Partnerships, Future, and Emerging Technologies - Industrial Crops: Breeding for BioEnergy and Bioproducts - page 404

When effectively coupled, pretreatment and enzymatic hydrolysis can be a very
efficient, cost-effective way to liberate monomeric sugars from the carbohydrate
portion of biomass with some organizations reporting over 90 % conversion of
structural polysaccharides to monomeric sugars [ 17 ]. There are a multitude of
pretreatment approaches being pursued that cover the gamut of pH ranges from
acidic to alkaline approaches as well as temperature ranges (~140-210 C). All of
these approaches have their plusses and minuses when compared on an efficiency
and cost basis [ 18 ]. However, comparison work has shown that there is really not a
“one size fits all” when it comes to a pretreatment approach across the spectrum of
suitable lignocellulosic feedstocks. In general, the alkaline or higher severity
approaches such as wet oxidation [ 19 ] that are more aggressive at depolymerizing
the lignin component tend to perform better on higher lignin feedstocks such as
softwoods, while the lower severity approaches such as dilute acid or hot water [ 20 ]
tend to perform better on the low lignin herbaceous feedstocks such as corn stover
or switchgrass.
Enzyme development with the goal of low cost-efficient hydrolysis to mono-
meric biomass sugars has been an area of extensive focus with considerable
progress being made [ 21 ]. This progress has been critically important in moving
the biochemical conversion process towards its goal of economic competitiveness;
however, further progress in both specific activity and costs to produce the enzymes
is still possible and desirable.
Similarly, with enzyme development, organism development for the cost-
effective efficient fermentation of biomass sugars to ethanol has been an area of
significant focus, again with impressive, high-impact progress being made
[ 22 ]. Humbird [ 15 ] reported for pilot scale results very effective fermentation
results of greater than 90 % of total biomass sugars to ethanol, which would
correspond to a total ethanol yield of 330 l/tonne of biomass. At these yields and
efficiencies, the overall economics compare favorably to first-generation ethanol
technologies. However, this needs to be caveated with the fact that these are pilot
plant numbers and the technology still needs to be proven out at commercial scale.
The final step in the biochemical lignocellulosic ethanol conversion process is
product recovery, which is envisioned to be standard fractional distillation to the
ethanol-water azeotrope, followed by molecular sieve concentration to anhydrous
ethanol. These techniques will be very similar to what is currently used in first-
generation ethanol processes and hence is a well-proven technology.
Since cellulosic ethanol technology utilizes only the carbohydrate portion of the
biomass, the lignin component of the biomass is available for other uses. Initial
process designs put forward by a number of organizations [ 23 ] dry the lignin and
then use it as a fuel for heat and power needs of the conversion process. The
Renewable Fuel Standard (RFS) requires that advanced biofuels show a 60 % ghg
reduction when compared to conventional gasoline to quality for the RFS credit.
Utilization of the lignin component as opposed to using a fossil fuel such as natural
gas or coal contributes significantly to ghg reductions for cellulosic ethanol [ 24 ].