Feb 2, 2008

Bioethanol: gasification and biofermentation

A recent USA Today article points out that Coskata, a bioethanol startup company has been backed by GM. Their ethanol production differs from others in that, they combine an initial biomass gasification step to produce syngas (carbon monoxide + hydrogen) and subsequent microbial production of ethanol. Some industry analysts believe that this represents an innovative combination of gasification and biofermentation technologies.

The reaction of carbon monoxide (CO) and hydrogen (H2) to produce ethanol (C2H5OH) is exergonic at standard temperature and pressure (STP). This reaction, however, suffers from problems of selectivity. Among the direct syntheses of alcohols from syngas, only methanol synthesis is 99% selective and also has high product formation rates. Ethanol formation is represented as:

3CO + 3H2 ---> C2H5OH + CO2 [Free energy of reaction at STP ~ -157 kJ/mol EtOH]

CO/H2 ratios of 1:1 are needed to produce ethanol. Each mole of ethanol produced generates a mole of CO2. Biomass (lignin-wood-cellulose) has atomic O/C ratios from 0.5-0.8 and H/C ratios from 1.1 to 1.8. Gasification of various feedstocks will result in different syngas compositions. The need for CO:H2 ratios of 1:1 necessitates water-gas shift reactions to convert some of the CO to CO2 to adjust the CO:H2 ratio in the feed.

CO + H2O ---> CO2 + H2

According to Coskata's website, the advantages of their biological process for ethanol conversion are the selectivity towards ethanol (lower separation costs), higher impurity tolerance of their microbes vis a vis chemical catalysts, lower operating pressures, and most significantly, IMO, variable CO:H2 ratios.

The use of gasification to produce a syngas stream makes it possible to process a variety of biomass feedstocks. A distinguishing feature of the biofermentation might be the use of bacteria to drive similar water-gas shift reactions in the culture medium itself. For example, recently researchers have found microbes (Carboxydothermus hydrogenoformans) which can convert carbon monoxide and water to hydrogen (and CO2).

The advantages of the process are that by regulating the concentration of the CO metabolizing bacteria, one can theoretically produce varied CO:H2 ratios (in the fermentor) for ethanol production. In this scenario, the use of bacteria like Carboxydothermus hydrogenoformans to shift CO and water to H2 is critical to the success of this process.

It therefore appears that this novel combination of gasification and biofermentation may have definite advantages in product selectivity, operating conditions, and also raises interesting questions of process design for other reforming processes in general. For example, one can think of methane reforming, bacteria producing methanol. I think the next big step in biocatalyst design will be the use of enzymes to drive reactions such as the conversion of CO2 to specific chemicals like ethanol or methanol upon reaction with hydrogen or through electrochemical reduction. Typically, reactions involving CO2 (and many other compounds) need specific metal catalysts for product specificity and yield. The aforementioned Carboxydothermus hydrogenoformans bacteria contains a collection of enzymes (CO dehydrogenases) , one of which has been shown to activate CO2 at electrodes. It all comes full circle at some point, after all.


Anonymous said...

Can you explain why they would want to turn syngas into ethanol rather than BTL synthetic diesel?
BTL produces aviation grade synthetic fuel with an energy content similar to fossil kerosene.
However, BTL is very capital intensive whereas conventional ethanol plants are require only low capital investment.
Is the advantage of gasification and biofermentation that it combines the flexible biomass feedstock of gasification with the low capital cost of fermentation?

TheLight said...

Thanks for the comment. I agree that turning syngas into diesel would produce much higher energy content than ethanol. The supposedly capital-intensive nature of BTL might be one reason. Economies of scale are really what matter here. Sasol's F-T technology operates on a ~50-80,000 barrels of liquids/d basis. However, recently, Oxford catalysts has supposedly come up with a small-scale F-T process.

"Is the advantage of gasification and biofermentation that it combines the flexible biomass feedstock of gasification with the low capital cost of fermentation?"

I think so. This definitely eliminates the need for having a (heterogeneous catalytic) water-gas shift reactor. However, I have not come across any engineering analyses comparing BTL and gasification-fermentation routes. The other reason why it is being adopted might be that it is a more socio-politically popular fuel choice in the US. I think that ethanol blender's tax credit, combined with poor US public perceptions about diesels are two of many contributing factors.
[Although, US gasoline consumption was 390 million gallons/d, whereas US diesel consumption was ~105 million gallons/d. ]

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