4.2 Biorefinery Concept

According to the Biomass Research and Development Technical Advisory Committee of the US Departments of Energy and Agriculture (2002) Report published by the U. S. Department of Energy and U. S. Department of Agriculture, the biorefinery is defined as "A processing and conversion facility that efficiently separates its biomass raw materials into individual components and converts these components into marketplace products including biofuels, biopower, and conventional and new bioproducts." Several papers discussed the major products and integrated biorefinery concept for syngas fermentation.

Ethanol is by far the most economical bio-product that is generated during syngas fermen­tation. Ethanol is currently being sold as a fuel additive to blend with gasoline. The existing gasoline engines can take up to 10% ethanol (known as E10) without modifying the engine. Biomass-derived syngas fermentation also produces other important bio-product such as acetic acid, butanol, and butyric acid (Datar et al., 2004). Acetic acid has numerous applications in chemical industries including synthesis of vinyl acetate and acetic anhydride. Butanol is con­sidered as a better transportation fuel compared to ethanol due its high energy content and high vapor pressure. In addition, butanol is used in the production of butyl acetate and butyl acrylate which can be used as fuel additives to enhance the octane value of gasoline. Butyric acid is being used as a flavoring agent in the food processing industry.

Apart from the main products, organic acids and alcohols, the growth of anaerobic microbes also produces valuable biochemical such as polyester which serves as an energy storage unit for the organism (Brown, 2006). Most of the syngas-fermenting microorganisms produce these polyesters under stressed conditions such as nutrient imbalances. Polyhydrox — yalkanoate (PHA) is one of the most known polyesters produced in the cells, and it is stored as a discrete granule. Polyester content of cell is as high as 80% (dry weight).

In conventional biochemical-based ethanol plants, lignin fraction of the biomass is consid­ered as a low-value residue. Usually, 10 to 30% of biomass feedstock contains lignin which has a higher heating value of 9,111 Btu/lb. Therefore, the lignin recovered from the diverse feedstocks should be integrated into the process. Thermal cracking of lignin at high temperatures ranging from 250 to 600 °C showed the potential of producing low molecular weight gaseous feedstocks for further processing.

In an integrated biorefinery, the process is optimized to produce biofuel, along with other high-value products such as biopower and bio-based materials for a long-term sustainability.