METHOD FOR SETTING AN OVEN ATMOSPHERE IN A HEAT-TREATMENT OVEN

Abstract

A method of establishing a furnace atmosphere in a directly heated heat treatment furnace is provided. A heat treatment furnace has at least one burner which is operated with a fuel gas and an oxygenous gas that are combusted to give a combustion gas. The combustion gas has a defined composition having a defined partial water vapor pressure. Hydrogen is used in the fuel gas with a proportion of at least 10% by volume. The heat treatment furnace is additionally flooded with a water vapor-free and/or hydrogen-free gas, as a result of which the water vapor-free and/or hydrogen-free gas mixes with the combustion gas so as to bring about a partial water vapor pressure of the mixture in the furnace atmosphere of the heat treatment furnace that is less than the defined partial water vapor pressure of the combustion gas.

Claims

1-9. (canceled)

10. A method of establishing a furnace atmosphere in a directly heated heat treatment furnace, where the heat treatment furnace has at least one burner which is operated with a fuel gas and an oxygenous gas that are combusted to give a combustion gas, where, depending on the composition of the fuel gas and the composition of the oxygenous gas and the mixture thereof, the combustion gas has a defined composition having a defined partial water vapor pressure, the method comprising: using hydrogen in the fuel gas with a proportion of at least 10% by volume, and flooding the heat treatment furnace with at least one of a water vapor-free and a hydrogen-free gas, as a result of which at least one of the water vapor-free and the hydrogen-free gas mixes with the combustion gas so as to bring about a partial water vapor pressure of the mixture in the furnace atmosphere of the heat treatment furnace that is less than the defined partial water vapor pressure of the combustion gas.

11. The method as claimed in claim 10, wherein hydrogen is present in the fuel gas with a proportion of at least 20% by volume.

12. The method as claimed in claim 10, wherein hydrogen is present in the fuel gas with a proportion of at least 40% by volume.

13. The method as claimed in claim 10, wherein hydrogen is present in the fuel gas with a proportion of at least 60% by volume.

14. The method as claimed in claim 10, wherein hydrogen is present in the fuel gas with a proportion of at least 80% by volume.

15. The method as claimed in claim 10, wherein hydrogen is present in the fuel gas with a proportion of at least 98% by volume.

16. The method as claimed in claim 15, wherein the at least one of the water vapor-free and the hydrogen-free gas is heated prior to the flooding of the heat treatment furnace.

17. The method as claimed in claim 16, wherein the heating is effected to a temperature corresponding to the temperature of the combustion gas +/300 C.

18. The method as claimed in either of claim 16, wherein an offgas removed from the heat treatment furnace is used partly or completely for heating of the water vapor-free and/or hydrogen-free gas.

19. The method as claimed in either of claim 17, wherein an offgas removed from the heat treatment furnace is used partly or completely for heating of the water vapor-free and/or hydrogen-free gas.

Description

[0031] The invention is elucidated in detail by the working examples that follow, in conjunction with the drawing.

[0032] FIG. 1 shows the invention 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 oxygenous gas (4) that are combusted to give a combustion gas (10) in the heat treatment furnace (1), where, depending on the composition of the fuel gas (3) and the composition of the oxygenous gas (4) and the mixture thereof, the combustion gas (10) has a defined composition having a defined partial water vapor pressure. Since hydrogen is used in the fuel gas (3) with a proportion of at least 10% by volume, the heat treatment furnace (1) is additionally flooded with a water vapor-free and/or hydrogen-free gas (5), as a result of which the water vapor-free and/or hydrogen-free gas (5) mixes with the combustion gas (10) so as to bring about a partial water vapor pressure of the mixture in the furnace atmosphere (9) of the heat treatment furnace (10) that is less than the defined partial water vapor pressure of the combustion gas (10). Before the flooding of the heat treatment furnace (1), the water vapor-free and/or hydrogen-free gas (5) may be heated. It is possible here to remove an offgas (7) from the heat treatment furnace (1), which can be utilized partly or completely for heating of the hydrogen-free gas (5) by means of a suitable heat carrier (6). Alternatively or additionally, the water vapor-free and/or hydrogen-free gas (5) may be heated, especially additionally, for example by an electrical heating device (11), shown by dashed lines, with which an increase in the temperature 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) established in accordance with the invention, heat treatment of a metal (8), for example a steel, preferably a steel alloy, is possible without the disadvantages of an altered or different scale formation on the surface of the metal/steel (8) in spite of the use of nonfossil fuels, when hydrogen is used with proportions between 10% and 100% by volume in the fuel gas (3).

[0033] FIGS. 2 and 3 each show a diagram when natural gas is used as fuel, proceeding from about 99% by volume of methane, with a proportion between 0% and 100% by volume of hydrogen (abscissa). On the left there is no hydrogen and 100% by volume of natural gas, whereas on the right there is no natural gas and 100% by volume of hydrogen in the fuel gas. The oxygen-containing gas envisaged for the burner was firstly ambient air (FIG. 2) and secondly oxygen (FIG. 3), considered in the calculation with an air ratio of 1.1.

[0034] The diagram also shows the constituents of the combustion gas (left-hand ordinate) against the composition of the fuel gas. On the right-hand ordinate, it is possible to determine the combustion gas volume generated in m.sup.3 per m.sup.3 of fuel gas used against the composition of the fuel gas.

[0035] The results shown in FIGS. 2 and 3 have been ascertained numerically and show the influence of nonfossil 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 oxygenous gas for the combustion, lowering of the CO.sub.2 content in the combustion gas is possible only with a hydrogen content of at least 35% by volume in the fuel gas; see FIG. 2. In addition, FIG. 2 shows clearly that a fuel gas consisting of 100% by volume of hydrogen cannot go below a combustion gas volume of 2.5 m.sup.3 per m.sup.3 of fuel gas (=hydrogen) used.

[0037] By contrast, FIG. 3 shows that, in the case of use of oxygen as oxygenous gas for combustion with 100% hydrogen as fuel gas, the volume of the combustion gas corresponds essentially 1:1 to the volume of the fuel gas used. A reduction in the CO.sub.2 content in the combustion gas is also apparent even at relatively low hydrogen contents (less than 35% by volume) in the fuel gas.

[0038] Over and above a hydrogen content of 60%, the partial water vapor pressure begins to rise significantly (FIG. 1). In FIG. 2, in the case of combustion of hydrogen with oxygen, the ratio is more extreme. An increasing proportion by volume of hydrogen in the fuel gas ultimately increases the partial water vapor pressure to a maximum level when 100% by volume of hydrogen is used in the fuel gas. If 100% by volume of hydrogen is burned without dilution of the furnace atmosphere, this has an adverse effect on the product properties of the metal, such that the water vapor content of the furnace atmosphere can be correspondingly lowered by addition for example of air, for example 20% by volume. This would lead to an improvement in further processing properties. Dilution with non-preheated air, for example, would result in a temperature drop, which would remove heating energy that is possibly needed from the metal.