530
~ n dEng. . chetn. p~ocess~ e s ~. e v 1903, . 22, 538-544
Chang, C. D. Chem. Eng. Sci. 1980, 35, 619. Chang, C. D. J . Catal. 1981, 69, 244. Chaw, C. D.; Kuo, J. C.; Lang, W. H.; Jacob, S. M.; Wise, J. J.; Sllvestrl, A. J. Ind. Eng. (2”.ProcessDes. Lbv. 1078, 17, 255. Chang, C. D.; Lang, W. H.; Smith, R. L. J . Catal. 1979, 56, 169. Chang, C. D.; SUvestrl, A. J. J . Catal. 1977, 47, 249. Chen, N. Y.; Reagan, W. J. J . Catel. 1079. 59, 123. Derouane, E. G.; Nagy. J. B.; DeJatfve,P.; van Mff, J. H. C.; Spekman, B. P.; Vedrlne, J. C.; Naccache, C. J . Catel. 1978, 53, 40. Fleckenstein. T.; Litterer. H.; FeMng, F. Chem. Ing. Techno/. 1080, 52, 816. Fletcher, R. Atomic Energy Research Establishmentdeport 6799. Harwell, 1971, Berkshire, England. Frey. H. M.; Voisey, M. A. Trans. Faraday Soc. 1068, 64, 954. Qlvens, E. N.; Pitman, C. J.; Plank, W.; Rosinskl, E. J.; Town, P. U S . Patent 4 079 095, 1978. Harney, B. M.; MNls. G. A. “ m t ) o n Process. Fob 1980, 67. Hina, J. “Divalent Carbon”; Ronald Press Co.: New York, 1964. Hlne, J. “Physical Organic Chemistry“, 2nd ed.; McGraw-Hill, Inc.: New York, 1962. Hinshelwood, C. N. “The Klnetics of Chemical Change”, Clarendon Press: Oxford, 1955. Khmse, W. “Carbene Chemistry”; Academic Press: New York. 1964. Kistiakowsky, 0. 8.; Sauer, K. J . Am. Chem. Soc. 1958, 78, 5699. L i m a n , D.; Jacob, S. M.; Voltz. S. E.;Wise, J. J. Ind. Eng. Chem. Process Des. Dev. 1978, 17, 340. Lin. F. N.: Anthony, R. G. 70th Meetlng of the American Institute of Chemical Engineering, New Ywk, 1977 (as cited In 011 Ges J . Mar 8, 1978, 92).
Marquardt. D. W. J . Soc. Ind. Appl. Math. 1963, 1 1 , 431. Morgan, C. R.; Warner, J. P.; Yurchak, S. Ind. Eng. Chem. Process Des. D e v . 1981, 20, 185. Pop, e.; Ivanus, Gh.;Boteanu, S.; Tomi, P.; Pop, E. U.S. Patent 4 172816, 1979. Pop, Gr.;Petre, C.; Ivanus, oh.;Toml, P. Nertehimle 1979. 19, 535. Salvador, P.; Kladnlg, W. J . Chem. Soc. Farahy Trans. 1 1977, 73, 1153. Senderens, J. B. Ann. Chlm. Fhys. 1912. XXV, 449. Swabb, E. A.; Gates, 8. C. Ind. Eng. Chem. Fundam. 1972, 1 1 , 540. van den Berg, J. P.; Wolthuben, J. P.; van Hooff, J. H. C. Proceedlngs of the 5th International Conference on Zeolites, Napoll, Italy, 1980. Voltz, S. E.; Wise, J. J. In ”Methand Technology and Application in Motor Fuels”, Paul, J. K., Ed.; Noyes Data Corporation; Park Ridge, NJ, 1978. Weisz, P. B. A&. Catal. 1982, 13, 137. Yurchak, S.; Vok. S. E.;Warner, J. P. Ind. Eng. Chem. Process Des. D e v . 1979, 18, 527.
Receiued for review October 30, 1981 Revised manuscript receioed June 4, 1982 Accepted November 4 , 1982
Supplementary Material Available: Table V. The experimental data for conversion of methanol to olefins (2 pages). Ordering information is given on any current masthead page.
Simple Field Test Model for Underground Coal Gasification Robert D. Galll, George E. Jones,’ and Fred E. Klvlat
oulf Research & Development Company, Pittsburgh,
Pennsylvanja
15230
A simple process model was used to study the effects certain variables had on the product gas composition from a subbitumlnous, steeply dipping underground coal gasificatlon field test. We found that two key model input parameters were (1) heat loss and (2) the ratlo CO/CO, in the carbon oxidation equation. An Increase in the heat loss parameter was generally accompanied by a decrease in the CO/CO2 parameter in order to fit the field test data. The heat loss parameter increased as the field test progressed, suggesting increased rock fall or ground water influx. Very late in the field test, the only way to fit the data was by substituting a “fictional” coal having a high char/vohme ratio. We conclude that as the process module aged, the production of char (relative to volatlles) increased.
I. Introduction Several mathematical models have been developed and used to study field test data from underground coal gasification. Gunn and Whitman (1976) developed a onedimensional model to study the gasification of a Hanna, Wy, subbituminous coal. Jennings et al. (1977) simplified the Gunn work by substituting algebraic relationships for the reaction rate expressions. Thorsness and Sherwood (1978) developed a model which includes more reactions than the Gunn model but requires more input information. We have used the Jennings model to evaluate results from a subbituminous, steeply dipping field test at Rawlins, WY. We have compared model calculations of product gas composition with the measured values. Using the model, we showed that observed changes in product gas composition could be related to changes in unmeasured, uncontrolled process variables such as heat loss. This paper presents such results. 11. Field Test Description A schematic of the Rawlins Steeply Dipping bed is shown in Figure 1. The test was done in a forward gasification mode. The consumption of coal began near the 0196-4305/83/1122-0538$01.50/0
injection well and advanced toward the product well. Air and water were injected for most of the field test, except for a five-day period of oxygen-steam injection at the end of the test. Neither groundwater influx, heat loss, nor reaction temperature was directly measured during the test. The absence of direct information on these variables led us to use the model to quantitatively estimate what effects they might have had. 111. Model Description A. One-Dimensional Model. The one-dimensional model developed by Jennings is illustrated in Figure 2. Energy and material balances are performed on a control volume of unit cross section. Coal with a given proximate analysis, density, and volatile composition enters the control volume at a temperature TcoL. Injected oxygen, nitrogen, and steam enter at a temperature TINJ. Liquid including both injected water and groundwater (WR), water, enters at TCob Ash leaves the control volume at TASH and product gas, at Tow. Heat loss is expressed by the term “LRn,which is defined as the quotient of heat losses divided by the heat generated by the combustion reaction of C with O2to produce CO and COP The model’s 0 1983 American Chemical Society
I d . Eng. Chem. Process Des. Dev., Vol. 22, No. 3, 1983 539 SEAM THICKNESS 1 3 WET
PRODUCTION
wrtt
INJICTION WELL
However, eq 6 , 7 , 8 , and 9 can be obtained by linear combinations of eq 1 , 2 , 3 or their reverse reactions; hence the model is more generally applicable than it appears at first glance. Methane formation reactions, such as CO + 3H2 CH4 + HzO (10) C + 2H2 CHI (11) are very slow in the absence of a catalyst; their exclusion from the model should not cause error in most cases. As shown in eq 3 and 5, char is consumed by reaction with both water and oxygen. The sum char consumption rate determines the calculated flame front velocity. The flame front is visualized as moving from injection to production well at this velocity. Further details of the model are given by Jennings et al. and in the Appendix. IV. Study of Field Test Results A. General Approach. We used the one-dimensional model to calculate dry product gas composition for comparison with the observed compositions. Measured, controlled process input such as water, air, steam, or oxygen injection were translated directly into their corresponding model variables and used in the calculations. The following model variables, which could not be directly related to measured values, were varied in efforts to fit the data: heat loss ratio, L,; CO/CO2 combustion ratio, CR; and liquid water/injected air or oxygen ratio, WR. The values we assumed for TINJ and TOUTwere gas injection and production temperatures measured a t the surface. These measurements did not change greatly during the field test. The values used are listed in Table VII. We used a coal volatile composition from Gunn and Whitman (1976), derived from lab devolatilization experiments a t 900 "C on subbituminous coal from Hanna. We used this coal volatiles composition for our study, since these data were not available for the Rawlins subbituminous coal. We assumed that it would provide a good approximation because the coals are of a similar grade. Coal proximate analysis data from the Rawlins seam were averaged and used in the calculations. The samples were taken prior to gasification. The model values assumed for coal volatile analysis, coal proximate analysis, coal density, and the various temperatures and basis flow rates are listed in Tables VI and W. The coal property data remained constant throughout the field test, except for the char/volatile ratio, which was changed on occasion in efforts to fit the experimental gas composition data (discussed later). In this study, the heating values referred to are calculated on the basis of CO, H2, and CHI contributions alone. The model assumes that most higher molecular weight components are cracked. Water concentrations in the wet product gas were ignored in this study, because the measured values were not as accurate as were those of CO, C02, CH4, and Hz. Further, the water content could be influenced by many factors which would have complicated this study. B. Results and Discussion. We studied four separate periods in the Rawlins field test using the model. We have named them: (1) akwater injection period; (2) high water injection period; (3) declining heating value period; (4) oxygen-steam injection period. 1. Air-Water Injection Period. After an initial process startup period, the process module reached an apparent steady-state condition, called such because the measured product gas composition was relatively constant (except for experimental noise). Water/air injection molar
-
+
Figure 1. Rawlins gasification module. Inl.cllon
w.11
Pieducllon mll
I
Figure 2. One-dimensional model.
user must specify the four temperatures and the fractional heat loss ratio. The model includes the following reactions
-
C + 1/202 CO (exothermic) C + O2
- +
C + H20 coal
char
COz (exothermic)
CO
ash
+ H2
(endothermic)
+ volatiles
(1)
(2) (3)
(endothermic) (4)
Equations 1 and 2 are combined into a single carbon oxidation (combustion) equation, in which the relative production of CO and COz is accounted for by the userspecified model parameter "CR" (1 + CR)C + (1 + CR/2)02 (CR)CO + C 0 2 (exothermic) (5)
-
The model does not directly include the following important reactions CO + H20 Cop + H2 (6)
coz + c - 2co co + -
-
1/2Oz
HzO
c02
H2 + 1/202
(7)
(8) (9)
Ind. Eng. Chem. Process Des. Dev., Vol. 22, No. 3, 1983
540
Table 11. Comparison of Rawlins Data with Model Calculation, High Water Injection Period
Table I. Comparison of Rawlins Data with Model Calculation, Air-Water Injection Period dry product gas compn, mol % component HZ
co COZ CH4 heating value, Btu/SCF
Rawlins dataa
model calcn b
15.9 25.7 6.70 4.26 177.0
14.9 25.9 6.70 4.30 174.8
dry product gas compn, mol % component H2
co COZ CH.l heating value, Btu/SCF
a Date, Process Days 11-13;injected water/injected air ratio: 0.319. Model Variables: heat loss ratio, LR = 0.10;CO/CO, combustion ratio, CR = 2.20;water/air molar ratio, W R = 0.319;other, see Tables VI and VII.
1
zoo, 190
1
17.3 24.7 8.3 4.57 181.5
8.7 22.4 7.8 3.93 139.8
a Data from Process Day 14,0-800 h; injected water/ injected air molar ratio: 0.75. Model variables: heat loss ratio, LR.= 0.10; CO/CO, combustion ratio, CR = 2.20;waterlair molar ratio, WR = 0.75;other, see Tables VI and VII.
Calculatrd Curvr Rawllne Data (Avo., PrOcaer Days 11-15)
Spread of Data Ovar Days 11-13
I
1SO
Model Varlablea:
120
LR CR
2.20
110
Other
Srr Tables A I and A l l
d
0.0
model calcn
T
x I
100
Rawlins dataa
0.10
I
I
I
I
0.1
0.2
0.3
0.4
I
0.6
1 0.0
WatrrlAlr Molar RIIIo.WR
Figure 3. Heating value vs. water injection.
ratios ranged between 0.32 and 0.46. We found that using a CO/C02 factor of 1.50 or greater and using a heat loss factor in the range of 0.0 to 0.1 permitted the model to fit the Rawlins data Table I shows the results of a calculation using CR = 2.20. In these calculations, we assumed that all the injected water reached the burn front and none is lost to the surrounding formation. Groundwater influx is also assumed negligible. Using the CO/C02 fador of 2.20, we calculated a heating value vs. water injection curve. The result is shown in Figure 3. Note that the experimental value is close to the estimated h u m obtainable value. The m e indicatea that a slight decrease in water injection could have improved heating value, but only slightly. 2. High Water Injection Period. On Process Day 13, the field test water injection rate was deliverately increased from 0.32 to about 0.75 lb-mol of H20/lb-mol of injected air to see what would happen. No other controllable process parameters were changed. After this increase, the measured product gas C02and H2concentration increased, while the CO content decreased. We tried to fit these data by continuing to use the parameters which fit the Initial Air Water Injection period (see Table I) and by using the new higher value of water injection rate. The results are shown in Table 11. The model failed to predict the new product gas composition. The strongest difference was between calculated and observed H2 concentration: 8.7 vs. 17.3%. Assuming that water injection was the only process variable that changed, these results suggest to us that the
0:oo
06:Oo
1o:oo
15:oo
20:oo
01:oo
0
00
HOUR
Figure 4. Data, declining heating value period.
increased water injection increased the rate of certain reactions that produce hydrogen, such as H2O
-
H2
H2O + CO
+ '/202
-+
COZ
+ Hz
Although these reactions are indirectly included in the model (see section IILA), it is possible that at high conversions, no combination of model variables can fit the data. Direct inclusion of these reactions in the model is needed for this part of the field test. 3. Declining Heating Value Period. The water injection ratio was decreased on Process Day 16 from 0.75 Ib-mol of H20/lb-mol of air to values under 0.60. The agreement between experimental and calculated results improved. Then, on Days 17 and 18 of the field test, the product gas heating value declined sharply from about 176 Btu/SCF at the beginning of the Day 17 to about 152 Btu/SCF early in Day 18. The reasons for this decline are unknown. The data for this period are shown in Figure 4. On Day 18, the product gas compositions leveled off and remained relatively constant for several days thereafter. We used the model to test several hypotheses for the cause of the decline in heating value. The hypotheses are as follows: (1)Part or all of the injection process water began bypassing the process module. The water was in-
Ind. Eng. Chem. Process Des. Dev., Vol. 22, No. 3, 1983 541
__
Model Varlibloa: LR
0.10
CR
2.20
Other
See Table8 A I and A I I
Model Variables :
CO
LR
0.00-0.30
CR
2.20-0.80
Other
8.0
Tablor AI,AII
\ ***
50
-
0O:OO
C"4
I
05:OO
1O:OO
20:OO
16:OO
01:OO
06:OO
Hour I
I
I
I
I
0.6
0.4
0.5
0.2
0.1
Figure 6. Comparison of calculated and experimental results. declining heating value period.
Water /AIR Molar Ratlo, WR
Figure 5. Calculated composition vs. water/air molar ratio.
creasingly lost to the surrounding formation. (2) The production of pyrolyzed coal decreased. (3) Heat losses increased. (4) Groundwater influx increased. To test these hypotheses, we sought to fit the curve of composition vs. time for Process Day 17. In other words, we attempted to fit the observed transient behavior of the system. As will be shown, this helped us to choose from among the different hypotheses. We tested hypothesis 1 by calculating composition vs. water injection curves and comparing these with the data. The model water injectiomwas decreased from the measured value of 0.57 to 0.0 lbmol of HzO/lb-mol of air. This provided a test of the hypothesis that the water reaching the process decreased gradually to 0.0. The calculated results are shown in Figure 5. Note that the calculated Hz and CO concentrations reach maxima at about 0.20 lb-mol of water/lb-mol of air. Such maxima were not observed in experiment. The predicted changes appear large enough to be observed above the experimental noise. Since they were not observed, we must reject hypothesis 1. We conducted a test of hypothesis 2 by increasing the assumed char/volatile ratio above that commensurate with the seam coal analysis. The calculated composition trends were the opposites of the observed trends. Therefore, we must reject hypothesis 2. We tested hypothesis 3 by gradually increasing the heat loss ratio to 0.20. This approximately fit the experimental Hz curve (results not shown). The correct changes in trends for CO and COzwere calculated, but these changes were not sharp enough to fit the data. The calculated final CO/CO2 ratio in the product gas was 2.9, compared with an experimental value of only 1.2 in the early part of Day 18. The above result led us directly to continue testing hypothesis 3 by simultaneously increasing the heat loss ratio and decreasing CR,the CO/CO2 combustion ratio. The results are shown in Figures 6 and 7. We were able to fit the data by increasing heat loss to 0.30 and decreasing the CO/COz combustion ratio to 0.60. We therefore accept the third hypothesis, that increased heat losses could cause the observed decline in heating
-
Calculated Curwe
Model Varlabloe:
:E I
LR
0.00-0.50
CR
2.20-0.80
WR
0.672
Icn;;.. .;..
I
0
Hour
Figure 7. Comparison of calculated and experimental results, declining heating value period.
value. These increased heat losses appear to reduce the CO/COz combustion ratio. A plausible explanation is that in the module, heat losses decrease the module temperature and shift the equilibria of the combustion reaction toward a lower CO/COz ratio. Published equilibria for the wbon-oxygen system (Gregg and Olness, 1976)show that the effect of increasing temperature is to decrease CO/C02 Increased heat losses could be caused by greater rock fall into the UCG cavity. Although we accept hypothesis 3, it was true that an increase in groundwater influx (hypothesis 4) could also explain the observed results. Therefore, we also found hypothesis 4 acceptable. We tested this hypothesis by varying the model water/air molar ratio from 0.572,the measured value, to 1.00,and decreasing CR from 2.20 to 0.60. The heat loss ratio was held constant at 0.0. The calculated results were similar to Figures 6 and 7. Additional liquid water introduces a higher latent heat requirement into the model, in effect a second form of heat
542
Ind. Eng. Chem. Process Des. Dev., Vol. 22, No. 3, i963
Table 111. Comparison of Rawlins Data with Model Calculation, Oxygen-Steam Injection Period
Table V. Test of Hypothesis 2, Oxygen-Steam Period dry product gas compn, mol %
dry product gas compn, mol % component
Rawlins data"
model calcn b
32.0 31.6 31.7 2.81 233.5
27.9 50.0 13.5 8.27 339
comDonent
co
COZ CH, heating value, Btu/SCF
Data from Process Day 43, 0.600-0.700 h; injected steam/injected oxygen molar ratio: 0.527. Model variables: heat losf ratio, LR = 0.10; CO/CO, combustion ratio, CR = 2.20; steam/oxygen molar ratio, SR = 0.527; other, See Tables VI and VII. a
model calcn b
32.0 31.6 31.7 2.81 233.5
23.3 56.9 16.6 3.17 292
HZ
~
HZ
Rawlins data"
co COZ CH, heating value, Btu/SCF
Data from Process Day 43, 0600-0700 h; injected steam/injected oxygen molar ratio: 0.527. Model variables: heat loss ratio, LR = 0.10; CO/CO, combustion ratio, CR = 2.20; steam/oxygen molar ratio, SR = 0.527; char/volatile ratio = 4.33; other, see Tables VI and VII. 0
Table IV. Test of Hypothesis 1. Oxygen-Steam Period dry product gas compn, mol % component HZ
co CO, CH, heating value, BtuISCF
Rawlins dataa
model calcn
32.0 31.6 31.7 2.81 233.5
30.7 29.2 31.8 7.95 278
Model Variable*:
. i
70
0.10
CharlVoIatllo
eo
4.33
Ratlo See Tables U,AU
Other
Data from Process Day 43, 0600-0700 h; injected steam/injected oxygen molar ratio: 0.527. Model variables: heat loss ratio, LR = 0.55; COlCO, combustion ratio, CR = 0.10; steam/lxygen molar ratio, SR = 0.527; other, see Tables VI and VII.
I
I
I
1.0
2.0
2.4
Ol 0
loss. This explains why either high heat loss ratios or high liquid water ratios can both explain the observed heating value decline. In any event, heat loss by some form is the key process variable. 4. Steam-Oxygen Injection Period. Steam and oxygen injection were used instead of water and air in the last five days of the field test. The steam/oxygen ratio was varied, and the experimental heating values ranged between 200 and 260 Btu/SCF. We first tried to fit these data by using the high CO/CO2 combustion ratios and low heat loss ratios used to fit data early in the field test. The calculated heating values were about 100 Btu/SCF higher than the observed values. A sample result is shown in Table 111. Note that the experimental CHI concentration is less than half the calculated value. We developed three hypotheaea to explain the steam-oxygen data: (1)heat losses occurred; (2) at this late stage of the test, the coal was approaching exhaustion, and less volatiles were produced; (3) a combination of 1 and 2. We tested hypothesis 1by increasing the model heat loss ratio and decreasing the model CO/CO2 combustion ratio. A sample result is shown in Table IV. We were able to fit the experimental CO, C02,and H2 concentrations, but could not fit the CHI data. The calculated CHI concentration was still higher than the experimental. We therefore reject the hypothesis that high heat losses alone caused the observed changes. We tested hypothesis 2 by substituting a fictional coal with a char/volatile ratio higher than that measured in the Rawlins seam. A sample result is shown in Table V. The table shows a calculated value obtained by adjusting the char/volatile ratio to 4.33 (as compared with the measured value of 1.261,high enough to fit the experimental CHI
Data Calculated
8teemIOxysen Molar Ratlo. S R
Figure 8. Comparison of calculated and experimental hydrogen concentration, oxygen-steam period. 100
5
i I
0
-
a0
eo
-
oat1 Calculated
Modal Verleble8
LR CR
0.66 0.10
-
CharlVOIatUa
4.33
Other
40
-
-
30
-
8
20
-
-:
70-
eo
B
60
t
s
f
e
Retlo 8ae Tabla8
AL An
0
0
0
I
1
Figure 9. Comparison of calculated and experimental carbon monoxide concentration, oxygen-steam period.
concentration. However, note that when this was done, the calculated product gas CO/CO2 ratio was still much higher than the experimental result. Therefore, the observed data cannot be explained by low devolatilization alone. To test hypothesis 3, we tried combinations of high heat loss, low CO/CO2 combustion ratio (CR), and high char/ volatile ratio. A good fit with all four experimental compositions was obtained using a heat loss ratio of 0.55, a
Ind. Eng. Chem. Process Des. Dev., Vol. 22, No. 3, 1983 543
.-- I
a
8 i d I
e
5
Table VI. Coal Prowrtv Data Used for Model Calculations
Data
O0I
- COIcuIOled
70
CR
coal proximate analysisa constituent wt fraction
Yodel Verlebler:
80
char volatiles ash tar moisture ratio char/volatiles
0.66 0.10
LR
eo -
CharlVolallle Ratlo
4.33
See Table8 AI, A l l
Olhar
60
40
-
30
-
0
$
6
co2
e
-
q 20
volatile composition (Gunn and Whitman, 1976)
to
CH4
0
-
I
.
-
I
t .o
0
2.0
a
Figure 10. Comparison of calculated and experimental carbon dioxide concentrationand methane concentration,oxygen-steam period. """
0
B
f2
Model Verleblor: 280
LR CR
270
0.66
0.10
CharlVolatlle
EI
250
P
250
t
f
Dale CeICuIated
4.33
1
220 I
210 200 0.0
1 .o
2.0
1 2.4
81aemlOxygen Molar Ratlo. SR
Figure 11. Comparison of calculated and experimental heating value, oxygenateam period.
CO/CO2 combustion ratio of 0.10, and a char/volatile ratio of 4.33. We therefore accept hypothesis 3. The results are shown in Figures 8-11. A liquid water influx of 1.20 lb-mol of H20/lb-mol of oxygen, combined with CR = 0.10, LR = 0.30, and char/ volatiles = 4.33, could also fit the data. As earlier discussed, the liquid water influx ratio and the heat loss ratio both introduce heat losses into the model. It is not surprising that a combination of the two could explain the observed result. In any event, heat loss, whether introduced by LR or by WR,must be high to fit the observed results. The higher char/volatile ratio is the only possible model adjustment which will fit the low observed CHI product gas content. We conclude, therefore, that the production of volatiles decreased as the module approached exhaustion. Increased consumption of volatiles relative to char could occur if oxygen has difficulty reaching the char, very possible in later stages of cavity growth. Further, the model's CO/CO2 combustion ratio which will fit the observed data has decreased to 0.10 from values as high as 2.20 early in the field test. When we decreased the CO/C02 ratio we invariably increased either or both the model heat loss and the model liquid water influx. As discussed earlier, heat losses might decrease the module
mol % b 0.423 0.004 0.285 0.038 0.244 0.007
2.4
SteamlOxyaen Molmr Rallo. 8~
0.446 0.354 0.051 0.0 0.149 1.26
a Coal density: 87.4 lb-mass/ft3. 900 "C.
Temperature:
Table VII. Values Used for Model Calculations Air-Water, High Water Injection, and Declined Heating Value Periods. Basis Air Injection: 1.0 lb-mol % air/ft2-h TINJ 77-85 O F TCOL 600 TASH 600 TOUT 456-470 Oxygen-Steam Injection Period. Basis Oxygen Injection: 1.0 lb-mol % air/ftz-h TINJ 243 "F TCOL 600 600 TASH TOUT 635
temperature and shift the equilibria of the combustion reaction toward a lower CO/CO2 ratio. V. Conclusions We conclude the following. (1)Most of the Rawlins field test data can be fit using a simple model having parameters which account for heat losses and for the CO/CO2 ratio resulting from the combustion of carbon. Although these parameters are independent of each other in the model, when we increased the heat loss factor we generally decreased the CO/CO2 ratio in order to fit the data. Therefore, heat loss and CO/CO2 combustion ratio are not independent in the real process. Further, this heat loss ratio increased (and the CO/C02 factor decreased) as the field test progressed. Latent heat effects could also account for process heat losses. (2) Late in the field test, the model could fit the data only if one substituted a fictional coal having a higher char/volatile ratio than that of the virgin coal. We conclude that the gasification process changed as the module aged; the production of volatiles decreased as the module approached exhaustion. Acknowledgment The authors wish to thank the U.S.Department of Energy (DOE) for funding this work, under DOE DEAC03-77ET13108. Appendix. One-Dimensional Model Table VI contains coal property data used for model calculations and Table VI1 lists values used for model calculations. I. Chemical Reactions. The one-dimensional model is described in detail by Jennings et al. (1977). The model includes the following reactions.
544
Ind. Eng. Chem. Process Des. Dev., Vol. 22, No. 3, 1983
-
combustion: (1+ CR)C + (1 + CR/2)02 (CR)CO + C 0 2 (exothermic) steam char: C
-
+ H20
devolatilization: coal char
-
CO + H2 (endothermic)
+ ash + volatiles (endothermic)
No reaction kinetics are included for the above reactions. The stoichiometry of combustion is accounted for by the term CR, which is specified by the user based on independent information such as combustion tube experiments. The model also assumes that all tars produced are cracked and become volatiles. The composition of coal volatiles is also specified by the user. 11. Energy Balance and Product Gas Composition Calculation. The energy balance is needed to calculate the flame front velocity and product gas component flow rates. In simplified form, it is energyin = energyout (Btu/ft2-h)
(Seethe Nomenclature section for defhtions of the terms.) In the model, both ground water influx and injected water are included in the term W,. Although injected water does not enter the process at the formation temperature TCOL, assumed for WR, the error introduced by assuming this is under 5 % . The flame front velocity, u, is given by u=
CMCHAR PCOAL wCHAR
where M C H=~ molecular weight of char (assumed that of carbon, 12 lb-mass/lb-mol, Wcw = weight fraction char in coal, and C = char consumption, Ib-mol char/ft2-h. The char consumption is given by ‘=
1 + CR w+(1+CR/2)N02
64.3)
Note that char is assumed to be 100% carbon in the calculation of C and V. Using eq A.l, the model calculates w ,the water consumed in the steam-char reaction, the only unknown term in the equation. All the other terms are built into the model or are supplied by the model’s user. The product gas fluxes in lb-mol/ft2-hare calculated from the following NH,of = NIN(WR+ SR)- w (A.4)
NCHdf
=
yvCH;Nv
(A.8)
where YVCO, YVH,, YVCO, YV,,, = weight fractions of components in coal volatifes, N, = rate of production of coal volatiles = upcom WvoL/MvoL, and MVOL = molecular weight of volatiles (14.8 Ib-mass/lb-mol). Nomenclature NIN = injected air or oxygen rate, lb-mol/ft2-h HIN = enthalpy of injected air or oxygen, Btu/lb-mol WR = injected water/air or oxygen ratio, lb-mol of water/ lb-mol of air SR = injected steam/oxygen ratio, lb-mol of steam/lb-mol of oxygen HLH20 = enthalpy of liquid water, Btu/lb-mol HH,O= enthalpy of gaseous water, Btu/lb-mol u = flame front velocity, ft coal/h PCOL = density of coal, lb mass/ft3 H C O= ~enthalpy of virgin coal, Btu/lb mass AHc = heat generated by combustion, Btu/lb-mol of oxygen consumed NO, = injected oxygen rate, lb-mol/ft2-h H ~ =Henthalpy of ash, Btu/lb-mass WmH = weight fraction ash in coal WVOL= weight fraction volatiles in coal HvoL = enthalpy of coal volatiles, Btu/lb-mass w = water consumed in steam-char reaction, lb-mol/ft2-h AH, = heat of steam-char reaction, Btu/lb-mol of water consumed LR = heat loss ratio AHVU = heat of vaporization of water, Btu/lb-mol NPROD = product gas flow rate, lb-mol/ft2-h H~RoD= enthalpy of product gas, Btu/lb-mol Registry No. CO,630-08-0; C02, 124-38-9. Literature Cited Gregg, D. W.: Olness, D. U. “Bask Principles of Underground Coal Qaslflcetlon”; Lawrence Livermore Laboratory, University of California, Livemre, CA, 1976. Gunn, R. D.; Whltmen, D. L. “An In Situ Coal QasfflceuOn Model for FeasibC ity Studles and Design”; Laramie Energy Research Center, Report of Investigatlons 76/2, Laramle, WY, 1976. Jennlngs, J. W.; Strlckiand, R. F.; Von Qonten, W. D. “Texas A&M Project Status: Underground Lignite Qasificatbn”; 3rd Annual Underground Coal Conversion Symposium, Fallen Leaf Lake, NV, 1977. Thorsness. C. B.; Sherwood, A. E. “Moving Equfflbrlum Front Model for In Situ Qaslfication”; Lawrence U v e r m e Laboratory, University of California, Livermore, CA, Report UCRL 52524, 1978.
Receiued for reuiew January 7,1982 Revised manuscript receioed September 2, 1982 Accepted September 29,1982
