Due to the excessive exploitation, the fossil fuels are facing scarce and they cannot be generated. On the other hand, most of the carbon emissions result from fossil fuel combustion. Reducing the use of fossil fuels will considerably reduce the amount of carbon dioxide and other pollutants produced. Renewable energy has the potential to provide energy services with low emissions of both air pollutants and greenhouse gases. Currently, renewable energy sources supply over 14% of the total world energy demand. Biofuels as the important renewable energy are generally considered as sustainability, reduction of greenhouse gas emissions, regional development, so­cial structure and agriculture, and security of supply (Reijnders, 2006). Biodiesel and bioethanol are presently produced as a fuel on an industrial scale, including ETBE partially made with bioe­thanol, these fuels make up most of the biofuel market (Antoni et al., 2007).

Biobutanol also has a promising future for the excellent fuel properties. It has been demon­strated that n-butanol can be used either 100% in unmodified 4-cycle ignition engines or blended up with diesel to at least 30% in a diesel compression engine or blended up with kerosene to 20% in a jet turbine engine in 2006 (Schwarz et al., 2006). The production of bio­butanol from lignocellulosic biomass is promising and has been paid attention by many companies. Dupont and BP announced a partnership to develop the next generation of bio­fuels, with biobutanol as first product (Cascone, 2007). In 2011, Cobalt Technologies Compa­ny and American Process Inc. (API) have been partnering to build an industrial-scale cellulosic biorefinery to produce biobutanol. Additionally, the companies agreed to jointly market a GreenPower+ biobutanol solution to biomass power facilities and other customers worldwide. The facility is expected to start ethanol production in early 2012 and switch to biobutanol in mid-2012. The annual production of biobutanol is estimated to 470, 000 gal­lons. (http://www. greencarcongress. com/2011/04/cobalt-20110419.html, http://www. renewa- bleenergyfocususa. com/view/17558/cobalt-and-api-cooperate-on-biobutanol/) Gevo, Inc. signed a Joint Development Agreement with Beta Renewables, a joint venture between Chemtex and TPG, to develop an integrated process for the production of bio-based isobuta­nol from cellulosic, non-food biomass, such as switch grass, miscanthus, agriculture resi­dues and other biomass will be readily available. (http://www. greencarcongress. com/ biobutanol/). Syntec company also is currently developing catalysts to produce bio-butanol from a range of waste biomass, including Municiple Solid Waste, agricultural and forestry wastes. (http://www. syntecbiofuel. com/butanol. php). Utilization the waste materials im­prove the economy of butanol production that makes biobutanol great potential to be the next new type of biofuel in spite of the existing drawbacks.

7. Conclusions

Biobutanol production has only recent years booming again after long time of silence. Quite a lot of progress has been made with the technology development of metabolic engineering in enhancing solvent production, increasing the solvent tolerance of bacteria, improving the selectivity for butanol. Fortunately, Clostridia have been tested being able to consume ligno — cellulosic biomass for ABE fermentation. The complex regulation mechanism of butanol synthesis is still need to be further study. For the strain improvement, for example, con­structing better butanol tolerance strains, more suitable hosts and genetic methods are re­quired to be set up. Furthermore, more efficient techniques for removing the inhibitors in the lignocellulosic hydrolysate need to be developed. In addition, from the economic point of view, the integrated system of hydrolysis, fermentation, and recovery process also are im­portant to be further developed to reduce the operation cost of butanol synthesis.

Author details

Hongjuan Liu*, Genyu Wang and Jianan Zhang

*Address all correspondence to: liuhongjuan@tsinghua. edu. cn; zhangja@tsinghua. edu. cn Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing, P. R., China