R. M. Worden1, M. D. Bredwell1, and A. J. Grethlein2

department of Chemical Engineering, Michigan State University,
East Lansing, MI 48824
2Athena Neurosciences, 800 Gateway Boulevard,

South San Francisco, CA 94080

Biomass-derived synthesis gas can be readily converted into fuels and chemicals by anaerobic microorganisms. However, synthesis — gas fermentations typically exhibit low volumetric productivities due, in part, to low cell densities, production of unwanted by­products, and slow transfer of the synthesis gas into the liquid phase. Engineering approaches to improve bioreactor productivities are discussed, and recent advances in this area are summarized. Particular emphasis is placed on the use of bioreactor design to increase biocatalyst concentrations, development of metabolic models to study pathway regulation and the use of microbubble dispersions to enhance synthesis-gas mass transfer.

Synthesis gas, which consists primarily of carbon monoxide (CO), and hydrogen (H2), is produced by the partial oxidation of an organic feedstock at high temperature in the presence of steam. Although coal and petroleum have historically been the most commonly used feedstocks for synthesis-gas production, several new gasification plants have recently been based on biomass (7). Biomass offers several advantages over the traditional feedstocks. First, it has a much lower sulfur content than many coals. Synthesis gas produced from wood chips at the GE gasification plant in Schenectady, NY contained 28 ppm H2S (2), compared to 1-2% for coal-derived synthesis gases (3). Purification steps to remove sulfur from coal-derived synthesis gas are energy-intensive and add significantly to the product costs (4). Second, biomass materials are more reactive and thus require lower gasification temperatures and/or residence times. Fluidized-bed coal gasifiers are typically run at 1000°C using residence times of 0.5-3.0 h. By comparison, a temperature of 850°C was sufficient to gasify biomass using a residence time of 30 s to 5.0 min (5). Third, gasification of wood waste has the potential to solve a disposal problem while producing a valuable product. When wood waste is in

© 1997 American Chemical Society

short supply, short-rotation forestry can serve as a steady source of feedstock.

Synthesis gas can be catalytically converted into chemical products (e. g., methanol) in reactors operated at high temperatures and pressures. The status of such catalytic processes has recently been reviewed (6). Production of higher molecular weight alcohols (e. g., butanol) from synthesis gas is problematic, because existing catalysts yield a broad mix of alcohols. Anaerobic bioconversion of synthesis gas into fuels and chemicals represents an alternative approach that offers the advantages of lower temperatures and pressures, higher reaction specificity of the biological catalysts, and higher tolerance to sulfur compounds. A variety of anaerobes are able to convert synthesis-gas components into fuels and chemicals, including ethanol, butanol, acetic acid, butyric acid, and methane. The pathways of most of these microbes involve the conversion of CO or CO2 and H2 to the intermediate acetyl-CoA, which serves as a branch point for production of cell mass and two- and four-carbon alcohols and acids. Several reviews of microbial CO metabolism have been published (7,8,9,10). Whole-cell biocatalysts capable of converting synthesis gas to fuels and chemicals can tolerate orders of magnitude higher H2S concentrations (greater than 1%) than iron and nickel catalysts (1-10 ppm) used for Fischer Tropsch conversion of synthesis gas (11,12,13).

Synthesis-gas fermentations have potential for commercial development, and some of the engineering issues have been addressed (14,15,16). Low bioreactor productivity is a major obstacle to commercialization. Several factors contribute to the low productivity, including low cell density, an inability to regulate branched pathways to obtain only the most desirable product, inhibition of the biocatalysts by the reactants and products, and low rates of transfer of CO and H2 from the gas to the liquid phase. Several engineering approaches have been used in the past few years to overcome these limitations. The purpose of this paper is to summarize these approaches and discuss their impact on the feasibility of producing fuels and chemicals via synthesis gas fermentations.