Gasification of lignite by BCR Two-Stage Super-Pressure Process Robert J. Grace' and Richard A. Glenn Bituminous Coal Research, Inc., Jlonroeville, Pa. 16146
Raymond 1. Zahradnik Carneyie-AlfellonUniversity, Pittsburyh, Pa. 15215
Optimization results are reported for the second stage of the BCR two-stage super-pressure gasifier b y operation of a 100 Ib/hr process and equipment development unit (PEDU) with lignite a t 1000 psi and exit temperatures from 1400-1 600°F. Hot synthesis gas is generated internally b y reacting benzene or cyclohexane, oxygen, and steam in a simulated first stage which entrains and gasifies a pulverized coal feed as it passes downward through an 8-in. diameter, 5 - f t long second stage. Second stage conditions closely approximate those expected in full-scale gasifier operation. Direct yields of methane range from 1 1-1 8% of the carbon in the lignite feed, depending upon the partial pressure of hydrogen in the Stage 1 gas. Overall carbon conversions are 35-5570. The unconverted carbon is completely gasified in Stage 1 to generate both the heat and synthesis gas required for Stage 2.
A major objective of this long-range program of gas geiierator research is the development of a n economical process for converting coal iiit'o a high 13tu pipeline gas. The basic component of this process is t,he Bituniiiious Coal Research (13CR) super-pressure two-st,age gasifier [Figure 1 as described by Doiiath arid Gleiiii (1965)l. Fresh coal arid steam are introduced into the upper section (Stage 2 ) where the fresh coal is contacted aiid entrained by a rising stream of hot synthesis gas produced in the lower section of the gasifier (Stage 1). The fresh coal is rapidly heated and partially gasified to inet'liane and additional synthesis gas in Stage 2 . The residual char is swept out of the gasifier along with the raw gas, and separated and returned to t'he bottom section where the char is completely gasified under slagging conditions by reaction with oxygen and steam t o produce both the synthesis gas and t'he heat required in Stage 2. T o meet pipeline specificat'ions, the product gas requires further processing. It is cleaned and subjected to partial water gas shift to adjust its H,/CO ratio scrubbed t o remove acid gases (COa, HZS), and finally subjected to catalyt'ic methanation to raise the heating value above 900 Btu/scf. The Stage 1 slagging section can be designed through use of existing technology and would not be unlike the corresponding section of the slagging pressurized gasifier described by Lacey (1966). The main emphasis of t'he BCR program has been on obtaining sufficient information on the physics and chemistry for optimization of the design parameters for St'age 2. Experiments were initially carried out' by Glenn et al. (1967) with coal slurries in rocking autoclaves at pressures of 3000-4000 psig aiid temperatures 1380-1400°F. Large amounts of methane could indeed be produced froin the coiit'act of coal wit,h super-heated steam. However, the tests were of necessity batch in nature and involved relatively slow heating rates aiid long residence times. Coiisequent'lg, t,he results could not be applied directly to an integrated entrained gasifier, T o whom correspondence should be addressed
and data from experiments under more realistic coiiditioiis were sought. The nest step consisted of tests continuous in nature and involved a short coal-st'earn-synthesis gas contact time with rapid heating. Over 100 experiments were conducted under conditions simulating those of Stage 2 by use of a 5 lb/lir continuous flow reactor (CPR). Lignite, a Wyoming subbituniinous coal, and Pittsburgh seam high volat'ile bituminous coal were tested. Appreciable amounts of iiiet,haiie could be produced during short contact' times of 2-20 see between steam,
I
v Cyclone
Stage 2
Gar Purificotion and Methanation
+
Fino1 Pipeline Gar
I
Recycle Solids
I Coal Steam
+
-+
Gasifier
-
4 Stage 1
Oxygen and Steam
Slag T
Figure 1. Simplified flow diagram for two-stage superpressure gasifier Ind. Eng. Chem. Process Des. Develop., Vol. 11, NO. 1, 1972
95
Coal Inlet
r - - y
I--I
1 gas under conditions more closely duplicating those in Stage 2. I n this PEDU, designed to process 100 lb/hr of coal, a hot synthesis gas (Stage 1 gas) is produced b y the partial oxidation of an "ashless," nonslagging fuel, such as benzene or cyclohexane, in the presence of steam. This paper describes this unit and reports the results obtained with lignite. PEDU
Figure 2. Vertical cross section of Stage 2 PEDU reactor
synthesis gas, and fresh coal a t about 1000 psi and 1750°F (Glenn and Grace, 1968). The experiments carried out in the 5 lb/hr C F R involved the simultaneous heating of the simulated Stage 1 gas, the superheated steam, and the fresh coal. Because of the limitations of the equipment, the reaction conditions did not exactly duplicate those conditions expected in the integrated gasifier. Nevertheless, the results warranted the construction of a process and equipment development unit (PEDC) in which fresh coal and steam can be contacted with hot Stage
-
A description and discussion of the process and equipment development unit has been presented b y Glenn and Grace (1968). The basic component is the main reactor shown in vertical cross-sectional view in Figure 2. The 2-ft 6-in. i.d. X 8-ft long shell is designed for operation a t pressures u p t o 1850 psi a t 750'F. The volume of the horizontal &in. i.d. section (Sbage 1) is 0.3 ft3,and that of the main 8-in. i.d. vertical section (Stage 2) is 1.96 ft3. A flow diagram of the total 100 lb/hr PEDU system is shown in Figure 3. The production of hot synthesis gas (Stage 1 gas) is accomplished by combustion of steamatomized ashless fuel (benzene or cyclohexane) with oxygen in a specially designed water-cooled burner. For both startup and standby a t atmospheric pressure, a water-cooled natural gas-air burner is provided as an integral part of the Stage 1 section. Pulverized coal is stored in a 1000-lb capacity steel tank equipped a t its base with a specially designed variable speed starwheel feeder. The entire assembly is mounted on a system of three load cells for indicating and recording the weight of coal fed. Injection of the coal into the reactor is accomplished by pneumatic transport through a specially designed water-cooled nozzle with either nitrogen, steam, or recycle gas as the t,ransport fluid. The coal is entrained by the gas flowing through the PEDLT,and the unreacted residue passes out of the unit along with the gas. The products leaving the reactor are mater-quenched to lower the temperature and remove most' of the unreacted solids. The quench pipe is fabricated from 2.9411. i.d. X 3.5in, o.d. inconel tubing and fully \vater-jacketed. Black liquor and product gases then enter tangentially a t t'he base of the scrubber. The black liquor passes on to a flash tank where dissolved gases are released, measured, and sampled.
I I! Product Gar Scrubber Superheater
Steam
Dissolved Gar Senling
Sample Filter
Solids Liquid 8016G19
Figure 3. Basic flow diagram of 100 Ib/hr Stage 2 PEDU 96
Ind. Eng. Chem. Process Des. Develop., Vol. 1 1 , No. 1, 1972
Table I.
Table II.
Typical Analyses of lignite Feedstock Proximate, % _ _~ _ _ _ -- -
As used
1I oi i t I ire Volatile iiiattei Filed cnrlioii Ad1 Total
24 32 35 7 100
0 2 9 9 0
42 4i 10 100
_
24 0 48 9 3 4
0 i 1 1 7 9 1.1 0 100 0 8.130
_
_
_
Cumulotive
4i 1 52 9
3 3 4 0
100 0
%
Ultimote, I
~
Dry, ash-free
Dry
~
64 4 4 5 0 9 1 5 10 4 1s 3 100 0 10,700
Typical Wet Screen Analysis of lignite Feed Stock
Mesh size
% retained
+50 +lo0 +200 f325 - 325 (by difference)
1.6 6 3 39 7 65.3 34 i
-~
71 5 1 1
9 0 0 7
20 -1 lo!, 0 11.940
Frovihioii is inndc for witlitlrnn-nl of :in aiinlyticnl saiiiplc of l)l:i(~kliquor at a poiiit tlon.iistre:iiii from tlic f l d i tank \vlicrr the> ni:gnificd solidh iii the slm :ire cdlcctctl i i i a :%)-ii:. Sut~clic-t\.lieI)rc-sui'c filter. Tlic niniii 1)rodurt gn; i t i'eain i i i o w c ul)n-:irtl from tlic basc of the +ri~ibbcrtlirougli t 1:c h1)r:iy tower for fwtlier clrniiiiig niitl cooliiig by recircu1:itctl \\-ntcr froiii n series of tlircc taiidciii-iiioiiiitctl full cone $])ray iiozzles. S c r t , tlie product gas strcain paqses through n filial filter befoie !ieiiig let dovii t o ntino,q)licric p w s u r e for iiicwiireiiwiit aiitl subsequeiit flariiig. Tliis filial filtei is a 1iigli-l)iw-urc imit ihigi:ciI to reinow Iimticnlcs dowii to 10 p. 'l7iy Anrc-stack ]iilot-h:.:icr :ismiibly i.. n 1ioi:mokeics:' t y l r lvitli ;.t:irl\r cstri:iioii, pilot liuriicr! n i i t l aii igiiitioii tcii: ivitli 1)roviiioii fur actuntiiig a coiitrol ~):iiielsigiinl al i i : r n i c of pilot flaiiie fnilurc. I'rotluc~t g:is sample Imts arc l)rovitlcd at three points i i i t 1i sj-5t r 1II tlo\vi i it rra i i i f 1.0 i 11 t lie rea ct 01'. T h e first is loc~at ed a t tlir ($sit of tlic rcxctor n-liere liot (1500°F) high-presi i i i ' c dirty g:ih iii:iy 11c olitniiictl. Tlic iecoiid is located at tlie csit of tht. q u c i i c ~ l i pilie ~vlicrccoolcr (30OOP) liigli-preswre c~lcaiig:ii iiiny wl~ttii-el~sit of the I)rotluct g urc s~il~:'t:~iitidly vlc ->trr:iin coirtiiiuou.: iiioiiitoriiig of ga? conipositioii is pro~iilctlfor four coiii1)oiiciits: C",, CO?, C'O, and H?. Results :ire aiitoni:itir:ill>- recortletl aiitl tliqilaytd oii the coiitrol paiiei. l'rovisioii i- alio inatlc for thc iiitcrinitteiit collectioii of ga. sanil~lcifor coiiililete aiial! for Hy, 0 2 , S,,C"4, C'O, ('yH.2. C'ZHF, sild CaH,. i c usu:il iiietlrods for cleterriiiiiiiig quailtities of gas produced arc suplileiiieiitcd b y w e of aii iiitcriial refereiice staiirlurtl, :i iii tl:c form of tagged xyleiie added t o the ashless fucl. After a gas satuple has been subjected to chroiiiato, a separate gas sample is analyzed to determiiie if any iiietliaiie was formed from the Stage 1 fuel rather (1
tlinii from ligiiite i i i Stage 2. Experiments with lignite
A scric~of te*ts a t :illout 1000 pressure n-as made with Sort11 I h l i o t a ligiiik. 'I'lie aiialj of tliis feed>tock is presciitctl i i i Table I. Tlic :is-rcreived ligiiite is air-dried aiid retiwrd t o - 100 i i i r 4 1 i:i size n-ith a viiiid-s\wpt Patterson ball mill ( 2 f t i i i tliaiiicter by 2 ft loiig) equipped with a Jf-iliiaiiis
classifier. The pulverizcd coal size coiisist as determiiied by is givcii i i i Tahle 11. Results of sigiiificaiit t w t s i i i the PEIII- with ligiiite are presented i i i Table 111. I t is iiot po.4ble to deteriiiiiie directly the water coiiteiit, of the gas leaviiig t,lie reactor siiice it is removed iii the cooliiig quciich ivater. However, if the total output osygeii r:ite is iioriiialized n-ith the total iiillut oxygen rat'e, the ainouiit of st cniii 1)remit i i i tlie discharge gases prior t o queiichiiig can be coml~utrd.This procedure, together n-it'li the use of C14 as nii iiiteriial staiidard. provided escrptioiially accurate compoiiciit and overall inaterial balaiices (Table IT'). Iiiput and output iiitrogcii rates agree to witliiii 27,; hydrogen ngreeiiiciit is witliiii 1.5yc, n i i d C14 bnlaiice n-itliiii 2 7 , also. K i t h tlie output stc:iiii rate tirtermiiicd, tlie product gas roiiipo4tioiiq ]xior to stcaiii rtnioval werc coiiiput~edin partial pressure unit.;. Gasificatjon Chemistry r .
t h e p h y h d aiid chcinicnl processes n-liicli take place be-1itliesis gas from Stage 1 i i i i t l the f i m h coal aiid steatii in Stage 2 are coiiiples, mid aiiy attciiil)t to iiiotlcl theiii must be regarded as npprosimnte. Soiictlieles.s, it' is pos&le to develop rensoiiahle correlatioiis i n terms of certaiii basic gasifier variables suggested by the gasificatioii cliciiii,4ry. Field et :il. (196i) linve sliowii t'liat as a result of the rapid Iicatiiig of the conl, a sigiiifieaiit, devolatilizatioii takes p h c e i i i a matter of iiiilliwcoiid~aiid produces a variety of gases iiiriutliiig liytlrogcii aiid methane. Tlir reiiiaiiidcr of the gasification ~ ) r o c elimy ~ be characterized by the carboii-hydrogen reaction
aiid b3- tlie carbon-steam reaction
C
+ HyO
-+
CO
+
H 3
(2)
T h c overall iiiethaiiatioii process lias been described b y IIoaeley aiid I'atersoii (1965a;b) as coiisistiiig of tlirec strps. The firit step is tlie rapid devolatilizatioii of coal viliicli 1)roduces, in addition t o volatile products, ai1 active cnrboii species wliicli reacts, i i i tlie second step, either with hydrogen to forrii more iiietliaiie or nitli itself in a cross-liiikiiig polyiiierizatioii to form an inactive char. The third step iiivolves ~ of hydrogen n-ith the inactive char. the s l o reaction In Stage 2 of t!ie UCR two-stage process, this third step is relativelj- uiiiniportaiit. Zaliradiiik aiid Gleiiii (19il) have slion-ii that the direct' methaiintion of conl iii Stage 2 enti be described adequately as a tTTo-step process iiidepeiideiit of resideiice time for greater tliaii a few secoiids. Tliey have liravided the following eslircsbioii for iiiethaiie yield (Jr.1.) esl)reased as the fractioii of carboii fed appeariiig a. niethaile (3) Ind. Eng. Chem. Process Des. Develop., Vol. 11, No. 1 , 1972
97
Table 111.
Data from PEDU Tests with lignite Test no.
Input rates, lb/hr Fuel Oxygen Steam Nit,rogen Lignite Output rates, scf/hr Output gas Reactor conditions Outlet temperature, O F Total pressure, psig Residence time, see Analytical data C in lignite, lb/hr Heat in lignite, Utu/hr x 108 D r y gas analysis, % Hz 02
15
2"
3"
4a
31.3 68.2 119.6 75.1 98.0
48.6 92.4 126.0 59.0 108.0
48.9 93.1 105.2 61.3 90.5
48.5 93.2 105,3 62.2 70.0
5b
42.6 80.9 73.7 58.1 94.0
1 Ob
6b
42.6 81.1 88.5 58.1 98.0
42.6 82.0 95.9 58.5 93.0
44.2 82.0 95.9 59.1 87.0
51.2 96.3 112.1 59.2 80.0
51.2 96.6 110.8 59.2 81 . o
3373
4618
4357
4265
3821
3935
3853
3787
4384
4399
1375 1020 16.2
1500 1025 13.2
1550 1005 15.6
1560 1015 14.7
1430 1020 16.3
1420 1020 15.4
1450 1020 15.0
1440 1020 15.2
1500 1020 13.5
1510 1020 13.4
41.2
51.2
43.1
33.4
46.7
48.7
46.2
43.2
39.8
40.3
985.3
852,7
720.3
557.2
774.5
807.4
766.2
716.8
659.1
667.4
33.0 0.05 18.7 16.6 26.7 4.9 0.0
27.7 0.05 29.6 10.5 26.9 5.0 0.2
NZ CO COZ CH4 CzHs Outlet gas partial pressures, psia
Hz
221.1 361.1 110.5 189.0 124.7 32.9
156,O 467.1 58.8 156.3 166.4 28.4
HzO
CO
32.9 0.04 18.9 19.0 24.5 4.7 0.0 220.8 341,O 127.5 172.0 126.6 31.6
32.3 0.06 21.3 17.8 24.7 3.8 0.0
36.8 0.05 20.7 15.5 21.6 5.2 0.1
220.6 338.2 121.2 177.9 145.1 26.3
COZ h-? CH4 Yields (70) 13.9 14.9 15.4 C in CHd 11.3 38.5 36.6 40.9 C in (CO Con) 24.6 C in tot'al g a s 36.9 52,4 51.5 56.3 B t u in CH4 21.7 29.4 26.6 28.5 45.4 B t u in (CO Hz) 25.2 34.3 39.4 74.8 B t u in total gas 48.7 60.9 67.9 45.4 Preformed methane 50.1 49.8 48.0 Denotes benzene fuel. Denotes cyclohexane fuel. Carbon gasified
+
+
36.6 0.06 20.5 14.3 22.7 5.8 0.1
37.1 0.06 20.4 13.9 22.7 5.7 0.06
36.5 0.06 21.0 13.7 22.8 5.8 0.1
38.3 0.07 19.0 15.0 22.6 5.0 0.01
38.8 0.07 18.6 15.0 22.5 4.9 0.16
241.6 369.5 94.5 154.7 135.3 38.1
239,l 385.1 89.8 151.4 131.6 36.8
234.3 388.4 87.9 151.4 134.2 37.5
247.3 384.8 96.7 150.9 122.3 32.2
250 8 384 0 97.1 149 3 120 2 31 8
14.6 13.3 20.9 19.1 36.0 33.1 28.2 25.6 18.0 18.5 45.3 47.1 65.2 61.8 (includes CIH,).
15.3 21.1 36.7 29.4 20.1 50.1 64.4
16.4 17.2 34.3 31.6 12.7 45.5 73.9
17.2 25.2 42.4 33 1 14.9 48.1 73.8
16.8 24.6 41.4 32.3 16.2 48.7 71.8
250,3 349.3 105.8 151.5 140.9 35.1
Table IV. Material Balances for PEDU Tests with lignite
Mol/hr Test no. Item
Oxygen in Oxygen out Hydrogen in Hydrogen out ?Nitrogen in Nitrogen out Carbon out in gas Carbon in fuel Carbon gasified Carbon in lignite
1
6.58 6.58 10.88 10.72 2.69 2.61 3.86 2.41 1.45 3.93
2
7.64 7.64 12.28 11.64 2.11 2.24 5.97 3.74 2.23 4.26
3
6.89 6.89 10.52 10.58 2.19 2.14 5.61 3.76 1.85 3.60
4
6.64 6.64 9.89 9.95 2.24 2.36 530 3.73 1.57 2.78
where a, b, and c are correlation constants related to the kinetic parameters of the process and P His~the hydrogen partial pressure. Equation 3 can be modified slightly to account for the observatioii by RIoseley and Patersoii (1967) that a t high hydrogen partial pressures, complete gasification of the carbon in coal to methane is possible. Hence, 98
Ind. Eng. Chem. Process Des. Develop., Vol. 1 1 , No. 1, 1972
5
6
7
8
9
10
5.58 5.58 10.0 9.9 2.10 2.07 4.33 3.04 1.29 3.90
6.04 6.04 10.95 10.76 2.10 2.11 4.50 3.04 1.46 4.06
6.22 6.22 11.21 11.17 2.11 2.10 4.46 3.04 1.42 3.85
6.16 6.16 11.14 11.05 2.13 2.12 4.39 3.16 1.23 3.60
6.98 6.98 12.32 12.40 2.14 2.18 4.98 3.66 1.37 3.31
6.96 6.96 12.29 12.48 2.14 2.14 4.96 3.66 1.30 3.36
J1.Y.
=
a
b(PHn) + -____ b(&) I f -
(4)
(1 - a)
The lignite data from the earlier 5 lb/hr CFR umt were correlated with the methane yield by means of Equation 4. The parameters obtained b y this eo1 relatioil a t 1750'F were:
1°1
15-
Figure 4. Effect of outlet hydrogen partial pressure on methane yield
METHANE YIELD, PERCENT OF CARBON IN 10FEED
51
CUNCS
Bared on Equation 4 Using CFR D a t a
PEDU Lignite Doto
"1 Figure 5. Effect of outlet temperature on direct methanation of lignite
4 1300
PEDU Lignite Data Curve Bared on Equation 5 Using CFR D a t a and A r r u m i n g on Activation Energy of 15 Kea1 I
I
I
1400
1500
1600
OUTLET TEMPERATURE, F
a = 0.07
b
=
0.0062 atm-'
Although no runs were made in the C F R with lignite a t temperatures other than 1750°F, it is reasonable to expect that' parameber b is temperature dependent. Zahradnik and Glenn (1971) also analyzed the direct methanation results of three different investigators by use of bituminous coal and concluded that the parameter b exhibited a n Arrhenius-type behavior with a n activation energy of 15 kcal. If a similar activation energy is assumed for lignite, it is possible t o extend the lignite correlation developed from the C F R data t o other temperatures. The curves in Figure 4 were obt'ained by adjusting parameter b for temperatures other than 1750'F. Figure 4 shows the methane yield data from t'he PEDU tests tabulated in Table I11 plotted against t'he outlet hydrogen partial pressure. The predicted curves bracket the P E D U lignite data in a general way. However, it is possible to rewrite Equat'ion 4 as follows:
(11f.Y. - a) -- b 1-a (Px2) (1 - M.Y.)
(5)
Then, if constant a is insensitive t o t'emperature, a plot of the kinetic group on the left-hand side of Equation 5 should display an Arrhenius behavior with a n activation energy of 15 kcal. Figure 5 is a plot of this kinetic group vs. outlet temperature with a taken to be 0.07. The curve corresponds t o 15 kcal activation energy and is not t o be regarded as a leastsquares fit of the data, but is included to show the temperature effect corresponding t o bhis activation energy.
One of the difficulties in attempting to develop a methane yield expression for the P E D U data is that neither the ternperabure nor hydrogen partial pressure is known in the vicinity where the direct methanation process occurs. The use of outlet values for these variables is the only alt'ernative a t this time. However, the outlet values for t'hese variables are no doubt related to their reaction zone values as the correlat,ioiidisplayed in Figure 5 shows. The reasoning used t o steer the analysis has been shown previously to have general validity in bench-scale experiments. T h a t Equation 5 predicts the proper temperature and hydrogen partial pressure effects in the P E D C is furt'her corroboration of the two-step model. *In alternate explanation often offered for direct methanation results is that the reaction represented b y Equation l reaches equilibrium. However, a plot of the outlet methane t o hydrogen ratio-Le., the apparent' equilibrium constant s . outlet temperature for Equation 1, ( f ' c ~ 4 ) / f ' ~ ~ ) ~ - - vthe shows the ratio t o be considerably below the equilibrium value corresponding to the outlet temperature (Figure 6). This is not t o say t'hat equilibrium is not reached a t a higher temperature corresponding to the reaction zone of Stage 2. There is no way to check this proposition wit'h current PED" instrumentation. On the other hand, the water-gas reaction
CO
+ HzO
COY
+ H?
(6)
generally achieves equilibrium a t the outlet temperature (Figure 7). Ind. Eng. Chem. Process Des. Develop., Vol. 1 1 , No. 1 , 1972
99
PEDU Lignite Data Curve Bared on Theoretical Equilibrium far Graphite in Equation 1
EQUATION 1 EQUILI6RIUM CONSTANT, (am)-'x10'
Figure 6. Apparent approach to carbonhydrogen equilibrium
.
1 1300
*.
*.
1400 IS00 OUTLET TEMPERATURE. I
1600
EQUATION 6 EQUILIBRIUM CONSTANT
rigure
I .
PEDU Lignite Data Curve Bared on Theoretical Equilibrium for Water G a r Shift 1300
1400
1600
1500
OUTLET TEMPERATURE, f
A generally agreed-upon rate expression for the carbonsteam reaction has been presented by Von Fredersdorff and Elliot (1963) : (7)
where C.Y. is the yield of carbon oxides as percent C, 7 is the residence time, and the subscript a denotes a n appropriate average. If the outlet steam to hydrogen ratio is used in place of the average ratio, the carbon oxide yield equation is
c.Y. where k and CY are kinetic parameters. I n the previously described tests in the 5 lb/hr C F R unit, this expression correlated the yields of carbon oxides with a good degree of accuracy. I n those instances, however, both inlet and outlet conditions were measured so that proper integration of the rate equation could be carried out to account for the change in gas composition as the steam-synthesis gaslignite mixture passed through the reactor. As there is no way at present to measure the conditions prevailing in the PEDU Stage 2 reaction zone, the outlet conditions must be used to approximate those in the gasification zone. First, two observations are in order. Under the high pressure conditions in the gasifier, the product (uPH~ is much larger than 1. The integrated rate expression may then be approximated as follows:
100
Ind. Eng. Chem. Process Des. Develop.,
Vol. 1 1 , No. 1 , 1972
The right-hand group has an Arrhenius behavior. But, because the temperature varies in a n unknown fashion as the lignite-gas mixture passes through the reactor, there is no apparent way to utilize this property. It is possible, however, t o plot the right-hand side of Equation 8 vs. reciprocal outlet absolute temperature. Figure 8 is such a plot and shows that the data do correlate in this way. Again, the correlation of the data in Figure 8 should be taken simply as indication of the nature of the gasification process and not as the consequence of a strict mechanistic explanation. The slope of the correlating line in Figure 8 corresponds to an activation energy of 28 kcal/g mole. Although this value is less than that normally reported for the steamcarbon reaction, it is not to be taken as a fundamental property but rather a convenient parameter for representing the PEDU data in a manner suggested by gasification chemistry.
'1
PEDU Lignito Data
Curve Bared o n Loart Squarer F i l
.020EQUATION 8 KINETIC GROUP,
B
Figure 8. Correlation of outlet temperature with carbon-steam kinetic
[ 4"cr) ,010-
1 .008-
,006
I
4.9
I
5.0
I
I
5.1
I
5.2
5.3
RECIPROCAL OUTLn TEMPERATURE,
5.5
x 10'
PEDU l i g n i t e Data
301
20
R-'
I
I
5.4
-I
Curve Bared on Theoretical Graphite-Steam E q u i l i b r i u m
/
Figure 9. Effect of outlet temperature on carbonsteam equilibrium
0
PERCENT CARBON I N FEED
I
1
I
4o
20 J
Figure 10. Effect of outlet temperature on total carbon yield and preformed methane
Preformed Methane PERCENT TOTAL POTENTIAL
._ 1300
1660
l5bO
14bo
OUllEl TEMPERATURE, F
The question of equilibrium for Equation 2 can be answered by examination of the apparent equilibrium constant' for this reactioii. Figure 9 is a plot of this apparent constant' K m. outlet' t'emperature, where
K =
( P C O ) (PH2) (PHrO)
'The outlet gases are not' in equilibrium with respect to Equation 2. In the two-stage super-pressure process, the product gases of priiicipnl concern are methane, hydrogen, and carbon monoxide. The greater the amount of methane in proportion to the amouiit of carbon monoxide and hydrogen, the smaller will be the filial cost of pipeline gas. To indes this effect, Gleiiii et al. (1967) introduced the term "preformed
methane" as the amount of methane in the gas relative to the total methane potentially available in this gas stream. Based on Stage 2 yields, this quantity is computed as follows: Preformed methane
=
%C K
+
'/4
% CHI (% H2
+ % CO)
!!here the percentages are with respect to the gas produced b> the Stage 2 reactions. For the hydrogen and carbon monoxide, the outlet gasifier compos1t:on for the amounts made 111 Stage 1 must be modified. Values of preformed methane are tabulated in Table 111for the PEDU tests. The preformed methane depends upon a number of gasificatioii parameterb, aiid the Stage 2 preformed methane value. vs outlet temperature are plotted (Figure 10). At low temperatuxes the preformed methane value IS low, because Ind. Eng. Chern. Process Des. Develop., Vol. 11, No. 1 , 1972
101
the direct methanation process is slow. At high temperatures the carbon-steam reaction proceeds a t a rapid rate, and tlie H2) is proportionately greater than the CH4 yield of (CO yield. Consequently, there is a region of intermediate temperature where preformed niet~hane is maximized, even though the amount of carbon gasified to both methane and carbon oxides continues t o increase with temperature (Figure 10). The gasification process is quite coniples and many other variables, including the composition of the Stage 1 gas, influence the preformed met’liane.
+
Discussion
The PEDU t’ests wit’li lignite hare showii that gasification proceeds smoothly and predictably in an internally fired unit in which coiidit’ioiis closely simulating those in Stage 2 are achieved. One of the distinctions between the PEDU and an integrated t,wo-stage gasifier, however, is in the amount of nitrogen required by the PEDU for purging aiid coal feeding. Such nitrogen will not be needed in the integrated gasifier so that the partial pressure of the reacting gases, particularly hydrogeii, will be increased proportioiiately. This in turn will lead to higher met’haiie yields and higher values of preformed methane. The PEDU tests have further shoivii conditions of operation more favorable t’lian others-namely, those at interniediate t,emperatures where maximum preformed methane is achieved. Actually, the tests have provided considerable information on the effect of temperature on the process. The previous experiments in the CFR varied gas composition and rate while maintaining a nearly constant temperature; the results established the validit’y of the two-step model and correlating equation for the direct methanation process. The current PEDU tests have extended the applicability of t’he model to other temperatures and prorided quantitative iiiformation on the effects of gasification variables on yields. The yield expressions for methane and carbon oxides have been inserted into a computer program simulating tlie integrated gasifier. This program is being used in conjuiictioii with economic informat’ion and gas processing requirement’s
to project cost aiid production figures for operation of t,he process on a commercial scale. A4dditional P E D P tests have already begun with R subbituminous Wyoming coal. Tests at’ higher pressures are plaiiiied to determine the effect of this variable on gasifier performance. The information aiid experience gained froin operation of the 100 lb/lir PEDU are being usrd in the design of a fully integrated 5 ton Air pilot plant for production of pipeline gas with the two-stage concept. This plant will esplore the problems of operating a slagging stage and entrained stage 15-ithin the same vessel aiid will provide design data for a commercial scale plant. literature Cited
I h i i a t h , E. E.j GIen~i~ I?. A , , “Pipeliiie (;as from Coal by TwnStage Entraiiied Gasific.ation,” i n “Operating Section Proceedi ~ i g s S~ e’ w ~ York, Amer. Gas Assoc., pp 65, 147, 151 (1965). Field, 11. .4.,Gill, 1). W.j Morgan, B. B.,Hawksley, P. (;. W., “Combustion of Pulverized Coal,” Leatherhead, England, Brit. Coal TJtil. Ile.;. .4\soc. (1967). Glenn, It. h.,Grace, 11. J., “hn Internally-Fired Process aiid 1)evelopment Unit for C ification of Coal under Condition. Simrilating Stage 2 of the BCIi Two-Stage Super-Pressure Procew,“ Amer. (;ai A s ~ o c . ,Synthetic Pipeline Gay Synip., Pittsburgh, Pa., 1968 Glenn, I? A , , lloiiath, E. E.j Grace, 1:. J., “Gaiification of Coal iiiider Conditions Simulating Stage 2 of the BCR Siiper-
Pressure Ga.C., Amer. Chem. Soc.. D D 81-103 11967). Lacey, J. A , , “The ’eisificatioii‘of Coal i n a Slaggiiig Preisiire Gaqifier,” d i j i e r . Chem. S O C . Diu. Fuel Cheiu. P r e p . , 10 (4), 151-67 (1966). lIosele!; F., Paterwn, I)., J . I n s f . F 7 ~ l38 , 128R), 13-23 (196%). on, I]., &fd., (296), 378-91 (1965h). (111, I ) . > z’bicl., 40, 523-30 (1967). 1-(!1i Fredersdorff, C. G.j Elliut, 11. h.,“Coal Ga.dkttioii,” i n “Chemistry of Coal Utilization,’’ IViley, Kew York, N.Y., 1963, pp 892-1022.
Zshradnik,11. I,,>Glenn, I?. .4.,Fitel, 50, 77-00 (1971). R ~ x i : r v e ofor review Fehrriary IT! I W l ACCI:PTI:D,Iiigrist 13j 1971
Based on work carried oiit at Bitnmiiiou.~Coal ReAearch, Iiic.) with support from the Office of Coal Research, U.S. ljepsrtmeiit of the Interior, tirider contract So. 14-01-0001-324. Pre.Geuted at the Symposium 011 Synthetic. Hydrocarbon Fiiels from \Vestern Coals, -4ICHE, Ileiiver, Colo., duguit 30-September 2, 1970.
Complex Method for Solving Variational Problems with State-Varia ble Inequality Constraints Tomio Umeda and Akio Shindo Chiyoda Chemical Engineering R. Construction Go., I’okohama, J a p a n
Atsunobu lchikawa T o k y o Institute of Technology, Tokyo, Japan
V a r i o u s methods for solving variational problems with or without inequality constraints have been developed. The maximum principle and dynamic programniiiig have received much attention. In applying the maximum principle, one must solve two-point boundary value problems, whereas in dynamic programming the curse of dimensionality restricts its applicability to simple problems. Thus, in most cases, the solu102
Ind. Eng. Chem. Process Des. Develop., Vol. 1 1 , No. 1 , 1972
tioii of variational problems requires application of a numerical method, such as gradient,or conjugate gradient. With nonlinear problems e>peciallj-. digital computers are necessary to solve optimization problems by a sequence of discrete optimization. T h a t state vuriahles must satkfy various inequality constrailits usually complicates solutioii of variationnl problems.
