As dedicated readers of Entropy Production will know I am researching the potential to use the byproducts of biodiesel crops to produce biogas -- a mixture of methane and carbon dioxide -- and fertilizer sludge. I'm still coming up short on researching the engineering fudamentals of biological methane production. I would like to lay out the biological basics to form a framework for future analysis. Breaking up complex biological polymers into methane gas is a multi-step process involving many different populations of bacteria. Biology has the habit of splicing words together like German vocabulary; they call the process of methane production methanogensis.
There are two basic pathways. The first is the production of hydrogen directly from polymers. The second is a longer process that breaks down polymers to acetate. Methanogenesis can consume either CO2 and H2 or acetate ion (CH3CO2) which also produces CO2 as a byproduct. In addition to hydrogen and acetate some other fermentation products such as methanol can also be transformed into methane.
The acetate pathway is the longer of the two. The first step is the basic fermentation of proteins, fats, and carbohydrates into smaller chunks. The next step is to break down the intermediate products into acetate(CH3COO-) and acetic acid (CH3COOH). The pH of the solution can become unbalanced at this point which is disadvantageous to the consumption of acetane into methane.
The basic reaction is:
C2H4O2 → CH4 + CO2
The hydrogen pathway can consume most forms of biological material. It basic disadvantage is that the enzyme used rapidly reduces in effectiveness in the presence of hydrogen. The very production of hydrogen acts to stall the process. As such the hydrogen must be quickly removed from the system. Diffusion is generally slow but feeding the hydrogen into another chemical reaction can consume it quickly. Because the product is hydrogen gas there has been considerable research into this pathway. Research has focused on genetically modifying the enzyme to operate in the presence of hydrogen or engineering improvements to increase the yield. Methanogens (methane production bacteria) can react H2 with CO2 to form methane. This will produce oxygen gas which is not desirable in the anaerobic environment.
The basic reaction is:
CO2 + 2H2 → CH4 + O2
Methanogensis can only take place in an anaerobic environment. Maintaining an anaerobic environment not only protects the oxygen adverse bacteria but also prevents the growth of oxygen breathing methane burners.
The rate of methane production is highly dependant on pH and temperature. Most methanogens (mesophiles) prefer a temperature between 30 - 40 o and a pH near neutral (7.0) or slightly alkaline. My inability to find temperature and pH dependence is one of my stumbling points at the moment.
Methane producers are not the only bacteria in the pond that consume hydrogen and acetate. In particular, sulfur and nitrogen fixing bacteria both out compete methanogens for reactants. Sulfur fixing bacteria like to produce Hydrogen Sulfide gas (H2S) which then bubbles off and should be separated from the biogas flow. On the other hand, nitrogen fixing bacteria tend to produce ammonium ion (NH4+) which then finds something else to bond to. Ideally one might want to search for a bacterium that fixes sulfur into a solid form that is suitable for soil fertilization. Principally one could find data on the N and S content for Canola meal and figure the number of moles of H2 consumed.
Just as an aside, I did find some research on the production of butanol from biomass instead of ethanol. I don't have on-line access to this research so I can't easily review it but it is interesting nonetheless.
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