Energy & Fuels 1996,9, 429-434
429
HsS Removal from Coal Gas at Elevated Temperature and Pressure in Fluidized Bed with Zinc Titanate Sorbents. 1. Cyclic Tests Wahab Mojtahedi” Enviropower Inc., Tekniikantie 12, 02150 Espoo, Finland
Javad Abbasian Institute of Gas Technology, 1700 S. Mount Prospect Road, Des Plaines, Illinois 60018 Received October 24, 1994@
Simplified integrated gasification combined cycle (IGCC)processes are considered t o be among the most efficient and environmentally acceptable technologies for power generation from coal. In such processes the coal is gasified a t pressure and the coal gas is cleaned and combusted in a gas turbine. Coal gas cleanup at elevated pressure and temperature in the IGCC processes offers advantages in higher power generation efficiency and simpler plant configuration. Regenerable mixed metal oxide sorbents are the prime candidates for removal of hydrogen sulfide (the main pollutant) from the hot coal gas in the simplified IGCC processes. In this paper, the results of cyclic sulfidatiodregeneration tests conducted with two zinc titanate sorbents are presented and discussed. These tests were carried out at high pressure and temperature (20 bar, 550-650 “C) in a bench-scale fluidized-bed reactor. The results indicate that the reactivity of both sorbents towards H2S gradually decline in cyclic sulfidatiodregeneration tests.
Introduction A “simplified” IGCC process is being developed by Enviropower Inc. which incorporates pressurized fluidized bed gasification of solid fuels such as coal and an advanced hot gas cleanup train.1,2 This IGCC process has the advantages of higher power generation efficiency, high power-to-heat ratio for cogeneration, excellent environmental performance, and simple plant configuration and modularity. The simplified IGCC process and hot gas cleanup train have been described in detail el~ewhere.l-~ Coal gas desulfurization to sufficiently low levels at temperatures of above 500 “C is now recognized as crucial t o efficient and economic coal utilization in the simplified IGCC system. The implementation of hot coal gas desulfurization heavily relies on the development of regenerable sorbent materials that have high sulfur capacity and can efficiently remove H2S (from several thousand ppm levels to a few ppmv) over many cycles of sulfidatiodregeneration. Structural stability and good mechanical strength are also desirable features in a sorbent. Only a few metal oxides can meet these stringent requirements. @Abstractpublished in Advance ACS Abstracts, April 15, 1995. (1)Mojtahedi, W.; Horvath, A.; Salo, K.; Gangwal, S. K. Development of Tampella Power IGCC Process. Presented at the 10th EPRI Conference: Coal Gasification Power Plants. San Francisco, CA, October 16-18, 1991. (2)Horvath, A,; Mojtahedi, W.; Salo, R;Patel, J.; Silvonen, R. Tampella IGCC Process: Cleaner and More Efficient Power from Solid Fuels. Presented at Power-Gen ’91 Conference, Tampa, FL, December 4-6, 1991. (3) Lehtovaara, A.; Mojtahedi, W. Ceramic filter behaviour in Gasification. Bioresour. Technol. 1993, 46, 113-118. (4) Salo, K.; Ghazanfari, R.; Feher, G.; Konttinen, J.; Lehtovaara, A,; Mojtahedi, W. Enviropower hot gas desulfurization Pilot. Proc. CoalFired Power Systems 94-Adu. IGCC PFBC Rev. Meet., Morgantown, WV, June 21-23, 1994, 1994, 236-245.
0887-0624/95/2509-0429$09.00/0
The basic reaction occurring during desulfurization of the hot coal gas is the reaction of H2S and the reactive metal oxide:
+
+
MYOX xH2S = MySx xH20
(1)
where MYOXand M,Sx are the metal oxide and metal sulfide, respectively. The sorbent is generally regenerated through reaction with oxygen:
MyS,
+ (3/2)xo2= x S 0 , + MYOX
(2)
In recent years it has been shown that certain mixed metal oxides have superior properties compared with - ~ ~ of single oxides for hot gas d e s u l f u r i ~ a t i o n . ~Oxides zinc and iron have been investigated as possible candidates for hot coal gas desulfurization. A compound of zinc and iron oxides, zinc ferrite, ZnFe204, has reached pilot-stage However, the rate of zinc losses (via ZnO reduction followed by zinc vaporiza(5) Westmoreland, P. R.; Gibson, J. B.; Harrison, D. Enuiron. Sci. Technol., 1977, 11, 488-491. (6) Haldipur, G. B.; Schmidt, D. K.; Smith, K. J. “A 50-Month Gasifier Mechanistic Study and Downstream Unit Process Development Program for the Pressurized Ash-Agglomerating Fluidized-bed Gasification System”. Final Report. DOEMC/21063-2740, Vols. 1-2, March 1989. ( 7 ) Tamhankar, S. S.; Hasatani, M.; Wen, C. Y . Chem. Eng. Sci. 1981,36, 1181-1191. (8) Grindley, T.; Steinfeld, G. “Development and Testing of Regenerable Hot Coal Gas Desulfurization Sorbents”. Final Report No. DOE/ MC/16545-1125, 1981. (9) Grindley, T.; Steinfeld, G. “Zinc Ferrite as Hydrogen Sulfide Absorbent”. Third Annual Contractors Review Meeting on Contaminant Control in Coal-Derived Gas Streams, Report No. DOEMETC /84-6, 1983. (lO)Anderson, G. L., et al. “Development of Hot Gas Cleanup System for Integrated Coal GasificatiodMolten Carbonate Fuel Cell Plants”. Final Report, No. DOEMC//19403-1816, 1985. (11)Tamhankar, S. S.; Bagajewicz, M.; Gavalas, G. R.; Sharma, P. K.; Flytzani-Stephanopoulos, M. Ind. Eng. Chem. Process Des. Deu. 1986,25, 429-437.
0 1995 American Chemical Society
430 Energy & Fuels, Vol. 9, No. 3, 1995
tion) has limited the application of zinc ferrite t o temperatures below 550 “C. Because of apparent limitations of zinc ferrite, research is being conducted t o develop a superior mixed metal oxide sorbent. Work on zinc titanates (ZnTiO3, has shown that titanium oxide is a better alternative to iron oxide additives in terms of higher stability of titanates over the ferrite compounds of zinc and their similar sulfidation equilibria. Because zinc titanate has also shown better attrition resistance than zinc ferrite in pilot tests and better regenerability and mechanical stability in the highly reducing coal gas, further development of zinc titanates is underway. Desulfurization of coal gas in fluidized bed offers advantages over the moving-bed and fwed-bed reactors due t o relative ease of temperature control during the highly exothermic regeneration step. However, a more durable and attrition resistant sorbent is needed t o minimize sorbent losses from the fluidized-bed reactor. The initial high-temperature high-pressure (HTHP) tests to evaluate zinc titanate sorbents for fluidized bed desulfurization has been reported ea1-1ier.l~This paper addresses the long-term physical and chemical durability of zinc titanate sorbents in cyclic fluidized-bed application.
Experimental Section Regenerable Zinc Titanate Sorbents. Among many sorbents tested, two zinc titanate sorbents (designated as UCI-2 and UCI-4))both with a nominal zincto-titanium ratio of 1.5 were selected for detailed testing in the state-of-the-art high-pressure high-temperature reactor (HPTR) system. The results of screen analyses of both sorbents are presented in Table 1 and their chemical and physical properties are shown in Table 2. The UCI-4 has smaller average particle size, lower mercury pore volume and surface area, and higher particle density compared with UCI-2 sorbent indicating that UCI-4 sorbent should be less reactive than UCI-2 sorbent. The results of sorbent evaluations in the preliminary sulfidatiodregeneration cyclic tests (3 cycles) conducted with both sorbents under identical operating conditions confirmed that the UCI-4 sorbent was slightly less reactive than UCI-2. The fluidization velocities of both sorbents were determined in a 10 cm diameter column at ambient (12) Focht, G. D.; Rande, P. V.; Harrison, D. P. Chem. Eng. Sci. 1988, 43 (111,3005-3013. (13) Lew, S. High-Temperature Regenerative H2S Removal by ZnOTi02 Systems. M.S. Thesis, Massachusetts Institute of Technology, January 1987. (14)Flytzani-Stephanopoulos, M.; Lew, S.; Sarofim, A. F. “Mechanistic and Kinetic Studies of High-Temperature Coal Gas Desulfurization Sorbents”. Quarterly Report to DOEPETC, DOEPC- 889271, December 1988. (15) Flytzani-Stephanopoulos, M.; Lew, S.; Sarofim, A. F. Hot Gas Desulfurization by Zinc Oxide-Titanium Dioxide Regenerable Sorbents. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1990,35 (11,77-86. (16) Gupta, R. P.; Gangwal, S. K. “Enhanced Durability of Desulfurization Sorbents for Fluidized-Bed Applications”. Topical Report, prepared for DOENETC under Contract No. AC21-88MC25006, June 1991. (17) Gupta, R. P.; Gangwal, S. K. “Enhanced Durability of Desulfurization Sorbents for Fluidized-Bed Applications”. Topical Report, prepared for DOEMETC under Contract No. AC21-88MC25006, November 1992. (18) Lew, S.; Sarofim, A. F.; Flytzani-Stephanopoulos, M. AIChE J. 1992,38 (a), 1161-1169. (19) Mojtahedi, W.; Salo, K.; Abbasian, J. Fuel Process. Technol. 1994, 37, 53-65.
Mojtahedi and Abbasian Table 1. Particle Size Distribution of the Sorbents size pm
sieve (mesh) UCI-2 (wt 5%)
595
500 420 297 250 177 149 120
105 88 74 53
0 1.0 13.5 41.6 9.2 22.2 10.9 0.7 0.2 0 0 0 0.7 100.0 308 254
30 35 40 50 60 80 100 120 140 170 200 270 Pan total
d p d = bL dpl, p m
dpm = l&Jdp,, p m
UCI-4
0 0 0 33.0 19.0 37.6 9.25 0.74 0.08 0 0 0 0.33 100.0 267 237
Table 2. Chemical and Physical Properties of the Sorbents zinc, o/o titanium, % Znfl’i bulk density, g/cm3 particle density, g/cm3 (mercury density) skeleton density, g/cm3 (He density) mercury pore volume, cmVg surface area, m2/g median pore diameter, A average size, p m
UCI-2
UCI-4
46.6 23.4 1.32 1.51
45.3 22.0 1.5 1.37 2.21
4.98
4.87
0.273 3.30 4500 380
0.240 1.90 5000 267
1.46
conditions. The minimum and complete fluidization velocities of both sorbents are about 6 and 20 c d s , respectively. Both sorbents showed excellent attrition resistance, making them suitable for fluidized bed application. Apparatus. A unique reactor system (HPTR) was used to evaluate candidate sorbents in cyclic sulfidatiod regeneration tests. The test unit includes simulated hot coal-gas-derived gas feed systems, a 7.5 cm diameter fluidized-bed reactor, and associated process instrumentation and control devices. A schematic diagram of the HPTR is shown in Figure 1. The reactor is based on a double-shell balanced pressure system. All the H2Swetted parts of the reactor and the sampling system are constructed of inert materials such as quartz or ceramic to prevent corrosion. A detailed description of the reactor system and operating procedure was reported earlier.20 Cold Flow Tests. Fluidized-bed tests were carried out with both sorbents at ambient pressure and temperature in a 7.5 cm diameter fluidization column to determine the desirable ranges of bed height and superficial gas velocity for the desulfurization experiments. Tests were carried out at fluidized-bed heights of 10-33 cm with superficial gas velocities of 20-30 c d s. At a bed height of higher than 15 cm (L/D >2) the bubble rise velocity appeared to be somewhat affected by the reactor wall. The fluidized bed appeared to be partially slugging at the bed heights above 30 cm (L/D >4). The superficial gas velocity, in the range investigated, did not have a significant effect on the fluidization behavior of the bed. (20)Abbasian, J.; Bachta, R. P.; Wangerow, J . R.; Mojtahedi, W.; Salo, K. Ind. Eng. Chem. Res. 1994, 33, 91-95.
Energy & Fuels, Vol. 9, No. 3, 1995 431
HrS Removal fiom Coal Gas ,EXIT
GAS TO CONDENSER
N2
IH2 L
-
I
J
Figure 1. Schematic diagram of the HPTR unit. Table 3. Composition of Simulated Coal Gas Used in Cyclic Tests compd volume % 13 18 ~~
~
>
En
250 -
2
0
11 8 2.5 0.15
Ga
balance
0
I-
z W
0
z
0 v)
U
Elutriation experiments were carried out with the two sorbents in a fluidization column equipped with a cyclone and a filter bag. The tests were conducted with gas superficial velocities in the range of 20-100 c d s and the elutriated solids were weighed after each test. The results indicate that, at superficial gas velocities of 50 c d s , a significant fraction (i.e. > 1%)of the sorbent particles are elutriated from the bed. The attrition resistance of the sorbents were determined in a conveying line with bends at velocities of 18-100 d s . Results indicate that both sorbents are more attrition resistant than limestone and FCC catalyst under similar operating conditions. Based on the results of the cold flow tests, to maintain good fluidization behavior and produce experimental data that can be used for scale-up, all the cyclic sulfidatiodregeneration tests that followed were conducted at a superficial gas velocity of 21 c d s with a sorbent fluidized-bed height of 12 cm. "Life-Cycle"Tests. The cyclic sulfidatiodregeneration tests were conducted in the HPTR unit at the temperature range of 550-650 "C a t 20 bar using a simulated coal gas mixture containing 1500 ppmv of hydrogen sulfide. The compositionof the simulated coal gas used in these tests is given in Table 3. Life-cycle tests were carried out with both sorbents to determine the effectiveness of the sorbent in the long cycling process. These sorbents would typically undergo hundreds of sulfidatiodregeneration cycles in a commercial plant. The physical properties of the sorbents were also determined after the life-cycle tests, particularly with
(3
c X W
Y
"
0
IO
20
30
40
50
60
70
80
90
100
SULFUR LOADING, g Slkg sorbent
Figure 2. Average H2S breakthrough concentrations with UCI-2 sorbent in cyclic tests.
regard to sorbent degradation due to zinc vaporization and loss of porosity and surface area. The H2S breakthrough curves for the UCI-2 sorbent, averaged over five cycles, are presented in Figure 2 for the 30 cycles of sulfidatiod regeneration tests. The results indicate that the reactivity of this sorbent gradually decreases during successive cyclic tests. The sulfur loading at H2S breakthrough concentrations of 50 and 100 ppmv in successive cycles for UCI-2 sorbent is shown in Figure 3, indicating that the sorbent reactivity deteriorates in the cyclic process and the rate of degradation has not reached a constant value after 30 cycles. The reduction in the sorbent reactivity toward H2S in the cyclic process implies that in order to maintain the sorbent reactivity at a constant value, fresh sorbent should be added to the reactor, while a fraction of spent sorbent is withdrawn from the reactor. Based on the results of the 30-cycle test and assuming a constant rate of reduction in sorbent reactivity, it appears that the rate of fresh make-up due to sorbent deactivation is much greater than the fresh make-up rate due to sorbent attrition in the fluidized bed.
Mojtahedi and Abbasian
432 Energy & Fuels, Vol. 9, No. 3, 1995 100 I go
E
t
I
. . . . . . 0 5o ppmv
......
*lOOppmv
I
.....................
.....................
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
...........................................................
wT io
t
10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
3
5
7
4b
30
60
50
80
70
90
SULFUR LOADING, grams SnCg SORBENT
Figure 5. Average H2S breakthrough concentrations with UCI-4 sorbent in LCT-1 cyclic tests
9 11 13 15 17 19 21 23 25 27 29 CYCLE NO.
100
Figure 3. Sulfur loading of UCI-2 sorbent in cyclic tests. 1.8,
0.4 I
BULK DENSITY
E w
0
2 4
0.1 6 8 10 12 14 16 18 20 22 24 26 28 30
Figure 6. Sulfur loading of UCI-4 sorbent in LCT-1 cyclic tests.
CYCLE NO. Corrected for Interparticle voids
Figure 4. Bulk density and pore volume of UCI-2 sorbent in cyclic tests.
To demonstrate sorbent stability, the H2S breakthrough curves as well as the relevant physical and chemical properties of the sorbent should be stable. The results of chemical analyses of fresh and regenerated sorbent indicate that the chemical composition of the sorbent is not generally affected by the cyclic process. However, the bulk density of the sorbent gradually increases while the sorbent porosity decreases in the cyclic tests as shown in Figure 4. The loss of sorbent porosity may be due to the chemical changes that the sorbent undergoes in the cyclic process resulting in an increase in the sorbent density. The increase in the bulk density of the sorbent may also be due to attrition in the fluidized bed. Because the results of the life-cycletest indicated that the UCI-2 sorbent does not stabilize after 30 cycles, cyclic sulfidatiodregeneration (life-cycle) test (LCT-1) were conducted with UCI-4 sorbent (which was produced by a different manufacturing technique) at 650 "C and 20 bar. A total of 50 cycles were carried out. The chemical and physical properties of the sorbent were also determined after each 10-cycleinterval as well as afker the first and fifth cycles. The average H2S breakthrough concentrations (average of five cycles) are presented in Figure 5, indicating that the overall reactivity of the UCI-4 sorbent also gradually decreases in the course of cyclic process. The
0
1
0
2
0
3
0
4
0
5
0
6
f c3840*
-145
0
8
7
0
-
c4650
0
9
0
1
SULFUR LOADING, grams S h g SORBENT
Figure 7. Average H2S breakthrough concentration in cycles 20-50.
sulfur loading at H2S breakthrough concentrations of 50 and 100 ppmv are presented vs cycle number in Figure 6 indicating a stepwise reduction in the overall reactivity of the sorbent after each 10-cycletest interval (specially during cycles 20-50) that coincides with the periodic sorbent cool-down and depressurization, necessitated by sampling of the sorbent. A comparison of the 5-cycle averages of the breakthrough curves for cycles 20-50 is shown in Figure 7 that clearly shows this stepwise reduction in the sorbent reactivity. This suggests that the periodic cooldown and depressurization may adversely affect the sorbent performance by accelerating sorbent deterioration. The observed higher reactivity of the UCI-4 sorbent at 650 "C compared to the reactivity of UCI-2 at 550 "C is mainly due to the effect of temperature, because as
Energy & Fuels, Vol. 9, No. 3, 1995 433
H2S Removal fivm Coal Gas LIV
r
I
....
'
220 0
1
I
1
I
I
I
I
I
I
I
5
10
15
20
25
30
35
40
45
50
CYCLE NO.
Figure 8. Average particle diameter of UCI-4 sorbent in cyclic tests.
0
5
10
15
20
25
30
35
40
45
50
CYCLE NO. Corroctod for intorporticle void.
Figure 9. Bulk density and pore volume of UCI-4 sorbent in cyclic tests.
indicated earlier, the reactivity of UCI-4 sorbent at 550 "C is slightly lower than that of UCI-2 under identical operating conditions. The average particle diameter of the UCI-4 sorbent, shown in Figure 8, indicates that, similar to UCI-2 sorbent, the sorbent particle diameter decreases in the cyclic process. The bulk density of the UCI-4 sorbent increases whereas mercury pore volume approaches a relatively constant value in the course of 50 cycles (Figure 9). This would suggest that the continued increase in the sorbent bulk density is mainly due to the sorbent attrition in the fluidized bed. The skeleton (true) density of both sorbents decreased from an initial value of 5 g/cm3 and stabilized around 4.5 gfcm3. The decrease in the skeleton density is probably due to formation of different zinc titanate crystalline structures during the course of the cyclic tests. This is consistent with the results obtained from X-ray diffraction (XRD) of the sorbent which will be discussed later. The molar zinc-to-titanium ratio of the sorbent essentially remained constant (within the range of accuracy of the measurements)in the course of cyclic tests. In order to eliminate the effects of the external variables such as periodic cooling and depressurizing, on the sorbent deterioration, the second life-cycle test (LCT-2) was carried out. In this test, the reactor
SULFUR LOADING, grams Skg SORBENT
Figure 10. Average H2S breakthrough concentrations with UCI-4 sorbent in LCT-2 cyclic tests.
Figure 11. Sulfur loading of UCI-4 sorbent in LCT-2 cyclic tests.
pressure was maintained a t 20 bar while the reactor temperature was set a t 650 and 750 "C during sulfidation and regeneration tests, respectively. The reactor temperature was also maintained a t 650 "C between successive cycles. A total of 45 cycles of high-pressure/ high-temperature tests were conducted in this second LCT-2 test series. The average H& breakthrough concentration (average of five cycles) vs sulfur loading in the LCT-2 test are presented in Figure 10. The sulfur loadings for all the runs conducted in this test are presented in Figure 11. The results indicate that after an initial large drop in sorbent reactivity during the first 15 cycles, the sorbent reactivity declined at a significantly slower and essentially constant rate over the remaining 30 cycles. The initial large decline in sorbent reactivity may be attributed to continuous exposure to high temperature over an extended period that can lead to deterioration of the physical properties of the sorbent. It should be noted that the sorbent was exposed to elevated temperatures (above 600 "C) for 200 and 1000 h in LCT-1 and LCT-2, respectively. Because the stepwise reduction in sorbent reactivity was not observed during LCT-2 test, it may be concluded that the periodic additional stress caused by cool-down and depressurization had an adverse effect on sorbent deterioration. Although the effect of continuous long-term exposure to elevated temperature appears to be initially significant,the lower slope of the loading curve in LCT-2 compared to LCT-1 indicates that the rate of deterioration is less than that of the sorbent in LCT-1 test.
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434 Energy & Fuels, Vol. 9, No. 3, 1995
Discussion of Results and Conclusion In general, the reactivities of both sorbents deteriorated in the cyclic sulfidatiodregeneration tests. The reactivity of the UCI-4 sorbent in the second life-cycle test was lower than in the first test (LCT-1) after the corresponding number of cycles. This may be attributed to the initial large drop in the sorbent reactivity during the first 15 cycles which may be due to long-term exposure t o high temperature leading to increased zinc migration and physical deterioration of the sorbent. This is generally consistent with the results of scanning electron microscopy (SEM) analyses performed on the regenerated sorbent particles which will be discussed in part 2 of this paper. A comparison of the properties of the fresh and regenerated sorbent after the two life-cycle tests are shown in Table 4. The results indicate that the Hg porosity, BET surface area, and the average particle size of regenerated sorbents from the two life cycle test series are essentially the same and the zinc-to-titanium ratio is within the experimental accuracy of the measurement. However, the densities (bulk, particle, and skeleton) of the regenerated sorbent from LCT-2 are higher than those from LCT-1.
Table 4. Changes in Sorbent Properties during the Life-Cycle Tests sorbent property
fresh
50th cycle, 1st series
45th cycle, 2nd series
bulk density, g/cm3 particle density, g/cm3 skeleton density, g/cm3 (He density) Hg pore volume, g/cm3 surface area, m2/g median pore diameter, A average particle diameter, pm ZnPTi (molar ratio)
1.37 2.21 4.87
1.62 2.51 4.5
1.68 2.7 4.78
0.24
0.153 1.3 4500 250 1.45
0.155 1.3 6500 252 1.47
1.9 5000 267 1.51
The results obtained in this study are similar to those reported by other investigators in that the sorbents do not appear to stabilize after a finite number of cycles. However, their excellent attrition resistance makes them physically suitable for fluidized bed application. Further research and development work is needed to improve the chemical integrity of the zinc titanate sorbents while maintaining their mechanical durability and attrition properties to make them suitable for IGCC application. EF940204Z
