A study of the role of alkali metal salts as char gasification catalysts by Knudsen cell mass spectrometry

Abstract

Knudsen cell mass spectrometry has been used to determine the gaseous species in thermodynamic equilibrium with admixtures of carbon and alkali metal salts. Both halides and carbonates were used as additives. In addition to measurements of equilibrium vapor pressures, the effect of added steam or carbon dioxide on the composition of the Knudsen cell effusate was examined. With the carbonates, the equilibrium pressure of the alkali metal over the solid phase is at least twice that of the decomposition pressure of the pure salt, but orders of magnitude below that appropriate to the carbothermic reduction of the carbonate. By way of contrast, the alkali halide-carbon admixtures exhibit no free metal vapor up to 1000 K, a temperature in the range employed for gasification. Apparently, the carbonates react with the carbon to form a stable phase with alkali metal activity substantially below unity. Inferences on the composition of such a phase and its role in promoting the carbon gasification rate are discussed.