—

Pradeep Chaminda Munasinghe, Samir Kumar Khanal

Department of Molecular Biosciences and Bioengineering (MBBE), University of Hawai’i at
Manoa, Agricultural Science Building 218, 1955 East-West Road, Honolulu, Hawaii 96822.
*Corresponding author: E-mail: khanal@hawaii. edu

1 BACKGROUND

Research on lignocellulosic biomass such as agri-residues (e. g., corn stover, wheat and barley straws, etc.), agri-processing byproducts (e. g., corn fiber, sugarcane bagasse, seed cake, etc.), and energy crops (e. g., switch grass, poplar, Napier grass, Miscanthus, etc.) has received considerable attention for bioenergy production, especially liquid transportation fuel in recent years. Lignocellulose is a renewable, nonfood feedstock with an annual availability of around 200 x 1012 kg (220 billion metric tons) globally. The United States alone has the potential of producing 1.3 billion dry tons of biomass annually, which could substitute more than 30% of the nation’s petroleum consumption (United States Department of Agriculture (USDA) and United States Department of Energy (USDOE) Joint Report, 2005). Thus, ligno­cellulosic biomass could play an important role in the bio-based economy to produce a variety of biofuels and bio-based products. Lignocellulosic biomass consists of 40-50% cellulose, 20-40% hemicellulose, and 10-30% lignin. Although multiple conversion technologies are avail­able for producing biofuels from biomass, there are two major pathways, namely biochemical and thermochemical. In biochemical conversion, the biomass is subjected to a combination of physical and chemical pretreatments to destruct the biomass structure. These pretreatments make the biomass accessible to enzymes. The pretreated biomass is subjected to enzyme hydro­lysis to obtain fermentable sugars, which are then fermented to biofuels (Takara and Khanal,

2011) . The biochemical route, however, has several drawbacks such as high pretreatment and enzymes costs, generation of inhibitory soluble compounds (acetic acid, furan derivatives, and various phenolic compounds), degradation of sugars, and low biomass to fuel conversion ratios (Lewis et al., 2010). On the other hand, in thermochemical conversion, the biomass is gasified to produce synthesis gas or syngas in short (a gas mixture predominantly consisting of CO, CO2, and H2). The syngas can be converted into liquid biofuels through Fischer-Tropsch (FT) synthesis (using metal catalysts) or direct microbial fermentation known as syngas fermen­tation (using microbial catalysts) (Henstra et al., 2007; Munasinghe and Khanal, 2010 (a)). The FT synthesis usually utilizes metal catalysts such as cobalt (Co), ferrous (Fe), copper (Cu), aluminum (Al), zinc (Zn), molybdenum (Mo), nickel (Ni), rubidium (Ru), and ruthenium (Rh) (Demirbas, 2007; Subramani and Gangwal, 2008). The major drawbacks of FT synthesis are the high costs of the metal catalyst, a fixed H2:CO ratio (2:1), catalyst poisoning due to inert gases and contaminants containing sulfur, and high operating temperature and pressure (Phillips et al., 1994; Vega et al., 1990; Worden et al., 1991).

Syngas fermentation via biocatalysts (such as Clostridium Ijungdahlii, C. autoethanogenum, C. carboxydivorans, Butyribacterium methylotrophicum, Methanosarcina barkeri, and Rhodospirillum rubrum) produces liquid/gaseous biofuels, and offers several advantages over the biochemical approach and the FT process. Some of the merits of syngas fermentation are the elimination of the need of expensive metal catalysts, a higher specificity of the biocatalysts, the independence of the H2:CO ratio for bioconversion, the operation of bioreactors at ambient conditions, and the elimination of issues concerning noble metal poisoning (Bredwell et al., 1999; Klasson et al.,

1990) . Poor solubility of syngas in the aqueous phase and low product yield are the major limitations of syngas fermentation. These limitations have been the bottlenecks to commercial­ization of the syngas fermentation process.

This chapter critically reviews the existing literature on biomass-derived syngas fermenta­tion into biofuels. Furthermore, relevant background information including pathways, micro­bial aspects, mass transfer, reactor design, and factors affecting syngas fermentation are also briefly discussed. In addition, the current developments, challenges, and future research directions in syngas fermentation to biofuels are also included.