Effect of torrefaction and ashless process on combustion and NOx emission behaviors of woody and herbaceous biomass

Highlights: • Impact of pretreatment methods on combustion and emission were studied herein. • Fuel-N decreased by ashless process, whereas increased by torrefaction for both fuels. • Combustion index and pore development improved by ashless process for both fuels. • Unburned carbon and NOx emission reduced by ashless process for both fuels. • Ashless process is preferable pretreatment method regarding combustion and emission. Fuel pretreatment is a type of thermal treatment that enhances biomass properties. This study investigates the impact of different pretreatment methods (torrefaction and ashless processes) on combustion and emission characteristics for two biomass fuel types, i.e., woody (pitch pine sawdust (pine)) and herbaceous (Hibiscus cannabinus L. (kenaf)) biomasses. The fundamental properties and structural pore characteristics as well as unburned carbon (UBC) and NO_x emissions following combustion tests, with both raw and pretreated biomasses, were investigated. The results show that torrefaction of both fuels improves the fuel ratio (FR), whereas the ashless process reduces the fuel ratio. For both fuels, torrefaction increased the fuel-N by approximately 30%–50%, whereas the ashless process decreased fuel-N. In fixed conditions, the combustion index (S) indicates the ashless process provides greater reactivity for both fuels. Upon torrefaction, combustion index (S) is higher for the pine samples but lower for the kenaf samples. In entrained conditions, the unburned carbon for both samples decreased after the ashless process, but increased following torrefaction of kenaf, which corresponds to the fuel ratios. The ashless process resulted in lower NOx emissions, whereas torrefaction resulted in higher NOx emission tendency for kenaf. These results indicate that the ashless process is a preferable pretreatment method for reducing combustion and NO_x emissions for woody and herbaceous biomasses.