Upgraded pongamia pod via torrefaction for the production of bioenergy

Abstract

Torrefaction can be used to reduce the oxygen content of biomass and improve the feedstock properties for thermochemical conversion. Pongamia (Millettia pinnata), a leguminous, oil-seed bearing tree, is a potential resource for sustainable aviation fuel production due to the high oil content of its seeds. The present work investigates thermochemical pretreatment of pongamia processing residues, i.e. pods. Torrefaction tests were performed with both a fixed bed reactor and a macro thermogravimetric analyzer (TGA) under nitrogen atmospheres. The effects of process conditions on feedstock properties relevant to thermochemical conversion technologies, proximate and ultimate composition, heating value, and Hardgrove grindability index (HGI), were measured. The chemical structure, reactivity, and changes in elemental composition of the torrefied materials were also investigated. The mass and energy yields decreased 43% and 25%, respectively, from the mildest (165 °C) to the most severe (281 °C) torrefaction conditions, while the energy densification index increased from 1.15 to 1.68. The HGIs of pods torrefied at temperatures >215 °C were found to equal or exceed the HGI of a reference bituminous coal sample. A LECO model TGA801 macro-TGA with a sample loading capacity of ~95 g was also used to torrefy pongamia pods. Products from the LECO and the fixed bed reactor were comparable, and the macro-TGA was demonstrated to be a useful fast screening tool to study effects of process parameters.