HK1143338A - Air pollution control - Google Patents

Description

Air pollution control

The present application is a divisional application of chinese patent application 200580017762.1 entitled "air pollution control" with the filing date of 29/4/2005.

Technical Field

The present invention relates generally to wide-gas pollution control technology, and more particularly to the reduction of mercury and other pollutants in combustion exhaust gases.

Background

Combustion exhaust gas generated from incineration, power plants, and coal-fired furnaces generally contains oxides of sulfur (SOx), oxides of nitrogen (NOx), and volatile heavy metals such as mercury. Typically, the mercury content of the coal is 0.05-0.25 mg/kg. The mercury is volatilized upon combustion and carried in the combustion exhaust gases.

Mercury can pose serious problems for human health and the environment. Com recently reported that Southeast United States alone could save costs of up to 20 billion dollars per year by reducing mercury pollution. Coal-fired power plants are the largest pollution source for mercury emissions, accounting for 40% of the total emissions, and coal-fired furnaces have mercury emissions accounting for 10% of the total emissions.

Previous methods of controlling SOx, NOx and mercury emissions have included injecting dispersions of calcium carbonate, activated carbon into the exhaust gas and/or passing the exhaust gas through a scrubber to neutralize SOx, NOx and absorb volatile mercury. After the scrubbing step, the solids entrained in the exhaust gas may be recovered by an electrostatic precipitator before the gas is discharged into the atmosphere. The use of a combination of scrubbers and electrostatic precipitators to condense gas dust typically removes 50-85% of the mercury in combustion flue gases.

Other methods of controlling mercury emissions are carried out using alkali metal sulfides such as sodium polysulfide solutions, in particular sodium tetrasulfide solutions. See, for example, U.S. Pat. Nos. 6214304 and BabcockPower Environmental Inc. technical Publication, "Multi-polar emulsions controls & Strategies, Coal-FiredPower Plant Mercury controls by Sodium catalysts"; licata A, Beittel R, Ake T, ICAC Forum, Nashville, TN.Oct.14-152003. In which mercury is converted into mercury sulfide which is then deposited by a dust separator. This treatment requires the introduction of an alkali metal sulphide solution into the flue gas and the simultaneous heating. An oxidizing agent, such as a chlorine-containing compound, may be added simultaneously to the burning coal or coke in the process, which may convert the elemental mercury to oxidized mercury, thereby making the agent more susceptible to reaction with the mercury. The disadvantage of this process is the harsh temperature requirements and the addition of an oxidant to the coal feed increases the corrosiveness of the system. In addition, because mercury sulfide is unstable and sensitive to pH or redox-induced reactions, the residue containing mercury sulfide deposits from the process (whether in black or β -crystal form) is susceptible to oxidation, thereby rendering the mercury sulfide deposits susceptible to re-oxidation and re-dissolution in water with an increased likelihood of bioavailability.

Disclosure of Invention

The present invention solves the problem of removing pollutants such as mercury and other heavy metals from combustion gases by providing an improved process for removing such pollutants from combustion gases using readily available reagents, techniques and equipment that can be used in conventional combustion gas scrubbers, such as Flue Gas Desulfurization (FGD) scrubbers. According to a first aspect of the invention, the combustion exhaust gas is contacted with a mixture of an alkaline earth metal sulfide and a buffering agent, preferably a buffering agent selected from the group consisting of phosphoric acid, phosphates, alkaline earth metal based pH buffering agents, and mixtures thereof. Advantageously, in one embodiment of the invention, the buffering agent comprises an alkaline earth metal pH buffering agent (e.g., calcium carbonate) and also a redox buffering agent (e.g., triple superphosphate) that stabilizes the mercury sulfide deposit. The reagent mixture may be formed into an aqueous dispersion or slurry and introduced into the combustion gas stream as an aerosol, preferably downstream of a particulate removal device such as a fabric filter or electrostatic precipitator (ESP).

In a second aspect of the invention, there is provided a composition for removing mercury or other heavy metals from combustion gases, the composition comprising an aerosolized aqueous dispersion or slurry of an alkaline earth metal sulfide and a buffering agent, for example an aerosolized solid dispersion of 20-50% w/w alkaline earth metal sulfide and buffering agent.

Drawings

The invention may be better understood with reference to the following drawings, in which:

FIG. 1 is a schematic diagram of a wet scrubber for removing mercury from combustion gases according to one embodiment of the present invention.

Fig. 2 is a schematic diagram of a dry scrubber for removing mercury from combustion gases according to an embodiment of the present invention.

Detailed Description

According to a first aspect of the invention, a method of controlling air pollution comprises contacting combustion exhaust gas with a mixture of an alkaline earth metal sulfide and a buffering agent. Preferred buffering agents are selected from the group consisting of phosphoric acid, phosphates, alkaline earth metal based pH buffers, and mixtures thereof. The alkaline earth metal sulfide and buffering agent (sometimes collectively referred to as remediation agent) may be introduced in an aerosol form into the combustion gas to react with and facilitate removal of mercury and/or other heavy metals from the combustion gas.

Non-limiting examples of alkaline earth metal sulfides include calcium sulfide, magnesium sulfide, calcium sulfide adducts, magnesium sulfide adducts, mixed calcium-magnesium sulfides, and mixtures thereof.

Non-limiting examples of alkaline earth metal-based pH buffers include calcium carbonate, calcium hydroxide, calcium phosphate, magnesium carbonate, magnesium hydroxide, magnesium phosphate, mixed calcium-magnesium carbonate, mixed calcium-magnesium hydroxide, mixed calcium-magnesium phosphate, triple superphosphate, apatite, and mixtures thereof. Triple superphosphate (also known as triple superphosphate, TSP, and superphosphate) is primarily monocalcium phosphate hydrate (CaH)₂PO₄)₂·H₂O(CAS NO.65996-95-4)。

In a preferred embodiment, the buffering agent comprises at least one redox buffer capable of stabilizing the mercury sulfide deposit against subsequent redox reactions and/or dissolution in water (redissolution). Non-limiting examples of such redox buffers include phosphoric acid, phosphates, and mixtures thereof. More preferably, the buffer comprises at least one alkaline earth metal-based compound and at least one redox buffer.

Alkaline earth metal carbonates, hydroxides, phosphates, and the like, as pH buffers for the acid gas stream stabilize the reagent mixture.

Phosphoric acid and/or its salts (e.g., phosphates) act as redox buffers to stabilize the eventually deposited mercury sulfide. In addition, the phosphate also acts as a mild acid to oxidize elemental mercury present in the combustion exhaust gas so that the mercury can be more easily deposited by the alkaline earth metal sulfide. Phosphates also have the advantage of inhibiting corrosion. The currently used coal feed additive calcium chloride reagent, which readily oxidizes elemental mercury, may increase the corrosiveness of the equipment.

The phosphate has both a pH buffering and redox buffering effect.

Provided herein are general and more specific examples of reagent mixtures useful in the practice of the present invention. In one general embodiment, the reagent mixture comprises an alkaline earth metal sulfide and a buffer. In a less general embodiment, the reagent mixture comprises an alkaline earth metal sulfide, an alkaline earth metal pH buffer, and a redox buffer. In a less general embodiment, the reagent mixture comprises an alkaline earth metal sulfide, an alkaline earth metal carbonate and a phosphate. In a more specific embodiment, the buffering agent comprises triple superphosphate and at least one alkaline earth metal carbonate or hydroxide (e.g., calcium carbonate, calcium hydroxide, etc.). In another embodiment, the reagent mixture comprises a mixture of calcium sulfide, triple superphosphate, and calcium carbonate, and even more particularly, in one embodiment of the present invention, the reagent mixture comprises calcium sulfide, triple superphosphate, and calcium carbonate in a relative amount of 3: 1: 2 weight/weight. Such mixtures are commercially available from Solucorp Industries, Ltd. (West Nyack, NY) under the trademark MBS 2.1^TMThe product of (1).

The reagent mixture may be prepared as an aqueous dispersion or slurry of fine particles in water and may be applied as an aerosol or spray or in other conventional ways to the combustion exhaust. Advantageously, the dispersion or slurry may comprise (preferably comprises) a small amount (parts per thousand) of a dispersant, preferably a hyperdispersant, more preferably a polymeric hyperdispersant such as a "Solsperse" adduct (available from Noveon Specialty Additives, Noveon Division, lubrizol ltd., Blackley, Manchester M98 ZS, United Kingdom). A specific example is "SolPlus D540". The solid dispersion can be prepared using a ball mill.

The concentration of the reagent dispersion or slurry is formulated to be suitable for the individual air pollution control device or type of device (e.g., wet scrubber or dry scrubber). For example, if a wet scrubber of the FGD type is employed, the mixture of alkaline earth metal sulfide and buffer may comprise a 20-50 wt/wt% solids dispersion that may be injected into the combustion gas stream with a nozzle. In the case of a dry scrubber, it is suitable to apply with the aid of a rotary atomizer a dispersion or slurry of relatively low concentration, for example a slurry of a mixture of 20 w/w% alkaline earth metal sulfide and a buffer. For dry scrubbing systems, typical particle sizes (solid state) are about < 325 mesh (e.g., about 45 microns). For wet scrubbing systems, larger particles (< 200 mesh, e.g., about 75 microns) may be used.

FIG. 1 illustrates one embodiment of a wet scrubber 10 suitable for use in FGD processes in which the present invention is practiced. The reagent mixture comprising the alkaline earth metal sulfide and the buffering agent is placed in the additive supply tank 12 with or without the addition of additional ground lime and/or limestone. The reagent mixture is directed through valve C into the wet scrubber sump 14. The spray injected into the tower 16 is controlled by means of a valve a and adjusted to meet the requirements of the upwardly flowing combustion exhaust gas, downstream particle removal devices such as fabric filters or electrostatic precipitators (ESP). The pH in the reaction tank 18 is maintained at about 6-8. The reaction is monitored by the sulphide content and when the sulphide content falls below the effective concentration, the reaction tank contents are discharged through valve B to the waste transfer tank 20 and the slurry is dewatered (concentrated). The discharged water is returned to the system for recycling. The concentrated slurry is transferred through valve H to a filter press or drum filter (not shown) and the dried solids are packaged for sale or disposal. After the reaction tank has been re-fed with the reagent mixture, the gas scrubbing process can be restarted. The gas scrubbing system can be operated with a single gas scrubber or, more preferably, in parallel with 3-6 wet scrubber banks, and can be operated with existing gypsum product manufacturing facilities or as a pre-treatment, upstream unit of a gypsum product manufacturing facility.

Gas scrubbers for air pollution control are well known. Non-limiting examples of such scrubbers are the wet Flue Gas Desulfurization (FGD) scrubbers sold by Babcock and Wilcox (a subsidiary of McDermott International). The scrubber has a spray level for introducing reagents and includes at least one nozzle, i.e., a slurry nozzle.

Figure 2 illustrates one embodiment of a dry scrubber 50 suitable for use in the practice of the present invention. The reagent mixture is introduced into the dry scrubber via a rotary atomizer 52 as a 20 wt/wt% slurry. The scrubber 54 is configured to maximize the wet contact time of the reagent aerosol with the combustion exhaust and to allow the dried aerosol to enter the baghouse 56 in a dry form without clogging the baghouse filter system. The dried reaction product can be recycled or sold.

In both wet scrubbing and dry scrubbing systems, mercury present in the combustion gases is converted to solid mercury sulfide deposits, thereby separating it from the exhaust gases. While not wishing to be bound by theory, it is believed that the present invention should facilitate the recovery of greater than 95% mercury as the combustion gases pass through the scrubber. Advantageously, the mercury-containing residue produced by this process is solid and substantially insoluble, and it is believed that the residue is stable against redissolution because the buffering agent prevents redox reactions that would otherwise lead to oxidation and redissolution.

It is known that, in addition to mercury, other heavy metals present in combustion gases can be recovered by carrying out the process of the invention. Most heavy metals form metal sulphide deposits. Since these sulfides are in contact with phosphate or similar buffering agents present in the remediation agent, they are stable and thereafter do not undergo oxidation and redissolution, and lead and other heavy metals can be recovered in a water-soluble form.

It is also known that the removal of mercury and other metals from combustion gases in accordance with the present invention is not inconsistent with existing air pollution control apparatus and methods for removing SOx and NOx gases from combustion gases.

In addition to the air pollution control methods described above, the present invention also provides remediation agents for use in wet and dry scrubbers. In accordance with this aspect of the invention, a composition for removing mercury or other heavy metals from combustion gases comprises an aerosolized, aqueous dispersion or slurry of an alkaline earth metal sulfide and a buffering agent, wherein the terms are as defined above. In one embodiment, the aerosolized dispersion or slurry comprises 20 to 50 wt/wt% of a solid dispersion of an alkaline earth metal sulfide and a buffering agent. The aerosol is formed in a conventional manner using, for example, a nozzle, rotary atomizer or other suitable apparatus.

The air pollution control methods and compositions described herein have the advantage of not having stringent temperature requirements and being readily reactive at room temperature with mercury and other heavy metals in both ionic and elemental forms. Whereas alkaline agents that rely solely on alkaline action cannot react with elemental mercury vapor to remove it. The sodium sulfide adopted by the method has strict temperature requirements and can act at high temperature.

The advantage of using alkaline earth metal sulfides is significant. Alkaline earth metal sulfides have the advantage of being solid, substantially insoluble, not as malodorous as alkali metal sulfides, and thus are convenient to handle, and they are also less susceptible to degradation and oxidation reactions as occur with alkali metal sulfides such as sodium polysulfide and sodium tetrasulfide.

The presence of alkaline earth metal carbonates, hydroxides, and/or similar compounds in the remediation agent can mitigate detrimental oxidation of alkaline earth metal sulfides with acidic compounds in the flue gas. Thus, any unreacted alkaline earth metal sulfide diluted with undegraded oxidation residue can be recovered while minimizing the emission of hydrogen sulfide gas.

In addition, the residue formed in the process is solid, insoluble, and stable (see Molecular Bonding Systems SITE Report EPA/540/R-97/507, incorporated herein by reference). These residues are maintained in this state by means of a redox buffer, such as triple superphosphate or calcium phosphate. Thus, another advantage and a further aspect of the present invention is that the by-products recovered from the scrubber and settler are in a stable, non-leaching form, and therefore, they can be disposed of as non-hazardous waste, or used as safe and non-hazardous raw materials in the manufacture of building products such as gypsum board, cement and cement board. Accordingly, the present invention also provides a solid residue as a by-product of the air pollution control process described herein, including alternatives to the above-listed products.

In one embodiment, the resulting residue is a substantially water-insoluble mercury-containing material in a solid state comprising a mercury-containing inorganic matrix in a substantially non-leachable state, the matrix being a by-product of the contact of mercury-containing combustion exhaust gas with a mixture of alkaline earth metal sulfide and buffering agent during air pollution control. The residues resulting from the scrubbing of sodium polysulphides and tetrasulphides are unstable, substantially soluble and therefore problematic in waste disposal, since they contain a large amount of sulphates, sulphites and thiosulphates.

The present invention has been described by way of various illustrative examples and preferred embodiments, but is not limited thereto. Various modifications may be made without departing from the invention, the scope of which is defined only by the appended claims and equivalents. The terms "a" and "an" and other singular articles, as used throughout the claims, do not exclude a plurality of components. Thus, more than one alkaline earth metal sulfide, more than one alkaline earth metal-based pH buffering agent, and the like are all employable.

Claims (30)

1. A method of controlling air pollution, the method comprising:

the combustion exhaust gas is contacted with a mixture of an alkaline earth metal sulfide and a buffer selected from the group consisting of phosphoric acid, phosphates, alkaline earth metal-based pH buffers, and mixtures thereof.

2. The process of claim 1 wherein the alkaline earth metal sulfide is selected from the group consisting of calcium sulfide, magnesium sulfide, calcium sulfide adducts, magnesium sulfide adducts, mixed calcium-magnesium sulfides, and mixtures thereof.

3. The method of claim 1, wherein the buffering agent comprises at least one alkaline earth metal-based pH buffering agent selected from the group consisting of calcium carbonate, calcium hydroxide, calcium phosphate, magnesium carbonate, magnesium hydroxide, magnesium phosphate, mixed calcium-magnesium carbonate, mixed calcium-magnesium hydroxide, mixed calcium-magnesium phosphate, triple superphosphate, apatite, and mixtures thereof.

4. The method of claim 1, wherein the buffer comprises at least one redox buffer capable of stabilizing the mercury sulfide deposit against re-dissolution in water.

5. The method of claim 4, wherein the at least one redox buffer comprises phosphoric acid or a phosphate salt.

6. The method of claim 1, wherein the buffer comprises triple superphosphate and at least one alkaline earth metal carbonate or hydroxide.

7. The process of claim 1, wherein the mixture of alkaline earth metal sulfide and buffer is in the form of an aqueous dispersion or slurry.

8. The method of claim 7, wherein the aqueous dispersion or slurry comprises a hyperdispersant.

9. The method of claim 7 wherein the hyperdispersant is a polymeric hyperdispersant.

10. The method of claim 1, wherein the mixture of alkaline earth metal sulfide and buffer comprises a mixture of calcium sulfide, triple superphosphate, and calcium carbonate.

11. The method of claim 10 wherein the relative amounts by weight of calcium sulfide, triple superphosphate, and calcium carbonate are 3: 1: 2, respectively.

12. The method of claim 1, wherein the mixture of alkaline earth metal sulfide and buffer comprises a 20-50% by weight solids dispersion.

13. The method of claim 9, wherein the solid dispersion is prepared by ball milling.

14. The method of claim 1, wherein the combustion exhaust gas is produced in an incinerator or power plant.

15. The method of claim 1 wherein the combustion gas is generated by a coal-fired furnace.

16. A method of controlling air pollution, the method comprising:

the combustion exhaust gas is contacted with a mixture of an alkaline earth metal sulfide and a buffering agent selected from phosphoric acid, a phosphate salt, and mixtures thereof.

17. A method of controlling air pollution, the method comprising:

the combustion exhaust gas is contacted with an aqueous dispersion or slurry comprising an alkaline earth metal sulfide, an alkaline earth metal carbonate and a phosphate.

18. The method of claim 17, wherein the alkaline earth metal sulfide comprises calcium sulfide, the alkaline earth metal carbonate comprises calcium carbonate, and the phosphate comprises triple superphosphate.

19. A method of controlling air pollution, the method comprising:

the mixture of alkaline earth metal sulfide and buffering agent is introduced into the combustion exhaust gas in a wet or dry scrubber.

20. The method of claim 19, wherein the buffer is selected from the group consisting of phosphoric acid, phosphates, alkaline earth metal based pH buffers, and mixtures thereof.

21. The process of claim 19, wherein the alkaline earth metal sulfide is selected from the group consisting of calcium sulfide, magnesium sulfide, calcium sulfide adducts, magnesium sulfide adducts, mixed calcium-magnesium sulfides, and mixtures thereof.

22. The method of claim 19, wherein the buffering agent comprises at least one alkaline earth metal-based pH buffering agent selected from the group consisting of calcium carbonate, calcium hydroxide, calcium phosphate, magnesium carbonate, magnesium hydroxide, magnesium phosphate, mixed calcium-magnesium carbonate, mixed calcium-magnesium hydroxide, mixed calcium-magnesium phosphate, triple superphosphate, apatite, and mixtures thereof.

23. The method of claim 19 wherein the buffer comprises at least one redox buffer capable of stabilizing the mercury sulfide deposit against water re-dissolution.

24. The method of claim 23, wherein the at least one redox buffer capable of stabilizing a mercury sulfide deposit against water re-dissolution comprises phosphoric acid or a phosphate salt.

25. The method of claim 19, wherein the mixture of alkaline earth metal sulfide and buffer is introduced through one or more nozzles.

26. A composition for removing heavy metals from combustion exhaust gas, the composition comprising:

an aerosolized aqueous dispersion or slurry of an alkaline earth metal sulfide and a buffering agent.

27. The composition of claim 26, wherein the aerosolized dispersion or slurry comprises 20 to 50 wt% of a dispersion of alkaline earth metal sulfide and buffer solids.

28. The composition of claim 26, wherein the aerosolized dispersion or slurry is formed using a nozzle or a rotary atomizer.

29. A solid residue produced as a byproduct of the process of any one of claims 1, 16, 17, and 19.

30. A substantially water-insoluble, mercury-containing solid material, the solid material comprising:

a substantially non-leaching mercury-containing inorganic substrate that is a by-product of an air pollution control process in which a mercury-containing combustion gas is contacted with a mixture of an alkaline earth metal sulfide and a buffering agent.