JPS5837010B2 - Method for treating gas mixtures for the removal of nitrogen oxides - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION This invention relates to the removal of nitrogen oxides from flue gases produced in stationary combustion sources.

Nitrogen oxides, which include NO and NO2 and are commonly represented by the general formula NOx, are recognized air pollutants.

These substances produce smog through photochemical reactions with hydrocarbons.

Common sources of nitrogen oxides are the exhaust gas of internal combustion engines, the tail gas of nitric acid plants, and stationary combustion sources such as power plants.

It is known in the art that flue gas from such stationary combustion sources contains 5QQppm or more of SO2 when using relatively high sulfur containing fuel oils.

As will be explained later, the method of the present invention uses approximately 90% of the SO2
Therefore, the method of the present invention is effective for removing nitrogen oxides from a gas containing at least 50 pp[l of SO2.

On the other hand, gases obtained from the combustion of typical low sulfur fuel oils are known to contain at least about 50 pI)m SO2.

In summary, the present invention is effective in removing nitrogen oxides from flue gases containing at least 50 ppm of sulfur oxides.

As will be explained later, even sulfur oxides resulting from the combustion of fuels containing even small amounts of sulfur will sulfate the catalyst, and the catalysts used in the present invention do not lose their activity in the sulfated form.

According to the method of this invention, hair ammonia is added to the flue gas stream in order to reduce nitrogen oxides in the flue gas, and this gas is supported on a porous refractory carrier under oxidizing conditions. The flue gas is treated by contacting the copper, copper oxide or copper compound catalyst at an inlet temperature of at least about 315°C.

A preferred catalyst is copper oxide.

According to a preferred embodiment of the invention, combustion gas (eg, flue gas) from a stationary source is treated in a one-step continuous process to reduce nitrogen oxides in this gas.

Adding hair ammonia to the hot combustion gas stream and oxidizing the flue gas stream containing the added ammonia with a free copper, copper oxide or copper compound supported catalyst for selectively reducing nitrogen oxides. bring into contact under sexual conditions.

The amount of ammonia added to the gas stream is at least about 0.5 moles per mole of nitrogen oxides (NOx) present.

This is at least about 0.7 times the stoichiometric amount required for reduction of nitrogen oxides, based on the assumption that nitrogen oxides consist of 90% NO and 10% NO2 by volume. means.

The reduction of nitrogen oxides is the formula 6NO + 4NH → 5N2 + 6H2
It is expressed as 0 3 3 NO 2,000 4 NH 3 → 7/2 N 2 + 6 H 20.

The preferred amount of ammonia is at least about 0.6 moles per mole of NOx; the most preferred amount of ammonia is about stoichiometric (per mole of NOx when the NOx is 90% NO and 10% NO2). Ammonia 0.7
3 moles) to about 5 moles of ammonia per mole of NOx.

Once the stoichiometric amount of ammonia is exceeded, NOx is reduced almost quantitatively, and even if the amount of ammonia increases,
The NOx outflow value does not become very low.

On the other hand, in the case of the catalyst used according to the method of the invention, catalytic oxidation of nitrogen oxides and ammonia by oxygen in the flue gas is avoided or at least minimized, so that a substantial excess of ammonia is NOx
It is not a substantial amount.

No ammonia is usually detected in the effluent flue gas, even if a substantial excess of ammonia is provided.

The flue gas stream containing added ammonia is heated to a gas population temperature of about 315°C to about 510°C, preferably about 34°C.
Contact with the catalyst at a temperature ranging from 3°C to about 454°C.

In the case of power plant operation, the flue gas is treated between an economizer and an air preheater (where the temperature is typically about 315° C. to 399° C.), or
It is convenient to treat the gas exiting the air preheater, usually at a temperature of about 149°C.

NOx reduction catalysts that are active at temperatures as low as 149° C. are susceptible to poisoning and have a limited lifetime.

Long-life catalysts require high temperatures.

According to this preferred embodiment, ammonia is added to the flue gas stream after passing through an economizer in a conventional power generation plant.

Therefore, the lower limit of the gas inlet temperature is not restricted to 315°C, and may even be 35°COW lower than this value, as long as the catalyst has a sufficiently high temperature for long life.

Catalytic materials include active materials such as copper oxide supported on an essentially inert porous support such as alumina, alumina silica, silica, and the like.

A preferred catalyst is copper oxide supported on alumina.

The support has a surface area (nitrogen adsorption method or BET method) of at least 40 m/9.

Copper (preferably in the form of copper oxide) is a preferred active material.

This may then be in the form of free metals, metal oxides or other metal compounds.

Usually the catalytically active material loaded into the reactor is in the form of a metal oxide.

Noble metals, such as platinum and palladium, are excluded because they are poisoned by the sulfur oxides normally present in the gases treated according to the invention, do not have an adequate lifetime, and are expensive.

The catalytic metal (or metals) must not only be susceptible to sulfur poisoning, but must also be active as a NOx reduction catalyst, even in the sulfate form.

The copper oxides contemplated herein form sulfates when exposed to sulfur dioxide and oxygen, and sulfation would occur in the continuous process contemplated herein.

Even small amounts of sulfur, such as those often present in what are referred to as sulfur-free gaseous fuels, are sufficient to sulfate the catalyst over time.

Conventionally used essentially inert refractory support materials such as alumina, alumina silica, silica, etc. can be used as supports for the catalyst.

The support has a surface area (B
It is a porous body with ET).

Preferred supports are high surface area porous materials having a surface area of at least 80, and more preferably at least 1 o tri'/&.

The choice of carrier depends to a large extent on the choice of active material.

Therefore, alumina or copper oxide on alumina is an effective catalyst, while copper oxide on silica is less effective.

Contacting the combustion gases with the catalyst is preferably carried out by passing the gases through a fixed bed of catalyst.

Space speed change is approximately 1000V/V/hr or more 100,000
It can vary in the range of 0V/V/hr or more.

A low power drop is required in power equipment, and in many other systems where combustion equipment is operated at substantially atmospheric pressure or only slightly above atmospheric pressure to avoid excessive energy losses. be.

In power equipment, the force drop through the catalyst bed should normally not exceed about 63.5 crrL of water, and sometimes must be less than this value.

For any given space velocity, grain deformation and shape, a reasonably low force drop, typically 63.5 Cr in the water column.
no greater than rL and more usually about 1.3 in the water column
A combination of gas velocity and catalyst bed depth is selected to give CrfL.

Copper oxide supported on alumina has an inlet temperature of approximately 315°C.
Provides surprisingly good results in removing nitrogen oxides at temperatures between -510°C, preferably between about 343°C and 454°C.

This makes copper oxide very suitable for use in power equipment.

Preferred space velocities using copper oxide on alumina range from about 5,000 to about 50,000 V/V/h.
It is r.

Although there are other materials known to be more active as NOx selective reduction catalysts, many of these other materials (particularly noble metals) have other desirable properties of the catalysts of this invention, namely: Their effectiveness even after prolonged exposure to sulfur oxides and oxygen (which converts the catalytic metal from the oxide form to the sulfate form, as described above) at temperatures above about 315°C and that they do not oxidize ammonia to NOx.

The copper and copper compounds used in the present invention do not lose their effectiveness even after being converted to the sulfate form.

Thus copper sulfate is itself an active catalyst. The catalyst gradually deactivates over a long period of time.

This may occur due to the adhesion of small amounts of carbonaceous material and/or fly ash to its surface and/or chemical changes in the catalyst surface.

When this occurs, the catalyst may be regenerated by treatment with air or flue gas containing greater than normal amounts of oxygen to remove carbonaceous material and/or fly ash. I can do it.

The catalyst may also be reacted with hydrogen, a reducing gas such as carbon monoxide or a hydrocarbon such as methane, ethane, furopane, phthane, hexane or decane, and mixtures thereof, to restore this substance to its active form. , can be regenerated by treatment with a reducing gas diluted with steam.

(After prolonged exposure to sulfur oxide-containing flue gases, the catalyst becomes mostly sulfate.

) Either or both of these regeneration steps can be performed when necessary (
This can be done for a given catalyst (usually only rarely or occasionally needed).

Copper is a particularly suitable active substance in those cases where regeneration with reducing gases is necessary to overcome the long-term effects of sulphation.

The catalyst can be prepared by techniques known in the art of catalyst and sorbent preparation.

The carrier may be of any desired shape, such as spherical or extruded shape (which is substantially cylindrical), and of any desired size.

However, preference is given to using catalysts whose support particles are shaped in such a way that the porosity of the catalyst bed is at least 0.5, preferably at least 0.6.

Saddles (e.g. U.S. Pat. No. 2,639,909)
No. 3,060,503) and rings are of a shape that provides the necessary void ratio.

By using a catalyst geometry that provides a catalyst bed with a high spatial volume, the ball drop for a given space velocity is
becomes very low.

Thus, when the catalyst bed has a void fraction of at least 0.50, it is very easy to achieve a maximum force drop of 63.5 cfrL (or less) in the water column through the catalyst bed.

Conventional methods for impregnating the carrier with the desired active material can be used.

Therefore, impregnating the previously burnt out carrier with a known impregnating solution, for example a copper oxide surface layer, if desired with an aqueous copper nitrate solution, by known means, followed by burning out the impregnation. The resulting salt can be converted to the corresponding oxide.

If desired, the oxide can also be reduced to the free metal with hydrogen or other reducing gas.

Conventional impregnation techniques provide a catalyst in which the active substance is dispersed throughout the support material and in its pores.

If desired, the active substance must be confined to a relatively small area near the surface of the carrier particle to facilitate contact between the active substance and the flowing gas.

External surfaces, as used herein, include those surfaces that are inherent to the particle shape and not due to internal pore structure.

In the case of a ring, which is the preferred carrier shape, the outer surface of the carrier includes the inner cylindrical surface as well as the outer cylindrical surface and the ends.

The area of impregnated material in this case typically extends from the external surface inwardly to about 0. It extends to a depth of not more than 9 mm and usually covers no more than about 70% of the carrier volume, and more typically about 20% to 40% of the carrier volume.
%.

Zone impregnation or "surface impregnation" is accomplished by spray painting the impregnating aqueous solution onto the carrier particles while kumpling, or by the techniques described in U.S. Patent Application No. 2,746,936, or by any other desired means. be able to.

A preferred surface impregnation technique is to coat the calcined carrier particles with a water-immiscible and volatile polar liquid, preferably such as n-penquinol or C611 oxo I+ alcohol (a mixture of isomers of C6 primary alcohols). immersing in a primary alcohol of C5CtO and transferring the carrier particles from the water-immiscible liquid to an aqueous impregnating solution such as copper nitrate and immersing in the aqueous solution for a limited time depending on the desired depth of impregnation;
The particles are then dried and calcined.

The advantage of surface-impregnated catalysts over catalysts in which the active substance is distributed in the pores of the support is that the active substance can be used more effectively.

Typically, the method of the invention is carried out in substantially positive atmospheric conditions, particularly in power equipment.

When the sulfur oxide content of the raw combustion gas is required or high enough to remove both nitrogen oxides and sulfur oxides, the sulfur oxides may be removed even before the nitrogen oxides are removed. It can be removed even after removal.

Note that the simultaneous removal of sulfur oxides and nitrogen oxides from a gas mixture is covered by a patent filed on the same day as the present application under the title "'Removal of nitrogen oxides and sulfur oxides from gases" 1. is the subject of the application.

According to one embodiment of the invention, sulfur oxides are first removed from a flue gas or other waste gas stream containing oxygen, SOx and NOx, and then ammonia is added to the desulfurized gas stream; The gas stream containing added ammonia is then contacted with a nitrogen oxide removal catalyst.

In this case, conventional means for the removal of sulfur oxides from the flue gas can be used for the first step.

For example, U-S Burean of Mines R
eportRL5735(1961)' (alkalinized alumina), U.S. Pat. No. 3,411,86
No. 5 (Alkalinized Alumina Activated with Iron Oxide and Antimony Oxide) or British Patent No. 1,089
, 7 16 (Copper Oxide on Alumina) are much preferred over wet methods using aqueous solutions.

This is because the wet process is carried out at a temperature below the atmospheric boiling point of water, and this requires the effluent gas to be brought to a temperature suitable for removing NOx using a reasonably long-life catalyst. This is because it requires reheating.

Addition of ammonia and catalytic reduction of nitrogen oxides into the desulfurized flue gas (which typically contains about 10% of the SO2 contained in the incoming flue gas) is carried out as described above. It will be done as it was done.

The amount of sulfur oxides contained in the desulfurized flue gas is sufficient to poison sulfur-sensitive catalysts, such as precious metal catalysts, making the use of sulfur-resistant catalysts such as those described above unnecessary. is necessary.

According to yet another embodiment of the invention, the nitrogen oxides are reduced in a first step and then in a second step the nitrogen oxides are reduced, such as a flue gas initially containing oxygen, nitrogen oxides and sulfur oxides. sulfur oxides are removed from a gas stream.

According to this embodiment of the invention, ammonia is added to the waste gas stream and the gas stream is contacted with a non-precious metal catalyst for selective reduction of NOx by ammonia under oxidizing conditions as described above. .

Removal of sulfur oxides can be accomplished by known means.

Either a wet method using an aqueous absorbent solution or a dry method using a solid sorbent can be used.

The invention will now be described in detail by specific embodiments of the invention as illustrated in the Examples below.

Example 1 This example describes the selective reduction of nitrogen oxides with ammonia in a continuous process carried out in a laboratory reactor.

In a series of experiments about 3-5% oxygen by volume, about 2700
Ammonia was added to a combined flue gas containing ppm by volume of sulfur oxides and 990 to 1010 ppm by volume of nitrogen oxides (approximately 90% NO and 10% NO2) and The gas mixture was passed through a 2.2 crfL diameter laboratory reactor containing a fixed bed (approximately 7.6 cm deep) of commercially available copper oxide supported on gamma (γ) alumina spheres approximately 3.2 cm in diameter. I let it happen.

The copper content of the catalyst was approximately 8% by weight and the catalyst was completely impregnated.

The space velocity was 5,000 V/V/hr.

Gas thermostats at 343°C and 399°C were used.

The amount of ammonia is 0.2 ammonia per 1 mole of NOx.
5 to 0.80 mol, and the NH3/NOx molar ratio was kept constant in each experiment.

Table 1 below shows the percentage conversion at various NH3/NOx molar ratios and indicated gas inlet temperature variations.

As Table 1 shows, substoichiometric amounts of ammonia (i.e., no more than 0.73 moles of ammonia per mole of NOx, based on the assumption that NOx is 90% NO and 10% NO2). Converted NOx when used
There was an essentially linear relationship between the percentage and the NH3/N0x molar ratio.

When stoichiometric or higher ammonia was used, NOx conversion was essentially quantitative.

There was no significant difference between the conversions obtained at gas inlet temperatures of 343°C and 399°C.

In other experiments under similar conditions, NOx conversion was found to be significantly lower when the gas population temperature was 315°C.

The nitrogen oxide content in the effluent was determined by Dynascienc
Measurements were made using a Dyna Sciences air pollution monitoring system equipped with an NX-1130 "Total Nitrogen Oxide 11 Detector" with a scale of 550 pI) In, 1500 ppI and 5000 pI) m.

The device was manufactured by Dynasciences Corporation, a subsidiary of Uittaker Corporation of Los Angeles, California, USA.

Example 2 In this example, a series of experiments were conducted using a catalyst in which 5% by weight of copper (initially in the form of copper oxide) was supported on alumina.
The effectiveness as a NOx reduction catalyst in the presence of ammonia was determined.

In this example, the catalyst was copper nitrate (Cu(NO3) on alumina.
)2.3H20) was prepared by impregnating it with an aqueous solution.

The impregnated alumina was air dried and then calcined at 427°C for 3 hours.

In a series of experiments, the gas composition to be treated was as follows.

CO2 (volume%) 15 02 (〃) 6.5H2
0( //) 10NOx (mainly NOXppm) 200so2 (ppm)
i o o N2
For the remainder of each experiment, the gas flow rate to the catalyst was maintained at approximately 4350 V/V/hr with variable space velocity, and the NH
The 3/NOX molar ratio was maintained at 1.125.

In the first run, 327°C was used as the inlet temperature of the catalyst bed, in the second run, 360°C, and in the third run, 388°C.
°C, and in the fourth run an inlet temperature of 427 °C was used.

Each separate experiment is continued until equilibrium operation is achieved, and a product gas sample is periodically withdrawn, and then the product gas sample is removed to quantify the amount of NOx actually reduced and/or converted. Analyzed.

The results obtained clearly show that copper (initially present in oxide form)
was shown to be an effective catalyst.

The achieved equilibrium conversions are shown in the table below.

Claims (1)

Claims: 1. In treating a gas mixture containing nitrogen oxides, sulfur dioxide and oxygen to reduce said nitrogen oxides, said gas mixture is enriched under oxidizing conditions and in the presence of ammonia. A method of treating a gas mixture comprising contacting it with a catalyst comprising a component selected from the group consisting of free copper and copper oxides and a refractory support at a gas inlet temperature of 2. In treating a gas stream containing nitrogen oxides, sulfur oxides, and oxygen to reduce nitrogen oxides and sulfur oxides, (1) contacting said gas stream with an adsorbent for the selective removal of sulfur oxides; and (2) extracting the degassed gas stream from the group consisting of free copper and copper oxides supported on a porous refractory support under oxidizing conditions and at elevated temperatures in the presence of ammonia. A method of treating a gas stream by contacting it with a catalyst comprising a catalyst component. 3. In treating a gas stream containing nitrogen oxides, sulfur oxides, and oxygen to reduce nitrogen oxides and sulfur oxides, (1) said gas stream is elevated under oxidizing conditions and in the presence of ammonia. (2) contacting the nitrogen oxide-depleted gas stream with a catalyst selected from the group of free copper and copper oxides supported on a porous refractory support; A method of treating a gas stream by contacting it with an adsorbent for selective removal of oxides.