Comminution Phenomena of Wood Pellets and Chips Under Combustion and Gasification Conditions

Abstract

This work reports on a study, carried out in a lab­scale fluidized bed apparatus, on fragmentation and attrition of two biomass fuels, namely wood chips and wood pellets, under combustion and gasification conditions. The aim was to highlight the effect of their different mechanical strength on the fuel particle size distribution and overall carbon conversion. Primary fragmentation tests showed that for wood pellets limited fragmentation occurred during devolatilization, 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. Results of char attrition experiments carried out under inert, combustion and gasification conditions showed that the carbon loss by elutriation is critical only during gasification, especially for the wood chips char. A gasification­assisted attrition mechanism was proposed to explain the experimental results, similar to the well known combustion­assisted attrition patterns already documented under oxidizing conditions. The higher mechanical strength of the wood pellets appears to be beneficial for reducing carbon elutriation and for obtaining a higher carbon conversion.