Fuel-fired furnace and method for controlling combustion in a fuel-fired furnace

Abstract

Fuel-fired furnace and a method for operating it, in which method: a main oxidizing agent is injected at a controlled flow rate into the combustion chamber (2) of the furnace; the combustible material is burnt in the combustion chamber (2) with the main oxidizing agent, producing thermal energy and flue gases (6) at a temperature higher than 600oC; the flue gases (6) are removed via an exhaust duct (13), said removed flue gases (6) possibly containing residual materials that could be oxidized, the exhaust duct (13) being equipped with an inlet (14) for a diluting oxidizing agent downstream of the combustion chamber (2); the residual materials that could be oxidized are burnt with the diluting oxidizing agent by means of a flame (12) at the inlet (14) for the diluting oxidizing agent; the flame intensity inside the exhaust duct (12) is detected; and the flow rate at which the main oxidizing agent is injected into the combustion chamber (2) is controlled according to the detected flame intensity.

Description

[0001] The present invention relates to the regulation of combustion in flame furnaces.

[0002] Flame furnaces are commonly used in industry for thermal energy generation and for high temperature processing of materials.

[0003] The term "flame furnace" refers to a furnace, such as a smelting furnace or an incinerator, in which at least some of the thermal energy is produced in the combustion chamber of the furnace by the combustion of a fuel with an oxidant present in the oxidizer. Thus, the term "flame furnace" also covers furnaces in which at least some of the thermal energy is produced by combustion without a visible flame, often referred to as "flameless combustion."

[0004] The fumes generated by combustion, generally containing CO2, CO and H2O, are evacuated from the combustion chamber of the flame furnace at a temperature above 600°C through an exhaust duct.

[0005] In theory, maximum thermal energy is generated by combustion when it is stoichiometric, that is, when the oxidant is injected into the combustion zone in a quantity that corresponds to the quantity of oxidant required for the complete combustion of the fuel present in the combustion zone. In this case, the carbon present in the fuel is completely oxidized to CO 2 , the hydrogen generally present in the fuel is completely oxidized to H 2 O, etc. In industrial practice, however, it is found that a slight excess of oxidant is necessary to achieve complete combustion of the fuel.

[0006] Insufficient injection of oxidant leads to a drop in furnace efficiency due to non-combustion or partial combustion of the fuel. Too much excess oxidant also leads to a drop in furnace efficiency (for example: greater loss of thermal energy through the evacuated fumes and, in the case of oxy-combustion, the evacuation with the fumes of the part of the oxygen that did not participate in the combustion, oxygen having a significant cost).

[0007] Among the other disadvantages of too high an oxidant flow rate, one can in particular point out a higher oxidation rate of the charge in the case of an oxidizable charge, as is the case in a melting furnace for oxidizable metals, such as aluminum, and certain reheating furnaces. It is notably known to operate flame furnaces in over- or under-stoichiometry regime, to avoid or limit harmful reduction or oxidation of the charge by the atmosphere in the combustion zone. Thus, for certain applications, optimal combustion differs from stoichiometric combustion.

[0008] Optimized operation of a flame furnace is generally possible in flame furnaces in which the fuel and oxidant inputs and their compositions are perfectly controlled.

[0009] However, in a significant number of industrial applications of flame furnaces, the quantity and / or composition of the combustible material available in the combustion zone is poorly or only slightly controlled.

[0010] This is for example the case: in flame furnaces in which the charge contains a variable quantity and / or quality of combustible materials, such as, for example, waste incinerators and secondary melting furnaces for the recycling of metals, in flame melting furnaces in which the charge contains inherent and / or added combustible materials, and in which the charge releases these combustible materials in an uncontrolled manner into the combustion zone generally located above the charge, such as, for example, secondary melting furnaces for the recycling of metals, in flame furnaces for the post-combustion of fumes from furnaces as described above, for example, post-combustion chambers of arc furnaces for the secondary melting of steel.

[0011] From JP-A-1314809 and JP-A-2001004116 it is known to equip an incinerator with a camera directed towards the interior of the combustion chamber and to regulate the post-combustion inside the combustion chamber above the main combustion depending on the image obtained of the combustion inside the chamber.

[0012] From WO-A-2005 / 024398, it is known to measure the quantity of chemical species contained in a gas from a metal treatment furnace, such as an electric arc furnace or a converter, by sampling a portion of the gas to be analyzed, cooling it to less than 300°C and measuring the quantity of CO and / or CO 2 present in the gas using the coherent light signal emitted by a laser diode, said method allowing measurement of said quantities with a response time of less than 10 seconds and real-time control of the furnace.

[0013] WO-A-03 / 056044 describes a process for melting aluminium, in which solid aluminium is introduced into a furnace, the aluminium is melted to form an aluminium bath, variations in the concentration of carbon monoxide (CO) and the temperature in the fumes leaving the furnace are detected, the formation of aluminium oxides on the surface of the aluminium bath is deduced and the melting process is regulated as a function of the formation of aluminium oxides.

[0014] However, measuring the concentration of certain species in the fumes from a flame oven is made difficult by the nature and quantities of pollutants, such as soot, in the fumes.

[0015] WO-A-2004 / 083469 describes an aluminium melting process in which the fuel / oxidant ratio injected by a burner into the flame furnace is regulated as a function of the temperature of the fumes in the fume evacuation duct provided with an air inlet known as "dilution air".

[0016] In such a process, the dilution air flow rate can vary depending on various parameters (size of openings, speed of flue extraction, condition of flue ducts, flow rate of other flue gas streams collected by the same extractor). This variable flow rate can have an influence on the flue gas temperature in the exhaust duct and thus have an impact on the furnace setting. Daily (day and night) and seasonal (summer and winter) variations in the temperature of the dilution air, which is generally ambient air, can also have an impact on the flue gas temperature in the exhaust duct.

[0017] The present invention aims to provide a combustion control in a flame furnace which does not have the disadvantages of the known methods described above.

[0018] The present invention thus relates to a method for operating an improved flame furnace. According to this method, an oxidant, called the "main oxidant", is injected at a regulated flow rate into a combustion chamber of the flame furnace. Combustible material is burned in the combustion chamber with the main oxidant thus injected, producing thermal energy and fumes having a temperature greater than 600°C in the combustion chamber. The fumes thus produced are evacuated from the combustion chamber through an exhaust duct. This exhaust duct is provided with an inlet for an oxidant called the "dilution oxidant", typically, but not necessarily, ambient air, downstream of the combustion chamber, so that the dilution oxidant comes into contact with the fumes at 600°C, or even higher.When the fumes still contain oxidizable materials, that is to say when the combustion of combustible material in the combustion chamber is not complete, a flame is thus obtained at the level of the dilution oxidant inlet inside the exhaust duct. Indeed, the contact between the dilution oxidant and the oxidizable materials in the fumes at high temperature generates self-combustion of said oxidizable materials, such as CO and / or H 2 , present in the evacuated fumes. According to the invention, the intensity of the flame is detected inside the exhaust duct, and therefore downstream of the combustion chamber, and the injection flow rate of main oxidant into the combustion chamber is regulated as a function of the detected flame intensity.

[0019] The combustible material may in particular be introduced into the combustion chamber in a controlled manner, for example, by injecting a jet of fuel into the combustion chamber by means of a lance or a burner. The combustible material may be present in the charge and therefore be introduced into the combustion chamber with the charge. The combustible material may also be introduced into the combustion chamber by a combination of controlled introduction and introduction with the charge into the combustion chamber.

[0020] Advantageously, the injection flow rate of main oxidant injected into the combustion chamber is reduced when the flame intensity thus detected is less than a predetermined lower limit and the flow rate of main oxidant injected into the combustion chamber is increased when the flame intensity thus detected is greater than a predetermined upper limit.

[0021] The presence of oxidizable materials, such as CO, in the fumes is thus detected by the intensity of their combustion with the dilution oxidant using a flame detector which returns a signal indicating the intensity of the combustion / flame inside the exhaust duct: (a) a high intensity being the sign of a significant presence of oxidizable materials in the evacuated fumes, and (b) a low intensity being the sign of a low presence of oxidizable materials in the evacuated fumes.

[0022] The invention thus makes it possible to determine the level of presence of oxidizable materials in the fumes and to apply in real time a correction to the combustion adjustment in the combustion zone.

[0023] The predetermined lower and upper limits are set according to the nature of the combustion process in the combustion chamber, as discussed above. When the combustion process aims at complete combustion of the combustible material in the combustion chamber, the predetermined lower limit is very low, but greater than zero. In this way, it is ensured that the main oxidant injection rate is neither excessive nor too low for the combustion process in the combustion chamber.

[0024] The invention makes it possible in particular to compensate for imperfect knowledge of the combustible material content of the furnace charge (typical case for recycling furnaces), of the quality of the combustible material and / or of its release into the combustion chamber by a real-time adaptation of the adjustment of the main oxidant flow rate and, as explained below, possibly also of the fuel flow rate injected into the combustion chamber.

[0025] Another advantage of the invention is that it can be implemented with an inexpensive and easy-to-implement flame intensity detector.

[0026] In certain combustion processes, the content of oxidizable materials in the discharged flue gases may exhibit frequent, but often short-term, variations. According to one embodiment, the flame intensity inside the discharge duct is detected for predetermined durations Δt1 and Δt2. The injection rate of main oxidant into the combustion chamber is reduced when the detected flame intensity has remained below the lower limit for the predetermined duration Δt1. Similarly, the injection rate of main oxidant into the combustion chamber is increased when the detected flame intensity has remained above the upper limit for the predetermined duration Δt2. Thus, excessive fluctuations in the combustion process are avoided.Another possibility is (a) to reduce the injection rate of main oxidant into the combustion chamber when the average value of the flame intensity detected during the predetermined time Δt1 is less than the lower limit, and (b) to increase the injection rate of main oxidant into the combustion chamber when the average value of the flame intensity detected during the predetermined time Δt2 is greater than the upper limit. In practice, the predetermined times Δt1 and Δt2 are typically identical.

[0027] According to one embodiment, the main oxidant and combustible material are injected into the combustion chamber at controlled flow rates, the combustible material is burned with the main oxidant in the combustion chamber, producing thermal energy and fumes at a temperature above 600°C in the combustion chamber, and the fumes thus produced are discharged from the combustion chamber through an exhaust duct. As indicated above, the discharged fumes may contain residual oxidizable materials. The exhaust duct is provided with a dilution oxidant inlet downstream of the combustion chamber. The residual oxidizable materials in the fumes are burned with the dilution oxidant, producing a flame inside the exhaust duct at the dilution oxidant inlet.According to the invention, the flame intensity inside the exhaust duct is detected and the injection flow rate of the main oxidant into the combustion zone is regulated as a function of the detected flame intensity.

[0028] It is also possible to regulate the main oxidant injection rate and the fuel injection rate into the combustion chamber based on the detected flame intensity.

[0029] Advantageously, the ratio between the main oxidant injection rate and the fuel material injection rate in the combustion chamber is reduced when the flame intensity detected inside the exhaust duct is less than a predetermined lower limit and the ratio between the main oxidant injection rate and the fuel material injection rate in the combustion chamber is increased when the detected flame intensity is greater than a predetermined upper limit.

[0030] In particular, it is possible (a) to reduce the ratio between the main oxidant injection flow rate and the fuel material injection flow rate in the combustion chamber when the detected flame intensity is lower than the lower limit for a predetermined time Δt1, and (b) to increase the ratio between the main oxidant injection flow rate and the fuel material injection flow rate in the combustion chamber when the detected flame intensity inside the exhaust duct is higher than the upper limit for a predetermined time Δt2.It is also possible (a) to reduce the ratio between the main oxidant injection rate and the fuel material injection rate into the combustion chamber when the average value of the flame intensity detected inside the exhaust duct during the predetermined time Δt1 is lower than the lower limit, and (b) to increase the ratio between the main oxidant injection rate and the fuel material injection rate into the combustion chamber when the average value of the flame intensity detected during the predetermined time Δt2 is higher than the upper limit.

[0031] The ratio of the main oxidant injection rate to the fuel injection rate into the combustion chamber can be changed by changing the main oxidant injection rate relative to the predetermined fuel injection rate, or by changing (a) the main oxidant injection rate and (b) the fuel injection rate. It should be noted, however, that the fuel injection rate into the combustion chamber is often regulated according to the thermal energy requirement in the combustion chamber.

[0032] According to one embodiment, the combustion chamber is equipped with at least one lance for injecting a regulated flow of main oxidant. The combustion chamber may also be equipped with at least one burner for injecting a regulated flow of main oxidant and a regulated flow of combustible material. The combustion chamber may also comprise at least one such lance and at least one such burner.

[0033] The process can be a batch process, a semi-batch process or a continuous feed process.

[0034] The combustion chamber can be the combustion chamber of an arc furnace, a rotary furnace, a fixed melting furnace, a reheating furnace, a boiler, an afterburner of gaseous effluents, etc.

[0035] The process may be a melting or vitrification process, and in particular a secondary melting process of recovered metals, a process of combustion of solid, liquid or gaseous waste, a process of post-combustion of gaseous effluents, a reheating process, such as the reheating of metallurgical products, etc.

[0036] The dilution oxidant inlet is typically an ambient air inlet in the exhaust duct (air gap), but can also be an oxidant injector, such as an oxygen-enriched air or oxygen injector.

[0037] The flame detector is advantageously an optical detector and in particular an optical detector chosen from ultraviolet detectors, infrared detectors and visible radiation detectors. The detector is preferably an infrared detector or an ultraviolet detector.

[0038] To avoid interference from combustion, known as main combustion, which takes place inside the combustion chamber, the flame is detected inside the exhaust duct, preferably in a location sheltered from the main combustion.

[0039] To better separate the detection zone inside the exhaust duct from the main chamber, the exhaust duct may be provided with an elbow. Flame detection then preferably takes place downstream of this elbow. The dilution oxidant inlet is advantageously located immediately upstream, in or downstream of the elbow, so that the flame generated by the combustion of the oxidizable materials in the flue gases with the dilution oxidant develops at least mainly downstream of the elbow.

[0040] When the furnace has a geometry that prevents interference between the main combustion and the flame detector or if the furnace has elements forming a screen between the main combustion and the flame detector, such an elbow is not necessary.

[0041] The present invention also relates to a flame furnace suitable for implementing the method described above.

[0042] Thus, the invention relates more particularly to a flame furnace comprising a combustion chamber, a means for injecting main oxidant at a regulated flow rate into this combustion chamber and a conduit for evacuating fumes from said combustion chamber. The evacuation conduit comprises a dilution oxidant inlet downstream of the combustion chamber. The flame furnace of the invention also comprises a detector for detecting a flame intensity inside the evacuation conduit at the dilution oxygen inlet. The detector is positioned and oriented so as to prevent the main combustion from distorting the detected flame intensity.

[0043] The exhaust duct may in particular comprise an elbow as mentioned above. Concerning the method according to the invention, the flame detector is then preferably positioned downstream of this elbow. Advantageously, the dilution oxidant inlet is positioned immediately upstream, in or downstream of the elbow of the exhaust duct.

[0044] The furnace comprises a control unit linked to the detector and to the means for injecting the main oxidant. According to the first embodiment of the furnace, this control unit is programmed: to compare the flame intensity detected by the detector inside the exhaust duct with a predetermined lower limit and a predetermined upper limit, to reduce the injection rate of main oxidant into the combustion chamber by the main oxidant injection means when the detected flame intensity is lower than the predetermined lower limit, and to increase the injection rate of main oxidant into the combustion chamber by the main oxidant injection means when the detected flame intensity is higher than a predetermined upper limit.

[0045] This control unit can be programmed in particular: to reduce the injection rate of main oxidant into the combustion chamber when the detected flame intensity is lower than the lower limit for a predetermined period Δt1 and / or when the average value of the detected flame intensity for a predetermined period Δt1 is lower than the lower limit for the predetermined period Δt1, and to increase the injection rate of main oxidant into the combustion chamber when the detected flame intensity is higher than the upper limit for a predetermined period Δt2 and / or when the average value of the detected flame intensity for the predetermined period Δt2 is higher than the upper limit for a predetermined period Δt2.

[0046] According to the second embodiment, the furnace according to the invention comprises means for injecting combustible material at a regulated flow rate into the combustion chamber and the flame furnace preferably comprises a control unit linked (a) to the detector, (b) to the means for injecting the main oxidant into the combustion chamber, and (c) to the means for injecting combustible material into the combustion chamber.This control unit is programmed (i) to compare the flame intensity detected by the detector inside the exhaust duct with a predetermined lower limit and a predetermined upper limit, (ii) to reduce the ratio between the main oxidant injection rate and the fuel material injection rate in the combustion chamber when the detected flame intensity is lower than the predetermined lower limit, and (iii) to increase the ratio between the main oxidant injection rate and the fuel material injection rate in the combustion chamber when the detected flame intensity is higher than a predetermined upper limit.

[0047] According to a preferred embodiment, this control unit is more particularly programmed: to reduce the ratio between the main oxidant injection rate and the combustible material injection rate into the combustion chamber when the flame intensity detected inside the exhaust duct is lower than the lower limit for a predetermined period Δt1 and / or when the average value of the flame intensity detected during the predetermined time Δt1 is lower than the lower limit, and to increase the ratio between the main oxidant injection rate and the combustible material injection rate into the combustion chamber when the detected flame intensity is higher than the upper limit for a predetermined period Δt2 and / or when the average value of the flame intensity detected during the predetermined time Δt2 is higher than the upper limit.

[0048] To vary the ratio between the main oxidant injection rate and the fuel material injection rate in the combustion chamber, the control unit will advantageously vary the main oxidant injection rate as a function of the fuel material injection rate. However, it is also possible for the control unit to vary the ratio between the main oxidant injection rate and the fuel material injection rate by regulating the main oxidant injection rate and the fuel material injection rate. In this case, the control unit may, for example, in the case of a flame intensity below the predetermined lower limit, reduce the ratio between the main oxidant injection rate and the fuel material injection rate by increasing the fuel material injection rate at an unchanged main oxidant injection rate.

[0049] The main oxidant injection means of the furnace may comprise one or more lances for injecting main oxidant into the combustion chamber.

[0050] The means for injecting combustible material into the furnace may comprise one or more lances for injecting combustible material into the combustion chamber.

[0051] The furnace may also comprise one or more burners for injecting combustible materials and main oxidant into the combustion chamber. Such a burner is therefore, on the one hand, part of the means for injecting main oxidant and, on the other hand, part of the means for injecting combustible material into the furnace.

[0052] The oven according to the invention can be an oven for a batch process, for a semi-batch process or for a continuous process.

[0053] The furnace may in particular be an arc furnace, a rotary furnace, a fixed melting furnace, a reheating furnace, such as a reheating furnace for metallurgical products, a boiler, a post-combustion chamber for gaseous effluents, etc.

[0054] The furnace may be a melting or vitrification furnace, and in particular a secondary melting furnace for recovered metals, an incinerator for solid, liquid or gaseous waste, etc.

[0055] The dilution oxidant inlet is typically an ambient air inlet in the exhaust duct (air gap), but can also be an oxidant injector, such as an oxygen-enriched air injector or an oxygen injector.

[0056] The flame detector is preferably an optical detector and in particular an optical detector chosen from ultraviolet detectors, infrared detectors and visible radiation detectors.

[0057] The combustible material injected into the combustion chamber may be a gaseous, liquid or solid fuel (e.g. natural gas, liquid fuel oil, propane, biofuel, pulverized coal) or a combination of several fuels. This combustible material may be injected in addition to the combustible material introduced into the combustion chamber with the charge, which may be mixed with the charge before its introduction into the combustion chamber and / or may be an intrinsic part of the charge.

[0058] The main oxidant can be air, oxygen-enriched air, pure oxygen (by definition having an oxygen content of 88% to 100% vol) or a mixture of oxygen with recycled fumes. In the latter cases (oxygen-enriched air and in particular pure oxygen or a mixture of oxygen with recycled fumes), the benefit is reduced fume volume and fuel consumption.

[0059] The invention is particularly useful for flame furnaces used for the secondary melting of metals. Secondary melting refers to the melting of recycled materials or materials from primary metallurgy (for example: cast iron from a blast furnace).

[0060] Examples of metals considered are: cast iron, lead, aluminum, copper, or any other metal that can be melted in a flame furnace.

[0061] The metal charge can also be loaded into the furnace in a mixture with combustible materials composed of a high proportion of carbon (plastic, coke, etc.). These combustible materials can be present in the metal charge (for example in the case of aluminum recycling) and / or intentionally added to the charge for the purposes of the melting process (for example in the case of the deoxidation reaction for lead recycling).

[0062] The present invention and its advantages appear more clearly in the following illustrative example, reference being made to the figure 1 which schematically represents a flame melting furnace according to the invention.

[0063] The furnace is more specifically a rotary furnace for secondary lead melting with a combustion chamber 2 with a capacity of 15t.

[0064] The furnace is equipped with a natural gas / oxygen burner 24 which generates the flame 11 in the combustion chamber 2. The power of the burner 24 and the oxygen / natural gas ratio are controlled by the furnace automation (control device 20 connected to the oxygen flow regulator 15 and to the natural gas flow regulator 17) according to the progress of the heating cycle, as described below.

[0065] Charge 30 consists of lead waste from the crushing of automobile batteries. A significant portion of this lead is in the form of a "paste" of oxide (PbO, PbO 2 ...) and lead sulfate (PbSO 4 ...). To this metallic charge are added materials necessary for the reduction of oxides partly made up of coke (containing a high carbon content), also called "reagents".

[0066] The lead recycling process involves heating the charge 30, and then keeping the charge hot in contact with the reactants in order to obtain liquid lead 4 and a slag which fixes the impurities and the sulfur present in the lead sulfate.

[0067] The furnace operates discontinuously. The combustion chamber 2 is loaded at the beginning of each cycle. The burner 24 is then ignited and its power modulated by the control device 20 so that the temperature of the load follows a heating cycle that has been determined empirically.

[0068] During the heating step, a significant portion of the carbon present in the solid charge 30 reacts with the atmosphere of the rotary combustion chamber 2 consisting essentially of the hot smoke produced by the burner 24.

[0069] This reaction produces CO and H 2 by the following reaction between part of the smoke and part of the carbon in the charge, the mechanisms of which can be schematically presented as follows: CO 2 + C → 2.CO H 2 O + C → H 2 + CO.

[0070] To limit the formation of CO in the atmosphere of chamber 2, it is possible to pre-set burner 24 in order to inject excess oxygen into chamber 2. However, the reaction rate of the carbon present in solid charge 30 with the atmosphere of the furnace varies according to the different parameters of the process, such as in particular the composition of the charge which varies according to the origin of the batches to be recycled.

[0071] For a load of 15t, the power of burner 24 will for example be set between 1 and 1.5 MW depending on the progress of the heating cycle. In the middle of the cycle, the burner is for example set for a power of 1.3 MW with the following flow rates: natural gas 130 Nm3 / h pure oxygen 270 Nm3 / h.

[0072] An analysis of the fumes 6 coming out of room 2 reveals the following composition: CO 2: 56% / CO: 25% / H 2: 4% remains N 2.

[0073] The CO and H2 in the fumes burn with dilution air in the flame 12 inside the chimney 13 which has a bend downstream and close to the chamber 2. The dilution air is ambient air which enters the chimney 13 through the opening 14 provided for this purpose downstream of the bend. This dilution air allows the combustion of the CO into CO2 and the cooling of the fumes before the filtration (not shown) which precedes the evacuation of the fumes. Too high a level of CO in the fumes 6 has several disadvantages: incomplete combustion of CO in the chimney 13, and therefore a residual CO emission at the chimney 13, a very significant rise in the temperature of the fumes in the chimney 13 not allowing the fumes to pass into the downstream filter (not shown), hence the forced reduction in the power of the burner 24, or even the stopping of the burner 24 on temperature safety, in order to allow filtration and compliance with environmental standards, and overconsumption of fuel and therefore a reduction in energy efficiency in the furnace, because the reactions CO 2 + C -> 2.CO and H 2 O + C -> H 2 + CO are endothermic.

[0074] The detection according to the invention by means of the UV detector 10 of the D-LX100 range marketed by the company Durag of the intensity of the combustion flame 12 of the CO + H 2 mixture with the dilution air just after the outlet 5 of the furnace makes it possible to correct the adjustment of the burner 24 by acting on the “Oxygen / Natural Gas” ratio. To this end, the detector 10 transmits to the control device 20 a signal corresponding to the detected flame intensity.

[0075] The elbow of the chimney 13 and the positioning of the UV10 detector relative to said elbow ensures that the UV10 detector detects only the intensity of the flame 12 inside the chimney 13 without interference from the UV radiation of the combustion inside the combustion chamber 2.

[0076] The invention allows, for example, the control device 20, in particular when the intensity of this combustion in the chimney 13 exceeds an experimentally predetermined upper limit: to increase by means of the oxygen flow regulator 15 the oxygen flow rate to 340 Nm3 / h and to keep the natural gas flow rate unchanged at 130 Nm3 / h, to decrease by means of the fuel flow regulator 17 the fuel flow rate to 95 Nm3 / h and to keep the oxygen flow rate unchanged at 270 Nm3 / h, and to modulate the two flow rates by increasing the oxygen flow rate to 300 Nm3 / h by means of the regulator 15 and by decreasing the natural gas flow rate to 110 Nm3 / h by means of the regulator 17.

[0077] In all three cases, burner 24 injects an excess of 70 Nm3 / h of oxygen, compared to the initial setting. This excess oxygen is then available for the combustion inside furnace 2 of the combustible materials released by the load.

[0078] As soon as the rate of release of combustible material by the load decreases, following the evolution of the cycle, the intensity of the combustion of CO and H 2 in the chimney 13 decreases and the intensity of the flame 12 detected by the detector 10 therefore also decreases.

[0079] It is then possible to gradually return the oxygen and natural gas flow rates of the burner 24 to the predetermined initial or basic flow rates and thus reduce the oxygen / natural gas ratio.

[0080] This adjustment of the oxygen / natural gas ratio is done dynamically depending on the intensity of the post-combustion of the smoke in the chimney 13 (intensity of the flame 12 detected).

[0081] The energy efficiency of furnace 2 is significantly improved and effective treatment of fumes, particularly their filtering, is ensured.

Claims

1. Method for operating a flame furnace comprising a combustion chamber (2), a method in which: - a main oxidant is injected at a regulated flow rate in the combustion chamber (2), - combustible material is burned in the combustion chamber (2) with the main oxidant, producing heat energy and fumes (6) in the combustion chamber at a temperature above 600°C, - the fumes (6) thus produced are discharged from the combustion chamber (2) through a discharge pipe (13), said fumes (6) discharged being able to contain residual oxidisable materials, the method being characterised in that: - the discharge pipe (13) is provided with an inlet (14) for dilution oxidant downstream of the combustion chamber (2), and in that: - the residual oxidisable materials are burned with the dilution oxidant by means of a flame (12) in the discharge pipe (13) at the dilution-oxidant inlet (14), - the flame intensity of said flame in the discharge pipe (13) is detected, - the rate of injection of main oxidant in the combustion chamber (2) is regulated according to the flame intensity detected.

2. Method according to claim 1, in which: - the rate of injection of main oxidant in the combustion chamber (2) is reduced when the flame intensity detected is below a predetermined lower limit, and - the flow rate of main oxidant injected in the combustion chamber (2) is increased when the flame intensity thus detected is above a predetermined upper limit.

3. Method according to claim 1, in which: - the rate of injection of main oxidant in the combustion chamber (2) is reduced when the flame intensity detected is below the lower limit for a predetermined period Δt1 or when the mean value of the flame intensity detected during the predetermined period Δt1 is below the lower limit, and - the flow rate of main oxidant injected in the combustion chamber (2) is increased when the flame intensity thus detected is above the upper limit for a predetermined period Δt2 or when the mean value of the flame intensity detected during the predetermined period Δt2 is above the upper limit.

4. Method according to claim 1, in which - combustible material is also injected at a regulated rate in the combustion chamber (2), and in which - optionally also the rate of injection of combustible material in the combustion chamber (2) is regulated according to the flame intensity detected.

5. Method according to claim 4, in which - the ratio between the rate of injection of main oxidant and the rate of injection of combustible material in the combustion chamber (2) is reduced when the flame intensity detected is below a predetermined lower limit, and - the ratio between the rate of injection of main oxidant and the rate of injection of combustible material in the combustion chamber (2) is increased when the flame intensity detected is above a predetermined upper limit.

6. Method according to claim 5, in which - - the ratio between the rate of injection of main oxidant and the rate of injection of combustible material in the combustion chamber (2) is reduced when the flame intensity detected is below the lower limit for a predetermined period Δt1 or when the mean value of the flame intensity detected during the predetermined period Δt1 is below the lower limit, and - - the ratio between the rate of injection of main oxidant and the rate of injection of combustible material in the combustion chamber (2) is increased when the flame intensity detected is above the upper limit for a predetermined period Δt2 or when the mean value of the flame intensity detected during the predetermined period Δt2 is above the upper limit.

7. Method according to any of claims 5 or claim 6, in which the rate of injection of combustible material in the combustion chamber (2) is varied according to the heat-energy requirement in the combustion chamber (2).

8. Method according to any of the preceding claims, in which the flame intensity is determined by means of an optical detector, preferably by means of an optical detector chosen from ultraviolet detectors, infrared detectors and visible-radiation detectors.

9. Flame furnace comprising - a combustion chamber (2), - a means (15, 24) for injecting main oxidant at a regulated rate in the combustion chamber (2), and - a pipe (13) for discharging fumes (6) from said combustion chamber (2), - the furnace being characterised in that: said pipe (13) comprises a dilution-oxidant inlet (14) downstream of the combustion chamber (2), - and in that the furnace also comprises: - a detector (10) for detecting a flame intensity at the dilution-oxygen inlet (14) inside the discharge pipe (13), and - a control unit (20) connected (a) to the detector (10) and (b) to the means for injecting main oxidant (15, 24), the control unit (20) being programmed: - -to compare the flame intensity detected by the detector (10) with a predetermined lower limit and a predetermined upper limit, - to reduce the rate of injection of main oxidant in the combustion chamber (2) by the means for injecting main oxidant (15, 24) when the flame intensity detected is below the predetermined lower limit, and - to increase the rate of injection of main oxidant in the combustion chamber (2) by the means for injecting main oxidant (15, 24) when the flame intensity detected is above the predetermined upper limit.

10. Flame furnace according to claim 9, in which the control unit (20) is programmed: - to reduce the rate of injection of main oxidant in the combustion chamber (2) when the flame intensity detected is below the lower limit for a predetermined period Δt1 or when the mean value of the flame intensity detected during the predetermined period Δt1 is below the lower limit, and - to increase the flow rate of main oxidant injected in the combustion chamber (2) when the flame intensity thus detected is above the upper limit for a predetermined period Δt2 or when the mean value of the flame intensity detected during the predetermined period Δt2 is above the upper limit.

11. Flame furnace comprising - a combustion chamber (2), - a means (15, 24) for injecting main oxidant at a regulated rate in the combustion chamber (2), and - a pipe (13) for discharging fumes (6) from said combustion chamber (2), - the furnace being characterised in that: - sa id pipe (13) comprises a dilution-oxidant inlet (14) downstream of the combustion chamber (2), - and in that the furnace also comprises: - a detector (10) for detecting a flame intensity at the dilution-oxygen inlet (14) inside the discharge pipe (13), - means (17, 24) for injecting combustible material at a regulated rate in the combustion chamber (2), and - a control unit (20) connected (a) to the detector (10), (b) to the means for injecting main oxidant (15, 24) and (c) to the means for injecting combustible material (17, 24), the control unit (20) being programmed: - to compare the flame intensity detected by the detector (10) with a predetermined lower limit and a predetermined upper limit, - to reduce the ratio between the rate of injection of main oxidant and the rate of injection of combustible material in the combustion chamber (2) when the flame intensity detected is below the predetermined lower limit, and - to increase the ratio between the rate of injection of main oxidant and the rate of injection of combustible material in the combustion chamber (2) when the flame intensity detected is above a predetermined upper limit.

12. Flame furnace according to claim 11, in which the control unit (20) is programmed: - to reduce the ratio between the rate of injection of main oxidant and the rate of injection of combustible material in the combustion chamber (2) when the flame intensity detected is below the lower limit during the predetermined period Δt1 or when the mean value of the flame intensity detected during the predetermined period Δt1 is below the lower limit value of the flame intensity detected during the predetermined period Δt1 is below the lower limit, and - to increase the ratio between the rate of injection of main oxidant and the rate of injection of combustible material in the combustion chamber (2) when the flame intensity detected is above the upper limit for a predetermined period Δt2 or when the mean value of the flame intensity detected during the predetermined period Δt2 is above the upper limit.

13. Furnace according to any of claims 9 to 12, in which the flame detector (10) is chosen from optical detectors, preferably from ultraviolet detectors, infrared detectors and visible-radiation detectors.

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