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Ind. Eng. Chem. Fundam., Vol. 18,
No. 2 ,
1979
Fluidized-Bed Combustion of Coal with Lime Additives. Kinetics and Mechanism of Regeneration of the Lime Sorbent James M. Chen and Ralph T. Yang" Brookhaven National Laboratory, Upton,New York 17973
The apparent solid-solid reaction between Cas and CaSO, is the rate-controlling reaction in the lime sorbent regeneration for fluidized-bed combustion of coal, as well as in some other important industrial processes such as the sulfuric acid-cement process. A two-step mechanism for this reaction involving SO3 was determined. The rate-controlling step in the mechanism is t h e decomposition of CaSO, to form CaO and SO3. Kinetics and rate expressions are established for the overall reaction and the separate steps. Results are also presented for the kinetics and the thermodynamics of all the possible reactions under the conditions of the aforementioned processes.
Introduction In fluidized-bed combustion processes it is desirable to regenerate the lime sorbent from the sulfated lime. The reaction schemes currently being considered are based on reductive decomposition of the sulfate, as shown in the following 4CaS04 + 2R 4Ca0 + 4soz+ 2R02 (1) where R is the reductant. Various reductants have been used successfully in laboratory-scale experiments, among which are CO (Jonke et al., 1977; Hoke et al., 1977; Wheelock and Boylan, 1960) and C (Yang et al., 1978; Turkdogan and Vinters, 1978). The temperatures for regeneration are in the range of 950 to 1200 "C. As pointed out by Ruth (1975),the regeneration reaction proceeds in the following two steps regardless of the reductants used CaSO, + 2R Cas + 2R02 (2) Cas + 3CaSo4 4Ca0 + 4 s 0 2 (3) In the two-step reaction mechanism, the second one is the common step. Reaction 3 is also the rate-controlling step, especially at relatively low temperatures. Thermodynamics of reaction 3 dictates the maximum attainable SOz concentration in the gas phase; its value increases from 0.05 atm at 950 "C to 0.1 atm at 1000 "C and to 0.5 atm at 1100 "C. Our experimental results (Yang et al., 1977) showed that over 7% SOzat 1 atm total pressure could be obtained at 1000 "C regeneration temperature, which is 30% below equilibrium. In the SO2-richreacting systems, reaction 3 may not be the only reaction occurring. Both Cas and CaO may also interact with SOp. Thermodynamics indicate that the following reactions are feasible in the range of the regeneration temperatures '/,Cas + 3/4CaS04* CaO + SOz (4)
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--
2/3Ca0 + SOz s 2/3CaS04+
+ 2SOZ * CaS04 + S2 2CaS + SOz * 2Ca0 + 3/zSp 3/zCaS + '/zCaS04 * 2Ca0 + S2 CaS04 9 CaO + SO3 Cas
(5) (6) (7) (8)
(9)
*Address correspondence to this author at the Department of Chemical Engineering, State University of New York at Buffalo, Amherst. N.Y. 14260. 0019-7874/79/1018-0134$01,00/0
The equilibrium constants, K's, for the above reactions are plotted against l / T in Figure 1. Reaction 3 is an apparent solid-solid reaction. However, it has been concluded that gaseous intermediates exist in the reaction (Yang et al., 1979). The conclusion was drawn by observing the formation of CaO in physically separated pellets of Cas and CaS04 (- 1mm separation) which were heated in a N2 stream at 900-1100 "C (Yang and Shen, 1979). This conclusion leads to the following generalized mechanism 3-x 3CaS04 3Ca0 + 3S0, + 3( T ) O ~ (0 Ix I3)
-
(10)
That is, the gaseous intermediates are formed from CaS04 decomposition and they react with Cas to form CaO and sop. Since the gases Oz, SOp,and SO3are all involved in the system, we must consider all of the possible reactions, Le., reactions 4-11. This paper reports the results of a systematic study of such a reaction system in order to improve our understanding of the high temperature lime regeneration reactions. Experimental Section The rate measurements, except for the reactions involving SO3, were performed by adopting the standard thermogravimetric method. The rate measurement and calculation procedures were the same as that previously described (Yang and Steinberg, 19761, except that a Cahn R-100 thermobalance was used. In all experiments, the gas flow rate was predetermined to be high enough that the effect of the gas-film diffusion was minimized. For the reactions involving SO3, a quartz tubular reactor was used, which will be described shortly. The reaction products were examined by X-ray diffraction using the standard powder technique. All chemicals used were of reagent grades. Gases were of prepurified grades. The SO2/N2gases were supplied by the Matheson Co. as custom-made, premixed gases a t various specified concentrations. The S03/Ar mixture was obtained by bubbling Ar through liquid SO3 (mixture of /3 and y forms). The bubbler was placed in a constanttemperature bath. Concentration of SO3 in Ar was calculated assuming saturation of SO3in Ar was reached. The quartz tubular reactor consisted of a preheating zone which was packed with alumina beads and a reaction zone in 0 1979
American Chemical Society
Ind. Eng. Chem. Fundam., Vol. 18, No. 2, 1979 4 3 - - - - v 7
42
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41
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36
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y
t-
, 65
I
l
l
' 3
3 7 -
/
/
--
Figure 1. Equilibrium constants vs. 1 / T (1) '/4CaS + 3/4CaS0, CaO + SO,; (2) 2/3Ca0 + SO2 ,/&aS04 + 1/6S2; (3) 3/zCaS + '/,CaS04 2Ca0 + S,; (4) CaS04 2Ca0 + SO3; ( 5 ) Cas + 2S02 CaS04 + Sz; (6) 2CaS + SOz 2Ca0 + 3/2S2.
-
-
'.L 20
40
(50
80
IO0
I20
140
I60
-
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36
39
IO
I I
O3
Figure 3. Equilibrium constants vs. 1/T for Cas with SO3reaction.
+lKix103
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07
-
/
+lKIx
BO
7 5
/ cas. 3 s a 3 -t30*4s0*
/
135
180
T I M E (mini
Figure 2. Reactions of C a s (21 mg), with SO2 (8%)and Cas (2.1 mg) with 0, (10%) a t 1000 "C; with 0, (a),with SOz (0).
which solid samples were placed. All the lines and connections were heated to prevent SO3solidification. Teflon connections and a special grease (KEL-F) for the glass joints were used. Results and Discussion The reaction system was studied by examining each of the possible simpler, not necessarily elementary, reactions. Along this approach, elimination and deduction were made, and a predominating or controlling mechanism was then reached. We first assumed that there were no other reactants beside CaS and CaSO,. The gaseous intermediates as shown by reactions 10 and 11 were determined by the following experimental deduction. Reaction between Cas and SOz. Results of the weight change of CaS with 8% SO2 in Nza t 1000 "C are given in Figure 2. Upon the reaction, a weight gain rather than weight loss was measured. This weight gain apparently indicated that SOz was not the gas intermediate. X-ray diffraction analysis of product showed formation of CaSO, and no CaO. I t was seen that reaction 6 was faster than reaction 7 under the condition, and that SOz was not an intermediate for reaction 3. Reaction between Cas and O p The reaction products between calcium sulfide and oxygen have been known to be calcium sulfate a t low temperatures but were reported to be calcium oxide and sulfur dioxide a t high temperatures (Hull et al., 1957). Since the Cas-CaS0, reaction starts from 900 "C, if oxygen is the intermediate, the reaction between CaS and O2 should form CaO and SO2 a t these temperatures. Experimental results of CaS with 0 2 (20% in N2) a t 1000 "C are also shown in Figure 2. Again, the CaS sample gained weight from oxygen. X-ray
diffraction indicated the formation of CaS04. I t was concluded that O2 was not an intermediate for reaction 3. Reaction between Cas and SO3. Equilibrium constants of the following reaction are given in Figure 3 C a s + 3 s 0 3 CaO + 4s02 (12) The high equilibrium constant values suggest the feasibility of this reaction. SO3can be formed from thermal decomposition of CaS0,. For instance, a t 1000 "C, the equilibrium partial pressure of SO3 for CaS04 decomatm. With this value, the equilibrium position is 0.8 X partial pressure of SO2in reaction 1 2 is 0.093 atm. Here, we did not consider the SO3-SO2 equilibrium because this reaction is a rather slow one, although the equilibrium S02/S03ratio is not low (-10 a t 1000 "C). The reaction of Cas with SO3was studied with a tubular reactor. The CaS samples were pelletized powder. To study this reaction, care was taken to avoid the possible reaction between CaS and CaSO, to produce CaO (if CaS04 could be formed from CaS + SO3), which may create misleading information. Since the reaction between C a s and CaSO, is extremely slow a t temperatures below 900 "C, the reaction temperature for the CaS SO3 system was controlled at 800 "C. The unavoidable side reaction here is the formation of CaS0, from CaO and SO3. The SO3 was introduced to the reactor for an interval of 15 s. Concentration of SO3 in Ar was 0.5%. The product pellets were analyzed by using a thymolphthalein indicator solution (pH 9.3-10.5). When a few drops of thymolphthalein solution were applied to the surface of these pellets, the color of product pellets immediately turned blue. However, the samples of pure CaS and pure CaS0, remained their own colors. This indicated that CaO was contained in the solid sample. X-ray diffraction showed that both CaS and CaSO, also existed in the sample beside CaO. The CaSO, product was believed to be due to the CaO and SO3 reaction. The above results lead us to believe that (1)CaO can be formed by C a s + SO3 in the high temperature range and (2) the CaO formation reaction is faster than the reaction: CaO + SO3 CaSO, at 800 "C. It becomes clear now that reaction 3 proceeds through the following mechanism CaS0, CaO + SO3 (13) CaS + 3 s 0 3 CaO + 4 s 0 2 (14) Thermodynamically, 9.3% of SOz may be formed a t a total pressure of 1 atm from the regeneration reactor a t 1000 "C. Rate-Controlling Step in Reaction 3. We now would like to decide which one of reactions 13 and 14 was the
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TIME Iminl
k
t
1
2
-It
\
0 0l0
-14
4
10
60
70
Figure 6. Plots of log (1 - X ) vs. time for CaS with CaS04reaction; T = 1000 "c (O), 950 "c (O),900 " c (A).
i 4
P
0
50
40
30
TIME (mini
Figure 4. Weight changes vs. time for CaS with CaS04 reaction a t lo00 "C; CaS (9.6 mg) + CaS04 (27 mg) ( A ) ,CaS (4.8 mg) + CaS04 (27 mg) (O), Cas (10 mg) + CaSO, (14 mg) (0).
0.7-
20
IO
1
20
30 4 0
50 60 70 TIME ( m i d
Figure 5. Reaction rates of 3CaS0, + CaS 1000 "c (01, 950 "c (A),900 "c (0).
80
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90
100 110
4Ca0 t 4S02; T =
rate-controlling step. If the CaSO, decomposition is the controlling step, the overall rate of reaction 3 would not be affected by increasing the amount of CaS above the stoichiometric ratio. If, however, reaction 14 is the controlling step, the overall rate would be increased by such an increase. With the above rationale in mind, overall rates were measured with samples of various ratios of Cas/ CaS0,. The results are presented in Figure 4. The samples were in the size range of 38 to 53 pm. The flow rate of N2 was 1000 sccm in these experiments. The stoichiometry of reaction 3 requires 4.8 mg of CaS for 27 mg of CaS0,. As shown in Figure 4, by doubling the stoichiometric amount of Cas, the overall rate was only slightly increased. On the other hand, reducing the amount of CaS04by one-half from the stoichiometric ratio, the overall rate was approximately halved. This interesting result clearly shows that the CaS0, decomposition is the rate-controlling step. Kinetics of Reaction 3. Figure 5 depicts the results of conversion, X , vs. time for the sulfide-sulfate (38-53 pm size) reaction at temperatures of 900, 950, and 1000 "C. The flow rates for these results were controlled at lo00 sccm. The plots of log (1- X)vs. time at the above three temperatures are shown in Figure 6. Linearity was seen for all the results. The line for the data at 1000 O C did not go through origin because the reaction started before the temperature reached 1000 "C. The linearity of the results indicates that the reaction is first order with respect to the solid concentration. The reaction rate may now be expressed as
where S is the solid concentration (or 1- X)and k is the
?
I
1
0 001 0 75
I
1
0 eo
1 0 85
-1
1
0 90
t ( K 1 x IO'
Figure 7. Temperature dependence of rate constant, k, for CaS with CaSO., reaction.
rate constant. The value of h can be obtained from the slope of the plot of log (1- X)vs. time (Figure 6). Using the Arrhenius expression, the temperature dependence of k was determined from the slope of the plot of log vs. 1/T as shown in Figure 7. The value was 82 kcal. In most of the decomposition reactions in which gas products are formed, reaction rates have been found to be proportional to the difference between the equilibrium partial pressure, P,, and the bulk gas partial pressure, P, i.e. h = h ( P , - P) (16) In our experiments, pure N2gas was flowing through the reactor. Thus, P is zero. Since P, is dependent upon temperature, the measured temperature dependence of k is the summation of those of k and P,. The temperature dependence of P, for the sulfate-sulfide reaction is 58 kcal (determined from thermodynamics). Therefore, the activation energy for k is 24 kcal. To be able to obtain a rate expression for reaction 3 in the temperature range of interest, one must first determine what is the actual driving force. For the heterogeneous reaction under consideration, the above results showed that the rate is first order in terms of the solid reactant concentration, S. The other part of the driving force is the product gaseous concentration, or approximately the partial pressure of the gaseous product. We have seen in the above results that in the Cas-CaS0, reaction, 100% completion of the reaction can be achieved. The completion is in line with the results that the sulfate decomposition is the rate-controlling step and that the reaction between Cas and SO3 is much faster than the
Ind. Eng. Chem. Fundam., Vol. 18, No. 2, 1979
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r....r,
-.I\-
0 3Or
A -I0-I2
20
40
6b
' 80 '
IbO' IhO' I i O ' IkO' l k O ' TIME ( m n l
Figure 8. Weight changes of CaSO., (27 mg) with CaS (9 mg) vs. time under SOz (4%) a t 1000 "C.
0.35r-I
I IO
20
l 30
1 40
,
.
I 60
50
I 70
1 80
l 90
TIME lrnin)
Figure 10. CaS with SOz reaction rates; T = 1000 "C (O), 950 "C
0.301
(O), 900
"c (A).
I 10
20 %J
I
40
50
60
70
80
I
90
IO0
T I M E (mini
Figure 9. Reaction rates of CaO with SO2 (4%); T = 950 " C (01, 1000
"c ( O ) , 1100 "c (A).
dt
IC
33
40
5C
T LIE
sulfate decomposition reaction. The above results apply only to the reaction systems wherein the CaS and CaS04 are in intimate contact. The true driving force due to the partial pressure may be consequently expressed as the deviation of the partial pressure of SO, from its equilibrium value, of P, - P. The overall rate expression for reaction 3 is
!E = -k(P,
'. A 20
-
p)
(17)
where
k = 3.3 x 10,exp(-12080/T)
(18)
An attempt was made to determine the overall rate dependence on the partial pressure of SO,. Experimental results of such measurements are shown in Figure 8. The sample first lost weight as expected. A steady weight gain was seen as the reaction progressed. The weight gain indicated that not only reaction 3 took place but some other reactions were competing. The two probably competing reactions are those of SO, with CaO and Cas which involve weight gain. To further understand the reaction system, we investigated these two reactions in the temperature range of interest. Reaction between CaO and SO,. Rate measurements were performed with the thermogravimetric reactor in which about 20 mg of CaO powder samples were exposed to a gas of constant composition (SO, in N,)at a total flow rate of 1000 sccm. In all the measurements, the SO2partial pressure was kept below the equilibrium value of the sulfide-sulfate reaction to prevent the formation of CaS which would in turn react with SO2 and complicate the results. The product samples were analyzed by X-ray diffraction, which indicated the formation of CaSO,. Also, deposition of elemental sulfur downstream of the reactor was observed. These results confirmed the occurrence of reaction 6. That is, calcium oxide reacts with SO2to form CaS04 and elemental sulfur. After determining the reaction product, the measured weight gain with respect to time was used to determine the extent of conversion vs. time. The results are shown in Figure 9.
6C
7C
80
90
l0C
min'
Figure 11. CaS with SO2 reaction rates at 900 "C; SO2 composition = 16% (U), 11% (01, 8% ( A ) , 6% (01,4% ( 0 ) .
Figure 9 shows that with constant SO2 concentration, the measured reaction rate decreases with increasing temperature. To explain this phenomenon, we examine the thermodynamics of this reaction. Increasing temperature lowers the equilibrium constant. With 0.04 atm of SO, partial pressure, the equilibrium partial pressure of elemental sulfur (S,) varies from 0.6 X atm a t 950 atm at 1000 "C, and to 0.1 X atm at "C, to 0.3 X 1100 "C. It is apparent that under the operating conditions the evolved gas (S,) from the reaction hindered the progress of the reaction, resulting in lower rates at higher temperatures. It is tempting to derive a rate equation, which would include both the forward and reverse rate constants, for the overall rate based on the data. Unfortunately, we did not control the partial pressure of S2 in the gas near the solid surface. Therefore, we could not derive a rate expression for this reaction based on the above results. Reaction of C a s with SO,. Reaction between CaS and SO, was measured under the same operating conditions as for CaO with SO, measurements. However, in these measurements, the SO2 partial pressure was kept above the equilibrium value of SO2 for the sulfate-sulfide reaction. The reason for this was to prevent the interference of the reaction between C a s and CaS04, the latter being formed by the Cas-SO, reaction. The product samples were analyzed by X-ray diffraction, the results of which indicated that the product contained CaS and CaSO,, but no CaO. Hence it was concluded that reaction 6 rather than 7 took place under the experimental conditions. Results of the measured weight change with respect to time were then converted to degree of conversion versus time. Figures 10 and 11 show the effects of temperature and SO2concentration, respectively, on the reaction rate. By plotting initial rate vs. SO2concentration on a log-log scale (Figure 12), the reaction was found to be 1.5 order with respect to the SOz concentration. The results on the reactions of SO, with CaO and CaS may be used to explain why the sample mixture of CaS
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1 1
I
1 1 1 101
I
I
I l I I 1
I .o
SO2 COMPOSITION
Figure 12. Rate dependence of CaS with SOz reaction on SO2 concentration at 900 "C.
and CaS04gained weight as shown in Figure 8. Originally, the solid sample contained only CaSO, and Cas. At 1000 "C, the initial rate of the sulfate-sulfide reaction was higher than that of the reaction of CaS with SOz (4%). As a result, the sample lost weight. As time progressed, the CaO content continued to increase and the CaS04content decreased accordingly. When the combined rates of the reaction between CaS and SOz and that between CaO and SOz, both forming CaS04, were higher than the rate of the sulfide-sulfate reaction, the solid sample started to gain weight. This series of reactions resulted in a minimum sample weight during the course of the reaction. Since the sulfide-sulfate reaction was retarded by SOz while CaS + SOz and CaO + SO2 reactions were enhanced by the SOz concentration, we also observed that by increasing the SOz concentration the solid sample gained weight in shorter periods of time. Nevertheless, both of the reactions formed CaS04 and are undesired ones in the regeneration of lime sorbent for fluidized-bed combustion of coal and in the sulfuric acid-cement process. However, because of the low values of thermodynamic limits, in that the CaS + SO2and the CaO + SOz reactions can proceed, they must be
considered in design and optimization of such industrial processes. A further note should be made on the difference between the conditions of our experiments and of industrial processes. Reactions 5 to 8 examined here have elemental sulfur as a product. In industrial processes, the elemental sulfur would be "scavenged" by either reducing or oxidizing gases to form H2S and COS or SO,. In our experiments, a flow system with a rather high gas velocity was used, and the partial pressure of elemental sulfur was kept low. If, however, the partial pressure of elemental sulfur in our experiments was significantly higher than in industrial processes, the reverse rates of reactions 5-8 would be higher in such processes. On the other hand, since the rates of reactions 5-8 are orders-of-magnitude lower than the rates of reactions 13 and 14, the conclusions drawn from this work should be applicable to industrial processes. Nomenclature k = rate constant with respect to the chemical driving force, atm-' s-l k = rate constant with respect to solid reactant concentration, S-1 P = partial pressure, atm P, = equilibrium partial pressure, atm S = solid reactant concentration (= 1 - X ) t = time, s T = absolute temperature, K X = fractional conversion of reaction Literature Cited Hoke, R. C., Bertrand, R. R., Nutkis, M. S., Kinzler, D. C., Ruth, L. A,, Gregory, M. W., Magee, E. M., "Studies of the PressuizedFluidzeccBedCoal combustion Process", Exxon Annual Report, EPA-600/7-77-107, 1977. Hull, W. Q.,Schon, R., Zirngibl, H., Ind. Eng. Chem., 49, 1204 (1953). Jonke, A., Vogel, G., Johnson, I., Lee, S., Lenc, J., Lescarret, A,, Montagna, J., Nunes, F., Shearer, J., Swyler, R., Smith, G., Swift, W., Teats, F., Turner, C.. Wilson, I.,Supportive Studies in Fluidized-Bed Combustion", Argonne Annual Report, EPA-600/7-7-138, 1977. Ruth, L. A., Proceedings, 4th International Conference on Fluidized Bed Combustion, Mitre, McLean, Va., p 425, 1975. Turkdogan, E. T., Vinters, J. V., Trans. Inst. Min. Mefall., in press, 1978. Wheelock, T. D.. Boylan, D. R., Ind. Eng. Chem., 52, 215 (1960). Yang. R. T., Albanese, A,, Chen. J. M.. Farber, G., Kainz, F. B., Pruzansky, J., Shen, M-S., Steinberg, M., "Regenerative Process for Desulfurization of High Temperature Combustion and Fuel Gases",Rogess Report No. 7, Brookkven National Laboratory, Upton, N.Y., 1977. Yang, R. T., Chen, J. M., Farber, G., Shen, M-S., Steinberg, M., Proceedings, Fifth International Conference on Fluidized Bed Combustion, DOE-MITRE Corp., McLean, Va., Vol. 3, p 798 1976. Yang, R. T., Shen, M-S., AIChE J., in press, 1979. Yang, R. T., Steinberg, M., J. Phys. Chem., 80, 965 (1976).
Received for review May 22, 1978 Accepted January 8, 1979
