JPS5825926B2 - Method and device for reducing NOx in combustion exhaust gas of cement firing equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method and apparatus for reducing nitrogen oxides in the combustion exhaust gas of a cement firing device.

In general, seeding methods have been proposed to reduce nitrogen oxides (NOx) contained in the combustion exhaust gas of rotary kilns for firing cement that burn liquid and gaseous fuels.

First, the first method for cement firing is to reduce the amount of NOx generated during combustion.
In this method, a part of the combustion exhaust gas is circulated by taking advantage of the fact that lowering the combustion temperature and reducing the oxygen concentration are effective, or the combustion is carried out in two stages, and first part of the combustion exhaust gas is recirculated with oxygen deficient or low excess air. There are two-stage combustion methods, in which fuel is burned at a low oxygen concentration and then the remainder is burned at a low oxygen concentration, a method in which fuel is injected tangentially into the furnace, and a method in which split flames are created by improving the burner. It has been devised.

In addition, the second method is to use reducing gas under the catalyst.
There is a method of reducing Ox to harmless N2 gas.
In this method, for example, C01H2, NH3 is used as a reducing agent.
, H2S1CmH2n (C3H8, C2H6, CH4, etc.) are used, and various catalysts have been developed.

Furthermore, as a third method, NO is removed by oxide under a catalyst.
There is a method of oxidizing and absorbing and removing NO2 as NO2. For example, oxidation with ozone and a combination with a catalyst are being considered.

However, the method of suppressing NOx through the first combustion in cement firing described above does not produce NOx from the viewpoint of the combustion reaction mechanism.
Conventionally, the amount of x generated cannot be suppressed below a certain limit.

In other words, it is not possible to control the generation of NOx stoichiometrically due to the effects of many groups, ions, atomic oxygen, etc. produced during the reaction, and if you try to suppress the generation of NOx by keeping it in a reduced state, , soot, NH3, C081, and other harmful substances may be generated concurrently, and thermal efficiency may be reduced, so there is a limit to the reduction state.

In addition, in the second and third methods, the reaction rate is accelerated by a catalyst to make them practical, and the temperature range is often relatively low, using gas that has undergone heat exchange in a rotary kiln.

This is thought to be due to the fact that the reaction rate of the reduction reaction is fast at low temperatures, that it is economically advantageous to use the gas after heat exchange, and that it is also related to the activity of the catalyst.

However, in reality, the high temperature, dust content, and gas that contains elements harmful to catalysts in rotary kilns for cement firing cause great difficulties, making it difficult to put them into practical use for stable use over long periods of time. It has drawbacks such as being expensive and uneconomical.

Therefore, in order to eliminate these conventional drawbacks, this invention supplies fuel into the combustion exhaust gas from a rotary kiln for firing cement that has undergone heat exchange with the object to be heated, and combusts it to produce reducing gas. Cement calcination characterized by supplying oxygen-containing gas into the reducing gas stream to burn the unburned gas after it is made into an air stream, and discharging the combustion gas through a heat recovery device during or after the combustion. The main purpose of this invention is to provide a method and device for reducing nitrogen oxides in the combustion exhaust gas of the device.

Generally, when firing cement, the fired product is heated to over 1,450°C, so the temperature of the flame reaches over 2,000°C. Gases containing other elements generate a large amount of NOx;
It is extremely difficult to reduce the
It is extremely difficult to reduce NOx with a catalyst even if the gas is used at temperatures as low as 0.degree.

Therefore, as an alternative to this, it is necessary to provide conditions that facilitate the occurrence of a reduction reaction corresponding to the use of a catalyst.

As a measure for this, raising the temperature of the exhaust gas increases the activity of the molecules, making it easier to reduce depending on the conditions.

Furthermore, in order to reduce the probability of oxidation reaction occurring as a competitive reaction, the exhaust gas is placed in a situation with a strong oxygen deficiency, and in order to increase the activity of the molecules of the gas used for reduction, NOx is kept in a group or ionic state. It is preferable to contact with a gas containing.

After that, the hazardous combustible gas is combusted while selecting conditions that generate less NOx, and then combined with a heat recovery device to increase the thermal efficiency of the entire system, so that almost no NOx is generated. It has been found that less harmful components are generated, the thermal efficiency is high, and a more economical method can be obtained.

This invention will be explained in detail below with reference to its embodiments.

FIG. 1 is a schematic diagram of an apparatus for implementing the method of reducing nitrogen oxides, which are harmful components in combustion exhaust gas, according to the present invention.

First, in the method of the present invention, fuel is supplied to the rotary kiln 2 and burned.

In this rotary kiln 2, 02 in the exhaust gas is suppressed to 1% or less in order to reduce the amount of NOx generated in the combustion exhaust gas.

This is necessary for effective progress of the reduction reaction in the next reduction chamber 3 and for energy saving in the entire system.

That is, when oxygen is present, the reducing gas in the reduction chamber first reacts with oxygen, and this excess oxygen impedes the progress of the reduction reaction and also increases the amount of oxidized NOx. This results in a significant decrease in the power of the gas and a decrease in the reducing ability itself.

On the other hand, if there is little oxygen and a large amount of soot is generated due to the reduction state, it will not be completely burned in the next oxidation chamber, causing problems with the dust collection equipment and processing of recovered materials, and making the state of the oxidation chamber unstable. This may lead to operational hazards.

Therefore, in order to avoid such a situation, the air ratio in the rotary kiln 2 is kept within the range of 0.90 to 1.10,
Particularly suitable is 1.05.

If the air ratio is below 0.90, there will be a lack of oxygen and incomplete combustion will occur, producing a large amount of soot.
．． If it is 10 or more, the generation of NOx increases rapidly.

The combustion exhaust gas generated in the rotary kiln 2 is reduced in the next reduction chamber 3 by supplying new fuel.

In the case of heavy oil, the amount of fuel injected into the reduction chamber 3 must be at least 1.5 times, preferably at least twice, the amount required to react with the residual oxygen in the high-temperature exhaust gas at the outlet of the heating furnace. be.

The relationship between (amount of fuel added in the reduction chamber)/(amount of fuel equivalent to residual oxygen in the rotary kiln exhaust gas) and the amount of NOx remaining at the exit of the reduction chamber was determined by the results of an experiment using heavy oil.
As shown in the figure.

These experimental results vary somewhat depending on the type of rotary kiln, the NOx 102 in the exhaust gas at the rotary kiln outlet, and the type of fuel.

If this fuel amount ratio is less than 1.5 times, NOx will increase rapidly, and if it is more than 2 times, it will be in the incomplete combustion region, so it is inappropriate. range is preferred.

In addition, the reaction temperature in the reduction chamber 3 inevitably becomes high,
It has been found that a temperature of 500°C or higher, more preferably 700°C or higher, increases the reducing ability and is therefore preferable.

Next, this NOx reducing gas is supplied with insufficient oxygen in the oxidation chamber 4, and the unburned gas is combusted.

In order to reduce the NOx generated at this time, it is possible to control the temperature rise by adjusting the introduction rate of oxygen supplied to the oxidation chamber 4, or to suppress the excessive rise in gas temperature by combining an appropriate heat recovery device. suitable.

In this way, the gas in which unburned substances are combusted in the oxidation chamber 4 is
For example, heat is recovered through a heat exchanger with the raw material powder or through the heat recovery device 5, and the thermal efficiency of the entire system is improved.

Further, the oxidation chamber 4 can also be used as a duct or this heat recovery device 5.

In this way, the nitrogen oxides contained in the combustion exhaust gas from the rotary kiln, which are harmful components, are suitably reduced in the reduction chamber, oxidation chamber, and heat recovery device, and the combustion exhaust gas is made into clean exhaust gas.

The above-mentioned method of the present invention is particularly suitable for the treatment of combustion exhaust gas from cement, coal, iron ore pellets, etc. using a rotary kiln. It can also be applied to glass and refractory kilns.

Next, several embodiments implementing the method of the present invention will be described.

3 to 6 show examples in which the method of the present invention is used to treat combustion exhaust gas from a rotary kiln of a cement firing apparatus.

Fig. 3 shows an example in which the present invention is applied to a so-called suspension preheater with an auxiliary burner, which has a combustion device in the preheater. A reduction chamber 13 is formed, and a fuel supply nozzle 21 is provided in this reduction chamber 13.
New fuel is supplied from the rotary kiln 12 and burned, and NOx contained in the combustion exhaust gas from the rotary kiln 12 is reduced.

The fuel combusted in the reduction chamber 13 is approximately 10% of the fuel combusted in the preheater section.

The upper duct part following the reduction chamber 13 is used as the oxidation chamber 14, and air for re-combustion is supplied from which air supply ports 22 at 2 to 4 locations around the duct, and the combustion furnace 18 is fully combusted. Heat exchange is performed with raw materials supplied from

The exhaust gas containing the heat-exchanged raw material is introduced into the cyclone 15, and the raw material is separated from the exhaust gas and supplied to the tip of the rotary kiln 12 to be fired.

The exhaust gas that has been heat exchanged with the raw material and separated in the cyclone 15 is led to another cyclone 16 through a duct 20.

Raw materials are supplied from the raw material supply port 19 in the middle of the duct 20, and heat exchange of the raw materials is performed.

The raw material is separated from the exhaust gas in this cyclone 16 and supplied to the combustion furnace 18.

Exhaust gas from the cyclone 16 is discharged into the atmosphere by a blower 24.

In this way, the combustion exhaust gas from the rotary kiln 12 is used to preheat and calcinate the raw material in the preheater section, and is also contained in the exhaust gas after passing through the reduction chamber 13, the oxidation chamber 14, and the cyclone 15, which is a heat recovery device. Nitrogen oxides, which are harmful components, are reduced.

In addition, in such a cement firing apparatus, approximately 40% of the heat used in the entire system is consumed by the rotary kiln, 55% is consumed by the combustion furnace 18 of the preheater, and 5% is consumed by the reduction chamber 13 of the riser duct. is burned.

Further, in the rotary kiln 12 and the combustion furnace 18, combustion is performed at an air ratio in the range of 0.95 to 1.0, and the reduction chamber 13
The reduction reaction is carried out effectively.

FIG. 4 shows an example in which the method of the present invention is applied to a conventional suspension preheater. In this example, the fuel distribution is approximately 95% in the rotary kiln 12 and approximately 5% in the reduction chamber 13.
This is the percentage of

This example differs from the previous example only in that there is no combustion furnace for preheating and calcination of the raw material, and the combustion exhaust gas from the rotary kiln 12 is passed through a plurality of fuel nozzles 21 at the inlet in the reduction chamber 13. The NOx in the combustion exhaust gas is uniformly mixed with the fuel sprayed from the combustion exhaust gas, and NOx in the combustion exhaust gas is reduced in the reduction chamber 13.

The exhaust gas is then led to an oxidation chamber 14 following the reduction chamber 13,
The air supplied from the air supply port 22 is sufficiently re-combusted and cycled.

Heat exchange is performed with the raw material introduced from the tube 16', and the material is introduced to the suicron 15.

The raw material is separated from the exhaust gas in this cyclone 15,
It is guided to the tip of the rotary kiln 12 to be fired.

The exhaust gas from which the raw materials have been separated is then guided through a duct 20 to a cyclone 16'.

A raw material is supplied from the raw material supply port 19 in the middle of this duct 20, and heat exchange is performed.

The raw material is separated from the exhaust gas in the cyclone 16' and guided to the oxidation chamber 14.

In such an example, it was found that the 300 to 400 ppm NOx measured at the inlet of the reduction chamber 13 was reduced to 1/6 at the outlet of the reduction chamber 13.

5 and 6 show an example in which the method of the present invention is applied to a dry boiler type cement firing apparatus.

In the example shown in FIG. 5, the combustion exhaust gas from the rotary kiln 32 is supplied with fuel from the fuel supply nozzle 38 in the reduction chamber 33 provided in the middle of the flue leading to the residual heat recovery boiler 35, and the NOx in the exhaust gas is will be returned.

Next, in the oxidation chamber 34, the air supply port 39 around the duct
Air is supplied from the exhaust gas, and the unburned portion of the exhaust gas is combusted.

Heat is recovered in a residual heat boiler 35 following the oxidation chamber 34, and can be used for power generation.

The amount of fuel burned in the reduction chamber 33 is about 5% of the total system.

When the amount of power generated by using the residual heat boiler exceeds the amount of power used in the factory, a method of adding one cyclone 36 for heat recovery can be used as shown in FIG.

In addition, in the examples shown in Figures 3, 4, and 6 above, the cyclone connected to the oxidation chamber collects a considerable amount of soot generated by incomplete combustion and sends it to the lorry kiln together with the raw materials. This has the advantage of eliminating the problem of soot flying out of the chimney.

As described above, by using the method and apparatus of the present invention, it is possible to greatly reduce NOx, which is a harmful component contained in combustion exhaust gas, to solve the pollution problem, and to make full and effective use of residual heat in the exhaust gas. be able to.

[Brief explanation of drawings]

Fig. 1 is a schematic diagram of an apparatus for carrying out the method of the present invention, and Fig. 2 shows the amount of fuel equivalent to the residual oxygen in the rotary kiln exhaust gas with respect to the amount of fuel added in the reduction chamber, and the amount of N at the outlet of the reduction chamber.
Graphs showing the relationship with the remaining amount of Ox, and FIGS. 3 to 6 are schematic diagrams showing examples in which the method of the present invention is implemented. In the diagram, 2°12.32: rotary kiln, 3.13,3
3: Reduction chamber, 4, 14, 34: Oxidation chamber, 5: Heat recovery device, 15.16.16': Cyclone, 19: Raw material supply port, 21.38: Fuel supply nozzle, 22° 39: Air supply port , 35: Residual heat boiler.

Claims (1)

[Claims] 1. Burning fuel at an air ratio of 0.90 to 1.10 in a rotary kiln equipped with a suspension preheater, and guiding the combustion exhaust gas to a reduction chamber following the rotary kiln,
In this reduction chamber, new fuel is added at least 1.5 times the amount of fuel that reacts with the residual oxygen in the combustion exhaust gas to maintain the temperature in the reduction chamber at 500°C or higher and reduce NOx in the combustion exhaust gas. A cement product that consists of introducing the reduction chamber into an oxidation chamber, mixing gas containing oxygen for re-combustion, combusting the unburned gas, and recovering the heat from the combustion exhaust gas after combustion and discharging it through a heat recovery device. NO in combustion exhaust gas from firing equipment
How to reduce x. 2. A rotary kiln that burns fuel to generate the main amount of heat, a reduction chamber that is formed in a rising duct at the tip of the rotary kiln and has a means for adding new fuel, and that reduces NOx in the rotary kiln combustion exhaust gas. An oxidizing chamber is formed in the upper duct portion following the chamber and has a means for introducing gas for re-combustion containing oxygen and burns the unburned portion of the combustion exhaust gas. A device for reducing NOx in combustion exhaust gas of a cement firing device, comprising a heat recovery device for recovering heat.