Optimization of a biomass torrefaction plant with near zero emissions

Recent studies have shown that the emissions from conventional torrefaction processes is the second largest contributor to the supply chain. This article presents a torrefaction unit that operates based on oxy-combustion concept, whereby preventing carbon dioxide and nitrogen oxides emissions. The oxygen required in the process is supplied from an Air Separation Unit (ASU) and the working fluid of the new system is carbon dioxide. The process model is implemented in Engineering Equation Solver (EES) and simulation is conducted using the design data of a conventional plant which torrefies wood at 553 K for 17.5 minutes. The overall efficiency of the plant which accounts for both thermal and electrical energy requirement of the process is found to be 88%. The total energy consumption of the system exhibits a minimum at an optimum torrefaction temperature. With willow as the feedstock, the optimum temperature is determined to be 536 K at a residence time of 20 minutes, at which the total equivalent thermal energy required is 2 MJ/kg dry biomass and the energy yield is as high as 91%. The results show that the optimum torrefaction temperature is feedstock dependent and it is lower for a longer residence time.