Process parameter optimization for upgrading rice straw by combined washing and torrefaction pretreatment [abstract]

Abstract

Biomass stands out as the most widely available renewable resource on a global scale which has benefits including its cost-effectiveness, widespread accessibility, short lifecycle, swift processability, and minimal climate impact. Incorporating biomass as fuel in boilers and furnaces poses challenges, notably in the form of ash-related issues and combustion problems such as slagging and fouling tendencies within combustion chambers. Addressing these operational hurdles necessitates the upgrading of biomass before its utilization as a fuel. Proven methods, such as washing and torrefaction pretreatments, have demonstrated their efficacy in enhancing the solid fuel quality of biomass by reducing ash content and elevating energy density. The objective of this research is to analyze and optimize the combined washing and torrefaction pretreatment of rice straw to improve combustion properties. Hence, rice straw washing was conducted using water and a dilute acetic acid medium by varying washing parameters to find the optimum washing conditions. Then torrefaction experiments were conducted to the optimized washed parameters to obtain the optimum torrefaction parameters. Thermogravimetric analysis of rice straw suggests that a portion of the volatile matter (VM) in the rice straw released between 200°C and 300°C which caused about 20% of weight loss. Compared to untreated rice straw samples acid and water-washed samples showed a significant thermal stabilization. For water washing, the optimum parameters obtained are 47°C, 30 minutes, and 23 L/S ratio. In these conditions, 25.24% of ash removal could be achieved. For the acid-washing pretreatment, optimized results were a 30 minutes of washing time and 40 L/S for the ash removal of 30.53%. According to the results of the torrefaction experiments, the solid yield was reduced with the increase in the temperature giving the highest mass loss (47.57%) at 300⁰C. Carbon content and HHV were increased with the torrefaction temperature while oxygen content and energy yield were decreased.