9.3.8 In Situ Methane Enrichment
In situ methane-enrichment method is based on the counterflow of gaseous
components such as nitrogen and oxygen, favouring desorption of the dissolved
carbon dioxide in the sludge by the recirculation concept. Additional carbon dioxide
present in the column is absorbed by sending back the circulated sludge to the
digester (Kadam and Panwar 2017). This compound has been tested efficiently at
pilot level (Sun et al. 2015). Increased recirculation rate causes increased methane
loss in the environment. The buffering capacity of the sludge tends to be altered in
this technique (Petersson and Wellinger 2009).
A small-scale plant of digester volume of 15 m
3 and with a bubble column of
140 dm
3 has been developed (Nordberg et al. 2005). Some amount of carbon dioxide
gets dissolved in the fluid phase of the digester tank. Continuous withdrawal of
sludge carbon dioxide generates high amount of methane (Lindberg 2003). Further
simulations can be worked out to reach high methane purity of up to 95%.
The advantages include economically suitable lower upgrading costs when compared to the conventional techniques and less requirement for ancillary equipment.
The disadvantages include limitation to pilot plants and mostly suitable for sludge.
9.3.9 Industrial Lung
Industrial lung is one of the hybrid processes that involves the use of enzymes such
as carbonic anhydrase to dissolve the carbon dioxide. The carbon dioxide is forced to
pass through the aqueous phase, and it is absorbed by the absorbent in the absorber
column. By applying heat, the absorbent can be regenerated (Petersson and
Wellinger 2009; Scholwin et al. 2013). Carbonic anhydrase enzyme is prepared
using six molecules of histidines for effective attachment of enzyme to the matrix by
a leading research organisation in Lund, Sweden, and this enzyme can enhance 99%
of methane recovery (Mattiasson 2005). CO 2 Solution Inc., a Canadian company,
used this technique and was patented for focussing on biogas upgradation, and their
current research is on bioreactor mechanics, enzyme immobilisation, production and
technology and enzyme cloning (Petersson and Wellinger 2009).
The advantages include 95–99% methane purity, withstanding higher
temperatures up to 85
C, and the disadvantages include increased enzyme production costs and constrained life time of enzymes (Sahota et al. 2018).
9.3.10 Supersonic Separation
Supersonic separation is one of the novel methods invented in the field of biogas
upgradation technology. The capacity of this method is wide with the facilities of
recompression, expansion and gas-liquid separation. In this method, the expansion
of the raw biogas to the supersonic velocity is facilitated by the convergent-divergent
nozzle that leads to the decrease in pressure and temperature, causing the
256
B. S. Dhanya et al.
In situ methane-enrichment method is based on the counterflow of gaseous
components such as nitrogen and oxygen, favouring desorption of the dissolved
carbon dioxide in the sludge by the recirculation concept. Additional carbon dioxide
present in the column is absorbed by sending back the circulated sludge to the
digester (Kadam and Panwar 2017). This compound has been tested efficiently at
pilot level (Sun et al. 2015). Increased recirculation rate causes increased methane
loss in the environment. The buffering capacity of the sludge tends to be altered in
this technique (Petersson and Wellinger 2009).
A small-scale plant of digester volume of 15 m
3 and with a bubble column of
140 dm
3 has been developed (Nordberg et al. 2005). Some amount of carbon dioxide
gets dissolved in the fluid phase of the digester tank. Continuous withdrawal of
sludge carbon dioxide generates high amount of methane (Lindberg 2003). Further
simulations can be worked out to reach high methane purity of up to 95%.
The advantages include economically suitable lower upgrading costs when compared to the conventional techniques and less requirement for ancillary equipment.
The disadvantages include limitation to pilot plants and mostly suitable for sludge.
9.3.9 Industrial Lung
Industrial lung is one of the hybrid processes that involves the use of enzymes such
as carbonic anhydrase to dissolve the carbon dioxide. The carbon dioxide is forced to
pass through the aqueous phase, and it is absorbed by the absorbent in the absorber
column. By applying heat, the absorbent can be regenerated (Petersson and
Wellinger 2009; Scholwin et al. 2013). Carbonic anhydrase enzyme is prepared
using six molecules of histidines for effective attachment of enzyme to the matrix by
a leading research organisation in Lund, Sweden, and this enzyme can enhance 99%
of methane recovery (Mattiasson 2005). CO 2 Solution Inc., a Canadian company,
used this technique and was patented for focussing on biogas upgradation, and their
current research is on bioreactor mechanics, enzyme immobilisation, production and
technology and enzyme cloning (Petersson and Wellinger 2009).
The advantages include 95–99% methane purity, withstanding higher
temperatures up to 85
C, and the disadvantages include increased enzyme production costs and constrained life time of enzymes (Sahota et al. 2018).
9.3.10 Supersonic Separation
Supersonic separation is one of the novel methods invented in the field of biogas
upgradation technology. The capacity of this method is wide with the facilities of
recompression, expansion and gas-liquid separation. In this method, the expansion
of the raw biogas to the supersonic velocity is facilitated by the convergent-divergent
nozzle that leads to the decrease in pressure and temperature, causing the
256
B. S. Dhanya et al.