Method for Reducing Nitrogen Oxides In Strip Heat Treatment Furnaces

Abstract

A method for treating a metal strip, where the metal strip undergoes heat treatment in a directly fired furnace and is subsequently heat-treated further in a radiant tube furnace. At least part of the exhaust gases from the radiant tubes is fed to the directly fired furnace.

Claims

1-10. (canceled)

11. A method for treating a metal strip (5), comprising: providing a metal strip (5); heat treating the metal strip (5) in a directly fired furnace (1) to yield a directly heated metal strip; heat treating the directly heated metal strip in a radiant tube furnace (10), thereby forming exhaust gases (16); and feeding at least a portion of the exhaust gases (16) from the radiant tubes to a burner in the directly fired furnace (1).

12. The method according to claim 11, comprising the step of cooling exhaust gases (16) prior to the step of feeding at least a portion of the exhaust gases to the directly fired furnace (1).

13. The method of claim 12, wherein the exhaust gases (16) are cooled with the aid of a heat exchanger.

14. The method according to claim 11, wherein the directly fired furnace (1) has an afterburner chamber (9) in which exhaust gases (14) formed in the directly fired furnace (1) undergo post-combustion, comprising the step of feeding at least sub-portion of the exhaust gases (16) fed from the radiant tubes to the afterburner chamber (9).

15. The method according to claim 12, wherein the directly fired furnace (1) has an afterburner chamber (9) in which exhaust gases (14) formed in the directly fired furnace (1) undergo post-combustion, comprising the step of feeding at least sub-portion of the exhaust gases (16) fed from the radiant tubes to the afterburner chamber (9).

16. The method according to claim 14, wherein a sub-portion of the exhaust gases (16) fed from the radiant tubes is fed directly to the afterburner chamber (9).

17. The method according to claim 14, wherein a sub-portion of the exhaust gases (16) fed from the radiant tubes is mixed with combustion air (18) for the afterburner (20) in the afterburner chamber (9).

18. The method according to claim 11, wherein a sub-portion of the exhaust gases (16) fed from the radiant tubes is mixed with combustion air (22) for the burners in the directly fired zone (2).

19. The method according to claim 12, wherein a sub-portion of the exhaust gases (16) fed from the radiant tubes is mixed with combustion air (22) for the burners in the directly fired zone (2).

20. The method according to claim 14, wherein a sub-portion of the exhaust gases (16) fed from the radiant tubes is mixed with combustion air (22) for the burners in the directly fired zone (2).

21. The method according to claim 18, wherein a sub-portion of the exhaust gases (16) coming from the radiant tubes is fed to a least one nozzle mix type burner.

22. The method according to claim 11, wherein the directly fired furnace (1) has a non-fired zone (7) rear of the directly fired zone (2) and an afterburner chamber (9) rear of the non-fired zone (7) relative to a direction (21) in which the metal strip (5) runs, exhaust gases (14) from the directly fired zone (2) flow through the non-fired zone (7) and pre-heat the metal strip (5), and the exhaust gases (14) undergo post-combustion in the afterburner chamber (9) after passing through the non-fired zone (7), comprising the step of blowing methane into the exhaust gases (14) in the non-fired zone (7), thereby causing at least a portion of any nitrogen oxides present in the exhaust gas (14) to be converted into hydrogen cyanide.

23. The method according to claim 12, wherein the directly fired furnace (1) has a non-fired zone (7) rear of the directly fired zone (2) and an afterburner chamber (9) rear of the non-fired zone (7) relative to a direction (21) in which the metal strip (5) runs, exhaust gases (14) from the directly fired zone (2) flow through the non-fired zone (7) and pre-heat the metal strip (5), and the exhaust gases (14) undergo post-combustion in the afterburner chamber (9) after passing through the non-fired zone (7), comprising the step of blowing methane into the exhaust gases (14) in the non-fired zone (7), thereby causing at least a portion of any nitrogen oxides present in the exhaust gas (14) to be converted into hydrogen cyanide.

24. The method according to claim 14, wherein the directly fired furnace (1) has a non-fired zone (7) rear of the directly fired zone (2) and an afterburner chamber (9) rear of the non-fired zone (7) relative to a direction (21) in which the metal strip (5) runs, exhaust gases (14) from the directly fired zone (2) flow through the non-fired zone (7) and pre-heat the metal strip (5), and the exhaust gases (14) undergo post-combustion in the afterburner chamber (9) after passing through the non-fired zone (7), comprising the step of blowing methane into the exhaust gases (14) in the non-fired zone (7), thereby causing at least a portion of any nitrogen oxides present in the exhaust gas (14) to be converted into hydrogen cyanide.

25. The method according to claim 22, comprising the step of providing a source of oxygen (25) to the post-combustion chamber (9), which degrades at least a portion of the hydrogen cyanide formed therein.

26. The method according to claim 22, comprising the step of adding nitrogen to the methane to form a nitrogen-methane mixture that is blown into the non-fired zone (7).

27. The method according to claim 11, wherein between 5% and 20% of the exhaust gases (16) present in the radiant tubes is fed to the directly fired furnace (1).

28. A method for treating a metal strip (5), comprising: providing a directly fired furnace (1) having a directly fired zone (2), a non-fired zone (7) adjacent the directly fired zone (2), and an afterburner chamber (9) adjacent the non-fired zone; providing a metal strip (5); heat treating the metal strip (5) in the directly fired furnace (1) by running the metal strip (5) through the non-fired zone (7) and then the directly fired zone (2) to yield a directly heated metal strip; heat treating the directly heated metal strip in a radiant tube furnace (10), thereby forming exhaust gases (16); and feeding at least a portion of the exhaust gases (16) from the radiant tubes to one or both of burners in the directly fired zone (2) and the afterburner chamber (9) of the directly fired furnace (1).

29. The method according to claim 28, comprising the step of cooling exhaust gases (16) prior to the step of feeding at least a portion of the exhaust gases to the directly fired furnace (1).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In the following, three embodiments of the invention are described on the basis of drawings. In these drawings:

[0028] FIG. 1 shows a schematic view of a directly fired furnace (DFF) in which the exhaust gases from the radiant tube furnace (RTF) are fed to the afterburner chamber;

[0029] FIG. 2 shows a schematic view in which the exhaust gases are fed to the burners of the directly fired furnace; and

[0030] FIG. 3 shows a combination of FIGS. 1 and 2, where methane is also injected or blown into the non-fired zone of the DFF.

DETAILED DESCRIPTION

[0031] Identical reference symbols in the individual figures refer to the same plant components in each case.

[0032] FIG. 1 contains a schematic diagram of a plant for heat-treating a metal strip 5. The metal strip 5 passes through a directly fired furnace (DFF) 1 first of all and then through a radiant tube furnace 10 (RTF). In the directly fired furnace 1, the metal strip 5 enters at the bottom through a gas lock 12 and runs upwards in direction 21.

[0033] In this zone, the metal strip 5 is pre-heated by the hot exhaust gases from the afterburner chamber 9. In the top furnace section, the metal strip 5 is deflected by deflection rolls 11 and passes through the non-fired zone 7, which is located directly ahead of the fired zone 2.

[0034] The non-fired zone 7 is several meters long and serves to pre-heat the metal strip 5, which also causes the hot burner exhaust gases 14 to cool down. The non-fired zone 7 here is the area before the fired zone 2, viewed in strip running direction 21, and in which there are no burners.

[0035] The metal strip 5 is heated up in the fired zone 2 of the furnace 1 with the aid of gas burners. Here, the metal strip 5 passes first of all through a zone 3 in which nozzle mix type burners are mounted in the furnace wall and then through a zone 4 with premix type burners.

[0036] The exhaust gas 14 forming due to the gas burners in the directly fired zone 2 flows upwards in the furnace 1 and is fed there through an opening 6, in a way that is known, to the afterburner chamber 9 containing an afterburner 20 for post-combustion of the exhaust gases 14. In this process, the carbon monoxide (CO) contained in the exhaust gases 14 and the hydrogen (H.sub.2) is essentially burned off (or oxidizes completely). The metal strip 5 does not pass through the afterburner chamber 9. The exhaust gases from the afterburner chamber 9 are then guided through the opening 8 again into the furnace area that the metal strip 5 passes through. In the bottom section of the furnace 1, the exhaust gases 14 are fed to a heat recovery system 13.

[0037] At the lower end of the furnace 1, the metal strip 5 is deflected with the aid of the deflection roll 11 and then fed to the radiant tube furnace 10. The strip path through the furnace 10 is not shown here.

[0038] In all disclosed embodiments, at least some of the exhaust gases 16 from the radiant tubes are fed to a portion of the directly fired furnace 1.

[0039] In the present example of FIG. 1, these exhaust gases 16 are collected in a collector 15 and fed to the afterburner chamber 20 via a fan 17. The exhaust gases 16 are mixed with combustion air 18 before reaching the afterburner 20. The combustion gas is supplied through the pipe 19. The exhaust gases 16 absorb part of the combustion heat, which lowers the peak temperatures during post-combustion, thus reducing the formation of NO.sub.x.

[0040] In FIG. 2, at least part of the exhaust gases 16 from the furnace 10 heated by radiant tubes is fed to the burners of the directly fired furnace 1. In the present example, they are mixed beforehand with combustion air 22. In addition, gaseous fuel 23 is fed to the burners. This also leads to a reduction in nitrogen oxide because the temperature peaks in the burners are reduced as a result of the supply of exhaust gas 16.

[0041] FIG. 3 shows an example of an embodiment in which the exhaust gases from the RTF 10 are fed to the afterburner chamber 9 and the burners of the directly heated furnace 1. In order to further reduce this nitrogen oxide content, methane (CH4) is injected in addition through the feed pipes 24 or blown with the aid of nitrogen into the non-fired zone 7 of the furnace 1. The methane blends with the hot exhaust gases, and the nitrogen oxides react with the methane to form hydrogen cyanide. It is also possible to use a conventional burner for this task, replacing the combustion air with nitrogen oxide.

[0042] The methane can be injected at several points at different distances from the directly fired zone 2, for example at a distance of 1 m, 2 m, and 3 m from the nearest burner.

[0043] Methane gas injection can be retrofitted easily to existing plants to thus reduce nitrogen oxide emissions. With the present method, NO.sub.x values can be achieved in the region of 100 mg/Nm.sup.3 or less.

[0044] The amounts of methane gas required can be relatively small here. A quantity of 5 m.sup.3/h may be sufficient for a standard furnace 1. It is useful if this non-fired zone 7 is largely free of oxygen (O.sub.2 content <0.05%) so that oxygen cannot react with the methane blown in. In order to guarantee that it remains oxygen-free, at least the burners nearest to it can be operated with excess fuel so that any oxygen present is burnt off beforehand.

[0045] In order to degrade the toxic hydrogen cyanide, oxygen (O.sub.2) or air is blown into the afterburner chamber 9 through pipes 25, causing a reaction in the hydrogen cyanide to form nitrogen (N2), carbon dioxide and hydrogen and/or steam.

[0046] Of course, the method according to the invention can also be used in a horizontal furnace configuration.

REFERENCE NUMERALS

[0047] 1 Directly fired furnace [0048] 2 Fired zone [0049] 3 Nozzle Mix type burner [0050] 4 Premix type burner [0051] 5 Metal strip [0052] 6 Opening to afterburner chamber [0053] 7 Non-fired zone [0054] 8 Opening from the afterburner chamber into the furnace [0055] 9 Afterburner chamber [0056] 10 RTF [0057] 11 Deflection roll [0058] 12 Gas lock [0059] 13 Heat recovery plant [0060] 14 Exhaust gases from the burners [0061] 15 Collector for the RTF exhaust gases [0062] 16 RTF exhaust gases [0063] 17 Fan [0064] 18 Combustion air [0065] 19 Gas supply [0066] 20 Afterburner [0067] 21 Strip running direction [0068] 22 Air supply [0069] 23 Combustion gas supply [0070] 24 Methane supply [0071] 25 Air supply