12.2 Syngas Fermentation
There is an unusual process for liquids production from biomass: gasification followed by fermentation of gases into liquids. During gasification, the gases of CO, H2, and CO2 are formed (as we have learned in past lessons), but instead of using something like FT or MTG, this is formation of liquids fuels through a fermentation process using a microbial catalyst. Products are typically ethanol, acetone, and butanol. Gasification was discussed in depth in Lesson 4, but I will cover it briefly here to remind you of the various processing aspects. Gasification takes place at temperatures of 750-900°C under partial oxidation. It happens in the following steps: drying; pyrolysis in absence of O2; gas-solid reactions to produce H2, CO, and CH4 from char; and gas-phase reactions that manage the amounts of H2, CO, and CH4. It is most often known as syngas, but if it contains N2, then it is called producer gas. Syngas can be generated from any hydrocarbon feed. The main cost associated with gas-to-liquid technologies has to do with the syngas production, which is over half of the capital costs. Costs can be improved using improved thermal efficiency through better heat utilization and process integration and by decreasing capital costs.
There are advantages to using fermentation as part of liquids generation rather than using something like Fischer-Tropsch:
- As with any gasification, it is independent of feedstock, and therefore, independent of biomass chemical composition.
- Microorganisms are very specific to ethanol production, whereas with chemical catalysts, there are a wide range of reaction products.
- No pretreatment is required as part of the biochemical platform.
- Complete conversion of biomass is achieved, including lignin conversion. This can reduce the environmental impact of waste disposal.
- Fermentation takes place at near ambient temperature and pressure, thus at a place where costs can be reduced significantly.
- The requirement for CO/H2 ratio is flexible.
Of course, there are disadvantages as well. These include:
- Gas-liquid mass transfer limitations.
- Low ethanol productivity, usually related to low cell density.
- Impurities in syngas generated from biomass.
- Sensitivity of microorganisms to environmental conditions (pH, oxygen concentration, and redox potential).
The microorganisms that are used for ethanol production from syngas are acetogens that can produce ethanol, acetic acid, and other products from CO and H2 in the presence of CO2. The organisms are: 1) Clostridium strain P11, 2) Clostridium ljungdahlii, 3) Clostridium woodii, 4) Clostridium thermoaceticum, and 5) Clostridium carboxidivorans P7. (Wilkens and Atiyeh, 2011) The bacteria are some of the same ones that occur during anaerobic digestion: acetogens and acidogens. I won’t go into great detail about the biochemistry, as it is a little beyond the scope of this class. The acetogens utilize the reductive acetyl-CoA (or Wood-Ljungdahl) pathway to grow carbons and hydrogens on single carbon substrates such as CO and CO2. Clostridium bacteria use H2 or organic compounds as the electron source for the reduction of CO2 to acetyl-CoA, which are further converted into acids and alcohols. The process proceeds in two phases: acidogenic and solventogenic phases. In the acidogenic phase, mainly acids are produced (i.e., acetic acid and butyric acid). In the solventogenic phase, mainly solvents are produced (i.e., alcohols such as ethanol and butanol). Reactions 14 and 15 show the reaction chemistry for acetic acid formation, and reactions 16 and 17 show the reaction chemistry for ethanol formation:
Acetic acid formation:
(14) 4CO + 2H2O → CH3COOH + 2CO2
(15) 2CO2 + 4H2 → CH3COOH + 2H2O
(16) 6CO + 3H2O → C2H5OH + 4CO2
(17) 2CO2 + 6H2 → C2H5OH + 3H2O
In summation, gasification-fermentation alternative is a method for biofuel production utilizing syngas generated from gasification of biomass feedstocks. Because it is biologically based, it has the potential for reducing costs compared to other syngas to liquid technologies, but there are several challenges related to this technology. Challenges include low alcohol productivity, low syngas conversion efficiency, and limitations in gas-liquid mass transfer. These challenges must be solved if this technology is to become economically viable.