34
after a solid liquid separation step are recovered and resuspended in fresh buffer
followed by addition of fresh substrate (Rodrigues et al. 2012). However, direct
recycling of residual substrate could be a problem, since with every cycle, the
amount of lignin and unhydrolyzable material increases with resultant decrease in
efficiency (Gomes et al. 2015). In both cases (recovery from hydrolysates and reuse
of residues), the reuse of enzyme requires the addition of fresh beta glucosidase
(BGL) since BGL do not have CBMs and hence recovery of this enzyme by adsorption is not practical. In a very recent work, Huang et al. (2016) have demonstrated a
workaround by using an engineered organism expressing BGL and using it in
simultaneous saccharification and fermentation with enzyme recovery by capture
using fresh substrate. Needless to say, the recovery of cellulase from hydrolysates/
residues is still a complicated art and needs to evolve to make it economically
feasible.
Apart from all the above strategies which are dependent on the technical aspects
of enzyme production and hydrolysis, one which is feasible and practiced is the
onsite production of enzymes. This can have serious impacts on the costs of hydrolysis operation, since onsite production can do away with all logistic costs and the
costs involved for purification and stabilization of enzymes and their storage, since
enzyme production can be scheduled as per demand and used directly (Cunha et al.
2016). Different studies have shown the cost advantages of onsite enzyme production. Hong et al. (2013) estimated a 30% reduction in enzyme production cost,
whereas Takimura et al. (2013) had estimated up to 70% reduction in cost compared
to offsite production. A recent study performing the cost evaluation of cellulases for
biomass hydrolysis based on Aspen Plus flow sheet simulation at a scale of 2000
tons of dry corn stover daily revealed that enzyme cost drives the price for bioethanol below the profitable margin, when enzyme is purchased from the market, even
with the cheapest enzyme currently available on sale (Liu et al. 2016). The authors
suggested that for profitable cellulosic ethanol production, novel and innovative
strategies including onsite production should be explored and tested.
1.7
Perspectives and Conclusions
Lignocellulose is considered as one of the best options as a renewable feedstock for
energy, especially liquid transportation fuels. Most of the technology options available today propose deconstruction of lignocellulosic biomass using enzymes to generate fermentable sugars which are then converted to ethanol or any other
value-added products through microbial fermentation. Major technical limitations
of the past are now being overcome and there are now companies claiming the commercial production of second-generation ethanol. While this is true, there is still no
bioethanol available as a commercial commodity, and the price of bioethanol is
publically disclosed. While a feasible pricing is claimed, several techno-economic
analyses performed on the price development of lignocellulosic ethanol point to the
high pricing needed to make the operation possible, and a major contributor to the
operation cost is the cost of enzymes. Biomass-hydrolyzing enzyme preparations
have evolved significantly over the past few decades and highly efficient cocktails
R.K. Sukumaran et al.
after a solid liquid separation step are recovered and resuspended in fresh buffer
followed by addition of fresh substrate (Rodrigues et al. 2012). However, direct
recycling of residual substrate could be a problem, since with every cycle, the
amount of lignin and unhydrolyzable material increases with resultant decrease in
efficiency (Gomes et al. 2015). In both cases (recovery from hydrolysates and reuse
of residues), the reuse of enzyme requires the addition of fresh beta glucosidase
(BGL) since BGL do not have CBMs and hence recovery of this enzyme by adsorption is not practical. In a very recent work, Huang et al. (2016) have demonstrated a
workaround by using an engineered organism expressing BGL and using it in
simultaneous saccharification and fermentation with enzyme recovery by capture
using fresh substrate. Needless to say, the recovery of cellulase from hydrolysates/
residues is still a complicated art and needs to evolve to make it economically
feasible.
Apart from all the above strategies which are dependent on the technical aspects
of enzyme production and hydrolysis, one which is feasible and practiced is the
onsite production of enzymes. This can have serious impacts on the costs of hydrolysis operation, since onsite production can do away with all logistic costs and the
costs involved for purification and stabilization of enzymes and their storage, since
enzyme production can be scheduled as per demand and used directly (Cunha et al.
2016). Different studies have shown the cost advantages of onsite enzyme production. Hong et al. (2013) estimated a 30% reduction in enzyme production cost,
whereas Takimura et al. (2013) had estimated up to 70% reduction in cost compared
to offsite production. A recent study performing the cost evaluation of cellulases for
biomass hydrolysis based on Aspen Plus flow sheet simulation at a scale of 2000
tons of dry corn stover daily revealed that enzyme cost drives the price for bioethanol below the profitable margin, when enzyme is purchased from the market, even
with the cheapest enzyme currently available on sale (Liu et al. 2016). The authors
suggested that for profitable cellulosic ethanol production, novel and innovative
strategies including onsite production should be explored and tested.
1.7
Perspectives and Conclusions
Lignocellulose is considered as one of the best options as a renewable feedstock for
energy, especially liquid transportation fuels. Most of the technology options available today propose deconstruction of lignocellulosic biomass using enzymes to generate fermentable sugars which are then converted to ethanol or any other
value-added products through microbial fermentation. Major technical limitations
of the past are now being overcome and there are now companies claiming the commercial production of second-generation ethanol. While this is true, there is still no
bioethanol available as a commercial commodity, and the price of bioethanol is
publically disclosed. While a feasible pricing is claimed, several techno-economic
analyses performed on the price development of lignocellulosic ethanol point to the
high pricing needed to make the operation possible, and a major contributor to the
operation cost is the cost of enzymes. Biomass-hydrolyzing enzyme preparations
have evolved significantly over the past few decades and highly efficient cocktails
R.K. Sukumaran et al.