1100
Energy & Fuels 1994,8, 1100-1105
Characterization of Reaction between Zinc Oxide and Hydrogen Sulfide Eiji Sasaoka* Faculty of Health and Welfare Science, Okayama Prefectural University, Kuboki-Ill, Soja} Okayama 719-11, Japan
Shigeru Hirano, Shigeaki Kasaoka, and Yusaku Sakata Faculty of Engineering, Okayama University, Tsushima-naka, Okayama 700, Japan Received January 8, 1994. Revised Manuscript Received May 31, 1994@
The reaction between zinc oxide and hydrogen sulfide was studied in the presence of coalderived gases. The reaction was inhibited by the presence of H2O. The effect of CO2 on the reaction was smaller than that of HzO. H2 accelerated the reaction at 500 "C and inhibited it at 400 "C in the presence of H2O. CO also inhibited the reaction in the presence of HzO. COS was formed from H2S and CO in the absence of Ha. Elemental sulfur was formed from COS only under the H2S-N2 system. In explanation of the experimental results, a reaction scheme has been presented.
Introduction Solid oxide fuel cells and molten carbonate fuel cells are receiving attention from thermal efficiency and/or environmental point of view as their high efficiency contributes to an abatement of CO2 emission per unit electric power. To establish these highly efficient processes, it is necessary to develop a high temperature process for the desulfurization of coal-derived fuel gas. The highly efficient removal of sulfur compounds from several thousand ppm down to ca.lppm has been ~laimed.l-~ From an economic point of view, iron oxide sorbent is attractive for desulfurization processes. However, iron oxide sorbent is not suitable for the highly efficient removal of hydrogen sulfide due to thermodynamic reasons. From the standpoint of desulfurization efficiency, zinc oxide is the most attractive sorbent among the sorbents reported, because of its favorable sulfidation thermodynamic^.^-^ For these reasons, current practical research seems to be concentrated on zinc oxide. In particular, a number of studies on desulfurization using zinc oxide stabilized by Ti02 and zinc ferrite have been r e p ~ r t e d . l , ~ - l ~ This experimental work focuses on characterization of the reaction between HzS and ZnO: the purpose of @Abstractpublished in Advance ACS Abstracts, July 1, 1994. (1)Lew, S.; Jothimurugesan, K.; Flytzani-Stephanopoulos, M. Ind. Eng. Chem. Res., 1989,28,535-541. (2) Lee, A. L. DOE Report MC/220045--2364: "Internal Reforming Development for Solid Oxide Fuel Cells-Final Report" 1987. (3) Minth, N. Q. CHEMTECH 1991,32-37. (4)Uchida, H. Nenryo Kyokaishi 1983,62,792-802. (5) Schrodt, J. T.; Hilton, G. E.; Rogge, C. A. Fuel 1975,54,269272. (6) Westmoreland, P. R.; Harrison, D. P. Enuiron. Sci. Technol. 1976, 10.659-661. (7) Woods, M. C.; Gangwal, S. K.; Jothimurugesan, K.; Harrison, D. P. Ind. Eng. Chem. Res. 1990,29,1160-1167. (8) Jothimurugesan, K.; Harrison, D. P. Ind. Eng. Chem. Res. 1990, 29. 1167-1172. ~. --(9) Silaban, A.; Harrison, D. P.; Berggren, M. H.; J h a , M. C. Chem. Eng. Commun. 1991,107,55-71. (10) Focht, G. D.; Ranade, P. V.; Harrison, D. P. Chem. Eng. Sci. 1988,43,3005-3013. I
0887-0624/94/2508-1100$04.50/0
this study is to clarify the reactions which occur in the H~S-HZ-CO-H~O-COZ-N~ system. A number of reports of the practical application of sorbents containing ZnO can be found in the previous reports as described above. However, the effects of the presence of coal-derived gases on the reaction between ZnO and H2S have not been sufficiently clarified. The complexity of the system may prevent the solving of the problem. Therefore, in this study, the reaction between zinc oxide and hydrogen sulfide was examined in relatively simple inlet gas systems.
Experimental Section Preparation of Zinc Oxide. Zinc oxide was prepared by precipitation using a 20 wt % aqueous solution of Zn(NO3)z and 14 wt % aqueous NaOH solution (containing 10% excess of the theoretical amount of NaOH required for precipitation). The precipitation was carried out by adding the raw salt solution to the NaOH aqueous solution under vigorous mixing a t room temperature. To remove the excess NaOH and produced NaN03, the product of the precipitation was washed with deionized water until the pH value of the waste water decreased to 7 (remaining Na was not analyzed). The product was separated by filtration, dried at 110 "C for 25 h, and then calcined in an air stream (300 cm3/min at STP) from room temperature to 800 "C (10 "C/min, total 3 h). The product thus obtained was crushed and sieved to 1.0 mm. The surface area of the sample obtained was 3.2 m2/g and its bulk density was 0.84 g/cm3. The XRD diffraction pattern (not illustrated) of the sample was almost the same as that of ZnO (ASTM No. 5-06641, and no other diffraction peaks were found. The temperature-programmed reduction (TPR) profile of ZnO was measured using a flow-type thermobalance: in these measurements, a mixture of Hz (25%) and Nz, or a (11)Gangwal, S. K.; Harkins, S. M.; Woods, M. C.; Jain, S. C.; Bossart, S. J . Enuiron. Prog. 1989,8, 265-269. (12) Sa, L. N.; Focht, G. D.; Ranade, P. V.; Harrison, D. P. Chem. Eng. Sci. 1989,44,215-224. (13) Ayala, R. E.; Marsh, D. W. Ind. Eng. Chem. Res. 1991,30,55fin (14) Lew, S.; Sarofim, A. F.; Flytzani-Stephanopoulos, M. Ind. Eng. Chem. Res. 1992,31, 1890-1899.
0 1994 American Chemical Society
Reaction of ZnO and H2S
.
Energy & Fuels, Vol. 8, No. 5, 1994 1101
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and H2S. mixture of CO (25%)and Nz, was fed to the reactor in the thermobalance at 200 cm3/min at STP and the reactor was heated from ca. 100 to 800 "C at a rate of 10 "C/min. The reduction of the ZnO sample, followed by the vaporization of elemental zinc, took place at temperatures above 400 "C in the case of both Hz and CO. The cumulated weight loss of the sample in the Hz-Nz system and CO-Nz system at 500 "C was 6 mg/(g of ZnO) and 4 mg/(g of ZnO), respectively. Details of the characterization of the ZnO sample for reduction under a simulated coal-derived gas will be reported in another article.l5 Apparatus and Procedure. The sulfidationexperiments were carried out using a flow-type packed-bed tubular reactor system16under atmospheric pressure at 500 "C (partially400 "C). The microreactor consisted of a quartz tube, 1.5 cm i d . , in which 0.5 mL of sorbent was packed. In these experiments, a mixture of HzS (500 ppm), Hz (18.1,36.2%),CO (10-36.2%), HzO (18.1%),COz (25%),and Nz was fed into the reactor at 200 cm3/min at STP. HzS concentrations of inlet and outlet gases were measured by an iodine method.I6 COS and C02 concentrationof outlet gases were measured using GC (equipped with TCD, column material: Porapak QS + Porapak T). The vaporization of Zn in the experiments using the packed-bed reactor was judged by whether or not Zn deposited on the wall of the reactor downstream from the bed. When the deposition occurred, a white (grayish)ring of zinc deposits was observed on the wall: this observation has been reported in a previous paper.l
Results and Discussion Effect of Presence of Ha0 or C02 on Reaction between ZnO and H2S. Initially, the reaction between ZnO and H2S in the H2S-N2 system was examined at 500 "C. Figure 1shows the fractional decrease of H2S against time on stream. The fractional decrease of H2S was calculated using the following equation: fractional decrease of H2S = [outlet concentration of H2Sl [inlet concentration of H,Sl
-
2
-
4
6
a-
Time on stream / h
Figure 1. Effect of HzO or COz on the reaction between ZnO
1-
-
Figure 2. Formation of COS from H2S by reaction with COz.
with the previous report.17 These reactions are expressed as follows:
+ H,S - ZnS + H,O H,S - (l/n)S, + H,
ZnO
-ZnS*
+ CO, - ZnO + COS
(3')
-Zn represents an active surface zinc species. The state of this zinc species will be discussed here after. COS formation consumes HzS in addition to the solid-gas reaction (eq 2). Therefore, if this reaction proceeds catalytically and more quickly than the solidgas reaction, it will enhance the reaction of H2S. The proceeding of the reaction of eq 3 also supports the assumption that surface active sulfur s' reacts with H2O. From the above results, the surface reactions of ZnO in these systems are considered as follows:
ZnO
+ H,S - -Zn-S' + H,O
As shown in Figure 1, the reaction proceeded and
(15) Sasaoka, E.; Hirano, S.;Kasaoka, S.;Sakata, Y. Energy Fuels, in press. (16)Kasaoka, S.;Sasaoka, E.; Inari, M.; Sada, N. Nenryo Kyokaishi 1987,66, 210-223.
(3)
As any coal-derived gas contains water and CO2, the effect of the presence of H20 and/or COZon the reaction between ZnO and HzS was examined. H2O inhibited the reaction of H2S as shown in Figure 1, and no elemental sulfur was found. These results suggest that active surface sulfur s', which may have been the precursor of the elemental sulfur in the H2SNz system, reacts with H20 and forms HzS. Furthermore, it is thought that this HzS formation from s' is a cause of the inhibition of the reaction by HzO. As HzO is a similar molecule to H2S, a competitive adsorption of H2O is also thought to be a cause of the inhibitation. C02 accelerated the reaction between ZnO and H2S somewhat, except in the initial stage of the reaction as shown in Figure 1. COS was found in the outlet gas from the reactor as shown in Figure 2. In this system, elemental sulfur was not observed. These results suggest that COS is formed via the following reaction:
(1)
elemental sulfur was produced as a byproduct. The elemental sulfur was deposited on the inside wall of downstream reactor tube which was cooled by atmosphere. From the production of elemental sulfur, it was confirmed that the catalytic decomposition of H2S proceeded over the sorbent. This result is consistent
(2)
-Zn-S'
X(-Zn-s') -ZnO
-
- -ZnS
-Zn,-S',-,
+ (l/x)S,(gas)
(2') (4) (5)
+ H20 = -ZnO-H,Oad (or Zn(OH)2) (6) -Zn-S' + H 2 0 - ZnO + H,S (7)
(17) Yumura, M.; Furimsky, E. Ind. Eng. Chem. Process Des. Dev. 1986,24, 1165-1168.
Sasaoka et al.
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By use of these reactions, the effects of H20 and C02 on the reaction between ZnO and H2S could be explained, but these effects on the desulfurization could not be evaluated in greater detail due to the lack of quantitative measurement of the elemental sulfur formation. Effect of H2 or CO on Reaction between ZnO and H2S in the Presence of H2O. As shown in Figure 3, H2 accelerated the reaction of H2S in the presence of H20, whereas CO inhibited the reaction of H2S at 500 "C. In the presence of CO, COS and C02 were produced as shown in Figure 4. Under these reaction conditions, elemental sulfur formation and Zn vaporization were not observed. The fractional decrease of H2S in the H2S-H2-H20-N2 system in Figure 3 was smaller than that in the H2S-N2 system in Figure 1. This indicates that the inhibition of the reaction by H2O was larger than the acceleration by Ha. The acceleration of the reaction by H2 may be connected to the reductive ability of Ha, and the difference in the effect of H2 and CO may also be caused by the fact that CO is a poor reductant. However, the inhibition of the reaction by CO cannot be explained by its reduction. As the formation of COS and COS was confirmed in the H~S-CO-HZO-N~ system, the following reactions are expected:
-
+ CO -Zn + H 2 0 COS + H 2 0
-Zn-S'
+ COS ZnO + H2 C 0 2 + H2S -Zn
7
10. CO chemisorption on the ZnO (or ZdZnS) may be a cause of the inhibition:
E, a
500%
,
-
(8)
(9) (10)
As the amount of C 0 2 production was smaller than that of the inhibition of the reaction of H2S, the inhibition by CO could not be explained clearly using eqs 8,9,and
+ CO - ZnO-CO,,
(11)
That is, this adsorbed CO blocks the surface of ZnO (or -Zn) and thus inhibits the reaction. It was also thought that a portion of the CO adsorbed on ZnS' stabilized the surface. At 400 "C, the effect of CO addition was examined by adding CO (32.6%). From this examination (not illustrated), it was confirmed that CO inhibited the reaction between ZnO and H2S, as was the case at 500 "C. On the other hand, in the case of H2 addition at 400 "C, the reaction was inhibited (as shown in Figure 51, contrary to the results obtained at 500 "C. This inhibition by Ha could be explained if the following reaction occurred: -Zn-S'
+ H, - -Zn + H2S
(12)
As vaporization of Zn was not observed in this system, it is thought that the -Zn formed is oxidized immediately to ZnO by H2O (eq 9). The causes of the difference in the effect of the H2 at 400 and 500 "C are thought to be as follows: at 500 "C, H2 reduces surface ZnO and hence accelerates the reaction between ZnO and HzS, but H2 cannot reduce surface ZnO at 400 "C and hence the reaction is inhibited as in eq 12. Effect of CO and COSon Reaction between ZnO and H2S in the Presence of H2 and H2O. As CO considerably inhibited the reaction in the presence of H2O and formed COS, the effect of CO on the reaction in the presence of both H20 and H2 was examined. The addition of a small amount of CO (10 or 18.1%)inhibited the reaction between ZnO and H2S. However, when the concentration of CO was increased to 36% the effect of the presence of CO became smaller as shown in Figure 6. Figure 7 shows the outlet concentration of COz in these experiments: the outlet concentration of C02 increased considerably when the inlet concentration of CO changed from 18.1 t o 36.2%. This dependency of the CO2 production on the CO concentration is similar to that of the reaction between ZnO and H2S on the CO concentration. In this 02-free system, the oxygen donor to CO is H20 or ZnO. Therefore, C 0 2 could be formed by either the following catalytic water gas shift reaction or the reduction of ZnO by CO:
CO
+ H,O
-.+CO,
+ H,
(13)
Reaction of ZnO and H2S
Energy & Fuels, Vol. 8, No. 5, 1994 1103
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Figure 6. Effect of CO on the reaction between ZnO and H2S in the presence of HZand HzO.
Figure 9. Effect of H2 and CO on the reaction between ZnO and HzS in the presence of C02.
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and HzS in the presence of Hz and HzO.
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From these considerations, it is suggested that the reduction of ZnO by CO or H2 formed from H20 by eq 13 accelerates the reaction between ZnO and H2S. In these systems, it is expected that COS would be formed according to eq 8, however, no COS formation was observed. This suggests that the reaction rate of eq 7 was considerably larger than that of eq 8 andlor the following catalytic reaction occurred:
COS
l
6
h8
Figure 10. Formation of COS in HzS-(CO or Hd-C02-N2.
Figure 8. Effect of CO and C02 on the reaction between ZnO
ZnO
4
Time on stream / h
Figure 7. Formation of COZin HzS-CO-HZ-HzO-NZ.
0.61
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i
O OO
+ H2-.CO + H2S
(15)
As coal-derived gases contain C02 in addition to Hz, CO, and HzO, the reaction between ZnO and H2S was examined using a mixed gas simulating a coal-derived gas. The gas composition could not be accurately adjusted for comparison with that in Figure 6 because of limitations in the experimental equipment. As shown in Figure 8, C02 inhibited the reaction. From this result, it may be suggested that the acceleration by CO at higher CO concentration (in Figure 8) was inhibited
by the presence of CO2, although the difference of the gas composition is neglected. The presence of C02 should inhibited the reaction of eq 13 and eq 14. Therefore, these results are not inconsistent with those of Figure 6. Effect of HZor CO on Reaction between ZnO and HzS in the Presence of Con. The effect of H2 or CO on the reaction between ZnO and HzS in the presence of C02 at 500 "C was examined. As shown in Figure 9, both the presence of H2 and CO accelerated the reaction. In both cases, the vaporization of Zn was not observed. COS was produced in the CO-CO2 system but was not found in the H2-CO2 system as shown in Figure 10. The absence of COS in the H2S-H2-C02 system is consistent with that in the H~S-CO-HZ-H~Osystem. The formation of COS in the HzS-CO-CO2 system occurred from the commencement of the reaction but the formation of COS in the presence of C 0 2 started at ca. 3.5 h time on stream. In both cases, the level of outlet concentration of COS was the same. These results indicate that the reaction of eq 8 proceeds predominantly to the reaction of eq 3. The CO acceleration is inconsistent with the result in the CO-H20 system. This acceleration may be induced by the reduction of surface ZnO by CO. Effect of HZor CO on Reaction between ZnO and H&. To understand the effect of H2 and CO on the reaction in complex systems, the effect of H2 and CO on the reaction in simple systems was examined. As shown in Figure 11, H2 accelerated the reaction of H2S, but the acceleration by CO was not found. In the presence of H2 or CO, elemental sulfur production was not observed; however, the vaporization of Zn was observed: a white (grayish) ring of zinc deposits was observed on the wall of the reactor downstream from the bed. This phenomenon has been reported in a
Sasaoka et al.
1104 Energy & Fuels, Vol. 8, No. 5, 1994
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Figure 12. Formation of COS in HzS-CO-HzO-Nz. previous report:l Zinc deposits were observed in a 1% HzS-13% H2-19% H20-N2 system at 650 "C. The experimental conditions of those experiments were not the same as those of the present work, but that observation is considered to be consistent with that of the present work when differences in the temperature and the system (the presence of H20) are taken into account. In the presence of CO, COS and CO2 were produced as shown in Figure 12. The production rate of C02 was considerably higher than that of COS at the beginning of the reaction. It is thought that C02 is produced by the reduction of ZnO with CO, as only ZnO could provide oxygen for oxidation of CO in the absence of HzO. If disproportionation of CO occurs over the sample, CO could provide oxygen, but the production of carbon from disproportionation was not found. The vaporization of zinc observed in this system also supports this conclusion. It is thought that COS is produced from the reaction between active surface sulfur Soand CO. These three processes (reduction of ZnO, COS formation, and Zn vaporization) can be described as follows:
+ CO - -Zn + C 0 2 -Zn-S' + CO -Zn + COS ZnO
-
-
(14) (8)
-Zn Zn (gas) (16) In the presence of H2, the following reactions in addition to eq 16 are supposed from the results:
+ H2 - -Zn + H,O -Zn-S' + H, - -Zn + H,S ZnO
1
2
3
Time on stream / h
Time on stream / h
0.5
500ppmH2S-18.1%Hz-N2
Lr
(17) (12)
The vaporization of Zn and the reproduction of H2S
Figure 13. Effect of HZand CO on the reaction between ZnO and HzS. could be explained using these equations. As it is possible that the vaporized Zn affects the reaction, the effect of the presence of CO and H2 was also examined at 400 "C, which is lower than the melting point of Zn (419 "C). As shown in Figure 13, it was confirmed that both CO and Ha accelerated the reaction. Zn vaporization was not observed (in the case of the presence of CO, the measurements of COS and C02 failed). From these results, it is suggested that the acceleration of the reaction by CO and H2 was induced by the reduction of the surface ZnO, although the effects of Zn vaporization on the reaction at 500 "C could not be evaluated because of a lack of quantitative analysis of the Zn vaporization. A comparison of the fractional decrease of H2S in Figures 11 and 9 confirms that the reaction between ZnO and H2S under the presence of H2 was inhibited by the presence of CO2, except in the latter stage of the reaction. On the contrary, it is found that the presence of C02 accelerated the reaction in the latter stage of the reaction, and also accelerated the reaction in the presence of CO. The reason for this acceleration by C02 is unknown, but it is possible that the inhibition of Zn vaporization contributed to these results. In the H2S(H2 or CO)-N2 system the following reaction, in addition to eq 2', may contribute to the overall reaction:
+ H2S- -Zn-S' + H2
(18) When the rate of reaction of eq 18 is larger than that of eq 2', the effect of H2 can be explained by the difference of eqs 2' and 18. To confirm that eq 18 does in fact occur, a comparison of the reactivity of ZnO and Zn with HzS was made at 400 "C. The Zn sample used was prepared from commercial Zn powder (Hakusui Chemical Co.; the surface area of the sample: 0.3 m2/g). We selected 400 "C as the reaction temperature to prevent the vaporization of Zn. As shown in Figure 14,the commercial Zn reacted with H2S, but the fractional decrease of H2S was considerably smaller than that of ZnO, even if the difference in the two sample's surface areas is taken into consideration. From this result, it was confirmed that Zn reacts with H2S, but it was not determined whether or not the rate of reaction of eq 18 was larger than that of eq 2'. To compare the rates of eq 2' and eq 18, a clarification of the difference between the chemical character of the surface -Zn produced from ZnO by the reduction and that of the commercial Zn is necessary. Mechanism of Reaction between ZnO and Has. From the above results, a reaction scheme can be -Zn
Energy & Fuels, Vol. 8, No. 5, 1994 1106
Reaction of ZnO and H2S 1
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had not been confirmed, the original ZnO and active ZnO were not treated as separate entities in the proposed reaction scheme. In the HzS-H2-CO-H20C 0 2 system, it is thought that the adsorption of H20, CO, and H2 plays an important roles in the reaction, but these adsorptions are omitted from the reaction scheme as it would be complicated by them. The acceleration of the reaction between ZnO and H2S by H2 and CO is explained by routes 2 and 3. The formation of COS and H2S from -ZnS' is explained by routes 4 and 6 (followed by route 5). The difference of the role of Ha at 400 and 500 "C in the presence of H2O is explained by route 1 6 and 2 3. At 400 "C, H2 cannot reduce ZnO in the presence of H20 and hence the reaction proceeds via routes 1, 6, and 5;as a result the reaction between ZnO and H2S is inhibited. At 500 "C, H2 can reduce ZnO to -Zn and hence accelerates the reaction via route 2 and 3. C 0 2 can inhibit the reduction of ZnO similar to H20, but it plays a complicated role in the reaction because its ability to prevent the reduction of ZnO is smaller than that of H20. This reaction scheme as described above can roughly explain the experimental results, even though the effect of adsorptions of coexisting gases is omitted. It is thought that H2O plays an important role in routes 1 and 2: H20 may adsorb on the ZnO surface and depress routes 1and 2. Adsorbed CO may play a role in routes 2, 3, and 6: if CO adsorption occurs stabilizing the surface, the surface reactions will be depressed. The adsorption of CO on -Zn has been previous reported.lg As this scheme involves some assumptions, the confirmation of those assumptions remains a problem. Further clarification of the catalytic activity of ZnO and ZnS for the hydrolysis of COS and water gas shift reaction is necessary.
-
Zn(d
Figure 16. Scheme of reaction between ZnO and HzS in H2SHz-CO-HzO-C02-N2.
presented as shown in Figure 15. In the H2S-H2 or CO-HzO-CO2 system, Zn vaporization was not found. If the reactions of eqs 8, 12, 14, and 17 occur, Zn vaporization should be observed; however, this was not the case. Therefore, it is possible that either the -Zn formed is immediately oxidized by H2O or C02 or the -Zn strongly is connected to the surface by the neighboring oxygen and is not easily removed by the reduction. It may be expressed as follows: 0
I
0
I
-0-Zn-0-Zn-0
I
I
+ H2
-
0
I
-0-Zn-0-Zn-0
I
I
+ H20
(19)
Therefore, it may be considered that the active surface zinc -Zn is not a free elemental zinc. A more detailed in situ analysis of the surface of the sample is needed to determine the valence of -Zn. From the previous experimental study of iron oxide sorbents,18 it was predicted that the ZnO regenerated from Zn was more reactive than the original ZnO. But, as this assumption (18)Sasaoka, E.; Sakamoto, M.; Ichio, T.; Kasaoka, S.; Sakata, Y. Energy &Fuels 1993,7, 632-638.
-
Conclusions As a basic study of zinc oxide high-temperature desulfurization sorbent, this work focuses on the characterization of the reaction between ZnO and H2S. Elemental sulfur was formed in the absence of other reactive gases. The reaction was inhibited by the presence of H20, and the effect of CO2 on the reaction was smaller than that of H2O. Ha accelerated the reaction at 500 "C but inhibited the reaction a t 400 "C. CO inhibited the reaction and produced COS when H2 was absent from the system. ZnO reduction with H2 or CO followed by the vaporization of Zn was found when H2O andor CO2 were absent a t 500 "C. The quantitative evaluation of the effect of the coexisting gases is a remaining problem.
Acknowledgment. We gratefully acknowledge that this work was supported by the Ministry of Education, Science and Culture, Japan, through the Grant in Aid for Scientific Research No. 04203111. (19) Ghiotti, G.; Boccuzzi, F.; Scala, B. J. Cutal. 1986,9,79-97.
