second unit is being fed with the off-gas obtained) for obtaining the maximum
technological feasibility that includes minimised amount of methane gas in the
off-gas stream, further avoiding the emission treatment step, thereby reducing the
cost associated with the technology (Augelletti et al. 2017). The PSA technology
can also be improved by implementing the use of promising adsorbents with
high selectivity and working capacity such as zeolitic imidazolate frameworks
and metal organic framework which are efficient in removing H 2 S from CO 2
(Liu et al. 2017).
In membrane separation technology, membranes, epoxyamine-based ion gel
membranes that favour adverse reactions (less compressed gas, humid feed), are in
need for an efficient membrane separation technology (Friess et al. 2017). A
polypropylene hollow fibre membrane contractor biogas upgrading unit is used in
membrane separation technology to yield high methane content and increased purity
(Park et al. 2017). The cost involved in membrane separation technology can be
greatly reduced by gaining knowledge about the effect of resonance radiation on
mass transfer (Levdansky and Izak 2017). The operational parameters such as the
pressure, retentate flow and temperature must be regulated to achieve high
biomethane concentration using polysulfone and polyimide fibre membranes
(Vrbova and Karel 2017).
Also, inclining towards hybridised form of upgrading technologies may show an
optimistic sign with high success rate, minimised operational cost, increased CO 2 ,
more S-capture efficiency and low energy consumption (Sahota et al. 2018). It is also
to be mentioned that the hybrid technologies, PWS/PMT, PMT/CT, are technoeconomically viable (Scholz et al. 2013); the hybrid technology, CT/PMT/TSA, is
less energy consumable (Song et al. 2017); and the hybrid technology, TSA/PMT, is
with high methane purity and reduced loss of CH 4 and CO 2 (Pinghai et al. 2012).
Further, exploration of more hybrid technologies is an urgent need by integrating the
good features of two or more technologies with improvised techno-economic
dimension for biogas upgradation.
The most predominant challenges that all the biogas upgrading technologies face
are to utilise the methane present in off-gas, to make small-scale upgrading plants
economical, the support policies (Sahota et al. 2018), the innovation requirements in
research and development, implementing novel biogas reforming technologies and
producing liquefied biogas. Those challenges are briefly described:
(a) Proper Utilisation of Methane Content Present in Off-Gas
The methane present in the off-gas may be released into the surrounding and pose
a serious threat to global warming, and hence the off-gas must be treated before
leaving the plant. The conventional techniques, PSW and PSA, and membrane
technologies suffer from the formation of a huge amount of methane (depending
upon the method, the methane content varies) in the off-gas, and oxidation process
must be performed to avoid the methane loss into the atmosphere. The oxidation
process generates heat which can be utilised at the anaerobic digestion plant or can
be wasted by cooling process. Alternatively, the liberated off-gas collated with raw
9 Recent Trends in Biogas Upgrading Technologies for Biomethane Production
275
technological feasibility that includes minimised amount of methane gas in the
off-gas stream, further avoiding the emission treatment step, thereby reducing the
cost associated with the technology (Augelletti et al. 2017). The PSA technology
can also be improved by implementing the use of promising adsorbents with
high selectivity and working capacity such as zeolitic imidazolate frameworks
and metal organic framework which are efficient in removing H 2 S from CO 2
(Liu et al. 2017).
In membrane separation technology, membranes, epoxyamine-based ion gel
membranes that favour adverse reactions (less compressed gas, humid feed), are in
need for an efficient membrane separation technology (Friess et al. 2017). A
polypropylene hollow fibre membrane contractor biogas upgrading unit is used in
membrane separation technology to yield high methane content and increased purity
(Park et al. 2017). The cost involved in membrane separation technology can be
greatly reduced by gaining knowledge about the effect of resonance radiation on
mass transfer (Levdansky and Izak 2017). The operational parameters such as the
pressure, retentate flow and temperature must be regulated to achieve high
biomethane concentration using polysulfone and polyimide fibre membranes
(Vrbova and Karel 2017).
Also, inclining towards hybridised form of upgrading technologies may show an
optimistic sign with high success rate, minimised operational cost, increased CO 2 ,
more S-capture efficiency and low energy consumption (Sahota et al. 2018). It is also
to be mentioned that the hybrid technologies, PWS/PMT, PMT/CT, are technoeconomically viable (Scholz et al. 2013); the hybrid technology, CT/PMT/TSA, is
less energy consumable (Song et al. 2017); and the hybrid technology, TSA/PMT, is
with high methane purity and reduced loss of CH 4 and CO 2 (Pinghai et al. 2012).
Further, exploration of more hybrid technologies is an urgent need by integrating the
good features of two or more technologies with improvised techno-economic
dimension for biogas upgradation.
The most predominant challenges that all the biogas upgrading technologies face
are to utilise the methane present in off-gas, to make small-scale upgrading plants
economical, the support policies (Sahota et al. 2018), the innovation requirements in
research and development, implementing novel biogas reforming technologies and
producing liquefied biogas. Those challenges are briefly described:
(a) Proper Utilisation of Methane Content Present in Off-Gas
The methane present in the off-gas may be released into the surrounding and pose
a serious threat to global warming, and hence the off-gas must be treated before
leaving the plant. The conventional techniques, PSW and PSA, and membrane
technologies suffer from the formation of a huge amount of methane (depending
upon the method, the methane content varies) in the off-gas, and oxidation process
must be performed to avoid the methane loss into the atmosphere. The oxidation
process generates heat which can be utilised at the anaerobic digestion plant or can
be wasted by cooling process. Alternatively, the liberated off-gas collated with raw
9 Recent Trends in Biogas Upgrading Technologies for Biomethane Production
275