WO1995017240A1

WO1995017240A1 - Method for inproving the mercury removal capability of a flue gas purification system

Info

Publication number: WO1995017240A1
Application number: PCT/DK1994/000474
Authority: WO
Inventors: Preston L. Veltman; Karsten S. Felsvang
Original assignee: Niro A/S
Priority date: 1993-12-20
Filing date: 1994-12-20
Publication date: 1995-06-29
Also published as: AU1309795A

Abstract

The mercury removal capability of a flue gas purification system is improved by introducing sulfur vapors into the flue gas stream where admixed flue gases and sulfur vapors contact solid particulate materials in the flue gas. The solid particulate materials adsorb mercury and sulfur vapors and catalyze reactions forming solid products comprising mercury. The solid products comprising mercury are separated thereby forming a purified flue gas stream.

Description

METHOD FOR IMPROVING THE MERCURY REMOVAL

CAPABILITY OF A FLUE GAS PURIFICATION SYSTEM This invention relates to a flue gas purification system and; more specifically, to a method for improving the mercury removal capability of a flue gas purification system. BACKGROUND OF THE INVENTION

Field of the Invention

As a result of environmental improvement and public health concerns over the past several years, it has been realized that with the rapid increase in the number and capacity of coal burning and refuse incinerator plants in the industrialized world, it is not adequate to limit the cleaning of flue gas from such plants to removal of the main pollutants therein, such as HCI, SO₂ and NO_x, alone. Other components occurring in substantially minor amounts also represent a risk to the environment due to their extreme toxicity. Mercury is regarded as presenting a risk to human and animal life even in very small concentrations. In several countries, legislation is being prepared with a view to reducing mercury emission.

Description of Prior Art

A number of different methods have been suggested for removing or recovering mercury from gases. However, the majority of the prior art processes have been created with the purpose of removing mercury from relatively small amounts of gas having high mercury concentrations. These processes are not suitable for cleaning flue gas since the cost of chemicals required would be prohibitive or operation of the process would be impractical in connection with large volumes of flue gas .

Processes for removing mercury from air of relatively low mercury content have also been suggested. Such a process is disclosed in published European Patent Application No. 1,456 (Akzo N.V.), published 18/04/1979. This process which is described as particularly suitable for the removal of mercury from air which is vented from buildings, is based on the principle that mercury vapor is absorbed as mercury chloride when passing through a bed of activated carbon having a specific chlorine content. According to the specification, high moisture content of the gas from which mercury is to be removed should be avoided since the effectiveness of the activated carbon is reduced thereby. From that specification, it also appears that activated carbon used in a stationary bed without chlorine is unsatisfactory as an absorbent for mercury. The process of that European application seems unsuitable for treating flue gas from a commercial plant since it would require the total amount of flue gas to be passed through a bed of activated carbon to which gaseous chlorine is added, which obviously involves the undesirable risk that any excessive amount of chlorine may be entrained with the flue gas exhausting to the atmosphere.

A process for removing mercury vapor from a hot hydrogen chloride-containing flue gas is disclosed in published European patent application No. 13,567, published 23/07/1980 (Svenska Flaktfabriken). According to that process, the gas which contains hydrogen chloride and minor amounts of mercury vapor, is contacted with powdered calcium hydroxide, preferably in a fluidized bed. The hydrogen chloride in the gas reacts with the calcium hydroxide to form calcium chloride which apparently is essential to the removal of mercury. However, that process will not reliably enable reduction of mercury vapor to the required low levels.

US Patent No. 4,273,747 to Rasmussen discloses removal of mercury from hot waste gases by atomizing an aqueous liquid into the waste gases in the presence of fly ash suspended in the gas and subsequently separating the fly ash together with a substantial part of the mercury originally present as vapor. In that treatment process, it is essential that the gas stream is cooled from a temperature of at least 200°C to a temperature below 160°C. The aqueous liquid may be water or it may be an aqueous solution or suspension of an alkaline compound, preferably calcium hydroxide. Obviously this process will not be suitable in these applications where it is not acceptable to cool the gas to the extent required or if the amount of fly ash present is insufficient due to the use of a preceding fly ash separation. Even when the conditions as to fly ash content of the flue gas and cooling are satisfied it would still be desirable in certain applications to increase the efficiency of the removal of mercury vapor in this process.

US Patent No. 4,061,476 discloses a gas purification method in which pulverulent solid sorption agent is injected into a stream of noxious contaminant-containing gas subjected to intensive turbulence and subsequently separated from the gas. Among other sorption agents, powdery filtering charcoal is suggested without indication of the contaminants for which this specific sorption agent is intended. According to the disclosure the absorbents are advantageously of grain sizes of less that 100μ, preferably less than 50μ. However, when the sorption agent is activated carbon in the form of fine particles it has been difficult to efficiently separate such fine carbon particles from the gas stream. This problem exists whether separation is attempted with electrostatic precipitators or fabric filters.

Canadian Patent No. 1,300,347 describes a spray absorption process for removal of mercury vapor and/or vapor of noxious organic compounds and/or nitrogen oxides from flue gas from an incinerator plant. The absorbent used in this process is an aqueous liquid, which in addition to alkaline components, contains suspended activated carbon. The flue gas at a temperature of 135-140°C is introduced into a spray absorption chamber wherein an aqueous liquid containing a basic absorbent is atomized to cool the flue gas to a temperature between 180°C and 90°C, to absorb acidic components from the flue gas and simultaneously to evaporate water in the aqueous liquid, thereby forming a particulate material containing reaction products of the basic absorbent with acidic components of the flue gas. The activated carbon is used in an amount of l-800mg/Nm flue gas.

Swedish patent application No. 8802093-8 discloses a process for the removal of mercury from flue gases from combustion plants designed for combustion of a fuel contaminated with small quantities of mercury. In this process a carrier gas containing vapor phase sulfur and/or selenium is added to the flue gas and the flue gases are by heat exchange means and gas admixture brought to a temperature required for the gas phase reaction between mercury and sulfur and selenium, respectively. The formed finely divided reaction products are removed from the flue gases by passing the gases through a filter. The required temperature for the gas phase reaction between mercury vapors and vapors and sulfur as disclosed by the Swedish patent application is a minimum of 300°C. The preferred reaction temperature is between 300-600°C.

The control of mercury and dioxin emissions from United States and European municipal solid waste incinerators is reviewed in an article published by B. Brown and K.S. Felsvang as reported at Session 7C - Mercury Control Second International Conference on Municipal Waste Combustion, Tampa, Florida, April 16-19, 1991, and an article by Karsten Felsvang, et al., entitled "Control of Air toxics by Dry FGD System", presented at the Power-Gen 1992 conference, Orlando, Florida, November 17-19, 1992. Both articles are included herein by reference.

Recent developments in mercury control by spray dryer absorption are described in a paper by Felsvang, et al., "Air Toxics Control by Spray Dryer Absorption", presented at the 1993 SO₂ Control Symposium, Boston, MA, August 24- 27, 1993. The paper describes the importance of mercury speciation in the control of total mercury emissions. Coals containing chlorine tend to have a higher percentage of mercury chloride in the flue gas, which is easier to capture than elemental mercury. The paper emphasizes that the amount of fly ash and the residual carbon content of the fly ash is important for achieving high mercury removal. Although a significant portion of the coal mercury content can be converted to the easier to capture mercury chloride by the coal chlorine content per se, or by adding a chloride containing additive to the coal, there remains a need for an inexpensive and effective method of improving the overall mercury capture, especially the capture of elemental mercury. The above paper is hereby incorporated by reference.

A need exists for an improved method for removing mercury vapor from flue gases.

A primary object of the present invention is to provide a cost effective process for improving the mercury removal capability of spray dryer absorption (SDA) flue gas purification systems.

A further object of the present invention is to provide a low cost reagent that will oxidize elemental mercury, react with oxidized mercury to form low volatile products and operate at temperature ranges encountered in SDA systems .

A still further object of the present invention is to provide a process which utilizes: (1) a low cost sorbent/ catalytic material that will absorb mercury; (2) a reagent from the gas phase; and (3) means to promote a reaction between mercury and the reagent.

A still further object of the present invention is to provide a process and/or apparatus that could be added to existing fuel gas treatment facilities and designs for such facilities that would not require costly heat exchange equipment, not require hazardous materials, and not interfere with the normal operation of the existing SDA system or system design.

Summary of the Invention

The present invention is applicable as an additional process or "add-on" to improve the mercury removal capability of spray dryer absorption flue gas purification processes wherein a solid particulate product is formed by the reaction between an atomized aqueous basic material and acidic materials in the flue gases. It has been found that the solid particulate products promote the reaction between elemental mercury and sulfur vapors to form solid mercury comprising products that can then be separated from the flue gas. It has also been found that the extent of reaction between elemental mercury and sulfur in the presence of the solid particulate product is critically dependent upon temperature. The preferred temperature is between 70 and 170°C. Sulfur vapors are preferably produced by vaporizing particulate sulfur in a gas which is non-reactive to sulfur. Sulfor vapors, in amounts based on the mercury and oxygen content of the flue gases are preferably introduced into the flue gas stream before entering the spray chamber.

The process according to the present invention includes, in a first stage, the production of sulfur vapors carried in a substantially inert gas stream which, in a second stage, are made to react with pollutants such as heavy metals, including mercury, in contact with solid particulate materials formed in-situ by reaction in a spray drying operation between an aqueous dispersion of calcium hydroxide and the acidic materials in the flue gas . The particulate materials function to absorb mercury and sulfur vapors and to catalyze reactions forming mercury comprising solid products of lowered volatility. The solid products comprising mercury are separated from the gas stream.

The present invention and the advantages provided thereby will be more fully understood with reference to the following detailed description of the preferred embodiment taken in conjunction with the accompanying drawings. Brief Description of the Drawings

Fig. 1 is a schematic illustration of a bench scale test apparatus used in the delopment of the process of the present invention;

Fig. 2 is a schematic illustration of a mercury vaporizer used to supply mercury vapor to the apparatus shown in Fig. 1;

Fig. 3 is a schematic illustration of a device used to supply sulfur vapor to the apparatus shown in Fig. 1;

Fig. 4 is a schematic illustration of a first embodiment of the present invention wherein a gas carrying sulfur vapors is introduced in a flue gas line prior to entering a spray dryer absorber apparatus;

Fig. 5 is a schematic illustration of another embodiment of the present invention wherein a gas carrying sulfur vapors is introduced directly into a spray dryer absorber apparatus;

Fig. 6 is a schematic illustration of a further embodiment of the present invention wherein a gas carrying sulfur vapors is introduced into a flue gas stream after passage through a spray dryer absorber but prior to entering a particle filtration device; and

Fig. 7 is a schematic illustration of a still further embodiment of the present inventiion which includes a partide separation device in the form of an electrostatic precipitator.

Description of the Preferred Embodiment (s)

With reference to the drawings, wherein like reference numerals designate the same or like parts throughout, there is shown in Fig. 1 a temperature controlled duct 10 wherein measured amounts of temperature controlled nitrogen gas can be introduced through a port 12, measured amounts of ter erature controlled mercury vapor containing gas can be introduced through a port 14, and measured amounts of temperature controlled sulfur vapor containing gas can be introduced through a port 16. A valve mechanism 18 is provided whereby the gases admixed in duct 10 can be by-passed to an activated carbon filter (not shown) as required during the experimental procedures . A filter device 20 is structured such that measured quantities of particulate materials can be placed in the filter so that gases passing through the device uniformly and effectively contact solids comprising a particulate material. A temperature controlled chamber 22 is provided to precisely control the temperature of the filtering device 20 and particulate materials therein. An absorption train 24 of the type used in the process of analytically determining the mercury content of gases is provided to measure the mercury content of gases passing the filter device.

Fig. 2 schematically illustrates an arrangement for producing a stream of mercury vapor in nitrogen gas. In Fig. 2 a source of liquid mercury 26 is delivered to a container 28 provided in a controlled temperature bath 30. Nitrogen gas delivered from a source (not shown) through line 32 passes a flow controller 34 and enters tank 29. Mercury vapor from container 28 mixes with the nitrogen gas and is carried by line 36 and coil 37 through a temperature controlled oil bath 38 and then by line 40 to the mercury vapor port 14 of duct 10 as shown in Fig. 1. Fig. 3 illustrates an arrangement by which a gas stream comprising sulfur vapors was produced in the conduct of this work. In Fig. 3 a temperature controlled chamber 42 is provided in which a particulate sulfur containing vessel 44 is located such that temperature and volume controlled nitrogen gas entering the chamber by a line 46 entrains sulfur vapors. The gas mixture passes through a filter 48 and exits the vaporizer by a line 50 connected to the sulfur vapor port 16 shown in Fig. 1.

The apparatus as schematically illustrated in Figs. 1-3 was utilized to conduct certain experiments to determine the effectiveness of certain particulate materials for removing mercury vapors from a simulated flue gas stream both in the absence and in the presence of sulfur vapors. The effect of mercury concentration in the gas, the gas temperature, and the filter temperature were also observed. A summary of the test results obtained with various materials being present and under different conditions as set forth in Table 1.

Hg Coacentratio n

C-3 PC/Na⁵ Gas Filter % Hg

Exasple Filter Mixture In Out Teap. ºc Temp. Removal,

1 Filter paper N₂. Hg 130 122 117 125-135 6 only

1a Filter paper N₂. Hg 152 140 1I9 14 0-14 7 9 only

2 93mg AC Darco N₂. Hg 189 137 117 111-I32 28 on filter

paper

2a 93mg AC Darco N₂. Hg 113 82 14 7 130 28 on filter

paper

3 1000mg SDA N₂. Hg 166 131 118 128-137 21 Product on

Filter Paper

3a 1000mg SDA N₂. Hg 205 171 117 121-131 20 Product on

Filter Paper

4 Filter paper N₂. Hg. S 188 102 119 123-133 45 only

5 100mg AC Darco N₂. Hg. S 85 29 148 120 62 on Filter

Paper

5a 100mg AC Darco N₂. Hg. S 105 37 118 122 62 on Filter

Paper

6 1000mg SDA N₂. Hg. S 129 1-i 119 120-125 89 Product on

Filter Paper

7 1000mg SDA N₂. Hg. S 118 <1 133 70-75 >99 Product on

Filter Paper 8 1000mg SDA N₂. Hg. S 135 77 150 120 13 Product on 8% O₂

Filter Paper

9 1000mg SDA N₂. Hg. S 140 32 150 75 77 Product on 1% O₂

Filter Paper

10 190mg AC B&S N₂. Hg 143 1 148 118 >99 Iodated

10a 190mg AC B&S N₂. Hg 180 <1 150 121 >99 Iodated With reference to Table 1, Examples 1 and la constitute reference tests against which the results of other examples can be compared. In Examples 1 and la simulated flue gases comprising nitrogen and mercury vapor only contacted filter paper. In Example 1, 6% of the mercury was removed from a simulated flue gas containing 130 μg/Nm mercury. The filter temperature varied during the test from 125 to 135°C. In Example la, 9% of the mercury was removed from a simulated flue gas containing 152 μg/Nm . The filter temperature varied from 140-147°C during the test.

Examples 2 and 2a tested the capability of a commercially available activated carbon known in the trade as AC Darco to remove mercury from a simulated flue gas. In Example 2, AC Darco in the amount of 93 mg was placed in the filter. Twenty eight percent of the mercury was removed from a simulated flue gas containing 189 μg/Nm mercury when the filter temperature was between 111-132°C. In Example 2a, wherein the simulated flue gas contained 113 μg/Nm 3 mercury, 28% of the mercury was removed' when the filter was operated at 130°C.

Examples 3 and 3a tested the mercury removal capability of a particulate material obtained as a product from a spray dryer absorber flue gas desulfurization operation (SDA). This material is formed when a calcium hydroxide comprising fluid is spray dried in the presence of flue gases comprising acidic materials . In Example 3, one thousand mg of the SDA solid product was placed in the filter device 20. Twenty one percent of the mercury contained in a simulated flue gas containing 166μg/Nm mercury was removed. During the test, the filter temperature varied between 128 and 137°C.

In Example 3a, 1,000 mg of the SDA product was placed in the filter device 20 and a simulated flue gas containing 205 μg/Nm mercury treated. Twenty percent of the mercury was removed. The filter temperature varied between 121 and 131°C.

Example 4 tested the capability of filter paper alone to remove mercury vapors from a flue gas containing sulfur vapors. Forty five percent of the mercury was removed from a simulated flue gas containing 188 μg/Nm mercury. The filter temperature varied between 123 and 133°C.

Examples 5 and 5a tested the capabiility of the AC Darco activated carbon to remove mercury from flue gases in the presence of sulfur vapors. In Example 5, 62% of the mercury was removed from the flue gas containing 85 μg/Nm³ mercury. The filter was operated at 120°C during the test.

In Example 5a, 62% of the mercury was removed from a flue gas containing 105 μg/Nm³ mercury. The filter was operated at 122°C during the test and contained 100 mg AC

Darco carbon.

Example 6 tested the mercury removal capability of a system comprising a flue gas containing sulfur vapors in contact with the same particulate SDA product used in Examples 3 and 3a. One thousand mg of the SDA product was placed in a filter. Eightynine percent of the mercury was removed from a flue gas containing 129 μg/Nm mercury. The filter was operated between 120 and 125°C.

Example 7 shows the effect of temperature on the mercury removal capability of a system compriising SDA particulates and sulfur vapors. In Example 7, one thousand mg of SDA product was placed in the filter. Greater than 99% of the mercury was removed from a simulated flue gas containing 118 μg/Nm³ mercury when the filter was operated between

70 and 75°C.

Examples 8 and 9 were carried out to determine the effect of oxygen and filter temperature on the SDA/sulfur vapor system.

In Example 8, eight percent oxygen was added to the flue gas. Notably, the percent mercury removal from a flue gas containing 135 μg/Nm mercury decreased to 43% removal from an expected removal rate near 90% as obtained in Example 6. The filter temperature was maintained at 120°C in this test.

Example 9 conditions essentially duplicated those of Example 8 except that the filter tempereature was held at 75°C. Notably the percent mercury removal increased to 77%.

Examples 10 and 10a were conducted to test the capability of a commercially available iodated activated carbon known as AC B&S to remove mercury from a simulated flue gas. In Example 10, one hundred ninety mg of AC B&S was placed in the filter. Greater than 99% of the mercury was removed from a flue gas containing 143 μg/Nm mercury when the filter temperature was 188°C. In Example 10a, greater than 99% of the mercury was -removed from a flue gas containing 180 μg/Nm mercury. The filter temperature was 121°C.

In summary, as shown by Examples l-3a of Table 1, the filter paper alone removed approximately 8% of the mercury. When the filter contained activated carbon, 28% of the mercury was removed indicating that the carbon increased the removal rate by 20%. When the SDA product was placed in the filter, 20% of the mercury was removed, indicating that the activated carbon was more effective for mercury removal than the SDA product.

As shown by Examples 4-7 of Table 1, the addition of sulfur vapors greatly enhanced the mercury removal capability of the systems. Removal of mercury using paper alone was about 8% whereas when the sulfur vapors were present with the paper, the removal increased to 45%. Examples 5 and 5a show that the presence of sulfur vapors with activated carbon increased the rate of mercury removal from 28% to 62%. Example 6 shows that sulfur vapors in the presence of SDA product increased the mercury removal to 89%.

Examples 1-6 were run using a filter temperature ranging from 120°C-147°C. Example 6 was run at a filter temperature from 120-125°C. In Example 7 the filter temperature was held between 70 and 75°C and, surprisingly, .the rate of mercury removal increased to greater than 99%. Examples 6 and 7 conclusively demonstrate that the SDA product in combination with sulfur vapors substantially improves the mercury removal capability of a spray dryer absorber flue gas purification system to near 100% removal of mercury.

Examples 8 and 9 show the effect of oxygen on a system as operated in Example 6. The mercury removal decreased from 89% to 43% showing that oxygen was interfering with the action of sulfur vapors . Example 9 was run with a filter temperature of 75°C and the rate of mercury removal increased to 77%. Examples 8 and 9 again demonstrate the criticality of temperature on the present mercury removal process as was demonstrated by Examples 6 and 7.

Examples 10 and 10a show results obtained with a commercially available costly absorbent for mercury.

Although the present invention will find application to improve the mercury removal capability of any of a number of flue gas purification systems, one such system where the present invention may be utilized as an add-on to improve the mercury removal capability is a spray dryer absorption flue gas purification process wherein a solid particulate product is formed by the reaction between an atomized aqueous basic material and acidic materials in the flue gases . A preferred embodiment incorporating the present invention in such a system is illustrated in Fig. 4.

In Fig. 4, a duct 52 delivers flue gases generated from an industrial or waste incinerator, not shown, to an inlet at the top of a spray dryer chamber 56. The temperature of the flue gases delivered to a commercial spray dryer absorption flue gas purification process is between 60°C and 220°C. Sulfur vapors, generated in a device, not shown, are delivered by a duct 53 to be mixed with the flue gases internally of the duct 52 before the flue gases are delivered to the spray dryer chamber 56. It is peferred that the mass ratio of the sulfur added to the flue gas

3

being treated to be between 1 and 200 mgS/Nm . A liquid feed material is provided from a source, not shown, through a feed line 58. The spray dryer chamber 56 includes a motor driven rotary atomizing wheel 60 which provides a conti nuous spray of liquid droplets at the top of the spray dryer chamber in the area where the mixed flue gases from duct 52 and sulfur vapors from duct 53 are introduced to the spray dryer chamber 56.

Typically, in a spray dryer absorption flue gas purification process, the liquid feed in the form of, for example, a slurry comprised of dispersed calcium hydroxide and recycled sorbent, is introduced in the form of small particles by the rotary atomizing wheel and a solid particulate product is formed -by the reaction between the atomized aqueous basic material and acidic materials in the flue gases. In the present invention, it has been found that the solid particulate product promotes a reaction between elemental mercury in the flue gases and sulfur vapors introduced through duct 53 to form products comprising solid mercury which can be separated from the flue gas. As evidenced by Examples 7 and 9 of Table 1, the extent of reaction between elemental mercury and sulfur in the presence of the solid particulate product is dependent upon temperature with the preferred temperature being in the range between 70 and 170°C. The temperature of the materials subject to the process internally of the spray drying chamber 56 can be controlled by controlling the temperature of the flue gases delivered through duct 52 and the temperature of the aqueous solution delivered through the duct 58. Flue gases and fine particles are drawn from the lower conical portion of the spray dryer chamber 56 by a motor driven fan 69 through a first exhaust duct 64 to a particle separator 65 and a second exhaust duct 68 to an exhaust duct 70 leading the purified gases to atmosphere. Large quantities of the solid particulate product and the products comprising mecury fall to the bottom of the spray dryer chamber 56 where they may be removed through a port 62. Less massive particles of the solid products which are carried through the first exhaust duct 64 are separated from the flue gases in the particle separator 65 where they can be removed through a port 66. Fig. 5 illustrates another embodiment of the invention in combination with a spray dryer absorption flue gas purification process. The components of the flue gas purification process illustrated in Fig. 5 are similar to the components of the process illustrated in Fig. 4. However, in Fig. 5 sulfur vapors generated in a device, not shown, are introduced through a duct 54 directly into the spray dryer chamber 56. In the embodiment illustrated in Fig. 5, the sulfur vapors are mixed internally of the spray dryer chamber 56 with the flue gases introduced through duct 52 by the swirlig action of the liquid droplets as produced by the rotary atomizing wheel 60.

A still further embodiment of the invention is illustrated in Fig. 6. The spray dryer absorption flue gas purification process of Fig. 6 is similar to the process of Fig. 4 with the exception that sulfur vapors are delivered through a duct 55 directly into the first exhaust pipe 64. The sulfur vapors mix internally of the first exhaust duct 64 with the flue gases and fine particulates withdrawn from the spray dryer chamber 56 and particulate products comprising solid mercury are separated from the flue gas in the particle separator 65.

Fig. 7 illustrates a still further embodiment of the present invention as utilized in a spray dryer absorption flue gas purification process. The embodiment illustrated in Fig. 7 is similar to the embodiment illustrated in Fig. 4 with the exception that an electrostatic precipitator 67 is utilized to separate fine particles withdrawn from the spray dryer chamber 56.

The embodiments illustrated by Figs. 4 and 7 are preferred since the sulfur vapors delivered through duct 53 can be controlled at a rate based on the mercury and oxygen content of the flue gases in duct 52 and readily mixed with the flue gases before the flue gases and sulfur vapors are delivered to the spray dryer chamber 56.

Although the mechanism of the present invention which improved the mercury removal capability of a flue gas puri fication process is not fully understood, the improvement is believed to result from the interaction of certain elements and materials which are present in the purification treatment process. For example, sulfur is known to react with mercury slowly at ambient temperatures. As previously discussed with reference to the examples summarized in Table 1, the process of the present invention proceeds rapidly at low temperatures. It has also been observed that an essentially quantitative reaction is obtained between mercury and sulfur in the system of the present invention. It is therefore reasonable to believe that particulates formed by reactions between calcium hydroxide and acidic materials in the flue gas form a dried particulate that absorbs sulfur and mercury from the flue gas and further catalyzes reactions to form low volatility compositions comprising mercury and sulfur which can be separated from the flue gas .

Although specific embodiments of the invention including several modifications with respect to where a sulfur vapor carrying gas stream may be introduced to a SDA flue gas purification system have been disclosed, the present invention is not to be construed as limited to the particulate embodiments and forms disclosed herein. The foregoing description is to be regarded as illustrative rather than restrictive and it should be understood that modifications and variations in details of construction may be made without departing from the spirit and scope of the invention as defined by the claims appended hereto.

Claims

WHAT IS CLAIMED IS:

1. A method for improving the mercury removal capability of a flue gas purification system comprising the steps of: introducing sulfur vapors into a flue gas stream to mix with mercury vapors in said flue gas;

contacting solid particulates in said flue gas with said sulfur vapors and mercury vapors to adsorb said mercury and sulfur vapors on said solid particulates and to catalyze reactions forming solid products comprising mercury; and

separating said solid products comprising mercury from said flue gas .

2. The method of claim 1 including the step of contacting particulate sulfur with a heated stream of a gas nonreactive to sulfur to produce said sulfur vapors.

3. The method of claim 1 wherein said sulfur vapors are introduced into said flue gas stream at a temperature between 60°C and 220°C.

4. The method of claim 1 wherein said sulfur vapors are introduced into said flue gas stream at a temperature between 70°C and 170°C.

5. The method of claim 1 wherein the mass ratio of sulfur contained in said sulfur vapors to said flue gas is between 1 and 200 mgS/Nm³.

6. The method of claim 1 including the step of separating said solid products comprising mercury from said flue gas by electrostatic precipitation.

8. A method for improving the mercury removal capability of a spray dryer absorber flue gas purification system comprising the steps of:

delivering a stream of flue gas to a spray dryer; mixing sulfur vapors with the flue gas;

spraying an aqueous dispersion of a basic reagent into the spray dryer in the form of liquid droplets;

evaporating water from said droplets and reacting said basic reagent with acidic constituents in the flue gas to form solid particles; contacting said solid particles with said sulfur vapors and mercury in said flue gas to absorb said mercury and sulfur vapors on said solid particles and catalyze reactions forming solid products comprising mercury; and separating said solid products comprising mercury from said flue gas.

9. The method of claim 8 including the step of recycling said solid particles and said step of spraying an aqueous dispersion into the spray dryer includes spraying an aqueous slurry of dispersed calcium hydroxide and said recycled solid particles.