JPH0415007B2 - - Google Patents

Description

[Detailed description of the invention]

FIELD OF THE INVENTION The present invention relates to a method for removing gaseous sulfur compounds, in particular sulfur dioxide gas, from the flue gas of a furnace burning sulfur-containing fuels such as coal or petroleum. PRIOR ART AND PROBLEMS SOLVED BY THE INVENTION Traditionally, calcium oxide, calcium carbonate or other alkaline compounds have been introduced into the furnace combustion chamber to reduce the sulfur dioxide content of the furnace flue gas. It is known to reduce In fluidized bed furnaces with circulating beds, the addition of calcium
Optimal temperature range for chemical reactions i.e. 800-1000℃
The sulfur dioxide content of flue gas can be reduced by as much as 90% when the furnace is operated in The sulfur dioxide gas thus absorbed leaves the furnace in the form of gypsum along with the flyash. In other types of furnaces, where higher temperatures than those mentioned above have to be employed, and where the nature of the combustion allows for a short residence time for the additives, the reduction in the sulfur dioxide content of the flue gas is substantially smaller, approx. Less than 50%. Therefore, on an industrial scale, this method
This does not apply to furnaces such as those mentioned above. On the other hand, it is also known that the sour gas content of the flue gas can be reduced by various absorption methods outside the furnace. One of these methods, which is known per se, is the so-called semi-dry method. In this method, the flue gas leaving the furnace is directed into a separate reactor where an aqueous slurry of calcium hydroxide is sprayed in the form of droplets through specific nozzles. The reactor is typically a fairly large vessel in which the flue gas velocity is reduced and the aqueous slurry is sprayed from above, ie from the top of the vessel downward. The temperature of the reactor is about 50-80°C at this time, and the atomization control of the aqueous slurry of calcium hydroxide is very important because water droplets that are too large will remain as liquid at the bottom of the reactor. The aqueous slurry concentration of calcium hydroxide is maintained such that the mature fraction in the flue gas can evaporate the moisture entering the reactor. As a result, the absorption product is recovered as a dry powder. With this method, as much as 90% of sulfur dioxide gas can be removed. Disadvantages of this method include a tendency to block the nozzle, the need for extra investment in preparation and batching equipment for the aqueous slurry of calcium hydroxide, and problems with droplet size control during spraying. . Therefore, the object of the present invention is to convert gaseous sulfur compounds into
Gaseous sulfur, such as sulfur dioxide gas, is removed from the furnace flue gas by converting it into solid sulfur compounds that can be easily separated from the gas and efficiently removed from the furnace flue gas in a simple, economical manner. An object of the present invention is to provide a method for removing compounds. Another object of the present invention is to provide a method that reliably decomposes hydroxide supplied to a furnace at a low temperature, allowing the temperature of the furnace to be set low and requiring less energy. (Means for Solving the Problems) In the method of the present invention, a substance that reacts with gaseous sulfur compounds, particularly sulfur dioxide gas, and water are supplied separately. In the present invention, problems with slurry preparation, handling and feeding are avoided in the following way. That is, (a) a powdered alkali metal hydroxide and/or alkaline earth metal hydroxide is supplied to a furnace together with a sulfur-containing material as a fuel and an oxygen-containing gas, and the hydroxide is dehydrated in the furnace. producing the oxide so that the freshly produced oxide is included in the sulfur dioxide-containing flue gas discharged from the furnace; Water and/or steam is fed into the reactor and/or into the flue gas for in-situ conversion to oxides, and finally (c) the alkali metals and/or alkaline earths obtained as reaction products. Solids containing metal sulfates and optionally their sulfites are separated from the gas. The basic idea of the invention is to feed the hydroxide in powder form into the flue gas and to mix this hydroxide with water and/or
or activated only in the flue gas by steam, thereby reacting with sulfur dioxide gas,
The idea is to form a solid sulfate/sulfite mixture that can be effectively separated from the flue gas by known physical separation methods. The powdered hydroxide is preferably such that the amount of alkali and/or alkaline earth metal is at least equivalent to sulfur in molar proportion according to the reaction equation.
Depending on the sulfur content in the fuel, an amount greater than that required for the reaction is introduced into the combustion chamber of the furnace or into the flue gas leaving the furnace. By introducing the hydroxide separately in powder form into the combustion chamber or into the flue gas duct, it is possible to use simple feed devices, such as pneumatic feed devices. This eliminates nozzle clogging and preparation and batch equipment for aqueous slurries. Conversely, the supply of water and steam from the nozzle is simple and easy. The supply of water or steam to the flue gas is usually
It is carried out at a temperature of 50 to 800°C, preferably 90 to 200°C. If the absorption product is desired to be recovered as a substantially dry powder, only enough water is used to be evaporated by the thermal energy of the flue gases. The invention will now be described with reference to the accompanying drawings, which schematically show suitable apparatus for carrying out the method of the invention.
This will be explained in more detail. In the figure, the furnace in general is designated by the numeral 1. The normally preheated sulfur-containing fuel 4, oxygen-containing gas 5, and calcium and/or magnesium hydroxide 6 are preferably added to the furnace in excess of an amount proportional to the amount of sulfur dioxide gas produced in the combustion chamber. 1 combustion chamber. The expression "in excess" here means that the amount of calcium, magnesium, or calcium and magnesium compound is greater than theoretically required by the reaction equation to react with all the sulfur fed to the combustion chamber. It means that. The hydroxide supplied to the furnace is first dehydrated in the furnace to become an oxide. The oxide can react with sulfite gas, first forming sulfite and then oxidation to form sulfate. Due to the short residence time in the furnace, only a small portion of the oxide can react with the sulfur dioxide gas, even at temperatures sufficient for the reaction.
For this reason, the flue gas 8 containing calcium oxide and/or magnesium oxide, including combustion residues, steam and unabsorbed sulfur dioxide gas, flows into the flue gas duct 7.
is discharged from the furnace combustion chamber through the Typically, the temperature of the flue gas 8 is very low, so that the reaction between calcium and/or magnesium oxides and sulfur oxides is relatively weak; under these conditions,
Oxides are considered relatively inert with respect to desulfurization. As the temperature of the flue gas decreases, the oxides react with the steam in the flue gas and revert to hydroxides. Therefore, the powdered hydroxide can be directly transferred to the flue gas duct 7 or to the reactor 2 after the flue gas duct.
It is preferable to supply the Further, the flue gas 8 is used in a heat exchanger 12 to supply air 5 to the furnace 1.
You can also preheat it. The sulfurous acid gas-containing flue gas 8, which leaves the combustion chamber of the furnace 1 and contains calcium and/or magnesium oxides and optionally calcium and/or magnesium hydroxides, is then fed to the reactor. The reactor is indicated schematically at 2. Water or steam is additionally sparged into the flue gas of reactor 2 in order to activate the oxides and/or hydroxides. This water or steam reacts with the calcium and/or magnesium oxides to form the respective hydroxides, activating the oxides or activating hydroxides already present in the flue gas. Hydroxide is flue gas 8
reacts with residual sulfurous acid gas in it to form the respective sulfites. This sulfite is at least partially further oxidized to the respective sulfate in the presence of oxygen. The amount of water 9 supplied to the reactor 2 is
The heat of the flue gas 8 is adjusted to such an extent that the water supplied to the reactor 2 can be sufficiently evaporated. As a result, the dried fly ash-like reaction product can be removed in the known fly ash separator 3 in the same manner as other dusts. The flue gas 11 is further fed from the flyash separator 3 to the chimney 13, and the separated flyash and reaction products may be subjected to further processing. The order of adding water and hydroxide is not limited in any way. Thus, for example, water or steam may be fed into the furnace and powdered hydroxide may be fed only downstream of the furnace, ie either into the flue gas duct or into a subsequent reactor. A further advantage of the method of the invention is that it can be applied to furnaces equipped with any type of burner. The size of the furnace is not a limiting factor, and there is no need to circulate calcium and/or magnesium hydroxide in the combustion chamber. As a result, expensive circulating beds or complex recirculation devices, and at the same time excessive dust due to the operating principle, as well as separation of the dust, can be avoided. Compared to conventional known spraying methods, spraying water or steam into the reactor 2 is less complex and easier to implement than using slurries that clog the nozzles and are difficult to mix. . (Example) Next, the present invention will be explained in more detail with reference to Examples. Example 1 Coal with a sulfur content of 1.4% is fed at a rate of 70 t/h to a pulverized coal furnace with a heat output of 600 MW. The furnace operates at maximum power. Excess combustion air is supplied, resulting in an oxygen content of 4% in the flue gas. Calcium hydroxide with a calcium hydroxide content of 90% is charged into the furnace in proportion to the amount of sulfur in the fuel entering the furnace. The theoretical equivalent is approximately 2.5 t/h calcium hydroxide. Calcium hydroxide and water and/or steam are injected into the flue gas in the flue gas duct or in a separate reactor following the flue gas duct. In terms of energy economy, it is most economical to increase the water content in the flue gas by blowing water into the flue gas in a separate reactor past the total heat recovery surface. It is. By increasing the moisture content of the flue gas, the calcium hydroxide can be made much more reactive in the furnace, and the calcium hydroxide reacts rapidly with the sulfur oxides in the flue gas. . The higher the moisture content of the flue gas leaving the furnace, the more effectively sulfur dioxide gas is removed from the flue gas. However, from the point of view of energy economy, it is advantageous to operate in such a way that the heat released by the chemical reaction is sufficient to evaporate the added water. If it is necessary to increase the final temperature of the flue gas, this can be achieved using external heat or by a warm flue gas stream. The results are shown in Table 1 below. In Table 1,
Figure 2 shows how much sulfur dioxide gas is removed from the flue gas in percentage when various amounts of calcium hydroxide are introduced into the furnace according to the invention.
The amount of calcium hydroxide is expressed as the molar ratio of the calcium content of the powdered calcium hydroxide to the sulfur content of the fuel fed into the furnace. Flue gas temperature was measured just before the water or steam feed point. However, when the temperature is 80℃, water or steam is supplied directly into the furnace.

【table】

[Table]
B) Immediately before the water supply point Example 2 Calcium-magnesium hydroxide containing 45% calcium hydroxide, 45% magnesium hydroxide and 10% impurities was prepared according to Example 1 with similar operating values. The powder is charged into a powdered coal furnace. Calcium-magnesium hydroxide and water and/or steam are injected into the flue gas in the furnace or in a separate reactor downstream of the flue gas duct. The increased water content increases the reactivity of especially calcium hydroxide, which reacts rapidly with sulfur oxides in the flue gas. When the molar ratio of calcium in calcium hydroxide to sulfur is at least 1, the reaction mainly occurs between calcium hydroxide and sulfur oxide, and the slow-reacting magnesium hydroxide remains mostly unreacted in the reactor. pass through. If calcium-magnesium hydroxide is fed into the hot flue gas, the result is that either the magnesium hydroxide decomposes into magnesium oxide and water, or the entire calcium-magnesium hydroxide is converted into calcium oxide and magnesium oxide. and water. In this case, each oxide can react with sulfur oxides, forming hydroxides again when the flue gas is cooled and the water content increases.
This hydroxide further reacts with sulfur oxides. When the molar ratio of calcium to sulfur is at least 1, the calcium compounds can react faster, so the reaction results and conditions are better than the first one.
This corresponds realistically to the corresponding figures shown in the table. (Effects of the Invention) As described above, according to the present invention, a spray method that can efficiently remove sulfur dioxide gas without the need for expensive equipment such as a fluidized bed furnace is adopted, while eliminating the drawbacks of the conventional spray method. Problems such as nozzle clogging due to hot slurry, droplet size control, adjustment and batch equipment use can be avoided. As described above, the present invention can provide a method for recovering and removing sulfite gas in flue gas in the form of solid sulfur compounds in a simple, economical, and highly efficient manner. In addition, compared to the conventional method of adding calcium carbonate to the furnace, the method of the present invention has the advantage that, for example, calcium hydroxide releases water at a lower temperature (100°C) than calcium carbonate (898°C) and completely decomposes at 580°C. ), it is completely decomposed into calcium oxide, so it has the advantage that the furnace temperature can be set low and requires less energy.

[Brief explanation of drawings]

The drawing schematically shows an apparatus suitable for carrying out the method of the invention. 1...Furnace, 2...Reactor, 3...Fly ash separator, 4...Sulfur-containing fuel, 5...Oxygen-containing gas, 6...Calcium hydroxide/magnesium, 7
... Flue gas duct, 8,11 ... Flue gas.

Claims (1)

[Scope of Claims] 1. A method for removing gaseous sulfur compounds from a flue gas of a furnace, comprising: The hydroxide is fed into a furnace together with the material and an oxygen-containing gas, and the hydroxide is dehydrated in the furnace to form its oxide, and the hydroxide is then transferred to the sulfur dioxide-containing flue gas discharged from the furnace. (b) supplying water and/or steam into the reactor and/or into the flue gas for the in-situ conversion of said freshly produced oxides to hydroxides which react with the sulfur dioxide gas; and, finally, (c) a furnace characterized in that the solids containing the alkali metal and/or alkaline earth metal sulfates and optionally their sulfites obtained as reaction products are separated from the gas. A method for removing gaseous sulfur compounds from flue gases. 2. A method according to claim 1, wherein the powdered hydroxide is supplied in excess of an amount proportional to the amount of sulfur in the flue gas. 3 Add water and/or steam to flue gas temperature
The method according to claim 1 or 2, wherein the spraying is carried out at a temperature of 50 to 800°C. 4. Spraying water into the flue gas to the maximum extent that is evaporated by the flue gas and the thermal energy generated by the reaction.
The method according to any of paragraph 3. 5. The method according to any one of claims 1 to 4, wherein the hydroxide supplied is calcium hydroxide or a calcium hydroxide-magnesium mixture.