JPS5813807B2 - Nitrogen oxide reduction method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a method for reducing nitrogen oxides Nox in combustion exhaust gas.

There are three types of nitrogen oxide reduction combustion methods (hereinafter also referred to as low NOx combustion methods), and these methods are currently being used alone or in combination to reduce NOx in combustion gas. .

(1) Over-fire air method (OFA method) In the combustion furnace 1 shown in Fig. 1, fuel is sent to the burner 4 through piping 3 and is ejected into the furnace, and combustion air A is sent from piping 2 to the burner section. It is divided into A2, which is sent to 2B, and A1, which is sent to the two people at the rear of the combustion zone.

In this way, the main combustion flame C becomes a low-temperature flame lacking air, suppressing NOx generation, and the unburned fuel is completely combusted in area 5 by air A1 from the two pipes, and is discharged through the flue 6. This reduces the amount of NOx in the exhaust gas G.

(2) Gas recirculation method (GR method) Fuel is sent to the combustion furnace 1 in Fig. 2 from the piping 3 to the burner 4.
When combustion is performed by the air A sent from the pipe 2 and a combustion flame C is formed, a part of the exhaust gas G flowing through the flue 6
A is blown into the furnace bottom 1a upstream of the combustion flame C by a recirculating gas fan 7 provided in the bypass W path 8.

By feeding this recirculated gas, the flame temperature is lowered and the NO
Suppress the occurrence of x.

(3) Recirculating gas mixing method (GM method) In the combustion furnace 1 shown in Fig. 3, the fuel sent from the pipe 3 is combusted in the furnace by the air A sent from the pipe 2.
A part of Ga of the exhaust gas G generated here is mixed into the air A from the pipe 2 by the recirculating gas fan 7.

The combustion flame C that burns due to the air Ag mixed with this recirculated gas is suppressed to a lower temperature than when combustion is performed only with air A not mixed with the recirculated gas.
The generation of Ox can also be suppressed.

however. The reduction level of NOx in the combustion zone 2 achieved by these methods is limited and cannot satisfy future circulation regulations, so the applicant has previously proposed an improved method as Japanese Patent Application No. 1983-23728. proposed an afterburning method.

The afterburner method is as follows.

(4) Afterburner method (Japanese Patent Application No. 51-23728) The fuel sent from fuel pipe 3 through burner 4 into combustion furnace 1 shown in Fig. 4 is normally combusted by air A sent from pipe 2. .

The exhaust gas G of the combustion flame C generated at this time is exhausted through the flue 6, and an afterburner 9 is provided in the middle of the flue 6 for the purpose of afterburning. Burning air Aa is pipe 2
From a, the air is divided into Aa2, which goes to the afterburner 9, and Aa1, which goes to the air blowing port 10.

The fuel for the AFNA burning is sent through the pipe 3a, and after completing primary combustion with the air Aa2, it undergoes secondary combustion with the oxygen in the exhaust gas G mixed with the air Aa1, and the combustion is completed.

To explain the effect at this time, the combustion flame C in the combustion furnace 1 can suppress NOx generation to the extent that it performs low excess oxygen combustion, but generally the residual oxygen is limited to 2 to 3%, and below this, CO generation or Look for smoke.

Therefore, part of the afterburning air Aa1 is mixed in advance with the remaining 022 to 3% exhaust gas G, and the afterburning air 9
A condition is created in which the combustion gas E can be combusted with a small air ratio (lower than the theoretical ratio).

The fuel and air A/a2 in the afterburner 9 are adjusted so that the combustion gas has an air ratio of 0.05 to 0.6, which is less than the theoretical air amount.

Note that when the air ratio is less than 0.05, CO generation or smoke occurs, and when it is more than 0.6, a sufficient NOx reduction effect cannot be obtained.

There are methods (1) to (4) mentioned above to reduce nitrogen oxide NOx in exhaust gas, but even in the afterburner method, which is said to have a particularly good denitrification rate, the method shown in Figure 5 is As shown in the experimental example shown, when the gas temperature is in the range of 200 to 600°C, the denitrification rate exceeds 90%, but CO and HC, which are harmful to the human body, are generated, and in the range of 800 to 1050°C, carbon
Although there is no generation of O and HC, the denitrification rate is about 50%.

In Fig. 5, the horizontal axis represents the gas temperature in the afterburning region, and the vertical axis represents the denitrification rate and the amount of CO and HC.
As a result of intensive research to find ways to reduce NOx, we discovered that it is possible to further reduce NOx by combining the afterburning method with the GR method, or further with the GM method, and based on this knowledge we have completed the present invention. Ta.

That is, the present invention has the following advantages: (1) Temperature 6 containing nitrogen oxides generated in the combustion system of a furnace
In combustion exhaust gas at a temperature of 50 to 1100°C, fuel is mixed at an air ratio of 0.
A method for reducing nitrogen oxides, characterized in that afterburning is performed under an air amount of 0.05 to 0.60, and a part of the afterburned exhaust gas is recirculated to the combustion system of the furnace via a recirculation path. .

(2) Temperature 6 containing nitrogen oxides generated in the combustion system of the furnace
In combustion exhaust gas at a temperature of 50 to 1100°C, fuel is mixed at an air ratio of 0.
In a method in which afterburning is performed under an air amount of 0.05 to 0.60, and a part of the afterburned exhaust gas is recirculated through the recirculation path of the combustion system of the furnace, the temperature in the recirculation path is 200 to 0.60. A method for reducing nitrogen oxides, characterized in that afterburning is further performed in the exhaust gas at a temperature of 600° C. under an air amount having an air ratio of 0.05 to 0.60.

It is related to.

The present invention can be applied to any boiler or general industrial heating furnace that uses gas fuel, liquid fuel, or solid fuel.

Hereinafter, the method for reducing nitrogen oxides of the present invention will be explained in detail with reference to FIG.

In Figure 6, 1 is a combustion furnace, 2 is a burner, 3 is a flue,
4 is a primary afterburner, and this afterburner 4
is provided in the middle of the flue 3 as detailed in Japanese Patent Application No. 51-23728.

Reference numeral 5 denotes a recirculation duct, which is a flow path branching from the downstream side of the afterburner 4 in the flue 3, and is installed downstream so as to configure a recirculation gas mixing method (GM method) and a gas recirculation method (GR method), respectively. It branches to form ducts 6 and 7.

A recirculation gas fan 8 is provided in the middle of the recirculation duct 5 to suck in exhaust gas from the flue 3 and blow it into the ducts 6 and 7.

9 is a secondary afterburner, which is connected to the recirculation duct 5.
It is provided upstream of the ducts 6 and 7 inside.

The meanings of the symbols enclosed in circles in the figure are as follows.

Go = Gas amount of main combustion exhaust gas GA: Gas amount of afterburner for main combustion gas GR: Recirculation gas amount GB: Gas amount of afterburner for recirculation gas No.: NOx concentration of main combustion exhaust gas NY: Combustion chamber outlet NOx concentration Nx : NOx concentration of exhaust gas released into the atmosphere NR: NOx concentration after afterburning of recirculated gas A: Air F: Fuel G: Exhaust gas C: Flame.

The exhaust gas from the flame C burned in the burner 2 generally contains 2 to 3% excess oxygen.

The NOx in this combustion exhaust gas is subjected to afterburning by a primary afterburner 4 provided in the flue 3 to reduce Nox.

The air ratio of this afterburner 4 is 0.05 to 0.60.

Some of the afterburned exhaust gas. It is recirculated through the recirculation duct 5 into the combustion furnace 1 via ducts 6 and 7.

The remainder of the exhaust gas is discharged to the atmosphere through the flue 3.

The temperature of the recirculated exhaust gas becomes 600° C. or lower, and the secondary afterburner 9 provided in the recirculation duct 5 burns the exhaust gas at a low temperature to further reduce NOx.

The combustion conditions at this time are the same as the first afterburning conditions, and the combustion is carried out under an air amount with an air ratio of 0.05 to 0.60.

Here, if the NOx reduction ratios of the primary afterburner 4 and the secondary afterburner 9 are η4 and η, then NY is determined by the following equation.

? Become.

-° Table 1 shows the NOx reduction ratio Nx/No obtained by substituting the numerical values obtained from the experimental results shown in FIG. 5 mentioned above into η4 and η.

As can be seen from the various NOx reduction measures in Table 1, the conventional afterburning method (■) alone had a NOx reduction effect of about 50%, but when combined with the recirculation method (◎), the NOx reduction effect was 33.3%. %.

Furthermore, by the method (2) of afterburning the recirculated exhaust gas with the secondary afterburner 9, the reduction rate is 25.6% or 17.2%.
It has become possible to reduce NOx up to %.

Therefore, it is possible to reduce NOx by % to % more than the afterburning method alone, which has conventionally been considered to have a good NOx reduction effect.

The fuel used in the afterburner is generally a hydrocarbon fuel, and the gas amount and distribution to the ducts 6 and 7 are arbitrary. or vice versa.

[Brief explanation of the drawing]

Figures 1 to 4 are explanatory diagrams of conventional nitrogen oxide reduction methods, and Figure 5 shows gas temperature and denitrification rate in the afterburning region. C
FIG. 6, a chart showing the relationship between O and HC amounts, is an explanatory diagram of the method of the present invention.

Claims (1)

[Claims] 1. Temperature 65 containing nitrogen oxides generated in the combustion system of the furnace
In combustion exhaust gas at 0 to 1100℃. Fuel to air ratio 0.0
A method for reducing nitrogen oxides, characterized in that afterburning is performed under an air volume of 5 to 0.60, and a part of the afterburned exhaust gas is recirculated to the combustion system of the furnace via a recirculation path. . 2 Temperature 65 including nitrogen oxides generated in the combustion system of the furnace
In the combustion exhaust gas at 0 to 1100℃, the fuel to air ratio is 0.0.
In a method in which afterburning is performed under an air amount of 5 to 0.60, and a part of the afterburned exhaust gas is recirculated through the recirculation path of the combustion system of the above-mentioned furnace, the temperature in the recirculation path is 200 to 200. A method for reducing nitrogen oxides, characterized in that afterburning is further performed in the exhaust gas at a temperature of 600° C. under an air amount having an air ratio of 0.05 to 0.60.