JPS5913245B2 - Method for reducing nitrogen oxides in exhaust gas - Google Patents

Description

[Detailed Description of the Invention] As is well known, methods for reducing nitrogen oxides (NoX) in combustion exhaust gas from boilers, etc. can be roughly divided into 1) reduction methods by improving combustion methods, 2) in-furnace high-temperature denitrification methods, Methods such as 3) dry catalytic denitrification method and 4) wet absorption treatment method are currently under development and research in various fields, but none of these methods have any problems in terms of economic efficiency, denitrification performance, operational stability, etc. I can not say.

According to the above classification, the present invention provides 1) improvement of combustion method, and 2.)
This method is a combination with the in-furnace high-temperature denitrification method, and provides a simple and effective method for highly reducing NOx in exhaust gas.

The following description will be made with reference to FIG. 1, which is a flow sheet showing a specific embodiment of the present invention.

In FIG. 1, numeral 1 denotes a normal boiler for power generation or steam generation, including a furnace 1a and a heat exchanger 1b.

1c, an air preheater 2, and a chimney 3, 4 is a supply line for combustion air, 5 is a fuel supply line, and 6 is a line for feeding 5 to 10 units of main fuel.

Nitrogen oxides (NOx, mainly composed of NO and including a small amount of N02) generated in the furnace 1a are the main cause of air pollution, especially photochemical smog, and are It is necessary to make it non-polluting and remove it in some way.

In the present invention, first, petroleum-based fuel is injected from line 6 to reduce NOx in the exhaust gas to N2 so that the fuel is in excess of the residual oxygen (usually 1 to 2 volume percent) in the exhaust gas after main combustion in the furnace 1a. Alternatively, it is converted to HCN, NH3, etc.

Here, the petroleum-based fuel is methane that combines with oxygen to generate reaction heat at 1,000 to 1,500°C.

Alcohols, including ethane, propane, and kerosene.

Any fuel that is commonly used as a fuel, such as aldehydes, or has the potential to be used as a fuel, may be used.

In this way, the number 80 to 95 of NOx generated in the furnace 1a is N
2. It is converted into HCN, NH3, etc., but the exhaust gas in this state contains a considerable amount of unburned substances such as carbon monoxide and hydrocarbons, and in order to completely burn them out,
Furthermore, oxygen (practically air) is present in a high temperature range (90°C).
(0°C or higher), and the residual oxygen concentration must be at least 0.5 to 2% by volume.

However, with such normal input, this results in HCN, N
There is a problem in that nitrogen compounds such as H3 are converted back to NOx, and the overall denitrification rate remains at about 40 to 50 points.

Therefore, first, before adding normal oxygen,
A very small amount of air or combustion exhaust gas (usually oxygen content: 1 to 3 percent by volume) is injected into the temperature range of ℃, and while there is still excess fuel, HCN and NH are produced by the effect of adding a small amount of oxygen.
Most of 3 is N.

After converting a small amount to NO, the unburned air is removed from the air in its wake, and normal oxygen is introduced so that the residual oxygen concentration is 20.5 to 3 percent by volume.

Although the overall denitrification rate has been improved to well over 70 to 75 by improving the combustion method in this way, it is still insufficient to meet the demand for suppressing nitrogen oxides in exhaust gas as much as possible.

In addition to improving the combustion method, the present inventors were also separately developing a denitrification method in which a chemical such as ammonia was introduced into the high-temperature exhaust gas.

While the present inventors are earnestly conducting research on more advanced denitrification methods, the downstream temperature of the above-mentioned improved combustion method was
Adding 0.5 to 3 times the mole of ammonia to the residual NOx in the temperature range (residual oxygen concentration 0.5 to 3 volume percent), that is, a combination of improving the combustion method and in-furnace high temperature denitrification method. It was discovered that an even higher overall denitrification rate could be obtained due to the synergistic effect of the above (see Examples below), leading to the present invention.

This can be understood as follows.

In other words, NH3°HCN, which is an intermediate product of the reaction in the combustion method improvement method mentioned above, and unburned matter are mixed with ■ and ■ during the two-stage oxygen injection process.
Depending on the formula, the 0 formula is given priority in decomposition, but
Small amounts of HC in areas where reaction time is not sufficient.

Unexpected amounts such as CO remain.

These contribute to formulas ■ and ■ in the next NH3 injection process,
This improves the denitrification rate compared to the case where NH3 is added alone in a region where unburned matter does not coexist (conventional non-catalytic NH3 addition denitrification method).

In this way, the effects of the combination of the improvement of the combustion method and the in-furnace high-temperature non-catalytic HN3 addition denitrification method will be realized.

NH3, HCN+02 → NOx, H2O, etc. ■NH3
, HCN+02→N2, H2O, etc. ■4NO+4
NH3+02→4N2+6H20■NO+2HCN+2
02→3/2N2+H20+2CO2■ In Figure 1, 9 is a fan for exhaust gas recirculation, 10 is a line that supplies the exhaust gas into the furnace, 7 is an air input line to remove residual unburned matter, and 8 is a residual NO 0.5-3 for
This is a double molar NH3 input line.

After leaving the furnace, the clean exhaust gas from which NOx and unburned components have been removed passes through the air preheater 2 and the chimney 3 and is released into the atmosphere.

Such a method, that is, the method of the present invention based on a combination of the combustion method improvement method and the in-furnace high-temperature ammonia addition denitrification method, has a denitrification rate of each alone, the former near 0 factor, the latter 40 to 70 L.
Compared to the simple multiplication of 1), the denitrification rate is 82-91, the synergistic effect is added, and the improvement of the combustion method and the in-furnace high-temperature ammonia addition denitrification method (NH
3/NOx molar ratio = 1.0 and 2.0).
When the molar ratio = 1.0, the coefficient is 85, and when the NH3/NOx molar ratio is 20, the coefficient is 96 (curve D in Figure 3, curve E))
It is effective and industrially useful as a simple and low-cost advanced denitrification method.

It goes without saying that the method of the present invention can be equally effectively applied to general combustion equipment, such as coal and gas combustion equipment, rubber incineration equipment, pulp recovery boilers, and the like.

Example: A test furnace as shown in Fig. 2 (inner diameter of flue 80 degrees, exhaust gas amount 1
．． 000 N m/Hr).

In Fig. 2, 101 is C heavy oil for fuel, 102 is a combustion air supply line, 103 is a combustion furnace, 104 is petroleum fuel (propane gas was used in this test), 105
106 is air, 107 is an ammonia supply line, and 109 is a chimney.

Exhaust gas measurement was performed at the flue 108, and the residual oxygen concentration at this point was set to be 3.0 in all experiments.

In addition, the oxygen concentration after main combustion in the furnace 103 is 1.0%,
The amount of propane supplied from line 104 is based on the heat of combustion.
10 of the main fuel supply amount from line 101.

When no supply is performed from lines 104, 105, 106, and 107, that is, when only main combustion is performed, the NOx value is 1.
NOx when propane is supplied from line 104, a small amount of air (0.01 to 0.5 times the oxygen concentration in the total exhaust gas) is supplied from line 105, and a large amount of air is supplied from line 106. The value is 44. llI)m.

Also line 104. When air is supplied from line 106 and ammonia is supplied from line 107 at a molar ratio (NH3/N0x) of 2 without any supply from '105, the NOx value is 45. It was ρ4.

In addition to the supplies from these lines 104 and 106, the results when a small amount of oxygen is supplied from line 105 and ammonia from line 107, which is the main focus of the present invention, are shown in the third table.
As shown in the figure.

The exhaust gas temperatures at the supply points from lines 104, 105, 106 and 107 are 1300°C, 1100°C, respectively.
1000℃, 950℃, in both cases point 107
No unburned substances such as CO2NH3 were observed.

In Figure 3, curve A) is an experiment in a denitrification method using an improved combustion method in which petroleum-based fuel is added to high-temperature exhaust gas after combustion, and oxygen is added in two stages downstream to eliminate unburned matter. The results are as follows: Points B) and 1C) normally contain NH in the exhaust gas after combustion.
In the experimental results of the in-furnace high-temperature non-catalytic NH3 addition denitrification method, point B) is when the NH3/NOx molar ratio is 1.0, and point C) is when the NH3/NOx molar ratio is 2.0. It is.

In addition, curves B/) and σ) are the results of the calculated NOx removal rate calculated using the formula (■) for the combined effect of both NOx removal methods, and curve D
), E) adds petroleum-based fuel to high-temperature exhaust gas after combustion,
Curve D) is the case when the NH3/NOx molar ratio is 1.0. This is an experimental result of a combination of an improved combustion method and a high-temperature non-catalytic denitrification method, in which NH3 is added after a small amount of oxygen is introduced in the downstream stream. , curve E) when the NH3/NOx molar ratio is 2.
This is the case of 0.

In this way, after supplying the gas so that the amount of oxygen in the gas supplied from line 105 is 0.5% or less with respect to the total amount of exhaust gas,
It can be seen that the combination of both denitrification methods, in which a large amount of air is introduced through line 106 and NH3 is introduced from its downstream side (line 107), is effective and a higher denitrification rate can be obtained.

[Brief explanation of drawings]

FIG. 1 is a flow sheet showing a specific embodiment of the present invention. Figure 2 is an apparatus flow sheet for an experiment demonstrating the usefulness of the present invention, and Figure 3 is an example of experimental results obtained using the apparatus shown in Figure 2.

Claims (1)

[Claims] 1 Petroleum-based fuel is added to the high temperature region where the exhaust gas temperature after combustion is 1000℃ to 1500℃, and the temperature of the exhaust gas after combustion is 900℃.
A two-stage oxygen process in which a small amount of flue gas and/or air is added to the temperature range from 1200°C to 0.5 volume percent or less to reduce the oxygen concentration to 0.5 volume percent or less, and then sufficient air is added to remove unburned substances. A method for reducing nitrogen oxides in exhaust gas, comprising an addition step and a further step of adding ammonia downstream thereof in a temperature range of 900° C. to 1100° C.