KR820000687B1 - Production of h,s from so2 obtained from flue gas - Google Patents

Abstract

Process for removing SO2 from flue gas and recovering it as H2S comprises (a) reacting the SO2 with aq. Na2CO3 to from Na2SO3; (b) reducing the Na2SO3 to Na2S; (c)reacting the Na2S with NaHCO3 to form H2S and Na2CO3 (d) recovering the H2S; (e) recycling part of the Na2CO3 to step(a); (f) reacting the remainder of the Na2SO3 with CO2 and H20 to form NaHCO3; and (g) recycling the NaHCO3 to step(c). Scaling of the scrubbing equipment is minimised. The recovered H2S can be used e.g. for manufacture of S or H2SO4.

Description

Recovery of hydrogen sulfide from sulfur dioxide in exhaust gas

1 is a flow chart illustrating the interrelationship of steps in accordance with the present invention.

The present invention relates to a process for the economic conversion of sulfurized sulfur contained in a mixed gas containing a small amount of sulfur dioxide (SO ₂ ) to hydrogen sulfide gas. The present invention can be used in a variety of environments but is particularly useful for exhaust gas desulfurization (FGD).

It is known that sulfur dioxide, even in small amounts in the atmosphere, is extremely harmful to the lives of animals and aquatic plants. Sulfurated sulfur is easily converted to sulfuric acid by oxygen and moisture in the air, which causes corrosion of many materials in buildings, including steel and concrete.

Emissions of large amounts of SO ₂ from the fuel combustor into the air were banned in 1967, when Congress passed the Air Purification Act and amended it in 1970. As a result of these legislation, the Environmental Protection Agency has established a standard limiting the emission of basic pollutants contained in gases. These contaminants are nitrogen oxides measured with particles, sulfur dioxide and nitrogen dioxide and their standard values are shown in Table 1.

TABLE 1

Heavy fuels in everyday use, such as bituminous coal and residual fuel oils, contain by weight at least about 3% sulfur.

When burning 12,000 BTU / pounds of coal containing 0.8% sulfur, the limits set by the EPA are exceeded. A device that burns large amounts of economical fuel to operate within EPA limits should be established to remove large amounts of SO ₂ contained in exhaust gases following the combustion zone. The most common removal agents are lime or limestone, which combines with SO ₂ directly or indirectly to purify the exhaust gases. The main purpose of using lime or limestone is that the contaminated formed deposits, mainly consisting of calcium sulfite and calcium sulfate, are no more harmful than SO ₂ in the air, which is buried except where it is allowed to be used locally.

It has long been anticipated that the release of large amounts of sulfur dioxide into the atmosphere will be banned at any time and has been banned in many parts of the world for many years. In the 1930s, limestone slurries were used to remove SO ₂ from exhaust gases, and research to address this problem began in the United States around 1935.

The results of this effort are described in a paper by Johnston and Shing in pages 1037-1049 of Volume 32, Volume 8, Industrial and Engineering Chemistry. A summary of all current status of FGD in the United States is contained in pages 101-103 of Cemical Engineering, published May 23, 1977. Of the methods listed closest to the methods of constructing the present invention, the so-called “Water-Soluble Carbonation Process (ACP)” is the Proceedings Volume 11 of the Symposium on Flue Gas Desulfurization held in March 1976 in New Udorians. Technology series).

Most fuel solids and liquids contain small amounts of nitrogen containing compounds. When these fuels burn, nitrogen appears as nitrogen oxide NO in the off-gases or is formed from nitrogen and oxygen as a whole reaction at the high temperatures that usually occur in boilers.

N ₂ + O ₂ = 2NO

Therefore, it was necessary to use a removal system capable of removing not only SO ₂ but also nitrogen and oxides. It is easier to remove NO ₂ from the gas stream with an aqueous removal solution than with NO.

Although the reaction is relatively slow, NO and oxygen combine to form NO ₂ at normal temperature. The reaction is as follows.

2NO + O ₂ = 2NO ₂

In a suitable method using this method, most of the NO in the off-gas has the opportunity to convert to NO ₂ . Fortunately, the removal liquid used in the present invention for removing SO ₂ is also effective as a NO ₂ adsorbent.

The method of contacting the exhaust gas with an adsorbent for SO ₂ captures certain particles dispersed in the exhaust gas in the removal slurry so that all contaminants contained in the exhaust gas are removed by the removal step of the present invention.

Many adsorbents for removing SO ₂ in the off-gas form slightly soluble compounds, and as a result, the apparatus used for effective contact between the gas and the adsorbent becomes large, making it difficult to operate on a standstill and require excessive maintenance costs.

When SO ₂ is removed indirectly or directly from the exhaust gas by the calcium compound, the resulting material has to be disposed of.

It is an object of the present invention to provide a solution of SO _{2 in the off-} gas by means of an adsorbent containing soluble sulfite salts which are adsorbed and then crystallized SO ₂ into a solution on dispersed thermal particles to minimize the size of the removal device. Provides a way to remove it.

Another object of the present invention is to solve the problem of disposal by converting SO ₂ absorbed in the mixture of diluent gas into hydrogen emulsion as a main component of the gas mixture by converting hydrogen emulsified economically to liquid hydrogen sulfide, sulfur element, sulfuric acid, etc. Provide a method.

Another object of the invention is to remove the SO ₂ in the exhaust gas as the adsorbents which can be completely reproduced by as to have a strong affinity for SO ₂ and then subjected to a processing step.

Another object of the present invention is to remove SO ₂ in the off-gas by a method of adsorbent which can be completely regenerated even if a significant fraction of the sulfurous oxide is oxidized to sulfur oxide while the removal solution is in the removal zone.

Another object of the present invention is to use an adsorbent slurry to remove SO ₂ in the exhaust gas, which does not reduce the ability to absorb SO ₂ as the slurry flows through the removal zone.

It is yet another object of the present invention to provide a method of using adsorbent crumbs that can be operated over a wide temperature range to remove SO ₂ in the exhaust gas.

The above objects and other advantages specified by the following description can be obtained by using a series of known chemical reactions in a novel order.

The reactions used in the process of the present invention consisted of the following rebounds.

The basic reaction in the step of absorbing SO ₂ in a solution containing sodium carbonate saturated or dissolved with sodium sulfite is as follows.

SO ₂ + Na ₂ CO ₃ = CO ₂ + NaSO ₃

This reaction occurs when the solution in the absorption zone contains some dissolved sodium carbonate. If sodium carbonate is not present, absorption is carried out using dissolved sodium sulfite as an absorbent.

Na ₂ SO ₃ + H ₂ O + SO ₂ = 2NaHSO ₃

The ability of the adsorbent to absorb SO ₂ continuously is maintained by the addition of sodium carbonate and sodium bisulfite reacts with carbonate as follows.

2NaHSO ₃ + NA ₂ CO ₃ = CO ₂ + 2Na ₂ SO ₃ + H ₂ O

An undesired reaction that occurs during the process is the oxidation of sulfur dioxide to sulfur oxides.

Na ₂ SO ₃ +1/2 O ₂ (air) = NaSO ₄

The second type of reaction involves the reduction of sodium sulfite and sodium sulfate to sodium sulfide by means of readily available reducing agents. In most cases, these reducing agents are inexpensive reducing agents such as bituminous coal and coke, wood or steam oil. These reductions can be represented by the following equation.

2Na ₂ SO ₃ + 3C = 2NaS + 3CO ₂

Na ₂ SO ₄ + 2C = Na ₂ S + 2CO ₂

If sodium nitrate is present, it is converted to sodium carbonate and nitrogen.

4NaNO ₃ + 3C = Na ₂ CO ₃ + 2N ₂ + 3CO ₂

The next reaction of the reduction reaction generates hydrogen emulsion from sodium emulsion and sodium bicarbonate.

Na ₂ S + 2NaHCO ₃ = H ₂ S + 2Na ₂ CO ₃

The reduction reaction sequence according to the hydrogen sulfide generation step results in regeneration of sodium carbonate used in the SO ₂ absorption step.

The final step is the known bicarbonate formation of sodium.

Na ₂ CO ₃ + H ₂ O + CO ₂ = 2NaHCO ₃

Inexpensive CO ₂ feedstocks can be used and the gas from the reduction stage becomes an exhaust gas free of SO ₂ .

In most cases the final reaction is the conversion of H ₂ S to sulfur using a traditional Krauss process as shown in the equation below.

2H ₂ S + O ₂ (air) = 2H ₂ O + 2S

The basic relationship between each reaction is as follows.

Na ₂ CO ₃ + H ₂ O + CO ₂ = 2NaHCO ₃

Na ₂ CO ₃ + SO ₂ = Na ₂ SO ₃ + CO ₂

1.5C + Na ₂ SO ₃ = Na ₂ S + 1.5CO ₂

2NaHCO ₃ + Na ₂ S = H ₂ S + 2Na ₂ CO ₃

Referring to the drawings, Figure 1 is a process diagram illustrating the interrelationships of the various steps when implementing the present invention using all the necessary equipment, and the exhaust gas containing SO ₂ is washed with water to remove particles before the SO ₂ removal step. Under certain circumstances, certain steps of the process may be physically separated. Although not preferred, in some cases certain steps of the process may be carried out at two different locations. One of the advantages of the present invention is that separation operations are technically possible.

The operating mode of the present invention provides an economical advantage over the use of any other process for removing contaminants before venting the exhaust gas to the atmosphere.

The present invention can be understood by each step starting with a sulfur compound, injected into the process as sulfur dioxide and finally in the form of concentrated hydrogen sulfide. Flue gas containing SO ₂ is washed with water to remove particulate solids. Referring to FIG. 1, the device of the conduit 2 enters into the gas inlet at the bottom of the first absorption zone 1, whereby the gas is in contact with the water soluble slurry and its liquid phase is mostly Aqueous solution containing dissolved sodium sulfite and small amounts of sodium sulfate, for example sodium nitrate and small amounts of sodium carbonate. The solid phase is essentially a mixed solid of sodium sulfite and sodium sulfate and the majority of the solid consists of sodium sulfite. The process of the cleaning step must be adjusted to crystallize the anhydrous salts, for example the temperature should be maintained at such a temperature that the precipitated solid salts are liberated from hydrogenated crystallization and the suitable temperature range is 40 ° -50 ° C. The purpose of maintaining the cleaning solution at this temperature is to minimize fuel costs in the next step of the process. This does not affect the ability of the solution to clean oxides of O ₂ or nitrogen, and unwanted oxidation of sulfur dioxide to sulfur oxides occurs essentially in the first absorption stage.

Oxidation range depends on many factors such as the amount of excess air in the exhaust gas, the presence / temperature of catalytic metals such as trace molten iron, and the like. Crystals formed when sodium sulfite and sodium sulfate are in solution at a suitable concentration and the solution is saturated to cause precipitation are mixed crystals of sulfite and sodium sulfate. If the absorbing solution is saturated with sodium sulfite and the ability to dissolve oxygen is limited, this reduces the extent to which the oxidation reactions occur compared to what would occur when using a smaller solution of sulfur dioxide.

Because of the nature of the absorbent sorbent, the solid in the thriller always contains some sodium sulfate mixed with sodium sulfite. As described above for sodium sulfite it is mixed with some sodium sulfate and similarly the liquid phase of the adsorbent contains some dissolved sodium sulfate but the main coagulation is sodium sulfite.

Oxides of nitrogen in the off-gas are absorbed more or less in the first absorption stage, depending on the amount of NO and NO ₂ present. If there is the same water concentration, most of the oxide is absorbed to form sodium nitrite. Excess nitrogen dioxide forms sodium nitrate and, as is known, NO present in the air is absorbed in an aqueous alkali solution containing dissolved sulfur oxides, but occurs more slowly than absorption of nitrogen dioxide. Since the oxides of nitrogen are always present in the off-gases containing the appropriate concentrations of SO ₂ , the liquid part of the absorbent crumb always contains sodium nitrite and sodium nitrate. As a result of the next stage of the process, these oxides of nitrogen are converted to nitrogen and vented into the atmosphere without any special steps to achieve the object of the present invention.

Solution to form a sodium sulfite anhydrous crystal from the SO ₂ absorption and maintain the activity of the solution to absorb the SO ₂ is thus supplemented by continuous or periodic addition of sodium carbonate.

The slurry used to contact the SO ₂ -containing gas in the first absorption zone is injected via pipe 3 to the end of the zone by means of a pump (not shown). The slurries flow countercurrently with the SO ₂ -containing gas flowing in the opposite direction through the band. SO ₂ is absorbed by the liquid phase of the slurry when the gas and the slurry are in contact. As the gas exits the zone through the exhaust pipe 22, the solution remains saturated when SO ₂ is absorbed, thus forming additional sodium sulfite solids. The slurry exits the first absorption zone and enters the settler 5 through the outlet pipe 4. More crystals in the slurry settle to the bottom of the settler, forming more dense masses.

A small amount of crystal and most of the liquid phase are discharged from the settler through the drain pipe 6 and flow into the tank 7. The mass formed in the settler and the base exits through the outlet and flows into the centrifuge 9 by a pipe 8. Most of the solids in the centrifuge 9 are separated from the liquid phase, whether with them or not. The liquid phase is directed into the tank 7 through the pipe 10 and impure sodium carbonate is injected into the tank 7 by means of the device of the belt 17. The mixture formed in the tank 7 is circulated through the absorption zone 1 by the pump and the device described above.

The moist solids separated from the centrifuge 9 are conveyed to the injection hopper 12 by the belt 11. The injection hopper 13 is periodically refilled with bituminous coal pulverized by the screw carrier 53 and the pulverized coal is removed from the hopper 13 by the screw carrier 14 and injected into the mixer 15. Impure and moist sodium sulfite is recovered from the injection hopper 12 by the injector 16 and injected into the mixer 15 whereby the impure and moist sodium sulfite and coal are immediately mixed in the mixer 15 and the mixture is screw conveyer 18. It is injected into the injection port of the rotary furnace 19 directly heated by the). By means of the device of the screw injector 20, powdered coal from the hopper 13 and combustion air from the pipe 21 are injected into the discharge arm 19 of the furnace. Coal is burned with a chemical equivalent of oxygen, so the gas atmosphere in the furnace is less reducing than oxidation. As the mixture of coal and sodium sulfite moves through the furnace to countercurrent with the hot products of combustion, sodium sulfite is reduced to sodium sulfide and carbon is oxidized to carbon dioxide.

Although not required, a ratio of sodium sulfite to coal of about 4: 1 is appropriate and most of the sodium is converted to sulfides. It is advisable to control the temperature so that the mixture in the furnace does not reach the application point, and by operating under these conditions, most of the sodium sulphide flows out of the furnace outlet.

The salt mixtures do not soften because the maximum temperature in the furnace ranges from 650 ° C to 750 ° C. Since excess carbon is present in the furnace-derived mixture, more than 90% of sodium sulfite is converted to sodium sulfide.

The solid mixture formed in the rotary furnace is recovered through the outlet of the furnace by means of a screw carrier 23 placed in an apparatus (not shown) to prevent air from contacting the mixture containing hot sodium sulfide solids. The screw carrier 23 delivers the mixture containing sodium sulfide to a continuous mixer-crusher 24 for mixing with a chemical equivalent of moist sodium bicarbonate in the mill. The mixer-crusher converts the mixture into small granules and mixes sufficiently. The mixer grinder 24 was equipped with a containment device to prevent the discharge of any vapors formed during the mixing operation.

The steam is kept at a slightly lower pressure to prevent exhausting in the mixer-crusher 24.

The mixture resulting from the mixing of the solid containing bicarbonate and sodium sulfide exits the screw carrier 25 from the mixer-crusher outlet and is passed to the steam tube rotary calciner 26.

Steam incinerators are heated by high pressure steam, eg steam at 400-450 psi.

In the calciner most of the sodium bicarbonate and sodium emulsified are heated to a temperature of about 200 ° C. under these conditions they react to produce impure sodium carbonate and hydrogen sulfide gas. Water in the mixture injected into the calciner is vaporized and steam and emulsified hydrogen gas exit the calciner's gas outlet and send it through a pipe 27 to the condenser where it is separated from most of the steam H ₂ S and separated from H _2. S is converted to sulfur in the Kraus process.

The solid containing the impure sodium carbonate formed in the calciner exits the calciner outlet so that impure sodium carbonate is introduced into the tank 7 and the dissolver 30 by means of the screw carriers 28 and 29, respectively, via the belt 17. A solution containing sodium bicarbonate, which is made of the following raw material, is also injected into the dissolver 30 through the pipe 32 and the sludge formed in the dissolver 30 is discharged into the filter 37 through the pipe 31. do. The thick filtrate separated from the solid by the filter 37 is sent to the tank 34 through a pipe 33 by a pump (not shown). The mixture of the remaining water soluble components, including the wash liquor entering the dissolver 30 by introducing the wash water into the washing section of the strainer 37 by means of a device of the water inlet tube 35, dissolves 30 through the strainer pipe 32. Enter The washed solid liberated from the water-soluble component is liberated from the filter 37, mixed with fuel by the screw carrier 38, and then injected into a boiler (not shown), in which a sulfur dioxide-containing product of combustion is formed. do.

The solution and suspended solids contained mostly in the tank 34, consisting of sodium bicarbonate and sodium carbonate, are circulated to the liquid inlet of the second absorption zone 40 by means of a pipe 54 device. The gas containing carbon dioxide is injected into the inlet of the absorption zone 40 by the pipe pipe 41 and the raw material of carbon dioxide is described next. In the absorption zone 40, carbon dioxide is absorbed by a solution containing sodium carbonate saturated with sodium bicarbonate. As a result of the absorption of carbon dioxide, sodium carbonate is converted to sodium bicarbonate and crystallized from the solution. The sludge leaving the absorption zone 40 is directed into the settler 42 by means of a pipe 43. Larger granules of sodium bicarbonate solid in the thrush settle into the bottom of the settler 42. Smaller granules and most of the solution come from the top outlet of the settler and are delivered to the tank 34 by means of a pipe 44 device. Ingots formed in the bottom of the settler 42 are injected into the centrifuge 45 by means of a pipe 46 and the centrifuge 45 is separated into two fractions, one of which is moistened with sodium bicarbonate that is largely moistened. It is composed of most of the solution contained in the slurry and the other is the centrifugal cake. The moist sodium bicarbonate cake is conveyed to the mixer-crusher 24 by a belt carrier 47 and the solution leaving the centrifuge flows into the tank 34 by means of a pipe 48 device.

Carbon dioxide injected into the absorption zone 40 is obtained as exhaust gases from each other 19 in the apparatus of the tube 49 also through the pipe 41. If this gas is not available as a result of local conditions or if the amount of CO ₂ is insufficient for some reason, carbon dioxide can be obtained from the gas leaving the absorption zone (1). When CO ₂ is injected into the absorption zone 40 from the gas leaving the absorption zone 40, it is pumped from the exhaust pipe into the pipe 41 connected to the absorption zone 40 by the device of the piping 50. Gas containing carbon dioxide from the absorption zone 1 which is not necessary for absorption in the zone 40 is discharged into the atmosphere by the device of the exhaust pipe 51. Whatever the source of carbon monoxide in the absorption zone, CO ₂ is mixed with a large amount of nitrogen. All remaining unabsorbed gas mixed with nitrogen exits zone 40 by means of an arrangement of exhaust pipes 52.

Many variations can be used for each of the steps described above and this process has many advantages.

It is necessary to remove particulate matter from the SO ₂ -containing gas before being treated by the process of the present invention. However, the process of the present invention can be carried out successfully even in the presence of particulate matter. Most particulate matter produced by the SO ₂ -containing gas is cleaned by the cleaning liquid and this solid leaves the cleaning system with solid sodium disulfide.

There is little tendency for scaling to occur on the inner surface of the scrubber by using an aqueous solution saturated with sodium sulfite, including soluble slurry adsorbents such as suspended sodium sulfite crystals. If scaling occurs, the soluble sulfite is rapidly redissolved by adjusting the concentration of sodium sulfite in the cleaning solution.

If desired, hydrocron may be used in place of the settler and similarly a filter may be used in place of a centrifuge.

It is good to reduce sodium disulfide to coal without melting the reaction mixture, but this is not essential. The reduction reaction is facilitated by reducing at a temperature sufficient to keep most sodium disulfide molten, but this method requires more refractory to align the inner surface of the vessel in which the reduction is carried out. The molten product should be solidified by cooling under inert atmosphere to minimize reoxidation before mixing with moist sodium bicarbonate.

When carrying out the reduction step, it is necessary to use a surplus reducing agent, and the ash from the unreacted reducing agent, the reacted coal, and the ash remaining as a result of the combustion of the coal is mixed with sodium carbonate formed in the steam engine rotary calciner. The filter cake is mixed with fuel injected into the boiler to remove ash from the system and recover valuable fuel remaining in the filter cake. The weight of the filter cake is only a small fraction of the weight of the fuel and the injection of the filter cake relative to the weight of the fuel does not affect the operation of the burner but prevents the fuel from being consumed.

This process has particular advantages when used to remove contaminants in the exhaust gas of a device that burns coal used to generate electricity to generate a large amount of steam. Although various sodium salts are formed and converted to salts other than coal, they are readily available and relatively inexpensive.

Prior to the present invention, no FED process has been used that is superior to limestone sludge cleaning. As a result, many limestone cleaning systems have been installed, most of which are continuously operated and built to meet the new rules, require sufficient space to build the scrubbers and the aids to use them.

The present invention is capable of reducing the cost of treating the off-gas in Yenal's steam generator equipped with limestone scrubbers. If there is room for the SO ₂ cleaning step, it can be achieved by replacing limestone with impure sodium carbonate. Most of these devices have a device for recycling the slurry and separating the filter cake from the slurry and require the installation of a centrifugal separator to obtain the filter cake with a low water content, which can save transportation costs.

Circulated thrush contains suspended water soluble sodium, sulfur salts and particulate matter.

The damp filter cake obtained by the centrifugation operation contains particulate matter and sodium salts, such as sodium sulfite, sodium sulfate and small amounts of sodium nitrite and sodium nitrate, which are water soluble in water which has been suspended in the exhaust gas.

The filter cake is conveyed to a place where there is enough land to install the remaining apparatus required to carry out the process by some conventional apparatus. Many devices have already been described and additional equipment is required, but these facilities consist essentially of devices for washing filter cakes and water-soluble sodium salts using a dissolver, a filter and minimal water.

Insoluble particulate matter is separated from the mixed sodium salts by conventional dissolution, evaporation crystallization and washing methods. The end result consisted of two filter cakes, one of which consisted of the material resulting from the combustion of a solid, mainly insoluble in water, and the other a damp filter cake consisting of sodium sulfite, sodium sulfate and sodium sulfate.

The filter cake of sodium salt is treated as described above to recover sulfur and regenerate sodium carbonate. A portion of the impure sodium carbonate formed in the H ₂ S formation step is sent back to the cleaning operation for additional SO ₂ uptake.

It is evident that it is technically possible to install and operate a device for treating slurries resulting from two or more FGD cleaning operations. From the above-described variant of separating particulate matter from soluble sodium salts, the present invention can be practiced by absorbing SO ₂ in an alkaline absorbing liquid whose sodium sulfite concentration is kept below its saturation value. Sodium carbonate and water are added to the adsorbed liquid to keep the composition within a narrow range. The solution containing dissolved sodium sulfite is recovered from the cleaning system and the solid sodium sulfite is recovered by conventional evaporation and crystallization processes. The recovered wet sodium sulfite filtration cake is treated as above.

In the above technique of the present invention, the exhaust gas and the absorption slurry are allowed to flow back. One aspect of the present invention is to provide an adsorbent in which all components are water soluble and have a constant high level SO ₂ absorption capacity. As a result, the absorbent medium can be effectively used in an air contactor such as a scrubber or an AC contactor.

The basic cost of any cleaning operation is the initial cost of the cleaning device and the energy required to achieve the gas-liquid contact required to absorb SO ₂ , and this energy consumption is measured by the pressure drop required to allow gas to flow through the system. . Some advantages of reducing pressure drop and capital investment in moderate SO ₂ removal provide the economic benefits of the material. Unlike most SO ₂ cleaning processes, this process can be operated at relatively high pH, eg pH 8 or above, without any side effects. This means that inexpensive materials can be used in the installation and additionally known devices can be used to improve the absorption of volatile acid-gases by alkaline solutions (the device is kept alkaline by a small amount of ammonia in the system). This device is useful for absorbing CO ₂ as well as SO ₂ . Ammonia enters the vapor phase and reacts with the acidic gas to form salt particles that quickly absorb water vapor so that they are easily wetted. The moist particles dissolve rapidly by the cleaning solution and when dissolved at high pH, ammonia evaporates and repeats the cycle. This eliminates the need for diffusion of acidic gas through the gas-liquid inner surface, ie the rate regulation step in the gas absorption operation. The resulting pressure drop resulting from maintaining a low concentration of ammonia in the cleaning system is greater than the cost of ammonia that must be supplied continuously.

If the already increased SO ₂ absorption capacity of the system when installed hayeoteul substituted for limestone in the scrubber with sodium carbonate and concentration of emergency a small amount of ammonia concentration that is less than 1% doeyeotda held in the SO ₂ absorption system is increased. By increasing the desulfurization capacity of already installed units, it is possible to burn fuels with increased sulfur content to limit the SO ₂ from the system up to the allowable amount. The price of the fuel changes inversely with the sulfur content of the fuel, thus providing additional savings in the operation of burning cheaper fuel to generate steam.

The present invention does not depend on the particular type of device described above, and any suitable slurry-gas contactor may be used for SO ₂ and CO ₂ absorption stages, and various types of centrifuges, filters, or settler may be used to separate the solids from the slurry. Can be used. This process has the advantage that there is no loss when any solids in the slurry are present in the filtrate and this apparatus may be replaced to yield a low moisture filter cake. The reduction step is as described above, which can be carried out at high temperatures at which the reactants and products remain solid or at which they melt.

When the reactants remain solid, the reduction can be carried out in a rotary furnace or in a multiple stove, which is directly heated. When the reaction was carried out at a temperature of melting the mixture containing sodium sulfide, the reactor was returned to a refractory vessel with a device for adding the reaction with sodium to be reduced. Air is blown into the mixture to combust some reducing material to provide the required heat. By operating the product outlet, the entire operation can be carried out continuously. H. The reaction between sodium sulphide and sodium bicarbonate to produce S and sodium carbonate is carried out at temperatures between 180 ° and 220 ° C. and lower temperatures may be used to increase the amount of water in the initial mixture. Rotary steam calciners are particularly useful devices for carrying out this reaction when a pressure transporter is to be used. Known high boiling point heat transfer fluids such as dowtherm can be used in place of high pressure steam and other devices can be used which can heat the initial mixture of Na ₂ S and NaHCO _{3 to} the reaction temperature under closed conditions. Multiple stoves can be used as well as indirectly heated rotary furnaces.

One of the factors influencing the economics of the present invention is the amount of energy required to recover H ₂ S. This is greatly affected by the moisture content of the mixture of Na ₂ S and NaHCO _{3 which} is heated to release H ₂ S. The higher the moisture content, the lower the temperature at which the mixture should be heated. Although under certain circumstances it is advisable to evaporate three times the amount, the moisture content is adjusted to evaporate 2-3 pounds of water per pound of released H ₂ S. If the mixture was heated to about 200 ° C., better conversion could be obtained when less water was present in the initial mixture.

In summary, the present invention provides an improved process for purifying SO ₂ -containing exhaust gases and evacuating them to the atmosphere as clean, harmless exhaust gases while recovering sulfur. Modifications to processes and proportions may be used without changing the scope of the invention as defined in the claims below.

Claims (1)

The exhaust gas as described above is contacted with a water-soluble alkaline sodium carbonate having a pH of 8 or above containing a reaction medium saturated with sodium sulfite to react most of sulfur dioxide and some oxygen with carbonate to disperse sodium sulfite and sodium sulfate in the reaction medium. Sodium and base are fed to the reaction medium by the addition of solid sodium carbonate recycled from this process. After forming a mixture consisting of the sodium sulfite and solid sodium bicarbonate separated in the process, water removal and sodium bicarbonate present in the mixture react with sodium sulfite in the mixture to form an emulsified hydrogen gas and form dry solid sodium anhydrous carbonate. The mixture is heated in the absence of air to a temperature of 180 ° C.-220 ° C. sufficient to recycle and the sodium carbonate formed in the process is recycled and the residual sodium carbonate formed in this process is added to a saturated sodium bicarbonate solution and carbonated with carbon dioxide and dispersed in the solution. Emulsify sulfur dioxide contained in the off-gas, which forms a slurry consisting of solid particles of sodium bicarbonate and separates the particles of sodium bicarbonate from the slurry formed above to recycle the separated sodium bicarbonate particles to the above-mentioned revolution. Recovery with hydrogen.

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