Lime kilns modern

Modern lime kilns operate at approximately 1,200°C to 1,300°C. Limestone, which has been crushed and screened into pieces with diameters of several inches, is fed into the top of the kiln. Air fed into the kilns bottom fluidizes the limestone, allowing for greater reaction... 

Except for very old mixed-feed vertical kilns, time kilns operate with countercurrent How of raw material anil heal. Modern lime kilns utilize coolers to preheat air by recuperating heal from the hot quicklime, Lime kilns may be tired directly with coal, oil, or gas. 

In practice, however, the achieved heat requirement can be much lower than the above value and modern lime kilns achieve net heat usages (see section 14.3.6) below 900 kcal/kg CaO. The above calculation makes two major omissions. 

Some modern designs of lime kiln have reduced this imbalance by transferring heat from the exhaust gases to the combustion air. 

Short dry kilns are usually accompanied by an external preheater or pre-calciner (Figure 1.7) in which the feed is dried, preheated, or even partially calcined prior to entering the main reactor (kiln). As a result the thermal load on the kiln proper is reduced. Hence kilns equipped with preheaters or precalciners tend to be short, on the order of 15-75 m (about 50-250 ft) depending on the process. The shorter kilns are those in which the entering feed material is almost calcined. Applications include cement and some lime kilns. Because of the large feed particle size encountered in limestone calcination, modern lime kilns are equipped with preheaters which function as a packed bed of stone with a countercurrent flow of kiln exhaust gas rather than the typical cyclone preheaters in cement kiln systems. 

Operation with a limited excess of air results in the emission of some volatiles as dark smoke. This, coupled with the relatively low reactivity of the lime has caused many oil-fired shaft kilns to be replaced by modern designs. 

The fuels used in the various designs of modern shaft kilns currently available (see section 16.8) are selected to produce medium to light-burned lime, at high thermal efficiencies, without the emission of dark smoke. All designs can use natural gas, most can also use fuel oil and some can operate on pulverised solid fuel. 

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