Fouling of the Cooling Surfaces in Biofuel-Fired Fluidized Bed Boilers

Fluidized bed combustion has the capability of burning low-grade fuels effectively. These fuels include wood, wood wastes and agrobiofuels, as well as demolition wood and recycled fuels. However, biofuel combustion has occasionally suffered e.g. from bed material sintering and fouling of superheaters and other cooling surfaces. These symptoms have restricted the wide utilization of new biofuels and energy fractions of material recycling. The fouling tendency of fuels is not dependent on their ash content only. More important factors are the composition of the ash formed in the combustion and the ash mineral reactions. The mechanisms of the deposit formation, origin of ash compounds and their vaporization were studied in cooperation with the Technical Research Centre of Finland (VTT ) and Åbo Akademi University. Deposits on tube heat exchangers were collected by using a temperature controlled deposit probe. Short probe tests were used for studying the deposit at the initial state of formation. The formation mechanisms of ash particles and deposits were studied with measurements of fly ash particle size distribution by VTT. The impaction of partly molten particles is anticipated to cause drastic fouling. Åbo Akademi University studied the ash reactions with their novel ash predictor, where the elementary composition of the fuel ash was used for equilibrium calculations. The tests were carried out at a 66 MWth BFB boiler firing wood chips, bark and saw dust as the main fuels and co-firing green forest residue, chipboard and peat. Peat firing tests were carried out to find out the effect of silicate minerals in ash as well as the effect of sulfur on deposition.