Impact of pre-treatment by torrefaction and carbonization on temperature field, energy efficiency and tar content during the gasification of cotton stalks

The present study focused on the gasification of raw and pre-treated cotton stalks (CS) by torrefaction and carbonization. Temperature fields, mass balance, energy balance, energy efficiency and tar content of the gas were investigated for the gasification of different types of biomass materials (raw, torrefied and carbonized CS). High temperature and thick reduction zone were obtained during the gasification of pre-treated CS comparatively to the gasification of raw CS. Thus, the thermal and catalytic cracking of the tars may be more pronounced for the gasification of pre-treated biomass particularly for the carbonized biomass. Mass and energy balances have shown a reduction of biomass conversion during the gasification of torrefied and carbonized CS. Indeed, the energy efficiency of 58.7, 46.5 and 38.4% were obtained for raw, carbonized and torrefied CS, respectively. The lowest energetic performances were found during the gasification of torrefied CS due probably to the severe degree of the torrefaction. However, the tar content in gas was drastically decreased by the pre-treatment of the CS. Indeed, the tar contents of 4.41, 2.24 and 0.10 g/Nm3 were obtained for the gasification of raw, torrefied and carbonized CS, respectively. Key words: Biomass gasification, pre-treatment, tar content, energy efficiency.