JPH0143596B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a method for adjusting the pH of ash treatment water generated when coal ash is transported as a water slurry in a coal-fired power plant. [Conventional technology] In general, when coal is burned at a coal-fired power plant, 10 to 20% of the amount of coal is generated as coal ash, of which about 85% becomes fly ash and is collected by an electrostatic precipitator. The remainder is discharged from the bottom of the boiler as clinker ash. Efforts are being made to use this coal ash as effectively as possible in cement admixtures, etc., but the proportion is only 20-30% at most, and the rest is disposed of in landfills. Recent coal-fired power plants are
This is a large-scale coal-fired power plant that uses coal imported from overseas, and is often located on the coast.Ash treatment is carried out by transporting water slurry to an ash disposal site (ash disposal pond) secured near the power plant, which is the most economical method. A format is often taken. When disposing of coal ash in a landfill, it is supposed to be disposed of in accordance with the Waste Disposal and Public Cleansing Act, and coal ash is treated as a controlled substance. When coal ash comes into contact with water and mixes,
The problem is that the pH of the eluate becomes high, and even during slurry transportation, the ash dumping site surplus water has a pH of 5 to 5, which is the drainage standard.
Values exceeding 9 often occurred. As a conventional technique, as shown in Japanese Patent Application Laid-Open No. 57-30594, sewage ash from an overflow tank 15 in a garbage incinerator and exhaust gas from the garbage incinerator are brought into gas-liquid contact with a gas-liquid contact device 17, and the exhaust gas is In
A method of processing with CO ₂ is known. However, in this method, the ash is washed with water in the ash washing tank 13, and then the ash is washed in the ash settling tank 1.
This method treats the overflow water after precipitation in step 4, and there is no description as to whether the gas before the desulfurization process or the gas after the desulfurization process is used as the exhaust gas. Furthermore, Japanese Patent Application Laid-open No. 57-32787 states that the purpose is to treat dust, acidic waste, and alkaline waste discharged from coal-fired power plants to a state that does not cause pollution, and to reduce water consumption as much as possible. A processing method is described. In other words, this publication states, ``Exhaust gas is led to the cooling tower 7a, and a liquid is sprayed at the throat to remove SO ₃ , SO ₂ , dust, HC1,
The HF is captured and the liquid accumulates at the bottom of the cooling tower 7a. This stored liquid is supplied by the circulation pump 21 from the spray nozzle to the throat portion of the cooling tower 7a. Since the liquid flowing through the circulation pump 21 is strongly acidic with a pH of 1 to 2, it is extracted in a pipe 23 and the acid is diluted with water captured in a pipe 24. Further, the gypsum extracted from the bottom of the absorption tower 7c is sent to a stylus tank 28 via a reaction tank 25, a pump 26, and an oxidation tower 27. The supernatant liquid is an alkaline liquid with a pH of 9 to 10, and is supplied to the wastewater treatment tank 29. (See page 3, upper left column, line 9 to upper right column, line 3). As described above, in the method described in this publication, the alkaline solution is the supernatant liquid during the concentration of gypsum, and it is assumed that the flyash is conveyed by air flow (from the second line from the bottom of the lower right column on page 2 to the upper left column on page 3). (See line 4). [Problems to be solved by the invention] However, the method described in JP-A No. 57-32787 does not provide a solution to the PH increase in ash dump water that can occur due to coal ash slurry transportation and dumping at ash dump sites. Although the scope is limited to power stations, there is no mention of problems at ash dumps due to transport of coal ash slurry. In view of the above-mentioned points, the present invention is based on the fact that the present inventors found and confirmed through experiments as described later _. When a gas containing 13 to 14% is introduced, the pH in the slurry increases to 6 to 6 after about 1 hour.
This was done by focusing on the fact that _CO2 , an acid gas in coal-fired boiler exhaust gas,
The present invention aims to provide a method for adjusting the pH of ash treatment water generated when coal ash is transported as a water slurry. [Means and effects for solving the problem] The method for adjusting the pH of ash treatment water of the present invention is such that when transporting the slurry of coal ash to the ash disposal pond in a coal-fired boiler for ash treatment, All or part of the flue gas after desulfurization is brought into gas-liquid contact in the reaction tower, and the CO ₂ in the flue gas is absorbed into the slurry to adjust the pH. This slurry is then stored in an ash disposal pond and the overflow water is drained. It is characterized by being used for circulation as water for discharge and/or ash transportation. The method of the present invention is characterized in that the coal ash/water slurry is brought into contact with the desulfurized exhaust gas that does not contain SO ₂ . _CO2 and _SO2 before desulfurization
This is because if the slurry is treated with exhaust gas containing CaCO 3 , CaCO ₃ once generated may be decomposed due to the pH as shown in the following equation, which is not preferable. CaCO ₃ +H ₂ SO ₃ →CaSO ₃ +H ₂ O+CO ₂ ↑ [Examples] Examples of the present invention will be described below based on the drawings. FIG. 1 shows an example of an apparatus for carrying out the method of the present invention, in which a dust collector 2 such as an electrostatic precipitator, a desulfurizer 3, a desulfurizer 3, a dust collector 2 such as an electrostatic precipitator, etc. Chimneys 4 and the like are connected in series. Instead of the electrostatic precipitator, it is also possible to use other devices such as a gravel filter dust collector, multi-cyclone, bag filter, etc. as the dust collector. Note that an air preheater is provided upstream or downstream of the dust collector 2, but is not shown. The exhaust gas from the coal-fired boiler 1 is introduced into a dust collector 2, where the dust is removed and then desulfurized. On the other hand, the coal ash is transported to the ash disposal pond 5 in a water slurry. The ash slurry concentration in this case is usually preferably 10-15
Weight%. The ash slurry is directly introduced into the CO ₂ absorption reaction tower 6, brought into gas-liquid contact with the desulfurized exhaust gas, and after adjusting the pH, is stored in the ash disposal pond 5, and overflow water (supernatant) is discharged and/or The water will be recycled and used for transporting ash. In addition, when simplifying the CO ₂ absorption reaction tower 6, since mist is included in the exhaust gas,
The exhaust gas is returned to the front of the desulfurization device 3 to remove mist. Next, an experimental example will be explained. Example 1 Coal ash with the composition shown in Table 1 (Samples A, B, C)
An eluate was prepared using seawater and distilled water, and the change in pH of the eluate over time was measured. The amount of coal ash mixed in the slurry was 10% by weight. A gas containing 13% CO ₂ and 87% air was used as the CO ₂ -containing gas, and was brought into contact with the eluate at a rate of 2/min. The results were as shown in FIG. In FIG. 2, circles indicate sample A, triangles indicate sample B, square marks indicate sample C, white marks indicate seawater, and black marks indicate distilled water.

〔Effect of the invention〕

As explained above, the method of the present invention aims to absorb _CO2 by bringing the coal ash/water slurry into direct contact with the flue gas after desulfurization, so the absorption efficiency is high and the PH of the ash treated water is reduced. This has the effect of reducing the amount of chemicals used to adjust the pH of ash treatment water to zero.

[Brief explanation of drawings]

Figure 1 is a flow sheet showing an example of an apparatus for carrying out the method of the present invention, and Figure 2 is a flow sheet showing an example of an apparatus for carrying out the method of the present invention.
It is a graph showing a change in pH over time when a gas containing CO ₂ is brought into contact. 1... Coal-fired boiler, 2... Dust collector, 3... Desulfurization device, 4... Chimney, 5... Ash disposal pond, 6... CO ₂ absorption reaction tower.

Claims (1)

[Claims] 1. When transporting coal ash as a slurry to an ash disposal pond in a coal-fired boiler for ash treatment, the coal ash/water slurry is brought into gas-liquid contact with all or part of the flue gas after desulfurization in a reaction tower, and CO in the flue gas is removed. ₂ is absorbed into the slurry to adjust the PH, the slurry is stored in an ash disposal pond, and the overflow water is recycled and used as water for discharge and/or ash transportation. Adjustment method.