Pet coke.pdf

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Reprint of article publishedin World Cement, April 2002.

By Gary R. Roy, F.L.Smidth Inc., USA.

Petcoke Combustion Characteristics

R E V I E W N O. 1 3 9



Introduction

Cement producers all over the world are

striving to lower their production costs,

one effective method of which is the

substitution of traditional fuels such as

coal, oil and natural gas with petcoke.

However, as many producers have expe-

rienced, the use of petcoke as a fuel pre-

sents several challenges that must be

addressed. Its high sulfur content can

present operational difficulties if not

properly addressed, and the fact that it

burns at a much slower rate than normal

coals means that for many existing kiln

systems it is not possible to fire 100%

petcoke in the kiln and calciner withoutspecial design considerations.

Many of the classical FLS calciners

have been successfully retrofitted to

enable petcoke firing, and new plants

specifically designed to enable 100%

petcoke firing have recently been com-

missioned. This article highlights the

equipment and the experience from sev-

eral plants that have been designed or

modified by F.L.Smidth to enable 100%

petcoke firing.

Petcoke as fuel

Petroleum coke is the solid residue that

remains after extraction of all valuable

liquid and gaseous components from

crude oil. The volatile content range is

typically 5 - 15%, depending on the cok-

ing process. The main difficulty in burn-

ing petcoke is its low reactivity due to

this low volatile content. This low reac-

tivity can be compensated for in a num-

ber of ways, such as finer grinding, high

momentum rotary kiln burner design

and calciner design.One traditional solution is to grind

the coke to a much finer residue than

standard coal. Figure 1 shows the

Reprinted from WORLD CEMENT April 2002

Figure 1. Recommended fineness for pulverised fuels.

Figure 2. Duoflex burner.

Gary R Roy, F.L.Smidth, USA, examines the way in which the combustion

of petcoke has had an effect on modern calciner and burner designs.



relationship between the volatile content of the fuel

and the required fineness to enable good combus-

tion in the kiln and calciner.

Most petcokes have a high sulfur content. High

sulfur petcoke is sold at a low price but may require

extra precautions in the cement kiln. Operational

problems, due to the increased internal sulfur circula-

tion in the kiln system that results, may often be

solved by burner design and operational mode, or by

returning dust to the upper end of the rotary kiln.

Ignition, combustion andburnout characteristicsThe initial de-volatilisation and com-

bustion of the volatile matter in the

coke particles is very fast, whereas the

combustion of the residual coke may

then require several seconds to com-

plete. The burning rate of an individual

char particle depends primarily on its

size (dp), the amount of oxygen presentin the local atmosphere and the local

temperature (T).

In the very high temperatures of

the flame in the rotary kiln, the com-

bustion rate is determined by the dif-

fusion of the oxygen molecules

towards the surface of the shrinking

char particle against the wind of CO

and CO2. At these temperatures, the

main influence of the burnout time

will be the size of the particle and the

amount of oxygen present. A highmomentum burner, such as the

Duoflex kiln burner depicted in Figure

2, which provides a vigorous mixing

of the particles with the local atmos-

phere, increases the probability that

the char particle will encounter oxy-

gen to facilitate the quick burnout.

In the calciner, the temperature

cannot exceed the equilibrium tem-

perature for calcination (850 - 900 ˚C),

as long as a reasonable amount of car-

bonate is present. At this moderatetemperature level, even a relatively

fine ground petcoke meal burns so

slowly that the heat produced is used

immediately for calcining. The reac-

tion rate is controlled by the speed in

which oxygen combines with carbon

on the particle surface. The reaction

between the oxygen and the char par-

ticle is the limiting factor, and the

burnout time will be roughly propor-

tional to dp. It will therefore require a

calciner with a considerable retention

time to complete the combustion

process before the exit of the calciner.

As the combustion rate doubles

every time T is raised by 70 ˚C, it can be

a great advantage to have a localised

‘hot zone’ in the calciner to promote

faster combustion.

Petcoke firing in the in-line calciner (ILC)In an ILC calciner, the kiln exhaust gases pass up

through the riser duct and into the calciner vessel.

Hot air from the cooler is brought to the calciner via

an air duct to provide oxygen for combustion of the

calciner fuel. As the initial average percentage of O 2

in the calciner is considerably lower than 21%, and

the average temperature is typically less than 900 ˚C,

Reprinted from W ORLD CEMENT April 2002

Figure 3. Operation of ILC with 100% petcoke firing.

%O2 ppm CO ppm NOx

Stage I 3.8 292 631

Stage IV 2.5 44 560

Loop duct outlet 3.2 150 560

Calciner outlet 3.8 765 530

Calciner ‘hot zone’ 4.3 1352 540

Riser duct 0.6 32 000 615

Kiln exit 2.5 7 955



it is very difficult to burn a low volatile fuel.

Furthermore, most older ILC calciners on existing sys-

tems, designed for coal firing, have a gas retention

time significantly less than 3 sec. Under these condi-

tions, it is not possible to burn 100% petcoke unless

ground to an extreme fineness, and then only if other

measures are taken.

FLS has used experience from several ILC plants

that have been converted to petcoke firing. In these

cases, a portion of the petcoke is introduced directlyinto the tertiary air duct before it has mixed with the

kiln gas. As such, it is an advantage to have a high effi-

ciency cooler so that the tertiary air temperature will

be as high as possible to promote ignition. The

remaining petcoke is distributed into the lower cone

section of the calciner. The raw meal is added above

the cone section and distributed along the walls of

the calciner by means of the double deflector in order

to provide a relatively raw meal free ‘hot zone’. A

small portion of the raw meal can be diverted to the

riser duct to limit build-ups. In these cases, the pet-

coke must still be ground to a fineness of approxi-

mately 1% retained on 90 microns.FLS recently commissioned a new ILC kiln specifically

designed to fire 100% petcoke. In this case, the reten-

tion time of the calciner was designed to be

7 sec by means of a long loop duct between the main

calciner vessel and the bottom stage. All of the fuel was

injected into the kiln riser duct. The petcoke had a

volatile content of 18%, and was ground to 7% retained

on 90 microns. The meal from the second lowest cyclone

is split between the riser duct and the top of the calcin-

er lower cone to create the ‘hot zone’ in order to facili-

tate the combustion of the petcoke. The notch in the

centre of the calciner and the sharp bend in the top ofthe loop duct promote mixing of the calciner gases to

aid in the complete burnout of the fuel. Gas measure-

ments reveal that although the CO level is very high in

the bottom of the calciner, the CO exiting the bottom

stage was less than 50 ppm, and the residual carbon (%

C) in the bottom stage material stream was 0.06%,

which indicates almost complete combustion (Figure 3).

Petcoke firing in separateline calciners (SLC)FLS has been able to burn pet-

coke in a large number of exist-ing SLC calciners. Again, a ‘hot

zone’ is created; this time by

dividing the calciner vessel into

two sections, with only a portion

of the raw meal flow directed to

the lower section. This results in a

lower raw meal concentration in

the bottom section, which facili-

tates a high temperature so that

the petcoke will burn at a suffi-

cient rate.

This ‘double calciner’ solution

is fairly easily implemented in

most SLC plants, whenever there

is ample height for the splitter

gate to divide the raw meal

from the second lowest cyclone of the C-string

between the lower and upper calciner sections.

F.L.Smidth has a reference of several SLC-S 100%

petcoke fired plants and a 100% anthracite-fired

plant, which is a fuel that is even more difficult to

burn than petcoke.

An SLC plant with two calciners was recently con-

verted to 100% petcoke firing. The modification

included the addition of diverter gates to split the

meal between the lower and upper portions of thecalciner and the riser duct, restrictions in the centre

of the calciner and lowering the solid fuel injection

points in the calciners (Figure 4).

In the kilns used today, the calciners are 100%

fired with petcoke containing 5% volatiles and

6.74% S, which is ground to a fineness of 0.5%

residue on the 90 micron sieve. The hot zones of the

calciners are operated at 1050 ˚C, and the gas reten-

tion time is 3 sec. The main burner fuel is a mixture of

petcoke, anthracite and lignite, adjusted so that the

SO3 of the clinker is close to 1.6%. The clinker pro-

duction rate is 10 200 tpd at a fuel consumption of

725 kcal/kg clinker.Two 5600 tpd SLC kilns are also firing 100% pet-

coke. In these kilns, the meal is again divided so that

the ‘hot zone’ is created, and they are able to have

stable operation even with the coke, which contains

only 11% volatiles and is ground to a fineness of only

9 - 12% retained on the 90 micron sieve.

Calciners tailor-made forlow volatile fuelsThe SLC-D system features a down draught calciner

(DDC), in which there is a downward flow of gas,

raw meal and fuel. The combustion air containing21% O2 carries the raw meal in suspension, where it

is introduced tangentially at the top of the calciner.

This tangential action causes the raw meal to travel in

a spiral movement close to the cylinder wall, protect-

ing the walls from the high temperature zone. The

fuel is fired from the top in a Duoflex burner, which

creates a well-mixed flame in the centre of the


Figure 4. Modification of SLC for 100% petcoke firing.



calciner vessel. The very high combustion rate is dueto the high flame temperature that is only possible in

the raw meal free atmosphere (Figure 5).

The outlet from the down draught calciner is con-

nected with the kiln riser pipe. The rotational motion

of the calciner output provides efficient mixing

between the hot combustion gases, raw meal and the

kiln gas flow. The calcined raw meal separated to the

kiln obtains a high degree of calcination, for example

in one plant 95% calcination was achieved, with only

845 ˚C in the lowermost cyclone.

The DDC arrangement is also easily retrofitted

into an existing SP or ILC system to help boost pro-

duction or adapt for the firing of low volatile fuels.

The first 100% petcoke fired DDC was inserted in

a kiln line producing white clinker. The high degree

of combustion 99.8% is attained, although the com-

bustion air/tertiary air is only 280 ˚C. Unlike grey

cement plants, white pyroprocessing systems cannotretrieve 800 - 900 ˚C hot air from the clinker cooler.

The calciner must make do with warm air generated

by a heat exchanger using preheater exit gas. The

preheated raw meal from the second lowest cyclone

is split between the kiln riser pipe and the tertiary

air riser leading up to the calciner. With this

arrangement, the raw meal is actually preheating

the combustion air prior to its entry into the DDC

(Figure 5). This is the ultimate case that shows that

the final design of the calciner, and the ability to

create and maintain a hot zone, are the critical com-

ponents in enabling a calciner to burn 100% pet-

coke efficiently.

Bibliography HUNDEBØL S., ‘Recent advancement in petroleum coke use for

cement manufacturing’, Proceedings of International Exhibitionand Seminar on Energy and Environment in Cement, Constructionand Allied Sectors, January 2002, New Delhi, India.

KAPLAN E. & NEDDER N., ‘Petroleum Coke Utilisation for CementKiln Firing’, Proceedings of 2001 IEEE meeting in Vancouver , 251.

KEEFE, B.P., ‘Plant Modernization Projects Employ New Calciner

Designs’, International Cement Journal , January 1997, 40-46.


Figure 5. Operating data from 100% petcoke fired SLC-D kiln.



DENMARK

F.L.Smidth

Vigerslev Allé 77DK-2500 Valby

Copenhagen

Tel: +45 - 36 18 10 00

Fax: +45 - 36 30 18 20

E-mail: [email protected]

INDIA

Fuller India Limited

Capital Towers

180, Kodambakkam High Road

Nungambakkam

Chennai 600 034

Tel: +91 - 44-252-191234

Fax: +91 - 44-2827-9393

E-mail: [email protected]

USA

F.L.Smidth Inc.

2040 Avenue C

Bethlehem, PA 18017-2188

Tel: +1 - 610-264-6011

Tel: +1 - 800-523-9482

Fax: +1 - 610-264-6170E-mail: [email protected]

Data in this brochure is intended for preliminary project planing only. Manufacturer reserves the right to modify equipment details and/or specifications without notice.

www.flsmidth.com

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