Rotary Kilns Operate 24/7 and are Stopped only for a Few Days for Maintenance
We performed a failure investigation on a rotary cement kiln after an accident which occurred during startup. Problems with the conveyor system feeding the kiln had occurred, and the kiln heatup was suspended. One day later, heatup resumed, but hot spots were observed on the kiln shell, indicating problems with the refractory lining.
Cement kilns are for the pyroprocessing stage of manufacturing various types of cement, which calcium carbonate reacts with silica bearing minerals to form a mixture of calcium silicates. A typical process of manufacture consists of three stages:
• Grinding a mixture of limestone and clay or shale to make a fine “rawmix”.
• Heating the rawmix to sintering temperature in a cement kiln.
• Grinding the resulting clinker to make cement.
A rotary cement kiln consists of a slightly inclined tube made from steel plate and lined with refractory brick. Refractories are special materials designed to withstand high temperatures. They are used to protect the walls and support structure of a vessel or chamber from very high temperatures. The refractory is used to insulate the steel structure, which becomes soft and weak above 800 C.
Rotary kiln tubes are sloped slightly (between 1 and 4 degrees) and slowly rotate between 30 and 250 revolutions per hour. Raw mix is fed in at the upper end, and the rotation of the kiln causes it to gradually move downhill to the other end of the kiln.
At the other end fuel – in the form of gas, oil, or pulverized solid fuel – is blown in through the “burner pipe”, producing a large concentric flame in the lower part of the kiln tube. As material moves under the flame, it reaches its peak temperature, before dropping out of the kiln tube into the cooler.
Air is drawn first through the cooler and then through the kiln for combustion of the fuel. In the cooler the air is heated by the cooling clinker, so that it may be 400 to 800 °C before it enters the kiln, thus causing intense and rapid combustion of the fuel.
Rotary kilns run 24/7 and are stopped only for a few days for essential maintenance. This is important because of heatup and cool down thermal transients. The process of starting a rotary kiln is complex – thermal differences between components must be carefully controlled to avoid the introduction of high thermally-induced stresses that can cause damage. A carefully orchestrated process of heating, kiln turning, and introducing feed to the kiln is necessary to bring the unit back into a full production mode.
There are three distinct liner zones in a kiln, the Upper Transition Zone (UTZ), the Burning Zone (BZ) and the Lower Transition Zone (LTZ). Each zone has different environmental conditions and as consequence uses different type bricks.
The process of starting a rotary kiln is more complicated than simply “turning it on.” Thermal differences between components and within components must be carefully controlled to avoid the introduction of high thermally-induced stresses that can cause damage. A carefully orchestrated process of heating, kiln turning, and introducing feed to the kiln is necessary to bring the unit back into a full production mode.
More can be learned about rotary kilns at ScienceDirect.
O’Donnell Consulting Performs Failure Analysis Services on Various Types of Equipment including Vessels, Heat Exchangers, Cyclones, Conveyors and Kilns.
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