Catalytic Hydrodesulfurization of Dibenzothiophene through Partial

Recently, Melius and Bergan (1990) and Katritzuky and his colleagues (1994) suggested that HCOOH may be an intermediate species in the water−gas shi...
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Ind. Eng. Chem. Res. 1998, 37, 2634-2638

Catalytic Hydrodesulfurization of Dibenzothiophene through Partial Oxidation and a Water-Gas Shift Reaction in Supercritical Water Tadafumi Adschiri,* Ryuji Shibata, Takafumi Sato, Masaru Watanabe, and Kunio Arai Department of Chemical Engineering, Faculty of Engineering, Tohoku University, Sendai, 980-8579 Japan

We conducted a comparative study of catalytic hydrodesulfurization of dibenzothiophene (DBT) with NiMo/Al2O3 at 673 K and 30 MPa, in various atmospheres (H2-SCW, CO-SCW, CO2H2-SCW, and HCOOH-SCW), using a tube bomb reactor. Higher conversion of DBT was obtained in CO-SCW, CO2-H2-SCW, and HCOOH-SCW than in H2-SCW. These results clearly indicate that a water-gas shift reaction in supercritical water (SCW) produces species which can hydrogenate DBT more effectively than H2 gas. We also conducted another experiment for the partial oxidation of a DBT-hexylbenzene mixture in SCW. Even in the presence of oxygen, effective hydrogenation of DBT took place. This result is probably because CO forms through the partial oxidation of hexylbenzene and converts to the hydrogenating species through a water-gas shift reaction. We think the catalytic desulfurization of heavy oils in SCW will be a promising new technology, since even by introducing oxygen or air instead of hydrogen, an excellent hydrogenating atmosphere can be supplied. Introduction Supercritical fluids (SCFs) are attractive media for chemical reactions, because of its unique characteristics (Subramaniam and McHugh, 1986; Savage et al., 1995; Saito, 1995). SCF’s solvent power to solutes which are insoluble under ambient conditions and miscibility with gases can supply a homogeneous reaction atmosphere and thus eliminate interphase transport limitations on reaction rates. Many of the physical and transport properties of SCF, including viscosity and diffusivity, are intermediate between those of a liquid or a gas and vary continuously and greatly. This suggests that reactions that are diffusion-limited in the liquid phase could become faster in an SCF phase. The intrinsic reaction rate and reaction equilibrium can also be controlled, since the solvent effect on the reaction (local and bulk dielectric constant, exchange rate of molecules in the cage, nonelectrostatic solvent-solute interactions, etc.) can be manipulated by changing pressure. Recently, quite a few application studies of reactions at supercritical conditions were reported, which include fuels processing, biomass conversion, biocatalysis, homogeneous and heterogeneous catalysis, environmental control, polymerization, materials synthesis, and chemical synthesis (Subramaniam and McHugh, 1986; Savage et al., 1995; Saito, 1995). Some researchers reported on solid-catalyzed reactions in supercritical fluids (Adschiri et al., 1991, 1994, 1995; Baptist-Ngyugen and Subramaniam, 1992; Saim and Subramaniam, 1991; Tilscher et al., 1981, 1984; Yokoto and Fujimoto, 1991; Yokota et al., 1989, 1991). In our previous works (Adschiri et al., 1991, 1994, 1995), we studied catalytic denitrogenation from coal tar pitch by hydrotreatment in a supercritical toluene-tetralin * Author to whom correspondence should be addresed. E-mail: [email protected]. Fax: +81-22-2177246.

mixture. A conventional method for the nitrogen removal from coal tar pitch is the catalytic hydrotreatment with H2 gas. Problems involved in this process are the terrible coke deposition on the catalyst and the slow reaction due to slow diffusion of H2 in the molten pitch onto the catalyst surface. In those papers we demonstrated that these two problems can be solved by using supercritical toluene-tetralin as a reaction media. Coke deposition could be suppressed due to the in situ extraction of a coke precursor during the reaction, and a faster reaction was achieved because of the homogeneous reaction atmosphere for H2-SCF pitch. Similar specific features of solid-catalyzed reactions in SCF have been reported for the other reactions. Tilscher (1981, 1984) first proposed to use SCF as a reaction solvent for a solid-catalyzed reaction with showing the result of in situ extraction of a catalyst inhibitor by SCF. Yokota et al. (1989, 1991a,b) intensively studied Fischer-Tropsch synthesis in supercritical n-hexane. The problem of the liquid-phase reaction is the slow reaction due to the limited mass-transfer rate. On the other hand, the problem of the gas-phase reaction is the deactivation of the catalyst by the deposition of wax formed. They reported that both fast reaction and in situ extraction of wax were performed in supercritical n-hexane. Saim and Subramaniam (1991), and BaptistNguyen and Subramaniam (1992) studied the catalytic isomerization of hydrocarbons. They not only reported the effective in situ extraction of the coke precursor from a catalyst surface but also performed a detailed kinetic study for explaining the results. In this study we intend to use supercritical water (SCW) as a reaction solvent for catalytic desulfurization from light or heavy oil, expecting similar results, as both hydrogen and oils are miscible in SCW. In this reaction, we expected another specific feature, that is, desulfurization by in situ hydrogen generation through a watergas shift reaction. Kumar, Akgerman, and Anthony (1984) and Hook and Akgerman (1986) conducted the

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experiments of desulfurization of dibenzothiophene by feeding CO and steam to a catalyst bed (CoO-MoO3/Al2O3) at 58-69 atm of pressure and 595-648 K of temperature. They reported that by the in situ generation of hydrogen through a water-gas shift reaction (CO + H2O ) CO2 + H2), 1 order of magnitude faster desulfurization took place (Hook and Akgerman, 1986). Stenberg et al. (1982) reported that H2S produced through this reaction can be a promoter for the watergas shift reaction. This suggests an autocatalytic reaction for the hydrodesulfurization by a water-gas shift reaction, which would be another advantage of desulfurization in SCW. A similar hydrogenating effect of a water-gas shift reaction was reported for the extraction of coal with a CO-water mixture at 400 °C (Ross et al., 1984) and extraction of oil sand at 400 °C and 14-24.5 MPa (Berkowitz et al., 1990). However, most of these studies are for the reaction in vapor steam or low-density SCW phase, but not for the dense SCW phase where the homogeneous reaction atmosphere for H2-oils-SCW and inhibition of coke deposition on catalysts are to be expected. Despite these supporting results, we could not have confidence that desulfurization of DBT proceeds through a water-gas shift reaction even in dense SCW, because controlling factors of the reaction, including elementary reaction rates, reaction equilibrium, sorption equilibrium on the catalyst, catalyst activity, and catalyst stability, should be significantly different from that in a low-density atmosphere, due to the great change of fluid properties and its solvent effects on the reaction around the critical point. Thus, in this paper, first of all, we show that, even in the dense SCW phase, desulfurization proceeds more effectively in CO-SCW than in H2-SCW. Next, we also report a new finding that the hydrogenation can be enhanced just by the addition of CO2 to H2-SCW, namely in a H2-CO2-SCW atmosphere. We also conducted the experiment in HCOOH-SCW. This series of experiments clarifies the role of a water-gas shift reaction (CO + H2O ) CO2 + H2) on the desulfurization in SCW. In this paper, we would like to stress more on the result of the experiment with O2-SCW. Even by introducing oxygen instead of hydrogen, excellent hydrodesulfurization took place.

CO, H2 + CO2, or O2) was introduced. The loaded amount of gases was evaluated from the pressure of loading (H2, 2.7 MPa; CO, 2.7 MPa; H2, 2.7 MPa + CO2, 2.7MPa; O2, 1.35 MPa). The molar amount of gas loaded was 20 times larger than that of DBT. HCOOH was also used as a reactant instead of these gases. In the HCOOH experiment, 18 mg of HCOOH was loaded with the DBT-toluene solution and distilled water. The amount of HCOOH corresponds to CO2 (2.7 MPa) + H2 (2.7 MPa) or CO (2.7 MPa) loading, if HCOOH decomposes completely to CO2 + H2 or CO + H2O. In the experiment of O2 loading, the DBT-hexylbenzene solution was used instead of a toluene solution. The loaded reactor was submerged in a molten salt bath (KNO3 (50 wt %)-NaNO3 (50 wt %) mixture, T1, Hoei kagaku) whose temperature was controlled to be 674 K, and the reaction had started. The temperature of the reactor was monitored by the thermocouple. Heat-up time required was around 30 s. After a reaction time of 5-60 min, the reactor was taken out of the bath and rapidly cooled in a cold water bath. The reactor was connected to a syringe (50 cm3) to collect the produced gas and measure its volume. To recover as much as possible of the gases dissolved in water, the reactor was submerged in a hot water bath with ultrasound vibration during the gas sampling. The produced gas was analyzed by GC-TCD (Shimadzu, model GC-8A and Hitachi, model GC-163). Liquid products with water were collected in HPLC-grade acetone. The reactor was then triple-rinsed with acetone, and an external standard, naphthalene, was added for analytical purpose. Composition of the solution was analyzed by GC-FID (Hewlett-Packard, model 5890 series II). Conversion of DBT was evaluated from the amount of DBT recovered and the amount of DBT loaded. Yield of the products, biphenyl and cyclohexylbenzene, was evaluated by a benzene structure basis.

Experimental Section

Results and Discussion

In this study dibenzothiophene (DBT) was used as a model sulfur compound. DBT of 98% purity was purchased from Aldrich. Toluene of 99.9% purity and hexylbenzene of 99.9% purity used for preparing the solution of DBT were purchased from Wako Chemicals. The NiMo catalyst supported on porous Al2O3 (KF-842, NiMo/Al2O3) used was from Nippon Ketjen. The size of the catalyst is 1.32 mm × 1.1 mm × 3.4 mm, and the surface area was 194 m2/g. The catalyst was used after 2 h of H2S (10%)-H2 gas treatment at 673 K and 0.5 MPa. Experiments were conducted with a SUS 316 tube bomb reactor (6 mL) with a high-pressure valve and a thermocouple. For the accurate loading of a small amount of DBT, the DBT (10 wt %)-toluene solution was prepared. About 0.3 g of the DBT-toluene solution and 2.52 g of distilled water were loaded with 1 g of the NiMo/Al2O3 catalyst in the reactor. After the air in the reactor was displaced with Ar gas, a reactant gas (H2,

Water-Gas Shift Reaction. Prior to the DBT desulfurization experiment, it was examined if the water-gas shift reaction proceeds in SCW with the employed catalyst (NiMo/Al2O3). The loaded amount of catalyst (1 g), CO (2.7 MPa), and water (2.52 g) were the same as for the following DBT desulfurization experiments (CO-SCW, H2-SCW, H2-CO2-SCW, HCOOH-SCW, and O2-SCW), but neither DBT nor toluene (nor hexylbenzene) was loaded. Because of the difficulty in the complete sampling of produced gases from the small reactor, accurate and quantitative analysis of the products could not be achieved. Nevertheless, we observed the tendencies that CO yield gradually decreased up to the conversion level of 50% at 60 min, while CO2 and H2 yield increased. This result suggests that the water-gas shift reaction proceeds under this condition. On the other hand, in the case that the catalyst was not loaded, the water-gas shift reaction did not take place. Also, when the catalyst

conversion ) 1 -

yield )

amount of DBT recovered × amount of DBT loaded 100 [mol %]

mol of benzene structure in product × mol of benzene structure in DBT loaded 100 [mol %]

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Figure 1. Yields of DBT and its decomposition products (biphenyl and cyclohexylbenzene) in the H2-SCW experiment at 673 K and 0.42 g/cm3 water density.

had not been treated with H2S, the reaction observed was negligible. Thus, in the following experiments, the H2S-treated catalyst (NiMo/Al2O3) was used. H2-SCW Experiment. Figure 1 shows the results of the H2-SCW experiment. The major products were biphenyl (BP) and cyclohexylbenzene (CHB). Prior to the experiment, an experiment in neat SCW was conducted (673 K, 0.42 g/cm3) and it was confirmed that a negligible reaction takes place in 30 min under the condition. Thus, the results shown in Figure 1 are due to the hydrogenation of DBT. Curves in this figure are drawn by free hand just for making clear the trends of variation for product yields. H2S gas formation was detected by a gas detect tube (Kitagawa, model APA), although its quantitative analysis had not been done. To check the products that originated from toluene, an experiment of toluene-H2-water (without DBT) was conducted. In this case, trace amounts of benzene, methylcyclohexane, and unidentified polymers were formed, but neither BP nor CHB was detected. In the DBT experiment, BP and CHB formed as the main products and the benzene structure mass balance (1DBT yield ) BP yield + CHB yield) was fairly good, as shown in Figure 1. Thus, it was confirmed that BP and CHB were produced by the hydrogenation of DBT. Then, for examining the effect of toluene on the DBT hydrogenation, experiments without toluene were also conducted as a supporting experiment. Both DBT conversion and yields of BP and CHB were more or less the same as those for the experiment with a tolueneDBT solution, which indicates the insignificant effect of toluene on the DBT hydrogenation. Therefore, the loading method of DBT-toluene solution was employed in the following experiments, for the accurate loading of DBT. CO-SCW Experiment. Figure 2 shows the results of the CO-SCW experiment. Also, in this case, BP and CHB formed as the main products and the benzene structure mass balance was fairly good. During this reaction H2 formed, which is obviously due to the water-gas shift reaction. The results of the H2-SCW experiment are also shown in this figure, which clearly show that the faster reaction took place in CO-SCW than in H2-SCW. Since the loaded amount of CO and H2 were the same in these two experiments, if the role of the water-gas shift reaction was just producing H2, the reaction rate in the CO-SCW experiment would have been equal to or less than that in H2-SCW. The obtained result of

Figure 2. Variation of major product yields in the CO-SCW experiment at 673 K and 0.42 g/cm3 water density and its comparison with the results of the H2-SCW experiment.

Figure 3. Variation of major product yields in the CO2-H2-SCW experiment at 673 K and 0.42 g/cm3 water density and its comparison with the results of the H2-SCW experiment.

the faster reaction in CO-SCW suggests that the species formed in the water-gas shift reaction from CO has higher hydrogenating reactivity than the H2 gas, as previously suggested by Hook and Akgerman (1986) for a vapor-phase reaction. CO2-H2-SCW Experiment. Next, the CO2-H2SCW experiment was conducted. Prior to this experiment, as a comparison, the CO2-SCW (without H2) experiment was also conducted to confirm that CO2 itself is inert for the reaction under the present conditions (673 K, CO2:2.7 MPa). However, in a CO2 (2.7 MPa)-H2 (2.7 MPa)-SCW atmosphere, as shown in Figure 3, a higher conversion of DBT and higher yields of DB and CHB were obtained than those in H2-SCW. In this case, a trace amount of CO was observed as a product. This is probably due to the reverse watergas shift reaction (CO2 + H2 ) CO + H2O), despite that the equilibrium of the reaction is on the H2 production side because of the excess amount of water. The analogy of the above-mentioned reaction in COSCW leads to the hypothesis that the enhancement of the desulfurization by the addition of CO2 is due to the formation of strong hydrogenating species through an H2O-H2 exchange reaction in the water-gas shift reaction. The difference between the H2-SCW and H2CO2-SCW experiments shown in this figure is attributed to the replacement of initially loaded H2 with active hydrogenating species through a water-gas shift reaction.

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Figure 4. Variation of major product yields in the HCOOH-SCW experiment at 673 K and 0.42 g/cm3 water density and its comparison with the results of the H2-SCW experiment.

If H2 were first converted to CO and then generated hydrogenating species, the hydrogenation rate observed in H2-CO2-SCW would have been slower than that in CO-SCW. However, the similar hydrogenation rates were observed in these two experiments. Although the detailed mechanistic and kinetic studies are necessary for explaining this result, at the moment we think this is because active hydrogenating species are formed also from CO2 through a reverse water-gas shift reaction. HCOOH-SCW Experiment. Recently, Melius and Bergan (1990) and Katritzuky and his colleagues (1994) suggested that HCOOH may be an intermediate species in the water-gas shift reaction in SCW. Melius and Bergan (1990) calculated the energy barrier for the water-gas shift reaction by assuming that watersolvated HCOOH is an intermediate species. They demonstrated that, with increasing the solvation around HCOOH molecule, the activation energy of the watergas shift reaction reduces. Although HCOOH is supposed to be just a candidate for intermediates of the water-gas shift reaction in SCW, as in our preliminary experiment it was observed that HCOOH decomposed to CO2 and H2 in SCW with the same catalyst, the experiment in HCOOH-SCW was also conducted. As shown in Figure 4, similar conversion of DBT and yields of BP and CHB to those in the CO-SCW or H2-CO2-SCW experiment were obtained. This result does not directly lead to the conclusion that HCOOH itself is the hydrogenating species. However, the result implies the similar reaction mechanism among CO-SCW, H2-CO2-SCW, and HCOOH-SCW. Hydrodesulfurization of DBT through Partial Oxidation. Next, we conducted another experiment in O2-SCW. In this case we prepared a DBT-hexylbenzene solution as a model oil, since alkylbenzene is a major component of light-heavy oils. The result of the experiment was much more than we expected. As shown in Figure 5, DBT conversion was reached around 50%, BP yield 27%, and CHB yield 3%. As in prior experiments, we also conducted an experiment of DBT oxidation without hexylbenzene and an experiment of hexylbenzene oxidation. In both cases, neither BP nor CHB was formed. Thus, we confirmed that BP and CHB was produced from DBT. In this experiment, any sulfoxide was not observed. The evidence of biphenyl, cyclohexylbenzene formation suggests the hydrogenative reaction. This means that even by introducing oxygen, effective hydrogenation of DBT takes place. In

Figure 5. Variation of major product yields in the O2-SCW experiment at 673 K and 0.42 g/cm3 water density.

Figure 6. Effect of water density on composition of produced gas in partial oxidation of hexylbenzene at 673 K (O/C molar ratio: 0.3).

this experiment, the hydrogenation of DBT to form DP and CHB in SCW is probably due to CO formation by the partial oxidation of hexylbenzene, followed by the formation of hydrogenating species through a watergas shift reaction. To examine CO formation from hexylbenzene, another series of experiments of hexylbenzene oxidation were conducted at 673 K and at different water density (loaded amount of water/reactor volume). The water density was varied in the range from 0 to 0.42 g/cm3 by changing the loaded amount of water. The reaction time was 5 min. Figure 6 shows the analysis result of the produced gas. The gas composition was evaluated as moles of gas divided by the total moles of collected gases. The results shown in this figure are the average of three or more runs. The reproducibility of the experiment is within the error of 2-3%. As shown in this figure, at 0 g/cm3of water density (without water loading) or at a very low water density (0.1 g/cm3), CO2 was the main products. However, with an increasing water density, the CO2 ratio tends to decrease while the CO ratio increases. At a water density of 0.25 and 0.42 g/cm3, CO became a major product. Under these conditions, H2 formation was observed. If the H2 is formed via the water-gas shift reaction from initially produced CO, the initial CO composition should be extremely high. The reason the CO ratio at 0.25 g/cm3 is higher than that at 0.42 g/cm3 is not clear. At the moment we think one reason is due to phase change around that water density. Brunner (1990) reported that fluid-fluid phase separation takes place for the hydrocarbon-SCW sys-

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tem at a higher pressure around the critical point. For example, critical loci of eicosane and tetradecane at 673 K is on 39 MPa and 47 MPa, respectively. The total pressure of hexylbenzene-SCW-O2 at 0.42 g/cm3 in this experiment is estimated to be nearly 40 MPa. If the hexylbenzene-SCW phase separation takes place at 0.42 g/cm3, the oxidation takes place in both the hexylbenzene-rich phase and SCW-rich phase and thus the result would be totally different from that in the homogeneous reaction at the lower pressure. Alkylbenzene is a major component of light or heavy oils. Therefore, even for using actual oils, probably effective CO formation takes place through its partial oxidation in SCW. Thus, we think the catalytic desulfurization of light-heavy oils in SCW will be a promising new technology, because even by introducing oxygen or air instead of costly hydrogen, an excellent hydrogenating atmosphere can be supplied. The corrosion of the reactor is a problem to be solved for the SCWO process, especially for a halogen-containing system. Under the present experimental conditions, significant corrosion has not been observed for the stainless steel reactor used. This is probably because of the absence of halogen in the system, very relatively low oxygen concentration against normal SCWO, and the reducing atmosphere due to the formation of CO or H2. Also, in the present experiment, the reactor after several runs was used to confirm that the catalytic effect of the reactor wall on the reaction is not significant. However, we think the selection of the reactor wall is also an important point in the development of the actual process. For the catalyst, we used a NiMo/Al2O3 catalyst for all the series of experiments in this study. However, we do not think this is the best catalyst for this system. Also, some sorts of activity change or catalyst characteristic change may take place in SCW for this catalyst after a long run. Under the present condition, clear catalytic activity change was not observed even after several runs. The appearance of the catalyst (shape, size, or morphology) after the reaction was not changed significantly, although the detailed evaluation of the catalyst characteristics after the reaction had not been conducted. Since the point of this study is not for the catalyst itself, but for the reaction atmosphere, detailed analyses on the catalyst had not been conducted. However, we think the study on the catalyst for an SCW atmosphere is also important for the development of this process. Conclusions We have completed a comparative study of dibenzothiophene (DBT) hydrogenation with NiMo/Al2O3 at 673 K and 30 MPa, in various atmospheres (H2-SCW, CO-SCW, CO2-H2-SCW, and HCOOH-SCW). Higher conversions of DBT and yields of BP and CHB were obtained in CO-SCW than those in H2-SCW. This result is due to the formation of active hydrogenating species through a water-gas shift reaction (CO + H2O

) CO2 + H2). Also, in CO2-H2-SCW, similarly higher DBT conversion and yields of BP and CHB were obtained than those in H2-SCW. This suggests that the hydrogenating species forms through the H2-H2O exchange in a water-gas shift reaction. Also, in HCOOH-SCW, similar results were obtained. The results of these series of experiments suggests the similarity of the reaction mechanism of producing hydrogenating species during a water-gas shift reaction in SCW. We also conducted another experiment for the partial oxidation of a DBT-hexylbenzene solution in SCW. Even in the presence of oxygen, effective hydrogenation of DBT took place. This result is due to the CO formation by the partial oxidation of hexylbenzene, followed by its conversion to the hydrogenating species through a water-gas shift reaction. Acknowledgment The authors thank for the Grant-in-Aid for Scientific Research on Priority Areas (07242207, 08232210, and 09218205) the Ministry of Education and Culture. Literature Cited Adschiri, T.; Suzuki, T.; Arai, K. Fuel 1991, 70 (12), 1483-1484. Adschiri, T.; Nakata, K.; Ogasawara, S.; Arai, K. Kagaku Kogaku Ronbunshu (Chem. Eng. Jpn.) 1994, 20, 965-970. Adschiri, T.; Nagashima, S.; Arai, K. Proc. Int. Conf. Coal Sci. 1995, 1407-1410. Baptist-Nguyen, S.; Subramaniam, B. AIChE J. 1992, 38, 10271037. Berkowitz, N.; Calderon, J. Fuel Process. Technol. 1990, 25, 3344. Brunner, E. J. Chem. Thermodyn. 1990, 22, 335-353. Hook, B.; Akgerman, A. Ind. Eng Chem. Process Des. Dev. 1986, 25, 278-284. Katritzuky, A. R.; Barcock, R. A.; Balasubramanian, M.; Greenhill, J. V.; Siskin, M.; Olmstead, W. N. Energy Fuels 1994, 8, 498506. Kumar, M.; Akgerman, A.; Anthony, R. G. 1984, 23, 88-93. Melius, C. F.; Bergan, N. E. 23rd Symp. (Int.) Combust. 1990, 217. Ross, D. S.; Blessing, J. E.; Nguyen, Q. C; Hum, G. P. Fuel 1984, 63, 1206-1210. Saim, S.; Subramaniam, B. J. Catal. 1991, 131 (2), 445-456. Saito, S. J. Supercrit. Fluids 1995, 8 (3), 177-204. Savage, P. E.; Gopalan, S.; Mizan, T. I.; Martino, C. J.; Brock, E. E. AIChE J. 1995, 41 (7), 1723-1778. Stenberg, V. I.; Raman, K.; Srinivas, V. R.; Baltisberger, R. J.; Woolsey, N. F. Angew. Chem., Int. Ed. Engl. 1982, 21 (8), 619620. Subramaniam, B.; McHugh, M. A. Ind. Eng. Chem. Process Des. Dev. 1986, 25 (1), 1-12. Tilscher, H.; Wolf, H.; Schelchshorn, J. Angew. Chem., Int. Ed. Engl. 1981, 20 (10), 892-894. Tilscher H.; Wolf, H.; Schelchshorn, J. Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 897-900. Yokota, K.; Fujimoto, K. Ind. Eng. Chem. Res. 1991, 30 (1), 95100. Yokota, K.; Hanakata, Y.; Fujimoto, K. Fuel 1989, 68, 255-256. Yokota, K.; Hanakata, Y.; Fujimoto, K. Fuel 1991, 70, 989-993.

Received for review October 27, 1997 Revised manuscript received April 4, 1998 Accepted April 7, 1998 IE970751I