JPH04141215A - Exhaust gas desulfurizing method - Google Patents

Abstract

PURPOSE:To achieve a high desulfurizing rate by humidifying the sulfur oxide containing an unreacted desulfurizing agent and reacted desulfurizing agent in exhaust gas after desulfurization at a specified temp. or less and drying the same at the specified temp. or higher. CONSTITUTION:The temp. of the exhaust gas from a boiler 1 is lowered by an air heater 2 to guide the exhaust gas to a desulfurizing tower 3. A desulferizing agent A is sprayed in the exhaust gas and the reacted desulfurizing agent A and the ash and unreacted desulfurizing agent A in the exhaust gas are collected by a dust collecting apparatus 5. Water or steam B is supplied to the collected desulfurizing A and ash-containing particles at 100 deg.C or higher from a line 7. A part thereof is disposed and the remainder is dried at 100 deg.C or higher by a heating apparatus 8. As the desulfurizing agent A, slaked lime, quick lime, caustic soda, caustic potash, magnesium hydroxide and sodium sulfate are used.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention relates to a flue gas desulfurization method using at least one compound selected from alkali metals and alkaline earth metal compounds as a desulfurization agent. It concerns the Reclamation Act.

[Conventional technology] Exhaust gas discharged from heavy oil-fired and coal-fired boilers in thermal power plants usually contains acidic harmful substances such as sulfur oxides (SOx) and (CI) at a rate of 100 to 3000 ppm. Since it is considered to be a causative agent of acid rain and photochemical smog, an effective treatment method is desired. Conventionally, wet methods (for example, limestone-gypsum method) or dry methods (
Activated carbon method) has been implemented, but while the wet method has a high removal rate of harmful substances, it is difficult to treat wastewater, it is necessary to reheat the exhaust gas, equipment costs and operating costs are high, and the dry method has a high removal rate. There was a problem that the rate could not be obtained.

Therefore, it is desired to develop a desulfurization method that can obtain a high removal rate with a wastewater-free, low-cost process.

In addition to the methods mentioned above, desulfurization methods for exhaust gas from boilers include a semi-dry method in which slaked lime or its slurry is sprayed into the exhaust gas, and a method in which limestone is directly dispersed in the high-temperature gas in the furnace or flue to remove acidic harmful substances. A dry method for removal has been proposed and is characterized by low equipment and operating costs, but both methods have the problem of low removal rates.

SOx in exhaust gas when slaked lime or quicklime is sprayed into exhaust gas
FIG. 5 shows a typical flow sheet of a method for reacting with the oxidation gas and removing it using a dust collector. The exhaust gas from the boiler 1 is lowered in temperature by the air heater 2, and is led to the desulfurization tower 3.

The desulfurizing agent A, such as slaked lime, is supplied as a spray into the flue 4 or the desulfurizing tower 3, and reacts with acidic harmful substances such as SOX in the exhaust gas.

When water B is also supplied at this time to lower the temperature and increase the humidity of the exhaust gas, water B may be supplied separately from desulfurizing agent A, or desulfurizing agent A may be supplied simultaneously as a slurry. The reacted desulfurizing agent A is collected by the dust collector 5 together with the ash in the exhaust gas, and a part of it is again supplied to the desulfurization tower 3 and reacts with acidic harmful substances such as S08 in the exhaust gas. The remaining desulfurization agent A and ash are discarded.

In such a method, the removal rate of acidic harmful substances is said to be dominated by moisture (relative humidity) in the exhaust gas.

That is, in order to increase the removal rate, it is necessary to lower the temperature of the exhaust gas and increase the moisture content. In order to increase the moisture concentration, methods of spraying water or slaked lime slurry have been proposed, but such methods of increasing the moisture concentration in the gas do not sufficiently improve the removal rate. If the removal rate is low,
Water or steam is added to the particles containing the unreacted desulfurizing agent collected by the dust collection bag f5 to destroy the reaction product shell formed on the surface, and then a part of this is sprayed into the exhaust gas again. A method has also been proposed to improve the removal rate by
For example, U.S. Pat.
No. 5827).

[Problems to be Solved by the Invention] However, in the above conventional technology, even if water or steam is added to particles containing unreacted desulfurizing agent collected by the dust collector 5, the shell of the reaction product is not completely removed. Since it is not destroyed, the recovery rate of removal is not sufficient. Therefore, there is a problem in that the proportion of particles to be recycled, that is, the amount of particles that must be treated by the dust collector 5 increases, and the processing capacity of the dust collector 5 has to be increased. As described above, in the above conventional technology, no consideration is given to the regeneration method of the desulfurization agent, especially the temperature during regeneration, and as a result, the removal rate of acidic harmful substances (desulfurization rate)
There was a problem that the cost of the device became high.

Therefore, an object of the present invention is to provide a flue gas desulfurization method that achieves a high desulfurization rate with a simple system while regenerating unreacted desulfurization agent.

[Means to solve the problem] The above object of the present invention is achieved by the following configuration.

In other words, a desulfurization tower that removes sulfur oxides from the exhaust gas by adding at least one type of alkali metal or alkaline earth metal compound as a desulfurization agent to the exhaust gas discharged from the combustion equipment. The flue gas desulfurization method used includes a step of adding water or steam to the desulfurization agent that has reacted with sulfur oxides in the flue gas containing unreacted desulfurization agent at a temperature of 100°C or lower, and a step of adding water or steam at a temperature of 100°C or higher. A flue gas desulfurization method comprising a step of drying a desulfurization agent;
It is.

Desulfurizing agents of the present invention include slaked lime, quicklime, caustic soda,
Caustic potash, magnesium hydroxide, sodium [11], etc. are used.

[Function] By adding water or steam to particles containing an unreacted desulfurizing agent after desulfurization treatment by adding it to exhaust gas at a temperature of 100°C or lower, and then drying at a temperature of 100°C or higher, Destruction of the reaction product shell on the particle surface is promoted. Therefore, the contact efficiency between the desulfurizing agent and acidic harmful substances such as SO8 in the exhaust gas is improved, and the removal rate of acidic harmful substances is increased.

Furthermore, by heating to 100° C. or higher, the reacted desulfurizing agent (Ca S O 3 ) contained in the particles is oxidized, making it easier to dispose of the used desulfurizing agent.

[Example] The present invention will be explained in more detail by the following example, but is not limited to the following example. Example 1 Exhaust gas from a coal-fired boiler is desulfurized using slaked lime as a desulfurizing agent. An example in which the method of the present invention is applied will be described.

In Figure 1, exhaust gas from boiler 1 is transferred to air heater 2.
The temperature is lowered in the step 3, and the product is led to the desulfurization tower 3.

Desulfurization agent A is sprayed and supplied into the flue 4 or desulfurization tower 3, and reacts with oxidized toxic gases such as SO□ in the exhaust gas, and the reacted desulfurization agent A is mixed with ash in the exhaust gas and unreacted desulfurization iA. The dust is collected by the dust bag W5.

When spraying desulfurization agent A, in order to further improve the desulfurization rate,
It is also possible to lower the temperature of the exhaust gas and increase the humidity by supplying water or steam B from the line 6 into the flue 4 or the desulfurization tower 3. At this time, water or steam B may be supplied at the same location as the desulfurizing agent A, or may be supplied at a different location. Particles containing unreacted and reacted desulfurizing agent A and ash collected by the dust collector 5 (hereinafter referred to as collected particles)
Water or water vapor B is supplied from the line 7, a part of the collected particles collected from the dust collector 5 is discarded, and the remainder is fluidized by air C or exhaust gas D in the heating device 8.
Dry while mixing. The dried collected particles are sent to a humidifier 10 through a line 9, and are again humidified with water or water vapor B in the humidifier 10. The collected particles subjected to such treatment are again sprayed and supplied from the line 11 into the flue 4 or the desulfurization tower 3, where they react with acidic toxic gases such as SO□ in the exhaust gas.

Using this device, the desulfurization performance was measured when coal A (sulfur content in coal was 08%) was burned. However, slaked lime was used as the desulfurization agent A and was sprayed into the desulfurization tower 3 together with water B. The molar ratio of slaked lime to S02 contained in exhaust gas is twice,
Water B was added in an amount of 3% by weight of the exhaust gas. In the dust collector 5, 5% by weight of water B was added to the collected particles, 50% by weight of the collected particles was discarded, and the remaining 50% was processed by the heating device 8. At this time, the average temperature inside the dust collector 5 was 90°C. In the heating device 8, the particles were dried with air C at 200°C, and in the humidifier 10, water B was added at a temperature of 70°C in an amount of 5% by weight to the collected particles. The desulfurizing agent A treated in this way was again spray-supplied to the desulfurizing tower 3.

After removing moisture from the gas at the boiler 1 outlet and the dust collector 5 outlet, the S02 concentration was measured and found to be 640 ppm and 50 ppm, respectively. In other words, 92% of SO2 in the exhaust gas is removed (hereinafter referred to as desulfurization rate 9).
2%).

Example 2 Using the same apparatus as in Example 1, the desulfurization rate was measured under the same conditions. However, in the heating device 8, instead of drying the collected particles with heated air, the economizer of the boiler 1
(Not shown) Ash collected from the bottom (temperature 350°C)
and humidified collected particles were mixed.

At this time, the SO2 concentration at the exit of the dust collector 5 is 55pp.
m, and almost the same desulfurization rate as in Example 1 was obtained.

Example 3 Using the same apparatus as in Example 1, the desulfurization rate was measured under the same conditions. However, the temperature inside the dust collector 5 should be set at 60 to 200°C.
The relationship between the temperature inside the dust collector 5 and the desulfurization rate was investigated. The results are shown in FIG. The lower the temperature inside the dust collector 5, the higher the desulfurization rate tends to be. However, if the temperature is too low, acid corrosion due to SOy etc. tends to occur, so the temperature is preferably 70 to 100°C. Similarly, humidifier 10
The temperature inside is also preferably 100°C or less.

Example 4 Using the same apparatus as in Example 1, the desulfurization rate was measured under the same conditions. However, the temperature inside the heating device 8 should be set at 60 to 400°C.
The relationship between drying temperature and desulfurization rate was investigated. The results are shown in FIG. The desulfurization rate decreases if the drying temperature is too low or too high, and it was found that a drying temperature of 100 to 300°C is preferable.

It is presumed that when the drying temperature is low, the water of crystallization in the reaction products (calcium sulfite and calcium sulfate in this case) is not removed, and the shell of the reaction products is not sufficiently destroyed due to volume change. If the drying temperature is too high, the water adsorbed on the surface of the unreacted desulfurizing agent particles (here, slaked lime Ca(OH)z) will completely evaporate, which is thought to reduce the desulfurization rate6. Even when the collected particles humidified in the chamber 10 are sprayed and supplied again, it is preferable that the gas temperature at the place where the spray is supplied is 100° C. or higher.

Example 5 Using the same apparatus as in Example 1, the desulfurization rate was measured under the same conditions. However, five types of coal with different sulfur contents were used. For each coal, the sulfur content and the SO2 concentration in the gas at the boiler outlet 1 and the dust collector 5 outlet, and among the reacted desulfurization agents (calcium sulfite and calcium sulfate) exhausted from the dust collector, High desulfurization and oxidation rates were obtained, the ratios (hereinafter referred to as oxidation rates) shown in Table 1.

Example 6 Using the same apparatus as in Example 1, the desulfurization rate was measured under the same conditions. However, Cab is used as a desulfurization agent.

The desulfurization performance of each of CaCO3, NaOH and Mg(OH)2 was investigated. The results are shown in Table 2.

Table 2 Comparative Example 1 Using the equipment based on the conventional technology shown in Figure 5, the desulfurization rate and oxidation rate were measured for six types of coal from A coal to F coal under the same conditions as Example 1 and Example 6. It was measured. However, water was added by spraying 3% by weight of the exhaust gas into the flue 4. The results are shown in Table 3. Compared to the desulfurization method according to the present invention, the desulfurization rate and oxidation rate are lower.

Table 3 Comparative Example 2 Using the apparatus based on the conventional technology shown in FIG. The desulfurization performance of The results are shown in Table 4. 0 The desulfurization rate is lower (the dust collector outlet SO2 is higher) than the desulfurization device according to the method of the present invention.

As shown in Table 4 and above, in the apparatus shown in FIG. 1, after the collected particles are humidified by the dust collector 5, they are heated and humidified again. In addition, by repeatedly humidifying the partially reacted desulfurizing agent A particles at temperatures below 100°C and heating them at temperatures above 100°C, the shell of the reaction product is destroyed and contact with acidic harmful substances such as SO2 is prevented. By improving the desulfurization rate, a higher desulfurization rate can be obtained.
The oxidation of the reacted decomposition is promoted. The same effect as the embodiment shown in FIG. 1 can be obtained from the desulfurization tower 3 as shown in the following FIG.
It can also be obtained by changing the structure of

In Fig. 4 (a) and (b), a part of the exhaust gas D is bypassed from the upstream side of the air heater 2 and supplied to the desulfurization tower 3, and the temperature is lowered once by the exhaust gas whose temperature has decreased via the air heater 2. The flow sheet of the example and the structure of the desulfurization tower 3 are shown in the case where the temperature inside the desulfurization tower 3 is raised again.
As shown in FIG. 4(b), the inside of the desulfurization tower 3 has a structure in which the gas flow meanderes due to vertical walls. Figure 4 (
a>, desulfurization agent A is supplied into the desulfurization tower 3 or flue 4, a part of the exhaust gas D is supplied to the rear part of the desulfurization tower 3 through line 12, and water B is sprayed from line 6. The gas temperature, which has been lowered by this, is raised again.

Water B is added to the particles collected by the dust collector 5 through a line 7, and a portion of the collected particles are supplied again to the flue 4 or the desulfurization tower 3.

[Effects of the Invention] According to the present invention, a high desulfurization rate can be obtained because shells of reaction products formed on the surfaces of unreacted desulfurization agent particles collected by a dust collector can be destroyed and removed. Furthermore, since the oxidation of the desulfurizing agent that has already reacted is promoted, its disposal becomes easier.

[Brief explanation of drawings]

Figure 1 is a flow sheet of a desulfurization apparatus in an example of the present invention, Figure 2 is a diagram showing the relationship between humidification temperature and desulfurization rate, Figure 3 is a diagram showing experimental data showing the relationship between drying temperature and desulfurization rate, and Figure 3 is a diagram showing experimental data showing the relationship between drying temperature and desulfurization rate. 4
Figures (a) and (b) are a flow sheet of a desulfurization apparatus and a structural diagram of a desulfurization tower in another embodiment of the present invention, and FIG. 5 is a flow sheet of a conventional technique. 1... Boiler, 3... Desulfurization tower, 4... Flue, 5...
・Dust collector, 8... Heating device, 10... Humidifying device, A
...Desulfurizing agent, B...Water or steam, C...Air,
D...Exhaust gas applicant Babcock Hitachi Co., Ltd. Agent Patent attorney Takayoshi Matsunaga

Claims (6)

[Claims] (1) A desulfurization tower that removes sulfur oxides from the exhaust gas by adding at least one type of alkali metal or alkaline earth metal compound as a desulfurization agent to the exhaust gas discharged from the combustion equipment. In the flue gas desulfurization method using a method, there are two steps: adding water or steam to the desulfurizing agent that has reacted with sulfur oxides in the flue gas containing unreacted desulfurizing agent at a temperature below 100°C, and adding water or steam at a temperature above 100°C. A flue gas desulfurization method comprising the step of drying a desulfurization agent. (2) Adding water or steam at 100°C or lower to the desulfurizing agent that has reacted with sulfur oxide in exhaust gas containing unreacted desulfurizing agent, and drying the desulfurizing agent to which water or steam has been added at 100°C or higher. The flue gas desulfurization method according to claim 1, characterized in that the step is repeated twice or more. (3) The step of drying the desulfurizing agent to which water or steam has been added at a temperature of 100° C. or higher includes drying the desulfurizing agent by mixing ash directly recovered from the combustion device with the desulfurizing agent. flue gas desulfurization method. (4) Unreacted desulfurization agent is the desulfurization agent recovered from the dust collector placed downstream of the desulfurization tower, and water or steam is added to the desulfurization agent at a temperature below 100°C until the exhaust gas temperature reaches Claims 1 and 2, characterized in that the mixture is sprayed into a flue between the combustion device and the desulfurization tower or into the desulfurization tower at a temperature of 100°C or higher.
Or the flue gas desulfurization method described in 3. (5) Water or steam is added to the unreacted desulfurization agent recovered in the dust collector, and inside the flue between the combustion device and the desulfurization tower where the exhaust gas temperature is 100°C or higher or inside the desulfurization tower. The flue gas desulfurization method according to claim 1, 2 or 3, characterized in that the flue gas desulfurization is supplied by spraying. (6) Supplying the unreacted desulfurization agent collected from the dust collector and added with water or steam at 100°C or less to the desulfurization tower,
2. The flue gas desulfurization method according to claim 1, wherein the unreacted desulfurization agent is dried with a portion of the flue gas that is directly supplied from the combustion device to the desulfurization tower without going through a heat exchange treatment step.