Gasification was originally developed for coal, biomass however
has a much lower heating value and a lower energy density. Coal
gasification is relatively insensitive to humidity in the coal, to the
point that some gasifiers are feed with a coal water slurry. For coal
with a Lower Heating Value of 40 MJ kg1 and a water mass fraction
of 50%, heating and evaporation of water at 1000 C at atmospheric
pressure represents 10% of the heating value of the coal. Humidity
in the fuel has more impact on the gasification yield in the case of
biomass. For woody biomass with a Lower Heating Value of
18 MJ kg1 and a water mass fraction of 50%, heating and evaporation
of water at 1000 C at atmospheric pressure represents 23%
of the heating value of the biomass. The net heat release during
combustion or gasification of wet biomass is too low. This deteriorates
the operability and yield of the process. Drying of the
feed is therefore an important issue in a BtL plant. The drying
section consumes a large amount of (low grade) energy.