Gas Phase Alkali Chlorides and Deposits during Co-Combustion of Coal and Biomass

Abstract

Straw pellets have been co-fired with a bituminous coal with a high chlorine content. The tests were carried out in a 12 MW circulating fluidized bed (CFB) boiler located at Chalmers University of Technology in Sweden. The tests focused on variation of the ratio of straw to coal in combination with different feeding rates of limestone to the bed. Three test series were carried out: Lime – the fuel was a constant mixture of coal and straw pellets, with an increasing lime supply; Coal – the fuel was coal only, with an increasing lime supply. Alkali – the fuel was coal and straw pellets and constant lime supply. The fraction of straw pellets (alkali) was increased. An In-situ Alkali Chloride Monitor (IACM) was used during the tests to measure the on-line concentration of alkali chlorides and SO2 before the convection section of the boiler. Deposit measurements were also carried out and the deposits were analysed by means of SEM-EDX. It was found that an increased fraction of straw pellets in the fuel mix results in increased level of alkali chlorides in the gas phase as well as in collected deposits.