Method for setting an oven atmosphere in a heat-treatment oven

Abstract

The invention relates to a method for setting an oven atmosphere in a directly heated heat-treatment oven, wherein the heat-treatment oven has at least one burner, which is operated with a fuel gas and an oxygen-containing gas, which are burned to form a combustion gas, wherein, dependent on the composition of the fuel gas and the composition of the oxygen-containing gas and the mixture thereof, the combustion gas has a defined composition with a defined water vapour partial pressure, wherein hydrogen is used in the fuel gas in a proportion of at least 10% by volume, and the heat-treatment oven is additionally flooded with a water-vapour-free and / or hydrogen-free gas, wherein, as a result, the water-vapour-free and / or hydrogen-free gas mixes with the combustion gas in such a way as to bring about a water vapour partial pressure of the mixture in the oven atmosphere of the heat-treatment oven that is less than the defined water vapour partial pressure of the combustion gas.

Description

[0001] Method for adjusting a furnace atmosphere in a heat treatment furnace

[0002] The invention relates to a method for adjusting a furnace atmosphere in a directly heated heat treatment furnace.

[0003] Heat treatment furnaces, such as direct-fired furnaces (DFFs), are established furnaces used in practice for the heat treatment of metals. These are typically fueled by fossil fuels such as natural gas. Since combustion takes place in the furnace, direct heating can be used to create a reducing or oxidizing furnace atmosphere, depending on the set air ratio (lambda value of the combustion gas). The furnace therefore contains the combustion gas from the burners, which contains a high proportion of water and, depending on the air ratio, oxygen (O2) and carbon dioxide (CO2) or hydrogen (H2) and carbon monoxide / carbon dioxide (CO / CO2).

[0004] As part of the globally demanded decarbonization, plants powered by fossil fuels are to be converted or converted to more environmentally friendly fuels or energy sources, such as hydrogen, in order to reduce or ultimately avoid the use of fossil energy.

[0005] Decarbonization requires a reduction in the use of fossil fuels and energy sources and, in turn, a reduction in CO2 emissions.

[0006] A change, particularly in the heat treatment of metals in a directly heated furnace, would result in a new furnace atmosphere with highly influential parameters regarding the material properties to be achieved in the final product or intermediate product of the heat-treated metal. If a heat treatment furnace were to be converted from a fossil fuel (natural gas) to an alternative, hydrogen-containing fuel, this would have a massive impact on the atmosphere during combustion of these fuels and thus also on the metals being heat-treated and their surface(s). When hydrogen-containing fuels are burned, a larger amount of water vapor is generated compared to natural gas, which would result in a higher water vapor partial pressure in the furnace atmosphere.This results in a higher tendency for oxidation (scale formation) during heat treatment due to oxygen-affine elements in the metal, which occurs particularly at the metal surface. The presence of a higher water vapor partial pressure affects the bond between the scale and the metal surface—in simple terms, the adhesion to the metal surface.

[0007] In particular, steel (as a metal) is very sensitive to an increase in the partial pressure of water vapor in furnace atmospheres during heat treatment. This can also promote undesirable hydrogen ingress into the steel, leading to problems in high-strength steels, for example, a condition known as "delayed fracture."

[0008] For example, heat treatment of a steel in a 100% steam atmosphere can reduce scale on the steel surface, such as FeO at a furnace chamber temperature of 1369 °C and Fe3O4 or Fe2O3 at a furnace chamber temperature of 1539 °C. Reducing the temperature in steam can increase the adhesion of the scale due to a shift in the phase proportions and make it more "sticky." Furthermore, the hot steam can intensify and accelerate scaling. Some of the scale is difficult to remove (approx. 20-60% [also depending on the alloy]), particularly the scale close to the substrate. The scale layer on top of the scale close to the substrate, however, is very brittle and can be removed with even minimal mechanical stress. It can be assumed that the increased steam partial pressure leads to increased material loss due to accelerated scale formation.

[0009] Heat treatment of steel in a steam atmosphere can alter the grain layers in the microstructure, which can lead to undesirable, premature grain boundary oxidation, which in turn can cause coating and / or surface defects. Due to the increased scale formation, grain boundary oxidation can also occur more rapidly and penetrate deeper into the substrate.

[0010] Heat treatment of steel in a steam atmosphere can also lead to a greater depth of decarburization, which means that the properties of the intermediate or final product are also affected, particularly adversely. This can manifest itself, for example, in mechanical properties that are outside the required range and can also lead to poorer surface properties or magnetic properties. Decarbonization in the application of heat treatment of metals in directly heated furnaces, especially steels, would therefore not only be a simple switch from fossil to non-fossil fuels, but would also involve a complex manipulation of the product parameters.

[0011] For example, EP 2 762 599 A1 and EP 3 109 338 A1 disclose the use of DFF furnaces in hot-dip coating lines for cold-rolled steel strip. Furthermore, DE 10 2011 053 698 B3 discloses the use of DFF furnaces for austenitization in hot-forming lines for press-hardened steels.

[0012] The object of the present invention is to further develop this process in such a way that it reduces the use of fossil fuels and does not have the aforementioned disadvantages.

[0013] This object is achieved by a method having the features of claim 1. Further embodiments are described in the subclaims.

[0014] The teaching thus relates to a method for adjusting a furnace atmosphere in a directly heated heat treatment furnace, wherein the heat treatment furnace has at least one burner operated with a fuel gas and an oxygen-containing gas, which are combusted to form a combustion gas. Depending on the composition of the fuel gas and the composition of the oxygen-containing gas and their mixture, the combustion gas has a defined composition with a defined water vapor partial pressure. Essential to the invention is that hydrogen is present in the fuel gas in a proportion of at least 10 vol.-% is used, and the heat treatment furnace is additionally flooded with a water vapor-free and / or hydrogen-free gas, whereby the water vapor-free and / or hydrogen-free gas mixes with the combustion gas in such a way that a water vapor partial pressure of the mixture in the furnace atmosphere of the heat treatment furnace is lower than the defined water vapor partial pressure of the combustion gas.

[0015] An increase in hydrogen in the fuel gas and thus an increase in the water vapor partial pressure in the resulting combustion gas must be counteracted by "diluting" the combustion gas by targeted mixing with a water vapor-free and / or hydrogen-free gas in order to create a furnace atmosphere in the heat treatment furnace which has a lower water vapor partial pressure compared to the (pure) combustion gas.

[0016] In particular, the determination or recording of the water vapor partial pressure is familiar to the person skilled in the art.

[0017] This measure makes it possible to set a furnace atmosphere that can correspond to the currently known level of natural gas-fired burners. The hydrogen used in the fuel gas can, for example, be produced and provided in water electrolysis using renewable energies such as wind, water and solar energy. Any oxygen required can also be produced and used by electrolysis using renewable energies (solar, wind, water, etc.). The water vapor-free and / or hydrogen-free gas for the mixture can contain or consist of dry air, nitrogen (N2), argon (Ar), carbon dioxide (CO2) or a mixture thereof. Other gases or mixtures of gases that do not contain hydrogen or hydrogen compounds and are suitable for the heat treatment of metals can also be used accordingly.

[0018] The oxygen-containing gas for operating the burner can be air, for example ambient air, oxygen or a combination of air and oxygen.

[0019] In particular, hydrogen may be present in the fuel gas in a proportion of at least 20 vol.%.

[0020] Preferably, hydrogen may be contained in the fuel gas in a proportion of at least 40 vol.%.

[0021] Hydrogen can preferably be contained in the fuel gas in a proportion of at least 60 vol.%.

[0022] Particularly preferably, hydrogen can be contained in the fuel gas in a proportion of at least 80 vol.%.

[0023] Further preferably, hydrogen can be present in the fuel gas in a proportion of at least 98 vol. This embodiment, for example, comprises 100% use of hydrogen; in other words, the fuel gas consists of hydrogen, with impurities in the fuel gas permitted at up to 0.5 vol.%, in particular up to 0.2 vol.%, preferably less than 0.1 vol.%. Impurities cannot be avoided technically or only with considerable equipment complexity.

[0024] If the fuel gas does not consist entirely of hydrogen, it contains additional amounts of methane (CH4) and / or carbon monoxide (CO) in addition to hydrogen to reach 100 vol.%, along with impurities, which are permitted up to 0.5 vol.%, in particular up to 0.2 vol.%, and preferably less than 0.1 vol.%. Particularly when using natural gas, the methane content can vary and thus also include other components, such as ethane, propane, ethene, and butane, individually or in combination.

[0025] In order to avoid negatively influencing and / or even increasing the energy of the combustion gas, it may be advantageous if, according to one embodiment, the water vapor-free and / or hydrogen-free gas is heated before flooding the heat treatment furnace and / or the burner. To essentially maintain the energy level of the combustion gas, heating takes place to a temperature that preferably corresponds to the temperature of the combustion gas + / - 300 °C. The temperature can thus correspond to a temperature window between minus and plus 300 °C relative to the temperature of the combustion gas. The temperature of the combustion gas can be measured using means known to those skilled in the art. Preheating the combustion gas and / or the oxidizing agent can lead to an increase in the adiabatic flame temperature.

[0026] In order to economically utilize the exhaust gas discharged from the heat treatment furnace, it may be advantageous to use part or all of the exhaust gas to heat the hydrogen-free and / or water vapor-free gas. In this case, too, the means for exhaust gas utilization or heat transfer are known to those skilled in the art.

[0027] Alternatively or in addition to the use of exhaust gases, the (additional) heating can also be carried out by other means, for example electrically, if a higher temperature level is required compared to the exhaust gas temperature.

[0028] The heat treatment furnace in question is particularly preferably used for steels or steel alloys in any form, whether as slabs, plates, sheets, strips, or (pre-)formed sheet metal components. The heat treatment temperature is generally between 200 °C and 1350 °C, in particular between 400 °C and 1260 °C, whereby this temperature refers to the temperature of the metal to which it is to be heated. The furnace atmosphere temperature or furnace chamber temperature can certainly be higher.

[0029] Furthermore, the temperature of the burner flame also influences the temperature of the furnace atmosphere, or rather, the temperature of the furnace chamber. The combustion temperature with ambient air and natural gas is approximately 1970 °C, with ambient air and hydrogen approximately 2130 °C, with oxygen and natural gas approximately 2860 °C, and with oxygen and hydrogen approximately 3080 °C.

[0030] Furthermore, the water content (water vapor and thus water vapor partial pressure) plays a crucial role in furnace atmospheres used for the heat treatment of metals. This determines, among other things, whether the furnace atmosphere has a reducing or oxidizing effect on metals. A common method for determining water content, well known to those skilled in the art, is the so-called dew point determination. Depending on the application, the dew point of a furnace atmosphere can range between -70°C and +35°C, especially for steels. Negative dew points generally indicate a reducing furnace atmosphere.

[0031] The invention is explained in more detail using the following embodiments in conjunction with the drawing.

[0032] In Figure 1, the invention is shown using the example of a schematic illustration. A directly heated heat treatment furnace (1) has at least one burner (2) which is operated with a fuel gas (3) and an oxygen-containing gas (4), which are combusted to form a combustion gas (10) in the heat treatment furnace (1), wherein, depending on the composition of the fuel gas (3) and the composition of the oxygen-containing gas (4) and their mixture, the combustion gas (10) has a defined composition with a defined water vapor partial pressure. Since the fuel gas (3) contains hydrogen with a proportion of at least 10 vol.-% is used, the heat treatment furnace (1) is additionally flooded with a water vapor-free and / or hydrogen-free gas (5), whereby the water vapor-free and / or hydrogen-free gas (5) mixes with the combustion gas (10) in such a way that a water vapor partial pressure of the mixture in the furnace atmosphere (9) of the heat treatment furnace (10) is lower than the defined water vapor partial pressure of the combustion gas (10). Before flooding the heat treatment furnace (1), the water vapor-free and / or hydrogen-free gas (5) can be heated. In this case, an exhaust gas (7) can be discharged from the heat treatment furnace (1), which can be used partially or completely to heat the hydrogen-free gas (5) by means of a suitable heat exchanger (6).Alternatively or additionally, the water vapor-free and / or hydrogen-free gas (5) can be heated, in particular additionally, for example by an electric heating device (11), shown in dashed lines, with which a temperature increase of the water vapor-free and / or hydrogen-free gas (5) above the temperature of the combustion gas (10) would also be possible. With the furnace atmosphere (9) set according to the invention, a heat treatment of a metal (8), for example a steel, preferably a steel alloy, is possible without the disadvantages of an altered or different type of scale formation on the surface of the metal / steel (8), despite the use of non-fossil fuels, if hydrogen is used in proportions between 10 and 100 vol.% in the fuel gas (3).

[0033] Figures 2 and 3 each show a diagram when natural gas, assuming approximately 99 vol.% methane, is used as the fuel with a proportion of between 0 and 100 vol.% hydrogen (abscissa). The left indicates no hydrogen and 100 vol.% natural gas; the right, however, indicates no natural gas and 100 vol.% hydrogen in the fuel gas. Ambient air (Figure 2) and oxygen (Figure 3) were used as the oxygen-containing gases for the burner, with an air / fuel ratio of 1.1 being considered in the calculation.

[0034] Depending on the composition of the fuel gas, the components of the combustion gas (left ordinate) are also shown in the diagram. On the right ordinate, the generated combustion gas volume in m can be calculated depending on the composition of the fuel gas. 3 per m 3 used fuel gas.

[0035] The results shown in Figures 2 and 3 were determined numerically and show the influence of non-fossil fuels, such as hydrogen in the fuel gas, on the composition of the combustion gas.

[0036] It is surprising that when ambient air is used as an oxygen-containing gas for combustion, a reduction in the CO2 content in the combustion gas is only possible with a hydrogen content of at least 35 vol.% in the fuel gas, see Figure 2. Furthermore, Figure 2 clearly shows that a fuel gas consisting of 100 vol.% hydrogen has a combustion gas volume of 2.5 m 3 per m used 3Fuel gas (=hydrogen) cannot be undershot. However, Figure 3 shows that when oxygen is used as the oxygen-containing gas for combustion with 100% hydrogen as the fuel gas, the volume of the combustion gas corresponds essentially 1:1 to the volume of fuel gas used. A reduction in the CO2 content in the combustion gas can also be seen even at lower hydrogen contents (less than 35 vol%) in the fuel gas.

[0037] From a hydrogen content of 60%, the water vapor partial pressure (Fig. 1) begins to rise significantly. In Figure 2, the ratio is more extreme when hydrogen is burned with oxygen. With increasing volume fraction of hydrogen in the fuel gas, the water vapor partial pressure ultimately increases to a maximum when 100 vol.% hydrogen is used in the fuel gas. If 100 vol.% hydrogen is burned without "dilution" of the furnace atmosphere, this has a negative effect on the product properties of the metal, so the water vapor content of the furnace atmosphere can be reduced accordingly by adding air, for example, to 20 vol.%. This would lead to an improvement in further processing properties. "Dilution", for example with unheated air, would result in a drop in temperature, which could potentially remove necessary heating energy from the metal.

Claims

Patent claims 1. A method for adjusting a furnace atmosphere in a directly heated heat treatment furnace, wherein the heat treatment furnace has at least one burner which is operated with a fuel gas and an oxygen-containing gas, which are burned to form a combustion gas, wherein, depending on the composition of the fuel gas and the composition of the oxygen-containing gas and their mixture, the combustion gas has a defined composition with a defined water vapor partial pressure, characterized in that - hydrogen is used in the fuel gas with a proportion of at least 10 vol.%, and - the heat treatment furnace is additionally flooded with a water vapor-free and / or hydrogen-free gas, whereby the water vapor-free and / or hydrogen-free gas mixes with the combustion gas in such a way that a water vapor partial pressure of the mixture in the furnace atmosphere of the heat treatment furnace is lower than the defined water vapor partial pressure of the combustion gas.

2. The process according to claim 1, wherein hydrogen is contained in the fuel gas in a proportion of at least 20 vol.%.

3. The process according to claim 1, wherein hydrogen is present in the fuel gas in a proportion of at least 40 vol.%.

4. The process according to claim 1, wherein hydrogen is present in the fuel gas in a proportion of at least 60 vol.%.

5. The process according to claim 1, wherein hydrogen is present in the fuel gas in a proportion of at least 80 vol.%.

6. The process according to claim 1, wherein hydrogen is present in the fuel gas in a proportion of at least 98 vol.%. Method according to one of the preceding claims, wherein the water vapor-free and / or hydrogen-free gas is heated before flooding the heat treatment furnace. Method according to claim 7, wherein the heating takes place to a temperature corresponding to the temperature of the combustion gas + / - 300°C. Method according to one of claims 7 or 8, wherein an exhaust gas is discharged from the heat treatment furnace, which is used partially or completely to heat the water vapor-free and / or hydrogen-free gas.

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