Higher moisture content in fuel leads to higher enthalpy loss. Efforts should be made to minimize addition of surface moisture content in fuel.
Enthalpy loss depends on added moisture & hydrogen present in the fuel. Hydrogen during combustion combines with the oxygen to convert to water. As they are in hot environment, this water and the moisture present in fuel get evaporated. A part of the energy of combustion is thus used for this evaporation and the heat carried away by the moisture is known as Enthalpy Loss.
Every fuel contains a percentage of inherent moisture and this percentage varies between fuels. The higher is this moisture content the higher is the enthalpy loss during combustion. The inherent moisture content is uncontrollable and hence we must focus our efforts on avoiding addition of surface moisture.
Most solid fuels are purchased by weight and thus there is little check on the moisture content of the fuel. Additionally these fuels are stored in the open thereby adding to the surface moisture content. Some of these may have moisture percentages as high as 30% to 50%! This greatly impacts the boiler efficiency also you end up paying for water at the price of fuel!
The table below presents the typical enthalpy losses seen in the different boilers.
Moisture content & enthalpy loss for biomass fuels