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EERC Update

A Road Map for Biofuels Research-Part II
By Joshua R. Strege
To modify a 1980's Oldsmobile slogan, "This is not your father's ethanol process!" Although consumers may see little difference, numerous processes are emerging into the commercial marketplace that are significantly different than traditional ethanol processes. As stated in last month's column, we feel that there are at least six broad categories of rapidly emerging advanced methods available to produce biomass-based transportation fuels. The first three involve the production of alcohols for use as gasoline additives. The others involve production of synthetic diesel from biomass. As part of our continuing series "Road Map for Biofuels Research," we'd like to further explore the advantages and disadvantages of each of the three alcohol pathways.

The first is enzyme hydrolysis, which involves the conversion of cellulose in biomass to sugars, followed by fermentation of the sugars to ethanol. This process is currently being used in plants being developed by Iogen Corp., Abengoa Bioenergy, BlueFire Ethanol Inc. and Poet LLC (formerly Broin Companies) under a recent multimillion-dollar U.S. DOE program for cellulosic ethanol plants. Each plant will produce about 20 MMgy of ethanol. The primary advantage of this pathway is that the fermentation step is similar to conventional ethanol production, which allows enzyme hydrolysis plants to be collocated with existing ethanol production facilities. However, the connection to conventional ethanol production also presents some disadvantages. In conventional fermentation, approximately one-third of the carbon available in the sugar is lost as carbon dioxide, and the water demand for hydrolysis and fermentation is high.

A second option is thermal gasification of biomass to produce syngas (consisting primarily of carbon monoxide, hydrogen, carbon dioxide and methane) followed by fermentation of the syngas to ethanol. To date, this process has only been tested in the laboratory, but it could prove to be a low-cost option in the future. This option could be problematic because the bacteria currently used for fermentation could occasionally produce products other than ethanol, leading to inconsistent product quality and yield. The bacteria may also require long gas residence times to achieve good conversion of syngas to ethanol at acceptable yields.

Lastly, thermal gasification of biomass followed by alcohol synthesis over a catalyst is a process being pursued by Range Fuels Inc. under the DOE award program. This process has an advantage over the first two pathways in that there is no biological component, allowing for higher temperatures and lower water demand. Similarly, the catalyst cannot mutate or alter its biology, so the alcohol product is of consistent quality over the catalyst lifetime. The main disadvantage of this process is that the product is not pure ethanol. It's a blend of alcohols containing a significant fraction of methanol, which could negatively impact vapor pressure.

There are, of course, hybrid variations to these three methods, such as novel pretreatment technologies, variations in enzymatic hydrolysis, combinations of biochemical and thermochemical processing steps, and other unique approaches such as converting cellulose to acetate esters, followed by further reaction with hydrogen to ethanol. Whatever the method for producing alcohols, 100 percent replacement of gasoline is not a near goal; rather, all of these processes involve the production of alcohols for blending with gasoline. When mixed with gasoline, the alcohol becomes an octane booster, limiting the amount of petroleum fuel that can be replaced by biofuel.

Stay tuned for next month's column, which will further explore the pathways to produce synthetic or "green" diesel from biomass.

Joshua R. Strege is a research engineer at the EERC in Grand Forks, N.D. He can be reached at jstrege@undeerc.org or (701) 777-3252.
 

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