Co-firing carbon dioxide-torrefied woody biomass with coal on emission characteristics

Torrefaction is a pre-treatment technique that involves the partial pyrolysis (200–300 °C) of biomass in an inert environment to improve its fuel properties. Effect of co-firing CO₂-torrefied mesquite and juniper (10 % on mass basis) with coal in a 30-kWt downfired burner was studied considering the grindability potential of CO₂-torrefied biomass (60 % higher grindability than N₂-torrefied biomass). It was observed that co-firing 10 % by mass of raw mesquite with coal reduced the NO _x emission from 420 to 280 ppm (33 % reduction) for an equivalence ratio (ER) of 0.9. However, co-firing TB with coal reduced the NO _x emission by 10 % when compared to base case NO _x emission from combustion of pure powder river basin coal (PRB) coal. Carbon monoxide percentage in the flue gas increased when biomass was co-fired with coal. An inverse relation was observed between CO and NO _x emissions due to NO _x reducing potential of CO in the presence of fixed carbon. Combining the advantages of using a greenhouse gas CO₂ for pre-treatment and potential of CO₂-torrefied biomass in reducing emissions with improved grindability, proves co-firing torrefied biomass with coal is an efficient technique.