Reaction Chemistry
The following major reactions take place in combustion and reduction zone
12
.
1. Combustion zone
The combustible substance of a solid fuel is usually composed of elements carbon, hydrogen and oxygen. In complete combustion carbon dioxide is obtained from carbon in fuel and water is obtained from the hydrogen, usually as steam. The combustion reaction is
exothermic and yields a theoretical oxidation temperature of 1450
0
C
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, The main reactions .
therefore, are:
C + O2 = CO2 (+ 393 MJ/kg mole) (1)
2H2 + O2 = 2H2 O (- 242 MJ/kg mole) (2)
2. Reaction zone
The products of partial combustion (water, carbon dioxide and uncombusted partially cracked
pyrolysis products) now pass through a red-hot charcoal bed where the following reduction
reactions take place
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C + CO2 = 2CO (- 164.9 MJ/kg mole) (3)
C + H2O = CO + H2 (- 122.6 MJ/kg mole) (4)
CO + H2O = CO + H2 (+ 42 MJ/kg mole) (5)
C + 2H2 = CH4 (+ 75 MJ/kg mole) (6)
CO2 + H2 = CO + H2O (- 42.3 MJ/kg mole) (7)
Reactions (3) and (4) are main reduction reactions and being endothermic have the capability
of reducing gas temperature. Consequently the temperatures in the reduction zone are
normally 800-1000
0
C. Lower the reduction zone temperature (~ 700-800
0
C), lower is the
calorific value of gas.
3. Pyrolysis zone
Wood pyrolysis is an intricate process that is still not completely understood
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The products .
depend upon temperature, pressure, residence time and heat losses. However following
general remarks can be made about them.
Upto the temperature of 200
0
C only water is driven off. Between 200 to 280
0
C carbon
dioxide, acetic acid and water are given off. The real pyrolysis, which takes place between
280 to 500
0
C, produces large quantities of tar and gases containing carbon dioxide. Besides
light tars, some methyl alcohol is also formed. Between 500 to 700
0
C the gas production is
small and contains hydrogen.
Thus it is easy to see that updraft gasifier will produce much more tar than downdraft one. In
downdraft gasifier the tars have to go through combustion and reduction zone and are
partially broken down. 6
Since majority of fuels like wood and biomass residue do have large quantities of tar,
downdraft gasifier is preferred over others. Indeed majority of gasifiers, both in World War
II and presently are of downdraft type.
Finally in the drying zone the main process is of drying of wood. Wood entering the gasifier
has moisture content of 10-30%. Various experiments on different gasifiers in different
conditions have shown that on an average the condensate formed is 6-10% of the weight of
gasified wood
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Some organic acids also come out during the drying process. These acids .
give rise to corrosion of gasifiers.
