The ignition of clouds of particles in shock-heated oxygen

Abstract

The ignition of clouds of coal particles in shock-heated oxygen has been studied. The requisite gas temperatures and pressures for ignition have been measured and have been related to particle ignition temperatures which are dependent on the volatile content of the coal and are in close agreement with the temperatures at which the particles lose volatile matter at an appreciable rate. Ignition delay times for various coals of different size ranges have been measured at oxygen pressures of 1.5 to 30 atm and temperatures of 700 to 1600°K. The experimental results indicate that the influence on the delays of the radiant heat transfer from a previously established flame at the shock-reflecting face is small. Some evidence that ignition is a surface catalysed process is presented. A mechanism for the ignition process is proposed. This relates the ignition delays with the times required to heat the particles solely by conduction to the appropriate particle ignition temperature. This theory is shown to describe the experimental delays satisfactorily. In testing it, similar experiments on completely volatile particles (anthracene) and non-volatile particles (graphite) have been carried out. Ignition of anthracene occurs when the particles approach their boiling point. Ignition of small graphite particles is a more complex process in which the first stage is the heating of the particles by conduction to a temperature just below that of the shock-heated gas. This is followed by a period in which heating due to chemical reaction overtakes heating by conduction.