14.7 Applications
325
14.7.2 Bioethanol Production
Aquatic plants fall into the category of third generation energy crops. Not only
are they non-fossil, but also they are sourced from biomass and even better non-edible
biomass that is grown without competing with food or space for human and livestock
habitation or food cultivation. In addition to this, their cultivation is beneficial to the
environment as they have the capacity to remove nitrogen, phosphorous, carbon
and microelements and heavy metal from wastewater. Third generation biofuels are
characterized by their ability to yield high biomass with lower resource requirements
than conventional feedstocks. Another advantage of aquatic sources such as algae
is the lower lignin content which reduces the complexity of separating lignin from
cellulose.
Different methods have been explored for the production of ethanol which takes
advantage of the cellulose content of aquatic plants, duckweed and water hyacinth
(Bayrakci and Kocar 2014). The key attraction here is the use of plants with bioremediating properties which contain polysaccharides which can be converted to glucose
for fermentation. The challenge of this is the right cocktail of enzymes and conditions
which can most effectively convert both the cellulose alongside other polysaccharides of the cell wall into glucose without costly separation processes. Cellulose
from aquatic source can be saccharified into glucose using the enzyme cellulase
(Shen and Xia 2003), and however, the cost of enzyme limits the commercial feasibility of such process. Thermochemical process such as pyrolysis is an alternative to
enzyme-based process for bioethanol production from cellulosic biomass. However,
this process requires high energy input which makes it commercially infeasible.
The high yield and relatively fast growth rate of algae compared to terrestrial
plants sourced as biomass made it a desirable third generation biofuel feedstock
option. However, a major drawback for biofuel production by algae is the high cost
of cultivation and harvest relative to the present price of fuel. For biomass to make
the cut as an ideal source of biofuel, it needs to be produced cheaply and contains
sufficient amount of organic compounds which can be converted to biofuel such as
ethanol or diesel.
As the world’s fossil resources begin to dwindle and the impact of using vast
land for cultivation of sources of second generation biofuels begin to manifest, third
generation biofuels are fast emerging. Aquatic-sourced cellulose is one of such.
Cellulose can be hydrolyzed to cellulase of cellobiose using enzymes such as
cellulase. In an example study, up to 65.9 g/L of glucose has been achieved after
enzymatic hydrolysis of cellulose from Azolla. This required use of a combination
of four different enzymes: cellulase and cellobiase for the hydrolysis of the cellulose and amylase and amyloglucosidase for the hydrolysis of the starch. Hydrolysis
of cellulose is more difficult than that of starch as cellulose has a more resilient
unbranched polymer structure.
Fermentation of glucose from Azolla-derived cellulose using Saccharomyces cerevisiae achieved an ethanol yield of 0.56 g/g. However, these yields vary for different
325
14.7.2 Bioethanol Production
Aquatic plants fall into the category of third generation energy crops. Not only
are they non-fossil, but also they are sourced from biomass and even better non-edible
biomass that is grown without competing with food or space for human and livestock
habitation or food cultivation. In addition to this, their cultivation is beneficial to the
environment as they have the capacity to remove nitrogen, phosphorous, carbon
and microelements and heavy metal from wastewater. Third generation biofuels are
characterized by their ability to yield high biomass with lower resource requirements
than conventional feedstocks. Another advantage of aquatic sources such as algae
is the lower lignin content which reduces the complexity of separating lignin from
cellulose.
Different methods have been explored for the production of ethanol which takes
advantage of the cellulose content of aquatic plants, duckweed and water hyacinth
(Bayrakci and Kocar 2014). The key attraction here is the use of plants with bioremediating properties which contain polysaccharides which can be converted to glucose
for fermentation. The challenge of this is the right cocktail of enzymes and conditions
which can most effectively convert both the cellulose alongside other polysaccharides of the cell wall into glucose without costly separation processes. Cellulose
from aquatic source can be saccharified into glucose using the enzyme cellulase
(Shen and Xia 2003), and however, the cost of enzyme limits the commercial feasibility of such process. Thermochemical process such as pyrolysis is an alternative to
enzyme-based process for bioethanol production from cellulosic biomass. However,
this process requires high energy input which makes it commercially infeasible.
The high yield and relatively fast growth rate of algae compared to terrestrial
plants sourced as biomass made it a desirable third generation biofuel feedstock
option. However, a major drawback for biofuel production by algae is the high cost
of cultivation and harvest relative to the present price of fuel. For biomass to make
the cut as an ideal source of biofuel, it needs to be produced cheaply and contains
sufficient amount of organic compounds which can be converted to biofuel such as
ethanol or diesel.
As the world’s fossil resources begin to dwindle and the impact of using vast
land for cultivation of sources of second generation biofuels begin to manifest, third
generation biofuels are fast emerging. Aquatic-sourced cellulose is one of such.
Cellulose can be hydrolyzed to cellulase of cellobiose using enzymes such as
cellulase. In an example study, up to 65.9 g/L of glucose has been achieved after
enzymatic hydrolysis of cellulose from Azolla. This required use of a combination
of four different enzymes: cellulase and cellobiase for the hydrolysis of the cellulose and amylase and amyloglucosidase for the hydrolysis of the starch. Hydrolysis
of cellulose is more difficult than that of starch as cellulose has a more resilient
unbranched polymer structure.
Fermentation of glucose from Azolla-derived cellulose using Saccharomyces cerevisiae achieved an ethanol yield of 0.56 g/g. However, these yields vary for different