produced by the above-mentioned scrubbing, membrane and adsorption technologies range from 95 to 98% (Bauer et al. 2013). However, the CO 2 footprint of these
technologies is high as a result of the direct release to the atmosphere of the CO 2
separated and their high energy demand (which represents 3–12% of the energy
content present in the raw biogas). Table 7 summarises the fundamentals and the
typical design-operating parameters of the main physical/chemical technologies for
CO 2 separation from biogas.
Biological CO 2 removal from biogas is still in an early stage of investigation,
hydrogenotrophic CO 2 reduction to CH 4 and photosynthetic CO 2 assimilation being
the two most promising technologies under scale up. Hydrogenotrophic CO 2
removal, also named power-to-gas, is based on the bioconversion of CO 2 to CH 4
using H 2 as an electron donor and CO 2 as a carbon source and electron acceptor by
hydrogenotrophic archaea. Equation 2 describes the stoichiometry of this CO 2
reduction, which can be conducted either directly into the anaerobic digestion (via
H 2 supplementation) or in an external bioreactor supplemented with H 2 and biogas:
4H 2 + CO 2 ! CH 4 þ 2H 2 O DnG
0
¼ À130:7 kJ/mol
ð2Þ
From an economic and environmental viewpoint, hydrogenotrophic CO 2
removal should be based on H 2 produced from water electrolysis using the excess
of renewable electricity (i.e. wind power generated during the night). The main
limitation of this technology derives from the limited gas–liquid H 2 mass transfer as
a result of the low aqueous solubility of this gas (Diaz et al. 2015). On the other
hand, photosynthetic CO 2 removal is based on the intensification of the symbiosis
between microalgae and quimioautotrophic bacteria at a high pH (=enhancement in
the CO 2 and H 2 S biogas–liquid mass transfer) in photobioreactors as a platform
technology to simultaneously remove CO 2 , H 2 S, NH 3 and VFAs from biogas at a
low energy cost and with a low environmental impact. In these systems, microalgae
use the solar energy to fix the CO 2 from biogas via photosynthesis (Meier et al.
2015). Residual nutrients from the effluents of the anaerobic digesters can be used
to support microalgae growth, which will significantly reduce the operation cost of
the upgrading process and partially mitigate the eutrophication potential of the
digestate. This technology has been successfully implemented in open high rate
algal ponds interconnected to external absorption columns at 2–3 times lower
operating costs than their physical/chemical counterparts (Toledo-Cervantes et al.
2017).
The other major biogas pollutant, H 2 S, can be removed using physical/chemical
and biological technologies already available at commercial scale (Abatzoglou and
Boivin 2009). Adsorption (with and without chemical reaction) and in situ chemical
precipitation still represent the two most widely implemented technologies worldwide despite their high operating cost (3.2 and 2.4 cts €/m
3 , respectively).
Similarly to their CO 2 removal counterparts, these physical/chemical technologies
exhibit high efficiencies and a high robustness. Likewise, biotechnologies such as
biotrickling filtration and microaerobic anaerobic digestion support high removal
efficiencies (>99%) at significantly lower operating cost (1.5 and 0.28 cts €/m
3 ,
12 Biofuels from Microalgae: Biomethane
263
technologies is high as a result of the direct release to the atmosphere of the CO 2
separated and their high energy demand (which represents 3–12% of the energy
content present in the raw biogas). Table 7 summarises the fundamentals and the
typical design-operating parameters of the main physical/chemical technologies for
CO 2 separation from biogas.
Biological CO 2 removal from biogas is still in an early stage of investigation,
hydrogenotrophic CO 2 reduction to CH 4 and photosynthetic CO 2 assimilation being
the two most promising technologies under scale up. Hydrogenotrophic CO 2
removal, also named power-to-gas, is based on the bioconversion of CO 2 to CH 4
using H 2 as an electron donor and CO 2 as a carbon source and electron acceptor by
hydrogenotrophic archaea. Equation 2 describes the stoichiometry of this CO 2
reduction, which can be conducted either directly into the anaerobic digestion (via
H 2 supplementation) or in an external bioreactor supplemented with H 2 and biogas:
4H 2 + CO 2 ! CH 4 þ 2H 2 O DnG
0
¼ À130:7 kJ/mol
ð2Þ
From an economic and environmental viewpoint, hydrogenotrophic CO 2
removal should be based on H 2 produced from water electrolysis using the excess
of renewable electricity (i.e. wind power generated during the night). The main
limitation of this technology derives from the limited gas–liquid H 2 mass transfer as
a result of the low aqueous solubility of this gas (Diaz et al. 2015). On the other
hand, photosynthetic CO 2 removal is based on the intensification of the symbiosis
between microalgae and quimioautotrophic bacteria at a high pH (=enhancement in
the CO 2 and H 2 S biogas–liquid mass transfer) in photobioreactors as a platform
technology to simultaneously remove CO 2 , H 2 S, NH 3 and VFAs from biogas at a
low energy cost and with a low environmental impact. In these systems, microalgae
use the solar energy to fix the CO 2 from biogas via photosynthesis (Meier et al.
2015). Residual nutrients from the effluents of the anaerobic digesters can be used
to support microalgae growth, which will significantly reduce the operation cost of
the upgrading process and partially mitigate the eutrophication potential of the
digestate. This technology has been successfully implemented in open high rate
algal ponds interconnected to external absorption columns at 2–3 times lower
operating costs than their physical/chemical counterparts (Toledo-Cervantes et al.
2017).
The other major biogas pollutant, H 2 S, can be removed using physical/chemical
and biological technologies already available at commercial scale (Abatzoglou and
Boivin 2009). Adsorption (with and without chemical reaction) and in situ chemical
precipitation still represent the two most widely implemented technologies worldwide despite their high operating cost (3.2 and 2.4 cts €/m
3 , respectively).
Similarly to their CO 2 removal counterparts, these physical/chemical technologies
exhibit high efficiencies and a high robustness. Likewise, biotechnologies such as
biotrickling filtration and microaerobic anaerobic digestion support high removal
efficiencies (>99%) at significantly lower operating cost (1.5 and 0.28 cts €/m
3 ,
12 Biofuels from Microalgae: Biomethane
263