»1 Comparison of temperature curves during firing of a setting of high-porosity backing bricks
1 Project definition
The sought-after characteristics of porous backing bricks, such as low body density and high heat insulation, are achieved for instance with the addition of porosity-enhancing materials, such as paper-making sludge, sawdust or polystyrene during manufacture. Besides their effects on porosity, these additional materials also advance the exothermic reaction enthalpy, which means energy is released as a result of the burning of the porosity-enhancing materials. Altogether, this brings with it a reduction in the enthalpy of the raw materials or, in other words, the higher the addition of porosity-enhancing materials the lower the additional energy requirement from the firing system for firing the raw materials. The objective is the optimum utilization of the available energy sources from the opening materials included to achieve porosity in the context of maximum energy and CO2 saving.However, the burn-out of the porosity-enhancing materials takes place within a very narrow temperature range, which could result in steep heating up of the materials in the heating-up zone and excessive porosity owing to the burn-out of the contained organic carbons at around 300º C (»1). The after-effects would be localized overheating leading to a reduction in the production quality as well as cracking and smelting. In extreme cases the firing aggregate becomes technologically uncontrollable. In any case, the energy released in the preheating zone is only of partial use in the process.Temperature sequences at varying positions of a porous setting during firing are shown in »1. The measured temperatures deviate significantly from the ideal temperature curve between 200 and 500º C. The lower brick layer remains at lower temperatures whereas the bricks in the upper layers experience a steep increase in temperature much sooner. This is caused by an improved hot air flow and subsequent faster heating in the area of the upper bricks. In comparison, the lower bricks are lagging behind where hot air temperature is concerned, but are then subjected to a drastic increase in heat, experiencing local excess temperatures of up to 1 000º C. Major disadvantages of rapid heating-up and subsequent quick cooling are high temperature stresses, resulting in adverse conditions for body formation. Additionally, it is not possible to technologically utilize a large part of the released energy in the firing system.
2 Background to the materials
To analyse this behaviour, it is necessary to realize the sequential processes during brick firing in the green brick. They are listed in »Table 1. A deciding factor for the described steep preheating during the preheating phase are the type and percentage of porosity-enhancing materials contained in the body and their conversion reactions.
2.1 Reaction sequences of organic components in the preheating zone
Porosity-enhancing materials are basically differentiated on the basis of biomass (sawdust, paper-making sludge) and synthetic materials (polystyrene). Comprehensive descriptions of the sequential reactions are contained in Rimpel 2010  und Rumpel 2000 . Essentially, depending on the availability of oxygen, the following three reactions ensue:
Pyrolysis describes the thermal decomposition of the organic components under the exclusion of oxygen (l = 0). Here organic polymers are converted into smaller, more simply constructed molecules. This results in liquid, gaseous and solid pyrolysis products. In the pyrolysis of biomass, the products are the pyrolysis gases such as CO, CO2, H2 and CH4, pyrolysis oil (methanol and acetic acid) or coal tar and charcoal.
During industrial gasification (0 < l < 1) solid fuels are converted in their entirety into combustible gases such as CO and H2 by adding a gasification medium (CO2, O2 or steam).
If there is excess oxygen present l > 1, then all pyrolysis and carbon gases, as well as any unburned residual carbon are oxidized in an exothermic reaction.
Broadly speaking, the organic  content of the raw materials undergo pyrolytic treatment in the tunnel kiln even though there is usually excess air l of > 0 available. In parallel, if the ambient temperature is sufficiently high and plenty of oxygen is available, the released highly volatile gaseous components are combusted indicating that the complete process is a combination of all three reactions.
It can be assumed that solid, liquid and gaseous pyrolysis products are formed during the conversion of the organic material in the bricks. Whilst pyrolytic gases leave the material, solid residues in the form of carbon are transported to higher temperature areas with the materials where full combustion takes place. Considering the above correlations the more pyrolytic products will be formed the closer the condition l = 0 is in the tunnel kiln.
Basically, the following reactions can occur in the preheating zone of the tunnel kiln during the conversion of the organic components .
It becomes apparent that essentially exothermic reactions occur, this means energy is released during the degassing and low-temperature carbonization process as well as during the actual combustion. The problem is that these energy releases are limited to two comparably narrow temperature ranges. The aforementioned degassing and low-temperature carbonization processes occur between 300 and 500º C and the formed residual carbon combusts from 700 to 800º C. The formed pyrolytic gases combust in the kiln between 300 and 500º C if sufficient oxygen is available. Most of the heat release in a conventional tunnel kiln process occurs during oxidizing conditions in the heating-up zone between 300 and 500º C , as the reaction enthalpy of combustion is substantially higher than of a degassing process (Equation 2). However, it is not possible to technologically utilize this completely within the kiln.
3 Best available technology to control higher organic content in raw materials
Basically, there are various technological solutions to control the described processes up to a certain degree of porosity. They are as follows.
3.1 Cooling with fresh air
One possibility is to use cool fresh air to take off excess heat from exothermic reactions in the low-temperature carbonization zone. A schematic showing the principle of this technology is shown in »2. On one hand, lowering the temperature in the firing chamber leads to unfavourable conditions for combustion. On the other, the cooler air in the kiln is able to incorporate exothermic heat from the low-temperature carbonization process [9, 10]. Junge  has already described the valid physical correlations. However, an adequate quantity of flue gas needs to be extracted from the kiln at the same time. Large amounts of energy are lost this way. Even though this supply of cool air reduces temperatures it simultaneously promotes burn-out of the porosity-enhancing materials owing to the higher O2 content in the additional air.
3.2 Operating the carbonization zone with low levels of oxygen
As an alternative to cooling with fresh air it is possible to suppress exothermic carbonization processes in the preheating zone with low levels of oxygen. To date, essentially there are two known recommendations.
3.2.1 Sealing off the carbonization zone with doors
In it is suggested that the preheating zone should be operated with installed doors in the temperature range between 250 and 650º C in combination with hypostoichiometrically operated burners and low oxygen. A diagram showing this principle is given in »3.
The resulting carbonization gases are extracted from the carbonization chamber with an additional suction system. Hot air is extracted behind the second door, it then goes around the carbonization chamber via a bypass and is injected into the front of the kiln. The air needs to travel through a heat exchanger as the temperature is too hot for the front of the kiln.
A disadvantage of this solution is the need to construct doors from high-temperature-resistant materials. Additionally, door maintenance is very difficult and they are also very susceptible to corrosion, which means condensation and high-temperature corrosion on the doors caused by exhaust gases need to be taken into consideration.
3.2.2 Extracting flue gas between 600 and 700° C
Another possibility to operate the preheating zone with a low level of oxygen would be to extract hot flue gas at about 600 to 700º C so that no oxygen is transported as far as the preheating zone. This solution is already practised in some brick plants. In addition to the hot air extraction, none or only very low quantities of flue gas are removed at the beginning of the preheating zone, making it possible to operate either the complete or a part of the preheating zone with direct-current (»4). This way no new oxygen is transported into the preheating zone for combustion.Disadvantages of this solution are the high temperatures of 600 to 700º C where extraction takes place and the materials to be used for the suction equipment. Furthermore, large quantities of exhaust air are realized when cooling the extracted flue gas with air. If recirculation of the energy of the extracted air is required then a heat exchanger, which also needs to be constructed of high-temperature-resistant materials, has to be installed.
From the aforementioned problem and the existing solutions, the objective to develop a technology to fire highly porous materials has been derived. The primary objective was to ensure a safe manufacturing process at degrees of porosity with a content of organic burn-out materials (TOC) > 3 mass% or an exothermic total raw material enthalpy > 1 000 kJ/kg. Additionally, fluctuating energy contents, which could be caused either by varying quantities or types of porosity-enhancing materials, need to be controlled. A further objective was to move the burn-out to higher temperatures so as to be able to optimize utilization of the energy contained within the raw materials.
5 Lingl PoroControl Technology
In an R&D project funded by Germany’s Federal Ministry of Economy and Technology and in conjunction with Wienerberger GmbH, an existing backing brick kiln was initially upgraded as necessary and thereafter equipped with the Lingl PoroControl Technology. Essentially, the upgrade consisted of the installation of two longitudinal recirculation circuits at the beginning of the preheating zone and four hot gas recirculations (Brakemeier) at around 700 to 900º C. All in all, these modifications achieved an even temperature progression with lower temperature differences in the tunnel kiln, a more uniform heating of the product and thus already a saving in energy. The next step within this research project was to increase porosity. Firing trials were conducted in which the degree of porosity was increased step-by-step. The mixtures produced were analysed according to their mineralogical composition and the TOC content and from this a total energy content in kJ/kg was calculated. Across the number of trials the TOC-content was increased from 2.2 to 3.85 mass%. Two trials are presented in the following as an example. In both trials a TOC content of 3.85 mass% was achieved with 50 vol% porosity of paper-making sludge and sawdust. The resulting measuring car curve for the first trial is applied across the tunnel kiln longitudinal section as shown in »5.
It was obvious that the steep preheating is still comparatively high in the preheating zone. The temperatures were at > 800º C. During this trial fresh air was recirculated in the preheating zone and only very little flue gas was extracted, retaining an oxygen content of 18% in the preheating zone. Further reductions in the oxygen content in the preheating zone were made during further trials as the selected setting obviously did not avoid burn-out of the porosity-enhancing materials.This was achieved with additional exhaust gas suction at about 400 to 500º C after the preheating zone (»6). These suction units are connected with the exhaust air installation as well as the two recirculation circuits. Additionally, fresh air can be injected through this, making it possible to objectively control the extraction or addition of flue gas depending on the oxygen content or temperature in the kiln.
Fresh air supply was reduced to an oxygen content of 6% and cooling was achieved with flue gas to avoid burn-out in the preheating zone. The measuring car curve across the longitudinal section of the kiln indicates substantially lower temperatures in the preheating zone.
At the same time burn-out of the porosity-enhancing materials was moved further along in the direction of the firing zone. This can be deduced from the reduction of the burner output and the substantial temperature differences across the cross-section of the kiln in the firing zone as shown in this example. Substantial temperature differences prove that the burners were switched off during this trial as the burners installed in the firing zones not only effect an increase in temperature but also air recirculation in the firing zone. Effective temperature homogenization can be achieved with varying methods depending on application, i.e. with burners with a reduced output, impulse burner, smaller burner jets, additional hot air recirculation or altered burner settings.
5.1 Waste heat utilization
An integral part of the Lingl PoroControl Technology is based on sensible utilization of excess energy derived during manufacturing as higher calorific exhaust air quantities are produced at higher degrees of porosity. Depending on plant and product, this utilization can be:
› Supplying the dryer with hot air
› Pre-heating combustion air
› Steam and/or hot water generation
› Generating power for own usage (i.e. ORC plant)
6 Conclusion and future prospects
With the Lingl PoroControl Technology a process for firing energy-rich raw materials has been developed that is suitable for both existing and new installations and is fundamentally characterized by the following points:
› Integrating recirculation into the process control to improve temperature homogenization
› Temperature and oxygen-controlled, safe process control
› Targeted flue gas supply instead of fresh air
› Substitution of HV burners with hot gas recirculation
› Energetic utilization of the exhaust gases after regenerative post-combustion
These measures ensure controlled low-temperature carbonization of the mixed-in organic materials and safe burn-out in the firing zone. This way, high-quality products with a high degree of porosity can be manufactured in a controlled firing process. Safe process management is guaranteed at all times. A greater proportion of the energy released during firing can now be reutilized than in the past. Excessive energy is transferred to a sensible waste heat utilization system. A positive side-effect is the conservation of resources, which is achieved by reducing fuel consumption and the substitution of fossil fuels.