Studies have been carried out on the explosions of coal dust-methane-air mixtures of relatively high dustconcentration, using an apparatus developed by authors, which can generate sufficiently uniform dust clouds in therequired range of dust concentration. Coals used are listed in Table 1.
The most characteristic phenomenon observed is that explosion occurs at higher dust concentrations thanthe upper explosion limit of a coal, while explosion never occurs at a higher methane concentration than the upper explosion limit of methane. Thus the explosion region forms a peninsula in the figure. A typical explosion limit curve isshown in Fig. 2. Effect of particle size, on the upper explosion limit of the mixture is large as shown in Fig. 3.
Figure 4 shows the curves of explosion limit for Horonai, Russel Fork and K-10 coals of 270-400 mesh insize. In the explosiye region of methane (5 to 15 pct), there is no considerable difference between them, whiledifference is found in the region of methane concentration less than 5 pct.
As an extreme case where volatile content is near 100 pct, Fig. 5 shows the upper explosion limit curve ofpotato starchmethane-air mixture. In this case the peninsula is not formed. This may suggest that further increase ofvolatile content of coals could cause the disappearance of the peninsula. This limit volatile content was estimated using theequation in the text, which was obtained from experimental results shown in Fig. 6, and ca. 60 pct volatile content was obtainedfor 270-400 mesh coal. As another extreme case, explosion limits of stone dust-methane-air mixtures are shown in Fig. 7.