69
hydrogen concentration decreases with increasing temperature [81]. The study of
pressure influence on fermentative processes requires in-depth analysis and could
be a prospective research topic.
6 Biogas Upgrading Methods
Methane is the component of biogas that decides its energy value; methane concentration in biogas varies between 50% and 70%. The second major component found
in high concentration in biogas is carbon dioxide which accounts the noncombustible share of biogas. Except for methane, the other biogas components contained in minor amounts (water, nitrogen, oxygen, hydrogen sulphide, ammonia,
siloxanes, volatile organic compounds, etc.) are practically unwanted components,
that may cause damage in various ways and have to be removed through cleaning
and upgrading processes, with the main scope to protect the metallic parts and raise
the calorific power of biogas [110].
Among the compounds to be removed from biogas so as to allow the use of biomethane in combustion and transport processes is hydrogen sulphide, which is produced from sulphate reduction and is found in biogas in concentrations up to
4000 ppmv. Hydrogen sulphide is corrosive to engines, pipes, gas storage tanks,
compressors, etc. but is also an inhibitor for catalytic reforming. Moreover, it is well
known that sulphur dioxide is generated by burning H 2 S, which is a pollutant for the
environment. Hence, the many potential problems that sulphur hydrogen can cause
require its removal from biogas through an upgrade process before its use as a fuel
[111]. Biogas produced from wastewaters can contain siloxanes formed from the
organic silicon compounds which arrive in sewage sludge due to the various uses of
silicon-containing compounds in households and industries [112]. Even if they are
found in low concentrations of up to 10.6 ppmv [112], the presence of siloxanes in
biogas is associated with serious problems since silicone oxides may generate sticky
residues by combustion. This unwanted residue can be deposited in the engines and
valves causing severe malfunction [110]. Biogas upgrading is aiming at obtaining
purified biogas, namely, biomethane, which is very close to natural gas standards
after removing carbon dioxide and other impurities such as water, hydrogen sulphide, siloxanes, etc. [113].
At present, the technologies for biogas upgrading are very diverse and very efficient; the choice of the most optimal process takes into account the application for
which the biomethane will be used. There are big differences between the quality
requirements of biogas for stationary CHP engines, where the concentrations of
water vapour and hydrogen sulphide in biogas must not exceed 1000 ppmv, while in
biogas for transport and biogas injected into the gas grid, carbon dioxide and other
impurities must be removed so as to upgrade the biogas to >95% methane, according to specific national standards [1].
Several conventional and commercially available technologies for biogas physical upgrading are currently used for various applications, water scrubbing, organic
Biochemical Conversion of Residual Biomass: An Approach to Fuel Gas and Green…
hydrogen concentration decreases with increasing temperature [81]. The study of
pressure influence on fermentative processes requires in-depth analysis and could
be a prospective research topic.
6 Biogas Upgrading Methods
Methane is the component of biogas that decides its energy value; methane concentration in biogas varies between 50% and 70%. The second major component found
in high concentration in biogas is carbon dioxide which accounts the noncombustible share of biogas. Except for methane, the other biogas components contained in minor amounts (water, nitrogen, oxygen, hydrogen sulphide, ammonia,
siloxanes, volatile organic compounds, etc.) are practically unwanted components,
that may cause damage in various ways and have to be removed through cleaning
and upgrading processes, with the main scope to protect the metallic parts and raise
the calorific power of biogas [110].
Among the compounds to be removed from biogas so as to allow the use of biomethane in combustion and transport processes is hydrogen sulphide, which is produced from sulphate reduction and is found in biogas in concentrations up to
4000 ppmv. Hydrogen sulphide is corrosive to engines, pipes, gas storage tanks,
compressors, etc. but is also an inhibitor for catalytic reforming. Moreover, it is well
known that sulphur dioxide is generated by burning H 2 S, which is a pollutant for the
environment. Hence, the many potential problems that sulphur hydrogen can cause
require its removal from biogas through an upgrade process before its use as a fuel
[111]. Biogas produced from wastewaters can contain siloxanes formed from the
organic silicon compounds which arrive in sewage sludge due to the various uses of
silicon-containing compounds in households and industries [112]. Even if they are
found in low concentrations of up to 10.6 ppmv [112], the presence of siloxanes in
biogas is associated with serious problems since silicone oxides may generate sticky
residues by combustion. This unwanted residue can be deposited in the engines and
valves causing severe malfunction [110]. Biogas upgrading is aiming at obtaining
purified biogas, namely, biomethane, which is very close to natural gas standards
after removing carbon dioxide and other impurities such as water, hydrogen sulphide, siloxanes, etc. [113].
At present, the technologies for biogas upgrading are very diverse and very efficient; the choice of the most optimal process takes into account the application for
which the biomethane will be used. There are big differences between the quality
requirements of biogas for stationary CHP engines, where the concentrations of
water vapour and hydrogen sulphide in biogas must not exceed 1000 ppmv, while in
biogas for transport and biogas injected into the gas grid, carbon dioxide and other
impurities must be removed so as to upgrade the biogas to >95% methane, according to specific national standards [1].
Several conventional and commercially available technologies for biogas physical upgrading are currently used for various applications, water scrubbing, organic
Biochemical Conversion of Residual Biomass: An Approach to Fuel Gas and Green…