Fate of some trace elements in fluidised-bed combustion and gasification processes

Coal and municipal waste (MSW) contain many trace elements which are released upon combustion and gasification and have toxic forms at certain concentrations.These pollutants must be removed, if increasingly stricter emission standards are to be met.Four metals, namely: cadmium (Cd), lead (Pb), zinc (Zn), and mercury (Hg), were selected for study and multicomponent, multiphase equilibrium calculations were carried out to determine which elements volatilise upon combustion and gasification and which ones condense upon cooling the flue gases.It was also intended to determine the distribution of the trace element species in the product streams.Both fluidised bed combustion and gasification processes were studied having coal and municipal solid waste as fuel.A temperature range of 200-1 300 °C and two operating pressures of 1 and 10 bar were employed.Air ratio was varied from 0.3 to 2.0, under both operating pressures.The results show some interesting changes in the composition of the combustion gases with changes in temperature as well as with the air ratio.Cd, Hg, and Pb seem to be totally volatilised at 700-900 °C temperature range, but Zn only partially.Upon cooling, sulphates are formed in oxidising, but sulphides in reducing atmosphere.