3 June 2005: Chemical engineers strive for fuel efficiency

A team of US chemical engineers from the University of Wisconsin at Madison, led by James Dumesic, has developed a novel method for generating large alkanes from plant-derived carbohydrates. 

These alkanes could form the basis for a novel, highly energy-efficient biodiesel that has the potential to offer an alternative to hydrogen as the optimum ’environmentally-friendly’ fuel.

The use of biofuels, such as ethanol and biodiesel, represents a small proportion of fuel sales but this is growing rapidly throughout the world, driven by environmental concerns and, more recently, by the high price of oil. Unfortunately, current biofuels are not actually that environmentally friendly. The total energy costs involved in generating ethanol from maize-derived glucose, including fermenting and distilling the maize, means that only 1.1 units of energy are produced for every unit of energy that is consumed.

Dumesic and his team realised that a more efficient production process might involve generating large alkanes, which are central components of petrol, from aqueous carbohydrate solutions, because alkanes spontaneously separate from water. They therefore designed a four-phase reactor in which water-soluble carbohydrate compounds are turned into alkanes by the action of a solid catalyst (Pt/SiO₂-Al₂O₃). The process involves the organic compounds becoming steadily more hydrophobic and therefore the reactor also includes a hexadecane alkane stream to remove the hydrophobic species and prevent them forming coke on the catalyst surface.

The researchers also discovered that in order to produce alkanes with large enough masses they needed to get the carbohydrate compounds to form extra carbon-carbon bonds before they entered the reactor. Fortunately, there are a number of catalysis-based means for doing this and the chemists found that by adding the extra bonds their reactor could generate liquid alkanes with a range of large masses (C₇ to C₁₅). 

’It’s a very efficient process,’ claims team member George Huber. ’If you look at a carbohydrate such as corn [maize], our new process has the potential to create twice the energy as is created in using corn to make ethanol’. With the efficiency of diesel engines now approaching that of fuel cells, this means that this novel production process could generate a fuel that offers similar environmental benefits to hydrogen but requires much less new technology. Jon Evans