JPH02192591A - Reduction of nox in plasma heating furnace - Google Patents

Abstract

PURPOSE:To reduce NOx in a plasma heating furnace by a method wherein reducing substance is supplied into the furnace and the exhaust gas of the heating furnace is flow into the furnace to react with the reducing substance until the inside of the furnace is heated to a uniform temperature. CONSTITUTION:A heating furnace 1 is provided with a plasma torch 2 for heating and the exhaust gas of the heating furnace 1 is induced by a blower 7 through a dust collector 5 and a cooler 6, then, is discharged as flare 8. One part of the exhaust gas is reflow into the furnace 1 by a blower 3 from the side part of the furnace through a valve 4. The flow exhaust gas contacts efficiently with reducing substance 10 or coke, for example, supplied into the furnace 1, while forming whirling stream. In this case, CO is generated and the inside of the furnace is kept in a reducing atmosphere. In an actual plant, the concentration of CO in exhaust gas is detected at all times by a CO detector 9 and the like and the exhaust gas flux valve 4 is opened when the concentration of CO is reduced.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for reducing NOx in a plasma heating furnace, and is used in a field that requires operations such as heating and melting, in which air is used as a working gas. The present invention relates to a method for reducing NOx generated when plasma is used.

[Prior Art] Conventionally, as a method for heating and melting operations, a plasma jet that performs uniform heating using a stable long arc has been put into practical use. From the viewpoint of versatility, air is often used as the plasma gas in this plasma jet. In this case, a large amount of NOx is generated within the high temperature and highly active plasma arc, and exhaust gas treatment becomes a problem.

Catalytic reductive decomposition using a catalyst is a common method for reducing N0% in exhaust gas, but deterioration of the catalyst due to scattered dust and the like is a problem. On the other hand, a method has been developed for plasma heating furnaces to reduce NOx by supplying a reducing substance such as coke into the furnace and making the atmosphere inside the furnace reducing without using a catalyst ( Special request 1986
-273193).

[Problem to be solved by the invention] In this case, when the temperature inside the furnace is not uniformly raised, that is, for several hours after the start of operation, the reaction area for gasifying the reducing substance is narrow, so it is difficult to create a sufficient reducing atmosphere. In some cases, it is difficult to reduce NOx.

The present invention relates to a plasma heating furnace, and an object of the present invention is to provide a method that can reduce NOx even in such a case.

[Means to solve the problem]

In heating using a plasma jet using air as a working gas, the present invention supplies a reducing substance, such as coke, into the furnace and, at the same time, pumps the exhaust gas from the heating furnace into the furnace until the inside of the furnace is evenly heated. This method of reducing NOx in a plasma heating furnace is characterized by reducing NOx by refluxing and reacting with a reducing substance to generate CO gas.

When the gasification reaction of reducing substances, such as coke, is insufficient, the CO concentration in the plasma heating furnace is low and CO21
1 degree is high. Therefore, high-temperature exhaust gas containing CO2 is returned to the furnace and reacted with coke to compensate for the lack of CO through the reaction of CO2+C-2GO, thereby stably reducing NOx.

[Function] FIGS. 1 and 2 show an example of an apparatus used to carry out the method of the present invention. The heating furnace 1 is equipped with a heating plasma torch 2, and the exhaust gas from the heating furnace 1 is collected through a dust collector 5. It passes through the cooler 6, is attracted to the blower 7, and is discharged as a flare 8.

As shown in Figures 1 and 2, part of the exhaust gas is transferred to the blower 3.
Then, the power from the furnace side is refluxed into Class 1 through valve 4. The refluxed exhaust gas efficiently contacts the reducing substance 10, such as coke, supplied to the furnace 1 while forming a swirling flow.

At that time, due to the reaction of CO2+C+2CO...(1),
generates co and maintains a reducing atmosphere in the furnace.

In an actual plant, as shown in Figure 1, the COa concentration in the wandering gas is constantly detected by a CO detector 9, etc.
Automatic control is performed to open the exhaust gas recirculation valve 4 when the concentration decreases.

The reaction of formula (1) above is expressed by Bouder ([3oudouard
This reaction is called 1 reaction and is common in the iron making field. In the blast furnace, the hot air blown from the tuyeres reacts with coke, producing C102
→ Combustion of CO2 occurs, and then this CO2 forms a reducing atmosphere in the furnace through the reaction of equation (1) above.

Now, regarding the reaction of the above formula (1), considering thermodynamic equilibrium, ΔG=40800−41.77.

Assuming that the partial pressure of CO2 in the gas supplied here is 15% (based on the exhaust gas analysis value), the 00 partial pressure caused by this reaction is plotted against temperature as shown in FIG.

From Figure 3, the 00 partial pressure increases from around 600℃ and reaches 800℃.
It can be seen that at ℃, CO2 almost completes the reaction. Therefore, a sufficient 00 partial pressure can be obtained at about the exhaust gas temperature without raising the furnace temperature to 1200 to 1300°C.

[Example 1] In a plasma melting experimental reactor as shown in Fig. 2, approximately 2
After preheating with LPG burner for an hour and a half, the temperature inside the furnace was 450℃.
After reaching ℃, the plasma was started. The experiment was conducted under the following conditions: plasma torch output of 150 KW, plasma gas of 2 OrIi''/h, coke supply rate of 6 kg/h, and exhaust gas recirculation valve opening degree of 50%.

FIG. 4 is a time chart of this experiment. As shown in FIG. 4, the NOX concentration 60 minutes after starting the plasma was 1100 pp or less.

For comparison, the same experiment was conducted with the exhaust gas recirculation valve 4 closed, and the amount decreased to <NNOx100 pp or less 3 hours after starting the plasma.

[Effect of the invention] In a plasma heating furnace, even before the temperature inside the furnace rises uniformly, by circulating the exhaust gas into the furnace, the reducing substances being supplied, such as coke, can be effectively gasified. Thus, NOx generated in the plasma can be reduced and reduced.

[Brief explanation of the drawing]

FIGS. 1 and 2 are system diagrams of examples of the present invention, FIG. 3 is a graph showing the equilibrium of the Buder reaction, and FIG. 4 is a time chart showing the progress of the examples.

Claims (1)

[Claims] 1. In heating using a plasma jet using air as a working gas, a reducing substance is supplied into the furnace, and the exhaust gas of the heating furnace is not allowed to flow into the furnace until the temperature inside the furnace is uniformly raised. A method for reducing NO_x in a plasma heating furnace, characterized by reducing NO_x by refluxing and reacting with a reducing substance to generate CO gas.