Flowsheeting, combined with estimates of the production cost, is a valuable
tool for the comparison of process alternatives and to determine bottlenecks
that require further improvement. It is, however, difficult to compare production
costs from different studies due to the many assumptions made in
the simulations, such as ethanol yield, productivity and concentration, as no
commercial-scale plants are in existence. Also, differences in capacity and
cost of raw material, as well as currency exchange rates, add to the uncertainty.
This is clearly illustrated by the large variation in the estimated ethanol
production cost, from 0.13 to 0.81 US$ L–1 ethanol.
The most important parameters for the economic outcome are the feedstock
cost, which varied between 30 and 90 US$ per metric ton, and the plant
capacity, which influences the capital cost. It is thus very important to reach
a high overall ethanol yield as this is directly related to feedstock and capital
costs for a given production capacity.
One of the major research challenges is to improve the hydrolysis of carbohydrates
throughmore efficient and less expensive pretreatment methods, but
also by enhanced enzymatic hydrolysis with superior enzymes at a reduced
enzyme production cost. The latter is one of the most uncertain costs in most
economic analyses.
It is also important to achieve a high ethanol concentration in the fermentation
or SSF steps to reduce the energy demand. This requires new
technology for enzymatic hydrolysis (or SSF) at high solids concentrations
and the development of robust fermenting organisms that are more tolerant
to inhibitors. They also have to be able to ferment all sugars in the raw
material in concentrated hydrolyzates, while maintaining high ethanol productivity
and a high ethanol concentration.
Finally, process integration within the process and with other types of industrial
processes, e.g. a combined heat and power plant or a starch-based
ethanol plant, will reduce the production cost further. Regarding the immediate
future, we believe that these integrated plant concepts will be used in the
first successful industrial-scale production of lignocellulosic fuel ethanol.