Just in case you haven't had enough in the recent threads about this, here
is more information.
Compression drying is basically a process of forcing the free water in
wood to move under high hydrostatic pressure through a solid structure.
Fundamental information regarding the time-dependent characteristic of
compression drying is necessary to develop efficient commercial processes.
The purpose of this study is to provide an initial evaluation of the effect
of some factors-pressure, wood density, and particle (chip) size-on free
Five species-aspen, balsam fir, jack pine, red maple, and red oak-were
tested in this study. For each species both typical pulp size chips and
particles from hammermilled chips were used. Drying rates were determined
under constant ram face pressures at 500 psi, 1,000 psi, 1,500 psi, and
2,000 psi, respectively. The concept of drying rate is one of the important
factors in dealing with compression drying, especially in designing
dewatering pressure cycles.
The most efficient compression drying is achieved during the first two
minutes. Drying rates are negligible after 3 to 4 minutes of constant
pressure in the 500 to 2,000 psi range. The analysis of variance for species
shows highly significant differences in final moisture contents. Size of
chips had a significant effect on final moisture contents. Compressed
density of hammermilled chips is slightly higher than that of unrefined
chips. High density chips require higher pressure to initiate effective
Moisture content affects the energy content of wood when burned and the
amount of wood fuel required to achieve desired heat or steam outputs.
Harvested wood or wood from landscaping has a moisture content between 35
and 50 percent and an energy content of approximately 5,500 BTUs (British
thermal units) per pound. Kiln-dried wood residue has a moisture content of
5 to 10 percent and an energy content of approximately 7,500 BTUs per pound.
Wood separated from a process waste stream and used for cogeneration is
typically air dried to a moisture content of 15 to 20 percent and has an
energy content of approximately 6,000 to 6,500 BTUs per pound.
a.. More efficient fuel than cordwood. Pellets have five to 10 percent
moisture content in comparison to 30 to 60 percent for cordwood and
woodchips. This means pellets are a more efficient fuel.
b.. Higher Btu content than cordwood. Wood pellets have a Btu output
content of 350,000 per cub. Ft. of fuel, versus 70,000 to 90,000 for
cordwood or wood chips. This means pellets produce more heat.
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