Effect of pyrolysis temperature and biomass particle size on the heating value of biocoal and optimization using response surface methodology

Abstract

The effect of pyrolysis temperature (T) and biomass particle size (z) on the biocoal's heating value of Oak acorn shell (OA), deseeded carob pods (CP) and olive mill solid waste (OMSW) were investigated. The higher heating value (HHV) increased with T. The effect of the particle size differed according to the biomass. Response surface methodology (RSM) was used to optimise the biocoal production system from technical, end-user and socio-environmental perspectives. The maximum HHV, representing the technical objective, is achieved at maximum T and minimum z for OA biocoal; at maximum T and minimum z for OMSW and at maximum T and maximum z for CP. The highest lower heating value (LHV), corresponding to end-user, for OA biocoal is achieved when T is greater than 450 °C and z < 1000 μm. For OMSW, particle size was the most important factor with the best LHV achieved when z > 1750 μm. The maximum net energy return per kg of biomass processed and maximum GHG offset (socio-economic) is realised at the lowest T for all biomass and particle sizes. This study is relevant to policy makers as it highlights potential conflict between the optimal conditions to produce high quality biocoal characterised by its high LHV and the optimal conditions to achieve the maximum utility from the resource and in turn potentially higher socio-environmental return.