The
production of ethanol from lignocellulose-rich materials such as wood
residues, waste paper, used cardboard and straw cannot yet be achieved
at the same efficiency and cost as from corn starch. A cost comparison
has concluded that using lignocellulose materials is unlikely to be
competitive with starch until 2020 at the earliest. The study, published
in the international journal Biofuels, Bioproducts & Biorefining,
did identify many opportunities for reducing costs and improving income
within the lignocellulose-to-ethanol process, and provides insight into
the priority areas that must be addressed in coming years.
Ethanol
can be blended with gasoline to reduce our dependency on fossil fuels.
The last 15 years has seen a massive growth of so-called
first-generation processes that use enzymes and bacteria to turn the
starch and sugars in corn and sugarcane into ethanol. But corn and
sugarcane are also important components of the human food web, so using
them for ethanol production has the potential to affect the price and
availability of these basic commodities.
On
the other hand, lignocellulose materials are often hard to dispose of,
but they are rich in sugars that can be fermented into ethanol following
appropriate processing. “Not only is cellulose the most abundant
polymer on Earth, it cannot be digested by humans, so using it for fuel
production does not compete directly with food supplies,” says the
study’s lead author Jamie Stephen, who works in the Department of Wood
Science at the University of British Columbia in Vancouver, Canada. The
race is on to commercialize this second generation ethanol.
Stephen’s
work focuses on the fact that the cost of building large scale
ethanol-producing facilities will likely be higher for second generation
ethanol compared to first generation technologies. One reason is that
sources of lignocellulose may require significant and costly
pre-treatment. “Researchers and companies are going to have to
concentrate on reducing the cost of pretreatment and increasing the
output of the digester in order to reduce the costs of the
lignocellulose-to-ethanol process,” says Stephen.
Another
reason costs are higher is that lignocellulose is made of multiple
kinds of sugar, while corn starch consists of pure glucose. Corn starch
can be reduced to glucose with low-cost amylase enzymes, while
pre-treated lignocellulose requires a cocktail of cellulase enzymes.
Providing these enzymes is one of the major costs of the whole process,
but you currently need 12 times more cellulase than amylase protein to
generate the same amount of ethanol from woody biomass. “Despite much
effort and progress over the last few years, the cost of using cellulase
enzymes is still significantly higher than for amylase-based processes,
and will need to be reduced substantially before lignocellulose starts
to become competitive with corn and sugarcane as a feedstock,” says
Stephen.
Finally,
while the input to sugarcane- and corn starch-based systems is fairly
constant, the feedstocks that go into lignocellulose systems are much
more variable. Different species of tree produce wood that has different
properties, and waste paper and agricultural wastes will have many
different types of material in them. To get maximum efficiency, each
type of biomass needs to be processed under different conditions, which
introduces another challenge for anyone wanting to make ethanol from
these materials.
Overall
Stephen believes we have a considerable way to go before
second-generation ethanol production will be ready for
commercialization. “Production requires significant cost reductions and
at least the same level of financial support that was given to the
first-generation systems if second-generation ethanol is going to be
fully competitive by 2020,” says Stephen.
