Self-Ignition of Combustible Bulk Materials Under Various Ambient Conditions

It has been known for a long time that combustible bulk materials may undergo selfignition if stored in sufficient amounts at adequate ambient temperatures. Most of the studies according to this problem refer to atmospheric ambient conditions (oxygen volume fraction of 21%). In some technical applications, however, bulk materials are processed or stored at a reduced level of oxygen concentration. Besides the oxygen volume fraction, the moisture content of the bulk material itself as well as the humidity of the ambient air effect self-ignition. The first part of the current paper presents experimental investigations on the influence of the volume fraction of oxygen on the self-ignition temperature of the bulk materials. The lower the oxygen volume fraction in the surrounding gas, the higher is the self-ignition temperature as a general trend. In the second part, tests under a normal atmosphere were carried out to investigate the influence of water in a liquid and vapour state on the self-ignition procedure of lignite coal. They showed that sub-critical deposits could become super-critical by pouring water into the bulk or by exposing a dry bulk sample to humid air. Besides the experiments, a numerical model was established which allows to compute the process of heating and self-ignition in bulk deposits. Technical applications of the model cover safe storage of dusts, granulate, bulk materials and wastes.