WASTE-GAS TREATMENT PROCESS

Abstract: In the desulfurization of flue gas and the like, the gas is preliminarily treated to remove sulfur trioxide and sulfuric acid by a first cooling of the gas, e.g. to about 250 DEG F to 400 DEG F in a combustion air preheater (14), and then simultaneously or thereafter introducing an aqueous stream (18) from an FGD system (20) containing dissolved chloride, said aqueous stream (18) being completely vaporized upon injection (22) into said gas, and recovering from the gas chloride salts which remain in the gas after the water fraction of the aqueous stream has been vaporized, and ammonium chloride which precipitates upon the fly ash in the gas or forms a finely-divided suspension. Ammonia may be added (16) to react with at least some of the Cl<-> values contained in the flue gas if further reduction of the chloride in the FGD system (20) is desired.

Description: -ι- Description Waste-Gas Treatment Process Technical Field This invention relates to the effective and efficient removal of chloride values from desulfurization (FGD) systems, and is more particularly concerned with limiting the chloride content in the FGD system to prevent the known adverse impact on the system performance. Background of Invention The combustion of fossil fuels in power plants, and by industries, and the combustion of various waste materials, e.g. in incinerators, represent one of the most serious causes of atmospheric pollution by reason of the gaseous effluents released. Unless the gaseous effluent streams are treated to remove deleterious components, serious problems can develop, such as smog, impairment of health, and the like. Thus, as man- kind has become increasingly aware of the need to preserve and protect the quality of the environment, more and more stringent laws have been placed on the books which -limit the amount of pollutants, if any, which may be released in effluent gases dur¬ ing the processing and utilization of various materials. Parti- cularly serious is the presence of sulfur compounds in gaseous effluents or discharges, such as flue gas from power plants. For example, an emission limit for sulfur dioxide from power plant flue gases became mandatory as a result of the 1970 amend¬ ment to the Clean Air Act. Consequently, various flue gas desulfurization systems have been proposed and are utilized. For the most part, flue gas desulfurization (FGD) technology has been used to treat flue gas from coal-fired utility boilers. In recent years, as the technology has matured, a major problem has been observed in some aqueous-based FGD systems resulting from the presence of chloride (Cl~) in the coal. Under typical operating conditions, chloritie present in the coal enters the FGD system as Hd gas which is rapidly absorbed. Depending on the chloride levels present and the nature of the FGD process, this can cause a number of problems. It will thus be apparent that many existing and planned flue gas desulfurization .systems have encountered or will encounter serious practical problems if there are objec¬ tionable levels of chloride, particularly in the form of HC1, in flue gas and like gaseous effluents, such as: 1. Corrosion of the area where the hot, untreated flue gas first contacts the aqueous scrubbing slurry/solution, or, if flue gas bypass is used, corrosion can occur in the zone where bypassed flue gas mixes with treated gas downstream of the FGD system. 2. In calcium-based systems, chloride can concentrate to high levels and cause the sulfur dioxide removal capability of the scrubbing slurry to deteriorate, requiring either equip¬ ment modifications or the use of expensive chemical additives to permit the system to remove the required quantity of sulfur dioxide. 3. In some systems, chloride can accumulate to levels which cause general corrosion of process equipment and piping, or, if the designers anticipate high chloride levels, equipment must be constructed of expensive alloys to prevent such corro¬ sion. 4. In some calcium-based systems, the solid waste produced is converted entirely to calcium sulfate dihydrate (gypsum) and sold for the manufacture of wallboard. In addi- tion to the other problems, if high chloride concentrations of the character indicated are encountered, special washing equip¬ ment and extensive washing are needed to remove any chloride from the gypsum by-product before it is in a condition to be sold. 5. In some sodium-based systems, a blowdown stream is required to purge chloride from the system. In addition to purging chloride, such a purge stream contains expensive sodium-containing reagent. In other sodium-based systems, special process equipment is installed upstream of the FGD sys¬ tem to remove HC1 by aqueous scrubbing. 6. In the case of power plants, many are subject to mandatory regulations which require operation with zero water discharge. As a result, when chloride concentrations build to undesired levels, the system must be purged. In some cases, this has led to the use of expensive water-treatment equipment just to control the chloride concentration. Moreover, at the pH levels found in many flue gas desulfurization systems (5.0 to 6.0), dissolved chloride can cause severe corrosion problems in the case of the usual materials of construction, such as mild steel or 300 series stainless steel, even when such chloride concentrations are as low as 2000 pp . Furthermore, HC1 concentrations above 3000 ppm, (expressed as Cl~) , in the quench stream for the desulfurization operation suppress the solubility of a major source of alkalin¬ ity in limestone flue gas desulfurization systems. This inter¬ feres with and inhibits S0 2 removal. In an effort to offset the effect of the above-mentioned high chloride concentrations, the engineers who design flue gas desulfurization plants provide for large absorbers, greater absorber liquor to gas ratios, increased amounts of limestone, organic acid additions, such as dibasic acids, or some other technique for improving the SO2 removal. It will be readily apparent from the foregoing that the presence of undesired levels of chloride in a flue gas desulfurization system increases dramatically both the capital and operating costs of such a system. The use of ammonia to treat waste gases has heretofore been proposed. For example, Cann U.S. 3,579,296 discloses the treatment of flue gas or other effluent gas to remove sulfur dioxide. Nothing is said about HC1 or chloride removal. The gas to be treated is. cooled to below its dew point, e.g. to 150°F, to condense water and SO3 and, since the condensate is highly acidic, a solution or suspension of lime is added. Thereafter ammonia is added to react with the SO2 present to form ammonium sulfite and ammonium bisulfite. In essence, therefore, the disclosed process is a solely desulfurization. Bradley U.S. 1,291,745 uses ammonia for treating gas from a smelter in connection with an electric precipitator. The ammonia is used to react with the sulfur dioxide in the smelter fume. The ammonia is converted into ammonium bisulfite, then into normal ammonium sulfite, and finally into sulfate by atmos¬ pheric oxidation. Williams U.S. 2,356,717 is also concerned with elec¬ tric precipitation and adds ammonia or ammonia-yielding com¬ pounds' to the gases or vapors from a catalytic petroleum crack¬ ing operation to increase the efficiency of the electric preci- pitator. The gas contains suspended particles and is at a tem¬ perature of 400-450°F. Humbert U.S. 3,523,407 has a similar objective and adds ammonia and water to a particle-laden gas stream. Optimum precipitation is said to occur when ammonia is added at the rate of 10 to 20 parts per million parts of gas and water is added at the rate of 4-8 gallons per 100,000 cubic feet of gas and the gas temperature is above 400°F. Russell U.S. 3,956,532 discloses the use of ammonia to treat a gas stream containing metal chlorides such as stannic chloride, TiCl4, VOCI3, and methyltricholorosilane, used in the coating of glass, all at a temperature ranging from 70° to 350°F« Japanese publications 147137/80 and 78619/81 discuss the removal of HC1 from incinerator effluents in a system directed to the removal of nitrogen>oxides but no conditions regarding HC1 removal are disclosed except that a table in the latter Japanese publication shows temperatures ranging from 235°F (113°C) to 392°F (200°C) for HC1 removal, the best removal apparently being at 235°F with an NH3/HCI ratio of 2:1. Japanese publication 52-054276 proposes the treat¬ ment of PVC-containing wastes by introducing gaseous ammonia into the smoke resulting from the combustion of PVC-containing wastes when at temperatures of 500°C to form NH4CI. Subsequently the waste smoke is cooled to 250°C to collect the NH4CI formed. Japanese publication 58-045721 describes a method of adding ammonia to a waste combustion gas containing NO x , SO x , and HC1 to convert the NO x to 2 and H2O, the equivalent amount of ammonia added being 5-12 times as much as that of the NO x in the gas. Reaction with HC1 or SO x in the gas obtains a reaction product which is then contacted with- water to absorb the product from the gas and to obtain an aqueous solution of the product. An alkali or alkaline earth metal compound is added to the aqueous solution to generate ammonia which is recycled to the waste combustion gas. Japanese publication 58-174222 shows, in the purifi¬ cation of combustion waste gas containing dust and SO3, etc., e.g., a discharge from a coal-fired boiler, the improvement of passing the waste gas through a first dust collector to remove dust, thereafter injecting ammonia into the waste gas at an appropriate temperature, e.g., 130°-180°C, to form a reaction product, collecting the reaction product in a secondary dust collector, and supplying the collected reaction product to a furnace from which the waste gas to be treated is discharged. Russian Patent No. 590005 of January 31, 1978, in the name of V. Baryshev, discloses the removal of oxides of sulfur and nitrogen and of HC1 from flue gases produced by burning sul¬ fur-containing fuels. The flue gases are first cooled to 150-200°C and then treated with gaseous ammonia in the amount of 1.6 to 5 weight percent in relation to the weight of fuel burned. Deposition of ammonium salts is said to be prevented in the Russian process. The example shows a flue gas treated with 0.3% ammonia. No salt deposits were formed. It is an object of this invention to provide a process and means effective for preventing the entry of undesired quantities of chlorides into flue gas desulfurization and like systems. It is another object of the invention to provide a process and means of the character indicated which are conven¬ ient to employ, and are inexpensive, yet are fully effective for their intended purpose of preventing chlorides from interfering with the desulfurization operation. Other objects and features of this invention will be readily apparent from the following detailed description of an illustrative embodiment thereof. Summary of the Invention In accordance with the invention, these and other objects are achieved by a two-step process wherein the flue gas or other effluent is treated with ammonia before it enters the flue-gas desulfurization (FGD) system under certain conditions and in a specified manner, as will be hereinafter detailed, and an aqueous stream from the FGD system is injected into the flue gas also at a location before the flue gas enters the FGD system. Briefly stated, for coal-fired utility boiler applica- tions, ammonia is injected into the flue gas just after the boiler air preheater, where the flue gas has been cooled below the ammonia decomposition temperature. Ammonia may be injected as an aqueous spray or mist or as a gas in an amount twice as great as needed stoichiometrically to react with any sulfur trioxide and any sulfuric acid present and may also be in a further amount to react with at least some of the chloride (Cl~) present. Thereafter, the injection of the aqueous stream from the FGD system controls the buildup of dissolved chloride in the FGD system. The aqueous fraction of the stream so injected as an atomized mist will vaporize, leaving behind as*jdried particles the dissolved solids, including chlorides, which were contained in this stream. Ammonia injection must occur at the air pre¬ heater exit, typically at 400°F or less, e.g., at 250-400°F. Injection upstream of the preheater will cause the preheater to plug with ammonium sulfate salts and eventually to corrode. Ammonia may be added in an amount above that amount required to react effectively with sulfur trioxide and sulfuric acid to achieve a reduction in the chloride concentration in the FGD system streams by removing at least some HC1 from the flue gas as ammonium chloride before the flue gas enters the FGD system, upon injection of the aqueous stream from the FGD system the flue gas is cooled to 180°F to 240°F, preferably to about 210°F. The ammonia is added either before or during the second cooling step. The ammonia must be added before the particulate removal and desulfurization steps. The second cooling is brought about by the above-mentioned injection of the aqueous stream from the FGD system. The quantity of the FGD stream is selected so that, as mentioned, the aqueous fraction of the stream in atomized form will completely vaporize, as it cools down the flue gas to the desired temperature, leaving behind as dried particles any * dissolved solids, including chloride, which were contained in this stream. Solid particles consisting of the dissolved chloride which remains after the aqueous stream is vaporized, and the reaction product ammonium chloride resulting from the gas phase reaction of ammonia and HC1, are subsequently removed in the particulate collection device which may be either an electro- static precipitator (ESP) or a fabric filter. Therefore, the injection of the ammonia and the aqueous stream must occur before the particulate collection device. In this way the chloride is removed from the system along with any fly ash solids collected in the particulate collection device. In contrast to the above-discussed prior art, the present invention utilizes, in combination, the addition of ammonia and the injection of an aqueous stream from the FGD system ahead of the particulate collection device to speci¬ fically remove chlorides from the FGD system. At a minimum, ammonia must be added to react with all sulfur trixoxide or sulfuric acid. This prevents damage of ductwork due to condensation of the corrosive sulfuric acid compound on the flue gas duct walls after the aqueous FGD system stream is injected. As mentioned, additional ammonia may be added to react with at least some of the HCl in the flue gas to accomplish an even lower chloride concentration in the FGD system as long as the flue gas temperature is lowered to 180°F to 240°F by the injection of the aqueous stream from the FGD system. Brief Description of the Drawing The single figure of the drawing shows diagrammati- cally, by way of a flow sheet, a representative system for carrying out an embodiment of the process of the invention. Description of the Preferred Embodiments This invention allows the practical and economical removal of HCl from a flue gas upstream of an aqueous-based FGD system. Use is made of a particulate control device, e.g. ESP, located upstream of the wet scrubber of the FGD system. HCl removal is accomplished through the injection of an aqueous stream from the wet scrubber system into the hot flue gas stream ahead of the particulate control device. The aqueous phase of the stream is evaporated by the hot gas, leaving behind the dissolved species which are made up partially of various chloride salts such as calcium chloride, magnesium chloride, and sodium chloride, as well as ammonium chloride. These chloride salts and the other dissolved species originally present in the aqueous stream are then removed in the particu- late control device along with any fly ash. The aqueous stream would most typically come from the solids dewatering area of the FGD system such as the overflow stream from the thickener or other primary solid/liquid separation device or the filtrate or centrate stream from the final dewatering step. Ammonia must be added prior to or with the aqueous stream injection point to react with all sulfur trioxide or sulfuric acid present in the flue gas, but not upstream of the boiler air preheater. This must be done to prevent the conden¬ sation of sulfuric acid on the flue gas ductwork after the aqueous stream is injected and the flue gas temperature lowered by the evaporation of the water in the aqueous stream. The reduction in temperature results in the condensation of sulfuric acid. Without the addition of ammonia, significant corrosion of the ductwork would be expected causing severe damage. Further chloride removal can be accomplished by adding more ammonia to the flue gas stream at the same location. HCl removal occurs through the known reaction of gaseous ammonia and HCl to form ammonium chloride. The solid NH4CI formed by this reaction may precipi¬ tate on fly ash particles or it may nucleate to form NH4CI crys¬ tals. In either case, the NH4CI formed will be collected in the particulate control device and is thus prevented from entering the FGD system. The process of the invention avoids the prob- lems associated with the presence of undesired amounts of chlorides in the FGD system. Some features of the invention which make the removal of chloride from the FGD system possible are: The aqueous stream from the FGD system is atomized into very fine droplets. This is an important feature of the invention. It prevents wetting and corrosion of the flue gas duct where the water is injected, facilitates temperature reduction and ensures optimum operation. The preferred atomiza- tion devices are two fluid-type spray nozzles in which air or some other gas (e.g., steam, compressed nitrogen, and the like) is used to cause the water to form fine droplets, or rotary-type atomizers in which a rapidly-rotating disc is used to effect the same formation of fine droplets. Water mists in other environ¬ ments are known, but such mists are not concerned with desulfuri- zation and chloride removal. Thus, Polyhala 2,740,693 shows spraying hot flue gases with a fine mist of water without pre- cooling for the purpose of removing insoluble gases such as nitrogen in a process for providing a source of nitrogen for the manufacture of synthetic nitrogen by the Haber process. Addition of the ammonia to react with sulfur trioxide and sulfuric acid in the flue gas must be added at a specific level to insure the formation of ammonium sulfate from the ammonia-sulfur trioxide reaction. To accomplish this, the amount of ammonia added must be at least twice the amount of sulfur trioxide present in the flue gas on a molar basis. If less than twice the amount of ammonia is added the formation of some ammonium bisulfate will occur. This species will condense at the flue gas temperatures typical of those down¬ stream of a boiler air preheater and collect on solid surfaces such as the duct walls and particulate collection device. Ammonium bisulfate is a very corrosive species and would severely damage this equipment. Ammonium sulfate does not possess corrosive characteristics to this equipment, however. For example, for a sulfur trioxide concentration of 15 ppmv in the flue gas, at least 30 ppmv of ammonia must be added to the flue gas to react with the sulfur trioxide to insure that no ammonium bisulfate is formed. To achieve further chloride removal beyond that possible from injection of an aqueous stream from the FGD system, additional ammonia is added to the flue gas and the temperature of the flue gas lowered to 180-240°F, prefer¬ ably about 210°F. For the reaction between ammonia and HCl in the flue gas to go to completion, and the minimum chloride concentration possible achieved in the FGD system, the addition of ammonia should be at least stoichiometrically equal to the chloride present. Some chloride removal .will occur at stoichio- metries less than 1.0, but -100 percent Cl removal requires a stoichiometry of at least 1.0. The reaction between ammonia and HCl in the flue gas is enhanced as the temperature is lowered. For all practical purposes, cooling the flue gas to a temperature of about 210° F. is optimum. Therefore, the rate of addition of the aqueous stream must be closely regulated to achieve a constant tempera¬ ture of no higher than 240 F and preferably 210° F. while ensuring complete vaporization of the aqueous fraction of the stream. The point of ammonia addition is also important. The ammonia is added either upstream of the point where the aqueous stream from the FGD system is added, and after the flue gas exits the air preheater, or it is injected at the point the cooling water is injected, either separately or in solution with the aqueous stream. In order to minimize the possibility of sulfuric acid condensation in the duct and to maximize ammonia dispersion prior to cooling the flue gas, the preferred point is upstream of the point at which the aqueous stream is added. Referring to the drawing, flue gas in line 10 from a coal-fired boiler 12 typically is cooled to between 250°F. to 400°F in a combustion air preheater 14 by heat-exchange. Ammonia in line 16 is injected at the air heater exit at a rate to provide at least twice the the amount, on a molar basis, of the sulfur trioxide and H2SO4 in the flue-gas stream being treated. The ammonia injection rate is controlled on the basis of the load of the boiler. Downstream of the ammonia injection point, clear liquor in line 18 from the FGD system 20 is atomized at 22 into the flue gas using two-fluid type nozzles (not shown) and compressed air from line 23. The water-injection rate is adjusted so that the water is completely evaporated by the flue gas and the desired chloride removal rate from the FGD system is maintained, the contained chloride com¬ pounds depositing as the water evaporates. Cooling of the flue gas results in precipitation of solid ammonium chloride which, along with the deposited compounds from the FGD stream, are separated in solid form from the flue gas in separator 24, which may be an electrostatic precipitator (ESP), fabric filter, or the like, subsequently removed through line 26, while the flue gas passes via lines 28 to the FGD system 20. As indicated above, ammonia is injected after initial cooling of the gas, which is effected by passage through the air preheater in the embodiment illustrated, but it will be under¬ stood from the foregoing description that standard items of equipment can be used for cooling, solids removal, and the like, such as the equipment described and used in the above-mentioned prior art. Similarly, various changes and modifications in the invention may be made without departing from the scope of the appended claims. It is intended, therefore, that all matter contained in the foregoing description and in the drawing shall be interpreted as illustrative only and not as limitative of the invention.

Claims: Claims 1. In a process for the desulfurization of a gas containing HCl values, said process comprising the steps of preliminarily cooling said gas to a temperature of about 400°F, or below treating the cooled gas with ammonia, to react with sulfur trioxide and sulfuric acid values in said gas, simul¬ taneously or thereafter injecting clarified liquor from an FGD process into said gas, and separating chloride-containing salts from said gas. 2. In a process as defined in claim 1, wherein said, gas is preliminarily cooled to a temperature of 250°F to 400°F. 3. In a process as defined in claim 1, wherein the gas is a flue gas. 4. In a process as defined in claim 1, where the gas is cooled to 180° to 240°F by injection of said liquor in the form of fine particles. 5. A process of removing HCl values from a gas con¬ taining the same which comprises cooling said gas to a tempera¬ ture of about 400°F, treating the cooled gas with ammonia, simultaneously or thereafter cooling said gas to about 180°F to 240°F, the addition of ammonia forming ammonium chloride, and separating said ammonium chloride from said gas. 6. In a process as defined in claim 5, where the gas is a flue gas. 7. In a process as defined in claim 5, where the gas is cooled to about 180°F to 240°F by introduction of fine particles of an aqueous stream. 8. In a process for the treatment of flue gas con¬ taining sulfur trioxide and sulfuric acid values, the steps of preliminarily cooling said gas to a temperature below the decomposition temperature of ammonia, treating the cooled gas with ammonia, in a molar amount to react with at least twice the amount of sulfur trioxide and sulfuric acid present in said gas plus an amount of ammonia to react with at least some of the HCl values in said gas. 9. A process of removing HCl values from a gas con¬ taining the same which comprises cooling said gas to a tempera¬ ture about 250°F to about 400°F, treating the cooled gas with ammonia, and simultaneously or thereafter cooling said ammonia- treated gas. 10. In a process for the desulfurization of atgas containing HCl values, the steps of preliminarily cooling said gas to a temperature of about 400°F or below, treating the cooled gas with ammonia, and simultaneously or thereafter cooling said gas to about 210°F, the addition of ammonia forming ammonium chloride, and separating said ammonium chloride from said gas. 11. A process as defined in claim 10, wherein said gas is preliminarily cooled to about 250°F to about 400°F. 12. In a process as defined in claim 10, wherein the gas is a flue gas. 13. In a process as defined in claim 10, wherein the gas is cooled to about 210°F by the introduction of fine particles of an aqueous system. 14. A process of removing HCl values from a gas containing the same which comprises cooling said gas to a temperature of about 400°F, treating the cooled gas with ammonia-, simultaneously or thereafter cooling said gas to about 210°F, the addition of ammonia forming ammonium chloride, and separating said ammonium chloride from said gas. 15. In a process as defined in claim 14, where the gas is a flue gas. 16. In a process as defined in claim 14, wherein the gas is cooled to about 210°F by the introduction of fine particles of an aqueous stream.

Inventors: BURKE JOHN MICHAEL, COLLEY JAMES DAVID, BURKE, JOHN, MICHAEL, COLLEY, JAMES, DAVID
Applicants: RADIAN CORP, RADIAN CORPORATION
Application year: 1988
Main classification: C01B7
IPC codes: B01D53, C01C1