Method for Reducing Nitrogen Oxides In Strip Treatment Furnaces

Abstract

The invention relates to a method for treating metal strip in a directly fired furnace through which the metal strip is guided. The furnace is fired directly by gas burners and has a non-fired zone through which the exhaust gases from the fired zone flow and thus heat the metal strip. After leaving the non-fired zone, the exhaust gases from the furnace undergo post-combustion in an afterburner chamber. According to the invention, methane is injected into the non-fired zone, which causes nitrogen oxides contained in the waste gas to be converted into hydrogen cyanide.

Claims

1-5. (canceled)

6. A method for treating a metal strip (5) in a directly fired furnace (1), comprising the steps of: providing a furnace (1) having a non-fired zone (7) rear of a directly fired zone (2) that is fired by gas burners and an afterburner chamber (9) rear of the non-fired zone (7), running a guided metal strip (5) forward through the furnace from the non-fired zone (7) and directly-fired zone (2), wherein exhaust gases (14) generated in the fired zone (2) flow and pre-heat the metal strip (5) and then undergo post-combustion the afterburner chamber (9), and injecting methane into the exhaust gas (14) in the non-fired zone (7), thereby converting nitrogen oxides present in the exhaust gas (14) to hydrogen cyanide.

7. The method according to claim 6, comprising injecting air or oxygen into the post-combustion chamber (9), thereby which degrading the hydrogen cyanide.

8. The method according to claim 7, comprising adding nitrogen to the methane to form a nitrogen-methane mixture that is injected into the non-fired zone (7).

9. The method according claim 8, wherein the methane is injected into the non-fired zone (7) at several different locations.

10. The method of claim 9, wherein the non-fired zone (7) has a plurality of nozzles (8), the nozzles (8) being configured to deliver nitrogen in the event of a fault condition to cool the metal strip (5) and deliver methane to reduce the nitrogen content during normal operation of the furnace (1).

11. The method according to claim 6, comprising adding nitrogen to the methane to form a nitrogen-methane mixture that is injected into the non-fired zone (7).

12. The method according to claim 7, wherein the methane is injected into the non-fired zone (7) at several different locations.

13. The method according to claim 8, wherein the methane is injected into the non-fired zone (7) at several different locations.

14. The method of claim 6, wherein the non-fired zone (7) has a plurality of nozzles (8), the nozzles (8) being configured to deliver nitrogen in the event of a fault condition to cool the metal strip (5) and deliver methane to reduce the nitrogen content during normal operation of the furnace (1).

15. The method of claim 7, wherein the non-fired zone (7) has a plurality of nozzles (8), the nozzles (8) being configured to deliver nitrogen in the event of a fault condition to cool the metal strip (5) and deliver methane to reduce the nitrogen content during normal operation of the furnace (1).

16. The method of claim 8, wherein the non-fired zone (7) has a plurality of nozzles (8), the nozzles (8) being configured to deliver nitrogen in the event of a fault condition to cool the metal strip (5) and deliver methane to reduce the nitrogen content during normal operation of the furnace (1).

17. The method of claim 16, wherein the ratio of methane to nitrogen in the mixture is approximately 1:10.

18. The method of claim 13, wherein the ratio of methane to nitrogen in the mixture is approximately 1:10.

19. The method of claim 9, wherein the ratio of methane to nitrogen in the mixture is approximately 1:10.

20. The method of claim 6, comprising the step of providing oxygen to the afterburner chamber (9), thereby converting hydrogen cyanide present in the afterburner chamber (9) to nitrogen, carbon dioxide and one or both of hydrogen and steam.

21. A method for treating a metal strip (5) in a directly fired furnace (1), comprising the steps of: providing a furnace (1) having a non-fired zone (7) rear of a directly fired zone (2) that is fired by gas burners and an afterburner chamber (9) rear of the non-fired zone (7); running a guided metal strip (5) forward through the furnace from the non-fired zone (7) and directly-fired zone (2), wherein exhaust gases (14) generated in the fired zone (2) flow and pre-heat the metal strip (5) and then undergo post-combustion the afterburner chamber (9); mixing methane and nitrogen to form a nitrogen-methane mixture; and injecting the nitrogen-methane mixture into the exhaust gas (14) in the non-fired zone (7), thereby converting nitrogen oxides present in the exhaust gas (14) to hydrogen cyanide.

22. The method of claim 21, comprising the step of providing oxygen to the afterburner chamber (9), thereby converting hydrogen cyanide present in the afterburner chamber (9) to nitrogen, carbon dioxide and one or both of hydrogen and steam.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In the following, an embodiment of the invention is described on the basis of drawings. In these drawings:

[0018] FIG. 1 shows a schematic view of a directly fired furnace for strip treatment;

[0019] FIG. 2 contains a side view of the furnace area 7 into which the methane is injected; and

[0020] FIG. 3 shows a sectional view of the non-fired furnace area 7.

DETAILED DESCRIPTION

[0021] Identical reference symbols in the individual figures refer to the same plant components in each case.

[0022] FIG. 1 shows a part of a directly fired furnace 1 in which a metal strip 5 undergoes heat treatment. The metal strip 5 is guided from above into the inside of the furnace and passes first of all through the non-fired zone 7, which is several meters long and is the area where the metal strip 5 is pre-heated. The non-fired zone 7 here is the area before the fired zone 2, viewed in strip running direction, and in which there are no burners.

[0023] 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 12 and then through a zone 4 with premix type burners. At the lower end of the furnace 1, the metal strip 5 is deflected with the aid of the deflector roll 11 and then fed to a radiant tube furnace (RTF), for example.

[0024] The exhaust gas 14 forming in the zone 2 fired directly by the gas burners flows upwards in the furnace 1 and is deflected there into direction 6 and fed, in a way that is known, to an afterburner chamber 9 containing an afterburner for post-combustion of the exhaust gases 14. The metal strip 5 does not pass through the afterburner chamber 9. The exhaust gases 14 also contain nitrogen oxides, mainly NO and NO.sub.2. In order to reduce this nitrogen oxide content, methane (CH.sub.4) is injected through the feed pipes 8 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.

[0025] 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.

[0026] In order to degrade the toxic hydrogen cyanide, oxygen (O.sub.2) or air is blown into the afterburner chamber 9 through pipes 10, causing a reaction in the hydrogen cyanide to form nitrogen (N.sub.2), carbon dioxide and hydrogen and/or steam. Finally, these exhaust gases are fed to a heat recovery plant 13 after they have been used once again for strip pre-heating.

[0027] FIG. 2 shows methane injection into the non-fired zone 7. It is shown here that the methane gas is mixed with nitrogen (N.sub.2) before being injected and is blown onto both sides of the metal strip 5.

[0028] FIG. 3 shows a sectional view through this zone 7. Here, the methane gas is supplied in such a way that both the area around the front side of the metal strip as well as the area around the rear side of the metal strip are enriched with methane so that all of the exhaust gas 14 comes into contact with methane if possible. It is feasible to inject the methane 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.

[0029] 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.

[0030] Of course, the method according to the invention can also be used in horizontal or L-shaped, directly fired furnaces.

REFERENCE NUMERALS

[0031] 1 Directly fired furnace [0032] 2 Fired zone [0033] 3 Nozzle mix [0034] 4 Premix [0035] 5 Metal strip [0036] 6 Direction [0037] 7 Non-fired zone [0038] 8 Methane injecting [0039] 9 Afterburner chamber [0040] 10 Oxygen injecting [0041] 11 Deflection roll [0042] 12 Furnace wall [0043] 13 Heat recovery plant [0044] 14 Exhaust gases from the burners