Measuring hot-surface minimum ignition temperatures of dust clouds - History, present, future

Hot surfaces capable of igniting dust clouds can exist in various situations in industry. Typical locations are inside furnaces, burners and dryers of various kinds. Hot surfaces can also arise accidentally e.g. by frictional overheating of bearings and other mechanical parts, and electrically. To avoid accidental ignition of dust clouds by hot surfaces, it is clearly essential to know the minimum temperature of any such surface, at which an explosible dust cloud making contact with the surface, will ignite. The apparatus that is currently used in most countries for this purpose is the Godbert-Greenwald (G-G) furnace. Two pioneering forerunners of the 1935 G-G furnace were developed by M. J. Taffanel and A. Dun (1911) in France, and subsequently by R.V. Wheeler (1913) in UK. However, none of the two methods were intended for assessing maximum permissible temperatures of hot surfaces in industrial plant to prevent hot surface ignition of dust clouds. The present paper reviews some standardized laboratory-scale apparatuses currently used for this purpose. However, when applying data from such tests, one has to use a safety factor. Current regulations in several countries require that maximum temperatures of hot surfaces in industry that may come in contact with explosible dust clouds, shall not exceed 2/3 of the MIT in degrees C of the actual dust in air, as measured by one of the current standard methods. It seems unlikely that a more sensible approach will become available in the near future. MIT of a cloud of a given combustible dust in air is not a physical constant. It varies both with dust concentration, turbulence and systematic flow of the dust cloud, and size and shape of the hot surface. Experimental and theoretical evidence shows that MIT decreases with increasing hot-surface size or increasing furnace diameter. Experiments have also shown that MIT increases with increasing dust cloud velocity past relatively small hot surfaces. However, some data indicate that increased dust cloud turbulence can lower MIT. In the literature the two terms minimum ignition temperature (MIT) and auto-ignition temperature (AIT) are sometimes used interchangeably. A possible difference between the two is discussed in the paper with reference to experimental data from hot-surface ignition of explosive gas mixtures. Development of numerical computer models for simulating ignition of dust clouds by hot surfaces is assumed to make further progress. It nevertheless seems unlikely that such models will be widely used in the near future for specifying maximum permissible temperatures of surfaces in industry. One obstacle to general practical use of numerical models seems to be identification of the credible worst-case scenario that can represent a given practical industrial ignition situation. Also, extensive use of sophisticated simulation models will probably be limited by unfavourable cost/benefit considerations.