JPH0374126B2 - - Google Patents

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to a method for denitrification and desulfurization of flue gas, and more particularly, to a method for denitrification and desulfurization of flue _gas , and more particularly, to
Some of the NO _X is oxidized to form NO ₂ , and this
This invention relates to a method for suppressing pitting caused by chlorine ions by desulfurizing flue gas in the presence of nitrate ions formed by NO ₂ absorption.

[Prior art]

Sulfur oxides _SOx ( _SO2 ,
Various methods have already been proposed for removing SO ₃ ) and nitrogen oxides NO _x (NO, NO _2, etc.), and the usual method is to denitrate flue gas and then desulfurize it.

By the way, the _NOx content of flue gas varies depending on the type of fuel, combustion method, etc., and is usually 150 to 300 ppm in a coal-fired boiler, for example.

However, with the above method of installing denitrification equipment on the upstream side and desulfurization equipment on the downstream side,
The amount of NO ₂ contained in the exhaust gas after denitrification, absorbed in the wet desulfurization equipment, and converted to nitrate ions in the absorption liquid, decreases significantly, and is approximately 0.5 mol% (0.5 to 0.5 _mol %) of the amount of NO 1.5ppm), or about 1/5 of the level before denitrification.

On the other hand, chlorine in the fuel is converted into hydrogen chloride through combustion, and is dissolved and accumulated in the absorption liquid in the system of the actual device along with the chloride in the make-up water.

In other words, the amount of hydrogen chloride in the flue gas of coal-fired boilers is 10 to 50 times that of heavy oil-fired boilers;
It is 50ppm.

Corrosive gases such as hydrogen chloride and hydrogen fluoride are absorbed, accumulated, and concentrated within the wet desulfurization equipment system, so the _SOx absorption equipment is exposed to an extremely harsh corrosive environment, and the occurrence of pitting in stainless steel is ignored. The problem is that it cannot be done.

In addition, in order to prevent pitting corrosion, the nitrate ion concentration in the desulfurization absorption liquid must be adjusted based on the results of corrosion tests.
Below 700 ppmW, depending on the concentration of hydroxide ions, the pitting potential decreases at the hydroxide ion concentration normally used, and the safe margin of use between the corrosion potential and the corrosion potential becomes narrower. It is known that about 700 ppm of ions is required, and the molar ratio of nitrate ions/chloride ions is 0.59 or more, preferably 0.85 or more considering the adhesion of scale or foreign matter to the metal surface.

However, as mentioned above, since the flue gas is desulfurized after denitrification, the amount of nitrate ions in the desulfurization absorption liquid decreases significantly, and prevention of pitting due to nitrate ions cannot be expected.

Therefore, carbon steel with organic lining such as glass flake lining or rubber lining, austenitic stainless steel with increased molybdenum content or nitrogen, or high-grade metals such as Hastelloy and Inconel are used as equipment materials depending on the absorption liquid conditions. It is used.

However, since organic linings are mainly hand-worked, there are problems in terms of quality assurance of running, and the maintainability of the lining is poor compared to metal, and it is time-consuming. Because they inherently have built-in defects, their expected lifespan is relatively short and they require full relining.

In addition, in the case of austenitic stainless steel, in the method described above where denitrification equipment is installed on the upstream side and desulfurization equipment on the downstream side, it is contained in the exhaust gas after denitrification, absorbed in the wet desulfurization equipment, and in the absorption liquid. NO ₂ , which becomes nitrate ions, decreases significantly, and the nitrate ion concentration in the desulfurization equipment absorption liquid decreases, which reduces the concentration of chlorine ions accumulated in the corrosive environment in the absorption system, coexisting nitrate ions, hydroxide ions, etc. Depending on the concentration of steel, pitting corrosion may occur, and in many cases, highly versatile austenitic stainless steels such as SUS-316L and SUS-317L cannot be used as equipment materials. On the other hand, the use of high-grade metals such as Hastelloy and Inconel has the drawback of causing a significant increase in equipment costs, so it is difficult to use them as general-purpose materials from an economic standpoint.

[Purpose of the invention]

The purpose of the present invention is to provide a method for denitrifying and desulfurizing flue gas, which eliminates the above-mentioned conventional drawbacks and allows the use of highly versatile SUS-316L stainless steel without using high-grade metals such as Hastelloy and Inconel. That is.

[Structure of the invention]

The flue gas denitrification and desulfurization method of the present invention that achieves the above objects removes a predetermined amount of NO _X by catalytic reduction of flue gas containing SO _X and NO _X , and oxidizes NO _X remaining in the flue gas. The method is characterized in that NO ₂ is formed so that the nitrate ion concentration in the downstream desulfurization absorption liquid becomes at least 70 ppmW, and then the flue gas containing the NO ₂ is desulfurized.

Here, the predetermined amount means an amount such that the concentration of nitrate ions dissolved in the absorption liquid in the desulfurization step described later does not become less than 700 ppmW.

First, in the present invention, exhaust gas containing SO _x and NO _x is catalytically reduced to remove a predetermined amount of NO _x .

Exhaust smoke varies depending on the type of fuel, combustion conditions, etc., but usually contains several hundred to several thousand ppm of _SO
Contains several hundred ppm of NO _X , more than 95% of which _is
NO and the rest is NO ₂ .

Many proposals have already been made for the reduction of NO _X , and various metal oxides, such as V ₂ O _{3
WO3 , MoO3} , _Cr2O3 , _{Mn2O3 , Fe2O3 , Co3O4 ,}
A method is known in which _NOx is reduced and decomposed into nitrogen using ammonia as a reducing agent in the presence of a catalyst in which NiO, CuO, ZnO, etc. are supported on TiO ₂ in a temperature range of about 200 to 450°C.

The present invention is not limited to the ammonia reduction method using a specific catalyst, and any known method can be employed as appropriate.

Through this catalytic reduction, most of the _NOx in the flue gas is removed, resulting in flue gas in which 40 to 200 ppmV of _NOx remains.

Next, in the present invention, NO in the flue gas from which most of the _NO
NO2 is formed from NO in _NOX , increasing the _NO2 content in flue gas.

Several proposals have already been made regarding the formation of NO ₂ by oxidizing NO, and catalytic oxidation methods and ozone oxidation methods are known.

The catalytic oxidation method supports single or composite systems of metals or metal oxides such as vanadium, iron, manganese, cobalt, chromium, copper, alkali metals, and alkaline earth metals on silica, alumina, titania, zirconia, diatomaceous earth, etc. 250-400
This is a method in which NO ₂ is formed by contacting with flue gas at a temperature of about °C.

Proposals have already been made regarding the ozone oxidation method, for example, in Japanese Patent Publication No. 56-43771, ozone-containing air is added to flue gas.

Formation of NO ₂ by oxidation of NO in the present invention may be carried out by either the above-mentioned catalytic oxidation method or ozone oxidation method, but since ozone is expensive in terms of oxidizing agent price, it is preferable to adopt the catalytic oxidation method. preferable.

What is important in the present invention is that the concentration of NO ₂ formed by the oxidation of NO in the flue gas is at least as low as the nitrate ion concentration in the desulfurization equipment absorption liquid.
700 ppmW, increasing the molar ratio of nitrate ions to chloride ions to preferably maintain it at 0.85 or higher.

By keeping the _NO2 concentration within this range,
Dissolved in the absorption liquid in the desulfurization process described later
The concentration of nitrate ions formed from NO ₂ is preferably maintained at 1000 ppm or higher, and the molar ratio of nitrate ions and chloride ions is maintained at a molar ratio of 0.85 or higher to suitably prevent pitting caused by chlorine ions by nitrate ions. can do.

Finally, in the present invention, NO ₂ in the flue gas containing NO ₂ formed by NO oxidation is absorbed and removed by the desulfurization absorption liquid, increasing the denitration rate of the flue gas treatment system and reducing SO Absorb and remove _X. That is, the present invention performs denitration in a flue gas treatment system in two stages, most of the denitration is performed by catalytic reduction, and part of the denitration is performed by oxidizing NO to NO ₂ and using a desulfurization device.

The type of desulfurization method and the composition of the absorption liquid are not particularly limited, and can be selected from various conventionally known wet desulfurization methods.

NO ₂ contained in flue gas is relatively easily dissolved in the desulfurization absorption liquid, and nitrate ions are formed.

By dissolving NO ₂ , the nitrate ion concentration is increased to at least 700 ppmW, and the molar ratio of nitrate ions to chloride ions is preferably 0.85 or more, thereby preventing pitting of SUS-316L by chloride ions.

The chlorine ion concentration in the absorption liquid depends on the fuel used, combustion conditions, presence or absence of cooling dust removal tower equipment upstream of the absorption tower,
It varies widely depending on the amount of drainage, etc., but usually
It is 500~1000ppmW.

In addition, in the denitrification and desulfurization method of the present invention,
_The heat is recovered and then sent to an electrostatic precipitator prior to desulfurization of the flue gas, which contains NO ₂ formed _by the catalytic oxidation of _NO Therefore, it is also possible to supply the product to the desulfurization process after removing the dust.

When ozone oxidation of NO is performed, it is preferable to use the low temperature range of the exhaust gas, and it is usually performed in the duct section after cooling and dust removal.

〔Effect of the invention〕

As described above, according to the present invention, after the flue gas is denitrified by catalytic reduction, the remaining _NO
Some of the NO in it is oxidized to form _NO2 .

Then, NO ₂ in the flue gas is absorbed and nitrate ions are formed in the desulfurization process, so a denitration effect is obtained and the nitrate ion concentration in the absorption liquid is kept at a predetermined level, that is, the nitrate ion concentration is at least
Can be controlled to 700ppmW.

Therefore, the nitrate ions can prevent pitting caused by chlorine ions formed by absorption of chlorine gas in flue gas.

That is, according to the present invention, there is no need to use high-grade metals such as Hastelloy and Inconel as the material for the desulfurization equipment, and it is possible to use general-purpose stainless steel such as SUS-316L, which significantly reduces the cost of the desulfurization equipment. I can do it.

Examples of the present invention will be described below.

〔Example〕

Example 1 Flue gas from a coal-fired boiler containing 150 to 300 ppm of _NOx was denitrified by an ammonia catalytic reduction method, and _NOx in the flue gas was decomposed and removed into nitrogen and water.

NO ₂ in flue gas is around 1/5 of that before denitrification (0.5~
1.0ppmW).

After this denitrified flue gas is supplied to the air heater and electric precipitator, the NO in the flue gas is
The flue gas containing this NO ₂ is supplied to _the desulfurization equipment to reduce the nitrate ion concentration in the absorption liquid.
It was maintained at 1000-1200ppmW.

Moreover, the nitrate ion/chloride ion molar ratio was 0.85 or more, and the pH of the absorption liquid was 3.5 to 4.5.

When the pH is less than 3.5, the nitrate ion/chloride ion molar ratio shifts toward a stricter direction due to the influence of OH ions in the absorption liquid, and when it exceeds 4.5, it shifts toward a more relaxed direction.

In addition, approximately 100% of the ozone supply is from NO to NO ₂
used for oxidation to

Under such desulfurization absorption liquid conditions, SUS-
It has been found that 316L austenitic stainless steel can be used as the internal structure material of the desulfurization equipment absorption system.

Example 2 The amount of _NOx in the exhaust gas of an oil-fired boiler depends on the type of oil,
Although it depends on the combustion method, the chlorine content in fuel is significantly lower than that of coal or asphalt.

However, in this case as well, air containing ozone is injected into the downstream duct of the exhaust gas cooling tower after heat exchange and dust collection, and NO ₂ is removed by ozone oxidation.
It has become clear that general-purpose SUS-316L stainless steel can be used as a material for desulfurization equipment by increasing the amount of nitrate ions in the absorption liquid.

Claims (1)

[Claims] 1 Remove a _{predetermined} amount of NO _X by catalytic reduction of flue gas containing SO _X and _NO 1. A method for denitrification and desulfurization of flue gas, characterized in that NO ₂ is formed so that the NO 2 is formed, and then the flue gas containing the NO ₂ is desulfurized.