This study relating to the steam drying of biofuels suggests that the
activities of water in air (defined as relative humidity) and of water
in superheated steam (defined as the ratio of the saturated pressure
and the saturated pressure at the superheated temperature) are identic
al. The dependence of the activity in superheated steam on the equilib
rium moisture content was studied in various experiments for different
wood materials at constant temperatures of 140 and 160 degrees C. The
equilibrium moisture content was found to depend on the following fac
tors: the activity of the superheated steam, the temperature and the m
aterials used, the first of these being the most important. The experi
mentally determined sorption isotherms were compared with different so
rption theories. The Dent model gave a good correlation with the exper
imental data. The sorbed water can be divided into primary water with
high binding energy and secondary water with low binding energy. If bi
ofuels are to be dried with superheated steam, an equilibrium moisture
fraction of similar to 0.05 seems to be relevant. It will probably be
possible to reach this with activities in the range 0.2-0.4 for all r
eal materials.