Effects of Combined Torrefaction and Pelletization on Particulate Matter Emission from Biomass Pellet Combustion

Combined torrefaction and pelletization can effectively improve the quality of the biomass fuel. In this work, particulate matter (PM) emissions from combustion of corn stalk pellets derived from the torrefaction and pelletization process were investigated using a fixed-bed reactor combined with a Dekati low-pressure impactor (DLPI). The obtained ash samples were subjected to various analytical techniques. Results indicated that, after torrefaction, the lower heating value (LHV) of the torrefied sample increased by 8-28% with an increase of the ash content by 9-30% and a decrease of the 0 content by 14-48%. Although 10-41% Cl and 27-60% S were released in the torrefaction, the absolute content of Cl increased from 7.6 mg/g for raw corn stalk to 9.2 mg/g for torrefied biomass at 300 degrees C. In comparison to the raw pellet, the total PM emission from torrefied pellets increased by 25-68% with the torrefaction temperature increasing, largely resulting from the increase of PM0.1-1. Chemical composition analysis of the PM showed that torrefaction changed PM1 composition from being dominated by KCl to mainly K2SO4. The variation of PM emission characteristics between the raw pellet and torrefied biomass pellet could be attributed to the changes of the contents and occurrence forms of ash-forming species in the torrefaction process. Although combined torrefaction and pelletization clearly enhanced the fuel properties of the corn stalk, it also increased PM emissions, especially PM1, to various degrees. Therefore, effective countermeasures are needed to mitigate the PM emission issues in industrial application.