Comparison of the Efficiency of the Reactors for Low-Temperature Pyrolysis of Biomass

Results from the comparative analysis of the energy and economic efficiencies of the torrefaction reactors (or low-temperature pyrolysis reactors) with the heat of the heating gas supplied directly to the processed biomass are presented. Upright reactors of two types are considered, viz., reactors with a bed of dense biomass moving by gravity towards the heating gas flow and fluidized-bed reactors. Advantages and drawbacks of the different types of torrefaction reactors are discussed. Criteria for the evaluation of the torrefaction reactors' energy efficiency have been defined based on the data of experimental research conducted on a pilot power-engineering plant at the Joint Institute for High Temperatures, Russian Academy of Sciences, equipped with a moving-bed reactor and the results of numerical modeling of a fluidized-bed reactor. The conditions for creating the thermal effect of the hemicellulose decomposition reaction and the mechanism that restricts the increase in the biomass temperature during the self-heating by the exothermic reaction heat are considered. The analysis performed has shown that a moving-bed reactor with a controlled exothermic biomass destruction reaction has the highest energy efficiency and the best criteria for estimating the financial and commercial efficiency of the investments. Conclusions have been made that moving-bed reactors are preferable for small-scale production for which granulated biomass (biomass pellets) is the optimal feedstock, while fluidized-bed reactors and fine-dispersed feedstock materials are more suitable for large-scale production.