FI78846B - FOERFARANDE FOER AVLAEGSNANDE AV GASFORMIGA SVAVELFOERENINGAR OCH SVAVELDIOXID UR ROEKGASER I EN PANNA. - Google Patents

Description

The present invention relates to a method for removing gaseous sulfur compounds, and in particular sulfur dioxide, from the flue gases of a boiler burning sulfur-containing fuels, such as coal dust or an oil boiler.

It is already known to reduce the sulfur dioxide content of boiler flue gases by feeding calcium oxide, calcium carbonate or some other alkaline compound to the boiler combustion chamber. In a fluidized bed furnace with a circulating bed, the addition of lime can reduce the sulfur dioxide content of the flue gases by up to 90% when the boiler is operating in the best possible temperature range for chemical reactions, namely 800-1000 ° C. The sulfur dioxide absorbed in this way leaves the boiler together with the air dust in the form of gypsum.

In other boilers, where higher temperatures than those mentioned above have to be used and where the residence time of the additive is short due to the nature of the combustion event, the reduction of flue gas sulfur dioxide is expected to be substantially lower, about 50% or less, so this method is not production scale. applied to such boilers.

On the other hand, it is known that the sulfur dioxide content of flue gases can be reduced by various off-boiler absorption methods. One such method known per se is the so-called spray method, in which the flue gases from the boiler are fed to a separate reactor, into which aqueous slurry of calcium hydroxide is sprayed in small droplets through special nozzles. The reactor is typically a larger vessel in which the flue gas velocity is allowed to decrease and water 2 78846 sludge is sprayed down from the top of the vessel. The temperature of the reactor o is then about 50-80 C and the control of the spraying of the calcium hydroxide aqueous slurry is very important, because too large droplets remain wet on the bottom of the reactor. The consistency of the aqueous slurry of calcium hydroxide is kept so high that the heat contained in the flue gases is sufficient to evaporate the water entering the reactor so that the absorption product is recovered in the form of a dry powder. The method makes it possible to remove up to 90% of sulfur dioxide. Disadvantages of the method include the sensitivity of the nozzle to clogging, the additional equipment for preparing and dosing the calcium hydroxide aqueous slurry, which increases the investment cost, and the difficulty in adjusting the droplet size during spraying.

The object of the present invention is to provide a method for removing gaseous sulfur compounds, such as sulfur dioxide, from boiler flue gases, by which gaseous sulfur compounds can be converted into solid sulfur compounds which can be easily separated from the gases and thus efficiently removed from boiler flue gases in a simple and economical manner.

The main features of the invention appear from the appended claims.

In the process of the present invention, hydroxide and water reactive with gaseous sulfur compounds, and in particular sulfur dioxide, are fed to the process separately, thus avoiding sludge preparation treatment and especially feed difficulties, so that a) powdered alkali and / or alkaline earth metal hydroxide is fed to the combustible sulfur-containing substance and oxygen. b) the boiler and / or flue gases are separately sprayed with water and / or steam and finally li 3 78846 c) the solid containing alkali and / or alkaline earth metal sulphate and possibly sulphite is separated from the gases.

The basic idea of the invention is thus that the hydroxide is fed to the boiler as a powder and activated only in situ in the flue gases with water and / or steam, whereby it reacts with sulfur dioxide to form a sulphate / sulphite mixture which can then be efficiently removed from the flue gases by conventional physical dust separation methods.

The powdered hydroxide is fed to the combustion chamber of the boiler according to the sulfur content of the fuel so that the amount of alkali and / or alkaline earth metal in the molar ratio of the reaction formula is at least equal to sulfur, but preferably greater than the amount required for the reaction. By feeding the hydroxide as a powder separately to the combustion chamber, simple e.g. pneumatic feeders can be used, thus avoiding clogging of the nozzles and the use of additional water slurry preparation and dosing equipment. Similarly, the supply of water and steam through the nozzles is simple and easy.

The supply of water or steam to the flue gases takes place in practice in the temperature range 50-800 ° C, but preferably in the temperature range 90-200 ° C. If it is desired to recover the absorption product as a dry powder, water is used in the injection only to the extent that the thermal energy of the flue gases is able to evaporate it into steam.

The invention will be described in more detail below with reference to the accompanying drawing, which schematically shows an apparatus suitable for carrying out the method according to the present invention.

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In the drawing, the boiler is generally indicated by the reference number 1. The combustion chamber 4, the oxygen-containing gas 5 and the powdered calcium and / or magnesium hydroxide 6 are fed to the combustion chamber of the boiler 1, preferably in excess of the sulfur dioxide gas produced in the combustion chamber. The word "excess" in this context means that the amount of calcium, magnesium or calcium and magnesium contained in the hydroxide is greater than what would theoretically be required by the reaction formula to react with all the sulfur fed to the combustion chamber.

The hydroxide fed to the boiler first dehydrates the oxide in the boiler. The oxide, in turn, can react with sulfur dioxide to form sulfite first and then sulfate upon oxidation. Due to the short residence time in the boiler, only part of the oxide has time to react with sulfur dioxide at a sufficiently high reaction temperature to remove calcium and / or magnesium oxide-containing flue gases 8 containing combustion waste 8 from the combustion chamber of the boiler along the flue gas duct 7.

The temperature of the flue gases is practically so low that the reaction between calcium and / or magnesium oxide and sulfur dioxide takes place relatively poorly and the oxides can be considered inactive to desulfurize under these conditions. As the temperature of the flue gases decreases, the oxides can react back to hydroxide with the water vapor in the flue gases. Thus, it is more preferable to feed the powdered hydroxide directly to the flue gas duct 7 or to the subsequent reactor 2. The flue gases can further heat the air supplied to the boiler 1 in the heat exchanger 12.

Calcium and / or magnesium oxide and possibly hydroxide-containing flue gases containing sulfur dioxide from the combustion chamber of boiler 1 are then passed to a reactor, generally designated 5 78. However, in order to activate the oxide and / or hydroxide, water or adds water vapor which reacts with calcium and / or magnesium oxide to form the corresponding hydroxide, activating it as well as any hydroxide already present in the flue gases. The hydroxide, in turn, reacts with the remaining sulfur dioxide in the flue gases 8 to form the corresponding sulfite, which is further at least partially oxidized to the corresponding sulfate in the presence of oxygen. The amount of water 9 fed to the reactor 2 is adjusted so small that the heat of the flue gases 8 is sufficient to evaporate the water fed to the reactor 2. In this case, the substantially dry dusty reaction product can be removed, like other dust, in a conventional dust separator 3, from which the flue gases 11 are passed to the barrel 13 and the separated dust 12 is passed to a possible further treatment.

The order of addition of the water is by no means critical, and thus, for example, water or steam can be fed to the boiler or only after the boiler to either the flue gas duct or the reactor following it.

As a further advantage of the method according to the present invention, it should be mentioned that the method can be applied to any type of boiler equipped with a combustion device. The size of the boiler is not a limiting factor and it is not necessary to recycle calcium and / or magnesium hydroxide in the combustion chamber, thus avoiding an expensive circulating bed solution with cumbersome recycling equipment and at the same time additional dust and dust separation due to its operating principle. In addition, compared with the known spray method, the spraying of water or steam into the reactor 2 is considerably simpler and easier to carry out than when using a slurry which clogs the nozzles and is difficult to mix.

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The invention is described in more detail below by means of examples. Example 1

A coal dust boiler with a capacity of 600 MW is fed with 70 tn / h of coal with a sulfur content of 1.4% at full capacity. Excess combustion air is supplied so that the oxygen content in the flue gases is 4%.

Calcium hydroxide with a calcium hydroxide content of 90% is fed to the boiler in a certain variable proportion to the amount of sulfur that enters the boiler with the fuel. The theoretical equivalent amount is about 2.5 tn / h p.o. of calcium hydroxide.

The flue gases are sprayed with calcium hydroxide and water and / or steam either in the flue or in a separate downstream reactor.

It is most economically advantageous to increase the humidity of the flue gases by spraying water on them after all the heat recovery surfaces in a separate reactor.

Adding moisture to the flue gases makes calcium hydroxide highly reactive, allowing it to react rapidly with the sulfur oxides contained in the flue gases. The wetter the flue gases as they exit, the more efficiently the sulfur dioxide will be removed from the flue gases. However, from an energy point of view, it is advantageous to operate in such a way that the heat released in the chemical reactions is sufficient to evaporate the added amount of water. If it is desired to increase the final temperature of the flue gases, this is done either with external heat or with a warm flue gas flow.

The results are shown in the table below, which shows the percentage of sulfur dioxide removed from the flue gases when varying amounts of calcium hydroxide were fed to the boiler in accordance with the present invention, expressed as the calcium content of powdered calcium hydroxide and the sulfur content of the fuel fed to the boiler. Flue gas temperatures are measured immediately before the water or steam supply point, except at 800 C where the water or steam is fed directly to the boiler.

. B)

Ca / S Flue gas Flue gas SO - 2 temperature temperature reduction, 2 o A) o 0.48 800 C 108 C 42% oo 0.52 50 C 65 C 56% 1.52 202 C 74 C 77% oo 1.56 90 C 68 C 82% O o 2.20 200 C 72 C 87% 2.22 120 C 62 C 96% oo 2.3 110 C 68 C 93% oo 2.5 90 C 66 C 97% 4.1 800 C 110 C 72% oo 4.0 120 C 68 C 98% A) water or steam supply to the boiler B) immediately before the water supply point

Example 2

Under the corresponding operating conditions, calcium-magnesium hydroxide having 45% calcium hydroxide, 45% magnesium hydroxide and 10% impurities is fed to the boiler according to Example 1. Calcium magnesium hydroxide and water and / or steam are fed to the flue gases either in a boiler or in a separate reactor downstream of the boiler.

The addition of moisture makes calcium hydroxide in particular highly reactive, whereupon it reacts rapidly with the sulfur oxides contained in the flue gases. If the molar ratio of calcium to sulfur in the calcium hydroxide is at least 8,78846, the reaction takes place mainly between the calcium hydroxide and the sulfur oxides and the slower magnesium hydroxide passes through the reaction zone substantially as such.

Claims (5)

9,78846 A method for removing gaseous sulfur compounds, such as sulfur dioxide, from boiler flue gases, characterized in that a) powdered alkali and / or alkaline earth metal hydroxide (6) is fed to the boiler (1) in addition to the combustible sulfur-containing substance (4) and oxygen-containing gas (5); b) separately (2) water (9) and / or steam are sprayed into the boiler (1) and / or the flue gas (8) and finally c) the alkali and / or alkaline earth metal sulphate obtained as a result of the reaction, and optionally -sulfite-containing solid (12) is separated (3) from gases (11). Method according to Claim 1, characterized in that the powdered hydroxide (6) is fed in excess of the sulfur in the flue gases. Method according to Claim 1 or 2, characterized in that the spraying of water (9) and / or steam is carried out at a temperature of the flue gases (8) of 50 to 800 ° C, preferably 90 to 200 ° C. Method according to one of Claims 1 to 3, characterized in that a maximum of as much water (9) is sprayed into the flue gas boiler (8) as the thermal energy produced by the flue gases and reactions is able to evaporate. Process according to one of Claims 1 to 4, characterized in that the hydroxide to be fed is calcium hydroxide or a mixture of calcium-magnesium hydroxide.