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Conference Article: Relation between gasification rates and gas desorption behavior with metallic impurities of carbon and graphite materials for the HTTR

Nomura, S.; Fujii, K.; Shindo, M. (Japan Atomic Energy Research Inst., Tokai, Ibaraki, (Japan). Dept. of Fuels and Materials Research); Imai, H. (Research Association for Nuclear Facilities Decommissioning, Tokai, Ibaraki (Japan))

Abstract

Gasification rates of carbonaceous materials by water vapor and carbon dioxide were strongly dependent upon both the kind of metallic impurities and their concentrations present in the materials. Similar relations were also held for gas desorption behavior from the carbonaceous materials. Seventeen kinds of carbons and graphite were oxidized in helium flow containing 0.65% of water vapor and carbon dioxide in the temperature range from 800 to 1000 deg. C. The rates of reaction with carbon dioxide were relatively equal to those with water vapor. The gasification rates increased with the sum of contents of transition metal impurities Fe, Ti, V and Ni. Structural parameters of the carbonaceous materials, the degree of graphitization, the crystallite size Co and c-axis lattice strain of the graphite crystallite hardly affected and gasification rates. The compensation effect between the activation energies and the pre-exponential factors of the reaction was found to exist in both gasifications. This means that the gasification is governed by catalytic action of the metallic impurities. Isokinetic temperatures, at which all catalyst lose their activity, were 1230 deg. C for the reaction with water vapor and 1120 deg. C with carbon dioxide. Six kinds of carbonaceous materials and some metallic impurities doped natural graphite powder compacts were used for the gas desorption experiments. Gases evolved from the materials up to 1000 deg. C were CO2, CO, H2, CH4 and physically adsorbed species, N2, O2 and H2O. Total volume of the former group of gases evolved from the graphite materials was correlated to both. Their ash contents and the gasification rates with water vapor at 1000 deg. C. The effect of metallic impurities Ca, Fe, Ni, Al and Si on the former gas species evolution behavior was also investigated.

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key words: activation energy; adsorption isotherms; carbon; carbonaceous materials; desorption; gases; gasification; graphite; httr reactor; impurities; materials testing; oxidation; reaction kinetics; enriched uranium reactors; experimental reactors; gas cooled reactors; graphite moderated reactors; helium cooled reactors; htgr type reactors; kinetics
Reference:
Specialists' meeting on the status of graphite development for gas cooled reactors. Tokai, Ibaraki (Japan). 9-12 Sep 1991
International Atomic Energy Agency, Vienna (Austria)
IAEA-TECDOC--690, pp:159-168