Abstract
In this paper, the influence on the system performance of different biomass pretreatment methods before gasification and subsequent production of Fischer-Tropsch crude is considered. Entrained flow gasification at high pressure is a well proven technology for coal as feedstock with the benefit of producing a practically tar free raw gas with a close to complete carbon conversion. The short residence time in this type of gasifier requires a fuel, which is relatively dry and has a small particle size, to be pressurized from ambient conditions up to 20-50 bar. The small particle size can be acquired by grinding but this is highly energy intensive and feeding is challenging. Torrefaction is a thermal pretreatment method carried out at around 300 °C which makes the biomass easier to grind, thus requiring less electricity, and makes the material less fibrous, while on the other hand requiring heat for the process. Pyrolysis is a process carried out at around 500 °C which lets the biomass decompose into a gaseous, a liquid and a solid part. The solid part can be grinded and fed into the gasifier together with the liquid and gaseous phases. This will further facilitate the feeding but with a further increased heat demand compared to torrefaction. The different pretreatment methods and their impact on the overall biorefinery energy system have been studied and evaluated using process integration methodology. The results show that the excess heat from an FT process with a biomass input of 300 MWHHV can replace the solid fuel boiler in a large chemical pulp and paper mill. With the preconditions given for this study, thermal pretreatment of the biomass can be beneficial in terms of system thermal efficiency.
