Kiln Technology by KELLER ICS

Tunnel kilns

The well-known KELLER ICS tunnel kilns are the heart of many heavy clay plants throughout the world.

Innovative control concepts, combined with modern burners and innovative environmental technology guarantee both optimal fired products of the highest quality and maximum economic efficiency and reliability.

Environmental protection technologies are the key to the future

Tailor-made supplies related to environmental protection and energy technology have been supplementing the KELLER ICS range for many years now.

Whether as part of a new plant or to retrofit an existing plant - measures such as cogeneration plants with combined electricity and waste heat recovery, the use of tunnel kiln waste heat for drying or the regenerative thermal oxidation of flue gas and low-temperature carbonisation gas are just three examples of many that define what KELLER ICS means by environmental protection technologies with a future for the heavy clay industry.

"Enviro Kiln“ – with our new countertravel kiln in the right direction

The turnaround in the energy policy is a task for society as a whole. It has been subject to discussion in an international scale for years and is meanwhile supported by a majority of the population and the political leaders. The plan is, for example, to switch the energy supply in Germany to the greatest possible extent to renewable energy sources until 2050.  Equally important in this context is energy efficiency, which is receiving increasing attention in public discussions for some time now. KELLER ICS has committed itself to increasing the energy efficiency in brick plants in a sustainable way with the aim of reducing energy-related CO2 emissions. The most energy-intensive processes in brick production are without doubt drying and firing. In order to drastically reduce energy requirements for firing, KELLER ICS radically revisited the principle of the countertravel kiln and created a new sustainable kiln concept. The energy consumption for a roof tile plant using H cassettes, for example, can thus be reduced by about 30 % compared to a modern conventional plant.

The firing process

The energy transfer in a conventional tunnel kiln follows the fluid-solid principle, i.e. air is used to transport energy.  In practice this means that the hot air available from the cooling zone of the kiln must be used elsewhere, and air continues to flow through the firing zone and leaves the kiln as flue gas loss. In order to significantly reduce the energy consumption during the firing process, these energy flows should be reduced or even eliminated. How can this be done?

The answer to this question is difficult and yet not unknown. In principle, the countertravel kiln combines numerous advantages. The energy transfer in the countertravel kiln follows the solid-solid principle (figure 1) and not the fluid-solid principle as with a traditional kiln. Therefore, this principle ensures that air in the form of longitudinal flow will not flow through the firing zone, thus considerably reducing the flue gas losses. Furthermore, there is no removal of energy from the cooling zone and a coupling with the dryer is therefore no longer necessary. Familiar countertravel kilns have two tracks with approximately 3 to 4 m wide kiln cars. The energy transfer from one track to the other is carried out by free convection (figure 2). These kilns have potential for improvement with regard to homogenisation and  intensification of convection by forced ventilation. A radiation exchange is not possible as the tracks are separated by a wall. Known are also theoretical considerations, for example from universities and institutes. KELLER ICS thoroughly examined the known results and basics of the countertravel kiln, then redesigned and modified them and developed the new countertravel kiln "Enviro". This kiln eliminates existing disadvantages and uses the well-known advantages, hence creating a completely new firing concept with a high energy saving potential.

The "Enviro kiln"

What is the design of the "Enviro kiln" and how does it work?

The most important purpose of the "Enviro kiln" is to ensure that the solid-solid energy transfer operates successfully. Therefore, it is obvious that the product flows travelling side by side are limited in their geometry, that the convection works as forced convection and that the heat transfer by means of radiation is used as efficiently as possible. In order to comply with these requirements, KELLER ICS theoretically cut the kiln cars into slices and lined them up next to each other, thus forming a large number of kiln cars where products to be heated up and to be cooled down are side by side moving in opposite directions. These are optimal conditions for a heat transfer. Recirculation fans are arranged in the convection area over the length of the kiln and provide a transverse circulation. From an approximate temperature of 700 °C upwards, these recirculation fans are no longer needed, as the radiation part is now predominant and takes over the main heat exchange. Final firing takes place in the firing zone as with any conventional kiln.

The pilot plant

Extensive theoretical considerations supported by the "Fluent" software in close cooperation with the University of Applied Sciences in Osnabrück preceded the studies for the redesign of the countertravel kiln. In order to review the findings and to obtain well-founded measurement data, KELLER ICS built a test kiln with the dimensions width 5 m, length 16 m and height 5 m (figure 5) and recorded extensive measurement data.

Fluid mechanics

In order to obtain high-quality firing results it is necessary to create a homogeneous flow profile in the kiln. We reached this goal by  installing specific components; measuring results from different levels provided the respective proof (figure 6). An air speed of less than 1 m/s is still sufficient to ensure the necessary heat transfer in the convective area.

Thermodynamics

The heat transfer according to the solid-solid principle works. After every kiln car push the "increase in heat" of one setting stack and the "decrease in heat" of the opposite setting stack can be measured and recorded, thus giving proof of the heat transfer (figure 7). As the products in the "Enviro kiln" are principally stacked in one row, the temperature distribution in the setting is excellent, and higher heating and cooling gradients can be applied at the critical holding point than this would be possible in conventional kilns – and the product quality is equally high. The new KELLER "Enviro kiln" fires other products as well, such as facing bricks, brick slips, common bricks or even non-ceramic products.

Saving energy – reduction of operating costs

The energy-saving potential offered by the new "Enviro kiln" is clearly illustrated in a Sankey diagram (Figures 8 and 9). The example of a  modern roof tile factory using H cassettes demonstrates that the total energy requirements for drying and firing can be reduced from 631 to 398 kcal/kg of roof tiles.

The new KELLER "Enviro kiln" succeeds to put an end to the coupling of kiln and dryer. Completely new ways to heat the dryer can be opened up, such as low-energy drying, cogeneration systems or heating with alternative fuels. The energy supply for the dryer can be tailored exactly to the needs of the customer. Thus, in many plants the current system-related oversupply of the dryer with kiln air and the "blow-off" of valuable energy will become a thing of the past. This protects the environment,  conserves resources and reduces operating costs – a worthwhile investment!