Repurposing Washingtonia filifera petiole and Sterculia foetida follicle waste biomass for renewable energy through torrefaction

Abstract

This study attempted to improve the properties and energy efficiency of torrefied biochar produced from the waste of two common street trees, namely, Washingtonia filifera and Sterculia foetida. The effects of various torrefaction temperatures (210–300 °C) and reaction times (30 and 60 min) were evaluated. W. filifera petiole (WFP) biochar exhibited a higher weight loss and energy yield than S. foetida follicle (SFF) biochar. As the torrefaction temperature and time increased, the higher heating values (HHVs) and fuel ratios of both types of biochar increased, and the H/C and O/C atomic ratios became similar to the atomic ratios of lignite and peat due to the decomposition of hemicellulose and cellulose in the biomass structures. Furans and acetic acid, respectively, were the major gaseous products of WFP and SFF torrefaction. The maximum energy return on investment (EROI) of 23.2 was achieved for WFP biochar torrefied at 210 °C for 30 min. Greenhouse gas (GHG) emissions can be reduced markedly by 74.7–91.6% if biochar is used as a substitute for bituminous coal. Moreover, cofiring WFP or SFF biochar with bituminous coal at power plants is sustainable and economically beneficial because of the high HHV, stability, thermal performance, sufficient EROI, and low GHG emissions.