars are appreciably less reactive than those

from low rank coals. In the temperature range in which fluidised bed

combustors usually operate, the chemical reactivity of anthracite char is

only about a fifth of that of low rank coal chars (4.4, 4.12, 4.13). This

means that if the reactions were completely chemically rate-controlled, for

a given heat release rate in the bed the reacting surface area of carbon

would have to be five times as great when burning anthracite as when

burning low rank coal. However, it is probably only the smallest particles

of a low rank coal that are subject to a large degree of chemical rate

control during in-bed combustion, so that the effect of changing to

anthracite will merely be to increase the critical particle size for

diffusion control. The net effect will be a much less than five fold

increase in bed carbon content.

Issue 002 Section 4

Page 23 of 96Copy No.'

A tenfold reduction in reactivity (graphitised coke compared with

ungraphitised coke) has been reported to result in a threefold increase in

bed carbon inventory (4.14). As an approximation, therefore, it can be

assumed that with anthracite fuel the concentration of carbon, and the

elutriation rate of carbon, will be 50% higher than that when burning low

rank coal. Apart from this effect of lower reactivity, the volatile

content of anthracite ranges from 5% to 10% (dry, ash-free basis) compared

with 35% to 40% (daf) for low rank coal. Since the volatiles may be

assumed to burn completely, this difference will also contribute to

decreased combustion efficiency when burning anthracite.

The extent of combustion in the freeboard will also be influenced

by char reactivity. In freeboard combustion, particle sizes are much

smaller than in the bed, and chemical rate control therefore plays a

greater role. The suggested correction in this case is a doubling of the

calculated value for Ct, except when Ct exceeds 0.25 when its value should

be multiplied by (2.33-1.33 Ct).

Lignites are very much more reactive than low rank bituminous

coals, and may contain as little as 40% of fixed carbon instead of the 60%

to 65% in low rank coals. The consequence of this is a lowering of both C0

and Ct.

To allow for varying reactivity it is recommended that, if

anthracite or semi-anthracite is the fuel, the value of C calculated as in0

section 4.2.1.1, step 3, be multiplied by 1.5 if the uncorrected value of

C is less than or equal to 0.5, and by (2-C ) if the uncorrected value is

greater than 0.5. The corrected value of CO should be used as input for

the calculation of freeboard combustion (section 4.2.1.2), and the derived

value of Ctshould be multiplied by 2.0 if the uncorrected value of Ct is

less than or equal to 0.25, and by (2.33-1.33 C t) if the uncorrected value

is greater than 0.25.

If the fuel is lignite, the derived value of CO should be

multiplied by 0.5, and that for Ct by 0.4.

Issue 002 Section 4

Page 24 of 96Copy No»>

Obviously, these calculations are only approximations, because in

fact coal compositions and reactivities vary continuously with coal rank

between the extremes of anthracite and lignite, or even peat. If an

estimate can be made of approximately where in this range the fuel

reactivity lies, it may be possible to interpolate appropriate correction

factors (applying only to natural solid fuels) for C and Ct from those

given above. Based on current information the following is suggested:

Wa= exp [-0.0477 (0 - 10)]

Wf = exp [-0.0815 (0 - 10)]

where Wa is the correction factor to be applied to C

W£ is the correction factor to be applied to Ct

0 is the coal oxygen content (dry, mineral-matter free basis).

4.3.1.4 The combustion of low grade, high ash coals

The combustion calculations in sections 4.2.1.1 and 4.2.1.2 are

based principally on experience with commercial grades of unwashed UK

coals, crushed usually to a top size of 6 mm (0.25 in.) or less. Ash

contents of these coals ranged from 15% to 35%, but they were nevertheless

"washable" coals, i.e. they consisted of a mixture of largely ash-free coal

particles together with a proportion of "dirt", by which is meant stony

material, pyrites and other sulphide and carbonate minerals, plus shaly

particles which contain some combustible matter, although not a large

proportion. It is these dirt particles which can provide suitable bed

solids, while the clean coal particles burn away leaving only very fine,

inherent ash, which is carried away with the gaseous products of

combustion.

The effects on combustion efficiency of changes in inert content

of the fuel depend to a very large extent on the form in which the inert

material exists in the fuel as fired. Provided the mineral matter content

is mainly in the form of dirt particles, as described above, combustion

Issue 002 Section 4

Page 25 of 96Copy No,'7-.

efficiency will not be significantly affected until the mineral matter

rises above about 60%; even at this level, the loss of unburnt carbon in

the bed offtake will be less than 1%, for crushed coal firing. (When

firing large coal, bed carbon contents are considerably higher than with

crushed coal firing, and loss of unburnt carbon in bed offtake may be a

cause of combustion inefficiency for high ash coals.) Use of high

limestone or dolomite feed rates have a similar effect.

In addition to the loss of unburnt carbon in the removed bed

material, the sensible heat of this material may represent a significant

loss of energy, unless use can be made of the heat, for example by

preheating the combustion air.

Some low grade fuels may, however, be more uniform in composition

and the fuel content may be more evenly mixed with the inert content. If

the latter possesses a structure which does not disintegrate when the

combustible burns away, the ash may offer physical hindrance to the air

needed for combustion of the innermost portion of the particle. In such

circumstances, combustion will, be much slower, and complete burn-out may

never occur because the particle is removed with bed offtake before the

centre has burnt. The combustion loss with the bed removal will therefore

be greater than when burning a normal untreated coal of the same ash

content. It is not known whether a high inherent ash content can similarly

inhibit freeboard combustion, and so lead to increased loss of carbon in

elutriated material.

When a low grade fuel is to be used as the principle fuel, it is

recommended that a combustion test be carried out in order to ascertain the

behaviour of the ash as the combustible burns away. Examination of bed ash

at the end of a short combustion pot test will also in many cases provide

an indication of the likely difficulty of achieving good particle burn-out.

4.3.2 Freeboard design

In the general situation when the freeboard volume is unknown it

Issue 002 Section 4

Page 26 of 96Copy No.v'

is necessary to assume an initial value. Either the exit freeboard

temperature or the cooling needed to hold the freeboard below a certain

maximum temperature are then calculated along with the combustion loss from

the freeboard, Ct If the values of these parameters are not acceptable

then the estimate of freeboard volume is revised and the calculations are

repeated.

Two alternative methods of carrying out these calculations are

available. The first treats the whole freeboard as a unit, and calculates

the combustion and heat release together with the fraction of the heat

release which is returned to the bed, without giving any information about

the temperature variation with height above the bed. The second is a

stepwise procedure, in which the freeboard is divided into a number of

slices and the calculation is progressive from the lowest to, the highest

slice.

4.3.2.1 Freeboard treated as a unit

This method is suitable when freeboard temperature is an input

design parameter, or for calculation where a general indication of

freeboard combustion and cooling requirements is needed. When the

freeboard temperature is a design parameter, this value is inserted into

equation 4.6 for the calculation of K. When the freeboard temperature is

unknown, a value of 1123 K (2021 R) should be used for calculation of K.

In order to determine the amount of freeboard cooling necessary

to achieve the design freeboard temperature a heat balance is performed on

the freeboard gases. Heat flows to be taken into account in the balance

are:

i). Heat introduced by gas leaving the bed surface.

ii). Heat released by combustion in the freeboard.

iii). Heat extracted by freeboard cooling.

iv). Heat transferred back to the bed.

v). The sensible heat flow in gas leaving the freeboard.

Issue 002 Section 4

Page 27 of 96Copy No.'

In the heat balances, the specific heat of the gas and of the

suspended particles may both be taken to be the same, 1050 J/kg K

(0.25 Btu/lb0 F), and the symbol c is used to denote the specific heat of

the gas/particle mixture. More precise specific heats for individual

components may be used if desired; the computer program calculates the

specific heat of the mixture from the predicted composition.

The combined mass flow up the freeboard is taken to be the mass

flow of gases leaving the bed surface (a fraction 1 - C of the coal having

been burnt) plus the elutriated bed material, as calculated by the

procedure described in Section 9. The gas is assumed to leave the bed

surface at the mean bed temperature. The heat released by combustion in

the freeboard is calculated as in the preceding paragraphs; it must be

remembered that Cf is expressed as a fraction of the potential heat input

to the combustor.

Where convective heat transfer surface is to be used for gas

cooling, conventional procedures for calculating heat transfer rates may be

used, the solids content of the gas stream being ignored. There will also

be a radiative component, and this can result in some cooling of the

freeboard gases before they reach the tube bank. Calculation of the

radiative component is an extremely uncertain procedure, and it will depend

on the general dimensions of the freeboard. For a rough calculation, an

effective gas/solids emissivity of 0.7 can be assumed. In some designs

radiation upwards to the freeboard exit may be significant and should be

included in the heat loss terms. The heat removed from the freeboard

should be calculated at the design freeboard temperature or at 1123 K

(2021 OR) as appropriate.

Heat transfer from the whole freeboard to the bed should be

estimated by using Figure 4.3 which is based on the analysis of reference

(4.10).

The effect of freeboard height on combustibles loss is shown in

Figures 4.4 and 4.5. Figure 4.4 illustrates the increase in losses which

Issue 002 Section 4

Page 28 of 96Copy Not

will result from drastic cooling at a given height in the freeboard; (at

6600C (1220°F) the reaction rate is only a tenth of that at 8500 C

(1560°F)). It is seen from Figure 4.4 that considerable reaction occurs

just above the bed surface, when the freeboard temperature is 900°C

(1650°F) or above. Except when C is small this causes the off-gases to

heat up rapidly due to the combustion of elutriated and splashing carbon,

and because of this, unless the freeboard is strongly cooled, a high

elutriation rate of carbon is often compensated for by high levels of

freeboard combustion.

Figure 4.5 shows that variations in the fluidising velocity also

have a significant effect on freeboard combustion. An increase in

fluidising velocity from 1 - 3 mls (3.3 - 9.8 ft/s) requires that the

height of the freeboard be increased by about 1.8 times to give similar

values of C /Cto0

When there is substantial freeboard cooling at a certain height,

this has two effects: firstly, by cooling the gases it reduces the rate of

combustion at all higher points; secondly, part of the heat removed by the

cooling will not be available in the bed. The fraction, 6, of the heat

present at a given height in the freeboard which is returned to the bed is

shown in Figure 4.6. It follows that removal of heat at this given height

is equivalent to removal from the bed of 6 times the heat removed. Thus

freeboard cooling, especially if it is positioned not far above the bed

surface, reduces the heat available in the bed.

It is important that excessive cooling surface is not installed

in the freeboard as incomplete combustion may then occur with excessive

carbon monoxide production. In extreme cases the combustion can be

quenched by freeboard cooling.

4.3.2.2 Stepwise calculation of freeboard combustion

This procedure is recommended when it is important that a

critical temperature is not exceeded at any height in the freeboard, and/or

Issue 002 Section 4

Page 29 of 96

Copy No..t.

100 :~ '"-1t - 1:':

10

i - ~ g :! Dj i m ! R 3 X :,' ii, ,il ~ti ~ . i ::' !!i, : -''. ::i~:!,:!'1 ~!i:.i:i ii"

..... ... .. .... :. .. ..70 +: ...: . . .:.... .... ;t:: :* L" , ..... . ii.....

Heat Transfer .................. .: . .. .:

% of Total Heat

Freeboard t . ! ,,,W g !:! :F i ,'':i :: ~::1:: i::'~

+1:: : : ~... .........

40 .L0" .... E 4 - = X::;

30 4

3ii0 0.5 i 1.5 2 2.i 5 3 3.5 4i

Variation of Heat Transfer from Freeboard to Bed with Freeboard Height and.9 l : : :::::::::Fluidising Velocity

Variatio of Hea Transfr from rueboardto Bed ith Freboard Hight au~~~~~~~~~~~~Fdisin VelocitO~~~~~~~~i: '" : ~iiiii

Issue 002 Section 4

Page 30 of 96

Copy No. ".

90

80 i I'

70

* Heat Transferi ::to Bed

60

% of Total HeatRelease inFreeboard

50

Fluidisin g Velocity40 E~aaaFq aKRR iaaBaiaffF iijl i ii#iiiB Velocity

0 2 4 6 8 10 12 14

Issue 002 Section 4

Page 31 of 96

Copv No.

Test conditions

Low rank bituminous coal CO= 0.3

Fluidising velocity 2 m/s Excess air 30%1.0

Freeboard DCtemperature

0.8

700

0.6

Ct

0.4

0.2

1100 1000

0 1 2 3 4

Height above bed m

Figure 4.4 (SI Units)

Effect of Freeboard Temperature on Freeboard Combustion

Issue 002 Section 4

Page 32 of 96

Copy No.H !

Test conditions

Low rank bituminous coal CO = 0.3

Fluidising velocity 6.5 ft/s. Excess air 30%1.0

CZ4 i! !:::: t,i H1 !Freeboard temperature

0.6

0.4

Figure 4.4 (British Units)

0.6

2 1::; i. iiiiiiiii i i~~fiiiw:h E;~~~~ii~

iii~~iiiliiiiliiiiiiiiji iiiiiiiiifiiiiiii sti#... ... . ... ~ ~ ~ ~ ~~ii' iiii ii: ii. Eilsfii 1~!O... ... . ...~~~iii "

Effect of Freeboard T6mperature on Freeboard Combustio n

Issue 002 Section 4

Page 33 of 96

Copy No.'- I

Test conditions

Low rank bituminous coal CO = 0.3

Freeboard temperature 900 GC Excess air 3096

1.0

0.8

0.6

Ct

CO

0.4

Fluidisingvelocity m/s

0.2

0.50O I II

0 1 2 3 4

Height above bed' m

Figure 4.5 (SI Units)

Effect of Fluidising Velocity on Freeboard Combustion

Issue 002 Section 4

Page 34 of 96

G Copy No',i

Test Conditions

Low rank bituminous coal CO= 0.3

Freeboard temperature 1650 OF Excess air 30%1.0

0.8 3 . ...

0:;: ::::i::::

--. 2 S X 0 W X W m iLS , Fluidising 7 m - *

0.6 4 X j

0.4 6 8 10 12

Height above bed ft

Figure 4.5 (British Units)

Effect of Fluidising Velocity on Freeboard Combustion

0.i~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~t;i

Issue 002 Section 4

Page 35 of 96

Copy No.!

when it is wished to position the freeboard cooling so as to ensure maximum

burn-out of elutriated carbon without quenching the combustion by

overcooling too soon.

The freeboard is divided into an arbitrary number of slices, and

each is treated as if it were a complete freeboard. For most calculations

5 equal slices will be found to be a suitable assumption. Starting at the

lowest, the amount of combustion and a heat balance are calculated for each

slice in order to find the gas exit temperature. The exit temperature and

unburnt carbon fraction are then used as the basis for calculations in the

next slice. For the lowest slice, the gas temperature is taken as that of

the bed.

Terms entering into the heat balance are:

Q1, = rate of heat generation by combustion in the slice,

Q2 rate of transfer of heat back to the bed,

Q3 = rate of heat loss for the slice without additional

cooling

Q4 = rate of transfer of heat due to additional cooling

surfaces in the slice

Q5 = rate of movement of sensible heat into the bottom of

the slice, and

Q6 rate of movement of sensible heat out of the top of the

slice

Q1 is evaluated as before, but it should be borne in mind that

the residence time, tf, is only a fraction of that for the whole freeboard.

Q2 is estimated by use of Figure 4.6, the proportion of heat

returned to the bed, 6, being evaluated at the mid-height of the slice;

then,

Q2= 6(Q1 Q3 Q4) ... ... ... ... ... ... 4.11

Issue 002 Section 4

Page 36 of 96

Copy No. !_

1.0 i i : lit

0.90. 1i i W .2 Fluidising velocity M

0,4

i*= D7iii~ S i X ;. i1: :

0.8

Wc: m g g i-' iwl I N gg R'1 1' 1 .1' 1' i! .3 1' 1i304

0.4

0.7

0

· 0 0.5 1 1.5 2 2.5 3 3.5 4

0P~~~ ~ ~~~~Figure 4.6 S Units)

?--, 0.5

CH .tih is er d - m Lf

F Si

Fhu Hreightabov bh ewould atwheich heAaistracted t- meL

Issue 002 Section 4

Page 37 of 96

Copv No.( .

'1.0 . ' - ' 71 :::-0,

Fluidising velocity ft/s f Ito ... S SiA > :.,, __. __ _-,. *.,..i~. ..... I ............... ~ ...... ~,....... _:_. · '- -,/n ''ff X

.; ' r r w W g X ii..l.. l. . 3 < X siS .S f i ,,, 1.......... . ...... .

3 t _ . ............. . . .-.......... .

0.

, .~-;. . . :4: :ti ; :::u::=:::::::: :::tjj: ::: :: : ....

o. ' S T 'i7. .. u ;W .. .~ ....... ..

...... +. ......I... :i: !;i: ;!;?i::;. .

Height above bedt whichheat ...is exr..... ..- .......

0.6 F"~t. . . ......... !.]!.! ........ ::1Fgr 4..6 . 4!:,.t

These curvec of be in Velocit y by th ................

~' 0.5 + ,,, .. ......... .... .

. l~ +r l :--: ::::-::::-:::::::::

0 2 4 6 8 10 12 14:::

Height above bed at which heat is extracted - ft if

,~~~~.·~~~~Figure 4.6 (British Units).

}'~~, HFreeboard which would otherwise have been Available in the Bed

Ij F r e eboard which would otherwise have been Available in the Bed

Issue 002 Section 4

Page 38 of 96

Copy No.( X

Q3 is calculated by standard methods and represents the heat loss

from the slice without additional cooling. Heat transfer to regions above

or below the slice is ignored. It is convenient to express Q3 as a

function of freeboard temperature for the particular operating conditions

and freeboard wall temperature of a given design. A linear approximation

to this function is suitable as input to the computer program.

Q5 and Q6 are calculated using the assumptions stated on page 27.

Q6 contains a term which may be unknown, the temperature at the top of the

slice, and is found from the heat balance equation:

Q6 = Q. + Q1 .Q2 .Q . ... ... ... ... ... ... 4.12

When equation 4.12 indicates that some critical temperature is

exceeded then equation 4.13, shown below, should be used.

=4 QQ -[ .. ... ... ... ' 4.13

Equation 4.13 gives the amount of extra cooling, Q4, which is

required to maintain that slice below the critical temperature.

The unburnt combustibles loss is found as C at the top of thet

last slice, and the heat returned to the bed is found by summing Q2 for

each slice.

4.4 Overbed Feeding

4.4.1 General considerations

Overbed firing has two particular advantages for fluidised

combustion. Firstly considerable simplification of the coal preparation

and feeding equipment, with resulting cost reduction, is possible if

relatively large coal particles can be fed directly by gravity to the top

of the combustor bed.

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The second advantage arises in connection with the adaption of

fluidised combustion to the retrofitting of medium scale industrial boilers

and heaters. If such equipment is to be competitive when converted to coal

firing then the fuel must be readily available and without any need for

further preparation on site. In the UK washed, graded coals such as

"singles" (coal with particles in the range 5 - 30 mm (0.2 - 1.2 in.))

fulfil this specification. The large particle size of such coals implies

the use of overbed feeding but it has been discovered that the fluid-like

properties of a bed of relatively dense material such as silica sand can be

used to "float" the larger particles of less dense coal and the fluidising

process then quickly disperses them uniformly over the bed. The use of

washed coals gives an additional advantage. Most of the ash in them is

inherent and is elutriated from the bed in the gas stream. The inert bed

of sand (or similar refractory) can then be easily maintained with periodic

topping up, although, even when firing washed coal, some large ash

particles tend to accumulate in the bed and it is necessary to remove these

periodically to ensure that the bed remains in a well fluidised state.

Mechanical and aerodynamic classifiers have been developed which withdraw a

stream of bed material, remove and reject oversize particles, and return

the sand to the bed. See Section 15.

The overbed feeding of washed, graded coals has been successfully

carried out in this way in many fluidised bed combustion industrial boiler

installations both in the UK and throughout the world. See, for example,

references (4.15-4.20) and (4.43-4.48). The slumped bed depths used are

typically in the range 0.1 - 0.3 m (4 - 12 in.) corresponding to fluidised

depths of 0.15 - 0.45 m (6 - 18 in.) because depths of this order can be

fitted more easily into existing shapes and sizes of combustion chamber, so

avoiding the necessity for positioning a pre-combustion chamber outside the

boiler when adapting existing designs of plant. A secondary attraction is

that the air delivery pressure can be much lower than for deeper beds, and

fan costs are thereby reduced.

The coals fited using overbed feeding include a wide range of. UK

washed coals with ash contents less than 10%. Washed "smalls" with a top

Issue 002 Section 4

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size of 12.5 mm (0.5 in.) have been burnt in beds 0.6 m (2 ft) deep using a

fluidising velocity of 2 mls (6.6 ft/s). Washed "smalls" sized 12.5 mm to

zero (0.5 - 0 in.), "singles" with particles in the range 5 - 30 mm

(0.2 - 1.2 in.) and "doubles" sized 30 - 50 mm (1.5-2 in.) have all been

burnt successfully in beds with static depths of 0.15-0.3 m (6-12 in.) at

fluidising velocities of 1.5-2.5 m/s (4.9-8.2 ft/s).

The overbed feeding method is not restricted to the feeding of

washed coals. The method has been used on the industrial scale

successfully for feeding coals with ash contents up to 18% and in research

equipment for low grade coals with ash contents up to 80%. Changes in the

design of the air distributor and ash removal system are required, however,

to handle the combustion of high ash coals. Once the ash content exceeds

about 15% sufficient ash particles are introduced to form the bed solids;

the start-up sand bed becomes replaced by an ash bed and continuous ash

removal and bed regrading becomes necessary. Various distributor designs

are available that are suitable for such duties; see Section 15.

Although the heating, devolatilisation and combustion of large

lumps of coal is slower thanrwith crushed coal (4.1), the evolution of

volatile matter in the vicinity of the coal feed point may be sufficiently

rapid to cause a deficiency of oxygen, particularly when the coal feed is

of an intermittent nature as might result from operation of a rotary valve

or ram feeder. In such cases, the delivery of a pulse of coal may be the

cause of a puff of dark smoke from the chimney unless an adequate supply of

air is available in the freeboard to ensure complete combustion of the

volatiles. This may be done by installing secondary air jets above the

bed. By increasing freeboard mixing these also generally improve the

burn-out of elutriated char particles. It is found that satisfactory

operation is obtained if excess air levels are in the range 20-30% and bed

areas up to 8.0 mi (86 ft 2) are supplied from a single source depending on

the coal feed arrangement.

The combustion rate depends on the surface area of the coal and

is lower, therefore, for larger coal particles. In consequence, above bed

feeding results in higher bed carbon contents than are found for the in-bed

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feeding of crushed coals. The bed carbon content can be as high as a few

percent, compared with about 0.1% in in-bed feeding of crushed coal. A

further result of the higher bed carbon content is that response times to

changes in coal feed rate are longer. During the overbed firing of a

"singles" coal time constants, t , of about 10 minutes are normal, compared

with 2 to 8 minutes when in-bed feeding. See also Section 13.1.5.

While the presence in the feed coal of fine particles, of a size

rapidly elutriated from the bed, does contribute to some extent to the

unburnt carbon in the gases leaving the combustor, a series of tests on a

vertical shell boiler, using a range of coal types and size distributions

has shown that coal rank (as indicated by oxygen content), bed temperature

and excess air level are the most important parameters governing combustion

efficiency (4.47).

4.4.2 Overbed feeding techniques

In overbed firing the coal feed is introduced into the combustor

very simply by gravity feeding through a vertical pipe or inclined chute

which may pass down through the freeboard or through the side of the

combustor. This feed tube may terminate at the combustor wall or be

extended to about 1 m (3.3 ft) above the expanded bed surface. Providing

the fluidising velocity is sufficient to provide vigorous bed particle

circulation and mixing, the location of the feed point is not critical.

Combustors have been operated successfully with a feed point at one end of

a rectangular bed (4.17) or offset from the centre of a circular bed

(4.15). Bed areas up to 4.0 m2 (43 ft2 ) have been fed from a single feed

point using a drop tube feeder.

Another possibility for overbed firing is to use a spreader

stoker or blast of air to distribute the coal over the bed. Beds up to

8.0 m (86 ft2) have been supplied from a single point by this means. It is

to be expected that distribution in this way of a coal containing a large

amount of fines would result in an unduly high unburnt carbon loss because

the fines would be more likely to become entrained by the off-gas than when

the coal is dropped in a compact mass into the bed.

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4.4.3 Estimation of combustion efficiency

No systematic investigation of the combustion taking place in the

bed and in the freeboard separately has been conducted for overbed feeding

of large coal or smalls. However, a series of tests has been carried out

with various coals on one vertical boiler (4.47), in which overall

combustion efficiencies were measured for a range of coal types and size

distributions. There is also a fair amount of test data for limited-range

operation of other industrial fluidised-bed boilers.

The most important controlling parameters have been found to be

the coal rank (indicated by the coal oxygen content), the excess air level

and the bed temperature. The dependance of the combustion efficiency on

each. of these is illustrated by Figures 4.7, 4.8 and 4.9. These Figures

show the variations in combustion efficiency found for one particular

industrial fluidised combustion installation (4.18, 4.47) and should be

considered as illustrative only, although the trends shown are likely to be

similar for other combustors.

Some work has been carried out on deriving a generalised

correlation for the combustion efficiency of bed and freeboard together

during overbed feeding (4.47). The correlating parameter used is the

fractional combustibles loss, (1 - n). The combustion efficiency as a

percentage is given by,

Combustion efficiency = 100 [1 - (1 - n). .. ... ... 4.14

4.4.3.1 Combustion efficiency correlation 1

The first correlating equation that has been derived is based on

just the three most important variables, bed temperature, excess air level

and coal reactivity (characterised by the coal oxygen content). The

equation is,

1-n x ( 0 - 1b- [) 0. 035 + exp (-3.7 + 0.17 0) (1 + ... 4

57 [0.008(Ox + 1) + 0.181

The constant, 0, is 1.0 for SI Units and 1.8 for British Units.

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Copy No.,i ')

Bed temperature OF

1300 1400 1500 1600 1700 1800100

Combustionefficiency

90

Test Conditions

No fines recycle.Excess air level 30%.

.... .......... ~~j i.....,~ !Fluidising velocity 1.8 m/s 15.9 ft/s).,+W : .:.,.,,::. - .. .........Static bed de th 160 mm (6.3 in.).

* 80

700 800 900 1000

Bed temperature °C

Figure 4.7

Typical Effects of Bed Temperature on Combustion Efficiencyfor Overbed Coal Feeding

Issue 002 Section 4

Page 44 of 96

B COPV No.

T eiciency

. ... ....

90 4 w

Combuo Obe t Conaediti n

I:~~m.. . . :;;;;;;...:..:;;; Bituminous coal, oxygen content 10% dmmf.i.. . . . ................ .. .. _ No finesrecycle.

!!T:~l:~q!!!.... ..3 !!'! !! Bed temperature 860 °C (1580 oF). ...?'}11~:. ._ 1~: : !! ...... _ l ? ii!!! Fluidising velocity 1.8 m/s (5.9 ft/s).

:H:[:[;................. _ _ _ _ Static bed depth 160 mm (6.3 in.).

-40 -20 0 20 40 60 80 100 120

Excess air level %

Figure 4.8

Typical Effects of Excess Air Level on Combustion Efficiencyfor Overbed Coal Feeding

Issue 002 Section 4

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Copy No.'

100

::. ii1ii_

Combustionefficiency ,

90 MMUMM KVWC o contenS Et :% :w: d i

forE9 IM1414:~ Test Conditions C Fie

... ~ No fines recycle.Bed temperature 860 oC (1580 OF).

M E ' ........... Excess air level 30%.~~ I~I~.0~~~~~~ Fluidising velocity 1.8 m/s (5.9 ft/s).

.,,, H t Static bed depth 160 mm (6.3 in.) .

80 , i Rt,0 2 4 6 8 10 12

Coal oxygen content % w/w dmmf basis

Figure 4.9

Typical Effects of Coal Reactivity on Combustion Efficiencyfor Overbed Coal Feeding

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4.4.3.2 Combustion efficiency correlation 2

When comparing predictions using correlation 1 with recent

experimental data a consistent small discrepancy was noted which has

subsequently been reconciled by the introduction of an expanded bed depth

term. While reformulating the correlation the equations have been modified

to enable combustion efficiencies of 98% to be predicted from high

reactivity fuels, under conditions of high excess air and high bed

temperature. This has been done to reconcile combustion efficiency

predictions and measurements for the operation of fluidised bed furnaces.

The correlation 2 equation is,

(1 - n)c exp [(8100 e)/T;]

(1 - n) = ... ....... ... 4.16

1273 [0.18 + 0.0064 (0 + 1) 2

The constant, E, is 1.0 for SI Units and 1.8 for British Units. The

parameter (1 - n) is found from,

1 - (1 - n)c = (1 + 0.01 Xc) [1 - 0.05 ... ... ... ... ... 4.17

and =- ... ... ... ... ... ... ... 4.180.05 (1 + 0.01 X )

Equation 4.17 has the same form as equation 4.4 for a Cj value of 0.05, and

may be solved similarly by trial and error, or by reference to Figure 4.2

or Table 4.1. In equation 4.17 the excess air is expressed as a corrected

value, X , which allows for the effects of the non-uniformity of coal

concentration in the bed and the rapid evolution of volatile matter close

to a coal feed point which may result in a local starvation of oxygen.

The corrected excess air level is calculated as follows.

For Lb4 650 mm (25.6 in.),

X = X - 0.00413 0 (650 - Lb) ... ... ... ... 4.19c X

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Copy No./

For Lb > 650 mm (25.6 in.),

X = Xc

The constant, 0, is 1000 for SI Units and 305 for British Units. If the

expanded bed depth, L, is unknown it can be estimated from the static bed

depth, L , using the following correlation.

L(1-= V.. ... ... ... ... ... ... ... 4.20(1 - V) (1 -VB)

where,

V = Uf Umf 4.21

B Uf Uf + 0.71 z + 4Ut

The minimum fluidising velocity, U can be calculated from

equation 3.1 or 3.2 in Section 3. The parameter z should be taken as the

horizontal gap between the in-bed heat transfer tubing. If no tubes are

present then z, represents the maximum bubble diameter. In that event

suitable arbitrary values for z are: for combustors with bed

cross-sectional areas up to 0.2, mn2 (2 ft2) - 50 mm (2 in.); for combustors

with bed cross-sectional areas' over 0.2 m2 (2 ft2) - 100 mm (4 in.). The

constant in equation 4.21, 0, equals 1000 for SI Units with z in mm and g

in mr/s2. For British Units with z in inches and g in ft/s2, e equals 12.

Equations 4.20 and 4.21 may be used to estimate Lb for most

materials used as bed solids. For 14/25 grade silica sand, which is

commonly used as bed solids in industrial boiler installations, the

following empirical correlation will give a more accurate estimate of

Lb (4.42).

For L >0.163 m (0.534 ft),

Lb = L (0.3 Uf + 1.12)

For 0.163 > L > 0.133 m (0.534 > L > 0.436 ft),S S

Lb =L (0.3 u + 1.88 -4.667 L 0)

where e = 1.0 for SI units and L in m and 0 = 0.305 for British units andS

L in ft.S

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For L < 0.133 m (Ls < 0.436 ft),

Lb = Ls (0-3 U f + 1.26)

For empirical correlations for other grades of sand and for

certain coal ashes reference (4.42) should be consulted.

4.4.3.3 Choice of correlation

Values of combustion efficiency predicted by the two correlations

are compared in Table 4.3 for typical operating conditions (excess air 25%,

fluidising velocity 2.2 m/s) of a boiler at the two extremes of coal

reactivity and at two expanded bed depths. The values predicted by

correlation 2 are higher. The correlation 2 values are supported by

experimental values for the high reactivity coal (10% oxygen) but

unfortunately no practical data is yet available for comparison from the

combustion of low reactivity fuels (1% oxygen) at excess air levels above

15%. It is recommended, therefore, that the more optimistic predictions of

correlation 2 for low reactivity fuels be treated with caution pending

experimental verification.

Table 4.3

Comparison of Predicted Combustion Etficienciesfor Overbed Feeding

Bed depth Coal oxygen content

Correlation 1% dmmf 10% dmmf

Static Expanded Bed temperature 'C

mm mm 900 950 900 950

1. Eq 4.15 Not bed depth 85.2 87.4 95.8 96.4dependent

2. Eq 4.16 150 267 87.8 90.8 96.8 97.6200 344 87.8 90.8 97.0 97.7

Note: Predicted efficiencies for excess air level of 25% and fluidising

velocity of 2.0 mis.

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4.5 Methods of Reducing Unburnt Carbon Loss

Maximum carbon utilisation is necessary for efficient and

competitive use of coal in fluidised bed combustors. In modern pulverised

coal fired water-tube boilers the combustion efficiency exceeds 99.5Z, and

it is probable that for this type of use, an efficiency much less than this

could not be tolerated. For the smaller, industrial boilers, however, a

lower combustion efficiency, down to say 97% could be accepted, and when

burning a very poor grade of fuel a loss of 5% could be tolerable.

Occasions can arise, therefore, when the combustion efficiencies of

fluidised bed combustion obtainable with once through operation may need to

be increased for economic operation. The causes and methods of reducing

carbon loss are reviewed in this section - the review of carbon loss

reduction given in reference (4.21) may also be consulted.

If the coal feed points are not too widely spaced, if adequate

gas mixing occurs in the freeboard, and if the excess air is not less than

10%, then the combustible losses in the form of carbon monoxide and unburnt

volatiles should be small, and the main concern is with elutriated unburnt

carbon.

The elutriated particles come from five different sources: (i)

the fines content ot the feed coal; (ii) thermal shattering of particles

when they are suddenly heated in the bed; (iii) reduction in coal particle

size due to combustion, causing the particles eventually to become small

enough to be elutriated; (iv) fine ash particles liberated by the burning

coal; and (v) breakage of coal and bed material due to the particle

jostling always encountered in fluidised beds. These sources of elutriable

material are all interdependent; an increase from one source may well

cause a decrease from another. Of particular interest are the carbonaceous

particles, and these come principally from sources (i), (ii), (iii) and

(v). Source (i) can be estimated directly from the coal size distribution,

and (iii) can be estimated indirectly by calculating the number of coal

particles per unit weight of feed, and thence finding the total weight of

these particles when they are just small enough to be elutriated. The

Issue 002 Section 4

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Copy No.,'>

rates of the other sources will depend on the coal hardness and resistance

to thermal shock.

The following courses may be considered for reducing the loss of

elutriated carbon particles.

1. Use of a higher bed temperature. This is limited to coals having

a higher ash fusion temperature than normal. Also with most

sulphur retention additives performance decreases above certain

bed temperatures. See Section 11.

2. Optimisation of combustor and freeboard design. Factors likely

to favour improved burn-out are: deeper freeboard; avoidance of

over-cooling of freeboard gases; use of baffles to facilitate

the return of particles to the bed; and use of a "flared"

freeboard to reduce gas velocities.

3. Recyling of fines collected by an internal or external cyclone.

4. Use of a separate bed to burn the carbon collected in a cyclone

collector.

5. Use of a lower fluidising velocity in the main bed with a smaller

coal size, at the expense of reduced heat release per unit area

of bed.

6. Increase in excess air level. The improvement in burn-out is

small above about 15% excess air with in bed feeding and 30%

excess air with over bed feeding, and there are other penalties:

reduced boiler efficiency, and excessive use of fan power. Scope

for improvement by this means is not great.

7. Reduction of fines content of coal feed.

Some of these measures will now be discussed in more detail.

Issue 002 Section 4

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Copy No.

4.5.1 Freeboard baffles

There is some evidence that the elutriation of unburnt carbon can

be reduced by positioning a baffle comprising one or more rows of tubes up

to about 0.5 m (19 in.) above the surface of the bed. It is likely that

this acts in two ways; firstly, by physically intercepting particles

ejected from the bed when bubbles burst, and causing them to fall back into

the bed and, secondly, by smoothing the gas velocity profile. It is

important, however, that if the baffle is to be cooled, the cooling must

not chill the freeboard gas to a point where combustion is unduly retarded.

4.5.2 Combustion of elutriated carbon in a separate bed

The material elutriated from a fluidised combustor fired with a

fines-containing small coal (for example with a top size of 1.7 mm

(0.067 in.)) consists of a mixture of ash (and additive, if used) and

incompletely burnt coal particles. Section 9 explains how the rate of

elutriation may be calculated, given a knowledge of the friability of the

ash (and additive). It should be noted here that particles of inert bed

material and additive have particle densities ranging from about 1800 to

2900 kg/m 3 (110-180 lb/ft3 ). Carbonaceous particles on the other hand, are

generally less dense at 1500 kg/m 3 (90 lb/ft3 ) or less. The top size of

carbonaceous particles elutriated will therefore be larger than the top

size of bed material elutriated, but otherwise the same principles apply.

The factors governing the distribution of carbon with particle

size of the elutriated fines are complex, and include the following:

(a) Combustion factors: The larger the particles the longer they are

likely to stay in the bed before they are elutriated. Hence they

have a greater chance of being burnt or broken into small

particles before they are elutriated. Very small particles burn

quickly, but may not meet sufficient oxygen during the short time

they are in the bed to allow them to burn completely. Therefore,

the particles leaving the bed surface will include many

Issue 002 Section 4

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Copy No.

carbonaceous particles which are very small. In the freeboard,

the finer particles burn away more quickly, but they spend a

shorter time in the freeboard because of their lower terminal

velocity (causing less `slip` through the rising gas stream).

All particles smaller than a certain size, (depending on particle

residence time, freeboard combustion rate constant and oxygen

concentration) are completely burnt, while a smaller number of

particles will be burnt down to this size from larger sizes. At

low or negative excess air values, the carbon content of the

finest size fractions is significantly increased.

(b) Elutriation of inert material: This affects the concentration of

carbon in different size fractions, but not the distribution of

the carbon with particle size. The net effect will depend on

whether or not fines recycle is practised. In the absence of

recycle, the elutriation rate of inert fines is the sum of the

rate of feeding fine mineral particles in the coal (and any

additive), plus the rate of production of fines due to thermal

effects or abrasion in the bed. When fines are being recycled to

the bed, the inert mineral content of the elutriated material is

much greater. Most of the additional material will be in the

form of particles larger than the cyclone "cut" size, but some of

the recycled particles will be broken during passage through the

recycle system and the bed. The net effect, however, is to

reduce the concentration of carbon in the coarser elutriated

fractions.

By way of illustration, for a primary cyclone designed to "cut"

at 10 pm (0.0004 in.) an average carbon content of primary

cyclone fines, as collected for refiring, is about 20%, roughly

half of the carbon being contained in particles smaller than

40 pm (0.0016 in.) for fluidising velocities of 0.6 - 2.4 m/s

(2 - 7.9 ft/s) and normal excess air levels in a combustor with a

3.7 m (12 ft) uncooled freeboard and using in-bed feeding. In

many industrial installations with overbed feeding the carbon

Issue 002 Section 4

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Copy No.q'

content of the elutriated material is in the range 30-50% with

about 20% of the chimney dusts containing carbon particles less

than 10 pm (0.0004 in.).

The efficient combustion of such fine carbon in a separate

burn-up bed usually requires either a very low fluidising velocity, to

increase residence time, or a very high bed temperature, to increase

reaction rate. The chief advantage of using a separate bed is that such

conditions can be obtained independently of the conditions in the main

combustor. The disadvantages of adopting a separate fines burn-out bed

are:

(a) The need to provide additional bed area and increased air

requirements.

(b) The probability that the heat release will be inadequate to

allow a worthwhile amount of heat transfer in the bed.

(c) Possible difficulties of control resulting from fluctuating

carbon input; it might prove necessary to provide a

supplementary fuel supply for control purposes.

4.5.2.1 Carbon burn-up cell design with in-bed feeding

The following stepwise procedure is proposed for the design of a

fines combustion bed using in-bed feeding. The procedure takes account of

the fact that the size distribution of the combustible matter is

considerably finer than that of coal normally fired at the same fluidising

velocity.

1. Calculate the combustible loss, CO which would be expected from

the surface of a bed at the same operating conditions, but fired

with coal.

2. Multiply this value by the factor [3(1 - C ) + CO = to obtain the

Issue 002 Section 4

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Copy No.&

combustible loss from the surface of the fines combustion bed,

C', i.e.

C' = C [3(1 - C ) + C 2. ... ... ... ... ... 4.220 0 0 0

3. Calculate the combustion in the freeboard and heat transfer to

the bed as in Section 4.3.1.2.

The procedure predicts that fines combustion is likely to be

extremely inefficient at fluidising velocities in excess of 2 mts, at

normal bed temperatures.

4.5.3 Refiring of fines to the main bed

It has been found possible to improve combustion efficiency by

refiring collected fines to the main combustion bed, though the improvement

obtained has varied widely with the design of the recycle system and with

the operating conditions. External cyclones are recommended and the fines

are fed using the overbed feeding method. Table 4.4 summarises the results

obtained during experiments using in-bed feeding. Limited data suggests

that similar improvements in combustion efficiency are possible when above

bed feeding is used (4.48).

Refiring or recycling of fines has three advantages over

combustion in a separate bed:

(a) It does not necessarily increase the required bed area or

combustion airflow.

(b) In cases where maintenance of the required bed depth is

difficult refiring of fines will help, as discussed in

Section 3.1. It is will also increase the heat transfer

coefficients to tubes in the bed because of the reduction in

mean bed particle size. A further consequence is that the

increased dust burden of the freeboard gases will improve

Issue

002 S

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Page

55 of

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Issue 002 Section 4

Page 56 of 96

Copy No. '

the emissivity and heat capacity of the gases and therefore

improve heat transfer rates in the freeboard. However, a

disadvantage is that the load on the gas cleaning equipment

will be increased.

(c) Where an additive is used to retain sulphur, the retention

efficiency may be improved by recycling solids.

The gains in combustion efficiency that can be obtained by

refiring fines to the main bed are limited by many factors, including:

(a) The maximum recycle rate is limited by the efficiency of the

dust collection equipment. The particles passing

uncollected through this equipment can contain substantial

amounts of unburnt carbon.

(b) Where elutriated solids are cooled before being collected

and refired, a limit on recycling would be imposed by the

acceptable amount of heat removed from the bed at the

expense of heat transfer in the bed. It would also be

necessary to dispense with freeboard cooling.

(c) Collection and recycling of large amounts of fine

particulate material may often present mechanical and

control problems, thus limiting the recycle ratio that can

be used.

There are strong attractions in fines recycling, although when a

very high combustion efficiency is required a separate burn-up cell may be

necessary.

4.5.3.1 Combustion loss estimation for fines recycle with in-bed feeding

When the in-bed feeding method is used the calculation procedure

outlined in Section 4.3 can be modified to account for fines recycle.

Issue 002 Section 4

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A rough estimation of the reduction in unburnt carbon heat loss

which can be obtained through fines recycling may be made by referring to

Figure 4.10. Detailed calculation of combustion losses with fines recycle

are carried out using the following stepwise procedure.

1. Calculate the combustible loss from the bed surface resulting

from incomplete combustion of freshly fired coal, C , using

steps 1-3 of Section 4.3.1.1.

2. Calculate the combustible loss in the off-gas, Ct, as for a

combustor without fines recycle, using Section 4.3.1.2 and 4.3.2.

3. Derive the probable size distribution of the unburnt carbon in

the elutriated particles from Figure 4.11, and apply the cyclone

grade efficiency to calculate X, the proportion of the carbon

leaving the bed that is collected by the recycle cyclone. It

should be noted that, whereas the size distribution of the total

fines changes considerably when recycling is practised (becoming

much finer), the size distribution of the carbon changes much

less, because a large fraction of this burns away during each

passage through the combustor. X may therefore be assumed to

remain constant through the calculations.

4. The rate of carbon refiring to the bed, C , is calculated by

multiplying X by Ct and by the fraction of the collected fines

which are to be refired to the bed. The quantity of carbon

rejected in the case of partial fines recycle can also be found.

5. Calculate X , the effective excess air, from:e

(100 + X)X = - 100 ... ... ... ... ... ... 4.23e (1 + Cr)

6. Calculate the combustible loss from the surface of a fines

combustion bed at the operating conditions of the main bed and

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Copy No.k

mo~ S.0a

oQ cli

oloomD ;3C

=

Figure 4.10

Estimation of the Reduction in Unburnt Carbon LossObtainable by Recycling Fines

Using In-bed Feeding

#r#t~~~~~mttmt~~~~~mhKh~~~~iittiWKf~~~~~~~~~mtittt~~~~~~KRmAKKR ~ ~ ~ 0

'# %~~~~~~~~~~~~~~~~~~~~~

i~~-t't~tFR4fAFRfFF~PFm;FmRsRFFF;·F~ Y,~

iiFf;~~~~fFi~~~Rm~~dfftmftffKK-·;#K~~~~~~~~RtX#~~~#t#####f ~ ~ C

c~~~~~~~~~~~~~~~~~~~~-*~~~~~~~E;FiEff-;rIu!~· s nr o ~~~~~~~~~~~~~~~~~~~~~~n o~~~~~~-

C~~~~~~~Otial -y 0eyln i

o~~~~~~~~sigI-e edn

Issue 002 Section 4

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excess air level Xe, by the method given in Section 4.5.2, steps

1 and 2.

7. Multiply C by this proportional loss from the surface of a fines

combustion bed to calculate C f' the quantity of recycled fines

leaving the bed surface after one recycle, as a fraction of the

carbon feed-rate with fresh coal.

8. Add Cof to C c to obtain C , the total loss of combustibles from

the bed surface after one recycle.

9. Calculate the total loss of carbon in the fines at the top of the

freeboard, Ct, as in Section 4.3.1, but using the excess air

level Xe

10. Repeat steps 4 to 9, using this new value of Ct, until the values

of parameters C , Ct and Cr converge to constant values.

11. Calculate the total carbon loss, namely the carbon fines passing

through the cyclone plus those rejected if a partial recycle

system is used.

12. Calculate the amount of combustion occurring in the freeboard, Cf

from,

C = C Ct

Because of the tedious nature of these repetitive calculations it

is recommended that they be performed on a computer. The computer program

(4.8) can be used to calculate combustion alone, or in conjunction with

other design aspects such as elutriation and abrasion of ash, tube-bank,

etc.

In addition'to the recycled carbon there will also be a large ash

recycle rate, the magnitude of which will depend on the ash abrasion

properties. This recycle rate should be calculated as in Section 9, and

_Issue 002 Section 4

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E

S t n

- | +XnE

Figure 4.11 (SI Units)

Size Distribution of Unburnt Carbon Fines

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Coypv No. , ,K_

' .

! u.

Figure 4.11 (British Units)

Size Distribution of Unburnt Carbon Fines

2°tSot 19g 2 ] 1 lb tSllZ#I t~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~a

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will to a large extent determine the design of the recycle systei for a

required improvement in combustion efticiency.

In order to design a fines recycling system for given

coal/combustion/operating parameters it is recommended that the above

procedure be applied for a range of different recycling rates, and the

amount of ash/additive recycle at each rate be also calculated. It will

then be possible to select an optimum recycle rate.

4.5.4 Reduction in fines content of feed

For in-bed feeding there is no apparent relationship between

overall (bed and freeboard) combustion efficiency and the fines content of

the feed. This may be because a lower bed combustion efficiency is, in

most instances, compensated by an increase in freeboard combustion.

For over-bed feeding, coals with appreciable fines content have

in some instances given significantly lower overall combustion

efficiencies, but more recent work has shown the coal size distribution to

be much less important than coal reactivity in determining overall

combustion efficiency.

4.5.5 Operation at higher temperatures or higher excess air levels

As discussed in Section 3, constraints imposed by such factors as

ash softening temperature and overall thermal efficiency and fan power

requirements limit to a large extent the freedom of choice in respect of

bed temperature and excess air. Nevertheless, there may be occasions when

a significant increase in combustion efficiency is possible by a moderate

increase in one or both of these operating parameters.

When firing unreactive fuels such as anthracite, the high.

elutriation rates of unburnt carbon result from the low specific reaction

rate of the fuel in the bed and the reduced burn-out rate of char in the

freeboard. This may be to some extent countered by operating at higher bed

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and freeboard temperatures, because the chemical reaction rate of the coal

increases with temperature according to the well-known Arrhenius

relationship although the mass transfer component of the overall combustion

process is little affected by changes in temperature. The use of higher

combustion temperatures is generally only practicable if the coal ash has

an unusually high fusion temperature. Where it has been possible to

operate at, say 1000°C instead of 900%C, (1830°F instead of 1650°F), marked

reductions in unburnt carbon loss have been found.

Up to a certain level of excess air (depending on the fuel type

and operating conditions), the combustion efficiency is increased by

increasing the amount of excess air. This critical level is about 15% for

crushed coal firing in static beds not less than 0.6 m deep, but it may be

as high as 30 or 40% for some shallow bed units in which uncrushed coals

are fed over-bed.

4.6 Assessment of Coals for Suitability in Fluidised Combustors

Although the procedure described above is adequate for the design

of combustion systems for well-characterised fuels, the need sometimes

arises for assessment of fuels of unusual properties. A number of test

facilities which may be used to obtain design data for use with such fuels

are described in Section 17.

4.7 Effect of Coal Characteristics on Combustor Design and Operation

4.7.1 General effects

A number of coal properties have- effects on the operation of

fluidised combustors, although as remarked earlier (Section 2.1) fluidised

combustion systems are more versatile as regards fuel properties than are

most conventional systems. Coal properties that may affect the operation

and design requirements are discussed in this Sub-Section.

Changes in some coal properties can affect the combustion process

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itself; e.g. coal reactivity. Equally importantly, however, are the

changes that can occur in the combustor heat balance and the bed

fluidisation characteristics through changes in coal properties.

A fluidised bed combustor is designed to have a precise heat

balance in the bed at MCR conditions so that the design bed temperature

remains constant at the design values of excess air, coal flow and

fluidising velocity. To obtain the necessary heat balance the following

conditions must be met.

1. A specified heat release must occur in the bed for specified

fluidising velocity. This implies certain design values for

bed temperature, excess air, coal flow and coal quality.

2. A fixed ratio must be maintained between the heat removed

from the bed as sensible heat in the gases and that removed

by heat transferred to immersed surfaces (cooled walls and

in-bed tubes).

3. The quality of fluidisation must be satisfactory at all

operating conditions.

If the fuel properties are altered by the substitution of one

coal for another then concomitant changes may occur in one or more of the

above conditions which may result in a bed MCR heat balance occurring at

conditions of bed temperature, fluidising velocity and excess air level

that differ from the design values. Some latitude in operating conditions

can normally be accommodated but extreme changes in fuel properties can

affect the combustor rating.

Some examples may clarify the above argument. For instance, an

increase in coal moisture content will alter condition 2. The combustor

will now have too much immersed surface heat transfer and a heat balance

will be obtained at a bed temperature less than the design value. Again, a

reduction in coal reactivity will reduce the combustion efficiency and

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hence the heat release, thereby changing condition 1. Increases in coal

ash content can produce several effects. An increase in the coal flow will

be needed to maintain condition 1. Bed cleaning methods may need

altefation to maintain condition 3. With ash contents above about 15% the

ash particles replace the sand as the bed solids. The flat plate type of

air distributor may no longer be suitable and continuous ash removal and

bed cleaning may become necessary. The fluidisation characteristics of

beds of coal ash are somewhat different from those of some beds. In

particular, the bed expansion characteristics are different, so condition 2

becomes changed. This is important if the bed expansion method of turndown

(see Section 4.10) is used.

The effects of particular coal characteristics are considered in

more detail below.

4.7.2 Moisture content

Moisture has no significant effect on the combustion process

itself but an increase in the moisture content replaces the coal calorific

value and alters condition 1 above.

Coals containing substantial proportions of fine particles are

often extremely difficult to handle if the free moisture content exceeds

the inherent moisture content and this must be taken into account when

designing the coal supply and feeding systems. The types of difficulty

likely to be encountered are: adhesion of fine coal to metal surfaces;

failure to flow down chutes and from bunkers; blockage of conveying

systems, etc.

On the other hand, coals with a high fines content should not be

too dry as the moisture has been found to stick the fines together so that

they remain in the combustor for long enough to be burnt and not instantly

elutriated. An optimum moisture content can be described by the phrase

"moist but not wet" and often corresponds to a free moisture content in the

range 5-7%.

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4.7.3 Swelling number

Certain coals, especially those in C.R.C. groups 200 to 600

exhibit swelling and caking on heating. The main design consideration is

to ensure that swelling and caking cannot occur while the coal is being

conveyed into the bed, thus causing blockage of the feed line.

4.7.4 Coal rank

It is known (4.13) that the chars derived from high rank coals,

such as anthracite and semi-anthracite, are less reactive than those

derived from low-rank coals. This factor is taken into account in the

combustion calculations of Section 4.3 by applying correction factors to

the combustion losses from the bed and freeboard.

Some comparisons of anthracite combustion with that of low-rank

coals have been obtained for over-bed firing of large coal (4.16, 4.18).

Using the coal oxygen content as an indicator of coal rank the results

obtained have been used to develop the correlation for combustion

efficiency given in Section 4.4.3. As the coal oxygen content is reduced

the coals become progressively less reactive and, if all other operating

conditions are kept constant, the combustion efficiency is decreased in the

manner illustrated by Figure 4.9.

Similarly a lignite has been burned with a combustion efficiency

of 99% compared with a 93% efficiency for a bituminous coal of similar ash

content fired at similar operating conditions (4.21). This increase in

efficiency is attributable to the higher reactivity of lignite.

Coal rank also has a marked influence on the temperature to which

a fluidised bed must be raised during start-up before self-sustaining coal

combustion begins. This temperature can vary from 400°C (7500 F) when

lignite is the fuel to 700°C (1290°F) for anthracite fuels or 800C

(1470°F) for petroleum coke. A simple laboratory test, described in

Section 17, has been developed to assess the reactivity of a given fuel.

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The test results and a knowledge of the heat losses expected from the

combustor during start-up can then be used with a computer program to

obtain an estimate of the fuel ignition temperature in the given combustor.

4.7.5 Ash content

The different ways in which the mineral matter content may be

distributed have been described in Section 4.1. Discrete ash usually

originates from the mining of some of the rock above and below the coal

seam along with the coal itself, and from pockets of mineral matter

included in the coal seam. Inherent ash arises from mineral matter

intimately mixed with the coal as it lies in the seam, and cannot be left

behind however carefully the coal is cut away. Whereas discrete ash can

usually be removed by coal preparation procedures due to its different

density, inherent ash cannot be removed. A washed coal therefore contains

inherent ash, but very little discrete ash. Coal may also contain

particles consisting of separate layers of coal and stone.

4.7.5.1 Effect of ash content on combustion

There is no evidence that discrete ash exerts any effect upon the

combustion process, other than as an inert substance. Heat gained by the

ash, and carried out of the combustor with it represents an overall loss of

efficiency, and this could reach undesirable proportions for a very high

ash coal. A high ash throughput will also tend to sweep out more carbon

from the bed. Combustion of a high sulphur coal with large amounts of

limestone additive will result in a similar state of affairs.

The position with regard to coal -containing a large amount of

inherent ash is much less certain, however. There is some evidence to

suggest that freeboard combustion may be hindered by the skeleton of ash

built up around a burning particle as the carbon content burns away. It is

also possible that somie coals with a high inherent ash content suffer more

size degradation in the bed than do coals of low inherent ash content.

Whether these are the true causes or not, carbon losses of two to three

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times the expected levels have been encountered when burning a very high

ash coal, (60%, dry basis). A bituminous coal of 30 - 40% ash content

could only be burnt with 93% efficiency on a CRE test rig which normally

gives 96 - 97% efficiency when burning lower ash bituminous coals under the

same conditions. (4.21).

Another direction in which the ash content affects the design and

operation of fluidised bed combustion systems is by imposing an upper limit

on the bed and freeboard temperatures. The mean bed temperature and the

freeboard gas temperature should not be allowed to exceed a temperature

about 220°C (400=F) lower than the ash sintering temperature. This is

because the temperature of individual burning particles can exceed the

surrounding temperature by this amount.

4.7.5.2 Effect of ash content on distributor design

The choice of air distributor type depends on both the feeding

method and the coal ash content. The distributor type used normally is a

flat plate with standpipe air nozzles and occasional bed drain ports. As

the ash content of the coal increases it becomes increasingly difficult

with a flat plate type distributor to ensure that oversize particles are

removed from the bed. Eventually an ash content is reached at which layers

of oversize material build up at the base of the bed and cause a loss of

good fluidisation. Such a limiting ash content varies with the top size of

the coal fed. Figure 4.12 gives an indication of the limits of operation

of the flat plate type of distributor.

If the coal is crushed, as is the case when the in-bed feeding

method is used, Figure 4.12 shows the coals with a top size of 6 mm

(0.25 in.) and ash content up to about 40% can be handled using the flat

plate distributor. On the other hand, an ash content of about 10% is a

limiting value for the overbed firing of coals with a top size of 25 mm

(1 in.).

Other types of distributor are available which are more suited

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for the firing of unwashed and high ash coals using the overbed feeding

method. A flat plate distributor with directed air nozzles in which the

air jets are arranged to sweep oversize ash particles towards the bed drain

ports (see Section 15) has been used successfully to handle ash contents up

to 25% (4.43). Coals with still higher ash contents require a bed

recirculation type of distributor which has a sloping base that induces the

bed solids to circulate towards ash offtake points. Ash removal can be

further improved by adding an additional air spout to help refluidise any

segregating ash. Spouting distributors of this type with a central spout

have been used successfully to handle low grade coals with ash contents up

to 807. Such fuels are more in the category of waste fuels and Section 7

should be consulted as well as Section 15.

4.7.5.3 Coal ash as a bed material

The ash formed on combustion can often form satisfactory bed

solids. It is necessary to evaluate the ash of any particular application

at the design stage because the fluidisation and bed expansion

characteristics of beds of sand and coal ash are sufficiently different to

influence the operation of the bed expansion method of turndown. An

estimating procedure is given in Section 4.8 below and Sections 3, 13 and

4.10 may also be consulted on turndown methods.

In general, inherent mineral matter does not result in large or

strong enough ash particles to form a durable bed material. Discrete

mineral matter can form such particles, but it cannot be taken for granted

that it will do so. Particles consisting of part stone and part coal will

burn away to leave solid ash particles with a size distribution ranging

from that of the original particles down to zero. Some particles of

mineral matter may undergo physical and/or chemical changes that cause

breakage, decrepitation, etc.

The ability: of the mineral matter to form particles suitable for

use as bed solids is not directly linked to the ash content of the feed

coal. As a generalisation, it is likely once the ash content exceeds 15%

Issue 002 Section 4

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-250

Coal40 Top

Size

30-O Coal Flat Plate Air DistributorTop Size with Occasional Bed Drain Ports. I

mm Fluidising Velocities Down to lm/s

20

in.

10

for Good Fudsto

a o0 10 20 30 40 50 60 70 80

% Ash in Coal

0

Figure 4.12

Tolerance of Flat Plate Air Distributor to Coal Sizeand Ash Content

Issue 002 Section 4

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that large ash particles will tend to accumulate in the bed. If the coal

feed is crushed such particles of large ash may be suitably sized to act as

bed solids. If uncrushed, unwashed coals are fed then much of the ash may

be oversize and require removal. Even washed coals fed using the above bed

method will contain the occasional large particle so equipment for at least

intermittent bed cleaning is always advisable.

4.8 Estimation of the Initial Ash Size Distribution Produced by

the Combustion of Coal

For prediction of the ability of a coal ash to maintain a

fluidised combustion bed it is strongly recommended that the test for this

purpose described in Section 17.3 be used. In the absence of the

information given by this test, an estimate of the initial size

distribution of the ash from the coal (i.e. ignoring any breakage due to

the action ot the fluidised bed) may be made in the following way. This

procedure only applies to crushed unwashed coals.

1. Estimate the percentage of the feed coal in each of the following

size ranges; (sizes are given in rim):

Fraction No. 1 2 3 4 5 6 7 8Size Range -3175 -1680 -1000 -500 -250 -125 -75

+3175 +1680 +1000 + 500 +250 +125 + 75

These size fractions, from left to right, will be referred to as

fractions 1 to 8, as shown, and the percentage of the coal in the ith

fraction is defined as c..

2. Estimate the percentage of the total ash occurring in each size

fraction as follows.

If the preparation system crushes the stone particles to the same

distribution as the 'pure coal' particles, then the ash content of all

eight size fractions will be equal to the overall ash content of the

feed. Thus the percentage of the total ash occurring in each size

fraction, pi, will be equal to the percentage feed coal in the

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fraction, i.e. for this special case p, = ci. This has been found to

be approximately the case for coal prepared in a disc attrition mill.

However, a hammer mill has been found to produce more fine stone than

fine 'coal', giving an ash content in the finest size fraction about

double the mean for the feed coal. For the hammer mill, the ash

contents of the eight size fractions may be taken to have the

following relative values:

1, 1, 1, 1, 1, 1.2, 1.5, 2.5

If the relative proportion of ash in the ith size fraction is q., the

percentage of the total ash occurring in the ith size fraction of feed

is calculated from

1Iciq iI

P. ... .. ... ... ... ...*- - ... ... 4.24

i=8

Z (ci qi)i= 1

Ideally the values of q should be obtained from a test using the

proposed type. of coal preparation equipment. It would not be

necessary to prepare feed of the specified size distribution, and the

values of qi obtained in a single test could be assumed to apply to

any size distribution prepared using the same type of preparation

system. It is clearly undesirable for the preparation system to

produce a high proportion of fine stone particles since this would

adversely affect both bed maintenance and particulate emission.

3. Estimate the size distribution of ash produced from each coal

feed size fraction as follows:

The size distribution of ash produced from a close size fraction

of coal feed will depend on the relative quantities derived from stone

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particles, stone/'coal' particles and 'coal' particles. In general a

feed of high ash content will have a high stone content, and hence a

high proportion of the ash produced from a size fraction of coal will

be in the same size fraction. Tables 4.5 and 4.6 give the size

distribution of ash produced from each of the eight size fractions of

coal for (a) Babbington coal, 35% ash, and (b) Newstead coal, 17% ash.

It can be seen in Table 4.5 that, for the high ash coal, about 90% of

the ash from each size fraction of coal is within the size fraction.

The proportion for Newstead coal, Table 4.6, is only about 60%. The

values in these tables are thought to be independent of the size

distribution to which the coal is prepared.

In the absence of other data, Tables 4.5 and 4.6.should be taken

to be typical of coals having ash contents of 35% and 17%

respectively. For coals with other ash contents, similar tables

should be constructed by interpolation or extrapolation from these

values. The computer program described in reference (4.8) carries out

these interpolations and extrapolations automatically in the absence

of information on the size distribution of the ash.

4. Calculate the size distribution of the ash produced on combustion

as follows:

Let the proportion of the ash prepared from coal of any one size

fraction denoted as j which is in size fraction i be defined as nij.

Multiply the percentage of total ash occurring in size fraction j

of the feed, p., by nij, the proportion of it which is within the ith

size range, to give the quantity of ash produced in size i from feed

of size j. Thus qi, the total percentage of ash produced in size

range i, is calculated by adding the quantities produced in this size

from the eight size fractions of feed. Thus:

j=8qi E (n.. p.) ... ... ... ... ... ... ... ... 4.25

:1=1 1

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Table 4.5 Size Distribution of Ash from Close Size Fractions

of Babbington Coal (35% ash)

Size Size range ot coal *range*of ash 1 2 3 4 5 6 7 8

1 91 0 0 0 0 0 0 0Size 2 2 92 O 0 0 0 0 0

distribution 3 1 2 94 0 0 0 0 0of ash 4 1 1 1.2 92.3 0 0 0 0

produced; 5 1 1 0.9 2.1 91.2 0 0 0% in range. 6 1 1 0.8 1.7 3.9 88.0 0 0

7 1 1 0.5 2.0 2.5 5.3 76.4 08 2 2 2.6 1.9 2.4 6.7 23.6 100

Table 4.6 Size Distribution of Ash from Close Size Fractions

of Newstead Coal (17% ash)

Size Size range of coal *range*of ash 1 . 2 3 4 5 6 7 8

1 50 0 0 0 0 0 0 0Size 2 7 54.0 0 0 0 0 0 0

distribution 3 3 6.8 61.5 0 0 0 0 0of ash 4 3 3.0 6.6 68.4 0 0 0 0

produced; 5 3 2.7 4.1 5.8 71.1 0 0 02 in range. 6 3 3.6 4.4 5.4 5.7 74.1 0 0

7 7 7.2 5.9 5.0 7.3 6.3 58.8 08 24 22.7 17.5 15.4 15.9 19.6 41.2 100

* The size ranges in Tables 4.5 and 4.6 are:

Range 1 2 3 4 5 6 7 8

Upper size, pm - 3175 1680 1000 500 250 125 75Lower size, pm 3175 1680 1000 500 250 125 75 0

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In order to estimate the particulate emission it is necessary to know

the detailed size distribution below 75 pm. Table 4.7 gives the size

distributions below 75 pm for Babbington coal with 35% ash and

Newstead coal with 17% ash. The size distribution in this range for

coals of other ash contents should be interpolated or extrapolated

from these values.

The ash size distribution is conveniently plotted as log (%

undersize) against log (size). It has been found that for many ashes a

straight line plot is obtained for the size range above about 200 jim.

Table 4.7

Size Distribution of Coal Ash below 75 pm

Size range, Percentage in range

jim Babbington, 35% ash Newstead, 17% ash

- 75 + 63 8.6 4.4- 63 + 45 13.0 6.5- 45 + 31 10.2 5.7- 31 + 22 11.5 9.4- 22 + 16 9.6 9.3- 16 + 11 11.7 12.9- 11 + 7.8 9.6 9.87.8 + 5.5 7.1 11.1

- 5.5 18.7 30.9

The above procedure gives an estimate of the size distribution of

the ash initially formed by combustion of the coal, and this information is

given by direct measurement in the Ash Degradability test described in

Section 17.3. In the absence of results of this test it is necessary also

to obtain a figure for the abrasion constant in order to assess the ability

of the ash to maintain a bed. An estimate of the abrasion constant can be

made from tests on a small combtistor (see Section 9.3).

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4.9 Conversion of Existing Equipment

Most existing equipment which might be considered for conversion

to fluidised coal-fired combustion falls into one of three categories:

(a) Water-tube boilers.

(b) Shell or cast-iron sectional boilers.

(c) Furnaces for generating hot gases, e.g. dryers or heaters.

As already mentioned in Section 2.4 retro-fitting applications are classed

as Group 3 because of the additional constraints imposed by the very

existence of the equipment. For example the equipment will have been

originally designed for firing by gas, oil, or some other mode of coal

firing, and will usually possess a basically different shape of combustion

chamber from the ideal for fluidised bed firing. Only certain designs of

shell boilers, and some water-tube boilers will therefore be conducive to

conversion to fluidised combustion, and even in converting these some

substantial rebuilding may be required. The distribution and duty of heat

transfer surfaces within the existing equipment will also have a major

bearing on the suitability for conversion.

Conditions which may be necessary or desirable, on converting an

existing boiler are as follows:

(i) The fuel should be burnt with a minimum loss of heat as

carbon in the ash, as unburnt combustion gases and as

sensible heat in the stack gas.

(ii) It may be necessary to maintain the maximum loading and the

turndown ratio for which the boiler was originally designed.

(iii) It is desirable that the boiler is capable of sustained

output and no undue frequency of shut-down for cleaning or

maintenance should occur.

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(iv) Statutory requirements with regard to emission of pollutants

must be met.

A precise design approach for conversion cannot be made as so

much will depend on the individual application. However, information and

experience is available on two main types of conversion. The first is the

conversion of water-tube boilers up to 27 000 kg/h (60 000 lb/h) steam for

stoker firing (4.27, 4.28) and the second is the conversion of smaller

equipment for steam raising, or for hot gas or hot water generation. In

the latter cases the space available for bed and freeboard can often be

limited and beds about 0.15 m (6 in.) static depth have been installed

without in-bed cooling. Examples of the successful conversion of a 2 MW

grass drying furnace and of a 350 kW hot water boiler are given in

references (4.17, 4.29 and 4.30).

For those applications where there is adequate headroom to

accommodate a fluidised bed with in-bed cooling and an ample freeboard the

design procedure outlined in Section 3.4.3 may be followed. For other

applications the work with overbed firing using low bed depths should be

consulted (4.13, 4.17-4.20, 4.30).

4.10 Turndown

In the operation of industrial boilers and furnaces in

particular, the heat demand often varies over a wide range; a turndown

requirement of 5:1 or 6:1 being quite common. The various lines of

approach for meeting these requirements with coal firing are summarised

below. The methods have been used successfully on the industrial scale.

Choice of the most suitable method will depend on the constraints of the

individual application and often a combination of methods may be

advantageous. Turndown and load variation are also discussed in Section 13

and, in general terms, in Section 3.6.

i) Variation of air and coal feeds in the same proportion.

This mode of operation can only be used when there is little

Issue 002 Section 4

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Copy No.

or no bed cooling (Group la). The high excess air levels

required for bed temperature control render the method more

suited for hot air heaters than for boilers.

ii) Use of compartmented beds with each compartment containing

cooling surfaces in propbrtion to its heat output. For a

given number of compartments operating, a narrow range of

outputs is obtainable by varying the bed temperature; the

number of compartments is chosen so that these ranges just

overlap.

iii) The installation of cooling tubes within and towards the top

of the bed, with variations of bed height caused by bed

expansion with increasing fluidising velocity, to give the

required variation in in-bed heat transfer rates (4.17-4.20,

4.30-4.32).

iv) Operation in an on-off cycle so that the bed, or a part of

the bed, is alternately fluidised and slumped (4.33).

Two methods, as yet untested on the industrial scale, are:

v) Use of a two-compartment bed in which one compartment is

uncooled and contains the coal feed, and the other

compartment is cooled. Particle circulation between the

combustion and the heat transfer zones would be varied to

meet the load demand (4.34).

vi) The installation of cooling tubes immediately above the

fluidised bed, with variation of bed height by withdrawing

or returning bed material (4.35).

Issue 002 Section 4

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Copy No. I

4.11 Firing of Coal/Liquid Mixtures

Aqueous or oil suspensions of finely ground coal are of

considerable interest in boiler firing, because they ofter many of the

advantages of oil - particularly in handling and metering. Against these

advantages must be set the costs of preparation and the reduction in boiler

efticiency due to the heat retained in the water vapour resulting from

evaporation of the water.

Coal/water mixtures have been fired successfully in an

experimental pressurised fluidised bed combustor (4.36). Such mixtures are

particularly attractive for pressurised combustion, because the pumping of

aqueous suspensions to high pressure is less expensive than compressed air

inijection of crushed coal. Whereas the preparation of crushed coal for

dry-coal feeding necessitates drying of the feed coal, wet grinding may be

used for the preparation of coal/water mixtures.

4.12 Example Calculation

4.12.1 Calculation basis ;

The following specimen calculations of combustion and heat

release relate to an atmospheric pressure fluidised bed combustor, fired

with a low rank coal of 60% carbon content, a calorific value of

23.26 MJ/kg (10 000 Btu/lb), and stoichiometric air requirement of 8 (wlw).

The coal is sized <3.2 mm (<0.12 inch) and in-bed feeding is specified,

with optional re-firing of primary cyclone fines.

Calculations are carried out:

1. without grit re-firing, and

2. with 80% re-firing of fines (cyclone cut size 30 pm).

Both general and stepwise calculation procedures are used for evaluating

the freeboard combustion in the 'no recycle' case, but only the general

procedure for the 'recycle' case. It is specified in advance that the

Issue 002 Section 4

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Copy No.{'

freeboard gas temperature must not exceed 950°C (1740°F).

Calculations are presented here only in British units to save

space. Use of S.I. units, of course, does not alter the general

calculation procedure in any way.

The operating data, listed below, have been chosen to correspond

to one of the tests on the NCB, CRE "three foot" combustor (reference

(4.37); test 3.3), but differ in some minor respects, namely:

i) the actual bed area was half that in this example, and there

was only one coal feed instead of two; thus J is unaltered.

ii) the freeboard cooling in the CRE test was by live, saturated

steam injection at 2.4 m (94 inches) above the air

distributor, in order to keep the freeboard temperature from

rising above 900C (1650°F).

The measured unburnt carbon loss at the top of the freeboard in

Test 3.3 was 9.2%; (there was,no recycling of fines in this test).

4.12.2 Design and operating data

Bed area 3 ft x 3 ft

Freeboard area 3 ft x 3 ft

Number of coal feed points, N 2

Expanded bed height, Lb 2.1 ft

Freeboard height 13 ft

Fraction of bed height occupied by tubes, Bh 0.42

Fraction of bed area occupied by tube bank 1.0

Tube diameter, D 2.375 inches

Tube pitch, Ph 4.5 inches

Pressure, P 1 atm.

Fluidising Velocity, Uf 8 ft/s

Excess air, X 10 %

Bed temperature, Tb 1560 OF

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Copy No.

Freeboard cooling is to consist of wall tubing extending from 6.5

feet above the air distributor to 10.5 ft above it. Heat transfer to the

wall is to be regarded as zero below this tubing, and to be represented by

equation (4.25) where wall tubing is present.

= 7.14 x 10 (T - 1230) L ... ... ... ... 4.26

Where Q3 is heat transfer to wall expressed as a fraction of heat input to

the combustor; T is the freeboard temperature ('F), and L is the height

of the section of cooled freeboard wall receiving heat from the gases

(feet). These tubes are to be supplemented, if necessary, by additional

tubing to prevent the gas temperature rising above 1740°F at any point in

the freeboard.

4.12.3 Calculation 1 - no recycle

The datum combustibles loss from bed surface by interpolation

from Figure 4.1 (or from equation 4.2) is,

Cd = 0.419

Calculate the coal mixing parameter from:

E) A b P

N Lb [1 - Bh (1.5 - 0.3 phID o)

j = 0.305 x 9 x 1

2 x 2.1 [1 - 0.42 (1.5 - 0.3 x 4.512.375)]

i.e. J = 1.074

The combustibles loss from the bed surface corrected for the coal mixing

parameter is:

Cj = Cd(0.7 + 0.185 J)

= 0.419 (0.7 + 0.185 x 1.07)

i.e. Cj = 0.376

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Copy No. '

The combustibles loss from the bed surface corrected for excess air (table

4.2 or equation 4.4) is,

C 0.3650

There is no correction for coal rank (low-rank coal).

Combustion occurring in bed is:

Cb = - C

= 0.635

Calculate the freeboard combustion:

(a) Simple calculation

Assume the freeboard temperature is 15600F, (2020°R).

Calcuate the combustion rate constant from:

K - 105 [(0.472x0.305x8) + 0.93] exp (-12 620x1.8/2020)

-1i.e. K = 2.72 s

Calculate the mean residence time of gas in freeboard:

tf = 13/8 ~ 1.625 s (Freeboard temp - bed temp)

Calculate the unburnt combustibles at the freeboard exit:

Equation 4.8 may be re-arranged for solution as follows:

(0.01 X) Co/YC = ... ... ... ... ... 4.27

(0.01 X) + C0 - CO/y

~where y= epf (0.01 X) K t 4.28where y = exp .................. 4.21 + (0.01 x)]

Thus = ex[ 0.01 x 2.72 x 1.625

1+0.1

= 1.494

Co/Y = 0.365/1.494 = 0.244

0.1 x 0.244

0.1 + 0.365 - 0.244i.e. C = 0.111

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G , Copy No.Y>

As in the calculation of C , there is no correction for coal rank.

Calculate the amount of combustion in the freeboard:

Cf e C C

= 0.365 - 0.111

i.e. Cf . 0.254

Estimate the proportion of this heat release which is transferred to the

bed, using Figure 4.3. From the figure it is found that 74% of the

freeboard heat release is transferred to the bed.

Calculate the heat removed by freeboard cooling:

S Cooled height of freeboard = 4 ft

Heat removed = 7.14 x 105 x(1560 - 1230) x 4 = 0.094

Calculate the proportion of the heat removed which would otherwise have

been transferred to the bed, using Figure 4.6:

If heat were extracted at 6.5 ft above the bed, 65.8% of it would otherwise

have been transferred to the bed, and if heat were extracted at 10.5 ft

above the bed, 56.6% of it would otherwise have been returned to the bed.

As an approximation, it may be assumed that heat is extracted uniformly

over this section of the freeboard, and an average value of ½w(65.8 + 56.6)

or 61.2% to the bed may be assumed.

The heat available in the bed is:

heat released by combustion in the bed, 1-C , plus,

fraction of heat from freeboard combustion which is

transferred to the bed, 0.74 x 0.254, minus,

fraction of heat extracted at wall which would otherwise

have been transferred to the bid, 0.612 x 0.094

i.e. Heat available in the bed:

= 1 - 0.365 + (0.74 x 0.254) - (0.612 x 0.094)

= 0.766

Calculate the heat available to raise the temperature of the

freeboard gases, which is the fraction of Cf not transferred to the bed,

1^~~~~~~~~~~~~~~~~~~~

Issue 002 Section 4

Page 84 of 96

S~~~~~~~~~Sgfminus the fraction of freeboard cooling which is not effectively lost by

the bed:

i.e. Heat available to off-gas = 0.254 (1 - 0.74) - 0.094 (1 - 0.612)

= 0.0296

Perform a heat balance on the freeboard gases. Assume that the

stoichiometric air ratio is 8.0 and let the coal feed rate be G lb/h.c

Hence the air rate is 8.8 G lb/h at 10% excess air. Assume thec

elutriation rate of inert solids (as calculated by the procedure described

in Section 9) is 0.25 Gc lb/h (this value is an arbitrary one introduced

here for convenience).

Hence the flow of gas up freeboard = 8.8 G + (1 - C ) G0

= 9.435 Gc lb/h

Elutriation rate of unburnt carbon = Gc x fraction of coal consisting of

carbon x C0

= 0.6 x 0.365 Gc

= 0.219 G lb/hc

Thus, total gas flow up the freeboard

= G (9.435 + 0.219 + 0.25)

= 9.904 G lb/hc

Terms in the heat balance are:

(i) Heat introduced in the gas leaving the surface at 1560°F.

= 9.90 Gc x 0.25 x (1560 - 60) *

c- 3714 x C Btu/h

(ii) Heat available to raise temperature of off gas

= 0.0296 x 10 000 Cc= 296 G Btu/h

c

(iii) Sensible heat flow in gas leaving the freeboard

= 9.90 G- x 0.25 (Te - 60)

= 2.48 G (T - 60)

* For an ambient temperature of 600 F

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Copy No. L

The freeboard exit temperature, T , is given by the heat balance as

follows:

2.48 (T - 60) = 3714 + 296 = 4010

i.e. T = 1679°Fe

It follows that no additional cooling will be required to prevent the

temperature rising above 1740°F.

Repeat the freeboard combustion calculation, if desired for

greater accuracy, using the mean of the bed temperature and the freeboard

exit temperature calculated above, which is 1607°F.

Following through the steps as before, this gives:

K = 3.82 s 1

tf = 1.58 s

y = 1.73

Cy/Y = 0.211

Ct = 0.083 instead of 0.111 as found previously and the freeboard

exit temperature becomes 1686°F.

(b) Stepwise calculation of 'freeboard combustion

Use five slices, each 2.6 feet in height.

The calculations for the first step are detailed here: those for the

remaining steps are summarised in Table 4.8. For each step after the

first, the effective temperature is that calculated as the exit temperature

of the previous step (for the first step it is the bed temperature), and

`C0 is the value calculated for Ct in the previous step.

Slice 1

Temperature, T = 1560°F

K = 2.72 s (as calculated in (a))

tf = 1.62515 = 0.325 s

y 1.084

CO ly 0.337

0.1 x 0.337(Ce) i = = 0.263

0.1 + 0.365 - 0.337

(Cf)1 0.365 - 0.263 m 0.102

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Copy No. ',L

Perform a heat balance; the terms are calculated as follows:

Q1 = rate of heat generation by combustion

= 0.102 x 10 000 G Btu/hc

= 1022 G Btu/hc

Q3 = rate of heat transfer to wall

0

Q4= additional cooling

0 (provisionally)

Q2 rate of transfer of heat back to bed

= 6 (Q1 - Q3) where 6 at 1.3 ft above the bed, from

Figure 4.6, is 0.85 t

i.e. Q2 = 866 Gc Btu/h

Q5 = sensible heat entering freeboard from bed

= mass flow x specific heat x temperature

= 9.90 G x 0.25 x (1560 - 60)C

= 3714 G Btu/hC

(The mass flow is calculated as in (a)).

Q6 = sensible heat leaving top of slice

= 9.90 Gc x 0.25 (T - 60)c aS= 2.48 Gc (Ta2 - 60);Btu/h

where Ta2 is the temperature of the top of the first slice.

The heat balance is:

Q6=.Q Q Q1 -Q2 -Q3 (Equation 4.11)Q6 .Q +1 ~ 2 - 3

i.e. 2.48 G (T a2 60) = 3714 G + 1022 G - 866 Gcc a2 c ~~~c c

whence T = 60 + 3794/2.48a2

= 1623°F

Thus no additional cooling is needed.

t If these calculations are carried out by computer the curves shown in

Figure 4.6 can be represented with sufficient accuracy over the normal

ranges of operating conditions by the empirical equation,

6= [(0.0145 Uf/e - 0.25) t/0 ] + 1where C = 0.305 for SI units with the fluidising velocity, Uf, in m/s

and the freeboard height, Lf, in m and 0 = 1.0 for British Units with Uf

and Lf in ft/s and ft respectively.

Issue 002 Section 4

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Copy No.t',

Remainder of slices (i = 2 to 5)

Q4 is found to be 0 in all slices, and is therefore not shown in the

following table. Q values are all in (Btu/h), and T in °F.(G) a

Calculated values for all 5 slices are shown in Table 4.8.

Table 4.8

Calculated Parameters for Freeboard Slices

Slice i i = 1 2 3 4 5

T a 1560 1623 1705 1728 1645

K, s-1 2.72 3.89 6.00 6.76 4.39

t, s 0.325 0.315 0.303 0.300 0.312

y 1.084 1.118 1.180 1.202 1.133

C 0.365 0.263 0.184 0.122 0.0840Co/Y 0.337 0.235 0.156 0.101 0.074

(Ct)i 0.263 0.184 0.122 0.084 0.068

(Cf)i 0.102 0.079 0.062 0.038 0.016

1022 787 622 377 165

Q3 0 0 441 925 30

6 0.85 0.74 0.68 0.62 0.59

Q2 866 584 123 -342 75

Q5 3714 3870 4073 4131 3925

Q6 3870 4073 4131 3925 3985Ta(i + 1) 1623 1705 1728 1645 1669a(i + 1)

The unburnt carbon loss predicted at the freeboard exit is 6.8%, the

maximum freeboard temperature is 17280F, and the heat available in the bed

is Cb + (Z(Q2))/10 000,

i.e. 0.635 + (866 + 584 + 123 - 342 + 75)/10 000 or 0.766 of the heat

input.

Issue 002 Section 4

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Copy No.

4.12.4 Calculation 2 - with recycle of 80% of the primary cyclone fines

The steps in the calculation when fines are recycled are as

follows:

1. Calculate the combustible loss from the bed surface resulting from

incomplete combustion of freshly-fired coal (calculation as in Section

4.9.3 above):

C 0.365oc

2. Calculate the combustible loss in the off-gas, as in Section 4.9.3

above:

t= 0.083 (using the freeboard temperature obtained in

the first approximation)

3. Interpolate between curves for 5 and 10 ft/s in Figure 4.8 to obtain

size distribution of elutriated carbon at 8 ft/s. Assume the cyclone

grade efficiency is zero for particles below 30 pm (1.18 x 10 in.)

and 100% for particles above this size. (Note: this is a

simplification of most cyclone grade efficiency curves, introduced to

avoid over-complication here which would tend to make it more

difficult to following the working.) Calculate the proportion of

elutriated carbon that is collected by the recycle cyclone:

X = 0.60

4. Calculate the rate of carbon refiring to the bed, assuming that 80% of

the fines are recycled:

C = X x C x 0.8 = 0.6 x 0.083 0.8r t

i.e. C = 0.040r5. Calculate the effective excess air:

X (100 + X)X lC 100

110 - 1001.040

i.e. X = 5.78%e

Issue 002 Section 4

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Copy No.* 2

6. Calculate the combustible loss from the-surface of a fines combustion

bed at the operating condition of the main bed and excess air X . Thee

combustible loss from the bed surface if the recycled fines were coal:

C = Cj x excess air correction. (Table 4.2)

where Cj, as before, is 0.376

i.e. C = 0.3700

Multiply by the factor, 3(1 - C ) + C ' to obtain the combustible

loss, C', from the surface of the bed when firing fines:

C' = 0.37 x [3(1 - 0.37) + 0.372]0

= 0.37 x 2.027

= 0.749

7. Calculate the quantity of recycled fines leaving the bed surface after

one recycle, as a fraction of the carbon feed-rate with fresh coal:

Cof = 0.043 x 0.75

= 0.030

8. Calculate the total loss of combustibles from the bed surface after

one recycle:

C 5 Cf + C = 0.030 + 0.365o of oci.e. C = 0.395

09. Calculate the total loss of carbon in the fines at the top of the

freeboard, as in (a); but using X instead of X:e

Temperature = 1686°F

K = 5.35 S-

t = 1.58

y = 1.59

Co/Y = 0.395/1.59 = 0.249

ct = 0.0578 x 0.249Ct=

0.0578 + 0.395 - 0.249

= 0.071

10. Repeat steps 4 to 9 as follows:

4. C 0.6 x 0.071 x 0.8r

0.034

Issue 002 Section 4

Page 90 of 96

Copy No.\.

5. X 110 1001.034

= 6.39%

6. C = 0.369

C' = 0.369 x [3(1 - 0.369) + 0.369'] = 0.748

7. Cof = 0.034 x 0.748 = 0.025

8. C = Cof + C = 0.025 + 0.365 = 0.39

9. K = 5.35 s

t = 1.578 s

y = 1.661

Co/Y = 0.39/1.661 = 0.235

0.064 x 0.235C t 0.064 + 0.39 - 0.235

= 0.069

This represents an adequate degree of convergence and final values

are:

C = 0.034rC = 0.390

Ct = 0.069

11. Calculate the total carbon loss:

Carbon fines passing through cyclone = (1 - x) Ct

= 0.4 x 0.069

= 0.028

Carbon fines rejected by partial recycle system

0.2 x C 0.2 x 0.034r

0.8 0.8

= 0.008

Total carbon loss = 0.028 + 0.008 0= .036

Issue 002 Section 4

Page 91 of 96

Copy No., _

12. Calculate the amount of combustion occurring in the freeboard:

cf = c - c

= 0.39 - 0.069

= 0.322

13. Repeat the freeboard heat balance, drawing on the results of

calculations of Section 9 to obtain the elutriation rate of inert

solids when recycling fines.

It is assumed here that this calculation shows the

elutriation rate of inert solids to be 5 G , where G (lb/h) is

the coal feed rate.

Mass flow of gas up freeboard = 8.8 0 + (1 - c ) Gc

= 9.41 G lb/h

Elutriation rate of unburnt carbon = 0.6 x 0.39 x G

= 0.234 G lb/h

Total flow up freeboard = G (9.41 + 0.237 + 5)

= 14.64 Gc

Terms in the heat balance are:

(i) Heat in gas leaving the bed surface at 1560uF:

= 14.64 G x 0.25 x (1560 - 60)

c= 54910G Btu/h

(ii) Heat available to raise temperature of off-gas, i.e.

fraction of Cf not transferred to the bed minus the fraction

of the freeboard cooling which is not effectively lost by

the bed. Proportion of freeboard heat release which is

transferred to the bed (as in 1 (a), above) is 74%.

Heat removed by freeboard cooling:

7.14 x 10 5 (1686 - 1230) x 4 = 0.13

Proportion of heat removed which would otherwise have been

transferred to the bed (as in l (a), above) is 61.2%.

Therefore, heat available to raise temperature of the

freeboard gases:

= (1 - 0.74) Cf - (1 - 0.612) x 0.13, where Cf is 0.322

i.e. heat available 0.034 of heat input with coal.

(iii) Sensible heat flow.in gas leaving the freeboard

= 14.64 Gc x 0.25 (T - 60)

= 3.66 GC (Te 60)c e

Issue 002 Section 4

Page 92 of 96

Copy No.

The freeboard exit temperature, T is given by the heat

balance:

3.66 (Te - 60) = 5491 + 0.034 x 10 000

i.e. T = 1654°FeSince the temperature in the freeboard when fines recycling

is used is below 1740°F, no additional cooling will be

required.

Finally, since the freeboard temperature is higher than

assumed, for increased accuracy the whole procedure should

be repeated for the new temperature,

i.e. ½(1560 + 1654) or 1607°F.

The net effect of fines recycle on the total combustion loss

has been to reduce the carbon loss from 0.089 without recycle to 0.043

with recycle.

Issue 002 Section 4

Page 93 of 96

4Copy No.t

4.13 References

4.1 Pillai, K.K. "Devolatilisation and combustion of large coal

particles in a fluidised bed". J. Inst. Energy, 54, 142-150,

(1981).

4.2 Chakraborty, R.K. & Howard, J.R. "Burning rates and

temperatures of carbon particles in a shallow fluidised bed."

J. Inst. Fuel, 51, (409), 220-224, (1978).

4.3 Pillai, K.K. "The influence of coal type on devolatilisation

and combustion in fluidised beds." J. Inst. Energy, 54, (420),

142-150, (1981).

4.4 Donsi, G., Massimilla; L. & Miccio, M. "Carbon fines production

and elutriation from the bed of a fluidised coal combustor."

Combust. Flame, 41, 57-69, (1981).

4.5 Newey, D.C. "The decrepitation and volatiles release of coal in

a fluidised bed at 900°C." BP Research Centre, Tech. Memo. No,

133 363, (Dec 1982)

4.6 Smith, I.W. "The combustion rates of pulverised coal char

particles." Paper to Conf. on Coal Combustion Technology and

Emission Control, California Inst. of Tech., Pasadena, (Feb

5-7. 1979).

4.7 Gill, D.W. NCB, CRE Control of Environment Report No. EC 25,

(Jan 1978).

4.8 Merrick, D. & Collop, D.J. NCB, CRE PADB Memorandum No. 79/15,

(Apl 1979).

4.9 Williams, D.F. et alii. NCB, CRE Fluidised Combustion Section

Reports Nos. 28, 29, 31, 32, 51, 52, 53, 54, 57, 58, 64 and 67.

4.10 Highley, J. NCB, CRE Fluidised Combustion Section Report No.

74, (Mar 1971).

4.11 Merrick, D. "Effect of excess air level on carbon loss." NCB,

CRE PADB Report No. PADB/78/33, (Nov 1978).

4.12 Field, M.A. Combustion and Flame, 14, 237-248, (1970).

4.13 Highley, J., Kaye, W.G., Chiba, T.i, Nienow, A.W. & Rowe, P.N.

Inst. of Fuel Symp. Ser. No. 1 "Fluidised Combustion", London,

(Sept 1975).

Issue 002 Section 4

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Copy No.' ©

4.14 D'Amore, D., Donsi, G. & Massimilla, L. "Bed carbon loading and

particle size distribution in fluidised combustion of fuels of

various reactivity." Proc. 6th. Int. Conf. on Fluidised

Combustion, Atlanta, Georgia, vol II, p. 675, (1980).

4.15 CURL. Report to CSL. "Gravity feeding on the vertical fire-tube

boiler at Leatherhead." (Dec 1977).

4.16 Highley, J. NCB, CRE Combustion Section Report No. 137, (Aug

1975).

4.17 Highley, J. & Vickers, M.A. NCB, CRE Combustion Section Report

No. 187 (Oct 1977).

4.18 Fisher, M.J. & Highley, J. NCB, CRE Combustion Section Report

No. 207, (June 1979).

4.19 Benson, R.A.C., Ellis, J.E. & Highley, J. NCB, CRE Combustion

Section Report No. 232, (May 1981).

4.20 Highley, J. "The development of fluidised bed combustion for

industry." Fluidised Bed Combustion and Applied Technology

Symposium, Peking, China (22-25 Aug 1983).

4.21 Rogers, E.A. & LaNauze, R.D. NCB, CRE Mineral Products Section

Report No. MP 102, (Dec 1977).

4.22 Williams, D.F., Stantan, J. & Highley, J. "Prediction of

combustion efficiency and freeboard temperature in fluid bed

boilers."; NCB Draft Design Document D3, (Mar 1971).

4.23 Ketley, H.C., Rogers, M.C. & Wright, S.J. "Pilot scale studies

on the combustion of coal in fluidised beds."; BCURA Members

information circular No. 341, (April 1966).

4.24 Hickman, R.G., Ketley, H.C. & Wright, S.J. "Combustion

efficiency and heat transfer patterns in a fluidised bed

burning untreated coals sized 0.25 - 0 in. (6 - 0 mm); BCURA

Members information circular No. 350, (Aug 1968), Fig 13.

4.25 Minchener, A.J. NCB, CRE Pollution Control Section Report No.

PC 13, (Aug 1979).

4.26 Stayte, M.R. & Sutcliffe, P.T. NCB, CRE Pollution Control

Section Report No. PC 72, (Nov 1981).

4.27 "Fluidised Bed Combustion - Industrial Applications; Test

Series 1 and 2." Report No.1/77/762A, (9 Mar 1977).

Issue 002 Section 4

Page 95 of 96

Copy No.C(,

4.28 "Fluidised Bed Combustion - Industrial Applications." Report

Nos. 1/73/593, (1973) and 1/74/634, (9 May 1974).

4.29 Highley, J. & Wheatley, P.C. NCB, CRE Combustion Section Report

No. 154, (Mar 1976).

4.30 Highley, J., Kaye, W.G. & Wheatley, G.P. Proc. 6th. Int. Conf.

on Fluidised Bed Combustion, Atlanta, Vol II, p. 584, (April

1980).

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Issue 002 Section 4

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