JP2018016832A - Method for producing molten iron with electric furnace - Google Patents

Abstract

In a method for producing molten iron by melting scrap in an electric furnace equipped with an auxiliary burner, the scrap is efficiently heated or melted by using solid fuel for the auxiliary burner. An auxiliary combustion burner that uses gaseous fuel and solid fuel as fuel, and has a plurality of concentric injection pipes for injecting gaseous fuel, solid fuel, and combustion-supporting gas, respectively. While using an auxiliary combustion burner that injects the combustion-supporting gas from the outermost peripheral injection pipe, when the scrap is heated or melted by this auxiliary combustion burner, the amount of solid fuel used is 50 to 50% of the total output energy in terms of burner output energy. 95%, the solid fuel is conveyed with an inert gas, and the flow rate of the carrier gas is 0.05 to 0.20 Nm 3 per kg of the solid fuel. [Selection] Figure 1

Description

  The present invention relates to a method for producing molten iron by melting iron-based scrap in an electric furnace equipped with an auxiliary burner.

When iron-based scrap is melted using an electric furnace, the iron-based scrap around the electrode melts quickly, but the iron-based scrap away from the electrode, that is, in the cold spot, dissolves slowly, and the iron-based scrap in the furnace Unevenness in scrap melting rate occurs. For this reason, the operation time of the entire furnace was limited by the melting rate of the iron-based scrap at the cold spot.
Therefore, in order to eliminate such non-uniformity in the melting rate of iron scrap and to dissolve the iron scrap in the entire furnace in a well-balanced manner, an auxiliary burner is installed at the cold spot, and the cold spot is used with this auxiliary burner. A method of preheating, cutting and melting iron-based scrap located in the area has been adopted.

As such an auxiliary combustion burner, for example, in Patent Document 1, oxygen gas for scattering of incombustibles and cutting of iron-based scrap is ejected from the center, and fuel is further supplied from the outer periphery of the oxygen gas. A burner having a triple tube structure for injecting combustion oxygen gas from the outer peripheral portion, and in order to increase the speed of the oxygen gas ejected from the central portion, a constricted portion at the tip of the central oxygen gas ejection tube High-speed pure oxygen for electric furnaces in which swirl vanes are installed in an annular space formed by a fuel jet pipe and a combustion oxygen gas jet pipe in order to impart a swirling force to the combustion oxygen gas jetted from the outermost periphery An auxiliary burner has been proposed.
Patent Document 2 proposes an electric furnace burner facility in which the nozzle tip of the auxiliary burner is eccentric and the burner is rotated to expand the directivity of the burner flame over a wide range.

Japanese Patent Laid-Open No. 10-9524 JP 2003-4382 A

  By using the techniques described in Patent Documents 1 and 2, iron-based scrap can be efficiently preheated and melted using an auxiliary burner, but there is a problem that the target of fuel is limited to expensive gaseous fuel. is there. Inexpensive fuels include solid fuels, especially coal, but it is difficult to burn coal faster than gaseous fuel, and it may be misfired depending on conditions, making it difficult to use coal as an auxiliary burner. .

  Accordingly, an object of the present invention is to solve the above-mentioned problems of the prior art, and in a method of melting molten iron in an electric furnace equipped with an auxiliary combustion burner to obtain molten iron, a solid fuel such as pulverized coal is added to the auxiliary combustion burner. It is in providing the manufacturing method of the molten iron which can heat or melt | dissolve an iron-type scrap efficiently.

    As a result of repeated investigations on an auxiliary furnace burner for an electric furnace that can use solid fuel such as coal and its use conditions, the present inventors used a specific auxiliary burner that uses solid fuel and gaseous fuel as the fuel, By optimizing the fuel usage ratio and the solid fuel carrier gas flow rate, the solid fuel can be combusted together with the gaseous fuel in an appropriate and efficient manner. As a result, it was found that the iron-based scrap can be efficiently heated or melted by the auxiliary burner. At that time, the total amount of carbon in the solid fuel to be used is measured in advance, and the amount of solid fuel injected in the auxiliary burner is adjusted appropriately according to the total amount of carbon. It turns out that it can be managed.

  According to the method of the present invention, (i) by changing the ratio of the gas fuel and solid fuel used in the auxiliary burner, the flame length can be arbitrarily set according to the distance from the iron-based scrap to be heated or melted. (Ii) In general, since the auxiliary burner has a relatively low gas flow rate, the spout of molten iron or molten slag may clog the gas discharge port. It has been found that since the splash is purged by the carrier gas, the gas discharge port is not easily clogged by the splash.

The present invention has been made on the basis of such knowledge and has the following gist.
[1] In a method for obtaining molten iron by melting iron scrap in an electric furnace equipped with an auxiliary burner,
An auxiliary combustion burner using gaseous fuel and solid fuel as fuel, having a plurality of concentric injection pipes for injecting gaseous fuel, solid fuel and combustion-supporting gas, respectively, Using an auxiliary burner that injects combustion-supporting gas from the injection pipe, and when heating or melting iron-based scrap with the auxiliary burner, the amount of solid fuel used is 50 to 95% of the total output energy in terms of burner output energy And a flow rate of the carrier gas of the solid fuel is 0.05 to 0.20 Nm ³ per kg of the solid fuel, and a method for producing molten iron using an electric furnace.

[2] In the manufacturing method of [1], the auxiliary combustion burner has a structure in which a solid fuel injection pipe, a gas fuel injection pipe, and a combustion-supporting gas injection pipe are arranged concentrically in order from the center side. A method for producing molten iron using an electric furnace, comprising:
[3] In the manufacturing method according to [1] or [2], the electric furnace includes a plurality of auxiliary burners, and a total output of all auxiliary burners is 20 Mcal / h or more per ton of scrap melting amount. A method for producing molten iron using an electric furnace.
[4] The method for producing molten iron using an electric furnace according to any one of the above [1] to [3], wherein the solid fuel is pulverized coal having an average particle diameter d90 of 50 to 500 μm.
[5] In the production method of any one of [1] to [4] above, the total carbon amount of the solid fuel to be used is measured in advance, and the amount of solid fuel used in the auxiliary burner is determined according to the total carbon amount. A method for producing molten iron by an electric furnace, characterized by adjusting.

  According to the present invention, when an iron furnace equipped with an auxiliary burner is used to melt iron-based scrap and produce molten iron, a specific auxiliary burner that uses solid fuel and gaseous fuel as fuel is used, and the use of solid fuel is used. By optimizing the ratio and the carrier gas flow rate of the solid fuel, the solid fuel can be combusted appropriately and efficiently together with the gaseous fuel, and it is possible to suppress troubles in carrying the solid fuel and a decrease in the flame temperature due to the carrier gas. As a result, the iron-based scrap can be efficiently heated or melted by the auxiliary burner. For this reason, the usage-amount of expensive gaseous fuel can be reduced, and the manufacturing cost of molten iron can be reduced significantly.

Further, in the present invention in which solid fuel can be combusted appropriately and efficiently with gaseous fuel in the auxiliary burner, by changing the ratio of gaseous fuel and solid fuel, the distance from the iron-based scrap to be heated or melted can be increased. Accordingly, the flame length can be arbitrarily adjusted, and from this point, the iron-based scrap can be efficiently heated or melted. In general, since the auxiliary burner has a relatively small gas flow rate, the spout of molten iron or molten slag that is scattered may clog the gas discharge port. However, in the auxiliary burner used in the present invention, the solid fuel Since the splash is purged by the carrier gas, there is an advantage that the gas discharge port is not easily clogged by the splash.
In the present invention, the total amount of carbon in the solid fuel to be used is measured in advance, and the amount of solid fuel injected is appropriately adjusted by adjusting the amount of solid fuel used in the auxiliary burner according to the total amount of carbon. Can be managed.

The longitudinal section which shows one embodiment of the auxiliary burner used by the method of the present invention Explanatory drawing which shows typically an example (vertical cross section in the electric furnace radial direction) of the implementation condition of this invention method A graph schematically showing a change in flame length when the ratio of gas fuel to solid fuel is changed for the auxiliary burner used in the method of the present invention. Explanatory drawing which shows the outline of the installation position of the auxiliary combustion burner in the electric furnace used in the Example Explanatory drawing which shows the use condition (longitudinal section in the electric furnace radial direction) of the auxiliary burner in the embodiment

  The present invention is a method for obtaining molten iron by melting iron-based scrap (hereinafter simply referred to as “scrap” for convenience of explanation) in an electric furnace equipped with an auxiliary burner. As the auxiliary burner, gaseous fuel and solid as fuel are used. An auxiliary combustion burner using fuel, having a plurality of concentric injection pipes for injecting gaseous fuel, solid fuel, and combustion-supporting gas, respectively, and supporting fuel from the outermost peripheral injection pipe An auxiliary burner that injects gas is used.

As elements necessary for combustion, there are three elements of a combustible substance, oxygen, and temperature (fire source). Moreover, as the state of the combustible substance, the ease of combustion is in the order of gas, liquid, and solid. This is because in the gaseous state, the combustible substance, oxygen, and temperature can be easily mixed, and combustion is continued (chain reaction).
When gas is burned as a flammable substance using an auxiliary burner, the gas generally burns immediately immediately after being injected from the tip of the burner, depending on the oxygen concentration, flow velocity, and burner tip shape. On the other hand, when a solid fuel typified by coal is used as a combustible substance, it is difficult to burn it quickly like a gas. This is due to the fact that the ignition temperature of coal is about 400 to 600 ° C., and that this ignition temperature is maintained and that the temperature rise time to the ignition temperature is necessary.
The temperature raising time to the ignition temperature depends on the particle size (specific surface area), and if the particles are made fine, the ignition time can be shortened. This is because the combustion reaction proceeds by maintaining the ignition temperature and reacting the combustible substance with oxygen. In order to make the combustion reaction proceed efficiently, it is important to sequentially generate an efficient heating of coal and a reaction between coal and oxygen.

Hereinafter, the case where LNG (liquefied natural gas) is used as the gaseous fuel of the auxiliary burner, coal (pulverized coal) is used as the solid fuel, and pure oxygen is used as the combustion-supporting gas will be described. The ignition temperature of these fuels is generally solid fuel> liquid fuel> gaseous fuel.
When LNG and coal are used as fuel for the auxiliary combustion burner, a combustion field above the ignition temperature of coal is created by the combustion of LNG and pure oxygen, and the temperature rises to the ignition temperature when coal is sent to this combustion field. Combustion (vaporization → ignition) occurs. The flame temperature decreases with the amount of heat required to increase the coal temperature, but the temperature increases in the region where coal ignition occurs.

Carbon dioxide, which is an incombustible gas, is generated by the reaction of LNG, which is fuel, and coal with oxygen. Nonflammable gas inhibits the continuation of combustion (chain reaction) and causes a decrease in combustibility. In addition, although coal is supplied by gas conveyance, if the flow rate of the conveyance gas is large, the temperature of the specific heat of the conveyance gas is lowered. Therefore, in general, the solid-gas ratio (solid supply rate per unit time / Combustibility is improved by increasing the carrier gas supply rate per unit time. However, the state where the solid-gas ratio is large is a condition in which coal is in a dense state, and the propagation of heat from the outside and the reaction with oxygen are hardly transmitted to the inside. In order to efficiently burn coal, it is important to create conditions in which heat and oxygen are sufficiently present around the coal in the combustion field after being discharged to the burner from a dense state during transportation.
On the other hand, when coal is blown with nitrogen or other gas, depending on the flow rate of the carrier gas, problems such as transportation troubles such as clogging of solid fuel in the flow path may occur, or the flame temperature may decrease due to the carrier gas. It is important not to cause such a problem.

Therefore, in the present invention, the specific combustion burner as described above is used, and the use ratio of coal (pulverized coal) and the carrier gas flow rate of coal are optimized. Specifically, when scrap is heated or melted with an auxiliary burner, the amount of coal used is 50 to 95% of the total output energy in terms of burner output energy, and the flow rate per kg of coal is 0.05 to 0.2 Nm. ^The coal is injected using the inert gas ³ as a carrier gas. As a result, coal can be combusted appropriately and efficiently together with LNG, and it is possible to suppress coal transport troubles (clogging in the flow path) and flame temperature drop due to the transport gas. The scrap can be efficiently heated or melted by the burner. The present invention is particularly effective in a large-sized electric furnace having a large scrap melting amount of 100 ton class or more.

FIG. 1 is a longitudinal sectional view showing an example of an auxiliary combustion burner used in the present invention.
In this auxiliary burner, the main body portion for supplying fuel and supporting gas has a triple tube structure in which three tubes are arranged concentrically. That is, this triple pipe structure is composed of a solid fuel injection pipe 1 at the center, a gaseous fuel injection pipe 2 arranged outside thereof, and a combustion-supporting gas injection pipe 3 arranged further outside thereof. Yes. The interior of the solid fuel injection pipe 1 constitutes a solid fuel flow path 10, and the gaseous fuel injection pipe 2 constitutes a gas fuel flow path 20 in the space between the solid fuel injection pipe 1, and provides fuel support. The space between the gas injection pipe 3 and the gaseous fuel injection pipe 2 constitutes a combustion-supporting gas flow path 30. The solid fuel injection pipe 1, the gas fuel injection pipe 2 and the combustion-supporting gas injection pipe 3 are each open at the tip, and the open ends thereof are a ring-shaped solid fuel discharge port 11 (injection port) and a gas fuel discharge port, respectively. 21 (injection port) and a combustion-supporting gas discharge port 31 (injection port).

Further, on the rear end side of the burner, the combustion-supporting gas injection pipe 3 is provided with a combustion-supporting gas supply port 32 for supplying the combustion-supporting gas to the combustion-supporting gas passage 30. Similarly, the gaseous fuel injection pipe 2 is provided with a gaseous fuel supply port 22 for supplying fuel to the gaseous fuel flow path 20. Similarly, the solid fuel injection pipe 1 is provided with a solid fuel supply port 12 for supplying solid fuel to the solid fuel flow path 10 via the carrier gas.
Moreover, although not shown in figure, the inner tube body and the outer tube body are further arranged concentrically outside the combustion-supporting gas injection tube 3, and between the outer tube body and the inner tube body, And the combustion-supporting gas injection pipe 3 are formed with a cooling fluid flow path (cooling fluid forward path and return path) communicating with each other.
Normally, a spacer (not shown) is arranged between the injection pipes of the triple pipe structure, and the interval between the injection pipes is maintained.
Further, swirl vanes for imparting swirl flow to the support gas or gaseous fuel may be provided in the combustion support gas channel 30 or the gas fuel channel 20. By imparting a swirl flow to the support gas or gaseous fuel, mixing of the injected support gas and fuel can be promoted.

Here, since the flow rate of the combustion-supporting gas is the largest among the supply gas amounts, the discharge area of the combustion-supporting gas discharge port 31 is set to the gaseous fuel discharge port 21 in order to match the flow rate with other supply gases. It is necessary to make it larger than the solid fuel discharge port 11, and from this point of view, it is optimal that the combustion-supporting gas injection tube 3 has the outermost periphery. Hereinafter, the case where oxygen is used as the combustion-supporting gas, LNG is used as the gaseous fuel, and pulverized coal is used as the solid fuel will be described as an example.
First, the amount of oxygen necessary for combustion is calculated by the following equation (1).
Oxygen required for combustion = oxygen ratio (coefficient) x [LNG flow rate x LNG theoretical oxygen quantity + pulverized coal supply quantity x pulverized coal theoretical oxygen quantity] (1)

The amount of oxygen necessary for combustion is specifically calculated under the following conditions. That is, as calculation conditions, the calorific value of LNG is 9700 kcal / Nm ^3, and the calorific value of pulverized coal as a solid fuel is 6250 kcal / kg. In addition, 90% of the total energy of the auxiliary burner is supplied from solid fuel and 10% from gaseous fuel. For example, when LNG is supplied at 10 Nm ³ / h, the calorific value is 97 Mcal / h, and it is necessary to supply 873 Mcal / h from pulverized coal, which is the difference from 970 Mcal / h, which is the target total calorific value of the burner. Yes, the supply amount is about 140 kg / h. Moreover, the theoretical amount of oxygen is calculated from the carbon content and hydrogen content in the fuel, the theoretical oxygen content of the LNG is 2.25Nm ³ / Nm ³ nm, the theoretical oxygen amount of the pulverized coal is When it is about 1.5 Nm / kg It is said.

The oxygen ratio is generally an oxygen excess condition of 1 to 1.1. The amount of oxygen necessary for combustion when the oxygen ratio is 1.1 is 233 Nm ³ / h (= 1.1 × [10 × 2.25 + 140 × 1.5]). Therefore, when pure oxygen is used, a flow rate 23.3 times that of LNG fuel is required. Even when compared with the transported nitrogen of pulverized coal, the nitrogen flow rate when the solid-gas ratio is 12 is about 12 Nm ³ / h, and a flow rate of about 20 times is required. Therefore, in order to make the discharge rate of oxygen the same as the discharge rate of fuel gas or pulverized coal, the support gas discharge port 31 has a discharge area that is 20 times or more that of the gaseous fuel discharge port 21 and the solid fuel discharge port 11. For this reason, it is reasonable to dispose the combustion-supporting gas discharge port 31 on the outermost peripheral portion of the burner. Further, when air is used as the combustion-supporting gas instead of pure oxygen, a flow rate five times higher is required. For the same reason, it is reasonable to arrange the combustion-supporting gas discharge port 31 at the outermost peripheral portion of the burner. Is.

  In the present invention, the fuel that can be used for the auxiliary burner is, for example, LPG (liquefied petroleum gas), LNG (liquefied natural gas), hydrogen, ironworks by-product gas (C gas, B gas, etc.), etc. These two or more mixed gases can be used, and one or more of these can be used. In addition, examples of the solid fuel include coal (pulverized coal), plastic (particulate or powdery, including waste plastic), and one or more of these can be used, but coal (pulverized coal) Is particularly preferred. As the combustion-supporting gas, pure oxygen (industrial oxygen), oxygen-enriched air, or air may be used. However, when scrap is dissolved, pure oxygen is preferably used.

  The amount of solid fuel used (injection amount) in the auxiliary burner has an optimum range from the viewpoint of energy cost and operability. In the present invention, the use ratio of solid fuel (the ratio of the whole fuel including gaseous fuel) is defined in terms of burner output energy. Since solid fuel is generally cheaper than gaseous fuel, it is desirable that the total amount of fuel be solid fuel from a cost perspective, but the amount of solid fuel used (injection amount) is the total output in terms of burner output energy. If it exceeds 95% of the energy (total output energy of solid fuel + gaseous fuel), the ratio of gaseous fuel is too small and the burner may misfire and not burn, so it is important to make it 95% or less . On the other hand, if the amount of solid fuel used (injection amount) is less than 50% of the total output energy in terms of burner output energy, the economic merit of using inexpensive solid fuel will be lost from the viewpoint of operation cost. For this reason, in the present invention, the amount of solid fuel used (injection amount) is set to 50 to 95% of the total output energy in terms of burner output energy. That is, the solid fuel is used in such an amount that the output energy due to the combustion of the solid fuel accounts for 50 to 90% of the total output energy of the auxiliary burner.

As the carrier gas for the solid fuel, for example, one or more kinds of inert gas such as nitrogen and argon and air can be used. Generally, in order to prevent self-ignition of the fuel, nitrogen or argon is not used. An active gas is used, and the solid fuel is conveyed by the carrier gas and is injected from the injection pipe. In addition, when air is used as the carrier gas, it is preferable to install a backfire prevention valve or the like to reduce the risk of solid fuel ignition / explosion. There is also an optimum range for the gas flow rate of the carrier gas of the solid fuel, and it is necessary to set it to 0.05 to 0.20 Nm ³ per kg of the solid fuel. If the gas flow rate per 1 kg of the solid fuel is less than 0.05 Nm ³ , the solid fuel cannot be properly transported, and operation troubles such as clogging of the solid fuel in the flow path may occur. On the other hand, when the gas flow rate exceeds 0.20 Nm ³ , the center temperature of the burner flame is lowered by the carrier gas, so that the scrap cannot be efficiently heated and the power consumption rate is deteriorated.

  In the present invention, the amount of solid fuel used (injection amount) is 50 to 95% of the total output energy in terms of burner output energy, but the amount of solid fuel used (injection amount) is stably controlled and managed. Therefore, it is preferable to measure the total carbon amount of the solid fuel to be used in advance and adjust (manage) the amount of solid fuel used in the auxiliary burner according to the total carbon amount. Hereinafter, a case where pulverized coal is used as the solid fuel will be described as an example. In the calculation of the amount of oxygen necessary for combustion described above, a typical calorific value of pulverized coal was used. However, the actual pulverized coal used varies in physical properties such as the calorific value, and is therefore used. It is preferable to measure the calorific value of pulverized coal in advance and adjust (manage) the amount of solid fuel used.

Generally, it is said that the calorific value of coal can be actually measured by a method defined in JIS M8814 (2003). When the water in the coal and the water produced by the combustion of hydrogen evaporate during the combustion of coal, the amount of heat of the latent heat is not used effectively, but the calorific value measured by the JIS method includes this latent heat. It is called the high calorific value (Gross Calorific Value). On the other hand, the calorific value excluding the latent heat of water vapor is called a low calorific value (Net Calorific Value), and this low calorific value is obtained by the following equation.
Low heating value (kcal / kg) = High heating value (kcal / kg) -6 (9H + W)
Here, H = amount (%) of hydrogen, which is a value obtained by elemental analysis. W = amount of moisture (%), which is a value determined by industrial analysis.
When coal is actually burned, the moisture is steam in the gas, and this latent heat of steam is not used. Therefore, in the combustion of coal, the lower heating value becomes the effective heating value. However, it is not efficient to measure the lower calorific value for each lot of pulverized coal used because of the high work load.

Therefore, it is preferable to use a method of simply estimating the effective heat generation amount from the total carbon amount of pulverized coal as follows. The effective calorific value can be easily estimated from the total carbon content of pulverized coal by the following equation.
Effective calorific value (kcal / kg) = 96 × (carbon content) −262
The total carbon content of the pulverized coal can be easily measured by elemental analysis. Therefore, the total carbon content of the pulverized coal to be used is measured in advance, and the pulverized coal in the auxiliary burner is determined according to the effective calorific value estimated from the total carbon content. It is preferable to adjust (manage) the amount of charcoal used. At this time, the required amount of pulverized coal (injection amount) may be calculated from the obtained effective calorific value based on the above-described concept for calculating the amount of oxygen necessary for combustion. Of course, instead of the total carbon amount of pulverized coal, the lower calorific value of pulverized coal may be measured, and the usage amount (injection amount) of pulverized coal may be calculated based on this lower calorific value.

  The pulverized coal used as fuel is generally more advantageous for combustion if the particle size is small. However, if the particle size is too small, clogging may occur in the piping supplying pulverized coal, or it may become scrap to be heated and melted. It does not reach and tends to cause problems such as being collected together with dust by a dust collecting device while floating in the air. Further, there is a problem that the smaller the particle size, the longer the time and cost for the pulverization. On the other hand, if the particle size is too large, there is a problem that the contact area with the gas becomes small, and it cannot burn in the flame and remains unburned. From the above viewpoint, the average particle diameter d90 of the pulverized coal is preferably about 50 to 500 μm. Here, for the definition of the particle size (average particle size), the particle size distribution of the pulverized coal is measured with a laser diffraction scattering type particle size distribution measuring device and displayed as a frequency distribution. In the present invention, the particle diameter corresponding to the weight is defined as d90 (average particle diameter d90).

FIG. 2 schematically shows an example of implementation of the method of the present invention (vertical cross section in the radial direction of an electric furnace), where 7 is a furnace body, 8 is an electrode, 9 is an auxiliary burner, and x is scrap. is there. The auxiliary burner 9 is installed with an appropriate depression angle. Usually, a plurality of such auxiliary burners 9 are installed so that scrap in a so-called cold spot in the electric furnace can be heated or melted.
In operation of an electric furnace, if the amount of scrap dissolved is large, it is natural that the burner output needs to be increased, but in order to supply a stable flame, the gas flow rate and cooling structure corresponding to it is necessary. Therefore, there is a limit to increasing the burner output. For this reason, it is preferable to install a plurality of auxiliary burners according to the required amount of scrap dissolution.
Further, if the auxiliary burner output is too small, the role as an auxiliary burner cannot be achieved. Therefore, a certain output or more is required. Specifically, the total output of all the auxiliary burners is 20 Mcal / t of scrap melting amount. It is preferable that it is h or more.

As is apparent from the above description, according to the method of the present invention, a specific auxiliary burner that uses solid fuel and gaseous fuel as the fuel is used, and the use ratio of the solid fuel and the carrier gas flow rate of the solid fuel are optimized. As a result, solid fuel can be combusted appropriately and efficiently with gaseous fuel, and it is possible to suppress solid fuel transport troubles and flame temperature drop due to transport gas. Can be heated or dissolved well. For this reason, the usage-amount of expensive gaseous fuel can be reduced, and the manufacturing cost of the molten iron in an electric furnace can be reduced significantly.
In general, since the auxiliary burner has a relatively low gas flow rate, the spout of molten iron or molten slag may clog the gas discharge port. Since the gas is purged, it is difficult for the gas discharge port to be clogged by the splash.

The flame length varies depending on the ignition temperature of the fuel used for the auxiliary burner. The solid fuel and the gaseous fuel have different ignition temperatures. For this reason, in the present invention in which the solid fuel can be appropriately and efficiently burned together with the gaseous fuel in the auxiliary combustion burner, by changing the ratio of the solid fuel and the gaseous fuel, The flame length of the burner (the flame temperature at a position away from the burner by a certain distance) can be arbitrarily adjusted.
As described above, in the auxiliary burner used in the present invention, a combustion field higher than the ignition temperature of solid fuel (coal, etc.) is created by combustion of gaseous fuel (LNG, etc.) and combustion-supporting gas (pure oxygen, etc.). When the solid fuel is sent to the combustion field, the temperature rises to the ignition temperature, and solid fuel combustion (vaporization → ignition) occurs. Although the flame temperature decreases with the amount of heat required for the temperature increase of the solid fuel, the temperature increases in the region where the solid fuel ignition occurs. Accordingly, in the present invention, when the ratio of gaseous fuel is higher than that of solid fuel, the flame generated by the auxiliary burner has a higher temperature near the burner tip (that is, a shorter flame), but the ratio of solid fuel than that of gaseous fuel. When the is increased, the heat generated after the endotherm of the solid fuel becomes high even at a position far from the tip of the burner (that is, a long flame is formed). Therefore, the flame length (flame temperature at a position away from the burner by a certain distance) can be controlled by changing the ratio of the gas fuel and the solid fuel.

  FIG. 3 schematically shows the change in flame length when the ratio of gaseous fuel and solid fuel is changed in the auxiliary burner used in the present invention. In the figure, the solid line is the flame temperature at a position 0.2 m away from the burner tip in the burner axis direction, the broken line is the flame temperature at a position 0.4 m away from the burner tip, and the horizontal axis is gaseous fuel + solid This is the ratio of solid fuel in the fuel. According to FIG. 3, the flame temperature at the 0.2 m position in the vicinity of the burner is high under the condition where the gas fuel ratio is high (the solid fuel ratio is low), but a sudden temperature drop occurs at the 0.4 m position. ing. That is, the flame length is short. On the other hand, under the condition where the ratio of the solid fuel is high, the flame temperature at the 0.2 m position in the vicinity of the burner is lower than that of the gaseous fuel 100%, but almost no temperature decrease occurs at the 0.4 m position. That is, the flame length is long. This is because the gaseous fuel is preferentially burned in the vicinity of the burner, and the solid fuel heated in the flame starts burning at a position of 0.4 m, and the temperature is maintained.

  In the operation of an electric furnace, the distance between the auxiliary burner and the scrap changes due to the charging, additional charging and melting of the scrap. In general, the distance between the auxiliary burner and the scrap is small at the start of operation or in the initial stage of addition, and increases with the progress of melting of the scrap. This is because the distance from the undissolved scrap to the auxiliary burner increases with the progress of melting of the scrap because the scrap is first melted in order from the scrap closest to the auxiliary burner. In the present invention, the flame length is adjusted (changed) by changing the ratio of the solid fuel and the gaseous fuel according to the distance from the scrap to be heated or melted by the auxiliary burner, and the relationship between the distance between the scrap and the auxiliary burner. Without the flames reaching the scrap. That is, when the distance between the auxiliary burner and scrap is small, the ratio of gaseous fuel is increased to shorten the flame length, and when the distance between the auxiliary burner and scrap is large, the ratio of solid fuel is increased to increase the flame length. To do. Thereby, a scrap can be heated or melt | dissolved efficiently.

  Specifically, in general operation of an electric furnace (operation of one charge), charging of scraps is performed about 2 to 3 times. The operation of the electric furnace begins with the start of energization and the start of use of the auxiliary burner after the initial scrap is charged. At the start of operation, some molten iron from the previous operation remains (residual hot water), and there is molten metal in the lower part, or the entire molten iron from the previous operation is discharged and the furnace may be empty. There is no big difference in the method. In the initial stage of scrap charging, the entire interior of the electric furnace is filled with scrap. Therefore, the distance between the scrap and the furnace wall is close. In many cases, the distance between the tip of the auxiliary burner and the scrap at the initial stage of scrap charging is about 0.5 m. Moreover, although the position of the auxiliary | assistant combustion burner front-end | tip part height is based also on the characteristic of a furnace, it is common that it is 1 m or more upwards from the hot-water surface height after scrapped off.

  As the operation proceeds, melting proceeds from the scrap in contact with the molten iron, near the electrodes, and near the auxiliary burner. In the scrap near the auxiliary burner, the scrap at the upper part falls with melting at the initial stage of scrap charging, so the distance between the auxiliary burner and the scrap is relatively close, but when the upper scrap disappears, the distance from the scrap increases. If the distance from the scrap increases, the heat of the auxiliary burner cannot be efficiently supplied to the scrap. Therefore, conventionally, an operation to stop the auxiliary burner has been performed. On the other hand, in the operation of the present invention, when the scrap is near, the ratio of gaseous fuel (such as LNG) is increased to dissolve the scrap with a short flame, and when the melting progresses and the distance of the scrap increases, the solid fuel By increasing the ratio of coal (such as coal), scrap is melted with a long flame. As a result, more scrap can be efficiently melted, and the operation time can be shortened and the power consumption can be reduced. Since the distance between the auxiliary burner and the scrap is changed by inserting the scraps about 2 to 3 times, the scrap can be efficiently dissolved by appropriately changing the ratio of the gas fuel and the solid fuel each time. it can.

  When using the auxiliary burner in the present invention, if the flame length is adjusted (changed) by changing the ratio of solid fuel to gaseous fuel according to the distance from the scrap to be heated or melted, the auxiliary burner and scrap However, for example, a laser distance meter is installed in the auxiliary burner, and the distance to the scrap can be measured with this laser distance meter. Moreover, the situation in the furnace can be observed with a monitoring camera through a window such as a discharge port. Depending on the structure of the electric furnace, the distance to the scrap can be grasped by observation in the furnace with the monitoring camera. In addition, information useful for grasping the distance may be obtained from the operation data.

  The test was conducted in an electric furnace provided with an auxiliary burner having the structure shown in FIG. FIG. 4 schematically shows a horizontal cross section of the electric furnace in which the experiment was performed. This electric furnace is a direct current type having a furnace diameter of about 6 m, a capacity of about 120 tons, and one electrode at the center. The furnace body includes a discharge port for discharging the slag on the molten steel and a steel outlet for discharging the molten iron. The auxiliary combustion burners are installed at four locations in the furnace body circumferential direction, and the output of each auxiliary combustion burner is 970 Mcal / h. FIG. 5 shows the state of use of the auxiliary burner in this embodiment (vertical cross section in the electric furnace radial direction). The auxiliary burner is installed at a position about 910 mm above the surface of the molten iron and is downward from the horizontal direction. A flame is emitted in the direction of about 25 °. The flame length of this auxiliary burner (output 970 Mcal / h) is 1.5 to 2.0 m, and the maximum flame reach length A in the furnace radial direction is A = 11921 mm.

LNG (gaseous fuel) and pulverized coal (solid fuel) are used as the fuel for the auxiliary burner, pure oxygen is used as the combustion-supporting gas, and pulverized coal is injected from the central solid fuel injection pipe using nitrogen as a carrier gas. LNG was injected from the outer gas fuel injection pipe, and pure oxygen was injected from the outer (outermost circumference) combustion-supporting gas injection pipe.
As pulverized coal, three types of lignite, MDT, and LVP were used. Table 1 shows the calorific value calculated from these components, the measured effective calorific value (low calorific value), and the total carbon content. In Invention Example 8 to be described later, MDT having a different component and total carbon amount from those shown in Table 1 was used.

Table 2 shows the operating conditions and test results of each example in this test.
Among the test results, the operability is evaluated as “○” (passed) if operation is possible without any trouble. The pulverized coal is clogged with piping during transportation, and the burner misfires without burning the pulverized coal. When operation trouble such as, etc. occurred, it was set as “×” (failed).
The cost was evaluated with “◯” (excellent), “△” (good), and “×” (impossible) depending on whether an economic merit occurred. The aim of the present invention is to reduce the amount of expensive gaseous fuel used in the auxiliary burner and to reduce the energy cost. Generally, it is desirable to reduce the power consumption of electric furnace operation. If an amount of solid fuel can be used, the economical cost merit is sufficient even if the electric power consumption of the electric furnace is the same. If there were any merits including that case, it was evaluated as “◯” (excellent) and “△” (good). The cost evaluation also took into account the pulverization costs for pulverizing coal to obtain pulverized coal.

In the evaluation of the flame temperature, it was determined whether or not the flame observed when the auxiliary burner burned was hot. The flame temperature is generally measured using a thermocouple, but in this example, it was simply measured using a radiation thermometer. Basically, it can be said that the higher the flame temperature, the more advantageous the melting of the scrap, but it is considered important that the melting point is 1600 ° C. or higher, which is the melting point at which the scrap begins to melt in the widest possible region of the flame. For this reason, the flame temperature is measured with a two-dimensional radiation thermometer from directly above the electric furnace with only the auxiliary burner burned before charging the scrap, and there is a temperature range of less than 1600 ° C in the flame area of 50% or more. In such a case, it was judged that the flame temperature was non-uniform, and “x” was evaluated. Further, the case where the temperature range of less than 1600 ° C. in the flame area was less than 50% to 20% or more was evaluated as “Δ”, and the case where it was less than 20% was evaluated as “◯”.
For comprehensive evaluation, if any of the operability, cost, and flame temperature is evaluated as “×”, the evaluation is “×” (fail), and any of operability, cost, or flame temperature is “△”. In the case of the evaluation, “△” was given.

In Comparative Example 1, pulverized coal is not used as a fuel, and only LNG is used. Naturally, the economic effect cannot be enjoyed. Moreover, since the output energy ratio by combustion of pulverized coal is as low as 40% and comparative example 3 uses expensive LNG as much as 60%, it cannot enjoy an economic effect. On the other hand, in Comparative Example 2, since the entire amount of fuel was pulverized coal, the pulverized coal did not combust properly, causing an operational trouble that caused a misfire.
In Comparative Example 5, since the flow rate of the carrier gas per 1 kg of pulverized coal was less than 0.05 Nm ³ , the pulverized coal was clogged in the flow path, and operation became impossible. In Comparative Example 4, since the flow rate of the carrier gas per 1 kg of pulverized coal exceeded 0.20 Nm ³ , the flame temperature was lowered and the scrap could not be heated efficiently, and the power consumption rate deteriorated.

On the other hand, good test results are obtained for all of the inventive examples. In other words, pulverized coal can be combusted appropriately and efficiently with LNG without causing trouble in conveying pulverized coal or flame temperature due to carrier gas, and iron-based scrap can be efficiently heated or melted by an auxiliary burner. Is done. As a result, the amount of expensive LNG used can be reduced, and the manufacturing cost of molten iron can be significantly reduced.
Invention Example 8 is an example using MDT, which is the same coal type as Invention Example 4, but since the same carbon type (MDT) as Invention Example 4 was used without analyzing the total carbon content, the total carbon content was also Assuming the same 81.7% by mass, the pulverized coal consumption was 115 kg / h. However, when the total carbon amount of the pulverized coal used in Invention Example 8 was analyzed, it was 85.8% by mass. The amount of pulverized coal calculated from the estimation formula was 109 kg / h, and 6 kg / h was excessively pulverized. It was found that charcoal was used. Therefore, the cost was evaluated as “Δ”.

Invention Example 13 uses two auxiliary burners out of the four auxiliary burners, and operates under the condition that the burner output is 17 Mcal / h per ton of dissolved amount. In this operation example, since the burner output is small, the effect of equalizing the dissolution rate by using the auxiliary burner is reduced, resulting in a slight reduction in the power consumption rate. Therefore, the cost was evaluated as “Δ”.
Invention Example 14 uses pulverized coal having an average particle diameter d90 of 20 μm. The average particle size d90: 20 μm is a particle size level that can be barely achieved by repeating the pulverization twice in the pulverization step, which not only increases the pulverization cost, but also increases the production time of pulverized coal. Evaluated as “△”.
Invention Example 15 uses pulverized coal having an average particle diameter d90 of 550 μm. Since the particle size of the pulverized coal is too coarse, unburned residue is generated, and the flame temperature is slightly lowered. Therefore, the flame temperature is evaluated as “Δ”. .

The basic unit of electric power when operating on various scraps under the conditions of Comparative Example 1 was in the range of about 250 to 400 kwh / t, but all the examples of the present invention should be in the basic unit of electric power within that range. I was able to.
In addition, when an experiment was performed with an actual machine under the operation conditions of the comparative example, almost the same result was obtained, and the power consumption rate was in a worsening trend except for Comparative Examples 1 and 3.

DESCRIPTION OF SYMBOLS 1 Solid fuel injection pipe 2 Gaseous fuel injection pipe 3 Combustion gas injection pipe 7 Furnace body 8 Electrode 9 Auxiliary burner x Iron-based scrap 10 Solid fuel flow path 11 Solid fuel discharge port 12 Solid fuel supply port 20 Gaseous fuel flow path 21 Gas fuel discharge port 22 Gas fuel supply port 30 Fuel-supporting gas flow path 31 Fuel-supporting gas discharge port 32 Fuel-supporting gas supply port

Claims (5)

In a method of melting iron scrap in an electric furnace equipped with an auxiliary burner to obtain molten iron,
An auxiliary combustion burner using gaseous fuel and solid fuel as fuel, having a plurality of concentric injection pipes for injecting gaseous fuel, solid fuel and combustion-supporting gas, respectively, Using an auxiliary burner that injects combustion-supporting gas from the injection pipe, and when heating or melting iron-based scrap with the auxiliary burner, the amount of solid fuel used is 50 to 95% of the total output energy in terms of burner output energy And a flow rate of the carrier gas of the solid fuel is 0.05 to 0.20 Nm ³ per kg of the solid fuel, and a method for producing molten iron using an electric furnace.   The auxiliary combustion burner has a structure in which a solid fuel injection pipe, a gaseous fuel injection pipe, and a combustion-supporting gas injection pipe are arranged concentrically in this order from the center side. A method for producing molten iron using an electric furnace.   The method for producing molten iron using an electric furnace according to claim 1 or 2, wherein the electric furnace comprises a plurality of auxiliary combustion burners, and the total output of all auxiliary combustion burners is 20 Mcal / h or more per ton of scrap melting.   The method for producing molten iron using an electric furnace according to any one of claims 1 to 3, wherein the solid fuel is pulverized coal having an average particle diameter d90 of 50 to 500 µm.   The total carbon amount of the solid fuel to be used is measured in advance, and the amount of the solid fuel used in the auxiliary burner is adjusted according to the total carbon amount. A method for producing molten iron using a furnace.

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