Coal Gasification Theory-01

8/6/2019 Coal Gasification Theory-01

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COAL GASIFICATION I N A LOW PRESSURE,LOW RESIDENCE TIME, ENTRAINED FLOW REACTORR . L . Coates, C. L. Chen, and B . J . Pope

Chemical Engineering DepartmentBrigham Young UniversityProvo , Utah 84601

INTRODUCTIONPr io r s tud ies i n which f ine ly-ground coal was heated very rap id ly haveshown th a t t h e f r ac t io n o f th e co a l th a t can be v o l a t i l i zed in c r eases wi thb o th th e r a t e o f h ea t in g and th e f i n a l t emp era tur e t o which t he coa l i sheated.entra ined f low rea ct or which show th a t v ol a t i l e products amounting t o 49.9percen t of t h e co al fe d may be produced from a finely-ground coal havingan ASIM v o l a t i l i t y of only 3 5 . 5 percent, even though maximum reactortemperature was less than the 95OoC reached i n th e s tandard vo l a t i l i t yt e s t . Kimber and Grayreac tor opera ted a t temperatures up t o 2200'K.much as 8 7 % g rea te r th an t ha t from th e s tandard t e s t and concluded both higherhea t ing ra tes and h igher f i n a l tempera tures increa se th e amount o f vo la t i leproducts.Another ch ar ac te r i s t ic o f h igh- ra t e , h igh- tempera ture pyro lys is o f coa lt h a t i s not found i n normal carboni zati on i s the product ion of s ig n i f i can tqua nt i t ie s of ac e ty lene and e thy lene i n th e pyro lys is gas . These p roductsare commonly observed durin g coa l p yr ol ys is i n a plasma o r by f l a s h h ea t -ing.The present study was u nd er ta ke n t o i n v e s t i g a t e t h e p o t e n t i a l for in-c re as ed v o l a t i l i t y and also the production of unsaturated hydrocarbons asa r e su l t o f r ap id p y ro ly s i s u s in g p a r t i a l combus tion as the source o fpyrolysis energy. This paper represen ts a progress repor t on the exper i -mental work accomplished t o d a te .

For example, Eddinger, e t a1 l , have presented data f rom an

rep o r t ed co a l p y ro ly s i s d a ta i n an en t r a in ed f lo wThey observed vo la t i le s as

EXPERIMENTAL PROCEDURESAn en tr ai ne d flow rea ct or was designed i n which t h e f inely-ground coa lcould be rapidly mixed with oxidizing combustion gases.gases were produced from a pre-mixed flame of pure oxygen with hydrogen.The reactor volume was d es ig ned fo r s h o r t r e s id en ce times, and the productswere quenched by water spray immediately downstream of the r eac to r .

The combustion

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A diagram of the reactor i s shown i n Figure 1. The rea ct io n tube wasmade of alumina.ing element fo r preheat ing and t o reduce heat los s dur ing the run. There ac ti on tube and heat ing elements were insulat ed with a f ibrous aluminain su la ti on and encased with a water-cooled se ct io n of 6-inch aluminumpipe.1 1 / 2 , and 2 inches were te st e d. The use of smaller diameter tubes per -mit ted te s t in g a t reduced res idence t imes.was made of aluminum.located 180 apart and a t an angle of 30 with the cen ter l in e of the re-ac t ion tube .3 inc hes below t he o r i f i c e th roug h which premixed combustion gas es were fedt o t h e r e a ct o r .near the base o f the r ea c to r t o r ecord the re ac to r t empera ture .The coal tested was a h i g h v o l a t i l e B Utah coal from the Orangeville, CarbonCounty, ar ea . Typical proximate and' ult i mate analyses of coal from t h i sa rea are l i s t ed i n T ab le 1. The coal was dr ie d, ball -mil led, and screenedt o minus 200 mesh for these te s t s . The mois ture as used in the te s t s wasless than one percent.The coal was entr aine d in to a s trea m of c ar ri er gas, ei th er hydrogen ornitr ogen , with an auger-driven fe eder . A variable-speed auger drive wasemployed to obtain feed rates ranging from 0.5 t o 5 . 0 pounds of coal perhour. Ent rai ni ng gas flows of from 13 t o 15 SCFH were used i n the 1/4 -inc hdiameter feed l ine .The product gas was separated from the quench water, passed through a f i l t e rand then through a gas met er. Samples of f i l t e r e d gas were withdrawn f o ran a ly s i s w i t h a gas chromatograph. The cha r w a s f i l t e r e d from the quenchwater, dr ie d and analyzed fo r ash content t o ve r i fy mater ia l balance cal -cu l a t i o n s .

This tube was placed ins i de an annu la r e l ec t r ic a l hea t -

Reaction tubes 4 5/8 inches i n length with ins ide diameters of 3 /4 ,The water-cooled i nj ec t or headThe co al was i nj ec te d through two copper in je ct or s

The impingement point for these injectors was a dis tance ofA pla tinu m/fJ% platinum-rhodium thermocouple was i ns er te d

The oper atin g parameters var ie d were the feed r at es of th e coa l and combus-ti on gases and the s toichi ometry of t he combustion gases.ing preheat ing of th e rea ct or ranged from 2 t o 2 2 minu tes . The range offeed ra t e var iab les t e s te d and the range o f r eac to r opera t ing ,cond i t ionst h a t r e s u l te d a r e l i s t e d i n Table 2 .

Run times follow-

RESULTSA t o t a l of th i r ty- two t e s t runs were made with th e 2-inch diameter reactiontube , twenty with hydrogen as th e c a rr ie r gas and twelve with ni t rogen.Twelve t e s t s were made with the 1 1/4-inch diameter reaction tube, andseven t e s t s were made with the 3/4-inch diameter tube.ta in ed f rom these te s t s a re p resen ted i n Table 3 . Typical data ob-

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Effect of Hydrogen Concentration.the hydrocarbon gases were generally higher the greater the concentrationof hydrogen in the reactor. Data illustrating this effect are presentedin Figure 6. In this figure the conversions to methane, ethylene, andacetylene at temperatures ranging from 1200 to 1400'K are plotted versusthe hydrogen partial pressure at the reactor outlet.methane is shown to be the most sensitive to this operating variable.Although the observed effect of hydrogen concentration on the methane yieldis in the direction expected from the hydrogenation reaction, i.e.,C + 2H2 = CH4, the equilibrium constant, Kp, for this reaction is much low-er than the observed ratio of PCH4/P2H2.with the curve representing hydrogenation equilibrium in Figure 7 .seems clear from this comparison that the hydrocarbon gases are nonequi-librium species resulting from pyrolysis reactions.Steam Carbon Reaction.dicated that a significant fraction of the steam produced by the combus-tion gases reacted with the coal to form hydrogen and carbon monoxide.calculated steam decomposition is plotted versus the oxygen/coal ratio inFigure 8.gen and nitrogen.use of hydrogen carrier gas is shown to suppress the steam decomposition.The approach of the reaction C + H20 = CO + H2 toward equilibrium is indi-cated by the data presented in Figure 9 .son that the steam-carbon reaction is far from equilibrium for all of therun conditions that were tested.

It was observed that conversions to

The conversion to

The observed ratios are comparedIt

The composition and volumes of the product gas in-The

This plot also shows the effect of the-two arrier gases, hydro-The higher hydrogen concentrations resulting from the

It is apparent from this compari-

Shift Reaction. The conversion of carbon to carbon dioxide was observed tobe rather low relative to the conversion to carbon monoxide, as mentionedabove.CO + H20 = C02 + H2, was observed to exceed the observed ratio ofPCOz pH2/pC0 pH20.observed ratios are seen to correspond to equilibrium Kp's at substantiallyhigher temperatures than those measured at the reactor outlet.Volume and Heating Value.of coal carriertion of the oxygen/coal ratio.uniformly with this ratio.carrier-free gas is shown in Figure 1.2.

In all cases the equilibrium constant, Kp, for the shift reaction,This is illustrated by the data shown in Figure 10. The

The volume of dry gas produced, less the volumegas fed to the reactor, is shown in Figure 11 as a func-The volume produced is seen to increaseThe corresponding heating value of the dry,

Combustion Equivalence Ratio.of combustion hydrogen to combustion oxygen was tested by operating thereactor with coal feed rate, oxygen to coal ratio, and carrier gas rateconstant, and varying the combustion hydrogen feed rate. Combustion hydro-gen was varied to give an equivalence ratio, i.e., the moles H2 per mole0, from 0.4 to 1.1. As illustrated in Figure 13, the weight of carbon inhydrocarbon gases per 100 pounds of coal increased and the molar ratio ofcarbon dioxide to carbon monoxide decreased with the equivalence ratio.

The effect of varying the equivalence ratio

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Reactor Temperature. Analysis of t he dat a showed th at th e primary va ri -able governing the composition o f the reactor products was the tempera-tu re . The temperature as indicated by the thermocouple measurements wasobserved t o increa se with t he amount of combustion gas fed t o t he re ac to rper pound of coal , Although the rea ct or tube was e l e c t r i c a l l y h ea te d, t h efeed rat es and heat t ra ns fe r a rea were such th at th e heati ng elements ex-er t ed a ra t he r smal l e f fe c t on the reac t io n tempera ture , se rv ing pr imar i lyt o reduce heat losses. Figure 2 pre sen ts t he measured t emperatures as afunction of the ratio of combustion oxygen per pound of coal . The ef fe ct sof reac t ion tube diameter and co a l f eed r a t e a r e a l so in d ica ted on th i sf i g u r e .Effect of Temperature.th e carbon i n th e coal t o th e hydrocarbon gases methane, acetylene, andethylene, and t o carbon monoxide and carbon di ox id e. The con ver si on d a t aare p lo t te d versus t he measured tempera ture wi thout regard f o r var ia t io nsin th e o th e r o p e ra t in g v a r i ab le s .th e measured volume and composition of t he gas produced, a f t e r cond ensa tionof the water vapor , and the fee d ra t e of th e coal . I t i s of i n t e r e s t t ono te f i r s t t h a t i n co n t r a s t t o g as i f i ca t io n pro d uc t s of co nv ent ion al low-pressure coa l gas i f ie r s , s i gn i f ic an t convers ion t o the th r ee hydrocarbongase s, methane, acetyl ene, and ethyl ene, were observed. Secondly, th e tre ndof conversion w i t h reac tor tempera ture i s c lea r ly ev id en t .conversion r i s e s t o a maximum and then decre ases with inc rea si ng tempera-tu r e , th e conversion t o ace ty lene increases wi th tempera ture , and th econversion t o ethylene decreases with temperature.The effect of replacing hydrogen as t h e co a l c a r r i e r w i t h ni t rogen on theconversion t o carbon oxides i s a l s o ind ica ted on th i s f ig u re .vers ion t o carbon monoxide appears t o be pr in ci pa ll y dependent on tempera-tu re ; however , the carbon dioxide yi el d i s s i g n i f i c a n t l y g r e a t e r wi th t h elower hydrogen concentrat ions result ing from the use of ni trogen carr iergas.r el a t iv e t o th e amount of carbon monoxide i s low as compared t o t he pro-ducts f rom convent ional gas i f ie r s .Effect of Residence Time.re ac to r on the conversion t o t he t hr ee hydrocarbon gases i s ind ica ted byth e d at a shown i n Figures 4 and 5.t h r e e d i f f e r e n t sizes of rea c t or tubes each opera ted a t a coa l feed ra t eof 1 . 2 pounds of coal pe r hour. These da ta show only a s l i g h t e f f e c t ofrea c to r s i z e on the p roduct y ie l ds . In Figure 5 the convers ions fo r re -ac to r temperatures in th e range 1200-1400K a re pl ot te d versus th e averageres idence t ime. Although th e da t a a re sca t t e re d , t her e is a t rend t o lowerconversions with increasing resid ence t ime. This trend i s most evident inthe convers ion t o ace ty lene .reac tor res idence t imes o f less t ha n f i f t y m il l is e co nd s a r e s u f f i c i e n t f o rga s i f ic a t ion t o occur in t h i s temperature range and t h a t these productst en d t o d i sapp ea r as the res idence t ime increases .

Figure 3 prese nts dat a showing th e conversion of

These con ve rs io ns were computed from

The methane

The con-

I t i s of i nt er es t t o not e th at th e amount of carbon dioxide produced

The ef fe ct of average residence t ime i n th eFigure 4 shows conversion data from

The d at a corresponding to Figure 4 and 5 show that

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SUMMARYThe observations and conclusions drawn from the experimental runs may besummarized as follows:1.

2 .

3.

4.

5.

6.

7.

Reactor temperature was found to be the most important parameteraffecting the overall gasification of coal.trolled principally by the combustion oxygen fed per pound of coal.As much as 57 percent of the mass of coal fed was gasified, and thespace time conversion corresponding to this yield was 408 lb carbongasified/ft3 hr.Significant yields of methane, ethylene and acetylene were produced.Up to 14 percent of the coal carbon was converted to these gases.The yield of ethylene was observed to decrease with increasingreactor temperature while the yield of acetylene increased.version to methane was observed to pass through a maximum at a re-actor temperature of about 1200'K. Maximum yields of methane,ethylene and acetylene were 6, , and 6 percent of the coal carbonrespectively.The yield of hydrocarbon gases was observed to increase with in-creasing hydrogen concentration and to decrease with increasing resi-dence time. Residence times shorter than 50 milliseconds are indi-cated for optimum yield of hydrocarbon gases.methane yield data with hydrogenation equilibrium indicates that thehydrocarbons in the product result not from hydrogenation reactionsbut from non-equilibrium pyrolysis reactions.Although significant steam decomposition was observed, conversion ofcarbon to carbon monoxide was substantially less than predicted forsteam, carbon equilibrium. The conversion of carbon to carbon dioxidewas observed to be much lower than predicted by the shift reactionequilibrium.The yield of hydrocarbon gases and the yield o f carbon dioxide relativeto carbon monoxide depends strongly on the stoichiometry of the combus-tion gas.carbon gas yield to be reduced and the ratio o f COz to CO to be in-creased from the yields with stoichiometric combustion gases.The volume of gas produced per pound of coal increases uniformly withthe oxygen/coal ratio.carbon gas yield the volume produced is 22 SCF per pound of coal.The carrier-free heating value of the product gas decreases uniformlywith increasing oxygen/coal ratio.maximum hydrocarbon gas yield, the heating value is in the range of

The temperature was con-

The con-

Comparison of the

Combustion gases lean in hydrogen fuel cause the hydro-

A t the ratio corresponding to maximum hydro-

At the ratio corresponding to370-420 BTU/SCF.

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ACKNOWLEDGEMENTThis work was carried out under contract to Bitwinous Coal Research, Inc.,with funds supplied by the U.S. Office of Coal Research.

REFERENCES1. Eddinger, R. T., Friedman, L. D., and Rau, E., "Devolatilization ofCoal in a Transport Reactor," Fue1.45, 245-252 (1966).2. Kimber, G. M., and Gray, M. D., "Rapid Devolatilization of Small CoalParticles," Combustion and Flame 2, 360-362 (1967).3. Bond, R. L., Ladner, W. R., and McConnell, G. I . , "Reactions of Coalin a Plasma Jet," Fuel 45, 381-395 (1966).


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TABLE 1.- Coal analysis - weight percent as receivedPROXIMATE ULTIMATE

Moisture 5.65Ash 6.20Vo la t i l e Matter 34.35Fixed Carbon 53.80100.00

Carbon 70.05Hydrogen 5.76Nitrogen 1.30Sulphur 0.64Oxygen 10.40Moisture 5.65Ash 6.20100.00

Coal s ize (-200 MESH)

TABLE 2.- Range of Feed Rate Variables

Coal Feed Rate, lbs /h r 0.7-4.1Oxygen/Coal Ratio 0.3-1.6Combustion Gas Equivalence Rat io 0.4-1.1Coal Carrier Gas (13-15 SCFH) Nitr og en or Hydrogen0

Range of reac tor opera t ing condi t ions

Average Tempera ture 1200-2500FAverage Res iden ce Time 0.012-0.343 sec.Space Time ConversionSteam P a r t i a l Pres sure (Reactor e x i t ) 0.125-0.255 atmHydrogen Pa rt i al Pres sure (Reactor e x i t ) 0.194-0.553 atm

13-408 lbs C gas i f ied / f t3 -hr

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1ABLE 3. - Ty pical d a t a o b ta ined f rom g as i f i ca t io n t e s t sRun No. 5 - 8 - 4 5 - 8 -2 5 - 8 - 1 6 - 9 - 4 7 - 3 1 - 1 8 - 2 3- 2React or Diameter, in chesFeed Rates, lbslhr

CoalHydrogen CarrierHydrogen CombustionOxygenNit rogen Car r ie rOxygen/Coal RatioCombustion Equivalence RatioReactor Temperature, O FVolume Gas Produced (Dry)

Total, SCFHC ar r i e r F ree Basis, SCF/lbCoal

Gas Analysis (Dry, Volume %)HydrogenOxygenNitrogenMethaneCarbon MonoxideEthaneEthyleneCarbon DioxideAcetylene

Carr ier Free Heating Value,BTU/CF

Steam Decomposed, Pe rc en tAsh i n Char, Weight pe rcen t

2 .ooo

1 . 5 9 00 .0820 .0690 . 5 4 0- -0 . 3 3 91 .022

1 4 4 5

3 0 . 91 0 . 2

74 .120 . 2 00 . 8 15 . 3 3

1 5 . 3 50 . 1 41 . 6 21 . 5 50 . 8 8

449 .4

1 8 . 2 89 . 3

2 .ooo

1.5900.0820.1020.800- -0 . 5 0 31.025

1750

37.614 .4

6 9 . 8 80.421 .335.02

1 8 . 4 10 .041 .262.061 .58

430.2

28.7914 .9

2.000

1 .5900.0820 .1401 .050--0.6601 .066

1955

45 .919.6

6 9 . 7 50.150.453 .85

20.850.010 .642 . 5 11 .79

395.0

3 8 . 8 11 5 . 8

2.000

2.0430.1671.2601.0200.6161.060

1966

- -

56.521.2

26.388.30

46.891 . 8 1

12.960.000.122.680.86

358.9

48.6716.5

1.250

1 . 6 7 00.0820.1671.340--0.8020 . 9 9 7

2157

60 .327.3

64 .231.654.122 .34

23.480.000.192.391 .60

365.7

46.8615.68

0 .750

I1.1800.1130.9000.082 11- -

I0.7621.008

1913

39.72 1 . 1

67.141.394.663.22

18.830 . 0 10.522.571.66

3 9 2 . 1 r

36.849

13.941

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7 Coal InjectorP r r l x e d Hydrogen-DxygenCoal Injutor

Alumina Tube

Electr icalt l e a t i n p Element

Cooling Coil

To Filter. Gas Meter

Figure 1. Schematic Diagram of Reactor .

...0. 2 0 . 4 0 . 6 0 . 8 1.0 1 . 2 1. 4 1. 6

Pounds Oxygen per Pound Coil

Figure 2 . Measured reactor outlet temp-er at ur e versus t he oxygen fedper pound o f coal .

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P e r c e n tC on ve r s i onof Carboni n Coal t o

b

Methane 4

U

4L t hy lune

C

tAcety lunr

b(

Carbon 4 (2 (c

M o n o x id e

c

Figure 3

C a r r i e r G as R e a c t o r d i n . i n c h221 . 2 50 . 7 5

0

aA A

Conversion of Carbon i n Coal t o Hydrocarbon Gases,carbon monoxide and carbon dioxide versus reactortemperature.

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8Percentconvers ionof Carbont oMethane

2

0PercentConversionof CarbonEthylenet O

0PercentConversionof CarbonAcety lene

t o2

0PercentConversion 60of CarbonCarbon 40Monoxidc

20

t o

0PercentConversion 8of Carbon

t O 4CarbonDioxide

Reactor Diameter0 2 i n c h e s0 1 1/4 inchesA 3/4 inches

- u - - L o %-- nc - . p0 -'9 \

0 . 2 0 .4 0 .6 0 .8 1. 0 1 . 2 1 . 4Lbs Oxygen / 1.b Coal

Figure 4 . Conversion d a t a showing small e f f e c to f v a r y in g t h e r eac to r s i z e .shown are for a co a l f eed r a t e o f1 . 2 l b s /h r .

Data

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8

I1

c

'Temperature R a n y eI:OO- iiInool;

- 0 0 0

00 A o

Figure 5. Conversion da ta showing small e f fe c tof average rea cto r res iden ce time.A l l da t a shown ar e fo r a coal feedr a t e o f 1 . 2 l b / h r .

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P e r c e n t 6C on ve r s i onof Carboni n C o a l t o

Methane

L r h s l e n e

A c e t y l e n e

Figure

e

4

2

C

4

2

0

4

2

0

R e a c to r d i a . Carrieri n c h (;as1'2

A 2 N0 1 . 2 5 1'2v 0.75 1120 2

Temperature Range1Z00-14009K

A A0 0AA0

A a "00.2 0.3 0.4 0 . 5 O.h

l l y d r o g e n P a r t i a l P r e s s u r e , A rm- . .6. Data showing the effect of hydrogenp ar t i a l pressu re on hydrocarbony i e ld s .

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1. 0

00 0

16000.001800 1000 1200 1400

Temperature I(

Figure 7 . Comparison o f equil ibr ium pressurer a t i o s for the hydrogenation reac-t ion with measured rat ios.

t 1

Figure 8. Calculated steam decomposition versusthe oxygen fed per pound o f coal .178


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i\ ' o lume ofC a r r i e r F r e eProduct Cas

2 I)Per Pound Coal ,S C I

n

0

A N 2 1 3V H 2 j / 4 3

IU. b 11.8 1 . 0 1 . 2 1 . 4 I .

1.1 11.4

000V

C a r r i e r R e a c t o r R~~0 H 2 1 1/4 3

310H 2 2

Gas Size ~i~~0 H ? 2b HZ 2 7 ,- _

Figure 11. Net volume o f prod uct gas p e r pound of coal fedas a func t ion of oxygen t o coal ra t i o .

5 5 l

500

I S 0

&- 40 0

2; 50

100

25 00.4 0.6 0 . 8 1.0 I . : 1 . 8

Pounds Oxygenl round coli

Figure 1 2 . Data showing th e va ri at io nof th e hea t ing va lue o f thed r y, c a r r i e r - f r ee p ro du ctgas w i t h the oxygen-coal ra t io .

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-I *o

A t m

Temperature. -I;Figure 9. Comparison of equilibrium pressureratios for the steam-carbon reactionwith measured ratios.

' 0.01(00 l U U U l 2 U U l J U U 1 6 0 0

.rcnpperaturc 01:Figure 10. Comparison of equilibrium pressure ratios

for the shift reaction with measured values.180

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8

POUn l SCarbon i nHydrocarbon 6G a s e s p e r10 0 PoundsCoalJ

2 n . I0 . 2 0 . 4 0. 6 0. 8 1. 0 I :

!lolcs H 2loles 0

Figure 13. Data showing th e e f f e c t of va rying combus-tion gas equivalence ra tes . The coal feedr a t e was 2 l b s /h r and t h e 0 2 /co a l r a t i owas 0.51.

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Coal Gasification Theory-01