Understanding how electric fields make flames cleaner could improve power stations

Burning hydrocarbons is a more complex process than it seems. If combustion is complete, then the products are carbon dioxide and water vapour, which may be undesirable in the atmosphere but are at least well understood and non-toxic. However, if the hydrocarbon is not completely consumed in the flame, then other products may form. These include carbon monoxide, oxides of nitrogen and soot, all of which are hazardous to health.

One technique that is used in power stations to ensure the combustion is clean is to apply an electrical field across the flame; this has been found to reduce the production of pollutants, although the mechanism for this is not clear. This led researchers at KAUST’s clean combustion research centre to embark on a study of exactly what compounds are formed when methane burns in air.

The team, led by Aamir Farooq, set up a device called a McKenna burner, which produces a flat flame from a well-mixed stream of methane, oxygen and argon; and above this they set a specially-designed molecular beam mass spectrometer, with which they hoped to detect positively-charged particles within the combustion products. There are much less of these than there are of neutral atoms and molecules, but tend to be reactive and so are undesirable.

"Controlling the charged particles allows a leaner, higher air-to-fuel mix, flame to be burned at lower temperatures, providing a more complete and efficient combustion of the fuel and a reduction in the production of pollutants," Farooq explained.

In a paper in the journal Proceedings of the Combustion Institute, Farooq and his colleagues explain that it has previously been very difficult to measure the production of charged particles, because they are orders of magnitude more scarce in the flame than uncharged species. The mass spectrometer was the key to identifying them in their experiment, and they were able to "measure a wide range of ions, allowing us to understand how they form and decay, leading to the formation of other ions," Farooq said. "A surprising and significant observation was a relatively large concentration of ions other than hydronium [water with an extra proton stuck to the oxygen]."

Analysing the combustion products of methane is only the first step in the research. The team intends to go on to study combustion products of more complex hydrocarbons, such as ethane and propane. These are also present in natural gas, the most common fuel in power stations, although in lower concentrations the methane. "Our work is the first attempt to identify the cations formed in the combustion of low-pressure methane, and represents a major step towards the use of external electric fields for generating clean power," Farooq said.