Torrefaction of Wheat Straw and Sunflower Shells Biomasses as Low-Cost Materials for Energy Co-Generation

In this work, the torrefaction process of low-cost agricultural wastes (e.g., wheat straw and sunflower shells) was studied for manufacturing high-calorific materials for energy co-generation. The renewable bio-coal materials were manufactured using a continuous multiple-hearth torrefaction furnace (so-called CENTORRE by CMI group technology). The possibility of using this type of reactor allows a better control of the process and a better monitoring of the operating temperature. Two different conditions were evaluated under nitrogen atmosphere at an average flow rate of 100 kg h−1: (i) 250 °C; (ii) 280 °C. An increase on the torrefaction severities involves a greater loss of mass (at 250 °C: 23% w/w for wheat straw and 17% for sunflower shells; at 280 °C: 52% w/w for wheat straw and 56% w/w for sunflower shells). As expected, the carbon/oxygen ratio also increases as a consequence of the decomposition of the main polymeric chains (i.e., hemicellulose, cellulose, and lignin). The raw and torrefied biomasses have been characterized using SEM-EDX, TGA, and elemental analysis (C, H, N, S, O). The energy content increases with the temperature, the higher heating values were determined as 20.4 MJ kg−1 and 25.8 MJ kg−1 at 280 °C, for wheat straw and sunflower shells, respectively. Torrefaction enhances the valorization of low-cost wastes into solid fuels for local energy production.