Changes in pore structure of raw lignite and dried lignite with and
without acoustic excitation were listed in Table 3. It can be seen that
the specific surface area, pore volume, and average pore diameter
were increased slightly during the nitrogen drying of lignite. This result
is inconsistent with those reported in the literature [3,4]. An effect of
introducing acoustic waves on the pore structure of dried lignite was
evident. In these cases acoustic waves led to slightly reduced specific
surface area, pore volume, and average pore diameter.