David Datukashvili and Pavel Lebedev, Skamol Rus LLC, and Leo F. Juhl and Peter H. Trane, Skamol A/S, Denmark, discuss the benefits of rotary kiln insulation.
There are several basic reasons why insulation for rotary kilns, such as those used in cement and lime production, should be considered. These include
The need for heavy duty refractory hot face lining in a kiln is clear. Any effective form of insulation should therefore serve as a backup between the dense refractory, which suffers from wear and the outer steel shell.
The insulation brick should provide high quality insulation and sufficient mechanical strength in order to withstand the ardous conditions in the rotary kiln. Thermal expansion of refractory and insulating bricks will create tremendous internal stresses on each layer of the lining, particularly in large diameter kilns.
Products such as Skamol's M-EXTRA, a grade of diatomaceous insulating brick, are considered suitable for withstanding such conditions. This brick has a high mechanical strength, cold crushing strength of 18 MPa and a high hot crushing strength of 40 MPa at 900° C as well as a good thermal conductivity of 0.24 W/(m x K) at temperatures of 400° C.
The rising cost of fuel, coupled with higher temperatures in kilns to ensure greater process efficiency, has made the need for fuel economy increasingly apparent. It is possible to calculate the percentage of fuel savings that can be achieved if full particulars of the kiln design is made available.
Leo Juhl and Peter Trane, Skamol, carried out an analysis of a 3.6 m. dia. x 75 m rotary kiln with a calcined kaolin capacity of 240.000 tpa. The following results were found:
Conversely, an increase in output from the kiln can be obtained at the same fuel consumption by incorporating an insulation back-up brick.
Incorporating an M-EXTRA backup brick to protect the shell life of the rotary kiln can prolong the shell's lifetime, as well as protecting the bearings and movings parts of the roller gear. In particular, it prevents distortion of the shell by protecting it from refractory heat thereby maintaining the efficiency of the driving mechanism. In an uninsulated kiln, the shell distorts due to overheating and, as a result, the drive rollers do not remain in full contact with the shell. This causes a frictional loss, which is detrimental to the whole kiln mechanism.
If the kiln is rotating clockwise, the minerale or clinker passing down the kiln towards the burning zone tend to mass at approximately the 8 o'clock point and at 4 o'clock if the kiln is rotatring anti-clockwise. This mass tends to circulate around its own centre, so that the core of the clinker does not come into direct contact with the heat passing down from the burning zone. For this reason, it is desireable to have a build-up of heat in the refractory lining. When the minerale or clinker are resting on the refractory, this would conduct heat to the core, which helps the heat to penetrate throughout the minerals or clinker as they pass down the kiln. The use of backup insulation keeps the mean temperature of the refractory at a higher level, considerably improving the calcination and heating process.
The ractice of using diatomaceous insulating bricks as backup insulation in rotary kilns is not new; in fact, it has been in practice for more that 80 years. Previously, ROTOL bricks were also installed as backup bricks. However, with a CCs of 10 MPa, these were too weak to take the stress in kilns with a diameter of more that 2 m. This problem led to the development of the diatomaceous brick known as M-EXTRA, which, as mentioned, has a hot crushing strength of 40 MPa at 900° C.
Once the insulating brick has been installed against the shell, it should last for many years, providing that due care is taken to ensure that the hot face lining is not permitted to wear too thin, thereby exposing the insulation. Skamol has installed Moler bricks as backup insulation layer in several rotary kilns. The oldest dates back to 1927, when two lime kilns (3.4 mdia x 100m) were provided with diatomaceous bricks as backup insulation. When the kilns were dismantled in 1960, the bricks were still in position and virtually undamaged. A more recent example is the installation of the M-EXTRA brick in a 4.75 m dia. x 135 m. kiln at Wisaforest Paper Mill, Pietarsaari, Finland, in 2005. Although the brick is not recommended for use in kilns with a diameter above 4 m., no maintenance of the insulating bricks has been required yet.
In the past, some installations involved springers in refractory material in the 12 o'clock, 3 o'clock, 6 o'clock and 9 o'clock positions, which go back to the shell itself. Experience, however, has shown that this can lead to considerable heat loss through the springers at the four points. This causes heat to reach the kiln shell and, over a period of time, to conduct heat along the kiln behing the insulation, thereby causing stresses in the brickwork. Instead, a continuous lining of insulation across the whole diameter and length of the kiln is recommended.
Using mortar between lining and the M-EXTRA lining should be avoided. If there is mortar joint between two materials the the insulation will incur damage when the refractory lining is due to be replaced. This may necessitate a complete reline back to the shell before the kiln begins operating again, which is both costly and time consuming. By discontinuing the previous practice of having springers going back to the shell, it is now possible to completely renew the refractory lining without disturbing the backup insulation.
Finally, it is important to ensure that the refractory contractor and client are fully aware of the techniques to be used when laying this type of lining. If there is any doubt, the it is advisable to seek advice from the insulation manufacturer's engineers prior to installation.