Torrefaction of Healthy and Beetle Kill Pine and Co-Combustion With Sub-Bituminous Coal

Combustion of biomass and co-combustion with fossil fuels are viable means of reducing emissions in electricity generation, and local biomass resources are appealing to minimize life cycle emissions. In the Rocky Mountain Region of the U.S., a bark beetle epidemic is causing widespread forest death and associated safety hazards. This biomass has potential to be a feedstock resource, thereby achieving dual goals of improving forest health while supplying biomass for uses such as co-firing with coal in power plants. In this study, combustion and co-combustion of healthy pine (HP) and beetle kill pine (BK) with coal were conducted to assess the interchangeability of these feedstocks in raw and torrefied forms. HP and BK pine were torrefied at 200, 250, and 300 °C to increase energy density and improve grindability, both of which aid in seamless integration into power plants. Grindability was assessed for both feedstocks at each torrefaction condition. The raw feedstocks were pyrolyzed to assess their relative compositions. Raw and torrefied feedstocks were then combusted alone and co-combusted with sub-bituminous Powder River Basin coal using thermogravimetric analysis (TGA). Modulated TGA was used to derive kinetic parameters of coal, raw and torrefied biomass, and coal-biomass blends. Results show increased grindability and pyrolysis mass loss of BK as compared to HP, which are attributed to the degraded state of the wood. Combustion and co-combustion show favorable interchangeability of the HP and BK, and additive behavior when co-combusted with coal.