A review of torrefaction for bioenergy feedstock production

Abstract

The torrefaction of biomass is a thermochemical decomposition process in which hemicellulose degradation is the dominant reaction, with the cellulose and lignin fractions largely unaffected. The primary product is a solid material that retains 75–95% of the original energy content. Properties of the torrefied solid include improved grindability, hydrophobicity, and energy density. Torrefied biomass has been processed successfully in batch‐mode and continuous process devices; net thermal efficiencies of the process as high as 90% have been reported. Torrefied biomass has been proposed as a feedstock for coal co‐combustion, as well as for gasification‐combustion and Fischer‐Tropsch fuel production. Analyses of supply chain impacts indicate that, in some scenarios, torrefaction can be the lowest cost and most energy efficient option for supplying fuel, especially when combined with pelletization of the material. Significant gaps still exist in our understanding of torrefaction; there is need to further study this important process for its potential benefits to bioenergy production. Some of the more pressing needs include characterization of chemical pathways of the torrefaction reaction, investigation of equipment performance and equipment‐related influences on the process, and elucidation of supply chain impacts. © 2011 Society of Chemical Industry and John Wiley & Sons, Ltd