DOE has recently funded research performed by a research team at the University of Arkansas which focused on the utilization of an autotrophic fermentation process for the conversion of waste gases from refinery operations into usable products, such as ethanol and acetic acid/acetate. The sources of these gases included the following refining unit operations: cat crackers, cokers, and catalyst regeneration units. Additionally, the fermentation of synthesis gas has been evaluated. Synthesis gas (or syngas) is common gas product produced from reformed natural gas (done catalytically) or from coal gasification. Syngas has been used to produce secondary fuel products and primary alcohols. Most of the primary alcohol work has been focused on the production of methanol using heterogeneous catalysts including ruthenium, moly-sulfide, and nickel based units. The composition of these gases is generally hydrogen (~50%), carbon monoxide (~25%), and carbon dioxide (~25%).

The microorganisms that were isolated from the past DOE ethanol from syngas efforts were found within the waste pits of poultry raising operations. It was reported that further culturing and acclimation resulted in the development of advanced strains that were derived from the "wild" strain. The mechanism used by all of these organisms for the production of ethanol is reported as:

6H₂ + 2CO₂ –> CH₃CH₂OH + 3H₂O

The above reaction scheme is reported to yield approximately 25 g/l of ethanol within the aqueous effluent exiting the fermenter which can be recovered as anhydrous ethanol using distillation and adsorption. It has also been reported that the carbon monoxide reaction is much more productive in terms of ethanol production. This mechanism is also more stable from a cell growth standpoint which allows for increased biocatalyst production, hence the higher yields of ethanol.

Studies indicate that under fermentation conditions favoring ethanol production, acetic acid is also produced. Approximately 5 g/l of acetic acid is formed following the mechanism detailed below, under conditions producing approximately 25 g/l of ethanol within the same fermenter (5:1 product yield):

Currently, the acetic acid formed within the fermentation step ends up as the major component of the aqueous stream produced from the ethanol separation step (as a distillation bottoms). This stream is often considered a wastewater to be treated within a treatment plant prior to discharge. Alternative uses of the acetate within the fermentation process, such as recirculation, was evaluated with little potential noted because of alcohol production inhibition within the fermenter. It is interesting to point out that slight modification of the fermentation process does result in a much higher yield of acetic acid with little ethanol being formed (this mechanism is of likely interest to federal agencies other than DOE, such as USDA).

As mentioned above, there are actually two processes for producing ethanol from synthesis gas. These are the use of inorganic heterogeneous catalyts (such molybdenum sulfide) or bioconversion using fermentation. The benefits of the fermentation process over the abiotic catalytic process are:

1 - The fermentation processes operates at low temperatures and pressures as opposed to those required for the catalytic process, thus, reducing capital costs and increasing the safety aspects of the plant.

2 - The fermentation process produces a higher ethanol yield than the catalytic process.

3 - Based on past DOE funded research using refinery gases, the fermentation process is more economically attractive.

4 - The functioning catalyst is essentially biological enzymes which are easily produced by viable organisms housed within the fermenter.

5 - The fermentation process is much more forgiving in terms of varying syngas flowrate and composition.

6 - The fermentation process is not poisoned by the presence of sulfur compounds which is particularly problematic with the catalytic process.