Process simulation of an integrated biomass torrefaction and pelletization (iBTP) plant to produce solid biofuels

Abstract

Abstract Torrefaction is an important thermal pretreatment method to produce solid biofuels from lignocellulosic biomass to replace coal for sustainable heat and power generation. In this study, a thermodynamic-based process simulation model was developed for an integrated biomass torrefaction and pelletization (iBTP) plant to conduct mass and energy balances and to determine equipment size and design parameters using an Aspen Plus simulation tool. The key unit operations of an iBTP plant consisted of a biomass drying, fuel combustion, torrefaction, grinding, pelletizing, and cooling operations. A solid convective rotary dryer was simulated to study the temperature profiles and moisture content of hot gas and solid biomass along the dryer length. A rotary kiln torrefaction reactor was modeled through pre-defined reactor modules to investigate the main thermal and electrical energy requirements. The reactor output parameters such as mass and energy yield, HHV, and torgas compositions were validated for a torrefaction temperature range of 230–290 °C. The other main unit operations including combustion, grinding, pelletization, and cooling were also simulated to conduct detailed thermodynamic mass and energy balances at an industrial scale (pellet production capacity of 100,000 Mg yr−1) from pine wood chip feedstock. When wood bark is used as an auxiliary fuel over natural gas (base case), a separate solid fuel burner unit was simulated. The total thermal energy consumption for the production of torrefied pellets were ranged from 5.2 to 14.1 MJ kg−1 with the torgas energy contributions of 30–90% for the torrefaction temperature range of 250–290 °C. For the base case torrefaction temperature of 270 °C, the overall thermal and electrical energy consumptions were 7.2 and 0.9, respectively for the Mg of torrefied pellets produced. The developed model can be used to conduct process economics and safety assessments of a commercial scale plant.