Hydrogen – a future fuel gas for brick plants? (part 3)

The use of renewably generated hydrogen instead of natural gas is regarded as an important element contributing to the transformation of the clay brick and tile industry to CO2-free production. In comparison with the electric heating of tunnel kilns, which is also widely discussed, using hydrogen as fuel gas in brick plants offers the advantage that only minor changes to the burner equipment are necessary. However, with regard to the application of hydrogen, wide-ranging questions arise in respect of its cost efficiency, combustion properties and the influence of the changed exhaust gas composition on product properties. Focussing on these aspects, a research project was conducted at the IAB Weimar (Weimar Institute of Applied Construction Research) from 04.2021 – 02.2023 specifically for clay brick and tile plants in Thuringia; the project was financially supported by the Thüringer Aufbaubank (development bank of the German state of Thuringia). (The article was published in three parts)

6 Results of the laboratory tests

6.1 Composition of the raw materials

Depending on the type of the deposit, the raw materials differ substantially from each other in some cases, and were assigned to kaolinitic, illitic or smectitic clay groups (»8 and »9). Some clays contained carbonates as well as organic components, three mixes contained poreformers. On the basis of the composition and grain size, the raw materials are suitable for the production of HMz, Dz as well as VMz.

The clays and the four HMz mixes have a high content of particles larger than 20 µm. The kaolinitic clays mostly contain a high content of particles smaller than 2 µm and can be found in the left part of the diagram. The illitic clays tend to be in the mid-size particle range 2 – 20 µm (»9).


6.2 Simultaneous thermal analysis (STA)

In »Table 17, the main exo- and endothermal reactions of heavy clay ceramic clays that release the gases H2O(g), CO2 and SO2 during firing are summarized. During oxidation, O2 is consumed, which can come from both the kiln atmosphere and the raw material. These reactions are recorded with the help of thermal analysis.

If – as in the case of H2 combustion – a relatively large content of water is present in the kiln atmosphere, these reactions can be influenced. These changes can be detected with the help of thermal analysis. Heavy clay ceramic bodies are widely known to consist of multimaterial mixes including clay and non-clay minerals, in which the above-listed reactions usually overlap. For this reason, besides the 17 raw materials, first as pure as possible materials underwent thermal analysis as a function of the water vapour content. These were:

Clays: predominantly kaolinitic, illitic or smectitic, respectively

Organics: Sawdust, coke, polyvinyl alcohol PVA

Carbonates: calcite, dolomite

The results are presented for selected examples.

On a kaolinitic clay containing 85 % kaolinite, 14 % illite/smectite and 1 % quartz, thermal analyses were conducted from 150 – 1 050 °C with a water vapour content of 1 – 95 vol%. The DSC curves are shown in »10.

The curves show that the peak temperatures of the endothermal clay mineral dehydroxylation and the exothermal metakaolin decomposition shift towards each other. Already at a water vapour content of 30 vol%, the peak temperature of the dehydroxylation increases in comparison to air by 51 K and that of the metakaolinite decomposition decreases by 20 K. At 87 vol% water vapour, the shift of the dehydroxylation peak was 71 K to higher temperatures and that of the metakaolinite decomposition by 31 K to lower temperatures. Accordingly, the temperature range between dehydroxylation and metakaolin-ite decomposition in high water vapour content is decreased by more than 100 K. Above 87 vol% water vapour, the temperature peaks shift only insignificantly.

The reaction enthalpies as well as the mass losses in the measurements were almost independent of the water vapour content, and high reproducibility of the results was shown. With increasing water vapour content, however, the temperature range of the dehydroxylation becomes narrower while that of the metakaolinite decomposition widens. The measured peak shift is shown for the kaolinitic clay as a function of the water vapour content in »11.

With the help of »11, as a function of the water vapour content, the temperature shift of the dehydroxylation and the metakaolinite decomposition can be estimated for the kaolin. Additionally, the content of water that can be found in the exhaust gases of tunnel kilns was inserted in the figure. With increasing temperatures in the preheating zone of the kiln, the water content decreases in the direction of 0 vol% (»4 and »5, in part 2 in ZI 2/2024). This makes it clear that the shifts of the peak temperatures in the preheating zone of a tunnel kiln shown in »10 and »11 tend to be minor.

The influence of the water vapour content on the dehydroxylation and mineral conversion established for the kaolinitic clay also occurred for the predominantly smectitic and illitic clays (»12).

In the illite and smectite, the reactions take place over a wi-der temperature range in comparison with kaolinite as well as in several sub-reactions. The dehydroxylation peaks shift in the case of illite and smectite to higher temperatures and those of the metakaolinite decomposition shift to lower temperatures.

In addition, all reactions are dependent on the preheating rate and the volume of supplied air. At a higher heating rate and increasing air volume, the temperature range of the reactions decreases, moreover they occur at somewhat higher temperatures.

By way of example for organic components in clay raw materials, Jeluxyl WEHO 500S sawdust was selected [39]. It consists of natural softwood chips. By means of predrying, grinding, dedusting and screening, a fraction with 100 % particles smaller than 500 µm and 35 % smaller than 180 µm was obtained. Accordingly, the sawdust was sufficiently homogeneous and fine-grained for the tests. To reduce the strongly exothermal reaction in the thermal analysis, 10 mass% sawdust was mixed with 90 mass% a-Al2O3 powder. The results are shown in »13.

The diagrams in »13 show two reactions. The first reaction of the volatile components is more pronounced in respect of both the enthalpy and in the mass loss. The second reaction can be attributed to the oxidation of the remaining fixed carbon. The exothermal enthalpy in water vapour is around 5 – 10 % lower than in air, the DSC curves, however, show a similar course. The water vapour in the kiln atmosphere acts via the endothermal water gas reaction as a gasification agent and replaces the pure oxidation of the organics from the atmospheric oxygen. The reactions and the mass losses occur in water vapour at higher temperatures than in the air atmosphere. The residual mass of 89 – 90 mass% means that the sawdust with its content of 10 mass% burned off almost without residue in air as well as in water vapour.

Comparative tests on polyvinyl alcohol (PVA) and coke showed that in water vapour the exothermal reactions – like for the sawdust – were shifted to higher temperatures. In the case of PVA, the exothermal enthalpy decreased in water vapour, for coke, on the other hand, it increased. Here, too, the mass losses in air and water vapour were almost the same. On account of its ash content, the coke burned with residue, in contrast to the sawdust and PVA.

As carbonate, calcium carbonate supplied by Carl Roth was used. It consists of more than 98.5 % calcite. The d50 particle size is 1.0 µm. The results of the thermal analysis are shown in »14.

The diagrams show the typical endothermal reaction of carbonate decarbonisation from around 650 °C. The enthalpies are almost the same in water vapour and air, the DSC curves are similar. The reactions and the mass losses take place in water vapour at lower temperatures than in the air atmosphere, however, in a same temperature interval of around 200 K. The residual mass of around 57 % shows that the calcite was almost completely decarbonised both in air and in water vapour.

The additionally tested dolomite behaved like the calcite with one exception. Dolomite had two decarbonisation reactions: at lower temperatures from the magnesite (MgCO3) followed by calcite (CaCO3) at higher temperatures. In water vapour, the two reactions shifted, as for the calcite, to lower temperatures. However, the decarbonisation of the magnesite shifted much more substantially than that of the calcite so that the two reactions took place clearly separate from each other.

As an example for the tested raw materials, »15 shows the results of a high-organics and high-carbonate HMz body. This mix consists in terms of mass of 19 % illite, 2 % smectite, 28 % kaolinite and fireclay, 2 % muscovite, 15 % quartz, 4 % feldspar, 11 % calcite, 6 % dolomite, 1 % iron minerals and 1 % rutile as well as 3.7 % TOC.

In the DSC curves, first an exothermal reaction with mass loss occurs, which can be attributed to the oxidation of the volatile organics. The exothermal enthalpies are smaller in water vapour than in air and decrease with increasing water vapour content. The following two endothermal reactions with mass loss come from the clay mineral dehydroxylation and the carbonate decarbonisation. Both enthalpies are somewhat smaller in water vapour than in air. Between 850 and 900 °C, the DSC curves become unsteady caused by the formation of new calcium silicates. These reactions are more pronounced in the case of higher carbonate contents as well as in air than in water vapour. The peak temperatures of the organics oxidation shift in water vapour to higher temperatures. The peak temperatures of the carbonate decarbonisation and the formation of new minerals shift to lower temperatures in atmospheres with higher water vapour contents. The mass losses show the same temperature dependence as the enthalpies. The mass losses in air and water vapour are almost the same.

To summarize, with the thermal analysis, as a function of the water content in the firing atmosphere, almost constant reaction enthalpies and mass losses were determined. The temperature ranges of the reactions, on the other hand, are dependent on the water vapour content, the firing conditions (heating rate, air vol-ume flow) and the mineral (type, structure, particle size). The DSC and DTG curves show in each case almost the same course as a function of the firing temperature. For real clays, the reactions overlap, as a result the enthalpies and mass losses cannot always be definitely identified. Possible causes are described in [40] and [41]. The peak temperatures and ranges of clay mineral dehydrox-ylation, carbonate decarbonisation and oxidation of organic components shift in the presence of water vapour as follows:

The clay mineral dehydroxylation shifts to higher temperatures, the enthalpy remains constant

The metakaolinite decomposition / spinel formation shifts to lower temperatures, the enthalpy remains constant

The decomposition of the organics shifts to higher temperatures, the (measurable) enthalpy decreases

The carbonate decarbonisation shifts to lower temperatures, the enthalpy remains constant

The thermal analyses were conducted with comparatively high water vapour contents, which were constant over the fir-ing time. As described in Section 4.4. (cf. part 2 in ZI 2/2024), the water vapour content in the tunnel kiln, however, is low in comparison with the thermal analysis measurements. Moreover, it decreases with increasing temperature. For this reason, it can be assumed that with the high air quantities in the tunnel kiln and the comparatively low contents of organics and carbonates in the raw material in practice, it will be hardly possible to verify the effects observed here. In addition, the much larger quantities of material and temperature differences in the setting in the tunnel kiln lead to extensive overlapping of the clearly demarcated reactions observed in the thermal analysis.

For conventional operation of tunnel kilns with an air-brick mass ratio > 2, therefore, for a switch from natural gas to H2, no major changes of the existing kiln technology (e.g. change in the burner capacity, shifting of the kiln equipment, additional air injections or similar) are expected. How far the established phenomena ultimately change the energy consumption of the kiln must be ascertained in further studies in the field.


6.3 Firings in the laboratory kilns

6.3.1 Ceramic-related properties

The different kiln atmospheres for the laboratory firing are summarized in »Table 18.

The results of the ceramic-properties-related studies are shown for three selected clays in Figs. »16 to »18. The Dz-clay 21.406 is predominantly illitic-chloritic with 28 % quartz and 9 % feldspar. The VMz-clay 21.394 is predominantly kaolinitic and contains  33 % quartz. The HMz-clay 21.395 is predominantly illitic and has a quartz content of 15 % (»8). The dotted-line arrows show in which general direction the respective property can be expected to go with increasing water vapour content. The values of the gradient firings (cross-hatched) are individual values, those of the firings in the MUT laboratory kiln are mean values from five individual samples, shown with standard deviations in each case.

The figures in »Table 19 show by way of example the fired colours of the three clays for the different kiln atmospheres. The top photos document both sample surfaces in each case (position during drying top and bottom) as well as efflorescence and spalling. The bottom photos show the fracture surfaces with the matrix, textures and reduction cores in each case.

For the tested raw materials, it was established:

During firing in water vapour, the strongest sintering took place in comparison with other kiln atmospheres in each case. This is shown by the lowest water absorption as well as the biggest firing shrinkage and flexural strength.

The materials fired in the H2 atmosphere showed an equal to slightly stronger sintering in comparison with those fired in natural gas and the oxidizing firing conditions.

For the mixes with the poreformers and some HMz clays, the values for flexural strength, water absorption and firing shrinkage were almost independent of the kiln atmosphere. Moreover, the results of the HMz materials differ insignificantly on account of the lower firing temperature.

The influence of the kiln atmosphere was most clearly manifest at a firing temperature of 1 030 °C and least at 900 °C.

The values of the mixes lay between the values of the starting materials.

The compressive strengths correlated with the values for flexural strength. The values scattered more widely, however, on account of pronounced textures within the cylinders.

The mass loss was independent of the kiln atmosphere for all materials.

The occurrence of isolated lime spot defects on the sample surface could be attributable to coarse-grained carbonates in the raw materials and occurred independent of the kiln atmosphere.

An influence of the kiln atmosphere on the firing in of existing drying efflorescence was not established.

The fired colour is dependent on the iron content in the raw material and kiln atmosphere. The raw materials with 2.86 – 6.65 % iron oxide (Fe2O3) fired to a light- to dark-grey colour in the water vapour atmosphere. For the lower water vapour contents (H2, natural gas) as well as in oxidizing firing, a light-red to red fired colour was achieved independent of the kiln atmosphere, the samples were slightly lighter after the gradient firing in isolated cases. The described colour development of the two red-firing clays was also shown for all other red-firing materials. One exception was the kaolinitic Clay 21.394 with just 1.69 % Fe2O3. It fired to a light colour both in water vapour as well as in oxidizing atmosphere, the colour was greyish in water vapour, while it tended to be reddish in other atmospheres.


6.3.2 Thermal conductivity

The thermal conductivity was determined on Clays 21.394 and 21.395. The results are shown as a function of the body density in »19. The grey arrow indicates the expected dependence of the thermal conductivity on the apparent density (cf. [42]).

The results show:

With increasing bulk density of the material, the thermal conductivity increased in an almost linear function independent of the material (grey arrow). This can be attributed to the widely known fact that fewer insulating pores are always present in a dense material.

The thermal conductivity and the bulk density increased for Clay 21.394 with increasing water vapour content. The values for these properties were lowest for the oxidizing firing conditions and highest for the water vapour atmosphere. The thermal conductivity values in the H2 and natural gas atmosphere were approximately equal.

For Clay 21.395, thermal conductivity and bulk density were almost independent of the kiln atmosphere.

The bulk density of Clay 21.394 is somewhat lower than that of Clay 21.395, its thermal conductivity, however, is much lower. This shows that the thermal conductivity as well as the bulk density resulting from the pore volume depend on other factors like the mineral content of the fired material, the pore form and the pore distribution. To determine this influence, on the fired material, the mineral content (XRD) and pore distribution (Hg-porosimeter) were determined and images captured with a scanning electron microscope (SEM).

The linear relationship between the thermal conductivity and the bulk density of fired bricks is known [43]. Also known is that, on account of the mineral content, considerable deviations from the linearity can sometimes occur. In tests on refractory materials it was established that the thermal conductivity increases with increasing Al2O3 and decreases with rising SiO2 content. Fired high-carbonate clays with a carbonate content between 15 – 25 % exhibited mostly higher thermal conductivities that expected based on the bulk density alone. With increasing mica content in the clay, the thermal conductivity decreased more strongly than expected [42]. In [43] it was also shown that the thermal conductivity increased with rising quartz content.

The mineral content of the tested clays differ significantly. The fine-grained Clay 21.394 is predominantly kaolinitic, has a high Al2O3 content, contains no carbonates, with twice as much quartz as Clay 21.395. The coarse-grained Clay 21.395, on the other hand, is predominantly illitic, contains less Al2O3 and quartz than Clay 21.394 as well as many carbonates. With the above-described opposite influences of the minerals, it is not possible to make any definite conclusions with regard to the different thermal conductivity based on mineral content of the clays along with their bulk density.


6.3.3 Pore distribution and volumes

On the plates of Clay 21.394 and Clay 21.395 fired at 975 °C in different atmospheres, following determination of the thermal conductivity, tests with the Hg-porosimeter were conducted. For this purpose, pieces were sawn out of the middle of the plates in each case and partial pieces of around 2 g and a size of 2 – 4 mm prepared. The results are summarized in »Table 20.

The pore distributions after firing of the two materials in different kiln atmospheres are shown in »20.

For Clay 21.394, it was established:

The intruded Hg-volume is lowest for the water vapour atmosphere. This can be explained based on the relatively large pores compared to in the H2, natural gas as well as oxidizing kiln atmosphere.

The intruded Hg-volumes, on the other hand, are almost the same for the H2, natural gas as well as oxidizing atmosphere and can be attributed to the similar pore distribution.

The inner surface decreases with increasing water vapour content in the kiln atmosphere. Here the values of water vapour and H2 differ by almost double the amount. For the H2, natural gas and oxidizing atmosphere, the difference is just 14 %.

The fired material shows a monomodal pore distribution in all firing conditions.

In the case of the water vapour atmosphere, both the pore maximum as well as the pore distribution curve move to larger pores, which can result in a higher bulk density and thermal conductivity.

For the kiln atmospheres oxidizing, natural gas and H2, the pore maximum and the curve shapes are similar. Accordingly, only minor differences in the bulk density and thermal conductivity are determined here.

For Clay 21.395, the following was established:

The intruded Hg-volume differs less as a function of the firing conditions than for Clay 21.394 and is almost independent of the water vapour content in the kiln atmosphere.

The inner surfaces also vary widely, without exhibiting any dependence on the kiln atmosphere.

The materials fired in the H2 and oxidizing kiln atmosphere exhibit the same bimodal distribution. The two curves are almost congruent. The pore distribution of the materials fired in a natural gas atmosphere, on the other hand, shows a somewhat less clear bimodal distribution. Here the two maxima of the pore sizes have shifted towards each other. The high water vapour content, on the other hand, leads to a merging of the two pore maxima to one maximum (monomodal pore distribution).

The results show a minor influence of the water vapour content on the intruded Hg-volume and the inner surface despite the change in the pore distribution. The influence of the kiln atmosphere is therefore just as low as on the thermal conductivity and the bulk density.

Overall, it was established that above all the high water vapour content for the two clays leads to a change in the pore distribution and pore sizes. Despite the same firing temperature, the intruded Hg-volume depends more strongly on the water vapour content in the kiln atmosphere for Clay 21.394 than for Clay 21.395.


6.3.4 Mineral content (XRD)

With the plates of Clays 21.394 and 21.395 fired in different atmospheres at 975 °C, following determination of the thermal conductivity, the mineral content was determined by means of XRD.

As expected, at the comparatively low firing temperatures, the two clays have a similar quartz content before and after firing. Feldspar and haematite in Clay 21.395 were also present in similar percentages before and after firing.

For Clay 21.394, after firing, the clay minerals kaolinite, illite and mica had transformed almost completely into X-ray amorphous phases. The content of amorphous phase is lower than in Clay 21.395. The peak heights of feldspar and quartz were reduced by the firing.

For Clay 21.395, after firing the clay minerals illite, mica and chlorite have almost completely converted into X-ray amorphous phases. The carbonates calcite and dolomite also formed amorphous phases. In addition, traces of diopside (calcium silicate) were found. The content of amorphous phase is higher than for Clay 21.394. The peak heights of feldspar and quartz also decreased slightly as a result of firing.

Overall, it was established that the different kiln atmospheres did not lead to any significant change in the mineral phases in the case of both clays. Both materials have, however, different starting minerals, on the basis of which the different values for the thermal conductivity together with the different bulk densities can be explained.


6.3.5 Microstructure (SEM)

On the plates of Clays 21.394 und 21.395 fired in the different atmospheres at 975 °C, following determination of the thermal conductivity, SEM analyses were conducted. For this purpose, pieces were sawn out of the middle of the plates, embedded in epoxide resin in vacuum and partially ground. During grinding of the samples, it was shown that the microstructure of Clay 21.394 was more unstable than that of Clay 21.395.

The minerals in Clay 21.394 are plate-like and oriented in the direction of pressing. The matrix shows slight compaction and seems loose and flaky, with a house-of-cards structure. The pores are correspondingly elongated. The quartz and feldspar grains are not firmly bonded with the matrix. Both can have an adverse influence on the strength. The pores can be attributed to the water evaporated out of the clay minerals as the clay does not contain any carbonates. From the microstructure, it can be concluded that the firing conditions were not sufficient to change the starting structure of the unfired clay and sinter the components firmly with each other. Reaction processes like conversion or melting cannot be definitely detected. No differences could be established between the individual firing conditions.

The minerals in Clay 21.395 do not appear platey compared with those in Clay 21.394, but already sintered together in a sort of flowing structure, which is reflected not only in the appearance of the microstructure but in higher strengths. Most of the quartz and feldspar grains are firmly enclosed by the matrix. This early sintering at 975 °C is typical for illitic clays. A difference in the microstructure depending on the kiln atmosphere could not be definitely detected. In both directions of measurement, the matrix contains on the one hand many round pores in a size of 10 – 20 µm. At its edges, CaO can be found, in the pores, needle-like carbonates were detected. In a water vapour atmosphere, the edge formed around the pores is somewhat more pronounced than in an oxidizing atmosphere and after the combustion of natural gas as well as H2. The matrix also appears somewhat denser in atmospheres containing water vapour than in the firing conditions with less as well as completely without water vapour.

The microstructures of the two clays differ clearly in respect of their pore structure. Parallel to the direction of measurement of the thermal conductivity, Clay 21.394 has many pores that are connected with each other and can therefore reduce thermal conductivity as a result of a kind of insulating effect. Clay 21.395, on the other hand, exhibits a comparatively large number of open coarse and fine pores that are mostly connec-ted with each other, and resin used for sample preparation could penetrate almost completely into these pores. This open pore structure can lead to an increase in the thermal conductivity.

Overall, it can be established that the high water vapour content only led to slightly stronger sintering of the microstructure for Clay 21.395. For Clay 21.394, this could not be observed.


7 Conclusion and outlook

The use of hydrogen as fuel gas for the decarbonization of brick plants is technically feasible. However, to effectively reduce the existing CO2 emissions from the fuel, the H2 content in the natural gas should be at least 80 %. H2 has different properties to those of natural gas and requires modification of the existing natural gas equipment such as pipelines, seals, valves, burners and safety equipment. Flame monitoring of the burners is possible by means of UV sensors. The higher flame temperature leads in conventional burners to a higher firing point load as well as to higher NOx content in the exhaust gas. In addition, during stoichiometric combustion of H2 with air, 67 % more water vapour is formed, increasing the dew point of the exhaust gas.

In the tunnel kiln, the pure exhaust gas is thinned by the cooling air so that the water content only increases by 30 – 40 % compared with natural gas. With conventional operation of tunnel kilns with an air/brick ratio of > 2, neither a change in the burner power nor the firing curve is to be expected, i.e. a shift of the existing kiln equipment does not appear necessary. The procurement and in-plant production of H2 is currently much more expensive than natural gas and so far economically inefficient for brick plants.

For every firing process, a certain process heat is necessary, accordingly the specific energy requirement is initially the same for both fuel gases. Hydrogen, however, requires 17 % less combustion air, while 9 % less exhaust gas is formed than with natural gas. As a result, in the entire process, approx. 2 % energy can be saved. This saving is, however, offset again with an increase of the exhaust gas temperature as a result of the higher dew point. Although the radiation of the two flames differs, the heat transfer is the same for the combustion of both gases. Because of a somewhat stronger sintering of the raw materials, lowering of the firing temperature and consequently an energy saving at the kiln are possible. This phenomenon, however, is dependent on the water vapour content and material composition.

The investigations into ceramic raw materials confirm that the fired properties are fundamentally influenced by the water vapour content in the kiln atmosphere. The results showed, however, that the fired properties with the expected low water vapour contents in the tunnel kiln for natural gas and H2 firing as well as in dry air differ little. The fired colour did not change at all or only by nuances. In contrast, a high water vapour content in the kiln atmosphere led to a stronger sintering with:

The increase of strength, firing shrinkage and bulk density, and accordingly the thermal conductivity,

The reduction of the water absorption and pore enlargement,

The change in the fired colour (reduction colours),

No change in the mass loss.

The influence of water vapour on the fired properties is additionally dependent on the clay composition (mineral content, grain size) and the firing temperature. It was established that especially in the case of a high water vapour content in the kiln atmosphere, the typical reactions like the oxidation of organic components, the clay mineral dehydroxylation and the carbonate decarbonisation shift to other temperatures. These shifts are, however, estimated to the be limited in the real conditions in the tunnel kiln.

A prediction of the firing behaviour of heavy clay raw materials is not possible on account of the presence of multimaterial mixes with different particle sizes and shaping properties. For that reason, the kiln equipment should always be adapted to the specific raw material, and prior to the design of a kiln the firing behaviour of the materials should be tested in the laboratory.

Under the current conditions prevailing in the tunnel kiln, the tested raw materials can be used without any restrictions also for firing with H2. Also for the raw materials with paper industry residue, sawdust and polystyrene for pore formation, no adverse properties could be determined in the case of firing with H2. So in terms of raw materials and the firing process, nothing stands in the way of the use of renewably produced H2. For the a next step, field tests on original brick settings and real air quantities in the tunnel kiln are conceivable. Here the composition of the kiln atmosphere in the tunnel kiln should be determined for both fuel gases and correlated with the ceramic properties.

The project from which these results were derived was funded by the Free State of Thuringia under the number 2021 WFN 0001 and co-financed with resources from the European Union within the framework of the European Regional Development Fund (EFRE).


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