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The best catalysts for promoting char gasification are Group I metals, particularly lithium and potassium, although other metals are active to a lesser extent. The most prevalent metal naturally in biomass char is potassium, which is not only inherently active, but volatilises to become finely distributed throughout the char mass. The formation of an active carbon/potassium complex is frequently proposed. Calcium is the other most common active metal found in biomass, but is far less effective and less volatile. In a gasification system the metals remain as carbonate due to the action of carbon dioxide. The alkali metals can react with silica to form silicates, which prevents catalytic action. Transition metals can also participate in catalysis of gasification; iron accelerates gasification and nickel prevents carbon deposition, which helps in conditioning biomass-derived syngas. Volatile iron pentacarbonyl has been identified as a promoter of the char gasification step, with catalytic activity related to the finely dispersed low-valency metal atoms generated during the thermo-decomposition of biomass.

The literature on the presence of heavy metals in contaminated wastes is reviewed. Various categories of materials produced from domestic and industrial activities are included, but municipal solid waste, which is a more complex material, is excluded. This review considers among the most abundant the following materials - wood waste including demolition wood, phytoremediation scavengers and chromated copper arsenate (CCA) timber, sludges including de-inking sludge and sewage sludge, chicken litter and spent pot liner. The partitioning of the metals in the ashes after combustion or gasification follows conventional behaviour, with most metals retained, and higher concentrations in the finer sizes due to vaporisation and recondensation. The alkali metals have been shown to catalyse the biomass conversion, particularly lithium and potassium, although other metals are active to a lesser extent. The most prevalent in biomass is potassium, which is not only inherently active, but volatilises to become finely distributed throughout the char mass. Because the metals are predominantly found in the ash, the effectiveness of their removal depends on the efficiency of the collection of particulates. The potential for disposal into soil depends on the initial concentration in the feed material.