Gasification Process
The essence of gasification process is the conversion of solid
carbon fuels into carbon monoxide by thermochemical process. The gasification of
solid fuel is accomplished in air sealed, closed chamber, under slight suction
or pressure relative to ambient pressure. Gasification process occuring in
general explained in this section.
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Gasification is quite complex thermochemical process. Splitting
of the gasifier into strictly separate zones is not realistic, but neverthless
conceptully essential. Gasification stages occurs at the same time in different
parts of gasifier.
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Drying
Biomass fuels consist of moisture ranging from 5 to 35%. At the
temperature above 100°C, the water is removed and converted into steam. In
the drying , fuels do not experience any kind of decomposition.
Pyrolysis
Pyrolysis is the themal decomposition of biomass fuels in the
absence of oxygen. Pyrolysis involves release of three kinds of products :
solid, liquid and gases. The ratio of products is influenced by the chemical
composition of biomass fuels and the operating conditions. The heating value of
gas produced during the pyrolysis process is low (3.5 - 8.9 MJ/m 3 ).
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It is noted that no matter how gasifier is built, there will
always be a low temperature zone, where pyrolysis takes place, generating
condensable hydrocarbon.
Oxidation
Introduced air in the oxidation zone contains, besides oxygen
and water vapours, inert gases such as nitrogen and argon. These inert gases are
considered to be non-reactive with fuel constituents. The oxidation takes place
at the temperature of 700-2000o c.
Heterogenous reaction takes place between oxygen in the air and
solid carbonized fuel, producing carbon monoxide. Plus and minus sign indicate
the release and supply of heat energy during the process respectively
C + O 2 = CO 2 + 406 [ MJ/kmol]
In reaction 12.01 kg of carbon is completely combusted with
22.39 m3 of oxygen supplied by air blast to yield 22.26 m 3
of carbon dioxide and 393.8 MJ of heat.
Hydrogen in fuel reacts with oxygen in the air blast, producing
steam .
H 2 + ½ O 2 = H 2 O + 242 [
MJ/kmol]
Reduction
In reduction zone, a number of high temperature chemical
reactions take place in the absence of oxygen. The principal reactions that
takes place in reduction are mentioned below.
Boudouard reaction
CO 2 + C = 2CO - 172.6
[MJ/kmol]
Water-gas reaction
C + H2 O = CO + H 2
- 131.4 [MJ/kmol]
Water shift reaction
CO 2 + H 2 = CO +
H 2 O + 41.2 [MJ/kmol]
Methane production reaction
C + 2H 2 = CH 4
+ 75 [MJ/kmol]
Main reactions show that heat is required during the reduction
process. Hence, the temperature of gas goes down during this stage. If complete
gasification takes place, all the carbon is burned or reduced to carbon
monoxide, a combustible gas and some other mineral matter is vaporized. The
remains are ash and some char (unburned
cabon)
