Large Scale Microbial Production of Advanced Biofuels: How big can we go?

Conventional biofuels, whether from starch, sugar or lignocellulosic materials, tend to be low energy density products such as ethanol or butanol.  These are made using catabolic (anaerobic), thermodynamically “downhill” processes. Such processes have been proven feasible in rather large fermenter volumes, in excess of 1 million gallons (~ 4 million liters). In order to make use of current infrastructure and engine design, DOE has shifted focus to higher energy density fuels that act more like oils or gasoline, such as hydrocarbons, long chain fatty acids, fatty alcohols, or terpene type molecules. These so-called “hydrocarbon biofuels” can be produced by a number of conversion pathways, including biochemical routes based on aerobic (anabolic; thermodynamically “uphill”) submerged cultivation. The principal technical issue for large scale fermentation is the need to provide sufficient oxygen to allow the organisms to thrive, and the size and power required of the agitators typically used to disperse air.

It is anticipated that the ability to economically maintain effective gas-liquid mass transfer will ultimately limit the size at which aerobic submerged cultivation processes for hydrocarbon production from sugars can be operated. Maximum bioreactor sizes will likely be smaller than what is possible for strictly anaerobic fermentation processes where aeration isn’t necessary. A key enabler of cost competitive biological hydrocarbon production is the ability to economically aerate at large scales. This author did a study for NREL detailing the size of equipment required for various scenarios, the feasibility of manufacturing such equipment, and approximate equipment costs.