JP2001241619A - Method for purifying flue gas in radiant tube burner - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for purifying flue gas at a high purification rate without carrying out a strict control and without reducing efficiency in removing nitrogen oxides in the flue gas in radiant tube burner combustion. SOLUTION: The combustion flue gas of a radiant tube burner is brought into contact with a catalyst for reduction of nitrogen oxides, air is added to the flue gas after the contact, and further the gas is brought into contact with an oxidation catalyst.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for removing nitrogen oxides and unburned components in exhaust gas from a radiant tube burner.

[0002]

2. Description of the Related Art Nitrogen oxides (NOx) are harmful to human health and also cause natural environmental problems such as acid rain. As a countermeasure, a so-called three-way catalyst is widely used for automobiles, and an ammonia selective reduction method is widely used for power generation facilities and the like.

[0003] However, the former is applicable only to exhaust gas having a stoichiometric air-fuel ratio in which excess oxygen does not remain in the exhaust gas, and the latter has a concern that toxic and strongly odorous ammonia is discharged.

[0004] On the other hand, efforts to suppress nitrogen oxides in radiant tube burner combustion exhaust gas have been made.
Most of the measures to improve the burner itself and to reduce the NOx itself in the combustion exhaust gas, such as the exhaust gas recirculation method.

On the other hand, a method of installing a NOx selective reduction catalyst using hydrocarbons in a flue gas of a flue gas and adding natural gas as a reducing agent, and a strict control of air ratio by installing a three-way catalyst in a flue gas of a flue gas. It is also working on how to do it.

In order to obtain a high NOx removal rate using a NOx selective reduction catalyst, a large amount of natural gas needs to be added, so that there is a disadvantage that the efficiency of the system is reduced. In the method using a three-way catalyst, strict air ratio control is required, so that additional equipment such as an oxygen sensor and a control valve is required, and there is a problem that initial costs increase.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of such circumstances, and purifies a radiant tube burner combustion exhaust gas with high efficiency without performing complicated control such as sensing with an oxygen sensor. It is intended to provide a method capable of doing so.

[0008]

Means for Solving the Problems As a result of intensive studies, the inventor has conducted strict air ratio control using an oxygen sensor or the like by bringing the combustion exhaust gas of a radiant tube burner into contact with a specific catalyst. At the very least, they found that the exhaust gas from the radiant tube burner could be purified with high efficiency.

That is, the present invention relates to the following radiant tube burner combustion exhaust gas purifying method. 1. It purifies the exhaust gas from the radiant tube burner using a nitrogen oxide (NOx) reduction catalyst, adds air to the obtained NOx purification gas, and oxidizes and removes unburned components using an oxidation catalyst. Radiant tube burner combustion exhaust gas purification method. 2. The air ratio of the supply gas to the radiant tube burner is less than 1.0, and the combustion exhaust gas from the radiant tube burner is purified using a nitrogen oxide (NOx) reduction catalyst,
A method for purifying a radiant tube burner combustion exhaust gas, comprising adding air to the obtained NOx purification gas so that the air ratio becomes 1.0 or more, and further oxidizing and removing unburned components using an oxidation catalyst.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The radiant tube burner flue gas purifying method according to the present invention purifies the flue gas from the radiant tube burner using a nitrogen oxide (NOx) reduction catalyst, and converts the obtained NOx purified gas into air. And add
Further, it is characterized in that unburned components are oxidized and removed by using an oxidation catalyst.

The nitrogen oxide reducing catalyst is not particularly limited as long as it has a nitrogen oxide reducing action, and a known catalyst can be used. Examples of such a catalyst include a catalyst in which at least one of noble metals such as Pd, Pt, and Rh is supported on a carrier as an active metal. Examples of the carrier include alumina, silica, and a mixture thereof. The nitrogen oxide reduction catalyst may be used alone or
More than one species can be used in combination. The amount of the active metal carried in the nitrogen oxide reduction catalyst can be appropriately set according to the active metal used, and is not particularly limited.
Usually about 0.01 to 5% by weight relative to the carrier, preferably
It is about 0.1 to 2%. The specific surface area of nitrogen oxide reduction catalyst is not particularly limited as long as the resulting catalyst can maintain the dispersion of the active metal can be used stably, usually 10 to 400 m ² / g approximately as measured by the BET method, preferably 20 to It is about 200 m ² / g. Such a nitrogen oxide reduction catalyst can be produced according to a conventional method. For example, a method in which a carrier is impregnated with an active metal may be used.

The oxidation catalyst is not particularly limited as long as unburned components such as carbon monoxide and hydrocarbons in the exhaust gas can be oxidized and removed, and known oxidation catalysts can be used. Examples of the oxidation catalyst include a catalyst in which at least one of noble metals such as Pd, Pt, and Rh is supported on a carrier; and metal oxides such as Cu-Mn and Fe-Mn. Examples of the carrier used for the noble metal-supported catalyst include alumina, silica, and a mixture thereof. These oxidation catalysts can be used alone or in combination of two or more. The amount of the active metal carried in the oxidation catalyst in which the active metal is carried on the carrier is not particularly limited and can be appropriately set according to the active metal used, but is usually about 0.01 to 5% by weight relative to the carrier,
Preferably, it is about 0.1 to 2%. The specific surface area of the oxidation catalyst supporting the noble metal is not particularly limited as long as the obtained catalyst can be used stably and the dispersion of the active metal can be maintained.
Usually 10 to 400 m ² / g approximately as measured by law, and preferably from 20 to 200 m ² / g approximately.

Such an oxidation catalyst can be produced according to a conventional method. For example, in the case of producing a catalyst in which a noble metal is supported on a carrier as an oxidation catalyst, a method of impregnating the carrier with an active metal may be used.

The nitrogen oxide reduction catalyst and the oxidation catalyst used in the present invention may be used in the form of pellets, for example, by adding a binder if necessary, or by wash-coating on a refractory honeycomb. it can.

The amount of the catalyst can be appropriately set according to the type of the catalyst used. The gas hourly space velocity (GHSV) of the nitrogen oxide reduction catalyst is usually about 1,000 to 500,000 h ^-1 , preferably about 5,000 to 300,000 h ^-1 . For example,
When using a Pt / Al ₂ 0 ₃ catalyst as a nitrogen oxide reduction catalyst is desirably used in order 1000～300000H ^-1 at GHSV, further preferred be about 5000~100000h ^-1. The gas hourly space velocity (GHSV) of the oxidation catalyst is usually 1000 to
500000H ^-1 mm, preferably 5000~300000h about ^-1. For example, in case of using a Pt / Al ₂ 0 ₃ catalyst as an oxidation catalyst is desirably used in order 1000～300000H ^-1 at GHSV, further preferred be about 5000~100000h ^-1. The lower the gas hourly space velocity (GHSV), the greater the amount of catalyst, so that the purification rate is improved. However, in addition to the problem of economy, there is a possibility that a problem of increased pressure loss in the catalyst layer may occur. On the other hand, if the amount of the catalyst is too small, an effective purification rate may not be obtained.

In both cases of the nitrogen oxide reduction catalyst and the oxidation catalyst, the catalyst temperature is desirably about 200 to 700 ° C. If the catalyst temperature is too low, effective activity cannot be obtained, and if it is too high, the durability of the catalyst may be shortened.

As a more preferred embodiment of the method of the present invention, for example, the air ratio of the gas supplied to the radiant tube burner is set to less than 1.0, and the exhaust gas from the radiant tube burner is purified using a nitrogen oxide (NOx) reduction catalyst. After adding air to the obtained NOx purification gas so that the air ratio becomes 1.0 or more, a method of oxidizing and removing unburned components using an oxidation catalyst can be mentioned.

The air ratio of the gas supplied to the radiant tube burner is desirably less than 1.0. The air ratio is
From the viewpoint of efficiency, it is preferably about 0.95 to 1.0, and 0.98 to about 0.95.
More preferably, it is about 1.0. The “air ratio of the supplied gas” is a ratio between the actual amount of air supplied to the radiant tube burner and the theoretical amount of air.

The fuel supplied to the radiant tube burner is not particularly limited. For example, usually city gas, LPG
Etc. can be used.

The flue gas from the radiant tube burner is first purified using a nitrogen oxide reduction catalyst.
The air ratio of the gas supplied to the radiant tube burner is 1.
Since it is less than 0, the combustion exhaust gas contains a large amount of unburned components (carbon monoxide, hydrocarbons, etc.) in addition to NOx. This unburned component works as a reducing agent, and NOx in the combustion exhaust gas
Is reduced to nitrogen over the nitrogen oxide reduction catalyst.

Air is added to the exhaust gas from which NOx has been removed (NOx purification gas) so that the air ratio becomes 1.0 or more. After the air is added, the unburned components contained in the NOx purification gas are oxidized and removed on an oxidation catalyst provided downstream of the nitrogen oxide reduction catalyst. It is desirable that the amount of air added be such that the air ratio combined with the combustion air becomes 1.0 or more.
More preferably, it is about 3.0. If the air amount is too small, unburned components may remain. If the air amount is too large, the temperature of the subsequent oxidation catalyst may be too low, and the catalytic activity of the oxidation catalyst may be reduced.

The method of the present invention includes an apparatus in which a nitrogen oxide reduction catalyst is provided on the upstream side of an exhaust gas flue and an oxidation catalyst is provided on the downstream side, and an air addition port is provided between the nitrogen oxide reduction catalyst and the oxidation catalyst. Can be implemented. For example, a radiant tube burner system as shown in FIG. 1 can be exemplified. Radiant tube burner is not particularly limited, for example, single-ended type, U
Known shapes such as a shape and a W shape can be applied.

Hereinafter, the present invention will be described in more detail with reference to FIG. First, fuel and combustion air whose air ratios are controlled as necessary are supplied into the inner tube of a radiant tube burner, and are burned at the burner top. The generated combustion exhaust gas is discharged along the outer tube of the radiant tube burner.

The flow rates of the fuel and air supplied to the radiant tube burner can be adjusted as required by, for example, a control valve such as a butterfly valve so as to have predetermined values. In addition, since the temperature in the furnace heated by the radiant tube is generally controlled by on-off control, the combustion amount and the air ratio do not significantly change during use of the radiant tube.

It is desirable that the generated combustion exhaust gas exchange heat with combustion air. For example, heat exchange can be performed between the exhaust gas and the combustion air in the upper part of the radiant tube burner, and the exhaust gas after the heat exchange is discharged to a flue.

In the exhaust gas flue, a nitrogen oxide reduction catalyst is provided on the upstream side and an oxidation catalyst is provided on the downstream side, and an air addition port is provided between the nitrogen oxide reduction catalyst and the oxidation catalyst. In the downstream of the oxidation catalyst, if necessary, exhaust gas may be discharged by ejector air or the like, and the inside of the radiant tube may be maintained at a negative pressure.

According to the present invention, even if the NOx concentration generated in the case of a radiant tube used at a high temperature is increased, the combustion exhaust gas can be purified for a long period of time without any influence.

[0028]

According to the present invention, NOx contained in the combustion exhaust gas is reduced to nitrogen on the nitrogen oxide reduction catalyst. At this time, the unburned components in the exhaust gas act as a reducing agent. Excess unburned components in the NOx purification gas are burned by excess oxygen on the oxidation catalyst. Ultimately, NOx
It also becomes a clean exhaust gas containing almost no unburned components such as carbon monoxide and hydrocarbons.

According to the present invention, since it is not necessary to add a reducing agent such as natural gas, it is possible to purify exhaust gas without lowering the efficiency.

In the method of the present invention, the flow rate of fuel or air may be controlled by a simple valve such as a butterfly valve.
Strict control such as sensing with an oxygen sensor is not required. Therefore, according to the present invention, it is possible to purify the combustion exhaust gas of the radiant tube burner at an extremely high purification rate without performing such strict control.

[Brief description of the drawings]

FIG. 1 schematically illustrates an example of a radiant tube burner system that can carry out the method of the present invention.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 3K017 BA01 BA06 BC10 BD01 3K070 DA02 DA06 DA25 DA26 3K091 AA01 AA03 BB08 BB26 CC06 CC22 DD01 DD08 EA14 EA25

Claims (2)

[Claims] 1. Purification of combustion exhaust gas from a radiant tube burner using a nitrogen oxide (NOx) reduction catalyst, adding air to the obtained NOx purification gas, and further oxidizing unburned components using an oxidation catalyst. A radiant tube burner combustion exhaust gas purifying method characterized by removing. 2. An air ratio of a gas supplied to a radiant tube burner is less than 1.0, and a combustion exhaust gas from the radiant tube burner is purified using a nitrogen oxide (NOx) reduction catalyst. A method for purifying exhaust gas from a radiant tube burner, which comprises adding air so that the ratio becomes 1.0 or more, and further oxidizing and removing unburned components using an oxidation catalyst.