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,
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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.
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