Energy & Fuels 1995,9, 344-353
344
Soot Formation over Zinc Ferrite High-Temperature Desulfurization Sorbent Eiji Sasaoka" Faculty of Health and Welfare Science, Okayama Prefectural University, Kuboki-Ill, Soja, Okayama 719-11, Japan
Yukimasa Iwamoto, Shigeru Hirano, Md. Azhar Uddin, and Yusaku Sakata Faculty of Engineering, Okayama University, Tsushima-naka, Okayama 700, Japan Received September 6, 1994@
Soot formation over zinc ferrite high-temperature desulfurization sorbents was examined. When active zinc ferrites were used in simulated coal-derived gases for fuel cells, a large amount of soot was formed over the ferrites. This soot formation decreased with the increase of reaction temperature. This soot formation was inhibited by H2O and C 0 2 and accelerated by H2 and CO. These dependencies of the rate of soot formation on the concentration of HzO, CO2, Hz,and CO can be expressed by six experimental equations. Carbides (Fe3C- and FesC-like compound, Fe,C) were found in the sample over which soot was formed. Fibrous ZnO was formed over the surface of the sample when the sample ZnFeaO4 was converted to carbides (Fe3C and Fe,C). It is thought that the carbides and the soot were produced via a surface active carbon.
Introduction Solid oxide fuel cells and molten carbonate fuel cells as new technologies using coal-derived gas are receiving attention from thermal efficiency and/or environmental point of view as their high efficiency contributes to an abatement of COz emission per unit electric power. To establish these processes, development of a hightemperature process for the desulfurization of coalderived fuel gas is an inevitable p r ~ b l e m . l - ~ Many kinds of metal oxide sorbents have been examined as high-temperature desulfurization sorbent^.^ From the standpoint of desulfurization efficiency, zinc oxide is attractive because of its favorable sulfidation thermodynamic^.^^^ For this reason, current research seems t o be concentrated on zinc oxide. In particular, a number of studies on desulfurization using zinc ferrite of them,lz7-13 the have been r e p ~ r t e d . l > ~In- ~some ~ sorbent was examined as a desulfurization sorbent for
* Abstract published in Advance ACS Abstracts, January 15, 1995.
(1)Lew, S. J.;Jothimurugesan, K.; Flytzani-Stephanopoulos,M. Ind. Eng. Chem. Res. 1989,223, 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. B.; 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.; Jha, M. C. Chem. Eng. Commun. 1991, 107, 55-71. (10) Focht, G. D.; Ranade, P. V.; Harrison, D. P. Chem. Eng. Sci. 1988, 443, 3005-3013. (111Gangwa1, 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. S C ~1989, . 44, 215-224. (13) Ayala, R. E.; Marsh, D. W. Ind. Eng. Chem. Res. 1991,30,5560.
0887-0624/95/2509-0344$09.00/0
an integrated gasification combined cycle (IGCC)power plant. In other cases, the sorbent was examined as a sorbent for fuel cells.14J5 Zinc ferrite breaks down to the constituent oxides and Fez03 is reduced to FesO4 (or FeO, Fe) in a reducing environment at a temperature which depends on the reducing environment.1° In the presence of CO, FeO or Fe forms iron carbide Fe3C.13 Thus, soot formation from the catalytic disproportionation of CO over zinc ferrite sorbents is expected; however, this has not been reported in previous reports.ll One of the reasons is that gas compositiom studied were set to prevent the formation of soot and Fe3C.13 Generally speaking, coalderived fuel gases for fuel cells are more fuel rich than those for an integrated gasification combined cycle.14J5 To prevent soot formation (carbon deposition) and carbide formation, it is important to understand the nature of the formation of carbon and carbide. This work focuses on evaluating the nature of soot formation over zinc ferrite under the high-temperature desulfurization conditions that exist in fuel cells.
Experimental Section Preparationof Zinc Oxide Sorbents. Zinc ferrite samples were prepared by coprecipitation using a 10 or 20 wt % aqueous solution of a mixed salt of Fe(N03)3 and Zn(NO& (mole ratio of Fe(N03)3/Zn(N03)2 = 21, and a 7 or 14 wt % aqueous NH3 or NaOH solution (containing 10% excess of the theoretical amount of NH3 or NaOH required for coprecipitation). The precipitation was carried out by adding the raw salt solution to the NH3 (or NaOH) aqueous solution under vigorous mixing a t 30 "C. The product of the coprecipitation was washed with deionized water until the pH of the waste (14) Sakurai, T.; Okamoto, M.; Miyazaki, H.; Nakao, K. Kagaku Kogaku Ronbunshu 1994,20, 268-274. (15) Sakurai, T.; Okamoto, M.; Miyazaki, H.; Nakao, K. Kagaku Kogaku Ronbunshu 1994,20, 275-282.
0 1995 American Chemical Society
Soot Formation over Zinc Ferrite
Energy & Fuels, Vol. 9, No. 2, 1995 345
Table 1. Zinc Ferrites Studied sample
bulk density, g/cm3
ZnFez04(10-7Na,600) ZnFe~04(10-7Na,700) ZnFez04(10-7Na,800) ZnFez04(20-14Na,700) ZnFez04(20-14Na,800) ZnFezO4(10-7NH,700) ZnFez04(10-7NH,800) ZnFe204(20-14NH,600) ZnFez04(20-14NH,650) ZnFez04(20-14NH,700) ZnFez04(20-14NH,750) ZnFez04(20-14NH,800)
1.7 1.9 2.2 2.3 2.3 2.2 2.6 1.8 2.1 2.4 2.4 2.5
BET surface area, m2/g 19 10
5.2 2.6 0.4 4.8 0.3 19 10 2.8 0.4 0.4
water declined t o 7. The washed product was separated by filtration and dried a t 110 "C for 25 h. I t was then heated in a n air stream (300 cm3/min a t STP) from room temperature to a set temperature (600-800 "C) a t the rate of 10 "C/min and held at this temperature (total time of heating and holding 3 h). The product thus obtained was crushed to granular particles using a mortar. The granular particules were sieved t o 0.7 mm size (-20/+28 meshes) and 0.5 mm size (-28/+35 meshes). Samples of 0.7 mm size were used to measure the reactivities of the samples for desulfurization, and the samples of 0.5 mm size were used to measure soot formation with a TGA thermobalance (Shimazu DT-30). Bulk density and Nz BET surface areas of the sorbents are listed in Table 1. ZnFez04(10-7Na,700) denotes the sample prepared from 10 wt % mixed metal salt (aq) and 7 wt % NaOH(aq) and calcined a t 700 "C. Similarly, ZnFezOd(20- 14NH,800) denotes the sample prepared from 20 wt % mixed salt (aq) and 14 wt % NHs(aq) and calcined at 800 "C. Apparatus and Procedure. The sulfidation experiments were carried out using a flow-type packed-bed reactor system16-18under atmospheric pressure a t 450-600 "C.The reactor consisted of a quartz tube of 6.5 mm i d . , in which 0.2 cm3 of sorbent was packed. In these experiments, a mixture of HzS (10-200 ppm), Hz (20%), CO (30%), COz (lo%), HzO (9.4, 14.1%), and Nz was fed into the reactor at 200 cm3/min a t STP. These gas compositions approximated the composition of coal-derived gases which will be used in fuel cells.I5 As the highly efficient removal of sulfur compounds from thousands of ppm down to ca. 1 ppm for fuel cells is demanded, it is necessary to evaluate the reactivity of a sorbent for low concentrations of sulfur compounds. Therefore, the concentration of HzS was set 10-200 ppm. The concentrations of HzS, COS, and SO2 were measured by a n on-line GC equipped with a flame photometric detector (column packing 10% PEG-6000/ TPA). The experimental studies of soot formation over zinc ferrite were carried out using a flow-type thermobalance equipped with a net sample basket. Sample particles (0.1 g) were packed t o form ca. two layers in the basket. The weight gain of a sample due to soot formation was measured: in these measurements, a mixture of Hz (0-30%), CO (0-40%), COz (0-25%), H20 (0-18.8%), and Nz (200 cm3/min at STP) was fed into the reactor a t a set temperature (400-550 "C). XRD was used to evaluate the change of sample structure and composition, and SEM (equipped with EDX, energy dispersive X-ray spectrometer) was used t o observe the morphology of the sample and the soot formed.
Results and Discussion Character of Reaction between ZnFe201 and H2S in Presence of Simulated Coal-DerivedGases. (16)Sasaoka, E.;Ichio, T.; Kasaoka, S. Energy Fuels 1992,6,603608.
(17) Sasaoka, E.;Sakamoto, M., Ichio, T.; Kasaoka, S.; Sakata, Y. Energy Fuels 1993,7 , 632-638. (18) Kasaoka, S.; Sasaoka, E.; Inari, M.; Sada, N. Nenryo Kyokaishi 1987,66,210-223.
2 3 4 5 6 7 Time on stream /h Figure 1. Reactivities of the ZnFezO4 samples calcined at 700 "C. Reaction conditions: 10 ppm HzS-20% Hz-307~ CO-10% C02-9.4% HzO-Nz, 500 "C. '0
1
t
Q
a
5 '
a 6
9, Time on stream /h Figure 2. Reactivities of the ZnFezO4 samples calcined a t 800 "C. Reaction conditions: 10 ppm HzS-20% Hz-30% CO-10% COz-9.4% Hz0-Nz, 500 "C.
The reactivity of the zinc ferrite sample depended on the preparation conditions. Some typical results using the samples calcined at 700 and 800 "C are shown in Figures 1 and 2. The samples prepared using NaOH were more reactive than the samples prepared using NH3. The reactivity time dependency of the sample varied according to the preparation conditions. In the cases of ZnFe204(10-7Na,700), ZnFezOd20- 14Na,700), ZnFe204(10-7NH,700), ZnFe204(20-14NH,700), and ZnFe204(20-14Na,800), the outlet concentration of sulfur compounds increased in the initial stage of the experiment and then decreased. These outlet concentration profiles of sulfur compounds suggest that the sample ZnFe204 was changed in the experiments: it was thought that ZnFezO4 was broken down to the constituent oxide or other reduced materials.1° The change of the samples will be discussed hereafter. In the cases of ZnFe204(20-14Na,700) and ZnFe204(107Na,800), the experiments could not be continued for 360 min (the elapsed time of 360 min was set as the standard experimental time) because the flow of the feed gas was obstructed. After the experiments, soot was found on the sorbent bed in the reactor; therefore, it was concluded that soot was formed over the samples and plugged the reactor tube. Soot Formation and Reactivity. To evaluate the soot formation in the experiments using a packed bed reactor, the height of the packed bed and the weight of the sample were measured before and after the experiment, and the ratio of final to initial values was calculated. If soot forms on the sorbent, the ratio of final to initial packed bed height will increase to more than 1.0, and the ratio of final to initial weight of the sample may increase (this will be discussed in more detail hereafter). Table 2 shows the soot formations and weight changes of the samples, and outlet concentration of sulfur
346 Energy & Fuels, Vol. 9, No. 2, 1995
sample ZnFezOd 10- 7Na,600) ZnFezO4(10- 7Na,700) ZnFe204(10-7Na,800) ZnFe204(20-14Na,700) ZnFez04(20-14Na,800) ZnFezOl(10- 7NH,700) ZnFe204( 10-7NH,800) ZnFez04(20-14NH,600) ZnFe204(20- 14NH,650) ZnFe204(20-14NH,700) ZnFe204(20-14NH,750) ZnFe204(20-14NH,800)
Sasaoka et al.
Table 2. Reactivity of Zinc Ferrite Samples and Soot Formation" reaction outlet concn of ratio of final to initial time,b min sulfur compd: ppm packed bed height 0 2.1 220 1.9 0 360 0 2.1 230 0 1.4 210 0 1.3 360 H2S, 3.0; COS, 0.5 1.0 360 1.0 HzS, 4.2; COS, 3.2; S02, 1.1 360 0 1.7 140 0 1.7 180 1.0 360 H2S, 2 1.0 HzS, 5; COS, 1.8;S02, 0.3 360 1.0 HzS, 4.2; COS, 3.0; SOz, 0.3 360
ratio of final to initial sample weight 1.10 1.09 1.11
1.04 0.97 0.99 1.05
1.03 1.00 0.98 0.99
a Reaction conditions: 10 ppm HzS-20% Hz-30% CO-10% C02-9.4% HzO-Nz, 500 "C for 360 min. * Time on stream when the reaction was stopped, because either the reactor was plugged by soot formed or the experimental run was over. Outlet concentration of sulfur compound just before the end of the reaction.
reaction temp, "C 450 500 525 550 575 600
Table 3. Effect of Reaction Temperature on Soot Formation over ZnFe204(10-7Na,700)a reaction outlet concn of ratio of final to initial ratio of final to initial time,b min sulfur compd; ppm packed bed height sample weight 80 1.5 1.00 360 1.9 1.09 1.2 360 0.91 1.2 360 0.85 1.0 360 0.83 1.0 360 0.84
Reaction conditions: 10 ppm HzS-20% &-3o% CO-10% co2-9.4% HzO-Nz, for 360 min. Reaction time when the reaction was stopped, because either the reactor was plugged by soot formed or the experimental run was over. Outlet concentration of sulfur compound just before the end of the reaction.
Table 4. Effect of H2S Concentration and/or H20 Concentration on Soot Formation over ZnFe~O4(10-7Na,700)" reaction outlet concn of ratio of final to initial ratio of final to initial time,b min sulfur compd: ppm packed bed height sample weight 360 0 1.6 0.97 360 0 2.2 1.03 360 0 2.1 1.07 360 0 1.9 1.09 360 H2S, 1.6 1.0 0.90
concn of H2S (ppm) and H2O (%) 200 &S-9.4 HzO 50 HzS-9.4 HzO 17.5 HzS-9.4 Hz0 10 HzS-9.4 HzO 10 HzS-14.1 Hz0
Reaction conditions: H2S-20% &--3o% CO-10% CO2-H20-N2,500 "C for 360 min. Reaction time when the reaction was stopped, because either the reactor was plugged by soot formed or the experimental run was over. Outlet concentration of sulfur compound just before the end of the reaction.
compounds at the end of the elapsed time for the each reaction. In some cases the reaction time shown is less than 360 min as the experiment could not be continued because of soot plugging the reactor tube. From the results listed in Table 2, the effect of the kind of precipitant and the calcination temperature can be evaluated. Results show that soot was formed over the samples reactive for the desulfurization and plugged the reactor tube; the reactivity of ZnFe-204 prepared using NH3 and calcined at the high temperature (2700 "C) was less than that of ZnFezO4 prepared using NaOH; the reactivity of ZnFezO4 prepared using NH3 increased with decreasing calcination temperature, and soot was formed over the samples calcined at the low temperature (5650 "C). In the case of ZnFe204(10-7Na,700), soot was formed but the plugging of the reactor did not occur. From the comparison of this result and other results of the reactive samples (see reaction time and ratio of packed-bed height in Table 2), it was thought that if the rate of soot formation was slow, the soot did not plug the reaction. The weight of sample over which a large amount of soot formed apparently increased as shown in Table 2. Theoretical ratios of weight change (product/ZnFe204) according to the reduction of ZnFe204 are as follows:
-
ZnFe204
ZnO
+ (2/3)Fe30,
0.978
ZnFe20, ZnFe,04 ZnFe20,
-
-
+ 2Fe0 ZnO + 2Fe
ZnO
ZnO
+ (2/3)Fe,C
0.934
0.801 0.834
Therefore, it is thought that the increase of sample weight was mainly due to the soot formation. Table 3 shows the effect of the reaction temperature on soot formation over ZnFe204(10-7Na,700). The outlet concentration of sulfur compound at the end of time on stream for every experiment run was zero. Soot formation increased with the decline in the reaction temperature; soot formation could not be found above 550 "C, but vaporization of Zn was observed in the cases of 575 and 600 'C.19 Table 4 shows the effect of the inlet concentration of HzO and H2S on soot formation over ZnFe204(10-7Na, 700). No effect of the concentrations of H2S was found in the 10-200 ppm range. The increase in concentration of HzO from 9.4 to 14.1% affected the soot formation: the presence of 14.1% H2O prevented the formation of the soot. Measurement of Soot Formation Using TGA. Generally, either a packed-bed reactor or a moving-bed (19) Sasaoka, E.; Hirano, S.; Kasaoka, S.; Sakata, Y. Energy Fuels 1994, 8, 763-769.
Soot Formation over Zinc Ferrite 1201
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Energy & Fuels, Vol. 9, No. 2, 1995 347
90
1
2
3
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Figure 3. Soot formation over the ZnFezO4 samples calcined at 700 "C. Reaction conditions: 20% Hz-30% CO-10% COz9.4% HzO-Nz, 500 "C.
reactor is used as for desulfurization processes, and the sorbent packed downstream from the bed of the reactor is exposed to sulfur-free coal-derived gas. Therefore, even if high concentrations of H2S prevent soot formation,14J5the formation of soot in the absence of sulfur compounds is an important problem in the practical use of ZnFe204. The formation of soot in the absence of H2S was measured using a thermobalance (thennogravimetric analysis, TGA). Figure 3 shows typical results from reacting the gas streams with the ZnFe204 samples: the weight of the samples over which soot was formed initially decreased and then increased. These results indicate that the decrease of the sample weight is induced by a reduction of the sample or the carbonization of iron oxide. Details of the change in the sample weight will be discussed later using XRD data. The soot formed over the samples could be confirmed by visual observation. Furthermore, the soot could be burned out with air and the weight of the sample burned out (at ca. 600 "C for 2 h) was almost same as that of the fresh sample (ratio of burned out to fresh sample weight was 1.02). These results obtained from the experiments using TGA are consistent with the results shown in Figure 1: over the two samples relatively more reactive with H2S [ZnFe204(10-7Na, 700),ZnFe20~(20-14Na,700)1, soot was formed, but over the other two samples [ZnFe204(10-7NH,700), ZnFe204 (20-14NH,700)1, soot formation was not observed. Effects of Coexistent Gases on Soot Formation. As shown in Table 4,the increase of the H2O concentration from 9.4 to 14.1% inhibited the soot formation. From this result, it was suggested that the presence of H2O affects soot formation, and therefore, the effect of the presences of gases which compose a coal-derived gas was examined using the thermobalance reactor. Figures 4 and 5 show typical results of the experiments using ZnFe204(10-7Na,700) samples. Figure 4 shows the effect of H20 on soot formation in the presence of a mixture of H2, CO, and C02: H2O depressed soot formation and soot formation was not observed at a concentration of 14.1% H2O. Figure 5 shows the effect of CO2 on soot formation in the presence of a mixture of H2, CO, and H2O. From these experiments (the effect of CO and H2 is not illustrated), the following results were obtained: (1)H2O and C02 reduce soot formation, and (2) CO and H2 accelerate soot formation. Rate of Soot Formation in the Presence of H2, CO, CO2, and HzO. In the above experiments, the weight of the samples over which soot was formed initially decreased and then increased linearly as shown in Figures 4 and 5 (the linear portion of the curve is
'
I
'
1 2 3 Time on stream / h
4
4
Figure 4. Effect of HzO on soot formation over ZnFe~04(107Na,700) in the presence of Hz, CO, and C02. Reaction conditions: 20% &-so% CO-10% COz-HzO-Nz, 500 "C. 1201
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Figure 5. Effect of COz on soot formation over ZnFezOd(107Na,700) in the presence of Hz, CO, and HzO. Reaction conditions: 20% H2-30% CO-cO2-9.4% HzO-NZ, 500 "C. o.6
7
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0.1 -
shown between the marks I and I). The rate of soot formation can be evaluated from the slope of the linear part of the line, if the weight change of the sample itself (Fez03 FesO4 or Fez03 Fe3C) is negligible. This assumption will be discussed hereafter. Figure 6 shows the dependency of the rate of soot formation (R,) over ZnFe204(10-7Na,700) on the reaction temperature in the 20% H2-30% CO-10% C029.4% H20-Nz system: the rate of soot formation increased with the decrease of the reaction temperature; soot formation was not observed at 550 "C, which may suggest that the soot formed is immediately gasified with H2O and/or C02. Figure 7 shows the effect of Hz concentration (CHJ on the rate of soot formation in the H2-30% CO-10% C02-9.4% H20-N2 system. Ha accelerated the soot formation as shown by the following equation:
-
-
R,= 0.0608 + 5.92
x 10-3CH2
(1)
348 Energy & Fuels, Vol. 9, No. 2, 1995 0.41
,
Sasaoka et al. 1
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30
/ %
Figure 10. Effect of COZ concentration on the rate of soot formation over ZnFe~04(10-7Na,700).Reaction conditions: 20% Hz-30% CO-COZ-(O or 9.41% HzO-Nz, 500 "C. 0, 0% Hz0; 0 , 9 . 4 % HzO.
in the presence of CO, (20% H2-30% CO-lO% C0,-H20-N,):
R, = 2.43 exp(-3.13 x 10-2C,p$)
(3')
in the absence of H,O (20% H2-30% CO-C02-N2):
R, = 5.60 exp(-7.69
cc:
/ ( %
in the presence of H20 (20% H,-30% co-co2-9.4% H,O-N,):
R,= 1.18 exp(-18.4
100 2
CH20 /
200
300
( %
Figure 9. Effect of HzO concentration on the rate of soot formation over ZnFe2O4(10-7Na,70O). Reaction conditions: 20% Hz-30% CO-(0 or 101% C O Z - H ~ O - N ~500 , "C. 0, 0% con; 0,10% co2.
This correlation was obtained from a line of best fit by trial and error. Figure 8 shows the effect of CO concentration (CCO) on the rate of soot formation in the 20% H2-CO-10% co2-9.4% H20-N2 system. CO also accelerated the rate of the soot formation as shown by the following equation:
R,= (4.54 - 1.35 x 102C,,-1)0.5
(2)
Figures 9 and 10 show the effect of H2O and C02 concentration (CH~O, CcoJ on the rate of soot formation. H20 and COa depressed the soot formation and their effect are expressed in the following equations: in the absence of CO, (20% H2-30% CO-H,O-N,):
R, = 5.60 exp(-2.03
(4)
)-l
Figure 8. Effect of CO concentration on the rate of soot formation over ZnFez04(10-7Na,700). Reaction conditions: 20% Hz-CO-10% COz-9.4% HzO-Nz, 500 "C.
0
x 10-2C,,2)
x 10-2CH202)
(3)
x 10-2Cco2)
(4')
These equations suggest that H2O and C02 synergistically reduce the soot formation: the effect of H2O is magnified by the presence of C02, and the effects of CO, are magnified by the presence of H20. From the above results, it is concluded that for soot formation H20 is the most important component and C02 is of secondary importance in the coal-derived gases. Observation of Samples by SEM. In order to investigate the morphology of the soot formed on the samples, the used samples in the thermobalance reactor were utilized, because the samples used in the packedbed reactor turned to powder when they were removed from the reactor. Figure 11A shows a SEM photograph of a fresh ZnFe204(10-7Na,700) sample, and Figure 11B shows a SEM photograph of the sample which was used in the 20%H2-30% CO-Nz system for 12 min at 500 "C in the thermobalance reactor. Both the sample shape and the surface state were drastically changed by the soot formation. Figure 12 shows a example of the mildly treated sample: the sample was used in 20% H2-30% CO-10% C02-9.4% HZO-N~,500 "C for 4 h. In this case, the sample maintained its particle shape, but the particle size expanded and its surface altered. In Figure 12, many fissures can be observed over the surface and also a fibrous or needle-like material is apparent. These fissures and fibrous material were also observed on the sample treated at 400 "C for 1.5 h (not illustrated). As a sample over which soot did not form, ZnFe204 (10-7Na,700) which was used in 20% H,-30% CO-10% co2-14.1% H20-N2, 500 "C for 4 h in the thermobalance reactor was measured. As shown in Figure 13, fibrous material is observed. As another sample which
Energy & Fuels, Vol. 9, NO. 2, 199ij 349
Soot Formation over Zinc Ferrite
Figure 13. Photomicrograph of treated %nE'c>,Ot( 10-7Na, 700) sample (over which soot was not observed). Conditions of treatment: 20% H2-30% CO-lO% C 0 ~ - 1 4 . l % H20-N2, 500 "C, for 4 h.
Figure 11. Photomicrographs of fresh and treated ZnFe204 ( 10-7Na,700) sample. A, Fresh sample. B, Treated sample (over which a large amount of soot was observed). Conditions of treatment: 20% H2-30% CO-N2, 500 "C, for 12 min.
b
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Figure 12. Photomicrograph of treated ZnFe.rO.&10-7Na, 700) sample (over which relatively a small amount of soot was observed ). Conditions of treatment: 20% &-30% CO- 10% co2-9.4% H20-N2, 500 "C, for 4 h.
Figure 14. Photomicrographs f treated ZnFea04(10- 7Na, 700) sample (over which soot was not observed). Conditions of treatment: 10 ppm HzS-20CTc H2-30% CO-10% Cor14.1% H20-N2, 500 "C, for 6 h.
was used in the presence of H2S, and over which soot formation was not found, the ZnFezO4( 10-7Na,700) sample used in 10 ppm H2S-2096 H2-3096 CO-10% co2-14.156 H20-N2, 500 "C for 6 h was observed by SEM. As shown in Figure 14, no fissures are found at
a magnification of 1000 times, but the surface is rough and fibrous material is found a t a higher magnification of 5000 times. From the above examinations, it was confirmed that the fibrous material was produced on the surface of the
350 Energy & Fuels, Vol. 9,No. 2,1995
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Sasaoka et al.
II
Figure 15. SEM photograph and EDX element maps of treated ZnFez04(10-7Na,'iOO) sample ( t h e same a s t h a t in Figurc 13). Conditions of treatment, 20r4 H2-30't CO-10% C02-14.1% H*O-N2,500 "C,for 4 h. Magnification of the photographs, 3 0 0 0 ~ .
sample exposed to the simulated coal-derived gas. To identify the fibrous material on the ZnFe204(10-7Na, 700)sample which was used in 20% H2-30% CO-10% c02-14.1% H20-N2 at 500 "C for 4 h in the thermobalance reactor, it was examined using EDX. In Figure 15, the SEM photograph and the EDX photographs (Zn, 0, and Fe dispersion maps) are shown. A comparison of SEM, Zn, and 0 confirms that the fibrous material over the sample is ZnO. XRD Spectra of Samples. XRD spectra of the samples were measured to understand the reactivity dependencies of the samples on time on stream in the packed-bed experiments. Figure 16 shows the XRD spectra of the samples used in the experiments shown in Figure 1. The samples prepared using NaOH show larger diffraction peaks for ZnO than those prepared using NH3, and diffraction peaks for FesC are found. Diffraction peaks for carbon are not observed in the XRD spectra of the carbon deposited samples prepared using NaOH, indicating that noncrystal carbon was formed over the samples.20 A comparison of Figures 1 and 16 suggests that the sample's reactivity with H2S depends on ZnO formation from ZnFe204. As soot was formed over the samples prepared using NaOH and not over the samples pre(20) Bianchi, D.; Tau, M.; Borcar, 84,375-385.
s.;Bennett, C. 0.J. Catal. 1983,
pared using NH3, it suggests that soot formation is connected to the production of FesC from ZnFezO4. Figure 17 shows the XRD spectra of the ZnFe204(2014NH) samples used in the experiments shown in Table 2. Diffraction peaks for FenC (and/or Fe:jC-like peaks) can be found in the spectra of the samples which were calcined at lower temperature (5650 "C), but it cannot be found in the spectra of high temperature calcined samples; diffraction peaks for ZnO and ZnFezO4 or Fe304 (it is dif'ficult to distinguish ZnFe2Od and FesO4 from the XRD spectra as their diffraction patterns are quite similar to each other) can be found in every sample. The strength of the diffraction peaks for ZnO of the sample increased with decrease of the calcination temperature. The strengths of the diffraction peaks for ZnFe204 (or Fen041 of the samples over which soot formation was observed were considerably lower than those of the samples over which soot formation was not observed. A comparison of Figure 17 and Table 2 suggests that the reactivity of the sample for desulfurization depends on ZnO formation from ZnFe204 and that soot formation is connected to the production of FenC. Figure 18 shows the XRD spectra of the two ZnFe204 (10-7Na,700) samples which were used in 20% H2-30% CO-10% co2-9.4% H20-N2,500 "C in the thermobalance reactor (see Figure 4) in comparison with that of
Soot Formation over Zinc Ferrite
Energy & Fuels, Vol. 9, No. 2, 1995 35 1 I
I
30
40
50
60
P
7
2 8 /degree 30
40
50
70
60
2 8 /degree Figure 16. XRD spectra of the samples used in the experiments shown in Figure 1. I
I
o
30
40
50 2 8 /degree
?
60
?
1
70
Figure 17. XRD spectra of the ZnFez04(20-14NH) samples used in the experiments shown in Table 2.
the unused sample. One of samples was used for 48 min and thermally quenched, and the other was used for 4 h: the time of the former is the minimum point of the sample weight curve and the time of the latter is the end point of the sample weight curve in Figure 4. In the XRD spectrum of the thermally quenched sample, diffraction peaks for ZnFe201 cannot be found but diffraction peaks for ZnO and Fe3C are present. Although the details of the XRD spectrum of the fully used sample differ from those of the thermally quenched sample, they seem t o be roughly similar. The difference
Figure 18. XRD spectra of the (10-7Na,700) samples used in 20% H2-30% CO-10% co2-9.4%H20-N2, 500 "C.
between the two samples may be caused by the soot covering of the fully used sample. As ZnFeaO4 was initially decomposed to ZnO and Fe&, and then the soot formation occurred, it is supposed that the initial reactivity dependency of the sample on time on stream shown in Figure 1 was induced by the initial decomposition of the sample ZnFezO4. Figure 19 shows the XRD spectra of the samples used in the experiment t o clarify the effect of reaction temperature on reactivity and soot formation (Table 3). In every spectrum for all the samples used, distinct diffraction peaks for ZnO are found; in the diffraction patterns of the samples used in the temperature range of 525-575 "C, distinct diffraction peaks for Fe3C are found; the diffraction peaks for Fe3C became smaller and broader with the decrease of the reaction temperature in the 450-525 "C range; in the spectra for the 450-500 "C range and at 600 "C, diffraction peak positions are similar to those of Fe&, but the patterns of the spectra are different from those of Fe3C. This may suggest that the carbide is composed of Fe3C and Fe,C, though the value of x is unknown. In the diffraction peaks of the sample which was used at 450 "C, the peaks of ZnFe204 (or Fe304) can be observed. A comparison of the strength of the peaks and those of the fresh sample peaks suggests that a large part of the sample used was converted to ZnO and Fe,C. Figure 20 shows the XRD spectra of the samples used to investigate the effect of the concentration of H2O on soot formation in the presence of C02, CO, and H2 (Figure 4). In the XRD spectrum of the samples used in the presence of 14.1% H2O (over which soot was not formed) the diffraction peaks of Fe3C can be found as is also the case with the sample used in the presence of 9.4% H2O (over which soot was formed). As the samples over which soot formation was not observed contained carbides, it is confirmed that carbide formation is not directly connected with soot formation. The above XRD results also suggest that the weight gain in the thermobalance experiments is mainly due to carbon deposition. Moreover, the weight change of
Sasaoka et al.
352 Energy & Fuels, Vol. 9, No. 2, 1995
position of ZnFez04,1° and that, as the reactivity of the sample depended on ZnO formation, ZnO predominantly reacts with HzS. The reaction may be expressed by the following scheme:
Soot formation can be described by the following reaction scheme20,21 using a surface-active iron, -Fe*, and a surface-active carbon, C*:
Fe3O4
j Fe3C *
CO,Hz
Fen-FeC* (or
FeA)
-
-Fe*, C, (soot)
u co
2 8 /degree
Figure 19. XRD spectra of the ZnFez04(10-7Na,700) samples used in the experiments shown in Table 3. 0, ZnO; A , Fe&; 0, ZnFez04,Fe304. I
I
I
I
0 1
: ZnFen04, Fe30, o : ZnO
// ,~,#""
14.1%H20
3
In the case of Figure 18, it is suggested that the formation of bulk carbide predominantly proceeds in the system, ZnFezO4 converts to ZnO and Fe3C and followed by soot formation. However, the spectra of some samples (ZnFe~04(20-14Na,700) in Figure 16,ZnFezO4 (20-14NH,600) in Figure 17, and ZnFez04(10-7Na, 700) used at 450 "C in Figure 19) over which the soot formation was observed, contained peaks for ZnFezO4 (or Fe304), albeit at a very lower intensity than in the fresh sample. These results suggest that the conversion of ZnFezO4 to carbide and soot formation simultaneously proceed under some reaction conditions. From these results and considerations, it was thought that soot and carbide formation may be connected to surface active species such as -FeC* shown in the above scheme. If the sample is easily reduced to Fe, carbon is fed into Fe via -FeC*, and therefore the carbide is predominantly produced; if the sample is only reduced to Fe with difficulty, soot is predominantly formed. At relatively high temperatures, the bulk of the sample can easily be reduced with reducing agents; therefore, the formation of Fe3C can predominantly occur; at relatively low temperatures, the reduction of the bulk of the sample is slow and soot formation occurs on the surface Fen-FeC* produced. In this case, active carbon C* may play the role of a reducing agent. The presence of the diffraction peaks for Fe3C in the XRD spectra of the samples over which the soot formation was not observed (see Figure 18)suggested that the active species -FeC* is removed by the following reactions before the carbon can be catalytically formed via the -FeC*: Fen-FeC*
Fen-FeC*
iresh
30
40
50
60
70
2 8 /degree
Figure 20. XRD spectra of the ZnFe~0~(10-7Na,700) samples used in the presence of HzO.
the sample itself is negligible except in the initial stage of the experiment. Mechanistic Consideration of Desulfurization and Soot Formation. From this work, it is suggested that the desulfurization reaction proceeds via decom-
-
+ CO + H, + CO, - Fen-Fe* + 2CO
+ H,O
Fen-Fe*
(5)
(6)
From the above scheme, it may be thought that the soot formed over a sample can react with HzO and COz. Therefore, the sample over which soot was formed in the 20% Hz-30% CO-10% C02-9.4% HzO-Nz system a t 500 "C for 3 h was heated up to 600 "C and held for 2 h in the same system. The weight of the sample decreased only a little. However, this weight change may be due to the vaporization of Zn which was also observed.lg These results suggest that the soot is stable. (21) Muranaga, K. Kogyo Kagaku Zasshi 1961, 64, 1001-1007.
Energy & Fuels, Vol. 9,No. 2, 1995 363
Soot Formation over Zinc Ferrite
That is, the following reactions are unlikely to occur in this system. C H,O -,CO H, (7)
1. Soot formation was accelerated by increases in the CO and H2 concentrations and depressed by increases of in the H2O and C02 concentrations.
c + co, 2co
(8)
2. H2O was the gas component which decreased the rate of soot formation the greatest.
(9) This result also suggests that eqs 5 and 6 are very important in this system for the inhibition of soot formation.
3. At 500 "C under atmospheric pressure, it was impossible to prevent the formation of carbides (Fe3C and Fe,C) when the active ZnFezO4 sample was used.
+
+
,
C
+ 2H,
CHI
,
Conclusion From this experimental study of soot formation over zinc ferrite under the desulfurization conditions of a coal-derived gas for fuel cells, the following results were obtained:
4. To prevent the formations of carbides and soot, the use of a ZnFezO4 sample of relatively low activity such as ZnFe204(20- 14NH,700) is suitable, but the viability of the sample has to be confirmed using the regenerated sample. EF940170V
