Attrition of wood pellets and chips during fluidized bed gasification

This work reports on a study, carried out in a lab-scale FB apparatus, on the fragmentation and attrition of two biomass fuels, wood chips and wood pellets, under gasification conditions. The aim was to highlight the possible effect of their different mechanical strength on the fuel particle size distribution and overall carbon conversion. Primary fragmentation tests were carried out by monitoring the number and size of fragments produced by each fuel particle upon FB devolatilization in nitrogen. Results showed that for wood pellets limited primary fragmentation occurred, with a fragmentation probability around 30% and particle multiplication factor of 1.4. On the contrary, wood chips were subject to extensive fragmentation as witnessed by large values of the particle multiplication factor and of the fragmentation probability. The decrease of the Sauter mean diameter upon devolatilization was caused by the combined effect of primary fragmentation and particle shrinkage. Results of char attrition experiments carried out under inert and gasification (60% inlet CO2 concentration) conditions showed that the carbon loss by elutriation is certainly one of the critical factors during the gasification process, especially for the wood chips char. The higher mechanical strength of the wood pellets appears to be somewhat beneficial for reducing carbon elutriation and for obtaining a larger carbon conversion during gasification. It was found that carbon consumption in the particles progressively weakens the char structure by pore enlargement. A gasification-assisted attrition mechanism was proposed to explain these experimental results, similar to the well known combustion-assisted attrition patterns already documented for coals under oxidizing conditions.