57
for increasing the efficiency of the bioconversion process; research and development may diminish the overall process cost [65]. In deciding for a pretreatment step,
it is important that it can justify its impact on the residual biomass processing cost,
capital costs, biomass feedstock costs and operating costs [56]. The pretreatment
method should be selective, efficient but also economical so as not to exceed 40%
of the overall cost of processing [66]. Prior to the biomass pretreatment step, the
mechanical processing and preconditioning like shredding, grinding, chipping,
blending, etc. should be avoided if possible, since this would add significant costs to
the whole biorefining process [67].
In the last decade, extensive research has been undertaken throughout the world
for developing various reliable and cost-effective pretreatment techniques for degradation of lignin from residual biomass and for improving process efficiency in
algal biomass recovery to produce renewable liquid and gaseous fuels. In scientific
papers, numerous classifications of the pretreatment techniques have been presented, including mechanical, thermal, chemical and biological techniques and
combinations thereof such as physico-chemical, bio-physico-chemical, mecanochemical and thermo-chemical, addressed and adapted to different types of substrates [58]. However, all these pretreatment methods may be broadly grouped into
three classes—physical, chemical and biological methods—with the mention that
the physical pretreatment techniques are the most diverse, including mechanical,
thermal, electrical pretreatment, irradiation, etc. [68]. Figure 2 shows a centralization of the biomass pretreatment methods for anaerobic fermentation processes with
the production of fuel gas and fermented residual material.
In the industrial practice, some combinations of the mentioned techniques have
become more efficient and advantageous options in terms of bioconversion efficiency and costs throughout the bioprocessing chain. Even a combination of three
pretreatments (e.g. mechanical, thermal and chemical pretreatment) can be efficient
while contributes to the cost optimization and to the improvement of the energy balance of the technological pretreatment stage [69].
Physical pretreatment aims at reducing the biomass particle size by applying an
external physical force and may include various techniques such as mechanical
beating, sonication, milling, cavitation, extruding, microwave irradiation, deflaking,
dispersing, refining, etc. [70]. The physical pretreatment methods are fairly ecofriendly, and in general they do not produce toxic compounds, but the major disadvantage is that they are high-energy consumption options [71].
Chemical pretreatments (acidic, alkaline, oxidative, ionic liquids, organosolv
process, deep eutectic solvents, etc.) result in the disruption of the inter- and intramolecular forces and chemical bonds that hold together the biomass components
[58, 71]. Although chemical pretreatment methods have proven effective in accelerating the hydrolysis step, they are less applied since they may increase the risk of
inhibition of microbial activity due to the effect of some compounds with toxic
effect. In addition, chemical pretreatment often generates environmentally harmful
chemicals, such as aldehydes, phenolic acids, furfural, etc. [64, 72]. Moreover,
chemical treatment can result in losses of organic matter causing decrease in the
Biochemical Conversion of Residual Biomass: An Approach to Fuel Gas and Green…
for increasing the efficiency of the bioconversion process; research and development may diminish the overall process cost [65]. In deciding for a pretreatment step,
it is important that it can justify its impact on the residual biomass processing cost,
capital costs, biomass feedstock costs and operating costs [56]. The pretreatment
method should be selective, efficient but also economical so as not to exceed 40%
of the overall cost of processing [66]. Prior to the biomass pretreatment step, the
mechanical processing and preconditioning like shredding, grinding, chipping,
blending, etc. should be avoided if possible, since this would add significant costs to
the whole biorefining process [67].
In the last decade, extensive research has been undertaken throughout the world
for developing various reliable and cost-effective pretreatment techniques for degradation of lignin from residual biomass and for improving process efficiency in
algal biomass recovery to produce renewable liquid and gaseous fuels. In scientific
papers, numerous classifications of the pretreatment techniques have been presented, including mechanical, thermal, chemical and biological techniques and
combinations thereof such as physico-chemical, bio-physico-chemical, mecanochemical and thermo-chemical, addressed and adapted to different types of substrates [58]. However, all these pretreatment methods may be broadly grouped into
three classes—physical, chemical and biological methods—with the mention that
the physical pretreatment techniques are the most diverse, including mechanical,
thermal, electrical pretreatment, irradiation, etc. [68]. Figure 2 shows a centralization of the biomass pretreatment methods for anaerobic fermentation processes with
the production of fuel gas and fermented residual material.
In the industrial practice, some combinations of the mentioned techniques have
become more efficient and advantageous options in terms of bioconversion efficiency and costs throughout the bioprocessing chain. Even a combination of three
pretreatments (e.g. mechanical, thermal and chemical pretreatment) can be efficient
while contributes to the cost optimization and to the improvement of the energy balance of the technological pretreatment stage [69].
Physical pretreatment aims at reducing the biomass particle size by applying an
external physical force and may include various techniques such as mechanical
beating, sonication, milling, cavitation, extruding, microwave irradiation, deflaking,
dispersing, refining, etc. [70]. The physical pretreatment methods are fairly ecofriendly, and in general they do not produce toxic compounds, but the major disadvantage is that they are high-energy consumption options [71].
Chemical pretreatments (acidic, alkaline, oxidative, ionic liquids, organosolv
process, deep eutectic solvents, etc.) result in the disruption of the inter- and intramolecular forces and chemical bonds that hold together the biomass components
[58, 71]. Although chemical pretreatment methods have proven effective in accelerating the hydrolysis step, they are less applied since they may increase the risk of
inhibition of microbial activity due to the effect of some compounds with toxic
effect. In addition, chemical pretreatment often generates environmentally harmful
chemicals, such as aldehydes, phenolic acids, furfural, etc. [64, 72]. Moreover,
chemical treatment can result in losses of organic matter causing decrease in the
Biochemical Conversion of Residual Biomass: An Approach to Fuel Gas and Green…