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Energy Resources Digest 6104 Item 9
28 June 07
The Cellulosic Ethanol Delusion
6/14/07
Here at the University of California at Berkeley, we have all heard denunciations of corn ethanol by prominent faculty members like Jay Keasling and Chris Somerville. But as soon as the denunciations are done, these same people immediately propose the grand solution to humanity's energy needs: ethanol from cellulosic biomass, preferably from their own patented superweeds. Despite repeated claims to the contrary, there is no energy-efficient and scalable industrial technology for producing ethanol from biomass. The Canadian biotech firm, Iogen Corporation, has a demonstration facility in Ottawa, with a nameplate capacity of 1 million gallons of ethanol per year. But it has only produced about 60,000 gallons of ethanol in 180 days from a 4-percent dilute beer. This demonstrates that Iogen has no viable technology beyond what was already achievable in the Soviet Union and Germany some 80 years ago.
The term cellulosic ethanol means that certain components of wood and green plant materials (cellulose, pectins, and hemicelluloses) are chemically separated and partially split into 6- and 5-carbon sugars, which are then fermented to produce dilute ethanol. Natural yeast does not ferment the 5-carbon sugars, further lowering conversion efficiency.
Two billion years of evolution have made cellulose incredibly strong and resistant to biochemical attacks. Cellulose can be quickly decomposed by mechanical grinding to 10-50 micron wood dust (try this on a tree trunk), heavy irradiation in a nuclear reactor (equivalent to burning 1/8 pound of gasoline per pound of cellulose), or steam exploding and severe chemical attack by hot concentrated sulfuric acid or sodium hydroxide. Biochemical enzymatic attacks take a long time and have low efficiencies.
Separating cellulose fibers from the rest of woody biomass is a very energy-intensive process. The best published energy efficiency of this process is about 30 megajoules (about 28,400 Btus) per kilogram of paper pulp, which is greater than the calorific value of a similar weight of pure ethanol. Therefore a much milder but less efficient enzymatic process must be used to obtain simple sugars from cellulose.
Production of cellulosic ethanol faces other obstacles, including the following.
Biomass availability. The natural productivity of a mature ecosystem is practically zero on a human time-scale. What is produced by plants, and by rock-weathering and floods, is consumed by bacteria, fungi, and animals, which are themselves recycled after death as nutrients for the plants. Therefore, "biowaste" is an engineering classification of plant (and animal) parts unused in an industrial process. This dated human concept is completely alien to natural ecosystems, which must recycle their matter completely in order to survive. Excessive biowaste removal robs ecosystems of vital nutrients.
Contamination. In corn ethanol production, normal fermentation times in batch mode are 48 hours; up to 72 hours is acceptable. Over 72 hours, the number of failures increases exponentially due to bacterial contamination. Typical enzyme processes for cellulosic alcohol take 5 to 7 days, or about 120 to 170 hours. This spells big problems if cellulosic ethanol producers ever go outside the laboratory or pilot scale (sterile fermenters) to a conventional fermentation vessel, which can not remain sterile for 120 to 170 hours.
Enzyme yield vs. rate. The rate of cellulose hydrolysis and fermentation can be increased by pre-treatment, but rates will slow down rather rapidly before high yields are obtained. It simply takes time and energy to chew into the sturdy lignocellulosic particles. Of course, one could run the biomass through a kraft-like process similar to paper-making. However, this cannot be done for cellulosic ethanol because energy losses would be severe.
In short, the current energy efficiency of producing cellulosic ethanol is so low that all other investigated paths to liquid biofuels are better. Some technical problems could be fixed with technology, so long as we do not violate the laws of physics. The cellulosic ethanol delusion as a whole violates these laws and gives us a false excuse to raid and destroy the last large, life-giving tropical ecosystems.
*** In response to other comments---
E R Digest 6107 Item 7b 30 June 07
~~~~~~~ EnergyResources Moderator Comment ~~~~~~~~
I agree, Patzek and Pimentel [Cornell] may have given us the best work on
ethanol there is.
The problem is not with their work but the lack of building a common
framework [on Energy Returned on Energy Invested] so all can tell the
relative merit of available energies, free of the advocacy noise we get from
the ethanuts--that will most likely end up wrecking a large part of the very
ag industry they are so eagerly stumping for.
~~~~~ EnergyResources Moderator Tom Robertson ~~~~~~
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