Thermal Effects of Plant Origin Biomass Torrefaction: Experiment and Mathematical Modeling

The results of experimental and computational studies of torrefaction (low-temperature pyrolysis) of granular plant biomass are presented. As a result of this process, high-quality solid hydrophobic biofuel with increased calorific value is obtained from biomass of various types (including waste). Unlike fossil coal, torrefied fuel contains virtually no sulfur and heavy metals, has a lower ash content, and is a carbon-neutral product. The carbon dioxide released during the combustion of plant biofuels is completely absorbed during the period of plant growth. Biofuels are a renewable resource. The annual increase in biomass is 400 billion t, and the rate of energy accumulation by terrestrial biomass is 3 × 10 21 J/year. The article considers a vertical reactor with a dense layer of pellets moving towards the heating gas flow under the action of its own weight and features of its operation, taking into account the possible implementation of the concept of a controlled exothermic reaction. To ensure the set mode, the plant has two control units: for controlling and maintaining the set temperature of the coolant at the reactor inlet and for controlling the opening of dampers for unloading finished products into the cooling zone. Experimental and calculated data are presented for three characteristic operating modes of the installation: with limitation of the exothermic reaction due to the control of the inlet temperature, with a controlled exothermic reaction, and with the production of biochar. The second mode turned out to be the most energy efficient. The deviation of the calculated data from the experimental data was no more than 4%. It is shown that the use of the energy of an exothermic reaction can significantly increase the energy efficiency of the process of obtaining carbon-neutral solid biofuel.