FR2969267A1 - DISCONTINUOUS LOAD FUSION PROCESS - Google Patents

Abstract

Batch loading process for melting materials to be melted in a flame furnace in which at least one thermal energy supply step comprises a conditioning phase during which - the at least one burner operates substantially in stoichiometry, - the temperature is reduced. power P supplied by the at least one burner, and an additional gas is injected through the at least one burner, in addition to the fuel and the oxidizing gas, so as to maintain constant or quasi-constant one of the following process parameters when one reduces the power P: o total impulse of the gases injected into the furnace by the at least one burner, o total impulse of the oxidizer and additional gas injected by the at least one burner, and o normal volumetric flow rate of the combustion gases generated by the at least one burner.

Description

DISCONTINUOUS LOAD FUSION PROCESS

The present invention relates to an optimized method for batchwise melting in a flame furnace, as well as to an installation adapted for the implementation of such a method.

A particularly interesting application of the invention is the second metal melting. For this reason, the invention is described below with reference mainly to metal second flame processes and furnaces. In recasting or second melting, liquid metal, often referred to as secondary metal, is obtained by melting a recycled metal charge or from primary metallurgy. The second fusion thus differs from the primary melting in which liquid metal is obtained from ore (for example: in a blast furnace for melting). It is known to use flame furnaces for the second melting of metals. The term "flame furnace" refers to a melting furnace in which at least a portion of the energy is produced by burning a fuel with an oxidant inside the furnace. The term "flame furnace" thus also covers melting furnaces in which at least a portion of the energy is produced by the combustion of a fuel with an oxidant without a visible flame, such combustion being sometimes called "flameless combustion". (In English "flameless combustion"). In a flame melting furnace, at least a portion of the fumes (generally containing CO 2, CO 2, H 2 O, O 2, ...) generated by the combustion come into direct contact with the metal filler, before to be removed from the oven. The flame oven is typically equipped with one or more burners for fuel and oxidant injection. Air, the main component of which is nitrogen, is the oxidizer traditionally used in flame furnaces. In this case, we are talking about "aérocombustion". When the oxidizer is oxygen enriched air, or even oxygen called "pure oxygen" (that is to say: having an oxygen content of 80% vol to 100% vol), we speak of "Oxycombustion". In this case, the oven operator has a reduced fuel consumption. In the case of oxycombustion, the flame is normally warmer and the volume of fumes generated is reduced compared with the aerocombustion. In batch-fed furnaces, the heating requirement varies according to the progress of the melting process. The thermal power to be supplied to the oven is generally greater at the beginning of the cycle, that is to say after loading a solid charge, than at the end when the oven has reached its operating temperature. During the final phase, the liquid charge must be maintained at a homogeneous temperature required for its treatment downstream of the melting furnace, avoiding cold or hot spots on the charge. It is also important to avoid any local overheating of the refractory surface of the oven. In the case of a rotary kiln or tilting rotary kiln, it is particularly important to keep the entire refractory surface warm by avoiding any local overheating, in order to optimize the heat transfer to the load. These obligations are delicate when the power of the furnace burner (s) is reduced. Given the specific properties of oxycombustion (hotter flame, reduced smoke), these obligations are even more delicate in the case of the latter. In view of conditions in melting furnaces, such as second melting furnaces (temperatures, highly polluted atmosphere, condensable materials, etc.) the burners must be particularly robust.

In practice, the most common burners of second melting furnaces have a range of operation, in terms of power, quite narrow. This operating range is around the nominal power for which the burner was designed. In order to reduce the power supplied, avoiding hot or cold spots, despite the narrow operating range of the burners, it has been proposed in the reference book "Handbook of Aluminum Recycling - Ch.Schmitz - Vulkan" (ISBN: 10: 3-8027-2936-6, ISBN 13: 978-3-8027-2936-2), at page 152, to use two distinct types of burners: (a) one or more high-power burners for use when furnace power is high and (b) one or more lower rated burners that remain on standby when the high-power burner (s) are active and used only when the power is to provide in the oven is reduced. The installation of two types of burners in an oven in order to allow a greater power variation is an expensive and complex solution (two burners, double fuel and oxidant supply, bulk, etc.), especially in the case of a Rotary kiln. The present invention aims to provide an improved solution to the problem described above. Thus, the invention relates to a discontinuous melting process in a flame furnace. The oven is equipped with at least one burner for injecting a fuel and an oxidizing gas into the furnace.

The method comprises: - one or more steps for charging solid material to be melted in the flame oven, - one or more steps for supplying thermal energy to the oven so as to melt the solid material and obtain melt, and one or more steps of unloading the melt from the furnace. At least a portion of the thermal energy is supplied to the furnace by combustion of the fuel with the oxidant gas in the furnace. The oxidizing gas has an oxygen content of 80% vol to 100% vol.

During the step of supplying thermal energy, said at least one burner operates at a power P and generates combustion gases in the furnace. In this method, at least one step of supplying thermal energy comprises a conditioning phase during which (a) the at least one burner operates substantially in stoichiometry, and (b) reducing the power P supplied by the at least one burner. According to the invention, a flow> 0 of a complementary gas having an oxygen content> 0% vol and <22 vol% during this phase of the invention is injected through the at least one burner, in addition to the fuel and the oxidizing gas. conditioning so as to maintain constant or almost constant one of the following process parameters when the power P is reduced: o total impulse of the gases injected into the furnace by the at least one burner, o total impulse of the injected gas and complementary gas injected by the at least one burner, and o normal volumetric flow of combustion gases generated by the at least one burner.

In this way, the invention makes it possible to reduce the power supplied by the at least one burner below the operating range for which the at least one burner has been designed. What is more, the invention thus makes it possible to reduce the power for a prolonged period without overheating the at least one burner and avoiding cold spots and hot spots on the charge or on the furnace refractories.

The oxidizing gas preferably has an oxygen content of 88% vol to 100% vol. When the fuel is a gaseous fuel, the total impulse of the gases injected into the furnace by the at least one burner is the sum of the injection pulses of the gaseous fuel, the oxidizing gas and the complementary gas.

It should be noted that, in the present context, powdered solid or liquid fuels sprayed propelled by a carrier gas are considered as gaseous fuels, and their injection pulse corresponds to the injection pulse of the carrier gas charged with the fuel solid or liquid. When the fuel is not a gaseous fuel, the total impulse of the gases injected by the at least one burner is the sum of the injection pulses of the oxidizing gas and the complementary gas. According to a simplified embodiment of the process, the injection pulse of the fuel is ignored and the complementary gas is injected so as to maintain constant or quasi-constant the total pulse of the oxidizing gas and the complementary gas injected by the fuel. at least one burner when the power P is reduced. By total impulse of the oxidizing gas and the complementary gas, the sum of the injection pulse of the oxidizing gas and the injection pulse of the complementary gas is understood. When the normal volume of combustion gases generated by the stoichiometric combustion of the fuel with the combustion gas is known, it is also possible, according to the invention, to inject the additional gas so as to keep the normal volumetric flow rate of the gases constant or quasi-constant. combustion generated by the burner, that is to say the combustion gases generated by the combustion of the fuel with the oxidizing gas and the complementary gas, when the latter is also injected. The normal volume of combustion gases generated by the combustion of the fuel with the oxidizing gas can be determined experimentally or on the basis of chemical formulas, when the composition of the fuel and the oxidant gas is known. In the present context, "normal volume" of a quantity of gas is understood to mean the volume occupied by this quantity of gas at a temperature of 0 ° C. and a pressure of 1.013 bar absolute.

It is considered that a parameter is quasi-constant when the variations in the value of said parameter are less than 10%. Preferably, these variations will be less than 7%. A burner operates stoichiometrically when it injects an oxygen flow rate that corresponds to the oxygen flow rate chemically necessary for the total combustion of the fuel flow rate injected by this burner with a zero excess of oxygen (O 2 content in the dry fumes of the burner). 0%).

A burner injects oxygen by injecting the oxidizing gas, and optionally also with the additional gas and / or with the carrier gas when these are also injected and contain oxygen.

In the present context, it is considered that a burner operates substantially in stoichiometry when the quantity of oxygen injected by the burner is between 0.95 and 1.25 times the stoichiometric amount of oxygen. In practice, the combustion stoichiometric substance results in a content of O 2 in dry fumes of the order of 0 to 5% and a content of unburnt in fumes low or zero (typically a CO content in dry fumes lower 10%, preferably less than 5%). As already indicated above, the invention is particularly interesting for the second melting of metals. In this case, the solid material to be melted and charged in the furnace is solid metal. In this case, the liquid material is molten metal. The method according to the invention may comprise a single charging step. A batch loading process with a single loading step is called a batch process. The process according to the invention may more particularly comprise a single charging step, a single thermal energy supply step and a single unloading step. The method according to the invention may also comprise several charging steps, each charging step being followed by a step of supplying thermal energy. A batch loading process with several loading steps is called a "semi-batch" process. Such a method comprising a plurality of charging steps and in which each charging step is followed by a thermal energy supply step may comprise several unloading steps or a single unloading step. The method according to the invention may in particular comprise several steps of loading by unloading step or else several unloading steps by loading step. According to one form of implementation, the thermal power supplied by the at least one burner is reduced continuously during the conditioning phase. According to an alternative embodiment, the power supplied by the at least one burner is reduced in a discontinuous manner during the conditioning phase, for example in a staircase. The conditioning phase may in particular comprise a refining phase of the melt before the unloading step.

The step of supplying thermal energy may comprise an over-stoichiometric phase which precedes the conditioning phase and during which the oxygen is injected by the at least one oxygen burner into over-stoichiometry, that is to say into excess of the stoichiometric amount corresponding to the amount of fuel injected by the at least one burner.

The over-stoichiometric phase may be more particularly the initial phase of the thermal energy supply step. Such an over-stoichiometric phase is particularly advantageous when the solid material to be melted itself comprises combustible material inherent in the solid material to be melted or intentionally added. The injection of oxygen over-stoichiometry with respect to the injected fuel then allows the complete combustion (1) of the injected fuel and (2) the combustible material released by the solid matter charge, so as to optimize the heating of the fuel. charge in order to limit or avoid unburned pollutants in fumes discharged from the oven. A method and a device for adjusting the fuel injection / combustion gas ratio by the at least one burner are described in the patent application FR 1053147 filed on April 23, 2010.

The oven may be a rotary kiln, and more particularly a tilting rotary kiln. The oven may comprise a single burner, which is particularly interesting in the case of a rotary kiln, including in the case of a tilting rotary kiln. As indicated above, the process may be a batch process or a semi-batch process.

The process may be a second melting process of a metal selected from cast iron, iron, lead, aluminum, copper, antimony and tin and alloys of said metals. The additional gas injected by the at least one burner is advantageously chosen from air, steam, recycled fumes and CO2. Air, as a complementary gas, comprises about 21% oxygen vol.

In the case of steam or fumes recycled as a complementary gas, the additional gas does not contain oxygen or a low oxygen level. According to one embodiment of the invention, the additional gas contains between 0% vol and 10% vol oxygen and preferably between 0% vol and 6% vol oxygen. The fuel injected by the at least one burner may be a gaseous fuel, such as natural gas or propane. The injected fuel may also be a liquid fuel, such as a liquid biofuel. The fuel may also be a powdery solid fuel, such as pulverulent coal. Liquid fuels and pulverulent fuels can be injected into the furnace by the at least one burner by means of a carrier gas. The carrier gas and the complementary gas may in particular have the same composition. The invention applies mainly to a melting furnace operating with pure oxygen (purity of oxygen between 80% and 100%). It allows the burner (s) of said furnace to operate in a range between 25% and 100% of its rated power or maximum power. It allows the use of a simple burner geometry, and can in some cases be applied with the existing oxy-fuel burner without changing the burner, for example, when the additional gas is mixed with the combustion gas upstream of the burner. This feature allows installation of the technology particularly simple and economical. In other cases, the burner will be specific with a first combustion gas circuit and a second complementary gas circuit. The invention permits the melt process to be carried out with a single burner in operation whose power is varied as described above. The invention makes it possible to keep the burner pulse constant without resorting to a so-called "variable impulse" burner which requires a more complex and more expensive construction. The management of the combustion gas / additional gas mixture to maintain constant the burner impulse (and in this case a slight reduction of the flue gas volume) or the flue gas volume (and in this case a slight increase in the burner impulse ) or make a compromise between these two solutions is easily automatable. The invention and its advantages are illustrated in the example below relating to the second recovery metal melting. Example In the second recovery metal melting, the metal charge often has a high proportion of carbon (plastic, paint, coke, etc.). These combustible materials may be de facto present in the metal filler (for example in the case of recycling aluminum cans) and / or may be added intentionally for the purpose of the melting process (for example for deoxidation during lead recycling).

The combustion of these combustible materials in the furnace saves money because it reduces the fuel consumption injected by the at least one burner. The pyrolysis or combustion of at least some of these combustible materials present in the feed generally starts as soon as the temperature in the furnace is sufficiently high. However, the combustion of these combustible materials in the oven is generally at a low speed that varies depending on the nature of the load, the time elapsed since the start of the heating cycle, the distribution of materials in the oven, or other process parameters.

At the end of the melting cycle, when all or most of the metal charge is present in molten form, the process requires less thermal energy and the power of the burner (s) can be reduced. This reduced power phase of the burner (s) can occupy a significant portion of the process time. Indeed, the (second) melting of certain metals, or even other solid materials to be melted, for example in the case of melting furnaces of glass or enamels, requires a refining phase of the melt during which one seeks to homogenize the composition and / or the temperature of the melt. Variations in compositions and / or temperatures in the molten metal (for example at the entrance to the casting plant) can seriously affect the quality of the final product. This can be manifested in a lack of mechanical or chemical resistance of the final product. What is more, the casting installations do not always operate at the same rate as the melting furnace. It may therefore be necessary to keep the metal to flow in the liquid state for a longer or shorter time before introducing the molten metal into the casting plant. As indicated above, it is important to keep the molten metal at a sufficiently homogeneous temperature and composition during this waiting period. Thus, the present invention, which makes it possible to reduce the power of the burner (s) reliably and without creating hot or cold spots, allows the melting furnace to be used as a holding furnace for a prolonged period of time. In the present example, the melting furnace is a tilting rotary kiln. The oven is equipped with a single burner that provides all the necessary thermal fusion energy.

The fact that the furnace does not need to be equipped with a second burner for low heating powers allows a lower cost and greater simplicity of the melting installation and its operation. The single burner of the furnace is of the oxy-fuel type.

The fuel injected by the burner is natural gas or NG. The oxidant gas injected by the burner is pure oxygen. The additional gas injected by the burner is air. The feed has a large proportion of organic material The second metal melting cycle is broken down into several steps or phases: a: loading the metal feed into the furnace; b: beginning of melting of the metal charge and pyrolysis of the combustible materials present in the charge; - c: following the melting of the metal charge; d: overheating and maintaining the temperature of the molten charge; and e: unloading the furnace from the molten charge. During the step or phase b, the power (fuel injection rate) and the stoichiometry (oxygen / fuel ratio) of the burner can be adjusted depending on the course of the pyrolysis of the fuel materials in the load. In fact, in the stage or phase b of the melting cycle, an excess of oxygen can be injected by the burner (with respect to the fuel flow rate injected by this burner) in order to ensure the combustion of the combustible materials of the charge. in the oven. For the step or the phase d and sometimes for the step or the phase c of the melting cycle, it is necessary to operate the low-power burner. In the latter case, it turned out, with rotary kilns tilting to a single burner according to the state of the art, that the distribution of the heater in the oven is not optimal, which very often leads to a overheating local, with a temperature too low in the rest of the oven. In the example according to the invention, the only burner of the furnace is a fuel / oxygen / air burner.

The ratio: air (as complementary gas) oxygen (as combustion gas) of said burner is adjusted, during the substantially stoichiometric operation of the burner, in order to maintain constant or almost constant one of the following parameters: the total impulse of the gases from the burner, or - the total impulse of the oxidizing gas and the complementary gas coming from the burner, or - the volume of fumes (in normal cubic meters) resulting from the combustion of the fuel injected by the burner. The characteristic aspects of the invention therefore apply for the phases or stages of the melting cycle during which a reduced power of the burner is required with a stoichiometric adjustment of the burner, or even an adjustment in stoichiometric substance (typically a slight excess of oxygen). . In practice ; the total oxygen flow injected by the burner will not exceed the oxygen flow required during a stoichiometric operation of the burner at the maximum power for which the burner was designed. The tables and the accompanying figures illustrate two embodiments of the invention: Table 1 and FIG. 1: Constant total impulse of the oxidizing gas and the additional gas injected by the burner; and - Table 2 and Figure 2: Constant impulse fumes generated by the burner. Example 1: Operating mode "Constant total impulse of the oxidizing gas and the additional gas (Table 1, Figure 1) The 02 content of all the oxidizing gas and the additional gas is calculated as follows:% O2 [oxidizing gas + additional gas] = QGN x% 02 QGN-Maxi oxidant where% O2 [oxidizing gas + additional gas]: 02 content of all the oxidizing gas and the complementary gas% 02 oxidant: 02 content of the oxidant QGN: flow rate of GN injected 2.15: ratio 02 / GN for stoichiometric combustion QGN-Maxi: GN flow at the rated power (or maximum) of the burner, this flow rate being 1000 Nm3GN / h in the example.

Example 2: Operating mode "Constant flue gas volume" (table 2, figure 2) The air flow rate (additional gas) required is: 3.2 x QGN-Maxi - 3.2 x QGN or QAtr - 0.791 QAir: QGN air flow rate: GN 3.2 flow rate: Flue volume / GN volume ratio for pure oxygen combustion, and a stoichiometric O2 / GN ratio (this value is specific to the fuel composition used) QGN-Max: fuel flow at the rated power (or maximum) of the burner 1000 15 Nm3GN / h in the example.

From these data can be calculated the flow of pure oxygen necessary for combustion. In Tables 1 and 2, case 1 represents the operation of the oxygen / natural gas burner at its nominal power with stoichiometric operation. Cases 2 to 6 correspond to decreasing burner capacities of 1000 Nm3 / h of natural gas at 250 Nm3 / h of natural gas. The power transmitted to the process (the load + the losses walls) thus goes from 8785 to 1472 kW, the latter value representing 17% of the original value. Tables 1 and 2 give the additional gas (air) and combustion gas (oxygen) flow rates required for the respective operation described. The use of a constant pulse mode of operation or a constant volume of flue gas makes it possible to reach the bottom of the rotating oven, regardless of the thermal power of the burner. The overall efficiency of the plant is maintained because the use of air is reserved for low power.

Case Case 1 Case 2 Case 3 Case 3 Cases Case 6 Natural gas flow Nm3 / h 1000 900 800 600 400 250 Air flow Nm3 / h 0 283 565 1131 1698 2123 Oxygen flow Nm3 / h 2150 1881 1611 1072 533 129 Oxidizer content 02% 100% 90% 79% 59% 40% 25% Flue gas flow Nm3 / h 3199 3108 3016 2833 2651 2514 Flue gas flow variation% ..., - .. ~, 89 ~, Flue gas flow kg / h 3852 3755 3657 3462 3268 3123 Flue gas flow variation 100% 97% 95% 90% 85% 81% Content 02 fumes% dry 2.05% 2.05% 2.05% 2.05% 2.05% 2.05% Energyprocess kW 8785 7810 6835 4885 2935 1472 Proportion energy% 100% 89% 78% 56% 33% 17% Air + 02 Nm3 / h 2150 2163 2177 2204 2231 2252 Variation 100% 101% 101% 102% 104% 105% Air + 02 kg / h 3071 3053 3034 2997 2960 2933 Variation 100% 99% 99% 98% 96% 95% Oxidizing impulse kg / hx Nm3 / s 1834 1834 1834 1834 1834 1834 Variation 100%%, 100%%, 100%%, 100% _10_,. i00% Table 1 Case Case 1 Case 2 Case 3 Case 4 Case 5 Case 6 Natural gas flow Nm3 / h 1000 900 800 600 400 250 Airflow Nm3 / h 0 396 792 1585 2377 2793 Oxygen flow Nm3 / h 2150 1859 1568 985 403 0 Content 02 oxidizer% 100% 86% 73% 51% 32% 21% Fume flow Nm3 / h 3199 3199 3199 3199 3199 3055 Fume flow variation% 10o '),;, 10o'),;, i X30? Fume flow kg / h 3852 3870 3888 3924 3959 3805 Fumes flow rate variation 100% 100% 101% 102% 103% 99% Content 02 fumes% dry 2.05% 2.05% 2.05% 2.05% 2 , 05% 2.05% Process energy kW 8785 7772 6758 4731 2703 1243 Proportion energy% 100% 88% 77% 54% 31% 14% Air + 02 Nm3 / h 2150 2255 2360 2570 2780 2793 Variation 100% 105% 110% 120% 129% 130% Air + 02 kg / h 3071 3168 3265 3459 3652 3616 Variation 100% 103% 106% 113% 119% 118% Combustive impurity kg / hr Nm3 / s 1834 1985 2140 2469 2820 2806 Variation 10 (.. 8% 35% 54% Table 2

Claims (14)

CLAIMS1) Discontinuous charging method for melting materials for melting in a flame oven equipped with at least one burner for injecting fuel and an oxidizing gas into the furnace, said method comprising: - one or more steps of loading solid material into the flame oven, - one or more steps of supplying thermal energy to the furnace so as to melt the solid material and to obtain molten metal, at least a portion of the thermal energy being supplied by the combustion in the furnace of the fuel with the oxidizing gas, this oxidizing gas having an oxygen content of 80% vol to 100% vol, and preferably 88% vol to 100% ovol, said at least one burner operating at a power P and generating combustion gases in the furnace, - one or more steps of discharging the melt from the furnace, wherein at least one step of supplying thermal energy comprises, a conditioning phase p wherein (a) the at least one burner operates in substantially stoichiometric manner, and (b) decreases the power P supplied by the at least one burner, the method being characterized in that when, during this conditioning phase, the through the at least one burner, in addition to the fuel and the oxidizing gas, a complementary gas having an oxygen content> 0% vol and 522vo1% so as to maintain constant or near constant one of the following process parameters when reduced the power P: o total impulse of the gases injected into the furnace by the at least one burner, o total impulse of the oxidant and the complementary gas injected by the at least one burner, and o normal volumetric flow rate of the combustion gases generated by the less a burner. The process of claim 1, wherein the solid material to be melted is solid metal. 3) Process according to claim 1 or 2, comprising a single charging step, a single step of supplying thermal energy and a single unloading step. 4) Process according to claim 1 or 2, comprising several charging steps, each charging step being followed by a step of supplying thermal energy. 5) Method according to one of claims 1 to 4, comprising a single unloading step. 6) Process according to any one of the preceding claims, wherein during the conditioning phase, the power supplied by the at least one burner is continuously reduced. 7) A method according to any one of claims 1 to 5, wherein during the conditioning phase, decreases the power provided by the at least one burner discontinuously. 20 The process of any one of the preceding claims, wherein the conditioning phase comprises a refining phase of the melt prior to the unloading step. 9) A method according to any one of the preceding claims, wherein the step of supplying thermal energy comprises an over-stoichiometric phase which precedes the conditioning phase and during which the at least one burner of the oxygen in over-stoichiometry. The method of claim 9, wherein the superstoichiometric phase is the initial phase of the thermal energy supply step. 11) A method according to any one of the preceding claims, wherein the furnace comprises a single burner. 15 12) Process according to the preceding claim, wherein the oven is a rotary kiln. 13) Process according to one of the preceding claims for the second melting of a metal selected from cast iron, iron, lead, aluminum, copper, antimony and tin and alloys of said metals. 14) Process for the second metal melting according to one of the preceding claims wherein the additional gas is selected from air, steam, recycled fumes, CO2.