9.4
Removal of Methane from the Off-Gas
During biogas production, the off-gas is generated which contains methane, and the
concentration of methane depends upon the amount of recovered methane. High
methane recovery is not acceptable because it requires high investment, maintenance
and operational cost. To avoid the cost problem, a certain amount of methane is left
out in most of the biogas plants (Report 2012).
Reducing the emission of methane content in to the environment is logically
important for an innovative biogas upgradation technology. In addition to the
methane emission reduction, methane slip too should be considered, since methane
is an effective greenhouse gas.
Hence, the emitted methane content that leaves a PSA column should be reduced
by off-gas treatment. There are few solutions that are mentioned below:
1. One of the solution to reduce the methane slip is to combine the air which is
utilised for combustion with off-gas (Petersson and Wellinger 2009). The basic
way to remove the methane in the off-gas is oxidation/combustion with production of excess heat that can be utilised in anaerobic digestion plants since this
plant requires heat (Report 2012).
2. The methane gas mission can be minimised by using the process of catalytic
combustion/thermal oxidation (Petersson and Wellinger 2009). Commercially,
many technologies have been developed by the manufacturers with combustion
of methane even at low content (wp3). Megtec developed VOCSIDIZER, a
device with ceramic media containing heat transfer bed developed for regenerative thermal oxidation. The off-gas containing methane is allowed to pass through
the ceramic media, and heat is applied. On the way of heating process, the
methane in the off-gas is oxidised with oxygen with the generation of carbon
dioxide and water vapour. The VOCSIDIZER can be maintained by the heat that
is generated during the process of oxidation, and also the off-gas flow can be
reversed periodically.
3. Flameless oxidation is another device based on the thermal oxidation process, in
which the biogas is passed through the oxidation chamber and heated at 650
C
using raw gas, and preheat is done at 450
C using the exhaust gas. Excess heat
from the exhaust gas is regenerated, and it is used for any heating purpose.
Catalyst can also be used to oxidise the methane content in such a way that the
energy and temperature required for the oxidation process are less. The oxidation
happens at the catalyst’s surface and palladium, and cobalt or platinum acts as the
active component (Petersson and Wellinger 2009; Report 2012).
A countable number of companies provide high methane recovery, leaving
off-gas into the atmosphere directly.
260
B. S. Dhanya et al.
Removal of Methane from the Off-Gas
During biogas production, the off-gas is generated which contains methane, and the
concentration of methane depends upon the amount of recovered methane. High
methane recovery is not acceptable because it requires high investment, maintenance
and operational cost. To avoid the cost problem, a certain amount of methane is left
out in most of the biogas plants (Report 2012).
Reducing the emission of methane content in to the environment is logically
important for an innovative biogas upgradation technology. In addition to the
methane emission reduction, methane slip too should be considered, since methane
is an effective greenhouse gas.
Hence, the emitted methane content that leaves a PSA column should be reduced
by off-gas treatment. There are few solutions that are mentioned below:
1. One of the solution to reduce the methane slip is to combine the air which is
utilised for combustion with off-gas (Petersson and Wellinger 2009). The basic
way to remove the methane in the off-gas is oxidation/combustion with production of excess heat that can be utilised in anaerobic digestion plants since this
plant requires heat (Report 2012).
2. The methane gas mission can be minimised by using the process of catalytic
combustion/thermal oxidation (Petersson and Wellinger 2009). Commercially,
many technologies have been developed by the manufacturers with combustion
of methane even at low content (wp3). Megtec developed VOCSIDIZER, a
device with ceramic media containing heat transfer bed developed for regenerative thermal oxidation. The off-gas containing methane is allowed to pass through
the ceramic media, and heat is applied. On the way of heating process, the
methane in the off-gas is oxidised with oxygen with the generation of carbon
dioxide and water vapour. The VOCSIDIZER can be maintained by the heat that
is generated during the process of oxidation, and also the off-gas flow can be
reversed periodically.
3. Flameless oxidation is another device based on the thermal oxidation process, in
which the biogas is passed through the oxidation chamber and heated at 650
C
using raw gas, and preheat is done at 450
C using the exhaust gas. Excess heat
from the exhaust gas is regenerated, and it is used for any heating purpose.
Catalyst can also be used to oxidise the methane content in such a way that the
energy and temperature required for the oxidation process are less. The oxidation
happens at the catalyst’s surface and palladium, and cobalt or platinum acts as the
active component (Petersson and Wellinger 2009; Report 2012).
A countable number of companies provide high methane recovery, leaving
off-gas into the atmosphere directly.
260
B. S. Dhanya et al.