Biomass pre-treatment by torrefaction

Abstract

Several biomass types have been studied as raw materials for torrefaction: wood species such as birch, pine, and salix, biomass residues from miscantus and densified biomass materials, wood and straw pellets. The effects of raw material, temperature (230°C, 250°C or 280°C), residence time (1, 2 or 3 hours), particle size (1-3,2 mm) and rate of nitrogen flow (5 and 10 L/h) are studied. The torrefied solid products are characterized with elemental composition, moisture and ash contents, atomic ratios H/C and O/C, degree of carbonisation, calorific value and total energy content. The gaseous products are analysed for CO2, CO, CH4, and C2 – hydrocarbons. The type of biomass influenced both the product distribution and the properties of the products. In treatment of densified biomass, straw and wood pellets the chemical changes seemed to proceed readily and the energy recovered in the solid torrefied products were high. The energy density of the solid product increases with increasing temperature and residence time, while the yields of solid products decrease. The process need to be optimised to produce solid product with high energy density and high energy content recovered in the solid product.