Microwave treatment combined with conventional heating of plant biomass pellets in a rotated reactor as a high rate process for solid biofuel manufacture

The effect of microwave treatment, combined with conventional heating of softwood and wheat straw pellets in inert media, on their properties as solid biofuel was investigated. The device (microwave power 0.9 kW at 2.45 GHz frequency) was equipped with a rotated reactor, radially placed in a tubular microwave cavity (resonator) and a coaxial waveguide attached to its bottom. The maximal temperature of pellets achieved by the subsequent microwave heating varied in the range of 120-320 °C. The outside convective heating of the reactor, before switching on the magnetron decreased the pellets' mechanical destruction. The differences between the yields of energy and masses were increased with increasing microwave assisted treatment temperature in the range of 230-320 °C. Consequently, the calorific values of pellets increased by 12-37% and by 14-34% for straw pellets and wood pellets, respectively. A non-thermal microwave induced effect was observed. The total input/output energies' balance was more beneficial for wood pellets. A drastic increase of the treated pellets' hydrophobicity was observed. The spraying of waste cooking rape oil onto the lipophilic surface of the treated soft wood pellets compensated the decrease of pellets' density, enhancing its volumetric energetic density.