Energy content of the fuel
The choice of a fuel for gasification will in part be decided by its heating value. The method of measurement of the fuel energy content will influence the estimate of efficiency of a given gasification system. Reporting of fuel heating values is often confusing since at least three different bases are used:
— fuel higher heating values as obtained in an adiabatic bomb calorimeter. These values include the heat of condensation of the water that is produced during combustion. Because it is very difficult to recover the heat of condensation in actual gasification operations these values present a too optimistic view of the fuel energy content;
— fuel higher heating values on a moisture-free basis, which disregard the actual moisture content of the fuel and so provide even more optimistic estimates of energy content;
— fuel higher heating values on a moisture and ash free basis, which disregard the incombustible components and consequently provide estimates of energy content too high for a given weight of fuel, especially in the case of some agricultural residues (rice husks).
The only realistic way therefore of presenting fuel heating values for gasification purposes is to give lower heating values (excluding the heat of condensation of the water produced) on an ash inclusive basis and with specific reference to the actual moisture content of the fuel. Average lower heating values of wood, charcoal and peat are given in Table 2.4.
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Table 2.4 Average lower heating values
1/ per cent of dry weight |