Torrefaction of agro-wastes (palmyra palm shell and redgram stalk): characterization of the physicochemical properties and mechanical strength of binderless pellets

To increase the utilization of agro-waste as solid fuel, torrefaction is an important process to improve the fuel properties. In the present investigation, palmyra palm shell (PPS) and redgram stalk (RS) were torrefied in a specially designed stainless-steel tubular reactor with varying temperatures (230 °C, 260 °C, and 300 °C) under a nitrogen atmosphere at two residence times of 30 and 60 min. The influence of torrefaction temperature and residence time on mechanical properties and moisture reabsorption tendency of the binderless pellets were investigated. With increasing the torrefaction temperature up to 300 °C (60-min residence), the mass and energy yield decreased, and the fixed carbon, high heating value, and energy density increased. The fuel ratio (FR) increased from 0.25 or 0.23 to 1.35 and 0.52 for PPS and RS, respectively. The combustibility index increased up to a certain temperature (260 °C); however, at 300 °C, it decreased. Grindability of the torrefied biomass increased; about 99.89% and 95.28% of the smallest particles’ fraction pass through a < 75-µm sieve for PPS and RS, respectively, at 300 °C and 60 min as compared to the raw agro-wastes. The moisture reabsorption tendency of untorrefied and torrefied PPS and RS pellets was measured under the controlled environment of 60% RH at 40 °C. FTIR study supported the hydrophobicity of the torrefied material than the raw biomass. This study demonstrated that the torrefied PPS and RS have fuel properties comparable with India lignite, and this renewable fuel could be sustainably used for power generation and domestic applications.