Carbon Dioxide Torrefaction of Woody Biomass

Abstract

The major drawback of biomass for direct combustion applications is its lower heating values and poor grindability when compared to conventional fossil fuels. Torrefaction is one of the thermochemical pretreatment techniques used to improve the properties of biomass. An inert environment is maintained to prevent oxidation of biomass during torrefaction. A novel method for utilization of carbon dioxide as the pretreatment medium for woody biomass has been investigated in the current study. Previous studies on smaller samples using thermogravimetric analysis (TGA) showed an increased mass loss in the CO2 environment, which was attributed to possible structural changes in the biomass and potential effect of ash constituents in the biomass. However, those claims were not validated. The current study on bigger batches of samples also resulted in increased mass loss when using CO2 compared to using N2 as the torrefaction medium. Scanning electron microscopy (SEM) studies and Brunauer–Emmett–Teller (BET) surface area investigation on the torrefied samples from different environments showed increased internal surface area indicating a mild effect of the CO2 reacting with the samples at temperatures normally employed for torrefaction conditions (200–300 °C). Further, grindability studies were performed on the samples pretreated in CO2 and torrefied in N2. The results on grindability showed improved grindability on using CO2 as the pretreatment medium. Proximate and heating value analysis on the pretreated samples showed an increasing trend in the heating value of the samples with increased temperature. Comparable mass loss at lower temparatures improved grindability and improved fuel properties, makes utilization of carbon dioxide as a torrefaction medium for pretreating biomass for combustion applications an attractive technology.