Technically, a variety of different liquid fuels and chemicals can be made from high quality syngas (Fig. 4). The production of liquid fuel, either a thermochemical — catalyzed conversion or a microbial fermentation process (under development),

Olefins

Gasoline

Aldehydes

Alcohols

Fig. 4 A diversity of chemicals can be produced from syngas (from page 3 of Drs. Spath and Dayton’s 2003 NREL Technical Report, NREL/TP-510-34929, with modification)

Data source [21]. EPI: Energy Products of Idaho. GTI: Gas Technology Institute. SEI: Southern Electric International. BCL/FERCO: Battelle Columbus Laboratory/ Future Energy Resources Corporation. MTCI: Manufacturing and Technology Conversion International.

may be used to convert syngas into liquid fuels (methanol, ethanol, gasoline, and FT diesel). The catalytic conversion of syngas to ethanol can occur under high — temperature and high-pressure conditions (~250°C, 60-100 atm) with a molar ratio of H2 to CO at 2-3:1. However, most syngas (Table 3) does not contain such a high H2/CO ratio. Also, the catalysis reaction is not specific, resulting in a final mixture of methanol, ethanol, some other higher alcohols, and reactant gases. Considerable technical progress is required to generate ethanol from syngas at a viable commercial scale and various projects continue to explore possible options. For example, Range Fuels in Georgia (Table 1), is in the process of building a 20 million gallon pilot plant to evaluate using this approach for lignocellulose to ethanol conversion. Syngas can also be converted into gasoline or diesel through the so called MTG (methanol-to-gasoline) or the more common FT process. While these methods have been utilized for many years in the fossil fuel industry (coal or natural gas feedstocks), the utilization of lignocellulosic biomass is not yet viewed as being commercial [23]. Two DOE-funded companies (Table 1) are in the pro­cess of building demonstration scale plants to further explore the feasibility of the gasification-FT process for biofuel production.

In the microbial fermentation process, anaerobic bacteria such as Clostridium ljungdahlii are used to convert cleaned syngas into ethanol [24]. Reactions involved in the biological conversion process are as the follows:

CO + 3H2O ^ C2H5OH + 4CO2

6H2+2CO2 ^ C2H5OH + 3H2O

In general, conditions for microbial conversion of syngas to ethanol are mild and specific, and the H2:CO ratio is not critical. However, microbial toler­ance to ethanol concentration in the fermentation broth is currently a limitation. Several public and private R&D projects are underway to address the issue (e. g. http://www. coskata. com; http://www. ineosbio. com).