Emissions and combustion characteristics of torrefied wood pellets

Abstract

The influence of torrefaction temperature on the durability, combustion characteristics, and emissions of CO, CO2, NOX, and particulate matter (PM) from biomass pellets was studied. The pellets were torrefied under inert conditions at 225, 250, and 300 °C for 60 min. Physical properties, such as weight loss, fines percentage, pellet durability index (PDI), and water absorption, were evaluated using ISO standards. The weight loss increased with higher torrefaction temperatures. Torrefied pellets had lower water absorption than untreated pellets. Fines percentage increased with torrefaction temperature while PDI decreased. Torrefied pellets at 300 °C had the lowest PDI (82.7%), while 225 °C had the highest (98.0%). The energy density and heating values increased with torrefaction temperature from 22.0 MJ/kg at 225 °C to 29.9 MJ/kg at 300 °C compared to 18.9 MJ/kg for untorrefied pellets. There were reductions in CO, CO2, and NOX emissions with an increase in torrefaction temperature while PM slightly reduced. This study found that torrefied biomass pellets had lower CO2 emissions than raw pellets.