Torrefaction, a process different from carbonization, is a mild pyrolysis process carried out in a temperature range of 230 to 300 °C in the absence of oxygen. This thermal pretreatment of biomass improves its energy density, reduces its oxygen-to-carbon (O/C) ratio, and reduces its hygroscopic nature. During this process the biomass dries and partially devolatilizes, decreasing its mass while largely preserving its energy content. The torrefaction process removes H2O and CO2 from the biomass. As a result, both the O/C and the H/C ratios of the biomass decrease. But Torrefaction will increases the relative carbon content of the biomass. The properties of a torrefied wood depends on torrefaction temperature, time, and on the type of wood feed. Torrefaction also modifies the structure of the biomass, making it more friable or brittle. This is caused by the depolymerization of hemicellulose. This makes it easier to co-fire biomass in a pulverized-coal fired boiler or gasify it in an entrained-flow reactor. There is a 29 to 33% increase in energy density (energy per unit mass) of the biomass through torrefaction. This increases its higher heating value (HHV) to about 20 MJ/kg. To know more advantages of the torrefaction, please click here.
In biomass, hemicellulose is like the cement in reinforced concrete, and cellulose is like the steel rods. The strands of microfibrils (cellulose) are supported by the hemicellulose. Decomposition of hemicellulose during torrefaction is like the melting away of the cement from the reinforced concrete. Thus, the size reduction of biomass consumes less energy after torrefaction.
During torrefaction the weight loss of biomass comes primarily from the decomposition of its hemicellulose constituents. Hemicellulose decomposes mostly within the temperature range 150 to 280 °C, which is the temperature window of torrefaction. As we can see from Figure below, the hemicellulose component undergoes the greatest amount of degradation within the 200 to 300 °C temperature window. Lignin, the binder component of biomass, starts softening above its glass-softening temperature (~130 °C), which helps densification (pelletization) of torrefied biomass. Unlike hemicellulose, cellulose shows limited devolatilzation and carbonization and that too does not start below 250 °C.
|Weight loss in wood cellulose, hemicellulose, and lignin during torrefaction|
Thus, hemicellulose decomposition is the primary mechanism of torrefaction. At lower temperatures (< 160 °C), as biomass dries it releases H2O and CO2. Water and carbon dioxide, which make no contribution to the energy in the product gas, constitute a dominant portion of the weight loss during torrefaction. Above 180 °C, the reaction becomes exothermic, releasing gas with small heating values. The initial stage (< 250 °C) involves hemicellulose depolymerization, leading to an altered and rearranged polysugar structures (Bergman et al., 2005a). At higher temperatures (250–300 °C) these form chars, CO, CO2, and H2O. The hygroscopic property of biomass is partly lost in torrefaction because of the destruction of OH groups through dehydration, which prevents the formation of hydrogen bonds.
A typical reaction time is about 30 minutes. The properties of torrefied wood depend on (1) the type of wood, (2) the reaction temperature, and (3) the reaction time. Pelletization may not increase the energy density on a mass basis, but it can increase the energy content of the fuel on a volume basis. Pelletization of torrefied biomass is better than torrefaction of pelletized wood from the standpoint of process energy consumption and product stability.This is because :a. Torrefied biomass (torrefied wood), for example using sawdust as feedstock, so the torrefaction process will consume less energy due to the smaller particle size than the pelletized wood (wood pellets). Surface material can be in contact with the process of torrefaction is also larger in general when the particle size is smaller, so that better product quality (product stability). Normally before entering the torrefaction process feedstock will be diminished to the size of a certain size and drying up to a certain moisture content.
b. Physical form of pelletized wood (wood pellets) will be damaged due to torrefaction so irregular and will tend to shrink. While torrefied biomass has no problem with it because the physical form of the final product after pelletization.