PHYSICAL PRETREATMENT OF BIOMASS - Biomass as a Sustainable Energy Source for the Future - page 251

thereby squeezing the moisture out. A drum press comprises a perforated drum with a
revolving press roll inside it that presses material against the drum. This kind of press
has been used with a wide range of biomasses.
A roller press is another apparatus for mechanical dewatering; in a way, it is similar
to a screw press. In a roller press, however, the pressure increase is accomplished by
two cylindrical rolls of which one or two are electrically driven with a small gap
between them through which the biomass is transported, simultaneously pressing
out the water. Advantages of a roller press are that the pressure can be set relatively
accurately and, more importantly, the energy use is comparatively low. In addition,
the equipment is relatively simple and cheap to produce. There are some disadvantages
though. Supply of biomass to the equipment is not very difficult for biomass with
longer stalks, but for, e.g., grass, this is a rather challenging task. This issue can be
solved by inclining the rolls and using a vertically mounted supply system so that grav-
ity helps the flow. Alternatively, a press can be placed in front of the roller to ensure that
biomass enters the system. Water removal in this way is limited, although it is some-
what improved (to ~50 wt% order of magnitude) by placing a few units in series.
Centrifuges can also be applied; these dewater based on centrifugal forces. An
example is the bowl centrifuge, in which the material enters a conical, spinning bowl
in which solids accumulate on the perimeter.

Example 8.1 Energy balance calculation for a biomass roller
press for dewatering

Wet biomass is pressed through a roller press; a schematic is shown in Figure 8.2.
The biomass contains 75 wt% moisture (ar basis). Drying takes place by once-
through pressing until a moisture content of 65 wt% (ar basis) is reached. Set up the
mass and energy balances. Assume steady-state operation and adiabatic compres-
sion of the plug and neglect changes in potential and kinetic energy related to the
biomass plug. Biomass enters the roller press at 25 C and leaves the press at a tem-
perature of 26 C; assume that the dried biomass has the same temperature as the

Wet biomass

Dried biomass

FIGURE 8.2 Schematic of the roller press.