Combustion of wood chips, produced by different harvesting methods, in fluidised bed boilers

The objective of this study was to determine safe combustion conditions in circulating and bubbling fluidised bed boilers for wood chips produced by different forest harvesting chains. The optimal mixing ratios for typical and commonly used forest chip qualities, as well as for mixtures of chips and other fuels, under steam temperatures typical for circulating and bubbling fluidised bed boilers are also determined. Emissions are also reviewed. The combustion and co-firing properties of fuels produced by different forest chains and their suitability were studied first in VTT's test facilities and later in industrial-scale power plant boilers. The formation of alkaline and chlorine compounds and their effects on boiler fouling were studied by deposit probes in the test facilities. Deposits and phase changes in their compounds, as well as the corrosion risks are analysed by SEM-EDX. Utilisation of logging residue chips can cause deposition on heat exchanger surfaces and corrosion due to chlorine of wood ash. The harmful formation of alkaline and chlorine compounds on boiler surfaces could be prevented by co-firing sulphur-containing fuel.