Comparative study on the co-combustion behavior of torrefied biomass blended with different rank coals

Co-combustion is a practical way to utilize biomass in utility boilers, and torrefaction can effectively improve the intrinsic drawbacks of biomass. In this study, the combustion experiments were conducted in both thermogravimetric analyzer (TGA) and circulating fluidized bed (CFB) test system to evaluate the co-combustion behavior of torrefied cornstalk (TC) blended with different rank coals, including lignite coal (LC), bituminous coal (BC), and anthracite coal (AC). The results indicate that TC can decrease the ignition temperatures of coals, while the burnout temperatures of blends vary with the coal rank. Both synergistic and anti-synergistic interactions occur during the co-combustion, which are affected by the combustion behavior of individual fuels, blending ratio, and combustion stage, etc. In the CFB test system, the CO concentration in flue gas and unburnt carbon content (UBC) in fly ash decrease with the TC blending ratio, which means the coal/TC blends have better combustion performance than coals. Additionally, the effect of TC on the NOx emissions of blends is related to the coal rank, and the SO2 emissions decrease with the TC blending ratio due to the low S content in TC. The feeding problem should be paid attention when more than 10% TC is blended with LC and BC. These achievements are helpful for the efficient utilization of biomass in the utility boilers.