DE2126818A1 - Process for the desulfurization of combustion gases - Google Patents

Description

(prio 1. 6. 70 US 41, 942-8249)

Esso Research and
Engineering Company

, NJ / V. St. A. Hamburg, May 28, 1971

Process for the desulphurization of combustion exhaust gases.

The invention relates to a method for the removal of sulfur dioxide from combustion exhaust gases, in particular a Naßve: iahren, in which the sulfur dioxide is removed by contact with an aqueous absorbent.

There are already different procedures for the removal of Sulfur dioxide from combustion exhaust gases has been proposed, but none of them has heretofore found general use in industry found. The previously known processes can be divided into wet processes and dry processes. In the wet process a usually aqueous absorption solution is used to separate the sulfur dioxide from a gas stream.

There are several requirements to be met by a desulfurization process for combustion exhaust gases. Once the procedure has to be in

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be able to absorb most of the sulfur dioxide content of the exhaust gas, preferably 90% or more of the SO, to be removed under strongly fluctuating conditions with regard to the content. Second, the process must not have any problems as regards bring about air or water pollution. Third, the procedure should be easy to perform and maintain be. Furthermore, the process should be as low as possible; for this it is necessary in many cases, that there is a salable by-product. It should be possible to use the process in existing energy generation plants, so that it can be used as widely as possible. Because of this latter requirement, wet processes are favored work at low temperatures and can therefore be inserted behind the conventional air preheater in which the incoming air is preheated for combustion with the help of the hot combustion exhaust gases. Require dry process in contrast, significantly higher operating temperatures, so that they are inserted into the process flow before the preheater and with must be integrated into the power generation system.

Various wet processes using aqueous ammonia or a not completely acidified aqueous ammonium salt solution, z. B. an ammonium sulfite solution have already been described

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for example in U.S. Patents 2,405,747, 2,134,431 and 2,810,627 and in Belgian Patent Specification 757 709. -

A problem with absorbing sulfur dioxide from combustion exhaust gases with the help of aqueous ammonia-containing solutions is that part of the tetravalent sulfur in the Absorber is oxidized to hexavalent sulfur. This phenomenon is due to the presence of small amounts of oxygen in the exhaust gases. The solutions coming from the absorber therefore contain a small one in the above processes Amount of ammonium bisulfate in addition to a much larger one Amount of ammonium bisulfite. When working up the absorption solution, the bisulfite is acidified in order to release sulfur dioxide. However, the bisulfate does not react with the acid, so that hexavalent sulfur is in the form of sulfate or bisulfate ions accumulates in the system.

Various means of eliminating the excess sulfate have already been proposed. For example, US Pat. No. 2,405,747 suggests removing the excess ammonium bisulfate to react with lime after separation from the system in order to obtain calcium sulphate in this way. According to the USA

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Patent Specification 2,810,627 is said to be the ammonium sulfite and ammonium bisulfite containing absorption liquid are oxidized with air to release sulfur dioxide and ammonium sulfate at the same time to create. However, since the demand for ammonium sulfate is currently low, the excess must be removed Ammonium sulfate a more valuable by-product can be produced.

The invention provides an improved process for converting the hexavalent sulfur formed in the absorber to Sulfur dioxide suggested. In this way, the accumulation of sulphate in the system is prevented and compared to the previous known method increases the yield of sulfur dioxide, which at the same time reduces the amount of valuable by-products, such as Sulfuric acid or sulphurous acid, increases.

According to the invention, a carbonaceous fuel contaminated by sulfur is burned with excess air, so that combustion gases are produced which contain small amounts of sulfur dioxide and oxygen. These exhaust gases are brought into contact with an aqueous, ammonia-containing absorption solution to remove the greater part of the sulfur dioxide to remove, at the same time a certain proportion of the tetravalent sulfur in the absorber to hexavalent sulfur oxidized and thereby a solution is formed, which both four and also contains hexavalent sulfur. At least a part

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the absorption solution is then acidified with ammonium sulfate to release sulfur dioxide and an aqueous, A flowable mixture, preferably a slurry, to obtain which contains ammonium sulfate and ammonium bisulfate. The water contained in the mixture is evaporated and the ammonium sulfate portion of the mixture becomes ammonium bisulfate and ammonia decomposes. A major part of the ammonium sulphate formed in this way is used to acidify further quantities the absorption solution is used, while a smaller proportion of the ammonium bisulphate is returned to the combustion chamber in which the combustion exhaust gases are generated, where there is a decomposition to a gaseous mixture of nitrogen, Sulfur dioxide and "water vapor" takes place.

The invention thus provides a new, better way of preventing the accumulation of sulfate in an exhaust gas desulfurization system proposed in which an aqueous, ammonia-containing absorption solution for removing the sulfur dioxide in the exhaust gases is used.

Combustion gases occur when carbon-containing fuels, z. B. oil or coal, containing chemically bound sulfur, usually with excess air in a combustion chamber to be burned. The combustion chamber can be a

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act conventional firing in a plant for the generation of electrical energy. The sulfur content of the combustion gases depends on the sulfur content of the fuel; it is typically around 0.2 to 0.3% by volume total exhaust gases. In addition, due to the use of excess air, the exhaust gases contain traces of oxygen of sulfur trioxide and significant amounts of nitrogen, carbon dioxide and water vapor.

According to the invention, the exhaust gases with an aqueous, ammonia-containing Treated absorption solution to separate the sulfur dioxide. First of all, the hot exhaust fumes on one for entry cooled in the sulfur dioxide absorber suitable temperature.

The sulfur dioxide absorption methods according to those mentioned above Patents may come within the scope of the present invention. Apply using the method of the Belgian patent specification 757 709 is preferred. The preferred absorption solution contains about 11 to 17 moles of ammonia and about 6.5 to 11 moles of sulfur dioxide for every 100 moles of water. The NH: SO molar ratio must be greater than 1: 1 and is preferably a little under 2: 1. In addition, the absorption solution generally contains some SO (mainly in the form of sulfate ions) due to the

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absorber occurring oxidation and as a result of the return the solution, as will be explained in more detail below.

Some oxidation of tetravalent sulfur to hexavalent Sulfur takes place in the sulfur dioxide absorber due to the presence of free oxygen in the exhaust gases. The amount of tetravalent sulfur which is oxidized is generally from about 5 to 20, mostly about 10%, based on on the total amount of tetravalent sulfur which gets into the absorption solution in the sulfur dioxide absorber. the Absorption solution thus contains ammonium bisulphite as a dissolved one Main component, but also smaller amounts of ammonium bisulfate. Combustion gases with a significantly reduced Sulfur dioxide content, i.e. with a content of no more than about 10% of the original amount, will eventually be added to the Atmosphere.

The feed rate for fresh absorbent solution into the Sulfur dioxide absorber is roughly proportional to the speed with which sulfur dioxide in exhaust gases enters the sulfur dioxide absorber is introduced. The sulfur dioxide feed rate in turn depends on the exhaust gas flow rate, which at a typical power plant fluctuates cyclically over the course of the day, because the demands on a power plant rise and fall,

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and further on the sulfur dioxide content of the exhaust gases, which in turn depends on the sulfur content of the fuel.

Fresh absorption solution can be fed into the sulfur dioxide absorber from a storage tank and the used one Absorbent solution can be drained into a second tank. The solution for the regeneration of the Sulfur dioxide can be withdrawn at a constant flow rate, so that the entire system with the exception of the sulfur dioxide absorber works at constant flow rate.

According to a preferred embodiment, the absorbed solution from the second tank is consumed at a constant flow rate deducted and divided into two parts. The first portion flows into an acid scrubber, where it is used to release Sulfur dioxide is treated. The second part is used without further treatment to supplement the fresh absorption solution, as described in more detail below.

The first portion of the used absorption solution is made with excess, hot, molten. Ammonium bisulfate in an acid zone or acidified in an acid scrubber, the temperature being about 93 to 107 C. Heat is added to the acid scrubber fed by the hot molten ammonium bisulfate.

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A gas stream containing sulfur dioxide and water vapor is drawn off overhead at the acid scrubber. This gas stream, which is considerably richer in sulfur dioxide compared to the original exhaust gases, can be treated in a conventional manner be to separate the water vapor, whereupon the sulfur dioxide either in sulfuric acid or in sulphurous acid can be transferred.

An aqueous mixture continues to form in the acid scrubber, which contains water, ammonium sulfate and ammonium bisulfate. This aqueous mixture is preferably a slurry which contains dissolved ammonium bisulfate and both dissolved ammonium as well as undissolved ammonium sulfate. The aqueous, ammonium sulfate and Ammoniumbi sulfate containing mixture that Although it forms in the acid scrubber, it may be a solution, but is generally a pumpable slurry preferred. The next step is to that the water is evaporated from the mixture formed in the acid scrubber, whereupon the ammonium sulfate contained therein to Ammonium bisulfate and water is decomposed. This is preferably done in a single step in which the aqueous slurry sprayed into a stream of hot combustion gases produced by burning a fossil fuel such as Natural gas, can be obtained. In the process, the water is first evaporated, which significantly cools the gas flow. Afterward

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the decomposition of the ammonium sulfate takes place at a temperature range from about 343 to 427 C. The ammonium sulfate becomes ammonium bisulfate and water at these temperatures decomposed; the extent of side reactions, d. H. the further decomposition of the ammonium bisulfate is at these temperatures relatively low. The dehydrated salt mixture is in the form of product droplets which are surrounded by a gas stream containing combustion products and ammonia. These droplets are melted together and collected at the bottom of the decomposer. The ammonia and hot combustion gases containing gas stream is withdrawn overhead.

The gaseous products from the decomposer are brought together with the previously described second part of the spent absorption solution. The product formed is freshness Absorption solution, which is returned to the first storage tank and is taken from this according to the respective need for the treatment of the exhaust gases. The ones in the ammonia tower The reaction taking place is essentially the conversion between the ammonia in the gaseous products of the decomposer with the ammonium bisulfite, which is the main component of the used absorption solution. This creates Ammonium sulfite, which is the main component of fresh Forms absorption solution. The bisulfate in the spent waste

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Sorption solution is converted into sulfate by reaction with ammonia convicted. The fresh absorption solution accordingly contains small amounts of ammonium sulfate in addition to much larger amounts Amounts of ammonium sulfite. Furthermore, lesser amounts of ammonium bisulfite are present in the preferred solution because the preferred NH: SO molar ratio is slightly less than 2: 1

J Lt

lies.

The ammonium bisulfate formed in the decomposer is drawn off, and the greater part of it is returned to the acid scrubber, where it is mixed with further quantities of used absorption solution is acidified. It is clear that none of the reactions described up to this point lead to decomposition of the hexavalent sulfur, which is formed in the sulfur dioxide absorber. To decompose the excess sulphate a smaller proportion of the ammonium bisulfate formed from the decomposer is fed back into the furnace according to the invention, from which the combustion exhaust gases originate. The amount of Ammoniumbi sulfate in this lower proportion is equivalent to that Amount of tetravalent sulfur which is oxidized to hexavalent sulfur in the sulfur dioxide absorber plus the small amount of hexavalent sulfur that enters the absorber from the incinerator.

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In your preferred mode of operation, the disintegrators are located and the furnace for the power plant so close together that the ammonium bisulphate in liquid form returned to the separation chamber State can be pumped into the combustion chamber and through nozzles made of a heat-resistant and corrosion-resistant material can introduce. If necessary, the decomposer in larger To arrange distance from the fire point of the power plant, then the ammonium bisulfate is preferably allowed to pass back harden in the oven and break it up into flakes, granules or a powder. The finely divided solid ammonium bisulfate can then be in the combustion gases in the combustion chamber with the help of suitable solid injection devices from heat and disperse corrosion-resistant material. The ammonium bisulfate can be returned to the furnace either continuously or intermittently. For example, it may be preferred at times of full load when the combustion chamber and the sulfur dioxide absorber are working at full capacity, no Ammonium bisulfate returned to the combustion chamber.

The ammonium bisulfate turns into a gaseous one in the furnace Product mixture, which contains nitrogen, water vapor and sulfur dioxide, decomposes. Furthermore, small amounts of Sulfur trioxide can be formed while no or only a very small amount of ash or solid residues is formed. These

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Gaseous decomposition products flow together with the exhaust gases into the sulfur dioxide absorber, where the sulfur dioxide is absorbed by the absorption solution. The nitrogen is released into the atmosphere together with the desulphurized exhaust gases blown. This is due to the decomposition of ammonium bisulfate The sulfur dioxide obtained is combined with the sulfur dioxide obtained from the combustion of fuels containing sulfur released in the acid scrubber as previously described and then converted into a useful by-product such as sulfuric acid or sulphurous acid. With the help of the present invention is thus excess sulfate from the System converted into an extremely valuable by-product and. not into much less useful by-products such as ammonium sulfate or calcium sulfate, which are the products of the known Procedures are.

The accompanying drawing is intended to explain the invention in greater detail serve, which reproduces a flow diagram of the process.

A sulfur-containing fuel is mixed with excess air in a combustion chamber 10, which becomes a conventional boiler of a power station, burned, with combustion exhaust gases that contain about 0.2 to 0.3 vol% sulfur dioxide as well as small amounts of sulfur trioxide and oxygen as well as larger ones

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Contain proportions of nitrogen, carbon dioxide and water vapor. The combustion air reaches the combustion chamber via line 11 10 and is preheated in the air preheater 12. A fuel, such as fuel oil, is introduced through line 13 at the bottom of the combustion chamber 10 where combustion

takes place with the formation of the aforementioned combustion exhaust gases. The combustion gases pass through the usual boiler and flue gas preheaters 14 and 15 to the outlet line 16 of the combustion chamber 10. The hot flue gases from the Line 16 are passed through the air preheater 12 where they pre-heat the incoming air and be cooled down by themselves. A further cooler (not shown), which is used to cool the flue gases to around 30 to 60 C, as this temperature range is preferred for the absorption of the sulfur dioxide from the exhaust gases. The exhaust gas is cooled in the cooler and saturated with water vapor, at the same time most of the sulfur trioxide is removed.

The cooled exhaust gas stream enters the sulfur dioxide absorber 20 through line 17. The exhaust gas flow is countercurrent brought into contact with a downward flowing aqueous absorption solution, which initially as the main dissolved component Contains ammonium sulphite. The fresh absorption

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solution is above the absorber 20 from the supply stank Zl through the line 22 is supplied for fresh solution. The absorber is preferably a multi-stage absorber, into which fresh absorption solution is fed in more than 1 stage.

The desulphurized exhaust gas, from which about 90 % of the original sulfur dioxide has been removed, is discharged overhead through the line into the atmosphere. A certain oxidation of the tetravalent sulfur to hexavalent sulfur takes place in the absorber 20. A stream of consumed absorption solution, which contains ammonium bisulfite as a dissolved main component and some ammonium bisulfate, is drawn off at the bottom of Ajsoxber 20 via line 24 and passed on to tank 25. The flow rate of the fresh solution and the used solution in the lines 22 and 24 is approximately proportional to the flow rate of the sulfur dioxide in the incoming exhaust gas stream 16. The used absorption solution is withdrawn from the tank 25 at an approximately constant flow rate via the line 26 and into the streams and 28 split.

The first stream 27 of spent absorption solution flows into the acid scrubber 30. Via line 31, an excess of hot molten Ammoniumbi sulfate is in the acid scrubber introduced. This ammonium sulphate is combined with the ammonium

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bisulfite in the stream 27 µm, releasing sulfur dioxide and evaporating a significant amount of water. A mixture from sulfur dioxide and water vapor is removed from the acid scrubber 30 overhead through the line 32. The water vapor can be removed by condensation and by drying according to known processes, while the sulfur dioxide is then removed can be converted into sulfuric acid or sulphurous acid as desired. The prevailing in the acid scrubber 30 The temperature is around 93 to 107 C. In addition, an aqueous suspension of ammonium sulfate is formed in the acid scrubber and ammonium bisulfate. The latter is more soluble and therefore lies completely in solution before, while the slurry both Contains dissolved as well as crystalline ammonium sulfate. The slurry leaves the acid scrubber 30 through the slurry line 33 deducted.

The ammonium sulfate / ammonium bisulfate slurry in line 33 is injected into a hot combustion gas stream 34 and passed along with this to a decomposer 35, where most of the ammonium sulfate is at temperatures is decomposed from about 343 to 427 C to ammonium bisulfate and ammonia. The decomposer 35 can also be used on others heat in a known manner, if desired, for example with

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Steam over heated water. .. That in the slurry The water contained is evaporated, whereupon the ammonium sulfate 7, u ammonia and droplets of ammonium sulfate and bisulfate decomposes, the latter being suspended in the gas stream are. The product of ammonium sulfate and bisulfate is collected at the bottom of the decomposer 35 and passed through the Line 36 withdrawn for molten salts. The gaseous products which arise in the decomposer 35 and are essentially formed by combustion gases and ammonia become withdrawn overhead through the line 37 from the decomposer 35.

The gases leaving the decomposer 35 reach the ammonia reactor 40. The second portion of the consumed absorption solution flows through line 28 and becomes as required diluted with water, which is supplied through line 41. The diluted used absorption solution passes through the line 41 from above into the ammonia reactor 40. Most of the ammonia in the gas stream 37 is from the Solution is absorbed and fresh absorbent solution is withdrawn through the bottom line 43 from the ammonia reactor. The temperature in the ammonia reactor 40 is kept at the desired operating value by a part of the content of the line 43 with the help a pump circuit is returned via a return line 44 and a cooler. The cooled solution occurs at the head of the ammonia

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-. 18 -

reactor 40 a. The remainder of the solution in line 43 flows through line 46 into storage tank 21. Unabsorbed gases, particularly nitrogen, carbon dioxide and traces Ammonia are withdrawn from the ammonia reactor 40 overhead through the line 47 and into the sulfur dioxide absorber 20 zta returned.

According to the invention, the ammonium sulfate and bisulfate product, which exits the decomposer 35 through line 36 in two Split up parts. The greater part is returned to the acid scrubber 30 through the line 31. A smaller proportion will returned through the line 50 into the combustion chamber 10 and there to sulfur dioxide, nitrogen, "water vapor and a small Amount of sulfur trioxide decomposed. The amount of ammonium sulfate and bisulfate returned to the combustion chamber via line 50 is so large as to prevent both accumulation and depletion of hexavalent sulfur in the system. These The amount of sulfur, expressed in gram atoms per hour, is essentially equivalent to the sum of the amount of sulfur in Gram atom per hour, which is oxidized in the sulfur dioxide absorber 20 from the value 4 to the value 6, plus the amount aa sulfur trioxide, which enters the absorber 20 with the incoming exhaust gas. The ammonium sulfate and bisulfate can either continuously or intermittently through line 50

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stream. This product preferably flows through the conduit in a molten state so that it passes into the combustion chamber 10 suitable spray nozzles (not shown) can be introduced, which are both heat and corrosion resistant. On the other hand, it can molten mixture of ammonium sulfate and bisulfate, in particular when the combustion chamber 10 and the decomposer 35 are arranged at a distance from each other, first solidified and then closed fine particles are broken up, which in turn in the flue gas flow in the combustion chamber 10 - with the help of a suitable Injection system for solids are introduced.

The example below is intended to further explain the invention.
example

A carbonaceous fuel is in the combustion chamber 10 under Application of a small excess of air burned, whereby a flue gas is produced, which about 0.23 vol% sulfur dioxide as well contains about 0.005 vol% sulfur trioxide. This combustion gas is cooled in the air preheater 12 and in an upward direction sent through the sulfur dioxide absorber 20 by contacting it with the downward flowing aqueous absorption solution comes. This solution essentially contains ammonium sulphite, ammonium bisulphite, ammonium sulphate and ammonium bisulphate are present, typically about 14.8 moles of NH, 7.4 moles of SO and 0.9 moles of SO, per 100 moles of water, the pH

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Value is 6, 6. This absorption solution is introduced into the absorber 20 via the line 17. The absorber 20 is operated at 5Q ^Q C and normal pressure. Dg.e entsehwef elte exhaust gas is withdrawn overhead through line 23, while the used absorption solution passes through line 24 into tank 25. The used absorption solution contains predominantly ammonium bisulfite and some ammonium bisulfate, ie typically about

14.3 moles NB, 10.4 MqI SQ, and 1.2 MqI SQ. on IQO moles of water, ο ί i

where the pH ^ value § ,. 2 is.

The consumed absorption solution is drawn off on the tank 25 through the line 16 and divided into the two streams 2, 1 and? E. Pe * stream 27 enters the S äurejwäseher 30, is introduced into the line 31 dureh geseHmcitlzenes Aminoniumbisulfat. The temperature of the solution in the acid scrub is about 93 to ⁰ G, sulfur dioxide and water vapor are removed via conduit 32, and in the acid scrub they also form watery residues ( g, which contains both dissolved and njijht dissolved Amneioniviriisulpha t together with some dissolved Ammntnium iisulphat; typically the solids content is about; 21 ycufo. This slurry is passed through the pipe

D | e. aqueous Aixfsehlämmung in the line 33 is in a hot Combustion gas stream 34 is injected and the mixture formed

then flows into the decomposer 35. The water of the Aufschlämrmuig is rapidly evaporated, resulting in the formation of salt particles which surround the hot combustion gas stream are. Most of the ammonium sulfate entering the decomposer is decomposed into ammonium bisulfate and ammonia. The molten ammonium bisulphate with a low content of undecomposed ammonium sulphate is placed in a pan collected at the bottom of the decomposer 35. The molten salt is withdrawn from here via line 36, while the hot Combustion gases with the ammonia formed in the decomposer 35 are passed on overhead through the line 37.

The gas mixture in the line 37 is fed together with fresh ammonia from the line 38 at the bottom into the ammonia reactor 40 initiated, in which the gases are brought in countercurrent with an aqueous ammonium bisulfite solution in contact, which through Diluting the second portion of the spent absorption solution in line 28 with water from line 41 is obtained. The dilute solution enters the ammonia reactor at the top through line 42 40 a. The ammonia is absorbed by this solution, whereby fresh absorption solution is formed, which through line 43 is withdrawn from the ammonia reactor. Part of this solution is through a pump circuit with a return

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line 44 and a cooler 45 sent to the desired Maintain operating temperature of about 50 C in the ammonia reactor. The remainder of the fresh absorption solution flows through line 41 into the storage tank 21. Non-condensed gases, mainly combustion gas, are from the ammonia reactor 40 overhead through the Line 47 withdrawn and returned to the sulfur dioxide absorber 20.

That withdrawn from the decomposer 35 through the line 36 Ammonium bisulfite is split into two parts. The greater part is returned to the acid scrubber 30 through the line 31. The smaller part flows back in molten form into the combustion chamber 10, where it is injected into the healthy flue gases will. This proportion of ammonium bisulfate turns completely into gaseous products, namely primarily nitrogen and sulfur dioxide and water vapor, decomposed. These gases flow together with the combustion exhaust gases into the sulfur dioxide absorber 20, where the sulfur dioxide is absorbed and the nitrogen is released through line 23 to the atmosphere.

In the following table 1 typical proportions are given in 454 g mol per hour.

1.058.51711.11

Currents in 454 g Table 1 . NH ₃ H ₂ O Moles per hour llezuge-
zilfer i * i
the character
tion Exhaust gases SO ₂ Components 2.196 14,840 Π 229,300
(1) 541 SQ ₃ 69 (2) 22nd 1, 097 2 2.127 14,840 23 54 127 610 4, 250 24. 1.539 1.517 IQ, 590 Vi 442 178 890
(8th) 890
(8th) 28 1.097 51 I. 250 ? .l 127 1,500 3, QQO 3c 442 890
(8th) Zl 941 941 34 6.280
(3) (4) 941 610 3, OQQ
(6) 36 37 6.280
O) - (5) 941

120

47- (2}

51 51 51

(1) Total current including SQ,

(2) Essentially saturated

(3) combustion gases

(4) Total flow including water vapor

(5) Including the water vapor formed by combustion, however without the ammonia and the water, which through the line 24 are fed

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Vi

(6) Only the water from stream 24, but not the water vapor of the combustion gas (stream 26).

(7) Water is added as required

(8) See also Table 2 with regard to the composition the salt components.

The SO in stream 16 consists of both the SO from the decomposition 2. &

Settlement of the ammonium sulphate as well as of the SO from the combustion of the fuel. The volumes of insoluble gases (ζ. Β. CO ^ and nitrogen) in lines 23 and 47 are not specified.

■ Et

All of the saline product in the decomposer is as ammonium bisutfate although small amounts of ammonium sulfate are usually also present.

The following table 2 shows the composition of the streams in lines 31 and 33 in 454 g mol per hour with regard to the salt content.

Table 2

Components

Ammonium bisulfate (ge s dhmol ζ en) Ammonium bisulphate (in solution) ammonium sulphate (in solution) Ammonium sulphate (solid) water

current

31
MUH « 33 - 382 890 242 317 3000

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Claims (3)

Claims 1. A method for the desulfurization of small amounts of sulfur dioxide and oxygen-containing combustion exhaust gases, which arise when burning carbonaceous fuels with a small to moderate excess of air in a combustion boiler, whereby the combustion exhaust gases are brought into contact with an ammonia-containing absorbent in a sulfur dioxide absorber in which a A small part of the tetravalent sulfur changes into the hexavalent state, whereupon an exhaust gas with a reduced sulfur dioxide content and a solution containing sulfur in a tetravalent and hexavalent state is withdrawn, at least part of the solution with ammonium bisulfate in an acid scrubber to release sulfur dioxide and to generate an aqueous one , flowable mixture containing ammonium sulphate and bisulphate is acidified, the water in the mixture evaporates and the greater part of the ammonium sulphate contained in the mixture decomposes to ammonium bisulphate and ammonia, thereby k Characterizes that a predominant part of the ammonium bisulfate thus obtained is returned to the acid scrubber to acidify the withdrawn solution and a smaller part of the ammonium bisulfate is returned to the combustion chamber, where the latter is decomposed into a gaseous mixture containing nitrogen, sulfur dioxide and water vapor. 10 9 8 5 1/1111 212P818 2. The method according to claim 1, characterized in that the aqueous ammonia-containing absorbent contains ammonium sulfite as the main dissolved component. - 3. The method according to claim 1, characterized in that the Amount of ammonium bisulfate returned to the combustion chamber about the sum of the amount that is absorbed in the sulfur dioxide from the tetravalent to the hexavalent state "is oxidized" and from the amount, which in the form of sulfur trioxide with the incoming Combustion exhaust gas is introduced into the absorber, equivalent :" . is. ugs: cm 10 9 8 5 1/1111