DE102010029648A1 - Method for melting e.g. aluminum scrap or glass, for heat treatment of steel in hearth furnace, involves operating burners of furnace using radiant flame, and supplying oxygen to combustion air flow to enrich air on pressure side of blower - Google Patents

Abstract

The method involves operating burners (2a, 2b) of a furnace e.g. hearth furnace and/or tank furnace (1), using radiant flame. Oxygen is supplied to combustion air flow for enriching the combustion air on a pressure side of a combustion air blower (7a). The oxygen-enriched combustion air is preheated at a temperature of approximately 100-200 degrees Centigrade below a furnace room temperature. The flame is tested and analyzed by an observation window with a camera i.e. personal communication device (PCD) camera with UV filter. Bright luminous color of the flame is recognized with a naked eye.

Description

The present invention relates to a process for melting metals or glass, preferably aluminum, or for heat treating metals, in particular steel.

State of the art

Methods for melting metals or glass or for heat treating metals such as steel have been known in principle for a long time. The metal or glass is melted or heated in an oven. For this purpose, either direct heating methods, in which the material to be melted or heated is melted or heated by the direct action of flames, or indirect melting methods in which the furnace wall and thereby the material is heated, for example, with electrically operated furnaces used.

In the direct melting method burners are used in which by combustion of a fuel, for example natural gas or petroleum, with the supply of an oxidizing agent, generally air or oxygen, a flame is generated, through the heat of which the material in the furnace is melted.

If the burners are operated with air as the oxidant, it is referred to as an "air-fuel burner", when using oxygen as an oxidant, the burners are referred to as "oxy-fuel burner".

Frequently, regenerative melting or heating methods are used. The exhaust gases generated during combustion are derived from the furnace and their heat is used in so-called regenerators to preheat the combustion air to save energy. When using oxygen instead of air as the oxidizing agent, no preheating is normally carried out, since it is technically very difficult to handle preheated oxygen, in particular for safety reasons.

When using air as an oxidizing agent, there is the problem that it contains about 79% by volume of nitrogen, which must be preheated in each case, which causes unnecessarily high energy consumption, and also by oxidation of this nitrogen during combustion, the unwanted toxic nitrogen oxides (NOx) are formed, which then have to be removed again consuming from the exhaust gas to prevent a release into the environment.

However, if oxygen is used as the oxidant instead of air, the exhaust gas temperature is extremely high compared to air burners, and undesirable oxidation of the metals to be melted or heated may possibly occur.

Some patent documents disclose the use of preheated oxygen in "oxy-fuel burners", even though this has not previously been established in practice.

EP 0 928 938 A1 describes a method and apparatus for supplying preheated oxygen to an oven operated with an "oxy-fuel burner". One or more regenerators are provided, each having a first and second regenerator bed. These two beds are alternately operated so that one is preheated with hot exhaust gas from the furnace, while the other oxygen preheats by being passed through an already preheated bed for supply to the burner. The oxygen is preheated to about 704 to about 1093 ° C (1300 to 2000 ° F), to a temperature that is at most 204 ° C (400 ° F) below the exhaust gas temperature. This is generally about 1315 ° C (2400 ° F). According to this document, oxygen is used as oxidizing agent with an oxygen concentration of at least 30%, but preferably 80%, or as technically pure oxygen with an oxygen concentration of at least 99.5%. By using oxygen instead of air, this document improves the energy efficiency and productivity of the smelting process and reduces the formation of unwanted nitrogen oxide (NOx) emissions in the exhaust. In contrast, there is the disadvantage that oxygen is relatively expensive compared to normal combustion air.

A number of documents describe the preheating of combustion air by using regenerators. This technique is common practice. Examples of such publications are US 2006/0129020 and GB 1 538 255 ,

DE 1 957 109 discloses a method and apparatus for melting aluminum using a combination of oxy-fuel and air-fuel burners. Something similar is also in EP 0 939 059 A2 disclosed.

Melting processes using oxygen that is not preheated are described, for example, in US Pat WO 02/103 067 A2 . EP 1 243 663 A2 . WO 2004/108 974 A1 . WO 02/088400 A1 . EP 2 098 603 A1 and DE 1 458 922 A1 disclosed.

DE 27 04 101 describes a furnace with various burner devices that use fuel and oxygen, the oxygen by direct mixing, without the use of a Regenerators, is preheated with the exhaust gases of the furnace chamber.

DE 36 90 574 C2 discloses a process for refining iron or aluminum in which preheated air or oxygen-enriched air is introduced directly into an oven. However, this air or oxygen-enriched air is not used to operate the burners.

US 3,207,493 describes regenerative furnaces where combustion air (or oxygen or oxygen-enriched air) is preheated by pairs of regenerators. There are neither temperatures nor oxygen levels disclosed.

WO 2007/048429 A1 describes an "oxy-fuel burner" with variable flame length, which is operated with preheated to 60 ° C oxygen or air.

WO 00/79182 A1 also describes "oxy-fuel burner" using preheated oxygen. There are claimed particular embodiments of the burner. In addition, the fuel is preheated according to this document.

M. Flamme and P. Grohmann, GASWÄRME International (56) No. 8/2007, pages 565 to 570 discusses increasing the efficiency of thermal process plants by using oxygen instead of using air. However, according to the article, the cost reduction is adversely affected by the cost of oxygen. The advantages of oxygen enrichment can be seen from page 569, left column in that the need for combustion air is reduced or eliminated and leads to lower nitrogen oxide emissions. Furthermore, an increase in the flame temperature is observed without additional fuel costs. This can increase productivity and reduce fuel consumption by up to 50%. Overall, a reduction in fuel consumption by 30 to 50%, a reduction of nitrogen oxide emissions by up to 90%, a reduction of dust emissions by 30 to 90% and an increase in production by 10 to 30% by the use of oxygen can be achieved. Furthermore, improved temperature uniformity, better heat transfer and better controllability are mentioned as advantages. Furthermore, the article on page 569, left column above, discloses that improving the heat output is achieved by influencing the flame shape and improving the radiation characteristics of the flame. It is particularly emphasized on page 566 bottom right that flameless combustion, ie a non-radiative flame, is advantageous.

R. Jeschar, E. Specht and H.-G. Bittner, Stahl and Eisen 108 (1988) No. 18, page 835 , also discusses the advantages and disadvantages of oxygen enrichment of combustion air in industrial furnaces. Here it is emphasized that an increase of the process temperature or the saving of fuel can be used. On the other hand, the additional costs for oxygen are a disadvantage. High exhaust gas temperatures are presented as a problem. Further, with an oxygen enrichment of the combustion air, there is a difficulty in recovering the heat from the exhaust gas because the exhaust fumes are often very dirty or aggressive.

Disclosure of the invention

In view of the above-described disadvantages and problems of the prior art, therefore, there was still a need for a method for melting metals or glass or heat treating metals, in which the disadvantages mentioned do not occur.

The object of the present invention is therefore to provide a method for melting metals or glass or heat treatment of metals, in which a high productivity, combined with less formation of harmful exhaust gases, in particular NOx and CO, can be achieved in a cost effective manner.

The object is achieved by the method according to claim 1. Further preferred embodiment of the method are defined in claims 2 to 7.

Description of the figures

1 is a schematic representation of a smelting plant for aluminum, in which the inventive method can be performed.

2 and 3 are schematic representations of this system to illustrate the two alternating modes.

Detailed description of the invention

The method according to the invention is used for melting metals or glass or for heat-treating metals, in particular steel. This heat treatment often takes place in cast steel moldings prior to further processing.

The process according to the invention is particularly preferably used for melting aluminum and here in particular aluminum scrap.

The method is characterized in that working with a radiant flame. In contrast, according to the known methods of the prior art, radiationless flames are used. Under "radiant flame" is understood according to the invention that the flame bright, almost white, and clearly visible lights. In contrast, a radiationless flame is hardly or not at all visible. The flame can through a provided in the furnace wall observation window with a special camera, z. As a PCD camera with UV filter, examined and analyzed. However, the bright bright color of the radiant flame is already clearly visible to the naked eye, it being important to avoid eye damage that you do not look directly into the flame.

According to the invention, oxygen-enriched air is used as the oxidant for the fuel. Under oxygen enrichment is understood according to the invention that the air contains more than about 22 vol .-% to less than 30 vol .-%, based on the volume of the combustion air, oxygen. Normal air contains oxygen in an amount of about 21% by volume. This proportion is increased according to the invention. However, a content of 30% by volume or more is excluded according to the invention as unfavorable. Namely, at a content of 30% by volume or more, excessive formation of nitrogen oxides by oxidation is observed. In addition, such a high oxygen content would be uneconomical and leads to safety risks in the operation of the furnace. Thus, with a high oxygen content of the combustion air to comply with special safety regulations, which greatly increases the cost. In addition, with a higher oxygen content of the combustion air than 30 vol .-%, a greatly increased risk of fire.

Commercially available oxygen (eg obtainable from Air Liquide or Linde AG) is used in accordance with the invention for enriching the combustion air, which oxygen is supplied to the combustion air stream at any point, but before it is introduced into the regenerator. In particular, the enrichment of the combustion air with oxygen takes place on the pressure side of the combustion air blower (( 10 ) in 1 to 3 ). The oxygen is inventively not before the passage through the combustion air blower ( 7a ) introduced into the system, as a fire risk can be avoided. The purity of the oxygen supplied corresponds to technical grade and is preferably at least about 99.5 vol .-%.

As furnace according to the invention all common directly heated furnace types in question. In particular, stove and pan furnaces are to be mentioned here as melting furnaces. For the heat treatment of metals in particular walking beam, shock, rotary hearth, - turntable and forging furnaces are used according to the invention. All mentioned oven types are commonly used in this field of technology. According to the invention, all common models and types can be used. A melting plant according to the invention for the melting of metals, in particular aluminum, is known in 1 shown.

The furnace can be operated continuously or discontinuously. Usual are discontinuous processes, and these are also preferred according to the invention. The metal or glass is completely melted or the metal is heated until reaching the preselected temperature, and then emptied the oven. Subsequently, new will be treated. Material introduced and the process repeated.

The furnace can be operated according to the invention with one or more burners and one or more regenerators: There are always used as many burners as regenerators, and the number of burner / regenerator pairs is even. Usually, the operation takes place in such a way that the fuel, preferably natural gas or mineral oil, such as light or heavy fuel oil, particularly preferably natural gas, is used to operate the one burner, the flame of which then melts the metal or glass in the furnace or the metal heated. The exhaust gas produced during the combustion is conducted into a regenerator and used there for preheating the regenerator bed consisting of thermally conductive material.

A regenerator according to the invention is a heat recovery system in which a heat storage medium, for example of ceramic material, is heated. The heat storage medium consists of temperature-resistant, thermal shock resistant and chemically resistant material. Preferably, it is in the form of a bed of balls with a diameter of about 10 to 50 mm, through the spaces between the exhaust gas and the oxygen-enriched air flows. But it is also possible, the material in the form of geometric body such. B. use tubes or honeycombs. Then, the combustion air is then passed through the storage medium and thereby preheated, the storage medium itself is cooled again.

The oxygen-enriched air is heated according to the invention to a value of about 200 to 100 ° C below the furnace chamber temperature.

The temperatures mentioned are measured in each case at the desired points of the system using conventional measuring devices.

So that the temperatures can be kept fairly constant, be preferably two regenerators and two burners operated alternately. While a Regenerotor is preheated, the preheating of the oxygen-enriched combustion air takes place in the other regenerator and vice versa. This switches between two operating modes of the regenerator. According to the invention, it is not necessary, when switching between the modes "passing the oxygen-enriched combustion air" and "passing exhaust" first flush the regenerator, such. In EP 0 928 938 A1 described. Instead, according to the invention, the supply of oxygen to the combustion air is interrupted for a short time, ie for 1 to 5 seconds, so that during this short time air with a normal oxygen content of about 21 vol .-% flows through the regenerator. Then it is switched to passing exhaust gas and the supply of oxygen to the combustion air, which is then fed to the second burner resumed.

The furnace according to the invention is always operated as usual with a slight overpressure of about 0.1 to 0.3 mbar (10 to 30 Pa) compared to the environment.

Surprisingly, it has been found that in this process management, d. H. by using a radiating flame and using oxygen-enriched air with the above-defined oxygen content, the furnace chamber temperature compared to that used in the conventional methods can be significantly reduced, in particular by about 50 to 150 ° C, compared with a conventional regenerative melting process.

It is possible, for example, by using a regenerative burner (burner according to the invention also occasionally referred to in combination with regenerators) instead of a burner without regenerator, the furnace chamber temperature when melting aluminum scrap by about 50 ° C to about 900 to 1250 ° C lower. When using regenerative burners, the furnace chamber temperature during melting of aluminum, depending on the furnace and burner, is usually approx. 950 to 1300 ° C. When melting glass, the furnace chamber temperature for regenerative burner use is about 1250 to 1450 ° C, for the heat treatment. Of steel normally about 800 to 1250 ° C. According to the invention it is now possible to work by oxygen enrichment of the combustion air at high radiant flame and thereby reduce the furnace chamber temperatures by about 50 to 150 ° C. Ie. when melting aluminum, the invention operates at furnace chamber temperatures between about 900 ° C and 1050 ° C, the melting of glass at about 1150 to 1350 ° C and the heat treatment of steel about 700 to 1150 ° C.

Surprisingly, however, the productivity of the process remains the same, d. H. The same amount of metal per hour is melted or heat treated as in the conventional regenerative processes, although the furnace chamber temperature is significantly reduced.

This finding is very surprising, since usually regenerative burners work with a non-radiant flame and thereby higher furnace chamber temperatures are achieved and this is generally regarded as advantageous and desirable.

According to the invention, a further advantage is that a longer service life of the furnaces is achieved by lowering the furnace chamber temperature and the total production output of the furnaces is increased, ie. H. The ovens are less affected by the lower heat and thus have a longer life. The service life of aluminum smelting furnaces is normally about 3 years. This time can be extended during operation according to the inventive method at least by about 1 year. In addition, the cost of the furnace is significantly reduced, since cheaper refractory material can be used for the lining than in furnaces that are operated at higher furnace chamber temperatures.

Furthermore, according to the invention, a saving in fuel, d. H. especially natural gas.

Detailed description of the figures

1 is a schematic representation of a plant for carrying out the method according to the invention in the melting of aluminum.

( 1 ) is a stove or furnace in which the aluminum to be melted is located. ( 2a ) and ( 2 B ) are alternately operated burners. Natural gas is fed through the supply lines ( 4a ) and ( 4b ). ( 3a ) and ( 3b ) are regenerators, is preheated by the present invention oxygen-enriched combustion air. ( 5a ) and ( 5b ) refer to changeover valves for diverting the exhaust gas flow or air flow, which is used for preheating in the regenerators ( 3a ) respectively. ( 3b ) is initiated. ( 6a ) and ( 6b ) are valves for controlling the oxygen-enriched combustion air supplied to the burners ( 2a ) and ( 2 B ) is supplied. ( 7a ) is a blower for the combustion air, and ( 7b ) is a blower for exhaust gas. ( 8th ) is a chimney for the discharge of excess exhaust gas. The pressure in the furnace is controlled by the exhaust control flap ( 9 ), which makes it possible to divert the desired amount of exhaust gas, so that the preselected slight overpressure in the oven can be adjusted. ( 19 ) is a mixer for introducing oxygen into the combustion air flow on the pressure side of the blower ( 7a ).

The enrichment of normal ambient air with oxygen to produce the oxygen-enriched combustion air, which is used in the process according to the invention, carried out with the aid of the mixer ( 10 ).

2 1 shows a schematic representation of the first step of the method according to the invention with alternating operation of the burners and regenerators ( 2a ) / ( 2 B ) and ( 3a ) / ( 3b ). Burner ( 2 B ) is in operation and fuel is supplied through the supply line ( 4b ) supplied to the burner. Approximately 10% by volume of the exhaust gas produced by the combustion is passed through the chimney ( 8th ) via the exhaust control flap ( 9 ), while about 90% by volume of the exhaust gas passes through the burner ( 2a ) with closed fuel supply by the regenerator ( 3a ) and only then through the chimney ( 8th ) are discharged. This will make the regenerator ( 3a ) preheated. The combustion air is removed by means of the mixer ( 10 ) enriched with oxygen and by the regenerator ( 3b ) preheated and the burner ( 2 B ) supplied as an oxidizing agent. Before carrying out the in 3 illustrated second step of the method, the oxygen supply to the combustion air is interrupted for about 1 to 5 sec, so that ambient air through the regenerator 3b ) flows. Then, the second step is switched.

3 schematically shows the second step of the method. Here is the burner ( 2a ) while supplying fuel through the supply line ( 4a ) and about 90% by volume of the exhaust gas is passed through the burner ( 2 B ) withdrawn from the oven and for preheating the regenerator ( 3b ) before they pass through the chimney ( 8th ) escape. The oxygen-enriched air is in turn supplied by the regenerator ( 3a ) preheated and the burner ( 2a ).

The temperature in the furnace chamber and at other selected positions of the plant is checked by suitable measuring devices. Such measuring devices are known. The oxygen concentration is also measured with known measuring devices at suitable positions of the system. The control of the entire system is computer-controlled and automatic by evaluating the respective measured values for temperatures and oxygen concentration.

In the 2 and 3 The steps shown are performed alternately until the aluminum is completely melted and removed from the oven.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 0928938 A1 [0009, 0036]
• US 2006/0129020 [0010]
• GB 1538255 [0010]
• DE 1957109 [0011]
• EP 0939059 A2 [0011]
• WO 02/103067 A2 [0012]
• EP 1243663 A2 [0012]
• WO 2004/108974 A1 [0012]
• WO 02/088400 Al [0012]
• EP 2098603 A1 [0012]
• DE 1458922 A1 [0012]
• DE 2704101 [0013]
• DE 3690574 C2 [0014]
• US 3207493 [0015]
• WO 2007/048429 [0016]
• WO 00/79182 A1 [0017]

Cited non-patent literature

• M. Flamme and P. Grohmann, GASWÄRME International (56) No. 8/2007, pages 565 to 570 [0018]
• R. Jeschar, E. Specht and H.-G. Bittner, Stahl and Eisen 108 (1988) No. 18, page 835 [0019]

Claims (7)

A method for melting metals or glass or for heat treating metals according to a conventional regenerative process in a directly heated furnace using one or more burners operated with a gaseous or liquid fuel with supply of combustion air, and using one or more Regenerators for preheating the combustion air, characterized in that the burner or burners are operated with a radiant flame. The method of claim 1, wherein the furnace room temperature is about 50 ° C to about 150 ° C lower than in a conventional regenerative melting or heat treatment process. The method for melting aluminum according to claim 1 or 2, wherein the metal is aluminum and the furnace room temperature is about 900 to 1050 ° C. Method according to one or more of claims 1 to 3, wherein the preheated combustion air is oxygen-enriched, d. H. Oxygen in a proportion of more than about 21 to less than about 30 vol .-%, based on the combustion air contains. A method of melting aluminum according to claim 4, wherein the oxygen-enriched combustion air is preheated to a temperature of about 100 to 200 ° C below the furnace chamber temperature. Method according to one of the preceding claims, wherein the metal is aluminum, preferably aluminum scrap. A method as claimed in any one of the preceding claims, wherein the furnace is a hearth furnace or a hearth furnace.

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