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 differential rates 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
-Fuel
Gasification
and over wide ranges of pressure, gas composition, and carbon burnoff. The preparation of the solid feed, derived from a low temperature char (trade name Disco), has been described (4). The raw, integral data were obtained from a fluid bed of char particles run batchwise with respect t o the solid and continuously with respect t o the fluidizing gas, and the unsolved problems of the mechanics of the fluid bed were bypassed bv the use of a graphical extrapolation procedure, which yielded the desired differential rate data. These data are invaluable for the understanding of the functioning of a commercial char gasification system, as well as for the elucidation of the basic mechanism of char-steam kinetics. However, this mechanism may not be strictly applicable to carbon-steam kinetics in general, because of the uncertainties introduced by the unknown effects of ash constituents and other trace materials present in the feed char.
Method of Investigation Theory of Graphical Extrapolation Procedure. The stoichiometry relating the chemical species that compose a given reaction system does not, of course, specify the absolute quantity of any one of these species. Similarly, the intrinsir rate (called the differential rate in this article) at which the reaction proceeds is also independent of the absolute quantities of any or all of the chemical species, and is a unique function of only the intensive variables-i.e., temperature, pressure, gas composition, etc. However, in a n experimental determination of the reaction rate it is necessary t o handle finite quantities of the reactants These quantities are called extensive variables, and the experimentally observed rate (called the integral rate in this report) will depend on both extensive and intensive variables. The mechanism of the effect of the extensive variables on the integral rate is closely associated with the physical characteristics of the equipment used to carry out the investigation. It is clear, therefore, that in order t o obtain information on the intrinsic kinetics of the rearting system, the effect.13 of the extensive variables must be divorced from the raw data-that is, the integral rate data must be processed, in some manner, to yield differential rates. The indirect determination of differential carbon gasification rates from the experimentally measured integral gasification rates, N , can be effected by extrapolation, to zero bed weight of carbon, of a plot of the specific integral gasification rate, N I W , versus the instantaneous weight of carbon, W . The slope of such a curve, at any value of W , is:
The specific integral gasification rate must be finite under all conditions, including that condition which obtains when W approaches zero. It follows, therefore, that the slope of the extrapolation curve, expressed by Equation 1, must also be finite a t TV = 0 and the numerator of the light,-hand term of Equation 1 must thus be equal to zero a t this point (in order t o maintain a finite quotient a t W = 0), or:
w --+
0
Equation 2 states rigorously that which is fairly obvious intuitively-namely, that a t TJ' = 0 the specific integral gasification rate is identically equal to the differential gasification rate. Independent Variables. Those specific integral gasification rates which define the points of an extrapolation curve are distinguished only by different values of the extensive variable of bed weight, TV, and must be identified by common values of all the other variables, extensive and intensive. The remaining estensive variable which affects these integral rates is the gas mass velocity. Therefore, the mass flow of feed gas was maintained constant for all runs of a given extrapolation series. The differen-
1058
tial gasification rates, resulting from the various extrapolation., are unique functions of specific values of the intensive variable. only. Table I lists these intensive variables, all of whirh weie held constant for any given extrapolation, and the first three of R hich were held constant, a t the indicated values, for all esprrinients reported here. The three last-named variables were systematically investigated, and the ranges covered are indicated. Complete details on the technique used to manipulate these vai iables are given below under Results.
Table I. Intensive Variables Affecting Carbon Gasification Rates Yariable Preyioiis temperature history" of casban feed
Value o r Range of Values Separate batch of char for each r u n subjected to fluidized N? prstreatment at, 1600' F. for 1 hourb prior to introduction of oxidizing gdS
Tpmperature, E'. Solid particle size range Total pressure atmospheres Inlet oxidizing gas composition
1600 63-1 50-menIP 1 to 30 H?/H20 ratio varied from 0.1 t u 1.0c 0 to 3 0 d
Solid composition (quantitatively expressed as 0 '7 of original carbon gasified and termed "carbon burnOff") Source and initial preparation of carbon feed, RS well as complete garticle size analvsin. are .~~~ incliided A_ i .~".. , ~.~ _ _ in _(. b Effect of varying length 'of nitrogen pretreatment on reabtivity of treated char to steam or carbon dioxide is considerable ( 4 ) . Those data furnished the basis for arbitrary choice of l-hour N 2 pretreatment period for 811 experimental runs in systematic study of char-steam kinetics. This value represents a balance bet,ween conservation of man-hours and complete elimination of effect of pretreatment time o n carbon reactivity (which woiild he accomplished by extended pretreat,ment). 0 This range of values does not include t x o runs made with 100% hydrogen, resu1t.s included a n d discussed in subsequent sections. d Extension of burnoff range to higher values would have required ronsiderable increase of time for each experiment. ~
~
~
~
Equipment. This was fully described in the previous paper
(4). Experimental and Operating Procedure. The final extrapolation plots were evolved from two successive cross-plotting operations start,ing with smoothed plots of the raw data. It was essent.ial, therefore, to maintain scrupulous control of all operating variables and to obtain primary measurements-i.e., composition and rate of evolution of dry make gas-of the highest attainable precision, as any aberrat'ion in t.he raw data would be magnified significantly by the subsequent graphical processing. Each extrapolation pldt, which yielded differential gasification rates over a carbon burnoff range of 0 t o 507,, required three to four separate runs made under ident,ical conditions of inlet gay composition and total pressure but with varying initial bed weight. Throughout a,ny given run, the operating variables of fluid bed temperature, reactor pressure, and flow rates of hydrogen and steam were maintained const'ant. The ratio of these flow rates was fixed a t some predetermined value bet'ween 0.1 and 1.0 (see Table I ) and their sum way such that the superficial fluidizing velocity was 0.44 rt 0.02 foot per second a t reactor conditions. The changes in kinetic characterist,ics of the batch solid charge of char, resulting from continuous imposition of the specified conditions, were then followed by continuous analysis of t,he dry make gas and continuous measurement of the rate of production of this gas. These raiv data were then processed in t8hemanner described below.
Results Evolution of Differential Gasification Rates from Raw Data. S M ~ O T HPLOTS E D OF Raw DATA.Figures 1 and 2 show the continuous change, with elapsed oxidizing time, of the directly memured quantities of dry exit gas composition and rate, rcspectively, for a tllpical run (run 68). The nominal values of the operating varia.bles for this run, as shown on the legends of the plots, were: a total pressure of 6 atmospheres and a n inlet gas composition of 50% hydrogen-50% steam. The gas compositionli a t the indicated values of elapsed oxidizing time were determined
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Vol, 44, No. Et
Fuel Gasification by specific complementary analyses of three gas samples taken simultaneously. The previous paper ( 4 ) describes the processing of these analyses (gravimetric, infrared, and Tutwiler) to yield values for the four plotted constituents. The remaining constituent in the dry exit gas was hydrogen, comprising the difference between 100% and the sum of the four shown. Each point defining the dry exit gas rate curve of Figure 2 was obtained from two successive wet-test meter readings taken over a measured time interval.
calculating the values of W , there is a small difference, in Table 11, between the calculated value of W at zero elapsed oxidizing time and measured initial bed weight recorded in the heading of the table. This was generally the case for all runs and merely reflects a small gap in the over-all carbon balance. The calculatioqs of the specific integral gasification rates, for the total carbon, N / W , and for methane formation alone, N c H ( / W ,are obvious. These will henceforth be designated as “specific integral total gasification rate” and “specific integral methane formation rate,” respectively. GRAPHICAL EXTRAPOLATION PROCEDURE. Figures 3 and 4 show, respectively, the specific integral total gasification rate and specific integral methane formation rate, as functions of per cent carbon gasified, for the three runs comprising the 5oy0 hydrogen -50% steam series a t 6 atmospheres’ total pressure. The only distinguishing variable between each of the three runs was the initial weight of carbon, which is the parameter in each plot. The raw data for the two other runs in the series (runs 67 and 69) were processed in exactly the same manner as was illustrated above for run 68.
I ah I 8.C 0
100
ELAPSED
I76
200 300 400 500 600 OXIDIZING TIME, MINUTES
Figure 1. Dry Exhaust Gas Composition as Function of Elapsed Oxidizing Time for Typical Run Run 68
,112 LL
0 v)
I6.8
16.4 The changes reflected in the data of Figures 1 and 2 were caused by two effects: the continuous change in the nature of the solid reactant surface as carbon was increasingly burned off, and the continuous depletion of the batch carbon charge. The former is an intrinsic property of the char-steam kinetic system, which must be separated from the latter, a n extensive variable, before the behavior of this property may be appraised correctly. The two effects are not separable a t this point, but are resolved in the subsequent processing of the raw data. TABULATED CALCULATIONS.The smoothed curves drawn through the data points in Figures 1 and 2 form the starting point for effecting the resolution mentioned above, as well as for calculating the relationship between certain relevant kinetic parameters. Table 11, B, lists values of the primary measurements read from the smoothed curves of Figures 1 and 2 at successive increments of elapsed oxidizing time. Table 11, C, lists quantities calculated, a t the given values of elapsed oxidizing time, from the smoothed raw data in part B. The integral gasification rate, N , is simply the sum of the fractions of carbon monoxide, carbon dioxide (corrected for carbon dioxide dissolved in the condensate), and methane in the dry exit gas multiplied by the dry exit gas rate in pound-moles per minute. Values for the cumulative carbon gasified, a t each specified value of elapsed oxidizing time, were then obtained by successive graphical integrations on a plot of iV versus the elapsed oxidizing time in minutes. The instantaneous weight of carbon in the bed, W , a t any time could then have been calculated by either of two methods: Starting with the initial weight of carbon in the bed and subtracting the appropriate value for the cumulative carbon gasified. Starting with the final weight of carbon in the bed, obtained from the weight and ultimate analysis of the residue, and adding the appropriate value for the cumulative carbon gasified. The latter method was used throughout, because, for reasons discussed in the section on material balances, the weight of residue carbon was more precisely measurable than the initial weight of carbon charged. Because of this “reversed” method for May 1952
16.C
15.6
Figure 2. Dr Exhaust Gas Rate as Function of Elapsed C h i z i n g Time for Typical Run Run 68
The final plots in the graphical extrapolation procedure for the typical series of runs (Figures 5 and 6) are quasi-cross plots of Figures 3 and 4,and show the specific integral total gasification rate and specific integral methane formation rate as functions of instantaneous bed weight with per cent carbon gasified as the parameter. (An exact cross plot would give the gasification rates versus initial bed weight. These plots give the gasification rates versus instantaneous bed weight, obtained by correcting the initial weight for the fraction of the carbon gasified.) The ordinate a t W = 0 for each curve in Figures 5 and 6 is the differential total gasification rate and differential methane formation rate, respectively, each being a unique function of the intensive variables of total pressure, inlet gas composition, and per cent carbon gasified. Both sets of differential rates are shown as functions of per cent carbon gasified in Figure 7. Processing of the data for the remaining six series of runs made in this study was the same as described above for the 50% hydrogen-50% steam series at 6 atmospheres. The extrapolation curves for these other series were, in most caws, fairly well defined, the radii of curvature being large enough to warrant estimation of a maximum error of =!=loyo for the extrapolated values. TABULATED VALUESOF DIFFERENTIAL RATES. Table I11 gives the differential rates of total gasification and methane formation, as functions of their three independent variables, for the sevgn extrapolation series. Also included in Table I11 are some rates
INDUSTRIAL AND ENGINEERING CHEMISTRY
1059
Fuel Gasification tion of the carbon balances relative t o the hydrogen and oxygen balances.
Table 11. Smoothed Data a n d Calculations for Typical Run (Run 68. Total pressure 6 atmospheres. Exac Inlet steam rate lh. moles/mii. X 104 Inlet Hz/HzO ratio Total pressure, atmospheres Smoothed raw data Dry exit gas analysis
coz
I ^
LU
CHa
HzS
Inlet gas 50% Hz-50% Hz0. Initial bed weight 0.0299 1h.-atom C)
6.58 1.01 6.02
----f
>
*
2.92 2.48 1 . 8 9 4.49 4.11 3.60 2.10 2.09 2 . 0 5 0.25 0.21 0.16 90.24 91.11 92.30
H Z (by difference) Dry exit gas rate, standard 18.24 17.76 cu. feet/hour Calculated quantities Integra! gasification rate ( N ) ,1b.-atoms C gasified/ min. X 106 74.7 6 6 . 4 Cumulative C gasified, 1h.atoms X lo4 0 14.1 Instantaneous weight of C in bed, ( W ) ,bb.-atoms X J04 296 282 Specific integral gasification rate ( N / W 1b.-atoms C C gasified/rni:./lb.-atom x 104 25.3 23.6 Specific methane formation rate ( N c H I / W )!b. moles CHI formed)min./lb.5.6 atom,C, X 10‘ 5.7 % of original C gasified 0 4.8
1.54 3.30 2.01 0.13 93.02
1.11 2.91 1.92 0.09 93.97
0.66 0 . 4 5 2.40 2.10 1.72 1.53 0.04 0 . 0 1 95.18 95.91
0.33 0.26 1 . 8 8 1.70 1.38 1.26 0.01 0.01 96.40 96.77
0.22 1.54 1.16 0.01 97.07
0.16 1.34 0.97 0.01 97.52
0.11 1.23 0.81 0.00 97.85
17.16 16.86 16.52 16.24 16.12 16.05 16.00 15.95 15.86 15.76 55.8 4 9 . 8
42.3
33.4
28.3
24.8
22.2
20.1
16.9
14.6
32.4
42.9
56.7
75.7
91.1
104
116
127
145
161
263
253
239
219
204
191
179
169
150
135
15.3 13.9
13.0
12.4
11.9
11.2
10.8
Discussion of Results Outline of Method Used to Discuss Differential Gasification Rates. The differential total gasification rates and the differential methane formation rates, a~ determined in this study, are unique functions of three independent variables. This statement is expressible by the following relationship:
R 21.2
1 9 . 7 17.7
5.8 11.0
5.8 5.7 5.5 5.2 1 4 . 5 1 9 . 2 25.6 3 0 . 8
for pure hydrogen at 1.5 and 7.5 atmospheres. These were not obtained by the extrapolation technique, as only single runs were made a t each of the two pressure levels. However, as the conversion of inlet hydrogen was never greater than 1%, the measured, integral rates from these two runs closely approximate differential rates and are thus included in the table. Material Balances. Table IV gives over-all carbon, hydrogen, and oxygen balances for all the runs made in this study. The figures for the latter two components reflect the high precision achieved in the analytical measurements and the general reliability of the data. The greater mean deviation for the carbon balances does not, it is felt, refute this general sanguine appraisal of the data. All carbon balances were based on a single average value for the carbon composition of the pretreated reactor solids at zero oxidizing time, as it was impractical t o determine this value for each run. The carbon content of the reactor solids at zero oxidizing time was somewhat less than the carbon content of the feed, because of slight (about 3.5%, evolution of gas phase carbon during the heating up and pretreatment periods. The former value was obtained by ultimate analysis of a charge subjected only t o heating up and pretreatment ( 4 ) . The large batch of source carbon, from which aliquots were taken for the feed t o each run, was carefully mixed before the start of the experimental program, but this still did not eliminate completely the inhomogeneities within the batch, as shown by ultimate analyses of spot samples. These deviations from the average feed composition, although slight, are sufficient to explain the higher average devia-
=
d B , T , Hz/HzO)
(3)
where R is either of the differential gasification rates and B is 5.0 4.8 4.7 4.4 4.1 the fraction of the original 3 5 . 3 3 9 . 3 42.9 49.1 54.4 carbon gasified. The quantitative form of the function + will not be presented in this paper. It may be said, however, that the value of + is not affected by the rate of mass transfer between bulk gas phase and particle surface. Calculations show that, under the experimental conditions used in this work, this mass transfer rate is of the order of 106 times as great as the gasification rates, thus making the latter completely independent of the former. These calculations do not preclude differential processes within the pore structure of the particle. Little or nothing is currently known about the nature of these processes. Their rate characteristics are inherently included in +, if, indeed, these rates are of the same order of magnitude as the over-all gasification rates. It is possible to anticipate a quantitative correlation by a discussion of the qualitative characteristics of each variable. This is accomplished, in ensuing sections, by considering the effect upon R of a change in one of the variables while maintaining the other two constant; or, mathematically, by considering separately the three following functions: Hz/HzO,s =
[ F I B . Hz/HzO =
91’ $2’
(4)
The discussion of each function is based on appropriate plots, and, in each case, the behavior of the function is examined a t various values (or levels) of the parameters. For example, although B is the principal Table 111. Differential Rates of Total Gasification a n d Methane Formation variable in the discussion of Per Cent C Gasified Per Cent C Gasified the effects on &’ of vari0 10 20 30 40 50 0 10 20 30, 40 50 ous values of the parameters, Total gasification rates Methane formation rates Total 1h.-moles CHI formed/min.)lb.1b.-atoms C gasified/rnin./lh:-atom Pressure, H2/H20and T, are also conGas Composition Atm. c x 104 atom C X 104 sidered. Pertinent work and 50% Hz-50% Hz0 1 15.6 1 2 . 6 10.2 8.4 7.5 7.0 1.1 2 . 5 2 . 7 2 . 3 1 . 5 . . . conclusions of other investigaHz-50% HzO 6 34 28 23 19 16 14 1 2 . 9 11.2 10.0 8 . 9 8 . 3 8 . 0 Hs-75% Ha0 1 33 39 35 29 26 24 1 . 5 2 . 5 3 . 3 3 . 7 3 . 6 3.ZS tors are appropriately fitted 6 25 0 Hz-75% Hz0 68 60 53 45 into this framework. This Hz-75’7 HzO 30 1% 1; 112 100 90 80 42 39 36 33 31 29 %‘ % HzQ-O~’! Hz0 1 55 60 63 65 67 70 1.0 2 . 2 3 . 3 4.0 4 . 2 4 . 2 method of discussing qualita6 122 122 122 120 112 105 6 . 5 11.6 1 5 . 3 1 7 . 3 1 7 . 8 1 7 . 3 10% Ha90% HzO . . . . . . . . . . . . 1.60.5 . . . . . . . . 100% Hz 1.5 t i v e l y t h e complexities of . . . . . . . . . . . . . 9 . 4 4.6 . . . . . . . . 100% Hz 7.5 kinetic data can be employed Differential methane formation rates not determinable in this series, as no direct measurement for CHI was effectively only for differential available at time runs were made. rates. Q
1060
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Vol. 44, No. 5
-Fuel Table IV.
Material Balances
(Expressed as
Series 10% Hz-90% HnO a t 1 atm.
10% H r 9 0 % HzO a t 6 atm. 25% Ha-75% HzO a t 1 atm. 25% Ha-75% H t 0 a t 6 atm. 25% Hz-75% Ha0 a t 30 atm. 50% Hi-60% Ha0 a t 1 atm.
60% Ha-50% Ha0 a t 6 atm.
Mean
Run 33B 32A 30A 31 63A 64 65 40A 39B 37 48 44 53 58 56 57 42A 34A 38A 41 67 69 68
ow
X 100) Component Carbon Hydrogen +3.02 +1.59 -4.14 4-0.80 +0.45 -0.91
$1.50
$3.88 +0.95 -1.31 +2.10 -0.33
+0.05
-0.27 4-0.67 -0.72 +4.73 +0.26 -2.89 +1.21 -1.34 -3.35 -0.63 i1.62
Oxygen
+0.08 -0.10 $1.09 +0.75 -1.26 +0.26 -0.36 $0.70 -2.09 -1.39 -0.14 $0.67 $0.42 -1.15
+l.Ol +0.64
50% burnoff being half the value for the unburned material. T h e rates corresponding to the median gas composition of 25% hydrogen-75% steam show a maximum value a t low burnoff and a steady decline with subsequent increases in burnoff. Figure 9 portrays the total gasification rate, a t 25% hydrogen-75% steam, as a function of burnoff, with total pressure as the parameter.
+0.59
101
$1.18 -0.60 +0.71 -0.63
I
8 7 6 5
I 0
I I 20 30
I 40
I
1
50
60
Figure 4. Specific Integral Methane Formation Rate
30
As function of er cent carbon gasified and initial bed weight for 50$hydrogen-50% steam series at 6 atmospheres
28
0 X
IO
PER CENT CARBON GASIFIED
32
0
1
I
9
+1.07
-2.55 -0.55 -0.38 +O 13 $0.26 -1.08 -0.15 -3.88 -1.28 -1.53 -0.80 -4-0.17 -0.85 SO.98 SO.11 iO.92
-0.19 -4.11 -0.33 -1.16 -0.55 +1.46 -0.65 $0.26 +0.01 10.84
Gasification-
These curves show that the relative depressing effect of burnoff increases with increasing total pressure, the percentage decreases in rate over the range 0 to 50% burnoff being 27, 40, and 46, respectively, for total pressures of 1, 6, and 30 atmospheres. In general, therefore, the change in nature of the char surface caused by increasing burnoff results in a decrease in the differential total gasification rate of the resulting carbon by hydrogensteam mixtures, and this depressing effect becomes more pronounced with increasing hydrogen-steam ratio and/or total pressure. Because the importance of the burnoff variable has only recently (within the past 5 years) been recognized by workers in the field of carbon-steam kinetics, 'only a few reported observations
26 24 22
PER CENT C GASIFIED
50 40
0
1
I
I
10 20 30 40 50 60 PER CENT CARBON GASIFIED
30
Specific Integral Total Gasification Rate
20
As function of per cent carbon gasified and initial bed weight for 50% hydrogen-50% steam series at 6 atmospheres
IO
Figure 3.
Properties of Variables Affecting Differential Total Gasification Rate. CARBON BURNOFF.The behavior of this variable is deducible from Figures 8 and 9, which were constructed from some of the values given in Table 111. Figure 8 shows the differential total gasification rate, a t a total pressure of 1 atmosphere, as a function of carbon burnoff, with gas composition (hydrogen-steam mixtures) as the parameter. It is clear from these curves that the nature of the burnoff-rate relationship is strongly dependent on hydrogen-steam ratio. At 10% hydrogen-90yo steam, for instance, the rate increases slightly, by a factor of about 1.18, over the range 0 to 50% burnoff. But a t 50% hydrogen-50% steam the rate suffers a sharp decrease over the same range, its value a t May 1952
JI
m
1 1 200 300 INSTANTANEOUS WEIGHT OF CARBON IN BED, LB. ATOMS x io4
o1
Figure 5.
I
IO0
Extrapolation Plot to Obtain Differential Total Gasification Rates
50% hydrogen-50% steam series at 6 atmospheres
of its effect, qualitative or quantitative, are available. The data of Johnstone and coworkers (6),who used graphite and hydrogensteam mixtures a t atmospheric pressure and 900' C . , show that the relative effect of burnoff was virtually independent of hydrogen-steam ratio and that the differential gasification rate in-
IN D U S T R I A L A N D E N G IN E E R I N G C H E M I S T R Y
1061
Fuel Gasification creased with increasing burnoff. Their data were taken over the fairly narrow burnoff range of 0 to 10%. Wu (S), in his atmospheric pressure study of the graphite-carbon dioxide system a t 1600" F., also found an increase in differential gasification rate with increasing burnoff over a burnoff range of 0 to 15%. In addition, t o support further the results of Johnstone and coworkers, Wu found the relative effect of burnoff on the gasification rate to be independent of carbon monoxide-carbon dioxide ratio. (Since carbon monoxide is a powerful inhibitor t o the carbon-carbon dioxide interaction rate, it holds an analogous position in this system t o hydrogen in the carbon-steam system.)
change in the number of active sites per unit of carbon; the increase in gasification rate with burnoff further implies a continuous increase in this number. The data in Figure 8, on the
70,
I
I
I
I
I
lO%H, - 90%&0
-06
I
I
50 40
30 20
f IO
f
B
IO
B* r
0
5 "8
I
O6 6s !94 y 0
Figure
I
I
10 20 30 40 50 60 PER CENT CARBON GASIFIED 8.
Differential Total Gasification Rates at 1 Atmosphere
As functions of gas composition and per cent carbon gasified
2
m 2 A
A comparison, beta een the common conclusions drawn by the two investigations mentioned above and those data from this investigation which are included in Figure 8, is instructive. (The d a t a of Figure 9 are not, of course, relevant i n this comparison, as
other hand, shoiv t,hat the relative effect of burnoff is a function of gas composition (as discussed above), thus implying that the mechanism of' the burnoff effect ie a change in the distribution of types as well as numbers of active sites. An alt,ernative description of the comparison would tie to say that hydrogen inhibit,ion of the total gasification rate increases with burnoff for the char used in this investigation, while, for the carbons used in the other two investigations, t,he effect of the inhibitor (hydrogen or carbon monoxide) was independent, of burnoff, This difference in the nature of the burnoff variable highlights the basic differences in the carbons used: the stable, highly graphitic types used in the t w o previous investigations versus t>he amorphous conglomerate used in this investigation. Because of these basic tliflerences, the latter carbon would be expected to exhibit the more drastic kinetic changes with increasing burnoff, and t.he results bear out, the anticipat>ion. The reactivity-i.e., differential gasification rate--of the amorphous char is not s t r i d y R point, funct,ion of b~irnoff,owing to the separate anti tiis-
4m
0
IO
20
30 40
50 60
FER CENT CARBON GASIFIED Figure 7.
Differential Gasification Rates
As functions of per cent carbon gasified for 50% hydrogen-50% steam series at 6 atmospheres
neither of the other two investigations included data a t superatmospheric pressures. ) Independence of gas composition arid the relative effect of burnoff, found by the two other groups of workers, implies t h a t the rnechanism of the effect is simply a
1062
1
0 Figure
IO 2 0 3 0 40 50 60 PER CENT CARBON GASIFIED 9.
Differential Total Rates
Gasification
At gas composition of 25% hydrogen-75% Steam as functions of total pressure and per cent of carbon gasified
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
Vol. 44, No. 5
-Fuel tinct effect of temperature history, including the residence time at reaction temperatures, on this reactivity (4)-that is, t h e effect of per cent carbon gasified and residence time were not separated experimentally, both being included in the "burnoff" parameter. AB was pointed out (4), the effect of residence time becomes negligible at some high value of burnoff, but the exact value of burnoff marking this point would depend, of course, upon the gasification rate (and thus the residence time) used t o attain this value. For the graphitic carbons, on the other hand, the
I50 100 70 50
* 30 0
JI
m
Gasification
course, a cross plot of Figure 9 at this burnoff value.) Figure 10 is discussed in greater detail below, but it is appropriate to note, at this point, the general increase of the total gasification rate with pressure for all gas compositions. Warner ( 7 ) ,on the other hand, not only claimed t h a t superatmospheric pressures would not cause an increase in carbonsteam gasification rates but also implied t h a t the rate would become of negative order with respect to steam a t pressures as low as 2 atmospheres. His generalization was based on differential data, obtained by passing steam a t high velocity over a machined graphite rod maintained a t constant temperature (range of 850' t o 950' C.),gasification rates being measured by weight differences in the rod. The data points at the upper limit of his pressure range (1200 mm. of mercury or about 1.6 atmospheres) show appreciable scattering, indicating t h a t their precision actually does not justify prediction of a negative order reaction. It is apparent, however, even from the limited pressure range of Warner's work, t h a t increasing pressure would have only a slight effect on the rate of gasification of graphite by steam. The widely different conclusions regarding the effect of pressure on gasification rate, reached by the two investigations discussed above, illustrates again, in trenchant fashion, the axiom which was highlighted in the discussion of burnoff and its effects on the gasification rates of various carbons-namely, t h a t the inherent character, as well as the magnitude, of the effect of various kinetic variables on the gasification rate varies widely for different types of carbon. It is clear, therefore, that sweeping generalizations based on data from one type of carbon are unwarranted.
- 0 7
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0.2
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:g Figure 10.
Differential Total Gasification Rates at 10% Carbon Gasified
As functions of gas composition and total pressure
ig!
G
m reactivity would be expected to approach closely a point function of burnoff, since their previous temperature histories included long residence times at temperature levels considerably above the reported reaction temperatures, and additional residence time at these latter temperatures should thus have a negligible effect on their reactivities. can be altered drastically The general form of the function by a change in temperature level. Data recently obtained in this laboratory, a t 1700' F., have demonstrated this. These later results will be presented in a subsequent paper. TOTAL PRESSURE.Figure 9, which was used above to complement discussion of carbon burnoff, also provides an index to the effect of total pressure on the differential total gasification rate, and the relation between this effect and carbon burnoff. The plot shows that, for a &as composition of 25% hydrogen-75'% steam, a general enhancement of this gasification rate is caused by increasing the total pressure, and t h a t this enhancement diminishes as burnoff increases. At zero burnoff, for instance, a pressure increase from 1 t o 30 atmospheres results in a 4.5-fold augmentation of the rate, while a t 50y0 burnoff the same pressure change causes about a 3.5-fold rate increase. Figure 10 relates the effect of total pressure on the differential total gasification rate and the dependence of this effect on gas composition, all points being characterized by a common burnoff of 10% carbon gasified. (The 25% hydrogen curve in Figure 10 represents, of
May 1952
J I
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u
Key ( 6 ) used a pressure range from 1 to 50 atmospheres and P temperature range of 900' to 1000" C. in his fixed bed study of gasification rates of various English cokes by steam, as well as carbon dioxide. His results show, qualitatively (based on integral gasification rate data) t h a t the total gasification rates were enhanced somewhat by increasing pressure, but not nearly to the extent shown by the data of this investigation. GAS COMPOSITION. Most of the characteristics of this variable have already been described, directly and indirectly, in concert with discussions of the two other independent variables. Figure 11, showing the differential total gasificat,ion rate at atmospheric pressure as a function of per cent hydrogen with burnoff as the parameter, is a cross plot of Figure 8, and gives a slightly different perspective on the relation between gas composition and this gasification rate. The sharply increasing negative slope of t h e curves with decreasing hydrogen-steam ratio is typical of a n inhibited reaction. It must be borne in mind, when interpreting
INDUSTRIAL AND ENGINEERING CHEMISTRY
1063
Fuel Gasification the meaning of these curves, that changes in abscissa1 values represent changes in both hydrogen partial pressure and steam partial pressure. Therefore, the marked decrease in total gasification rate with increased hydrogen concentration cannot be attributed to hydrogen inhibition alone, a part of this decrease being due t o decrease in steam pressure. The two effects may be resolved, however, in semiquantitative fashion, by closer inspection of tjhe
0 IO 20 30 40 50 60 51 PER CENT CARBON GASIFIED Figure:iP. Differential Methane Formation Rates at 1 Atmosphere As functions of gas composition and per cent carbon gasified
curves-for example, at zero burnoff, the decrease in the gasification rate accompanying a n increase in hydrogen consumption from 10 to 25% is 57/30, or about twofold. The concurrent decrease in steam concentration (or steam partial pressure) is 90/75 or 1.2. Now the twofold decrease in rate could be explained in terms of steam partial pressure alone, or if it were assumed t h a t the total gasification rate is of fourth order with respect to steami.e., (l.2)4 s 2. The absurdity of this assumption, however, is quickly manifested by similar inspection of other portions of the same curve and, of course, from the behavior of the superatmospheric pressure data. The order of the total gasification rate with respect t o steam is probably close t o unit,y, which implies (using the above figures) a n appreciable hydrogen inhibition. However, quantitative correlation of the whole matrix of data in this paper would be necessary before the magnitude of the two effects could be properly apportioned.
Properties of Variables AfIecthg Differential Methane Formation Rate. CARBON BURNOFFThe effect of burnoff on the differential methane formation rate is somewhat more complicated than its analogous relation t o differential total gasification rate. Figures 12 and 13 are plots of this methane formation rate, at total pressures of 1 and 6 atmospheres, respectively, as functions of carbon burnoff with gas composition as the parameter. Figure 14 relates the methane formation rate with burnoff and total pressure a t a constant hydrogen-steam ratio of 1 to 3. The curves of Figure 12 are all characterized by maxima, which move to the left as the hydrogen-steam ratio increases. An increase in total pressuie at constant gas composition also tends to move the characteristic maxima t o the left, as shown by comparison of the 10% hydrogen curves a t 1 atmosphere (Figure 11) and 6 atmospheres (Figure 13). It follows rationally, therefore, that combinations of increasing pressure and hydrogen-steam ratio should eventually eliminate the maxima, leaving rate curves which show continuous downward trends v i t h increasing burnoff. Thie hypothesis is borne out by the 50% hydrogen curve a t 6 atmospheres (Figure 13) and the 25% hydrogen curve a t 30 atmospheres (Figure 14). The radical behavior of the burnoff variable a t low pressures is of academic interest, and will certainly have an important bearing on any quantitative correlation of the methane formation rate. From a more practical standpoint, hen-ever, the most important qualitative characteristic relating burnoff and methane formation rate is that which obtains at the higher pressure levels, under which condition a fuel gas producer would probably operate. At these high pressures, increasing burnoff has a continuous depressing effect on the methane formation rate similar to its effect on total gasification rate.
rn -11
PER CENT CARBON GASIFIED
Figure 14. Differential Methane Formation Rates at Gas Composition of !?5y0Hydrogen75% ’ Steam As functions of total pressure and per cent carbon gasified
Figure 13. Differential Methane Formation Rates at 6 Atmospheres AB functions of gas composition and per cent carbon gasified
The phenomenon of hydrogen inhibition of the carbon-steam interaction#rate was found and measured by several previous investigators*(l-d, 6 ) for various types of carbon. I n order to compare quantitatively the inhibition exhibited by the data of Figure 11 with t h e same effect from other investigations, it would be necessary to correlate all these sets of data by t h e same rate equation. This will not be attempted here.
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TOTALPRESSCRE.Inspection of Figure 14 shows that the differential methane formation rate has a much larger pressure cocafficient than was evidenced by the differential total gasification rate. For a pressure increase from 1 t o 30 atmospheres, the methane formation rate increases about twentyfold at zero burnoff and twelvefold a t 5070 burnoff, whereas the total gasification rate averages only a fourfold increase over this burnoff range and between the same pressure limits. The integral data of Key (6),the only other reported experimental investigation of carbon-steam kinetics at superatmospheric pressure, indicate, in general, a similar relation between the pressure coefficients of methane formation and total gasification rate. GAS COMPOSITION.Figure 15, which is analogous t o Figure 10 for the total gasification rate, relates the differential methane formation rate t o total pressure and gas composition a t 10 % burnoff. Under these conditions, surprisingly, the methane formation rate is virtually independent of gas composition as long as some steam is present. The radical departure from this pattern of the gasification rates by pure hydrogen is also notable and is discussed in this paper.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 5
Fuel Gasification A similar plot at 40% burnoff (Figure 16) shows t h a t the independence of rate' and hydrogen-steam ratio does not persist at higher burnoffs. Furthermore, at a given total pressure, the methane formation rates increase with increasing steam concentration-i.e., decreasing hydrogen-steam ratio. The data of Figure 16 show, therefore, that under certain conditions the methane formation rate with respect t o steam is higher than with respect t o hydrogen. This gives an unprecedented insight into the mechanism of methane formation in the carbon-steam system. It is notable t h a t the points of Figure 16 are enclosed by an envelope whose boundaries converge increasingly with increasing total pressure (as shown). It may thus be stated that, a t t h e higher pressure levels, the rate of methane formation from hydrogen-steam mixtures tends t o become independent of hydrogensteam ratio for all burnoffs in the range 0 t o 40%. It may also be predicted, from the trends of Figures 15aand 16, that at burnoffs exceeding 40% an increasingly higher pressure would be required t o attain the point where the effect of hydrogen-steam ratio would disappear, and furthermore, at pressures below this point, methane formation rates increase with increasing steam concentration-Le., decreasing hydrogen-steam ratio. The reason for the deviation of the pure hydrogen points from the general pattern discussed above is not clear a t present, but it appears that hydrogen plays two entirely different roles when steam is present or absent. Key (6) observed, qualitatively, t h e same phenomenon. A conceptual explanation of this anomaly would require additional data.
Summary The salient features of char-steam kinetics at 1600" F., as r e vealed by the above analysis, may be summarized as follows: I n general, increasing carbon burnoff causes a decrease in char reactivity, as evidenced by decreases in both total gasification rate and methane formation rate. The decrease in methane formation rate is, however, less pronounced.
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Figure 16.
50
2 3 5 7 IO 20 30 50 TOTAL PRESSURE, ATM. Differential Methane Formation Ratea at 40% Carbon Gasified
As functions of gas composition and total pressure
An increase in total pressure causes appreciable increase in both total gasification rate and methane formation rate. Also, significantly, the pressure coefficient for the latter is considerably greater than for the former. An increase in steam concentration-i.e., a decrease in hydrogen-steam ratio-increases both the total gasification rate and methane formation rate.
Acknowledgment
.
51
The authors wish t o thank C. W. Zielke for performing some of the calculations and J. P. Caldwell for the continuous operation and maintenance of the equipment.
,
Nomenclature N I
2
3
5 7 IO
20 30 50
TOTAL PRESSURE, ATM. Figure 15.
Differential Methane Formation Rates at 10% Carbon Gasified
As functions of gas composition and total presaure
Key's conclusion as to the mechanism of methane formation in the carbon-steam system, based o n the above observation as well as his other data, was entirely different from the conclusion reached in this investigation. H e stated:
It was concluded t h a t the methane produced in the (C-H2.0) reaction probably arose from the direct action of hydrogen, which was resent as a result of steam decomposition, on a coke surface whici was at the same time undergoing erosion by steam. T h e statement means, in essence, that steam provides some sort of catalytic action for the reaction C 2Hz = CHa, and implies that this action is independent of steam concentration. The data of this investigation, on' the other hand, show t h a t the methane formation rate is proportional to steam concentration. Key's data were taken over a considerably narrower hydrogen-steam range, however, which necessarily limited his interpretation.
+
May 1952
= integral gasification rate (or integral total gasification
rate), pound-atoms of carbon gasified per minute integral methane formation rate, pound-moles of methane formed per minute = solid carbon present at any time, pound-atoms = general symbol for differential gasification rate (either total gasification or methane formation)
=
W
R
+,+ I f ,
+ 2 r , +3f
B ir
= general functional notations associated with differen-
tial gasification rates fraction of original carbon gasified = total pressure, atmospheres
=
Literature Cited (1) Chang, T. H., Sc.D. thesis in chemical engineering, Massachusetts Institute of Technology, 1950. (2) Gadsby, J., Hinshelwood, C. N., and Sykes, K. W., Proc. Rov. SOC.(London), 187, 129-51 (1946). (3) Gadsby, J., Long, F. J., Sleightholm, C. A,, and Sykes, K. W., Ibid., 193, 357 (1948). (4) Goring, G.E.,Curran, G. P., Tarbox, R. P., and Gorin, E., IND. ENO.CHEM.,44,1051 (1952). ( 6 ) Johnstone, H. F., Chen, C. Y . ,and Scott, D. S., Ibid., in press. (6) Key, A., Gas Research Board, London, Publ. 40 (July 1948). (7) Warner, B. R.,J. Am. Chem. Soc., 66, 1306-9 (1944).
(8)Wu, P. C., Sc.D. thesis in chemical engineering, Massachusetts Institute of Technology, 1950.
REC~IVE forDreview Deoember 17, 1951.
IN D U S T R I A L A N D, E N G IN E E R IN 0 C H E M IS T R Y
ACCEPTED Maroh 18, 1952.
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