JPH09208966A - Heating method for tubular heating furnace - Google Patents

Abstract

PROBLEM TO BE SOLVED: To uniformize the heat flux without increasing a furnace volume of a tubular heating furnace while keeping the close arrangement of heating pipes. SOLUTION: A hot diluted air preheated at a temperature above a critical blow-off temperature and having remarkably low oxygen concentration relative to normal air is produced by mixing air with a part of combustion exhaust gas discharged from a tubular heating furnace. The hot diluted air and a fuel are subjected to diffusion combustion in a tubular heating furnace without changing the chemical equivalent ratio of the optimum air ratio and the hot gas flow is intermittently, continuously or periodically flowed around the heating pipes to heat the fluid in the heating pipes.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a tubular heating furnace. More specifically, the present invention relates to improvement of a method for heating a heated tube of a tubular heating furnace.

[0002]

2. Description of the Related Art A conventional tubular heating furnace, for example, an ISOFLOW TYPE tubular heating furnace, has a single-pass or multi-pass heated pipe through which a fluid to be heated such as petroleum passes and is stacked along the furnace wall. Is also folded back and arranged, and a combustion chamber is formed inside the furnace bottom. A diffusion burner is installed in the combustion chamber, a flame is formed in parallel with the heated pipe, and combustion gas flows toward the top of the furnace to heat the heated pipe by radiant heat transfer and convective heat transfer. ing.

[0003]

However, in the conventional tube-type heating furnace, since the flame exists in the fixed region in the combustion chamber, the heat flux inevitably becomes nonuniform in the heated tube. In order to correct this, in the case of an isoflo tube heating furnace, it is necessary to prevent the heated pipe from blocking the direct radiation from the flame by other heated pipes inside the furnace, and to prevent the radiation from reflection from the wall surface. The number should be increased, but as a result, the number of tubes to be heated installed in the furnace is reduced, and a large furnace volume is required to perform necessary heating. In other words, the ratio between the maximum heat flux and the average heat flux of the heated pipe depends on the density of the number of the heated pipes, that is, the ratio of the heated pipe surface area to the furnace volume. It is known that it becomes about 1.6 as shown in FIG. For this reason, there is no choice but to set the average heat flux that is considerably lower than the allowable heat flux of the heated tube, and there is a problem that the heating efficiency deteriorates.
Even if the heated tubes are arranged in three or more rows in order to increase the number of tubes to be heated without increasing the furnace volume, the maximum heat flux of the tubes close to the flame exceeds the allowable value, and the radiant heat from the flame is increased. Since a heated pipe that blocks the heat is generated and the heat flux becomes non-uniform, it is not possible in practice because the average heat flux of the heated pipe decreases.

In the case of a convection heat transfer dominant type tube heating furnace,
The amount of heat transfer, that is, the heat flux was restricted by the flow velocity of the gas passing around the heated tube. Therefore, since the average heat flux is small and the heating efficiency is not good, a large heating furnace is required to increase the throughput.

The present invention minimizes the ratio of the maximum heat flux to the average heat flux without reducing the surface area of the heated tube per furnace volume, that is, without increasing the furnace volume of the tubular heating furnace. An object of the present invention is to provide a combustion method for a tube-type heating furnace, which enables uniform heat flux while keeping the heating tubes densely arranged.

[0006]

In order to achieve such an object, the heating method of the tubular heating furnace of the present invention is much more than ordinary air by adding a part of the combustion exhaust gas discharged from the tubular heating furnace. In addition, the high-temperature diluted air and the fuel, which have a low oxygen concentration and are at least blown off at the oxygen concentration, are pre-heated and diffuse-combusted in a tubular heating furnace without changing the chemical equivalence ratio at the proper air ratio. The gas flow is caused to flow around the heated pipe intermittently, continuously or periodically to heat the fluid in the heated pipe.

As a result of the observation of the combustion state by the present inventors, the influences of the air temperature and the oxygen concentration on the combustion stability tend to be as shown in FIG. Therefore, the high-temperature diluted air and fuel preheated above the blowout limit temperature (the temperature at which it blows off when the temperature becomes lower) at each oxygen concentration diffuse-mixes and flame-holds when it enters the combustible range. Initiate combustion with the help of the mechanism or spontaneously.
However, since the oxygen concentration is much lower than that of normal air and the volume of dilution air is considerably large, the heat generation rate is sufficiently slower than that of normal combustion, accompanied by an oxidative exothermic reaction. Therefore, even while flowing around the pipe to be heated intermittently, continuously, or periodically, the oxidative exothermic reaction is continuously maintained and continues to burn in a wide range in the furnace, and heat is generated in the process of generating sensible heat. It is robbed and heats the fluid in the heated pipe. Moreover, since the flame volume is significantly increased by the high-temperature dilution air having a low oxygen concentration and a large volume, the flow velocity is increased. That is, a flame is formed in which the flow velocity of the hot gas flow is fast and the combustion is stable with a very large volume.

The heating method of the tubular heating furnace of the present invention is
The preheating temperature of the high-temperature dilution air is set to the auto-ignition temperature of the fuel at the oxygen concentration or higher, more preferably 600 ° C or higher. In this case, a flame holding mechanism is not required, and when the fuel diffusely mixes and enters the combustible range, spontaneous combustion occurs.

Further, the heating method of the tubular heating furnace of the present invention is
A part of the exhaust gas from the tube heating furnace is recirculated to the tube heating furnace and mixed with the combustion air that has been preheated to a high temperature by heat exchange with the exhaust gas that has not been recirculated to generate dilution air. I have to.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The structure of the present invention will be described below in detail based on an example of an embodiment shown in the drawings.

FIG. 1 schematically shows an example of a tubular heating furnace in which the heating method of the present invention is carried out. The tubular heating furnace is not particularly limited in type, and is applicable to an isoflow type tubular heating furnace or a convection heat transfer control type tubular heating furnace. For example, in the case of applying to an isoflow type tube heating furnace, three paths or more of heated tubes are concentrically arranged in a cylindrical heat-insulating and heat-resistant furnace body and folded at the upper end and the lower end, respectively. ing. At least one diffusion burner 2 is arranged in the combustion chamber at the center of the bottom of the tube heating furnace 1. Diffusion burner 2
Is not particularly limited to its structure, but for example, a portion of the combustion gas exhausted from inside the tube heating furnace 1 is circulated through the external circulation path 3 and the sensible heat of the remaining combustion gas is increased to a high level. Efficient heat exchanger 4 recovers heat to preheat combustion air, and the preheated combustion air and recirculated combustion gas are mixed to have a lower oxygen concentration than normal air and the chemical equivalent ratio is not changed. A burner system is employed in which hot diluted air preheated to a capacity and at least above the blowout limit temperature at its oxygen concentration is obtained, and the fuel is diffused and burned therewith.

The external circulation system passage 3 connects the exhaust pipe 5 connected to the top of the furnace and the combustion air supply pipe 6 connected to the burner 2 at the bottom of the furnace, and is exhausted from the tubular heating furnace 1. It is provided to circulate a part of the combustion gas as it is.

On the other hand, the combustion air exchanges heat with a part of the combustion gas exhausted through the exhaust pipe 5 of the tubular heating furnace 1 through the heat exchanger 4 and is preheated by the sensible heat of the combustion gas. It In addition,
Reference numerals 7 and 8 are fans.

According to the tubular heating furnace configured as described above, a part of the combustion gas discharged from the furnace top of the tubular heating furnace 1 remains at the same temperature through the circulation path 3 and the diffusion burner 2 at the bottom of the furnace. Is recirculated to the combustion air supply pipe 6 connected to.
At the same time, the combustion air is preheated to a temperature commensurate with the thermal efficiency by exchanging heat with a part of the combustion gas that has not been branched to the outside of the furnace circulation path 3 side in the heat exchanger 4. The high temperature air and the combustion exhaust gas are mixed before being supplied to the burner and are supplied as high temperature diluted air diluted to a predetermined temperature and a predetermined oxygen concentration. For example, the hot dilution air is preheated to at least the blowout limit temperature, more preferably the autoignition temperature or higher, and has a much lower oxygen concentration than that of normal air, and more preferably 1 or more.
It is adjusted to 5% or less. Here, the hot dilution air is
For example, if preheated to 600 ° C. or higher, when the oxygen concentration is 15% or less, at which the volume of the flame remarkably increases, the blowout limit temperature is always exceeded and blowout does not occur.

The dilution air is supplied with a capacity that does not change the chemical equivalent ratio at the proper air ratio. Because hot diluted air has a lower oxygen concentration than normal air and is hot,
If the chemical equivalence ratio is not changed, the capacity of the oxidant increases considerably.

Therefore, the oxidation exothermic reaction gas that flows into each burner throat and mixes with the fuel injected from each fuel nozzle to generate is much larger than the conventional one and flows at a high speed. For example, as shown in FIG.
Compared to the case of diffusive combustion of the same fuel using combustion air having an oxygen concentration of 21% and a temperature of 50 ° C. (flame indicated by the solid line), diffusion combustion is performed using high temperature diluted air having an oxygen concentration of 3% and a temperature of 1010 ° C. In the case of making it (flame dampened by virtual line), the flame volume reached 20 times or more.

At this time, as shown in FIG. 2, the high-temperature diluted air and the fuel preheated to at least the blow-off limit temperature are diffusively mixed and combusted even if the oxygen concentration is lower than that of normal air (21%). When it enters the range, it starts combustion spontaneously with the help of the flame holding mechanism. However, since the oxygen concentration is much lower than that of normal air and the volume of dilution air is considerably large, the heat generation rate is sufficiently slower than that of normal combustion, accompanied by an oxidative exothermic reaction. And the combustion gas, which has a high flow velocity and continues to burn in a wide range, accelerates the flow velocity significantly in the furnace with many tubes and improves convective heat transfer,
While flowing in a wide area in the furnace, it continuously burns and heat is taken in the process of generating sensible heat to heat the fluid in the heated pipe. Furthermore, since the flow rate of hot gas uses high-temperature diluted air whose oxygen concentration is much lower than that of normal air, the flow rate of hot gas is much higher than during normal combustion, and the gas flow in the furnace becomes violent. Local temperature difference is eliminated by promoting and increasing convective heat transfer. Therefore, it is possible to heat the pipe to be heated by convective heat transfer and radiant heat transfer while generating sensible heat in almost the entire area of the furnace without creating a heat flux peak. When the volume as a combustion oxidant is increased without changing the air ratio to make the oxygen concentration much lower than that of normal air, and when the temperature is blown out and the temperature rises above the critical temperature, the oxidation exothermic reaction changes the normal air. The phenomenon of stable combustion occurs although it is much slower than when used.

On the other hand, the heat exchanger 4 connected to the exhaust system
In this, the heat of the combustion exhaust gas after being used for heating the boiler water, which is the fluid to be heated, is highly efficiently recovered to preheat the combustion air. At the same time, the combustion gas in the furnace is taken out to the external circulation system passage 3 by the negative pressure generated by the circulation fan 7, and the sensible heat of the exhaust gas is recovered by the highly efficient heat exchange to preheat the combustion air to a high temperature. It is mixed with and is injected again at high speed into the combustion chamber as diluted air having a predetermined oxygen concentration and temperature.

The above-described embodiment is an example of the preferred embodiment of the present invention, but the present invention is not limited to this, and various modifications can be made without departing from the gist of the present invention. For example, the maximum flame temperature may be further lowered by injecting steam or the like into the exhaust gas to be recirculated.

[0020]

As is clear from the above description, according to the tubular heating equipment of the present invention, a markedly increased flame can be formed in the furnace, which has a high flow velocity and continues to burn in a wide range. Peak does not occur. Therefore, the ratio between the maximum heat flux and the average heat flux of the heated tubes is extremely small and approaches 1 as shown in FIG. 4, and therefore the density of the number of the heated tubes, that is, the surface area of the heated tubes and the furnace volume It does not depend on the ratio, and the number of heated tubes installed in the furnace can be increased. Therefore, in order to perform the necessary heating, the volume of the furnace can be reduced and the number of tubes to be heated can be made dense, so that the furnace can be made compact. For example,
In the case of the Isoflo type tube type heating furnace, 3
Since it is possible to increase the number of tubes to be heated by arranging the tubes to be heated in rows or more, even if the tube heating furnace of the same size, the heat treatment amount increases dramatically, and in the case of the same treatment amount, The furnace can be made smaller.

Also, in the case of the convection heat transfer type tube heating furnace, the flame volume remarkably increases and the flow velocity of the gas passing around the heated tube increases, so that the amount of heat transfer by convection heat transfer increases. At the same time, while continuing to burn in a wide area, it flows around the heated pipe, so that it also receives radiant heat transfer. Therefore, the average heat flux of the heated tube can be increased, the heating efficiency can be improved, and the furnace can be made compact or the heat treatment time can be shortened.

Further, by making the temperature inside the furnace uniform, it is possible to reduce the thermal stress on the furnace structure and to make the heat flux to the tubes throughout the furnace uniform. Furthermore, since the oxidation exothermic reaction is sufficiently slow, combustion noise and oscillatory combustion can be suppressed.

Further, the heating method of the tubular heating furnace of the present invention is
The preheating temperature of the high-temperature diluted air is set to the self-ignition temperature of the fuel or higher, more preferably 600 ° C. or higher. In this case, when the fuel diffusely mixes and enters the combustible range, spontaneous combustion occurs, and more stable combustion occurs.

Further, the heating method of the tubular heating furnace of the present invention is
A part of the exhaust gas from the tube heating furnace is recirculated to the tube heating furnace and mixed with the combustion air that has been preheated to a high temperature by heat exchange with the exhaust gas that has not been recirculated to generate dilution air. I have to.

[Brief description of drawings]

FIG. 1 is a schematic view showing an example of a tubular heating furnace for carrying out a heating method of the present invention.

FIG. 2 is a graph showing the effects of combustion air temperature and oxygen concentration on stable combustion.

FIG. 3 is an explanatory diagram showing changes in flame volume when the oxygen concentration and temperature of combustion air are changed.

FIG. 4 is a graph showing the relationship between the ratio of the maximum heat flux and the average heat flux to the ratio of the surface area to be heated and the furnace volume.

[Explanation of symbols]

 1 Tube type heating furnace 2 Diffusion burner 3 External circulation system path 4 Heat exchanger 5 Exhaust pipe 6 Air supply pipe

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Jun Nishimura 1-1-1, Shibaura, Minato-ku, Tokyo Cosmo Stone Oil Co., Ltd. (72) Inventor Yoshio Moriguchi 1-1-1, Shibaura, Minato-ku, Tokyo Cosmo Stone Oil Co., Ltd.

Claims (4)

[Claims] 1. High-temperature diluted air which has a much lower oxygen concentration than ordinary air and is preheated to at least the blow-out limit temperature at that oxygen concentration by adding a part of combustion exhaust gas discharged from a tubular heating furnace. And fuel are diffused and burned in the tube heating furnace without changing the chemical equivalence ratio at the proper air ratio, and the hot gas flow is intermittently or continuously or periodically flowed around the heated tube to be heated. A method for heating a tubular heating furnace, comprising heating a fluid in a tube. 2. The heating method for a tubular heating furnace according to claim 1, wherein the preheating temperature of the high-temperature diluted air is equal to or higher than the self-ignition temperature of the fuel at the oxygen concentration thereof. 3. The heating method for a tubular heating furnace according to claim 1, wherein the high-temperature dilution air has a preheating temperature of 600 ° C. or higher and an oxygen concentration of 17% or lower. 4. A part of the exhaust gas from the tube-type heating furnace is recirculated to the tube-type heating furnace and mixed with combustion air that has been preheated to a high temperature by heat exchange with exhaust gas that has not been recirculated. The heating method for a tubular heating furnace according to claim 1, wherein the dilution air is generated.