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. In raw biomass, high oxygen content prompts its over­oxidation during gasification, increasing the thermodynamic losses of the process. Torrefaction could reduce this loss by reducing the oxygen in the biomass. Torrefaction also 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.

A popular example of torrefaction is the process of roasting coffee beans. As the green beans are heated to 200 to 300 °C, their surface darkens (www. coffeeresearch. org/coffee). Figure 3.10 contains photographs of rice husk, peanut husk, bagasse, and water hyacinth before and after torrefaction. The color change is present in all biomass but to different degrees.

Torrefaction also modifies the structure of the biomass, making it more friable or brittle. This is caused by the depolymerization of hemicellulose. As a result, the process of size reduction becomes easier, lowering its energy con­sumption and the cost of handling. This makes it easier to co-fire biomass in a pulverized-coal fired boiler or gasify it in an entrained-flow reactor.

Torrefaction causes some reduction in the energy content of the biomass because of partial devolatilization, but given the much higher reduction in mass, the energy density of the biomass increases. Table 3.7 shows an example of torrefaction. Here, we note that by losing only 11 to 17% energy, the biomass

(bagasse) lost 31 to 38% of its original mass. Thus, there is a 29 to 33% increase in energy density (energy per unit mass) of the biomass. This increases its higher heating value (HHV) to about 20 MJ/kg. Even if we take into account the energy used in the torrefaction process, we can see from Table 3.7 that there is a net rise in the energy density of the fuel.

Another special feature of torrefaction is that it reduces the hygroscopic property of biomass; therefore, when torrefied biomass is stored, it absorbs less moisture than that absorbed by fresh biomass. For example, while raw bagasse absorbed 186% moisture when immersed in water for two hours, it absorbed only 7.6% moisture under this condition after torrefying the bagasse for 60 minutes at 250 °C (Pimchua et al., 2009). The reduced hygroscopic (or enhanced hydrophobic) nature of torrefied biomass mitigates one of the major shortcom­ings for energy use of biomass.