Combustion of the CO in secondary metallurgical exhaust gas, with calorific value control and volume flow control

Abstract

A method for the post-combustion of exhaust gases comprising carbon monoxide from metallurgical processes includes conditioning the exhaust gas prior to post-combustion by metering a combustion gas and/or one additional gas in feedback-controlled fashion. The feedback control depends on the composition of the exhaust gas dependent on the exhaust gas volume flow. A device for post-combustion of exhaust gas during vacuum treatment of liquid steel comprises a flare stack at an exhaust outlet, means for feeding combustion gas to the flare stack, means for feeding an inert gas into the exhaust gas channel of the vacuum pump, means for ascertaining the exhaust gas volume flow and/or for measuring the exhaust gas velocity within the exhaust gas channel, means for analyzing the exhaust gas composition, means for metering the combustion gas and the inert gas, and means for feedback control of the metering of the combustion gas and/or the inert gas dependent on the exhaust gas composition.

Claims

1-17. (canceled)

18. A method for post-combustion of an exhaust gas, wherein the exhaust gas comprises carbon monoxide from metallurgical processes, and wherein the exhaust gas has a discontinuously generated exhaust gas volume, and wherein a composition of the exhaust gas and/or a volume flow of the exhaust gas varies during a period within which the exhaust gas is generated, the method comprising: conditioning the exhaust gas prior to post-combustion by introducing a combustion gas and an inert gas in feedback-controlled fashion into the exhaust gas upstream of the post-combustion, wherein the feedback control is performed dependent on the composition of the exhaust gas and dependent on the volume flow of the exhaust gas.

19. The method according to claim 18, wherein the inert gas is nitrogen.

20. The method according to claim 18, further comprising: determining a calorific value of the exhaust gas indirectly via a carbon monoxide content of the exhaust gas using a gas analyzer (12).

21. The method according to claim 18, wherein the feedback control takes place dependent on a carbon monoxide content of the exhaust gas, and wherein a feedback control objective is achieving a maximum conversion of carbon monoxide to carbon dioxide.

22. The method according to claim 18, wherein the feedback control is configured such that a calorific value of the exhaust gas does not fall below 200 BTU/scf.

23. The method according to claim 18, wherein the feedback control is configured such that the volume flow of the exhaust gas does not fall below a given minimum volume flow.

24. The method according to claim 23, wherein the minimum volume flow of the exhaust gas is determined dependent on a flow velocity of the exhaust gas in a given flow cross-section, such that the flow velocity is greater than a flame propagation velocity of the exhaust gas during combustion.

25. The method according to claim 18, wherein the post-combustion is carried out by a supporting gas flare stack (2) arranged in or on a flue.

26. The method according to claim 25, wherein the introducing the combustion gas and the inert gas is performed via feed lines (8, 9) with valves (13, 14) that can be regulated in terms of volume flow.

27. A method for exhaust gas aftertreatment during vacuum treatment of liquid steel in a metallurgical process comprising post-combustion of exhaust gas stemming from vacuum treatment of a metal melt by a flare stack (2) in or at an exhaust gas channel (5) of a vacuum pump, wherein the method comprises: conditioning the exhaust gas prior to the post-combustion by introducing a combustion gas and an additional gas in feedback-controlled fashion to the exhaust gas upstream of the post-combustion, wherein the feedback control is performed dependent on a composition of the exhaust gas and dependent on a volume flow of the exhaust gas.

28. The method according to claim 27, wherein the post-combustion is carried out periodically only during a decarbonization phase of the metal melt.

29. A post-combustion device for post-combustion of exhaust gas during vacuum treatment of liquid steel in a secondary metallurgical process, comprising: at least one flare stack (2) at an exhaust outlet (4) of an exhaust gas channel (5) of a vacuum pump of a secondary metallurgical plant; a first valve for introducing combustion gas to the flare stack; a second valve for introducing an inert gas into the exhaust gas channel of the vacuum pump upstream of the flare stack (2); a sensor for measuring a volume flow of the exhaust gas and/or for measuring a velocity of the exhaust gas within the exhaust gas channel (5); an analyzer for analyzing an exhaust gas composition; and a controller for feedback control of the first valve and the second valve dependent on the exhaust gas composition.

30. The post-combustion device according to claim 29, wherein the first valve is a volume-flow controllable valve (13) arranged in a first feed line (8) for the combustion gas, the first feed line (8) being connected to the exhaust gas channel (5), and wherein the second valve is a volume-flow controllable valve (14) arranged in a second feed line (9) for the inert gas, the second feed line (9) being connected to the exhaust gas channel (5).

31. The post-combustion device according to claim 29, wherein the controller processes the following input variables: the exhaust gas composition, a volume flow of the exhaust gas, a quantity of combustion gas fed, and a quantity of inert gas fed.

32. The post-combustion device according to claim 29, wherein the controller is a programmable logic controller.

33. The post-combustion device according to claim 29, wherein the controller controls a support burner (3) of the flare stack (2).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] FIG. 1 is a schematic representation of the post-combustion device at a secondary metallurgical facility.

[0029] FIG. 2 shows a control scheme of the method for the post-combustion of exhaust gases.

[0030] FIG. 3 is a schematic representation of the controller used in the feedback control method.

[0031] FIG. 4 is a representation showing the exhaust gas composition and the exhaust gas quantity during a degassing process, wherein the feedback control intervention prior to post-combustion is also shown.

DETAILED DESCRIPTION

[0032] Reference is initially made to the post-combustion device 1 shown in FIG. 1, which comprises a flare stack 2 with a support burner 3, which is connected to the exhaust outlet 4 of an exhaust gas channel 5 of a vacuum pump (not shown) of a metallurgical plant. For example, the metallurgical plant may include a casting ladle and devices for degassing the metal melt contained in the casting ladle. The degassing of the metal melt can be performed, for example, by a partial-quantity degassing process, such as the Ruhrstahl-Heraeus process, with which a vacuum vessel is immersed in the melt for degassing, wherein negative pressure is generated in the vacuum vessel via vacuum pumps designed as steam jet pumps to degas the melt. Typically, multi-stage vacuum pumps, which are connected to an exhaust gas channel 5, are used for this purpose. For reasons of simplification, the term “vacuum pump” is predominantly used in the singular in the present application. However, “vacuum pump” also refers to an arrangement of vacuum pumps or a pump with a plurality of pump stages.

[0033] The support burner 3 of the flare stack 2 can be put into and out of operation or ignited and extinguished, as the case may be, via a control device 6.

[0034] Upstream of the flare stack 2, the exhaust gas channel 5 is connected to an extinguishing line 7, a feed line 8 for combustion gas and a feed line 9 for nitrogen. Via the extinguishing line 7, nitrogen can be fed as an extinguishing agent from an extinguishing agent tank 10 to the exhaust gas channel 5.

[0035] A flow measuring device 11 for determining the exhaust gas volume flow is arranged in the exhaust gas channel 5 upstream of the mouth of the feed line 8 for combustion gas into the exhaust gas channel 5 and downstream of the mouth of the feed line 9 for nitrogen. Upstream of the mouth of the feed lines 9 into the exhaust gas channel, a gas analysis device 12, which is preferably used to continuously determine the exhaust gas composition, is also provided. Depending on the exhaust gas composition and the flow through the exhaust gas channel 5 the feed of combustion gas and nitrogen as inert gas into the exhaust gas channel 5 is controlled by means of a control device 21, the feedback control scheme of which is explained below on the basis of the representation in FIG. 2. The feedback control device 21, which is shown in simplified form in FIG. 3, controls valves 13, 14 provided in the feed lines 8, 9, each of which meters more or less combustion gas or inert gas or nitrogen, as the case may be, into the exhaust gas channel 5.

[0036] The feedback control scheme shown in FIG. 2 comprises two interdependent control loops 15, 16, wherein a first control loop 15 controls the calorific value of the exhaust gas determined on the basis of the gas composition as a reference variable, and the second control loop 16 shown below in FIG. 2 controls the exhaust gas volume flow as a reference variable. The calorific value of the exhaust gas is determined from the measured values from the gas analysis device 12 via the CO content. The gas analysis device 12 provides, among other things, the oxygen content and the carbon monoxide content of the exhaust gas. The CO content or carbon monoxide content, as the case may be, of the exhaust gas determines its calorific value.

[0037] The calorific value of the exhaust gas further depends on the nitrogen content of the exhaust gas. The exhaust gas volume flow must not fall below a certain minimum value, in order to ensure sufficient gas velocity and thus prevent a possible re-ignition in the exhaust gas channel. To ensure this, an appropriate quantity of inert gas or nitrogen, as the case may be, is fed to the exhaust gas channel, which in turn has a feedback effect on the calorific value of the exhaust gas. The calorific value of the exhaust gas should not fall below a specified minimum value, for example in the order of magnitude of ≥2 kWh/Nm.sup.3 (200 BTU/scf). This value corresponds to a stoichiometrically complete combustion of the CO.

[0038] The first control loop 15 includes a first control device 17 for combustion gas feed, which acts on the volume flow controllable valve 13 in the feed line 8 for combustion gas. The reference variable for the calorific value is specified via a calorific value calculator 18, which uses the actual calorific value, the exhaust gas volume flow, the exhaust gas composition and the actual nitrogen volume flow from the second control loop 16 as input variables.

[0039] The second control loop 16 includes a second control device 19 for the metering of nitrogen, which acts on the volume-flow controllable valve 14. The second control loop 16 further includes a volume flow calculator 20, which uses the actually fed nitrogen volume flow and the combustion gas volume flow as input variables. The volume flow calculator 20 specifies the reference variable for the minimum exhaust gas volume flow and supplies this value in parallel to the calorific value calculator 18.

[0040] FIG. 4 illustrates the exhaust gas composition and the exhaust gas quantity during a typical degassing process of a secondary metallurgical treatment of a molten steel, wherein the pressure prevailing during decarbonization, the exhaust gas quantity, the inert gas quantity, the natural gas quantity and the CO content of the exhaust gas are plotted over time. The pressure drop (vacuum/thin solid line) at the beginning of the degassing process and the pressure increase at the end of the degassing process can be easily recognized. This is accompanied by an initially high and then decreasing formation of CO. The dotted line illustrates the calorific value of the exhaust gas supported by the metering of natural gas (CH.sub.4), whereas the bold solid curve illustrates the metering of nitrogen.

LIST OF REFERENCE SIGNS

[0041] 1 Post-combustion device [0042] 2 Flare stack [0043] 3 Support burner [0044] 4 Exhaust outlet [0045] 5 Exhaust gas channel [0046] 6 Control device [0047] 7 Extinguishing line [0048] 8 Feed line for combustion gas [0049] 9 Feed line for nitrogen [0050] 10 Extinguishing agent tank [0051] 11 Flow measuring device [0052] 12 Gas analysis device [0053] 13, 14 Valves [0054] 15 First control loop [0055] 16 Second control loop [0056] 17 First control device [0057] 18 Calorific value calculator [0058] 19 Second control device [0059] 20 Volume flow calculator [0060] 21 Control device