MXPA98005774A

MXPA98005774A - Process to remove sulfur dioxide from combustion gases, in particular from exhaust gases from an electrical plant and residential gases from resident planters

Info

Publication number: MXPA98005774A
Application number: MXPA/A/1998/005774A
Authority: MX
Inventors: Risse Theo; Ferrao Lourenco
Original assignee: Ae&Ampe Lentjes Gmbh
Priority date: 1997-07-19
Filing date: 1998-07-17
Publication date: 1999-06-01

Abstract

The present invention relates to a process for removing SO2 from combustion gas, wherein the combustion gas is treated with an absorption fluid in the wash zone of a washing tower and scrubbing of gases in countercurrent. According to the invention, the absorption fluid consists of a mixture of ammonia water, a fluid consisting of ammonium sulphite which is extracted from the washing tower and cleaned of gases below the washing zone, and is returned without intermediate treatment, and a solution of ammonium sulfate which is returned from an oxidation device. The mixing ratio between ammonium sulphite and ammonium sulfate is in the range between 15: 1 and 3

Description

PROCESS TO REMOVE DIOXIDE FROM SULFURS OF COMBUSTION GASES.

IN PARTICULAR OF GASES OF EXHAUST OF A POWER PLANT AND ESCAPE GASES OF INCINERATE PLANTS WASTE WASTE DESCRIPTION OF THE INVENTION The invention relates to a process for removing sulfur dioxide from combustion gases, in particular exhaust gases from a spraying station and exhaust gases from waste incineration plants, where: the combustion gas is directed from the bottom to the upper part through a gas scrubbing and cleaning tower that comprises at least one washing area and is treated in countercurrent with an absorption fluid which is added to the upper side of the washing zone and is extracted outside the washing and scrubbing tower by, under the washing area, and the absorption fluid is returned in a washing fluid circuit to the upper side of the washing area, where ammonia water is supplied to the circuit of washing fluid and a fluid flows branching away from the washing fluid circuit and is directed to an oxidation device in which the ammonium sulfide (NH4), SO and the acid sulfite of Ammonium (NH4HS03) present in the fluid is oxidized to become ammonium sulfate (NH4) 2S04). When sulfur dioxide is removed from the combustion gases by means of a gas washing process using an aqueous solution of ammonia as the absorption fluid, essentially the following reactions occur: S02 + NH3 + H20 = NH4H03 NH4H03 + NH3 = (NH4) 2S03 (NH4) 2S03 + S02 + H20 = 2NH4H03 (NH4) S03 + 1/2 02 = (NH4) 2S04 During the process, ammonium sulfate is produced, which can be sold as a byproduct and which can be used as fertilizer. A process comprising the features described in the introduction is known from DE-C 37 33 319. A washing fluid circuit is connected to the middle wash zone of a three-stage scrubbing and scrubbing tower, add ammonia water as an absorbent for the wash fluid circuit and add water to compensate for the balance of matter. A partial flow is withdrawn from the washing fluid circuit and directed to an oxidation device, where oxidation is carried out to form an ammonium sulfate solution which is further processed by concentration and granulation to form a product which can be sold as fertilizer. The absorption fluid guided in the circuit includes a high concentration of dissolved salts, in particular (NH4) 2S03 and NH4HS03. The absorption fluid used and the washing zone comprises a high partial pressure of H3 which favors the formation of impure aerosols. In the case of the known processes, a third washing zone downstream is connected, in which the ammonia washing zone present in the gas is washed out to a permissible limit value using a washing fluid which has been returned acid by the addition of sulfuric acid. The additional washing zone to remove NH3 by separation is expensive. In addition, the formation of aerosols has not yet been avoided in a completely satisfactory manner. In the case of a process known from EP-A-0 778 067, an ammonium sulfate solution is used as the absorption fluid which is extracted from an aerated sedimentation container of a scrubbing and scrubbing tower, and is directed to nozzle devices in the upper part of the washing tower and gas cleaning. The challenge is to obtain as complete oxidation as possible of the absorption fluid in the sedimentation container of the washing towers. Too much absorption fluid is extracted from the wash fluid circuit where the salt content of the absorption fluid constitutes approximately 25 to 40% by weight. The The absorption of S02 by the ammonium sulfate solution used in the process is unsatisfactory. It is necessary to provide a larger wash zone in order to remove S02 from the flue gas until a permissible limit value has been reached. further, the established level of aerosol formation is no longer acceptable, if the smoke to be cleaned contains a high concentration of S02 of more than 2700 mg / N in the bucket. Similar processes are known from EP-A-0 212 523 and EP-A-0 620 187. An ammonium sulfate solution is also used in the case of these processes as the absorption fluid. The aim of the invention is to improve the process described in the introduction in such a way that it is possible to separate S02 effectively while keeping the aerosol formation as low as possible. In order to obtain this objective, the invention describes that the washing fluid added to the upper side of the washing zone consists of a mixture of: a) ammonia water, b) a fluid containing ammonium sulfide which is extracted from the tower for washing and purifying gases below the washing area and returning without intermediate treatment, and c) an ammonium sulfate solution which is returned from the oxidation device, where the proportion of a mixture between ammonium sulfite (NH4) 2S03) and ammonium sulfate ((NH4) 2S04) is adjusted to be in a range of 15: 1 to 3: 1. The ratio of the ammonium sulphite / ammonium sulfate mixture is preferably in the range between 10: 1 to 5: 1. It has been unexpectedly established that the washing fluid containing ammonium sulfite in the mixture with ammonia water and an ammonium sulfate solution produces an absorption fluid which is characterized by a high capacity to absorb S02 and does not show a tendency to aerosol formation. S02 is absorbed very rapidly by the ammonium sulfite, which is present in a high concentration in the absorption fluid, according to the invention, to form NH4HS03. A balance is established between S02 of the combustion gas, the NH3 added as ammonia water and (NH4) 2S03. S02 is absorbed more rapidly by the absorption agent used according to the invention compared to the absorption that is carried out with ammonia water alone or by a mixture of a solution of ammonia and ammonium sulfate. It is possible by means of a faster and more effective absorption of S02 to increase the through flow rate of the combustion gases and / or to operate with a single short wash. With the aid of the process according to the invention, it is possible to clean combustion gases comprising a high content of sulfur oxide. The aerosol content in the cleaned combustion gases can be maintained constantly below 15 mg / Nm3 and in most cases it can even be reduced to less than 10 mg / Nm3. The proportion of the ammonium sulfate solution contributes to decrease the aerosol formation potential of the absorption fluid used according to the invention. This may be due to the fact that the ammonia partial pressure of ammonium sulfate is markedly lower than that of ammonium sulphite / ammonium acid sulfite. The ammonia water used generally comprises an ammonia content between 10 and 30% by weight. It can be formed directly in situ by mixing ammonia with water, but other commercially available products can also be used. The amount of addition of ammonia water is controlled by the pH value of the absorption fluid. The pH value can be adjusted in the range between pH 4, 5 and 7. The pH value of the absorption fluid on the addition side is preferably adjusted between pH 5 and pH 6.5, where the pH value is adjusted to add ammonia water. The amount of fluid flow containing ammonium sulfite that returns directly to the wash zone while the oxidation step is derived, is adjusted so that the absorption fluid added to the wash zone contains a hyperesthoichiometric amount of (NH4) 2S03 that corresponds to the reaction of the equation: (NH4) 2S03 + S02 + H20 = 2NH4HS03 for the absorption of the amount of S02 introduced with the combustion gas into the washing zone. In a rapid manner, the flow amount of the washing fluid containing ammonium sulfite is controlled so that the absorption fluid contains a concentration of ammonium sulfite which is greater by about 25% compared to the stoichiometric amount necessary of according to the reaction equation mentioned above for a stoichiometric conversion. According to a preferred embodiment of the invention, the washing and purifying gas tower comprises a packed bed in the washing area which is sprayed on the upper side with absorption fluids. The amount of addition of the absorption fluid is determined according to a value obtained empirically for the packed bed. The amount of fluid flow from the ammonium sulfate solution that returns from the oxidation device in the wash zone is produced from the following material equilibrium of the flowing fluids: mox = m - mWF = mNH3 where : mox: flow amount of the ammonium sulfate solution returning from the oxidation device to the wash zone; m: amount of addition flow of the absorption fluid mWF: flow amount of the washing fluid containing ammonium sulfide that returns to the wash zone; Q mm2: quantity of ammonia water flow supplied fresh.

A partial flow of ammonium sulphate is excluded from the oxidation device. By virtue of the amount of flow 5 excluded, it is possible to control the salt content of the absorption fluid. Preferably, the process proceeds such that the ammonium sulfate solution comprises a concentration of (NH4) 2S04 of 25% to 30% by weight and is extracted from the oxidation device. 0 The combustion gas to be cleaned is generally produced by burning anthracite, lignite, petroleum coke, petroleum, refinery waste, waste and similar combustible substances at power stations. Generally, the gas has a temperature of about 135 to 280 ° C and is directed to the apparatus by means of corresponding conduits. To the Upon entering the apparatus, the gas, which often contains other toxic substances such as hydrogen halides such as HCl, is sprayed in the same stream with alkaline solution, for example, NaOH or KOH in order to neutralize the hydrogen halides. According to a preferred embodiment of the invention, the combustion gas is cooled before entering the washing zone by spraying it with cooling water and / or an ammonium sulfate solution and in doing so it is simultaneously saturated with water. In a further preferred embodiment of the process according to the invention, an absorption fluid is provided which is extracted from a washing / purifying tower of bases in a gas permeable base below the washing zone and where the solution of ammonium sulfate used for rapid cooling purposes is separated in the wash settling vessel. The ammonium sulfate solution is again removed from the wash settling tank and sprayed as a quench fluid into the flue gas stream. The ammonium sulfate solution is concentrated by evaporation of water. By controlling the incoming flow of ammonium sulfate solution, which is extracted from the oxidation device, and extracted from a partial flow of a concentrated ammonium sulfate solution from the wash settler, the concentration of ( NH4) 2S04 in the wash settling vessel. In this way, it is possible to obtain ammonium sulfate solutions of a concentration - lu up to about 42% by weight. Unwanted crystallization occurs at a higher concentration. It has been established that the crystals of (NH4) 2S04, which are formed in the case of high concentrations, are not absorbed in the scrubbing and gas scrubbing tower and form aerosols which can only be separated using expensive filtering apparatuses. At least part of the concentrated ammonium sulfate solution, which is extracted from the wash settling tank, can be directed to the ammonium sulfate recovery plant and reprocessed to form fertilizers. The invention is explained in the following with reference only to an exemplified embodiment illustrated in the drawing. The only figure shows the schematic illustration of the apparatus for the purpose of implementing the process according to the invention. The combustion gas directed to the apparatus is sprayed in a rapid cooling section 1 in the same stream with a solution of NaOH or KOH, which is supplied via conduit 2, in order to remove any hydrogen halide present by separation. . In addition, water is introduced into the combustion gas by means of a supply device 3 and / or an ammonium sulfate solution by means of a pipe 4 cooled and saturated with steam. The fluid temperature is maintained in the region between 45 ° C and 60 ° C, where the gas saturation temperature determines the temperature of the rapid cooling Preferably, an ammonium sulfate solution is used as the quench fluid and is withdrawn from an oxidation device at a concentration from 25 to 30% by weight and is supplied to the settling tank 6 of the scrubbing and purification tower. of gases 7. The combustion gas which has been cooled, saturated with water vapor and released from hydrogen halide, subsequently flows into the washing area 8 of the washing and gas scrubbing tower 7. The combustion gas , which is substantially loaded with S02, is brought into contact with an absorption fluid in the washing zone 8, absorption fluid which, according to the invention, consists of a mixture of ammonia water, an aqueous solution of sulphate of ammonium and a solution of newly formed ammonium sulphite. A partial flow of the absorption fluid is extracted in a gas-permeable base 9 below the washing zone 8 and is supplied via the conduit 10 to the oxidation device. Oxidation of the ammonium sulfite and the ammonium acid sulfite present in the fluid to form ammonium sulfate occurs in the oxidation device when an oxygenated gas is added. The other absorption of the absorption fluid is directed through the conduit 11 in the circuit, mixed with ammonia water from the conduit 12 and with the ammonium sulfate solution supplied via the conduit 13 outside the oxidation device 5 and is supplied to the spray installation 14 above the washing zone 8.

The washing zone 8 contains a packed bed, which increases the contact surface between the combustion gas and the absorption fluid, and improves the absorption of S02. It is possible through the use of the packing to reduce the height of the washing zone 8 and therefore to reduce the total height of the washing and gas cleaning tower installation 7. The commercially available packing consists of material which is inert with regarding acid gases and can be used in the packed bed. In order to obtain an effective absorption of S02, the pH value of the absorption fluid is adjusted on the addition side between pH 5 and pH 6.5. The pH adjustment is made by adding ammonia water. An air line 15 comprising a fan to the oxidation device is connected. The air supplied via line 15 is brought into intense contact with the fluid containing NH4HS03 / (NH4) 2S03. The ammonium sulfate solution produced is collected in the settling vessel of the oxidation device 5. The concentration of ammonium sulfate is determined, for example, by measuring the density. With an ammonium sulfate concentration of between 25 to 30% by weight, the ammonium sulfate solution is extracted from the settling vessel of the oxidation device 5. The partial flow extracted through conduit 16 can be directed to an ammonium sulfate recovery plant in which sulphate is produced of pure ammonium when removing the water. A partial flow is withdrawn via conduit 17 and directed to the wash settling vessel and used in a fluid circuit 18, which is connected to the wash settling vessel, as a quench fluid for the purpose of cooling the combustion gas, wherein the ammonium sulfate solution is concentrated to form concentrations of ammonium sulfate of up to 42% by weight. The amount of fluid in the settling vessel of the oxidation device 5 can be kept constant by means of a corresponding supply line by supplying water or washing fluid from a buffer vessel 20. In the exemplary embodiment illustrated in the figure, the gas scrubbing and washing tower 7 comprises a second washing zone 21, and likewise comprises a packed bed. A washing solution with a pH value between pH 4 and pH 6 is added by means of a fluid distributor 22 to the bed of the second washing zone 21, and gets in contact with the cleaned gas. It is within the scope of the invention that the combustion gas of the invention in which its acid components have been removed, that is directed through an aerosol separation device [not shown], but preferably, such a filter installation omitted. A wet electronic filter placed above the second wash zone can be used as an aerosol separation filter.

The washing fluid charged from the second washing zone is withdrawn in a fluid collection tray 23 placed under the second washing zone 21 and is directed via the line 24 to the buffer vessel. The pH value of the washing fluid is adjusted by supplying ammonia water via conduit 12. The fluid level in the settling vessel of damper vessel 20 is controlled by adding water via conduit 25. The combustion gas cleaned subsequently it goes to the chimney and discharges to the environment.

Claims

1. A process for removing sulfur dioxide from combustion gases, in particular exhaust gases from a power plant and exhaust gases from waste incineration plants, where: the combustion gas is directed from the bottom to the top to through a gas scrubbing and scrubbing tower comprising at least one wash zone and counter-currently treated with an absorption fluid which is added to the upper side of the wash zone and is extracted outside the tower. washing and purifying of gases below the washing area, and the absorption fluid is returned in a washing fluid circuit to the upper side of the washing zone, where ammonia water is supplied to the washing fluid circuit and a fluid flows branching away from the wash fluid circuit and is directed to an oxidation device in which the ammonium sulfite (NH4) 2S03) and the ammonium acid sulfite (NH4HS03) present in the fluid are oxidized to become ammonium sulfate (NH4) 2S04), the method is characterized in that the absorption fluid added on the upper side of the washing zone consists of a mixture of: a) ammonia water, b) a washing fluid containing ammonium sulphite which is extracted from the scrubbing and scrubbing tower below the washing zone and returned without intermediate treatment, and c) an ammonium sulfate solution which is returned from the oxidation device, wherein the mixing ratio between ammonium sulphite and ammonium sulfate is adjusted in the range of 15: 1 to 3: 1.

2. The process according to claim 1, characterized in that the mixing ratio between the ammonium sulphite and the ammonium sulfate in the absorption solution is adjusted in the range between 10: 1 and 5: 1.

3. The process according to any of claims 1 or 2, characterized in that the pH value of the absorption fluid is adjusted on the addition side between pH 5 and pH 6.5, where the pH value adjustment is made by the addition of ammonia water.

. The process according to any of claims 1 to 3, characterized in that the flow amount of the washing fluid containing ammonium sulfite that returns directly to the washing zone while the oxidation step is derived, is adjusted in such a way that the absorption fluid added to the wash zone comprises a hyperestichrometric amount of ammonium sulfite corresponding to the reaction of the equation (NH4) 2S03 + S02 + H20 = 2NH4HS03 for the absorption of the quantity of S02 supplied with the combustion gas to the washing area.

5. The process according to any of claims 1 to 4, characterized in that the washing and purifying gas tower contains in the washing area a packed bed which is sprayed on the upper side by the absorption fluid, wherein the of addition of the absorption fluid is determined according to a value obtained empirically by the packed bed, and in which the flow amount of the ammonium sulfate solution returned to the washing zone from the oxidation device is adjusted according to to the value of the predetermined addition amount.

6. The process according to any of claims 1 to 5, characterized in that the ammonium sulfate solution is extracted with an ammonium sulfate concentration of 25% by weight to 30% by weight of the oxidation device.

7. The process according to any of claims 1 to 6, characterized in that the combustion gas is cooled before entering the washing area when it is sprayed with fast cooling water and / or a solution of ammonium sulfate and the gas of Combustion is saturated with water.

8. The process according to claim 7, characterized in that the absorption fluid is extracted outside the wash tower and cleaned of gases in a gas permeable base below the washing zone and where the ammonium sulfate solution used For rapid cooling purposes it is separated in the washing sedimentation container, where the ammonium sulfate solution is again removed from the wash settling tank and sprayed into the flue gas stream as the quench fluid, and where a concentration is set in the settling wash tank. of ammonium sulfate of maximum 42% by weight.