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Abstract
Biomass is a promising renewable energy source for providing a significant contribution to an ever-increasing global energy demand, reducing the GHG emissions, and improving the energy security. However, it is necessary to overcome challenges related to using biomass as a solid biofuel. Torrefaction is a thermal treatment process, where the biomass constituents, mainly hemicellulose, are thermally decomposed under atmospheric pressure. This results in reducing the biomass heterogeneity, changing the fibrous biomass to a more brittle structure like coal, and removing volatiles including hydroxyl and carboxyl groups from the biomass structure. The remaining torrefied solid has higher energy content and hydrophobic nature. The densification process such as pelletization would further increase the energy density of the torrefied pellets, reducing the costs of handling, storage, and transportation. The final torrefied pellet is a potential feedstock for several processes but specifically suited for co-firing with coal in power generation plants. Since the conventional combined torrefaction and pelletization (TOP) plant has not fully developed yet, in this study, an integrated torrefaction and pelletization process with a new configuration of torrefaction after pelletization (TAP) was proposed and investigated. A comprehensive process simulation framework was modeled with detailed unit operations, incorporating thermochemical conversion kinetics to study different lignocellulosic biomass feedstocks. The model was validated with the experimental data from torrefaction of two types of commercial wood pellets at a temperature range of 200 to 300C and process data from the literature. The developed model was used to explore the mass and energy balances, system efficiencies, design parameters for unit operations, and emissions to the environment at different industrial scales. The techno-economic analysis showed that for a 100,000 Mg yr-1 production capacity torrefied pellet plant, the total capital investment of a TAP configuration was around $29.6 million, which was 12% lower than the TOP approach. The production cost and minimum selling price were $166 Mg-1 ($6.9 GJ-1) and $197 Mg-1 ($8.1 GJ-1) respectively. If the price of torrefied pellets were equivalent to price of wood pellets ($185 Mg-1), the return on investment (ROI) was 15% for a torrefied pellet plant capacity of 200,000 Mg yr-1.