7.3.7.2 Bioethanol Production Using Lignocellulosic Biomass
The conversion of the lignocellulosic biomass to bioethanol involves several steps:
(i) particle size reduction, (ii) mixing of biomass with water to make slurry (solid
load of 10–15%), (iii) pretreatment to break the recalcitrance of the biomass, i.e.,
removal of lignin and depolymerization of hemicelluloses and cellulose,
(iv) chemical or enzymatic hydrolysis of the pretreated solid or liquid (based on
carbohydrate composition) for breaking down of polymers to monomers,
(v) fermentation of reducing sugars to ethanol, and (vi) distillation for ethanol
recovery (Wyman 2018). The different by-products of 2G ethanol production
process are usually lignin residues and wastewater generated during different steps
of ethanol generation (Zabed et al. 2017). The lignin can be put to different
applications, i.e., combustion for generation of heat and electricity for making
overall process more economical. Recent trends show that lignin can be effectively
used as raw material for production of different value-added products (Chaturvedi
and Verma 2013). The wastewater generated can be recycled where a portion of
wastewater can be recirculated as backset or the wastewater may be used for
recovery of some organic compounds generated during the different ethanol generation steps, which is of great economical importance (Mathew et al. 2018). Due to
morphological and physiochemical complexities of the lignocellulosic biomass and
varying chemical constituents of the different lignocellulosic biomasses, the scientific community is struggling to come up with uniform conversion method or
optimum production condition (Hassan et al. 2019). A summary of different
pretreatment and fermentation methods used for bioethanol production using various
lignocellulosic feedstocks is presented in Table 7.3.
7.3.7.3 Bioethanol Production Using Algal Biomass
The generation of ethanol from algal biomass involves harvesting, dehydration,
pretreatment, hydrolysis, saccharification, and distillation. The first step involves
harvesting of algal biomass from the open ponds or photo-bioreactors. The second
step involves dehydration using sun drying or specialized drying (sprig, freeze, and
fluidized bed drying) for up to 50% removal of water content (McKendry 2002;
Grima et al. 2003). Different drying techniques have its own advantages and
limitations, for example, sun drying is cheap but requires longer duration and larger
surface area. Spig drying is used for costly product isolation; however, the process
results in pigment loss and is usually a very expensive technique (Bibi et al. 2017).
Freeze-drying facilities extraction of oils but are very expensive and difficult to
upscale. The third step involved the extraction of by-product using different crushing
or pretreatment techniques for the enhanced sugar and lipid yield resulting in better
ethanol yield. The starch and cellulose are hydrolyzed using chemical or enzyme and
fermented using yeast in container called fermentors (Singh et al. 2011). After
alcoholic fermentation, the distillation is performed to recover the ethanol from
fermentation broth by removing water and other components. The lipid part of
algal biomass is used for generation of biodiesel. A summary of different
pretreatment and fermentation methods used for bioethanol production using various
micro -and macroalgae is presented in Table 7.4.
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B. Kumar et al.
The conversion of the lignocellulosic biomass to bioethanol involves several steps:
(i) particle size reduction, (ii) mixing of biomass with water to make slurry (solid
load of 10–15%), (iii) pretreatment to break the recalcitrance of the biomass, i.e.,
removal of lignin and depolymerization of hemicelluloses and cellulose,
(iv) chemical or enzymatic hydrolysis of the pretreated solid or liquid (based on
carbohydrate composition) for breaking down of polymers to monomers,
(v) fermentation of reducing sugars to ethanol, and (vi) distillation for ethanol
recovery (Wyman 2018). The different by-products of 2G ethanol production
process are usually lignin residues and wastewater generated during different steps
of ethanol generation (Zabed et al. 2017). The lignin can be put to different
applications, i.e., combustion for generation of heat and electricity for making
overall process more economical. Recent trends show that lignin can be effectively
used as raw material for production of different value-added products (Chaturvedi
and Verma 2013). The wastewater generated can be recycled where a portion of
wastewater can be recirculated as backset or the wastewater may be used for
recovery of some organic compounds generated during the different ethanol generation steps, which is of great economical importance (Mathew et al. 2018). Due to
morphological and physiochemical complexities of the lignocellulosic biomass and
varying chemical constituents of the different lignocellulosic biomasses, the scientific community is struggling to come up with uniform conversion method or
optimum production condition (Hassan et al. 2019). A summary of different
pretreatment and fermentation methods used for bioethanol production using various
lignocellulosic feedstocks is presented in Table 7.3.
7.3.7.3 Bioethanol Production Using Algal Biomass
The generation of ethanol from algal biomass involves harvesting, dehydration,
pretreatment, hydrolysis, saccharification, and distillation. The first step involves
harvesting of algal biomass from the open ponds or photo-bioreactors. The second
step involves dehydration using sun drying or specialized drying (sprig, freeze, and
fluidized bed drying) for up to 50% removal of water content (McKendry 2002;
Grima et al. 2003). Different drying techniques have its own advantages and
limitations, for example, sun drying is cheap but requires longer duration and larger
surface area. Spig drying is used for costly product isolation; however, the process
results in pigment loss and is usually a very expensive technique (Bibi et al. 2017).
Freeze-drying facilities extraction of oils but are very expensive and difficult to
upscale. The third step involved the extraction of by-product using different crushing
or pretreatment techniques for the enhanced sugar and lipid yield resulting in better
ethanol yield. The starch and cellulose are hydrolyzed using chemical or enzyme and
fermented using yeast in container called fermentors (Singh et al. 2011). After
alcoholic fermentation, the distillation is performed to recover the ethanol from
fermentation broth by removing water and other components. The lipid part of
algal biomass is used for generation of biodiesel. A summary of different
pretreatment and fermentation methods used for bioethanol production using various
micro -and macroalgae is presented in Table 7.4.
186
B. Kumar et al.