Optimisation of multifuel-based bioenergy production

Utilisation of solid biofuels and wastes sets new demands for fuel handling and feeding systems, as well as for boiler design and combustion technologies. Especially ash quality of wood fuels differs remarkably chemically from ashes of peat or coals and other fossil fuels. The alkaline metals, which are usually responsible for fouling of heat transfer surfaces, are abundant in wood and will be released during combustion easily in gas phase. The combustion and co-firing properties of fuels have been studied both in VTT's test facilities and in industrial-scale power plant boilers. The formation of alkaline and chlorine compounds and their effect on boiler fouling and corrosion have been monitored by cooled temperature controlled deposit and material probes. The experimental studies clearly showed that if sulphur existed in combustion environment, chlorine was released as HCl and did not drift into deposits. Fouling of the boiler effects on several factors at the plant. These factors, e.g. the increment of operating and maintenance costs caused by fouling of the boiler, can increase the energy production costs significantly. In addition to increased need for soot blowing and cleaning, as well as the possible renovation need of the super heaters, the efficiency of the boiler decreases as thermally insulating ash deposits on the heat transfer surfaces of the boiler. By the side of these factors costs are caused by more often replaced bed material, the increased need for power consumption of the plants, and storage costs of wood fuels