Environ. Sci. Technol. 2004, 38, 5803-5808
Development of a Mercury Transformation Model in Coal Combustion Flue Gas YE ZHUANG,* JEFFREY S. THOMPSON, CHRISTOPHER J. ZYGARLICKE, AND JOHN H. PAVLISH Energy & Environmental Research Center, University of North Dakota, P.O. Box 9018, Grand Forks, North Dakota 58202-9018
A bench-scale entrained-flow reactor was used to extract flue gas produced by burning a subbituminous Belle Ayr coal in a 580-MJ/h combustion system. The reactor was operated at 400°, 275°, and 150 °C with a flow rate corresponding to residence times of 0-7 s. Transformations of elemental mercury (Hg0) and total gas mercury (Hggas) in the reactor were evaluated as functions of temperature and residence time. The most significant mercury transformations (Hg0 to Hgp and Hg0 to Hg2+) occurred at 150 °C, while virtually no obvious mercury transformations were observed at 275° and 400 °C. Approximately 30% of total mercury has been oxidized at temperatures higher than 400 °C. A mass transfer-capacity limit model was developed to quantify in-flight mercury sorption on fly ash in flue gas at different temperatures. A more sophisticated model was developed to demonstrate not only the temperature and residence time effects but also to consider the effective surface area of fly ash and dependence of mercury vapor concentration on mercury transformations in flue gas. The reaction orders were 0.02 and 0.55 for Hg0 and Hggas, respectively. Only a few percent of the total surface area of the fly ash, in the range of 1%-3%, can effectively adsorb mercury vapor.
Introduction Coal combustion is a significant source of mercury emissions to the atmosphere. It is estimated that about one-third of the known anthropogenic mercury air emissions in the United States is from coal combustion (1, 2). Because of its high toxicity and tendency to bioaccumulate in the food chain, the U.S. Environmental Protection Agency (EPA) has recently decided to regulate mercury emissions from coal-fired electric power plants. For coal-fired utilities, these impending mercury regulations require that control strategies be investigated and developed. The efficacy of mercury control methods depends largely on the form of mercury (gas vs particulate) and species of mercury (elemental vs oxidized) formed upstream of the control devices. Mercury emissions include gaseous elemental form (Hg0), mercury in association with particulate matter (Hgp), and various gaseous mercuric compounds (Hg2+). Particulate-associated mercury (Hgp) can be removed from flue gas by conventional air pollutant control devices such as an electrostatic precipitator (ESP) or a bag house. Oxidized mercuric compounds (Hg2+) are readily * Corresponding author phone: (701)777-5326; fax: (701)777-5181; e-mail:
[email protected]. 10.1021/es030683t CCC: $27.50 Published on Web 10/01/2004
2004 American Chemical Society
captured in flue gas desulfurization units. Hg0 is most likely to escape air pollution control devices (APCDs) and be emitted into the atmosphere. Understanding the speciation of mercury is critical because control options rely heavily on its form and species. Also, a thorough understanding of mercury species transformation in flue gas is beneficial for seeking more cost-effective mercury control technologies. Over the past several years, researchers have collected extensive descriptive information on the physical and chemical factors that govern mercury speciation in coal combustion flue gas. It is generally agreed that all the mercury in coal is vaporized in the combustion and expected to be Hg0 in the high-temperature zone (3). Mercury transformations in postcombustion flue gas involve both homogeneous gas-phase and heterogeneous reactions and are kinetically limited (4), depending on the concentrations of flue gas constituents, fuel compositions, and the conversion processes and operating conditions that affect the time-temperature profile. Some researchers suggest that Hg0 is oxidized by gaseous oxidants in flue gas (5, 6), most likely by chlorine species, to form gaseous mercuric compounds (Hg2+). Chemical kinetic models suggest that atomic chlorine (Cl) in flue gas is the dominant reactant for mercury oxidation, whose concentration is controlled by the interactions with other flue gas constituents including HCl, CO, H2O, and NO (7-9). HCl and CO promote Cl as well as HgCl2 formation, while H2O in flue gas impedes their formation. NO can either promote or inhibit Cl and HgCl2 formation, depending on the NO levels (8). The mercuric compounds (Hg2+) formed are either maintained as gas phase or sorbed on fly ash. The predicted mercury oxidation levels based on homogeneous gas-phase reactions (5) were lower than the field data, indicating a solely gas-phase reaction is not sufficient to describe mercury transformation in flue gas. A better understanding of gas-to-particle conversion is needed, particularly the relationships between fly ash properties and oxidation and sorption of gaseous mercury. The fraction of mercury associated with particulate matter varies from 0% to 90% for some western U.S. coals (10). The fly ash compounds including residual carbon and flue gas constituents play major roles in determining mercury partitioning. Numerous studies have been performed to evaluate mercury sorption capacity of actual and simulated fly ash and various mineral compounds. Bench-scale data indicate that certain metallic constituents of fly ash, such as CuO and Fe2O3, promote mercury oxidation, especially in the presence of HCl and NOx (11). Both SiO2 and Al2O3 were inactive in mercury capture (11), while Ca(OH)2 was only effective in capturing HgCl2 (12). Other experimental data (13) demonstrated that residual carbon in fly ash enhanced the sorption of mercury. Mercury capture by fly ash was dramatically increased when flue gas temperature was reduced to below 400 °C (12) and increased by extending the contact time between flue gas and fly ash (10). X-ray absorption fine structure spectroscopy provides some insight to the chemical bonding for mercury capture, suggesting a Hg-Cl compound as one of the species on the carbonaceous fraction of the fly ash (14). Most studies on mercury gas-solid partitioning and speciation in coal combustion flue gases have been performed in fixed-bed reactors instead of an in-flight state as in coal combustion. In this paper, our goal is to examine mercury reactions with fly ash in real coal combustion flue gas; quantify fly ash reactivity with mercury in flue gas; and quantify the effects of time, temperature, surface area, and VOL. 38, NO. 21, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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mercury vapor concentration on mercury transformations in flue gas. The experiments were conducted in an entrained-flow reactor where fly ash particles were in suspended state, which is more representative as the interactions between fly ash and flue gas are more typical of that in a real system. Unlike in a real system where flue gas temperature and residence time usually vary simultaneously, the designed experiments were aimed at distinguishing temperature and residence effects on mercury transformation by controlling them separately. Overall rates of transformation of total gaseous mercury (Hggas ) Hg0 + Hg2+) to Hgp and Hg0 to Hg2+ and Hgp were determined using the entrained-flow reactor and an on-line Hg analyzer on a slipstream of flue gas from a pilot-scale (580-MJ/h) pulverized coal (pc)-fired combustion system. Preliminary results from burning a subbituminous Belle Ayr coal demonstrate the importance of evaluating Hg transformation rates using actual coal combustion flue gases. The overall conversion of Hggas to fly ash may be limited by either macroscale mass transfer from bulk gas phase to fly ash surface, equilibrium sorption capacity, or rates of reactions occurring on fly ash. Modeling efforts (15-17) have been pursued to predict mercury capture by sorbent as functions of sorbent size and injection rate and gas-sorbent contact time. However, experimental data on heterogeneous reactions and sorption are very limited to further verify these models. Since the entrained-flow reactor tests were conducted in an environment where temperature and residence time were separately controlled, the obtained experimental data provided more intrinsic information of mercury transformation as functions of temperature and residence time. Herein, an incorporated mass transfer-capacity limit model was developed to describe the Hggas-to-Hgp conversion based on the obtained entrainedflow reactor test data. The “real” mercury in-flight sorption capacity of fly ash in a flue gas environment was determined, which can be further used to evaluate and quantify fly ash mercury capture under different temperatures.
Experimental Description Coal Combustion Facility. A 580-MJ/h pc-fired unit was used to generate representative flue gas and fly ash that are produced in a full-scale utility boiler. The combustor is oriented vertically to minimize ash deposition on the wall. A refractory lining ensures adequate combustion zone temperature for complete combustion of fuel and prevents rapid quenching of the coalescing or condensing fly ash. Based on the superficial gas velocity, the mean residence time of a particle in the combustor is approximately 3 s. The coal nozzle of the unit fires axially upward from the bottom of the combustor, and secondary air is introduced concentrically to the primary air with turbulent mixing. Coal is introduced to the primary air stream via a screw feeder and ejector. An electric air preheater is used for precise control of the combustion air temperature. The temperature in combustion zone is around 1510 °C, and flue gas temperature at the combustor outlet is about 1037 °C. The hightemperature flue gas then flows through a series of heat exhangers to cool to 150 °C before entering an APCD such as an ESP. The residence time of flue gas in the whole system is approximately 6.5 s, resulting in a quenching rate of 500 °C/s, which is quite close to a full-scale system. Entrained-Flow Reactor. A portable bench-scale entrained-flow reactor, shown schematically in Figure 1, was used to examine mercury reactions with fly ash under controlled environments to determine the transformation rates of mercury in coal combustion flue gas. Since mercury species had been reported to be associated with particulate matter at higher temperatures (>400 °C) (18), the reactor was isothermally heated at 400°, 275°, and 150 °C to evaluate 5804
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FIGURE 1. Schematic diagram of the entrained-flow reactor setup. mercury transformation as a function of time at these temperatures. The reactor, made of stainless steel, is 122-cm long with an internal diameter of 6.35 cm. The reactor was oriented vertically, and the inner surface of the reactor was polished to minimize fly ash deposition on the wall surface. Ten sampling ports are located along the reactor at intervals of 12.7 cm as indicated in Figure 1. A slipstream from the 580-MJ/h combustion system was extracted at 450 °C (after the heat exchanger), assuming most mercury is in the elemental state beyond that temperature. Different-sized nozzles were selected to isokinetically extract coal combustion flue gas at various flow rates corresponding to 0-7 s of residence time in the reactor (under different temperatures). The extracted flue gas was then introduced from the top of the reactor using a stainless steel tube to connect the pilotscale combustion system with the reactor. The flue gas temperature into the reactor was monitored by a thermocouple (as shown in Figure 1). Forced-air convection was applied to ensure the temperature of flue gas into the reactor matched the preset reactor temperature. The flue gas residence time in the connecting tube was around 0.2-0.5 s, resulting in cooling rates in the range of 250-600 °C/s. Electric heating tapes (Thermolyne) combined with a temperature controller were used to heat the reactor to the designated temperatures. Flue gas exiting the reactor was then conditioned to remove fly ash, acid gases, and moisture prior to a MISCO Control Box (Model 7200) to measure the flow rate. Mercury Measurement. An online continuous mercury monitor (CMM), Tekran 2537A, was used to measure the Hggas and Hg0 concentrations at different sampling locations on the entrained-flow reactor. The Tekran 2537A instrument uses gold amalgamation cold-vapor atomic fluorescence spectroscopy to measure Hg0 concentrations. A glass probe was used to extract flue gas from the entrained-flow reactor at different locations. The stream intake in the sampling probe was cross-flow with the main gas stream in the reactor, aiming to minimize the particle intake in the sampling probe. The sampling probe was externally heated to 250 °C to prevent any possible condensation. The sampled flue gas was quenched and treated in the iced-cooled conversion unit. Because of the short residence time in the sampling probe, no mercury oxidation was assumed to take place. The minimum level of fly ash carried in the sampling flue gas was retained on a glass wool plug in the probe. The glass wool
plug was replaced regularly (approximately every 30-40 min) to minimize the mercury species bias caused by the formed filter cake. Since no significant ash built up on the glass wool, no mercury measurement was performed on the ash. The flue gas was treated in a conversion unit to remove all acid gases and either remove Hg2+ or convert Hg2+ to Hg0 for analysis. The pretreated flue gas at ambient temperature was then sampled by the CMM for either Hg0 or Hggas measurement, depending on which side of the conversion system the flue gas was passed. The Hg2+ concentration was calculated as the difference between the Hggas and Hg0 concentrations.
TABLE 1. Proximate and Ultimate Analyses and Mercury and Chlorine Contents for Belle Ayr Coal
Model Description The heterogeneous conversions of gas-phase mercury species to particulate phase involve (1) mass transfer from bulk gas phase to the external surface of fly ash, (2) physical and/or chemical adsorption of mercury vapor species on the fly ash, and (3) further mercury transportation within fly ash (intraparticle diffusion). (1) The external mass transfer of mercury species from the bulk flue gas to the fly ash surface was described as (17)
dCg a ) kg ‚ ‚ (Cg - C*) N)dt V
(1)
where N is the variation rate of mercury concentration (µg/ cm3‚s); kg is mercury mass transfer coefficient in flue gas in cm/s; a is the total external surface area of fly ash in duct in cm2; V is the total volume of the duct in cm3; Cg is the mercury concentration in bulk flue gas in µg/m3; and C* is the mercury gas-phase concentration in equilibrium with the mercury concentration in fly ash in µg/m3. The term (Cg - C*) represents the driven force from bulk gas phase to fly ash surface. The interfacial area per unit volume flue gas, a/V, was calculated based on dust loading in flue gas m (g/m3) fly ash density F (g/cm3) and fly ash diameter dp (cm) (19):
a 6‚m ) v Fp‚dp
(2)
Assuming a zero-drift velocity between fly ash particles and flue gas stream, the mass transfer coefficient, kg, was estimated based on the empirical correlation for force convective around a solid sphere (20)
2‚DHg kg ) dp
(3)
where dp is the aerodynamic diameter of fly ash particles in cm and DHg is the mercury diffusivity in flue gas in cm2/s. The fly ash aerodynamic diameter was obtained by aerodynamic particle sizer measurement. The mercury diffusivity in flue gas was calculated from Chapman-Enskog Theory (20)
DHg ) 0.001853 ‚
x ( T3 ‚
)
1 1 1 + ‚ (4) MHg Mfluegas P ‚ σ Hg Hg,fluegasΩHg,fluegas
where T is the flue gas temperature in K; MHg is the mercury molecule weight in g/mol; Mfluegas is the molecular weight of flue gas in g/mol; P is the atmospheric pressure in atm; σHg,fluegas is the average collision diameter between mercury and flue gas molecules in D; ΩHg,fluegas is the dimensionless collision integral. (2) The physical and/or chemical adsorption of mercury vapor species on the fly ash is very complex, and limited data are available for understanding the detailed mechanisms.
a
proximate analysis, %
as received
moisture volatile matter fixed carbon ash heating value, MJ/kg
24.8 35.9 34.4 4.83 20.9
ultimate analysis, %
as received
carbon hydrogena nitrogen sulfur ash oxygena mercury concentration, µg/g chlorine concentration, µg/g
51.4 6.29 0.80 0.29 4.83 36.4 0.077-0.089 22.5
Hydrogen and oxygen do not include H and O in sample moisture.
Langmuir isotherm has been used to describe the adsorption procedure, which required kinetic and sorption parameters that were obtained in a pure nitrogen environment (16). Therefore, those parameters neglect all other chemical reactions typical in coal combustion flue gas. Herein, Henry’s law isotherm is used and expressed as C* ) HCs, to describe the relationship between mercury concentration in solid phase, Cs (µg Hg/g fly ash), and the corresponding equilibrium gas-phase mercury concentration, C* (µg/m3). The equilibrium sorption parameter H, having units of g/m3, indicates the minimum needed fly ash to reach equilibrium capacity and reflects in-flight mercury absorption in flue gas. The parameter H is a quantitative criteria to evaluate mercury capture by fly ash. The value of H was derived from the experimental data of the entrained-flow reactor tests and will be discussed in detail later. (3) Mercury transportation within fly ash (intraparticle diffusion) is a diffusion process and can be described as
(
)
∂Cs ∂2Cs 2 ∂Cs + ) DHg-surf ∂t r ∂r ∂r2
(5)
where Cs is the mercury concentration in solid phase in µg/g; t is the time in s; DHg-surf is the mercury surface diffusion coefficient in cm2/s; and r is the radius of fly ash particle in cm. The fly ash in flue gas is capable of sorbing mercury vapor species to some extent, which is proved by the measurable mercury in fly ash. The conversion of mercury vapor to fly ash in flue gas is limited by capacity as well as mass transfer. According to other research results (17), the intraparticle diffusion was neglected in this model. Therefore, an analytical solution to describe the heterogeneous transformation of mercury vapor (limited by both mass transfer and capacity) was obtained as below
Cg ) C0
H m H 1+ m
+
1 H 1+ m
[
(
exp -kga 1 +
H t m
)]
(6)
where C0 is the initial mercury vapor concentration. The Henry’s law constant H was then inferred by fitting the model directly to the experimental data from the entrained-flow reactor tests that were conducted in real coal combustion flue gas under isothermal condition. The calculated values of H under different temperatures quantify the in-flight mercury sorption reactivity of fly ash at various temperatures. VOL. 38, NO. 21, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Elemental Analysis of Ash from Belle Ayr Coal Combustion (%) SiO2
Al2O3
Fe2O3
CaO
MgO
Na2O
K2O
P2O5
TiO2
BaO
MnO
SrO
LOIa
20.9
16.1
7.3
34.8
5.93
3.14
0.8
2.87
1.58
1.18
0.07
0.53
0.15
a
Loss on ignition.
TABLE 3. Experimental Conditions temp, °C 150 275 400 a
residence time, s 0 0 0
0.5 0.5 NTa
1 1 NTa
3 3 NTa
5 5 5
7 7 7
Not tested.
Because of the lack of critical information such as Hg speciation on fly ash that is needed for a detailed chemical reaction model, we have developed an approach and practical model that could be used to evaluate Hg-fly ash adsorption rates for different coals and combustors and provide insightful information to optimize Hg control technologies.
FIGURE 2. Total mercury vapor transformation as a function of temperature and residence time.
Results and Discussion Coal and Fly Ash Characteristics and Analyses. A subbituminous coal from the Belle Ayr Mine was selected for this investigation. Coal samples were collected from the feeder for proximate and ultimate analyses. The results are presented in Table 1, showing the Belle Ayr coal is a low-sulfur coal with a low ash content. The collected coal samples were analyzed for Hg and Cl contents. The data (also listed in Table 1) show 22.5 µg/g of Cl and 0.077-0.089 µg/g of Hg in coal (dry basis), which corresponds to 3.23 ppmv and 11.7 µg/Nm3 of Cl and Hg in a dry flue gas. The low chlorine content in the coal suggests that chlorination of Hg0 may not be significant. The mercury concentrations reported in this paper were normalized to 20 °C under 1 atm to compare mercury concentrations at different temperatures. The fly ash was collected for elemental analyses, and the results are presented in Table 2. The loss on ignition of the ash was also measured and listed in Table 2, indicating a complete coal combustion and low residual carbon level in the fly ash. Entrained-Flow Reactor Tests. Mercury in Belle Ayr coal flue gas was reported as elemental dominance. By extracting flue gas at 450 °C with a conventional quenching rate, mercury species into the flow reactor were expected to be representative. The entrained-flow reactor test was designed to simulate mercury sorption on fly ash through an isothermal duct with extended time to obtain mercury transformation rates. Mercury vapor concentrations, both Hggas and Hg0, were measured as a function of residence time at three different temperatures. Temperature and residence time conditions employed in these tests are summarized in Table 3. Figure 2 shows the Hggas variations with residence time at 400°, 275°, and 150 °C. At both 400° and 275 °C, Hggas-to-Hgp conversion was insignificant, indicated by the slight reduction of Hggas from 10.36 µg/Nm3 at the 0 s to 10.06 µg/Nm3 at 7 s and from 9.84 µg/Nm3 at the 0 s to 9.38 µg/Nm3 at 7 s for 400° and 275 °C, respectively. During the 150 °C test, however, Hggas was 8.3 µg/Nm3 at the reactor inlet and reduced to 5.3 µg/Nm3 at 5 s, showing a more obvious Hggas-to-Hgp conversion at 150 °C than 275° and 400 °C. The continuous decrease of Hggas concentrations at the reactor inlet from 10.36 µg/Nm3 at 400 °C to 9.84 µg/Nm3 at 275 °C and 8.3 µg/Nm3 at 150 °C indicated that additional Hggas-to-Hgp conversion occurred in the connecting tube from the 580MJ/h combustion unit to the flow reactor, which might be 5806
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FIGURE 3. Elemental mercury vapor transformation as a function of temperature and residence time. induced by the faster quench rate of flue gas since more atomic Cl was expected to be generated (21). Hg0 was also measured at different residence times during these tests to evaluate Hg0 transformation rates at these temperatures. The experimental data are plotted in Figure 3. Again, there was virtually no decrease of Hg0 with extended contact time between in-flight fly ash and Hg0 at 275° and 400 °C, meaning no significant Hg0-to-Hgp conversion and oxidation occurred at these temperatures. Compared to the corresponding Hggas concentration, however, it is noted that there was 32%-35% mercury oxidation in the flue gas before it entered into the reactor. The ratios of Hg0 to Hggas were constant during the testing residence times. The observed mercury oxidation at the reactor inlet and subsequent lack of continuous oxidation in the reactor indicates a possible homogeneous and/or heterogeneous gas oxidation occurred at high temperature (>400 °C), which might be induced by atomic Cl momentarily existing in combustion zone (22). For the 150 °C test, Hg0 was 5.62 µg/m3 at the reactor inlet and decreased to 2.75 µg/m3 at the 5-s residence time, demonstrating an effective Hg0 conversion and/or oxidation with the extended residence time at the lower temperature of 150 °C. The experimental data show no significant Hg transformation in the low-sulfur and -chlorine flue gas at 400° and 275 °C. The observed Hg0 and Hggas changes with time at 150 °C are the results of interactions among fly ash, mercury, and other flue gas constituents. To quantify the fly ash
TABLE 5. Calculated H, n, and A at 150 °C - Freundlich Isotherm
TABLE 4. Parameters Used in Model Calculation parameter
value
fly ash diameter, µm dust load in flue gas, g/cm3 fly ash density, g/cm3 pressure, atm molecule weight of flue gas, g/mol molecule weight of mercury, g/mol collision diameter of flue gas, Å collision diameter of mercury, Å temperature, °C residence time, s
8 3.28 2.4 1 28.97 200.59 3.617 2.969 150-400 0-7
Hggas Hg0
reactivity for mercury sorption and/or oxidation, the mass transfer-capacity limit model (eq 6) was used to best fit with the experimental data. The parameters used in the modeling calculation are listed in Table 4. Levenberg-Marguardt iterative method was used to solve the nonlinear equation to determine H at different temperatures. As discussed previously, parameter H quantifies reactivity of fly ash for in-flight mercury sorption. A smaller value of H means that the fly ash has a higher reactivity for mercury sorption. For Hg0, the H was 6.78 g/cm3 at 150 °C, dramatically increased to 73.4 g/cm3 at 275 °C, and recessed to 44.8 g/cm3 at 400 °C. There was also a similar nonmonotonic correlation between the in-flight mercury sorption reactivity H and flue gas temperature for Hggas. The calculated H demonstrated that the fly ash was most effective for mercury capture at 150 °C, and this efficacy was significantly attenuated with increasing temperature. The observed fly ash reactivity with mercury at 400 °C was somewhat overestimated because of the Hg transformation occurring at higher temperature which was not included in the present heterogeneous model. The model fit data are also plotted in Figures 2 and 3 to compare with the experimental data. For the 150 °C test, experimental data from 3 to 5 s were within the CMM sensitivity range, meaning no significant further Hg transformation occurred with extended residence times, which matches the model calculation results. Henry’s law isotherm is a very simple description for mercury sorption on fly ash; all the factors are lumped into the parameter H. In a real system, however, mercury sorption on fly ash is affected by the effective surface area, bulk-phase mercury concentration, etc. Herein, Freundlich equation was further applied to describe mercury vapor sorption on fly ash more precisely:
C* ) H‚Cns
(7)
Instead of assuming a first-order reaction between sorbed mercury on fly ash and the corresponding mercury vapor in equilibrium, the reaction order, n, was to be determined by best fitting with the experimental data to further understand the effect of mercury vapor concentration on mercury sorption. According to other research results (23), mercury vapor sorption also depends on the availability of the activated site on fly ash, in which the detailed mechanisms are not well understood yet. Therefore, a more sophisticated model was further developed to describe heterogeneous mercury transformation in coal combustion flue gas. The final model is
[
(
)]
dCg C0 - C g a ) Cg ‚ ‚ A ‚ Cg - H dt V m
n
(8)
where A stands for the fraction of surface area on fly ash that is reactive for mercury sorption, n is the reaction order, and H is the in flight mercury adsorption reactivity of fly ash. By applying the same approach, the parameters of A, H, and n
H, g/m3
n
A, %
4.98 2.16
0.55 0.02
1.0 3.0
FIGURE 4. Mercury vapor transformation rate as a function of mercury vapor concentration. were determined by best fitting with the experimental data obtained in the 150 °C test, which indicated the most significant mercury transformation. The results are summarized in Table 5 for both Hg0 and Hggas. The experimental data are replotted in Figure 4 as mercury transformation rates vs mercury vapor concentration in flue gas, and the modeling results are also plotted in Figure 4 for comparison. For Hg0, the reaction order was 0.02, indicating a slight dependence on Hg0 vapor concentration in flue gas for Hg0 transformation. The in-flight mercury sorption reactivity H was 2.16 g/m3. Results showed only 3.16% of the total surface area of fly ash was involved in the heterogeneous Hg0 transformation, which agrees with other research results (23) that only specific activated sites on fly ash were responsible for mercury sorption. More research is needed to bring light to the detailed mechanisms. The corresponding values of H, A, and n for Hggas are also listed in Table 5 to compare with Hg0 results. The overall reaction order for Hggas was 0.55, showing a slightly stronger dependence on mercury vapor concentration in flue gas for Hggas than Hg0. The in-flight mercury sorption reactivity of fly ash with Hggas, H, was 4.98 g/m3, which was lower than the 2.16 g/m3, meaning more fly ash was needed to reach equilibrium for Hggas than Hg0. Again, only a very small fraction of (1%) total surface area of fly ash was activated sites for mercury vapor transformation.
Acknowledgments The authors acknowledge the financial support of the EPA under Assistance Agreement R827649-01. The content of this paper does not necessarily reflect the views of EPA, and no official endorsement should be inferred.
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(16) Flora, J. R. V.; Vidic, R. D.; Liu, W.; Thurnau, R. C. Modeling Powdered Activated Carbon Injection for the Uptake of Elemental Mercury Vapor. J. Air Waste Manage. Assoc. 1998, 48, 1051-1059. (17) Chen, S.; Rostam-Abadi, M.; Chang, R. Mercury Removal From Combustion Flue Gas by Activated Carbon Injection: Mass Transfer Effects. Prepr. Pap. s Am. Chem. Soc., Div. Fuel Chem. 1996, 41 (1), 442-446. (18) U.S. Environmental Protection Agency. Selective Catalytic Reduction Mercury Field Sampling Project; National Risk Management Research Laboratory, Dec 2002. (19) Meserole, F. B.; Chang, R.; Carey, T. R.; Machac, J.; Richardson, C. F. J. Air Waste Manage. Assoc. 1999, 49, 694. (20) Cussler, E. L. Diffusion: Mass Transfer in Fluid System; Cambridge University Press: New York, 1984. (21) Niksa, S.; Fujiwara, N. Predicting Mercury Speciation in CoalDerived Flue Gas. Combined Power Plant Air Pollutant Control Mega Symposium, Washington, DC, May 19-22, 2003. (22) Senior, C. L.; Bool III, L. E.; Huffman, G. P.; Huggins, F. E.; Shah, N.; Sarofim, A.; Olmez, I.; Zeng, T. 90th Annual Meeting of the Air & Waste Management Association, Toronto, Ontario, Canada, 1998. (23) Olson, E. S.; Laumb, J. D.; Benson, S. A,; Dunham, G. E.; Sharma, R. K.; Mibeck, B. A.; Miller, S. J.; Holmes; M. J.; Pavlish, J. H. An Improved Model for Flue Gas-Mercury Interactions on Activated Carbons. Combined Power Plant Air Pollutant Control Mega Symposium, Washington, DC, May 19B22, 2003.
Received for review November 3, 2003. Revised manuscript received July 29, 2004. Accepted August 4, 2004. ES030683T
