Biogas upgrading to biomethane - The Biogas: Science, Production and Applications - page 347

In old systems, methane recovery rates were ~ 94% (methane slip of ~ 6%).

Current methane recovery rates range between 97.5 and 98.5% (methane

slip of 1.5-2.5%) [10]. Depending on the technology provider, there are also
strategies to set up significantly lower methane recovery rates. On the one
hand, this is done to be able to use the off-gas flow more efficiently and, on
the other, to attain increased methane concentrations in the product gas.
Product gas qualities of

~

99% methane have been reached by accepting
methane concentrations of 17-18% in the off-gas flow [32].
Because the off-gas flow contains significant amounts of CH 4 it can be
necessary to oxidize it in an off-gas treatment step. The off-gas does not
include significant amounts of sulphur, so 'catalytic oxidation' and
'flameless oxidation' as off-gas treatment technologies are mostly used in
large-scale applications [33]. Regenerative thermal oxidation (RTO) would
also be possible if sufficiently low methane concentrations were reached in
the off-gas flow (e.g. by dilution with air).

>

Water scrubber

Water scrubbing (see Figs 15.6 and 15.7) is an absorptive method for biogas
upgrading using only the inorganic solvent water [13]. Unlike adsorption,
absorption implies the dissolution of gas or vapour in a liquid (absorption
agent) [27]. Basically, it has to be distinguished between physisorption and
chemisorption. Water scrubbing is a typical physisorption based on the
reversible absorption by physical bonding forces (Van der Waals force).
Low temperatures and high pressures increase the absorption rate [27].
In the water scrubber process, CO 2 and other acidic (e.g. H 2 S) and basic
(e.g. NH 3 ) gas components are absorbed in parallel [11]. As a side effect of
this method, a precision desulphurization is not necessary because H 2 Sis
also removed from the biogas in the absorption column sufficiently.
However, it has to be removed from the off-gas. Typical methane
concentrations in the biomethane flow are

￿ ￿ ￿ ￿ ￿ ￿

96% [9, 13, 34].
After condensate separation, the raw biogas normally passes through two
compression stages up to approximately 4-8 bar [13, 15, 35], but partly also
up to 10 bar [7]. The applied operation pressures and temperatures in the
absorption column depend on the particular provider's strategy. One system
provider adjusts the temperature to a range of 5-10

>

C [34]. After each
compression step, cooling takes place. At this stage, heat uncoupling is also
possible. The raw gas enters the absorption column at the bottom. The
water flowing through the column from top down is loaded with the
absorbed gas (primarily CO 2 and H 2 S). Saturated with water, the product
gas leaves the column from the top and is sent for adsorption drying [13].
The scrubbing solution (also carrying some CH 4 ) is, in a first step,
decompressed in the so-called flash column to pressure levels of

8

~ 2-4 bar