MX2008003648A - Method of removing sulfur trioxide from a flue gas stream - Google Patents

Abstract

A method of removing SO3from a flue gas stream includes providing a reaction compound selected from the group consisting of sodium carbonate, sodium bicarbonate, sodium sesquicarbonate, and mixtures thereof. The reaction compound is injected into the flue gas stream. The temperature of the flue gas is between about 500°F and about 850°F. The reaction compound is maintained in contact with the flue gas for a time sufficient to react a portion of the reaction compound with a portion of the SO3to reduce the concentration of the SO3in the flue gas stream.

Description

METHOD FOR EXTRACTING SULFUR TRIOXIDE FROM A COMBUSTION GAS CURRENT The present invention relates to the purification of gases, and more particularly to a method for purifying combustion gases containing noxious gases, such as S03. S03 is a noxious gas produced by the combustion of sulfur-containing fuel. When present in the flue gas, S03 can form an acid mist that condenses in electrostatic precipitators, ducts or baghouses, and causes corrosion. S03 at concentrations of just 5-10 ppm in exhaust gas can also result in white, blue, purple or black feathers by cooling the hot strata gas in the colder air of the atmosphere. The effort to reduce NOx emissions from power plants fed to coal through selective catalytic reactors (SCR) has resulted in the unexpected consequence of oxidizing S02 to S03 and consequently increasing the total emissions of S03. The SCR employ a catalyst (usually vanadium pentoxide) in order to convert N0X to N2 and H20 with the addition of NH3, but there is also an unexpected oxidation of S02 to S03. Although through the highest strata the S03 concentrations are still relatively low, sometimes emissions can produce a highly visible secondary plume, which, although not regulated, however, is perceived by many authors as a source of problems. Efforts to reduce S03 levels to the point that secondary S03 pens are not visualized may impede the collection of particles for stations that employ electrostatic precipitators (ESP). S03 in the combustion gas not only absorbs volatile ash particles and reduces the resistivity of fly ash, so it allows ESP to capture the particles by electrostatic means. At present, some manufacturing plants inject S03 to reduce the resilience of fly ash, when the ash resistivity is too high. S03 reacts with steam in the flue gas ducts of the coal-fired power plant and forms H2S0 as steam. A portion of it condenses in the air warming baskets. Another portion of the sulfuric acid vapor can condense in the duct if the duct temperature is too high, corroding the duct. The rest of the acid vapor condenses when the plume is tempered when it is brought into contact with the relatively cold atmosphere or when wet cleaners are used for the desulphurization of gas. combustion (FGD), in the tempering zone of the cleaner. The rapid tempering of the acid vapor in the FGD tower results in a fine acid mist. Often the droplets are too thin to be absorbed in the FGD tower or captured in the nebular eliminator. Consequently, there is only limited elimination of S03 by the FGD towers. If the levels of sulfuric acid emitted from the strata are high enough, a secondary plume appears. Dry absorbent injection (DSI) with various absorbers has been used to extract S03 and other gases from the combustion gas. However, in the past DSI has generally been made at temperatures well below 370 ° F since the equipment material, eg baghouse means, does not withstand higher temperatures. In addition, many absorbent materials are sintered or melted at temperatures near or above 400 ° C, making them less effective for gas removal. The reaction products of many absorbent materials also adhere to equipment and ducts at higher temperatures, which requires frequent cleaning of process equipment. In one aspect, a method is provided for extracting S03 from a flue gas stream that includes S03. The method includes providing a reaction compound selected from the group consisting of sodium carbonate, sodium bicarbonate, sodium sesquicarbonate and their mixtures. The reaction compound is injected into the flue gas stream. The temperature of the combustion gas is between about 500 ° F and about 850 ° F. The reaction compound is kept in contact with the flue gas for a sufficient time to react a portion of the reaction compound with a portion of S03 to reduce the concentration of S03 in the flue gas stream. In another aspect, a system is provided for the removal of S03 from a flue gas stream that includes at least about 3 ppm of S03 which includes providing a trona source having an average particle size between about 10 microns and about 40 microns. microns. The highchair is injected as dry granular material in the flue gas stream. The temperature of the combustion gas is between about 275 ° F and about 365 ° F. The chair is kept in contact with the combustion gas for a sufficient time to react a portion of the sodium absorber with a portion of the S03 to reduce the concentration of S03 in the flue gas stream. The reaction product comprises Na2S0.

The foregoing paragraphs are provided by way of general introduction and are not intended to limit the scope of the following claims. The presently preferred embodiments, together with the further advantages, will be better understood by reference to the following detailed description taken in conjunction with the accompanying drawings. FIG. 1 is a phase diagram showing the reaction products of trona with S03 as a function of the combustion gas temperature and S03 concentration. FIG. 2 is a diagram of an embodiment of a flue gas desulfurization system. The invention is described with reference to the drawings, in which like elements are referred by equal numbers. The relationship and operation of the various elements of the present invention are better understood through the following detailed description. However, the embodiments of the present invention are described below only by way of example, and the invention is not limited to the embodiments illustrated in the drawings. Dry absorbent injection (DSI) has been used as a low cost alternative to dry spray or wet cleaning systems for the extraction of S03. In the DSI process, the absorbent is stored and injected dry in the combustion duct, where it reacts with the acid gas. Under certain processing conditions, the reaction product of the absorbent and the acid gas is a sticky ash. Sticky ash tends to adhere to equipment and process ducts, requiring frequent cleaning. Consequently, it would be beneficial to have a process that minimizes the amount of reaction product of the sticky ashes. The present invention provides a method for extracting S03 from a flue gas stream comprising S03 by injecting a reaction compound such as sodium sesquicarbonate, sodium bicarbonate, or soda ash into a flue gas stream to react with S03. Sodium sesquicarbonate of trona is preferably provided. The trona is a mineral that contains approximately 85-95% sodium sesquicarbonate (Na2C03 -NaHC03-2H20). A vast deposit of the mineral trona is located in southwestern Wyoming, near Green River. As used herein, the term "trona" includes other sources of sodium sesquicarbonate. However, embodiments are preferred in which the sesquicarbonate source is mine trona. The term "combustion gas" includes the exhaust gas of any kind of combustion process (including coal, oil, natural gas, crude glass material, etc.). He Combustion gas geney includes S02 together with other acid gases such as S02, HC1, S03, and N0X. When heated to 275 ° F or above this value, sodium sesquicarbonate undergoes rapid calcination of the sodium bicarbonate contained in sodium carbonate, as shown in the following reaction: 2 [Na2C03 • NaHC03 • 2H20]? 3Na2C03 + 5H20 + C02 Sodium bicarbonate undergoes a similar reaction at elevated temperatures: 2 NaHC03? 3Na2C03 + H20 + C02 A chemical reaction of preference of the reaction compound with S03 is shown below: Na2C03 + S03? Na2S04 + C02 However, under certain conditions, unwanted reactions that produce sodium bisulfate may occur. If sodium sesquicarbonate or sodium bicarbonate is not completely calcined before the reaction with S03, the following reaction occurs: NaHCO3 + S03? NaHS04 + S03 Under certain conditions, another undesired reaction produces sodium bisulfate as shown below: Na2C03 + 2S03 + H20? 2NaHS04 + C02 Sodium bisulfate is an acid salt with low melting temperature and is unstable at high temperatures, so it is broken down as indicated in the following reaction: 2NaHS04? Na2S207 The type of reaction product of Na2C03 and S03 depends on the concentration of S03 and the temperature of the combustion gas. FIG. 1 is a phase diagram showing the typical products of trona reaction with S03 as a function of the temperature of the combustion gas and the concentration of S03. In particular, above a certain concentration of S03, the reaction product can be solid NaHS04, liquid NaHS04, Na2SO4, or Na2S207, depending on the temperature of the combustion gas. The boundary between liquid NaHS04 and solid Na2SO4 at a temperature above 370 ° F can be represented by the equation log [S03] = 0.009135T-2,456, where [S03] is the base log 10 of the concentration of S03 in ppm and T is the temperature of the combustion gas in ° F. Liquid NaHS04 is particularly undesirable because it is "sticky" and tends to adhere to the process equipment and causes other particles, such as fly ash, to also adhere to the equipment. Accordingly, it is desirable to operate the process under conditions in which the amount of liquid NaHS04 reaction product is minimal. Consequently, the process can be operated at a temperature below about 370 ° F, above about 525 ° F, or at a temperature and concentration of S03 where log [S03] < 0, 009135T-2, 456. The temperature of the combustion gas varies with the location in the injection system and may also vary in part with time during operation. As the temperature of the combustion gas increases, the reaction product of the sodium compound and S03 ranges from solid NaHS04, to liquid NaHS0, to solid Na2SO4 or Na2S207. Consequently, to avoid the formation of sticky ashes, preferably the process is operated in a suitable temperature range. In one embodiment, the temperature of the combustion gas when the highchair is injected is between about 500 ° F and about 850 ° F. The chair is kept in contact with the combustion gas for a time sufficient to react a portion of the chair with a portion of S03 to reduce the concentration of S03 in the flue gas stream. The temperature of the combustion gas is preferably greater than about 500 ° F. The temperature of the combustion gas is preferably less than about 800 ° F, and more preferably less than about 750 ° F. The temperature of the combustion gas is most preferably between about 525 ° F and about 750 ° F. In another embodiment, the temperature of the combustion gas is between approximately 275 ° F and approximately 365 ° F. This temperature range is lower than the temperature for the formation of sticky NaHS0. The S03 concentration of the combustion gas stream that is geney treated is at least about 3 ppm, and commonly between about 10 ppm and about 200 ppm. In order to avoid adhesion of residual material to process equipment, when operating at combustion gas temperatures greater than about 500 ° F the non-gaseous reaction product is preferably less than about 5% NaHS0, and most preferably less than about 1% NaHS04. The preferred outlet S03 concentration of the gas stratum is preferably less than about 50 ppm, more preferably less than about 20 ppm, still more preferably less than about 10 ppm, and most preferably less than about 5 ppm. The by-product of the reaction is combined with fly ash. The trona, like most alkaline reactants, tends to react more quickly with the stronger acids first the gas stream, and then, after a certain residence time, it reacts with the weaker acids. Said gaseous constituents such as HC1 and S03 are strong acids and the trona reacts with much faster with these acids than you would with a weak acid such as S02. Accordingly, the injected reaction compound can be used to selectively remove S03 without substantially reducing the amount of S02 in the flue gas stream. In FIG. 2 shows a diagram of one embodiment of the process The furnace or combustor 10 is fed with a fuel source 12, such as coal, and with air 14 in order to burn the fuel source 12. From the combustor 10, the Combustion gases are conducted to a heat exchanger or air heater 30. Room air 32 can be injected to reduce the temperature of the combustion gas. A selective catalytic reduction (SCR) 20 device can be used to extract N0X gases. A transverse vaporizer 22 can be opened to divert combustion gas from the SCR. The outlet of the heat exchanger or air heater 30 is connected to a particle collection device 50. The particulate collection device 50 extracts the particles formed during the combustion process, such as fly ash, from the combustion gas before drive it to an optional wet cleaning cup 54 and then to the gas stratum 60 for ventilation. The particle collection device 50 can be an electrostatic precipitator (ESP). Other types of devices Collecting particles, such as a baghouse, can also be used for the extraction of solids. The baghouse contains filters to separate the particles formed during the combustion process of the combustion gas. The S03 removal system includes a source of reaction compound 40. The reaction compound is selected from sodium sesquicarbonate, sodium bicarbonate, and soda ash. The reaction compound is preferably provided as particles with an average particle size of between about 10 microns and about 40 microns, more preferably between about 24 microns and about 28 microns. The reaction compound is preferably in dry granular form. The reaction compound is preferably sodium sesquicarbonate in the trona form. A suitable source of trona is Trona T-200®, which is the product of mechanically refined trona vein available from Solvay Chemicals. The T-200® trona contains approximately 97.5% sodium sesquicarbonate and has an average particle size of approximately 24-28 microns. The S03 removal system may also include a ball mill sprayer, or other type of mill, to reduce and / or otherwise control the particle size of trona or other reaction compound.

The trona is transported from the source of reaction compound 40 to injector 42. The reaction compound can be transported rheumatically or by any other suitable means. The apparatus for injecting the reaction compound is illustrated schematically in FIG. 2. The injection apparatus 42 introduces the reaction compound into the section of combustion gas conduits 44, which are preferably arranged in an upstream position of the air heater 30. The injection system is preferably designed to maximize the contact of the reaction compound with S03 in the flue gas stream. Any type of injection apparatus known in the art can be used to introduce the reaction compound into the gas conduit. For example, injection can be achieved directly by means of an eductor driven by compressed air. The ambient air 32 can be injected to reduce the combustion gas temperature before the injection point 42. The process does not require slurry equipment or reactor vessel if the reaction compound is stored and injected dry in the combustion duct 44 where it reacts with the acid gas. However, the process can also be used with humidification of the combustion gas or wet injection of the reaction compound. In addition, the particles can be joined wet by a wet cleaning cup existing 54 if the process was used for the fine cleaning of acid mist. In particular, the flue gas desulphurisation system can be operated in such a way that the removal of S03 is achieved by injecting the reaction compound with S03, while the majority of S02 is extracted by the wet cleaner 54. The The process can also be varied to control the temperature of the combustion gas. For example, the temperature of the combustion gas stream upstream of the trona can be adjusted to obtain the desired temperature of the combustion gas when the reaction compound is injected. In addition, ambient air 32 can be introduced into the combustion gas stream to reduce the temperature of the combustion gas and monitor the temperature of the combustion gas when the reaction compound is injected. Other possible methods of controlling the combustion gas temperature include the use of heat exchangers and / or air coolers. The process can also vary the location of the trona injection or include multiple injection sites of reaction compound. To achieve desulfurization, preferably reaction compound is injected at a rate relative to the flow rate of S03 in order to provide a standardized stoichiometric ratio (NSR) between sodium and sulfur. between about 1.0 or greater. The NSR is a measure of the amount of reagent injected relative to the theoretically required amount. The NSR expresses the stoichiometric amount of absorbent required to react with all of the acid gas. For example, an NSR of 1.0 would mean that sufficient material was injected to theoretically produce 100% removal of S03 in the incoming combustion gas; an NSR of 0.5 would give a theoretical yield of 50 percent removal of S03. The reaction of S03 with sodium carbonate is very fast and efficient, so in general only one NSR of one is required for the removal of S03. The reaction compound preferably reacts with S03 on S02, so that S03 is extracted even in the presence of large amounts of S02. Preferably, an NSR less than 2.0 or more preferably less than 1.5 is used so there is no substantial reduction in the concentration of S0 in the combustion gas caused by reaction with excess absorbent. In one embodiment, the flue gas stream also comprises S02, and sufficient reaction compound is added to also extract part of the SO2. The reaction compound is kept in contact with the flue gas for a sufficient time to react a portion of the reaction compound with a portion of S02 to reduce the concentration of S02 in the combustion gas stream. This can be particularly useful in small manufacturing plants, where it is more economical to have a single system for extracting S02 and S03 instead of adding a wet cleaner to extract S02. Since the N0X removal systems tend to oxidize the existing S02 to S03, the injection system can also be combined with a N0X removal system. The trona injection system can also be combined with other SOx removal systems such as sodium bicarbonate, lime, limestone, etc., in order to improve the performance or extract other additional hazardous gases such as HC1, NOx and the like. Surprisingly, it has been observed that when the temperature of the combustion gas is between about 500 ° F and about 850 ° F (preferably between about 550 ° F and about 750 ° F), or between about 275 ° F and about 365 ° F, the reaction product is not sticky. The accumulation of solids in the filter is avoided, particularly when it is an ESP. This effect is particularly pronounced in the range of elevated temperatures. Accordingly, the invention also relates to the use of the method for extracting S03 from a combustion gas according to the invention and its embodiments preferred, in order to avoid the formation of tacky reaction products.

EXAMPLES Studies were conducted at an Ohio power generation plant using an electrostatic hot-side precipitator (ESP) and not a baghouse. The plant used a catalyst for the removal of N0X, which caused elevated levels of S03 in the flue gas. The concentration of S03 in the flue gas was between about 100 ppm and about 125 ppm. The used high chair was T-200® from Solvay Chemicals.

EXAMPLE 1 The T-200® trona was injected into the combustion gas at a combustion gas temperature of 367 ° F. A perforated plate of ESP in the plant had significant aulation of solids after the operation of the S03 removal system for approximately two weeks.

EXAMPLE 2 The operation of Example 1 was repeated with the change that the highchair was injected at a combustion gas temperature below 365 ° F. In comparison with the perforated plate of Example 1, a perforated ESP plate in the plant had significantly less amount of solids aulation after the operation of the S03 removal system for two weeks.

EXAMPLE 3 The operation of Example 1 was repeated with the change that the high chair was injected at a lower combustion gas temperature of about 500 ° F. A perforated ESP plate in the plant was relatively free of solids aulation after the operation of the S03 removal system for two weeks with the T-200® trona. The embodiments described above and shown herein are illustrative and not restrictive. The scope of the invention is indicated by the claims rather than by the foregoing description and the accompanying drawings. The invention may have other embodiments without departing from the spirit of the invention. Accordingly, these and other changes that are covered by the scope of the present claims are intended to be encompassed therein.

Claims (26)

1. NOVELTY OF THE INVENTION Having described the invention as above, property is claimed as contained in the following: CLAIMS 1. A method for extracting S03 from a flue gas stream comprising S03, characterized in that it comprises: "providing a reaction compound selected from the group consisting of sodium carbonate, sodium bicarbonate, sodium sesquicarbonate, and mixtures thereof "injecting the reaction compound into the combustion gas stream, wherein the temperature of the combustion gas is between about 500 ° F and about 850 ° F; and 1 keeping the reaction compound in contact with the combustion gas for a sufficient time for a portion of the reaction compound to react with a portion of S03 to reduce the concentration of S03 in the flue gas stream, where the product of reaction of the reaction compound and S03 is selected from the group consisting of Na2SO4, Na2S207, and mixtures thereof.
2. 2. The method according to claim 1, characterized in that it further comprises providing a system of NOx removal upstream of the location in which the reaction compound is injected.
3. The method according to claim 1, characterized in that the combustion gas stream comprises at least about 3 ppm S03 upstream of the location where the reaction compound is injected.
4. The method according to claim 3, characterized in that the combustion gas stream comprises between about 10 ppm and about 200 ppm S03 upstream of the location, where the reaction compound is injected.
5. 5. The method according to claim 1, characterized in that the reaction compound is sodium sesquicarbonate.
6. The method according to claim 1, characterized in that the reaction compound is provided in the form of particles with an average particle size of less than about 40 microns.
7. The method according to claim 6, characterized in that the average particle size of the reaction compound is between about 10 microns and about 40 microns.
8. 8. The method according to claim 7, characterized in that the average particle size of the The reaction compound is between about 24 microns and about 28 microns.
9. The method according to claim 1, characterized in that the temperature of the combustion gas is greater than about 550 ° F.
10. The method according to claim 1, characterized in that the temperature of the combustion gas is less than about 750 ° F.
11. The method according to claim 1, characterized in that the temperature of the combustion gas is between about 500 ° F and about 750 ° F.
12. 12. The method according to claim 1, characterized in that the reaction compound is injected at a rate with respect to the flow rate of S03 to provide a normalized stoichiometric ratio of sodium to sulfur of between about 1.0 and 1.5.
13. The method according to claim 1, characterized in that the reaction compound is injected as a dry material.
14. The method according to claim 1, characterized in that it further comprises grinding the reaction compound to a desired average particle size at a location close to the flue gas stream.
15. 15. The method according to claim 1, characterized in that the reaction product of the compound Reaction and S03 are selected from the group consisting of Na2SO, Na2S207, and mixtures thereof.
16. The method according to claim 1, wherein the combustion gas stream also comprises SO2, characterized in that the method also comprises keeping the reaction compound in contact with the combustion gas for a sufficient time to react a portion of the reaction compound with a portion of S02 to reduce the concentration of S02 in the flue gas stream.
17. 17. A method for extracting S03 from a combustion gas stream, characterized in that it comprises: "providing a trona source; m injecting the chair as dry granular material into the flue gas stream, wherein the temperature of the flue gas is between about 500 ° F and about 850 ° F and wherein the flue gas stream comprises at least about 3 ppm from S03; and "keeping the chair in contact with the combustion gas for a time sufficient to react a portion of the trona with a portion of S03 in order to reduce the concentration of S03 in the flue gas stream.
18. 18. The method according to claim 17, characterized in that the combustion gas stream comprises between about 10 ppm and about 200 ppm of S03 upstream of the location where the high chair was injected.
19. 19. The method according to claim 17, characterized in that the highchair is provided in the form of particles with an average particle size of between about 10 microns and about 40 microns.
20. The method according to claim 17, characterized in that the temperature of the combustion gas is between about 500 ° F and about 750 ° F.
21. 21. The method according to claim 17, characterized in that the reaction product of the reaction compound and S03 are selected from the group consisting of Na2SO4, Na2S207, and mixtures thereof.
22. The method according to claim 17, characterized in that it further comprises adjusting the temperature of the combustion gas upstream of the trona in order to obtain the desired temperature of combustion gas where the trona is injected.
23. The method according to claim 17, characterized in that the adjustment also comprises introducing ambient air into the combustion gas stream and monitor the temperature of combustion gas where the highchair is injected.
24. 24. The method according to claim 17, characterized in that the adjustment also comprises controlling the flow of a material through a heat exchanger in communication with the combustion gas.
25. 25. A method for extracting S03 from a flue gas stream comprising at least about 3 ppm S03, characterized in that it comprises: "providing a trona source having an average particle size between about 10 microns and about 40 microns;" introducing ambient air into the flue gas stream in order to reduce the temperature of the flue gas to less than about 365 ° F; • injecting the trona as dry granular material into the flue gas stream, wherein the temperature of the combustion gas is between about 275 ° F and about 365 ° F; and "keeping the chair in contact with the combustion gas for a sufficient time to react a portion of the sodium absorbent with a portion of S03 in order to reduce the concentration of S03 in the flue gas stream, where the product of reaction select from solid phase Na2SO4, solid phase NaHS04 and mixtures thereof.
26. 26. The method according to claim 25, characterized in that the combustion gas stream comprises between about 10 ppm and about 200 ppm S03 upstream of the location where the highchair is injected.