Valorization of rice straw via torrefaction and its effect on the physicochemical properties

The surplus biomass residue arising from biomass harvesting has enough potential for generating bioenergy and is a promising energy source emerging for the future. Biomass possesses high moisture content and low calorific value and, therefore, needs improvement to convert into solid biofuel. In the present study, torrefaction of lignocellulosic biomass (rice straw) of different geographical locations has been carried out to enhance its physicochemical characteristics. For three sets of temperatures (200, 250, and 300 degrees C) and residence times (30 min, 45 min, and 60 min), experiments were conducted in a batch reactor at a heating rate of 10 degrees C.min-1 in an inert environment (50 mL.min-1 nitrogen). The torrefied product obtained was analysed using various analytical techniques such as proximate and ultimate analysis, calorific value measurement, XRD, FTIR, and Raman spectroscopy. Results revealed that torrefaction at a mild temperature of 200 degrees C and 30 min of residence time resulted in a maximum mass yield and energy yield, which subsequently decreased at higher temperatures. The calorific value increased with a rise in torrefaction temperature, with values ranging between 18.98 and 19.83 MJ.kg-1 for different rice straw varieties. Since H2O, CO, and CO2 content gets released upon torrefaction, a significant decrease in the hydroxyl group was observed through FTIR spectra. Herein, from proximate, ultimate, and ANOVA analysis, it is found that the biomass properties can be gradually improved with increasing torrefaction severity, and the rice straw of different geographical locations can be used interchangeably. Thus, 250 degrees C is considered the optimum torrefaction condition for rice straw due to the significant mass and energy yield as well as calorific value, which is in the range of peat and lignite and thus can serve as an alternative to coal or can be co-fired with conventional fossil fuels. This would reduce the burden of excess biomass accumulation on fields and could act as a considerable solution for the environment. This study will further aid in the investigation of ash reduction strategies as well as the kinetic study of the torrefied rice straw.