(b) Technological Indicators
Technological indicators include the methane purity, removal of O 2 and N 2 along
with CO 2 and power consumption (Sahota et al. 2018). High amount of methane is
required for utilising biogas as a fuel to be used in the transport systems. High
methane purity is ensured in amine scrubbing method (>98%), organic physical
scrubbing method (93–98%), pressurised water scrubbing method (95–98%), polymeric membrane technology (90–99%), adsorption methods (>96–98%), in situ
methane enrichment technique (up to 95%), chemical dehydrogenation method
(97–99%), industrial lung method (99%) and cryogenic technology (99%). But the
cryogenic technology and amine scrubbing method require high energy which is a
major disadvantage of this method. Industrial lung method provides methane purity
of about 95–99%.
The upgrading technologies, membrane technology and pressure swing adsorption techniques, have the ability to remove nitrogen and oxygen along with carbon
dioxide (Bauer et al. 2013a, b), and hence the purity of the gas is pronounced more.
The disadvantage of polymeric membrane separation technology is high energy
demand. 15–18% methane loss is observed, and compression is required for adsorption method. Removal of hydrogen sulphide gas is possible in pressurised water
scrubbing, organic physical scrubbing, polymeric membrane technology and cryogenic technology. Nitrogen/oxygen removal is possible and partially possible in
cryogenic and polymeric membrane technology, respectively. H 2 S/N 2 /O 2 removal is
impossible in chemical dehydrogenation method (Sahota et al. 2018).
The chemical dehydrogenation method involves in CO 2 and H 2 conversion
completely but with a drawback of catalyst degeneration by the trace gases present
(Guebitz et al. 2015). The advantages for the hybrid technologies cryogenic/membrane technology and membrane technology/pressurised water scrubbing method
include enhanced energy performance, low energy intensity, high methane purity
and increased CO 2 and S-capturing effect (Belaissaoui et al. 2012).
In addition to this, pretreatment is required for the removal of H 2 S in all the
upgrading technologies with the exception of pressurised water scrubbing method
(Bauer et al. 2013a, b). The nitrogen and H 2 S removal are a necessary step to be
executed in the conventional physical/chemical method, which is an additional
barrier during the operation, but in contrast to this, the nitrogen generated is utilised
as a nutrient in the biological method (Toledo-Cervantes et al. 2017).
The technological indicators point out that the suitable and highly efficient
method is the biological method which can overcome the disadvantages of many
techniques by providing high methane purity (95%) and CO 2 consumption rate
(in fermentation method), 80–97% CH 4 recovery (no need for H 2 S/N 2 /O 2 removal)
in algal-bacterial photobioreactor, 65–98.9% CH 4 recovery (no need for H 2 S/N 2 /O 2
removal) in in situ chemoautotrophic method, 88–89% CH 4 recovery in ex situ
chemoautotrophic method, and 80% CH 4 recovery in microbial electrochemical
method (Angelidaki et al. 2018).
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B. S. Dhanya et al.
Technological indicators include the methane purity, removal of O 2 and N 2 along
with CO 2 and power consumption (Sahota et al. 2018). High amount of methane is
required for utilising biogas as a fuel to be used in the transport systems. High
methane purity is ensured in amine scrubbing method (>98%), organic physical
scrubbing method (93–98%), pressurised water scrubbing method (95–98%), polymeric membrane technology (90–99%), adsorption methods (>96–98%), in situ
methane enrichment technique (up to 95%), chemical dehydrogenation method
(97–99%), industrial lung method (99%) and cryogenic technology (99%). But the
cryogenic technology and amine scrubbing method require high energy which is a
major disadvantage of this method. Industrial lung method provides methane purity
of about 95–99%.
The upgrading technologies, membrane technology and pressure swing adsorption techniques, have the ability to remove nitrogen and oxygen along with carbon
dioxide (Bauer et al. 2013a, b), and hence the purity of the gas is pronounced more.
The disadvantage of polymeric membrane separation technology is high energy
demand. 15–18% methane loss is observed, and compression is required for adsorption method. Removal of hydrogen sulphide gas is possible in pressurised water
scrubbing, organic physical scrubbing, polymeric membrane technology and cryogenic technology. Nitrogen/oxygen removal is possible and partially possible in
cryogenic and polymeric membrane technology, respectively. H 2 S/N 2 /O 2 removal is
impossible in chemical dehydrogenation method (Sahota et al. 2018).
The chemical dehydrogenation method involves in CO 2 and H 2 conversion
completely but with a drawback of catalyst degeneration by the trace gases present
(Guebitz et al. 2015). The advantages for the hybrid technologies cryogenic/membrane technology and membrane technology/pressurised water scrubbing method
include enhanced energy performance, low energy intensity, high methane purity
and increased CO 2 and S-capturing effect (Belaissaoui et al. 2012).
In addition to this, pretreatment is required for the removal of H 2 S in all the
upgrading technologies with the exception of pressurised water scrubbing method
(Bauer et al. 2013a, b). The nitrogen and H 2 S removal are a necessary step to be
executed in the conventional physical/chemical method, which is an additional
barrier during the operation, but in contrast to this, the nitrogen generated is utilised
as a nutrient in the biological method (Toledo-Cervantes et al. 2017).
The technological indicators point out that the suitable and highly efficient
method is the biological method which can overcome the disadvantages of many
techniques by providing high methane purity (95%) and CO 2 consumption rate
(in fermentation method), 80–97% CH 4 recovery (no need for H 2 S/N 2 /O 2 removal)
in algal-bacterial photobioreactor, 65–98.9% CH 4 recovery (no need for H 2 S/N 2 /O 2
removal) in in situ chemoautotrophic method, 88–89% CH 4 recovery in ex situ
chemoautotrophic method, and 80% CH 4 recovery in microbial electrochemical
method (Angelidaki et al. 2018).
270
B. S. Dhanya et al.