Numerical studies of mid-kiln tyre combustion

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 NO_x 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 NO_x 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 NO_x emissions. When the tyre replacement is 40% or more, NO_x 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.