Environmental Engineering Reference
In-Depth Information
temperatures of 135-145
C [17]. Another plant manufacturer reports typical
heat consumptions of 0.55-0.70 kWh th /m n 3 raw biogas [26]. Heat recovery is
also possible. One provider states that 20% of the supplied heat can be
uncoupled at temperature levels of
8
C [17].
Methane recovery rates are stated to be around 99.9% [11, 17, 26].
Because of very low methane losses in the system (compared to all the other
methods), the off-gas does not normally require further treatment.
~
50
8
Membrane separation
Membrane separation, also known as gas permeation, takes advantage of
the different permeabilities of gas compounds through polymer membranes
[8]. Three different flows can be defined in membrane systems: the feed (raw
biogas), the permeate (CO 2 -rich gas) and the retentate (CH 4 -rich gas).
Different partial pressures of respective compounds between the feed and
permeate side can be defined as the driving force of the system [42].
Increased pressures on the feed side and decreased pressures on the
permeate side cause high flux rates [42]. Candidate polymers include
cellulose acetate and aromatic polyimides [13], which have high perme-
abilities of CO 2 ,H 2 O, NH 3 and H 2 S compared to CH 4 [13]. Permeabilities
for N 2 in particular but also for O 2 , are significantly lower [43]. An essential
parameter for economic operation of membrane systems for biogas
upgrading (separation CO 2 /CH 4 ) is the selectivity of these two gas
compounds. For polyimide/polyaramide membranes, CO 2 /CH 4 selectivities
range between 20 and 25 [44]. In the past, CO 2 /CH 4 selectivities had been
~
20 but there are now membrane materials available with selectivities of
~
50
[43]. As of 2012, membrane materials with CO 2 /CH 4 selectivities of
~
70 were
in the testing phase [43].
To extend the life of the membranes and obtain optimum separation, the
raw gas is dried and precision desulphurized, and dust and aerosols are
separated, before the gas enters the membrane [8]. The biogas is compressed
(depending on the manufacturer) to 7-20 bar (in systems of the 1980s and
1990s, pressures greater than 20 bar were used) and precision desulphurized
either before or after compression; it then enters the membrane modules [7,
18, 21, 45]. Pressure losses in the system can be assumed to be
￿ ￿ ￿ ￿ ￿ ￿
1 bar [18].
Inside the membrane module, the CO 2 permeates the membrane while the
majority of CH 4 is not permitted to pass. Most practical applications use
processes involving at least two-stage systems (see Fig. 15.12) [13, 42]. The
permeate flow still contains some methane so that the off-gas flow (e.g. of
the second stage) is recirculated or is passed through an additional stage [13,
42]. Because membrane upgrading plants consist of a number of single
membrane modules, these systems show excellent part load behaviours.
Electricity demand can vary from 0.18 to 0.35 kWh el /m n 3
~
raw biogas
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