JP3290312B2 - Flue gas desulfurization method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to a method for treating sulfur oxides in combustion flue gas, and more particularly to a method of injecting ammonia gas into exhaust gas from a boiler or the like to produce coal combustion fly ash (hereinafter referred to as fly ash). The present invention relates to a method of reacting sulfur oxides above and recovering the produced ammonium sulfate together with fly ash.

[0002]

2. Description of the Related Art Exhaust gas discharged from a boiler using coal, heavy oil, oil coke, or the like contains a high concentration of sulfur dioxide, which causes acid rain and respiratory tract disorders, and the smoke is discharged. Desulfurization is an essential process for pollution control.

Conventionally, as a flue gas desulfurization method, a lime gypsum method for injecting lime slurry into flue gas and wet air oxidation of sulfur oxide to recover gypsum, a water mag method for injecting magnesium hydroxide, and sodium carbonate are also used. Wet methods such as the soda method of pouring, the ammonium ammonium sulfate method of absorbing ammonium water to generate ammonium sulfate, the spray drying method of spraying slaked lime slurry into a spray dryer to recover gypsum, and denitration in an activated coke packed tower An active coke method, a dry method that simultaneously performs desulfurization, is operating in actual equipment.

[0004] Here, the wet method is widely used because it has a high desulfurization rate and can effectively use by-products such as gypsum and ammonium sulfate. However, there are drawbacks in that the process is complicated and wastewater treatment equipment is required, so that the equipment becomes large and the treatment cost increases.

On the other hand, the dry method requires no wastewater treatment equipment,
In addition, since the operation of cooling and heating the exhaust gas can be simplified, there is an advantage that the processing cost can be significantly reduced. As an example, there is a method in which sulfur dioxide (SO ₂ ) is oxidized in the gas phase on a catalyst, and then reacted with ammonia to recover ammonium sulfate (Kiyoura method; Masahiko Ogata et al., “Heavy oil and flue gas desulfurization technology”, Nikkan Kogyo) Newspaper, 1971). In the former gas phase catalytic oxidation step, a catalyst containing vanadium pentoxide as a main component is often used to oxidize SO ₂ to sulfur trioxide (SO ₃ ) in a temperature range of 450 ° C. to 500 ° C. Next, ammonia and water are injected at 250 ° C. to neutralize the solution to produce ammonium sulfate. According to this method, since the reaction product is ammonium sulfate, there is an advantage that it can be used as a fertilizer. However, although the catalyst containing vanadium pentoxide as a main component has high activity, it requires a heat source due to the high temperature at which the activity develops, and the catalyst surface does not function effectively unless used after dust removal. the device by SO ₃ to the exhaust gas cooling process is generated because it requires between the sum reaction easily corroded, the price of the catalyst has drawbacks such as high that, in order to practically solve the many problems Must. As a catalyst for oxidizing SO ₂ at a low temperature, carbonaceous catalysts such as activated carbon and activated semi-coke are known (JP-B-62-60132, JP-B-62-132).
No. 1-53086), which also has the disadvantage that it is expensive and that it is difficult to desorb the product because it has an adsorbent element.

Further, a method has been proposed in which ammonia is injected into flue gas containing fly ash and sulfur oxides to remove the reaction product together with dust (Japanese Patent Laid-Open No. 54-167).
No. 37, `` Mitsubishi Heavy Industries Technical Report '' Vol.10, No.5, p211-218 (197
3) etc.). The method of sulfur oxides, there is focused on the creation of removal of SO ₃ which cause acid corrosion of the apparatus, does not mention the removal of SO ₂ which is a main component of sulfur oxides, The injection of ammonia is also performed in a high temperature range of 300 ° C to 400 ° C. Further, in this method, since ammonium is injected at a high temperature, ammonium ammonium sulfate ((NH
₄ ) There is a disadvantage that HSO ₄ ) is partially produced as a by-product and adheres to the device to accelerate corrosion.

[0007]

In order to solve this problem, Japanese Unexamined Patent Publication No. 1-310721 discloses a method for cooling exhaust gas containing sulfur oxides to a temperature slightly higher than the acid dew point.
A method is proposed in which a mixture of ammonia and cooling air is injected, cooled to an acid dew point, and sulfuric acid gas in flue gas is fixed as ammonium sulfate and separated by a dust collector.
At this time, a method has been proposed in which fly ash is added as a precoat material to the upstream side of the dust collector in order to improve the removal of dust by the dust collector. However, even with this method, it is difficult to remove SO ₂ , and it is not a comprehensive desulfurization method.

The present inventor has conducted intensive studies to solve this problem. As a result, when ammonia and water are blown into the flue gas containing SO ₂ , the reaction represented by the first formula occurs, and desulfurization is directly performed in the gas phase. We focused on the progress of the reaction.

2NH ₃ (g) + SO ₂ (g) + H ₂ O (g) → (NH ₄ ) ₂ SO ₃ (s) (1) Although this reaction occurs equilibrium below 152 ° C. In the phase, the reaction rate is low and the reaction does not take place unless the temperature is 65 ° C. or lower, so that a catalyst for promoting the reaction is required. Therefore, when searching for various substances that could give a reaction field,
Fly ash, the waste of coal-fired boilers, becomes SO
₃ acid dew point or more can prevent acid corrosion by (usually 110
(° C. or higher), it was found that ammonium sulfite was efficiently produced on the surface.

The produced ammonium sulfite can be converted into ammonium sulfate by separately oxidizing it in the gas phase (see formula (2)), and the fly ash to which ammonium sulfate is attached can be extracted as it is or with water if necessary. It can be effectively used as fertilizer.

(NH ₄ ) ₂ SO ₃ (g) + 1 / 2O ₂ (g) → (NH ₄ ) ₂ SO ₄ (2) The present invention has been made based on the above findings and has an oxidizing activity. Sulfur oxides in combustion flue gas using inexpensive materials,
In particular, it is an object of the present invention to provide a low-cost flue gas desulfurization method by a dry ammonium ammonium sulfate method that oxidizes SO ₂ in a relatively low temperature gas phase to form ammonium sulfate on the surface and makes it usable as a fertilizer.

[0012]

That is, the present invention provides a method of reacting a mixed gas of an exhaust gas containing sulfur oxides and moisture and an ammonia gas on a fly ash surface at an acid dew point or higher and 152 ° C. or lower to form ammonium sulfate or an ammonium sulfate or the like. This is a flue gas desulfurization method comprising a step of generating a precursor (mainly ammonium sulfite) and a step of oxidizing a reaction product on fly ash to generate and recover ammonium sulfate.

[0013]

In the dry desulfurization method according to the present invention, in the first step,
SO ₂ contained in combustion flue gas, and water is also inevitably contained in combustion flue gas by ammonia separately injected, the SO ₃ contained in the exhaust gas in the gas phase of ammonium sulfate ((NH ₄ ) ₂ SO ₄ ) (see equation (3)). SO ₃ contained in exhaust gas in the first stage
By first removing H ₂ SO ₄ , it is possible to prevent H ₂ SO ₄ from liquefying even if the temperature is lowered in the next second stage reaction (see equation (1)). The reaction temperature in the first stage is preferably higher than the acid dew point, particularly higher than the acid dew point of SO ₃ , and lower than 250 ° C.

Next, in the reaction of the second step according to the present invention,
SO ₂ contained in combustion flue gas, like water which is inevitably contained in combustion flue gas by ammonia separately injected, by contacting the fly ash and the combustion flue in a gas phase, fly Ammonium sulfite ((N
H ₄ ) ₂ SO ₃ ) (see the above formula (1)). In this reaction, the lower limit of the reaction temperature is set at a value equal to or higher than the acid dew point from the viewpoint of preventing acid corrosion of the apparatus, and preferably, SO _3.
Above the acid dew point. The progress of the reaction of the formula (1) is 15
Since the process proceeds at 2 ° C. or less, the upper limit of the temperature is set to 152 ° C.

Further, SO ₃ contained in the combustion exhaust gas, which is partially left in the sulfur oxide, adheres to fly ash as ammonium sulfate ((NH ₄ ) ₂ SO ₄ ) (formula (3) below). )reference).

2NH ₃ (g) + SO ₃ (g) + H ₂ O (g) → (NH ₄ ) ₂ SO ₄ (s) (3) Then, fly ash is removed by a dust removing device and contained in the combustion exhaust gas. The SO ₂ to be removed is removed as ammonium sulfite ((NH ₄ ) ₂ SO ₃ ), and the SO ₃ is removed as ammonium sulfate ((NH ₄ ) ₂ SO ₄ ). The desulfurized gas is released to the outside.

Since it is necessary to efficiently remove the fly ash to which the reaction product has adhered at a temperature of 152 ° C. or less, it is preferable to use a filtration type dust collector. The contact between the fly ash and the reaction gas may be any of a fluidized bed system, a packed moving bed system, or a system in which fly ash is blown into a filtration type dust collector inlet and reacted with a filter cloth surface deposition layer,
Any method can be used as long as a sufficient residence time for the reaction can be obtained, but it is preferable to employ a method in which the fly ash in which the reaction products have accumulated can be continuously taken out of the system.

The main component of the reaction product adhering to fly ash is ammonium sulfite, which is separately oxidized by air or the like (see the above formula (2)). Thereby, ammonium sulfate precipitated on fly ash can be used as a fertilizer together with fly ash. In this case, if necessary, it can be extracted with water and concentrated before use.

According to this method, after oxidizing SO ₂ ,
Compared to the known dry method of reacting with H ₃ and H ₂ O, no catalyst is required, resulting in lower costs and no clogging due to fly ash. Further, it is not necessary to treat SO ₃ at a high temperature and a high concentration, so that corrosion can be prevented.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, experimental examples of the present invention will be described with reference to the accompanying drawings. FIG. 1 shows the reactor used to confirm that SO ₂ , ammonia, and water react on fly ash to cause a desulfurization reaction. This reactor is provided with a reactor 2, which is filled with fly ash 3 and silica wool 4 which supports the fly ash 3, and is heated to a predetermined temperature by a heater 7 disposed around the reactor 2. . In the reactor 2, SO ₂ and ammonia (NH ₃ ) are passed through the evaporator 1 to saturate water (Air) and nitrogen (N ₂ ).
And mixed in the reactor. The reactor is heated to a predetermined temperature, and the reactions shown in the above formulas (1) and (3) are performed.

For the gas after passing through the reactor, trap 5
After removing the water with, the water was led to an ultraviolet absorption spectrophotometer 6 where the SO ₂ slipped in the reactor ₂ was measured. further,
After the reaction, sulfur oxides trapped and dissolved in water were analyzed by ion chromatography, and ammonium sulfate precipitated on fly ash was weighed. Based on the above three analysis values, the mass balance of the sulfur content was calculated, and the production amount of ammonium sulfate and the desulfurization rate were calculated.
Table 1 shows the composition of fly ash used in the experiment.

[0022]

[Table 1]

Using the apparatus of FIG. 1, a reaction temperature of 140 ° C.
Dwell temperature 3 seconds, SO ₂ 1500 ppm, ammonia 30
After reacting for 2 hours at 00 ppm, 10% by volume of water, 6% by volume of oxygen and the remaining nitrogen, fly ash to which the reaction product was attached was recovered. FIG. 2 shows an XRD pattern of the recovered fly ash obtained by oxygen oxidation. The desulfurization rate at this time was 42.4%. According to the XRD pattern, ammonium sulfate was precipitated on the fly ash after the reaction, and a part of the sulfate was considered to be considered to have reacted with the alkali component in the fly ash and SO ₂ . From the results of this example, it was confirmed that SO ₂ in flue gas reacted with ammonia and water on fly ash to produce ammonium sulfate.

[0024]

As described above, according to the present invention, sulfur oxides containing SO ₂ in flue gas are reacted with ammonia on fly ash, which is a cheap material having oxidizing activity,
The reaction product, ammonium sulfate, is efficiently immobilized on fly ash. Therefore, according to the present invention, the flue gas desulfurization treatment can be performed at low cost. In addition, an effect is obtained that ammonium sulfate as a precipitate can be effectively used as a fertilizer.

[Brief description of the drawings]

FIG. 1 is an experimental apparatus diagram of a flue gas desulfurization method showing one embodiment of the present invention.

FIG. 2 is an XRD pattern diagram of fly ash performed for identifying a precipitate after the reaction.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Evaporator 2 ... Reactor 3 ... Fly ash 4 ... Silica wool 5 ... Trap 6 ... Ultraviolet absorption spectrophotometer

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) B01D 53/50 B01D 53/34

Claims (1)

(57) [Claims] The method according to claim 1 mixed gas of exhaust gas and the ammonia gas containing SO ₂ and water, and reacted on fly ash surface above the acid dew point 152 ° C. or less, a step of generating ammonium sulfite, on fly ash nitrous generate ammonium sulfate to oxidize ammonium sulfate, flue gas desulfurization method and a recovering.