Method for controlling a gas-fueled furnace

Abstract

To provide a control method for stable combustion of high efficiency, in a gas fuel heating furnace using a composite nozzle in which an outer nozzle for discharging a combustion supporting gas such as oxygen and air is arranged around the circumference of a center nozzle discharging a fuel gas.SOLUTION: In performing a control to reduce generation of CO and NOx by obtaining flame by burning a fuel gas from a center nozzle while adding oxygen so as to be in a state in which the amount of oxygen is reduced from a complete combustion state, and supplying oxygen and air to a combustion reactive interface around the frame from an outer nozzle, a high combustion control step of decreasing a ratio of oxygen supply amount from the outer nozzle while increasing a flow rate of the combustion gas from the center nozzle, and a low combustion control step of increasing a ratio of oxygen supply amount from the outer nozzle while decreasing the flow rate of the fuel gas from the center nozzle are switched and controlled.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to a control method for a gas-fuel heating furnace for uniformly heating a steel ingot with a flame extending approximately horizontally, and more particularly to a control method for a gas-fuel heating furnace in which outer nozzles for discharging a combustion-supporting gas such as oxygen or air are arranged around a center nozzle for discharging a fuel gas. [Background technology]

[0002] Gas-fuel heating furnaces are used, for example, as soaking furnaces for reducing the temperature difference between the inside and outside of steel ingots before they are subjected to processing steps such as slabs and billets, thereby allowing for uniform processing, and are also used in other heating furnaces for steel ingots, such as continuous furnaces. For example, multiple steel ingots are placed inside a furnace chamber, and a gas burner is installed on one end wall in the longitudinal direction of the approximately rectangular parallelepiped furnace chamber, and a flame is formed from the gas burner, which is installed on one end wall in the longitudinal direction of the approximately rectangular parallelepiped furnace chamber, so as to extend approximately horizontally above the steel ingots while not directly hitting the steel ingots, to heat them.

[0003] Here, Patent Document 1 discloses a "regenerative burner system" that uses a pair of gas burners each having a heat storage body to improve the combustion efficiency of a soaking furnace as a gas fuel heating furnace, and is composed of a fuel, air, and exhaust gas switching valve, a hot-burning air blower, an exhaust fan, and a control device for controlling them. When one gas burner is burning, the other gas burner exhausts combustion exhaust gas to heat the heat storage body. Next, by switching between combustion and exhaust of the pair of gas burners, the heat stored in the heat storage body is recovered by the combustion air and returned to the combustion chamber. By repeating this switching in a short cycle of several tens of seconds to several minutes, it is possible to obtain a system that preheats the combustion air to a high temperature and burns the air while recovering waste heat with high efficiency.

[0004] As a highly efficient gas-fueled heating furnace system, attempts are being made to increase the combustion temperature of the gas burner by burning air with a higher oxygen ratio (oxygen-enriched air) or pure oxygen, thereby achieving higher combustion efficiency.

[0005] For example, Patent Document 2 describes the flame of a gas burner as follows: In a tubular flame that supplies fuel and oxidizer from a tangential direction to a cylindrical combustion chamber, the inside is covered with high-temperature combustion gas and the outside is covered with unburned mixed gas, which gives the tubular flame high insulation properties and makes it possible to form a high-temperature field close to the adiabatic flame temperature.The document then discloses a gas burner structure and furnace system that can suppress intermittent combustion (oscillating combustion) and ensure stable combustion when using an oxygen-enriched combustion tubular flame gas burner that forms a tubular flame by burning fuel gas in a state where the oxygen concentration in the oxidizer gas introduced into the combustion chamber is 60% or more. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2000-282137 A [Patent Document 2] JP 2017-3220 A Summary of the Invention [Problem to be solved by the invention]

[0007] There is a gas-fueled heating furnace that allows control of the combustion reaction interface of the flame by arranging outer nozzles that discharge combustion support gas such as oxygen or air around a center nozzle that discharges fuel gas almost horizontally. In such a gas-fueled heating furnace, it is expected that thermal NOx can be reduced by controlling the combustion reaction interface to suppress local temperature rise along the flame, resulting in highly efficient and stable combustion.

[0008] The present invention has been made in consideration of the above-mentioned circumstances, and its object is to provide a control method that provides highly efficient and stable combustion in a gas-fuel heating furnace that uses a composite nozzle in which outer nozzles that spray combustion-supporting gases such as oxygen or air are arranged around a center nozzle that sprays fuel gas. [Means for solving the problem]

[0009] The control method for a gas-fuel heating furnace according to the present invention is a control method for a gas-fuel heating furnace equipped with a composite nozzle in which outer nozzles are arranged around a center nozzle that discharges fuel gas approximately horizontally and discharges air with an adjusted amount of oxygen, and the method adds oxygen to the fuel gas from the center nozzle so that the oxygen amount is reduced from a complete combustion state, and then combusts the fuel gas to obtain a flame, and supplies oxygen and air from the outer nozzle to the combustion reaction interface around the flame to reduce the generation of CO and NOx, characterized by switching between a high combustion control step in which the flow rate of the fuel gas from the center nozzle is increased while the supply ratio of oxygen from the outer nozzle is reduced, and a low combustion control step in which the flow rate of the fuel gas from the center nozzle is reduced while the supply ratio of oxygen from the outer nozzle is increased.

[0010] According to these features, the use of an oxygen enrichment burner in the center nozzle reduces the generation of CO and NOx, and allows for highly efficient and stable combustion. In addition, as a gas-fueled heating furnace, the temperature inside the furnace can be raised quickly to the soaking treatment temperature, resulting in improved treatment efficiency.

[0011] In the above-mentioned invention, a duct for discharging exhaust gas may be provided at the lower part of the furnace wall surface on which the center nozzle and the outer nozzle are provided. The high combustion control step may be characterized in that entrained air is supplied from the duct together with the supply of air from the outer nozzle. The air may be heated by heat exchange in the duct and discharged from the outer nozzle. According to this feature, the flame is stabilized through oxygen control, and highly efficient and stable combustion can be obtained. [Brief description of the drawings]

[0012] [Figure 1] 1 is a cross-sectional view showing one embodiment of a gas fuel heating furnace used in the present invention. [Diagram 2] FIG. 2 is a front view of a composite nozzle provided in a gas-fired furnace. [Diagram 3] FIG. 1 is a block diagram of a gas-fired furnace. [Figure 4] 1 is a table showing the concentration of CO and NOx according to the fuel flow rate and oxygen ratio. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] A method for controlling a gas fuel heating furnace as one embodiment of the present invention will be described with reference to FIGS. 1 to 4. FIG.

[0014] As shown in Fig. 1, a gas-fuel heating furnace 1 is a horizontal atmospheric furnace equipped with a furnace chamber 2 having a substantially rectangular parallelepiped shape and a composite nozzle 10 which is a gas burner for obtaining a flame f, and is used, for example, as a soaking furnace. The composite nozzle 10 is provided on the upper side of the furnace wall surface 3 at one end in the longitudinal direction of the furnace chamber 2. This allows the flame f to be formed so as to extend substantially horizontally above the steel ingot S inserted in the furnace chamber 2 without directly impinging on it. In addition, a gas exhaust port 4 which leads to a duct for discharging exhaust gas is provided at the lower part of the furnace wall surface 3 on which the composite nozzle 10 is provided. The duct will be described later.

[0015] Referring also to FIG. 2, the composite nozzle 10 includes a center nozzle 11 that discharges fuel gas and an outer nozzle 16 that discharges air with an adjusted oxygen content around the center nozzle 11. The center nozzle 11 is an annular nozzle made of a tube that draws a plurality of substantially concentric circles, and is arranged in this order from the inside to the outside, with a central air outlet 12, a fuel gas outlet 13, an oxygen outlet 14, and an outer air outlet 15. In other words, the center nozzle 11 is an oxygen-enriched burner that can discharge fuel gas, air as a combustion-supporting gas, and oxygen, and can obtain a flame f by burning the fuel gas. In particular, the control of the flame f can be stabilized by discharging the fuel gas from the annular nozzle portion that is the fuel gas outlet 13 and discharging air from the central air outlet 12 provided at the center. For example, city gas can be used as the fuel gas.

[0016] Furthermore, a plurality of outer nozzles 16 are provided, for example, at equal intervals on a concentric circle centered on center nozzle 11, and here, they are arranged at four locations on a circle centered on center nozzle 11. Furthermore, each of the outer nozzles 16 has a double-tube structure having an oxygen outlet 17 on the inside and an air outlet 18 on the outside. In other words, both the center nozzle 11 and the outer nozzle 16 can eject oxygen, and the composite nozzle 10 formed as a whole by these also forms an oxygen-enriched burner.

[0017] The flame f formed in the furnace by the combustion of the fuel gas from the composite nozzle 10 is controlled to extend almost horizontally and heat a wide area in the furnace uniformly. As described above, the center nozzle 11 can discharge oxygen, but the energy of the flame f can be increased by increasing the oxygen supply ratio. On the other hand, if the oxygen supply ratio is too large, the fuel gas will burn out in a short time, and the flame f cannot be extended for a long time. Therefore, oxygen is supplied to the fuel gas discharged from the fuel gas discharge port 13 of the center nozzle 11 so that the amount of oxygen is reduced from the complete combustion state in which the fuel gas is completely burned. That is, the ratio of the oxygen supply from the central air discharge port 12 and the oxygen discharge port 14 is adjusted with respect to the discharge amount of the fuel gas, so that the fuel gas cannot be completely burned by the oxygen supplied from the center nozzle 11 alone, and the fuel gas is moved toward the tip side of the flame f in an incomplete combustion state.

[0018] Here, oxygen is supplied to the flame f from the oxygen outlet 17 and the air outlet 18 of the outer nozzle 16. The oxygen is supplied from the combustion reaction interface f' of the flame f toward the inside of the flame f, and the unburned fuel gas can be burned before it reaches the tip of the flame f. In other words, while the center nozzle 11 causes incomplete combustion of the fuel gas so as to lengthen the length of the flame f in order to heat the inside of the furnace uniformly, the fuel gas is completely burned with oxygen from the outer nozzle 16, thereby reducing the generation of CO.

[0019] As shown in FIG. 3, the gas-fueled heating furnace 1 is provided with a duct 5 continuing from the gas exhaust port 4 to form a flue through which the exhaust gas passes. The duct 5 is provided with a sensor 6 for detecting the components of the exhaust gas inside the duct 5, and the concentrations of CO and NOx can be monitored. In addition, a recuperator 7 is provided in the duct 5 to exchange heat between the air to be led to the composite nozzle 10 and the exhaust gas. That is, an air introduction pipe 12' is connected to the recuperator 7, and the air heated by heat exchange with the exhaust gas can be led to the central air outlet 12 of the composite nozzle 10. This can improve the combustion efficiency of the fuel gas. In addition, a damper 8 is provided at the outlet of the duct 5, and the pressure inside the furnace chamber 2 can be controlled by adjusting the amount of exhaust gas discharged.

[0020] In such a soaking furnace, the output of the burner is controlled to heat the steel ingot S inserted in the furnace chamber 2 and maintain the temperature at a predetermined level. For example, when raising the temperature, the output of the burner is increased to shorten the heating time, and when maintaining the temperature, the output is reduced to operate efficiently. The output of the burner is controlled by adjusting the combustion state of the flame f from the composite nozzle 10, which is the burner, by adjusting the discharge amounts of fuel gas, air, and oxygen.

[0021] Here, if the discharge amount of fuel gas is changed to change the output, the oxygen supply ratio (hereinafter referred to as the oxygen ratio), which is the volume ratio of the discharged oxygen to the discharged fuel gas, is changed, and the combustion state of the flame f is changed. For example, when the oxygen ratio is large, the fuel gas is easily combusted completely and the generation of CO is suppressed, but the oxygen-rich atmosphere makes it easier to generate NOx. On the other hand, when the oxygen ratio is small, the generation of NOx can be suppressed, but the fuel gas-rich atmosphere makes it easier to generate CO. In other words, it is necessary to balance the oxygen ratio with the amount of fuel gas to suppress the generation of CO and NOx.

[0022] Incidentally, the discharged oxygen is supplied to the inside of the flame f and mixed with the fuel gas, but since it is a system open to the furnace chamber 2, it is difficult to completely mix all of the oxygen, especially from the outer nozzle 16. For example, the state of oxygen supply changes depending on the flow rate of the discharged fuel gas. Therefore, the ratio of fuel gas and oxygen actually supplied to the flame f changes depending on the output of the burner. Therefore, the inventors investigated the relationship between the discharge amount of fuel gas (flow rate per hour) and the oxygen ratio for the gas-fuel heating furnace 1 from the viewpoint of suppressing the generation of NOx and CO.

[0023] That is, the discharge amount of fuel gas and the amount of oxygen discharged by air and oxygen were adjusted, the concentrations of NOx and CO in the exhaust gas inside the duct 5 were detected by the sensor 6, and the relationship between the flow rate (fuel flow rate) of the fuel gas and the oxygen ratio, and the concentrations of NOx and CO was investigated. The results of this investigation were shown in Figure 4, with "O" recorded when the NOx and CO concentrations were below a predetermined concentration, and "X" recorded when they exceeded the predetermined concentration. Here, the predetermined concentrations were set to 150 ppm or less for NOx and below the detection limit for CO.

[0024] As shown in Figure 4, when the fuel gas discharge rate (fuel flow rate) is constant, increasing the oxygen ratio tends to decrease the concentration of CO produced and increase the concentration of NOx. In other words, as described above, the concentration of products increases or decreases depending on whether the mixture is fuel gas rich or oxygen rich.

[0025] Furthermore, when the oxygen ratios for "small," "medium," and "large" were checked for the cases where both CO and NOx were below the specified concentrations (represented as "O") for each of the fuel flow rates "small," "medium," and "large," the results were "small"-"large," "medium"-"medium," and "large"-"small." In other words, it was found that the cases where both CO and NOx could be kept below the specified concentrations tend to be when the fuel flow rate was increased and the oxygen ratio was decreased.

[0026] According to this, the generation of CO and NOx can be suppressed by controlling the amount of oxygen so that the oxygen ratio is decreased in the high combustion region where the fuel gas flow rate is increased and the oxygen ratio is increased in the low combustion region where the fuel gas flow rate is decreased. Therefore, by switching control to control as a high combustion control step in which the fuel gas flow rate is increased and the oxygen ratio is decreased when the burner output is increased, and as a low combustion control step in which the fuel gas flow rate is decreased and the oxygen ratio is increased when the burner output is decreased, the burner output can be controlled while suppressing the generation of CO and NOx.

[0027] Incidentally, the magnitude of the oxygen ratio as described above is considered to be influenced by the flow velocity of the gas discharged from the center nozzle 11 forming the flame f. That is, in the high combustion control step in which the flow velocity is fast, the pressure in the flame f is lowered to promote the supply of oxygen from the combustion reaction interface f' of the flame f and also promote the mixing and stirring of the fuel gas and oxygen. That is, a relatively large amount of oxygen supplied from the outer nozzle 16 can be used for combustion. On the other hand, in the low combustion control step in which the flow velocity is slow, the pressure in the flame f is increased compared to when the flow velocity is fast, delaying the supply, mixing and stirring of oxygen, and reducing the amount of oxygen supplied from the outer nozzle 16 that can be used for combustion. Therefore, the adjustment of the increase and decrease of the oxygen ratio described above is mainly performed on the oxygen discharged from the outer nozzle 16.

[0028] As described above, the pressure inside the flame f is lowered in the high combustion control step. Therefore, a part of the exhaust gas inside the duct 5 may be sucked from the gas outlet 4 located directly below the composite nozzle 10 and entrained in the flow of air and oxygen from the outer nozzle 16. In this case, a part of the entrained exhaust gas is supplied to the flame f as entrained air together with the air and oxygen from the outer nozzle 16. Depending on the pressure inside the furnace chamber 2, air may enter through a gap in a door (not shown). It is also preferable to perform the above-mentioned control in consideration of these factors.

[0029] Although typical embodiments of the present invention have been described above, the present invention is not necessarily limited thereto, and a person skilled in the art will be able to find various alternative embodiments and modifications without departing from the spirit of the present invention or the scope of the appended claims. [Explanation of symbols]

[0030] 1 Gas-fueled heating furnace 2 Furnace room 3 Furnace wall 4 Gas exhaust port 5 Duct 10 Composite nozzle 11 Center nozzle 16 Outer nozzle f flame f' combustion reaction interface

Claims

1. A method for controlling a gas-fueled heating furnace having a composite nozzle in which outer nozzles for discharging air having an adjusted amount of oxygen are arranged around a center nozzle for discharging a fuel gas and a combustion supporting gas substantially horizontally, comprising the steps of: In the center nozzle, oxygen is discharged together with the fuel gas so that the amount of oxygen is reduced from a complete combustion state, and then the fuel gas is burned to obtain a flame. At the same time, oxygen and air are supplied from the outer nozzle to the combustion reaction interface around the flame to reduce the generation of CO and NOx. a high combustion control step of increasing a flow rate of the fuel gas while keeping a flow rate of the combustion supporting gas from the center nozzle constant, and decreasing a supply rate ratio of oxygen from the outer nozzle; a low combustion control step of increasing the oxygen supply ratio at the outer nozzles while keeping the flow rate of the combustion supporting gas from the center nozzle constant; and a low combustion control step of increasing the oxygen supply ratio at the outer nozzles while keeping the flow rate of the combustion supporting gas from the center nozzle constant.

2. 2. The method for controlling a gas-fuel heating furnace according to claim 1, wherein a duct for discharging exhaust gas is provided at a lower portion of the furnace wall surface on which the center nozzle and the outer nozzles are provided.

3. 3. The method for controlling a gas-fuel heating furnace according to claim 2, characterized in that the high combustion control step includes, in addition to the supply of air from the outer nozzle, the supply of air containing the exhaust gas that is sucked from inside the duct due to a pressure drop within the flame.

4. 4. The method for controlling a gas-fueled heating furnace according to claim 3, wherein the air is heated by heat exchange within the duct and discharged from the outer nozzle.

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