1.6.3.1.2 Torrefaction

Torrefaction, a process different from carbonization, which takes place at a much higher temperature, is a mild pyrolysis process carried out in a temperature range of 200 to 350 °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 over-oxidation 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 [4].

An example of torrefaction is the process of roasting coffee beans. As the green beans are heated to 200 to 300 °C, their surface darkens. The color changes 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 2 shows an example of torrefaction. Here, we note that by losing only from 11 to 17% of energy, the biomass 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 to HHV ≈ 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. The reduced hygroscopic (or enhanced hydrophobic) nature of torrefied biomass mitigates one of the major shortcomings for energy use of biomass. Table 2. Changes in the biomass (bagasse) properties after torrefaction at 250 °C [4]

Property	Torrefaction time (min)
	15	30	45
Mass yield (%)	69	68.33	62
Energy yield (%)	88.86	91.06	83.23
Energy density (% energy yield / % mass yield)	1.29	1.33	1.34
Energy required (MJ/kg product)	2.34	2.58	2.99
Higher heating value, HHV (MJ/kg product)	19.88	20.57	20.72
Rise in HHV (%)	22.35	24.96	25.51
HHV (MJ/kg raw material)	15.44	15.44	15.44
Net energy (MJ/kg product)	17.54	17.99	17.73

The advantages of torrefaction are as follows [4]:

• it increases the O/C ratio of the wood, which improves its gasification efficiency,

• it reduces power requirements for size reduction, and improves handling,

• it offers cleaner-burning fuel with little acid in the smoke,

• torrefied wood absorbs less moisture when stored,

• a fuel gas that has an enhanced heating value may be obtained through gasification,

• one can produce superior-quality biomass pellets with higher volumetric energy density.

Mechanism of torrefaction. 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 [4].

During torrefaction the weight loss of biomass comes primarily from the decomposition of its hemicellulose constituents within the temperature range 150 to 280 °C. Lignin starts softening above the temperature ~130 °C, which helps densification (pelletization) of torrefied biomass. Cellulose devolatilization and carbonization does not start below 250 °C. During torrefaction H₂O and CO₂, which make no contribution to the energy in the product gas, constitute a dominant portion of the weight loss. The reaction becomes exothermic above 180 °C. The formation of chars, CO, CO₂, and H₂O takes place at 250–300 °C. 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.

The torrefaction temperature (T _tor, ^oC) and the reaction time (_heating, s) are the most important parameters of the torrefaction process. The design norm for torrefaction [4] is

Torrefaction loses more oxygen and hydrogen than carbon. Hence, the H/C and O/C ratios decrease. However, it should not be confused with carbonization, which takes place at a much higher temperature and produces charcoal with even lower H/C and O/C ratios.

Torrefied pellets. The densification process resolves some typical problems of biomass fuels: transport and storing costs are minimized, handling is improved, and the volumetric calorific value is increased. Pelletization or briquetting may not increase the energy density on a mass basis, but it can increase the energy content of the fuel on a volume basis. For example, while the energy density on a mass basis for raw wood, torrefied wood, wood pellet, and torrefied pellet was 10.5, 19.9, 16.2, and 21.6 kJ/kg (LHV as received basis), respectively, it was 5.0, 4.6, 10.5, and 18.4 GJ/m3, respectively, on a volume basis. Thus, pelletization of torrefied wood greatly increases the transportation and handling cost of biomass. Pelletization of torrefied biomass is better than torrefaction of pelletized wood from the standpoint of process energy consumption and product stability.