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

Abstract The surplus biomass residue generated from biomass harvesting has enough potential for generating bioenergy and is a promising energy source for future use. Biomass possesses a high moisture content and low calorific value and therefore needs improvement to convert it into solid biofuel. In the present study, torrefaction of lignocellulosic biomass (rice straw) was carried out to enhance its physicochemical characteristics for producing high-grade biofuels and chemicals. For three sets of temperatures (200, 250, and 300°C) and residence times (30 minutes, 45 minutes, and 60 minutes), experiments were conducted in a batch reactor at a heating rate of 10°C.min− 1 in an inert environment. The torrefied products obtained were analyzed using various analytical techniques, such as proximate and ultimate analysis, calorific value measurement, and FTIR analysis. The results revealed that torrefaction at a mild temperature of 200°C and 30 minutes of residence time resulted in a maximum mass yield of 87% and an energy yield of 97%, which subsequently decreased at higher temperatures. The calorific value increased with increasing torrefaction temperature, with a maximum value of 19.50 MJ.kg− 1 occurring at 300°C and 60 minutes of residence time. Since H2O, CO, and CO2 are released upon torrefaction, a significant decrease in the number of hydroxyl groups was observed in the FTIR spectra. Despite the high calorific value at 300°C, 250°C and 30 minutes of residence time are the optimum torrefaction conditions for rice straw due to the significant mass and energy yield and the significant presence of amorphous carbon, as confirmed by Raman spectroscopy. This study will improve the physicochemical properties of rice straw for the production of high-value fuels, chemicals, and other high-strength materials.