Introduction - Biomass Gasification, Pyrolysis and Torrefaction - page 13

In pyrolysis, large hydrocarbon molecules of biomass are broken down
into smaller molecules. Fast pyrolysis produces mainly liquid fuel, known as
bio-oil, whereas slow pyrolysis produces some gas and solid charcoal (one of
the most ancient fuels, used for heating and metal extraction before the dis-
covery of coal). Pyrolysis is promising for conversion of waste biomass into
useful liquid fuels. Unlike combustion, it is not exothermic.

1.2.2.3 Torrefaction
Torrefaction is being considered for effective utilization of biomass as a clean
and convenient solid fuel. In this process, the biomass is slowly heated to
200 300 C without or little contact with oxygen. Torrefaction alters the

chemical structure of biomass hydrocarbon to increase its carbon content while
reducing its oxygen. Torrefaction also increases the energy density of the bio-
mass and makes the biomass hygroscopic. These attributes thus enhance the
commercial value of wood for energy production and transportation.

1.2.2.4 Gasification
Gasification converts fossil or nonfossil fuels (solid, liquid, or gaseous) into
useful gases and chemicals. It requires a medium for reaction, which can be
gas or supercritical water (not to be confused with ordinary water at subcriti-
cal condition). Gaseous mediums include air, oxygen, subcritical steam, or a
mixture of these.
Presently, gasification of fossil fuels is more common than that of nonfossil
fuels like biomass for production of synthetic gases. It essentially converts a
potential fuel from one form to another. There are several major motivations
for such a transformation and are as follows:

To increase the heating value of the fuel by rejecting noncombustible
components like nitrogen and water.
To strip the fuel gas of sulfur such that it is not released into the atmo-
sphere when the gas is burnt.
To increase the H/C mass ratio in the fuel.
To reduce the oxygen content of the fuel.

In general, the higher the hydrogen content of a fuel, the lower the vapor-
ization temperature and the higher the probability of the fuel being in a gas-
eous state. Gasification or pyrolysis increases the relative hydrogen content
(H/C ratio) in the product through one the following means:

1. Direct: Direct exposure to hydrogen at high pressure.
2. Indirect: Exposure to steam at high temperature and pressure, where
hydrogen, an intermediate product, is added to the product. This process
also includes steam reforming.
3. Pyrolysis or devolatilization: Reduction in carbon content by rejecting it
via solid char or CO 2 gas.