Partnerships, Future, and Emerging Technologies - Industrial Crops: Breeding for BioEnergy and Bioproducts - page 405

Feedstock Considerations
Since the biochemical lignocellulosic ethanol conversion process hinges on high
sugar conversion to ethanol, desirable feedstocks will have a high carbohydrate
content that is amenable to conversion to monomeric sugars that in turn are readily
fermented to ethanol. Although this statement is fairly straightforward and may be a
bit obvious, it is in reality overly simplistic and may not be a good metric for
judging the relative value of feedstocks. Lignocellulosic feedstocks have a great
variety as to both physical and chemical properties, and these variations can have
significant impacts on the yields and efficiency of the conversion process.
In the early days of biochemical conversion process development, the process
was commonly referred to as the sugar platform and organizations and researchers
focused on cost of sugars [ 25 ]. Research predominantly focused on maximizing
sugar yields per mass unit of feedstock. However, since the ultimate goal in any
conversion process is to produce the final product at the highest efficiency and yield
at the lowest possible cost, history and experience has shown that this singular focus
on sugar yields could be misleading. Fermentation inhibitors such as furfural,
hydroxymethylfurfural (HMF), or other inhibitory compounds can significantly
affect the fermentability of the sugar solution [ 26 ]. The presence and concentration
of these inhibitory compounds is very much a function of the feedstock type, the
pretreatment technology, and finally the severity of the pretreatment. Since some
inhibitory compounds such as furfural and HMF arise from thermal degradation of
sugars, the higher severity pretreatment approaches that lead to higher sugar yields
also lead to high inhibitory compound concentrations. Hence final ethanol yields
could actually be lower for higher sugar concentration hydrolysates with these
compounds present, then they would be for lower sugar concentration hydrolysates
without these compounds being present. Although these inhibitory compounds can
be reduced or eliminated by hydrolysate conditioning [ 27 ], these processes add cost
and complexity to the process as well as lead to sugar losses; hence, it is best to
avoid these processes if possible.
For these reasons, there is really not a preferred feedstock since the best choice
will be the feedstock that is available at the desired quantities at the lowest cost. The
conversion process will need to be well suited for the feedstock. Adding to the
complexity will be that feedstock availabilities and cost are geographically depen-
dent as well as weather and time of year dependent. Therefore, it might be cost
effective to build robustness into the process conversion plant so that a range of
feedstocks can be accommodated based on weather or seasonal variations. Simply
building a plant capable of processing the predominant feedstock in the geograph-
ical area (i.e., corn stover in the US Midwest or Sugar Cane Bagasse in Brazil) may
make the plant difficult to operate economically year in and year out over the 30+
year life of the plant if the feedstock availability drops significantly due to a
prolonged drought or other condition that affects the availability of that particular
feedstock.