Shaft rotary kiln
The theoretical heat (the standard enthalpy) of reaction required to make high-calcium lime is around 3.15 MJ per kg of lime, so the batch kilns were only around 20% efficient. The key to development in efficiency was the invention of continuous kilns, avoiding the wasteful heat-up and cool-down cycles of the batch kilns. The first were simple shaft kilns, similar in construction to blast furnaces. These are counter-current shaft kilns. Modern variants include regenerative and annular kilns. Output is usually in the range 100-500 tons per day. Counter-current shaft kilns
The fuel is injected part-way up the shaft, producing maximum temperature at this point. The fresh feed fed in at the top is first dried then heated to 800°C, where de-carbonation begins, and proceeds progressively faster as the temperature rises. Below the burner, the hot lime transfers heat to, and are cooled by, the combustion air. A mechanical grate withdraws the lime at the bottom. A fan draws the gases through the kiln, and the level in the kiln is kept constant by adding feed through an airlock. The degree of burning can be adjusted by changing the rate of withdrawal of lime. Heat consumption as low as 4 MJ/kg is possible, but 4,5 to 5 MJ/kg is more typical. Due to temperature peak at the burners up to 1200c°in a shaft kiln conditions are ideal to produce medium and hard burned lime. Regenerative kilns
These typically consist of a pair of shafts, operated alternately. Fitst, when shaft A is the "primary" and B the "secondary" shaft, the combustion air is added from the top of shaft A, while fuel somewhat below via burner lances. The flame is top-button. The hot gases pass downward, cross to shaft B via the so called "channel" and pass upward to exhaust of shaft B. At same time in both shafts cooling air is added from the button to cool the lime and to make exhaust of gases via the bottom of the kiln impossible via maintaining always a positive pressure. The combustion...
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