Today, hydrogen is typically produced from natural gas (methane) or coal using metal catalysts as part of the process. However, the production of biofuels (such as hydrogen) 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 700°C and under conditions of steam or oxygen where the amounts of oxygen (from air or pure oxygen) 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. The use of microorganisms 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 hydrogen. Examples of these microorganisms include Rhodospirillum rubrum, Rhodopseudomonas palustris and Rhodopseudomonas gelatinosa, and Rubrivivax gelatinosus CBS.
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 hydrogen, ethanol, or other products in biofuel reactors. Several types of microbial reactors can also be used (e.g., trickle bed, continuous stirred tank, bubble column, and packed bed reactors). Each type of gasifier and microbial reactor has advantages and disadvantages relative to each other which must be considered in the selection of the technologies.
Hydrogen is not currently produced on a commercial basis using microbial fermentation of syngas. This is a new technology still in the research phase.