Biochemical Reactions in Syngas Fermentation
The biochemical conversion of CO, CO2 and H2 in syngas to ethanol following the acetyl-CoA pathway under anaerobic conditions produces acetic acid and ethanol as the major products. The overall stoichiometry for ethanol and acetic acid production from these gaseous reactants can be represented by Equations 12.6-12.9 [31].
6CO + 3H2O ^ C2H5OH + 4CO2 AG° = — 216.0 kJ/mol (12.6) 2CO2 + 6H2 ^ C2H5OH + 3H2O AG° = — 97.1 kJ/mol (12.7) 4CO + 2H2O ^ CH3COOH + 2CO2 AG° = — 135.0 kJ/mol (12.8) 2CO2 + 4H2 ^ CH3COOH + 2H2O AG° = — 54.8 kJ/mol (12.9)
CSTR = Continuous Stirred Tank Reactor TBR = Trickle Bed Reactor na = not available
The combination of Equations 12.6 and 12.7 shows that two — thirds of the carbon from CO is converted to ethanol. During the acetyl-CoA pathway, hydrogen provides the required reducing equivalents and electrons when hydrogenase enzyme is present in the fermentation media as shown in Equation 12.10.
H2 ^ 2H+ + 2e (12.10)
If the hydrogenase enzyme is inhibited or hydrogen is not present in the fermentation broth, the required electrons are obtained from CO in the presence of carbonyl dehydrogenase (CODH) enzyme. In other words, CO is used in supplying electrons, rather than in the biofuel production. This obviously results in a drastic reduction in alcohol yields. It is therefore vital to maintain healthy concentrations of both hydrogen and CO in the culture medium during the fermentation. It is very important to operate under optimum growth conditions of the syngas-fermenting microorganisms whose conditions can vary from organism to organism. Many of the known microbes tend to produce significant fraction of acetate products in addition to alcohol products as well. Therefore, in order to shift the product formation from acetogenesis to solven — togenesis, researchers have investigated nutrient limitations, pH shifts, reducing agent addition, hydrogen addition and many other modifications.
12.2 The Effects of Operation Parameters on Ethanol Yield
Several operation parameters can affect the ethanol yield. The most important ones are: culture media, pH, carbon source, inhibitors, gas impurities, and gas pressure.