Modeling and optimization of chemical-treated torrefaction of wheat straw to improve energy density by response surface methodology

Abstract

Today, torrefaction is important technique for extending the potential of biomass for improvement of energy density. The independent variables investigated for torrefaction study were temperature, retention time, acid concentration, and particle size. The experiment was designed by central composite design (CCD) method using design expert (version 11). The three dependent variables were higher heating value (HHV), energy enhancement factor (EEF), and mass yield (MY) were carried out. Numerical optimization using response surface method (RSM) to maximize the HHV and EEF with lowest MY was carried out. The maximum HHV of 25.05MJ/kg, lowest MY of 60.15%wt and highest EEF of 1.593 were obtained at 299.99°C, 31.89min of retention time, 0.75g/l of acid concentration and 0.20mm of particle size. Proximate analysis, bulk density, hydrophobicity, energy density, and ultimate analysis of raw and torrefied biomass were determined to analyze the physiochemical properties of the fuel. In addition, thermo-gravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), and X-ray diffractometer (XRD) were done to distinguish the behavior of raw and torrefied wheat straw. The results implied that energy density of torrefied biomass was well improved when compared with raw biomass. All in all, energy density of torrefied biomass was improved, which can be used as an alternative energy.