Rotary kilns are some of the largest pieces of equipment found in industry. Their resilience and versatility makes them suitable for a wide variety of applications, such as the calcination of lime and alumina, reduction of iron ore, cement clinkering and solid waste incineration. Common to all of their uses is the need to thermally process a material, typically particulate, by directly firing a fossil fuel. A kiln consists of a refractory lined cylindrical furnace with its axis at a slight

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... to the horizontal. Raw material is fed at the upper end and, by virtue of the kiln's rotation, it travels along its length. The burner is located at the lower end. As such, the kiln is a counter current heat exchanger, with hot combustion gases flowing upwards over a bed of particulate material traveling down. To ensure efficient and quality processing, all of the material in a kiln needs to be heated up to temperature, without any of it overheating. This requires efficient heat transfer from the combustion gases to the bed and efficient heat transfer within the bed, to ensure all of the material is evenly heated. However, this is not always achieved. For example, it has been found in industry that fine material often remains unreacted. The reason has been attributed to the phenomenon of segregation, whereby the fines collect towards the center of the bed and so are shielded from the hot combustion gases. Experiments on laboratory kilns have confirmed that segregation in the cross section of a kiln occurs rapidly. Within a few revolutions, the fines collect in the middle of the bed forming a core, often referred to as a "kidney". Since a kiln typically rotates at 1-2 rpm and material residence time is 1-4 hours, a bed that contains fines will be segregated for almost all of its time in the kiln. The fines will not be exposed to the hot gases and so may not react. On the other hand, a bed that contains mono-sized particles has been found to intimately mix. In such a bed all of the particles have an eq [...]