Propagation of Coal Dust Explosions in Pipes

This paper discusses the possibilities of pulverized coal explosions in pneumatic conveying. To characterize the coal dust explosions, we measured, at different points on rectilinear pipes, possibly connected to a 1-m3 vessel, explosion overpressures, Pmax, and flame speeds. Trials in an isolated, 250-mm-diameter, 20-m-long pipeline and ignition near a closed end showed the difficulty of the flame propagation that did not surpass 10 m with Pmax values at the maximum equal to 0.6 bar (60 kPa) for a one-end-open duct and 0.95 bar (95 kPa) for a closed duct. On the other hand, with the pipeline connected to a 1-m3 vessel, with ignition in the vessel, the Pmax values inside the pipe can reach 10 bar (1000 kPa) and flame speeds 1500 to 2000 m/s. When exchanging the 250-mm-diameter pipe for a 440-mm one, such a character is not obtained and flame speeds are 600 m/s at the maximum with overpressures around 2.5 bar (250 kPa). If the open end, 250-mm-diameter pipe connected to a 1-m3 vessel is only 10 m long, overpressures inside remain under that obtained in the vessel, that is, around 4 bar (400 kPa). With a 40-m-long pipe, the high speed flames seem unable to be sustained. In the 1-m3 vessel and 150-mm-diameter open end pipe system, the overpressures in the pipe have similar values to that obtained in the vessel (Pmax: 5 bar [500 kPa]) and the flame propagates up to 20 m. When the pipe is 40 m long, Pmax values remain the same, but the flame stops generally after 20 m. With closed end, 150- and 250-mm pipe connected to a 1-m3 vessel, flames cannot propagate the full length of the duct, but explosion overpressures may be 10 to 20 bar (1000 to 2000 kPa). For low concentration it has been noticed, outside the duct, that the concentration is heterogeneous but shows periodical figures; when the flame appears the same periodicity is found again. This may be due to the geometry of the studied system. All these trials demonstrate the difficulty of a pulverized coal explosion in a pipe with a diameter less than or equal to 250 mm and the influence of the dust cloud local concentration.