Today, fuel ethanol is produced from the fermentation of sugars that are typically derived from corn grain or sugarcane. However, the production of biofuels (such as ethanol) from syngas is an emerging technology. Syngas is a gaseous mixture consisting primarily of carbon monoxide (CO), carbon dioxide (CO2), and hydrogen (H2). It is produced from the gasification of feedstocks at temperatures in excess of 1100°F and under conditions where the amounts of oxygen (from air, pure oxygen, or steam) are less than what is needed for complete combustion. The chemicals in syngas can then be used as building blocks in other processes to produce compounds such as methanol, ethanol, and hydrogen using either a metal catalyst or microorganisms.
A number of feedstocks can be used to produce syngas including natural gas, coal, petroleum coke and biomass. Though many of these processes are well-established, interest in producing syngas from biomass is increasing due to its abundant availability and renewable nature. The production of syngas from biomass resources has great potential as it can readily use a wide variety of non-food biomass resources (e.g., prairie grasses, wood chips, solid municipal wastes, paper wastes, softwood trees) and uses the entire biomass feedstock which increases the conversion efficiency.
Metal catalysts can be used to convert syngas to biofuels, but this approach has several limitations (e.g., the need for high pressures and temperatures; low selectivity in producing products; sensitivity to, and poisoning of, catalysts by contaminants in the syngas; and the need to remove the high levels of heat generated during reactions) which increases the costs of producing biofuels from syngas. An alternative method to produce biofuels from syngas is to use microbial organisms rather than metal catalysts. This method overcomes some of the drawbacks of using metal catalysts, but has limitations of its own such as low productivity and long reaction times, and research is ongoing to improve this technology.
Several types of microorganisms are capable of consuming syngas as part of their metabolism and producing useful end-products including ethanol. Examples of these microorganisms include Clostridium ljungdahlii, Clostridium autoethanogenum, Eubacterium limosum, Peptostreptococcus productus, and Clostridium carboxidivorans P7T. These microorganisms (called acetogens) grow in the absence of oxygen and first produce a compound (called acetyl-CoA) which can serve as a precursor for the subsequent production of many other organic compounds including ethanol and butanol (another potential biofuel). Acetogens are a versatile group of microorganisms that can use gases like carbon dioxide, hydrogen, and carbon monoxide, as well as sugars, to produce organic chemicals.
A number of different gasifier types can be used to produce syngas (e.g., counter-current fixed bed, co-current fixed bed, fluid bed and entrained flow gasifiers) which is then converted to ethanol or other products in biofuel reactors. Several types of biofuel reactors can also be used (e.g., trickle bed, continuous stirred tank, bubble column, and packed bed reactors). Each type of gasifier and biofuel reactor has advantages and disadvantages relative to each other which must be considered in the selection of the technologies.
Ethanol is not currently produced on a commercial basis using microbial fermentation of syngas. This is a new technology still in the research phase, with most research conducted at a laboratory scale using synthetic syngas mixed from commercial gases.