Torrefaction for fuel switching from coal to pure biomass in power plants

Torrefaction changes the elementary composition of biomass and moves it towards to coal, and accordingly, torrefaction based co-firing system in a pulverized coal boiler have been proved as a promising option for direct co-firing with a large percentage of biomass. This work examined and assessed various torrefaction degrees influencing on emissions reductions and system performances in a 200MWe power plant. The raw PKS was torrefied at four different torrefaction temperatures, namely, 200°C, 250°C, 270°C, and 300°C, respectively. A series of analyses were performed to understand the impacts of torrefaction temperatures on mill power requirements, pollutions emissions, and boiler efficiency. According to the results, an enhanced torrefaction caused more energy consumption on biomass pretreatment process but less energy consumption on biomass grinding. When considering the boiler efficiency and emissions in together, torrefaction temperature of 275 °C is property option both for high boiler efficiency of 95% and a lower NOx emission of 100 mg/Nm³. A high torrefaction temperate is required when expecting an even lower NOx emission, but the boiler efficiency would be reduced at the same time. Therefore, a balance consideration of combustion efficiency and emission should be made for torrefaction based biomass-fired boiler.