Ind. Eng. Chem. Res. 2008, 47, 5825–5831
5825
Simultaneous Removal of SO2, NOX, and Hg from Coal Flue Gas Using a NaClO2-Enhanced Wet Scrubber Nick D. Hutson,*,† Renata Krzyzynska,†,§ and Ravi K. Srivastava‡ Office of Research & DeVelopment and Office of Air and Radiation, U.S. EnVironmental Protection Agency, 109 TW Alexander DriVe, Research Triangle Park, North Carolina 27711
A bench-scale study was conducted on the simultaneous removal of SO2, NOX, and mercury (both Hg0 and Hg2+) from a simulated coal flue gas using a wet calcium carbonate scrubber. The multipollutant capacity of the scrubber was enhanced with the addition of the oxidizing salt, sodium chlorite. The results showed a maximum scrubbing of 100% for SO2 and Hg species and near complete NO oxidation with about 60% scrubbing of the resulting NOX species. The chlorite additive was less effective as an oxidant in the absence of SO2 and NO in the flue gas. Oxidation of NO and mercury were only about 50% and 80%, respectively, in the case of no SO2 in the simulated flue gas. The mercury oxidation was similarly affected by the absence of NO in the flue gas. Introduction On March 10, 2005, the EPA issued the Clean Air Interstate Rule (CAIR)1 which, when fully implemented in 2015, will reduce sulfur dioxide (SO2) and nitrogen oxides (NOX) emissions in the eastern United States by over 70% and 60%, respectively, from 2003 levels. The Agency has also suggested a regulatory approach for the control of mercury emissions from coal-fired electric utility boilers. Wet flue gas desulfurization (FGD) technologies, particularly limestone-based wet scrubbers, have been shown to provide high SO2 removal efficiencysin excess of 95% removalswith good reliability.2 In 2005, it was estimated that ≈100 GW of the U.S. coal-fired utility capacity (≈305 GW) was utilizing some type of FGD technology.3 About 86% of those systems are wetscrubber based, and almost 70% of those wet scrubbers use the limestone process.2 Removal of flue gas SO2 using the limestone process takes place in the scrubber as follows: SO2(g) + H2O f 2H+ + SO32-
(1)
2H+ + SO32- + CaCO3 f CaSO3 + CO2(g) + H2O
(2)
And, assuming there is an adequate supply of oxygen (air), the calcium sulfite may be then be oxidized to calcium sulfate (gypsum) as follows: CaSO3 + 1/2O2 f CaSO4
(3)
Because of the deep emission reduction requirements of CAIR, a significant increase in the use of wet-FGD technology is expected in the next decade. It is estimated that, in 2020, about 60% of the total coal-fired capacity will utilize some sort of wet-FGD technology.3 While wet-FGD scrubbers are installed specifically for SO2 control, they may also effectively remove oxidized forms of mercury from flue gas.3 Deep emission reductions will likely require additional Hg-specific control technologies, such as * To whom correspondence should be addressed. E-mail: hutson.nick@ epa.gov. Tel: (919) 541-2968. † Office of Research & Development. ‡ Office of Air and Radiation. § Oak Ridge Institute of Science and Education (ORISE) postdoctoral research fellow
activated carbon injection (ACI). Wet-FGD scrubbers have the potential to provide a high level of mercury control because of their substantial gas-liquid mass transfer and the high solubility of most oxidized mercury compounds (such as HgCl2). This cobenefit control is only effective, however, for flue gas streams containing oxidized forms of mercury because the elemental form is not soluble and tends to pass through the wet scrubber. In general, facilities that burn coals with higher chlorine content (as often seen in eastern bituminous coals) tend to produce more oxidized mercury. Conversely, facilities that utilize coals with lower chlorine content (e.g., western sub-bituminous coals) tend to produce flue gases containing mostly elemental mercury. Emissions of nitrogen oxides (NOX) in the U.S. are largely controlled by combustion controls (e.g., low-NOX burners, staged combustion) and selective catalytic reduction (SCR) or selective noncatalytic reduction (SNCR) technologies. As mentioned earlier, CAIR mandates additional reductions in NOX emissions. As a result, it is predicted that the use of selective catalytic reduction (SCR) systems will significantly increase and will be used on slightly more than 50% of the total coal-firing capacity by 2020.3 With the number of wet-FGD scrubbers scheduled to be constructed in response to CAIR, it is desirable to develop wetscrubber-based technologies capable of providing simultaneous multipollutant (SO2, NOX, and both Hg0 and Hg2+) control. Such technologies could make wet-FGD scrubbers more costeffective and could obviate the need for installation of additional costly control equipment, such as an SCR system. A considerable amount of work has been done in an effort to remove NO in a wet scrubber. However, because NO has a very low solubility, it is practically impossible to remove NO gas by wet scrubbing if the gas does not contain NO2.4 So, effective scrubbing of NO has typically involved oxidation of NO to NO2 as a first step. This has generally been undertaken using two approaches: (1) gas-phase oxidation of NO to NO2 prior to entering the wet scrubber and (2) gas-liquid oxidation of NO to NO2 via addition of a soluble oxidant to the scrubber liquor. Zhao and Rochelle studied absorption of elemental mercury vapor in aqueous oxidants (HNO3 and HNO3/H2O2) catalyzed by Hg(II).5 The same authors also studied elemental mercury absorption in aqueous sodium hypochlorite (NaOCl) and determined that it strongly absorbs Hg even at high pH, though low pH, high [Cl-], and high temperature (55 °C) were
10.1021/ie800339p CCC: $40.75 2008 American Chemical Society Published on Web 06/25/2008
5826 Ind. Eng. Chem. Res., Vol. 47, No. 16, 2008
Figure 1. Bench-scale experimental apparatus.
the most favorable conditions.6 Chen et al. have also studied oxidation and absorption of NO using NaOCl solutions in a packed tower.7 Hydrogen peroxide (H2O2) has also been studied as an additive for enhanced oxidation and removal of NO from flue gas.8,9 Many other additives have been studied.10–15 Much work has been done looking at the use of aqueous solutions of sodium chlorite (NaClO2) for NO oxidation and scrubbing. In some of the earliest work in this area, Sada et al. investigated NO absorption kinetics in mixed alkaline solutions of NaClO2 and NaOH using a stirred vessel with plain gas-liquid interface.16 More recent tests have been performed under similar conditions using other adsorption vessels, such as packed column, stirred tank, bubble column, spray scrubber, etc.17–19 Adewuyi et al. studied the simultaneous removal of NO and SO2 in a laboratory bubble column containing an aqueous solution of NaClO2 buffered with NaHPO4 and KH2PO4.20 The results suggested that the buffered solution is more effective in absorbing NOX and SO2 and in controlling chlorine dioxide (ClO2) leakage. Much work has also been done by researchers at the National Cheng Kung University (Tainan, Taiwan) in looking at absorption of NO and SO2 in NaClO2/ NaOH solutions21,22 and by NaClO2 solutions under acidic conditions.23,24 Lee et al. have studied the simultaneous removal of SO2 and NO by a NaClO2 solution in a wetted-wall column.25 In this work, we have studied the addition of oxidizing salts (primarily NaClO2) to a bench-scale FGD scrubber to give the scrubber multipollutant capacity. The goal was to modify the scrubber chemistry so that some level of control could be expected for SO2, NOX, and vapors of both Hg0 and Hg2+. Experimental Section Bench-Scale Apparatus. The bench-scale experimental setup, shown in Figure 1, used a flow-through gas-liquid impinger to simulate a wet flue gas desulfurization scrubber. The system
included a gas blending system for makeup of the synthetic flue gas, the flow-through gas-liquid impinger, and online gas analyzers for measurement of the gases of interest. The scrubber slurry was pumped into the impinger which overflowed to maintain a constant slurry level and volume of ≈380 mL. The scrubber temperature was maintained at 55 °C by immersing the scrubber, to the overflow, in a controlled temperature water bath. Oxidant and a calcium carbonate solution were blended prior to introduction to the scrubber using peristaltic pumps. The addition rate of the oxidant solution was monitored via weight loss in the beaker. The alkali solution addition rate was monitored via the weight gain from the overflow of the scrubber. The alkali scrubber solution was constantly mixed to ensure a homogeneous 10 wt % solids slurry. The majority of the work in these tests involved NaClO2 as the oxidant additive. However, other oxidantssNaClO3, H2O2, KMnO4, and Ca(OCl)2swere also studied. Simulated flue gas was generated from a controlled mixture of cylinder gases. Mass flow controllers and valves were used to control the flow of the component gases to provide a standard simulated flue gas comprised of approximately 81 vol % N2, 11 vol % CO2, 8% O2, 1500 ppmv SO2, 200 ppmv NO, and 206 µg/m3 Hg0. The total flow of the simulated flue gas was 2 L/min (STP). Elemental mercury vapor was supplied using a VICI Metronics Dynacalibrator permeation oven held at 100 °C and using N2 as the carrier gas. The mercury concentration was 10 times higher than is typical for a coal combustion flue gas to accommodate the sensitivity of the mercury analyzer. The SO2 concentration was varied from 0 to 2500 ppm, and the NO concentration was varied from 0 to 470 ppm. These values represent the extreme ranges of what might be seen in the flue gas of a coal-fired power plant. The exhaust from the impinger passed through a miniimpinger containing quartz wool to remove any water mist prior to analysis. The scrubbed gas then passed through heat traced
Ind. Eng. Chem. Res., Vol. 47, No. 16, 2008 5827 Table 1. Summary of Experimental Conditions experimental condition
Figure 2. Concentration profile for pollutants during experimental period for NaClO2 added to 10 wt % CaCO3 slurry at 2.5 mM. The initial concentration of Hg in the scrubber inlet was 206 µg/m3. Initial concentrations of NO and SO2 were 200 and 1500 ppm, respectively.
lines to a Nafion dryer which removed remaining any remaining water vapor. A gas sample was then pulled through a series of analyzers while the excess gas is vented to the hood. The NOX species were measured using a continuous chemiluminescence NOX analyzer (API model 200AH). The SO2 was measured using a continuous fluorescence analyzer (API model 100AH). Hg0 vapor was measured using a continuous cold vapor atomic absorption (CVAA) analyzer (BUCK model 400A). This instrument uses atomic absorption to quantify elemental mercury by producing an output signal between 0 and 250 mV that is proportional to the concentration. Both water and SO2 interfere with mercury measurement by absorbing light at the wavelength being measured. Water was eliminated from the sample with the in-line Permapure Nafion dryer. SO2 was an integral part of the flue gas being tested, and it produces a CVAA signal proportional to the SO2 concentration. The SO2 concentration was determined using the same sample stream as analyzed for mercury concentration (i.e., the SO2 analyzer was installed in series, downstream of the CVAA analyzer). The CVAA response to SO2 was determined daily, and the mercury concentrations are subsequently corrected for SO2 interference. To prevent long-term instrument drift, the sample cell from the CVAA analyzer was removed, soaked in a 10% nitric acid solution for at least 4 h, rinsed, and dried between each experiment. All analyzer results were automatically logged to a data acquisition system every 10 s. An example concentration profilesin this case for NaClO2 added at a concentration of 2.5 mM in the CaCO3 slurrysis shown in Figure 2. A summary of experimental conditions for the bench-scale tests is given in Table 1. Experimental Procedure. The experiments were conducted by allowing the impinger to fill to overflowing while the simulated flue gas stream bypassed the system (see Figure 1). The flue gas was then introduced to the impinger and scrubbed for 60 min. This time was inadequate to allow the scrubber
value
remark
SO2 NO CO2 O2 N2
0-2500 ppm 0-470 ppm 11 vol % 8 vol % balance
Hg0
206 µg/m3
total flue gas flow
2 L/min
scrubber slurry solids wt % scrubber slurry addition rate oxidant concentration scrubber temperature mean residence time L/G
10 wt %
SO2 in nitrogen NO in nitrogen compressed CO2 compressed air (N2/O2) compressed N2, air, other Hg0 permeation tube at 100 °C std conditions, 1 atm and 20 °C alkali and oxidant solutions combined alkali and oxidant solutions combined type and concentration varied controlled by water bath slurry residence time liquid-to-gas ratio
550 g/h varied 55 °C 0.76 h 4.17
liquor to reach steady state. The scrubber vessel was assumed to be a continuously stirred tank reactor (CSTR). Using the CSTR model, it was predicted that added components in the slurry were at 73% of their steady-state value at the end of the 60 min experiments. These predicted values were used in scrubber mass balance calculations. The fresh oxidant/CaCO3 slurry was added at a consistent rate during the scrubbing period. The scrubber effluent was collected in a glass sample bottle during a timed 10 min interval near the end of the 60 min scrubber operation. The sample was immediately weighed, and it was placed on ice to quench or slow any oxidation reactions. The sample was then analyzed as soon as practical for chloride, sulfate, nitrate, and nitrite by ion chromatography (IC) using EPA reference method 300.0. The sample was also analyzed for total Hg content. The samples were prepared for Hg analysis by aliquoting 10 mL of the solids slurry into digestion tubes and digesting according to ASTM method D6784-02 (Ontario Hydro) as described for the potassium chloride fraction. Mercury analysis of each digest was carried out by CVAA according to EPA SW846 method 7470A “Mercury in Liquid Waste (Manual Cold Vapor Technique).” A Perkin-Elmer FIMS 100 flow injection mercury system was used for this analysis. The instrument was calibrated with known standards ranging from 0.025 to 1 µg/L mercury for low range sample and with known standards ranging from 0.25 to 10.0 µg/L for high range samples. The method detection limit for mercury in aqueous samples is 0.01 µg/L. Samples with known additions of mercury for analytical spikes also were digested as described above prior to CVAA analysis. A second independent sample was also collected in a glass beaker for measurement of final pH. Experimental Results Blank, Baseline, and Oxidant Screening Tests. Blank tests were conducted using only a deionized water solutionswith no oxidizer and or alkali sorbent added. These tests showed approximately no removal of Hg0 vapor, less than 2% removal of NOX, and about 24% removal of SO2. Baseline tests were conducted with a 10 wt % CaCO3 slurry with no added oxidizing agent. These tests again showed no removal of Hg0 vapor and minimal NOX removal with nearly 100% removal of SO2. Different slurry concentrations of CaCO3 were also testedsfrom 0.5 up to 10 wt %sin order to see the influence of CaCO3 concentration on pollutant removal. The results were consistent, including the removal of SO2, indicating that the CaCO3 concentration, in the range tested, did not significantly affect the ultimate results.
5828 Ind. Eng. Chem. Res., Vol. 47, No. 16, 2008 Table 2. Removal of Pollutants (% at 1 h) in Bench-Scale Tests Using Oxidant Additives Other Than NaClO2 pollutant removal (% at 1 h) a
oxidant
oxidant concn [mM]
SO2
NOX
NO
Hg
NaClO3 NaClO3 Ca(OCl)2 KMnO4 H2O2 H2O2 H2O2 NaClO2
10 50 10 20 10 10 50 2.5
100 100 99 100 100 100 99 100
7 6 3 34 5 6 5 62
4 3 2 33 2 3 4 36
3 0 56 100 0 3 0 95
a
Oxidant concentration in a 10 wt % CaCO3 slurry.
Table 3. Removal of Pollutants (% at 1 h) in Baseline Bench-Scale Tests pollutant removal (% at 1 h) -
slurry
ClO2 concn [mM]
SO2
NO
NOX
Hg
DI water (no CaCO3) CaCO3 slurry DI water (no CaCO3) DI water (no CaCO3)
0 0 2.5 8.8
24 100 66 100
