Tetsuya Haga

Composite catalysts for carbon gasification

Abstract The activities of a variety of binary and a ternary catalysts for carbon gasification have been compared. Equimolar mixtures of various metal salts, 0.5 mol-% in total cation concentration, were loaded onto an activated carbon by simultaneous impregnation of their nitrates, and the samples were subjected to gasification at 800°C in 50% steam—helium and/or at 850°C in 50% steam—hydrogen. It was found that some composite catalysts such as Na + Ca, Na + Fe, Ca + Fe and Na + Ca + Fe showed higher activities than the sum of the individual constituents. The nature of the gasifying agent influenced the catalytic effects. The Na + Ca, Ca + Fe, Na + Fe and also Na + Ca + Fe catalysts are generally effective for gasification with oxygen-containing gases (steam, carbon dioxide and a mixture of the two); and Fe + Ca and Fe + Ca + Na also enhance hydrogasification. Some dual catalysts were successfully applied to steam gasification of coal char. The results are discussed in association with the catalytic features of sodium, calcium and iron group metals.

Promotion of nickel-catalyzed hydrogasification of carbon by alkaline earth compounds

Abstract A pitch coke was impregnated with nickel nitrate and some additives and gasified in an atmospheric hydrogen flow, to investigate the possibility of promoting nickel-catalyzed hydrogasification by the addition of foreign components. Alkaline earths and aluminum nitrate, which were not direct catalysts for the hydrogasification of carbon, enhanced methane formation markedly, while nitrates of potassium, chromium, and iron and also potassium carbonate had almost no effect. Additives which are effective are believed to be in the form of an oxide. In the presence of nickel, methane is formed at two separate regions: 400–700 °C and above 750 °C. All the promoters increased conversion in the lower temperature reaction while only calcium salt enhanced it in the higher temperature reaction. Promoters, especially magnesium, extensively suppressed the sintering of nickel. The enhancement of methane formation at lower temperatures is ascribed to the increased dispersion of nickel. Possible reasons for the promotions are discussed.

Influence of structural parameters on coal char gasification: 1. Non-catalytic steam gasification

Various coal chars prepared at 750 °C from a series of coals (wt% C = 66.1–88.1daf) and at 240–1000 °C from one coal (%C = 81.9), were gasified at 800–850 °C with an equimolar H2OHe or equimolar H2OH2 mixture. The structure of the char was characterized by X-ray diffraction and gas adsorption measurements and compared with the reactivity. It was found that a crystalline structure parameter obtained by the X-ray method sensitively reflected the reactivity of the char.

Role of MgO and CaO promoters in Ni-catalyzed hydrogenation reactions of CO and carbon

Abstract The roles of Mg and Ca promoters in Ni-catalyzed hydrogasification of carbon were investigated with emphasis on nickel dispersion and nickel-carbon interactions during gasification. The dispersion of nickel on carbon, which was estimated by means of XRD, H 2 -TPD, and CO methanation test, was (Ni + Mg)a(Ni + Ca)>Ni. The increased dispersions of nickel by the Mg and Ca added were found to result in the enhanced NiC interaction, which was monitored through the desorption of CO from oxygen remaining on carbon, and, therefore, in the promotions of Ni-catalyzed lower-temperature gasification at 400 ≈ 700°C. The Ca promoter was likely to give surface CaO(OOO) species upon heating up to 550°C and release CO 2 above ≈650°C. The CO 2 released is efficiently converted by the nickel adjacent to CaO into CO to create Ni(OC species above 700°C, resulting in acceleration of Ni-catalyzed gasification above 700°C. In situ oxygen transfer to Ni/C during reaction is the most important role played by the Ca promoter.

Enhancements in nickel-catalyzed hydrogasification of carbon by surface treatments

Abstract A pitch coke was given 4 types of pretreatment to modify the surface state; air oxidation, heatingquenching, steam treatment and hydrogen treatment. The treated cokes were impregnated with nickel and gasified in an atmospheric hydrogen flow. The catalytic reactivity of the pitch coke was enhanced by these pretreatments. Several properties of pretreated cokes were compared and it seemed that highly hydrophilic or acidic surfaces of carbon were unfavorable to metal-catalyzed hydrogasification. The reactivity enhancement was ascribed to an increase in the hydrogen adsorption capacity of nickel on the pretreated coke. The oxygen-containing surface groups are presumed to inhibit the spillover of atomic hydrogen from nickel to carbon. The effect of 1 type of pretreatment. heating-quenching, seems to relate to the expansion in pore volume.

A kinetic feature of catalytic gasification of carbons—Activation of nickel and iron catalysts during gasification

Abstract A unique rate increase with time in catalytic gasification of carbons by hydrogen was examined as to its dependence on reaction conditions, catalyst species, carbon substrates and so on, to clarify the reason of the phenomenon. It was marked with a porous substrate at relatively low temperature when nickel or iron was the main catalyst. By heating carbon substrate in hydrogen at 850 to 900°C before catalyst impregnation, the rate increase disappeared. Comparing several possible explanations, it is concluded that the rate increase with conversion is due to the recovery of metal catalyst from deactivation caused by heteroatoms on carbon surface. The hydrogen pretreatment resulted in a high rate of gasification with hydrogen as well as steam.

Studies on catalytic gasification of coal chars. Part 1. Development of pores during gasification

Abstract Chars prepared from three coals were impregnated with nickel catalyst and gasified with hydrogen at 850°C and 10 atm. The surface area and porosity of gasified residues at several conversions were measured. Blair Athol char, which had a relatively large surface area, showed a unique gasification pattern, i.e., the rate increased with time until a maximum was attained. Catalytic gasification resulted in an increase in the volume of macropores of a given diameter range, which was characteristic of the catalyst system used. In non-catalytic gasification and in cases when activity of the catalyst was low, mainly pores smaller than 6 nm were enlarged. Possible explanations of the acceleration of the rate are discussed and some consequences of the porosity measurements for this explanation are indicated. As the macropores are enlarged during catalytic gasification, the kinetic pattern could be modified when intraparticle diffusion comes into play.