Microwave Assisted Torrefaction of Plant Biomass of Different Origin with a Focus on Solid Products Valorisation for Energy and Beyond

Abstract

In the present paper, the microwave-assisted torrefaction of three types of the most widely spread lignocellulosic biomass feedstock, namely, wood, wheat straw, and peat, in the form of commercial fuel pellets, was studied using a laboratory-scale microwave torrefactor of original design. The torrefaction process, at temperatures varied in the range of 175-300 °C, was on-line monitored in terms of energy consumption and the temperature of the treated pellets. Elemental analysis, thermogravimetric analysis, Py-GC/MS and FTIR spectroscopy were used to characterize the biomass transformation and confirmed the simultaneous development of destruction and condensation processes in lignocarbohydrate complexes at microwave treatment. Torrefaction promoted the increase in the calorific value of all treated biomass. Higher heating values of the treated solid biomass compared to non-treated softwood, wheat straw, and peat increased by 24 %, 42 %, and 31 %, respectively. Biomass weight loss significantly depends on the structure and chemical composition of lignocellulosic matrixes of different origin. Wheat straw revealed itself as the most thermodegradable material. Therefore, although wheat straw had the highest calorific value, the overall output/input energy balance for the process of wheat straw torrefaction was less beneficial in comparison with that of peat and softwood. The microwave-assisted torrefaction promoted the increase in the hydrophobicity of the material and the improvement of its surface characteristics. Therefore, the possibility of torrefied biomass application as a natural sorbent for oil-derived products from the water surface was proposed.