61
Methanogenesis, the final stage of the anaerobic digestion process, where ca. 2/3
of the biomethane is produced by acetoclastic methanogenesis starting from acetic
acid but the remaining 1/3 of the biomethane is produced from hydrogen and CO 2
by hydrogenotrophic methanogenesis. Biochemical pathway studies showed that
methanogenesis from methanol, methylamines and formate also occurs [86]. Some
other gases besides methane and carbon dioxide, which resulted during the second
stage of the fermentation process (hydrogen sulphide and ammonia), may be found
in the biogas mixture. It was proved that methanogenic species represent microbial
groups most sensitive to the environment conditions (acidity, temperature, inhibitors) in anaerobic digestion. Also, they have a significantly slower regeneration time
of 5–16 days compared to acidogenic bacteria which have a regeneration time of
less than 36 h [87]. The main chemical reactions describing the biochemical steps
and the enzymes involved in the metabolic reactions are briefly presented in Table 3.
In anaerobic fermentative processes for biomethane, various microbial consortiums must co-exist in the fermentation mass in a proper biochemical balance,
although they disturb each other in terms of optimal growth conditions. Methanogens
may become inactive or even die if the environment conditions in the fermentation
reactor are not appropriate or constant. When any process unbalance occurs, the
acidogenic bacteria start growing fast and make the volatile fatty acids increase
sharply, automatically leading to slowing down or even stopping biogas production [91].
Due to the complex and interactive dynamics of the different microbial groups in
anaerobic digestion, with a disproportionate quantity of various microbial groups
that have a direct influence on the overall process reaction rate, the anaerobic digestion for producing biomethane is a very much unstable biochemical process, with
the risk of process failure in case of any slight variations of the environmental
parameters [81].
Hydrogen may be regarded as one of the most interesting fuels of the future since
hydrogen combustion is an eco-friendly energy generation process as it releases
only water vapours. Currently, hydrogen is produced largely by chemical processes
from natural gases, hydrocarbons or coal and also by water electrolysis, but only a
small part is generated from biomass, as seen in Fig. 4. The main beneficiaries of
hydrogen synthesis are chemical industries, mostly for producing fertilizers and
petroleum derivatives [34].
However, using non-renewable sources to obtain hydrogen is not a very attractive
option in terms of environmental impact; sustainability of hydrogen production and
storage still remains an important challenge. As a cleaner solution, biochemical
hydrogen synthesis has been adopted to minimize CO 2 emissions. Biological processes for producing hydrogen are less energy consuming and have a great potential
to be implemented as a more advantageous alternative to regular methods. Moreover,
they could use low value feedstock, such as waste biomass and other industrial or
municipal organic residuals [24].
Biological pathways to produce biohydrogen can use both autotrophic and heterotrophic microorganisms. The process is catalysed by anaerobic bacteria and/or
algae, and it runs in aqueous environment at normal temperature and pressure
Biochemical Conversion of Residual Biomass: An Approach to Fuel Gas and Green…
Methanogenesis, the final stage of the anaerobic digestion process, where ca. 2/3
of the biomethane is produced by acetoclastic methanogenesis starting from acetic
acid but the remaining 1/3 of the biomethane is produced from hydrogen and CO 2
by hydrogenotrophic methanogenesis. Biochemical pathway studies showed that
methanogenesis from methanol, methylamines and formate also occurs [86]. Some
other gases besides methane and carbon dioxide, which resulted during the second
stage of the fermentation process (hydrogen sulphide and ammonia), may be found
in the biogas mixture. It was proved that methanogenic species represent microbial
groups most sensitive to the environment conditions (acidity, temperature, inhibitors) in anaerobic digestion. Also, they have a significantly slower regeneration time
of 5–16 days compared to acidogenic bacteria which have a regeneration time of
less than 36 h [87]. The main chemical reactions describing the biochemical steps
and the enzymes involved in the metabolic reactions are briefly presented in Table 3.
In anaerobic fermentative processes for biomethane, various microbial consortiums must co-exist in the fermentation mass in a proper biochemical balance,
although they disturb each other in terms of optimal growth conditions. Methanogens
may become inactive or even die if the environment conditions in the fermentation
reactor are not appropriate or constant. When any process unbalance occurs, the
acidogenic bacteria start growing fast and make the volatile fatty acids increase
sharply, automatically leading to slowing down or even stopping biogas production [91].
Due to the complex and interactive dynamics of the different microbial groups in
anaerobic digestion, with a disproportionate quantity of various microbial groups
that have a direct influence on the overall process reaction rate, the anaerobic digestion for producing biomethane is a very much unstable biochemical process, with
the risk of process failure in case of any slight variations of the environmental
parameters [81].
Hydrogen may be regarded as one of the most interesting fuels of the future since
hydrogen combustion is an eco-friendly energy generation process as it releases
only water vapours. Currently, hydrogen is produced largely by chemical processes
from natural gases, hydrocarbons or coal and also by water electrolysis, but only a
small part is generated from biomass, as seen in Fig. 4. The main beneficiaries of
hydrogen synthesis are chemical industries, mostly for producing fertilizers and
petroleum derivatives [34].
However, using non-renewable sources to obtain hydrogen is not a very attractive
option in terms of environmental impact; sustainability of hydrogen production and
storage still remains an important challenge. As a cleaner solution, biochemical
hydrogen synthesis has been adopted to minimize CO 2 emissions. Biological processes for producing hydrogen are less energy consuming and have a great potential
to be implemented as a more advantageous alternative to regular methods. Moreover,
they could use low value feedstock, such as waste biomass and other industrial or
municipal organic residuals [24].
Biological pathways to produce biohydrogen can use both autotrophic and heterotrophic microorganisms. The process is catalysed by anaerobic bacteria and/or
algae, and it runs in aqueous environment at normal temperature and pressure
Biochemical Conversion of Residual Biomass: An Approach to Fuel Gas and Green…