Solidification characteristics of solid biofuel densified by two-step torrefaction process

Coal coke, which is used widely in industrial furnaces, emits large amounts of CO₂. To utilize solid biofuels as alternatives to coal coke, the fuel ratio of the biofuels must be improved to generate functions, such as deoxidization, permeability, and carbon pickup. In this study, an innovative densification molding method is proposed; it uses a two-step torrefaction process with a high CO₂ reduction effect. The molding method consists of the following two-step torrefaction process at torrefaction temperatures of 463-773 K: In the first step, raw biomass is torrefied to remove some of the volatile matter that inhibits densified molding. In the second step, the torrefied biomass is densified at the above temperature. The purpose of the second torrefaction step is to further enhance the fuel ratio due to the conversion of volatile matter to fixed carbon and to develop the thermal softening of lignin. Solid biofuel densified using a two-step torrefaction process was produced from a Japanese cedar sample, and it was found that its fuel ratio was significantly improved. Furthermore, the mechanism of the adhesive effect during carbonization was elucidated by analyzing the structure of the densified solid biofuel using Raman spectroscopy.