full scale evaluation of mercury reemission in wet …

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FINAL TECHNICAL REPORT September 1, 2008, through December 31, 2009

Project Title: FULL SCALE EVALUATION OF MERCURY REEMISSION IN

WET FLUE GAS DESULFURIZATION SYSTEMS ICCI Project Number: 08-1/6.1B-1 Principal Investigator: Dr. Wei-Ping Pan, Western Kentucky University Other Investigators: Dr. Yan Cao and Dr. Chin-Min Cheng, Western Kentucky

University Project Manager: Dr. Francois Botha, ICCI

ABSTRACT

The objective of this research is to investigate the potential reduction of oxidized mercury (Hg2+) to elemental mercury (Hg0) and subsequent emission of Hg0 under wet flue gas desulfurization (WFGD) operating conditions. Experiments were performed in a bench- scale WFGD system and three full scale WFGD systems. In the bench-scale study, the effects of different FGD operation parameters (i.e., types of reagents, slurry temperature, slurry pH, oxygen content in the carrier gas) were investigated. It was found that the highest reemission occurred when the system was operated under sodium sulfite (Na2SO3) reagent slurry with low pH (4.5) at high temperature (60oC). The simulated Hg0 reemission was suppressed with increase amount of oxygen introduced into the simulated bench-scale FGD scrubber system. In addition, three FGD additives (sodium sulfide (Na2S), TMT15, and hydrogen iodine (HI)) were also tested in the bench-scale study. With 0.025% wt/wt of Na2S, 94.5% of reemission was suppressed. At the same concentration, 84 and 86% of reemission reduction were achieved by TMT15 and HI, respectively. In one of the full-scale studies, a commercial available FGD additive, i.e., sodium hydrogen sulfide (NaHS), was tested in Power Plant C (Plant C) located in central Illinois Units 31/32/33. An overall 80% of mercury removal efficiency was achieved at Unit 33 with 120 gal/hr of NaHS injection, which was improved from 70% before injection. The reemission was reduced from 46% to no reemission. Compared to the results obtained from the previous 2007 study, the Hg emission at the Units 31/32 common stack was reduced significantly even without the injection of NaHS, which was likely due to the change of FGD operation parameters (e.g., increase of oxidation air flow and slurry discharge rate). To further investigate the effects of FGD operation conditions on the reemission of Hg0, a full-scale study was carried out at a utility located (Plant O) at Western Kentucky, which encountered over 450% of reemission problem. It was found that both oxidation air flow rate and sulfur load in the FGD scrubber showed significant impact on controlling the stack mercury emission. The effects of pH, which was maintained in a range from 5.0 to 6.0, and slurry retention time were not observable. Another full scale investigation was carried out at the Unit 4 of Power Plant S (Plant S), whose WFGD system was changed from natural to forced oxidation in September, 2008. No change on the stack mercury emission was observed in the year-long monitoring after the system was modified. However, the effect on the modification was not conclusive as the FGD system encountered several operation difficulties during the testing period.

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EXECUTIVE SUMMARY

In the project, the potential reduction of Hg2+ to Hg0 and subsequent emission of Hg0 under wet FGD operating conditions was investigated. Experiments were performed in a bench-scale simulated WFGD system and three full scale WFGD systems to answer the following: (1) which FGD parameters play a key role on Hg removal through the FGD? (2) To what degree does flue gas composition contribute to the oxidized Hg remission and Hg removal efficiency? (3) How can the FGD be modified to optimize the Hg removal efficiency by controlling the flue gas composition and scrubber chemistry?

Three tasks were carried out. In Task One, a bench-scale WFGD scrubber system was set up to evaluate the effect of FGD operation parameters, such as types of reagents, slurry pH, absorber temperature, and amount of oxygen introduced into slurry. In addition, three additives (i.e. sodium tetrasulfide (Na2S4), TMT15 (Trimercapto-s-triazine, C3N3S3Na3), and hydrogen iodide (HI)), were tested with different dosages. The effects of transition metals (i.e., Fe2+, Pb2+, Cr3+, AsO-2, Cu+2, and Ni2+) were also investigated. A wet-chemistry continuous mercury monitoring system was used to measure the change of total and elemental mercury concentrations during each experiment batch.

In Task Two, a full-scale WFGD testing was carried out at Power Station C (Plant C) Units 31/32/33 to investigate the effect of a commercial available FGD additive (i.e., sodium hydrogen sulfide, NaHS) on the reduction of Hg0 reemission. Units 31/32 were pulverized coal (PC) cyclone boilers. Both units were equipped with selective catalytic reduction (SCR) and cold side electrostatic precipitators (ESP). The flue gases from Units 31/32 were fed into a common duct after the ESP units. One common FGD process was employed to remove SO2 for both flue gases. Unit 33 is a wall-fired PC boiler with SCR, ESP, and WFGD processes. During the testing period, all units were operated in a constant- and/or full-load condition. The utilities used limestone to prepare FGD reagent slurry. While the testing was carried out two continuous emission monitoring systems were set up at the inlet of the FGD process and stack to continuously collect mercury concentrations in the flue gas across the FGD system. For quality control purposes, both Ontario Hydro method and sorbent trap method were also used at both locations to provide reference values. During the testing period, coal, fly ash, reagent slurry, FGD gypsum, wastewater, makeup water were also collected and analyzed for mercury to establish mass balance of the FGD process. In addition to mercury, other trace elements, such as arsenic and selenium, were also studied. EPA Method 29 was used to collected flue gas trace element concentrations. To further investigate the effect of FGD operation parameters on the reemission of Hg0, a three-week long continuous mercury monitoring was carried out at Plant O located at western Kentucky. The utility operated two units (i.e., Units One and Two). The two units used a common cold side ESP and WFGD processes. Unit one is a PC cyclone boiler with a full load of 150 MWe. Unit Two is a wall-fired PC boiler with a full-load of 280 MWe. Plant O and Plant C used same type of bituminous coal. While the testing was carried out at Plant O, a continuous emission monitoring system was installed at the stack. Plant information (PI) data was provided by the utility, which include, unit load, coal flow, SO2 concentration at the inlet and outlet of the FGD process, lime feed rate, absorber blowdown rate, chloride

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blowdown, recycle pump status, force oxidation flow rate, recycle slurry density, absorber pH, slurry height in each absorber, total air flow rate, flue gas oxygen concentration before FGD process.

In Task Three, a full-Scale WFGD testing at Power Station S (Plant S) Unit 4 was carried out to evaluate the effect of change of FGD process from natural to forced oxidation operation on the mercury emission. A continuous emission monitoring system was installed at the Unit 4 stack at the beginning of the project and started the monitoring before the FGD process was modified. Weekly sorbent trap mercury measurement was also carried out started form July 2009 to provide reference mercury readings.

Results obtained from the bench-scale study suggest:

(1) The greatest Hg0 reemission occurred when the system was operated under high slurry temperature (60oC) and low slurry pH (4.0) conditions with the sodium based sulfite (Na2SO3) reagent. The observation was due to the promotion effect for the reduction of Hg2+ to Hg0 under acidic condition as indicated in the following equation. HSO3

- + H2O + Hg2+ Hg0 + SO42- + 3H+

The concentration of HSO3- in the slurries might be increased under acidic

conditions as Na2SO3 has higher solubility than CaSO3.

(2) The reemission of Hg0 can be suppressed by increase the amount of oxygen introduced into FGD slurry. The introduced oxygen reacted with SO3

-2 and form SO4

-2. As a result, the above reaction was inhibited.

(3) A general trend showing decrease of Hg0 reemission when additives were added into the slurry. Three precipitates HgS, Hg3TMT2 and HgI2 were produced when additives were used to suppress Hg0 reemission. Over 94% of reemission was reduced when 0.025% wt/wt of Na2S4 was used as FGD additives. With the same of dosage, 84 and 86.3% of reduction was achieved when TMT15 and HI was used, respectively. It was found that the byproducts produced from Na2S4 and HI additions released H2S and HI at low pH and high temperature. However, Hg3TMT2 was found to be stable and it unlikely releases H2S and less likely releases mercury during wallboard production. Considering economy and environment factors, TMT15 is a more effective additive to suppress Hg0 reemission compared to the two other additives tested in this study.

(4) Transition trace elements, such as divalent iron (Fe2+), lead (Pb2+), trivalent chromium (Cr3+), arsenite (AsO2

-), mono-valent copper (Cu+), and nickel (Ni2+) were also found to affect the reemission of Hg0. In general, the higher the concentration of these trace elements the higher reemission of Hg0 can be found.

Results obtained from the full-scale testing at Plant C and Plant O indicates:

(1) The commercial available NaHS FGD additives can effectively suppress the reemission of Hg0. The injection of NaHS at a rate of 120 gallon/hr effectively reduced the Hg reemission from 46% to no reemission, which improved the total

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Hg removal from 70 to over 80%. The lower than expected mercury removal was likely due to the malfunction of by-pass damper at the FGD inlet duct. According to the plant, approximately over 10% of the flue gas bypassed the FGD unit, which likely resulted in the higher Hg emission at the stack.

(2) The reemission of Hg0 can be improved by modifying FGD process operation. Compared to the results obtained from the previous 2007 (ICCI report 06-1/4.1 C-1) study, the Hg emission at the Units 31/32 common stack was reduced significantly even without the injection of NaHS. It is found that (1) the overall Hg removal efficiency measured from this study was much greater (73% in 2007 and 92% in this study) as a result of much lower mercury emissions at the stack (0.028 lb/GWh in 2007 and 0.006 lb/GWh in this study); (2) the elemental mercury to total mercury ratio (Hg0/Hg(T)) at the FGD inlet did not significantly change (9±4% in 2007 and 11±5% in this study); (3) the Hg0/Hg(T) ratio at the stack was higher in this study (78%) to what was observed in 2007 (64%). The much improved Hg emission was likely due to the several modifications done at Units 31 and/or 32 during the outage of Unit 31. The most significant change of operation between the 2007 study and this study was likely the retention time of reagent slurry in the absorbers and the addition of oxidation air.

(3) The effect of oxidation air on the reemission of Hg0 was also observed at the full-scale testing carried out at Plant O. It was observed that the oxidation air flow affected the reemission of Hg0 by three different ways. During the period of when oxidation air was momentary turned off, the emission of Hg at the stack decrease immediately. Once the oxidation air was turned on, mercury concentration started increasing until the oxidation air flow increased to a certain flow rate. When the oxidation air flow rate increased to a certain level, the mercury concentration was found to decrease sharply. However, the effect of oxidation air flow rate seemed to be more significant when only Unit 1 was in operation. When both units were in full-load, the correlation between oxidation air flow rate and mercury emission was not observable, which is likely due to a higher load of sulfur in the scrubber slurry at the higher load condition.

(4) The load of sulfur in FGD scrubber (i.e., kg of S/m3 of slurry/hour) was found has observable effect on the mercury emission at both low and high load conditions. In general, the higher S load the lower mercury concentration was found at the stack. With higher sulfur load in the FGD scrubber, more mercuric disulfite might be formed and, as a result, reduce the presence of mercuric sulfite, which is suggested as the precursor for Hg0 reemission. Therefore, less reemission was observed. In addition, with higher load of sulfur, disproportionation of sulfite might also occur, which produces reduced sulfur. The reduced sulfur ions may react with mercury and form mercuric sulfide, which is highly insoluble and could be removed from the liquid phase reaction as a solid.

(5) While no oxidation air is available, all SO2 removed from the flue gas become sulfite (SO3

-2) at the operation pH, which promotes the formation of mercuric disulfite, and therefore, less or no mercury reemission. With insufficient

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oxidation air, the available oxygen might react with mercuric disulfite and form mercuric sulfite or compete with mercury for sulfite, which inhibits the formation of mercuric disulfite and promotes reemission. However, when the oxidation air flow rate increase, the oxidation of sulfite further inhibits the formation of mercuric sulfite under lower sulfur load condition. But when both units were running, the sulfur load increased and the supplied oxygen was not sufficient to have the same effect as at a lower sulfur load when only Unit 1 was in operation.

Results obtained from the full-scale testing at Plant S Unit 4 implied the change of FGD process from natural to forced oxidation might not have effect on the emission of mercury. However, the results were not conclusive due to the operation difficulties encountered by the utility. During the testing period, the FGD process was not able to produce high purity FGD gypsum. ICSET have helped the plant indentify the problem, which was related to addition of the FGD additive and pH control.

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OBJECTIVES

The main objective of this research is to investigate the potential reduction of Hg2+ to Hg0 and subsequent emission of Hg0 under wet FGD operating conditions. Experiments were performed in a pilot scale WFGD system and three full scale WFGD systems on site. To this end, the following tasks were accomplished.

1. Task One: Bench WFGD Testing.

2. Task Two: Full-Scale WFGD Testing at Plant C and Plant O on the effect of FGD Additive and Operation Conditions.

3. Task Three: Full-Scale WFGD Testing at Plant S Unit 4 on the effect of change from natural to forced oxidation operation.

INTRODUCTION AND BACKGOUND

Coal-fired electric power generation remain the largest man-made mercury source, which has not yet been efficiently controlled in part because this is one of the most expensive to control. In general, the combination of selective catalytic reduction (i.e., SCR) and flue gas desulfurization (i.e., FGD) processes for nitrogen oxide (i.e., NOx) and sulfur dioxide (i.e., SO2) emission controls, respectively, affords more than an eighty percent reduction in mercury emissions in the case of high chlorine content coals. However, the control efficiency of mercury by wet FGD process is hindered by the potential of elemental mercury reemission in the wet FGD scrubber when the process is operated under forced oxidation. In the previous ICCI project (06-1/4.1C-1) carried out by Institute for Combustion Science and Environmental Technology (ICSET) of Western Kentucky University (WKU), it was found that the amount of elemental Hg increased 157% and 40%, respectively, in the stack gases of two tested facilities when compared to the concentrations in the flue gases at the FGD inlets. The emissions of Hg of these two facilities did not meet the Hg emission limit set by the Illinois Mercury Rule, which was implemented in July 2009. It was concluded that the mercury emission limit can be met for the two tested facilities if the elemental mercury reemission phenomenon can be reduced or eliminated.

The reemission of elemental mercury refers to the phenomenon of when the concentration of elemental mercury at the stack gas is higher than what is found at the FGD inlet. It occurs as oxidized mercury (Hg2+) either in the flue gas or captured by FGD slurry is converted to elemental mercury (Hg0) in the FGD wet scrubbers. The mechanism of how the conversion occurs in the FGD scrubber has not yet been extensively explored, and therefore, they are relatively unknown. There is a need to understand (1) which FGD parameters play a key role on Hg removal through the FGD. (2) To what degree does flue gas composition contribute to the oxidized Hg remission and Hg removal efficiency? (3) How can the FGD be modified to optimize the Hg removal efficiency by controlling the flue gas composition and scrubber chemistry?

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EXPERIMENTAL PROCEDURES

Task One: Bench-Scale WFGD Testing

The bench-scale experiment was carried out in a lab-scale wet-FGD simulating system, which is schematic described in Figure 1. The simulated scrubbing system was composed of a 500mL, round bottom flask with three necks, water bath, magnetic stirring system and temperature controllers. The three necks were used for oxidized mercury solution injection, inlet and outlet of carrier gas. The water bath was used to keep the slurry at the range of operation temperature. A magnetic stirring machine and a magnetic bar were used to keep the solids uniformly suspended in the slurry during tests. The desired flow rates of carrier gases were controlled by calibrated MFCs. The outlet gas was transported to a speciated mercury analyzer with heated lines, which kept the carrier gas heated at about 160oC to prevent mercury loss during delivery.

Figure 1. Schematic of lab-scale simulated scrubber system and mercury analyzer system.

Mercury reemission study was carried out according to the test matrix indicated in Table 1. For comparison purposes, the normal operation parameters for a full-scale FGD process are also included in the table. As shown, in addition to calcium oxide, calcium sulfite, and calcium sulfate, sodium sulfite, sodium tetrasulfide (Na2S4), hydroiodic acid (HI), and TMT15 were also used as the reagent in the simulated scrubbing system. The TMT15 additive was obtained from Evonic-Degussa Corporation.

At the beginning of each experiment, 400 mL of the slurry with desired concentration (0.5% wt/wt) was prepared and poured into the flask, which was placed in a water bath set up at a desired temperature (i.e., 40, 50, 60, 70oC). Carrier gas was then introduced into the scrubber at a rate of 800 mL/min. After the system was stabilized, a HgCl2 solution with a concentration of 1.06×10-7 mol/L was then injected into the system at a

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rate of 10 mL/hr using a syringe and syringe pump. Each experimental batch was run for at least 110 minutes but no more than 125 minutes, which dependent on actual conditions. Oxidation reduction potential (i.e., ORP) values of the slurries were tested before each experimental batch. Real-time vapor phase mercury measurements from the simulated scrubber were conducted by using a PS Analytical Sir Galahad Hg analyzer, and mercury concentration was monitored before and after HgCl2 injection.

Table 1. Typical operation parameter comparison between full-scale and lab-scale wet FGD.

Operation parameter Full-scale Laboratory-scale

reagent Limestone/lime CaO/CaSO3/CaSO4/Na2SO3/ Na2S4/HI/ TMT15

Slurry pH 4-6.5 4.0-7.0

Absorber temperature(oC) 50-60 40, 50, 60

L/G(gal/1000acf) 50-130 Batch operation

Gas flow rate (ml/min) ----- 800

Gas temperature at absorber inlet(oC) 130-160 160

Gas temperature at absorber outlet(oC) 50-60 40-60

Hg2+ concentration at FGD inlet 4-15 4.40

Task Two: Full-Scale WFGD Testing to evaluate the effect of FGD operation on the reemission of elemental mercury

In this task, the effects of sodium hydrogen sulfide (NaHS) injection on the emissions of mercury, selected trace elements, and total reduced sulfur at Plant C Units 31/32/33 located at central Illinois were investigated. The testing was carried out from 5/12-5/30/2009. An extended study aimed to investigate the effect of ESP field operation on Hg emissions without NaHS injection was conducted on 6/8 and 6/9 at Units 31/32. In addition, the effect of FGD operation on the reemission of elemental mercury was carried out at a Plant O located at Western Kentucky. The configurations of tested units can be seen in Table 2. Schematic descriptions of the tested units can be seen in Figure 2.

As shown in the figure, flue gases from Plant C Units 31/32 were fed into a common duct after the ESP units. One common FGD process was employed to remove SO2 for both flue gases. Limestone (CaCO3) was used to prepare FGD reagent slurry. The flue gases generated from Unit One and Unit Two of power station Plant O flew into a common duct before the electrostatic precipitator. After the ESP unit, the flue gas was divided into two streams before the FGD process. The two streams flew into Module A and Module B wet scrubbers. Both Units One and Two are PC boilers with a total gross of approximate 400 MWe.

In the Plant C study, the mercury concentrations at the FGD inlet and stack were continuously monitored before, during, and after the NaHS injection periods. To provide reference mercury readings, during the testing period, the ASTM 6784-02 method, also known as Ontario Hydro (OH) Method, was carried out for mercury measurement at the FGD inlet and stack.

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Table 2. Configurations of Test Sites.

Boiler

Capacity (MWe)

Boiler Types APCD Configurations Testing Period CEM participated9

Plant C Units 31/32

76 each Cyclone SCR1+ESP2+FGD3 5/17-5/23 PSA (FGD inlet) Thermo (Stack)

Unit 33 188 Wall-fired SCR+ESP+FGD 5/26-5/31 Thermo (FGD inlet)

PSA (Stack) Plant O

Unit1 150 Cyclone SNCR4+ESP+FGD 7/24-8/11 Thermo at the common Stack Unit 2 250 Wall-fired SCR+ESP+FGD 7/24-8/11

1 selective catalytic reduction 2 electrostatic precipitator 3 flue gas desulfurization 4 selective non-catalytic reduction

(a) (b)

(c)

Figure 2. Schematic description of Plant C (a) Units 31/32, (b) Unit 33, and (c) Plant O

Unit 4.

The stack mercury concentration was also measured using a sorbent trap method at the stack. In addition to mercury, the concentrations of trace elements (e.g., arsenic, selenium, and boron) at the FGD inlet and stack during testing period were also measured

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using the EPA Method 29. Coal, bottom ash, ESP ash, FGD reagent slurry, FGD gypsum, and filtrates were also collected during the testing period for trace element analysis. A Thermo Mercury Freedom and Wet-chemistry PS Analytical Hg continuous emission monitoring (CEM) systems were applied to collect continuous emission monitoring data at the FGD inlet and stack during the testing period. The testing matrix of NaHS injection at Plant C can be seen in Table 3. The analytical methods applied during the Plant C NaHS injection testing can be seen in Table 4.

In the Plant O study, depending on electricity demand, the power station was operated under several different scenarios during the testing period. The operation of the units and FGD modules of each scenario was not under constant conditions, for example, the load of each boiler, the flow rate of oxidation air, the load of reagent slurry, the discharge rate of scrubber slurry, and the height of the slurry in each module might change during each scenario. The Thermo mercury CEM system was installed at the Units One and Two common stack to monitor the change of Hg emission under the listed operation scenarios.

Daily calibration was carried out every day when Hg CEM systems were in operation. Plant Information (PI) data during the testing period was provided by the tested facilities.

Table 3. Overview of NaHS Injection Testing Scope and Schedule at Plant C Units 31/32/33.

Units 31/32 Date in May, 2009

Injection Rate (gph)

Test FGD In Stack

15-16 Safety Training, and set up

17-18 0 Hg by OHM, sorbent trap, and CMM

Trace Element by EPA M29 X X

19 10 Hg by OHM, sorbent trap, and CMM

Trace Element by EPA M29 Total Reduced Sulfur by M16B

X X







X X



24 Teardown Units 33

Date in May, 2009

Injection Rate (gph)

Test FGD In Stack (FGD

outlet) 25 set up



X X

29 60/90/120 Hg by OHM, sorbent trap, and CMM


X X

30 120/90/0 Hg by OHM, sorbent trap, and CMM


29-30 Contingency and Teardown

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Table 4. Analytical Methods and Instruments Used for Coal, Ash, and FGD Slurry Samples.

Task Three: Full-Scale WFGD Testing at Plant S Unit 4 to evaluate if reemission of elemental mercury occurred when the FGD process was changed from natural to forced oxidation

The modification of the FGD process was scheduled to be carried out from 9/2008 to 11/2008. ICSET installed a Tekran Series 3300 at the Unit 4 stack in June 2008 and started the continuous mercury monitoring before the FGD process was modified. During the FGD modification, the Hg CEM system was removed from service until the finish of the modification in February, 2009. The continuous monitoring was stop in December, 2009. The concentration and speciation of Hg across the FGD system of Plant S Unit 4 were studied on 8/6/2008 and 6/19/2008 before and after the FGD system was retrofitted to be operated under forced oxidation mode. Weekly sorbent trap mercury measurement was also carried out started form July 2009 to provide reference mercury readings.

RESULTS AND DISCUSSION

Task One: Laboratory-Scale Mercury Reemission Study

Laboratory Study

(1) Simulation of elemental mercury reemission

Figure 3 demonstrates the Hg0 emission curves observed during the testing when HgCl2 was injected into four different slurry reagents (i.e., CaO, CaSO3, CaSO4, and Na2SO3). As shown, no Hg0 reemission took place from CaO slurry as the elemental Hg concentration was low (~200 ng/dscm) and remained constant after HgCl2 was introduced into the system. However, increase in the concentrations of elemental Hg was found in the other three slurry reagents after HgCl2 was injected into the simulated

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scrubber. The highest elemental mercury concentration was found in the Na2SO3 slurry batch followed by the CaSO3 and CaSO4 slurry batches.

The observed trend was due to the reduction of oxidized mercury to elemental mercury by sulfite presenting in the slurry. The Hg0 reemission reaction mechanism was explained by the following chemical reaction described in eq.1 (Ghorishi et al, 2006),

HSO3- + H2O + Hg2+ Hg0 + SO4

2- + 3H+ eq.1

Concentration of HSO3- in the slurries affected Hg0 reemission rate under the acid

condition because Na2SO3 had higher solubility than CaSO3 had. But Hg0 reemission also occurred in CaSO4 slurry, which indicated the reemission of Hg0 is not solely controlled by the presence of sulfite.

0 20 40 60 80 100 1200

500

1000

1500

2000

2500

3000H gC l

2 10m l/hr, 800m l/m in nitrogen ,

T em perature 60 0C , pH =6.0

Hg(

0) c

once

ntra

tion,

ng/N

m3

H gC l2 in jection tim e/m in

0 .5% C aO 0 .5% N a

2S O

3

0 .5% C aS O3

0 .5% C aS O4

Figure 3. Effects of reagents on Hg0 reemission.

(2) Effects of operation conditions on Hg0 reemission

A series of tests under the different operation conditions were carried out to investigate some operation parameters of full-scale wet FGD. Effects of operation conditions including temperature, pH and oxygen concentration in carrier gas on Hg0 reemission were performed and the results can be seen in Figure 4. It was found that the reemission of Hg0 increases as the pH and temperature of the slurry increased. It was also observed that the reemission was able to be suppressed by increased the amount of oxygen in the carrier gas. All the experiments were carried out in a simulated inhibited oxidation scrubber, in which CaSO3 slurry was kept at a constant solid content of 0.5% (wt/wt) in the slurry, and HgCl2 injection rate and injection time was the same as 3.1.

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0 2 0 4 0 6 0 8 0 1 0 0 1 2 00

2 0 0

4 0 0

6 0 0

8 0 0

1 0 0 0

1 2 0 0

1 4 0 0

1 6 0 0

0 .5 % C a S O3, H g C l

2 1 0 m l/h r ,

8 0 0 m l/m in n i t ro g e n , p H = 6 .0

Hg(

0) c

once

ntra

tion,

ng/N

m3

H g C l2 in je c t io n t im e /m in

4 0 0 C

5 0 0 C

6 0 0 C

0 2 0 4 0 6 0 8 0 1 0 0 1 2 0

2 0 0

4 0 0

6 0 0

8 0 0

1 0 0 00 .5 % C aS O

3, H g C l

2 1 0 m l/h r,

8 0 0 m l/m in n itro g en , tem p era tu re 5 0 0C

Hg(

0) c

once

ntra

tion,

ng/N

m3

H g C l2 in jec tio n tim e/m in

p H = 4 .5 p H = 5 .0 p H = 6 .0 p H = 6 .5

0 2 0 4 0 6 0 8 0 1 0 0 1 2 00

2 0 0

4 0 0

6 0 0

8 0 0

1 0 0 0

1 2 0 0

1 4 0 0

1 6 0 0

1 8 0 0

2 0 0 0

2 2 0 0 0 .5 % C a S O3, H g C l

2 1 0 m l/h r ,

8 0 0 m l/m in n i tro g e n ,

te m p e ra tu re 6 0 0C , p H = 6 .0

Hg(

0) c

once

ntra

tion

,ng/

Nm

3

H g C l2 in je c tio n t im e /m in

N itro g e n 6 % O

2 in N

2

1 4 % O2 in N

2

C o m p re s s e d a ir

Figure 4. Effects of (a) temperature, (b) pH, and (c) oxygen concentration in the carrier

gas on the reemission of Hg0.

(a)

(b)

(c)

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(3) Evaluation of additives on suppressing Hg0 reemission

Based on existing problem of Hg0 reemission through full-scale wet scrubber, one of the effective technologies to suppress Hg0 reemission was additive injection technology. Three additives (i.e. sodium tetrasulfide (Na2S4), TMT15 (Trimercapto-s-triazine, C3N3S3Na3), and hydrogen iodide (HI)), were tested with different dosages in this study. All the experimental batch were carried out in the 0.5% (wt/wt) CaSO3 slurry. The pH value of the slurry was maintained at 6.0 and temperature was set at 60oC. The 1.06×10-7 mol/L HgCl2 injection rate was 10 mL/hr and injection time was 110 min. Results obtained from the testing can be seen in Table 5.

Based on effects of three additives on suppressing Hg0 reemission, the area below the curves of Hg0 concentration with HgCl2 injection time were integrated, and the area denoted the mass of Hg0 emission from the slurry. Suppressing rates of Hg0 reemission

with addition of additives were calculated as0 0

0baseline addtive

baseline

Hg Hg

Hg , and the results

are shown in Table 5. The table qualitatively indicated that a general decrease of trend of Hg0 reemission rates when additives were added into the slurry. Because Na2S4 and HI were toxic and harmful, care should be taken when they were used, while TMT15 was not a harmful additive. Three precipitates HgS, Hg3TMT2 and HgI2 were produced when additives were used to suppress Hg0 reemission. Byproducts released H2S and HI at low pH and high temperature when Na2S4 and HI were used, while Hg3TMT2 was stable in the byproducts, and it unlikely release H2S and less likely release mercury during wallboard production. The effects of suppressing Hg0 reemission rates were apparent from 80.8 to 94.5% when Na2S4 were used. More dosages of HI were added into the slurry to achieve the lower Hg0 reemission rate than 10 times dosage of TMT15. In contrast to Na2S4 and HI, TMT15 was an environment friendly additive, and it had as high as 70.4% of suppressing Hg0 reemission rate with addition of 0.0005% (v/v) TMT15. Considering economy and environment factors, TMT15 was an effective additive to suppress Hg(0) reemission.

Table 5. Comparisons of additives on suppressing Hg0 reemission.

Additive and dosage Mass of Hg2+ injection, ng

Mass of Hg0 reemission, g Suppress Hg0

reemission rate, η /% No additive

388.70

84.5 baseline 0.025% Na2S4 4.6 94.5 0.005% Na2S4 11.8 86

0.00125% Na2S4 16.2 80.8 No additive 75 baseline

0.0025% TMT15 12 84 0.00125% TMT15 17.9 76.1 0.0005% TMT15 22.2 70.4

No additive 75.4 baseline 0.025% HI 10.5 86.3 0.0125% HI 15.4 79.6 0.005% HI 41 45.6

15

(4) Effect of Transition Metals on Hg0 reemission

Some transition trace metals presented in the wet FGD slurries, and their ions is soluble in the slurry. Although their concentrations in the slurry were relative low and existed in ppm grade, effects of some transition metal ions on elemental mercury reemission were evaluated. Tests should be studied in the single factor, and some transition metal ions were prepared in the DI water respectively. Results obtained from the experiment can be seen in Figure 5. As shown in the figure. The tested trace elements were found to influence Hg0 reemission.

Task Two: Full-Scale WFGD Testing to evaluate the effect of FGD operation on the reemission of elemental mercury

Effect of NaHS injection on the control of Hg0 reemission

The first objective of this task is to investigate the effects of NaHS injection on the emissions of mercury at Plant C Units 31/32/33. The testing was carried out from 5/12-5/30/2009.

(1) Hg emission at Units 31/32:

Table 6 presents the concentrations of Hg in the flue gas at the FGD inlet and stack of Units 31/32 when the injection of NaHS to the FGD absorbers was operated under different rates. For comparison purposes, results obtained from the previous study carried out in 2007 are also included in the figure. As can be seen, the effect of NaHS injection on the reduction of Hg emissions at the stack was not observed. The baseline emission (without NaHS injection) was 0.0051 lb/GWh before the injection. The emission did not decrease under the 3 injection rates tested in this study. Baseline measurement was carried out after the 30 gallon/hour injection (the highest injection rate tested in Units 31/32) was stopped and the emission was determined to be 0.0029 lb/GWh. The overall Hg removal efficiency ranged from 86.4 to 95.2%. Compared to the results obtained from the previous 2007 study, the Hg emission at the Units 31/32 common stack was reduced significantly even without the injection of NaHS.

By comparing the results obtained from the 2007 study and this study, it is found that (1) the overall Hg removal efficiency measured from this study was much greater (73% in 2007 and 92% in this study) as a result of much lower mercury emissions at the stack (0.028 lb/Gwh in 2007 and 0.006 lb/GWh in this study); (2) the elemental mercury to total mercury ratio (Hg0 /Hg(T)) at the FGD inlet did not significantly change (9±4% in 2007 and 11±5% in this study); (3) the Hg0/Hg(T) ratio at the stack was higher in this study (78%) to what was observed in 2007 (64%). The much improved Hg emission was likely due to the several modifications done at Units 31/32 during the outage of Unit 31, which are listed in Table 7. Based on the modification, the relative PI data between the two testing period is listed in Table 8, which suggests that the most significant change of operation between the 2007 study and this study was likely the retention time of reagent slurry in the absorbers and the addition of oxidation air. By the increase discharge rate of

16

reagent slurry observed in this study and similar absorber tank slurry level, it can be calculated that the absorber slurry was replaced about 1.5 times faster than in 2007.

Mercury Concentration at lab-scale wet FGD on 11/10/08

0

1000

2000

3000

4000

5000

11/10/20088:00

11/10/20089:12

11/10/200810:24

11/10/200811:36

11/10/200812:48

11/10/200814:00

11/10/200815:12

11/10/200816:24

11/10/200817:36

11/10/200818:48

Calendar Date

Mer

cury

Con

cent

rati

on, n

g/N

m3

Hg(0)

Hg(T)

pH=5, temperature 60C, 1000ml/min carrier gas, 0.1ppm HgCl2 5ml/hr injection

1. 10mMol/L FeSO4 in slurry

2. 5 mMol/L FeSO4 in slurry

3. 2 mMol/L FeSO4 in slurry

1 2

Hg2+ injection Hg2+ injection Hg2+ injection


0

1000

2000

3000

4000

5000

11/23/20088:43

11/23/20089:55

11/23/200811:07

11/23/200812:19

11/23/200813:31

11/23/200814:43

11/23/200815:55

11/23/200817:07

11/23/200818:19

11/23/200819:31

Calendar Date

Mer

cury

Con

cent

rati

on, n

g/N

m3

Hg(0)

Hg(T)


1. 1mMol/L Pb2+ in 400mL DI water;

2. 0.5 mMol/L Pb2+ in 400mL DI water;

3. 2 mMol/L Pb2+ in 400mL DI water;

12

Hg2+ injection Hg2+ injection

Hg2+ injection

3


0

100

200

300

400

500

600

700

800

900

1000

11/12/2008 9:55 11/12/2008 11:07 11/12/2008 12:19 11/12/2008 13:31 11/12/2008 14:43 11/12/2008 15:55 11/12/2008 17:07

Calendar Date

Mer

cury

Con

cent

rati

on, n

g/N

m3

Hg(0)

Hg(T)


1. 10mMol/L Cr(NO3)3 in slurry

2. 5 mMol/L Cr(NO3)3 in slurry

1 2

Hg2+ injection Hg2+ injection

17


0

500

1000

1500

2000

11/24/20088:14

11/24/20089:26

11/24/200810:38

11/24/200811:50

11/24/200813:02

11/24/200814:14

11/24/200815:26

11/24/200816:38

11/24/200817:50

Calendar Date

Mer

cury

Con

cent

rati

on, n

g/N

m3 Hg(0)

Hg(T)pH=5, temperature 60C, 1000ml/min carrier gas, 0.1ppm HgCl2 5ml/hr injection

1. 1mMol/L NaAsO2 in slu rry2. 0.5 mMol/L NaAsO2 in slurry3. 2 mMol/L NaAsO2 in slurry

1 2

Hg2+ injection Hg2+ injection Hg2+ injection

3


0

1000

2000

3000

4000

5000

11/7/20088:00

11/7/20089:12

11/7/200810:24

11/7/200811:36

11/7/200812:48

11/7/200814:00

11/7/200815:12

11/7/200816:24

11/7/200817:36

11/7/200818:48

Calendar Date

Mer

cury

Con

cent

rati

on, n

g/N

m3

Hg(0)

Hg(T)


1. 10mMol/L CuCl slurry2. 5 mMol/L CuCl slurry

1

2

Hg2+ injection

Hg2+ injection


0

200

400

600

800

1000

1200

12/31/20088:28

12/31/20089:40

12/31/200810:52

12/31/200812:04

12/31/200813:16

12/31/200814:28

12/31/200815:40

12/31/200816:52

12/31/200818:04

12/31/200819:16

Calendar Date

Mer

cury

Con

cent

rati

on, n

g/N

m3

Hg(0)

1. 10mM Ni 2+ in 400mL DI water, pH 6.5, temperature 60C, 800ml/min Nitrogen gas, 0.1ppm HgCl2 5ml/hr injection



1

2

3

. Figure 5. Effect of (a) Fe2+, (b) Pb2+, (c) Cr3+, (d) AsO¯2, (e)Cu+, (f) Ni2+ on the

reemission of Hg0. .

18

Based on the fact that no significant change of properties of coal used between the two studies, it plausible to conclude that the two changes likely contribute to the reduction of Hg0 reemission.

Table 6. Summary of Testing Results at Units 31/32.

Load Hg in Coal FGD inlet Stack Reemission Removal Efficiency by FGD Overall

Mwe lb/GWh lb/GWh lb/GWh % % % 2007 Results 160 0.075 0.074 0.028 157 62.2 62.7 5/17 Baseline 145 0.060 0.053 0.0051 NO 90.4 91.5 5/19 10 Gal/hr 152 0.059 0.053 0.0062 NO 88.3 89.5 5/20 20 Gal/hr 152 0.061 0.054 0.0083 NO 84.6 86.4 5/21 30 Gal/hr 152 0.060 0.050 0.0065 75 87.0 89.2 5/23 Baseline 152 0.060 0.060 0.0029 NO 95.2 95.2

The redox potential of the slurry collected from wet well was monitored throughout the testing period and the results can be seen in Table 9. The results suggest that the absorber was in oxidizing condition during the pre-injection baseline testing period. The redox potential was higher on 5/17 than on 5/18, which likely resulted in the higher reduction of elemental Hg across the FGD process. The redox potential became negative as the injection of NaHS started, which might explain the slight increase of elemental Hg at the stack at 20 and 30 gal/hour of NaHS injection. After the injection was stopped, it was found that the redox potential of the wet well slurry increased. The change of redox potential was likely to have direct correlation with the elemental and total mercury concentration level at each testing conditions.

Table 7. Modifications of Operations.

Changes Boiler 1 modified secondary air flow 2 Higher boiler temperature 3 combust more effectively SCR 1 re-generated catalyst 2 higher temperature 3 air temperature after air heater becomes lower ESP 1 Operated under different field operation

FGD 1 both two oxidation air blower are running; only 1 was working in year 2007.

2 FGD blow down interval becomes shorter

Table 8. Comparison of Selected Operation Conditions of Units 31/32 during the Testing Periods in the 2007 Study and This Study.

Operation Conditions Unit 2007 2009

Load (Units 31/32) MWe 150.6 146.4

Flow SCFH 64138270.4 58045308.6

Boiler O2 % 2.81 2.80

Ammonia Injection

311.3 333.2

FGD Absorber Level

53.4 52.8

Absorber Slurry pH

5.01 4.99

Slurry Density

1.14 1.14

Reagent Slurry Discharge Rate GPM 53.1 77.1

19

Table 9. Change of Redox Potential of Wet Well Slurry during NaHS injection.

Sample Type Unit Date Time

ReDox (mv)

Wet Well Slurry 31/32 05-17-09 19:00 293.5 Wet Well Slurry 31/32 05-18-09 15:00 190.0 Wet Well Slurry 31/32 05-19-09 18:00 112.1 Wet Well Slurry 31/32 05-20-09 18:30 -120.1 Wet Well Slurry 31/32 05-21-09 16:40 -185.6 Wet Well Slurry 31/32 05-22-09 18:00 -246.7 Wet Well Slurry 31/32 05-23-09 18:30 -142.4 Wet Well Slurry 31/32 05-24-09 11:00 -56.3

(2) Hg emission at Unit 33

Table 10 shows the change of Hg concentrations in the flue gas at Unit 33 FGD inlet and stack as a result of NaHS injection. For comparison purposes, results obtained from the previous study carried out in 2007 are also included in the table. As shown, the emission of Hg at the stack was reduced as the NaHS injection rate was increased from 0 to 120 gallon/hr. With an injection rate of 120 gallon/hr, the highest injection rate tested in this study, the total Hg emission was 0.012 lb/GWh, compared to 0.022 lb/GWh before the injection. An overall Hg removal efficiency of 80% was achieved with 120 gal/hr of NaHS injection. In addition, in general, the reemission phenomenon became less significant as the injection rate increased. No reemission was observed at 120 gallon/hr.

Table 10. Summary of Testing Results at Unit 33.

Load Hg in Coal FGD inlet Stack Reemission Removal Efficiency

by FGD Overall Mwe lb/GWh lb/GWh lb/GWh % % % 2007 Results 200 0.067 0.059 0.0110 33 81.4 83.6 5/26 Baseline 188 0.071 0.057 0.0215 46 62.3 69.7 5/29 60 Gal/hr 188 0.061 0.069 0.0190 55 72.5 68.9 5/29 90 Gal/hr 185 0.061 0.058 0.0180 35 69.0 70.5 5/29 120 Gal/hr 185 0.061 0.057 0.0120 NO 78.9 80.3 5/30 Baseline 187.5 0.053 0.050 0.0220 37 56.0 58.5

Effect of NaHS injection on the Emission of Trace Elements

The concentrations of 21 elements in the flue gas (both gaseous and particulate-bound) were also measured at the inlet of the FGD process and stack. It was found that Al, B, Fe, P, K, Se, and Mn were the only elements whose gaseous phase concentrations were detectable at both the inlet and outlet of the FGD process. In the case of particulate-bound trace element, the emission of most elements decreased after the flue gas passed through the FGD unit. The concentrations of vapor phase selenium at the stack varied during NaHS injection and seemed to be affected by the injection rates. As shown in Figure 6, in which the concentrations of gaseous Se at the Units 31/32/33 are demonstrated, the injection of NaHS noticeably increased the concentration of vapor phase Se at the stack.

20

(a) (b)

on Baseline 2009

10 Gal/hr

20 Gal/hr

30 Gal/hr

Post-injection Baseline

Vap

or S

e C

once

ntra

tion,

g/

dscm

0

10

20

30

40

50

60

70

Rem

oval

Effi

cien

cy A

ccro

ss F

GD

, %

-40

-20

0

20

40

60

80

100Vapor Se at FGD inlet Vapor Se at Stack Batch vs Col 15

Unit 31/32

Baseline

60 Gal/Hr

90 Gal/Hr

120-90 Gal/Hr

Va

por

Se

Con

cen

trat

ion,

g

/dsc

m

0

20

40

60

80

100

Rem

oval

Effi

cien

cy A

ccro

ss F

GD

, %

-40

-20

0

20

40

60

80

Vapor Se at FGD inlet Vapor Se at Stack Batch vs Col 14

Unit 33

Figure 6. Concentrations of (a) gaseous and (b) particulate-bound Se at Unit 33 stack.

Effect of FGD Operation on the Hg0 Reemission

A full-scale study on the effect of FGD operation on the reemission of Hg0 was carried out at a power station located at Western Kentucky. The utility was chosen because of its significant reemission phenomenon, which was observed in a previous study carried out by ICSET. The reemission of the utility can be seen in Table 11. As shown, the reemission of Hg was found to be higher than 200%.

Table 11. Emission of Mercury at the Power Plant Located at Western Kentucky tested in 2008.

Date

Starting Time

Hg0 FGD Inlet Hg0 Stack Reemission

Hg(0)1 Hg(2+)2 Hg(VT)3 Hg(0) Hg(2+) Hg(VT) %

5-Jun, 2008

13:35 2.30 15.30 17.6 7.78 0.88 8.66 238

8-Jun, 2008

10:10 1.79 16.42 18.21 9.03 1.22 10.25 404

13:03 1.39 17.22 18.61 8.01 1.06 9.07 476

10-Jun, 2008

7:00 2.27 17.45 19.72 10.35 1.27 11.62 356

In this study, depending on electricity demand, the power station was operated under several different scenarios, summarized in Table 12, during the testing period. The operation of the two boilers and the common FGD modules was not under the same conditions among each scenario, for example, the load of each boiler, the flow rate of oxidation air, the load of reagent slurry, the discharge rate of scrubber slurry, and the height of the slurry in each module might change during each scenario.

Removal Efficiency

Removal Efficiency

21

Table 12. Power Plant Operation Scenarios during Full-scale FGD Operation Testing Period.

Boiler FGD Scrubber

Operation Scenario

Unit 1 Unit 2 Module A Module B

1 2 3 4 5 6

Figure 7 demonstrates the averages of the Hg concentrations collected by the Hg CEM system from each operation conditions described in Table 12. As shown in the figure, the two highest mercury emissions was found during 7/25 00:00-7/25 14:25 and 7/29 14:00-7/31 20:00, during which the power plant was operated under Scenario 6 with about 8000 scfh of oxidation air flowing through Module B scrubber. However, with a higher oxidation air flow rate of 11500 scfh, the concentration of Hg at the stack was found about 1/3 under the same boiler operation scenario. The lowest Hg emission was observed from in the Operation Scenario 3 when only Unit 1 was in operation with a total of 17000 scfh of oxidation air running through both modules. The observations suggested the oxidation air flow rate was important in controlling the reemission of Hg when only Unit 1 is in operation. While the power plant was operated with both boilers and the two FGD scrubber modules (i.e., 7/25 16:00-7/29 12:00, 8/3 0:00-8/5 21:15, and 8/7 13:50-8/10 7:30), the concentration level of Hg remained relatively constant of 3.5 g/dscm. The figure demonstrates that the reemission of mercury was greatly affected by the boiler and FGD module operation conditions.

The correlation between the load of boilers and mercury emission can be seen in Figure 8. As shown, with an exception of the spike observed at the total of over 400 MWe, higher stack mercury concentrations were normally observed when the facility was operated under lower load conditions, especially when only Unit 1 was in operation.

In addition to the observation found in Figure 7, both laboratory and Plant C Units 31/32 testing results also suggested that oxidation air flow rate was one of the potential factors that affects the reemission of elemental mercury. The effect of oxidation air flow is, therefore, examined. To take the volume of FGD slurry in the modules into account, a A/S ratio was calculated, which is the ratio of oxidation air flow rate (i.e., ft3/minute) to total slurry liquid volume in the wet scrubber tank (i.e., ft3). The oxidation air flow showed significant effect when only Unit 1 was in operation. For example, as shown in Figure 9, the concentration of Hg remained constantly at the level of 2 g/dscm before the A/S ratio sharply decreased from above 1.2 to below 0.8. The concentration of Hg sharply increased to the level of 4 g/dscm when the A/S ratio decrease occurred. The increase in Hg emission was unlikely associated with the change of load. The boiler load slightly decreased from 145 to 120 MWe when the A/S ratio decreased. However, when the load returned to 145 MWe, the mercury concentration ration remained at the 1.2 g/dscm level.

22

Date/Time

7/25 00:00-7/25 14:25

7/25 16:00-7/29 12:00

7/29 11:55-7/29 14:00

7/29 14:00-7/31 11:20

7/31 11:25-7/31 20:05

7/31/21:00-8/2 20:50

8/3 00:00-8/5 21:15

8/5 21:20-8/6 1:15

8/6 1:20- 8/6 8:15

8/6 8:20-8/7 13:45

8/7 13:50-8/10 7:30

Load

, MW

e

0

100

200

300

400

Oxi

datio

n A

ir, S

CF

H

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

Hg

Em

issi

on,

g/

dsc

m

0

2

4

6

8

10

12

14

Unit 1 Load Unit 2 Load Module A FOC Module B FOC Hg(T)

Figure 7. Emission of mercury at the stack under different operation scenarios.

Plant Load, MW

100 200 300 400

Hg(

T)

Con

c.

g/ds

cm

0

5

10

15

20

25

30

35

Figure 8. Correlation between boiler load and mercury concentration in stack gas.

It was also observed that the oxidation air flow affected the reemission of mercury in three stages. As shown in Figure 10, during the period of when oxidation air was momentary turned off (i.e., A/S ratio decreased to 0), the emission of Hg at the stack decrease immediately. Once the oxidation air was turned on, the A/S ratio and mercury concentration started increasing. The concentration of mercury sharply decreased when the A/S ratio increased to a level above 1.15. Interestingly, as the A/S ratio started decreasing below 1.15, the mercury concentration increased. During this period, the boiler load remained relatively constant. The boiler load dropped from 150 MWe to 135

23

MWe at the midnight of 8/7 and came back to full load at 10:00. With a higher A/S ratio after the boiler was running at full-load, the mercury concentration remained at about 2 g/dscm.

The correlation between the oxidation air flow rate, which is demonstrated as OA index, and mercury emission concentration when only Unit 1 was in operation (90% or higher of full load of 150 MWe) is shown in Figure 11. The OA index was calculated by dividing the A/S ratio with boiler load. As shown, the higher OA index value the lower chance that high mercury emission can be found. However, the effect of oxidation air flow rate was not as significant when both units were in operation. As also shown in Figure 10, the correlation between the OA index and stack mercury concentration was not the same when only Unit 1 was in operation, indicating other factors might be more important when the utility was operated at higher load.

The other factor that might affect the reemission of mercury was the load of sulfur in the FGD scrubbers. As demonstrated in Figure 12, the Hg concentration shows an opposite trend as the SO2 concentration at the stack. For example, the Hg concentration suddenly decreased at 6:30 on 7/25. At the same time, a spike on stack SO2 concentration was also observed. The observation indicates the S concentration in the FGD slurry might also play important role in controlling mercury reemission. To evaluate the effect, “load of sulfur in FGD slurry”, which was the total mass of sulfur that was removed by FGD process in a period of time, was calculated and was plotted as a function of mercury concentration. Figure 13 demonstrates the correlation between the load of sulfur in FGD wet scrubber (i.e., kilogram of sulfur per cubic meter of FGD slurry per hour) and mercury concentration at the stack when (1) Unit 1 was over 90% of full load and (2) both Units 1 and 2 were over 90% of full load. A general trend showing the lower sulfur load the higher chance that high mercury concentration can be observed at the stack is found. Same observation was found in a bench-scale studies carried out by Blythe et al. (2008) and Chang and Ghorishi (2003). Both studies suggest that the reemission of Hg0 can be reduced by increase the concentration of sulfite in the slurry.

The effect of sulfur load on the reemission of mercury might be likely associated with the mercury-sulfur compounds formed in the slurry. The main reaction steps for SO2 to be removed from the flue gas in a forced oxidation FGD process can be written as (Liu and Xiao, 2006):

)aq()g( OHSOOHSO 2222 eq.2 HHSOOHOHSO )l()aq( 3222 eq.3

HSOOHHSO )aq(2

323 eq.4

OHCaSOSOOHCaCO 232

323 5.05.0 eq.5 2

422

3 22 SOOSO eq.6

OHCaSOSOOHCaCO 242

423 22 eq.7

24

Date/Time

8/6 1:00

8/6 3:00

8/6 5:00

8/6 7:00

Hg

Con

c.

g/ds

cm, 2

0oC

loca

l O2

0

2

4

6

8

Ga

s/L

iqu

id R

atio

, ft3

air

/ft3 li

quid

/min

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Loa

d, M

We

0

100

200

300

400

Hg(0)Oxidation Air/Liquid Ratio of Module AOxidation Air/Liquid Ratio of Module BUnit 1 Load

load remains constant

Oxidation air/liquid ratio decreased

Oxidation air flow was decreased to half

Figure 9. Change of mercury emission when the oxidation air flow decreased.

Date/Time

8/6 8:00

8/6 16:00

8/7 0:00

8/7 8:00

HgC

onc.

g/

dscm

, 20

o C lo

cal O

2

0

2

4

6

8

10

Gas

/Liq

uid

Ra

tio, f

t3 air/

ft3 liqu

id/m

in

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Lo

ad, M

We

0

50

100

150

200

Hg(T)Oxidation Air/Liquid Ratio of Module AOxidation Air/Liquid Ratio of Module BUnit 1 LoadUnit 2 Load

no forced oxidation,no re-emission

Due to sudden change of load

The index reach 1.1

Figure 10. Effect of oxidation air flow on mercury emission.

25

Oxidation air flow rate index,ft3/min/ft3/MWe

0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016 0.018

Hg

Con

cent

ratio

n,

g/ds

cm

0

2

4

6

8

10

12

14

16

18

20

Unit 1>90% of full loadBoth Units >90% of full load

Figure 11. Correlation of oxidation air flow index and mercury emission when Unit 1 and/or Unit 2 were in operation with 90% of full load.

The oxidized mercury may react with sulfite ion and form mercuric-sulfur complex, such as sulfite (i.e., eq. 8), which is not stable and decomposed into elemental mercury, where the reemission of mercury occurs.

HHSOHgHgSOHgSO OH43

223 )0(2 eq.8

However, at the same time, a more stable form, such as mercuric disulfite complex, might also form (eq. 9).

223

2232 )SO(HgHgSO eq.9

With higher sulfur load in the FGD scrubber, more mercuric disulfite (Blythe et al., 2008) might form and reduce the presence of mercuric sulfite. As a result, less reemission was observed. In addition, with higher load of sulfur, disproportionation of sulfite might also occur (eq. 10). The reduced sulfur ions may react with mercury and form mercuric sulfide (eq.11), which is highly insoluble and could be removed from the liquid phase reaction as a solid (Blythe et al., 2008, Hargrove et al., 1997).

224

23 34 SSOSO eq.10

HgSSHg 22 eq.11

These possible mechanisms can also be applied to explain the effect of oxidation air flow rate on the reemission of elemental mercury. While no oxidation air is available, all SO2 removed from the flue gas become sulfite (SO3

-2) at the operation pH, which promotes the formation of mercuric disulfite, and therefore, less or no mercury reemission as observed in Figure 10. With insufficient oxidation air, the available oxygen might react

26

with mercuric disulfite and form mercuric sulfite or compete with mercury for sulfite, which inhibits the formation of mercuric disulfite and promotes reemission. However, when the oxidation air flow rate increases, the oxidation of sulfite further inhibits the formation of mercuric sulfite under lower sulfur load condition, which might have resulted in the phenomenon observed in Figure 10. But when both units were running, the sulfur load increased and the supplied oxygen was not sufficient to have the same effect as at a lower sulfur load when only Unit 1 was in operation.

Date/Time

7/25 0:00

7/25 2:00

7/25 4:00

7/25 6:00

7/25 8:00

7/25 10:00

7/25 12:00

7/25 14:00

HgC

onc

. g

/dsc

m, 2

0oC

loca

l O2

0

5

10

15

20

25

30

35

SO

2 In

let a

nd

ou

tlet,

pp

mv

0

100

200

300

400

500

600

700

2000

3000

Lo

ad

, MW

e

0

50

100

150

200

Hg0

HgT

SO2 at FGD inlet

SO2 at Stack

Unit 1 Load

Figure 12. Correlation of oxidation air flow index and mercury emission.

Sulfur Load,kg/m3/hr

5 10 15 25 30 35

Hg

Con

cent

ratio

n,

g/d

scm

0

2

4

6

8

10

12

14

16

18

20

Units 1 and 2 >90% of full loadUnit 1>90% of full load

Figure 13. Correlation of FGD sulfur load and mercury emission.

27

Results obtained from Plant C Units 31/32 study suggest that slurry retention time might be one of the potential factors affect the reemission of mercury. The slurry retention time was calculated from the slurry volume in activate FGD module and the discharge of absorber slurry. As shown in Figure 14, the variation of FGD slurry discharge did not have observable effect on the emission of mercury, which indicates the reaction time for reemission to occur was likely shorter than the retention time.

Slurry Retention Time,hr

0.1 1 10 100 1000 10000

Hg

Con

cent

ratio

n,

g/d

scm

0

2

4

6

8

10

12

14

16

18

20

Unit 1>90% full loadCol 14 vs Hg(T) Units 1,2, >90%

Figure 14. Correlation of slurry retention time and mercury emission.

The correlation between the slurry pH and mercury emission can be seen in Figure 15. As shown, the pH range of the slurry was mostly maintained in a range from 5.5 to 5.7, where high mercury concentrations were observed. No mercury emission higher than 4 g/dscm was observed when pH was lower than 5.3.

pH of the Slurry

5.0 5.2 5.4 5.6 5.8 6.0

Hg

Con

cen

tra

tion

, g

/dsc

m

0

2

4

6

8

10

12

14

16

18

20pH of Module A slurry at Unit 1>90% full loadpH of Module B at Unit 1>90% of full loadpH of Module A slurry at Units 1 and 2 >90% of full loadpH of Module B slurry at Units 1 and 2 >90% of full load

Figure 15. Correlation of slurry retention time and mercury emission.

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FGD Operation Simulation

The simulation of FGD operation on Hg0 reemission was performed based on the parameters listed below.

Inlet boundary conditions (experiment data at 7/28/09 13:10): Mass flow rate: 304.4 kg/s Temperature: 433.4 K H2O concentration: 0.059 (mass fraction) CO2 concentration: 0.0289 (mass fraction) O2 concentration: 0.0277 (mass fraction) Hg2+ concentration: 1.81 x 10-4(mass fraction) (14 µg/m3/s) Source terms in the FGD:

Energy: -49000*ln(8.197 x 10-4*(T-293)+1) CO2: 0.0 H2O: 0.005163 kg/m3/s

The three-dimension model and CFD mesh set up can be seen in Figure 16. The 3-D model includes the absorber and inlet, outlet duct pipe. It has the same geometry as the drawing. No spray nozzles and support pipes were modeled. The mist eliminators were simulated by the porous material. The mesh includes 823397 cells which are comprised of tetrahedral and hexahedral type cells. Most of the cells have the hexahedral type except the elbow which connects the absorber to the outlet duct. The tetrahedral type cells are chosen in the elbow because of complicated geometry. In the simulation, absorber was divided into different zones, such as the zone above the spray nozzles, no mercury reemission was simulated. The bottom of the absorber is the reservoir of the slurry, whose volume was not included in the CFD simulation.

Navier-Stockes equations, energy equation and species transfer equations were applied. Mercury reemission rate calculation was based on the solution of a transport equation for species mass fractions. The following is the specie equation related the Hg reemission:

hg hg hg hgY vY J St

eq. 13

,hg hg m hgJ D Y in laminar flow case, or ,t

hg hg m hgt

J D YSc

in turbulent

flow case. 39.9

2.35 RThg oS C e

eq. 14

Where Yhg is the Hg concentration in the flue gas. J is the diffusion flux of Hg specie. Dhg,m is the diffusion coefficient for Hg in the mixture. Shg is the net rate of production of Hg by chemical reaction. Sct is the Schmidt number which equal to 0.7 in default. Co is the mercury concentration in W-FGD system. In the current numerical model, the parameter Co was assumed as a constant, that means the oxidized mercury is uniformly distributed in the whole absorber. The value of the Co was assumed as a known parameter during the simulation. The spray vaporization is simulated by following ways:

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a. The flue gas temperature decreased due to vaporization of the water in the slurry

was simulated by a heat sink source in the energy equation. The value of the heat sink was calculated from the water latent heat. This number was assumed to be a constant in the absorber.

b. The spray vaporization was simulated by adding a mass source term to the H2O mass transfer equation. The value of this source term can be calculated from the temperature difference between inlet and outlet.

The standard k-ɛ turbulence model was chosen for it's robustness, economy, and reasonable accuracy for a wide range of turbulent flows. The second-order upwind schemes were used for discretization of the governing equations to provide a high accuracy scheme. The SIMPLEC algorithm was used for the pressure-velocity coupling. Since the mercury concentration is much less than other major species such as H2O, O2, the simulation starts with the solving momentum, energy and O2, H2O species equations. After a convergent result was reached, the Hg reemission transfer equation was solved separately. The current simulation results can be seen in Figure 17. ICSET applied data observed from the FGD operation effect on the reemission of Hg0 study to perform the simulation work.

Figure 16. CFD three-dimension model and mesh set up.

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Figure 17. CFD simulation results.

Task Three: Full-Scale WFGD Testing at Plant S Unit 4 to evaluate if reemission of elemental mercury occurred when the FGD process was changed from natural to forced oxidation

To evaluate the change of SICP Unit 4 FGD process from natural to forced oxidation operation on the mercury emission, ICSET carried out mercury measurement using Ontario Hydro method, a continuous emission monitoring system, and a sorbent trap method to continuously monitored the stack mercury emission. The first series of measurement was carried out from 8/5 to 8/6/2008 before the FGD process was undergone modification. The results obtained from the first series of the measurement can be seen in Table 13 and Figure 18. The second series of the measurement was carried out after the modification, which was started from 6/19 and continued until the end of November, 2009. The results are shown in Table 13 and Figure 19.

As shown in the table, there is no observable difference in the stack mercury emission between the two series of measurements. Although no change on the mercury emission behavior was observed before and after the modification of the FGD process, the effect of change of oxidation operation was not conclusive. The FGD process encountered several operation difficulties after the modification,which are summarized in Table 14. The FGD slurry dewatering system was not in operation until the middle of June. Also, during this period, it was found that the oxidation air blowers were incorrectly installed. During the period from June to August, 2009, low FGD gypsum quality was observed. ICSET carried out FGD performance test and helped the plant identify the problem. It was found that the over-dosed sodium formate additive inhibited the sulfite oxidation process in the scrubber, which resulted in low FGD gypsum purity. After a month long boiler annual outage (10-11/2009), the thickner of the FGD process was not in operation. Some modifications on the thickeners are needed. As shown in Table 14, the Plant S Unit 4 boiler was not operated under normal condition during the testing period.

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Table 13. Mercury Emission Results from OHM Measurement before and after FGD Modification at Plant S Unit 4.

FGD inlet Stack Hg(T) Hg0 Hg(T) Hg0

3%, 20oC, Dry mg/dscm Before Modification

8/6/2008 Run 1 8.57 2.14 1.28 0.97 8/6/2008 Run 2 7.62 0.96 1.30 1.19 8/6/2008 Run 3 7.92 0.75 1.19 1.04

Average 8.04 1.28 1.25 1.07 Std. Dev. 0.48 0.75 0.06 0.11

After Modification 6/16/2009 Run 1 12.65 1.21 0.77 0.60 6/19/2009 Run 2 11.40 0.46 0.49 0.42 6/19/2009 Run 3 12.13 0.56 0.89 0.74 6/19/2009 Run 4 10.95 0.77 1.12 0.96 6/19/2009 Run 5 8.92 0.76 0.99 0.93

Average 11.21 0.75 0.85 0.73 Std. Dev. 1.44 0.29 0.24 0.23

Table 14. Operation Difficulties occurred in Plant S Unit 4 after FGD Process Modification.

Period Operation Difficulties

1/2009-5/2009 1. Dewatering system not in operation 2. Oxidation air pump malfunction

6/2009-10/2009 1. Low FGD gypsum purity 2. FGD pH meter malfunction

11/2009-12/2009 1. Annual outage 2. Thickener malfunction

Date/Time

8/5 18:00 8/5 22:00 8/6 02:00 8/6 06:00 8/6 10:00 8/6 14:00 8/6 18:00

Hg

Con

cent

ratio

n,

g/ds

cm20

o C, 3

% O

2

0

2

4

6

8

10

Hg(T) at FGD inletHg(0) at FGD inletHg(T) at StackHg(0) at StackHg(T) at FGD inlet from OHMHg(0) at FGD inlet from OHMHg(T) at Stack from OHMHg(0) at Stack from OHM

Figure 18. Continuous emission monitoring results at Plant S Unit 4 before FGD modification.

32

Instrument Maintenance

Date/Time

6/19 0:00

7/3 0:00

7/17 0:00

7/31 0:00

8/14 0:00

8/28 0:00

9/11 0:00

9/25 0:00

10/9 0:00

10/23 0:00

11/6 0:00

11/20 0:00

Hg

Co

nc. m

g/d

scm

, 20o

C, l

ocal

O2

0

2

4

6

8

10

Hg(0)Hg(T)Sorbent trap Data

boiler tube leak

Boilershut down OutageInstrument

Maintenance

Instrument Maintenance

Figure 19. Continuous emission monitoring results at Plant S Unit 4 after FGD modification.

CONCLUSIONS AND RECOMMENDATIONS

Hg0 reemission across a lab-scale simulating scrubber was confirmed in the slurries of CaSO3 and Na2SO3 containing HSO3

-. Effects of operation conditions including temperature, pH and oxygen concentration on Hg0 reemission were investigated. Results showed that Hg0 reemission rates increased with increase of operation temperature. Hg0 reemission rates decreased with the increase of O2 concentration. Effects of additives including Na2S4, TMT15 and HI on the suppressing Hg0 reemission were evaluated in the simulated scrubber. Considering economy and environmental friendliness factors, TMT15 was an effective additive to suppress Hg0 reemission across wet FGD systems at coal-fired power plants.

A commercial available NaHS FGD additive showed it can effectively suppress the reemission of Hg0. With an injection rate of 120 gallon/hour, the reemission of Hg0 was reduced from 46% to no reemission at Plant C Unit 33. A more effective injection rate was likely to be achieved by injecting the additive in the location where better mixing of the solution and slurry can be obtained. In addition to chemical additive, modification of FGD process was found to be able to reduce Hg0 reemission. By comparing the results obtained from the previous 2007 study and this study, the Hg emission at the Units 31/32 common stack was reduced significantly even without the injection of NaHS. The PI data obtained from Plant C Units 31/32 between the two studies shows that the oxidation air was

33

increased 50% with an addition oxidation air pump. Due to better oxidation in the slurry, the slurry discharge rate was also increased. The effect of oxidation air on the reemission of Hg0 was also observed at the full-scale testing carried out at Plant O. It was observed that the oxidation air flow affected the reemission of Hg0 in three different ways; (1) during the period of when oxidation air was momentary turned off, the emission of Hg at the stack decrease immediately; (2) Once the oxidation air was turned on, mercury concentration started increasing until the oxidation air flow increased to a certain flow rate; (3) When the oxidation air flow rate increase to a certain level, the mercury concentration decreased sharply. However, the effect of oxidation air flow rate seemed to be more significant when only Unit 1 was in operation. When both units were in full-load, the correlation between oxidation air flow rate and mercury emission was not observable, which is likely due to a higher load of sulfur in the scrubber slurry. The distribution of different mercuric-sulfite complexes (e.g., mercuric sulfite and mercuric disulfite) under different sulfur load and oxidation air flow rate was likely the controlling mechanism. The Injection of NaHS might affect the the concentrations of vapor phase selenium at the stack. It was found varied during NaHS injection and seemed to be affected by the injection rates. The injection of NaHS noticeably increased the concentration of vapor phase Se at the stack. Further investigation is necessary to verify the effect. Two series of continuous mercury measurement were carried out before and after the FGD process of Plant S Unit 4 was modified from natural to forced oxidation process. No observable difference in the stack mercury emission between the two series of measurements. Although no change on the mercury emission behavior was observed before and after the modification of the FGD process, the effect of change of oxidation operation was not conclusive. During the year-long continuous monitoring, the FGD process was not able to produce high purity gypsum, indicating the system was not in optimum operation condition.

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REFERENCES

Blythe G., Currie J., and DeBerry D., Bench-scale Kinetic Study of Mercury Reactions in FGD liquors, Final Report, DOE-NETL DE-FC26-04NT42314, Pittsburgh, PA, 2008.

Chang, J. and Ghorishi, S.B., “Simulation and Evaluation of Elemental Mercury Concentration Increase in Flue Gas Across a Wet Scrubber,” Environ. Sci. Technical, 2003, 37, 5763-5766.

Ghorishi, B., Downs, B., and Renninger, S., Role of Sulfide in the Sequestration of Mercury by Wet Scrubbers, Presented in EPRI-DOE-EPA-AWWA Combined Power Plant Air Pollution Control Mega Symposium, August 28-31, 2006, Baltimore, MD.

Hargrove, O.W. Jr., T.R. Carey, C.F. Richardson, R.C. Skarupa, F.B. Meserole, R.G. Rhudy, and T.D. Brown. “Factors Affecting Control of Mercury by Wet FGD,” Presented at the EPRI/DOE/EPA Combined Utility Air Pollutant Control Symposium, Washington, DC. August 1997.

Liu, S-Y and Xiao W-D, Modeling and simulation of a bubbling SO2 absorber with granular limestone slurry and organic acid additive, Chem, Eng., Tehnol., 2006, 29, 1167-1173.

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DISCLAIMER STATEMENT

This report was prepared by Wei-Ping Pan & Institute for Combustion Science and Environmental Technology (ICSET) of Western Kentucky University (WKU) with support, in part, by grants made possible by the Illinois Department of Commerce and Economic Opportunity through the Office of Coal Development and the Illinois Clean Coal Institute. Neither Wei-Ping Pan & ICSET of WKU, nor any of its subcontractors, nor the Illinois Department of Commerce and Economic Opportunity, Office of Coal Development, the Illinois Clean Coal Institute, nor any person acting on behalf of either:

(A) Makes any warranty of representation, express or implied, with respect to the accuracy, completeness, or usefulness of the information contained in this report, or that the use of any information, apparatus, method, or process disclosed in this report may not infringe privately-owned rights; or

(B) Assumes any liabilities with respect to the use of, or for damages resulting from the use of, any information, apparatus, method or process disclosed in this report.

Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring; nor do the views and opinions of authors expressed herein necessarily state or reflect those of the Illinois Department of Commerce and Economic Opportunity, Office of Coal Development, or the Illinois Clean Coal Institute.

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