Every operator should aim to obtain fuel with the lowest possible water content (Zárybnická, 2011). Whilst freshly felled wood has a moisture content of up to 60–80 per cent, air-dried wood with a moisture content below 20 per cent is ideal for energy use (Furka et al., 2011). An interesting aspect of gasification compared to direct combustion, however, is the fact that completely dry feedstock is not essential, as water (in the form of water vapour) can act as a reactant in the reactor, contributing to hydrogen production (Geffert, 2013).

A high moisture content in the fuel dramatically increases the energy requirements of the process (Furka et al., 2011). The main reason for the low efficiency is that a large proportion of the energy is consumed in evaporating the water rather than in the actual gasification process (Furka et al., 2011). When the water content is up to 20 per cent, the calorific value of wood (approx. 14.3 MJ/kg) is almost double that of fuel with 50 per cent moisture content (Furka et al., 2011).

Moisture is a key factor in the formation of undesirable tars. Experimental measurements confirm that, when using damp wood chips (25 per cent moisture content), demonstrably more tars are produced during the process than when using pre-dried fuel (Furka et al., 2011). Tars are undesirable in the gas because they condense at temperatures below 300 °C and cause serious technical problems (Furka et al., 2011).

High fuel moisture content has a negative impact on the resulting gas by increasing the volume of water vapour in the product (Furka et al., 2011). In addition to reducing the energy value of the gas, damp fuel also poses a risk of the feed mechanism jamming in the hopper, an increase in the dew point temperature, and increased susceptibility of equipment components to corrosion (Furka et al., 2011).