Abstract
The modern semi-wet Lepol process of Portland Cement manufacture relies on a filter pressed, 13-15% moisture, mix of clay/shale plus chalk/limestone, being first dried and then partially calcined on a travelling grate. The cement clinkering reaction then takes place in a shortened version of a conventional rotary kiln. A significant improvement in both process efficiency and clinker output is envisaged if a larger proportion of the calcination reaction could be removed from the kiln and transferred to the grate section. This process alteration, of course, needs extra heat and this can be supplied by some form of fuel injection to the grate chamber. Such a process modification is known as pre-calcining. The research described in this thesis involves the establishment of the fundamental aerodynamic mixing patterns which occur in real Lepol kilns, through the aid of a 1:24th scale Perspex replica, backed-up by "cold" trials on a working prototype. These results were then used to develop a compatible coal fired burner system, which would deliver approximately 30% of the total process heat requirement, direct to the grate calcination chamber. The flow regimes, exhibited in the model by polystyrene beads, suggested that approximately 20% of Lepol grate area was not being utilised for heat transfer. Areas of slow moving gases were observed, their position coinciding with areas of alkali dust build up in the real kiln. These aerodynamic flow maldistributions were, effectively, cured by the installation of under grate baffles in the kiln model. The modification of the Lepol grate system to accommodate auxiliary pre-calcination firing proved to be a far more intractable problem. The high momentum flows required to ensure adequate burn out for conventional coal burners only succeeded in exasperating the existing aerodynamic problems. Oil or gas burners would have produced better results but would have been quite uneconomic, excepting only that very cheap waste fuels might have tipped the balance. The ultimate solution to the problem proved to be a vortex combustion chamber, fitted over the grate, as a source of clean, low momentum hot gases.