EP4558650A1 - Method for setting an oven atmosphere in a heat-treatment oven - Google Patents

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

Method for adjusting a furnace atmosphere in a heat treatment furnace

The invention relates to a method for setting a furnace atmosphere in a directly heated heat treatment furnace.

Heat treatment furnaces, such as direct fired furnaces (also called DFF), are established furnaces in practice that are used for the heat treatment of metals. As standard, these are powered by fossil fuels such as natural gas. Since combustion takes place in the oven, a reducing or oxidizing oven atmosphere can be set using direct heating, depending on the air ratio (lambda value of the fuel gas) set. The furnace contains the combustion gas from the burners, which contains a high proportion of water and, depending on the air ratio, oxygen (0 ₂ ) and carbon dioxide (C0 ₂ ) or hydrogen (H ₂ ) and carbon monoxide/carbon dioxide (CO/CO ₂ ).

As part of the worldwide demand for decarbonization, plants powered by fossil fuels should in the future be converted to more environmentally friendly fuels or energy sources, such as hydrogen, in order to reduce or ultimately avoid the use of fossil energy.

Decarbonization requires a reduction in the use of fossil fuels or energy sources and, in turn, a reduction in CO ₂ emissions.

A change, particularly when heat treating metals in a directly heated furnace, would result in a new furnace atmosphere with very influential parameters with regard to the material properties to be achieved later on the end product or intermediate product of the heat-treated metal. If a heat treatment furnace is converted to an alternative, hydrogen-containing fuel with regard to its fossil fuel (natural gas), this will have a massive impact on the atmosphere when these fuels are burned and thus also on the metals to be heat-treated or their surface(s). When burning the hydrogen-containing fuels, a larger amount of water vapor is generated compared to natural gas, which ensures that there would be a higher water vapor partial pressure in the furnace atmosphere. The result of this is that there is a higher tendency for oxidation (scale formation) during heat treatment due to oxygen-affinous elements in the metal, which are particularly present on the surface of the metal metal is created. The presence of a higher water vapor partial pressure influences the connection between the scale and the metal surface, or, to put it simply, the adhesion to the metal surface.

In particular, steel (as a metal) is very sensitive to an increase in the water vapor partial pressure in furnace atmospheres during heat treatments. This can also promote an undesirable introduction of hydrogen into the steel, among other things. lead to problems with high-strength steels, which is known as a “delayed fracture”.

For example, by heat treating a steel in a 100% water vapor atmosphere, scale on the surface of the steel such as FeO at a furnace room temperature of 1369 °C and Fe ₃ O ₄ or Fe ₂ O ₃ at a furnace room temperature of 1539 °C can be reduced. By reducing in steam, the adhesion of the scale can increase and become more “sticky” due to the shift in the phase proportions. In addition, the scaling can be increased and accelerated by the hot steam. Parts of the scale are difficult to remove (approx. 20 - 60% [also depending on the alloy]), especially the scale close to the substrate. The scale layer, on the other hand, which lies on the scale close to the substrate, is very brittle and can be removed with even slight mechanical action. It can be assumed that the increased water vapor partial pressure results in increased material loss due to the accelerated scale formation.

By heat treating the steel in a steam atmosphere, the grain layers in the structure can be changed, which can lead to undesirable, advanced 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 quickly and also penetrate deeper into the substrate.

Heat treatment of a steel in a steam atmosphere can also lead to a higher decarburization depth, which means that the properties of the intermediate or final product are also influenced, in particular adversely. This can manifest itself, for example, in that the mechanical characteristics lie 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 influence on the product parameters.

For example, from EP 2 762 599 Al and EP 3 109 338 Al it is known to use DFF ovens in hot-dip coating lines for cold steel strips. Furthermore, it is known from DE 10 2011 053 698 B3, for example, to use DFF furnaces for austenitization in hot forming lines for steels to be press-hardened.

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

This task is solved by a method with the features of claim 1. Further refinements are described in the subclaims.

The teaching thus relates to a method for setting a furnace atmosphere in a directly heated heat treatment furnace, the heat treatment furnace having at least one burner which is operated with a fuel gas and an oxygen-containing gas, which are burned 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. It is essential for the invention that hydrogen is used in the fuel gas in a proportion of at least 10% by volume, and the heat treatment furnace is additionally flooded with a steam-free and/or hydrogen-free gas, whereby the steam-free and/or hydrogen-free gas is mixed with the Combustion gas mixes in such a way that a water vapor partial pressure of the mixture in the furnace atmosphere of the heat treatment furnace is caused to be smaller than the defined water vapor partial pressure of the combustion gas.

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 through 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.

In particular, the determination or detection of the water vapor partial pressure is familiar to those skilled in the art.

This measure makes it possible to set a furnace atmosphere that can correspond to the currently known level for burners fired with natural gas. The hydrogen used in the fuel gas can, for example, be generated and provided in water electrolysis using renewable energies such as wind, water and sun. Any oxygen that may be required can also be generated and used using electrolysis using renewable energies (sun, wind, water, etc.). The steam-free and/or hydrogen-free gas for the mixture can contain or consist of dry air, nitrogen (N ₂ ), argon (Ar), carbon dioxide (C0 ₂ ) 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.

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

In particular, hydrogen can be contained in the fuel gas in a proportion of at least 20% by volume.

Hydrogen can preferably be contained in the fuel gas in a proportion of at least 40% by volume.

Hydrogen can preferably be contained in the fuel gas in a proportion of at least 60% by volume.

Hydrogen can particularly preferably be contained in the fuel gas in a proportion of at least 80% by volume.

More preferably, hydrogen can be contained in the fuel gas in a proportion of at least 98% by volume. This embodiment includes, for example, a 100% use of Hydrogen, in other words, the fuel gas consists of hydrogen, with impurities in the fuel gas being permitted up to 0.5% by volume, in particular up to 0.2% by volume, preferably less than 0.1% by volume, Whereby contamination cannot be technically avoided or can only be avoided with a great deal of equipment expenditure.

If the fuel gas does not consist entirely of hydrogen, it contains, in addition to hydrogen, further amounts of methane (CH ₄ ) and/or carbon monoxide (CO), around 100% by volume, as well as impurities of up to 0.5% by volume, in particular up to 0.2% by volume, preferably less than 0.1% by volume, can be permitted. Particularly when using natural gas, the proportions of methane can vary and can therefore also include other components, such as ethane, propane, ethene and butane, individually or in combination.

In order not to negatively influence and/or even increase the energy of the combustion gas, it can be advantageous if, according to one embodiment, the steam-free and/or hydrogen-free gas is heated before flooding the heat treatment furnace and/or the burner. In order to essentially maintain the energy level of the combustion gas, the heating takes place to a temperature which preferably corresponds to the temperature of the combustion gas +/- 300 ° C. The temperature can therefore correspond to a temperature window between minus and plus 300 ° C based on the temperature of the combustion gas. The temperature of the combustion gas can be recorded using means known to those skilled in the art. Preheating the fuel gas and/or the oxidizing agent can lead to an increase in the adiabatic flame temperature.

In order to be able to use the exhaust gas removed from the heat treatment furnace economically, 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 using exhaust gases or heat transfer are known to those skilled in the art.

Alternatively or in addition to using the exhaust gas, the (additional) heating can also be carried out using other means, for example electrically, if in particular a higher temperature level is required compared to the exhaust gas temperature.

The heat treatment furnace in question here is particularly preferably used for steels or steel alloys in any form, whether as a slab, plate, sheet, strip or (pre-)formed sheet metal component. The temperature for the heat treatment is in Essentially between 200 °C and 1350 °C, in particular between 400 °C and 1260 °C, this temperature referring to the temperature of the metal to which it is to be heated. The oven atmosphere temperature or oven chamber temperature can certainly be higher.

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

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

The invention is explained in more detail using the following exemplary embodiments in conjunction with the drawing.

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 burned to form a combustion gas (10) in the heat treatment furnace (1), in 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 hydrogen is used in a proportion of at least 10% by volume in the fuel gas (3), the heat treatment furnace (1) is additionally flooded with a steam-free and/or hydrogen-free gas (5), thereby producing the steam-free and/or hydrogen-free gas (5) mixes with the combustion gas (10), such that a water vapor partial pressure of the mixture in the furnace atmosphere (9) of the heat treatment furnace (10) is smaller than the defined water vapor partial pressure of the combustion gas (10). Before flooding the heat treatment furnace (1), the steam-free and/or hydrogen-free gas (5) be heated. An exhaust gas (7) can be removed 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 steam-free and/or hydrogen-free gas (5), in particular additionally, can be heated, for example by an electrical heating device (11), shown in dashed lines, with which the temperature of the steam-free and/or hydrogen-free gas (5) can also be increased above the Temperature of the combustion gas (10) would be possible. With the furnace atmosphere (9) set according to the invention, heat treatment of a metal (8), for example a steel, preferably a steel alloy, is possible without the disadvantages of a changed 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% by volume in the fuel gas (3), possible.

Figures 2 and 3 each show a diagram when natural gas is used as fuel, assuming approximately 99% by volume of methane, with a proportion of between 0 and 100% by volume of hydrogen (abscissa). Left means no hydrogen and 100% by volume of natural gas, while on the right means no natural gas and 100% by volume of hydrogen in the fuel gas. The oxygen-containing gas for the burner was ambient air (Figure 2) and oxygen (Figure 3), taken into account in the calculation with an air ratio of 1.1.

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, depending on the composition of the fuel gas, the volume of combustion gas produced can be determined in m ³ per m ³ of fuel gas used.

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.

What is surprising, when using ambient air as an oxygen-containing gas for combustion, is that a reduction in the CO ₂ content in the combustion gas is only possible with a hydrogen content of at least 35 vol.% in the fuel gas, see Figure 2. Figure 2 also shows It is clear that a fuel gas consisting of 100% by volume of hydrogen cannot fall below a combustion gas volume of 2.5 m ³ per m ³ of fuel gas (=hydrogen) used. On the other hand, Figure 3 shows, when using oxygen as an oxygen-containing gas for combustion with 100% hydrogen as the fuel gas, that the volume of the combustion gas corresponds essentially 1 to 1 to the volume of the fuel gas used. A reduction in the CO ₂ content in the combustion gas can also be seen with lower hydrogen proportions (less than 35% by volume) in the combustion gas.

From a hydrogen content of 60%, the water vapor partial pressure (Fig. 1) begins to rise significantly. In Figure 2, when hydrogen is burned with oxygen, the ratio is more extreme. As the volume fraction of hydrogen in the fuel gas increases, the water vapor partial pressure ultimately increases to a maximum when 100% by volume of hydrogen is used in the fuel gas. If 100% by volume of hydrogen is burned without “diluting” the furnace atmosphere, this has a negative effect on the product properties of the metal, so that the water vapor content of the furnace atmosphere can be reduced accordingly by adding air, for example, to 20% by volume. This would lead to an improvement in the further processing properties. “Diluting”, for example with air that has not been preheated, would result in a drop in temperature, which would potentially deprive the metal of necessary heating energy.

Claims

Patent claims 1. A method for setting a furnace atmosphere in a directly heated heat treatment furnace, the heat treatment furnace having at least one burner which is operated with a fuel gas and an oxygen-containing gas, which are burned to form a combustion gas, depending on the composition of the fuel gas and the Composition of the oxygen-containing gas and its 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 in a proportion of at least 10% by volume, and - the heat treatment furnace is additionally flooded with a steam-free and/or hydrogen-free gas, whereby the steam-free and/or hydrogen-free gas mixes with the combustion gas, such that a water vapor partial pressure of the mixture in the furnace atmosphere of the heat treatment furnace is smaller than the defined water vapor partial pressure of the combustion gas becomes. 2. The method according to claim 1, wherein hydrogen is contained in the fuel gas in a proportion of at least 20% by volume. 3. The method according to claim 1, wherein hydrogen is contained in the fuel gas in a proportion of at least 40% by volume. 4. The method according to claim 1, wherein hydrogen is contained in the fuel gas in a proportion of at least 60% by volume. 5. The method according to claim 1, wherein hydrogen is contained in the fuel gas in a proportion of at least 80% by volume. 6. The method according to claim 1, wherein hydrogen is contained in the fuel gas in a proportion of at least 98% by volume. Method according to one of the preceding claims, wherein the steam-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 which corresponds to the temperature of the combustion gas +/- 300 °C. Method according to one of claims 7 or 8, wherein an exhaust gas is removed from the heat treatment furnace, which is used partly or completely for heating the steam-free and / or hydrogen-free gas.