Prediction Hg Removals With Activated Carbon Injection in Utility Gas Cleaning Systems

Transcription

1 Prediction Hg Removals With Activated Carbon Injection in Utility Gas Cleaning Systems Paper No. 15 Balaji Krishnakumar and Stephen Niksa Niksa Energy Associates LLC, 1745 Terrace Drive, Belmont, CA ABSTRACT This paper validates the predicted Hg removals from NEA s MercuRator package for cases in the Phase II NETL Hg field test database with activated carbon injection (ACI). Datasets from six full-scale sites characterized ACI with Darco Hg into one hot-side electrostatic precipitator/fabric filter (HESP+FF) combination (Toxecon -I), four coldside ESPs (CESPs), and one into the plate banks in a CESP (Toxecon -II). Datasets from three sites characterized ACI with brominated Darco Hg-LH into two CESPs and one ToxeconÔ-II configuration. The measured Hg removals varied from 7 to 95 %, yet the predictions were uniformly accurate to within the measurement uncertainty across the domain of test conditions, and depicted the test-to-test variations at all but one of the sites. Hg removals were predicted within 12.6 % (std. dev.), excluding the evaluation with the data from Conesville, which had large systematic discrepancies. MercuRator s satisfactory performance does not rest upon parameter adjustments. None of the parameters in the Hg/Cl in-flight reaction mechanism were changed from the values set in NEA s interpretation of the Phase I NETL database. Hence, MercuRator can now be recognized as a truly comprehensive mechanistic framework for Hg transformations throughout the complete domain of utility gas cleaning conditions. INTRODUCTION The electric power industry in the U.S. will soon face regulations on Hg emissions. Even though EPA s Clean Air Mercury Rule (CAMR) was vacated by the U.S. Court of Appeals of the D.C. Circuit, the EPA is required to formulate regulations for Hg emissions under section 112 of the Clean Air Act. Also, several states have enacted laws or proposed legislation to reduce Hg emissions from coal-fired power plants. Activated carbon injection (ACI) in the flue gas is currently the most important Hg control strategy in utility gas cleaning systems, especially for plants firing low rank coals. Since these coals have inherently low Cl and generate very little unburned carbon (UBC) in their flyash, only minimal amounts of Hg are inherently captured on particulates and removed in ESPs. ACI directly alleviates the difficulties for low UBC levels, and the scarcity of Cl can be remedied by either co-injecting halogens with the coal or by using a halogenated carbon in the ACI. Even when these issues are managed, ACI performance can be highly variable. Several tests reported in NETL s Phase II Hg field-test database suggested that SO 3 in the flue gas, either in situ or added as a flue gas conditioning agent, 1

2 may have a significant detrimental effect on Hg removal by ACI. The testing did not demonstrate whether a threshold SO 3 concentration determined whether or not SO 3 interference would come into play. Compounding matters, measurements of SO 3 concentrations are subject to large uncertainties and are almost never reported with Hg speciation in field tests. To address these issues quantitatively and to help utility operators identify the best Hg control strategy for their diverse assortments of coals and gas cleaning configurations, NEA developed MercuRator, a collection of detailed chemical reaction mechanisms that describe the essential Hg transformations in utility gas cleaning systems, and can accurately rank-order any of the most popular Hg control options. This paper completes our demonstration of MercuRator s performance across the full range of ACI configurations and applications. Following our quantitative interpretations of NETL s Phase I Hg field test database 1, which includes several ACI performance evaluations, we developed a mechanism for SO 3 production and integrated it into MecuRator to quantify SO 3 inhibition on Hg removals by UBC and ACI, and to interpret NETL s Phase II test data 2. Quantitative interpretations of the test data with halogen injection and halogen + ACI were reported for applications with Cl 3 and Br 4 species, so this paper focuses on the tests with ACI only with conventional (Darco Hg) and Br-enhanced (Darco Hg-LH) activated carbons under different injection configurations and SO 3 levels. The validation cases reported here cover gas cleaning configurations with ESPs; fabric filters (FFs); SCR/ESP combinations, and ESP/FGD combinations. VALIDATION DATABASE Table 1 shows the validation database for this study of 66 tests at 6 plants representing 3 gas cleaning configurations classified as PCD only, SCR+ESP, and ESP+FGD. Fortyfive tests at 6 plants evaluated Darco Hg at various injection locations, including 5 tests at Presque Isle applying Toxecon-IÔ with ACI between a HESP and FF, and 4 tests at Independence applying Toxecon-IIÔ with ACI within the ESP plates. Of the 21 cases with Darco Hg-LH addition, 5 tests at Independence applied Toxecon -II and the remaining 16 cases had ACI upstream of the ESP. The data were qualified wherever possible by ensuring that the total Hg inventory was uniform at all sampling locations upstream of the first PCD, and also with several consistency checks on Hg speciation along the gas cleaning system. The best evaluation of the Hg inventory is as the average of total measured Hg at multiple sampling locations ahead of the first Hg removal position. Any test with total Hg that exceeded the inventory by ± 20 % was discarded. Tests with increases in Hg 0 levels across an ESP greater than 20 % were also discarded. Of the 66 tests in this database, only 20 from Meramac and Monroe included Hg speciation measurements whereas the remaining tests from Daniel, Conesville, Independence, Presque Isle, and three tests each at Monroe and Meramac gave only total Hg removals. Consequently, no data qualification was possible for the latter six sites. Even for tests that reported Hg speciation data, a Hg inventory could not be evaluated from more than one measurement location prior to the first PCD. The inordinately low measured Hg removals at Conesville for high ACI concentrations 2

3 Table 1. The Ph. II NETL validation database on external Hg controls. External Hg Control ACI: Power Plant Configuration Injection Location [ACI] (lb/mmacf) Darco Hg Meramac ESP Upstream of ESP Monroe SCR+ESP Upstream of ESP 1 6 Daniel ESP Upstream of ESP 3 9 Conesville ESP+FGD Upstream of ESP Presque Isle HESP+FF Upstream of FF (Toxecon -I) Independence ESP Within ESP 1 8 (Toxecon -II) Darco Hg-LH Meramac ESP Upstream of ESP Daniel ESP Upstream of ESP 3 9 Independence ESP Within ESP (Toxecon -II) 1 9 coupled with large temperature stratification across the ESP prevent us from satisfactorily applying the SO 3 inhibition mechanism at this site. SIMULATION PROTOCOL MercuRator predicts complete Hg speciation at the outlets of all units and air pollution control devices (APCDs) from the furnace outlet to the smokestack, including back-end heat exchangers, SCRs, APHs, ESPs and FFs, wet FGDs and SDAs. Gas cleaning configurations can be re-configured at will to easily assess the performance benefits of adding an SCR and/or FGD, or of applying ACI. We simulate each individual test based on the coal quality and gas-cleaning conditions in effect while the Hg speciation data were recorded, to the extent possible. Users must provide the following input data: The fuel properties consist of a proximate analysis and an ultimate analysis expanded with the Hg- and Cl-contents. A nominal flue gas composition is assigned from the fuel properties, fuel feedrate, and an economizer O 2 level. A continuous thermal history is constructed from the temperatures recorded at all Hg sampling locations; temperatures upstream of the measurement positions and transit times are almost always assigned from default specifications or in calibration adjustments. UBC characteristics are assigned from LOI measurements and coal quality, and are also evaluated as a continuous function of residence time along the gas cleaning system. The flyash loading is determined by subtracting away the bottom ash, and expressed as a suspension loading in the whole flue gas. LOI is first used to estimate UBC, and then added to the flyash loading. Sorbent injections are added to the flue gas at the residence time associated with the injection location. Hence, all the necessary gas cleaning conditions are specified as continuous functions of residence time along the entire gas cleaning system. The sets of differential equations associated with the elementary reaction mechanisms 3

4 are solved as one-dimensional initial value problems in transit time, beginning with the equilibrium flue gas composition at the furnace exit. Each simulation of a full-scale gascleaning system takes several seconds on a mobile workstation operating at 2.9 GHz. For most of the tests in this study, test-specific coal-cl levels were not reported. Variations in the flue gas Cl level could factor into the interpretations for the parametric and long-term tests at Monroe Unit 4 (MORU4P1-P3 & MORU4LT2); for the tests at Daniel when fired with a 60/40 bituminous/sub-bituminous blend (DSU1PHG1-3) and an 80/20 blend (DSU1PHG4a-6b). In addition to Cl, LOI, and ACI rates, the effect of SO 3 on the capture of Hg by ACI was also evaluated. NEA s SO 3 formation mechanism is first used to estimate the SO 3 concentration along the gas cleaning system, accounting for SO 2 oxidation across a SCR and any injected SO 3. The acid dew point is then evaluated for the flue gas moisture and SO 3 concentrations. Whereas most APHs and PCDs operated with flue gas temperatures above the acid dew point, the inhibition of Hg removal by SO 3 condensation on ACI was apparent in several cases at Monroe Unit 4 and Daniel Unit 1 where the operating temperature fell below the acid dew point. Parameters in the thermal histories were not adjusted to compensate for any defects in the predicted performance of any of the different Hg control strategies. Moreover, none of the reaction rate parameters in any of the chemical reaction mechanisms were adjusted from their previously assigned values in any of the cases reported here. SIMULATION RESULTS The predicted overall Hg removals will be expressed as the following percentage: C C = 100 Hg T Hg - C OUT T Hg C Hg where C Hg T is the total Hg inventory and OUT C Hg is the fractional level of Hg 0 and Hg 2+ vapors at the outlet of the last PCD in the cleaning system. Meramac Unit 2 This unit consists of a T-fired furnace with an ESP only configuration burning a PRB coal with less than 100 ppmw coal-cl and an inherent LOI of 0.8 wt. %. The ESP operated above the calculated acid dew point. Both Darco Hg and Darco Hg-LH were injected upstream of the ESP. The predicted Hg removals are evaluated with the measured values in Fig. 1. The measured Hg removals in Fig. 1 increase from 45 to 75 % as the Darco Hg ACI concentration was increased from 0.6 to 10 lb/mmacf, and saturate at an ACI concentration of 3.2 lb/mmacf, at which point increasing the ACI concentration to 10 lb/mmacf removes only about 7 % more Hg. At ACI concentrations below 2.0 lb/mmacf, the Hg removals are under-predicted by % but are within ±10 % for the higher sorbent injection concentrations. The discrepancy between measured and predicted Hg removals may be partly due to the absence of test-specific Cl values, because slightly higher Cl levels at the lower ACI concentrations would enhance 4

5 Figure 1. Comparison of measured (hollow bars) and predicted (solid bars) Hg removals (%) for the Darco Hg (left five) and Darco Hg-LH (right three) ACI tests at Meramac. the predicted Hg removals. In comparison to the Darco Hg, both measured and predicted Hg removals using Darco Hg-LH are significantly higher. Whereas the highest Hg removal of 76 % is achieved by injecting Darco Hg at a concentration of 10 lb/mmacf, only 3 lb/mmacf of Darco Hg-LH resulted in greater than 90 % Hg removal. The predicted Hg removals are typically within 20 % of the measured values, which is better than the predictions for Darco Hg at the lower ACI concentrations. Monroe Unit 4 Unit 4 at Plant Monroe consists of a wall-fired furnace with an SCR+ESP combination. The plant fires a 60/40 blend of PRB and bituminous coals with coal-cl varying from 760 to 980 daf ppmw. Test-specific coal-cl values were available and the variation in Cl levels does factor into the interpretations of this dataset. The plant employs SO 3 and NH 3 flue gas conditioning for particulate capture and the measured SO 3 concentration of 13.7 ppmv confirmed the predicted value of 14.2 ppmv. The calculated acid gas dew point was 140ºC which was higher than the ESP inlet temperature of 125ºC; therefore, SO 3 interfered with Hg capture on ACI under these conditions. The inherent LOI at this site was 3.2 % and the baseline removals ranged from 18 to32 %. Darco Hg was injected upstream of the ESP. The measured and predicted Hg removals are compared in Fig. 2. Notice that flue gas HCl concentrations appear on the left axis. The predicted Hg removals compare well with the measured values for three of the four ACI concentrations. At the higher ACI concentrations the predictions are within 2-15 % of the measured values. The higher measured Hg removal during the long-term test MORU4LT2 (5 lb/mmacf ACI) compared to the brief parametric test MORU4P3 (6lb/MMacf ACI) is partly due to the higher HCl level in the former test. Given that there 5

6 Figure 2. Comparison of measured and predicted Hg removals (%) for Darco Hg ACI at Monroe. was sufficient HCl in the gas phase in both tests, the higher Hg removal in the long-term test could be due to carbon build-up along the gas cleaning ductwork. In additional MercuRator predictions without any inhibitory effect of SO 3, the Hg removals exceeded 90 % with even the lowest ACI concentration of 1 lb/mmacf. Clearly, SO 3 interference was important in this ACI evaluation. The discrepancy for the lowest ACI concentration at MORU4P1 does not reflect a problem with the predicted ACI performance; rather, it is due to an over-prediction for the inherent Hg removal. The inherent LOI of 3.2 % and the elevated HCl levels would not remove much Hg if this was a bituminous-fired gas cleaning system. But for a blend of mostly PRB coal, the UBC surface area would be almost five times greater than the area of bituminous-derived UBC. Consequently, MercuRator over-predicts the inherent removal, perhaps because the actual UBC surface area was lower than our assignment. Since test-specific LOI values were not reported, and since the predictions are accurate the all three higher ACI concentrations, an LOI variation could also be responsible for the poor prediction at the lowest ACI concentration. Daniel Unit 1 Daniel Unit 1 is a T-fired furnace with an ESP only cleaning configuration. The plant fires a 60/40 or 80/20 blend of bituminous/prb coals with the blends having average coal-cl of approximately 100 and 430 daf ppmw, respectively. Only an average inherent LOI of 3.2 % was reported. Test-specific coal-cl were not available. The measured baseline Hg removal at this site was 5 % when firing a 60/40 bituminous/prb blend and unknown for the 80/20 blend. The calculated baseline SO 3 concentrations at Daniel were 7 and 9 ppmv, respectively, for 60/40 and 80/20 coal blends and no measured SO 3 concentrations were reported. Both Darco Hg and Darco Hg-LH were injected upstream 6

7 Figure 3. Comparison of measured and predicted mercury removals (%) for Darco Hg (left) and Darco Hg-LH (right) tests at Daniel. SO3+ represents SO 3 added to the baseline flue gas. of the ESP at concentrations of 3, 5 and 9 lb/mmacf. Tests at Daniel evaluated SO 3 interference on Darco Hg performance by addition of 6 ppmv SO 3 upstream of the ESP in all cases involving the 60/40 blend and in select cases involving the 80/20 blend. For the Darco Hg-LH tests, SO 3 was injected upstream of the ESP at a concentration of 6 ppmv in cases involving the 80/20 blend, and at 3 and 6 ppmv for the 60/40 blend. The calculated acid dew point of 137ºC is greater than the temperatures across the ESP. The reported temperatures at the ESP inlet and outlet were 137 and 121ºC for the 60/40 blend, and 141 and 130ºC for the 80/20 blends. The measured and predicted Hg removals are compared in Fig. 3 for the (left) Darco Hg and (right) Darco Hg-LH tests. The tests involving Darco Hg, labeled as DSU1PHG1 3 in Fig. 3, fired the 60/40 blend while the rest fired the 80/20 blend. The suffixes a and b in the 80/20 blend test labels (DSU1PHG4 6) represent 0 and 6 ppmv added SO 3, respectively. For the Darco Hg-LH injection tests, all tests fired the 60/40 blend except DSU1PLH4a b, which fired the 80/20 blend. The suffixes a, c and b in the test labels in Fig. 3 (right) represent 0, 6 and 3 ppmv added SO 3, respectively. For the tests DSU1PLHI1 3, the ACI injection location was shifted upstream resulting in additional sorbent residence time. For the Darco Hg tests with the 60/40 blend (DSU1PHG1 3), the predicted Hg removals are within 5 % of the measured values. The measured Hg removals increased from 10 to 29 % while the predicted values increased from 9 to 32 % as the ACI concentrations were increased from 3 to 9 lb/mmacf. With no SO 3 interference, MercuRator predicts that the Hg removals would have increased from 28 to 38 % as the ACI concentration were increased from 3 to 9 lb/mmacf, which is % greater than measured values. For the Darco Hg tests involving the 80/20 blend (DSU1PHG4 6) with and without added SO 3, the predicted Hg removals are within 10 % of the measured values for four of the six cases. The extent of inhibition of Hg capture by ACI due to SO 3 is accurately predicted at all ACI concentrations while the actual removals are over-predicted by at 7

8 least 15 % for 9 lb/mmacf. Whereas the measured extents of SO 3 interference hardly change with ACI concentration, the predictions show progressively less interference for increasing ACI concentrations. For the cases involving Darco Hg-LH, the predictions are accurate for all cases except two tests involving the 80/20 blend. Although the predictions show the inhibitory effect of SO 3 for these tests, the predicted Hg removals are significantly greater than the measurements. This discrepancy can only be explained if the actual operating temperatures were lower than the nominal values by at least 10 degrees, which promotes stronger inhibition by SO 3. The predicted extents of Hg removal were within approximately 10 % of the measured values for all the remaining tests. MercuRator clearly predicts the extent of inhibition of SO 3 on Hg capture by Darco Hg-LH at all three ACI concentrations and three added SO 3 concentrations for the 60/40 blend. Hg removals at tests DSU1PLHI1 3, where the sorbent injection location was shifted upstream, are also predicted within 15 % by accounting for the additional sorbent residence time in the flue gas. At comparable flue gas conditions, the brominated Darco Hg-LH resulted in 6, 9 and 27 % more Hg removal than the untreated Darco Hg at injection concentrations of 3, 5 and 9 lb/mmacf, respectively, in the presence of 6 ppmv added SO 3. The predictions at Daniel clearly show the accuracy of the mechanisms applied in MercuRator to predict SO 3 inhibition of Hg capture by ACI because the predictions are within 10 % of the measurements for seven of the nine tests involving Darco Hg and for nine of the thirteen involving Darco Hg-LH. These tests include added SO 3 upstream of the ESP and comprise a relatively broad range of Cl and ACI concentrations within one gas cleaning system. The correlation coefficient for the predicted removals for all tests at Daniel except two Darco Hg-LH tests with the 80/20 blend was 0.93 with a std. dev. of 8.7 % of the Hg inventory, which is well within the measurement uncertainties. Presque Isle (Toxecon -I) The gas cleaning configuration at Presque Isle treats the flue gas from three furnace units representing a 270 MW equivalent stream. The furnaces fire a subbituminous coal with an average coal-cl of 100 ppmw (daf) and the gas cleaning system is an HESP+FF combination. Darco Hg was injected between the HESP and FF representing a Toxecon -I configuration with an HESP particulate collection efficiency of 99.2 %. Mercury measurements at this plant were made only across the FF. The measured baseline Hg removal across the FF was 18 %, which appears excessive because essentially all of the particulate matter is captured in the HESP upstream of the FF. The inherent LOI was 0.7 %. The calculated baseline SO 3 concentration was 5.8 ppmv and no measured SO 3 concentration was reported. The FF operated above the calculated acid dew point, so SO 3 interference was minimal. Darco Hg was injected upstream of the FF. The measured and predicted Hg removals for Presque Isle are presented in Fig. 4. Predicted Hg removals are accurate at all ACI concentrations. The maximum deviation from the measured values is 7 %, which is well within the measurement uncertainties. The correlation coefficient was and the std. dev. was merely 1.5 % of the Hg 8

9 Figure 4. Comparison of measured and predicted Hg removals (%) for the Toxecon -I evaluations at Presque Isle with Darco Hg. inventory. Since most of the particulate is captured in the HESP, hardly any Hg oxidized across the APH so the Hg speciation fractions remained essentially unchanged until the ACI location. The ultimate predicted Hg-P level varied from 34 to 85 % of the Hg inventory for ACI concentrations from 0.4 to 2 lb/mmacf. Independence Unit 2 (Toxecon -II) Independence Unit 2 is a T-fired furnace fired with PRB subbituminous coal with an ESP only cleaning configuration. Test specific coal-cl were not available. The inherent LOI was 0.45 %, which removed approximately 7 % Hg. The calculated SO 3 concentration was 5.4 ppmv and no SO 3 measurements were reported. The ESP operated well above the calculated acid dew point, so SO 3 interference would have been minimal. Darco Hg and Darco Hg-LH were injected into the last field of the ESP representing a Toxecon -II configuration. The Toxecon -II configuration was simulated in MercuRator by using two ESPs in series. The first ESP represents the fields that are upstream of sorbent injection while the second ESP represents the downstream fields. Because flue gas residence times in the upstream and downstream fields of the ESP were not reported, the same default ESP transit times were used in MercuRator. Since default transit times are much shorter than actual ESP transit times, this approach would not introduce significant uncertainties. The measured and predicted Hg removals are presented in Fig. 5 for (left) Darco Hg and (right) Darco Hg-LH injection. Mercury removals at Independence were calculated from ESP inlet and outlet measurements and therefore include the inherent Hg removal in the fields upstream of the ACI location plus the removal on sorbents in the downstream ACI 9

10 Figure 5. Comparison of measured and predicted Hg removals (%) for the Toxecon -II evaluation at Independence with Darco Hg (left four) and Darco Hg-LH (right five). fields. The predicted total Hg removals for both Darco Hg and Darco Hg-LH are within about 10 % of the measured values except for the test with the lowest Darco Hg concentration (INDU2PHG3), where the Hg removal was under-predicted by 13.6 %. The correlation coefficient was 0.98 and the std. dev. was 5.8 % of the Hg inventory. The accuracy of the predictions could be further improved if we had additional details regarding the flue gas residence times in the ESP after the sorbent injection location. DISCUSSION Examining the Hg removals by Darco Hg from the validation database, the Toxecon -I configuration at Presque Isle achieved the highest Hg removals with the lowest sorbent injection concentrations. Specifically, 2 lb/mmacf achieved greater than 90 % Hg removal at Presque Isle. Tests at the four remaining test sites showed similar Hg removals at comparable ACI concentrations that were much lower than the ACI performance at Presque Isle. Moreover, the superior performance at Presque Isle and the common Hg removals at the other four sites are clearly apparent in the MercuRator predictions. All datasets with the exception of Meramac showed a continuous increase in both the measured and predicted Hg removals for progressively greater ACI concentrations. At Meramac, the Hg removals saturated above 3.2 lb/mmacf and increasing the ACI concentration to 10 lb/mmacf increased the Hg removal by only 7 %. The predictions at Meramac also show saturation in Hg removal albeit at a much higher ACI concentration. In comparison to Darco Hg, the Darco Hg-LH injection at Meramac at 3 lb/mmacf concentration resulted in greater than 95 % Hg removal. The Hg removals for the Toxecon -II configuration at Independence saturate at an ACI concentration of 6 10

11 lb/mmacf, for which the Hg removal approached 90 %. The Hg removals at Daniel when firing the 60/40 coal blend and without any added SO 3 increased from 50 to 83 % as the ACI concentration was increased from 3 to 9 lb/mmacf. The lone measurement at Daniel when firing the 80/20 blend resulted in comparable Hg removal to the 60/40 blend at an ACI concentration of 9 lb/mmacf. MercuRator underpredicted the Hg removals at Meramac by approximately 20 % and overpredicted the removal by a similar extent for the lone test at Daniel firing the 80/20 blend. For the tests at Daniel firing the 60/40 blend and the Toxecon -II at Independence, the predicted Hg removals are well within 10 % of the measured values for all tests. MercuRator also accurately predicts the saturation in Hg removal with increasing ACI concentrations at Independence. Both measured and predicted Hg removals using brominated Darco Hg-LH sorbent were higher than those for the standard Darco Hg by about 20 % at comparable ACI concentrations unless the removals saturated in the limit for elevated ACI concentrations. Overall, the Hg removals for ACI for the entire validation database were predicted within a standard deviation of 12.6 % of the measurements. The predictions also accurately depict SO 3 inhibition of Hg capture by ACI. Moreover, none of the reactivity parameters were adjusted from the values set in NEA s interpretation of Phase I NETL database. ACKNOWLEDGEMENT This study was sponsored by the Electric Power Research Institute under their program entitled, Understanding Mercury Chemistry Through Modeling. REFERENCES 1. Naik, C. V. ; Krishnakumar, B. ; Niksa, S., Predicting Hg emissions from utility gas cleaning systems. Fuel 2010, 89, Krishnakumar, B.; Niksa, S., Predicting the impact of SO 3 on mercury removal by carbon sorbents. Proc. Int. Combust. Symp. 2010, 33, to appear. 3. Niksa, S.; Hou, Y., Proc. U. S. EPA-DoE-EPRI Combined Power Plant Air Pollutant Control Symp.: The MEGA Symp., 2008, Baltimore, MD, August Niksa, S. ; Padak, B.; Krishnakumar, B. ; Naik, C. K., Process chemistry of Br addition to utility flue gas for Hg emissions control. Energy Fuels 2010, 24(2),