JPH03213122A - Method and apparatus for desulfurizing exhaust gas - Google Patents

Abstract

PURPOSE:To efficiently perform inexpensive desulfurization by spraying water to the coal ash coming flying from a combustion furnace to humidify the same within the unsaturation range of the water in gas and performing desulfurization only by the coal ash. CONSTITUTION:When sulfur oxide contained in the exhaust gas discharged from a pulverized coal firing furnace 1 is removed, water is sprayed to the coal ash coming flying from the firing furnace 1 from a water spray nozzle 12 without supplying a desulfurizing agent to the firing furnace 1 and the coal ash is humidified within the unsaturation range of the water in the gas to perform desulfurization only by the coal ash. As a result, desulfurization can be performed efficiently and inexpensively.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a flue gas desulfurization method and apparatus for removing sulfur oxides (SOx) contained in the flue gas of a pulverized coal combustion furnace such as a pulverized coal boiler; Specifically, by not adding an alkaline agent from outside the system, or by adding a small amount of alkaline agent to the coal ash, the sulfur oxides in the exhaust gas are removed mainly by the alkali content in the coal ash, and the sulfur oxides are absorbed in powder form. The present invention relates to a method and apparatus for desulfurizing exhaust gas from which ash can be extracted.

[Conventional technology]

Conventional flue gas desulfurization equipment typically uses an alkaline absorbent in the form of a solution or slurry mixed with water in an amount equal to or more than the molar equivalent of the sulfur oxides to be removed from the flue gas. For this reason, the sulfur compounds generated after desulfurization are in an aqueous solution or slurry state, and in order to separate the sulfur compounds, they can be separated into solid-liquid as gypsum, etc., or crystallized by firing/condensing as sodium sulfite or sodium sulfate. It is necessary to separate.

Transporting such absorbents from another location to the flue gas desulfurization equipment also increases desulfurization costs.

Conventionally, in JP-A-60-22920, a slurry containing a Ca-based desulfurizing agent is sprayed into the exhaust gas in the flue, and the desulfurizing agent is dried and powdered by the sensible heat of the exhaust gas, and SO,
A dry desulfurization method is disclosed in which the reaction is performed with

In addition, conventionally, there is a method of desulfurization by charging limestone into the combustion chamber of a boiler, etc., but in this case, even if the amount of limestone added is more than the molar equivalent of sulfur, the desulfurization rate is as low as 60%.
Moreover, the amount of ash increases due to the limestone added, and the product is a mixture of coal ash, gypsum, calcium sulfite, unreacted limestone, etc., and has low utility value.

Furthermore, Japanese Patent Application Laid-Open No. 62-200107 describes a method in which a desulfurizing agent is directly introduced into a furnace, and after the flue gas is rapidly cooled by water spray, the collected fly ash lumps containing unreacted desulfurizing agent are crushed and separated. , discloses an in-furnace desulfurization method in which unreacted desulfurization agent is circulated to the furnace.

Furthermore, JP-A-63-141629 discloses that in the exhaust gas generated from a combustion furnace in which an alkaline agent is blown into the furnace,
A desulfurization method has been disclosed in which only water is sprayed without adding an alkaline agent, the humidity is increased within the unsaturated range of the water, and SOX in the exhaust gas is reacted with the scattered alkaline agent. There is. However, this Japanese Patent Application Publication No. 63-1416
In the method described in No. 29, the amount of sulfur oxides removed by coal ash is greater than the amount of sulfur oxides removed by the added alkali. This JP-A-63-
In the method described in Japanese Patent No. 141629, a desulfurization rate of 80 to 90% is achieved, but the utilization rate of the alkaline agent introduced is low.

[Problem to be solved by the invention]

As mentioned above, the conventional methods involve charging an alkaline agent into the furnace or spraying a slurry of the alkaline agent into the furnace or into the exhaust gas, and therefore have various problems. In other words, the amount of waste increases, and the cost of raw materials and transportation increases. Furthermore, when using slurry, there are problems such as wear and blockage of the device.

On the other hand, although some coal ash is mixed into cement and used as building materials, most of it is disposed of in landfills. Therefore, a method that can extract coal ash used as a desulfurization agent in the same powder form as conventional coal ash is easy to handle without changing conventional equipment, and there are no problems with wastewater treatment. , has many advantages.

The present invention was made in view of the above points and to solve the above problems, and in order to remove sulfur oxides from exhaust gas to a predetermined desulfurization rate or higher, the desulfurization ability of coal ash is fully utilized. The object of the present invention is to provide a method and apparatus for desulfurizing exhaust gas, which desulfurizes efficiently and inexpensively.

[Means and effects for solving the problem] In order to achieve the above object, the method of claim 1, as shown in FIG. In the method of removing sulfur oxides contained in the gas, water is sprayed onto the coal ash coming from the combustion furnace 1 without supplying a desulfurizing agent to the combustion furnace 1, and the water in the gas is humidified within the unsaturated range. As a result, it is characterized by desulfurization using only coal ash.

Further, the method of claim 2 is a method for removing sulfur oxides contained in exhaust gas discharged from a pulverized coal combustion furnace 1, as shown in FIG. After the coal ash collected by a3 is activated by further pulverization into fine powder, it is reinjected into the exhaust gas, and the desulfurization agent is supplied to the combustion furnace 1 in a powdered state dispersed in the exhaust gas. without doing,
It is characterized in that it is used as a desulfurizing agent together with the coal ash coming from the combustion furnace 1, and by spraying water onto these powders and humidifying the gas within the unsaturated range of water in the gas, desulfurization is carried out.

Moreover, as shown in FIG. 3, in the method of claim 3, instead of activating the coal ash collected by the dust collector 3 in FIG. 2 by further pulverizing it into fine powder,
The coal ash collected is activated by adding an alkali in an amount less than the molar equivalent of the sulfur oxide in the exhaust gas to coat the coal ash, or by pulverizing the coated material into a fine powder. It is characterized by 6
is an exhaust gas heater that reheats the gas leaving the desulfurization tower as a measure to prevent blockages and corrosion in dust collectors and flues.

Moreover, as shown in FIG. 4, in the method of claim 4, instead of activating the coal ash collected by the dust collector Ia3 in FIG. The collected coal ash is reacted with alkali in an amount less than the molar equivalent of sulfur oxides in the exhaust gas, as well as gypsum and lime, which transforms the coal ash into a porous or acicular microstructure and activates desulfurization. It is characterized in that it is activated by performing a treatment to increase the oxidation rate or by pulverizing the substance into a fine powder.

Furthermore, the method of claim 5 is the method of claim 1.2.3 or 4, in which a desulfurization tower 2 is used as shown in FIGS.
Water is sprayed in at least two stages by the water spray nozzle 12, and after the upstream spray water has completely evaporated, the next stage of water spray is carried out.

As shown in FIG. 1, the apparatus of claim 6 includes a desulfurization tower 2 equipped with a water spray nozzle 12 installed in the exhaust gas duct 11 of the pulverized coal combustion furnace 1, and It is characterized in that a coal ash circulation line 13 is connected to the exhaust gas duct 11 or the desulfurization tower 2 for reinjecting a portion of the ash collected by the dust collector.

Further, the apparatus according to claim 7 is characterized in that, as shown in FIG. 2, a crusher 7 is installed in the coal ash circulation line 13 according to claim B.

Moreover, as shown in FIG. 3 or FIG. 4, the apparatus of claim 8 is an activation processing device 5a for adding alkali to the coal ash circulation line 13 in claim B and mixing it with coal ash.
It is characterized by installing an activation treatment device 5b for adding or reacting alkali and gypsum/lime with coal ash.

In the method and apparatus of the present invention, as the alkali, C
a(OFI)z, CaO, NaOH, Mg(OH), etc. are used.

Furthermore, "activation" in the present invention refers to the following.

(1) Crushing of collected coal ash.

(2) Coating the surface of the collected coal ash particles with a small amount of alkali, or pulverizing this material.

(3) A small amount of alkali and a small amount of gypsum/lime are added to the collected coal ash and reacted to form a structure such as ettringite or zeolite, which expands the surface area of the ash and increases its reactivity.

Or crush this thing.

In the method of the present invention, if a relatively low desulfurization rate is sufficient, the amount of alkali input is set to 0 and desulfurization is performed only with coal ash in the combustion exhaust gas, or the collected coal ash is pulverized and reactivated. After decomposition, it is mixed with coal ash coming from the combustion exhaust gas and desulfurized.

In order to ensure a higher desulfurization rate, in order to further activate the collected coal ash, before crushing the collected coal ash,
Alternatively, remove the water-soluble alkaline agent after crushing, add a molar amount below the stoichiometric amount for the chemical reaction of sulfur, sprinkle it on coal ash, and re-input it in dry powder form, and mix it with the coal ash flying from the combustion exhaust gas. By desulfurizing, the total desulfurization rate is improved. Furthermore, alkali and gypsum/lime are added to the collected coal ash to cause a reaction, and once the reaction activity has been improved, the coal ash is reinjected in the same manner to improve the desulfurization rate.

Furthermore, since it is possible to improve the desulfurization rate by efficiently bringing the powder into contact with the sprayed water, repeating water spraying leads to an improvement in the overall efficiency.

The desulfurization method of the present invention is based on the fact that the distribution of alkaline content contained in coal ash particles and that all the metal elements contained as components of coal ash have a desulfurization effect to a greater or lesser extent, and that the origin of coal ash is This is thought to be due to the desulfurization effect caused by structural micropores etc.

Normally, when coal ash is in a dry state without being sprayed with water, its desulfurization ability is low even when the temperature drops, and it cannot be used in desulfurization equipment. However, by wetting the particle surface with water spray, the desulfurization ability is greatly increased, and desulfurization of 30 to 60% becomes possible. This varies depending on the type of coal, the composition of the ash, the combustion conditions, etc., but for several types of coal ash tested, all of them demonstrated an absorption capacity of around 50% (see Table 1 below). .

The following has been revealed about various types of coal through photographs of the microscopic features of combustion ash of specific coal varieties and analysis of the distribution of elements in the ash. In other words, the elements in coal ash are almost uniformly distributed in the particles;
There are few things that are unevenly distributed.

On the other hand, the immobilization reaction of sulfur oxides occurs relatively only on the surface of coal ash particles. Therefore, by pulverizing the absorbent coal ash to increase its surface area and reintroducing it, the surface area available for immobilization reactions is increased and the absorption capacity per unit weight is increased. Components in the ash include Fetus, CaO,
In addition to MgO, NagOlK, and O, AI, Si, and Ti
Depending on the ash production conditions, some of these oxides have reactivity with sulfur oxides and are useful for fixing sulfur content. In the flue, the exhaust gas flows in a state where ash is dispersed in the exhaust gas. When water is sprayed into the exhaust gas, the water evaporates and the overall temperature of the exhaust gas gradually drops, but above the saturation temperature,
Preferably, the amount to be sprayed is 5° to 15° C. higher than the saturation temperature. Furthermore, during spraying, it is preferable to wet the coal ash particles as uniformly as possible. For this purpose, a plurality of spray nozzles are arranged evenly across the entire flow path cross section, and as shown in FIGS. 5 to 7,
It is advantageous to increase absorption performance by arranging at least two or more stages in the flow direction to increase opportunities for particles to get wet. Furthermore, the next stage of spraying is performed after the previous stage's spray water has completely evaporated. This is because the absorption reaction is increased by the presence of water.

As shown in the above-mentioned Japanese Unexamined Patent Publication No. 60-22920, alkali is made into a slurry and sprayed, and the amount of water is such that at the outlet of the absorption section, the gas is unsaturated with water and the powder is in a dry state. There is a method of introducing a slurry, but compared to this conventional method, it is possible to significantly increase the amount and surface area of particles dispersed in the exhaust gas, which is effective in increasing the absorption rate. The reason is that when the amount of powder is large relative to the amount of water, it becomes difficult to disperse the particles, and the amount of powder is limited.

The desulfurization section requires space for water to completely evaporate due to the relationship between flow rate and residence time, so increasing the cross-sectional area will shorten the flow path.

Next, if the amount of reactive alkali in coal ash is insufficient, N
A soluble alkali such as aOH is added in the form of an aqueous solution before re-grinding to compensate for the insufficient amount of alkali. This amount may be supplemented by the remaining amount of sulfur oxides absorbed by coal ash.

The molar equivalent of the amount of alkali used relative to the sulfur oxide is usually 1 or more, but in this case it may be 1 or less.

In addition, one method to improve the activity of coal ash is to add an alkali agent, a small amount of Ca (OH), and gypsum to the coal ash, add water, and cure it at 80 to 100°C for 5 to 15 hours. Ettringite (3CaO・AI,
O,'3CaSOm'32HtO) or calcium monosulfoaluminate hydrate (3CaOHAlzol
・CaSO4・1211.0), and if this is used after drying and pulverizing, the alkali component in coal ash can be used extremely effectively. The reason is that the needle-like structure develops and the surface area is extremely expanded (10 times or more).

Therefore, when the alkaline component in the ash reacts with SO□, the crystals are broken and the surface is renewed, creating a new reaction surface. This improves desulfurization performance. In this case, Ca (Off) z to be added is 10 to 30-t% of the coal ash coming from the combustion furnace side,
It is desirable to add dihydrate gypsum in an amount of 10 to 30-t% based on the coal ash coming from the combustion furnace.

Note that CaO may be used instead of Ca (OH) x.

The above reaction is performed with Na, K, M other than calcium.
There are also elements such as g, Fe, etc. (different substances), and for example, substances such as zeolite are generated to increase the adsorption surface. The present invention also includes the reuse of reactants, including such.

Powder suspended in exhaust gas has the effect of agglomerating due to water spray, and relatively fine powder agglomerates into thick particles (and becomes easier to settle). Therefore, the amount of coal ash that settles in the desulfurization section increases. I will do it.

Although this phenomenon is an advantage in reducing the load on the subsequent dust collector, it is a disadvantage for desulfurization in the absorption section because it shortens the space retention time. In order to reduce this drawback, the flow rate should be set a little higher to reduce sedimentation, or the flow should be flowed upward to extend the retention time. Therefore, it is desirable that the absorber allows the exhaust gas to flow in the vertical direction.

In the method of the present invention, the condition of the exhaust gas is below the saturated humidity, and the structural material can be of a slightly lower grade and cheaper material than in wet desulfurization equipment, but desirably, the absorption part It is desirable to protect the material by exchanging heat between the incoming exhaust gas and the exhaust gas at the outlet of the absorber, or by heating the outlet exhaust gas. By heating, cheaper materials can be used for subsequent equipment.

〔Example〕

Examples of the present invention will be described below.

Example 1 As shown in Table 1, ash content 15.5wt%, 8% 0.3
4 wt% of coal (referred to as coal type A) was burned in a pulverized coal combustion boiler 15 shown in FIG. The exhaust gas was introduced into the desulfurization tower 2, water was sprayed thereon, and the gas was heated within the unsaturated range of the water in the gas, and the desulfurization performance was measured.

The generated 5Ox4 degree in the exhaust gas before entering the desulfurization tower 2 is 249
It was ppm. The desulfurization efficiency was as shown in Table 1 when the exhaust gas temperature was set to 60° C. and 70° C. after desulfurization by spraying water to this exhaust gas.

Example 2 Coal with ash content of 4.3 wt% and ash content of 0.47 wt% (coal type B
) under the same conditions as Example 1.

The results were as shown in Table 1.

Table 1 Example 3 Coal ash of the coal type in Example 1 was heated in a ball mill for 20 hours.
The pulverized coal ash that has been pulverized to increase the Blaine specific surface area from 3000c+J/g to 6000e+J/g is mixed into the exhaust gas from the combustion furnace according to the flow shown in FIG.
Note that the ratio (weight) of coal ash in the exhaust gas from the combustion furnace and crushed coal ash was 1:2. As shown in Table 2, if the desulfurization rate in the case of only unpulverized coal ash is 45 to 55%, the desulfurization ability in this example was 60 to 70%.

Example 4 An aqueous solution containing 0.5 to 0.8 molar equivalent of NaOH, which is an oxide of sulfur in exhaust gas, was added to the coal ash of the coal type in Example 1 to coat the coal ash. The ash is added to the exhaust gas from the combustion furnace according to the flow shown in Figure 3.
The gas is mixed, introduced into a desulfurization tower, and sprayed with water to add Y! to within the unsaturation range of the water in the gas! Desulfurization treatment was carried out by doing this.

Note that the ratio (weight) of coal ash in the exhaust gas from the combustion furnace to coal ash coated with alkali (excluding alkali) was 1:2. As shown in Table 2, the desulfurization ability in this example was 60 to 95%.

Example 5 Coal ash of the coal type in Example 1 was mixed with 10 wt% of gypsum based on the coal ash coming from the combustion furnace and 0.8 molar equivalent of NaOH of sulfur oxide in the exhaust gas, and coal ash coming from the combustion furnace. According to the flow shown in Fig. 4, the coal ash produced by adding 20ht% of Ca (OH) z and water to react with ettringite is mixed with the exhaust gas from the combustion furnace, introduced into the desulfurization tower, and sprayed with water. Desulfurization treatment was carried out by humidifying the gas within the unsaturated range of water in the gas. Note that the ratio (heavy M) of coal ash in the exhaust gas from the combustion furnace and coal ash subjected to ettringite reaction (excluding alkali, gypsum, and Ca(OH) 2 ) was 1:2. As shown in Table 2, the desulfurization ability in this example was 80 to 90%.

Table 2 [Note] Desulfurization ability is the desulfurization rate of coal ash only. The desulfurization rate is shown as above.

〔Effect of the invention〕

Since the present invention is configured as described above, it exhibits the following excellent effects.

(1) Desulfurization using only coal ash or desulfurization mainly using coal ash saves raw material costs and reduces operating costs.

(2) Only water is sprayed into the exhaust gas,
Since the exhaust gas is unsaturated, the amount of heat required for reheating may be small.

(3) Although the coal ash is somewhat moist, it is in the form of dry bundles, and conventional ash processing equipment can be used as is.

(4) Moisture content evaporates due to the heat generated by pulverizing the coal ash, so it is easy to re-inject the coal ash.

(5) Re-injection (crushed ash can be put into the flue, so
There is almost no effect on the boiler.

(6) Since the equipment can be manufactured using inexpensive materials, equipment costs are reduced.

(7) There is no wastewater and no wastewater treatment equipment is required.

[Brief explanation of drawings]

1 [N is a flow sheet showing one embodiment of the exhaust gas desulfurization apparatus of the present invention, FIGS. 2 to 4 are flow sheets showing other embodiments of the exhaust gas desulfurization apparatus of the present invention, and FIGS. Figure 7 is an explanatory diagram showing an example of a desulfurization tower, and Figure 8 is Example 1 of the present invention.
．． This is a flow sheet of the device used in 2. 1... Combustion furnace, 2... Desulfurization tower, 3... Dust collector, 4
...Chimney, 5a, 5b...Activation processing device, 6...
・Exhaust gas heater, 7... Pulverizer, 11... Exhaust gas duct, 12... Water spray nozzle, 13... Coal ash circulation line, 15... Pulverized coal combustion boiler

Claims (1)

[Claims] 1. A method for removing sulfur oxides contained in exhaust gas discharged from a pulverized coal combustion furnace, in which water is added to coal ash flying from the combustion furnace without supplying a desulfurization agent to the combustion furnace. A method for desulfurizing exhaust gas characterized by desulfurizing using only coal ash by spraying and humidifying water within the unsaturated range of gas. 2 In a method for removing sulfur oxides contained in exhaust gas discharged from a pulverized coal combustion furnace, coal ash collected by a dust collector is activated by further pulverization into fine powder, and then reinjected into the exhaust gas. The desulfurization agent is dispersed in powder form in the exhaust gas and used as a desulfurization agent together with the coal ash that comes from the combustion furnace without being supplied to the combustion furnace, and these powders are sprayed with water. , a method for desulfurizing exhaust gas, characterized in that desulfurization is carried out by humidifying water within the unsaturated range of the gas. 3. Instead of activating the coal ash collected by the dust collector by further pulverizing it into a fine powder, alkali is added to the coal ash collected by the dust collector in an amount less than the molar equivalent of sulfur oxides in the exhaust gas. 3. The method for desulfurizing exhaust gas according to claim 2, further comprising activating the method by coating coal ash or by pulverizing the coated material into fine powder. 4. Instead of activating the coal ash collected by the dust collector by further pulverizing it into fine powder, the coal ash collected by the dust collector is mixed with alkali in an amount less than the molar equivalent of the sulfur oxide in the exhaust gas, and add gypsum and lime to react,
3. The activated coal ash is activated by altering the coal ash into a porous or acicular microstructure to increase the desulfurization activity, or by pulverizing the coal ash into fine powder. Method for desulfurizing exhaust gas. 5. The method of reducing exhaust gas according to claim 1, 2, 3 or 4, wherein the water spraying step is performed in at least two stages, and the next stage of water spraying is performed after the upstream spray water has completely evaporated. Desulfurization method. 6. A desulfurization tower (2) equipped with a water spray nozzle (12) is installed in the exhaust gas duct (11) of the pulverized coal combustion furnace (1), and the exhaust gas duct (11) or An exhaust gas desulfurization device characterized in that a coal ash circulation line (13) for reinjecting a portion of ash collected by a dust collector is connected to a desulfurization tower (2). 7. The exhaust gas desulfurization device according to claim 6, characterized in that a crusher (7) is installed in the coal ash circulation line (13). 8 An activation treatment device (5a) for adding an alkali to the coal ash circulation line (13) and mixing it with the coal ash, or adding an alkali and gypsum/lime to react with the coal ash.
7. The exhaust gas desulfurization apparatus according to claim 6, further comprising: or 5b).