|
One of the major limitations of large-scale ethanol
production is the availability of the raw materials it requires; namely corn
and sugarcane crops. Cellulosic ethanol is ethanol derived from the breakdown
of cellulose, incorporating other types of feedstock; namely wood, grass, and
non-edible plant parts. For years this has been an intriguing sustainable alternative
to corn-based ethanol because it can be made from agricultural wastes rather
than food crops. Effective utilization of these materials would make ethanol a
much more economically viable alternative fuel.
How It Works
Cellulosic ethanol production involves the breakdown of
lignocellulose, a structural material in plants. Lignocellulose is comprised of
cellulose, hemicellulose, and lignin. Cellulose and hemicellulose contain sugar
molecules (glucose and xylose, respectively) which, when freed, can then be fermented
into ethanol (in the case of glucose) or sold for other processes (in the case
of xylose).
Cellulose
visualization. Image Credit: OLCF
The lignin component in these compounds is a large and
complex molecule made of hydrogen, oxygen, and carbon. It acts as a binding and
protective element to the microfibril structure of cellulose and hemicellulose.
In order to produce fermentable sugars, enzymes (e.g. cellulase) or acids must
break through this lignin wall in order to interact with the cellulose and
hemicellulose.
In addition, lignocellulose is a compact crystalline
structure, making it hard for enzymes to navigate effectively. The reality is
that glucose polymer chains in cellulose are largely insoluble due to these
factors.
Because of these difficulties, industrial scale production
of cellulosic ethanol is just not cost effective. It generally requires
expensive specialized enzymes or toxic and corrosive acids. Both processes are
slow and require costly equipment to handle these substances.
Utilizing Supercritical Water
To counter this feasibility issue, a startup company name
Renmatix is using supercritical water for the transformation process. The company
says its method makes sugar with wood chips for the same price as with
sugarcane, which has become a profitable market in Brazil.
Lignocellulosic
biomass in the form of wood chips. Image
Credit: Nature.com
Supercritical water is water pushed above its critical point
through high temperatures and pressures into a physical state beyond liquid and
gas distinction. For a fun visual example of a supercritical fluid, check out
this video by Nottingham ScienceCity:
http://www.youtube.com/watch?v=yBRdBrnIlTQ
In this supercritical state, the fluid is an incredible
solvent, capable of breaking down the lignocellulose in a matter of seconds. The
high speed allows the plant to produce more sugar with less equipment than other
methods.
There are a number of challenges with supercritical water
however. The fast reactions are much harder to control, and can result in
undesirable by-products if carried too far. Materials which can handle
supercritical water are limited, and likely expensive. Yields of sugar/biomass
also tend to be relatively small.
In the past, these problems with supercritical water have
labeled it as uneconomical, so vouching for its viability may prove difficult.
Renmatix has not released any details on its process, except to say that it has
overcome these problems and uses two steps to break down the cellulose and
hemicellulose.
Storage container
for sugar production. Image Credit: Technology
Review
Renmatix is pushing investors for funding to build a larger
production facility to prove its process's commercial viability. Currently, the
operation has only been carried out on a small scale, but Renmatix wants to
raise enough money to create a plant capable of producing 100,000 tons of sugar
per year.
Researcher Gary Aurand at the University of Iowa says that
the process could work if Renmatix has engineered a system that incorporates
only supercritical water for the dissolving step. Separating the dissolved materials into
an area of lower temperature and pressure would slow down the process and could
provide the control necessary to minimize some of the problems.
Regardless, the feasibility of a large-scale plant using
supercritical water is questionable. There are a lot of question-marks
surrounding its operation, but just as many regarding the possibilities surrounding
increased biofuel production in the future.
Resources:
Technology
Review (MIT)
Oak
Ridge Leadership Computing Facility (OLCF)
Center
for Environmental Research & Technology at UCR
| 
Re: A "Super" Solution for Biofuel Production
