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Show For the trials related in the present paper the following initial parameters have been varied : grid temperature, oxygen concentration, particle size . Coals of different rank as well as there chars obtained by rapid devolatilization have been compared from the point of view of ignition and oxidation behaviour. The physical and chemical characteristics of coals and chars treated in the present paper are listed on table I . 2. DISCUSSION OF THE EXPERIMENTAL RESULTS 2.1. The competition between pyrolysis and ignition. Figure 2 shows a typical series of CO and CO measurements obtained at increasing grid temperatures on a 200/250 microns lignite coal in air. At temperatures below the ignition temperature (T. = 750 K) one observes a slow release of CO and CO which is partly due to pyrolysis (the amounts of CO and CO released by pyrolysis under nitrogen flow are indicated by the dash-dotted curves at 725 K), partly also to slow oxidation of other pyrolysis products as well as to slow oxidation of the solid. We shall refer to this small CO and CO peak as "volatiles "oxidation peak" , since it marks the onset of CO and CO pyrolysis and of volatiles oxidation in the gas phase. At temperatures equal to or larger than the ignition temperature, a further, sharp rising CO and CO peak accompanishes ignition of the solid matrix (IG = ignition). The ignition delay becoming shorter when the grid temperature is increased, the ignition peaks shift progressively closer to the volatiles oxidation peak (V. O.) and finally both peaks coincide. When this happens, the devolatilization rate accelerates substantially, since, due to ignition and subsequent fast combustion of the solid matrix, the solid temperature T exceeds now by several hundreds of degrees the grid temperature (see T (time) curve on figure 3). For the same reason the volatiles oxidation in the gas phase may become very fast and bear an explosive character ; in this case, when the volatiles concentration reaches the lower flammability limit, a 7-6 |
