Increasing requirements relating to energy consumption, productivity, emissions and operational costs have led to a range of ways to optimise cement production. Here KIMA Echtzeitsysteme describes its KilnCooler system, which uses water to reduce the impact of hot spots on cement rotary kilns. When we speak about putting water on an, admittedly hot, rotary kiln, many people have concerns. These should be thrown overboard. A rainstorm will bring far more water onto the kiln than the system described below.
As the margins of cement producers become more squeezed, especially in developed markets, there is increased pressure on cement plant kilns. The dark side of this drive towards operational optimisation is an increased need for maintenance. Over the year maintenance teams have to keep the kiln running from one planned stoppage to the next, without unplanned halts.
In this context one parameter is becoming particularly difficult: keeping the kiln shell temperature under control and avoiding uneven temperature profiles / hot spots when looking at the entire circumference of a kiln section. In part due to the introduction of alternative fuels, kiln temperatures now vary more rapidly and with less predictability than in the past. Formerly less critical areas of the kiln shell can also heat up unexpectedly.
To help create even and predictable kiln shell temperature profiles, KIMA Echtzeitsysteme has developed the KilnCooler™, a water evaporation-based system for kiln shells. This idea often meets with the reaction that water should never be put on the kiln. This article will show that this assertion is incorrect. The system does not provide a miracle but it is very effective at cooling the kiln under certain conditions.
Introduction to the KilnCooler
When experienced kiln operators are asked which circumstances they think have the biggest influence on temperature increases within kiln shells, there are four main answers:
1. Increased use of alternative fuels, leading to higher wear on the refractory and widely varying coatings due to changing conditions;
2. Larger kilns with higher mechanical tension, which leads to higher kiln ovality and damage to the refractory lining;
3. Use of inappropriate refractories;
4. Uneven coatings.
As with so much else in the cement sector, the exact reasons for a critical temperature increase differ from kiln to kiln, plant to plant and country to country. However, the main possible effects are the same everywhere: Increasing shell temperatures can lead to hot spots and, often, the need to stop the kiln. This can take a kiln line down for a week or more.
When calculating the costs of the resulting production losses even under conservative assumptions, one can easily see the big commercial potential underlying the prevention of an unplanned emergency stop. Table 1 shows an example calculation for a 4000t/day kiln. The example does not take into account the additional costs that occur, for example the salaries of the people who have to take care of the unplanned kiln maintenance works, the costs of additional fuel needed for heating up the kiln again and other smaller costs.
| Parameter | Value |
| Kiln prodution rate | 4000t/day |
| Sales price | US$77/t |
| Assumed sales margin | US$11/t |
| Minimum duration of kiln stop | 6 days |
| Cost of production losses because of an emergency kiln stop | 4000t/day x US$11/t x 6 days = US$264,000 |
Above - Table 1: Example calculation of the costs of production loss due to an unplanned kiln stop, assuming two days for stop/cool down, two days to work on the refractories, one day for drying and one day for start-up.
This simple calculation already shows the steep increase in the cost of production loss when simply replacing the 4000t/day by higher production rates like 10,000t/day or more. If plants were able to save all of these costs by keeping the kiln in operation until the next planned shutdown, the savings could be used for other important optimisation projects.
To avoid such unnecessary production losses, it is critical to cool down the kiln shell, bring the temperatures back into balance and keep them within the desired range. The key to success in this case is controlled, punctual and efficient cooling. Due to it being a liquid, the cooling efficiency of water is several orders of magnitude higher than that of air. The following example calculation (under the assumption provided in Table 2) shows this big difference in cooling efficiency. Under the assumption that the air blown to the complete kiln can be heated up by 40°C, one can dissipate roughly 2MW of thermal power by using 150,000m3/hr of air
(Figure 2). The same amount of thermal power can be dissipated with 0.9L/s of water, just 3.2m3/hr water for a complete 60m kiln (Figure 3).
| Parameter | Value |
| Kiln length | 60m |
| Kiln diameter | 5m |
| Ambient temperature | 22°C |
| Temperature difference by which the blower air can be heated up | 40°C |
Above - Table 2: Assumptions for comparison calculation of air versus water.
When spraying water to a hot metal surface one needs to take care to use the correct amount. Too much water means the cooling rate might be too high. Mechanical tensions might also occur leading to a risk of damage within the metal structures. Too little water, and the temperatures might come down too slowly or, in the worst case, continue to increase. When using the right amount of water, one can cool down temperatures as fast as necessary and stabilise them at the desired set point. To do this a water cooling system needs four main features:
1. Precise and reliable measurement of the temperature of the area that needs to be kept under control;
2. Water nozzles that allow the water to be sprayed in the right shape, thus allowing it to reach the surface as a droplet and then evaporate completely;
3. A control system to correctly dose the water;
4. A continuous self-checking feature to ensure operational safety.
A basic KIMA KilnCooler unit has four nozzles, each combined with an infrared (IR) pyrometer (range of 120 - 1000°C (248 - 1832°F)) and operated by a high level control. The total cooling length for one of these portable units is 2.6m. For larger areas the same systems can be daisy-chained.
The big advantage of a water cooling system is, besides its efficiency, the possibility of punctual cooling. The water flow can be switched off and on within milliseconds, which is not possible with air blowers. This is important to reduce the temperature of a hot spot, or even a ‘warm’ area, back into balance with the lower temperature of the surrounding areas, which reduces the mechanical tension arising due to the difference.
While this is very effective, one also has to be aware of the risks of excessive cooling. As the water-based system cools a smaller area, there is a risk for high tension in the steel if areas are cooled to much, possibly causing damage to the refractory lining. A blower, by comparison, will cool not only the hotter areas, but also the complete circumference. While this reduces the circumferential differences, it can lead to shrinking of an entire kiln section, leading to enhanced stress for the refractory.
From evaluation, calculation and on-site experience, it was found that a cooling rate of 2°C per minute (Figure 5) is a good rate at which to quickly quench hot spots while keeping mechanical tensions low. This rate also leaves enough space for adjustments by the high level control if necessary. As long as the metal temperature does not reach 600°C (1112°F), spraying water onto the hot metal does not harm the micro-structure of the steel. The KilnCooler controller itself is adjusted from KIMA E’s site to an upper limit temperature of 500°C, as above this temperature the kiln should be stopped in any case.
Practical report from HeidelbergCement
The practical proof of a safe and highly efficient cooling of the kiln shell was particularly provided with the development partner HeidelbergCement at Ennigerloh Factory in Germany. Accompanied by the Association of the German Cement Industry (VDZ), the thermal scan taken and illustrated in Figure 6 shows a 24-hour plot observation.
The use of the system is shown on a kiln shell section being irregularly heated by more than 50°C. Starting with the commissioning of the system at about 14:00 hours, cooling down by about 50°C was achieved within two hours in this real situation. As the set point of 350°C is approached, the amount of water spray is automatically reduced significantly. Only 10 hours after start-up, the cooling system using water evaporation was switched off. The only slight increase in temperature and the stabilised status after switching off observed at that time, suggests that some coating has been newly formed in the kiln, so that further forced cooling is no longer necessary. Further case-studies in this article confirm that the treatment of a hot spot by means of the system avoids unexpected kiln stops.
Considering that the resistance of steel against alternating stress depends on many parameters, the use of the KilnCooler can be seen under another important light. Steel alters its strength depending on: Temperature; Surface finish; Metallurgical microstructure; Presence of oxidising or inert chemicals and; Residual stresses.
However, the steel is most affected by the number of cycles it undergoes. As the number of cycles is determined by the kiln’s rotation, the other parameters, especially the temperature, should be observed carefully and kept under control. To avoid fatigue of the shell, it is recommended that the temperature of the shell is kept below 400°C (752°F).
The Wöhler-Curve (Figure 7) shows, in principle, the fatigue behaviour of steel under stress, induced by an alternating load. It shows the magnitude of a cyclic stress against the logarithmic scale of cycles to failure. The number of cycles to failure is significantly reduced if the steel has an enhanced temperature. Therefore, if the steel temperature is controlled and harmonised at the entire circumference by the KilnCooler, the system can be an important tool for extending kiln life. Indeed, it makes sense to bring the KilnCooler into operation even at temperatures of 250-300°C and not just after hot spots are formed.
It is important to state at this point that when KIMA Echtzeitsysteme describes ‘hot spots’ we mean temperatures below 500°C and not red spots at all. If the kiln shell reaches 480°C the refractory might already be heavily damaged or totally removed. In this case the KilnCooler cannot help and the kiln should be stopped. The system should not be used to circumvent problems arising from poor maintenance or the choice of wrong or low performance refractories.
Three more questions
Finally three more questions arise when talking about spraying hard water (containing lime) onto the hot kiln shell:
1. Does the resultant lime layer on the shell have any influence on the heat conduction?
Answer: There is a theoretical critical thickness at which the heat conduction from the inside of the kiln through the shell to the outside would be disrupted. However, experience has shown that this thickness is never attained due to the very dry conditions on the shell and the movement of the kiln itself. The layer breaks off the kiln well before it reaches the critical thickness.
2. Could the white stripes of the lime layer influence the reading of the kiln scanners employed?
Answer: The emissivity of limestone is 0.95, much higher than rusty steel, which is 0.69. Due to this, there is no negative influence of the lime layer on the kiln scanners.
3. Can the lime block the nozzles or tubes?
Answer: Of course the lime content in the water can lead to blockages. To avoid this, the system continuously monitors the water pressure and flow rate. In any case, blockages have not been reported so far, even with systems that have been in operation for several months. If a blockage is detected, the system will send an error message to inform operators. Removing a blockage and exchanging / cleaning the dirt trap, the nozzles or valves can be carried out within minutes.
Some case studies
Leube Cement, Gartenau, Salzburg, Austria:
“In Salzburg, directly located in the Alps, a rain shower is usually heavy and puts more water onto the kiln than the KilnCooler system,” said Klaus Czepl, Production Manager.
“We have used the system since August 2015. During its first winter, the system gave us more than two months of full additional production time due to quelling the first hot spots arising from the use of new alternative fuels and thinner than expected refractories. Just in front of the burning zone we have had serious problems creating a coating and balls were created for the first time ever in
our production.”
“The system enabled us to create a stable coating in that locality once more and we reached the scheduled maintenance stop in January under full production. Previously, with air cooling fans we were not able to solve the problem. Today the system is in daily use when it comes to managing coating drops and
hot spots.”
Holcim plant Hoever, Hannover, Germany:
“When we bought the system in November 2015 we were extremely lucky,” said Matthias Heuer, Production Manager. “Only a few weeks later and just before the Christmas holidays we had fundamental problems with hot spots.”
“A massive hot spot as a result of very thin refractory lining and permanent coating drops were treated with the water spray kiln cooler. Within hours we developed a new coating and stabilised the temperature profile of the kiln surface.”
“Without the system we would have had to stop the kiln over the two week holiday as our maintenance team was limited during this period. This meant that we were able to operate until our scheduled kiln stop in January 2016.”
Afrisam, Dudfield, Lichtenburg South Africa:
Theo Conradie, Process Engineer, said, “At 12.0m we had some hotter areas in which the KilnCooler reduced the temperature from 350°C to 250°C within a short time. Only a few hours later we saw a second spot at 9.0m with a temperature of 390°C. We moved the system and the temperature dropped from 390°C to 330°C in around four hours.”
Summary
The past two years of operation have shown that cement plants that suffer from temperature problems on the kiln shell were able to prolong kiln runtimes for weeks or months by using the KIMA KilnCooler. The system can take care of hot areas and keep the temperature within those areas at the desired set point temperatures. Also it has shown that, where possible, new coatings have been built up in the cooling area to provide a more even internal coating.
The system supports maintenance teams by giving them enough time to plan the necessary maintenance works and talking to the refractory suppliers. Due to the very low energy consumption, the operational costs are very low. It has been shown that the amount of water sprayed onto kiln is very low compared to the amount that drops from a rainstorm. It does not have any negative influence on the kiln steel as the system is intended for operation below 500°C only.
The cooling efficiency of water combined with the possibility of controlled punctual spraying leads to a game-changing technology when it comes to increasing temperatures on kilns at the end of a production campaign. The earlier it is used in lower temperature regions, the more effective it is at increasing the lifetime of steel and refractory.
