JPH07112116A - Method for reducing nitrogen oxides in waste gas of recovery boiler - Google Patents

Abstract

PURPOSE:To easily and surely reduce NOx in waste gas in a furnace of a boiler for recovering pulp spent liquor by throwing black liquor into combustion waste gas having a specified temp. range in downstream of a black liquor burner, in the furnace of pulp spent liquor recovery boiler. CONSTITUTION:In a boiler for recovering chemicals and the amount of heat in a pulp plant, that is, a furnace of a boiler for recovering pulp spent liquor, black liquor is thrown into combustion waste gas having a temp. range of 850-1050 deg.C in downstream of a black liquor burner. For example, a main fuel black liquor feeding line 105 is opened to around the bottom part of a boiler furnace body 101. A plurality of black liquor feeding nozzles 104a, 104b from which the black liquor is thrown in are arranged above the opening of the boiler furnace body 101 and a secondary air feeding line 106. Further, an NOx meter 109 is arranged in a flue 108 to measure NOx in the waste gas. In this way, NOx in the waste gas is easily and surely reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for reducing nitrogen oxides in exhaust gas of a boiler (herein referred to as a recovery boiler) for recovering chemicals and heat of a pulp mill.

[0002]

In conventional recovery boiler, nitrogen oxides in the exhaust gas for (hereinafter referred to as NO _X), only the correspondence have been made by improved combustion of the input methods such as a so-called combustion air, produced in the combustion unit No measures were taken to reduce NO _x in the subsequent part. The catalytic flue gas denitration method, which is usually adopted when using gas / liquid fuel or coal as fuel, is difficult to be adopted in the recovery boiler because the amount of dust contained therein is extremely large.

[0003]

Conventionally, the NO _X produced in the recovery boiler is as low as 70 to 110 ppm, and it has not been a serious problem. However, recently the concentration of black liquor, which is the fuel, rises, and the fixation of low SO _X combustion (SO
_Since the emission concentrations of _X and NO _X are in a reciprocal relationship), etc., the produced NO _X concentration tends to rise. The adjustment of the combustion air, although it is possible low NO _X reduction in the combustion section, So can not suppress the emission of SO _X and reducing sulfur.

The present invention uses SO in the combustion section._XAnd reducing sulfur
To some extent by giving priority to the suppression of _XI accept the generation
Generated NO that enables combustion with high air ratio and high temperature.
_XIn the recovery boiler exhaust gas to reduce the N in the wake of the combustion section
O₂It is intended to provide a reduction method.

[0005]

The method for reducing NO _x in the exhaust gas of a recovery boiler of the present invention is a method for producing black liquor in a combustion exhaust gas in a temperature range of 850 to 1050 ° C., which is downstream of a black liquor burner of a pulp waste liquid recovery boiler furnace. It is characterized by throwing in.

[0006]

When black liquor is put into high temperature exhaust gas, HCN and NH ₃ are produced by the following equation (1) due to thermal decomposition of the black liquor.

Black liquor + O ₂ → HCN + NH ₃ + CO + CO ₂ + H ₂ O etc. (1) The HCN and NH ₃ produced here are NO in the exhaust gas of the recovery boiler and the following equations (2) and (3). And is reduced to harmless N ₂ .

4NO + 4HCN + 3O ₂ → 4N ₂ + 2H ₂ O + 4CO ₂ (2) 4NH ₃ + 4NO + O ₂ → 4N ₂ + 6H ₂ O (3) Optimal temperature range for the reactions of the above equations (2) and (3) (8
50-1100 ° C) is present. Although the reactions of the above formulas (2) and (3) are known, it was not clear that HCN and NH ₃ are produced during the thermal decomposition of black liquor.

The inventors of the present invention have found that when black liquor is burnt in a temperature range of 850 ° C. to 1050 ° C. during the combustion test of black liquor, NO _x generated in the combustion exhaust gas is small, and this temperature The present inventors have found that HCN and NH ₃ are contained in the pyrolyzed gas of black liquor in the region, and arrived at the present invention.

In an actual recovery boiler, the combustion exhaust gas temperature is 1100 to 1200 ° C. immediately after the secondary air is injected after the black liquor burner and 900 to 1000 ° C. at the superheater tube group inlet. There is a temperature range of the combustion exhaust gas of 1050 ° C. The black liquor is introduced in the temperature range of this combustion exhaust gas and brought into contact with the combustion exhaust gas and the pyrolysis gas of the black liquor. In the recovery boiler furnace, the combustion exhaust gas and the black liquor are sufficiently connected, and a long reaction time of about 2 seconds is obtained.
HCN in the pyrolysis gas of black liquor according to formula, it is possible to reduce of the NO _X in the exhaust gas by the denitration reaction by the by NH ₃ (2) and Equation (3) below.

In the actual application to the boiler, it is needless to say that the combustion exhaust gas and the injected black liquor are sufficiently mixed, and further it is possible to cope with changes in the load (black liquor combustion amount in the hearth). Need to be considered. Therefore, it is natural that it is preferable to use a plurality of nozzles rather than a single nozzle. Also, in practice, even if there are multiple furnaces, if they are set to the same level in the furnace of the recovery boiler, the temperature at the charging point (and the subsequent flow) will change when the load changes, so it is possible to charge from multiple levels. It is desirable to keep it.

[0012]

[Example] With an experimental apparatus whose flow sheet is shown in FIG.
The effect of the present invention was investigated. In FIG. 1, 1 is an electric furnace,
1a is an electric furnace temperature controller, 2 is a furnace body penetrating the electric furnace 1, 3
Is a simulated exhaust gas to the furnace body 2 (O ₂ : 7.4%, NO: 10
0 ppm, balance N ₂ ) supply line. Reference numeral 4 denotes a black liquor tank. The black liquor is supplied from the black liquor tank 4 through a pump 5 into a furnace of a furnace body 2 from a black liquor supply nozzle 6 and, after burning, is deposited as a smelt 7 in a lower part of the furnace body. . Denitration effect measures the NO _X and O ₂ concentration in the exhaust gas taken from the rear flue 2a via sampling line 9 consecutive analyzer 8.

In this test, the furnace body 2 has an inner diameter of 80 mm,
A magnetic tube having a thickness of 5 mm was used, and the length of the heating portion of the electric furnace 1 was set to 1000 mm. Further, the supply amount of the simulated exhaust gas from the simulated exhaust gas supply line 3 is set to 20 Nl / min, and the O ₂ concentration of the supplied simulated exhaust gas: 7.4% is O in the rear flue.
Black liquor was supplied from the black liquor supply nozzle 6 so that the concentration was ₂ %. Then, the temperature of the electric furnace 1 is changed from 800 ° C. to 110
The temperature in the rear flue 2a is changed between 0 ° C and N
The O _X was measured and evaluated.

The test results are shown in Table 1. When no additive is added, N
O _X The concentration was about 100 ppm, the test temperature is 950
A maximum denitration rate of 68% was obtained at 0 ° C. The composition of the black liquor used in the test is shown in Table 2.

[0015]

[Table 1]

[0016]

[Table 2]

By the above test, it was confirmed that a denitration rate of 15% or more can be obtained at a test temperature of 850 ° C to 1050 ° C.

FIG. 2 is a flow sheet diagram showing an embodiment of the present invention applied to a pulp waste liquid recovery boiler.
Is a boiler furnace body of a recovery boiler, 102 is a black liquor tank, 10
3 is a black liquor supply pump, 104a and 104b are black liquor supply nozzles into the furnace where black liquor is supplied from the supply pump 103, 105 is a main fuel black liquor supply line into the boiler furnace body 101, and 106 is secondary air supply line for black liquor combustion in the boiler furnace body 101, 107 superheater tube bank of the boiler furnace body 101, 108 is flue, 109 denotes NO _X meter, respectively.

The main fuel black liquor supply line 105 is opened near the bottom of the boiler furnace body 101, and the black liquor supply nozzles 104a and 104b are connected to the boiler furnace body 101 of the main fuel black liquor supply line 105. The nozzle 1 is arranged above the opening and the secondary air supply line 106.
From 04 and 104b, the temperature of the exhaust gas in which the black liquor supplied from the main fuel black liquor supply line 105 burned is 850 to 10
Black liquor is added to the region of 50 ° C.

In addition, O ₂ in the exhaust gas from the flue 108
The concentration is set to 2-3%, and NO _X is measured by the NO _X meter 109 installed in the flue 108.

In the recovery boiler having the flow chart shown in FIG. 2, when the load of the boiler was 97%, NO _X was about 100 ppm when black liquor was not injected from the black liquor supply nozzles 104a and 104b. Black liquor supply nozzle 10
Black liquor from 4a and 104b is about 25
Table 3 shows the results when the contents were added so as to be 20%, 20%, 15% and 10%. As shown in Table 3, in this example, a denitration rate of 27% or more and a maximum of 53% could be obtained.

[0022]

[Table 3]

[0023]

According to the present invention, by introducing the black liquor, which is the fuel of the recovery boiler, into the combustion exhaust gas in the temperature range of 850 to 1050 ° C., which is the downstream of the black liquor burner of the furnace of the pulp waste liquid recovery boiler. it is possible to effectively reduce the concentration of NO _X in the exhaust gas of spent liquor recovery boiler.

Further, the present invention can be carried out by a simple apparatus and is industrially very useful.

[Brief description of drawings]

FIG. 1 is a flow sheet diagram of a test apparatus of the present invention.

FIG. 2 is a flow sheet diagram showing an embodiment of the present invention.

[Explanation of symbols]

1 electric furnace 1a electric furnace temperature controller 2 furnace body 2a rear flue 3 simulated exhaust gas supply line 4 black liquor tank 5 pump 6 black liquor supply nozzle 7 smelt 8 continuous analyzer 9 sampling line 101 boiler furnace body 102 black liquor tank 103 Supply pumps 104a, 104b Black liquor supply nozzle 105 Main fuel black liquor supply line 106 Secondary air supply line 107 Superheater tube group 108 Flue 109 NO _X meter

─────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] May 27, 1994

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0004

[Correction method] Change

[Correction content]

The present invention uses SO in the combustion section._xFu reducing sulfur
To some extent by giving priority to the suppression of _xI accept the generation
Generated NO that enables combustion with high air ratio and high temperature.
_xIn the recovery boiler exhaust gas to reduce the N in the wake of the combustion section
O _x It is intended to provide a reduction method.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Noriaki Uchimura 1-1, Atsunoura-machi, Nagasaki City Mitsubishi Heavy Industries Ltd. Nagasaki Shipyard Co., Ltd. (72) Inventor Michimasa Yagi 1-1, Atsunoura-machi, Nagasaki City Mitsubishi Heavy Industries Ltd. Nagasaki Shipyard

Claims (1)

[Claims] 1. A method for reducing nitrogen oxides in a recovered boiler exhaust gas, which comprises charging black liquor into a combustion exhaust gas in a temperature range of 850 to 1050 ° C. downstream of a black liquor burner of a pulp waste liquid recovery boiler furnace.