Fuel blend characteristics and performance of co-fired fluidised bed boilers

When co-firing biomass fuels with fossil fuels, following implications may be expected: increased rate of deposit formation, shorter sootblowing interval, bed material agglomeration, increased risk of corrosion, higher in-house power consumption etc. Deposit formation inhibits heat transfer and reduces boiler efficiency, additionally chlorine rich deposits induce hot corrosion of heat transfer surfaces. Due to this operating and maintenance costs can increase, but this can be diminished or even avoided with appropriate fuel blend and process control. Effect of different fuels on deposition and corrosion have been studied by deposit and material testing probes, both in VTT Processes' test facilities and in power plant boilers. To support plant operators in daily decision-making and in plant operation optimisation advanced methods for combining data from deposit formation monitoring and process calculations are being developed. These methods will help operators to achieve better boiler performance with improved fuel flexibility. Use of biomass fuels reduces emissions but may cause harmful forming of deposition. Depending on the boiler type, it is recommended to combust 10-20% of peat with wood fuels in order to reduce above mentioned operational problems. If wood fuels consist mainly of new tree growth, like logging residue, it is favourable to use even higher share of peat, 30-40%, as co-firing fuel.