Method for setting the oven atmosphere inside a heat treatment oven

Abstract

The present invention relates to a method for setting an oven atmosphere in a direct heating type heat treatment oven, wherein the heat treatment oven has at least one burner operated by a fuel gas and an oxygen-containing gas that are combusted to form combustion gas, and depending on the composition of the fuel gas, the composition of the oxygen-containing gas, and their mixture, the combustion gas has a defined composition with a defined water vapor partial pressure, hydrogen is used in the fuel gas at a rate of at least 10% by volume, the heat treatment oven is further filled with a gas that does not contain water vapor and / or does not contain hydrogen, and as a result, the gas that does not contain water vapor and / or does not contain hydrogen is mixed with the combustion gas such that the water vapor partial pressure of the mixture in the oven atmosphere of the heat treatment oven is lower than the defined water vapor partial pressure of the combustion gas.

Description

Technical Field

[0001] The present invention relates to a method for establishing an in-furnace atmosphere in a direct heating type heat treatment furnace.

Background Art

[0002] A heat treatment furnace, for example, a direct heating furnace (also called a direct firing furnace, DFF), is a furnace for established practical use in the heat treatment of metals. These are supplied with fossil fuels, such as natural gas, in a standard manner. Since combustion takes place inside the furnace, it is possible to establish a reducing or oxidizing in-furnace atmosphere using direct heating according to the established air ratio (lambda value of the fuel gas). Therefore, what exists in the furnace is a combustion gas for a burner containing 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).

[0003] In view of the global demand for decarbonization, plants operated by fossil fuels should be retrofitted or converted in the future to more environmentally friendly fuels or energy carriers, such as hydrogen, in order to reduce or ultimately avoid the use of fossil energy.

[0004] Decarbonization requires a reduction in the use of fossil raw materials or energy carriers and, consequently, a reduction in the associated CO2 emissions.

[0005] In particular, by conversion when heat-treating metals in a directly heated furnace, a new furnace atmosphere can be brought about that has parameters which are very influential regarding the physical properties achieved at a later stage of the final or intermediate product of the heat-treated metal. Thus, when a heat treatment furnace is converted to an alternative hydrogen-containing fuel with respect to its fossil fuel (natural gas), this also has a major impact on the atmosphere during the combustion of these fuels and thus on the metal being heat-treated or its (one or more) surfaces. In the combustion of hydrogen-containing fuels, a large amount of water vapor is produced compared to natural gas, which ensures that a higher partial water vapor pressure can exist in the furnace atmosphere. As a result, elements with an oxygen affinity in the metal are formed especially on the surface of the metal, increasing the tendency for oxidation (scale formation) during heat treatment. The presence of a higher partial water vapor pressure affects the bond between the scale and the metal surface, i.e., the adhesion on the metal surface, to put it simply.

[0006] In particular, steel (as a metal) is very sensitive to any increase in the partial water vapor pressure in the furnace atmosphere during heat treatment. This also promotes the unwanted introduction of hydrogen into the steel and can lead to problems especially in high-strength steels known as "delayed fracture".

[0007] For example, the heat treatment of steel in a 100% water vapor atmosphere can reduce the scale on the surface of the steel such as FeO at a furnace temperature of 1369 °C and Fe3O4 or Fe2O3 at a furnace temperature of 1539 °C. As the water vapor decreases, the adhesion of the scale can increase due to the movement of the phase fraction and thus become more "sticky". Furthermore, scale formation can be enhanced by hot water vapor and proceed in an accelerated manner. Here, part of the scale, especially the scale near the substrate, can only be removed with difficulty (about 20 - 60% [also depending on the alloy in some cases]). In contrast, the scale layer on the scale near the substrate is very brittle and can also be removed by gentle mechanical action. An increase in the partial water vapor pressure can be considered to result in an increase in material loss due to the acceleration of scale formation.

[0008] The heat treatment of steel in a steam atmosphere can change the particle positions within the structure, which can lead to undesirable early grain boundary oxidation, and as a result, may cause coating defects and / or surface defects. Since the degree of scale formation increases, the formation of grain boundary oxidation can similarly progress more rapidly and may also penetrate deeper into the substrate.

[0009] The heat treatment of steel in a steam atmosphere can also result in a higher decarburization depth, which means that the properties of the intermediate or final product are similarly affected, particularly adversely. This can be evident, for example, in that the mechanical indicators are outside the required range and the surface or magnetic properties may be further reduced.

[0010] Therefore, decarburization in the case of applying heat treatment to metals, particularly steel, in a directly heated furnace can involve not only a simple change from fossil fuels to non-fossil fuels but also complex effects on product parameters.

[0011] European Patent Application Publication No. 2762599 and European Patent Application Publication No. 3109338 disclose, for example, the use of a DFF furnace in a molten plating coating line for cold steel strips. In addition, for example, German Patent No. 102011053698 discloses the use of a DFF furnace for austenitization in a hot forming line for press-hardened steel.

Prior Art Documents

Patent Documents

[0012]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0013] The object of the present invention is to reduce the use of fossil fuels and to develop a method that does not have the above-mentioned drawbacks.

[0014] This object is achieved by a method having the features of claim 1. Further configurations are described in the dependent claims.

[0015] Accordingly, the present teaching is a method for establishing an in-furnace atmosphere in a direct heating type heat treatment furnace, the heat treatment furnace having at least one burner operated by a fuel gas and an oxygen gas that are combusted to produce combustion gas, and depending on the composition of the fuel gas, the composition of the oxygen gas, and their mixture, the combustion gas having a defined composition with a defined partial water vapor pressure. The essential feature of the present invention is that hydrogen is used in the fuel gas at a rate of at least 10% by volume, and the heat treatment furnace is further filled with a gas that does not contain water vapor and / or does not contain hydrogen, so that a gas that does not contain water vapor and / or does not contain hydrogen is mixed with the combustion gas such that the partial water vapor pressure of the mixture in the in-furnace atmosphere of the heat treatment furnace is lower than the defined partial water vapor pressure of the combustion gas.

[0016] The increase in hydrogen in the fuel gas, and thus the resulting increase in the partial water vapor pressure in the combustion gas, must each be offset by the controlled mixing of the combustion gas with a gas that does not contain water vapor and / or does not contain hydrogen in order to establish an in-furnace atmosphere in the heat treatment furnace having a lower partial water vapor pressure compared to (pure) combustion gas.

[0017] In particular, the determination or detection of the partial water vapor pressure is well known to those skilled in the art.

[0018] This approach makes it possible to establish a furnace atmosphere that can cope with currently known levels using a natural gas combustion burner. The hydrogen used as the fuel gas may be produced and provided, for example, by water electrolysis using renewable energy such as wind power, hydropower, and sunlight. Any necessary oxygen may likewise be produced and utilized by electrolysis using renewable energy (such as the sun, wind, water, etc.). The gas that does not contain steam for mixing and / or does not contain hydrogen may contain or consist of dry air, nitrogen (N2), argon (Ar), carbon dioxide (CO2), or a mixture thereof. Correspondingly, it is also possible to use a further gas or a gas mixture that does not contain hydrogen or hydrogen compounds and is suitable for the heat treatment of metals.

[0019] The oxygen-containing gas for the operation of the burner may be air, for example ambient air, oxygen, or a combination of air and oxygen.

[0020] In particular, hydrogen may be present in the fuel gas at a ratio of at least 20% by volume.

[0021] Preferably, hydrogen may be present in the fuel gas at a ratio of at least 40% by volume.

[0022] More preferably, hydrogen may be present in the fuel gas at a ratio of at least 60% by volume.

[0023] Particularly preferably, hydrogen may be present in the fuel gas at a ratio of at least 80% by volume.

[0024] Even more preferably, hydrogen may be present in the fuel gas at a ratio of at least 98% by volume. This configuration includes, for example, the use of 100% hydrogen, which means that, considering that the fuel gas consists of hydrogen and impurities, if any, can only be industrially avoided by the complexity of high-level equipment, impurities of up to 0.5% by volume, particularly up to 0.2% by volume, and preferably less than 0.1% by volume are allowed in the fuel gas.

[0025] Except when the fuel gas consists entirely of hydrogen, additional proportions of methane (CH4) and / or carbon monoxide (CO) are present together with hydrogen in order to be added up to 100% by volume together with impurities, and the impurities are tolerated up to less than 0.5% by volume, in particular 0.2% by volume, preferably less than 0.1% by volume. In particular when using natural gas, the proportion of methane may vary and thus additional components such as ethane, propane, ethene, and butane may also be included individually or in combination.

[0026] In one configuration, in order not to adversely affect the energy of the combustion gas and / or to increase the energy of the combustion gas, it may be advantageous to heat a gas that does not contain steam and / or does not contain hydrogen before filling the heat treatment furnace and / or the burner. In order to essentially maintain the energy level of the combustion gas, the combustion gas is preferably heated to a temperature corresponding to the temperature of the combustion gas + / - 300 °C. Thus, the temperature may correspond to a temperature range from minus 300 °C to plus 300 °C based on the temperature of the combustion gas. The temperature of the combustion gas can be detected by means known to those skilled in the art. Preheating of the fuel gas and / or the oxidant can result in an increase in the adiabatic flame temperature.

[0027] In order to make the off-gas removed from the heat treatment furnace economically utilizable, it may be advantageous to use part or all of the off-gas to heat a gas that does not contain hydrogen and / or does not contain steam. Also in this case, the means of off-gas utilization or heat transfer are known to those skilled in the art.

[0028] Instead of or in addition to off-gas utilization, especially when a higher temperature level is required compared to the off-gas temperature, the (additional) heating can also be carried out by other means, for example electrically.

[0029] More preferably, here the heat treatment furnace in question is used for any form of steel or alloy steel, regardless of whether it is a slab, plate, sheet, strip, or (previously) formed sheet metal part. The temperature for heat treatment is essentially from 200°C to 1350°C, particularly from 400°C to 1260°C, and this temperature relates to the temperature of the metal being heated. The furnace atmosphere temperature or the furnace space temperature can be considerably higher.

[0030] In addition, the temperature of the burner flame also affects the temperature of the furnace atmosphere or the furnace space. The combustion temperature with ambient air and natural gas is about 1970°C, the combustion temperature with ambient air and hydrogen is about 2130°C, in the case of combustion with oxygen and natural gas it is about 2860°C, and in the case of combustion with oxygen and hydrogen it is about 3080°C.

[0031] In addition, in the case of heat treatment of metals, the moisture content (water vapor, thus partial water vapor pressure) in the furnace atmosphere plays an important role. This controls, inter alia, whether the furnace atmosphere has a reducing or oxidizing effect on the metal. A standard method known to those skilled in the art for detecting the moisture content is called dew point measurement. The dew point of the furnace atmosphere can be from -70°C to +35°C, particularly in the case of steel, depending on the application. A negative dew point generally indicates a reducing furnace atmosphere.

Brief Description of the Drawings

[0032]

Figure 1

Figure 2

Figure 3

Embodiments for Carrying out the Invention

[0033] The present invention will be described in detail by the following examples in conjunction with the drawings.

[0034] FIG. 1 shows the present invention using an example of a schematic diagram. The direct heating type heat treatment furnace (1) has at least one burner (2) operated by a fuel gas (3) and an oxygen gas (4) that are combusted to bring combustion gas (10) into the heat treatment furnace (1). Depending on the composition of the fuel gas (3), the composition of the oxygen gas (4), and their mixture, the combustion gas (10) has a defined composition with a defined partial water vapor pressure. Since at least 10% by volume of hydrogen is used in the fuel gas (3), the heat treatment furnace (1) is further filled with a gas (5) that does not contain steam and / or does not contain hydrogen. As a result, the gas (5) that does not contain steam and / or does not contain hydrogen is mixed with the combustion gas (10) such that the partial water vapor pressure of the mixture in the furnace atmosphere (9) of the heat treatment furnace (10) is lower than the defined partial water vapor pressure of the combustion gas (10). Before filling the heat treatment furnace (1), the gas (5) that does not contain steam and / or does not contain hydrogen may be heated. Here, it is possible to remove the off-gas (7) from the heat treatment furnace (1), which may be partially or fully utilized to heat the hydrogen-free gas (5) by a suitable heat medium (6). Alternatively or additionally, the gas (5) that does not contain steam and / or does not contain hydrogen may be heated, in particular additionally, for example, by an electric heating device (11) shown by a dashed line, whereby an increase in the temperature of the gas (5) that does not contain steam and / or does not contain hydrogen exceeding the temperature of the combustion gas (10) may also be possible. According to the furnace atmosphere (9) established in accordance with the present invention, when hydrogen is used in the fuel gas (3) at a ratio of 10% to 100% by volume, heat treatment of a metal (8), such as steel, preferably a steel alloy, is possible without the drawback that a changed or different scale is formed on the surface of the metal / steel (8) despite using a non-fossil fuel.

[0035] Figures 2 and 3 each show a diagram indicating the proportion of hydrogen (horizontal axis) in the range of 0% to 100% starting from approximately 99% by volume of methane when natural gas is used as fuel. The left side has no hydrogen and is 100% natural gas, while the right side has no natural gas in the fuel gas and is 100% hydrogen. The oxygen-containing gas assumed for the burner was, firstly, ambient air (Figure 2), and secondly, oxygen considered in the calculation with an air ratio of 1.1 (Figure 3).

[0036] This diagram also shows the components of the combustion gas (left vertical axis) with respect to the composition of the fuel gas. On the right vertical axis, it is possible to determine the volume of the combustion gas generated per 1 m 3 per m 3 of the fuel gas used with respect to the composition of the fuel gas.

[0037] The results shown in Figures 2 and 3 have been numerically confirmed and show the influence of non-fossil fuels such as hydrogen in the fuel gas on the composition of the combustion gas.

[0038] Surprisingly, when ambient air is used as the oxygen gas for combustion, the reduction in the CO2 content in the combustion gas is possible only when the hydrogen content in the fuel gas is at least 35% by volume, as seen in Figure 2. In addition, Figure 2 clearly shows that for a fuel gas consisting of 100% hydrogen, the volume of the combustion gas cannot be less than 2.5 m 3 per 1 m 3 of the fuel gas (= hydrogen) used.

[0039] In contrast, Figure 3 shows that when oxygen is used as the oxygen gas for combustion together with 100% hydrogen as the fuel gas, the volume of the combustion gas essentially corresponds to 1:1 with respect to the volume of the fuel gas used. The reduction in the CO2 content in the combustion gas is also evident even at a relatively low hydrogen content (less than 35% by volume) in the fuel gas.

[0040] When the hydrogen content exceeds 60%, the partial vapor pressure begins to increase significantly (Figure 1). In the case of combustion of hydrogen and oxygen in Figure 2, the ratio is more extreme. As the volume fraction of hydrogen in the fuel gas increases, ultimately, when 100% by volume of hydrogen is used in the fuel gas, the partial vapor pressure increases to the maximum level. If 100% by volume of hydrogen is combusted without "diluting" the furnace atmosphere, it will have an adverse effect on the product characteristics of the metal. Therefore, for example, by adding, for example, 20% by volume of air, the water vapor content of the furnace atmosphere can be reduced accordingly. This can lead to further improvement of the processing characteristics. For example, "dilution" with non-preheated air can result in a temperature drop, which can remove the heating energy that may be required from the metal.

Claims

1. A method for establishing an in-furnace atmosphere in a directly heated heat treatment furnace, wherein the heat treatment furnace has at least one burner operated by a fuel gas and an oxygen gas that are combusted to provide a combustion gas, and wherein the combustion gas has a defined composition with a defined partial water vapor pressure depending on the composition of the fuel gas, the composition of the oxygen gas, and their mixture, in which - hydrogen is used in the fuel gas at a rate of at least 10% by volume, - the heat treatment furnace is further filled with a gas that does not contain water vapor and / or does not contain hydrogen, such that the gas that does not contain water vapor and / or does not contain hydrogen is mixed with the combustion gas such that the partial water vapor pressure of the mixture in the in-furnace atmosphere of the heat treatment furnace is lower than the defined partial water vapor pressure of the combustion gas characterizes the method.

2. The method according to claim 1, wherein the hydrogen is present in the fuel gas at a rate of at least 20% by volume.

3. The method according to claim 1, wherein the hydrogen is present in the fuel gas at a rate of at least 40% by volume.

4. The method according to claim 1, wherein the hydrogen is present in the fuel gas at a rate of at least 60% by volume.

5. The method according to claim 1, wherein the hydrogen is present in the fuel gas at a rate of at least 80% by volume.

6. The method according to claim 1, wherein the hydrogen is present in the fuel gas at a rate of at least 98% by volume.

7. The method according to any one of claims 1 to 6, wherein the gas that does not contain water vapor and / or does not contain hydrogen is heated before the filling of the heat treatment furnace.

8. The method according to claim 7, wherein the heating is carried out to a temperature corresponding to the temperature of the combustion gas ± 300 °C.

9. The method according to claim 7 or 8, wherein the off-gas removed from the heat treatment furnace is used partially or completely for the heating of the gas that does not contain water vapor and / or does not contain hydrogen.

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