Biogas upgrading to biomethane - The Biogas: Science, Production and Applications - page 360

formed. Depending on the SO x load and potential emission requirements, an
additional SO x cleaning step might be required. For H 2 S reduction in off-
gas flows, activated carbon filters are usually used. If there is a demand for
reduction of SO x in off-gas flows, basic (lye) scrubbers (using, for example,
NaOH as absorbent) are normally used [33].
Depending on the applied biogas upgrading method and its operational
mode, methane loads in the off-gas flow can vary from 0.1% up to 15%

(see Table 15.1) of the concentration in the raw biogas at the inlet of the
upgrading plant. In this context two terms have to be defined.

~

. Methane loss or methane slip is the ratio of the quantity of methane that

does not appear in the product gas to the quantity of methane in the raw
biogas at the inlet to the upgrading plant [8].

.

Methane emissions to the atmosphere are the ratio of the quantity of
methane that is emitted unoxidized to the atmosphere to the quantity of
methane in the raw biogas at the inlet to the biogas upgrading plant [8].

For degradation of CH 4 in the off-gas, the methods of practical relevance
are [33]

. regenerative thermal oxidation (RTO)

. catalytic oxidation

flameless oxidation (in Flox ® burners) .

. co-firing in combustion engines (e.g. micro turbines).

Regenerative thermal oxidation (RTO) is a suitable treatment method for
off-gas flows with low methane concentrations [8] and is thus mostly applied
in off-gas streams of water scrubbers and Genosorb ® scrubbers [33]. In the
future, it may also become an advantageous treatment method for off-gases
from PSA and membrane systems, provided that sufficiently low methane
losses (and therefore low methane concentrations in the off-gas) will be
reached in those processes. Autothermal operation (without co-firing of
natural gas, liquefied petroleum gas (LPG) or raw biogas) is already possible
with methane concentrations of 0.2% Vol , which is equivalent to 1.3 g CH 4 /
m n 3 [48]. However, for the start-up phase, heating with higher calorific gases
is needed [9]. Compared with other off-gas treatment methods, RTO can be
made resistant to corrosive media such as H 2 S and its degradation products
[9]. The operation temperature inside the RTO system is

￿ ￿ ￿ ￿ ￿ ￿

C [48]. To
guarantee complete oxidation, an oxygen concentration of minimum
10% Vol has to be ensured. To ensure low heat losses, heat is stored in
ceramic materials (the shaded boxes in Fig. 15.14) and the flow direction is
switched continuously. As illustrated in Fig. 15.14, in phase 1 the ceramic
bed on the left-hand side preheats the gas before oxidation takes place.
Before the exhaust gas leaves the second chamber, this hot gas heats the
ceramic bed on the right-hand side. Phase 2 illustrates the process after the

~ 800 8