TORREFACTION OF BIOMASS
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Torrefaction is a thermo-chemical pre-treatment of biomass within a narrow temperature range from 200°C to 300°C, where mostly the hemicellulose components of a biomass depolymerise. This treatment is carried out under atmospheric conditions in a non-oxidizing environment at low heating rates (< 50°C/min) and for a relatively long reactor residence time. Torrefaction increases the energy density of a biomass and reduces its O/C and H/C ratio, so its properties approach to that of coal. Biomass is usually referred to as lignocellulose, as its major mass constituents are cellulose, hemicelluloses and lignin. Research on torrefaction carried out to date deals solely with lignocellulose biomasses, and their degradation mechanism is explained primarily in terms of hemicellulose. However, there are biomasses which are non-lignocellulosic, have a small fraction of fibres in them or could possibly benefit from torrefaction. These include municipal solid waste, sewage sludge, animal waste, etc. Experiments were conducted on three non-cellulose biomasses (poultry waste, digested sludge, and undigested sludge) along with three typical lignocellulose biomasses (wood pellet and switchgrass and an agricultural waste – coffee bean husks). Results showed that non-lignocellulose biomasses torrefy similarly to their lignocellulose counterparts. Due to the immense potential of the torrefaction process, numerous manufacturers have developed their own patented technology for torrefying. Nevertheless, choosing the right torrefaction technology has become exceptionally difficult because of a near absence of a comparative assessment of different types of reactors. An experimental work was conducted to review the major generic types of reactors such as rotating drum, convective bed, fluidized bed and microwave, delineating the essential features of generic types of reactors. According to the results of this study, biomass torrefaction in a rotating drum gave the highest energy dense product, followed by fluidized bed and convective bed; the microwave reactor showed over-torrefaction at the core, while leaving the exterior green. To help effective design of a torrefier, several systematic experiments were conducted to investigate the effects of some of the more important operating parameters, such as torrefaction temperature, residence time and biomass particles size on the torrefaction yield. Although the mass yield decreased with the torrefaction temperature, energy density increased with it. Moreover, torrefaction yield varied for different biomass particle sizes depending on the type of reactor used, but the particle size did not have any clear effect on the energy density of the torrefied product.