Biogas upgrading to biomethane - The Biogas: Science, Production and Applications - page 362

phase requires heating with higher calorific gases or electricity [9]. The
operation temperature inside the reaction chamber is only C because

of the reduced activation energy. To guarantee complete oxidation, an
increased oxygen concentration of 2% Vol (compared with stoichiometric)
has to be ensured [48].
Lean gas burners such as flameless oxidation processes have practical
relevance, especially for off-gas streams of PSA systems and membrane
separations [33]. This is a suitable technology for high methane concentra-
tions and if there is sufficient heat demand (e.g. for the provision of digester
heat). Autothermal operation is possible with methane concentrations of 4-
5% Vol (27-34 g CH 4 /m n 3 ) [9, 48, 49]. Because most biogas upgrading
methods have much lower CH 4 concentrations in the off-gas, either
methane slip has to be set intentionally high or a dosing of high calorific
gas (e.g. raw biogas) is necessary [8]. The operation temperature of

~ 400 8

Cis
significantly higher than for the other methods described above [49]. With
this high process temperature it is appropriate to preheat air and lean gas to
temperature levels of

~ 900 8

~ 500 8 C [49]. The exhaust gas leaves the plant at a

~ 8

temperature of C [49] and therefore this method is suitable for the

provision of process heat or external heat utilization.
Oxidation of off-gas by co-incineration in combustion engines or boilers
is also possible and has been applied in early installations in the USA [2].
However, utilization of pure off-gas is not possible so the off-gas must be
mixed with a higher calorific gas (e.g. raw biogas). The most suitable
modules for this method are micro turbines because of the lowest required
calorific value of 3.8 kWh/m n 3 (Hs) [9]. This means (if only methane is the
energy carrier as fuel) that a minimum methane concentration of 35% Vol has
to be reached at the fuel inlet of the micro turbine.

600

15.4 Costs of biogas upgrading
Because the cost of biogas upgrading is the decisive criterion in a project,
relevant financial parameters are now discussed. It is important to note that
all data stated here should not be used alone as decision criterion within a
project.
Figure 15.16 shows specific investment costs as a function of raw biogas
upgrading capacity for five upgrading technologies. The costs do not include
planning, permission or other further construction costs. All costs are based
on price indications of current plant generations provided by manufacturers
[14-17, 45]. These costs are therefore not representative for the respective
technologies but can be considered as indications only. All costs except
those for amine scrubbers include an off-gas treatment step for CH 4
degradation. This means that for some technologies, the specific investment

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