Laser shines light on flame composition

A researcher from the Technology Foundation STW, a Dutch funding agency for university research, has introduced a new method for measuring the composition of fuel flames. Researcher Roger Evertsen’s method makes use of laser light in a technique that is said to be quick and extremely sensitive.

During the combustion of fossil fuels such as natural gas and oil, combustion gases such as hydrogen and carbon dioxide are released. During the combustion process various partially converted fuel molecules of OH, CH, HCO, CH2, NO and NO2 are found in the flame.

However, where and in what quantities these substances occur in the combustion chain is not clear. To gain more insight into this, Roger Evertsen carried out measurements in the flames of combusting natural gas.

The measurements were carried out using a technique that makes use of laser light that is directed through the flame. Mirrors are present on both sides of the flame so that the light is almost continually reflected through it. During each passage the glowing combustion products absorb a bit of the laser light and as a result of this the clarity of the laser gradually decreases.

If the flame contains a high concentration of an absorbent material such as OH, the intensity of the laser light rapidly decreases. If the concentration of the intermediate product is small, as is the case for CH and CH2, less laser light will be absorbed.

The level of this absorption as well as the rate at which it occurs is a measure of the quantity of combustion gas in the flame. Each substance is sensitive to a certain wavelength (colour) of the laser light and therefore a different laser is used for each substance.

Evertsen discovered that this cavity ring-down spectroscopy could measure HCO concentrations of 430 parts per million and CH2 concentrations of 80 parts per million.

The toxic gases NO and NO2, which are said to be difficult to measure using other techniques, could be detected at concentrations of 25 parts per million and 3 parts per million respectively. CH molecules could be detected at concentrations of just a few parts per billion. The measurements were performed at both low and normal air pressure.

At normal air pressure the area in which all chemical reactions occur is often no thicker than a millimetre. Measurements at low air pressure are therefore much more convenient because the flames are then much larger (the size of the flame is inversely proportional to the pressure).

The experiments were carried out at normal pressure so as to model the real situation in a gas water heater or gas cooker.