39 1
Ind. Eng. Chem. Res. 1987,26, 391-397
C, = constant-pressure specific heat D = diameter of pipe g = gravitational acceleration k = ratio of specific heats L = lift distance Ma= air mass flow rate M,., = water mass flow rate Pa= actual power Pi = ideal power Po = output power PI = atmospheric pressure PC = plateau height of probability density function shown in Figure 3 p ( u ) = probability density function as shown in Figure 3 Q, = air volumetric flow rate Q, = water volumetric flow rate r, R = pipe radius SL= area shown in Figure 3 SG = hatched area shown in Figure 3 T1= air atmospheric temperature T2 = compressed air temperature u1-u4 = signal amplitudes as shown in Figure 3 yL= liquid velocity obtained from probe calibration VL = time-averaged local liquid velocity X = axial distance down stream
Greek Symbols (Y
9
= void fraction = air lift pump efficiency
Literature Cited Allen, T. H. World Min. 1976, 29, 34. Delhaye, J. M.; Galaup, J. P. Turbul. Liq. 1975, 83-90. Duns, H.; Ros, N. C. J. R o c . World Pet. Congr., 6th 1963, 2, 451. Gibbs, C. W. Air Lift Pumping, New Compressed Air and Gas Data, 2nd ed.; Ingersoll-Rand: Phillipsburg, NH, 1971; p 31-1. Griffith, P.; Wallis G. B. J. Heat Transfer 1961, 83, 307. Henriksen, M.; Goplen, S. P.; Barron, T. F. Presented a t the Annual AIME Meeting, New York, 1977, paper 77-F-319. Orkiszewski, J. J . Pet. Technol. 1967, 19, 829.
* Author to whom correspondence
should be addressed.
Gerald L. Morrison,* Tala1 I. Zeineddine Mogens Henriksen, Gary B. Tatterson Mechanical Engineering Department Texas A&M University College Station, Texas 77843 Received for review January 23, 1986 Revised manuscript received September 29, 1986 Accepted October 29, 1986
Steam Decomposition in Underground Coal Gasification An idea for probing the combustion zone temperature (CZT) in underground coal gasification (UCG) by using the hydrogen amount in product gas coming from the steam decomposition (HS) was proposed. It seems, from both experimental and theoretical analyses of three generators, that the increase in CZT results in the increase of HS and vice versa. Experimental results of UCG in single well and multiwell generators, blind and open, constructed in bituminous coal seam by the shaft method with ambient and heated air and oxygen as oxidizing agents were presented, and good agreement with theoretical predictions was reached after taking into account the carbon dioxide trapping and hydrogen burning processes. Underground coal gasification (UCG) experiments were carried out in bituminous coal seam. The methods used were the shaft type with open and blind wells (recuperative type). Air, recuperated air (in the blind well generator), and steamloxygen were used as oxidizing agents. The experimental results as well as theoretical calculations made possible the formulation of an idea that the amount of hydrogen from the decomposed steam, HS, formed a t a given period of gasification, can be used for a characterization of the combustion zone temperature (CZT) changes. The goal of the remaining part of the present work is to prove that idea for the three investigated generators by presenting the experimental as well as theoretical results. Experimental Section The coal analysis and dry product gas compositions together with HS and product gas heating values (HV), water constant (W) in the product gas, and heat losses (LC) during three experiments, in generators 5 (G5) (air, open well), G8 (recuperated air, blind well), and G3/1 (open, three-well, steam/oxygen) are presented in Table I and Figures 1, 2, and 3 for G5, G8, and G3/1, respectively. Figure 4 presents HS and CZT changes vs. time during the above experiments. The temperature was predicted on the basis of theoretical considerations, mainly, which, together with calculated results included and the experimental ones 0888-588518712626-0391$01.50/0
Table I. Coal Analysis in Air Dry State parameter water, W", w/o ash, A", w/o carbon, C", w/o hydrogen, Ha, w/o nitrogen, N", w/o oxygen, On, w/o total sulfur, Sta,w/o volatile matter, V', w/o lower heating value, LHV, kJ/kg
value G8 G3/1 G5 10.50 4.48 6.80 2.60 4.83 13.20 69.63 69.48 63.34 4.68 4.23 4.58 1.50 1.30 2.40 10.08 9.91 12.00 1.01 1.22 2.23 35.81 30.16 38.21 24 670 22 740 27 100
in Figures 1-3, will be discussed in later sections of this work. The experimental HS's, called HSE in the figures, were calculated from eq 1, based on the experimental composition of dry product gas (Figure 4), and transformed into volume percentages (Figures 1-3) where A = 1.297, 1.182, and 1.2998 and B = 0.5130, 0.4738, and 0.5242 for G5, G8, and G3/1, respectively. HSE = ([M,(HE + 2CH4E)Ca]/[100Mc(C02E+ COE + CH4E)I) - [ ( M J P ) / ( l O O M ~ J ]= A[(HE + 2CH4E)/(C02E COE + CH4E)I - B (1)
+
After the experiments, the cross sections of postgasification cavities and the analysis of roof rock swelling, re0 1987 American Chemical Society
392 Ind. Eng. Chem. Res., Vol. 26, No. 2, 1987
?."
r17
!I C OE
CH4C-26
1'O I 150
/--
,I I
,COZTC
0
- 40 L 2
10
I
Figure 3. Gasification history of G3/1.
Figure 1. Gasification history of G5.
C02TC
a
-....\i,l----------PI
4.5 I
I
Figure 4. Comparison of HSE and HSC with TC, GT, and PT; all vs. time, for all three generators.
Figure 2. Gasification history of G8.
duced degassing depth of coal, geometry of cavity, and amount of slag and rubble in the cavity were performed to obtain information on the temperature existing in the gasification zone during experiments (Rauk, 1971, 1978). Results of cavity analyses for G5 and G8 are presented in Table I1 and Figure 4 together with estimated temperatures, G T and PT, where K, and d, were calculated from eq 2 and 3, respectively. K , = ( K m , J / ( K , m - mrl?
(2)
+ Aia + VI")])/ (SV1a[lOO- (W," + A," + V,")])] (3)
d, = d - [(VV,a[lOO - (Wla
,
In G8, the above indirect methods of temperature estimation were supported with three thermocouples installed in the recuperator pipeline to measure air temperatures. The thermocouples indicated decreasing temperatures as the gasification front was moving forward and the recuperator was becoming shorter, Table I1 and Figure 4. The roof rock swelling and degassing depth in Table I1 for G5 and G8, also, agree with the temperature tendency to decrease, gradually, during the gasification course (the decreasing swelling and degassing mean the decreasing temperature and vice versa). The crosscut at 159 h in G8 showed an unusually high K , due to the pipeline failure
Ind. Eng. Chem. Res., Vol. 26, No. 2, 1987 393 Table 11. Parameters from the Postgasification Cross Sections and Thermocouple Readings GT, gas HV, av T , time, L, K MJ/m3 h m K, K, K G5 9 2.7 3.1 26 7.9 2.3 2.93-4.60 1473-1573 66 20.0 4.2 1673 95 28.8 3.1 1523 123 37.5 3.4 1523 150 45.4 2.0 1423 G8
26 42 79 102
2.2 1.9 2.2 1.7 1.4 3.0
3.0 4.7 8.9 11.5 12.5 17.9
111
159
1473 1423 1473 1373 1323 ?
and the product gas burning. The highest temperature existed in G3/1, reaching a maximum a t 1800 "C, probably (as estimated from gasification under identical conditions in the surface model generator), and were the most uniform ones, which was proven by the analysis of the postgasification cavity geometry. In the case of each generator, an ignition period existed during which temperatures were rising and the ratio of slag to rubble was increasing. Later on, the trend was reversed.
Theoretical Sect ion T o make sure that the above results agree with theoretical predictions and, if so, to get more information on the temperature course in respective generators, calculations were done by using the method of Natarajan et al. (1980). In the case of G5 and G8, the assumption was made that CHI comes mostly from pyrolysis, and methanation reaction 7 was deleted from the set of reactions, the products of which form the combustion product gas. Methane production is decreased by air injection due to the lowering of hydrogen partial pressure, and the major methane contribution, over 90%, is from carbonization (Natarajan et al., 1980). Neglecting the methane contribution from combustion, we made the error smaller than 0.4 v/o (v/o = volume percent) in the CH4 concentration, assuming 4 v/o as the upper limit of CH4 content in the dry experimental product gas; see Figures 1 and 2. In G3/1, we also deleted reaction 7 for simplicity. It is true that the amount of CH4 is higher here despite an increase in T, due to the lack of N , but p would be almost in all cases equal to 0.06 which was compensated for by adding an appropriate amount of CHI to the pyrolysis gas composition described below. The complete set of reactions is presented as eq 4-7. Equations 8-11 are needed to find c + 02 co, (4)
-
+ HzO CO + H20 C + 2H2 C
+ H, COP + Hz
CO
-
-+
-+
(5) (6)
CH4
(7)
m, n, p , and z , where eq lla-c are heat balances for G5, 8G, and G3/1, respectively. Having m, n, p, and z , one z-1-m-p=O Kpw.g.sh.
Kpmeth.
= (1 + n)(m =AdCqi
527m - 41n
+ n - 2p)/(m
(8) - n)(s5 - m - n) (9)
+ m - p + l)/$(m + n - 2
~
)(10) ~
+ 16 + LC = 0
(lla)
+ 316p - 4672 + 4q5 + 44TWI
2.84-4.20
4, cm
PT, K
28.6 19.9 20.2 15.3
1323-1473
883 923 653
+ 313p - 4662 + 8q5 + 44TWI + 38 + LC = 0 ( l l b ) 527m - 41n + 316p - 3922 + 2q5 + 44TWI + 2 + LC = 0 ( l l c ) log Kpw,g,sh, = (2203.24/7') + 5.1588 X m 5 T + 531m - 39n
2.5426
X
l O - ' P - 7.461
X
l O - l 1 P - 2.3 (12)
can figure out the product gas compositions by using eq 12 for Kr.g.sh.. Since in eq 4-7 C meant the fixed carbon, the pro uct gas of pyrolysis has to be added to the combustion product gas to get the complete composition of the outlet gas. In this case, Wen and Lee's (1979) slow pyrolysis gas composition a t 1000 "C was chosen (H, 47, CO + CO, 34, CH, + C,H6 19 v/o) for coal with a similar volatile matter content which agrees with Massey's (1974) results. Assuming from Massey (1974) that C,H, equals about 3 v/o and subtracting it from the above pyrolysis gas composition (in our experimental compositions C,H, = 0) and assuming from Massey (1974) that CO,/CO = 0.54 and from Wei (1979) that about one-sixth of the product gas comes from pyrolysis, the following mixture was added to the gasification product and the resulting compositions were adjusted to 100%: CHI 2.8 (5 for G3/1 to make up for the assumption that p = 0; see the beginning of this section), CO 3.83, COz 2.07, and H2 8.1 v/o. The parameters chosen in eq 8-11, the calculated conversions, as well as the resulting gas compositions compared against the experimental ones, and other parameters are presented in Figures 1,2, and 3 for G5, G8, and G3/1, respectively. The total water influx (TWI) is understood as the sum of the q5 steam taking part in gasification, which can be introduced with oxidizing agent or formed underground from influxing water, causing extra heat losses and water influx (WI). TWI is an important fitting parameter in eq 11for enabling one to get LC comparable with experiment, while q5 in eq 9 and 11 is important for getting a comparable amount of H 2 0 with experiment in the calculated product gas. TWI equals 6,5, and 5 for G5, G8, and G3/1, respectively. The q5 parameter for G3/1 agrees with the experimental steam/oxygen ratio. The gas compositions (C and E) in Figures 1-3 are for dry gas. WC is the water in the final gas (corrected for pyrolysis, hydrogen burning, and C02trapping). The same corrections were made for HSC from which an amount of H proportional to the portion of HSC in H was subtracted to account for hydrogen burning; see the remaining part of this section for the explanation of hydrogen burning and COz trapping processes. Both HSE and HSC are for dry product gas. The resulting theoretical gas compositions, after taking into account only reactions 4-7 and subtracting H 2 0 to get
394 Ind. Eng. Chem. Res., Vol. 26, No. 2, 1987
dry product gas and an addition of pyrolysis products, do not agree with experimental dry gas compositions. The C02 content and, in some cases, e.g., G5 and G8, H content are too high. The H excess can be eliminated by taking into account the H burning, specially, in G5 and G8 along with the increasing heat losses. During experiments, about 25-70% H was burned out in G5, with less in G8 and almost none in G3/1. Analogous losses were taken into account in the calculations; see HBC (hydrogen burned calculated) in Figures 1-3. To cope with the COz excess, cooling the gas by water was taken into account. The experimental dry product gas compositions in Figures 1-3 were obtained after subtracting water from the water-cooled gas where the water content (W) is the water in the product gas before cooling. Due to a big difference between COP and the remaining gas components solubilities in water to C02’s advantage, we believe that by subtracting water from experimental water-cooled gas (rich in, evaporated during cooling, water) we subtracted, also, a large portion of COSwhich dissolved in it during cooling. To support such a claim, let us cite the absorption of C 0 2 in water under increased pressure, from 10-30 atm, as a method to remove COPcompletely from a raw gas, containing up to 40 v/o C02 initially, and as a way to make it feasible for ammonia synthesis. Also, the C 0 2 trapping was, probably, taking place on dolomitic calcines and aluminosilicates which could be present in coal, roof, floor, and piling rubble based on the floor and roof rock analysis (Rauk, 1971). The results of that analysis show for the floor rocks calcination losses of 9.62, Si0260.01, A1,0, 20.78, FezOB5.97, CaO 0.75, MgO 2.5, and SO, 0.37 w/o (w/o = weight percent); and for the roof rocks losses of 18.52, 53.8, 16.38, 7.82, 0.7, 2.39, and 0.39 w/o, respectively. Therefore, a portion of C02, believed to be lost with cooling water and on dolomitic calcines and aluminosilicates, called C02TC (C02 trapped calculated), was subtracted from the calculated dry gas composition to fit the experimental results. The amount of water used for cooling and the amount of COz in the product gas justify the C02TC values. Accounting for hydrogen burning and C 0 2 trapping in the calculations allowed us to obtain a rather good agreement with experimental gas compositions which will be discussed in detail in the next section. Natarajan et al. (1980) did not have to account for COz trapping because the experimental compositions with which they compared calculated results were not after water cooling. Figure 4 presents experimental values of HS, HSE, as well as theoretical ones, and HSC, together with calculated temperature vs. time for all generators. The HSE values were of an “oscillatory” character, except for G8 where averaging over longer time periods was done earlier when readings were taken during the gasification experiment. HSE values oscillate around some curve, the shape of which was found by using the third-order polynomial least-squares curve fitting (C-F) procedure. Consequently, the same procedure was adapted for TC and HSC which resulted in TCC-F and HSCC-F in Figure 4. Also, WE of G3/1 seemed to form a “smooth” curve, and to get easy access to WE for any value of time, WE was curve fitted with the result of WEC-F in Figure 3. This curve fit was rather successful with a high coefficient of determination, a high correlation coefficient, and a low standard error of estimate (0.97, 0.99, and 0.61, respectively). Agreement of curve-fitted curves with original ones was rather poor for HSE and HSC in G5 and G3/1, as shown by a low coefficient of determination, a low correlation coefficient, and a high standard error of estimate (G5, HSE 0.064,0.25,
0.088, HSC 0.79,0.9, 0.99; G3/1, HSE 0.23,0.47, 0.11, HSC 0.36, 0.6, 2.6, respectively) due to a large scattering of points, but increasing the order to sixth order did not help much. Much better agreement between curve fitted and non-curve-fitted HS functions was reached for G8 (HSE 0.89, 0.94, 0.017; HSC 0.94, 0.97, 0.62, respectively), as expected, because averaging was done earlier. The temperature curve fitting was more successful in all cases (G5, 0.9, 0.96, 41.5; G8 0.8, 0.9, 56.9; G3/1 0.8, 0.9, 33). In any case, curve fitting provided an average course of a given value as a function of time. The curve-fitted HSE (HSEC-F) were drawn together with TCC-F in Figure 4 which allows for direct comparisons.
Discussion and Conclusions Figures 1-3 present actually a complete experimental and calculated history of G5, G8, and G3/1, respectively. Each starts with m, n, and p as conversions of the reactions taking place and then the compositions follow, calculated and experimental, in the forms H, CO, COP,CH4,and N (except for G3/ 1) meaning hydrogen, carbon monoxide, carbon dioxide, methane, and nitrogen. Next are HS’s, HBC’s, C02TC’s, heating values (HV’s), water contents (W’s), calculated heat losses (LC’s), and calculated CZT changes (TC’s). Actually, TC represents the temperature of reaction 6 since Kpw,g,sh, was used in the calculations. The extent of this reaction directly in CZ is small; it proceeds faster inside the link zone where temperatures are lower (Harloff et al., 1980). Hence, TC is rather smaller than CZT; nevertheless, it changes in parallel way toward the actual CZT, because the link zone temperature depends on the temperature of the gas coming from CZ. The real parameter characterizing the CZT should come from reaction 5 , which takes place mainly on the carbonized coal wall of CZ and strongly depends on CZT. It could be its conversion m or amount of H, called HS, from this reaction, for example. It should be emphasized that HS is calculated only from hydrogen coming from reaction 5 taking place at CZT. Including the hydrogen from reaction 6 in HS worsens the agreement of HSE and HSC and, thinking in terms of characterizing the temperature course of reaction 5 by HS, introduces a kind of desinformation by adding the effects of another reaction occurring at lower 5”. It is specially visible for G3/1 where conversion of reaction 6 is higher. The above comments on TC are the main justification for an attempt of using HS as the relative changes of the CZT indicator from a theoretical point of view. Later on, it will be shown that it also has a support in experimental data; mainly, HSE will follow the course of TC. In Figure 1,conversion m increases with increasing T and vice versa, while n changes on the opposite as expected. The calculated compositions follow experimental ones rather closely. In the case of H only, occasional differences reach more than 3 v/o; the same type of agreement was reached for CO, even better, because differences were less than 2.5 v/o for CH4 and slightly worse for N and C02. We could “trap” more COz and improve the C02C and C02E relations as well as NE and NC (would increase) but at the cost of worsening the agreement of H and CO. Similar comments apply to the compositions in Figures 2 and 3 except that the highest compositional discrepancies were obtained for CO, very good agreement was obtained for the remaining constituents in G8, and the worst disagreement obtained in G3/1 was CH,, the situation being identical when p # 0. Experimental readings in G8 were averaged over longer periods of time, hence more clear pictures in Figure 2.
Ind. Eng. Chem. Res., Vol. 26, No. 2, 1987 395 Commonly, we notice that with a decrease of temperature we saw a drop in H (almost constant in G3/1 where the temperature was most uniform) as well as in HS, CO, HV, and CO2TC and an increase in COz,N, HBC, and LC, which is rather obvious because most of the H and CO comes from reaction 5. At lower T this reaction is less productive, so the role of COz and N increases. Increased heat losses are partially compensated for by burning more hydrogen. HSC follows the general course of HSE but is always (except short periods in G5, Figure 1, and G3/1, Figure 3) higher. The reason for this lies, probably, in the fact that some of H E is lost with tar which is not accounted for in eq 1 nor is it taken into account in theoretical calculations. The situation repeats itself in all three generators; see HS’s in Figures 1-3. Looking at calculated hydrogen losses due to burning (HBC, percentages of burned H in relation to the HC in a dry product gas, in Figures 1 and 2), one sees the tendency of the increasing burning with the decreasing temperature of the gasification front, and it is true for all generators. Burning stays on the same level where the temperature is most stable; see Figure 3 for G3/1. This level is lower than in G5 and G8, except for the ignition period. It agrees well with experimental facts. C02TC (percentage of C 0 2 trapped in relation to the COz in dry product gas) was assumed to be more extensive at higher temperatures due to the higher amount of evaporated water present as steam in gas, better contact between gas and water phases allowing for better solubility of COz in HzO during cooling. HVC follows the experimental one rather well for all three generators. WE and WC courses agree in general but differences during short periods at the beginning of gasification in G5 and at the end in G3/1 reach almost the 6 v/o level; see WC and WE in Figures 1 and 3. We could improve it by making the TCC-F and HSEC-F agreements in Figure 4 slightly worse. The higher TWI in G5 reflects the higher water influx during gasification. LC (heat losses calculated as a percentage of the coal’s lower heating value) shows the increasing tendency with decreasing temperature which agrees with experiment. LC reached levels of 25%, 27% and 9% for G5, G8, and G3/1, respectively, in the worst cases (not counting 23% at the beginning of G3/ 1 where gasification was just starting). The last and most important parameter in the figures is the calculated temperature, TC. In G5 and G8, a continuous decrease of TC was observed, while in G3/1 the temperature differences during gasification were smaller. The reason for a gradually lowering of T i n G5 and G8 was the increase of heat loss to overburden caused by changing the geometry of the cavity. It was, particularly, visible in G5 where the geometry of the combustion channel was changing by increasing its width in the roof area. The slope of the side walls was changing from the vertical slope to a more and more inclined one. Ashes, chips of rubble, and slag collected on those slopes to isolate coal from the oxidizing agent, decrease the active coal surface in the gasification process, and cause heat losses by absorbing heat from the combustion zone. In G8, also, combustion moved to the roof areas; besides, the recuperative system was becoming shorter as the gasification front was moving forward, causing increased heat losses. Such changes of the cavity geometry were not present in G3/1, resulting in lower losses and high and uniform temperature and gas heating values. The type of oxidizing agent was, also, a reason for the highest thermal state in
G3/1. Oxygen and steam were applied in that generator, while in G5 and G8 air was used, carrying withint itself the nitrogen ballast which contributed to heat losses. On the surface, it appears contradictory to discuss the higher thermal state in G3/ 1 while TC’s for this generator are lower than for G5 and 8 (Figures 1-41, but in view of the previous comments and interpretation of TC, as representing the temperature in the link zone, it is not. Simply in G3/1, we have gotten a lot more C 0 2 and Hz from reaction 6 in the link zone (higher n and no N); hence, Kpw.g.sh. and TC’s take values closer to the link zone temperature (than to CZT) than they did in G5 and 8, where the effect of reaction 6 is smaller. Another proof of substantially higher T‘s in G3/1 are much higher values of HSE and HSC for this generator (Figures 1-4); coming only from reaction 5, HS’s were results of a higher T. Besides that T‘s are higher in G3/1, we believe also that the TC course is parallel to CZT, because it depends on it, but lower by about 400 deg for G5 and G8 and at least 600-800 deg for G3/1. The relative changes of TC’s within the temperature curse for each generator were reflected in the changes in experimental conditions (conversions, relative amounts of constituents, HS’s, HBC’s, C02TC’s, HV’s, and LC’s), so we think that we have gotten a good description of the gasification temperature from the relative point of view but not the absolute one. This is important when comparing the temperature course with HSEC-F and HSCC-F for each generator which is done in Figure 4. We see that the idea of using the HS to approximate the relative changes of temperature during gasification, formulated in the introduction, is not without reason. Both quantities follow each other very closely for G5 and G8 with the exception of the initial periods when gasification is not fully developed yet and the final ones when extinguishing is taking place. The worst fit was reached in G3/1 in the first half of the experiment; Le., HSEC-F follows the TCC-F course but not so closely as in G5 and G8. In our opinion, this is not an argument against the idea of HS as a T indicator but is the result of a poor description of the gasification process during its first half in G3/1 by the model. It is remarkable that in this case, HSCC-F follows TCC-F rather closely, and the same may be said about G5 and G8. It seems that from a theoretical point of view we have proven the idea. So, HSCC-F and TCC-F follow usually each other and HSEC-F if the model fits experimental conditions, but in G3/1, where the opposite takes place, HSEC-F does not follow HSCC-F well. Let us take the point at t = 2.5 h, for example, where HSE is about 1and HSC 11v/o, Figure 3. Attempts to correct this by lowering m failed because the agreement of gas compositions would worsen substantially. This disagreement is reflected in the differences of HSCC-F and HSEC-F. Hence, if we fit well HSCC-F to HSEC-F (which means we have chosen a good model), we usually prove the idea of parallel TC and HSEC-F, because TCC-F and HSCC-F are usually parallel. To confirm these facts from an experimental point of view, we would like to introduce some experimental temperature readings, although the above TC courses were also based on experimental compositions with which calculated compositions had to be matched. We do not have straight, point by point, thermocouple temperature readings except three thermocouple readings for G8, circled points .in Figure 4, and indirect temperature estimations by the method of “geothermometry” (Rich and Youngberg, 1983; Perry and Gillott, 1982), crossed points in Figure 4 for G5 and 8 and also presented in Table 11. Geothermometry
396 Ind. Eng. Chem. Res., Vol. 26, No. 2, 1987
may be up to 50-deg accurate (Perry and Gillott, 1982). Indeed, the results of Table I1 confirm the theoretical course of temperature. The temperature falls with the course of gasification in G5 and G8 after the initial increase; see K,, d,, and thermocouple readings in Table I1 and Figure 4 for crossed and circled points. The drop in T is about 250 and 270 deg for G5 and G8, respectively, where for G8 the thermocouple values (PT) were used. In TCC-F, Figure 4, it amounts to about 338 and 223 deg for G5 and G8, respectively. The TCC-F drop in G3/1 is smallest and amounts to 167 deg. The most uniform T course in G3/1 is supported mainly by the postgasification cavity inspection. To prove fully the idea of using HSE as a temperature probe for in situ gasification, one must measure simultaneously the temperature and HSE and compare them. So far, the comparison of HSE and TC deduced from experimental compositions as well as sporadic temperatures from thermocouple readings and indirect geothermal estimations seems to support the idea. During T deduction as a result of the calculations, assumptions were made about hydrogen burning and carbon dioxide trapping, mainly, by gas cooling water and partially by surrounding dolomitic calcines and aluminosilicates whose sorption and selectivity can rival zeolites after appropriate thermal treatment (Barrer, 1978). This would be specially the case in G5 where the postgasification cross sections showed the highest exposure of roof and side walls slopes, covered with coal ash minerals, rubble, and slag, to gas and C02TC turned out to be the highest one. In conclusion, we state that the idea of using hydrogen from steam decomposition, directly in the combustion zone and not in the linking zone of the in situ coal gasification generators, as a CZT changes indicator seems to be correct for the three generators presented in this work. The whole work is an example of a good description of the processes in three underground generators by using the relatively simple model of Natarajan et al. (1980). A good match of compositions, hydrogen from steam decomposition, heating values, heat losses, hydrogen burned, and, most of all, hydrogen from steam decomposition and temperature, calculated and sporadically experimental, was obtained, actually, thanks to the four main assumptions: 1. HS comes from CZ and not from the linking zone. 2. Cooling water traps much more COz than other gas components, but dolomitic calcines and aluminosilicates can also have their share in trapping. The amount of trapped COz became consistent with the COZ-H,O solubility data for a given T. 3. A decrease in CZT causes an increase in heat losses and burning of H (including HS), the amount of which has to be consistent with experimental data. 4. Methane in the product gas comes mostly from pyrolysis. The good agreement of TC and HSE in G5 and G8, and not bad agreement in G3/1, makes the idea very attractive, and we strongly encourage further experimental testing, specially, for the comparison between TE and HSE; the agreement between HSC and TC seems to be satisfactory. The above conclusions are based on the results of three experimental tests; therefore, more work has to be done to confirm the T E and HSE dependences for a higher number of cases and to calibrate or find a functional dependence between the combustion temperature changes and, associated with them, changes of HS for a given type of coal, geological, and process conditions. If the idea will stand the test, it may save a lot of effort and money connected with installing thermocouples, taking care to protect them against breaking or burn-out, and may help to control and better understand the underground coal gasification process.
Also, the oxygenisteam gasification guarantees the production of gas of a higher and more uniform heating value as compared with air gasification; a great amount of hydrogen from steam decomposition may play an important role in the case of switching the UCG process to 'the production of synthetic gas. However, any future applications should be preceded by an economic evaluation.
Acknowledgment The idea presented and discussed above was first formulated by the late Prof. Jerzy Rauk, the leading figure in the Polish underground coal gasification. The experimental results came from his Ph.D. thesis and a thesis presented to qualify him for an assistant professorship and were based on tests financed and carried out by Central Mining Institute in the past. M.S.M. supported the idea with theoretical estimation and interpretation, which was possible thanks to a computer in the Institute of Physical Chemistry, Polish Academy of Sciences a t Warsaw. Obtaining a curve-fitting program from Dr. Tadeusz Zakroczymski and Krzysztof Wyrzykowski is highly appreciated.
Nomenclature A l = fitting coefficient in eq 10 Ala, Aza = ash content in coal before and after gasification,
respectively, w/o Ca = carbon content in a gasified coal seam, w/o
C-F = curve fitted (by the third-order least-squares method) C02TC = calculated relative amount of trapped carbon dioxide, % d = coal sample length, cm d, = reduced depth of degassing in eq 3, cm GT = experimental temperature from geothermometry in Table I1 and Figure 4 (crossed points), K H, CH,, COz,CO, and N = hydrogen, methane, carbon dioxide, carbon monoxide, and nitrogen, experimental or calculated (E or C) product gas components, v/o Ha = hydrogen content in coal seam, w/o HBC = calculated relative amount of burned hydrogen, % HSC = calculated HS from reaction 5 and corrected for pyrolysis, hydrogen burning, and COPtrapping, v/o HSE = experimental hydrogen from steam decomposition in Figures 1-4 and in eq 1,m3/kg of coal, or experimental dry product gas, v/o HV (E or C) = heating value of the product gas, MJ/m3 Kpmeth= equilibrium constant for methanation in eq 10 Kpy,g,sh = equilibrium constant for the water gas shift reaction in eq 9 K , = degree of rock looseness K , = degree of roof rock swelling in eq 2 L = cross-sectional distance from the ignition well in Table 11, m LC = calculated heat losses, 70 of LHV of coal m = total roof thickness transformed into rubble and slag, cm mrl = thickness of the rubble layer in the postgasification cavity, cm mSl = thickness of slag layer in the postgasification cavity, cm m, n,p = conversions of reactions 5, 6, and 7 , respectively, in eq 8-11 M , = carbon molecular weight, kg/kmol MH2= molecular weight of hydrogen, kg/kmol M , = molecular volume of ideal gas at normal conditions, m3/kmol @ = pressure in the system, atm
Ind. Eng. Chem. Res. 1987,26, 397-398 PT = recuperator pipeline air temperature in G8 measured by thermocouples in Table I1 and Figure 4 (circled points),
K N / 0 2 = 4 for air and 0 for oxygen/steam in eq 10, mol/mol of O2 q5 = H20/Oz = steam/oxygen ratio in eq 9-11, mol/mol of
q1 =
0 2
c4i = 41 + q 5 S = surface area of sample base (base of cuboid or cylinder),
cm2 TC = calculated temperature, an intermediate between CZT and link zone temperature, K V = volume of coal sample, cm3 VIa, Vza = volatile matter in coal sample before and after gasification, respectively, w/o W (E or C) = water content in product gas, v/o Wla, WZa= water content in coal before and after gasification, respectively, w/o z = amount of fixed carbon reacting with 1 mol of 02,in eq 8 and 11, mol/mol of O2 Registry No. H2, 1333-74-0.
397
Harloff, G. J.; Eddy, T. L.; Schwartz, S. H. Presented at the Proceedings of the 6th Underground Coal Conversion Symposium, Shangri-La, OK, 1980;p 111-40. Massey, L. G. Adu. Chem. Ser. 131, 1974,15. Natarajan, R.; Edgar, T. F.; Savins, J. G. Presented a t the Proceedings of the 6th Underground Coal Conversion Symposium, Shangri-La, OK 1980;p 111-15. Perry, C.; Gillott, J. E. Bull. Can. Pet. Geol. 1982,30(1),34. Rauk, J. Ph.D. Dissertation, Central Mining Institute, Katowice, Poland, 1971 (in Polish). Rauk, J. Thesis for Assistant Professorship, Central Mining Institute, Katowice, Poland, 1978 (in Polish). Rich, F. J.; Youngberg, A. D. Prepr. Pap-Am. Chem. SOC.,Diu. Fuel Chem. 1983,28(1),141. Wei, J. Ind. Eng. Chem. Process Des. Dev. 1979,18, 554. Wen, C. Y.; Lee, E. Stanley Coal Conversion Technology; AddisonWesley: Reading, MA, 1979;p 72. *Author to whom correspondence should be addressed a t Marchlewskiego 8/2,40-129Katowice, Poland.
Jerzy Mastalerz, Maciej S. Matyjaszczyk,* Jerzy Rauk Central Mining Institute 40-951 Katowice, Poland
Literature Cited Barrer, R. M. Zeolites and Clay Minerals as Sorbents and Molecular Sieves; Academic: London, 1978.
Received for review April 3, 1984 Revised manuscript received July 31, 1986 Accepted September 18, 1986
CORRESPONDENCE Comments on “Predicting the Maximum Spoutable Height in Spouted Beds of Irregularly Shaped Particles” Sir: Morgan and Littman (1982) have presented a correlation for predicting the maximum spoutable height in spouted beds of irregularly shaped particles. The correlation for spherical particles proposed by Littman et al. (1978) is modified to account for the effect of particle shape by introducing a factor g(@s),a function of particle shape factor, qbS. Hence, modified correlation
should fit both spherical and nonspherical particle data. For the nonspherical particle data given in Table I1 (Morgan and Littman, 1982) with a value of A,, > 0.014 (for 24 data points), in eq 1 the standard deviation of m is 0.226 and an average deviation 37.4%. To improve the accuracy of the fit and to retain similarity with the correlation for spherical particles, Morgan and Littman, using data for both spherical and nonspherical particles, have made an asymptotically similar fit to eq 1 to get the quadratic m = 0.218 + 5.13
10-3A,;1 + 2.54 A,, > 0.014 X
X
10-5A,,-2 (2)
Equation 2 fits the nonspherical partical data mentioned above with a standard deviation of m of 0.185 and an average deviation of 27.1 % . Introduction of the third parameter has not resulted in any substantial improvement in accuracy of the fit, while simplicity is lost. Also, eq 1 and 2 differ with regard to the effect of variables on H,. 0888-5885 / 87/ 2626-0397$01.50/ 0
For example, the effect of di on H, in eq 1 and 2 is different. In view of the above observations, the significance of parameters in eq 1 and the need to retain them in the development of an equation for nonspherical particles are checked. In the process, it is observed that nonspherical particle data of Malek and Lu (1965) for air-polyethylene and air-brucite systems were used along with the data for spherical particles by Littman et al. (1978), without any correction factor for shape as proposed by Morgan and Littman (1982), in order to get the correlation m = 0.218 + 0.005/A (3) for spherical particles. Littman et al. (1979) have given additional data on maximum spoutable height in spouted beds of spherical particles and a mA vs. A plot. The data for A > 0.02 in that plot are said to fit eq 3 within about 12% average deviation. The 62 data points identified from the plot include those for air-polythene and air-brucite systems also and fit eq 3 with a multiple correlation coefficient squared (R2)of 0.636, a standard deviation of m of 0.053, and an average deviation of 12.25%. Based on a leastsquares estimation, the same data give m = 0.180 + 0.00645/A (4) with R2 = 0.68, a standard deviation of m of 0.05, and an average deviation of 11.6%. The above analysis indicates that no significance can be attached to the parameters in eq 3, as the data used for 0 1987 American Chemical Society
