Integration of torrefaction and in-situ pelletization for biodried products derived from municipal organic wastes: The influences of temperature on fuel properties and combustion behaviours

Abstract

Biodried products (BPs), derived from municipal organic wastes (MOWs), are generally accepted as carbon-neutral and renewable energy sources. To upgrade their combustion utilization as solid fuels, pelletization subsequent to torrefaction was conducted for BPs by activating the inherent binders using in-situ heat. In this study, the effects of temperature on the fuel properties and combustion characteristics of torrefied BP (TBP) pellets were comprehensively investigated. The results indicated that torrefaction promoted the emissions of O-containing volatiles, including CO2, CO, water and some tar (acids, esters, phenols, etc.), and dehydration might be dominant with higher ratios of deoxygenation and dehydrogenation. High torrefaction temperatures improved the energy densities (1.03–1.27) of TBPs, but decreased the energy yields (99.61%–68.46%). Meanwhile, high temperatures (above 200 °C) for pelletization facilitated the porosity structures and energy consumption of TBP pellets, which also upgraded the pellet properties, especially for TBP250, which exhibited a low wettability index (5.08%), high hardness (10.89 N/mm2) and durability (>96%). Moreover, the TBP pellets were found to retard moisture absorption and volume expansion significantly. In addition, torrefaction enhanced the decomposition of residual lignocellulose and char during the combustion of TBPs. Compared with lignite, TBPs were more flammable with a lower average activation energy. The integration of torrefaction and pelletization effectively decreased the emissions of CO, NO and SO2 during combustion of TBP pellets, and the combustion slags also exhibited favourable potentials for soil regulation. By integrating torrefaction with in-situ pelletization for BP pretreatment, this study provided an efficient alternative for MOW management.