JPS6153105B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for removing ammonia from coal ash to which ammonium salts have adhered. In recent years, the use of coal-fired boilers is predicted to increase due to energy diversification policies. Coal-fired boilers can be divided into those that burn coal exclusively and those that co-fire coal with heavy oil, but in both cases, the exhaust gas generated contains nitrogen oxides (NOx), sulfur oxides (SOx), and dust (ash). ), denitrification,
Exhaust gas purification measures such as desulfurization and dust removal are being considered. Catalytic ammonia catalytic reduction is a common method for denitrating boiler exhaust gas, and the catalyst layer is installed before the air heater, where the exhaust gas is denitrified (denitrified).
NOx) and is discharged through an air heater and dust collector. Of the ammonia injected at this time,
There is a small amount of so-called leak ammonia that passes through the catalyst layer without reacting with Nox, and this is in the exhaust gas.
It is known that it combines with SOx to produce ammonium salts that adhere to air heaters, etc. Therefore, when this system is applied to coal-fired boiler exhaust gas, the above-mentioned ammonium salt adheres to dust (ash) in the exhaust gas, and this ammonium salt-adhered dust is collected by a dust collector. Conventional coal-fired boilers do not have denitrification equipment installed, so ammonium salts do not adhere to them, and the dust collected by the dust collector is called fly ash and is used as fly ash cement raw material or cement admixture. Currently, the ash is being dumped into an ash dump. However, when coal ash with ammonium salts attached is mixed with cement, when water is added and kneaded, the pH value of the cement paste rises to around 12, causing ammonia to be liberated and its odor to be used during work. The environment will deteriorate significantly. In addition, if coal ash with ammonium salts is dumped, there are concerns about contamination of public water by elution of ammonium salts into nearby odors and rainwater. In order to solve these problems, methods such as washing with water and thermal decomposition of coal ash with ammonium salt adhesion have been proposed. In addition to processing equipment, dehydration equipment is also required to remove as much water as possible from the ash. In addition, in the thermal decomposition method, for example, extracting a part of the boiler exhaust gas and bringing the hot air into contact with the ash requires a dust collector again. Methods such as bringing the heat medium into contact with the ash are used, but this method not only requires duct routing and a heat medium heating device, but also causes heat loss, so this method is by no means preferable in terms of equipment and running costs. It can not be said. As described above, the treatment and disposal of fly ash adhering to ammonium salts is a major problem, and an important issue for promoting energy diversification policies and preventing pollution in the future. The present invention was made against this background, and its gist is to add an alkaline substance such as quicklime or slaked lime to coal ash adhering to ammonium salts and water at a ratio of 10 to 30 parts by weight per 100 parts by weight of coal ash. This is a coal ash treatment method in which ammonium salts are decomposed by stirring and mixing, and the liberated ammonia is returned to the flue. The decomposition reaction of ammonium salt is expressed by the following reaction formula. CaO＋H ₂ O→Ca(OH) ₂ (1) NH ₄ HSO ₄ +Ca(OH) ₂ →CaSO ₄・2H ₂ O＋NH ₃ (2) (NH ₄ ) ₂ SO ₄ +Ca(OH) ₂ →CaSO ₄・2H ₂ O+2NH ₃ (3) The amount of water added to the ash is 10 to 10 parts by weight of ash.
30 parts by weight, so that the ash is moist and does not scatter. If it exceeds 30 parts by weight, the ash will become fluid and difficult to handle, and if it is less than 10 parts by weight, it will not be moist enough and there is a risk of scattering. If the purpose of adding quicklime or slaked lime is only to decompose ammonium salts, it is acceptable to add the same molar amount as ammonium salts, but if a certain amount of ground strength is expected to be used for the purpose of using the site of an ash dump, add 100 ml of quicklime or slaked lime.
The upper limit is 5 parts by weight. If the upper limit is exceeded, not only the strength will decrease but also the running cost will increase, which is not preferable. In addition, as embodiments of the present invention, a coal-fired boiler 1, a denitrification device 2, an air heater 3, and a dust collector 4 are provided.
A flowchart in which the present invention is applied to a system consisting of the following is shown as an attached diagram and will be explained below. Although water and alkaline substances such as quicklime and nitric lime can be added separately, it is effective to mix them in the adjustment tank 6 as shown in the figure and add them as a slurry or solution in order to achieve uniform dispersion. The ash from which the ammonium salt has been decomposed still has adhering water and remains moist to prevent it from scattering, but as shown in the reaction equation, ammonia (NH ₃ ) is partially dissolved in the adhering water. If the amount of water added is large and the state becomes fluidized, the amount of ammonia dissolved will also be large. This dissolved ammonia is also actively dissipated NH ₃ + H ₂ O → NH ₄ OH (4) Therefore, the higher the temperature of the ash, the more effective it is. It is preferable to add water and an alkaline substance such as quicklime or slaked lime. The liberated ammonia is led from the stirring and mixing zone to the flue. The most preferable introduction point is at the front stage of the denitrification device 2 as shown in the figure, and the introduced ammonia is effectively reused as a reducing agent for denitrification together with the ammonia from the injection line a. be done. In this way, the ammonium salts on the fly ash are decomposed, the ammonia is removed to a sufficiently low concentration, the mixture is stirred, the fly ash is discharged from the mixing zone, and the fly ash is treated in a state where it can be dumped directly into the ash dump in the same way as before. This is the reason.
The removed ammonia is also returned to the flue and reused. According to the method of the present invention, the equipment includes, as shown in the figure, a mixing and stirring device 5 for mixing ash with quicklime or slaked lime and water, and a line c for introducing the removed ammonia into the flue.
Also, if quicklime or slaked lime is used only to decompose ammonium salts, only a small amount is required and running costs can be kept low, making it possible to process coal ash with ammonium salts at an unprecedented cost. The decomposition reaction of ammonium salt carried out in the present invention is (2)(3)
As shown in the formula, the reaction is between calcium hydroxide and ammonium salt, but calcium may also be magnesium, sodium, or potassium, and cement that exhibits a high PH value due to hydration reaction can be reacted with calcium.
(OH) May be replaced with ₂ . Next, examples will be given to clarify the effects of the present invention. Example 1 When water was added to the coal ash collected by the dust collector and the state of the ash was observed, it was as follows.

【table】

[Table] Example 2 Coal ash with ammonium salts attached was heated to various temperatures, and quicklime, slaked lime and water were added to it.
After stirring and mixing for 30 minutes, the amount of residual ammonia was determined as follows.

[Table] Ammonia odor was generated during stirring and mixing, and ammonia emission was observed. Quantitative results also support this, and the ammonia reduction effect is remarkable. Example 3 Supernatant liquid of slaked lime slurry was added to coal ash with ammonium salt attached, mixed and stirred for 5 minutes, and left to stand for 30 minutes.
Residual ammonia was determined after 1 minute, 1 hour and 24 hours. The results are shown in the table below.

[Table] Example 4 Quicklime slurry and slaked lime slurry supernatant liquid were added to coal ash to which ammonium salts had adhered, and the mixture was stirred and mixed for 30 minutes.The mixture was then filled into a mold, and after 7 days of moist air curing, the compressive strength was measured. The results are shown in the table below.

[Table] Example 5 Coal ash with ammonium salts attached was heated to 80°C, caustic soda, caustic potash, magnesium hydroxide, cement, and water were added to it, and after stirring for 30 minutes, the residual ammonia was quantified, and the results are shown in the following table. Obtained.

【table】 [Brief explanation of the drawing]

The figure is a flowchart of an embodiment of the invention. 1...Coal-fired boiler, 2...Denitration equipment, 3...
...Air heater, 4...Dust collector, 5...Stirring mixer, 6...Alkaline substance slurry (or solution) preparation tank, a...Ammonia injection line, b...
...Denitrified and dust-removed exhaust gas line, c...Recovered ammonia return line, A...Ammonium salt-adhered coal ash, B...Water, C...Alkaline substances, D...
...Coal ash treated to remove ammonium salts.

Claims (1)

[Claims] 1 In the process of removing nitrogen oxides from coal-fired boiler exhaust gas using ammonia as a reducing agent, the coal ash collected and removed by the dust collector after the denitrification device is mixed with alkaline substances such as quicklime and slaked lime and 100 parts by weight of the coal ash. A method for treating coal ash, which is characterized by adding 10 to 30 parts by weight of water to the coal ash, stirring and mixing the mixture, and returning the generated ammonia to the flue.