EP0120109A1 - Method and apparatus to control the combustion of escaped gas from a hot air cupola furnace - Google Patents

Abstract

Method to control the combustion of the hot uncleaned escaped gases of a hot-air cupola furnace, which have a vary variable calorific value, in a combustion chamber which is arranged directly subsequent to the furnace and is supplied with combustion air, the quantity of combustion air being regulated in such a manner that the excess air of the combustion is constant. <IMAGE>

Description

The invention relates to a working method for operating a hot-wind cupola, more precisely to control the combustion of the warm, unpurified hot, unpurified gaseous gases of a hot-wind cupola, which are preferably sucked out of an annular chamber below the opening, in a combustion chamber immediately downstream of this and charged with combustion air Combustion chamber, the wind being heated by the flue gases generated during the combustion and excess flue gas heat possibly being used in a waste heat boiler, for example.

The invention further relates to a device for operating a hot-wind cupola, more precisely to control the combustion of the blast furnace gas of a hot-wind cupola, which has a charging opening and an annular chamber, preferably arranged below the charging opening, from which the warm, uncleaned heat fluctuating calorific value is obtained Top gases must be extracted into a combustion chamber located outside the furnace and burned in the combustion air supplied together with the combustion chamber, the wind to be directed into the furnace being heated by the flue gases generated during the combustion by means of a heating device and excess flue gas heat by means of a heat extraction device , such as a waste heat boiler, is to be used for other purposes.

The cupola furnace, which has long been known in its basic structure, is the most common and therefore dominant melting unit for the production of cast iron, malleable iron, etc. in foundries. A distinction is made in principle between the so-called cold wind cupola furnace, in which wind (air) is introduced into the lower section of the cupola furnace from the so-called wind ring via nozzles distributed around the periphery of the cupola furnace, and the basically similarly constructed hot wind cupola furnace, the wind of which is introduced before the oven is heated.

A cupola furnace generally consists of a cylindrical, vertical furnace shaft, in which, as so-called iron carriers, pig iron pellets, steel scrap, cast iron and its own cycle, as well as relatively coarse coke as fuel and limestone as a slag generator be used. The combustion air for burning the coke located in the furnace shaft is usually blown into the furnace shaft through a plurality of radial openings arranged approximately 1.0 to 1.5 m above the floor. The burning coke heats and melts and overheats the iron carriers in its vicinity. The liquid iron and the slag eventually run out of the furnace from a common or separate so-called siphon, while the top gas is sucked off either above or below the charging opening, burned and used to preheat the wind, with larger modern smelting plants before Combustion, the top gas is cleaned.

With regard to the energy balance of a cupola furnace, an optimal utilization of the energy supplied to the cupola furnace with the coke is to be striven for, so that extensive combustion of the carbon contained in the coke to carbon dioxide and extensive heat transfer of the sensible heat of the blast furnace gas to the charged feedstocks is desired.

Optimal utilization of the energy supplied to the cupola furnace with the coke is, however, subject to metallurgical limits, since the burn-off of the alloy elements increases with the oxidation of the iron supports in the upper section of the furnace shaft and therefore a minimum content of incompletely burned carbon in the form of carbon monoxide to maintain a reducing furnace atmosphere is required.

An economic optimum must therefore be found from case to case, in which the sum of the expenditures Cost of the coke and the alloying materials is as minimal as possible. Since the oxidation of the iron carriers essentially depends on their so-called specific surface area related to the respective weight, which is usually specified in dm ² / kg, this optimum shifts with increasing average thickness of the iron carriers used to a lower CO content at a low temperature of the top gases . For this reason, in many cases there is an economic optimum for furnace operation with blast furnace gases with such low CO contents and so low temperature that a reliable ignition of the blast furnace gases is no longer guaranteed under all circumstances. The temperature of the top gases fluctuates with the unavoidable fluctuating composition of the iron carriers, the mean wall thickness of the iron carrier particles and the bulk density being of particular importance. As the wall thickness decreases, the heat transfer from the blast furnace gas to the scrap improves. With increasing bulk density, the dwell time of the feed materials in the furnace increases, which improves the heat transfer.

The complete distribution taking place in the melting zone. Burning of carbon to carbon dioxide is partially reversed above the melting zone by reaction of the gases with the surface of the pieces of coke, producing carbon monoxide. As has already been mentioned above, the fluctuating composition of the iron carrier in practice, as well as the different specific surface area of the coke, lead to a correspondingly fluctuating temperature and a different composition of the blast furnace gas. If, for the above reasons, the top gases are not ignited in all conditions, this leads to a short-term extinguishing of the flame and then to reignite to deflagration.

The composition of a top gas of a cupola furnace also fluctuates with the amount of so-called false air sucked in through the charging opening, which leads to an uncontrolled cooling and dilution of the top gas, this influence on the top gas composition being decisively determined by the pouring column of the insert, which drops in rhythm with the charging . In addition, the quantity of false air is also determined by the flow resistance of the material column, that is to say by the composition of the charged material, so that for this reason too, limit cases occur in which the ignition of the blast furnace gas is not guaranteed.

The general prejudice against exposure to a tube recuperator or the like, which is expressed in numerous publications by the relevant experts. With unpurified burnt top gases has resulted in the fact that, especially in larger cupola melting plants with recuperation, practically only the wet dedusting devices that can only be used in front of a recuperator have been used, and accordingly the cleaned top gases which have been cooled during wet dedusting have only been burned. Since a cold ignition stack gases with temperatures of about 30 - 50 ⁰ C abuts particular difficulties, besides, additional heat exchangers are used to the cooled top gas and / or the combustion air to temperatures of 200 - 300 ° C preheat.

Such wet dedusting devices consist of a so-called saturator, in which the top gas is sprayed with so much water that it is completely saturated with water vapor is, with the larger part of the easily wettable dust particles being discharged with the excess water and a downstream fine washer, in which the fine. Coagulate dust particles with water droplets in such a way that the dust-laden droplets can then finally be separated out in a droplet separator.

A wide variety of constructions are used as fine washers, which work according to the principle of the Venturi throat with water, or so-called disintegrators are used in which small water drops are formed by a rotating rotor, which coagulate with the very fine dust particles of the blast furnace gas.

Such wet washers can remove the very fine dusts of the cupola furnace, which consist predominantly of metal oxides, to a reasonably satisfactory degree only with high (generally electrical) energy expenditure, with residual amounts of dust generally remaining, which are considered to be very high, in particular in view of today's emission protection efforts.

A large number of wet scrubbers, which are operated with a so-called clean gas dust content of 200 to 300 mg / m ^{3 of} flue gas, are still in operation today. This clean gas dust content can be improved to values below 50 mg / m ³ with disintegrators and / or high-performance venturi scrubbers if you take a correspondingly large amount of energy into account for the acceleration of the dust particles or for the formation of fine droplets, but it is today no longer responsible from an energy policy point of view, to pursue an arbitrarily high expenditure of energy for certain technical goals, which also has a decisive impact on costs.

With regard to the ignition problems above mentioned, it should be noted that the ignition is more difficult in a wet scrubbing purified stack gases in addition by the in the furnace top gas at saturation temperatures of 50 contained up to 60 ⁰ C water vapor from, so that usually gas coolers are used in such installations where the Cool the top gases after saturation to temperatures of 30 to 40 ° C in order to reduce the water vapor content which hinders the ignition by condensation.

Finally, even in such systems, the ignition of the top gases is made more difficult by the fluctuating proportion of the false air sucked in through the charging opening of the top, whereby it has been shown that the amount of false air is not kept at a fixed value with the usual control loops dependent on the furnace pressure in the annular suction chamber can.

For these reasons, special burners have been developed by a manufacturer for the safe combustion of the exhaust gases cleaned in this way, which allow natural gas to be blended with the blast furnace gas in those work phases in which the blast furnace gas does not have sufficient heat content to continuously maintain the combustion.

In contrast, in recent years, cupola furnaces with an immediately adjacent combustion chamber for burning the unpurified blast furnace gases and downstream heat exchangers for preheating the wind and for further utilization of excess waste heat with subsequent gas cleaning have only been put into operation very rarely, because of uncontrolled combustion and cooling of the flue gases there is a risk that softened fly ash on the tube wall of the heat exchanger adheres, and because the stabilization of the combustion with the fluctuating temperature and composition of the top gases is difficult.

Now, however, cupola furnaces with a directly adjacent combustion chamber have significant advantages, which are particularly important when it comes to recovering energy and cleaning the exhaust gases as far as possible, since the sensible heat of the top gas is not lost in such systems because harmful hydrocarbons are cracked in the combustion chamber, the carbon contained in the blast furnace gas burns in the combustion chamber without stressing the dust management, and the largely water-vapor-free flue gases reduce condensation and corrosion problems for heat exchangers, subsequent fabric filters and the like. cause.

The present invention has for its object to improve the known methods and devices of the type described above while avoiding their described and further disadvantages to the extent that the difficulties in controlling the combustion of the low-energy top gases with fluctuating temperature and composition are eliminated or at least largely eliminated are what creates a prerequisite for using the energetically, operationally and environmentally advantageous stoves of this type, the combustion control, in particular with regard to the allocation of combustion air, to be carried out with the greatest accuracy at the same time. Furthermore, the quality of the exhaust gas, ie the clean gas dust content, is to be improved considerably compared to the previously practiced methods and equipment and the amount of false air required for the smoke-free operation of the open top manhole is to be controlled by a reliable control circuit.

As a solution to the procedural part of the task it is provided that the amount of combustion air is regulated so that the excess air of the combustion is always kept constant.

In the combustion control according to the invention, it is possible to operate with a relatively low excess of air, which must be kept constant despite the fluctuating composition of the blast furnace gas.

A continuously measured component of the blast furnace gas (for example the CO content) can be used as a control variable for determining the excess air of combustion. In a particularly preferred embodiment of the present invention, a constantly measured component of the flue gas is used to determine the excess air, and preferably the excess oxygen content in the flue gas, this method of operation having proven to be particularly useful because for the determination of the 0 ₂ Content measuring devices with a short reaction time of about 0.1 to 5 seconds are available, with a measuring device with a ceramic oxygen ion line being particularly suitable as an oxygen detector, as described in the "Reports of the German Ceramic Society" 52, 1975, no. 10, pages 321-324.

The method according to the invention, taking into account the lockless furnace construction and the resulting quantity of false air, is based on the fact that this is appropriately taken into account as a proportion of the quantity of combustion air, the quantity of false air being used as a fixed or preferably variable proportion of the quantity of combustion air to meet the needs In this way, it is necessary to take into account any fluctuations in the quantity of false air that might counteract the combustion.

Accordingly, in the method according to the invention, the combustion air can consist of a first air portion mixed with the blast furnace gas at the inlet of the combustion chamber and a second air portion having false air through the charging shaft, the two air portions generally being of different sizes and an air portion also becoming zero .

According to one embodiment of the present invention, the second air fraction can be controlled in such a way that it ensures that, even in the event of pressure fluctuations in the furnace, no top gas escapes from the charging opening, the first air fraction then being controlled by a control circuit in such a way that the excess air from the combustion is always constant.

The first air fraction can expediently be chosen to be constant and large enough that with the lowest calorific value of the resulting blast furnace gas there is just enough combustion air, in which case no second air fraction is supplied, and that the second air fraction is only supplied at the higher calorific value of the blast furnace gas.

According to the invention, the combustion air can also consist only of the false air sucked in through the inspection shaft, the amount of combustion air then being controlled in turn by a corresponding control circuit so that the excess air of the combustion is kept constant.

Otherwise, the proposed according to the invention allows Process for the controlled combustion of the top gases occurring with a strongly fluctuating calorific value in a combustion chamber charged with unpurified top gas due to the surprisingly possible elimination of an upstream wet scrubbing with considerably less energy expenditure than the wet processes described, the achievement of a clean gas dust content below 10 mg / m ³ if subsequently cloth filter o . Like. be used for gas cleaning, there being no danger of overheating ash particles on the pipe walls of the wind heater. There is thus the possibility of cleaning the burnt flue gas, which is indirectly cooled for the purpose of waste heat recovery without increasing volume, with a cloth filter and in this way to make use of the significantly better gas purification with, at the same time, considerably lower energy expenditure, as explained above.

The equipment part of the object of the present invention, specified above, is achieved according to the invention by a control device with which the amount of combustion air to be supplied to the combustion chamber is to be controlled in such a way that the air excess of the combustion is constant.

Further preferred configurations of the device according to the invention result from the subclaims.

The drawing shows a simplified, schematic D position of a hot-wind cupola furnace which is operated by means of the device according to the invention using the working method according to the invention. The cupola furnace designated as a whole is a Heißw cupola furnace, which has an insertion or inspection opening 2, one below the inspection opening 2 arranged annular chamber 3 for collecting the rising top gases, a wind ring 4 with nozzles 6 and a tap opening 7.

The furnace shaft 8 is essentially cylindrical and extends vertically. The blast furnace gas rises in the furnace shaft according to arrow 9 and passes from this or the annular chamber 3 of the furnace 1 via a short connecting line 11 into the combustion chamber 16 immediately downstream of the cupola furnace 1.

According to arrow 12, false air enters the cupola furnace 1 from above, which partly mixes with the blast furnace gas.

At the inlet of the combustion chamber 16 there is an air supply line 13 by means of which a first air portion, which is indicated by an arrow 14, is to be added to the top gas flowing into the combustion chamber.

As a control variable for determining the excess air of the combustion, the ° ₂ content is constantly measured as a component of the flue gas, specifically with an oxygen probe 17 which is arranged in the combustion chamber 16 and works with a ceramic oxygen-ion line with a short reaction time.

The combustion air for the combustion chamber 16 consists not only of the false air sucked in through the chute 18, as may be the case in the embodiment of the present invention, but also of another supplied to the blast furnace gas via the air supply line 13 and mixed with it Air fraction, which is referred to below as "first air fraction" (during the air fraction of the combustion caused by the false air air is referred to as "second air fraction"), the second air fraction is controlled so that it ensures that, even with slight pressure fluctuations in the furnace, no top gas escapes from the charging opening 2, during the arrow 14 through the air supply line 13 admixed first air portion is controlled by a control loop so that the excess air of the combustion is always constant. The constant first air fraction is chosen so large that at the lowest heating value of the blast furnace gas there is just enough combustion air, whereby under such conditions no second air fraction is then supplied, while the second air fraction is only supplied at a higher heating value of the blast furnace gas.

The wind indicated by the arrow 19 in the drawing is fed to the cupola furnace 1 via a wind pipe 21, but is still heated in a recuperator 22 before entering the furnace 1, specifically in a hot air heater 23, from which it is then heated reaches the furnace 1 via the horizontally represented section 21 'of the wind pipe 21. The recuperator 22 also has a waste heat boiler 24, in which excess flue gas heat, which is not required for heating the wind, is used for further use, for example to create hot water in a hot water boiler 26.

The exemplary embodiment shown schematically is a lined furnace with a melting rate of 10 t / h, a wind temperature of 500 ° C, a top gas temperature of 250 ° C, a wind volume of 5,800 m ³ / h and a top gas quantity of 6.80 ₀ m ³ / h with 600 m ³ / h of false air, 17.1% N _{2 '14} .4% CO, 10.5% C0 ₂ and 1.7% 0 ₂ , with a fixed amount of combustion air of 1,300 m ³ / h before the blast furnace gas is mixed into the combustion chamber 16 and a quantity of combustion air (valid for the above numerical example) of approximately 800 m ³ / h is sucked in through the charging opening 2 in the form of false air. The second proportion of air is reduced to 300 m ³ / h with a CO content of 10% and increases to 2,000 m ³ / h with a CO content of 21%. By keeping a proportion of 1.2% 0 ₂ in the flue gas constant, the excess air in the combustion is approx. 25% in all cases.

In the additional control circuit for the gas management of the cupola furnace already mentioned, in addition to the oxygen probe 17 determining the controlled variable 0 ₂ , there is an actuator designed as a swirl controller 27, which is arranged in front of the suction fan 28, the swirl controller 27 also having a filter 29 and a cooler 31 are upstream.

When an oxygen deficit is signaled, that is to say an actual oxygen value which is less than the predetermined oxygen target value, the swirl controller 27 on the suction fan 28 is raised, as a result of which the amount of combustion air sucked in as false air through the opening 2 is increased until the actual oxygen value in the flue gas corresponds to the specified oxygen setpoint.

Apart from the considerably higher energy yield compared to known methods, the method according to the invention also results in a considerably more environmentally friendly quality of the exhaust gas 32, which is 10 times and more pure compared to the previously practiced methods.

Claims (17)

1. A method for controlling the combustion of the warm, unpurified top gases of a hot-wind cupola furnace which have a strongly fluctuating calorific value in a combustion chamber which is immediately downstream of this and is charged with combustion air, characterized in that the amount of combustion air is regulated so that the excess air of the combustion is constant is. 2. The method according to claim 1, characterized in that a continuously measured component of the blast furnace gas, for example the CO content, is used as a control variable for determining the excess air of combustion. 3. The method according to claim 1, characterized in that a continuously measured component of the flue gas, for example the O ₂ component, is used as a control variable for determining the excess air of combustion. 4. The method according to claim 2 or 3, characterized in that the controlled variable for determining the excess air is determined by means of a measuring device with a short reaction time. 5. The method according to claim 4, characterized in that the reaction time of the measuring device is 0, 1- 5 seconds. 6. The method according to claims 3 to 5, characterized in that a measuring device with a ceramic oxygen-ion line is used for a control variable consisting of the 0 ₂ portion of the flue gas for determining the excess air of combustion. 7. The method according to one or more of the preceding claims, characterized in that the combustion air consists of a first air portion mixed with the blast furnace gas at the inlet of the combustion chamber and a second air portion flowing in as false air through the inspection shaft. 8. The method according to claim 7, characterized in that the second proportion of air is controlled so that it ensures that even in the event of pressure fluctuations in the furnace, no top gas escapes from the loading opening; and that the first proportion of air is controlled so that the excess air of combustion is always constant. 9. The method according to claim 7 or 8, characterized in that the first air portion is chosen to be constant and so large that just enough combustion air is available at the lowest heating value of the blast furnace gas, with no second air portion being supplied; and that the second proportion of air is only supplied at a higher calorific value of the blast furnace gas. 10. The method according to one or more of claims 1 to 6, characterized in that the combustion air (only) consists of the false air sucked in through the inspection shaft. 11. Device for controlling the combustion of the blast furnace gases of a hot-wind cupola furnace, which has a charging opening and has an annular chamber, preferably arranged below the charging opening, from which the warm, unpurified blast furnace gases, which have a strongly fluctuating calorific value, are sucked off into a combustion chamber arranged outside the furnace and are to be burned therein together with the combustion air supplied in front of the combustion chamber, the combustion air to be conducted into the furnace Wind is to be heated by the flue gases generated during the combustion by means of a heating device and excess flue gas heat is to be used elsewhere by means of a heat extraction device, such as a waste heat boiler, for carrying out a method according to one or more of the preceding claims, characterized by a control device which is to regulate the amount of combustion air (12, 14) to be supplied to the combustion chamber (16) so that the excess air of the combustion is constant. 12. Device according to claim 11, characterized in that the control device has a measuring device - :( 17) with a short reaction time of about 0.1-5 seconds. 13. The device according to claim 12, characterized by a measuring device (17) with a ceramic oxygen-ion line. 14. Device according to one or more of claims 11 to 13, characterized in that an air supply line (13) is provided at the inlet of the combustion chamber (16), by means of which the top gas flowing into the combustion chamber (16) is admixed with a first air portion is. 15. The device according to claim 14, characterized in that the first proportion of air in a control loop to control is that the excess air is constant when the combustion air also consists of a second air portion flowing in as false air through the inspection shaft (18). 16. Device according to one or more of claims 11 to 13, characterized in that the second air portion is to be controlled with a control loop so that the excess air is constant when the combustion air consists only of the second air portion. 17. The device according to one or more of claims 11 to 16, characterized in that a swirl regulator (27) arranged upstream of the exhaust gas suction fan (28) as an actuator for the control circuit. Like. Is provided.

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