JPH06100427B2 - NOx reduction method for plasma heating furnace - Google Patents

Description

The present invention relates to a method for reducing NO _{x in a} plasma heating furnace, and
The present invention relates to a method for reducing NO _x generated when plasma using air as a working gas is used, which is one of the methods in a field requiring an operation such as melting.

[Prior Art] Conventionally, a plasma jet that performs uniform heating by a stable long arc has been put into practical use as a method of heating and melting operation. From the viewpoint of versatility, air is often used as the plasma gas for the plasma jet. in this case,
A large amount of NO _x is generated in a high-temperature and high-activity plasma arc, and exhaust gas treatment becomes a problem.

As a method for reducing NO _x in exhaust gas, catalytic reduction decomposition with a catalyst is generally used, but deterioration of the catalyst due to scattered dust is a problem. On the other hand, in a plasma heating furnace, a reducing substance such as coke is supplied into the furnace without using a catalyst to make the atmosphere in the furnace reducible.
A method for reducing NO _x has been developed (Japanese Patent Application No. 63-27319).
3).

[Problems to be Solved by the Invention]

In this case, when the inside of the furnace is not heated uniformly, that is, for several hours after the start of operation, it may not be possible to form a sufficient reducing atmosphere because the reaction region of gasification of the reducing substance is narrow. It may be difficult to reduce NO _x .

The present invention relates to a plasma heating furnace, and an object thereof is to provide a method capable of reducing NO _x even in such a case.

[Means for Solving the Problems]

The present invention, in heating using a plasma jet using air as a working gas, supplies a reducing substance, for example, coke into the furnace, and discharges exhaust gas from the heating furnace into the furnace until the inside of the furnace is uniformly heated. A method for reducing NO _{x in} a plasma heating furnace, which is characterized by refluxing and reacting with a reducing substance to generate CO gas by the reaction of CO ₂ + C ₂ CO, and reducing NO _x while supplementing insufficient CO gas. .

When the gasification reaction of the reducing substance such as coke is insufficient, the CO concentration in the plasma heating furnace is low and the CO ₂ concentration is high. Therefore, high-temperature exhaust gas containing CO ₂ is recirculated into the furnace and reacted with coke to supplement the deficient CO by the reaction of CO ₂ + C → 2CO, thereby stably reducing NO _x .

[Action]

FIG. 1 and FIG. 2 show examples of the apparatus used for carrying out the method of the present invention. The heating furnace 1 is provided with a heating plasma torch 2, and the exhaust gas of the heating furnace 1 is attracted to the blower 7 via the dust collector 5 and the cooler 6 and is discharged as flares 8.

As shown in FIGS. 1 and 2, part of the exhaust gas is blower 3
Then, the gas is recirculated into the furnace 1 from the side of the furnace through the valve 4. The recirculated exhaust gas efficiently contacts the reducing substance 10, for example, coke supplied to the furnace 1 while forming a swirling flow.

At that time, CO ₂ is generated by the reaction of CO ₂ + C → 2CO (1), and the furnace is kept in a reducing atmosphere.

In an actual plant, the CO concentration in exhaust gas is constantly detected by a CO detector 9 as shown in FIG. 1, and automatic control is performed to open the exhaust gas recirculation valve 4 when the CO concentration decreases.

The reaction of the above formula (1) is called Boudouard reaction and is general in the field of pig iron making. In the blast furnace, the hot air blown from the tuyere reacts with coke to cause combustion of C + O ₂ → CO ₂ , and this CO ₂ forms a reducing atmosphere in the furnace by the reaction of the above formula (1).

Now, regarding the reaction of the above equation (1), considering thermodynamic equilibrium, Is.

The partial pressure of CO _{2 in} the gas supplied here is set to 15% (from the exhaust gas analysis value), and the CO partial pressure due to this reaction is plotted against temperature, as shown in FIG.

It can be seen from Fig. 3 that the CO partial pressure increases from about 600 ° C, and that at 800 ° C, almost all the CO ₂ completes the reaction. Therefore, a sufficient CO partial pressure can be obtained at about the exhaust gas temperature without raising the furnace temperature to 1200 to 1300 ° C.

〔Example〕

In a plasma melting experimental furnace as shown in FIG.
After preheating with the LPG burner for half an hour, the plasma was started after the temperature in the furnace reached 450 ° C. Plasma torch output 150K
W, plasma gas 20 m ³ / h, coke supply 2 kg / 20 minutes (6 kg
/ h) and the exhaust gas recirculation valve opening was 50%. FIG. 4 is a time chart of this experiment, and the horizontal axis shows the elapsed time. As shown in the progress of FIG. 4, the NO _x concentration 60 minutes after plasma startup was 100 ppm or less.

For comparison, in the closed state of the exhaust gas recirculation valve 4, was subjected to the same experiment, it drops below finally NO _x 100 ppm 3 hours after the plasma start.

〔The invention's effect〕

In the plasma heating furnace, even before the temperature inside the furnace is raised uniformly, the exhaust gas is recirculated into the furnace to effectively gasify the reducing substances, such as coke, etc. Reduces NO _x generated,
The reduction can be achieved.

[Brief description of drawings]

1 and 2 are system diagrams of the examples of the present invention, FIG. 3 is a graph showing the equilibrium of the Bouder reaction, and FIG. 4 is a time chart showing the progress of the examples. 1 ... Furnace, 2 ... Plasma torch, 3 ... Blower, 4 ... Valve, 5 ... Dust collector, 6 ... Cooler, 7 ... Blower, 8 ... Flare, 9 ... CO detector, 10 ... Reducing property material

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-61-231314 (JP, A) JP-A-62-190317 (JP, A)

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 exhaust gas from the heating furnace is recirculated into the furnace until the inside of the furnace is uniformly heated. is reacted with a reducing substance _{Te, CO 2 + C → 2CO NO} x reduction method of a plasma furnace, characterized in that to reduce NO _x while replenishing the CO gas quantity is insufficient to produce a CO gas by the reaction.