Effect of Feed Rate During Comminution of Coal by High Energy Waterjet

Abstract

High pressure waterjets attack, fragment and comminute coal. The purpose of this research is to determine the effect of selected waterjet parameters on the specific energy required to create particles less than 75 μm in size. The selected parameters are waterjet pressure, nozzle diameter, relative velocity between moving coal sample and stationary waterjet nozzle and nozzle standoff distance, defined as the distance between nozzle and target. This information is necessary as a starting point for the design of coal comminution equipment based on waterjet technology. Over 980 individual specimens of bituminous coal from Moberly, Missouri USA, were subjected to controlled attack by waterjet. Trials were executed with waterjet pressures up to 100 MPa, and with nozzle standoff distances between 0.80- to 44.5 × 10⁻³ m. Nozzle diameters of 1.14-, 0.81 -, and 0.41 × 10⁻³ m were used, and coal sample feedrates varied up to 0.127ms⁻¹ relative to a stationary nozzle. The most important parameter was found to be feed rate. At feedrates less than 0.254ms⁻¹, specific energy is quite high, more than 200kWg⁻¹ s⁻¹, in some trials. At feedrates of 0.51 ms⁻¹ or more, the specific energies are typically in the range of 20-50 kWg⁻¹s⁻¹. These results are attributed to stagnation pressure within existing fractures in the coal. At high feedrates, the jet plays on the surface of the coal for a short time, thereby permitting only shallow fractures – to form. Low feedrates permit deep cutting of the coal and much larger particles thereby form, consuming much more energy.

Key words:

• Comminution
• coal particles
• high energy waterjet