Combustion Physics

Lund University

What is combustion?

Human beings have always been fascinated by fire, but it is during the last decades we have actually started to understand the detailed physics and chemistry in combustion. The reason for this breakthrough is the use of modern scientific tools such as advanced laser spectroscopy and fast computers to be used for solving complex numerical problems. Such tools are developed and utilized in the scientific work at our division.

In a combustion process fuel is oxidized and large amounts of chemically bound energy is released. This energy heats the products and combustion of a fuel such as methane (the main component of natural gas) with air leads to a flame temperature of around 1900 ºC. In the combustion of methane and air, the main products are carbon dioxide and water. However, the formation of these products is very complex and hundreds of different species, such as H, O, OH, CH, CH3, etc., are intermediates in the combustion process. Besides carbon dioxide and water, a large number of pollutants are formed as, for instance, nitrogen oxides, carbon monoxide and soot particles. The efficiency and cleanliness of a combustion process depend on several parameters, such as oxygen supply, temperature history, and mixing properties.

In the following text, two very common flames are described.

The candle flame

The candle flame bring us in a good mood, but do you know what happens in the burning of a candle flame? The solid fuel (stearin) melts and is transported up along the wick through a capillary force. The stearin is vaporized and the fuel is first thermally dissociated to smaller fuel fragments in the transport upwards the flame. Part of the fuel fragments build up three-dimensional solid structures of carbon, i.e. soot particles. They thermally radiate yellow light, the so-called Planck radiation. The combustion of fuel and soot occur on a surface outside the yellow part of the flame. This type of combustion where fuel enters from one side and the oxidant from the opposite side is called a diffusion flame.

The Bunsen flame

Maybe you remember the Bunsen flame from the chemistry lectures at school where it was used to heat chemical compounds. A Bunsen flame is an example of a premixed flame. The cold mixture of fuel and air flows up the pipe, and is mainly combusted in the inner blue cone of the flame. The combustion is fuel-rich and leads to incomplete combustion. It maens that the final burning of carbon monoxide and hydrogen occur in an outer blue diffusion flame.