Novel Mercury Oxidant and Sorbent for Mercury ... - Springer Link

Water Air Soil Pollut: Focus (2008) 8:333–341 DOI 10.1007/s11267-007-9146-6

Novel Mercury Oxidant and Sorbent for Mercury Emissions Control from Coal-fired Power Plants Joo-Youp Lee & Yuhong Ju & Sang-Sup Lee & Tim C. Keener & Rajender S. Varma

Received: 14 February 2007 / Revised: 12 July 2007 / Accepted: 6 August 2007 / Published online: 27 October 2007 # Springer Science + Business Media B.V. 2007

Abstract The authors have successfully developed novel efficient and cost-effective sorbent and oxidant for removing mercury from power plant flue gases. These sorbent and oxidant offer great promise for controlling mercury emissions from coal-fired power plants burning a wide range of coals including bituminous, sub-bituminous, and lignite coals. A preliminary analysis from the bench-scale test results shows that this new sorbent will be thermally more stable and costeffective in comparison with any promoted mercury sorbents currently available in the marketplace. In addition to the sorbent, an excellent elemental mercury (Hg(0)) oxidant has also been developed, and will enable coal-fired power plants equipped with wet scrubbers to simultaneously control their mercury J.-Y. Lee : S.-S. Lee : T. C. Keener (*) Department of Civil and Environmental Engineering, University of Cincinnati, Cincinnati, OH 45221-0071, USA e-mail: [email protected] Y. Ju Analytical Science, Pacific Core R&D, Dow Chemical (China) Investment Co., Ltd. Shanghai Branch, 24/F Aurora Plaza, 99 Fucheng Rd, Shanghai Pudong 200120, China R. S. Varma Sustainable Technology Division, National Risk Management Research Laboratory, U. S. Environmental Protection Agency, MS 443, 26 W. Martin Luther King Drive, Cincinnati, OH 45268, USA

emissions as well as their sulfur oxides emissions. This will work by converting all elemental mercury to an oxidized form which will be removed by the wet scrubber. This will result in significant cost savings for mercury emissions control to the atmosphere, and will help in keeping electric costs low. The sorbent and oxidant will benefit from the utilization of a waste stream from the printed circuit board (PCB) industry, and would thus be environmentally beneficial to both of the utility and electronics industries. The sorbent also demonstrated thermal stability up to 350°C, suggesting a possibility of an application in pulverized coal-fired power plants equipped with hot-side electrostatic precipitators and coal gasification plants. Keywords Clean coal technologies . Coal-fired utility plants . Fixed-bed test . Mercury emissions control . Oxidant . Sorbent

1 Introduction On March 15, 2005, US EPA announced the Clean Air Mercury Rule (US EPA 2005) to permanently limit mercury emissions from coal-fired power plants. The first-phase cap is 38 tons annually beginning in 2010, with a final cap set at 15 tons starting in 2018, resulting in nearly 70% reductions from 1999 emission levels. Sorbent injection is one of the most promising technologies for application to the utility industry as virtually all coal-fired boilers are equipped

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with either an electrostatic precipitator (ESP) or a baghouse (US EPA 2005). Temperature is known to affect the adsorption capacity of sorbents, and in most cases, the gas temperature window at the available injection location upstream of particulate matter control device is 130~150°C. Among various sorbents tested under the Department of Energy (DOE)’s Field Testing Program (Feeley et al. 2005) the most widely tested and promising sorbent is found to be raw and chemically promoted activated carbon (AC) which has demonstrated the capability of capturing both elemental and oxidized mercury from flue gas streams. Among the chemically promoted carbonaceous sorbents, halogenated activated carbons have demonstrated excellent performance in Hg(0) removal, especially in flue gases with low HCl content such as subbituminous and lignite coals, and can achieve 90%+ mercury removal (Feeley et al. 2005; Granite 2005). These halogenated activated carbons are at least 50% more expensive than unpromoted raw activated carbons in the current marketplace (Granite 2005), and additional halogen off-gassing from the halogenated sorbents could cause another serious health and corrosion problem to public health and utility plants, respectively (Feeley et al. 2005; Granite 2005). In addition, the DOE’s preliminary cost estimate for disposal of fly ash containing spent activated carbon is approximately $3 billion per year, which is approximately three times as much as the utility industry makes a year from their fly ash sales (US DOE 2006). The preliminary estimate is also projected to increase the cost of electricity by a factor of as much as four for some coal-fired generating units (US DOE 2006). To date, the best non-carbonaceous sorbent showing the highest mercury removal capacity is Amended Silicates (Butz and Broderick 2005). Other noncarbonaceous sorbents based on fly ash, zeolite, and fly ash-derived zeolite were also evaluated (Dombrowski et al. 2003; Trever et al. 2004), but were less effective than the Amended Silicates. In comparison with activated carbon sorbents, it was determined that noncarbonaceous sorbents were still able to preserve the quality of fly ash. Consequently, there is a strong desire to develop efficient and cost-effective non-carbonaceous or carbonaceous sorbents, which may not have an adverse impact on fly ash sales, public health, landfill disposal, and long-term operation of utility plants. The primary objective of this study is to develop advanced solid sorbent materials suitable for remov-

Water Air Soil Pollut: Focus (2008) 8:333–341

ing mercury in the elemental form from power plant emissions, preferably as a discrete waste to minimize formation of toxic waste generation. The authors developed efficient and cost-effective novel Hg(0) oxidant and sorbent (Varma et al. 2007), and these materials were evaluated using fixed-bed systems located at the University of Cincinnati (UC) and the Energy & Environment Research Center (EERC) of the University of North Dakota (UND), and their results are presented in this paper.

2 Experimental Method 2.1 Synthesis of Novel Oxidant and Sorbent The novel oxidant was synthesized with cupric chloride dihydrate and Montmorillonite clay using an acetone solvent. A predetermined amount in the range of 5~ 10%(wt.) of cupric chloride dihydrate, CuCl2 ·2H2O (Aldrich Chemical. Co., Milwaukee, WI, USA) was added to acetone under a vigorous stirring condition for about 10 min until its complete dissolution. Montmorillonite K 10 clay (Aldrich Chemical Co. Milwaukee, WI, USA) was then added slowly under constant stirring to form suspension, and the mixture is kept at 50°C for 2 h. The solvent was then removed from the resulting suspension below 50°C using a rotary evaporator (BÜCHI Rotavapor R-200, BÜCHI Laboratory Equipment, Switzerland). The resulting blue solid was further dried at 100°C under vacuum for 16 h to afford cupric chloride-impregnated montmorillonite clay as light blue solid. The novel sorbent was also prepared in the same manner with Norit’s FGD activated carbon. 2.2 Fixed-bed System at the University of Cincinnati (UC) 2.2.1 Mercury Measurement An elemental mercury permeation tube (VICI Metronics, Inc., Poulsbo, WA) was used for steady Hg(0) vapor generation. A pre-purified nitrogen carrier gas flow rate was set to 100 mL/min to steadily inject Hg(0) vapor into the fixed-bed system, and was maintained at all times with a mass flow controller (MFC, Model GFC 171, Aalborg Instruments and Controls, Inc., Orangeburg, NY). The 3-cm long Hg(0) permeation tube


immersed in a water bath was set to release Hg(0) vapor at a rate of 78 ng/min (=9 ppbv) inlet Hg(0) concentration. The water bath was successfully operated at 48°C within ±0.2°C to meet the specified inlet Hg(0) concentration. The influent Hg(0) vapor concentration was repeatedly measured with 4% (w/v) KMnO4/10% (v/v) H2SO4 impinger solutions used in the Ontario Hydro Method (ASTM Method D6784-02 2007), and the variations were confirmed within ±0.5 ppbv. An Ontario Hydro impinger train was used to obtain mercury speciation data from the effluent gas stream after the sorbent bed. Ontario Hydro Method encompasses several components needed to measure mercury in the effluent gas, including three types of impinger solutions to absorb Hg species from the gas phase: (1) 1 2+ M KCl solution for capture of oxidized (Hg2þ 2 or Hg ) mercury; (2) 5% (v/v) HNO3/10% (v/v) H2O2 solution impingers; (3) 4% (w/v) KMnO4/10% (v/v) H2SO4 solution impingers for capture of elemental mercury, as well as from the digestion of the spent sorbents and the filter used to capture particulates escaping from the sorbent bed and the rinse of the entire apparatus with

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dilute nitric acid. A cold vapor atomic absorption spectrophotometer (CVAAS, Model 400A, Buck Scientific Inc., East Norwalk, CT) was used to analyze mercury concentrations in the impinger solutions and solid phase digestion solutions. A blank test was carried out in order to examine the adsorption of mercury vapor on the tubing, reactor, and blank glass fiber filter prior to the main experimental study on mercury uptake by sorbents. The system was cleaned with 10%(v/v) nitric acid and de-ionized water before each experiment to remove residual mercury in the system as described in the Ontario Hydro Method. The amount of residual mercury in the tubing and the reactor wall turned out to be negligible in comparison with the amount of mercury recovered at the outlet of the system for a 10-min testing period. Mercury speciation results were obtained using an impinger train as described in the Ontario Hydro Method. The impinger train was placed on the outlet side of the system for obtaining speciated mercury samples as shown in Fig. 1. The effluent mercury vapor can be fully or partially oxidized due to reactions

Fig. 1 Schematic of a fixed-bed system at the University of Cincinnati

336 Table 1 UC simulated flue gas conditions

Water Air Soil Pollut: Focus (2008) 8:333–341 H2O

O2

CO2

SO2

NO

HCl

Hg(0)

7%(v)

3%(v)

12%(v)

500 ppmv

200 ppmv

0 ppmv

85 μg/Nm3

between elemental mercury, a sorbent, and other flue gas components. A 1 M KCl was used to capture potentially oxidized mercury, and an additional water trap was used to remove condensed water vapor as shown in Fig. 1. Prior to the main experiments, blank tests confirmed that the amount of Hg(0) captured in the 1 M KCl solution and water trap was negligible. 5% (v/v) HNO3/10% (v/v) H2O2 and 4% (w/v) KMnO4/10% (v/v) H2SO4 solution impingers were used to collect Hg(0), as well as the digestion fluid from digestion of the sorbents adsorbing mercury vapors and the filter used to capture particulates escaping from the sorbent bed and the rinse of the entire apparatus with dilute nitric acid. 2.2.2 Fixed-bed Adsorption Tests A fixed-bed system shown in Fig. 1 was used to test a set of new sorbents and oxidants under a simulated flue gas condition listed in Table 1. A sorbent sample was mixed in a silica (SiO2, Fisher Scientific, fine granules, particle size: 149-420 μm) diluent prior to being packed in the reactor. Approximately 50 mg of each sorbent in 6g of silica was used, and the bed material was supported by a fritted quartz disk with a Teflon™ o-ring and a glass fiber filter with a nominal 1 μm pore diameter in order to minimize channeling

Table 2 Summary of testing conditions

and prevent the sorbent from escaping through the bed. All testing conditions are summarized in Table 2. An additional filter system with a glass fiber filter with a nominal 0.7 μm pore diameter was used at the outlet of the reactor to capture sorbent particles potentially escaping from the bed. The fixed-bed reactor was constructed to allow for a total flow of 1 L/min gas throughput at 23°C. The inside diameter of the reactor (1.27 cm=1/2 in.) made of borosilicate glass was selected to meet a superficial velocity of 13 cm/s at 23°C in the empty bed reactor. The superficial velocity of the simulated flue gas was chosen to simulate a flow pattern in the ductwork of coal-fired utility plants (~50 ft/s=~1,500 cm/s), an electrostatic precipitator (ESP) (~5 ft/s=~152 cm/s), and a fabric filter (~3 ft/min=~2 cm/s). During each test, the mercury-laden inlet gas bypassed the sorbent bed, and passed to the analytical system until the desired inlet mercury concentration was established. Then, the adsorption test was initiated by diverting the gas flow through the sorbent column in downflow mode to minimize the potential for fluidization of the bed. All of the tubing and valves in contact with Hg(0) were constructed from Teflon™, which has been demonstrated to have good chemical resistance and inertness toward elemental mercury. The sorbent bed and filter system was placed

Item

Testing conditions

Reactor Temperature (°C) Flow rate (cm3/min) Flow mode Superficial velocity in an empty reactor (cm/s) Residence time in an empty reactor (s) Sorbent Gas Inlet Hg(0) concentration Adsorption capacity determination (1-h testing)

1/2-in. (1.28 cm) i.d. borosilicate 140 1,000 at 23°C; 1,395 at 140°C Downflow 13 at 23°C; 18 at 140°C 0.23 at 23°C; 0.17 at 140°C 50 mg in 6 g of a sand bed Simulated flue gas 78 ng/min=9 ppbv=85 μg/Nm3 Ontario Hydro Method (spent sorbent analysis and impinger solution analysis)


in a temperature-controllable convection oven (StabilTherm® Electric Utility Oven, Model OV-500C-2, Blue M Electric Company, Blue Island, IL), which can maintain the system temperature within ±0.5°C. A Teflon™-coated thermocouple was installed inside the fixed-bed reactor to control the gas temperature at the inlet of the sorbent bed. Tests were carried out in the reactor that was maintained at 140°C under Hg(0)-laden nitrogen flow for screening purpose. A temperature, 140°C, was also chosen to simulate similar conditions between an air preheater and a particulate control device for sorbent injection. The total gas flow rate of 1 L/min was monitored at the outlet of the impinger system using a bubble flow meter. It was confirmed that consistently reproducible results were obtained with this experimental setup. 2.3 Fixed-bed System at the EERC The sorbent and oxidant were also tested for their Hg(0) adsorption capacities in a fixed-bed bench-

Fig. 2 Schematic of a bench-scale system at EERC

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scale system located at the Energy & Environment Research Center (EERC) of the University of North Dakota as shown in Fig. 2. A nominal 150 mg of each sample was vacuum-loaded onto a quartz filter in a Silcosteel®-CR treated 6.4-cm-diameter US Environmental Protection Agency (EPA) Method 5 dustloading filter holder. The samples were tested at 135°C under a typical simulated flue gas of eastern bituminous coal as shown in Table 3. A P.S. Analytical (PSA) continuous mercury monitor (CMM) with a wetchemistry conditioning/conversion unit was used upstream of the filter-sorbent assembly to measure both the elemental and total mercury at the inlet and outlet of the fixed-bed reactor. Before the start of each test, the inlet mercury concentration was measured by the CMM. The sample was then plumbed into the system while ambient air was analyzed for mercury until the amount of mercury measured was less than 0.3 μg/m3. At the start of the test, flue gas was applied to the fixed bed at 29.9 scfm (=0.847 STD m3/min). Each sample was run until mercury breakthrough reached at least 90% of the inlet concentration.

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Table 3 EERC simulated flue gas conditions

H2O

O2

CO2

SO2

NO

NO2

HCl

Hg(0)

8%(v)

6%(v)

12%(v)

1600 ppmv

400 ppmv

20 ppmv

50 ppmv

14 μg/m3

3 Results and Discussion 3.1 Evaluation of Oxidants with Raw Activated Carbon in the UC Fixed-bed System One-hour tests were conducted for a set of a novel oxidant, raw Norit FGD activated carbon, a mixture of the oxidant and the FGD AC at 140°C using the UC fixed-bed system, and their results are summarized in Table 4. The mass balance closure for all runs was in a reasonably acceptable range (87-106%), and the amount of mercury captured in spent sorbents was determined after performing the digestion procedures described in the Ontario Hydro Method. Please note that HCl gas was not added to the simulated flue gas for all the Runs listed in order to eliminate the wellknown heterogeneous mercury oxidation (mercury adsorption on activated carbon after in-situ HCl gas impregnation on activated carbon, Carey et al. 1998). However, the performance of the novel oxidant was not affected by the absence of HCl. Run 1 showed 11% of mercury capture in the novel oxidant (UC-O: 10%(wt.) CuCl2·2H2O impregnated Montmorillonite clay), and approximately 74% of the mercury emissions from the bed were captured as oxidized mercury in the second filter placed in a filter holder, a water condensation impinger, and KCl

solution impingers. Run 1 showed that mercury adsorption in the novel oxidant was relatively small (approximately 10% of total Hg(0) injected into the system), and a significant majority of the inlet Hg(0) vapor was converted to the oxidized mercury form and adsorbed onto the solid phase (filter) or absorbed into the aqueous phase bubbler where water or KCl solution was placed. Therefore, the novel oxidant was found to be an excellent Hg(0) oxidant. In Run 2, raw Norit FGD activated carbon (DOE’s benchmark sorbent) was tested in order to determine its Hg(0) adsorption capacity in the absence of HCl in the gas phase. These tests showed 26% sorption capacity and negligible oxidation capability. In our early tests, raw activated carbon did not work well for Hg(0) removal without HCl gas in any type of simulated flue gases. These results obtained in the absence of HCl gas corroborate low mercury removal observed from the flue gases of PRB subbituminous and lignite coals in the DOE’s Mercury Control Field Testing Program (Feeley et al. 2005) and a previous fixed-bed study (Carey et al. 1998). Runs 3 and 4 were performed to examine the possibility of capturing the oxidized mercury created from the use of the novel oxidant (UC-O: 10%(wt.) CuCl2·2H2O impregnated Montmorillonite clay) by the in-situ adsorption onto activated carbon. A stan-

Table 4 1-h testing results in a fixed-bed reactor at 140°C Run

Sorbent/ oxidant

Loading (mg sorbent in 6 g sand)

Hg from spent sorbent + filter (%)

Hg from 2nd filter in filter holder (%)

Hg in water condensation (%)

Hg in tubing (%)

Hg in KCl (%)

Hg in KMnO4 (%)

Mass balance closure based on inlet Hg (%)

1 2 3 4 5

UC-Ob ACb UC-O + ACa UC-O + ACb UC-Sb

50 50 40+10 45+5 31

11 26 98 98 87

19 N/A N/A 1 N/A

28 0 0 0 0

0 0 0 0 0

27 0.5 N/A 1 3

14 73 2 2 3

100 106 87 90 93

UC-O=10% CuCl2·2H2O impregnated Montmorillonite K10 clay AC=Norit’s FGD activated carbon UC-S=5% CuCl2·2H2O impregnated Norit FGD activated carbon N/A not available a

Impinger configuration for total mercury analysis: SnCl2 → HNO3/H2O2 → water trap → KMnO4/H2SO4

b

Impinger configuration for mercury speciation analysis: KCl → HNO3/H2O2 → water trap → KMnO4/H2SO4


339

Fig. 3 Mercury speciation results obtained from fixedbed tests at UC

nous chloride (a reducing agent; SnCl2) solution was used in place of the KCl solution for Run 3 for total mercury analysis after the fixed-bed reactor so that all the mercury emitted after the bed would be converted to Hg(0), and could be collected in the downstream KMnO4/H2SO4 solution impingers. Run 3 employed 20% (10 mg) of the FGD activated carbon after uniformly mixing it with 80% (40 mg) the novel oxidant in 6 g of sand. Its result showed that almost all Hg(0) (98% of the total 87% recovered mercury from all impinger solutions, and digestions of filters and solids) was captured in the mixture of the two materials (FGD activated carbon and novel oxidant) with 20% addition of the FGD activated carbon. In

Fig. 4 Bench-scale Hg(0) sorption results for novel sorbents and oxidants using a fixed-bed system at EERC

Run 4, the amount of FGD activated carbon was reduced to the half that of Run 3, 10% (5 mg), and was tested under the same conditions. The 10% addition of the activated carbon also demonstrated almost the same performance in Hg(0) removal as that of Run 3 under the same test conditions. Since the majority of the sorbent cost comes from that of the activated carbon (current Norit’s FGD activated carbon’s cost is $0.42/lb), it is very crucial to minimize the amount of activated carbon used. Since the residence time in an empty bed reactor was less than 0.2 s at 140°C, the reaction kinetics involved in Hg(0) reactions seem to be fast enough to utilize in most of coal-fired power plant configurations. A novel sorbent

340

(UC-S: 5% CuCl2·2H2O impregnated Norit FGD activated carbon) was prepared and tested in Run 5 under the same operating conditions. It also showed that almost all of Hg(0) could be captured from the flue gas conditions. All of these mercury speciation results are summarized in Fig. 3. 3.2 Evaluation of Sorbents and Oxidants in the EERC Fixed-bed System These tests were carried out under the simulated flue gas conditions of burning high-sulfur bituminous coal. The test results for all four samples (UC-A and UC-B: 5 and 10%(wt.) CuCl2·2H2O impregnated Montmorillonite clay, respectively; UC-C and UC-D: 5 and 10% CuCl2·2H2O impregnated Norit FGD activated carbon, respectively) are plotted in Fig. 4 alongside a baseline carbon sample of the EERC’s lignite activated carbon (LAC), a benchmark sorbent used for mercury capture. The data from all the tests have been normalized to fit breakthrough curves which demonstrate the percent of the inlet Hg(0) concentration not captured by the sorbent. Figure 4 shows that both of the novel oxidant samples of UCA and UC-B have a slight adsorption capacity for Hg (0) capture as denoted by the breakthrough curves, Sample UC-A/B Total Hg. However, both of them showed the good oxidizing capability as denoted by the breakthrough curves, Sample UC-A/B Elemental Hg. Sample UC-C performed much like the EERC’s baseline lignite-derived activated carbon (LAC) sample, with breakthrough beginning about 20 min into the test and 50% breakthrough occurring at 1 h into the test. Sample UC-D provided the best Hg(0) capture performance, surpassing the mercury capture obtained using the baseline LAC. Sample UC-D captured 100% of the mercury for 30 min before breakthrough started. Fifty percent breakthrough occurred at approximately 1.5 h into the test. Near the end of each test, Hg(0) was measured to determine the amount of oxidation occurring.

4 Conclusions Novel oxidant and sorbent were developed and tested in the fixed-bed systems. The novel sorbent demonstrated its potential to capture Hg(0) from almost all types of flue gases of burning bituminous and PRB


sub-bituminous/lignite coals. It is also expected to be able to remove oxidized forms of mercury as it is a chemically-promoted activated carbon sorbent. In addition to the sorbent, an elemental mercury oxidant was also developed by impregnating the same chemical onto a clay material. Although it showed some Hg(0) sorbent capability, however, worked primarily as an Hg(0) oxidant. The difference between the adsorption capabilities (or the extent of desorption after chemical reactions) of the novel sorbent and oxidant seems to be attributed to the affinity of products formed as a result of chemical reactions and the surfaces of the substrate materials. Fixed-bed results show that a coinjection scenario of the oxidant and raw activated carbon may be applicable to Hg(0) removal in flue gases with relatively high Hg(0) content such as PRB subbituminous and/or lignite coal-firing sites as an economical and viable option while minimizing the use of activated carbon. The success of the scenario will depend on the mass transfer resistance of the oxidized mercury desorbed from the oxidant and its re-adsorption onto raw activated carbon. This oxidant will also enable coal-fired power plants equipped with wet scrubbers to simultaneously control their mercury emissions as well as their sulfur oxides emissions. This will work by converting Hg(0) into the oxidized form in the flue gas, which will be removed by the wet scrubber. This is expected to result in tremendous cost savings for the control of mercury emissions to the atmosphere, and help in keeping electrical generating costs low. The sorbent and oxidant will benefit from the utilization of a waste stream from the printed circuit board (PCB) industry, and would thus be environmentally beneficial to both of the utility and electronics industries. Acknowledgements The authors sincerely appreciate the Energy & Environment Research Center of the University of North Dakota for their complimentary evaluation of our sorbent and oxidant materials in their bench-scale fixed-bed facility.

References ASTM Method D6784-02. (2007). Standard test method for elemental, oxidized, particle-bound and total mercury in flue gas generated from coal-fired stationary sources (Ontario Hydro Method). Butz, J. R., & Broderick, T. E. (2005). Demonstration of Amended Silicates™ sorbents for mercury control at Miami Fort 6. (Paper presented at the DOE/NETL Mercury Control Technology R&D Program Review Meeting, Pittsburgh, PA), July 12-14.

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341 tive on the status of mercury control technologies for coal-fired power plants. Retrieved July 11, 2007, from the National Energy Technology Laboratory of the Department of Energy web site: http://www.netl.doe. gov/technologies/coalpower/ewr/mercury/pubs/NETL% 20Clarification%20on%20Mercury%20FINAL%200406. pdf., April 25. Office of Air Quality Planning and Standards and Office of Research and Development, US Environmental Protection Agency. (2005). Clean Air Mercury Rule. Retrieved July 11, 2007, from the US Environmental Protection Agency web site: http://www.epa.gov/air/mercuryrule/pdfs/camr_ final_preamble.pdf., March 15. Trever, L., Slye, R. & Ebner, T. (2004). Assessment of low cost novel sorbents for coal-fired power plant mercury control, Final Report, DOE award number: DE-FC26-01NT41180, March. Varma, R. S., Ju, Y., Sikdar, S., Lee, J.-Y. & Keener, T. C. (2007). compositions and methods for removing mercury from mercury-containing fluids, US Patent 0140940, 21 June 2007.