The combustible gases from the gasifier can be used a) in internal combustion engines, b) for direct heat applications and c) as feedstock for production of chemicals like methanol.
However in order for the gas to be used for any of the above applications it should be cleaned of tar and dust and be cooled. As previously mentioned cooling and cleaning of the gas is one of the most important processes in the whole gasification system. The failure or the success of producer gas units depends completely on their ability to provide a clean and cool 12 gas to the engines or for burners. Thus the importance of cleaning and cooling systems cannot be overemphasized.
A. Cooling and Cleaning of Gas
The temperature of gas coming out of generator is normally between 300-500
0
C. This gas has to be cooled in order to raise its energy density. Various types of cooling equipment have been used to achieve this end
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Most coolers are gas to air heat exchangers where the cooling .
is done by free convection of air on the outside surface of heat exchanger. Since the gas also
contains moisture and tar, some heat exchangers provide partial scrubbing of gas
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Thus .
ideally the gas going to an internal combustion engine should be cooled to nearly ambient
temperature.
Cleaning of the gas is trickier and is very critical. Normally three types of filters are used in
this process. They are classified as dry, moist and wet
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.
In the dry category are cyclone filters. They are designed according to the rate of gas
production and its dust content
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The cyclone filters are useful for particle size of 5 μm and .
greater
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Since 60-65% of the producer gas contains particles above 60 μm in size the .
cyclone filter is an excellent cleaning device
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.
After passing through cyclone filter the gas still contains fine dust, particles and tar. It is
further cleaned by passing through either a wet scrubber or dry cloth filter. In the wet
scrubber the gas is washed by water in countercurrent mode. The scrubber also acts like a
cooler
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, from where the gas goes to cloth or cork filter for final cleaning.
Since cloth filter is a fine filter, any condensation of water on it stops the gas flow because of
increase in pressure drop across it. Thus in quite a number of gasification systems the hot
gases are passed through the cloth filter and then only do they go to the cooler
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Since the .
gases are still above dew point, no condensation takes place in filter. Figure 8 shows
schematically a downdraft gasification system with cleaning and cooling train. 13
There is quite a substantial pressure drop across the whole gasification system and the design
e pressure drop should not exceed 100 cm of water
7
.
to run completely or partly on the gas. However in actual practice running the
