Bed agglomeration characteristics during fluidized olivine bed combustion of typical biofuels

Controlled combustion and agglomeration experiments were conducted during eight hours in a bench– scale bubbling fluidized bed (5 kW) to determine the bed agglomeration tendencies of four different typical biomass fuels, namely: willow, logging residues, wheat straw, and wheat grain distillation residue (DDGS). Olivine and quartz were used as bed materials. Bed material samples and agglomerates were studied by means of scanning electron microscopy (SEM) in combination with energy–dispersive X–ray spectroscopy (EDX), in order to analyse the morphological and compositional changes of coating– / reaction–layers and necks between agglomerated bed particles. Significant differences in agglomeration tendency were found between the olivine and quartz bed material when combusting logging residues- and willow, i.e., biomass fuels with fuel ash rich in Ca and K and lesser amounts of Si. The results reveal no significant differences in bed agglomeration tendency between olivine and quartz during combustion of the two fuels that have relatively high phosphorus (DDGS) and silicon content (wheat straw). Willow and logging residues resulted in relatively continuous layers on the bed particles for both bed materials. The inner (reaction) layer was rather homogeneous whereas the outer coating layer had a more granular structure. The elemental composition of the inner layers around the quartz particles were dominated by Si, K and Ca whereas the inner layers around the olivine bed particles mainly consisted of Mg, Si and Ca. Only discontinuous (outer) coating layers resembling lumps of residual ash particles were formed on the bed particles in combustion of the silicon- (wheat straw) and the phosphorous rich fuel (DDGS). For these fuels no major differences in the elemental composition of the bed particle coatings / agglomeration necks were found between quartz and olivine bed materials.Godkänd; 2011; 20111019 (ohmmar)