Energy-Efficient Tunnel Kiln Car Systems in the Heavy Clay Industry

In the clay brick industry, the development of tunnel kiln car systems has always been a major topic for manufacturers of clay bricks and roofing tiles. This article presents some views on this subject from Burton-Werke, a supplier of tunnel kiln car systems for most brick and roofing tile plants in Germany.

From the perspective of overall kiln technology development, the trend is towards automated firing equipment to meet the growing demands for clay products, with more precise raw material preparation and more uniform green bodies. This discussion includes roller kilns, Monker kilns, high-frequency technology, etc.

However, alongside these developments, the traditional tunnel kiln will certainly retain its place, and it has evolved in many respects, not only in terms of firing components.

Before deciding on a specific firing technology, a cost-benefit analysis is usually performed, taking into account the necessary products and raw materials to be used.

With regard to the development of tunnel kiln cars, the following aspects deserve special attention.

This involves not only technical and economic calculations but also the user’s expectations. For a system supplier, the task is not to select one standard solution or another, but to create a solution for the user that meets their requirements, aligns with their own considerations, and satisfies their ultimate needs.

Nevertheless, irrespective of the above, the following general criteria for selecting a tunnel kiln system are commonly used, mainly for cost reasons.

• Wear (depreciation)

• Energy consumption

• Maintenance and cleaning effort

• Repair

When analysing consumption factors, it is easy to see that the energy consumption of a tunnel kiln car is an important factor, but far from being the only principle for deciding on a specific tunnel kiln car system. The kiln car is a structural component of the entire kiln system and is subject to significant loads. If this structural component is considered as an independent system, the respective functions must first be examined.

• Good product quality

• Minimal energy consumption due to reduced weight and thermal insulation (heat storage and heat transfer)

• Chemical resistance to the tunnel kiln atmosphere and energy media under firing conditions

• Thermal stability (under thermal shock and rapid temperature drops)

• Mechanical strength (influenced by human factors)

• Dimensional stability (interchangeability of refractory components, affected by reversible expansion)

• Ease of maintenance and repair (replacement of wear parts)

• Low investment and maintenance costs (short maintenance time)

• Long service life

From the table it is clear that perfection cannot be achieved, but it is easy to maximise the fulfilment of the target functions of the kiln car while neglecting secondary functions. If the car weight is drastically reduced, the mechanical stability of the system inevitably decreases, which can of course be improved by using higher-quality materials, but this increases depreciation costs and maintenance risks.

Although the above is not fundamentally new, it should be kept firmly in mind when making relevant decisions. Because when the priority factor “energy saving" is set for the tunnel kiln car, other equally important functions should not be overlooked.

Figure 1 Two-layer corner U‑blocks, hollow pillars and various insulation methods with columns and protective panels (for side firing, e.g., single-layer roofing tile firing), thin protective panels

Today, up to 15 different materials are used in tunnel kiln car systems, ranging from various special materials with thermal shock resistance to refractory concrete and mortars, various fibre materials, and high-performance ceramics based on mullite and silicon carbide. Since no manufacturer produces all of these materials themselves, the user usually receives a complete solution from a single source, which can provide the same guarantee and service. At the design stage, the combination of different materials plays a very important role.

In designing a tunnel kiln car, the basic objectives are threefold: the car perimeter, the car lining, and the supporting structure or kiln furniture for setting the bricks.

For example, for a kiln car of size 7*6 m, the perimeter area accounts for 10%, the supporting structure area for 5%, and the lining area for 85%. This is common for modern kiln car designs.

In recent years, with the continuous development of firing technology, especially in material selection, the proportions of each of the above parts have been changing. A trend can be observed: materials that have already proven successful in the fine ceramics sector are also increasingly being applied in the clay brick industry (as shown in Figure 1).

The perimeter of a tunnel kiln car mainly serves the following functions:

• Labyrinth sealing (dependent on dimensional stability!)

• Mechanical protection of the car lining

• Protection of the car chassis from temperature effects

To this end, the following properties are required:

• Dimensional stability

• Strength under cold and hot conditions

• Resistance to thermal shock or temperature changes

From a technical point of view, lightweight refractory concrete blocks are required to achieve these functions. Extruded large-format blocks based on cordierite and dry-pressed large-format blocks also based on cordierite – each possible solution has its advantages and disadvantages. The dry-pressed large blocks for the kiln car perimeter are discussed in more detail below.

This type of block has a number of important advantages, such as high dimensional stability, eliminating the need for secondary processing of the blocks. Under current raw material and production technology, its defined mineral composition can be more easily obtained.

In modern kilns, the pushing cycle of kiln cars is becoming increasingly shorter, making the thermal shock resistance of the materials increasingly important. Burcclight 12/25H, a recently developed material, fully meets these requirements.

The test results for this material are as follows:

It is evident that this material has a higher bulk density than traditional lightweight refractory blocks, but in comparison it can be used to produce larger products and thinner interlocking blocks with thermal shock resistance. Although the weight of the kiln car perimeter made of Burcclight material differs significantly from that using lightweight refractories, its thermal shock resistance and ease of assembly are greatly improved.

Even in a modern, fully automated brick plant, the perimeter of the tunnel kiln car is subjected to high thermal and mechanical stresses. In addition to requiring high durability of the material, it is even more important that when a perimeter part is damaged, it can be replaced quickly. For this reason, the perimeter blocks are not bonded or mortared, but dry-laid, with connections only through toothed mechanical interlocking – which is obviously a very good method.

Naturally, this requires a certain dimensional accuracy of the blocks. Normally, only dry pressing can produce dimensionally stable blocks; otherwise, dimensional accuracy can only be achieved through secondary processing.

The function of a modern tunnel kiln car lining is thermal insulation, while the load is usually borne by the metal chassis of the car. This function determines the choice of materials: almost exclusively lightweight, highly insulating materials. The first to mention here are ceramic fibres, now available in ready-to-use grades. For economic reasons, depending on the service temperature, these fibres can be replaced by lightweight concrete or various aggregates, such as silica, lightweight grog, pumice, etc. It should be noted that these insulating materials cannot be directly exposed to the flame; they must be protected by a suitable surface covering, for example a thermal shock-resistant thin panel. Even though this slightly increases the weight of the kiln car, this method prevents corrosion of the insulating material, especially in side-fired kilns. Moreover, a hard surface layer is necessary for effective cleaning of the car deck, which can be a significant factor causing severe wear, dust, sand, and accidents. Today it is already possible to produce such protective panels with a thickness of 10 cm and dimensions of 500*600 mm.

As the level of automation in modern brickworks increases and the number of operators decreases, problems involving tunnel kiln protective panels are diminishing. Nevertheless, in practice we often see that the cover layers used in many cases are later reinforced and placed on the kiln car columns to facilitate loading and unloading. This is also a typical example of the serious divergence between energy saving and maintenance according to production requirements.

Comparison of properties of different kiln car insulation lining materials:

Another example is the placement of front and rear end protections on the kiln car chassis. Such protections are unnecessary when the pushing cycle is 10 hours or less. If, for process reasons, the kiln car has to remain in the tunnel kiln (e.g., after a collapse or reduced pushing speed), the advantage of this protection is to keep the bottom of the car cooler. The use of this method is ultimately a user decision.

The function of the column structure is to bear all loads from the products and kiln furniture during firing and transfer the forces to the metal chassis of the kiln car. This requires relatively high cold and hot strength values, as well as compressive and flexural strength, and some deformation behaviour at the service temperature. In addition to the weight of the refractory components should be minimised. For this reason, most components of the kiln car are subjected to the greatest stresses. Naturally, the column structure must be designed strictly according to the firing load and firing temperature. However, analysis of recent kiln car system projects shows a growing departure from traditional refractory systems, i.e., systems consisting of dedicated flues, high transverse supports, special columns with perforated panels (called “Bensen"), and kiln furniture placed on specially shaped slabs supported by core columns. In fact, in the production of fired paving bricks, thinner and more refined systems have already been adopted, using extruded columns on which large-format load-bearing bricks or slabs or beam structures can be placed. Figure 2 shows an example of such a system.

Figure 2

Such refined systems no longer use traditional refractory clay materials. For this reason, the clay is crushed to a grain size of 0–0.2 mm, then slip-cast, pressed into granules or extruded into shapes, and such materials are still in use. This also concerns the production technology of high-grade refractory components with special requirements. In this area, high-performance materials are continuously being introduced: materials based on mullite‑nitride‑bonded silicon carbide, recrystallised silicon carbide, and silicon‑infiltrated silicon carbide. These materials have very high strength values, allowing a significant reduction in the thickness of ceramic components and thus a marked reduction in the weight of refractory components. With the help of advanced side-fired kilns using high-velocity burners, the setting height can be continuously reduced to single-layer firing, and the corresponding supporting structures (kiln furniture) will be further developed. Due to the reduced weight of refractory components, suitable mechanical stability against displacement and vibration can be achieved through dovetail joints, interlocking, or clever bolted connections such as locking strips, caps, rods, and strong component tolerance restrictions.

This has also greatly stimulated the demand for higher production technology levels from refractory product manufacturers. For such products, the permissible dimensional tolerance is 1 mm, which represents the current state of the art. The prerequisites for meeting the above requirements are the production of dimensionally accurate products using high-quality raw materials; the development of advanced pressing tools, such as programmable hydraulic presses with multi‑stage moulds; and precise control of drying chambers and kilns.

In some cases, when designing kiln cars with combinations of the various materials mentioned above, attention should be paid to the great variation in physical properties, which is decisive for the continuous operation and trouble‑free performance of the tunnel kiln car system. Therefore, while previous kiln car designs were mainly based on numerical values, today calculations of energy, mechanical, and thermal performance during the production of each component play an increasingly important role. Figure 3 shows an optimal load design achieved through structural and thermal calculations.

Figure 3

This shows the importance of the physical properties of materials in kiln car design. For example, considering the reversible thermal expansion of materials, an analysis of the coefficient of thermal expansion shows that the values vary greatly in some cases. If this is overlooked, it will inevitably lead to consequences detrimental to the kiln car system.

A tunnel kiln car system is always linked to the user and the product. Knowing the future process parameters of the plant, such as firing temperature, firing cycle, and kiln atmosphere, and taking into account various production conditions at the design stage, is essential for making the right choice to extend the service life of the system. Only in this way can adverse factors and unnecessary consumption be avoided, and the system be optimised.

Dr. Volker Hesse is Deputy Technical Director at the Burton-Werke, Melle/Buer