Complexation and removal of mercury from flue gas desulfurization systems

Abstract

A method for the reduction and prevention of mercury emissions into the environment from combusted fossil fuels or other off-gases with the use of hypobromite is disclosed. The hypobromite is used for the capture of mercury from the resulting flue gases using a flue gas desulfurization system or scrubber. The method uses hypobromite in conjunction with a scrubber to capture mercury and lower its emission and/or re-emission with stack gases. The method allows the use of coal as a cleaner and environmentally friendlier fuel source as well as capturing mercury from other processing systems.

Claims

1. A method for reducing mercury emissions, comprising: providing a gas stream comprising mercury; and passing the gas stream into a scrubber comprising a scrubber liquor and hypobromite.

2. The method of claim 1, wherein the hypobromite is mixed with a carrying agent selected from: limestone slurry, lime slurry, sodium-based alkali solution, trona-based solution, sodium carbonate solution, sodium hydroxide solution, and water.

3. The method of claim 2, wherein the carrying agent is limestone slurry.

4. The method of claim 2, wherein the carrying agent is water.

5. The method of claim 1, further comprising mixing a bromine salt and an oxidant to form the hypobromite.

6. The method of claim 5, wherein the oxidant is sodium hypochlorite.

7. The method of claim 1, further comprising providing an antimicrobial agent which includes sodium bromide, sodium hypochlorite, and sulfamic acid.

8. The method of claim 1, wherein the mercury is from combusted coal.

9. The method of claim 1, wherein the scrubber is a wet scrubber selected from a spray tower system, a jet bubblers system, and a co-current packed tower system.

10. The method of claim 1, wherein the hypobromite is added to the scrubber liquor and then added to the scrubber.

11. The method of claim 1, wherein the hypobromite is added to the scrubber containing the scrubber liquor.

12. The method of claim 1, wherein the hypobromite is added to virgin liquor then added to the scrubber.

13. The method of claim 1, wherein the hypobromite is added to a make-up liquor then added to the scrubber.

14. The method of claim 1, wherein the hypobromite is added to a return liquor then added to the scrubber.

15. The method of claim 1, wherein the hypobromite is added to a reclaimed liquor then added to the scrubber.

16. The method of claim 1, wherein the hypobromite is added to a liquor injected directly into flue gases then added to the scrubber.

17. The method of claim 1, wherein the hypobromite is added to a recirculation loop of the scrubber liquor.

18. The method of claim 1, wherein the hypobromite is added to a low solids return to the scrubber from a scrubber purge stream.

19. The method of claim 1, wherein the hypobromite is added to an aqueous stream introduced into the scrubber, wherein the aqueous stream is selected from a demister and make-up water stream.

20. The method of claim 1, wherein the mercury is from an incinerator, cement kiln, or an ore refinery.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a schematic of an experimental setup of impingers to measure simulated mercury emission capture.

(2) FIG. 2A is a plot that shows the percent of mercury removal as a function of sodium hypochlorite concentration with and without hypobromite using an embodiment of the invention.

(3) FIG. 2B is an expansion view of the plot in FIG. 2A between a narrower range of sodium hypochlorite concentration with and without hypobromite.

(4) FIG. 3 is a plot that shows the percent of mercury removal as a function of sodium hypochlorite concentration with and without hypobromite using an embodiment of the invention.

DETAILED DESCRIPTION

(5) Unless expressly stated to the contrary, use of the term a is intended to include at least one or one or more. For example, a device is intended to include at least one device or one or more devices.

(6) Any ranges given either in absolute terms or in approximate terms are intended to encompass both, and any definitions used herein are intended to be clarifying and not limiting. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements. Moreover, all ranges disclosed herein are to be understood to encompass any and all subranges (including all fractional and whole values) subsumed therein.

(7) One of skill in the art would also understand that salts of bromine refer to bromide ions typically seen as Br.sup. but which can also exist in a different oxidation state. Hypobromite is an ion known as BrO.sup. which is oxidized more than bromide (Br.sup.).

(8) The present disclosure describes the use of hypobromite to unexpectedly improve the capture of mercury emissions across a flue gas desulfurizer (FGD) in coal-fired flue gas streams or other processing system where mercury vapor is present or released. Examples include municipal solid waste (MSW) incinerators, medical waste combustors, ore roaster and refineries, and cement kilns.

(9) The scrubbers currently used in the industry include spray towers, jet bubblers, and co-current packed towers as examples. These types of air pollution control devices (APCDs) are provided as examples and are not meant to represent or suggest any limitation. The hypobromite may be added to virgin limestone or lime slurry prior to addition to the scrubber, the recirculation loop of the scrubber liquor, the low solids return to the scrubber from the scrubber purge stream, demister water, make-up water, or the scrubber liquor. Semi-dry FGDs can also be adapted, including spray dryer absorbers (i.e., SDAs), circulating dry scrubbers (CDS) or TURBBOSORP available from Von Roll. The hypobromite may be added to semi-dry FGDs so that the hypobromite contacts mercury passing through the scrubber.

(10) Typically, the hypobromite is applied at a ratio of 0.5:1 to 20000000:1 weight hypobromite to weight of mercury being captured. The preferred ratio is from 1:1 to 2000000:1 and the most preferred range is from 5:1 to 200000:1. In some embodiments, the hypobromite is prepared by mixing a bromide salt, for example, calcium bromide (CaBr.sub.2) with an oxidant source. The oxidant source may be any oxidant that converts a bromide ion (Br) to hypobromite (BrO.sup.) such as hypochlorite, e.g., sodium hypochlorite.

(11) In some embodiments, the hypobromite may be augmented with an antimicrobial supplement. An example of such an antimicrobial agent is StaBrEx that includes hypochlorite, sodium bromide, and sulfamic acid, available from Nalco Water, 1601 W. Diehl Road, Naperville, Ill. 60563-1198, and as further described in U.S. Pat. No. 4,992,209, which is incorporated herein by reference in its entirety.

(12) In general, hypobromite may be introduced into the scrubber and thereby into the scrubber liquor via several routes. The following will serve as just some of the variations that are available to introduce the hypobromite into the scrubber liquor. The scrubber liquor is defined as the water-based dispersion of calcium carbonate (limestone) or calcium oxide (lime) used in a wet or dry flue gas scrubber to capture SO.sub.x emissions. The liquor may also contain other additives such as magnesium and low-molecular weight organic acids, which function to improve sulfur capture. One example of such an additive is a mixture of low-molecular weight organic acids known as dibasic acid (DBA). DBA typically consists of a blend of adipic, succinic, and glutaric acids. Each of these organic acids can also be used individually. In addition, another low-molecular weight organic acid that can be used to improve sulfur capture in a wet scrubber is formic acid. Finally, the scrubber liquor will also contain byproducts of the interaction between the lime or limestone and flue gas, which leads to the presence of various amounts of calcium sulfite or calcium sulfate as well as anions such as halides (i.e., chlorides, bromides, and iodides) and other cations such as iron, zinc, sodium, or copper. The scrubber liquor includes but is not limited to the make-up liquor, return liquor, the reclaimed liquor, virgin liquor, and liquor injected directly into flue gases.

(13) Another addition point for the hypobromite to the wet scrubber is at the low solids liquor return. A portion of the liquor is often continuously removed from the scrubber for separating reaction byproducts from unused lime or limestone. One such means of separation is centrifugation. In this process, the scrubber liquor is separated into a high solids and low solids stream. The high solids stream is diverted to wastewater processing. The low solids fraction returns to the wet scrubber and can be considered reclaimed dilute liquor. The hypobromite may be added to the reclaimed low solids stream prior to returning to the scrubber.

(14) Another feed liquor found in the operation of a wet FGD is called virgin liquor. Virgin liquor is the water-based dispersion of either lime or limestone prior to exposure to flue gas and is used to add fresh lime or limestone while maintaining the scrubber liquor level and efficiency of the wet FGD. This is prepared by dispersing the lime or limestone in water. Here, the hypobromite can be added either to the dispersion water or to the virgin liquor directly or to the demister water.

(15) Finally, some scrubber installations use scrubber liquor and/or water (fresh or recycled) injected directly into the flue gas prior to the scrubber for the purpose of controlling relative humidity of the flue gas or its temperature. The excess liquid is then carried into the scrubber. Here also are two potential addition points for the introduction of the hypobromite.

(16) The addition of the hypobromite can be made in any of these locations, wholly or fractionally (i.e., a single feed point or multiple feed points), including but not limited to the make-up water for the lime or limestone slurry or the scrubber liquor.

(17) In some embodiments, bromide is available as bromide salt or hydrobromic acid (HBr) from an upstream process to the scrubber. The bromide salt may be in the form of calcium bromide (CaBr.sub.2), sodium bromide (NaBr), or other bromide salts or as a mixture of various bromide salts. Hypobromite is formed by adding an oxidant to the stream containing the bromide salt to generate hypobromite in situ. The oxidant may be added to the scrubber liquor, low solids liquor return, virgin liquor, dispersion water, or other liquid existing during the capture, recovery, and treatment process.

(18) Often, the recovery of desirable ore products involves refining the ore from materials that contain mercury. For example, phosphate is often extracted from phosphorite which contains mercury as a trace element. During refinement of the desirable phosphorous mineral, mercury can be liberated such during fertilizer manufacture. In such cases, the mercury passes into a scrubber fluid, for example, a sodium-based alkali that is used to capture sulfur dioxide (SO.sub.2). The mercury can be removed using the processes described herein.

(19) As another example, gold ore processing often involves roasting the gold ore to oxidize and remove sulfide. The gas generated by sulfur burning in the roaster is scrubbed to remove the sulfur dioxide and other components which can be contaminated with mercury. Mercury can be removed from these off-gases to make the gold processing more environmentally desirable.

(20) Thus, techniques described herein can be used to remove contaminating amounts of mercury from off-gases arising from various ore processing and ore refineries processing those ores.

(21) The invention is illustrated by the proceeding descriptions and the following examples which are not meant to limit the invention unless otherwise stated in the claims appended hereto.

EXAMPLES

(22) Testing for elemental mercury oxidation or absorption was performed using a multiple impinger setup with an inline mercury analyzer. The inline mercury analyzer for this testing was an Ohio Lumex RA-915 Portable Zeeman Mercury Analyzer.

(23) Referring to FIG. 1, a multiple impinger setup (with impingers serially connected: 1 to 2, 2 to 3, 3 to 4, etc.) was used to expose samples with elemental mercury. In Impinger 1, elemental mercury was added (200 ppt, 5 mL) and combined with stannous chloride (2 mL) to evolve elemental mercury. In Impinger 2, 30 mL of solution was added. The solution can be a diluted sample, synthetic slurries, test slurries, deionized (DI) water (for calibration and reference) or any combination of the above. In Impingers 3-4, various solutions (30 mL) were added to reduce the amount of volatile material reaching the detector and not affect the mercury signal. These various solutions included HNO.sub.3 and NaOH solutions at concentrations from 0.1 to 1 M. During calibration, these materials are replaced with the same volume of DI water. In the last impingerImpinger 3, 4, or 5, depending on the systemwas left empty to catch any liquid overflow. The mercury detector was then connected to the last impinge, followed by a carbon trap and pump. The pump draws ambient air from the room where the impingers are located.

(24) The second impinger used for this application was not a typical bottom draining impinger. Instead, a 100 mL round bottom flask was fitted at the bottom of the impinger so that the flask could be lowered into a heating bath for variable temperature measurements and was large enough for a variety of test solution volumes. The solution was bubbled with gas through a disposable pipette.

(25) Test slurries used were obtained from a commercial limestone forced oxidation wet flue gas desulfurization scrubber at a coal-fired electrical generator unit firing eastern bituminous coal. The pH of the slurry is typically between 5.5 and 6.5.

Example 1

(26) Adding calcium bromide (CaBr.sub.2) in a ratio of 0.5:10 wt % of CaBr.sub.2 to hypochlorite in solution generated hypobromite. Table 1 lists the measured concentrations of sodium hypochlorite alone, sodium hypochlorite with calcium bromide and hypobromite from the sodium hypochlorite/calcium bromide solutions associated with the percent of mercury removal shown in FIGS. 2A and 2B from a synthetic slurry comprising 20 wt % CaSO.sub.4. The generation of hypobromite increased elemental mercury removal from flue gas when compared to hypochlorite alone.

(27) TABLE-US-00001 TABLE I NaClO (ppm) NaClO:CaBr.sub.2 (ppm:ppm) Hypobromite (ppm) 50 50:2.5 5 5 5:0.25 0.5 0.5 0.5:0.025 0.05 0.05 0.05:0.0025 0.005 0.005 0.005:0.00025 0.0005

(28) Mercury removal in FIGS. 2A and 2B is calculated using the amount of elemental mercury not captured ([Hg.sup.0].sub.NC, mercury detected at the detector) compared to the amount of mercury initially in the system ([Hg.sup.0].sub.I (Equation 1).

(29) $\begin{array}{cc}\mathrm{Hg}\mathrm{Removal}\left(\%\right)=\left(1-\frac{{\left[{\mathrm{Hg}}^0\right]}_{\mathrm{NC}}}{{\left[{\mathrm{Hg}}^0\right]}_I}\right)100& \left(\mathrm{Equation}1\right)\end{array}$

(30) Both hypobromite and hypochlorite were also tested with a commercial test slurry treated with a mercury re-emission product (Nalco MerControl 8034). The observed results were nearly identical to the data shown in FIGS. 2A and 2B. The addition of hypobromite increased the capacity of the solution to promote elemental mercury capture by water-based scrubber liquor over hypochlorite alone.

Comparative Example 1

(31) To test the saturation of elemental mercury in water, a known amount of elemental mercury vapor was bubbled through deionized water in a plastic jug using a Mercury Instruments MC-3000 Mercury Calibrator. A mercury generator was employed to create vapor phase elemental mercury (29 or 270 g/m.sup.3) with a N.sub.2 carrier gas (2.5 L/min). This gaseous mixture was then bubbled through deionized water (0.9 L) for a varying amount of time (1-20 min). A small amount of water was then removed for analysis on an Ohio Lumex RA-915 Portable Zeeman Mercury Analyzer.

(32) The retention of elemental mercury in water over time was also tested using the same testing setup and procedure as above with one addition. After bubbling mercury through the deionized water for a set amount of time, the mercury ampule was bypassed so that only N.sub.2 gas would flow through the deionized water.

(33) Elemental mercury was bubbled through deionized water to determine the saturation of elemental mercury in a system more similar to the dynamic system in a wet flue gas desulfurizer scrubber. The results shown in Table II indicate that the system was driving towards a steady state of 60 ppt and 700 ppt, at 29 g/m.sup.3 and 270 g/m.sup.3 respectively, rather than increasing up to the saturation limit of 60 ppb (60,000 ppt) all at 25 C. The initial concentrations at short times are higher than expected due to an oversaturation of the nitrogen carrier gas in the head space of the mercury ampoule, causing a burst of elemental mercury at the beginning of each test. Over time in both the low and high concentration systems, the concentration of elemental mercury drives to an equilibrium value. The theoretical values in Table II refer to the total amount of elemental mercury flow through the testing system based on concentration, flow rate, and time.

(34) TABLE-US-00002 TABLE II Hg.sup.0 in water, (ppt) Elapse Time (min) Theoretical* Actual Gas: 29 g/m.sup.3 and 2.5 L/min 1.0 146 235 2.5 309 137 5.0 521 93 10.0 937 61 15.0 1,342 58 Gas: 270 g/m.sup.3 and 2.5 L/min 1 349 601 10 6,139 701 20 21,139 667

(35) The saturation of elemental mercury in water was driving to equilibrium as governed by Henry's Law (Equation 2 below). Henry's law is defined as Henry's constant (K.sub.H, 376 atm at 25 C.), mole fraction of elemental mercury in solution (x), and partial pressure of elemental mercury (p, atm). Below in Table III are the theoretical values of elemental mercury in water based on Henry's Law and the concentration of the gas flowing through the system. The values are not exactly the same as the actual measured values but are on the same order of magnitude. These predictions are significantly lower than the solubility limit of 60 ppb for elemental mercury in pure water.

(36) $\begin{array}{cc}{K}_H=\frac{p}{x}& \left(\mathrm{Equation}2\right)\end{array}$

(37) TABLE-US-00003 TABLE III Elemental Mercury Concentration Gas Phase Water Phase (ppt) (g/m.sup.3) Actual Theoretical* 29 60 103 270 700 963 0.3 2 5.0 18 16.823 60,000

(38) Next, these same solutions were tested for mercury retention or stability by first adding elemental mercury to the water in the same manner as above and then bypassing the elemental mercury ampoule so that only N.sub.2 gas (elemental mercury content of zero) bubbles through the water for a varying amount of time. This would approximate the effect of a forced oxidation system in the wet flue gas desulfurizer scrubbers. In Table IV, it can be seen that the elemental mercury was quickly removed from the deionized water by the pure N.sub.2 gas. This behavior is also consistent with Henry's Law as the elemental mercury in the water phase transfers to the gas phase, the system continuously shifts in order to reach equilibrium. Elemental mercury was not readily soluble or retained in deionized water.

(39) TABLE-US-00004 TABLE IV N.sub.2 flow time (min) Hg (ppt) % retention 29 g/m.sup.3 at 2.5 L/min for 1 min 0 297 1 174 59% 2.5 71 24% 5 14 5% 29 g/m.sup.3 at 2.5 L/min for 10 min 0 76 1 46 61% 2.5 18 24% 5 3.4 4% 270 g/m.sup.3 at 2.5 L/min for 1 min 0 601 1 374 62% 2.5 157 26% 5 35.7 6% 270 g/m.sup.3 at 2.5 L/min for 10 min 0 701 1 439 63% 5 90 13% 10 5.4 1%

(40) The data in these tables and figures clearly define the problem by demonstrating that water-based scrubber liquors do not effectively decrease elemental mercury concentration in combustion flue gas. It is also clear from these bench-scale results that the hypobromite successfully and unexpectedly controls the emission of mercury from a scrubber by decreasing the elemental mercury flue gas concentration and does so more efficiently than conventional techniques.

(41) It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the invention and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the appended claims.