The importance of fuel control in improving the availability of biomass-fired power plants

Combustion of biomass-based fuels and wastes may cause unexpected problems in power plant operation. In addition to operational disturbances in fuel pre-treatment and feeding systems, several observations have shown that alkali metal compounds in biomass cause hot corrosion of heat transfer surfaces - especially at low sulphur conditions. The behaviour of biomass fuel is influenced during combustion by existence of other fuels. It has been noticed in the previous studies that even small concentration of chlorine in logging residue chips will result in harmful formation of alkali and chlorine compounds on boiler heat transfer surfaces. This could be prevented by co-firing sulphur-containing fuel like peat. In this case vaporised alkali metals of wood ash are bound in sulphur, the amount of alkali chlorides in deposits is reduced or these are not formed at all, and chlorine is released as HCl into flue gases. The formation of alkali and chlorine compounds in biomass combustion and their contribution to boiler fouling and corrosion has been monitored by specific temperature controlled probes both at VTT Processes' test facilities and at full-scale power plant boilers. These studies have been supplemented by examining the availability and operation history of selected Finnish and Swedish wood-fired power plants. As a result of these studies, and in co-operation with steam boiler manufacturers and operators, VTT Processes has developed new methods for defining the optimum share of biomass fuels in different fuel blends and for improving power plant availability.