Fuel Gasification carbon dioxide. However, after a weight loss corresponding to an attack of an average depth of 0.143 A. the rate of reaction of the normal sample was the same as the sample reacted with oxygen. The amount of surface oxide formed in this phase of the reaction could be only a very small fraction of a monolayer. The presence of impurities in the form of elemental iron or one of its carbides greatly changes the nature of the reaction. If one assumes, as is likely, t h a t a carbide is formed, these carbon atoms must react readily with the carbon dioxide molecules. This result may be expected from the nature of the binding forces involved in the carbide structures.
Literature Cited (1) Baker, C. J., J . Chem. Soc. (London), 51, 249 (1887). (2)
Beebe, R. A., Beckwith, J. B., and Honig, J. M., J. Am. Chem. SOC.,6 7 , 1554-8 (1945).
(3) (4) (5)
Boersch, H., and Meyer, L., 2. phys. Chem., B29, 59 (1935). Bonner, F., and Turkevich, J., J . Am. Chem. Soc., 7 3 , 5 6 1 (1951). Broom, W. E. J., and Travers, M. W., Proc. Roy. Soc. (London), 135A, 512 (1932).
( 6 ) Brunauer, S., Emmett, P. H., and Teller, E., J . Am. Chem. Soc., 60, 309-19 (1938). (7) Dewar, J., Proc. Roy. Soc. (London), 74, 127 (1904). (8) Duval, X., J. chim. phys., 4 7 , 339-45 (1950). (9) Emmett, P. H., and Brunauer, J . Am. Chem. SOC.,5 9 , 155364 (1937);
s.,
(10) Eyring, H., Colburn, C. B., and Zwolinski, B. J., Discussions Faraday Soc., No. 8 , 39-46 (1950). (11) Gadsby, J., Long, F. J., Sleightholm, P., and Sykes, K. W., Proc. .Roy. Soc. (London),A193, 357 (1948). (12) Giauque, W. F., and Clayton, J. O., J . Am. Chem. SOC..5 5 , 4 8 8 0 (1933).
Glasstone, S., Laidler, K. J., and Eyring, H., “Theory of Rate Processes,” New York, McGraw-Hill Book Co., 1941. (14) Gulbransen, E. A., Rev. Sci. Instruments, 15, 201 (1944). (15) GuIbransen, E. A,, Trans. Electrochem. SOC.,81, 187 (1942). ENG.CHEM.,4 1 , 2762 (16) Gulbransen, E. A., and Andrew, K., IND. (13)
(1 949). (17)
Gulbransen, E. A., and Andrew, K., J . Metals (Trans. Am. Inst. Mining Met. Engrs.), 185, 515 (1949).
(18)
Hirschfelder, J. O., and Wigner, E., J . Chem. Phys., 7 , 616
(19) (20)
Hulburt, H. M., and Hirschfelder, J. O., Ibid., 11, 276 (1943). Jones, R. E., and Townend, D. T. A., J . chim. phys., 4 7 , 348-52
(21)
Keyes, F. G., and Marshall, M. J., J . Am. Chem. Soc., 4 9 , 1 5 6 - 7 3
(22)
Laidler, K. J., Glasstone, S,,and Eyring, H., J . Chem. Phys.. 8 ,
(1939). (1950).
(1927). 659-76 (1940).
Lambert, J. D., Trans. Faraday SOC.,3 2 , 452 (1936). Langmuir, I., J . Am. Chem. Soc., 3 7 , 1154 (1915). Letort, M., and Martin, J., Compt. rend., 222, 1049-51 (1946). Letort, M., and Martin, J., Nature, 157, 874-5 (1946). Letort, M., Petry, J., Barret, P., Collart, F., Labaeye, P., and Martin, J . , J . chim. phys., 4 7 , 548-56 (1950). (28) Lewis, B., and von Elbe, G., “Combustion, Flames and Explosions of Gases,” London, Cambridge University Press, 1938. (29) Lowry, H. H., and Hulett, G. A., J . Am. Chem. SOC.,4 2 , 1413
(23) (24) (25) (26) (27)
(1920).
(38)
McLean, S., Trans. Roy. SOC.Can., 15, 73-84 (1921). Martin, M., and Meyer, L., 2. Elektrochem., 4 1 , 136 (1935). Mellor, J. W., “Comprehensive Treatise on Inorganic and Theoretical Chemistry,” London, Longmans, Green and Co., 1924. Ibid., Vol. 6 , 1925. Meyer, L., Trans. Faraday Soe., 3 4 , 1056 (1938). Meyer, L., 2. phys. Chem., B17, 385 (1932). National Bureau of Standards, Washington, D. C., “Tables of SelectedValues of Chemical Thermodynamic Properties,” 1949. Pauling, L., “Nature of the Chemical Bond,” Ithaca, K. Y . , Cornel1 Universitv Press. 1939. Porter, H. C , and Ralston,’O. C., U. S. Bur. Mines, Tech. Paper,
(39)
Rhead, T . F. E., and Wheeler, R. V., J . Chem. Soc., 101, 846
(40) (41)
Riley, H. L., J . chim. phys., 47, 565-73 (1950). Semechkova, A. F., and Frank-Kamenetzky, D. A,. Acta Physicochem. (U.R.S.S.), 12, 829 (1940). Shah, M. S., J . Chem. Soc. (London), 1929, 2670, 2685. Sihvonen, V., Trans. Faraday Soc., 3 4 , 1062 (1938). Smith, A,, Proc. Roy. SOC.(London), 12, 424 (1863). Strickland-Constable, R. F., Trans. Faraday Soc., 4 0 , 3 3 3 (1944).
(30) (31)
(32) (33) (34) (35) (36) (37)
409 (1928). (1912); 103, 461, 1210 (1913).
(42) (43) (44) (45) (46) Ibid., 4 3 , 769 (1947).
RECEIVED for review January 8, 1952. Scientific paper 1629.
ACCEPTED March 15, 1952.
Kinetics of Carbon Gasification by Steam Effect of High Temperature Pretreatment on Reactivity of Low Temperature Char to Steam and Carbon Dioxide G. E. Goring, G. P. Cursan, R. P. Tarbox, and Everett Gorin PITTSBURGH CONSOLIDATION COAL CO., LIBRARY, PA.
Using the integral gasification rate by steam or carbon dioxide at 1600’ F. and 1 atmosphere as an index of reactivity, the effect on a low temperature char of various pretreatment times (0 to 24 hours) in nitrogen at 1 6 0 0 ’ F. was determhed. This efFect was appreciable, differences in gasification rate of over twofold being observed, although the differences diminished asymptotically with increasing pretreatment time. The criteria furnished by steam and carbon dioxide were similar. All experimental work was carried out in a fluidized bed, operated batchwise with respect to the charge of 65- to 150-mesh char and continuously with respect to the fluidizing gas. May 1952
T
4 effect of temperature history on the reactivity of carbons derived from coal has been studied by many previous workers. Using various indexes of reactivity-i.e., adsorption of vapors, electrical resistivity, crystallite size, reaction with various oxidizing gases under controlled conditions, etc.-they have agreed that this reactivity is a path function of temperature history. It is clear then that investigation of this variable should be included in any study of the high temperature kinetics of a system involving carbon. Failure to do this could result in masking the effects of other, more important, kinetic variables. The plethora of literature devoted t o the more general subject of carbon reactivity t o oxidizing gases shows that many of these investigators recognized t h e significance of previous tzmperature history to that reactivity. I n only a few instances, however, was
INDUSTRIAL AND ENGINEERING CHEMISTRY
1051
Fuel Gasification a n effort made to evaluate the relationship quantitatively, and then the number of experiments was usually minimized, just enough work being done on this corollary subject to support the validity of the main body of kinetic data ( 8 ) . The most comprehensive study found in the literature was the work of King and Jones ( 4 ) ,who used small amounts of adventitious iron t o elucidate the mechanism of the effect of residence time on the reactivity of coke to carbon dioxide. Whereas most previous workers in this field studied the effect of temperature level on carbon reactivity, the investigation reported here featured the study of 1
'+IO \
'I
Figure 1. Reactor Feed gas inlet (from prehearer) 2. Oval ring joints 3. Alloy steel reactor 1.5 inches in inside diameter, 3.5 inches in outside diameter 4 . Grid retainer 5. Micrometallic grid 6. Axial thermowell, 1 / r inch standard pipe 7. Alundum furnace bed 8 . Nichrome ribbon resistance winding (maximum output approximately 10 kw.) 9. Stuffing box arrangement for thermowell 10. 3 thermocouple pairs (20-gage Chromel-Alumel set in 6-hole porcelain insulators) 11. Exhaust gases, to condensing, throttling, and sampling system 1.
I-
residence time a t a given temperature. I t preceded the undertaking of an extensive experimental program on the kinetics of the char-steam system. The effect of pretreatment time in a neutral atmosphere a t 1600" F. was determined, using the gasification rate of the treated char, by either steam or carbon dioxide a t 1600" F. and atmospheric pressure, as the index of reactivity. ,411 experimental work was carried out in a fluidized bed, operated batchwise with respect to the solid charge and continuously with respect t o the fluidizing gas.
Experimental Technique The source of carbonaceous material used in this investigation was a large batch of commercial Disco, a low temperature char made from 3/8 x 0 inch washed Pittsburgh Seam coal. The Disco was ground t o -35 Tyler mesh and fluidized in nitrogen for 30 minutes a t 1100" F. to remove the residual tar. Several hundred pounds of this material were then mixed and dried by fluidizing with air a t about 200" F. in a large, steam-heated vessel. The 65to 150-mesh fraction of this dried material furnished the feed for all experiments.
Apparatus. REACTOR. Details of the reactor assembly are shown in a cross-sectional drawing (Figure I). The reactor tube was machined from a forged hillet of alloy steel (called K-155 or Unilog commercially) GO inches in length. The thermowell stuffing box, 9, also served as the retainer for the tubes (not shown) used for introduction of the batch solid charge and removd of the solid residue; these devices were temporarily
1052
substituted for the thermowell a t the beginning and end of each run. Distribution of the fluidizing gas was effected by a sintered disk (Type 309 steel, porosity of 35 microns and 0.25 inch thick), which lyas swaged into the head of a bottom axial insert to the reactor as shown. The reactor furnace consisted of three coils of Sichrome V ribbon ( 3 / 1 6 X 0.030 inch), the electrical energy to each of which was separately controlled, embedded in Alundum cement, and surrounded by layers of granular Sil-0-Cel and finally 85 % magnesia.
FLOWSYSTEM. Figure 2 shows the flow relationship of the reactor and necessary auxiliary equipment, and also indicates the means of controlling the four principal operating variables: fixed gas (nitrogen or carbon dioxide) flow rate, reactor pressure, stcam flow rate, and bed temperature. Precise metering of carbon dioxide or nitrogen was accoinplished through standard rotameters, carefully calibrated under operating conditions. The fixed gas purification train, which removed moisture and oxygen by bringing the high pressure cylindcr gas in contact with reduced nickel catalyst and silica gel, is not shoxn in Figure 2. The purified gases contained less than 0.005% oxygen and less than 0.001% moisture. Maintenance of a conshnt reactor pressure for the atmospheric pressure runs discussed in this paper presented no difficulty, sporadic adjustment of an exhaust line valve being sufficient. For the superatmospheric data discussed in the following paper (a), however, automatic control of reactor pressure was required. This was effected by a diaphragm-operated Hammel-Dah1 MicroFlow valve, 21, and a Brown Air-0-Line recorder controller, 25. The steam-metering device was a second diaphragm-operated Hammel-Dah1 Micro-Flow valve, 10, which, in this case, served as a variable orifice. The orifice aperture size was selected by manual.adjustment of a lead screw which acted against the valve stem spring. The pressure downstream of the orifice-Le., reactor pressure-was maintained as described above. Upstream pressure-Le., boiler pressure-was controlled within similar narrow limits by a series of instruments which automatically throttled the electrical energy input. to the boiler. Therefore, for a given orifice setting of t'he metering valve, a constant steam flow resulted. The steam system was capable of maintaining flows ovcr the range of 0.1 to 30 pounds per hour within 1 to 2%, provided that careful prerun calibration procedures were observed. The steam calibration system, 11, not shown in detail in Figure 2 , was operated only when the steam-generating system and metering valve were isolated from the rest of t8heequipment by closing the steam shutoff valve, 12. The calibration train, leading to a watcrcooled condeneer and finally to a weighing receiver, had requisitc valves and gages to maintain a downstream orifice pressure equal to reactor pressure. Maintenance of a constant fluid bed temperature was semiautomatic; control of the electrical energy input t o bhe bottom reactor winding was effected by an automatic throttling system (not shown in Figure 2), similar to t'hat described for the boiler, actuated by a bed thermocouple. Inputs to the ot,her two windings of the reactor furnace and to the preheater, 15, winding were manually controlled. The complementary operation of the reactor and preheater furnaces resulted in a maximum vertical fluid bed temperature gradicnt of I t 4 O F. a t lGOOo F. The remaiuder of Figure 2 should be self-explanatory. For the sake of simplicity many minor control feat'ures-i.e., t'hermocouple stations-have been omitted. Procedure. In order to evaluate the effect of high temperature pretreatment time on the reactivity of the carbon toward carbon dioxide or steam, it was necemary to maintain constant the following independent variables, all of which affect the cxpcrimentally observed gasificat'ion rate: 1600O F. I atmosphere
10.39 foot/second (COPruns)
10.44 foot/second (HnO runs)
Weight of solid feed charged t o reactor R a t e of heating f;om charging temperature (1000 F.) t o reaction temperature (1600' F.)
1.00 pound, equivalent t o a b o u t 2 5 inches of fluid bed height 6 O E'. per minute
For the carbon dioxide runs the inlet gas was pure carbon dioxide; for the steam runs the inlet gas was 90% steam-lO% nitrogen. After the batch of char feed had been charged to the reactor at the standby temperature of 1000" F., the bed was fluidized with
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 5
Fuel Gasification purified nitrogen $nd its temperature raised t o 1600' F. under the controlled conditions outlined above. Nitrogen fluidization was then continued at 1600' F. for some selected value of the pretreatment time variable (within the range 0 t o 24 hours), at the end of which time the oxidizing gas was admitted and a series of successive kinetic data points taken. A data point consisted, effectively, in the measurement of the dry exhaust gas rate (by consecutive wet meter readings) and the determination of the composition of this gas (by analytical methods described below). Both the rate and composition of the dry make gas varied
and low, but a p reciable, methane content; the mixture was found t o be total& unsuited for analysis by the Orsat method t o a precision comparable with the other experimental measurements. A special gravimetric method, combined with a separate infrared spectrophotometric analysis for methane and a separate Tutwiler analysis for hydrogen sulfide, was applied with success. Thip required taking triplicate gas samples for each data point. The gravimetric analysis gave the percentage sums COZ HzS, CO CH!, and HZ 2CH4. The results of the separate infrared and Tutwiler analyses permitted resolution of the gravimetric sums into their components.
+
,+
+
24
,
Figure 2. 1. 2. 3. 4. 5. 6.
7. 8. 9.
10. 11. 12. 13.
Distilled feed water inlet Feed water su ply tank Boiler shell, 38gallon capacity Immersion heater, 30-kw. capacity Powerstat Mechanical linkage Diaphragm motor Steam pressure control instrument Steam superheater Valve used as variable orifice Outlet to steam flow calibrating system Steam shutoff valve Cylinders of various fixed gases
Flow Diagram 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26.
RQtametersfor fixed gases Mixed gas preheater Reactor Entrained solids filter Condenser Condensate separator Condensate accumulator Back-pressure throttling valve Outlet to gas-sampling system Wet meter Exhaust Control instrument for back-pressure valve Recording instrument for AF' across reactor
smoothly with increasing oxidizing time, the changes in these dependent variables reflecting both the depletion of the total carbon in the batch bed and the change in the nature of the carbon surface. Separation of these two effects from the effect of pretreatment time was achieved, semiquantitatively, by plotting the per cent of the original carbon gasified (usually termed carbon burnoff) against the observed specific integral gasification rate. This burnoff variable, a n intrinsic property of the carbon, is of considerable importance in both char-carbon dioxide and charsteam kinetics. However, for the particular phase of the investigation reported here, it was experimentally varied only to such an extent t h a t the effect of pretreatment time would not be masked. Both the carbon burnoff and the specific integral gasification rate were calculable, a t any point in the run, from the two continuous measurements made on the dry exit gas (using the method described under Results).
Analyses. Gas samples were taken into evacuated bottles through magnesium perchlorate desiccant, which removed water vapor and traces of ammonia. During analysis they came into contact only with mercury, which was the confining fluid in both the Orsat method used for the carbon dioxide series and the specially developed gravimetric method used for the steam series. Gases from the dioxide series were analyzed only for carbon dioxide in a n Orsat unit, precision for carbon monoxide and hydrogen being of low order. The other constituents in the exit as were calculated by a method shown in the Results section, a h w i n g for hydrogen evolution from the char. Hydrogen sulfide content was negligible (less than 0.05%) in all samples from this series. STEAM SERIES.Gases from the steam series presented problems stemming from their high hydrogen and carbon monoxide content May 1952
Table I.
The solid feed material and all residues were analyzed by standard ultimate analysis. Analyses of the original coal and the prepared feed char are shown in Table I. Typical residue analyses are included in Tables I1 and 111. In order t o ascertain whether the length of the pretreatment period a t 1600" F. significantly changed the solid composition, a series of feed charges was fluidized in nitrogen in the reactor, heated at the standard rate t o 1600' F., and maintained at that temperature for i n t e r v a l s ranging from 0 t o 9 hours. Analysis of the residues a f t e r 1 - h o u r and 3-hour p r e t r e a t ment, included in Table I, indicate a n inappreciable change in composition.
Solid Feed Compositions
Ultimate AnaIysis, Dry Basis, Weight % ' prepared 1600' F. Devolatilized Char Coal t o After 1 hour After 3 hours Disco planta charb H 5.0 2.47 1.00 0.65 C 74.8 79.13 81.05 82.15 N 1.5 1.69 1.30 1.17 0 7.2 2.00 0.13 0.08 S 2.3 1.95 1.92 1.80 Ash 9.3 12.76 14.10 14.15 b size Distribution a Volatile 37.1 Fixed carbon 53.6 Tyler mesh % by wt. 48-65 2.8 65-100 75.3 100-150 20.4 150-200 0.4 200-270 1.1 -270 0.0
Results Tables I1 and 111, representing typical runs for the carbon dioxide and steam series, respectively, are divided into two main sections: smoothed raw data and calculated quantities. The methods for obtaining some of these calculated quantities require extensive explanation, which is given below, particularly for the carbon dioxide series. T o augment the explanations, the tables are arranged in such a manner that the calculations are in Stepwise fashion. T h e "smoothed raw data" points were read, at chosen increments of elapsed oxidizing time, from smooth
INDUSTRIAL AND ENGINEERING CHEMISTRY
1053
Fuel Gasification Table 11. Smoothed Data a n d Calculations for Typical Carbon Dioxide Run ( R u n 6. Inlet gas = 100% COz. Smoothed ~ R Wdntn e' = elapsed-ixidising time, minutes Mole fraction COa in dry exit g a s R = d r y exit gas rate, standard C I I . feetjhour Inlet COz rate, standard cu. feetjhour Toral pressure, atmospheres
Pretreatment time 0 hour)
A.
B. Calculated quantities Exit Con. standard cu. feet/honr Hi.= Hz HzO, standard cu. feet,' hour (using Eq. 3) Hz, standard cu. feet/hour (from Eq. 4) HzO, standard cu. feet/hour (from E y . 4) CO, standard cu. fcet/hour (from Eci. 4) Wet exit gas composition, mole fraction
+
con
CO
H2
HzO
R'
=
A
= integral gasification rate,
ZII'
=
It'
=
>V/W
=
lOOB =
wet exit gas rate, mole/min. x 104
0 0.477
40 0.577
80 0,625
5 89
5.47
5.29
4.34 0.974
120
0.648
183 0,667
5.19
5.09
2.81
3.15
3.30
3.37
3.39
0 41
0.30
0.22
0.19
0.11
0.23
0.13
0.00
0.07
0.04
0.21
0.17
0.13
0.12
0 07
2.83
2.19
1.90
1.76
1.66
0.460 0.487 0.038 0,034
0.559 0.387 0.023 0.031
0.609 0,351 0.016 0.028
0.633 0,331 0 013 0.023
2.63
2.43
2.34
2.29
1b.-atoms C gasified/niin. X 106 6G.9 cumulative carbon gasified, 1b.-atoms X 10' (by graphical integration of S 218. Q ' plot) . o instantaneous weight of carbon in bed, 1b.-atoms X 104 597 specific integral gasification rate X lo4 9.53 % of original carbon hurned 0
43.4
20.0 577 7.52 3.4
38.1
36.3 561 6.79 6.1
33.3
51.0 546 G.47 8.5
to measure directly the rate of evolved hydrogen under some median condition, using the data from this experiment t'o refine the observations for the four runs of the carbon dioxide series. A pretreatment period of 3 hours was chosen for this special esperiment, all other conditions being kept the same as in the regular carbon dioxide runs. To determine the evolved hydrogen, the sampling system of the experimcntal unit was modified to permit diversion of the exit gas, for a series of measured periods throughout the entire run, through a special gravimetric train. These gravimetric measurements permitted calculation of both the hydrogen and steam content of the exit gas, and, therefore, the rate of total hydrogen evolution ( 1 1 2 HYO)from the char.
+
In As C in char 8' = 0 As inlet COz Total out I n exit gas
-
72.9
6.53 12.2
W
t-n
2 #20
pnz 16 w o
&jl2 >t-
",Z
8
X I
gg4 2
x
+
0 . 0 5 4 2 1b.-atom, b y weight of reiiidue and ultimate analysis 0.0954 100 = 1 . 5 % lligll 0 . 0 3 4 3 ib.-niole Or
-1
0.0343 - 0.0341 o,0341
524
0 0412 1b.-atom, by graphical integratio: of r(c0 COdR'I I". e plot
I n bed residue
0.656 Total 0 , 3 2 2 0 , 0 9 5 4 - 0.0940 0.008 0,0940 0 . 0 1 4 Oxygen Balance In 2.22 As inlet COz n,,t In exit gas 34.2
0 0597 1b.-atom 0 0343 1b.-atom 0 0940 1b.-atom
-
0 0341 1b.-mole
0 2 , b y graphical i n t e v a tion of [ ( c o ~CO/H I I ~ o / ~ ) RUS. ' ] e' plot
x
100
=
0 . 6 % low
+
2 Table 111. A.
(Run 27.
Snioothed raw data = elapsed oxidizing time, minutes Dry exit gas analysis, mole fraction Con
e’
0
CH4 Hz HzS
Nz
Inlet gas
0.131 0.224 0.020 0.540 0.001 0.084
=
40
9 0 % HzO-lOYo Nz. 120
80
0.145
0.210
0.024 0.529 0.001 0.091
R = dry exit gas rate, standard cu. feet/hour 5.90 5.20 4.77 Inlet steam rate, standard cu. feet/ hour 4.36 Inlet Nz rate, standard cu. feet/ 0.446 hour Total pressure, atmospheres 0.977 B. Calculated quantities N = integral gasification rate, 1b.-atoms C gasified/min. x 106 95.9 85.7 79.5 SW = cumulative carbon gasified,lb.-atoms X 104 (by graphical integration of N 9s. e’ plot) 0 18.2 34.7 W = instantaneous weight of C in bed, 1b.-atoms X 104 595 577 561 IV/W = specific integral gasifica16.1 14.9 14.2 tion rate, X 1 0 4 lOOR = Yo of original carbon burned 0 3.1 5.8 Steam conversion, 65.0 * 59.1 55.9
0.161 0.192 0.030 0.514 0.002 0.101
-
(
14.0 B
0.506
0.002 0.108
0.175 0.187 0.019 0.498 0.003 0.120
4.32
4.05
3.80
3.64
72.8
68.4
63.9
60.8
65.1
93.4
530
502
120
137
476
458
13.6
13.4
13.3
10.9 52.0
15.7 49.7
20.1 47.3
23.1
(2)
);
--
(3)
Bed Residue Composition, %, Dry Basis H 0.7 C 80.1 N 0.7 0 0.0 S
45.7
Total 100 X 0 .
(Expressed as
+
was used in calculations for carbon dioxide runs (see below). It was previously stated that the precision of the Orsat determination for carbon monoxide was too low to be used as a primary measurement. Use of the Orsat carbon monoxide values to obtain a water gas shift,constant was justified, however, as each successive gas sample was characterized by approximately the same value of this constant. Therefore, use of several samples to determine this value greatly reduced the effect of the inherent error in a single carbon monoxide determination. Using the correlation for H E , the known value for the water gas shift ratio and the measured composition of carbon dioxide in the dry exit gas, the complete composition of the “wet” exit gas, a t any point, for the four regular carbon dioxide runs was calculahle by solution of the following three equations:
t6 7
COz Series t l . 5 +1.3 $0.7 +0.7
t23 25 27
H20 Series +0.8 +0.8
$1.2
(HzO)
Oxygen
+
All four components in the water gas shift ratio,
component quantities above were expressed in standard cubic feet per hour (see Table 11). Knowing the composition of the wet exit gas, the carbon gasification rate was calculated from the following expression:
-0.2 0.0
+0.3
--+ -1.3 -0.3 0.0
(5)
(Y%0 )(YCO) (YCOn)(YH,)
+ (H2) = H;
R is the observed dry exit gas rate; for calculation purposes, all
-0.6
The cuive representing this equation is plotted in Figure 3, using an auxiliary, graphical relation (not shown) between B and 0 obtained from the data of the special hydrogen evolution run. The experimental points are fairly well described by this curve, as shown. Therefore Equation 3 v a s used to calculate H E a t any point in the four regular runs of the carbon dioxide series.
’
were determined, at several data points, in the special hydrogen evolution run: Y H ~ Oand Y H by ~ the gravimetric train arrangement described above and yco2 and yco by Orsat analysis. The value for the ratio showed no trend with burnoff and averaged 0.91 (the equilibrium value a t 1600” F. is 1.20). This average value May 1952
+ (CO) + (Hz) = R
‘U in xn 100)
Carbon
8
819
Component Balances
Run
9
__
0 , 0 6 0 2 1b.-atom
0 6 0 2 ~=51 . 2 % high
0.0595 Oxygen Balance In As inlet steam 0 . 0 1 7 7 1b.-mole OZ out In exit gas 0 . 0 0 9 3 1b.-mole On b y graphical integration of [(C?z CO/Z) R’] US. e In condensate 0.0084 1b.-mole 0% Total 0 . 0 1 7 7 1b.-mole 02 0.0177 - 0,0171 = 0.0% 0.0177
(COz) Table IV.
1.8
Ash 16.7 Carbon balance In 4 As C in char 0‘ = 0 0 . 0 5 9 5 1b.-atom -., out 0 . 0 1 3 7 1b.-atom = ZW at end of run In exit gas In bed residue 0 , 0 4 6 5 1b.-atom by weight of residue and ultimate analy-
13.7
-3
+ 7.0 - 14.0 (B
188
0.173 0.187 0.023 0.500 0.002 0.115
0.028
B being the fraction of original carbon gasified. The total evolved hydrogen rate during the oxidizing period is then
H E X 106 = 25e+.012e
l Gasification
Pretreatment time 0 hour)
160
0.169 0.188
dotted line in Figure 3) thus giving values, during the oxidizing period, for that portion of the hydrogen released “thermally.” Subtraction of these extrapolated values of HTfrom the measured total evolution rate during the oxidizing period, H E , gave that portion of the hydrogen released b y “complex disruption,” called Hc. These values of Hc were correlated by the following empirical equation:
Hc X 1W = 7.0
e
Smoothed Data and Calculations for Typical Steam Run
20
0.112 0,247 0.011 0.556 0 0.074
co
u
where N is the pound-atoms of carbon gasified per minute and R‘ is the [‘wet” exit gas rate in pound-moles per minute. The specific integral carbon gasification rates, N / W , expressed as pound-atoms of carbon gasified per minute per pound-atom of solid carbon present, for the four regular carbon dioxide runs are plotted as functions of burnoff in Figurk4. Pretreatment times of 0, 1, 3, and 7.3 hours are represented. Steam Series. Computations for the three steam runs are straightforward, relative to the carbon dioxide series, as calculation of gasification rates from the observed data was not coniplicated by the hydrogen evolution from the char. The integral gasification rate, N , a t any value of the elapsed oxidizng time, is simply the product of the dry exit gas rate and the sum of the mole fractions of the three carbon-bearing constituents in this
INDUSTRIAL AND ENGINEERING CHEMISTRY
1055
Fuel Gasification gas. The subsequent procedure for calculating N / W and B , indicated in Table 111, is similar to the carbon dioxide series. Figure 5 is a plot of N / W versus per cent carbon burnoff for the steam runs, representing pretreatment times of 0, 3.7, and 24 hours.
(2)(0.03)]. Thus, the increase in observed gasification rate a t zero burnoff between 7.3 and 0 hours prdtreatment time, due to increased hydrogen evolution only, was approximately (1.00 - 0.10) (1.00 - 0.03)
Discussion of Results The similarity of the effect of nitrogen residence time (pretreatment time) on the gasification rate in both systems is evident. I n general, summarizing the common implications of both families of curves, the effect of pretreatment time (1) is appreciable a t low burnoff; ( 2 ) becomes increasingly less significant as burnoff increases, but does not disappear entirely within the range of burnoff (0 to 25y0) covered; and (3) at a given burnoff, approaches an asymptotic limit with increasing pretreatment time.
I
0
1
$[O‘
9
9
*4’
A
6
3
8 7.’IO
X
2w
%u
Although carbons formed a t low temperature tend to graphitize continuously as they are exposed to increasing temperature, Riley and coworkers ( I ) , in an x-ray diffraction study, found that crystallographic changes in the carbon are very rapid and essentially a function of temperature only. Thus the general decrease in reactivity with increasing pretreatment time a t constant burnoffs, shown in Figures 4 and 5, apparently cannot be ascribed to increasing graphitization. For the carbon dioxide series, the accelerating effect of hydrogen evolution on the gasification rate must be considered as a possible explanation of the decrease in this rate with increasing pretreatment time, a t constant burnoff. The gasification rates for these carbon dioxide runs are greater than the rates that would have been measured in the absence of evolved hydrogen, due t o the following cycle:
adding,
+C
c +co,
= =
+ HLO + Hr
CO 2CO
+
The rate of the C HzO reaction is considerably less than the rate of the r ~ a t e rgas shift reaction and thus would control the rate of the above cycle. Also, under the experimental conditions employed, the C H20rate was roughly twice that of the C COPrate-viz., Figures 4 and 5 . Therefore, at zero burnoff in the zero-hour pretreatment run of the carbon dioxide series, where total hydrogen evolution (calculated from Equation 3) was 10% of the inlet carbon dioxide (see Table 11),the maximum enhancement of the gasification rate relative to pure carbon dioxide must (2)(0.10)]. have been roughly proportional to [LOO - 0.10) This assumes that all evolved hydrogen was converted to steam by the water gas shift reaction. Similarly, a t zero burnoff in the 7.3-hour pretreatment run, where total hydrogen evolution was 3y0of the inlet carbon dioxide (calculated from Equation 3) the maximum increase in the gasification rate relative to pure carbon dioxide must have been roughly proportional to [( 1.00 - 0.03)
+
+
+
+
1056
This calculated increase of about 7% is orders of magnitude less than the observed increase from 4.37 to 9.53 (Figure 4). Indeed, the fact that the contribution of hydrogen evolution to the overall decrease in reactivity a t constant burnoffs is small is implied in the steam series curves (Figure 5)-that is, if hydrogen evolution had been the principal factor in causing the depressing effect of pretreatment time on gasification rate in the carbon dioxide series, then, in all probability, there would have been little or no effect of pretreatment time in the steam series, as evolved hydrogen could not augment the carbon-steam gasification rate. [In fact, hydrogen is a powerful inhibitor to the carbon-steam interaction rate (8, s).] Using the same reasoning as above, it may be shown that only a small fraction of the over-all decrease in gasification rate with increasing burnoff, a t constant pretreatment time, observed in the zero-hour pretreatment time runs for both carbon dioxide and steam series, may be attributed to the decrease in hydrogen evolution rate.
z -
Plot of N / W us. C . Burnoff with pretreatment time as parameter
HzO
+ (2)(0.03)
A
Ib
PER CENT CARBON GASIFIED Figure 4. Carbon Dioxide Series
Hz(evolved) + COZ = CO
+ (2)(0.10) = 1*07
6T
c30
us
9:
Gm _I
.4 possible explanation of the observed decreases in reactivity sh0n.n in Figures 4 and 5 may be based on the catalytic effect of small quantities of iron present in the ash, and the gradual deactivation of this iron by continuous thermal treatment. King and Jones (4)have studied the catalysis by small amounts (0.05 to 1.0 veight yo]of iron in the carbon-carbon dioxide system. They found that only the .‘reducible” portion of the iron is catalytic, while that portion combined v i t h the acidic ash components is essentially noncatalytic, and furthermore, that the addition of acidic substances like silica and titania to the coke along with the small amounts of added iron resulted in substantially complete vitiation of the catalytic effect of the latter. They defined “reducible” iron as the iron existing as the metal plus the iron in those forms which are easily reducible to the metal under the specified carbon gasification conditions. The total iron in the carbonaceous feed used in the present investigation was 2 0 weight %. The state of this iron in char which had been subjected to zero-hour pretreatment-i.e., char that had been heated from the feeding temperature of 1000O F. to the reaction temperature of 1600 O F., under previously described conditions-was determined to be the following: Fe as metal Fe as FeS Fe as oxides
INDUSTRIAL AND ENGINEERING CHEMISTRY
1.5% 46 Yo 11 %
Vol. 44, No. 5
Fuel GasificationThe remaining iron (roughly 40%) was probably combined with acidic ash constituents in some inert form. The constituents listed above, however, might all be termed "reducible," certainly for the steam runs where large amounts of hydrogen were present and probably for the carbon dioxide runs where small but b i t e amounts of hydrogen were present. Unfortunately, no determination was made of the percentage of reducible iron in the gasification run residues. However, there was more than enough silica present in the ash to combine with all of the iron. T h y , a possible explanation of the observed decrease in reactivity to an asymptotic value, either with increasing pretreatment time a t a constant burnoff or with increasing burnoff for the zero-hour pretreatment runs, is that the reducible iron was gradually deactivated by the acidic oxides, such as silica, in the ash. Those runs which had a pretreatment time of 1 hour or longer exhibited a slowly increasing gasification rate with burnoff. Although this was partially due t o an intrinsic change in the condition of the carbon surface, most of this increase is only apparent, and is caused by the use of the integral gasification rate as the dependent variable. Whereas the differential rate would be identified with a single gas phase composition (at a given total pressure), the integral rate is a function of some average between the inlet and outlet compositions. Thus, for a given initial bed weight, continuous increase in burnoff caused, of course, a continuous' decrease in total carbon and a si-multaneous decrease in steam or carbon dioxide conversion. The result was a continuous increase in the average steam or carbon dioxide partial pressure per unit of carbon, thus tending to increase the integral gasification rate, which is expressed on a unit carbon basis.
Conclusions In addition to revealing some striking information on the effect of pretreatment time, this investigation furnished data which have permitted circumspect control of this relatively minor variable in the broad study of char-steam kinetics now in progress.
No further work on this variable is planned for the immediate future. In order to obtain greater insight into the mechanism of the effect of pretreatment time on the char reactivity an extentensivo, systematic series of experiments would be required. Such a program would be designed to separate quantitatively the variables of gas composition, carbon burnoff, the state of the iron in the ash, and pretreatment time, an interpretative task which could be accomplished only through differential gasification rate data.
Nomenclature HT
= hydrogen evolution rate during nitrogen pretreatment
HC
=
HE =
HA B
= =
h R'
= =:
= =
N
W
= =
8 8'
=
period, pound-moles/minute/pound-atom of initial carbon in bed incremental hydrogen evolution rate over and above HT occurring after introduction of oxidizing atmosphere of carbon dioxide, pound-moles/minute/pound-atom of initial carbon in bed HT Hc,or total hydrogen evolution rate in oxidizing atmosphere of carbon dioxide H a expressed as standard cubic feet per hour fraction of original carbon burned mole fraction, subscript denoting constituent dry exit gas rate, standard cubic feet per hour wet exit gas rate, pound-moles per minute pound-moles of gas phase carbon passing any point per minute instantaneous weight of carbon present, pound-atoms total elapsed time a t 1600" F., minutes elapsed oxidizing time a t 1600" F., minutes
+
Literature Cited (1) Blayden, H. E., Gibson, J., and Riley, H. L., Proc. Conf. Ultrafine Structure of Coals and Coke, Brit. Coal Utilisation Research Assoc., 1944, 176-231.
(2) Gadsby, J., Hinshelwood, C. N., and Sykes, R. W., PTOC. Roy. SOC.(London), 187, 129-50 (1948). (3) Goring, G. E., Curran, G . P., Tarbox, R. P., and Gorin, E., IND. E m . CHEM.,44, 1067 (1952). (4) Xing, J. G., and Jones, J. H., J. Inst. Fuel, 5 , 39-55 (1932).
RECEIVED for review August 4,
1951.
ACCEFTED March 18, 1952.
(KINETICS OF CARBON GASIFICATION BY STEAM) Effect of Pressure and Carbon Burnoff on Rate of Interaction of Low Temperature Char with Steam-Hydrogen Mixtures at
1600"F. G. E. Goring, G. P. Curran, R. P. Tarbox, and Everett Gorin
.
PITTSBURGH CONSOLIDATION COAL CO., LIBRARY, PA.
The first phase of a comprehensive experimental program on the kinetics of the char-steam system has been completed. The data are all characterized by a temperature of 1600' F., and cover a study of the effects of three independent variableson the gasification rates. These variables, and the ranges over which they were systematically investigated, are: (1) per cent carbon gasified (carbon burnoff), 0 to 50%; (2) total pressure, 1 to 30 atmospheres; (3) gas composition, H2/H20 ratio varied from 0.1 to - 1.0. Sporadic runs were also made with a feed gas of pure hydrogen. Using a graphical extrapolation technique, the directly measured integral gasification rates were processed to yield, indirectly, a set of differential gasification rates. Two types of differentialrates were determined, COZ CH4) and the methane the total gasification rate (CO formation rate. Analysis of these differential rates, as they are unique functions of specific values of the three variables mentioned, evolved basic information on the mechanism of cliar-steam interaction.
+
May 1952
+
T
HE rational design of a commercial carbon gasification sys-
tem requires a detailed knowledge of the kinetics of the carbon-steam reaction. However, in spite of the large amount of research carried out in this field in the past 30 years, no generally reliable rate correlation is available. One of the main reasons for this has been the failure of some investigators to separate the effects of intensive and extensive variables (these terms are defined below) on their observed rate data. T h a t is, the reported data have been waiped by the specific configurations of the various experimental equipment from which they were taken, thus greatly limiting not only their applicability to commercial processes but also their utility in defining the basic mechanism of the carbon-steam reaction. The need for determination and correlation of differential gasification rates, which are functions only of intensive variables and thus universally applicable to any gasification scheme, is therefore evident. Over-all object of this present investigation was to obtain differential gasification rates for the char-steam system at 1600" F.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1052
