How to Save Energy and Money

8/14/2019 How to Save Energy and Money

1/93

BOILERS &

FURNACES

G u i d e B o o k 2

3E STRATEGY

STRATEGY

EFFICIENCY

ENERGY

EARNINGS

EUROPEAN COMMISSION

N e t h e r l a n ds M i n i s t e r y o f E c o n o m i c A f f a i r s

TSITechnical Services International

MY

IN GRE ER

A NL ES DAN

H

ow

to

save

energy

and

m

oney


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HOW TO SAVE

ENERGY AND MONEY

IN BOILERS AND FURNACE SYSTEMS

This booklet is part of the 3E strategy series. It provides advice on

practical ways of improving energy efficiency in boilers and furnace

systems.

Prepared for the European Commission DG TREN by:

The Energy Research Institute

Department of Mechanical Engineering

University of Cape Town

Rondebosch 7700

Cape Town

South Africa

www.eri.uct.ac.za

This project is funded by the European Commission and co-funded by

the Dutch Ministry of Economics, the South African Department of

Minerals and Energy and Technology Services International , with the

Chief contractor being ETSU.

Neither the European Commission, nor any person acting on behalf of

the commission, nor NOVEM, ETSU, ERI, nor any of the information

sources is responsible for the use of the information contained in this

publication

The views and judgements given in this publication do not necessarily

represent the views of the European Commission


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HOW TO SAVEENERGY AND MONEY

IN BOILERS AND FURNACESYSTEMS


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HOW TO SAVE

ENERGY AND MONEY

IN BOILERS AND FURNACE SYSTEMS

Other titles in the 3E strategy series:

HOW TO SAVE ENERGY AND MONEY:THE 3E STRATEGY

HOW TO SAVE ENERGY AND MONEY IN ELECTRICITY USE

HOW TO SAVE ENERGY AND MONEY IN STEAM SYSTEAMS

HOW TO SAVE ENERGY AND MONEY IN COMPRESSED AIR SYSTEMS

HOW TO SAVE ENERGY AND MONEY IN REFRIGERATION

HOW TO SAVE ENERGY AND MONEY IN INSULATION

Copies of these guides may be obtained from:

The Energy Research Institute

Department of Mechanical Engineering

University of Cape Town

Rondebosch 7700

Cape Town

South Africa

Tel No: (+27 21) 650 3892

Fax No: (+27 21) 686 4838

Email: [email protected]

Website: http://www.3e.uct.ac.za

ACKNOWLEDGEMENTS

The Energy Research Institute would like to acknowledge the following for their contribution in the production of

this guide:

Energy Technology Support Unit (ETSU), UK, for permission to use information from the Energy

Efficiency Best Practice series of handbooks.

Wilma Walden of Studio.com for graphic design work ([email protected]).

Doug Geddes of South African Breweries for the cover colour photography.

Canadian gov. See other guides.


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G u i d e B o o k E s s e n t i a l s :QUICK CHECK-LIST FOR SAVING ENERGY

and MONEY IN BOILERS AND FURNACE

SYSTEMS

This list is a selected summary of energy and cost savings opportunities outline in the text. Many

more are detailed in the body of the booklet.These are intended to be a quickchecklist.

BOILERS (CHAPTER 9)

Maintain efficient combustion.

Maintain good water treatment. Repair water and steam leaks.

Recover heat from flue gas and boiler blowdown whenever possible (see Steamguidebook).

Ensure good operational control and consider sequence control for multi-plantinstallations).

Attempt to match boilers to heat demand. Valve off idle boilers to reduce radiation

losses. Use flue dampers where appropriate to minimize flue losses when the plant is not firing. Ensure that boilers and heat distribution systems are adequately insulated.

Blowdown steam boilers only when necessary (see Steam guidebook). Ensure as much condensate as practicable is recovered from steam systems.

Insulate oil tanks and keep steam or electric heating to the minimum required.

FURNACES (CHAPTER 12)

Minimise heat losses from openings on sealed units such as doors. Use high efficiency insulating materials to reduce losses from the plant fabric.

Attempt to recover as much heat as possible from flue gases. The pre-heating of

combustion air or stock or its use in other services such as space heating is well worthconsidering.

Reduce stock residence time to a minimum to eliminate unnecessary holding periods. Ensure efficient combustion of fuels where applicable.

Avoid excessive pressure in controlled atmosphere units. If maintaining stock at high temperature for long periods, consider the use of specialized

holding furnaces. Make sure excessive cooling of furnace equipment

is not occurring. Ensure the minimum amount of stock supporting

equipment is used.

Ensure there is effective control over furnaceoperating parameters computerized controlshould be considered for larger units.


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Ta b l e o f C o n t e n t s1. INTRODUCTION ............................................................................................................................................................................................1

2. COMBUSTION ..................................................................................................................................................................................................1

2.1 Combustion air .........................................................................................................................................................................................1

2.1.1 Excess Air.....................................................................................................................................................................................4

2.1.2 Glue Gas Analysis....................................................................................................................................................................42.1.3 Determination of Excess Air ............................................................................................................................................5

2.2 Heat losses ..................................................................................................................................................................................................7

2.2.1 Heat loss due to incomplete combustion................................................................................................................8

3. HEAT TRANSFER ...........................................................................................................................................................................................10

3.1 Conduction ...............................................................................................................................................................................................10

3.2 Convection................................................................................................................................................................................................11

3.3 Radiation.....................................................................................................................................................................................................12

4.THE FUELS...................................................................................................................................................................13

4.1 Pipeline gas................................................................................................................................................................................................13

4.2 Liquid Petroleum Gas ........................................................................................................................................................................14

4.3 Fuel Oil ........................................................................................................................................................................................................14

4.4 Coal .........................................................................................................................................................................................................15

4.5 Choice of Fuel ........................................................................................................................................................................................16

5. COMBUSTION EQUIPMENT: OIL AND GAS BURNERS..............................................................................18

5.1 Gas Burners .............................................................................................................................................................................................18

5.2 Oil Burners ...............................................................................................................................................................................................18

5.2.1 Pressure Jet ..............................................................................................................................................................................18

5.2.2 Air or Steam Blast Atomiser............................................................................................................... 19

5.2.3 Rotary Cup ..............................................................................................................................................................................19

5.2.4 Low Excess Air Burners ...................................................................................................................................................19

5.3 Burner Controls ....................................................................................................................................................................................19

6. COMBUSTION EQUIPMENT: SOLID FUEL COMBUSTION....................... .......................... ......................21

6.1 Stokers .........................................................................................................................................................................................................21

6.2 Chain Grate Stoker .............................................................................................................................................................................21

6.3 Sprinkler Stoker.....................................................................................................................................................................................22

6.4 Fluidised Bed Combustion..............................................................................................................................................................22


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7. ENERGY SAVING EQUIPMENT ........................................................................................................................................................23

7.1 Flue gas heat exchangers ................................................................................................................................................................23

7.1.1 Economiser (Feedwater heater)..................................................................................................................................26

7.1.2 Recuperator (Air heater) ................................................................................................................................................26

7.2 Accumulators ..........................................................................................................................................................................................26

7.3 Insulation ....................................................................................................................................................................................................26

7.4 O2 Analysers ............................................................................................................................................................................................27

7.5 Variable speed fan drives ................................................................................................................................................................28

7.6 Flue gas dampers ..................................................................................................................................................................................28

7.7 Waste heat boilers ..............................................................................................................................................................................28

8. POLLUTION ....................................................................................................................................................................................................29

8.1 Environmental Equipment ..............................................................................................................................................................30

8.1.1 Ash Handling Equipment ................................................................................................................................................30

8.1.2 Air Pollution Control Equipment................................................................................................................................30

9. BOILERS ........................................................................................................................................................................................................31

9.1 Types of boilers......................................................................................................................................................................................31

9.1.1 Water Tube Boilers..............................................................................................................................................................32

9.1.2 Multi-Tubular Shell Boilers ..............................................................................................................................................34

9.1.3 Reverse Flame or Thimble Boilers..............................................................................................................................36

9.1.4 Steam generators ................................................................................................................................................................37

9.1.5 Sectional Boilers ....................................................................................................................................................................38

9.1.6 Condensing Boilers..............................................................................................................................................................39

9.1.7 Modular Boilers ....................................................................................................................................................................409.1.8 Composite Boilers ..............................................................................................................................................................41

9.2 Boiler system selection ....................................................................................................................................................................42

10. ENERGY AND COST SAVING FOR BOILERS ..............................................................................................43

10.1 Potential Losses ..............................................................................................................................................................................43

10.2 Boiler Energy Balance ................................................................................................................................................................43

10.3 Minimizing Boiler Losses ..........................................................................................................................................................44

10.3.1 Maintenance saving opportunities ..............................................................................................................................44

10.3.2 Blowdown Heat Loss ........................................................................................................................................................45

10.3.3 Heat Transfer ..........................................................................................................................................................................46

10.3.4 Excess Air Reduction..........................................................................................................................................................48


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10.3.5 Flue gas heat recovery ......................................................................................................................................................49

10.3.6 Combustion air pre-heat ................................................................................................................................................53

10.3.7 Load Scheduling ....................................................................................................................................................................54

10.3.8 On-Line Cleaning ................................................................................................................................................................56

10.3.9 Flue Shut-Off Dampers ....................................................................................................................................................56

10.3.10 Variable speed fan drives ................................................................................................................................................56

10.3.11 Integrated control ................................................................................................................................................................57

10.4 What to do first a quick checklist ................................................................................................................................58

10.4.1 Check list ..................................................................................................................................................................................58

11.TYPES OF FURNACES ............................................................................................................................................................................59

11.1 Batch Furnaces ................................................................................................................................................................................59

11.2 Continuous Furnaces ..................................................................................................................................................................59

11.3 Direct Fired Furnaces ................................................................................................................................................................60

11.4 Indirect Heated Furnaces ........................................................................................................................................................61

12. ENERGY AND COST SAVINGS FOR FURNACES ............................................................................................................62

12.1 Potential Losses ..............................................................................................................................................................................62

12.1.1 Furnace Energy Balance....................................................................................................................................................62

12.2 Minimizing Furnace Losses ......................................................................................................................................................63

12.2.1 Flue gas heat loss..................................................................................................................................................................63

12.2.2 Heat Loss to incomplete combustion......................................................................................................................66

12.2.3 Radiation Heat Loss............................................................................................................................................................66

12.2.4 Furnace pressure control ................................................................................................................................................67

12.2.5 Furnace efficiencies and Monitoring and targeting ..........................................................................................68

12.3 What to do firsta quick checklist ................................................................................................................................69

APPENDIX ........................................................................................................................................................................................................70

Conversion Tables ................................................................................................................................................................................................70

Boiler Efficiency Test ............................................................................................................................................................................................71

Furnace Efficiency Test ........................................................................................................................................................................................83


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This guide examines the energy savings potentials

for boilers and selected furnaces. The boiler

section starts with a description of different

boilers plant, combustion equipment used and

fuels available. Environmental impacts are

described, boilers selection processes outlined and

finally a list of measures and a strategy outline for

saving energy in boiler operation.

2. COMBUSTION

In all aspects of boilers and furnaces (including

dryers and kilns) heat is produced from

combustion or by the use of electrical energy.The

heat is transferred to the product or water toproduce stream in the case of a boiler.

The fuel (with the exception of electricity which

heats an element) burns in the combustion

chamber, which varies in shape and size

depending on the application. Common fuels

include pipeline gas, liquid petroleum gas, heavy

fuel oil, lighter oils and solid fuels such as biomass

or coal. If gas is produced on sitethis can also be

used.

The in the case of a furnace the product is then

exposed directly to the heat generated in the

combustion chamber, flue gas heat or a gas/fluid

that has been heated by the combustion process.

2.1 COMBUSTION AIR

Stoichiometric air represents the amount of air

required for complete combustion with the

perfect mixing of the fuel and air Stoichiometric air

is sometimes called theoretical air. If perfect mixing

is achieved, every molecule of fuel and air takes

part in the combustion process. Excess air must be

supplied to ensure complete combustion of thefuel because perfect mixing of fuel and air does

not occur. Percentage excess air is defined as the

The guide then moves on to savings in furnaces.

Various types of furnaces and energy saving

measures are described.The emphasis here is on

savings from excess air reduction, combustion air

preheat, correct insulation and furnace pressure

control.

1. INTRODUCTION


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total amount of combustion air supplied in excess

of the stoichiometric air, expressed as a

percentage of the stoichiometric air.

Total air = Stoichiometric air x (1 % Excess Airr)Total air = Stoichiometric air(x (1 +( 100 )The minimum amount of excess air required

varies with the fuel used and the efficiency of

mixing the air and fuel. If less than the minimum

quantity of air is supplied, some of the fuel will not

burn completely and there is a waste of fuel

energy. Evidence of incomplete combustion usually

shows up as carbon monoxide (CO) in the

products of combustion (flue gas). A continuous

gas analyser, or a manually operated Orsat, can be

used to check for CO in the flue gas.

Too much air also wastes energy. The gases leaving

the furnace are hot and contain heat energy. If

excessive amounts of air are supplied to the

furnace, the excess will also be heated.The effect

on heat losses by varying the amount of air

supplied to the furnace is shown in Figure 1.The

minimum losses occur when the amount of air

supplied is slightly greater than the

stoichiometric amount.

The weight or volume of each element or

compound in the fuel is required to determine the

stoichiometric air. It is often inconvenient to

determine stoichiometric air in this manner, as in

many instances the precise fuel analysis is

unknown or varies. A more convenient method

is to determine the quantity of air per unit of heat

in the fuel, i.e. kilograms of air per gigajoule of heat

in the fuel as fired (kg/GJ). Expressed in this

manner, the stoichiometric air required forcommon types of fuel is almost constant. Table 1

provides values for several different types of fuel,

which may be used in boilers or furnaces.

It may be suspected that a supply air fan, air inlet

louvers, ducting or the air flow control method is

inadequate. Knowledge of the required amount of

furnace combustion air enables checking the

adequacy of the air supply system.The combustion

air requirements can be calculated and compared

Figure 1: Zone of maximum combustion efficiency (Source:

Canadian Gov.) (Energy Management Series 7. Page 4. Figure 2)


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to the capacity of the components in the air

supply system.

Combustion air can be supplied to the equipment

by natural or forced draft systems. Natural draft

uses the negative pressure (draft) produced by the

furnace stack to draw combustion air into the

furnace and the resulting flue gases out of thefurnace.The most common example of this is the

ordinary domestic gas furnace. Natural draft is

usually applied only to small furnaces with less

than about one GJ/h heat input.

There are several disadvantages related to natural

draft firing. The amount of combustion air drawn

into the furnace cannot be controlled accurately

and the fuel and air mixing is inefficient.This means

that higher levels of excess air must be maintainedto ensure that complete combustion is achieved

under all conditions.The furnace pressure is always

negative which allows air to leak into the furnace,

and create additional flue gas volume and heat

losses.

Forced draft firing uses a fan to supply combustion

air to the equipment. Airflow is regulated by

means of dampers so that accurate control of the

proportion of air to fuel for various firing rates ispossible. A common method used to achieve this

is to operate the fuel valve and the damper with a

common mechanical linkage. Some form of

adjustable cam is used to vary the relative

positions of the fuel valve and damper to provide

proper fuel/air ratios at all firing rates.

The combustion air fan also provides bettermixing of the fuel and the air. The air is introduced

into the furnace around the burner(s) and vanes,

which produce a swirling motion in the air as it

enters the furnace, can create turbulence. A high-

pressure drop between the air supply and the

furnace is required to produce turbulence, and this

can only be achieved with a forced draft system.

These advantages mean that the excess air for a

forced draft system can be lower than for natural

draft firing, with resulting lower heat losses to theflue gas.

Forced draft firing permits a slightly positive

furnace pressure at all times. Leaks will then be

from the furnace outwards, which may lead to a

dangerous situation when a furnace door is

opened. Therefore, it is desirable to control

furnace pressure at a slight positive value of not

more than about 10 Pa.This is normally achieved

by regulating a damper in the breeching betweenthe furnace flue gas exit and the base of the stack.

Example: Combustion air requirements for a furnace using 700 l/h of Number 6 fuel oil, at 15 per centexcess air can be calculated. From Table 1, theoretical combustion air is 327 kg/GJ.The heating value of fuel

oil with 2.5 per cent sulphur is about 42.3 MJ/L (sulphur content can usually be obtained from the fuel

supplier).

Combustion air requirement = 700L/h x 42.3 MJ / L x 327 kg / GJ x 1.15Combustion air requirement =

Combustion air requirement = 1000 MJ / GJ

= 11135 kg/h

11135 kg/hor

1.204 kg / m3

= 9248 m3

/h at standard conditions.


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It may not be possible to maintain furnace

pressure as low as desired if heat recovery

equipment is installed in the flue gas system or if

the stack provides insufficient draft.

2.1.1 EXCESS AIR

The actual percentage of excess air supplied to

the furnace is one of the most informative items

of information to the furnace operator.The most

accurate way of determining this is to analyse the

flue gas leaving the furnace.

2.1.2 FLUE GAS ANALYSIS

A furnace in which heat is produced by the

combustion of fuel can be considered to have fuel

and combustion air as inputs, and flue gas as the

output (Figure 2). Practically all fuels used in

furnaces are hydrocarbons, which contain the

elements hydrogen and carbon. Although some

fuels contain other constituents they are not

usually important to the combustion process.The

hydrogen in the fuel burns to form water vapour,

and the carbon burns to form carbon dioxide

(CO2), or a mixture of carbon dioxide and carbon

monoxide (CO).Air contains nitrogen (N2) as well

as oxygen (O2).The N2 does not take part in the

combustion process, except for the formation of

small quantities of nitrogen oxides (NOx).

The major constituents of the products of

combustion are water vapour, CO2, CO, N2, and

any excess O2 left over from the combustion

process. Not all of the constituents will be present

in all instances. The presence of CO indicates

incomplete combustion.

Flue gas analysis can be determined by the use of

a continuous analyser or by periodic sampling.The

sample should be taken as close to the furnace

exit as possible to reduce air infiltration errors.

Some continuous analysers measure O2 content

and record or indicate the results. Other

continuous analysers measure the combustibles

content of the flue gas, which is mostly CO but

may also include some unburned fuel in gaseous

form. If a continuous flue gas analyser is not

available, a sample of the flue gas can be taken and

analysed with the use of an Orsat. The Orsat

determines the percentage by volume of O2, CO2,

and CO in the flue gas. The remaining gas is

assumed to be N2, plus a small quantity of water

Figure 2: Combustion process. (Source: Canadian Gov.) (Energy Management Series 7.

Page 6. Figure 3)


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vapour, which did not condense out of the sample.

There are other manually operated analysers

available, which measure either CO2 or O2 in the

flue gas.These are simpler to use and can be useful

as a cross check against an Orsat.

2.1.3 DETERMINATION OF EXCESSAIR

Flue gas analysis provides sufficient data to

calculate the excess air to the furnace. In most

furnaces, CO is absent or very low because of high

levels of excess air. For natural gas or fuel oil firing

with no CO in the flue gas, the per cent excess air

can be determined from Figure 3. If other fuels are

used or if CO is present, the following equation

can be used:

% Excess air = O2 0.5CO% Excess air = x 100% Excess air = 0.2682N2 (O2 0.5CO)

Where O2 = oxygen by volume in flue gas (%)

CO = carbon monoxide by volume (%)

N2 = nitrogen by volume (%)

Examples: The flue gas analysis by volume on a

furnace burning natural gas gives the following

results:

O2 = 9.8%

CO2 = 6.2%

CO = 0%

From Figure 3, excess air is approximately 79 per

cent. This number can be compared to the

following calculation.

%N2 = 100% - (9.8% + 6.2% + 0%)

= 84%

Figure 3:Excess air versus flue gas analysis. (Source: Canadian Gov.) (Energy Management Series 7.

Page 7. Figure 4)


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% Excess Air = 9.8(0.5 x 0)% Excess Air = x 100% Excess Air = (0.2682 x 84)[9.8(0.5 x 0)]

= 77%

This value is very high for a furnace burning

natural gas, and the possibility of reducing the

excess air level should be investigated.

Another example will provide greater familiarity

with the calculation procedures. A furnace is

burning coke-oven gas with the following flue gas

analysis.

O2 = 2.1%

CO2 = 10%

CO = 0%

N2 = 87.9% (by difference)

The equation should be used to calculate the

excess air since Figure 3 is not applicable for coke-

oven gas.

% Excess Air = 2.1(0.5 x 0)% Excess Air = x 100% Excess Air = (0.2682 x 87.9)[2.1(0.5 x 0)]

= 9.8%

This excess air is quite acceptable for a furnace

burning coke-oven gas.

In a furnace burning natural gas with a deficiency

of air, the flue gas analysis is as follows.

O2 = 0%

CO2 = 11%

CO = 2%

N2 = 87% (by difference)

Figure 3 cannot be used because of the presence

of CO.

% Excess Air = 01(0.5 x 2)% Excess Air = x 100% Excess Air = (0.2682 x 87)[0(0.5 x 2)]

= 4.1%

Table 1: Combustion Air Requirements

Fuel Stoichiometric Air Typical Excess Air Total Air kg/GJ As

kg/GJ As Fired (minimum as a %) Fired

Natural Gas 318 5 334

#2 Fuel Oil 323 10 355

#6 Fuel Oil 327 10 360

Coke-oven Gas 1 295 15 340

Refinery Gas 2 312 10 343

Propane 314 5 330

CO 12%

H2 42%

CH4 37%

C2H4 and higher 5%

CO2 Remainder

1

Analysis by volume CH4 31%

C2H6 20%

C3H8 38%

H2 5.6%

C4H10 and higher 1.0%

Inert Gases Remainder

2

Analysis by volume


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This means that approximately 4 per cent less

than the theoretical air required for complete

combustion is being supplied to the burners. If the

type of process permits it, increasing the

combustion air supply should reduce the carbon

monoxide.

Occasionally, CO occurs with high O2. This is

usually an indication of poor mixing of the fuel and

combustion air. Sometimes improvements can be

made by adjusting the burner air dampers to

create more turbulence where the fuel and air

mix. In other instances it may be necessary to

replace the burner assembly.

2.2 HEAT LOSSES

The heat discharged from the stack, is usually the

largest loss in a fuel fired boiler or furnace. Flue gas

analysis and flue gas temperature can be used to

calculate the loss. If there is no heat recoveryequipment on the furnace or boiler, these

measurements should be taken at the outlet to

minimize the possibility of the readings being

affected by air infiltration. With heat recovery

equipment the readings should be taken

immediately downstream of the equipment.

The flue gas heat loss has four components, which

can be calculated separately.

Dry gas heat loss.

Heat loss from the water vapour

contained in the combustion air1

.


produced by the combustion of the

hydrogen in the fuel2.


produced by the evaporation of moisture

in the fuel3

.

For natural gas and oil, the moisture in the fuel is

minimal, and the evaporation of the moisture heat

loss can be ignored.The values for flue gas losses

can be calculated using figures from the appendix,

which gives a boiler efficiency test. Figure 4 below

shows this graphically for fuel oil.

1 This is often very small and is a function of atmospheric humidity.2 This quantity is a function of the fuel and therefore cannot be changed by

operation. It is therefore not included in this discussion.3 As above this quantity is primarily a function of the fuel and therefore cannot

be changed by operation. It is therefore not included in this discussion.

Figure 4: Flue-gas loss for fuel oil. (Source: Canadian Gov.) (Energy Management Series 6.

Page 12. Figure 10)


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In practice this loss can vary from 8% to 50%

depending on the fuel. The major influencing

factors are the exit flue gas temperature and the

degree of excess air present.To minimize losses in

coal-fired plant, correct combustion is essential

including better fuel preparation, better stoking

practices and improved control of combustion air

both the undergrate and the overgrate draughts.

The same factors apply to oil-fired boilers. Fuel

preparation should be correct (uncontaminated

and at the right temperature),burners undamaged

and properly maintained, and combustion air

(both primary and secondary) should be

introduced at the right rate and with adequate

turbulence.

For fuels such as coal, biomass, and industrial waste

or municipal refuse, the heat loss from the

moisture in the fuel can be considerable. Wood,

for instance, could have a moisture content of up

to 60 per cent, depending on the source and

capability of the wood burning equipment. Figure

5 shows the variations in the moisture heat loss

for a typical biomass fuel having different moisture

contents at a flue gas temperature of 200 C. At

30 per cent moisture, this fuel heat loss is 5.5 per

cent of the fuel heat content. At 60 per cent

moisture, the loss increases to 21 per cent.

2.2.1 HEAT LOSS DUE TO

INCOMPLETE COMBUSTION

Heat can also be lost by the incomplete

combustion of fuel, this is indicated by the

presence of CO and, in the case of coal,

combustible material left in the ash.

2.2.1.1 HEAT LOSS TO CO

By controlling the amount of dark smoke

produced, the level of CO can be kept to a

practical minimum. The three influencing factors

are insufficient combustion air, inadequate fuel/air

mixing, or the ingress of cold air freezing the

combustion reaction. The heat loss, which is

measured in terms of the non-conversion of

Figure 5:Flue-gas loss with moisture content for biomass fuel. (Source: Canadian Gov.)

(Energy Management Series 6. Page 13. Figure 11)


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9

carbon into carbon dioxide, is relatively small, but

the rapid fouling of heat transfer surfaces under

these conditions adversely influences the boilers

performance.

2.2.1.2 HEAT LOSS TO

COMBUSTIBLES IN THE ASH

(COAL APPLIANCES)

This loss generally varies from 2% to 5%. It is a

clear indication of combustion air starvation for

which there are three possible causes: poor air

distribution under the grate: too thick a fire bed: or

uneven bed thickness resulting from poor stoking

practices.

The unburned combustibles heat loss is not

significant for properly operating oil and gas fired

installations, but it can be for solid fuel units. Figure

1 demonstrates that there could be a minor

unburned fuel loss at the maximum efficiency

point, but the real significance of this figure is that

the losses increase very rapidly as the total air is

decreased. The measure of this condition is

reflected by the presence of significant

combustibles in the flue gas.

In coal, biomass and other solid fuels, unburned

combustible material will be found in the refuse

collected in the ash pit and the fly ash hopper.The

loss should be determined when the boiler is

tested for efficiency.To do so requires a method of

collecting and weighing the refuse under

controlled conditions and laboratory testing the

refuse for its HHV. The loss can be calculated as

shown.

Unburned combustible heat loss = Dry refuse

quantity x Refuse heat content

Where units are:

Heat loss (MJ/kg fuel as-fired)

Dry refuse (kg of refuse/kg of as-fired fuel)

Refuse heat content (MJ/kg of refuse)


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The transfer of heat from the burner flame to the

product can be by conduction, convection, or

radiation, and in most instances a combination of

all three.

3.1 CONDUCTION

Heat transfer to the product by conduction is only

significant in indirect heated equipment, where the

product is isolated from the flame by a heat

exchange surface. Muffle furnaces and furnaces

using radiant tube heaters (Figure 6) are examples

of indirect heating arrangements. Heat conducted

through a solid can be calculated.

Q = k x A x T x 3.6Q =Q = t

Where, Q = Heat conducted (kJ/h)

k = Thermal conductivity of solid

[W/(mC)]

A = Surface area (m2

)

T = Mean temperature differen-

tial across solid (C)

T = Thickness of solid (m)

3.6 = Conversion factor from watts

to kilojoules per hour.

The foregoing equation shows that rate of heat

transfer increases in proportion to surface area,

and to temperature differential across the solid,

and is inversely proportional to material thickness.

3. HEAT TRANSFER

Figure 6: Radiant Tube Gas-Fired Rotary Furnace. (Source: Canadian Gov.)

(Energy Management Series 7. Page 13. Figure 7)

10


19/93

11

Example: A muffle furnace has a 10 mm thick, high nickel steel enclosure with a surface area of 55 m2

.

Useful heat to the product, all of which is transmitted through the wall, is 1.9 GJ/h.The thermal conductivity

of high nickel steel is 31 W/(mC).The temperature drop through the muffle wall can be determined asfollows:

Heat Conducted = 31W / (mC) x 55m2

x DT x 3.6Heat Conducted =Heat Conducted = 0.01 m

Heat conducted is 1.9 GJ/h, or 1.9 x 106

kJ/h

Rearranging the equation,

T = 1.9 X 106

X 0.01T =T = 31 X 55 X 3.6

= 3.1C

The temperature drop across the enclosure is 3.1C at the specified rate of heat transfer.

surface increases, but not proportionally. The

following equation can be used for gases:

Q = 23.46 x A x T x V0.78

x d

Where, Q = Rate of convection heat transfer

(KJ/h)

A = Area of heat transfer (m2

)

T = Temperature differential between

solid and fluid (C)

V = Fluid velocity (m/s)

d = Gas density (kg/m3

)

3.2 CONVECTION

Heat transfer by convection takes place at the

boundary between a solid wall and a gas or liquid.

Intermingling takes place between the stagnant

layer of fluid at the wall and the moving fluid

stream next to the stagnant layer.Tests on rate of

heat transfer by convection show that the rate is

proportional to surface area and temperature

differential between the solid and the fluid. It also

increases as the velocity of the fluid over the wall

Example: A furnace is 3 metres long and has a 1 metre by 1 metre cross-section. Flue gas flows through

the furnace at an average velocity of 0.5 m/s with a gas temperature of 500C.The temperature differential

between the furnace walls and the flue gas averages 150C. For most practical purposes, the density of air

can be used for flue gas. From standard references, the density of air at 500C is 0.458 kg/m3

.The average

rate of heat transfer by convection to the walls, floor and roof can be determined as follows.

Furnace area swept by flue gas = (1 + 1 + 1 + 1) m x 3m

= 12 m2

Q = 23.46 x 12m2

x 150C x (0.5m/s)0.78

x 0.458kg/m3

= 11 263 kJ/h


20/93

12

3.3 RADIATION

Heat transfer by radiation becomes significant for temperatures above 600C. A hot body emits

radiation in the form of heat, which can be

received by another solid body in the path of heat

radiation. In an electric furnace or boiler, the walls

or tank, which are heated by the electrodes, emit

heat radiation to the furnace contents.

The amount of heat radiated from a solid body is

proportional to the fourth power of its absolute

temperature, and directly proportional to itsemissivity. Absolute temperature is the number of

degrees above absolute zero and is measured in

Kelvin (K), which is equivalent to degrees Celsius

plus 273.

Emissivity is a measure of the heat radiated from

an object compared to that radiated from a similar

sized black body at the same temperature.The

maximum value of emissivity is that of the black

body; which is 1. Typical emissivity values forfurnace walls and oxidized steel are 0.8 to 0.9.

Because both the hot body, (the furnace wall) and

the cooler body, (the furnace contents) are

emitting radiation, the net total heat received by

the contents is the difference between the heat

emissions of the two bodies. The equation for a

furnace is:

Q = K x F x [( T14

( T24

]Q = K x F xQ = K x F x [(100) (100) ]

Where, Q = Rate of radiation heat transfer

(kJ/h)

K = Black body coefficient (20.6)

F = Overall radiation factor

depending on emissivity and

surface areas of the furnace

walls and contents

T1,T2 = Absolute temperature of hot

and colder bodies respec-

tively (K)

F = A1F =F = 1 + ( A1 ) ( 11)F = 1 + ( A1 ) ( 1 1F = e1 + ( A2)( e21)

Where,A1 = Surface area of furnace

contents exposed to walls(m

2

)

A2 = Surface area of furnace walls

(m2

)

e1

= Emissivity of furnace contents

e2

= Emissivity of furnace walls

Example: A furnace with a square cross section of 1 metre by 1 metre is heating carbon steel billets

100mm by 100mm.The furnace wall temperature is 1000C.The furnace floor does not radiate heat. From

Table 3, the emissivity of a fireclay brick furnace wall is 0.75, and the emissivity of oxidized carbon steel is

0.80.The heat input to the billet per metre of length when the steel is heated to 650C can be calculated.

A1 = (0.1 + 0.1 + 0.1) x 1

= 0.3m2

A2 = (1 + 1+1) x 1

= 3m2

F = 0.3F =F = 1 + ( 0.3 ) ( 11)F = 1 + ( A1 ) ( 1 1F = 0.8 + ( 3 )( 0.751)

= 0.234

T1 = 1000C + 273

= 1273K

T2 = 650C + 273= 923K


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13

Heat radiated/metre length

Q = K x F x [( 12734

(9234

]= 20.6 x 0.234 x

Q = K x F x

[(100

) (100

) ]= 91 604 kJ/h

Radiation also takes place from hot gases to the

furnace contents. This method of heat transfer

does not follow the same laws as the radiation

from solid bodies.Radiation from a luminous flame

is higher than from a clear flame of hot gases.

4. the fuels

Each conventional fuel differs from the others in

its combustion characteristics, and this influences

heat transfer. Fuels may be solid, liquid or gaseous,

and either commercial or waste. Commercial

fuels are fossil fuels, which are extracted,

treated/refined to varying degree and sold

nationwide by organizations such as oil companies.

Waste fuels are by-products or adjuncts of

processing or domestic activities and are,

obviously, only economically available locally.

Factors other than simple conversion to heat must

also be considered, including those relating to: the

storage and handling of the fuels, maintenance,

environmental impact etc. All of these influence

the overall efficiency and true cost of burning a

fuel.

4.1 PIPELINE GAS

Because gas mixes so readily with air and burns

without producing smoke and soot, boiler and

furnace maintenance costs are low. Natural gas

burners tend to be simpler with fewer mechanical

parts and are also therefore cheaper to maintain.

Natural gas would normally be the preferred fuel

for burning in boiler plant if convenience alone is

considered. It does not have to be stored; in

common with all the gaseous hydrocarbons it

mixes readily with combustion air to burn clearly;

and, ideally, the products of combustion are just

water and carbon dioxide. These basic arguments

would seem to carry a great deal of weight because

globally the majority of new boiler and furnace

installations in recent years have been gas tired.

The availability of an adequate gas supply at

individual sites needs to be checked in advance as

local constraints in the distribution system can

sometimes lead to delays in providing a

connection. A second factor is safety. Complying

with legislation regarding the supply and use of gas

involves some specialised equipment that has to

be maintained.


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14

Thirdly, burning gas does cause pollution. While

the pollutants do not include smoke or noxious

substances, they do include gases that contribute

to the so-called greenhouse effect. Gas, being

composed predominantly of methane, is in itself

one such gas. Carbon dioxide, which is produced

by the combustion of all fuels, is another: its

production is not only unavoidable but also

desirable as its presence indicates complete

combustion of the gas. However, pipeline gas also

produces oxides of nitrogen (NOx). This is

because the gas burns at high temperatures and

this provides the additional energy necessary to

make the oxygen and nitrogen in the air combine.

As regards the pricing of gas, the actual price that

a customer will pay, as for any fuel, depends on the

amount used and the type of supply, and can vary

over a wide range. Prices are generally competitive

with oil products, for example with gas oil for firm

gas supplies and with heavy fuel oil for

interruptible supplies. Continued plant operation

during interruptions of an interruptible supply

requires a boiler to be dual-fuel fired usually with

oil as an alternative. In firing these two fuels the

burner would normally be set to achieve the most

effective results on gas, because gas is used for

most of the year, with oil firing only on the few

days of interruption sometimes experienced.

4.2 LIQUID PETROLEUM GAS

Liquid Petroleum Gas (LPG) is used to describe

two fuels: propane and butane. In practice the vast

majority of installations use propane. All the

general comments about natural gas apply equally

to LPG.

One major difference between the two fuels is

that LPG requires both storage facilities and the

special precautions needed in relation to leakages.

The first can be very significant in terms of both

the capital cost of a project and its overall

operational and maintenance costs. The storage

tanks involved are pressure vessels and therefore

subject to both annual and long-term inspection

and testing. If a customer owns his own tanks he is

responsible for carrying out all inspections and

tests at his own expense. In practice, many

customers lease or rent the tanks from the fuel

suppliers, eliminating both this responsibility and

also that of general maintenance.

The second major difference is that LPG is heavier

than air. If natural gas, which is lighter than air,

escapes, all sources of ignition should be removed

and windows opened: it will then disperse

naturally. LPG, on the other hand, may find its way

down into pipe ducts, cable tunnels, drains, cellars

etc., and will not disperse unless forced to using a

fan. This characteristic influences the location of

storage tanks in relation to buildings, hollows,

drains, cellars etc. and plant location may be

affected.

4.3 FUEL OIL

Crude oil is a complex mixture of hydrocarbons.

The other fuel users mainly require the lighter

fuelspetrol, kerosene, diesel, oil, gas oil etc.This

end of the barrel also provides the main

feedstock requirement for the petrochemicals and

plastics industries. However, the primary

separation of oil provides mainly the heavier more

viscous fuel oils, which potentially cause problemsin storage , handling, combustion and

environmental pollution.The main advance of fuel

oil, on the other hand, derives from the fact that

these heavier fractions tend to be cheaper.

Problems relating to fuel oil storage include both

the capital cost of the storage tanks and the

problem of handling the oil. Fuel oils are viscous

liquids, which become thicker and more

intransigent the colder they become. Gas oil, the

lightest and least viscous of the fuels, will usually

remain in liquid form no matter how cold the


23/93

15

winter. This either allows it to flow under gravity

from the tank to the burner or enables it to be

easily pumped. This holds true unless prolonged

periods of cold weather occur where the

temperature remains below freezing for a week or

more. Under these conditions, some of the waxes

contained in the oil begin to alter into sticky solids.

Typically, these solids build up on the filters in the

burner supply line, eventually blocking them.

Although this is an infrequent occurrence, some

exposed sites have installed electric trace heating

on the filters and/or the external distribution

pipework as a precaution.

The heavier grades of oil require heating in order

to remove them from the tank at all.To reduce the

amount of energy required for pumping the oil to

the burners, an appropriate pumping temperature

should be maintained.

Table 2 shows the recommended minimum

storage temperatures for the different grades of

oil and also the minimum temperatures for

optimising pumping costs.The temperatures given

in this table, especially for the heaviest oils are only

meant as an indication.With the exception of gas

oil, the general trend is for the heavier and more

viscous oil grades to require higher storage and

pumping temperatures.

The oil is heated either electrically or by taking

steam from the boiler, thereby reducing its overall

efficiency. The uncontrolled overheating of oil can

be very expensive, and uninsulated or poorly

insulated tanks or pipes are also a major waster of

energy.

Considerable energy is wasted if all the oil in a

tank is heated to the required pumping

temperature, and it is also bad practice to have

too much hot oil circulating and not being used by

the burners. A well designed hot oil ring main

circulates sufficient oil plus about 10% in order to

meet the maximum demand for all the burners it

serves. Fresh oil is drawn from the storage tank as

required, but the storage tank never forms part of

the basic circulation system thereby allowing all

the oil to heat up to the pumping temperature.

This ensures that both the size and the capital and

running costs of the oil heaters are kept to a

practical minimum.

The penalty of this oil heating requirement is that

it is uneconomic to use these heavier grades of

fuel oil on small boiler plant. Below 3 MW heavy

oil would be inefficient and, for bunker oil, 20 MW

is probably the lower limit. However, the market

price for the heavier fuel oils over recent years has

encouraged their greater use.

Provided that a grade of fuel oil is delivered to the

burner in good condition and at the correct

temperature for the burner, the production of

smoke or carbon monoxide should be minimal.

Table 2: Recommended Minimum Storage Temperatures for Different Grades of Oil

Fuel Oil Grade Viscosity Minimum Storage Typical Pumping

Type * *Cst @ 100C Temperature C Temperature C

Gas/Oil D 1.0 None stated None stated

Light E 8.2 10 10-12

Medium F 20.0 25 30-35

Heavy G 40.0 40 55-60

Bunker H 56.0 45 70

* Refers to BS 2869 - 1986.


24/93

16

The fact that all fuel oils contain some sulphur

means that sulphur oxides (SOx) are produced

during combustion. Such gases are now

considered to contribute to the global pollution

problem. Oil , however, burns at a lower

temperature than the gaseous fuels and therefore

produces less NOx gases.

4.4 COAL

The clean burning of solid fuels presents a

problem because the air required for combustionis less readily available to the mass of fuel,

compared with atomised liquid fuels and gas.As a

result, coal burning has been responsible for most

of the traditional forms of air pollution smoke,

soot, grit and dust. Modern coal plant using

microprocessor control, on boilers with improved

stoker design, has eliminated this problem.

Stringent control of SOx and particulates can be

achieved through the use of limestone injection,

cyclones and bag filters.

Throughout the sub-tropical and temperate

regions of the world coal deposits are generally

significantly larger than crude oil or natural gas

deposits. As crude oil prices have risen, many oil-

importing countries with significant coal deposits

have undertaken considerable research into coal

burning and, in some cases, have implemented

policy decisions promoting the use of coal for

boiler firing.

Coal is the cheapest of the available conventional

fuels. Furthermore, coal prices tend to be more

stable than prices for other fuels, and long-term

price contracts with only moderate built-in

increases are available.

A coal-fired plant does, however, incur higher

capital and operating costs.As well as the boiler or

furnace plant itself, the capital cost incurred

includes bunkerage, coal handling equipment, and

facilities for ash removal, handling and storage.

Operational costs are high because, despite

considerable development efforts by plant

manufacturers to reduce the labour component, it

is rare that coal fired plants are ever fully

automated and unmanned.

Maintenance costs are also significantly higher than

for the other fossil fuel.The difficulty of achieving

clean combustion means that the boilers require

more frequent cleaning. Both the fuel and the ash

are very hard and abrasive so levels of wear and

tear on coal and ash handling equipment are high.

The disposal of ash in a manner that avoids

pollution is a significant operational component

and, in some regions of the country,can be a costly

business.

Low combustion temperatures limit pollution

from NOx, but the SOx released by coal

combustion must be considered. Both the calorific

value and the sulphur content of coal vary from

source to source.The average South African coal

sold into the industrial market has low sulphur

content and is less polluting than the heavier fuel

oils.

4.5 CHOICE OF FUEL

The choice of fuel is not a simple matter. It involves

balancing a number of factors including the capital

cost of the plant, the price of the fuel, and

operating and maintenance costs. Some

consideration should also be given to likely future

changes in fuel and pricing policies and to

pollution control legislation.


25/93

17

Table 5 summarises those advantages and

disadvantages that can be estimated and quantified

for each fuel.

Table 4: Calorific values of Some Fuels

Fuel Calorific ValueMJ/Unit

GasNatural Gas 38.0/cu m

LPG Propane 50.0/kg

LPG Butane 49.3/kg

Fuel Oil

Gas Oil 38.0/liter

Heavy Oil 41.0/litre

Coal 29.0/kg

Table 5:The pros and cons of various fuels.

COAL FUEL OIL NATURAL GAS LPGDisadvantages Disadvantages Disadvantages Disadvantages

Capital Capital Cost For: Capital Cost For:Cost For:

Tanks Storage Tank (or Bunkerage leased)

InsulationFuel Handling

Heavy Fuel OilAsh Handling

Running Cost For: Running Cost For: Running Cost For:

Tank Heating Fuel (Especially for Small Fuel Cost Installations)

Heavy Fuel Oil Interrupt TariffHeavy Oil as Second Fuel

Maintenance Maintenance Costs Maintanance Costs For: Maintenance CostsCosts For: For: For:

Safety Equipment Safety Equipment Wear from Abrasive Boiler/FurnaceFuel & Ash Cleaning

Boiler Cleaning Burners

Environmental Costs: Environmental Environmental Costs: EnvironmentalCosts: Cost:

Smoke Emission High NOxSmoke Emission High NOx

Grit & Dust EmissionSulphur Emission

Sulphur Emission

Clean up Heavy Fuel Oil

Ash Disposal Cost Higher NO x

Advantages

Advantages

Advantages

Advantages

LowC

ost

CheaperThanGas

NoStorage

NoSulphur

NoSulphur


26/93

18

In order to ensure the proper mixing of fuels with

combustion air and the correct flame shape, for

maximum heat transfer from the flame to the

water/steam or heated product, specialized

equipment is used. The type of equipment is

dependent on the furnace/boiler conditions and

the fuel or fuels of choice. (Boilers and furnacescan be set up to fire more than one fuel.)

5.1 GAS BURNERS

Apart from the safety requirements in their

design, gas burners are essentially simple. Very

small boilers use a simple atmospheric burner,which entrains its combustion air from its

surroundings. However, as the air and gas are not

forced to mix, surplus air is required to ensure

complete combustion. This surplus is heated and

then passes out via the flue, thereby reducing

boiler efficiency.

A larger boiler with a fully enclosed combustion

chamber needs a burner that will force the air and

gas to mix thereby controlling the length andshape of the flame.The quantity of combustion air

can be precisely controlled to maximise

combustion efficiency.

Natural gas mixes readily with air.The ring-type gas

burner consists of a circular barrel ringed with

multiple outlet ports. The spud type burner

consists of a ring of 4 to 8 single barrels, each with

a widened end containing multiple outlet ports. In

either case the register surrounds the barrels with

air.

Many boilers are equipped with combination

natural gas and oil burners with the second fuel

used as back up for the prime fuel.

5.2 OIL BURNERS

Oil burners are more complicated because the

fuel has to be in the right condition for clean and

rapid combustion. This entails atomising the oil

into small droplets of the correct size, which can

only be done if the oil is at the right temperature

and therefore the right viscosity. At too low a

temperature the droplets are too big: combustion

is poor and produces soot and smoke.At too high

a temperature the droplets can be too small,passing through the flame too rapidly to burn. In

neither case is the full energy content of the fuel

being used: furthermore, the heat transfer surfaces

become fouled.

Oil burners are of three basic types.The simplest

and most widely used is the pressure jet where

the oil is pumped at pressure through a nozzle.

The air or steam blast type uses gas pressure to

shatter the oil into droplets, while the Rotary Cupuses centrifugal force to break the oil up. Each

type of burner has its benefits and disadvantages.

5.2.1 PRESSURE JET

Advantages:

Very simple in construction and cheap to

replace.

Comes in many sizes to suit most

applications.

5. combustion equipment: oil and

gas burners


27/93

19

Can produce all flame shapes from long

and thin to short and fat so can fit all

types of boiler or furnace combustion

chamber.

Disadvantages:

Prone to clogging by dirty oil so needs

fine filtration.

Limited turndown ratio of only 2:1.

Easily damaged during cleaning.

Highest oil pre-heat temperature requi-

red for atomisation.

5.2.2 AIR OR STEAM BLAST

ATOMISER

Advantages:

Very robust in construction.

Good turndown ratio of 4:1.

Good control of the combustion air/fuel

over the whole firing range.

Good combustion of the heavier fuel oils.

Disadvantages:

Energy used either as compressed air or

as steam for atomisation.

5.2.3 ROTARY CUP

Advantages:

Good turndown ratio of better than 4:1.

Good atomisation of heavy fuel oils.

Lowest oil pre-heat temperature required

for atomisation.

Disadvantages:

Most complex and costly to maintain.

Electrical consumption required for the

cup drive.

Oil and gas burners produced or sold in this

country have to meet statutory safety and

emission standards.

5.2.4 LOW EXCESS AIR BURNERS

Standard natural gas and oil burners operate at 10

to 15 per cent excess air at full capacity and higher

excess values at lower firing rates.The increasing

excess air with decreasing firing rate phenomenon

results from burner registers, which are fixed at

settings that provide best results at full capacity.

Low excess air burners permit operation at 2 to 5per cent excess air. A reduction of excess air from

15 to 5 per cent would reduce fuel costs by

almost 1 per cent.These savings result from higher

cost features as follows:

Better design of the air diffusers, air

register, and burner, which achieve better

mixing and combustion.

Burner registers which are modulated

with the tiring rate to provide bettercombustion at firing rates below 100 per

cent.

5.3 BURNER CONTROLS

In conjunction with the choice of burner type,

consideration must be given to the control system

required. The simplest ON/OFF control means

either that the burner is firing at full rate or that it

is off.The major disadvantage with this method of

control is that the boiler is subject to large and

often frequent thermal shocks every time the

boiler tires. Its use is therefore limited to small

boilers with an output up to 300 kW.

Slightly more complex is the HIGH/LOW/OFF

system where the burner has two firing rates.The

burner operates first at the lower tiring rate and

then switches to full firing as needed, thereby

overcoming the worst of the thermal shock.The


28/93

20

burner can also revert to the low-fire position at

reduced loads, again limiting thermal stresses

within the boiler. Typically this type of system is

fitted to boilers with an output of up to 3.5 MW.

A modulating burner control will alter the firing rate

to match the boiler load over the whole turndown

ratio. Every time a burner shuts down and restarts,

the system must be purged by blowing cold air

through the boiler passages: this wastes energy and

reduces efficiency. Full modulation, however, means

that the boiler keeps firing, and fuel and air are

carefully matched over the whole firing range to

maximise thermal efficiency and minimise thermal

stresses.Typically this type of control can be fitted to

boilers above 1 MW.

In matching a burner and a control system to a

boiler three factors must be taken into

consideration.

The maximum output of the plant:

Whether the load is steady or fluctuating:

The fuel being used.

An ON/OFF control, for instance, is not suitable

for heavy fuel oil

The basic choices as they relate to oil burners are

summarised in Figure 7. There is always some

overlap between burner types and control system

types but the preferred combinations are outlined.

Figure 7:Type of fuel oil with recommended burners and controls. (Source: ETSU)

(Good Practice Guide 30.Page 67. Figure 38.)


29/93

21

Because carbon burns fairly slowly and coal needs

to be in the combustion chamber for a relatively

long period for the air to reach it and cause

complete combustion, many forms of stoker (for

transferring coal to the grate) have been

developed. Some have experienced periods of

popularity and have now declined, while othershave stood the test of time.

Coals from different pits or washeries can have

very different combustion properties.

Furthermore, coals from the same pit that have

been stocked for long periods are very different

from newly mined coal. As a result a boiler

combustion system must be regularly adjusted to

maximise energy conversion. In the following

section only those types of stoker that would befitted to a boiler with an output of 1.5 MW and

above are considered. Below this level there is

limited choice: each boiler comes with its own

proprietary form of stoker, screw feeding the coal

either onto the top of the fire or pushing it up

from below.

Three basic types of stoking system are commonly

used with the larger boilers - two of them

traditional designs and one a relatively modern

development.

6.1 STOKERS

Stokers are mechanical devices that burn solid fuel

in a bed at the bottom of a combustion chamber.

They are designed to permit continuous or

intermittent fuel feed, fuel ignition, adequate

supply of combustion air, release of gaseous

products, and disposal of ash.

Stokers are classified according to the manner in

which the fuel reaches the fuel bed. In an underfed

stoker, the fuel and air enter the burning zone

from beneath the bed. Overfed stokers have the

fuel entering the combustion zone from above, in

the opposite direction to the airflow. The

spreader-type overfeed stoker delivers fuel so thata portion burns in suspension while the remainder

falls and burns on the moving grate.

6.2 CHAIN GRATE STOKER

The chain grate stoker has for many years been

the most widely used method for firing coal on

medium sized industrial and commercial boilers,

even though it is relatively expensive to buy,

operate and maintain. To reduce operating costs

equipment manufacturers are working to develop

a fully automatic system requiring little or no

intervention from trained operators.

The coal is fed onto one end of a moving steel

belt. As the belt moves along the length of the

furnace, the coal burns before dropping off the

end as ash. Some degree of skill is required,

particularly when setting up the grate, air dampers

and baffles, to ensure clean combustion leaving the

minimum of unburnt carbon in the ash and to

achieve maximum heat transfer in the furnace

chamber.

This type of stoker will only operate effectively

using certain types and qualities of coal. Coal must

be uniform in size, as large lumps will not burn out

completely by the time they reach the cod of the

grate. Furthermore, small pieces or fines may

block the air passages in the grate and make it

6. combustion equipment:

solid fuel combustion


30/93

22

more difficult for combustion air to reach the coal.

The grate also relies on having a layer of ash on

top of it to protect it from the highest

temperatures of the burning coal, so using coals

with a very low ash content will result in rapid

grate damage.

6.3 SPRINKLER STOKER

The sprinkler stoker is an original mechanical

stoker system,which has been brought up to date.

The principle is to spread fresh coal on top of an

already, burning firebed.Once the system has been

set up to spread this coal evenly it is simple to

operate and has many fewer mechanical parts to

maintain than the chain grate stoker.

Many units of this type have been manufactured

with control systems very similar to those for gas

or oil-fired boilers. Fuel feed rate and combustion

air are adjusted in parallel to give a turndown ratio

of 3:1.The chain crate stoker can also achieve this

but the sprinkler can be regulated much more

quickly.

This type of stoker was popular initially because it

was very much cheaper than the chain grate

equivalent. Its main drawback was that it had to be

de-ashed by hand. Effort has been put into

developing an automatic de-ashing system but,

obviously, this has considerably eroded the

sprinkler stokers price advantage.

Like the chain grate stoker, this type of stoker is

selective with regard to fuel size. Fines in the coal

are picked up by the combustion air and flue gases

and carried through the boiler. This can cause

considerable erosion within the boiler and result

in high grit emissions from the stack.

6.4 FLUIDISED BED

COMBUSTION

Fluidised bed combustion is the most recent coal-

burning technology, the fuel being fed onto a hot,

air-agitated bed of refractory sand.This system has

two main advantages:

1. It is much less selective in terms of fuel quality

and can burn not only very poor coal with ahigh ash content but even industrial or

commercial waste.

2. The lower combustion temperature involved

allows cheaper materials and refractories to be

used in its construction.

However, this technology is still new and is in the

experimental stage in South Africa.


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23

A short description of common equipment used

for saving energy in boilers and furnaces follow. In

some cases these are discussed further under the

energy savings sections of either boilers or

furnaces.

7.1 FLUE GAS HEAT

EXCHANGERS

Since most of the heat losses from a fuel fired

furnace appear as heat in the flue gas, the recovery

of this heat can result in substantial energy savings.

A common method is to install a heat exchanger

at the furnace exit.

A heat exchanger can be used to transfer heat

from the hot flue gas to the incoming combustion

air, or to the heat water used elsewhere in the

plant.The rate of heat transfer is proportional to

the surface area of the heat exchanger, and to the

mean temperature differential between the flue

gas and the combustion air.

Q = U x A x LMTD x 3.6

Where, Q = Rate of heat transfer (kJ/h)

U = Heat transfer coefficient of

heat exchanger [W/(m2

C)]

A = Surface area of heat ex-

changer (m2

)

LMTD = Logarithmic mean tempe-

rature difference (C)

3.6 = Conversion factor from

watts to kilojoules per hour

LMTD = T1T2LMTD =LMTD = T1

LMTD = LnLMTD =

(T2

) Where, LMTD = Log mean temperature dif-ference (C)

T1 = Greater temperature differ-

ence between the flue gas

and the heated air or water

(C)

T2 = Lesser temperature differ-

ence between the flue gas

and the air or water (C)

Ln is the natural logarithm

A heat exchanger may be used to heat water with

the heat from flue gases. An important design

consideration is how close the heated water

temperature should be to the temperature of the

hot gas entering the exchanger. It is not possible to

heat the fluid to a temperature above the

temperature of the hot gas entering, regardless of

the relative fluid and hot gas flows. Small

temperature differentials imply large heat

exchanger surfaces. This is illustrated by the

following example.

7. energy saving equipment


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Figure 8:Tempering Air Heat Exchanger. (Source: Canadian Gov.)

(Energy Management Series 7.Page 18. Figure 11.)

Example of savings

A heat exchanger is to be added to a dryer which is exhausting 450 000 m3

/h of moist air at 100C.The

exhausted air is used to heat up 350 000 m3

/h of incoming air from an ambient temperature of 10C to

85C, which is within 15C of the hot exhausted air (Figure 8). The heat exchanger design has a heat

transfer coefficient quoted by the manufacturer of 28 W/(m2

C). Heat given up by the exhausted air is

equal to the heat gained by the incoming air, since there are no significant heat losses in a heat exchanger

of this type. Density of air at standard conditions is 1.204 kg/m3

, and specific heat is 1.006 kJ/(kgC).The

surface area of the heat exchanger required can be calculated as follows:

Cold air heat gain (Q) = Volumetric flow x Density x Specific heat x Temperature rise

= 350 000 m3

/h x 1.204 kg/m3

x 1.006 kJ/(kgC) x (85-10)C.

= 31.79 x 106

kJ/h

Exhaust air heat loss = Volumetric flow x Density x Specific heat x Temperature drop

= 450 000 x 1.204 x 1.006 x (100CTout) kJ/h

Cold air heat gain = Exhaust air heat loss

This can be expressed as:

31.79 x 106

= 450 000 x 1.204 x 1.006 x (100CTout) kJ/h

Rearranging the equation:

(100C - Tout) = 31.79 x 106

(100C - Tout) =

(100C - Tout) = 450 000 x 1.204 x 1.006= 58.3C


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Heat exchanger exhaust temperature,Tout = 100C58.3C = 41.7C

Maximum temperature differential, T1 = 41.7C10C

= 31.7C

Minimum temperature differential, T2 = 100C85C = 15C

The logarithmic temperature difference (LMTD) is:

LMTD = 31.7 C15 CLMTD =LMTD = 31.7 CLMTD = InLMTD = ( 15 C )

= 22.3C

Cold air heat gain (Q) = 31.79 x 106

kJ/h = 28 W/(m2

C) x A x 22.3C x 3.6 kJ/Wh

Surface area, A = 31.79 x 106

Surface area, A =Surface area, A = 28 x 22.3 x 3.6

= 14 142m2

If the cold air is heated to within 5C of the exhausted moist air instead of 15C, the size of the heat

exchanger required in increased considerably.The calculations are as follows:

Temperature of heated air = 100C5C

= 95C

Cold air heat gain = 350 000 m3

/h x 1.204 kg.m3

x 1.006 kJ/(kgC) x (9510)C

= 36.03 X 106

kJ/h

(100CTout) = 36.03 x 106

(100CTout) =(100CTout) = 450 000 x 1.204 x 1.006

= 66.1C

Tout = 100C66.1C

= 33.9C

T1 = 33.9C10C

= 23.9CT2 = 100C95C

= 5C

LMTD = 23.9 C5 CLMTD =LMTD = 23.9 CLMTD = InLMTD = ( 5 C )

= 12.1C

Surface Area (A) = 36.03 x 106

Surface Area (A) =Surface Area (A) = 28 x 12.1 x 3.6

= 29 541 m2


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7.1.1 ECONOMISER (FEEDWATERHEATER)

This is applicable mostly to boilers, and is an

option used for heating incoming boiler water by

cooling the flue gases. The equipment is a gas-

liquid heat exchanger. Care must be taken not to

allow the flue gases to cool below the sulphur

dew point. Economizers can be considered where

hot water is required. For furnaces, if the use ofhot water and the operation of the furnace do not

always occur simultaneously, it may be practical to

install an insulated hot water storage tank. This

would level out the effect of variations in the hot

water supply and demand.

7.1.2 RECUPERATOR (AIR HEATER)

In a recuperator air entering the combustion

chamber is preheated using the heat of the hot

exhaust flue. This is an important measure for

furnaces where preheating the feed with flue gases

is more difficult that for boilers.The hot gas passes

inside tubes arranged in bundles.The combustion

air is directed over the outside of the tubes by

means of a series of baffle plates. Combustion air

pre-heat has always been regarded as the poor

cousin of the economizer for boilers because air

pre-heaters are large and less efficient than a gas-

liquid heat exchanger - or economizer - used toheat boiler feed water.

7.2 ACCUMULATORS

Boilers produce steam to meet demand. When

spikes in this demand occur, or the load is uneven,

it is often the case that an extra boiler would have

to be used intermittently, or output of severalboilers would rise to meet this demand. In the first

case this can be inefficient due to losses associated

with the heating and cooling of the boiler shell. In

both cases, some of the required boiler capacity

(and running and capital outlay) could have been

avoided by using an accumulator.

An accumulator effectivelystores oraccumulates

steam from boilers during times of low demand

and then can release it during short high demandintervals.

7.3 INSULATION

Insulation is used to retain heat within the furnace

or boiler enclosure. Common insulation materials

include calcium silicate, mineral fibre, ceramic fibre,

cements, cellular glass and glass fibre.An indication

of the heat loss from the hot walls of a furnace or

boiler is given in figure 9.

It should be noted that the reduction in the temperature differential to 5C would require the heat

exchanger area to be slightly more than doubled. An increase in design temperature rise of the incoming

air from (85C10 0C) = 750C to (95C10C) = 85C results in an increase in heat recovery of

(85 C75 C)(85 C75 C) x 100 = 13%

75 C

A careful analysis of capital costs and savings in fuel costs for different possible heat exchanger sizes is

important.


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A significant development in this field, for furnaces,

has been the use of ceramic fibre insulation, which

is a better insulator than solid refractory material

and also requires less heat to reach the operating

temperature. The disadvantages are higher initial

cost and low resistance to physical damage.A layer

of refractory on the bottom of the furnace and

other areas subject to damage is normally used to

protect the ceramic fibre. Further layers ofceramic fibre insulation can be installed on the

outside of the refractory as required.

7.4 O2 ANALYSERS

Systems for checking the O2 or CO2 content of a

boiler flue gas have been available for a long time

but, historically, none have been sufficiently reliable

to be incorporated in an automatic control

strategy. Portable or permanently installed O2 or

CO2 monitoring equipment used by a well trained

and intelligent boiler operator is still the best

method of limiting excess air and hence increasing

efficiency.

The production of the zirconium cell for O2

detection has made available a reliable measuring

system, and this has resulted in the development

of various systems,which automatically control theamount of excess air, thereby overcoming

variations in the fuel and air parameters. Using

these oxygen detection feedback controllers,

usually termed oxygen trim control, allows much

lower excess air levels to be achieved throughout

the operating range.

The simplest systems use the feedback signal

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