High performance and low emissions - optimisation of multifuel-based bioenergy production

There is a growing international interest in utilising renewable fuels, also in multifuel applications. Main reasons for this are the objective to reduce CO₂ emissions and meet emission limits for NO_x and SO₂. On one hand cofiring, defined as simultaneous combustion of different fuels in the same boiler, provides an alternative to achieve emission reductions. This is not only accomplished by replacing fossil fuel with biomass, but also as a result of the interaction of fuel reactants of different origin (e.g. biomass vs. coal). On the other hand, utilisation of solid biofuels and wastes sets new demands for process control and boiler design, as well as for combustion technologies, fuel blend control and fuel handling systems. In the case of wood-based fuels this is because of their high reactivity, high moisture content and combustion residues' high alkaline metal content. Combustion and cofiring properties of fuels have been studied both in VTT Processes' test facilities and in industrial-scale power plant boilers. The formation of alkaline and chlorine compounds in biomass combustion and their effect on boiler fouling and corrosion have been monitored by temperature controlled deposit formation and material monitoring probes. Deposit formation monitoring at full-scale boilers provides unique information on the rate of deposit formation, the effect of sootblowing and consequent changes in heat transfer. Additionally, the data from deposit formation monitoring has been shown to correlate with boiler performance, which gives basis for studying the interrelation of: fuel blend characteristics - deposit formation - boiler performance. If biomass fuels are blended with coal or peat, following implications may be expected: increased rate of deposit formation, shorter sootblowing interval, cleaning of heat transfer surfaces in revisions may be required, bed material agglomeration (in fluidised beds), increased risk of corrosion, higher in-house power consumption, higher flue gas temperature. These factors affect operating and maintenance costs, but their effect can be reduced or even avoided with appropriate fuel blend control. By the aid of the aforementioned means, optimum share of biomass fuel in different fuel blends can be defined.