The advantages of co-firong peat and wood in improving boiler operation and performance

VTT Processes has studied co-firing of peat and wood based fuels in several national and international research projects. Utilisation of solid biofuels sets new demands for fuel handling and feeding systems as well as for combustion technologies. Wood fuels differ from peat in many properties; moisture content, heating value, energy density, particle size, ash content and ash quality etc. All these factors effect greatly on fuel combustion behaviour. One of the most important differences between wood fuels and peat is ash behaviour during combustion. Typically wood fuels contain higher amount of weak acid soluble alkaline metals, potassium and sodium, when compared to peat. These alkaline metals exist in different forms of compounds (sulphates, chlorides, nitrates, etc.) or they are attached directly to organic matter in growing plants. Easily soluble alkalies are released during combustion into gas phase and cause risk for boiler bed material agglomeration, fouling of heat transfer surfaces and superheater corrosion. These phenomena have effect on power plant availability and production economics and thus they should be carefully recognised. Fouling inhibits heat transfer and reduces boiler efficiency, additionally; chlorine rich deposits induce hot corrosion of heat transfer surfaces. Effect of different fuels on deposition and corrosion have been studied by deposit and material testing probes, both in VTT's test facilities and in power plant boilers. Studies show that with co-firing peat with wood a great deal of benefits is achieved in controlling emissions and 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.