Head of Technical Department, Peter H. Trane, and Application Manager, Niels I. Jacobsen, Skamol A/S Denmark, present new test results proving that the diatomite insulating brick, M-EXTRA, is applicable as insulating layer in rotary kilns with diameters above 4 m.
Skamol A/S has extensive experience with production and development of back up insulation materials for rotary kilns within the drying and calcination industries - especially MESA kilns. In an endeavor to get a better understanding of the forces in an operating rotary kiln, Skamol has teamed up with an independent test institute to examine the behavior of M-EXTRA during operation. The scope included finite analysis modeling and extensive material testing.
Apart from the more obvious advantages of saving energy and reducing the CO2 outlet, there are many other good reasons for insulating rotary kilns. Experience has shown that when a kiln is insulated with M-EXTRA the refractory layer has a longer service life. There are several reasons for this. Firstly, as this article will show, the M-EXTRA is a unique back up insulator that will absorb some of the forces in the lining that would normally stress the refractory brick. As the rotary kiln rotates there are many forces influencing the refractory lining, such as thermal expansion and shell deformation, to name a few. By adding a layer between the refractory and the shell that can absorb some of these dynamic forces, the condition of the refractory can be improved.
With M-EXTRA as insulation, refractory bricks will have a lower thermal load and, therefore, less internal stress due to the difference in thermal expansion through the brick.
The reduced stress in the refractory will result in less wear as it will be less prone to chemical attacks, which again can lead to spalling. Spalling in a refractory brick can occur in a position where the refractory brick is under high stress. The physical properties of M-EXTRA ensures that these bricks will absorb some of the stress in the lining and relieve the refractory brick.
Double layer linings with the M-EXTRA brick has been installed successfully in smaller diameter kilns (< Ø 4 m.) for several years whereas the use of this brick for kilns with larger diameters has previously been limited. These limits were determined by experiences and assumptions of the forces affecting the insulation layer in a rotary kiln. M-EXTRA has been installed in larger diameter rotary kilns with positive results, but in the past there has been no test results supporting these good installation results from larger diameter rotary kilns.
New test results combined with modeling of the rotary kiln lining, however, reveals that the diameter is no longer an obstacle for dual lining installation. This opens up for an array of new possibilities within the wet cement industry where rotary kiln diameters are often found to be 4 m (13.1 ft.) and above.
A rotating kiln is a very complex unit that undergoes consecutive changes of stresses along every cycle. The use of M-EXTRA insulation bricks as back up lining for rotary kilns evidently relies on the material’s ability to withstand the exposure to these stresses, which is created by the movement of the kiln. A finite analysis of the refractory lining in a 5 m. (16.4 ft.) kiln with a deformation of 0.5 % showed that the forces in the boundary between the refractory layer and the layer of back up insulation with M-EXTRA were between 0.5 and 1.2 MPa, depending on the position in the rotation.
As the kiln rotates past a roller, the steel shell is compressed inwards, while the thermal expansion of the refractory is pressing outwards. These combined forces will stress the M-EXTRA at those points. The finite analysis determined that the maximum force in the M-EXTRA brick, near the roller, is 0.5 MPa under maximum deformation at recommended maximum service temperature. During the service life of a normal lining, insulated with M-EXTRA in the inlet-zone of a cement kiln, the kiln will rotate 2.5 million times. In the inlet zone of a wet process kiln, the M-EXTRA back up lining will sustain approximately 5 million impacts at any point in the lining during the service life.
The effects of this strain on the lining was examined by compressing the M-EXTRA samples at different temperatures at maximum calculated force in a cyclic environment. The below figure shows the result of these tests. Tests were carried out with samples that were heated through and kept at a constant temperature throughout the test.
At 750° C the curve stabilises after 5000 load cycles, hereafter the creep is considered stable (a creep of 0,5 % corresponds to 0.32 mm). In the actual conditions in a rotary kiln the creep will be significantly less due to the temperature gradient in the M-EXTRA as seen in figure 2.
At 900° C the curve stabilises at a higher level of creep, which suggests that this is near the maximum service temperature in a dynamic environment. The stress/strain test (see figure 1) supports these findings. Skamol, therefore, recommends that the expected wear of the refractory lining should be taken into consideration when designing the lining.
The series of tests included ‘Refractoriness Under Load’ (see figure 3), which also concluded that the optimised temperature in the boundary layer between the refractory and the M-EXTRA is 900° C. At this temperature, the M-EXTRA will start to shrink. This shrinkage is however less than 10 % of the thermal expansion of the hot face refractory brick which means that the dual layer linings’ integrity is not compromised by the shrinkage of the M-EXTRA when it has reached its max. service temperature. When the maximum service temperature for M-EXTRA is reached, a relining should be considered at the next planned outage.
Depending on the hot face refractory type, M-EXTRA can be installed in the kiln zones where the operation temperature is below 1200° C. With an operating temperature of 1200° C the refractory brick will normally be worn to 50 % or less of the original thickness before the M-EXTRA reaches its max service temperature.
Correct installation is essential in order to achieve the maximum benefits of a dual layer lining design. The insulating bricks must be installed in stretcher bond in order to prevent the distortion of the lining and chipping of the corners of the M-EXTRA, which can lead to lining failure. Installation in stretcher bond prevents the buildup of static tension in the insulating layer under start up and operation of the kiln.
During installation, the bricks must also be fitted to the surface, laid in the mortar applied at the shell. It is important that all voids between the shell and the insulating bricks are filled with mortar, as the bricks will otherwise break when stress is applied - either by closing the ring or during reheating. At larger deformations and at kiln weldings, a castable patch can be used instead.
The installation of M-EXTRA also gives a better surface for the installation of the hot face refractory making the installation easier and giving a better integrity in the lining.
With increasing energy costs and a high focus on carbon dioxide emissions, energy savings are a natural priority for any high energy industry. By using proper insulation, the external heat loss is minimised and huge energy savings can be achieved. It is not uncommon to reduce the external heat loss by 50 % in the areas where the kiln is insulated.
The reduced energy loss in kilns not only affects energy consumption, but also provides for a better control of the kiln temperature under adverse weather conditions e.g. in heavy rain where the insulating layer will prevent excessive cooling of the kiln. This affects the quality of the product being processed in the kiln as it becomes possible to maintain a stable process temperature.
Another one of the added benefits, of lowering the temperature of the shell, is that it makes the shell more resistant to ovality due to higher structural integrity.
The modelling and tests have provided a better understanding of the stresses in a refractory lining. The modelling proved that the radial stresses are significant and is a determining factor when choosing refractory lining. From the results of the tests, it can be concluded that installation of M-EXTRA is applicable in rotary kilns regardless of their diameter. The creep of the M-EXTRA ensures a longer service life of the refractory lining by absorbing stress from the hot face refractory and giving a better alignment for the refractory during installation. M-EXTRA lowers the shell temperature, thereby ensuring a better structural integrity of the shell. Significant energy savings can be found when insulating a wet process cement kiln, which makes the payback time on the investment very short and worthy of consideration.
Skamol have a series of other insulating products for the cement industry. Including calcium silicate boards for insulation in preheater towers and air ducts. This solution is widespread and Skamol have many years of experience with good results.
Skamol also carries a series of acid resisting bricks with excellent insulating capabilities. Especially in stage 1 and 2 of the preheater where acid attacks on the shell is a problem. A lining design where the condensation point of acid gasses is in the acid restraint brick, i.e. POROS 500, is an excellent solution for protection against acid attacks.