Physicochemical and Fuel Characteristics of Torrefied Agricultural Residues for Sustainable Fuel Production

Torrefaction as a thermal pretreatment was conducted on oat hull, canola hull, and barley straw in a fixed-bed reactor at temperatures in the range of 220–300 °C and residence times of 30-60 min to study impacts of these two parameters on physicochemical and fuel properties of biomass. The chemical nature of torrefied biomass was analyzed by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction, solid-state ¹³C nuclear magnetic resonance spectrometry (¹³C NMR), and X-ray photoelectron spectroscopy, while the morphology of the sample was analyzed by scanning electron microscopy (SEM) and Brunauer–Emmett–Teller (BET) analyses. The results indicated that torrefaction pretreatment significantly elevated the fuel ratio in all biomasses and reduced cellulose crystallinity. Thermogravimetric analysis (TGA) showed that significant decomposition of biomass occurred under the most severe conditions in the following order: canola hull, barley straw, and oat hull. A remarkable decrease in volatile matter and increase in fixed carbon content are also observed from the TGA curve for torrefied biomasses. ¹³C NMR and FTIR analysis of torrefied biomass samples demonstrated that the cross-linking, devolatilization, and carbonizing of biomass during the torrefaction process might be accountable for low mass yield. According to SEM results, a more fragmented and tubular structure was obtained at higher temperature because of thermal cracking and degradation of lignin, respectively, which made the biomass easy to grind. Inductively coupled plasma–mass spectrometry analysis showed that barley straw contains the highest amount of minerals (44.5 mg/g) followed by oat hull (15.5 mg/g) and canola hull (8.3 mg/g), when torrefied at 260 °C for 60 min. In addition, torrefaction significantly increases the alkaline and other essential element concentrations in torrefied samples. The BET surface area of torrefied biomass was found to be two to three times higher than that of raw biomass. Torrefied canola hull possesses the highest heating value (25.26 MJ/kg) followed by oat hull (23.31 MJ/kg) and barley straw (22.89 MJ/kg) torrefied at 300 °C. Torrefaction severely reduced the atomic ratio of C/H and O/H. Consequently, the equilibrium moisture content and moisture uptake rate of torrefied biomass decreased significantly and thus increased hydrophobicity. Thus, it is evident that torrefaction is an efficient pretreatment method for enhancing the quality of solid biomass as fuel.