Biogas upgrading – Review of commercial technologies - SGC

SGC Rapport 2013:270
Only the amine scrubber system has a heat requirement. The heat is required for
reversing the chemical reaction and liberating the CO2. The amount of heat depends
to some extent on the inlet methane concentration, but since system heat
loss is constant this effect is to some extent buffered. A typical value for the heat
demand is 0.55 kWh/Nm 3 , which is required in heating the stripper. The heat is
added at 150-120°C and 80% of the heat may be reused in low-temperature
(65°C) applications such as substrate pre-heating or AD heating. Catalytic or
thermal oxidation of methane in the off-gas from both physical scrubbers, membrane
and PSA can also be used to heat the AD or the substrate as well as the
excess heat from the compressors.
The chemical consumption of the water scrubber and amine scrubber is mainly
anti-foaming agents and, in the amine scrubber case, make-up of amine. The requirement
is small, with the highest reported by the amine scrubber system
(0.00003 kg/Nm 3 ) with the water scrubber being in the same vicinity. Hydrogen
sulphide removal requires activated charcoal, both for PSA, genosorb, membranes
and amines; there is also an option in the case of water scrubbers to use sulphur
removal on the off-gas to meet environmental permits. All of the technologies have
approximately the same need for lubricant oil for the compressors.
3.3 Methane slip
The methane slip is quite high in the PSA case with 1.8-2% reported as mean and
median values. Higher values are most probably due to temporary technical problems.
The water scrubber has a slip of about 1% in modern plants. Values much
higher than this are not probable in a well-functioning plant. Both these technologies
require tail-end solutions to decrease the methane slip to meet stricter regulations.
This may be done by thermal or catalytic oxidation of the methane slip. The
amine scrubber system has a much lower amine slip with 0.1% guaranteed. Organic
physical scrubbers have a higher slip than the other technologies, but the
methane is oxidized to supply heat to the process to enable the desorption and
thus it is utilized internally. Membranes also seem to enable operation with a very
low methane slip, about 0.5%, with design (iii) in Figure 15. In some membrane
applications on the market, liquefaction of the carbon dioxide in the waste gas is
used to recover 100% of the methane in the waste gas by cryogenic separation.
3.4 Investment costs
The investment cost of the various technologies has been reported earlier in the
text, but in summary the different upgrading methods does not differ much; especially
not at higher throughputs. The specific investment costs are shown together
in Figure 31.
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