CATALYTIC REACTIONS OF
Carbon with Steam-Oxygen Mixtures ALFRED W. FLEER1 AND ALFRED H. WHITE University of Michigan, Ann Arbor, Mich.
Powdered coke (20 to 40 mesh) treate with 5 per cent of sodium carbonate as catalyst was gasified with steam-oxygen mixtures while falling freely through a steel reaction tube, 2 inches in diameter and 8 feet long, heated to 900-1000" C. (1650-1830' F.). The rate of steam decomposition was independent of oxygen concentration and proceeded a t a rate which empirically followed the law of a first-order reaction. The reaction rate doubled for each 15" C. (135" F.)interval, and the energy of activation was computed to be 26,800 calories per gram mole. A t 1000" C. practically complete decomposition of steam was obtained with a contact time of approximately 5 seconds. HzO = Catalysis of the reaction, CO COZ Hg,occurred to such an extent t h a t equilibrium conditions were operative which controlled the carbon dioxide content of the gas produced and set the limits for over-all decomposition of the inlet mixtures a t a given percentage of steam decomposition. Experiments were also made with a fuel bed 4 feet deep, using catalyst-treated coke particles 0.15 to 0.5 inch in size. The effect of oxygen in these runs indicated t h a t the rate of heat transfer rather than the rate of reaction limited the extent of decomposition of the reactants. The steady-state grate temperatures attained in either case were proportional to the oxygen content of the inlet mixture and the total inlet rate t o the 0.16 power.
+
*
+
Thirty-fourth holder of the Michigan G a s Association Fellowship i n Gas Engineering, 1933 to 1935. Present address, Shell Petroleum Corporation, St. Louis, Mo.
COXTINUOUS GASIFICATION APPARATUS
T
HE desirability of carrying out the gasification of fuels in the presence of a catalyst such as sodium carbonate has been well established. Taylor and Seville (18) in 1921 published the first data on the catalysis of the reactions of carbon with steam and carbon dioxide a t 490" to 570" C. (914" to 1058" F.). They found that the carbonates of sodium and potassium were the only efficient catalysts in these reactions and that a maximum effect was obtained with about 20 per cent by weight of the catalyst in either case. Vsing steam a t these temperatures, they obtained gases which were very low in carbon monoxide. Beginning in 1926, Cobb and colleagues (2,7-11,13,15,17)published a series of papers dealing with the reactions of carbon with steam, carbon dioxide, and oxygen in the presence of various catalysts. They investigated the effect of catalysts with varying temperatures, times of contact, and concentrations of catalyst, and obtained data which clearly demonstrated the superiority of sodium carbonate as a catalyst. When oxygen was the reacting gas, the combustion rate was controlled by diffusion, and the effect of the catalyst was to lower the maximum temperature attained 1301
INDUSTRIAL AND ENGINEERING CHEMISTRY
1302
by about 100" C. (180" F.) and to effect an increase in the carbon monoxide content of the gas produced. I n 1931 Fox and White (12) published the results of experiments designed t o show the mechanism of the catalytic action of sodium carbonate on the gasification of carbon. They determined that sodium carbonate reacted with carbon a t temperatures above 800" C. (1472' F.) to give sodium vapor according to the equation: Na2C03
+ 2C = 3CO + 2Na
(1) and that the reaction pressure a t 1025' C. (1877" F.) exceeded the atmospheric. They explained the catalytic act,ion of sodium carbonate as due t o the rupturing of the condensed film of carbon monomide by the sodium vapor and its transporting of the reaction to the gas phase where the reacting gases did not have t o diffuse through a gas film against adverse flow. They postulated that reaction 1 and the following reactions took place t o regenerate the catalyst and begin the cycle anew: 2Na COz = NasO CO (2) rag0 Cog = NagCOs (3) Weiss and White (21) investigated the reversal of reaction 2 and showed that it was quantitative in the range 750' to 900" C . (1382" to 1652' F.). They also studied the effect of sodium carbonate on the gasification of graphite with dry air and steam-air mixtures as used in producer gas operations and found that, in general, sodium carbonate lowered the temperature required for comparable decomposition by about 150" C. (270" F.). The desire t o carry out the gasification of fuels in a continuous manner has led t o numerous proposals advocating the use of powdered fuels in water gas manufacture. Little success has attended the efforts made in this direction with the exception of the Heller process in Germany. I n this process (4,19) the entire heat for the steam-carbon reactions is transferred through the walls of a refractory reaction chamber. The use of oxygen along with steam for continuous gasification was proposed as early as 1920 by Cobb and Hodsman (6) and later by Jefferies (Id), Willien (22), and others. The first experimental work in this regard was made by Vandaveer and Parr (20) using solid fuel beds. Their results, however, as well as those of others were not wholly favorable, and there has been a lag of interest since that time. There is no record of a n attempt to carry out the continuous gasification of powdered fuel using steam-oxygen mixtures and employing a catalyst such as sodium carbonate. The present paper presents the results of an investigation made to determine the effect of contact time, temperature, and oxygen concentration, t o study the gasification reactions involved, and t o establish the limiting conditions pertinent in the gasification of carbon with steam-oxygen mixtures.
++
+
Apparatus The a paratus used in this investigation consisted essentially of a K 1 - 2 steel reaction tube, 2 inches in diameter and 8 feet long, which was freely suspended within an electrically heated furnace consisting of three separate electrical circuits. Each of the circuits was thermostatically controlled by means of an alumelchrome1 thermocouple which was in contact with the external wall of the reaction tube at a point corresponding t o the mid-position of the furnace circuit. Four other thermocouples were placed on the reaction tube so that the distance between adjacent thermocouples was 16 inches. Steam was supplied to the reaction tube from a main carrying 35 pounds per square inch pressure which was held a t constant pressure by means of a vent system. It was first superheated to about 300-400°C. (570750" F.), and its rate controlled by means of a convergent-divergent type of steam nozzle. The oxygen flowing from a cylinder was controlled by a needle valve and was then saturated with water vapor and measured in a wet test meter. The oxygen was introduced into the feed line a t the outlet of the steam nozzle, and the steam-oxygen mixture, preheated to 175-200" C. (347-392' F.)
VOL. 28, NO. 11
entered the lower end of the reaction tube. The bottom of the reaction t'ube was filled with pieces of chrome brick to within 1.5 inches of the lower thermocouple. A 2-inch layer of 0.75 X 0.5 inch coke was placed on top of the chrome brick at the beginning of the run. Pulverized coke, screened to 20-40 mesh, was introduced at the top of the reaction furnace and allowed to fall countercurrent to the rising gas stream. Feeding was accomplished by means of a screw-type feeder, which was driven by a variablespeed d. c. motor t,hrough a variable-speed transmission system. The feed was introduced through a tube 1 inch in diameter, projecting 1 foot into the reaction furnace. The feed tube was provided with scraper arms which were rotated by an auxiliary motor to prevent plugging of t'he feed lines. The entire feeder assembly was surrounded by an electric oven kept at 125' C. to prevent condensation of steam in the feeder and hopper with the subsequent cessation of feeding. The feed hopper was also provided with rabble arms to stir the powdered fuel in order to prevent bridging. The gases were removed from the top of the reaction tube and in turn passed through condensers, filters, and scrubbers. The gas was then metered, sampled, and stored in a 25-cubic-foot holder or vented to a flue. The condensate from the condensers was weighed and also analyzed for sodium carbonate.
Accuracy of Results The temperature controllers maintained the temperature within 5' C. (9'F.) at the three control points located on the external wall of the nietal reaction tube. Because of the linear expansion of the reaction tube it was necessary t o ''cramp" the windings of the electric furnace in the region where the thermocouple protection tubes projected through the alundum core. This cramping resulted in local hot bands, above and below each thermocouple position which, when measured under static conditions by means of a radiation pyrometer, showed a t some points a maximum deviation of about 100" C. (180' F.). The need of providing a centrally located scraper arm in the feed tube precluded the possibility of measuring the internal temperature while the system was "in flow." However, in a preliminary experiment (not reported here) a centrally located traveling thermocouple was used while the system was in flow. In this case, when operating with the control thermocouples a t 850" C. (1562' F.), the maximum temperature recorded by the traveling thermocouple while being mechanically moved through the reaction zone was 862" C. (1583.6" F.), and the minimum temperature 838' C. (1540.4' F.). Considering the length of reaction tube and the small area of the hot bands, the temperature control may be considered precise and the furnace characteristics good under flow conditions. The steam flow from the control nozzle was calibrated under definite conditions of temperature and pressure previous to a run by weighing the condensate obtained over a definite time interval. This rate was then corrected for temperature and pressure variations during the run and was accurate within 1 per cent. Although a water balance could he obtained within 5 per cent and the over-all accuracy of the oxygen meter was well within 3 per cent, it was impossible to obtain a material balance for oxygen based on the gases made. For this reason the amount of net free oxygen introduced during the runs was calculated as the difference between the total quantity of oxygen in the final gas and the quantity equivalent t o that obtained from the decomposition of steam, as determined by a hydrogen balance. It was obvious that the oxygen loss, averaging about 25 per cent of the free inlet oxygen fed, was not due t o errors in metering or analysis. It was definitely indicated, and there is much evidence ( 3 ) t o support the contention that part of the oxygen reacted with the fuel t o form solid oxides of carbon. The remainder of the oxygen unaccounted for was due either to the reaction of oxygen or steam or both, with the metal reaction tube. The rate of fuel fed during the run was obtained from readings of the revolution counter on the feeder screw and was
NOVEMBER, 1936
INDUSTRIAL AND ENGINEERING CHEMISTRY
1303
checked against the initial and final weighings of the feed hopper. The maximum variation of fuel delivery per revolution was about 7 per cent from the mean. The rate of gas flow was determined from the readings of a l a r g e A m e r i c a n type of bellows meter and was accurate within 3 per cent.
c
5 g1O
w
d
m
N
i
0
Range of Investigation The powdered fuel runs were made on a coke sample ranging from 20 to 40 mesh in size. S e v e r a l s e r i e s of runs were made on a deep coke bed for c o m p a r i s o n . The fuel size in this case was 0.5 to 0.75 inch, and a 4-fOOt bed was used in all cases, The concentration of sodium carbonate in the powdered coke runs was about 5 per cent in all cases, the actual variation being from 4.53 to 5.01 per cent. During the runs on a deep fuel bed,sodium carbonate concentrations of 3.45 and 5.87 per cent were used in separate series of runs. The temperatures s t u d i e d in this work wereQOO",950", and 1000" C. (1652", 1742O, and 1832°F.). Reaction rates below 900" C. were too slow to be worthy of attention; a t temperatures above 1000° C. there was extreme danger of burning out the electrical circuits of the furnace and of forming a thick scale on the reaction tube. The contact time was calcul a t e d f r o m a gas volume taken as the average of the net total inlet rate and the wet gas outlet rate, and a tube volume based upon a 7-foot tube length. It varied from 2 t o 7 seconds. During each series of e x p e r i m e n t s runs were usually made a t the net free inlet oxygen concentration of 40, 20, and 0 per cent. An oxygen concentration of about 45 per cent is the thermal optimum, as will be subsequently shown. Another variable was the molal ratio of the carbon and gas reactants. Most of the experimental runs were made a t an equimolal ratio where the rate of carbon fed in moles was equal to the number of moles of steam plus twice the number of moles of oxygen
INDUSTRIAL ASD ENGIKEERISG CHEMISTRY
1304
introduced during the same period. A series of runs a t fourtenths of the equimolal ratio were inadvertently made because of an alteration in the fuel feeding rate and are included in the tabulated data. The experimental and calculated data are shown in Table I.
Thermally Optimum Oxygen Concentration The thermally optimum concentration of oxygen in the steam-oxygen mixture is that amount which will react with carbon to form carbon monoxide and furnish just sufficient heat to balance the endothermic reaction of steam with carbon to form carbon monoxide and hydrogen and also supply the sensible heat required to raise the reactants to reaction temperature. This concentration was calculated by means of a heat balance, using the latest specific heat data and the specific heat equation of the form suggested by Chipman and Fontana (5), employing concordant values for heats of reaction computed by indirect methods. The data and set-up of the heat balance follow. For the reaction :
AH* =
c+ -28,763 -
'/2
0 2
6.54T
=
co
(4)
+ 0.00050T2 + 264T1/'2
For the reaction:
+
+
C H20 (g.) = CO Hz AHt = $28,369 3.23 T - 0.00021 T Z C p of 0 2 = 9.32 0.00036 T - 44 T-li2 C p of HIO 7.11 0.00264 T C p of C (beta graphite) = 9.62 - 0.000442'
+ +
-
(5)
+ 250 Tij2
- 130T-'/i
Then:
[
SmJ;;"
7.11 dT
+ 0.00264TdT
9.32 dT
1
j-
+ 0.00036 TdT
-
44 T-'/'¶dT
+ +
react r On loo = vol. or mole per cent net free oxygen om
Effect of Carbon Accumulations During any run the carbon which failed t o react as it fell through the tube accumulated a t the bottom above the supporting layer of chrome brick. It may be queried whether the steam-oxygen mixture reacted with the powdered coke in the suspended condition or with the coke which had fallen to the bottom of the reaction tube. Observation of the accumulated concentrated ash-coke product together with the ash-sodium carbonate-sodium oxide fusion product showed that glomerates were formed of indefinite size, distribution, and si Face conditions, and of unknown reactivity. Evidently t- A, a direct comparison with results obtained using a powdered coke bed of comparable depth would have been of no significance in determining the effect of the accumulated mass. Assuming the accumulated particles to retain their original size and distribution, it could be computed that contact times would be of the order of 0.02 to 0.10 second. Although it is evident that no great effect on decomposition could be expected with such relatively short contact times, this calculation is of doubtful value for determining the effect of coke accumulation in the powdered fuel runs. Therefore, in order t o show indirectly that the effect of the coke accumulated in the bottom of the tube was negligible, two runs of longer duration (50 and 30 minutes) were made a t 950" and 900" C., respectively. Gas analyses were made on samples during the first and second halves of the runs, as well as on a sample collected during the whole period. The original data on cumulative gas and oxygen flow with respect to time are plotted for run 20, lasting 50minutes at 950" C. (Figure l), and for run 30, lasting 30 minutes a t 900" C. (Figure 1). The data on steam decomposition, carbon accumulations, and change of fuel bed depth are given in Table 11. TABLE
11. DATAON EFFECTO F
+ snl
in steam-oxygen mixture Thesteam and oxygenwereusually preheated to 175-200°C. (347-392" F.); if this preheat temperature is assumed to be 200" C. (473" K.), if the reaction temperature is taken as 900" C. (1173" K.), and if the heat loss term is neglected (since in the experimental work the electrical furnace circuits supplied the radiant heat loss), we may integrate and calculate the thermally optimum oxygen concentration to be 45.1 per cent. Similarly the value of the thermally optimum concentration a t a reaction temperature of 950" C. is 45.6 per cent; a t 1000" C. the concentration is 46.0 per cent. I n both the powdered fuel runs and the deep coke bed runs, the carbon fuel was already at reaction temperature by heat supplied from the furnace; therefore, the item representing
CARBON
R u n 20, 50 min., 950' C. 10725 25750 10-50 min. min. min. av.
+
Where S, = steam, gram moles 0, = oxygen, gram moles T , = temp. of inlet steam, ' K. To = temp. of inlet oxygen, ' K. TR = temp. of reaction, K Q = heat loss from furnace at reaction temp. during the same period that S , gram moles of steam
x, =
the sensible heat required for raising this reactant to temperature was omitted from the left-hand side of the heat balance equation.
1++
Sm(28,369 - 3 . 2 3 T ~- 0.00021T~' 2 5 0 T ~ '2) Q = -2 0, (-28,763 - 6.54 T R 0.00050 TR* 264 T R ' I ~ )
VOL. 28, NO. 11
ACCUMULATION
R u n 30 30 min., 900'0 5-16 16r27 5-27 min. min. min. a v .
c.
Dry gas analysis. %: ~~
con
38.2 0.3 0.1 32.5 co 27.3 H¶ 0.5 CHk 1.2 Nn Steam decomposed, % 44.6 At Start of R u n 43 Carbon accumulation, grams 2 Fuel bed depth, inches
CnHm 0 2
39.8 0.4 0.2 30.1 26.3 0.7 2.5 42.5
38.5 0.4 0.1 31.6 27.1 0.6 1.7 44.4 At E n d of R u n 319 9
33.0 35.1 31.2 0.7 0.6 0.6 0.4 0.4 0.2 3 5 . 2 32.7 30.7 29.1 2 8 . 8 29.0 0.5 0.5 0.6 2.9 4.0 3.8 47.5 49.2 48.7 At Start At End of R u n of Run 43 334 2 9.5
Figure 1shows that, after an initial period of 4 t o 6 minutes, there was practically no change in the slope of the lines of gas make us. time, which indicates that constant or steady-state conditions had been obtained. Since the steam flow, as well as the oxygen flow, was held constant during each run, the gas make was an indication of the over-all decomposition obtained. Since the amount of coke accumulated on the fuel bed increased over four times during each run, it must be concluded that the over-all decomposition or gasification was affected only negligibly by carbon accumulation, and that the principal reaction was with the coke particles while they were falling freely through the reaction tube. The maximum coke accumulation in any series of runs gave a measured fuel bed depth of 16 inches; the average was only
NOVEMBER, 1936
INDUSTRIAL AKD ENGINEERING CHEMISTRY
10 inches, similar to the 9- and 9.5-inch depths obtained during the special test runs already reported. The slight change in the slope of the gas make lis. time curves for these two runs is equivalent t o a lag of about 1 or 2 minutes in the case of the 30- and 50-minute runs, respectively, or an error in overall decomposition of about 5 per cent in either case, which may be ascribed to both carbon accumulation and time lag required to attain steady-state conditions. An idea as to the actual magnitude of the effect of coke accumulation on steam decomposition may be obtained from Table 11, where gas samples were obtained after steady-state conditions had been reached. Computations based on the steam decomposition a t the average time for the t-vvo sample periods in either run and the coke accumulation a t this average time shorn that, for a fourfold increase in coke accumulation, a 7 per cent increase in steam decomposition is expected. Since large changes in coke accumulation effected only slight changes in both steam decomposition and over-all decomposition, it is logical to assume that the oxygen-steam mixtures reacted to an overwhelming extent with the fuel while it was in suspension. Further evidence t o support this contention is obtained from Figure 1 and Table I, column 7 . Under the initial conditions of flow, comparable decompositions were obtained with the 20-40 mesh coke in suspension, plus a few inches of 0.5-0.75 inch coke, t o those with a coke bed 0.5-0.75 inch in size and 4 feet deep. This argument is conclusive since, under the initial conditions in the powdered fuel runs, decomposition was nearly as high as a t any time during the run. The evidence that reaction occurred while the fuel was in the suspended condition leads to the assumption that the surface of the powdered coke particles must have attained the approximate reaction temperature of the tube wall during part of the time while in suspension, owing to radiant heat transfer from the furnace walls. This could only obtain when the rate of feeding fuel was not so great that individual particles were completely obscured. The actual time of contact of the reacting gases with the fuel after the particles had reached full reaction temperature could not be determined, but the contact times calculated on the basis of the total tube volume are nearly proportional to the contact time a t reaction temperatures. It is also practically impossible to determine the actual time required for particles to fall through the reaction tube under the experimental conditions without first determining empirical constants. With the upward velocity of the gas varying between 1 and 8 feet per second, the total time of fall is estimated a t from 0.5 to 2 seconds for the 7-foot reaction tube, based upon the effect of gravity as compared t o the buoyant force of the rising gas stream. The contact times for the powdered fuel and deep coke bed runs as calculated have no comparable significance, because of the inability to calculate the time at reaction temperature in the case of the powdered fuel runs. I n addition, the calculated contact times for the deep fuel bed runs are based on the volume of the empty tube.
Over-All Decomposition of Steam-Oxygen Mixture in Powdered Fuel Runs When gasification of carbon is carried out by using a steamoxygen mixture, the over-all decomposition of the combined reactants is a better criterion of gasification efficiency than is steam decomposition, since it is a measure of the completeness of reduction of the combined inlet components to carbon monoxide and hydrogen. This is conveniently calculated as the ratio of total wet gas outlet rate to total net inlet rate, minus one, times 100. I n Figure 2 is plotted the over-all decomposition us. contact time. The comparative displacement and slope of the lines
1305
at goo", 950", and 1000" C. gi\-e an indication of the effect of temperature and time on the combined decomposition reactions. The 900" C. isotherm as plotted is not curved to fit the experimental point for run 19 a t a contact time of 2.38 seconds. This run gave anomalous results, both with regard to a higher steam decomposition and a lower concentration of carbon dioxide, than would be expected, with a resultant appreciable effect on over-all decomposition. Since this run was the first of the powdered coke series, the authors feel justified in giving this point less weight in drawing the isotherm. Also, this correlation was made on a basis of contact time only without taking into account the variations in the ratio of oxygen to steam in the inlet mixture. If the time required to reach the 50 per cent over-all decomposition point is computed a t the three temperatures and the time a t 1000" C. is considered as unity, the ratio of the times a t 950" and 900" C. are, respectively, 1.7 and 2.2. Therefore, from either ratio it may be computed that the temperature interval required to double the over-all decomposition rate is about 87.5" C. in this temperature range, as foll0w.s: 950"
.7 c.:11.o x
50 = 85" c.
900"
.0 c.:22x .2
100 = 90"
c.
If the temperature interval required to double the rate were calculated a t a higher decomposition point, values less than 87.5" C. would be obtained, indicating that the courses of the reactions occurring a t all temperatures are not the same. If the ratio of time required to attain 75 per cent over-all decomposition t o the time required t o attain 50 per cent decomposition is calculated a t the three t e m p e r a tures, the results are 1.68, 1.45, and 1.25, respectively, for temperatures of 1000", 950 O , and 900" C. As will b e s h o w n later, since the reduction of steam is a react i o n w h i c h conforms to the laws of a first-order reaction, in that the time required to reach 75 per cent decomposition is twice the time required for 50 per c e n t decomposition, it may be c o n c l u d e d that the reduction of carbon d i o x i d e , though similar in nature, is a reaction whose rate is c o n t r 011 e d b y other factors, such as an approach t o FIGUREl . EFFECTOF CARBONiicequilibrium condiCU?dUL%TION IN R U N S 20 (ABOVE) .IND tions. 30 (BELOW)
INDUSTRIAL AND ENGIKEERING CHEMISTRY
1306
Decomposition of Steam in Powdered Fuel Runs Although in the present work the efficiency of gasification is best measured in terms of over-all decomposition, the decomposition of steam plays an important role in determining the final gas quality and establishing the decomposition limits necessary when using sodium carbonate as a catalyst. The study of the data also throws light on the mechanism of the reactions. Steam decomposition, as calculated, represents the primary reduction of steam and also any secondary reaction with carbon monoxide. Steam decomposition may be correlated on the basis of contact time only, not taking into account the changes in oxygen concentration in the inlet mixture. This is not meant to imply that the presence of oxygen is without effect upon the gasification reactions, but rather that, when using powdered fuel in approximately equimolal ratio t o the reacting gases, the particles do not obscure each other sufficiently a t the rate of feeding used in these tests to prevent the transfer of radiant heat from the furnace walls to an extent comparable t o the heat requirement based purely upon chemical reaction rates.
Thus it is apparent why the presence of oxygen in the gases has a negligible effect on percentage of steam decomposition as long as the rate of reduction is limited by the speed of chemical reaction and not by the rate of heat transfer. The decomposition as here represented includes decomposition also according to the reaction: CO
d- M = dt
t = contact time k = reaction rate constant Y = M/Mo
The integrated expression of this equation is -In (Mo - M ) = kt + I since a t t = 0, M = 0; then I from which follows :
=
where (I
- y)
=
2.303 log (1 - y) = -kt Fraction of steam undecomposed
If the experimental data on fraction of steam undecomposed is plotted against contact time on a semi-log scale (Figure 3), straight lines may be drawn emanating from a common origin [zero seconds contact time and (1 - y) = 1.01 which represent the data at the three temperatures very well. The values of +L as determined from the plot are as follows : 900" C. : +k = 0.174 950" C. : +k = 0.247 1000° C. : +k = 0.430
Thus, the reaction rate of steam decomposition, principally according to reaction 5 , empirically obeys the law for a firstorder reaction. Also the plots show that the time required to attain 75 per cent decomposition, a t any temperature, is twice that required t o attain 50 per cent decomposition, or the time required t o decompose a given fraction of the remaining molecules of steam is always the same. 2 This is used in preference t o the more exact equation of Benton for a pure reactant undergoing a given volume change on reaction. With t h e other gases undergoing volume changes as is the case here, t h e calculation of cont a c t time is more readily made on the basis of t h e average of the inlet a n d outlet rates.
+ Hz
(6)
spectively, make it possible to calculate the apparent energy of activation, A , by means of the Arrhenius formula, dlnk dT
A RT2
the integrated form of which is k A 1 2.303 log 2 kl = R (E-?k)
Using the three sets of values for k and T shown in Figure 3, the average value of the energy of activation, A , is computed to be 26,800 calories per gram mole in the range 900' to 1000' C. From this value of activation energy the temperature interval required t o double the reaction rate of steam decomposition is calculated to be 75' C. in this same temperature range.
Mechanism of Reaction
-In M ,
which by definition may be written in the form:
COz
Apparent Energy of Activation The values of k obtained a t goo", 950°, and 1000" C., re-
k (Mo - M )
where M = moles steam decomposed M , = original moles of steam
+ Hz0
However, the tabulated data show that, because of an approach to equilibrium conditions in reaction 6, the net extent of this reaction in the forward direction when steam alone is reacting will not affect more than about 5 to 15 per cent of the total steam decomposed. Moreover, when oxygen is present, the net extent of steam decomposition according t o reaction 6 may possibly be negligible.
Apparent Order of Reaction The reaction rate for a first-order reaction according to Benton (1)may be represented by the equation?
VOL. 28, NO. 11
Benton ( 1 ) derived an equation for a reaction which is of first order owing to catalysis of the reaction by a solid surface, where the velocity is determined by the rate of collision of activated molecules with the surface, or which is of fist order owing to transformation of adsorbed molecules, for the case of slight adsorption. Although the rate equation may be formally integrated, Benton suggests the use of the approximate equation:
where N A = reactant A , moles
t = contact time (av. of inlet and outlet rates), sec. = area of catalyst surface Ca = av. concentration of reactant A k = reaction rate constant
-8
Using the same data presented in Figure 3 and plotting the fraction of steam decomposed times the steam rate per unit contact time vs. the average concentration of steam, it was found that no correlation could be made. This indicated that the rate of steam decomposition was practically independent of the concentration of steam and that the mechanism of the catalysis was not merely a surface effect, even assuming the reactant t o be weakly adsorbed. Fox and White (12) postulated t h a t the catalytic action of the sodium carbonate catalyst at temperatures above 800" c. (1472' F.) is due t o the formation of sodium vapor which then reacts according t o the equation: 2Na
+ HZO = NazO + HZ
(7)
NOVEMBER, 1936
INDUSTRIAL AND ENGINEERISG
1307
CHEMISTRY-
From top t o bottom, left
FIGURE 2. OVER-ALLDECOMPOSITIOS OF INLET GASESUS. COXT.4CT TIMEFOR POWDERED FUEL Rms FIGURE 3. SEMI-LOGPLOT O F FRACTION OF STEAM UNDECOMPOSED US. CONTACT TInm FOR
POWDERED FUELRUXS
FIGURE 4. CARBON DIOXIDE19 DRYGAS us. CONTACT ' r I M E FOR POWDERED FUELRTJNS
A
F r o m top to bottom, iaght
FIGURE 5.
EQUILIBRICM
COSSTCNT US
TEM-
MONOXIDE
PERATURE FOR STEAM-CARBON RE~ C T I O N AND VALUES OF EQUILIBRIUM R ~ T I FROM DATAO F W E I S S AND W H I T E (21) FOR CATALYZED AND UNCAT~LYZED RUNS
O
FIGURE 6. CARBOX DIOXIDEIN D R Y Gas US STE&M DECOMPOSED FOR POWDERED FUELRUNS ASD DEEP COKEBED RUN^ FIGURE7 . STEADY-ST.4TE GRATE T E M P E R & TURE US. EQTSIVALENT M O L E F R 4 C T I O S O F 0x1QEN I?; INLET Upper curve, b ( 3 h d coke bed runs with steam and oxygen preheated t o run temperature. lower curve, powdered fuel runs with steam and oxygkn preheated to temperature of l o u e r end of furnace
In the present experiments it has been noted that with a lowered fuel-catalyst feeding rate the extent of steam deconiposition decreases for the sarnr contact time. Thus the extent of pseudo-monomolecular reaction 7 seems to be dependent on the effective concentration of sodium vapor; it is probably a chain reaction. the .lowest reaction in the chain being monomolecular
Effect of Steam-Carbon Monoxide Reaction The present experimental work shoved that carbon dioxide concentration, although high initially, decreased rapidly during the first few seconds of contact time, but then decreased slo~vlywith longer contact times as shown in Figure 4 for the three isotherms of 1000", 950". and 900" C. It is impossible to explain this on the basis of the chemical rate of reduction of carbon dioxide, since the initial concentrations of carbon dioxide and carbon monoxide are unknown when oxygen is also used as a reactant. The relatire concentrations of car-
bon inoiloxide and carbon dioxide in this case depend upon the relative extent of the following reactions:
c + ll202 = co c + 0 2 = co, co + 1/*02 = co, The mechanism of the first two reactions according to Rhead and Wheeler (16) is:
c, + 0,
=
c,o,
C,O, = uCO
+ bCOp
The physico-chemical complex of oxygen and carbon which is initially formed decomposes to give carbon monoxide and carbon dioxide in ratios depending on the temperature. Furthermore, after an initial concentration of carbon dioxide is built up, reactions are occurring simultaneously, both to decrease and increase the carbon dioxide concentration as follows :
IKDUSTRIAL AND ESGINEERING CHERIISTRI-
1308
VOL. 28. NO. 11
Catalysis of Steam-Carbon Monoxide Reaction The experimental results obtained are explainable on the basis of the effective approach to equilibrium conditions in reaction 6. This was confirmed experimentally in the present work while gasification a a s carried out in the presence of the sodium carbonate catalyst. The relation between carbon dioxide concentration and the extent of steam decomposition is shown in Figure 6, which includes the data on the deep fuel bed runs as well as the powdered fuel runs. This plot indicates that there must be high steam decomposition t o lower the carbon dioxide content enough to produce a gas of high quality. It is particularly significant that this approximate correlation is independent of temperature, contact time, or oxygen concentration in the inlet gases. The expression for the equilibrium constant in reaction 6 may be formulated as:
Where p p POWDERED FUELFEEDING S Y ~ T EWITH M OVES REMOVED COa
+ c = 2co
(12) CO Hz0 = COz Hz (6) If it may be assumed that reaction 6 is operative to such an extent that equilibrium is approached a t any given temperature, then the concentration of carbon dioxide a t any time will be determined by the extent of steam decomposition, the equilibrium constant at the temperature in question, and the initial oxygen concentration. The percentage over-all decomposition of the inlet gases, however, \\-ill be determined largely by the extent of steam decomposition and the equilibrium concentration, and is only slightly dependent upon the oxygen concentration as was pointed out in reviewing the experimental results. In order to illustrate the effect of over-all decomposition, equilibrium constants for reaction 6 may be selected at: 1.0and 0.5, corresponding to an extreme temperature variation of 820" and 1080" C. (1508" and 1976" F.), respectively (Figure 5 ) . From the value of the equilibrium constant of 1.0 it may be calculated that, as the steam decomposition goes from 50 to 75 per cent (a 50 per cent increase), the change in over-all decomposition varies from 34 to 60 per cent (an increase of 76 per cent) regardless of changes in the concentration of oxygen in the inlet mixture of 0 to 40 per cent. At the higher temperature where the equilibrium constant is 0.5, the change in over-all decomposition is as follows (in per cent): Oxygen in Inlet 0
+
Steam Decomposed 50 75
Ovei- 411
Decompn.
37.5 66.0
+
Increase
in Steam Decompn 50
Increase Over-All Decompn. 66
1x1
Figures 2 and 3 show that a t the higher isotherms (1000" C.) the ratio of times required to effect a 50 per cent increase in over-all decomposition is less than the ratio required to effect an increase of 50 per cent in steam decomposition; the comparable time ratios are 1.68and 2 , respectively. Under the conditions corresponding to the lower temperature (900 C.) the effect is even more marked; the ratio of times being 1.25 compared to 2 . In either case, the effect is practically independent of the oxygen concentration in the inlet mixture.
=
partial pressure
The calculation of this ratio for the experimental runs is shown in Table I, column 10. The value of the free energy for reaction 6 is given by the expression: AFR = -8130 - 8.58 T In
where T AF
Since
= =
T
+ 0.00107 T 2- 484 TI/%+ 80.93 2'
temperature, K. calories per gram mole log K =
- AF' 4.575T
+
then log K6 = 1777 T-1 f 1.875 1nT - 0.000234T 105.79T-'/2 - 17.694 From this equation, values of K are plotted against temperature in degrees Centigrade as shown in Figure 5 . Equilibrium values in the experimental range 900" to 1000" C. lie between 0.776 and 0.599; at 800" C. the equilibrium value is 1.068. For the most part the values of the equilibrium ratio obtained from the present experimental data lie near or within the range of the theoretical equilibrium values corresponding to the operating temperature. It is t o be expected that, as the reaction approaches the equilibrium value, the driving force is small so that only a t infinite time can equilibrium conditions be attained. The actual experimental ratio is therefore expected to lie above or below the equilibrium value corresponding to that temperature, depending upon from which side the equilibrium is being approached. Furthermore, in the present work the final gases were in contact with the sodium-carbonate-treated coke particles before they reached full reaction temperature. Confirmation of the effect of sodium carbonate in promoting this reaction is obtained when the data of others are examined. If the ratio (COz)(H2)/(CO)(H20)is calculated from the data of Weiss and White (21) for runs on untreated graphite and on graphite treated with sodium carbonate, the results shown in Figure 5 are obtained. With treated graphite the ratios lie near the equilibrium value for the corresponding temperature; with untreated graphite they are generally much lower. The data of hiarson and Cobb (16) and of Cobb and Sutcliffe (7, 17) also show that, for coke treated with catalysts, the ratios agree closely v i t h the calculated equilibrium values; with pure cokes the ratio is much lower than that corresponding to equilibrium.
Effect of Oxygen o n Gasification i n a Deep Coke Bed For the powdered fuel runs the presence of oxygen in various concentrations in the inlet mixture is without great effect on steam decomposition and over-all decomposition, whereas
NOVEMBER, 1936
INDUSTRIAL ASD ENGINEERING CHEMISTRY
1309
By plotting f, against ,Yo:Ru.l6, the curves of Figure 7 are obtained: the data are given in Table I, columns 11 and 12. The upper curve on the deep coke bed runs represents the condition where all the fuel and the gases are preheated to the run temperature, The lower curve represents the powdered fuel runs, where considerable heat was required to compensate for heat losses and preheat' the gas entering the furnace to reaction temperature. The curves clearly indicate the effect of an increase in oxygen concentration on the maximum steady-state grate temperature. Correlation between grate temperature and oxygen concentration alone could not be made without taking into account the inlet rate factor. This indicates that the exothermic reactions of oxygen with carbon are apparently faster when the contact time is fractionally shorter, as compared to the extent of the endothermic reactions of carbon dioxide and steam, which must necessarily decrease as the contact time is shortened. Thus, the combustion of oxygen under these conditions is shown not to be controlled by chemical reaction T.4BLE 111. EFFECTO F OXYGES ON STEAM DECOMPOSITION rate but by the rate of diffusion of oxygen through the adIX A DEEP COKEBED versely flowing gas film. If we accept the theory of the forNet Free Contact Steam mation and decomposition of the complex solid oxides of XatCOs R u n No. Temp. 02 Time Decomposed carbon as presented by Rhead and Wheeler (16),we may con% c. % Sec. 5% 5.87 49 950 32.2 3 33 87 3 clude that the reactions 10 and 11 are faster than the rate a t 5.87 50 22.6 2 29 72 7 which the diffusion of oxygen takes place under these circum5 87 47 33.1 2 72 93 4 5.87 48 0.0 4 46 83 5 st'ances.
the carbon dioxide concentration in the gases produced under the same equilibrium conditions increases as the total oxygen content in the exit gases increases. I n the case of the deep fuel bed runs, however, the data in Table I, columns 5, 7 , and 9, indicate that correlation between steam decomposition and contact time cannot be made unless the percentage of net free oxygen in the inlet mixture is also considered. Thus, using a coke impregnated with 5.87 per cent sodium carbonate a t 950" C., runs 49 and 50 (Table 111) correlate with contact time at approximately 22 per cent of oxygen. Run 47, at a higher concentration of 33 per cent, gives a much higher percentage decomposition a t a n intermediate contact time. Run 48, however, using steam only in the inlet, gives a comparatively lower percentage of steam decomposed, when the contact time is considered, than any of the other three runs. The same situation occurred at 1000' C. (runs 50 through 53).
5.87 5.87 5.87 5.87
50 51 52 53
1000
22.6 25.7 31.4 0.0
2.29 2.05 2.82 4.42
7'2.7 82.7 85.5 76.8
The data on a coke with 3.45 per cent sodium carbonate content do not develop these facts, since the oxygen concentrations and the contact times are too nearly alike. The data on over-all decomposition (Table I, column 8) also show the relatively greater efficacy of oxygen in gasification when using a solid fuel bed where much of the reactant surface is obscured from the radiant heat source; therefore, the rate, of heat transfer rather than the rate of chemical reaction is the controlling factor on decomposition.
Effect of Oxygen on Steady-State Grate Temperature It was found that the maximum grate temperature which was attained under steady-state conditions nas a function of the concentration of oxygen in the inlet, the inlet steam-oxygen rate, the preheating temperature, and the heat loss from the furnace. The preheating temperature and heat loss are a function of deep fuel bed or powdered fuel operation, and the data on each can be separatrly considered on the basis of the other variables. At constant heat loss and preheat conditions: where
t, = grate temp. at steady btate R = net total inlet rate, cu. ft./min. X , = mole fraction net free oxygen in inlet
Then a t a constant mole fraction of oxygen, (t,)x, = (f)a'R By plotting tu in degrees Centigrade m. the cubic feet of inlet gases per minute a t various constant concentrations of oxygen, a band of parallel curves on log-log paper are obtained with a slope of 0.16 for At,/'AR. Thus (.f)a' R = R0.16 Then tu = (f)uX,.R"16
where X;Ra.l6 may be defined as the equivalent mole fraction of oxygen.
Acknowledgment The authors wish to express their appreciation for the cooperation of Fred w. Batten, Llewellyn s. Howe, and John Lapin in carrying out the experimental work and to the Mirhigan Gas Association for their financial assistance.
Literature Cited Benton, J . Am. Chem. Soc., 53, 2984 (1931). Branson and Cobb, Gas J., 178, 901-5 (1927). Brewer and Ryerson, IND. ENG.CHEM.,26, 1002 (1934). Brownlie, Gas Age-Record, 68, 220 (1931). Chipman and Fontana, J . Am. Chem. SOC.,57, 48 (1935j. Cobb and Hodsman, Gas J . , 150, 640 (1920). Cobb and Sutcljffe, Ibid., 178, 895-901 (1927). Dent, Gas W o r l d , Nov. 3, 1928, p. 435. Dent and Cobb, Gas J . , 178, 908-12 (1927). Ibid., 182, 946-54 (1928). Ibid., 186, 776-82 (1929). Fox and White, ISD. ENG.CHEM.,23, 259 (1931). Inst. Gas Eng. (Univ. of Leeds), Gas J., 186,766-75 (1929). Jefferies, Gas Age, 47, 145-50 (1921); Gas Record, N o . 5 , 45-55 (1921). (15) hlarson and Cobb. Gas J . , 175, 889-91 (1926). (16) Rhead and Wheeler, J . Chem. Soc., 103, 461-89 (1912). (17) Surcliffe and Cobb, Gas J . , 182, 946-54 (1928). (18) Taylor and Neville, J . Am. Chem. Soc., 43, 2055-71 (1921). (19) Thau, A., Colliery Guardian, 142, 1709-12 (1931). (20) Yandaveer and Parr, ISD.ESG. CHEX, 17, 1123 (1925). (21) Weiss and White, I b i d . , 26, 83 (1934). (22) Willien, .4m. Gas I s s o c . JIonthly, 5, 565 (Sept., 1923). (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14)
RECEIVEDMay 12, 1936. Presented before the Division of Gas and Fuel Chemistry a t the 9 l s t Meeting of the American Chemioal Societg, Kansas City, &Io., April 13 t o 17, 1936. This paper forms a portion of B dissertation submitted to the Department of Chemical Engineering a t the University of Michigan by A W. Fleer for the degree of doctor of philosophy.
