Published on 22 May, the study shows that in order to achieve energy and greenhouse-gas emissions benefits, researchers need to closely examine and differentiate among the types of plants used to produce corn ethanol – so that production can become more sustainable.
Since the US began a programme to develop ethanol as a transportation fuel during the second oil crisis in 1979, its use has increased from 175 million gallons in 1980 to 4.9 billion gallons in 2006, and is still increasing – with virtually all the ethanol used for transportation being produced from corn, observe the authors.
During this period of fuel ethanol growth, corn farming productivity has increased dramatically, and energy use in ethanol plants has been reduced almost by half, they find. The majority of corn ethanol plants are powered by natural gas, but as gas prices have shot up over recent years, efforts have been made to further reduce the energy used in ethanol plants or switch from natural gas to other fuels such as coal and wood chips.
The paper examines nine corn ethanol plant types, categorised according to the type of process fuel employed, use of combined heat and power, and wet or dry milling plants – finding that they can have distinctly different energy and greenhouse-gas emission effects.
The researchers conclude that of the nine options included in the study, all achieve positive energy balances and reduce petroleum use relative to gasoline – though at the expense of increasing natural gas or coal use depending on which is the process fuel. Switching from natural gas to coal as a process fuel in corn ethanol plants may eliminate the greenhouse-gas reduction benefits, and eliminating the need for drying has a significant positive effect on corn ethanol's energy and greenhouse-gas emission benefits, they report.
The authors also conclude that cellulosic ethanol produced from switch-grass offers by far the greatest energy and greenhouse-gas emission benefits and may represent a long-term, sustainable ethanol production pathway. They hope that the information provided will help the corn ethanol industry select the most energy- and greenhouse-gas emission-friendly way forward.