A coal dust explosion occurs when, by some means, a combustible concentration of coal dust is lifted into mine roadways and ignited.
For example, the ignition of a small quantity of methane gas generates a pressure wave which lifts coal dust that has accumulated on the roadway surfaces into the air. The associated methane flame ignites the coal dust.
Without some means of limiting the reaction, coal dust will continue to lift and burn and creating an even larger pressure wave, and a coal dust explosion propagates through the mine.
This behaviour can be seen in the images below. The explosion was staged in a surface gallery about 150m long. There is an initial methane explosion which lifts coal dust into the air which is ignited (frame 3) The ensuing explosion propels dust into the air and out of the tunnel mouth and ignites the airborne coal dust. The coal dust burns and drives a pressure wave ahead of itself causing more dust to be ejected from the tunnel and eventually burning all the way out of the gallery.