A simplified mathematical model is proposed to determine the agglomeration tendency of bitumen-coated coke particles in fluid cokers. The model calculates a theoretical critical velocity that depends on key parameters such as the particle size, bitumen viscosity, and bitumen thickness; it also accounts for the temperature- and reaction-dependent variations of the bitumen thickness and viscosity. A peak theoretical critical velocity at the intermediate reaction times for all coking temperatures is predicted. By comparing this peak critical velocity with the estimated inter-particle collision velocities within an industrial-scale reactor, the agglomeration tendency of coke particles is determined within fluid cokers. The results show that at low temperature regions (T=400 °C), there is no agglomeration tendency; however, at high coking temperatures (T=503 and 530 °C), substantial agglomeration tendency is expected. It is also found that the number of coke particles constituting an agglomerate could be as high as a few hundreds.
A one-dimensional mathematical model of tyre combustion is developed and incorporated in a computational fluid dynamics (CFD) code with the objective of simulating industrial-scale cement kilns and studying the midkiln firing of whole tyres. The simulation without tyre combustion shows a high flame temperature, high NOx emission, and good quality product. Replacing 20% of the total kiln heat with tyres as fuel and dropping them in to the kiln at 50 m from the burner tip results in a reduced flame temperature, a 33% reduction of NOx emission and good clinker quality. Further investigations show that up to 30% replacement of fuel with tyres is acceptable to maintain the clinker quality with reduced NOx emissions. When the tyre replacement is 40% or more, NOx emission can be further reduced, but the clinker quality becomes poor. The tyre drop-in location is another critical parameter. For the kiln studied tyres can be dropped in within 50 m of the burner tip. Further downstream dropping-in reduces kiln efficiency and clinker quality. The actual replacement of tyre combustion and the drop-in location depends on the kiln design and operating conditions. The developed comprehensive simulation tool can be used to reach an optimised kiln design under a specified operating condition.
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