Torrefaction ? What ’ s that ?

Torrefaction is a thermo-chemical process that reduces the moisture content of wood and transforms it into a brittle, char-type material. The thermo-chemical process can reduce the mass of wood by 20-30% resulting in a denser, higher-valued product that can be transported more economically than traditional wood chips. Through torrefaction, wood may retain 90% of the energy value. This energy dense end-product can be used as a coal replacement or co-fired/co-milled with coal in electricity generating power plants. Torrefied wood can be used as a soil amendment, for backyard grilling, residential heating, or as a feedstock in gasification processes. This paper is a literature synthesis that will present (1) the torrefaction process, (2) current developments in commercial torrefaction equipment, (3) characteristics of and markets for torrefied wood, and (4) feedstock specifications for torrefaction. Introduction Woody biomass torrefaction is a process of heating biomass in a low-oxygen environment. There is some variation in the reported temperatures used in torrefaction. In existing literature, torrefaction ranges were found from 220 3000C (428 5720F). These temperatures are much lower than those often related to fast pyrolysis (4006000C) or gasification (9000C or higher) (Table 1). Wood properties undergo changes when processed at temperatures associated with torrefaction. Woody biomass consists of hemicelluloses, cellulose, lignins, and extractives. Torrefaction releases water and volatile organic compounds. Some of the lignin is devalitized and the remaining lignin is loosened. Hemicellulose is released and the remaining bio-char is a product of the torrefaction process. It is an intermediate product between wood and charcoal and has most of the advantages of both products. Compared to the coal it replaces, biomass reduces sulfur dioxide (SO2), nitrogen oxides (NOx), and net greenhouse gas emissions of CO2 (Lipinski et al, 2002). Co-firing torrefied wood is more attractive than using raw biomass such as wood chips because the torrefied wood is friable and can be blended, pulverized and co-fired with coal. The