Thermal Biomass Conversion: A Review

Abstract

In this paper, the most important methods of thermal conversion of biomass, such as: hydrothermal carbonization (180–250 °C), torrefaction (200–300 °C), slow pyrolysis (carbonization) (300–450 °C), fast pyrolysis (500–800 °C), gasification (800–1000 °C), supercritical steam gasification, high temperature steam gasification (>1000 °C) and combustion, were gathered, compared and ranked according to increasing temperature. A comprehensive model of thermal conversion as a function of temperature, pressure and heating rate of biomass has been provided. For the most important, basic process, which is pyrolysis, five mechanisms of thermal decomposition kinetics of its components (lignin, cellulose, hemicellulose) were presented. The most important apparatuses and implementing devices have been provided for all biomass conversion methods excluding combustion. The process of combustion, which is energy recycling, was omitted in this review of biomass thermal conversion methods for two reasons. Firstly, the range of knowledge on combustion is too extensive and there is not enough space in this study to fully discuss it. Secondly, the authors believe that combustion is not an environmentally-friendly method of waste biomass utilization, and, in the case of valuable biomass, it is downright harmful. Chemical compounds contained in biomass, such as biochar, oils and gases, should be recovered and reused instead of being simply burnt—this way, non-renewable fuel consumption can be reduced.