Environmental Engineering Reference
In-Depth Information
15.13
Membrane separation plant (Copyright: Fraunhofer IWES).
depending on applied operation pressures, recirculation volume flows and
membrane qualities [18, 19, 21, 45]. In new systems, specific energy
consumptions are significantly lower than the upper value of 0.35 kWh
el
/
m
n
3
. One membrane provider
states
specific electricity demands of
0.2 kW
el
/m
n
3
related to raw biogas upgrading capacity (for operation
pressures of 10-20 bar) [21]. A plant provider has stated specific electricity
demands in the range of 0.29-0.35 kWh
el
/m
n
3
depending on raw gas
constitution and methane recovery rate (related to upgraded biomethane
with a CH
4
concentration of 97%) [43]. In a large-scale operation plant
located in Germany, a specific electricity consumption of 0.20 kWh
el
/m
n
3
raw biogas has been reached [18].
Methane recovery rates stated in the literature range from 85 to 99%
(methane losses of 1-15%) [19, 21]. In the past, economic methane recovery
rates could be expected in the range 95-96% [18]. Increased yields were
possible, but led to increased recirculation rates and therefore to increased
electricity consumption.
Because the off-gas flow contains significant amounts of CH
4
it can be
necessary to oxidize it in an off-gas treatment step. Suitable treatment
methods are comparable to those of the PSA systems described earlier. A
membrane separation plant is shown in Fig. 15.13.
<
Cryogenic upgrading
Cryogenic upgrading uses the effect of different boiling points of methane
(
C) [46].
Depending on the temperature level in the process, theoretically other gas
161
8
C) and sublimation points of carbon dioxide (
78.48
8
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