Pretreatment methods for gasification of biomass and Fischer–Tropsch crude production integrated with a pulp and paper mill

In this paper, the influence on the system performance and greenhouse gas (GHG) emissions of different biomass pretreatment methods before gasification and Fischer–Tropsch (FT) crude production was evaluated. Entrained flow gasification has the benefit of producing a practically tar-free synthesis gas with nearly complete carbon conversion. This gasifier type requires a relatively dry fuel, with small particle size, at high pressure. The size can be acquired by milling, which is energy intensive and feeding is challenging. Torrefaction of biomass facilitates milling; it thus requires less electricity, however, the torrefaction process requires heat. Pyrolysis decomposes the biomass into gaseous, liquid, and solid parts, respectively. This further makes feeding easier, but comes with a greater heat demand than torrefaction. The impact of the different pretreatment methods on the overall energy system has been evaluated using process integration methodology. The results show that the excess heat from an FT process with a biomass input of 300 MW_HHV can replace the bark boiler in a large chemical pulp and paper mill, producing 350,000 tonnes of bleached paperboard annually. With the preconditions given for this study, thermal pretreatment of biomass may be beneficial in terms of wood-to-FT crude efficiency, with efficiencies up to 68 %, assuming 40 % electrical efficiency. Pretreatment using pyrolysis performed the best in regards to GHG emissions, if CO₂ from acid gas removal was vented, while milling, with an annual reduction of around 700,000 tonnes of CO₂,_eq, had the best results if the CO₂ was captured and sequestrated.