Moisture content of the fuel
The heating value of the gas produced by any type of gasifier depends at least in part on the moisture content of the feedstock.
Moisture content can be determined on a dry basis as well as on a wet basis. In this chapter the moisture content (M. C.) on a dry basis will be used.
Moisture content is defined as: diy weight
Alternatively the moisture content on a wet basis is defined as:
wet weight
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Conversions from one to another can be obtained by:
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and:
High moisture contents reduce the thermal efficiency since heat is used to drive off the water and consequently this energy is not available for the reduction reactions and for converting thermal energy into chemical bound energy in the gas. Therefore high moisture contents result in low gas heating values.
When the gas is used for direct combustion purposes, low heating values can be tolerated and the use of feedstocks with moisture contents (dry basis) of up to 40 — 50 percent is feasible, especially when using updraught gasifiers.
In downdraught gasifiers high moisture contents give rise not only to low gas heating values, but also to low temperatures in the oxidation zone, and this can lead to insufficient tar converting capability if the gas is used for engine applications.
Both because of the gas heating value (engines need gas of at least 4200 kJ/m3 in order to maintain a reasonable efficiency) and of the tar entrainment problem, downdraught gasifiers need reasonably dry fuels (less than 25 percent moisture dry basis).