Production of ethanol from lignocellulosic materials using thermophilic bacteria: Critical evaluation of potential and review

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Abstract

Resource and technological aspects of ethanol production are considered. Conversion of lignocellulosic substrates to ethanol via thermophilic bacteria is then addressed, with particular emphasis on evaluation from an engineering perspective.

The biological conversion of lignocellulosic materials to ethanol is a versatile process which can be used in various applications for replacing or improving petroleum products, treating wastes, or reducing air pollution. Petroleum replacement can be in relation to neat fuels, fuel additives, or raw materials. Waste treatment applications may be either for wastes which require treatment (e.g. municipal solid waste) or wastes which do not (e.g. many forestry and agricultural residues). Biological treatment of solid wastes with concomitant ethanol production may become attractive in that solid wastes represent less expensive substrates than those usually considered for ethanol production. In addition, the potential energetic yield of ethanol production is about twice that of electricity generation, and electricity and ethanol have comparable value per unit energy.

Estimated recoverable oil reserves represent a resource approximately 75 times the current annual consumption on a world-wide basis. However, some countries are in a particularly poor position with regard to petroleum supply and demand. For example the U.S. estimated recoverable oil reserves represent approximately 15 times the current annual consumption. The annual ethanol production potential in the U.S. achievable within 20 years is estimated at 1.3 × 1013 MJ based on a compilation of estimates for the rates of production and availability of various biomass materials. Relative contributions to this potential are: 41 % for wastes, 39 % for energy-devoted forestry, and 19 % for energy-devoted agriculture. Notably only 6% of the total ethanol production potential is derived from corn. Pentose sugars represent 28 % of the total potential with hexose sugars the remainder. Ethanol can displace gasoline at a ratio of about 1∶1.3 on an energetic basis, thus 1.3 × 1013 MJ of ethanol can displace about 1.7 × 1013 MJ of gasoline. The U.S. ethanol production potential of 1.3 × 1013 MJ, or 1.7 × 1013 MJ of displaced gasoline, can be compared to the yearly U.S. consumption of 7.5 × 1013 MJ for energy of all kinds, 2 × 1013 MJ for the transportation sector, and 1.2 × 1013 MJ for gasoline.

Four distinguishing features of thermophilic bacteria for ethanol production in comparison to yeast systems are identified. These include the advantages of pentose utilization and in situ cellulase production and cellulose utilization, and the disadvantages of low ethanol tolerance and low ethanol yield. Many frequently-cited advantages are not considered to be of great significance from an economic viewpoint, including facilitated product recovery and high conversion rates. The economic impacts of the distinguishing features of thermophiles for ethanol production are evaluated relative to a base-case process for ethanol production consisting of pretreated hardwood hydrolysis using Trichoderma reesei cellulase followed by conversion of soluble hexose sugars by yeast and reaction of xylose to furfural. Relative to the base case, the impact of in situ cellulase production and substrate hydrolysis is to lower the ethanol selling cost by 37%, and the impact of pentose utilization is to lower the cost by 23%. These two features together increase the ethanol yield per unit wood substrate by 47% over the base case. The increased cost of ethanol separation at low concentrations appears to be relatively small if energy-efficient processes are used, however such processes have not yet been implemented on a large scale. High ethanol yields must be obtained if thermophilic ethanol production is to be practiced on a significant scale.

Research results pertaining to the distinguishing features of thermophiles for ethanol production are reviewed. Critical research areas are proposed for closing the large gap between the potential of thermophilic bacteria for ethanol production and that which has been experimentally realized to date. These include process-oriented studies utilizing potentially realistic substrates and conditions, and both biological and engineering approaches to increasing ethanol yields.