ENGINEERING AND PROCESS DEVELOPMENT
Desulfurization of l o w Temperature Char by Partial Gasification C. W. ZIELKE, G.
P. CURRAN, EVERETT GORIN,
AND
G. E. GORING'
Research and Development Division, Pittsburgh Consofidation C o d Co., librory, Pa.
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T WAS noted in the course of the 1600" F. carbon-steam kinetics study carried out in these laboratories that sulfur is usually eliminated preferentially during the early stages of the gasification process. Unfortunately, little or no information is available, from the earlier kinetics study, relating the sulfur content of the char residues to the process variables. Since the production of a low sulfur char is of commercial interest due to its potential metallurgical applications, it was decided t o investigate the kinetics of the desulfurization of char in the low burnoff range, A considerable amount of literature has developed on the desulfurization of coke by treatment with hydrogen, steam, and other gases. This work has been reviexed by Lowry (6). The published work has been concerned largely with the desulfurization of high temperature coke in contradistinction to the work recorded here on low temperature char. The previous investigators were chiefly interested in the development of procedures for desulfurizing hot coke before it was pushed from the oven. These procedures were empirical, and no systematic investigation has been made of the effect of process variables on the kinetics of the desulfurization process. In particular, the effecte of three important variables have been lackingnamely, pressure, carbon burnoff, and hydrogen-steam ratio. Some of the more pertinent of the previous work is reviewed below. Gurarii (4)investigated the effect of temperature over the range of 800" to 1200" C. and particle size over the range of 2 to 15 mm. on the removal of sulfur from coke with steam and hydrogen. The efficiency of removal of sulfur increased with increasing temperature and decreasing particle size but in no case was more than 54% of the original sulfur removed. Mirev ( 6 ) showed the desulfurization of coke vas less effective with hydrogen than with hydrogen-steam mixtures. Monkhouse and Cobb ( 7 ) removed 94% of the sulfur from coke a t 1000" C. with hydrogen. They were able to desulfurize coke to the same extent a t 800' C. with steam and hydrogen. Pexton and Cobb (8)investigated the effect of carbon burnoff on the desulfurization of 1 / 3 ~ X inch coke with steam-nitrogen inlet gas mixtures in the temperature range of 800" to 1000" C. Their work showed that, a t low carbon burnoffs, pure steam is an efficient agent for the removal of sulfide sulfur but not for the removal of organic sulfur. The present investigation is concerned with a study of the effect of the process variables on char desulfurization by partial gasification with steam-hydrogen mixtures and pure hydrogen. The extrapolation technique described earlier ( 2 ) in connection with the original char-steam kinetics work was also employed in this work. Data were thus obtained that relate the efficiency of desulfurization to a definite value of the gas composition and that are independent of the physical characteristics of the equipment employed. The variables studied and ranges over which they were investigated are as follows: I
Present address, Standard Oil Company of Indiana, Whiting, Ind.
January 1954
Carbon burnoff, % carbon gasified Total pressure, atmospheres Gas composition, mole fraction hydrogen
0-40 1-6 0 1-1.0
The use of the extrapolation procedure necessarily yields data on the efficiency of desulfurization a t zero concentration of hydrogen sulfide, but a semiquantitative picture of the inhibiting effect of hydrogen sulfide was obtained by processing the desulfurization data as a function of bed weight and also by the results of runs where hydrogen sulfide was added directly to the feed stream. The present investigation was conducted in the same experimental equipment that was described previously and that was used in the study of the kinetics of the carbon-steam reaction (2). The majority of the runs were carried out with the same char that had previously been used in the aforementioned kinetics study. The char was devolatilized a t 1600" F. by fluidizing with nitrogen before admission of the reactant gas, h few runs were also carried out with char briquem. These briquets were composed of 90% of the standard 65- to 100-mesh char, using 5% of a coal-derived pitch and 5% of biturmnous coal as a binder. The briquets were first calcined a t 1100' F. for hour in a stream of purified nitrogen followed by a similar treatment a t 1600" 1". for 1 hour. The calcined briquets were '5/32 inch in diameter, The analysis of the feed materials is given in Table I. Table I.
Solid Feed Compositions (Dry basis,
Devolatilized Char, 1 Hr., 1600' F. 1.22 H 81.77 C x 1.59 0.16 0 8 1.92 (3.40 Ash Sulfide S 0.70 3.08 Iron
%I
Prepared Briquet Feed 2,l5 78.35 1.65 3.27 1.95 12.93
Devolatilized Briquets, 1 H r . , !600° F.
om
52 33
1,31 0.23 1.94 13.56
Except for their duration, the desulfurization experiments using hydrogen-steam mixtures were all replicas of 1600" F. carbonsteam kinetic runs that had been carried out previously. Runs a t each inlet condition of pressure and hydrogen-steam ratio were made a t three different initial bed weights. These runs were each terminated a t the value of the gasifying time that gave the desired carbon burnoff, this value being known from the calculations on the original kinetics runs. The exact carbon burnoff, determined by analysis of the solid residue, usually checked quite closely with the calculated burnoff. The pure hydrogen runs were conducted in such a manner that differential data were obtained directly. This was accomplished by using very small initial weights of char (10 grams, bed height 1/2 inch), so that the hydrogen underwent negligible conversion. Both the pure hydrogen and hydrogen-steam runs were conducted a t a fluidizing velocity of 0.44 foot per second.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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ENGINEERING AND PROCESS DEVELOPMENT The runs using the briquetted char were conducted in an analogous manner to the fluid bed runs, except that a k e d bed was employed. Under the conditions employed no difficulty was encountered in eliminating vertical and horizontal temperature gradients, owing to the fact that the over-all reaction was substantially thermoneutral. Carbon Burnoff, Total Sulfur, Sulfide Sulfur, and Organic Sulfur are Determined on Char Residues
r l
Standard ultimate analyses were run on all the bed residues. The carbon burnoff was calculated from the difference between the initial and the final weight of carbon in the bed solids.
3 2 . 0 1 PRESSURE, CARBdN BURNOFF, IATM. 20%
+
INLET QAS COMPOSITION
w
0
3 1.6
A
IObH, -9OxH.O 26%H, -75% H,O
x
50%Ht - 5 0 % H 2 0
2
values. Work by Powell (9) and Wibaut (11) indicates that the organic sulfur in coke is absorbed free sulfur and sulfur in solid solution in the carbonaceous mass. For convenience, such sulfur is termed “organic sulfur” in this paper. The residues were not analyzed for sulfate sulfur since considerations of equilibria and dissociation pressures, and the findings of previous workers (8),show that the concentration of this type of sulfur is negligible a t the experimental conditions. No attempt was made to determine the hydrogen sulfide content of the effluent gas. Earlier attempts to obtain sulfur balances in this way failed, undoubtedly because of the reaction of the hydrogen sulfide with the moist lines downstream from the reactor, the stainless steel wool filter, and solution of hydrogen sulfide in the ammoniacal condensate. The possibility that an appreciable part of the sulfur was eliminated as carbonyl sulfide (COS) was rejected on the basis that equilibrium for the reaction H2S GOz = HzO COS is far to the left for the conditions in question.
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Desulfurization Results with HydrogenSteam Mixtures and Pure Hydrogen
The experimental results on the desulfurization of both the 65-
to 100-mesh char and the char briquets with hydrogen-steam mix-
1
I
20
0
40
60
80
too
WEIGHT OF CARBON. LB. ATOMS x io4 Figure 1. Extrapolation Plot to Obtain Residue Char Sulfur Contents a t Zero Bed Weight
Total sulfur was determined by the Eschka method (10) while the sulfide sulfur was determined by the evolution method (IO). Organic sulfur was calculated as the difference between these two
Table 11.
Series 10% Ha-QO% HzO, 1 atmosphere 25% H2-76% Hz0, 6 atmospheres 10% Hr9O% HzO, 1 atmosphere 25% He-75% HaO, 1 atmosphere 60% Hz-50% HzO, 1 atmosphere 25% H r 7 5 % HzO, 6 atmospheres 50% Hz-50% HeO, 6 atmospheres
HxS addition runs,
0.5% HzS-10% Ha-89.5’7 H a , 1 atmospiere Briquetfeedruns 50% H r 5 0 % dzO, Gatmospheres
54
tures are given in Table 11. Included in Table I1 are figures for the maximum possible organic and sulfide sulfur contents and their respective per cent removal, The maximum sulfur content of the char residue is the sulfur content it would have had if none of the sulfur was removed. The graphical extrapolation procedure to obtain the sulfur content of the bed residues a t zero bed weight is illustrated in Figure 1 for the I-atmosphere runs. The extrapolated sulfur contents a t zero bed weight as functions of burnoff, pressure, and gas composition are illustrated graphically in Figure 2. Some points are given in the high burnoff range-Le., 35 to 70%. These points were measured previously in the course of carbonsteam kinetics study for the case of the lowest bed weights. These data were obtained a t a condition sufficiently close to zero bed weight such that no extrapolation procedure was necessary.
Desulfurization Data for Runs Using Hydrogen-Steam Mixtures as Inlet Gas
Run No.
Gasifying Time Min.’
S-19 S-20 8-21 8-22 9-23 8-24 s-2 9-1 9-3 8-7 6-8 S-9 8-4 S-5 S-6 8-13 S-14 S-15 S-10 S-11 5-12 S-16 8-17 9-18
24 30 39 17 22 25 48 63 80 81 93 109 197 231 242 38 48 57 90 96 107 48 63 80
Weight Dry Feed Char Gram’s 22.07 43.24 87.33 43.02 87.59 134.7 22.32 43.64 88.28 22.26 42.31 88.02 22.33 41.65 88.11 43.36 87.61 135.2 88.08 134.3 219.1 21.97 42.93 87.07
115 116 117
90 96 107
89.8 138.2 225.5
Wt. Carbon, Initial 16.5 32.4 65.4 32.2 65.6 100.9 16.7 32.7 66.2 16.7 31.7 66.0 16.7 31.2 66.0 32.5 65.7 101.3 66.0 100.6 164.2 16.5 32.2 65.3
Final 14.5 28.4 57.4 28.7 67.8 89.1 13.4 25.9 53.1 13.2 24.4 53.5 13.1 24.1 52.7 26.1 52.4 81.0 52.6 80.6 131.2 13.3 25.9 53.3
68.9 104.1 165.0
56.1 83.4 134.5
Sulfide Sulfur, % ’ Organic Siilfur, % Total Sulfur, % Ao- Removed hlaxiAc- Removed MaxiAc- Removed MaxiB ~ r n o f f% , mum tu. from mum tufrom mum tufrom Nominal Exact possible a1 char possible a1 cEar possible a1 char 78.2 0.78 0.17 1 . 3 5 1 . 2 0 11.1 35.7 1.37 2.13 10 12.1 0.78 0.14 82.1 1.35 1.25 1.39 34.7 7.4 10 12.3 2.13 0.78 0.24 69.2 1.35 1.36 2.13 1 . 6 0 24.9 0 10 12.2 0.77 68.9 0.24 1.00 52.4 1 . 3 3 0.76 10.9 2.10 42.9 10 0.77 0.25 67.5 1.35 0.82 1.07 49.5 39.3 11.9 2.12 10 0.77 0.38 60.6 1.35 0.79 1 . 1 7 44.8 41.5 11.7 2.12 10 0.84 0.12 85.7 1.45 2.29 1 . 2 4 45.9 1 . 1 2 22.8 19.8 20 0.85 0.20 76.5 1 . 4 5 1.14 21.4 2.30 1 . 3 4 41.7 20.8 20 0.84 0 . 2 0 76.2 1.45 1.22 15.9 1 . 4 2 38.0 19.8 2.29 20 0.85 0.17 80.0 1 . 4 5 0.66 2.30 0.83 6 3 . 9 54.5 20 21.0 80.5 1 . 4 8 0.77 0.87 0.17 48.0 23.0 2 . 3 5 0.94 20 60.0 64.7 0.85 0.30 1 . 4 4 0.84 20.5 2.29 1.14 50.2 41.7 20 84.9 0.86 0.13 1 . 4 6 0 . 2 6 82.2 20 21.6 2.32 0.39 83.2 86.2 0.87 0.12 1 . 4 8 0.23 2.35 0.35 85.1 84.5 20 22.8 71.6 0.84 0.28 6 6 . 7 1 . 4 5 0.37 74.5 20.2 2.29 0.65 20 90.5 0.84 0.08 1.45 0.41 0.49 78.6 71.7 19.7 2.29 20 0.84 0.07 91.7 1.45 0.45 69.0 20.2 2.29 0.52 77.3 20 0.84 0.08 9 0 . 5 1.45 20.0 2.29 0.56 0.48 66.g 75.5 20 0.12 85.7 87.8 0.84 1.46 0.16 20.3 2.29 89.0 20 0.28 0.08 9 0 . 5 1.46 0.18 87.6 88.6 0.84 19.9 2.29 20 0.26 0.84 88.1 1.45 0.21 85.5 20.1 2.29 0.31 0.10 20 86.5 20 19.4 2.27 1.85 18.5 0.84 0.23 72.6 1.43 1.62 , 20 19.6 2 . 2 8 2.32 0.84 0.40 52.4 1.44 1.92 , . 20 18.4 2.26 2.19 311 0.83 0.45 45.8 1.43 1.74 , , Carbon
.
20 20 20
18.6 19.9 18.5
2.30 2.32 2.24
0.27 0.39 0.40
88.3 83.2 82.5
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
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Vol. 46, No. 1
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ENGINEERING AND PROCESS DEVELOPMENT
It appears from Figure 3 that the effect of pressure disappears when pure hydroC Sulfide gen is used as a desulfurization agent, Run Initial W t , Preesure, Hydrogenation Gasified, Char Residue Totai S, and that the sulfur content is a unique No. Char, Grams .4tnl. Time, Min, % Sulfur Content, % 7c 1 1GO 0.85 1.30 0.36 function. of the per cent carbon gasified 6-28 9.73 s-29 9.74 1 200 1.35 1.00 0.30 and is independent of the pressure. It 5-30 9.73 1 300 1.80 0.97 .. was shown, however, by a single run in 0.55 0.28 6 50 3 17 S-34 9.71 which sufficient methane was added to 0.46 0.17 6 100 5.50 S-32 9.74 s-31 9.74 6 200 8.90 0.36 0.15 the inlet gas to suppress gasification of 49Aa 89.36 1.5 1436 9.21 0.36 carbon with hydrogen, that desulfurizaS-35 9.66 6 atm. H? 100 0 36 0.54 0:33 tion can be achieved without concomi0.9 a t m CHI tant carbon burnoff. The data for this a Large bed used in this run. run (S35) and the corresponding run 1532) without methane addition are shown in Table 111. There ww substantially no carbon burnoff obtained in the former run as against The data on desulfurization of the char with pure hydrogen are 5.50% in the latter and yet the sulfur contents of the two bed resigiven in Table I11 as a function of pressure, residence time, and per cent carbon gasified. Also included in this table are the data dues vary little. Thus Figure 3 simply states that the dependfor one run in which methane was added to the inlet hydrogen. ence of the desulfurization rate on hydrogen partial pressure is quantitatively very similar to that of methanization of char. The two reactions are independent, however, such that desulfurization can be achieved even when methanization is suppressed. MAXIMUM POSSIBLE SULFUR CONTENT If one considers the gasification time required to reach a given sulfur content, rather than the per cent gasification, the situation 3 is more complicated. On this bask an examination of the data (Table 11)shows that a t atmospheric pressure a hydrogen-steam ratio in the neighborhood of 1.0 corresponds to a minimum reaction time. At higher pressures, the gasification time is not a strong function of the gas composition, a t least over the range of mole fractions of hydrogen from 0.3 to 1.0 where no minimum is apparent. Further examination of the data on the reduction of the sulfide sulfur content as opposed to the total sulfur content reveals that this type of sulfur is much more readily removed than organic sulfur, The efficiency of removal is in almost all cases better than 70% and does not vary in a regular manner either with bed weight or gas composition. The sulfide sulfur is undoubtedly present largely as ferrous sulfide and is apparently removed about equally well by either steam or hydrogen by the Figure 2. Sulfur Contents at Zero Bed reactions (1), Weight as Functions of Per Cent Carbon Table 111.
Pure Hydrogen Desulfurization Data
Gasified, Pressure, and Inlet Gas Cornposition
The relationship between the sulfur content of the bed residue and the per cent carbon gasified for the pure hydrogen series-is shown in Figure 3.
FeS
+ H20= FeO + H?S
2.4
1
I
Kiwm'
F.
= PH's = 0.014
I
MAXIMUM POSSIBLE
Desulfurization Rate of Char Depends on Particle Size Carbon Gasified, Presence of Hydrogen Sulfide, and Bed Depth
The effect of the independent variables on the sulfur content of the bed residues a t zero bed weight is evident from Figure 2. The dotted curve represents the maximum sulfur content that the char would have had if there had been no sulfur removal a t all. It is obvious from Figure 2 that, a t a given per cent carbon gasified, the sulfur content of the char residue is lower the higher the hydrogen-steam ratio a t a given pressure and is lower the higher the pressure a t a given hydrogen-steam ratio. The effect of particle size can be noted from the results obtained in the desulfurization of the '/*-inch briquets. The reduction in sulfur content is substantially the same as in the case of the 65- to 100-mesh char. The sulfur content of the peripheral material was actually higher (0.53%) than the core material (0.36%). This may be due to preferential diffusion of hydrogen to the center of the briquet. Comparison of Figures 2 and 3 shows that pure hydrogen is a more effective desulfurization agent a t a given per cent carbon gasified than hydrogen-steam mixtures. January 1954
ow 2
e
PER CENT CARBON GAS1 FI ED
Figure 3. Variation of Residue Char SUIfur Content with Per Cent Carbon Gasified for Pure Hydrogen Runs
INDUSTRIAL AND ENGINEERING CHEMISTRY
ENGINEERING AND PROCESS DEVELOPMENT d
(
V
)
X= d(W_W) where S , and W,, and S and ?I‘are the weights of sulfur and carbon a t zero gasifying time and a t time t, respectively. It can be readily shown that the ratio of the specific rate of desulfurization l/S d~ dS to the specific rate of gasification l / W dW is given by (2) *
I I I I t 0.I ‘PER CENT HYDROGEN SEFIDE IN EFftUENT GAS 0’4
4L
Figure 4.
Ratio of Gasification and Desulfurization Rates to Process Variables Per cent carbon gasified as parameter
The lack of variation with bed height indicates that hydrogen sulfide is not a strong inhibitor for the above reactions as long as the hydrogen sulfide concentration is well below the equilibrium values given above. On the other hand, both the variation with bed height and the effect of hydrogen sulfide addition indicate that hydrogen sulfide is a strong inhibitor for removal of the organic sulfur. The hydrogen sulfide runs were carried out with the addition of 0.5% hydrogen sulfide to a 10% hydrogen-90% steam mixture. The data for these runs and the corresponding runs carried out in the absence of hydrogen sulfide addition are included in Table 11. The organic sulfur content in the presence of large amounts of hydrogen sulfide, in the hydrogen sulfide addition runs, increases beyond the maximum possible value. It is not clear how this increase in organic sulfur comes about, but possibly active carbon centers are formed during the gasification of carbon with steam that are capable of fixing sulfur from hydrogen sulfide. The 0.5% hydrogen sulfide addition is large enough to reverse the reaction between ferrous sulfide and hydrogen to form metallic iron and hydrogen sulfide but not sufficient to reverse the reaction between ferrous sulfide and steam. This is reflected in a net increase in sulfide sulfur in the char residue as compared with the corresponding run without hydrogen sulfide addition, but a net decrease in sulfide sulfur as compared with the feed char. A semiquantitative picture of the magnitude of hydrogen sulfide inhibition may be obtained from an analysis of the relative rates of sulfur removal and carbon gasification. Point data from which the instantaneous rate of sulfur removal could be obtained are unfortunately not available because of the inability to analyze precisely for hydrogen sulfide in the exit gas. Approximate values for the sulfur removal rate were obtained by a graphical differentiation procedure under the assumption that all of the sulfur is eliminated as hydrogen sulfide. The slope X of a curve relating the per cent of the original sulfur removed from the char to the per cent carbon gasified was obtained by graphical differentiation.
where B is the fraction of the carbon gasified. The specific carbon gasification rate had previously been measured (3)which, therefore, permitted the specificdesulfurization rate to be calculated from Equation 2 and consequently, by means of a point sulfur balance, the hydrogen sulfide concentration in the effluent gas. Some typical curves relating the ratio of the gasification and desulfurization rates to the independent variables are given in Figure 4. The points shown a t zero hydroge sulfide concentration are the ratio of the differential rates of sulfur removal to carbon gasification. The other values are the ratio of the integral rates a t the given values for the outlet hydrogen sulfide concentration, per cent carbon gasified, and inlet hydrogen-steam ratio, The ratios of the integral rates shown are given as functions of inlet hydrogen-steam ratio, even though, as a result of the carbon gasification reaction, there is some additional hydrogen produced that results in a higher hydrogen-steam ratio a t the outlet from the bed. The inhibiting effect of hydrogen sulfide thus shown is probably actually a minimum value and is in actuality somewhat higher. It is also subject to accuracy limitations inherent in the use of integral rather than differential data. The inhibiting effect of hydrogen sulfide is m a r k e d 4 . e , an outlet hydrogen sulfide concentration of the order of 0.2% causes a decrease in the relative desulfurization rate of the order of 10 to 40% depending on the conditions employed. The inhibiting effect decreases as the pressure is increased. The relative desulfurization rate in general decreases strongly with increasing per cent carbon gasified. literature Cited
Britzke, E. V., and Kapustinsky $. F., 2. anorg. Chem., 194,323 (1930).
Goring, G. E., Curran, G. P., Tarbox, R. P., and Gorin, E., IND. ENC.CHEM.,44, 1051-7 (1952). Ibid., p. 1057-65. Gurarii, Yu S., Ukrain. Khem. Zhur., 6, No. 2,49-83 (1931). Lowry, H. H., “Chemistry of Coal Utilization,” Val. I, pp. 4489, New York, John M‘iley and Sons, Inc., 1945. Mirev, D., Annuaire unizl. Sofia, 11, Fae. phys. math. Livre 2, 32, 315-79 (1936).
Monkhouse, A. C., and Cobb, J. W., Gas J., 158,828-33 (1922). Pexton, S., and Cobb, J. W., Ibid., 163, 160-73 (1923). Powell, A. R., J . Am. Chem. Soe., 45, 1-15 (1923). U. S. Steel Co., Handbook, “Methods of Analysis of Coal Coke and By-products,” pp. 77-84, 1929. Wibaut, J. P., Proc. 3rd Intern. Conf. Bituminous Coal, 1, 657-73 (1931). ACCEPTED September 24, 1953. RECEIVED for review May 6, 1953. Presented before the Division of Gas and Fuel Chemistry, AMERICAN CHEMICAL SOCIETY, Pittsburgh, Pa., April 1953.
END OF ENGINEERING AND PROCESS DEVELOPMENT SECTION
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