Environmental Engineering Reference
In-Depth Information
.
absorption
.
(gas) permeation
.
cryogenic upgrading (to LBG or CBG).
The six most widespread technologies are pressure swing adsorption,
water scrubber, physical absorption (using organic solvents), chemical
absorption (using organic solvents), high-pressure membrane separation
and cryogenic upgrading. Each will be described in turn. Table 15.1 (on next
page) compiles the key parameters of these six upgrading technologies and
the data in the table will be discussed in the following sections. It is
important to note that the upper levels of the specific ranges for electricity
and heat demand, as well as for methane losses, are mostly related to old
systems. Therefore, these whole ranges do not reflect the state-of-the-art of
the current generation of upgrading plants.
Pressure swing adsorption (PSA)
The PSA method (using molecular sieves) is an adsorptive biogas upgrading
technology. Adsorption means that gas compounds (adsorbates) are
retained on the surface of solids (adsorbents) [27] and are trapped by the
size of the molecular sieve. As adsorbents, activated carbon, zeolites or
carbon molecular sieves can be used [11]. Low temperatures and high
pressures increase the adsorption rate [27]. PSA technology was introduced
at a commercial scale in the 1960s [28].
Besides CO
2
, other molecules such as H
2
S, NH
3
and H
2
O can be co-
adsorbed. In practical use, H
2
O as well as H
2
S are removed before the
biogas is injected into the adsorption columns. Water dew points in the
product gas of
C can be reached [10]. N
2
and O
2
can also be
adsorbed, partly in parallel to CO
2
. From large-scale plant operation it is
known that around 50% of the N
2
is separated into the off-gas flow [29].
Typical methane concentrations in the biomethane flow are
100 to
80
8
96% [9].
Operation pressures may be in a range of 1 to 10 bar [10]. In most PSA
systems the biogas is compressed from 4 to 7 bar [11]. The pressure loss in
the system is
>
~
1 bar [16]. After precision desulphurization, cooling and
removal of water, the cleaned raw gas is injected into an adsorption column
where the molecular sieve is located. The operation temperature ranges from
5to35
C [10]. The predominant part of the CO
2
is adsorbed on the surface
of the molecular sieve while most parts of CH
4
pass it without adsorption.
Only a small part of methane is also adsorbed. The product gas leaves the
adsorber nearly continuously and the subsequent desorption is done by
release of pressure. The first part of the flow is led into an empty column,
primarily to build up the pressure there. A further advantage compared with
direct outlet of this gas flow as off-gas flow of the overall plant is that a main
8
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