Butanol is of interest as a fuel for internal combustion engines. Butanol has a higher energy density and lower vapour pressure than ethanol, which makes it more attractive as fuel or blending agent. Butanol is produced during fermentation by solvent producing bacteria (e. g. Clostridia acetobutylicum) in a process that is generally referred to as ABE (i. e. acetone, butanol, ethanol fermentation). Production of butanol and acetone from biomass via fermentation started during World War I, but declined in the course of the twentieth century primarily due the lower production cost of non-renewable butanol produced by the petrochemical industry (Lopez-Contreras, 2003). However, with the increasing demand for renewable biofuels there is great renewed interest in fermentative production of butanol. Currently, a number of industrial facilities are producing butanol (Johnson, 2008), although uniquely from starch and sugar feedstocks such as corn and molasses. Production of ABE from lignocellulosic feedstocks (i. e. cellulosic butanol) is currently at the R&D stages. One of the main advantages of cellulosic butanol fermentation is that most solvent-producing bacteria can convert both pentose sugars (a main component of lignocellulose) as well as hexose sugars to butanol. Major challenges in further development of ABE processes at industrial scale are overcoming the low volumetric productivity of the fermentation, which requires development of new microorganisms for ABE fermentation that have a higher tolerance for the end products. In addition, a particular challenge in butanol fermentation is the efficient separation of the three end products acetone, butanol and ethanol (Ezeji et al, 2007). It is expected that with advances in cellulosic ethanol and, in particular, pre-treatment of lignocellulosic biomass, butanol production from lignocellulosic biomass will get further implemented.

Cellulosic hydrogen

Hydrogen is predicted to be an important energy carrier in the future. It can be produced from renewable biomass feedstocks either by thermo-chemical conversion or by biological conversion. The use of microorganisms for biological hydrogen production via fermentation is increasingly attracting attention recently (Hagen, 2006). Carbohydrates, such as sugars, starch or (hemi) cellulose, are the prime substrates for fermentative processes, including biohydrogen. For future sustainability of the energy supply, the utilisation of (hemi)cellulose is of prime interest, as this component is most abundant in crops that can be grown for the purpose of energy supply (de Vrije et al., 2009). In the proposed bioprocess, thermophilic and phototrophic bacteria are employed consecutively, producing clean hydrogen at small scale (Claassen and de Vrije, 2006). The utilisation of a great variety of biomass feedstocks has been studied within the last decade for biohydrogen production, in particular for thermophilic bacteria. Lignocellulosic biomass types that were evaluated for application to bio-hydrogen production include Miscanthus, delignified wood fibres (de Vrije et al., 2009), Sweet Sorghum Bagasse (Panagiotopoulos and Bakker, 2008) and barley straw and corn stalks (Panagiotopoulos et al., 2009). In general, thermophilic bacteria, including Caldicellulosiruptor saccharolyticus and Thermotoga Neapolitana, appeared to be able to simultaneously and completely utilise all soluble monomeric C5 and C6 sugars derived from pre-treated lignocellulosic biomass. In addition, these bacteria may also convert di — and oligosaccharides. Simultaneous and complete substrate utilisation from pre-treated lignocellulosic biomass will add to an energy-efficient process and would be a major advantage in industrial scale production facilities. As with cellulosic ethanol and butanol, advances in pre-treatment of lignocellulosic biomass achieved in the near future will greatly accelerate prospects for producing biohydrogen at the demo — or industrial scale.

Biofuel-driven biorefinery

The costs of lignocellulosic biomass derived advanced biofuels (i. e. ethanol, ABE and hydrogen) produced by biochemical conversion in general are too high to be market competitive without any governmental support. The production of added- value Bio-based Products from process residues like hemicellulose and lignin/ stillage has the potential to significantly increase the market competitiveness of the total biomass-to-products value chain. Currently, a lot of effort is put in the development processes potentially being part of biofuel-driven biorefineries. Examples are technology developments for the conversion of hemicellulose to furfural-derived chemicals and pentoside surfactants; lignin/stillage to phenolics — derived wood adhesives, resins and thermosets; and cellulose to HMF and xylonic acids (Reith et al., 2009).