Light refractory linings save energy
Analytical comparison. Two cases: short firing cycle with H-cassettes and long cycle for bricks
It is essential for both economic and environmental reasons to seek maximum energy efficiency in all processes within the heavy clay industry. The kiln is one of the most energy-demanding parts of the process, and the kiln cars play a crucial role in its performance and its thermal efficiency.
The design of the kiln cars can significantly contribute to making the installation more efficient. Many aspects are involved in this: the airtightness to the kiln pit, the interlocking with the kiln walls, the chassis design, and the refractory lining design. However, we will focus on the last factor: the refractory lining design concerning two characteristics:
Insulating Capacity (Conductivity):
The insulating capacity, measured as conductivity, is important to minimize heat transmission through the lining (primarily when the kiln car is travelling inside the kiln but also when they are running outside the kiln). Conductivity becomes more important in the case of long cycles (for example, with solid thick bricks or pavers) than in short cycles (for example, in H-cassette installations for roofing tiles). This is because heat takes some time to penetrate into the lining and reach the bottom, therefore, the long cycles allow the conduction through the lining, completely stablished, longer time.
Heat Retention Capacity (Thermal Inertia):
The capacity of the kiln car lining to retain heat is the result of a combination of the materials used, how they are combined in the lining, and particularly how the refractory pieces themselves are designed. A high capacity to retain heat is, in principle, negative; the kiln car accumulates energy inside the kiln, and when it comes out, it still retains some heat that will mainly be lost outside the kiln before the next cycle begins. This energy stored at the exit of the kiln is essentially the product of the thermal capacity (closely linked to weight) and the temperature. At first glance, lighter linings retain less heat, but it is also important to make them easier to cool down in the cooling section of the kiln in order to have the lining at lower temperature at the kiln exit.
These two factors are not as independent as they might appear. The kiln cars operate in a transient flow, so thermal inertia plays a role in the „apparent“ conductivity of the lining and, conversely, the presence of insulation can influence the lining‘s ability to cool down both inside and outside the kiln.
To accurately assess energy savings in a particular case, it is also important to consider the time that the kiln cars will remain outside the kiln between cycles. Longer time between cycles will always mean higher losses; however, the design of the lining can minimize them.
The two cases will show two different situations:
1. H-cassette plant (hydrocasing kiln) with relatively short firing cycle
2. Brick plant (traditional sand seal kiln) with long firing cycle
The calculations have been made using the FEM (finite element method), using the real kiln curve, and supposing the air speeds in the different areas of the kiln and pit. The graphics show the third passage through the kiln in order to have a stabilized situation.
Case 1: H-cassette Hydrocasing kiln.
This installation is a typical Hydrocasing kiln designed some decades ago, with the sealing of the kiln cars made with water. The firing cycle is 24 hours, which is not very short for an H-cassette installation. The time outside the kiln between cycles is 22 hours and 14 minutes.
Firstly, we calculate the temperatures and heat of the Original Design linings (»Figure 1).
This lining is composed of a large mass of concrete in the lower half, insulated at the bottom with calcium silicate. Refractory supports are placed on top, to support the H-cassettes. There is ceramic fibre filling the area between supports. Applying the Finite Element calculation, we obtain the temperatures and accumulated heat throughout the process. The graphic belongs to the 3rd passage through the kiln in order to stablish a steady situation (»Fig. 2).
Regarding the temperatures, we have the temperature of the kiln in blue, and the average temperatures at different depths of the lining, from “level 1” at the top to “level 6” at the bottom. The graphic shows how the temperatures penetrate into the lining with a delay because of the thermal inertia of the lining.
Regarding the heat curves we have:
In green, the “Remaining heat” in the lining. It represents the energy which is stored in the kiln car at certain moment. The reference level (zero energy) is when all the lining is at 25ºC. This heat accumulation gives us a very relevant information to analyse the thermal efficiency of the lining. At the exit of the kiln in the second passage, the energy stored per kiln cars was 431.608 Kcal. Later, at the entry of the kiln at the next cycle, the energy is 115.545 Kcal. This means that the difference, 316.063 Kcal, is the energy that the lining has lost outside kiln during the period of 22 hours and 14 minutes between cycles. On the third cycle, it can be seen that the energy stored peaks till 792.526 at the end of the firing zone. From that point starts the cooling part of the kiln where part of the energy accumulated is recovered by the kiln. The remaining energy which has not been possible to recover will be lost outside the kiln.
In grey, “Heat underneath”, the accumulated heat which has been transmitted to the channel underneath the kiln cars. At the beginning of the third cycle the accumulated heat till that point is 370. 911 Kcal, while at the end is 464.838 Kcal. The difference, 93.927 Kcal, is the energy lost by conduction through the lining inside the kiln (the energy lost through the bottom outside the kiln, is already included in the calculations of energy lost outside the kiln). This energy is less that one third of the one lost outside, basically because is a short cycle kiln. This means that the energy lost outside the kiln is much higher than the one inside.
Now, we are going to make the same calculations to the light linings designed by Forgestal-Campo, in order to compare the thermal efficiency and calculate the eventual energy savings using this light design (»Fig. 3).
The lining proposed by Forgestal-Campo is built with extruded hollow refractory blocks. The dense concrete is only used in the base of the lining to get a good level. There is also a calcium silicate layer on the very bottom of the lining. The filling concrete at the core of the lining is light and insulating.
The graphic with temperatures and heat for the Forgestal-Campo solution, using the same kiln conditions as for the previous lining design is the following: »Fig. 4.
Like the previous calculation we have:
In green, “Acc. Heat Car”, the remaining heat in the lining. The accumulated heat here at the exit is 291.677 Kcal which compared to the original solution (431.608 Kcal) is 1/3 smaller. At the entry of kiln in the next cycle, the Forgestal-Campo solution still keeps 51.245 Kcal, less than 50 percent of the 115.545 Kcal in the case of the original solution, which is heavier and in consequence keeps more heat. In conclusion, the energy lost outside the kiln, the difference of heat between kiln exit and kiln entry, is 316.063 Kcal for the original design and 240.432 Kcal. This means a saving of 25 percent energy in favour of the light design from Forgestal-Campo.
In grey, “Acc. Heat Underneath”, the accumulated heat which has transmitted to the channel underneath the kiln cars. Comparing the energy lost with both systems we found a reduction of the 20 percent in the case of Forgestal-Campo solution (388.663 - 313.795 = 74.868 Kcal in comparison to 93.927.
Case 2: Brick Plant, long cycle.
Now, we are going to make the same calculations in the case of a facing brick plant, with longer cycle compared to the H-cassette case. The firing cycle is 43,5 hours and the kiln cars remain an average of 67 hours outside the kiln.
On one hand, we are going to simulate a classical solid refractory solution (»Fig. 6):
In this solution all refractory parts are solid. There is a dense concrete layer at the bottom of the lining. The supports have a calcium silicate base. The infill is light and insulating
Applying the Finite Elements Method, we got the following graphic of temperatures and heat: »Fig. 7.
On the other hand, we analyse the solution by Forgestal-Campo. This is a lighter solution which is easier to heat up and cool down: »Fig. 8.
Also in this system, the refractory pieces are all extruded and hollow, in order to make them lighter but keeping a good mechanical strength. The base of the lining is dense concrete, and the core filling, light and insulating.
We follow the same methodology and kiln conditions used for the heavy solution to get the results of temperature and heat (»Fig. 9).
When we compare the results of both systems, the solid solution and the hollow blocks Forgestal-Campo solution we see:
The accumulated heat at the exit of the kiln, is 45 percent less in the case of Forgestal-Campo design in comparison with the heavier solution. This is a remarkable figure and represents a saving of 458.121 Kcal in favour of the Forgestal-Campo solution. In this plant, the remaining time outside the kiln is much longer than the H-cassette case. In consequence, outside, the kiln cars almost lose all the energy remaining at the exit of the kiln (around 92 percent). The fact that the hollow solution retains less energy inside the kiln (less mass and less temperature at the exit), makes it more efficient.
However, the amount of energy lost inside the kiln (conductivity) is quite similar but favourable in a 16 percent for the heavy solution. The reason behind this is that the higher mass delays the penetration of the heat into the lining. The bigger thermal inertia of the lining slows the heat penetration by absorbing more energy which will penalize later. On the other hand, the presence of holes in the hollow pieces does not help to insulate at high temperatures where the radiation takes an important role. In any case, this advantage of the heavy solution represents only a saving of 42.602 Kcal while the difference of the accumulated heat in favour of the light solution is 458.121 Kcal.
Once we have the savings per kiln car, we can get the annual savings for the whole kiln car fleet. Estimating a production of 13.104 kiln cars/year for the H-cassette plant, and 7.028 kiln cars/year, using light linings with the Forgestal-Campo design, we would get energy savings of 1.443 MWh and 3.397 MWh respectively. This represents a reduction of CO2 emissions of 291 t and 686 t respectively, per year (»Fig. 11).
In conclusion, this study shows that the design of the lining can strongly contribute to save energy in a kiln for heavy clay production. Traditionally, the conductivity of the lining has been the main feature to compare the thermal performance of a kiln car. This would be right if the flow were steady, but kiln cars have a transient flow. Therefore, the thermal inertia plays a very important role, even more than the conductivity, as shown. But the thermal inertia cannot be compared only taking the height of two linings. The distribution of the weight inside the lining also plays an important role in the behaviour.
