Coal as Energy in the Steel Industry - American Chemical Society

Coal as Energy in the Steel Industry D A N P. M A N K A

Downloaded by GEORGETOWN UNIV on February 19, 2015 | http://pubs.acs.org Publication Date: November 12, 1982 | doi: 10.1021/bk-1982-0205.ch014

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The iron and steel industry uses up to 20 percent of the energy consumed by industry in the United States. Coal is a major source of this energy. Turning coal into coke has three major advantages to the industry: coke is essential as a source of heat in the blast furnace. One-third of the gas generated during the coking of coal is a source of heat for underfiring of the coke ovens and two-thirds is used in steel reheating furnaces. The coal-byproduct chemicals derived from the gas are increasingly valuable. Washed metallurgical bituminous coal is charged into the ovens. It must have low sulfur and low silica content; it must have proper volatile matter and expansion properties, otherwise it will stick in the ovens. Analyses of the various coals charged to the ovens will be discussed, as well as the composition of the gases liberated during the carbonisation cycle. Recovery of the various chemicals from the gas will be discussed. The mixture of iron ore and limestone in the blast furnace is heated by the coke to form liquid pig iron. The gases liberated in the blast furnaces are used in stoves to preheat the air which is blown into the furnace to maintain combustion of the coke. The liquid pig iron is refined to liquid steel by direct reaction with oxygen in the Basic Oxygen Furnace. Some boiler coal is used as a source of heat in boilers to produce steam used throughout the coke and steel plants. The iron and steel industry uses up to 20 percent of the energy consumed by all industry in the United States. Coal is a major source of this energy. Converting coal Into coke serves this steel industry in three ways. 1. Coke is essential as a source of heat and as a reactant in the blast furnace to convert iron ore into pig iron. 0097-6156/82/0205-0281$06.00/0 © 1982 American Chemical Society

In Coal and Coal Products: Analytical Characterization Techniques; Fuller, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


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2. One-third o f the gas generated d u r i n g the coking o f c o a l i s a source o f heat f o r u n d e r f i r i n g o f the coke ovens. 3 . The remaining two-thirds o f t h e gas i s used i n the r e heating furnaces i n the s t e e l p l a n t . Valuable chemicals are recovered from the gas i n the coke p l a n t . Benaene i s used i n the production o f nylon, toluene and xylene a r e used as s o l v e n t s and i n the making o f v a r i o u s chemic a l s . Phenols recovered from t a r and the gas f i n d uses i n making thermosetting p h e n o l i c p l a s t i e s . Ammonia i s used as f e r t i l i s e r i n a g r i c u l t u r e . Coumarone-indene recovered from the high b o i l i n g s o l v e n t i s used i n the r e s i n i n d u s t r y . Naphthalene recovered from t a r i s used t o make p h t h a l i c anhydride. There a r e l i t e r a l l y hundreds o f compounds i n the t a r and the chemicals recovered from the coke oven gas. S e v e r a l o f these a r e i n s u f f i c i e n t l y l a r g e concentrations t o make recovery i n pure form economically f e a s ible. In the production o f coke c o a l i s heated f o r s e v e r a l hours i n ovens i n the absence o f a i r a t temperatures above 1000°C t o remove the v o l a t i l e matter i n the c o a l . C o a l must be washed with water t o remove many i m p u r i t i e s . I t i s charged t o the ovens i n the wet o r d r y form. A coke oven i s constructed from high temperature, s t r o n g b r i c k s . The oven dimensions a r e 1 5 t o 18 inches i n width, 1 5 f t . high, and f t . i n l e n g t h . The l a r g e s t width o f 18 inches i s on the s o - c a l l e d coke s i d e where the coke i s pushed out o f the oven i n t o coke c a r s . Subsequently, the r e d hot coke i s cooled d i r e c t l y with water. The narrow, o r 1 5 i n c h width, i s on the other end of the oven g e n e r a l l y c a l l e d the pusher s i d e where a l a r g e ram from a pusher c a r i s guided i n t o the oven t o push out the coke. Between the outer w a l l s o f the ovens i s an opening extending the f u l l height and l e n g t h o f the oven which i s the f l u e . Coke oven gas i s burned i n these f l u e s t o supply heat t o the charged c o a l . The w a l l on one s i d e o f the oven i s heated f o r a predetermined time and then the other w a l l i s heated f o r the same p e r i o d o f time. I n t h i s manner, heat i s s u p p l i e d t o the c o a l t o coke the coal uniformly. G e n e r a l l y , there a r e 5 9 ovens t o a b a t t e r y . A l l the v o l a t i l e l i q u i d f l o w i n g from a b a t t e r y i s c o l l e c t e d i n a separate c o l l e c t ing main before the l i q u i d s from a l l the b a t t e r i e s a r e mixed t o gether ahead o f the t a r - l i q u o r s e p a r a t i n g tanks. Some b a t t e r i e s may c o n t a i n as many as 7 9 ovens. I n t h i s d i s c u s s i o n we w i l l cons i d e r a coke p l a n t c o n s i s t i n g o f f o u r b a t t e r i e s with 5 9 ovens each and one b a t t e r y c o n t a i n i n g 7 9 ovens f o r a t o t a l o f 3 1 5 ovens. Not every c o a l can be charged t o the ovens. The concentrat i o n o f a s h and s u l f u r must be considered t o make good, s t r o n g coke. Too much s u l f u r i n the coke w i l l f i n d i t s way i n t o the s t e e l . Some c o a l s expand when they a r e heated, while others cont r a c t . Some c o a l s have a high content o f v o l a t i l e matter, while others have a low c o n c e n t r a t i o n . Therefore, blends o f v a r i o u s c o a l mixes a r e coked i n s m a l l experimental ovens t o determine the f



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expansion or contraction of the coal and the physical character istics of the final coke. If the coal expands too much, it will be detrimental to the walls of the ovens and also cause sticking of the coke, so that it cannot be pushed out of the ovens. Stick ing ovens removes them from normal production, thereby lowering the daily production of coke. The sticking ovens in many cases have to be cooled down to facilitate removal of coke and again must be reheated before charging with eoal. Too much contrac tion of the coal forms an undesirable, dense coke. Table I lists the analysis of a typical coal mixture charged to the ovens. The lower the sulfur content, the lower the HgS concentration in the coke oven gas that must be removed from the gas before it is burned to meet the pollution standards set by EPA. Note that the contracting coals expand somewhat during the coking process before finally contracting. The expansion and eontraction of the various coals was determined on the individual coals charged to the small experimental oven. The coal mixture is determined on the quality of the final coke, whieh must meet certain hardness, volatile matter, ash and sulfur content to be suitable for use in the blast furnace. The oven is charged through three openings located on top of the oven. The oven is filled to a heighth of 12 feet with 16.2 tons. A leveling bar on the pusher machine levels the eoal in the oven. As noted previously, a l l doors and openings on the oven are closed so that the coal is heated in the absenee of air, otherwise the eoal would only burn and not form coke. Oxygen analysis of the gas exiting the oven during the 16-17 hour coking cycle is 0 . 1 # or less. A total of 695° tons of coal is charged per day, forming 46Θ0 tons of coke per day, and 7 3 , 3 0 0 , 0 0 0 cubic feet of coke oven gas per day. Coke Oven Gas Flow Diagram The flow diagram of gas is practically the same in a l l coke plants. As more desulfuriaation processes are installed, these will vary in location depending on the type of system being i n stalled. In the flow diagram in Figure 1, coke oven gas rises from the coke oven, a, through standpipe, b, to gooseneck, c, where it is contacted with flushing liquor (ammonia liquor). Tar and moisture are condensed. Ammonium ehloride, and a portion of the ammonia, fixed gases, hydrogen cyanide and hydrogen sulfide are dissolved by the liquor. The gas, liquor and tar enter the gas collecting main, d, which is connected to a l l the ovens of a battery. In some cases there may be two gas collecting mains to a battery. The gas, liquor and tar are separated in tar decanters. The tar, f, separated from the liquor, e, flows to tar storage, g. A por tion of the liquor, e, is pumped to the gooseneck, c, on the top of each oven. The remainder of the liquor is pumped to the ammonia liquor s t i l l , h, where it is contacted with live steam to drive off free ammonia, fixed gases, hydrogen cyanide, and


In Coal and Coal Products: Analytical Characterization Techniques; Fuller, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982. MIXTURE

HIGH VOLATILE W. VA.

PA.

HIGH VOLATILE

PA.

MID VOLATILE

W. VA.

LOW VOLATILE

27

46

15

12

% IN MIX

31.4

33.6

36.4

24.7

16.3

% VOLATILE MATTER

62.9

61.4

56.7

68.4

75.7

% FIXED CARBON

5.7

5.0

6.9

6.9

8.0

% ASH

1.03

0.64

1.35

0.89

0.7

%S

6.3

7.4

7.0

7.5

6.5

2

% H0

10% CONTRACTION 1.7 psig PRESSURE BEFORE CONTRACTION

2 psig PRESSURE BEFORE CONTRACTION

15.5% CONTRACTION

0.9 psig PRESSURE BEFORE CONTRACTION

26% CONTRACTION

4 psig PRESSURE

8.0% EXPANSION

13 psig PRESSURE

12.5% EXPANSION

Table I. Analysis of Typical Coal Mixture Charged to Ovens.


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hydrogen s u l f i d e . As the l i q u o r flows from the f r e e s t i l l , h t o the f i x e d s t i l l , i , i t i s contacted with lime o r sodium hydroxide, k, t o l i b e r a t e f r e e ammonia from ammonium c h l o r i d e . L i v e steam, admitted a t j , flows up through the f i x e d and f r e e s t i l l s and the ammonia, f i x e d gases, hydrogen cyanide and hydrogen s u l f i d e are added through, 1, t o the main coke oven gas stream ahead o f the ammonia s a t u r a t o r , r . The coke oven gas, separated from l i q u o r and t a r i n , e, i s cooled i n d i r e c t l y with water i n the primary c o o l e r s , m. The f i n e t a r t h a t separates from the gas i s pumped through, n, t o the t a r storage tank. The cooled gas i s pumped by exhausters, o, t o the e l e c t r o s t a t i c p r e c i p i t a t o r s , p, where a d d i t i o n a l f i n e t a r i s condensed and pumped through, q, t o the t a r storage tanks. The gas i s contacted with a d i l u t e s o l u t i o n o f s u l f u r i c a c i d i n the ammonia s a t u r a t o r , r , t o remove f r e e ammonia. The ammonium s u l f a t e - l a d e n a c i d flows through, s, t o the ammonia c r y s t a l l i z e r (not shown) where c r y s t a l s o f ammonium s u l f a t e are separated, and the remaining s u l f u r i c a c i d i s pumped back t o the ammonia s a t u r a t o r through, t . The ammonia-free gas flows t o the f i n a l c o o l e r s , u, where i t i s f u r t h e r cooled by d i r e c t water c o n t a c t . The water p l u s condensed naphthalene flows from the c o o l e r through t a r , which absorbs the naphthalene. The water i s cooled and r e c i r c u l a t e d i n t o the f i n a l c o o l e r through, v. The cooled gas enters the wash o i l scrubbers, w, a l s o known as benzole scrubbers, where i t contacts wash o i l , a petroleum o i l , pumped i n t o the scrubbers through y. The a l i p h a t i c and aromatic compounds a r e e x t r a c t e d from the gas by the wash o i l . The p r i n c i p a l components a r e benzene, toluene, xylenes, indene, and s o l vent, a l s o known c o l l e c t i v e l y as l i g h t o i l . The b e n z o l i z e d wash o i l i s pumped through, x, t o the wash o i l s t i l l (not shown) where l i v e steam s t r i p e out the l i g h t o i l compounds. The debenzolized wash o i l i s cooled and returned t o the wash o i l scrubber. I n some p l a n t s the l i g h t o i l i s f u r t h e r processed and f r a c t i o n a t e d i n t o benzene, toluene, and xylenes and i n t o a h i g h - b o i l i n g s o l v e n t f r a c t i o n . Naphthalene i s a l s o present i n the l i g h t o i l . Plants with low volumes o f l i g h t o i l do not have f a c i l i t i e s f o r r e f i n i n g , t h e r e f o r e , the o i l i s s o l d t o l a r g e r e f i n e r i e s . The gas from the wash o i l scrubbers flows through, z, t o a gas h o l d e r which tends t o e q u a l i z e the pressure. Booster pumps d i s t r i b u t e o n e - t h i r d o f the gas f o r u n d e r f i r i n g o f the coke ovens, and two-thirds t o the s t e e l p l a n t where i t i s used as a f u e l i n the many f u r n a c e s .


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Determining End o f Coking Cycle By Gas A n a l y s i s The l e n g t h o f a coking c y c l e f o r c a r b o n i z i n g o f c o a l i n an oven i s determined from experience and the range o f the f l u e temp e r a t u r e . Most coke p l a n t s a l s o have experimental coke ovens t o determine coking c y c l e s f o r v a r i a b l e c o a l mixes and a t d i f f e r e n t f l u e temperatures. Another method t o determine the end o f t h i s


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c y c l e i s by a n a l y s i s o f the gas f l o w i n g from the oven i n t o the standpipe. T h i s s e e t i o n w i l l d e s c r i b e t h i s approach. A. Sampling System. The gas sample from the standpipe must be eooled, separated from t a r and water and f i l t e r e d before i t i s analyzed f o r Ho, 0^, N » CH/. CO, CO? and i l l u m i n a n t s . The sampling t r a i n i n F i g u r e 2 has been used s u c c e s s f u l l y f o r the continuous pumping o f gas t o a sample b o t t l e o r t o a gas chromâtograph. I t i s designed t o operate f o r s e v e r a l hours so t h a t many samples can be analyzed before there i s an accumulation of t a r i n probe. c o r i n the s e p a r a t o r s , d. Standpipe, b i s l o c a t e d on the gas discharge and on t o p o f coke oven. a . Normally, there i s an opening on the s i d e o f the standpipe where l i v e steam i s admitted t o remove accumulated t a r i n the p i p e . T h i s opening i s i d e a l l y l o c a t e d f o r gas sampling, s i n c e i t i s g e n e r a l l y about two t o three f e e t above the oven. The probe, c, extending t o approximately the center o f the standpipe. i s a 3A i n c h o r 1 i n c h heavy w a l l s t a i n l e s s s t e e l p i p e . The end o f t h i s pipe i n the gas stream i s sealed and the other end has a plug f o r removing accumulated t a r . A l / 4 - i n c h by a p p r o x i mately 3 - i n c h s l o t i s c u t along the l e n g t h o f the pipe i n the gas stream extending from the c l o s e d end. When the probe i s i n s e r t e d i n t o the standpipe. i t i s important t h a t the s l o t face the l l r e c t i o n o f gas flow. T h i s p o s i t i o n decreases the amount o f t a r pumped i n with the sample gas. The probe i s i n s e r t e d i n t o a plug s i m i l a r t o the one used f o r the steam l i n e and pushed through the opening i n the standpipe u n t i l the plug s e a l s the opening. This i n s e r t i o n should be done r a p i d l y t o prevent excessive flow o f coke oven gas through the opening. The probe extending outside o f the pipe and the l / t f - i n e h pipe from the probe t o the f i r s t separator, d should be i n s u l a t e d . The two s e p a r a t o r s , d, f o r c o o l i n g the gas and s e p a r a t i o n o f t a r and water, a r e i d e n t i c a l . These a r e made from *f-ineh pipe, 10- t o 12-inch long, one and sealed and a flange with a gasket on the t o p end. The i n l e t pipe extends approximately half-way i n t o the s e p a r a t o r . Both separators a r e kept i n a bucket o f water, e. The separators should be pressure t e s t e d before they are used. The remaining p i p i n g i s l/Jf-inch copper o r s t a i n l e s s t u b i n g with Swaglock f i t t i n g s . Although most o f the t a r i s condensed i n the s e p a r a t o r s , the gas i s drawn through g l a s s wool i n a g l a s s tube, f , t o remove the l i g h t e r t a r o i l . T h i s g l a s s tube can be a g l a s s b o t t l e normally used i n the l a b o r a t o r y t o d r y gases with d r i e r i t e . I t has an i n l e t connection on the bottom, an o u t l e t connection near the top, and a metal screw cap with a s e a l on the top. Tygon t u b i n g i s used f o r the metal tube t o g l a s s connection. The gas flows through a f i n a l f i l t e r , &, t h a t removes submicron p a r t i c l e s , such as the P a l l T r i n i t y " J u n i o r S i z e " E p o c e l c a r t r i d g e . The gas i s drawn from the standpipe by a p e r i s t a l t i c pump, i . Good r e s u l t s a r e obtained with t h i s type o f pump u s i n g a l / 2 - o r 5 / 8 - i n c h diameter p l a s t i c tube f o r conveying the gas. The t u b i n g i s e a s i l y 2

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COAL AND COAL


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PRODUCTS

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Coal as Energy in the Steel Industry - American Chemical Society

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