The mathematical model of the coal grinding process in a pneumatic transport mill
Recently, a tendency toward constructing machines for grinding materials by high-speed impact has emerged. This method is applied in the jet and pneumatic mills used in coal powder processing systems at thermal power plants. The article presents design versions of the developed pneumatic transport mills that make it possible to combine grinding and pneumatic transportation of friable materials. The operating principle of such devices is based on accelerating the particles by air stream, which are then collided against the baffles installed in the pneumatic transport pipeline elbows. The grain grinding degree is determined by the stream velocity and the number of pipe elbows. The study results have demonstrated full workability of the proposed devices, a significant decrease in the grinding process energy intensity, and sufficient throughput capacity at the necessary grinding fineness. The aim of theoretical investigations was to reveal coal grinding regularities during the collision of particles against the baffle elements installed in the transport pipeline. A mathematical model has been developed, which establishes the dependence between the increase of coal particle surface area at the impact against a hard obstacle and the main influencing factors: the particle size and the motion velocity at the moment of impact, the incidence angle, and the physical and mechanical properties of coal. The influence of additional factors caused by the motion of particles in the air stream and their size distribution on the increase of specific surface area was taken into account by the destruction work coefficient. The obtained results make it possible to estimate the influence of basic design and technological parameters of pneumatic transport mills on the increase of specific surface area, which characterizes the size distribution of the grinded material.