Energy & Fuels 2006, 20, 1039-1043
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Impact of Supplemental Firing of Tire-Derived Fuel (TDF) on Mercury Species and Mercury Capture with the Advanced Hybrid Filter in a Western Subbituminous Coal Flue Gas Ye Zhuang* and Stanley J. Miller Energy & EnVironmental Research Center, UniVersity of North Dakota, Grand Forks, North Dakota 58202-9018 ReceiVed September 27, 2005. ReVised Manuscript ReceiVed NoVember 3, 2005
Pilot-scale experimental studies were carried out to evaluate the impacts of cofiring tire-derived fuel and a western subbituminous coal on mercury species in flue gas. Mercury samples were collected at the inlet and outlet of the Advanced Hybrid filter to determine mercury concentrations in the flue gas with and without TDF cofiring, respectively. Cofiring of TDF with a subbituminous coal had a significant effect on mercury speciation in the flue gas. With 100% coal firing, there was only 16.8% oxidized mercury in the flue gas compared to 47.7% when 5% TDF (mass basis) was fired and 84.8% when 10% TDF was cofired. The significantly enhanced mercury oxidation may be the result of additional homogeneous gas reactions between Hg0 and the reactive chlorine generated in the TDF-cofiring flue gas and the in situ improved reactivity of unburned carbon in ash by the reactive chlorine species. Although the cofiring of TDF demonstrated limited improvement on mercury-emission control with the Advanced Hybrid filter, it proved to be a very cost-effective mercury control approach for power plants equipped with wet or dry flue gas desulfurization (FGD) systems because of the enhanced mercury oxidation.
Introduction Mercury is an immediate concern for the U.S. electric power industry because of the U.S. Environmental Protection Agency’s (EPA) conclusion that mercury emissions from power plants pose significant hazards to public health. The EPA Mercury Study Report to Congress1 and the Utility Hazardous Air Pollutant Report to Congress2 both identified coal-fired boilers as the largest single category of atmospheric mercury emissions in the United States, accounting for one-third of the total anthropogenic emissions. Researchers have carried out extensive studies to develop mercury control technologies for coal-fired power plants. Among the most-promising approaches are coal cleaning, mercury capture in a wet scrubber, and dry sorbent injection upstream of the particle-control devices. The efficacy of mercury control methods depends largely on the form of mercury (gas vs particulate) and the species of mercury (elemental vs oxidized mercury) formed upstream of the control devices. Mercury emissions are in the form of gaseous elemental mercury (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 baghouse. Oxidized mercuric compounds (Hg2+) are readily captured in flue gas desulfurization (FGD) units. Hg0 is most likely to escape air pollution control devices and be emitted to the atmosphere. * To whom correspondence should be addressed. E-Mail Address:
[email protected]. (1) Mercury Study Report to Congress; Report EPA-452/R-97-010; U.S. Environmental Protection Agency: Washington, DC, Dec 1997. (2) Study of Hazardous Air Pollutant Emission from Electric Utility Steam Generating UnitssFinal Report to Congress; Report EPA-453/R-980004a; U.S. Environmental Protection Agency: Washington, DC, Feb 1998.
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 combustion and is expected to be Hg0 in the hightemperature zone.3 Mercury transformations in postcombustion flue gas are complicated and depend on the concentrations of flue gas constituents, fuel compositions, and the conversion processes and operating conditions that affect the timetemperature profile. It is believed that Hg0 can be oxidized by gaseous oxidants in flue gas,4,5 most likely by chlorine species, to form gaseous mercuric compounds (Hg2+). Chemical kinetic models suggest that atomic chlorine (Cl) in the flue gas is the dominant reactant for mercury oxidation, whose concentration is controlled by interactions with other flue gas constituents, including HCl, CO, H2O, and NO.6-8 Fly ash compounds, including residual carbon, also play important roles in mercury partitioning in flue gas. Bench-scale data indicated that certain metallic constituents of fly ash, such as CuO and Fe2O3, promoted mercury oxidation, especially in the presence of HCl and NOx.9 Experimental data10 demonstrated that residual carbon in fly ash enhanced the sorption of mercury. (3) Toole-O’Neil, B.; Tewalt, S. J.; Finkelman, R. B.; Akers, D. J. Fuel 1999, 78, 45-54. (4) Senior, C. L.; Sarofim, A. F.; Zeng, T.; Helble, J. H.; Mamani-Paco, R. Fuel Process. Technol. 2000, 63, 197-213. (5) Chen, Z.; Senior, C. L.; Sarofim, A. F. In Proceedings of the 27th International Technical Conference on Coal Utilization & Fuel Systems, Clearwater, FL, March 4-7, 2002; Coal Technology Association: Gaithersburg, MD. (6) Sliger, R. N.; Kramlich, J. C.; Marinov, N. M. Fuel Process. Technol. 2000, 65-66, 423-438. (7) Niksa, S.; Helble, J. J.; Fujiwara, N. EnViron. Sci. Technol. 2001, 35, 3701-3706. (8) Edwards, J. R.; Srivastava, R. K.; Kilgroe, J. D. J. Air Waste Manage. Assoc. 2001, 51, 869-877.
10.1021/ef050317q CCC: $33.50 © 2006 American Chemical Society Published on Web 03/23/2006
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X-ray absorption fine-structure spectroscopy (XAFS) analysis suggested a Hg-Cl compound as one of the species on the carbonaceous fraction of the fly ash.11 On the basis of the current EPA regulation on mercury emission for utility power plants, there is an urgent need to develop a very cost-effective mercury control technology that is able to reduce mercury emission to 40% with minor impacts on plant operation. Experimental results from recent field studies12 at the Big Stone Power Plant, which burned a western subbituminous coal with tire-derived fuel (TDF) cofiring, showed a correlation between the concentration of oxidized mercury in the flue gas and the amount of TDF fed into the boilers. The mercury species in the flue gas from the cofiring of TDF and the western subbituminous coal was 44.2% oxidized mercury, 49.2% particle-bound mercury, and only 6.6% elemental mercury, which is in contrast with the dominance of elemental mercury in a typical western subbituminous coal flue gas. The inherent mercury capture across the field of the Advanced Hybrid filter under 5-10% TDF cofiring was up to 49% compared to nearly 0% with 100% subbituminous coal combustion. The above data suggest that the low rate of TDF cofiring changed the mercury chemistry in the flue gas and resulted in some improvement in mercury collection with the Advanced Hybrid filter. The Advanced Hybrid filter is a unique particle-control device developed by the Energy & Environmental Research Center (EERC) under the sponsorship of the U.S. Department of Energy (DOE) and W. L. Gore & Associates, Inc. It combines electrostatic precipitation and fabric filtration and provides an ultrahigh particle collection efficiency of >99.99%. Detailed information about the Advanced Hybrid filter can be found elsewhere.13 However, because of intermittent TDF cofiring and a varied TDF feed rate in the power plant, it is difficult to quantify the effect of TDF cofiring on mercury species and mercury removal. Subsequently, a pilot-scale experiment was conducted at the EERC to evaluate cofiring TDF and a western subbituminous coal on mercury species and mercury capture with a 55 kW Advanced Hybrid filter. Experimental Section A series of pilot-scale experiments were designed and completed at the EERC to evaluate the effects of TDF cofiring on mercury species and mercury capture with the Advanced Hybrid filter with a western subbituminous coal flue gas. Figure 1 shows a schematic diagram of the pilot-scale system. A 55 kW (200 acfm) pc-fired combustor was used to produce flue gas from coal combustion. The combustor was oriented vertically to minimize wall deposits. A refractory lining helps to ensure an adequate combustion-zone temperature for complete combustion of fuel and prevents rapid quenching of the coalescing or condensing fly ash. On the basis of the superficial gas velocity, we determined (9) Ghorishi, S. B.; Lee, C. W.; Kilgroe, J. D. In Proceedings of the Air & Waste Management Association 92nd Annual Meeting & Exhibition, St. Louis, MO, June 20-24, 1999; Air & Waste Management Association: Pittsburgh, PA; Paper 99-651. (10) Hassett, D. J.; Eylands, K. E. Fuel 1999, 78, 243-248. (11) Huggins, F. E.; Yap, N.; Huffman, G. P. Proceedings of the Air & Waste Management Association 93rd Annual Meeting & Exhibition, Salt Lake City, UT, June 18-22, 2000; Air & Waste Management Association: Pittsburgh, PA; Paper 528. (12) Zhuang, Y.; Miller, S. J.; Dunham, G. E.; Olderbak, M. R. Mercury Control with the AdVanced Hybrid Particulate Collector; Technical Progress Report, DOE NETL Cooperative Agreement DE-FC26-01NT41184; U.S. Department of Energy, National Energy Technology Laboratory: Pittsburgh, PA, Oct-Dec 2001. (13) Miller, S. J.; Collings, M. E.; Zhuang, Y.; Gebert, R.; Davis, D.; Rinschler, C. In The Combined Power Plant Air Pollutant Control Symposium: The Mega Symposium, Chicago, Aug 20-23, 2001.
Zhuang and Miller
Figure 1. Schematic drawing of the 55 kW pilot-scale system.
Figure 2. Front view of the 55 kW (200 acfm) Advanced Hybrid filter at the EERC.
the mean residence time of a particle in the combustor was approximately 3 s. The coal nozzle of the unit fired axially upward from the bottom of the combustor, and secondary air was introduced concentrically to the primary air with turbulent mixing. The western subbituminous coal was introduced to the primary air stream via a screw feeder and ejector. An electric air preheater was used for precise control of the combustion air temperature. The flue gas temperature at the combustor outlet cools to approximately 1000 °C (1832 °F). The coal combustion flue gas exiting the combustor was further cooled to a temperature of approximately 135 °C (275 °F) and then introduced into the pilotscale Advanced Hybrid filter. As shown in Figure 2, the pilot-scale Advanced Hybrid filter has one row of four filter bags inside the chamber, and perforated plates are placed on each side of the bags to separate the bags from the discharge electrodes. The perforated plates serve as a collection area for capturing most of the particulate matter (PM) entering the Advanced Hybrid filter unit, and the filter bags collect the remaining PM when the flue gas flows through the bags. Combining the best features of ESPs and fabric filters in a unique approach, the Advanced Hybrid filter provides >99.99% collection efficiency for fine particles at high filtration velocities with reasonable pressure drops. Detailed information on PM capture with the Advanced Hybrid filter has been reported elsewhere.13,14 Instrumentation enables the continuous monitoring of system temperature, pressure, flow rates, flue gas constituent concentrations, and particle-control device operating data. During the test, the TDF was fired as a supplemental fuel. The TDF was preshredded, screened to -40 mesh, and then fed with a separate feeder directly into the pneumatic coal feed line at rates of 1.4 and 2.8 kg/hr, corresponding to 5 and 10% of the feed coal (14) Zhuang, Y.; Miller, S. J.; Olderbak, M. R.; Gebert, R. AdVanced Hybrid Particulate CollectorsDOE Final Report for Phase III; Report DEFC26-99FT40719; U.S. Department of Energy: Washington, DC, Sept 2001.
Impact of TDF Firing on Mercury Species and Capture
Energy & Fuels, Vol. 20, No. 3, 2006 1041
Table 1. Coal Analysis Results
proximate analysis (wt %) moisture content volatile matter fixed carbon ash ultimate analysis (wt %) hydrogen carbon nitrogen sulfur oxygen ash heating value (Btu/lb) chlorine in coal, dry basis (µg/g) mercury in coal, dry basis (µg/g) sample 1 sample 2 sample 3 mean
as-received
moisture-free
24.8 35.94 34.42 4.83
47.83 45.74 6.43
6.29 51.42 0.8 0.29 36.37 4.83 8999
4.69 68.42 1.07 0.38 19.01 6.43 11 975 22.5 0.077 0.089 0.115 0.094
Table 2. Summary of the Subbituminous Coal Flue Gas Compositions (dry basis) O2 (%)
CO2 (%)
CO (ppm)
NO (ppm)
NO2 (ppm)
SO2 (ppm)
HCl (ppm)
4.5
14.9
3.9
583
5.8
315
0-0.75
(mass-based), respectively. Ontario Hydro samples were collected at the Advanced Hybrid filter inlet and outlet, during the baseline test (no TDF cofiring) and TDF cofiring periods to quantify the impact of TDF cofiring on mercury species in the flue gas and to determine the corresponding beneficial effect on mercury removal with the Advanced Hybrid filter. EPA Method 26 sampling was conducted to determine the increase in HCl in the flue gas with TDF cofiring.
Results and Discussion Fuel and Flue Gas Analyses. Table 1 lists the proximate and ultimate analysis data and mercury content of the subbituminous coal tested. The analysis data show a mercury concentration of 0.077-0.115 µg/g (dry basis), with a mean value of 0.094 µg/g in the raw coal. On the basis of the proximate and ultimate analysis data, 1 kg of the subbituminous coal would produce 6.7 m3 of dry flue gas normalized to a 3.0% oxygen level. Combining the mercury content in raw coal and combustion calculation results, the total mercury concentration in the flue gas was expected to be approximately 14 µg/m3 of dry flue gas (at a 3% oxygen level). Table 2 summarizes the typical flue gas concentrations of O2, CO2, CO, NO, NO2, SO2, and HCl in the western subbituminous coal flue gas without TDF cofiring. The CO concentration was around 4 ppm, indicating complete coal combustion. The low-sulfur subbituminous coal produced 315 ppm SO2 in the flue gas. The NO and NO2 concentrations in the flue gas were 583 ppm for NO and only 5.8 ppm for NO2. The EPA Method 26 samples were completed to determine HCl concentration in the baseline coal flue gas (No TDF cofiring), and the results (also listed in Table 2) showed extremely low levels of HCl concentration (0-0.75 ppm) in the flue gas. Because of the low levels of CO, SO2, NO, NO2, and HCl in the flue gas, most mercury in the baseline subbituminous coal flue gas was expected to be in the elemental vapor phase. Table 3 lists the proximate-ultimate analysis results of the TDF, indicating higher carbon content and heating values than those of the subbituminous coal tested. The collected TDF sample was ashed in a 500 °C environment for 2 h for the X-ray fluorescence spectroscopy (XRF) analysis to determine the
Figure 3. Mercury species in the flue gas in baseline and TDF cofiring tests. Table 3. Analysis Results of the TDF
proximate analysis (wt %) moisture content volatile matter fixed carbon ash ultimate analysis (wt %) hydrogen carbon nitrogen sulfur oxygen ash heating value (Btu/lb) XRF Analysis (%) Si Al Fe Ti P Ca Mg Na K S Zn Cu C Cl concentration in TDF, as received (µg/g)
as-received
moisture-free
0.6 66.22 28.06 5.12
66.61 28.24 5.15
7.58 83.23 0.80 2.69 0.58 5.12 16 485
7.56 83.72 0.8 2.71 0.05 5.15 17.6 3.4 5.8 0.2 0.5 3.9 1.1 31.8 1.2 4.2 19.6 0.3 3.7 598
elemental compositions in the ashed TDF. The results are included in Table 3. The major elements in the ashed TDF are Na, 31.0%; Zn, 19.6%; Si, 17.6%; and Fe, 5.8%. The TDF sample was analyzed separately for chlorine content, and the analysis data showed 598 µg/g chlorine in the TDF, which is much higher than the 22.5 µg/g chlorine in the subbituminous coal. The additional chlorine in the combustion zone may enhance mercury oxidation in the coal flue gas. Mercury Results Discussion. To clarify the effect of cofiring TDF on mercury species in the flue gas, we started pilot-scale tests with coal combustion only (without cofiring of TDF) to establish mercury species in the subbituminous coal flue gas in the baseline condition. The TDF, as a supplemental fuel, was then fed into the furnace at feed rates of 1.4 and 2.8 kg/hr, corresponding to 5 and 10% (mass basis) of the feeding coal, respectively. Ontario Hydro sampling was completed at both the Advanced Hybrid filter inlet and outlet during the baseline and TDF cofiring tests to determine mercury species across the Advanced Hybrid filter. Plotted in Figure 3 are mercury species results from Ontario Hydro measurement. In the subbituminous coal baseline test without TDF cofiring, the flue gas contained 11.2 µg/m3
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Zhuang and Miller Table 4. Chloride Concentrations in Flue Gas (in ppm, dry basis, 3% O2) baseline
5% TDF (1.4 kg/hr)
10% TDF (2.8 kg/hr)
0-0.75
2.54
2.47
Table 5. LOI and Mercury Content in Combustion Ash
LOI (%) Hg (µg/g)
Figure 4. Normalized mercury species distributions in flue gas in baseline and TDF cofiring test.
elemental mercury, 2.3 µg/m3 oxidized mercury, and 0.2 µg/ m3 (a very low level) particle-bound mercury. The high level of elemental mercury in the subbituminous coal flue gas is typical and caused by the low levels of chlorine in the raw coal. Because all of the particulate-associated mercury was captured within the Advanced Hybrid filter, the outlet flue gas had only gaseous mercury with 7.8 µg/m3 elemental mercury and 2.9 µg/ m3 oxidized mercury. There was 21.8% inherent mercury capture in the subbituminous coal flue gas with the Advanced Hybrid filter. With 5% TDF cofiring (1.4 kg/hr), mercury partitioning in the flue gas changed, as shown in Figure 3. Although the levels of particle-bound mercury in the flue gas were still low, there was more oxidized mercury (6.4 µg/m3) in the flue gas than in the baseline test (2.3 µg/m3). The corresponding elemental mercury was reduced from 11.2 µg/m3 in the baseline to 7.0 µg/m3 at 5% TDF cofiring. Additional mercury oxidation occurred across the Advanced Hybrid filter, resulting in 9.4 µg/ m3 of oxidized mercury in the Advanced Hybrid filter outlet flue gas. When the TDF cofiring was increased to 10% of the coal feed rate, corresponding to 2.8 kg/hr, most of the mercury was oxidized before it entered the Advanced Hybrid filter. The total mercury concentration in the Advanced Hybrid filter inlet flue gas was 14.4 µg/m3, including 1.2 µg/m3 particle-bound mercury, 12.2 µg/m3 oxidized mercury, and only 0.9 µg/m3 elemental mercury. After the gas flowed through the Advanced Hybrid filter, there was 10 µg/m3 oxidized mercury and a trace level of elemental mercury in the outlet flue gas. To further clarify the effect of TDF cofiring on mercury oxidation in flue gas, we normalized and replotted the above mercury species data in Figure 4. It clearly shows that more oxidized mercury in the flue gas enters the Advanced Hybrid filter when TDF is cofired with the subbituminous coal than when the coal is combusted alone: 47.7% oxidized mercury with 5% TDF cofiring compared to 16.8% without, indicating that enhanced mercury oxidation occurred upstream of the Advanced Hybrid filter. The 10% TDF cofiring converted even more mercury into oxidized and particle-bound mercury: 6.7% particle-bound mercury, 84.8% oxidized mercury, and only 8.4% elemental mercury. The outlet mercury speciation data (Figure 4) showed that additional oxidation occurred across the Advanced Hybrid filter with TDF cofiring; 97.3% of the mercury emission was oxidized mercury during the 10% TDF cofiring. TDF cofiring, however, did not significantly improve mercury removal with the Advanced Hybrid filter, because the mercury remained in the gaseous phase when going through the Advanced Hybrid filter. The 10% TDF cofiring provided 28% mercury capture with the Advanced Hybrid filter compared to 21% without TDF cofiring.
baseline
5% TDF (1.4 kg/hr)
10% TDF (2.7 kg/hr)
0.16 0.0076
0.14 0.0094
0.48 0.17
The above experimental data indicate that cofiring TDF enhances mercury oxidation, which may start in the combustion zone and continue in the Advanced Hybrid filter unit. The possible reasons could be the additional chlorine in the feed TDF, the unburned carbon from the TDF combustion, or a combination of these effects. During combustion, the chlorine compound in the TDF thermally decomposes and most of the Cl released reacts with water vapor to produce HCl. It is anticipated that some of the volatilized Cl recombines with Na and Ca to form NaCl and CaCl2. A very small proportion of the Cl is expected to remain in its atomic form, react to form HOCl, or form Cl2 through catalysis reactions with metals such as zinc. Theoretically, Cl, HOCl, and Cl2 are chemically reactive with Hg0.6-8 In addition, Cl attached to a catalytic site on fly ash or unburned carbon surfaces can oxidize Hg0 and/or enhance Hg capture.15 The analysis results from EPA Method 26 samplings showed a 2.54 ppm HCl concentration in flue gas with 5% TDF cofiring compared to 0-0.75 ppm measured in the baseline test (listed in Table 4). The 2.54 ppm HCl in flue gas is still low, but the TDF combustion may enhance the formation of atomic chlorine in the combustion zone, which is regarded as the chlorine species responsible for mercury oxidation. The 10% TDF cofiring did not double HCl concentration in the flue gas. The combustion ashes were collected for loss on ignition (LOI) and mercury content analyses. Listed in Table 5 are the analysis results of the baseline and TDF cofiring tests. The LOI levels in the baseline and the 5% TDF cofiring tests were very close: 0.16 and 0.14% for baseline and 5% TDF cofiring, respectively. The corresponding mercury contents in the ashes were also similar: 0.0076 µg/g (baseline) and 0.0094 µg/g (5% TDF cofiring). The 10% TDF cofiring test generated ash with a 0.48% LOI, three times as much as that in the baseline test. The mercury content in the 10% TDF cofiring ash also increased to 0.17 µg/g. The above analysis data indicate that the modestly improved mercury capture observed in the 10% TDF cofiring test was caused by the elevated LOI in the ash. The LOI in the ash not only improves mercury oxidation but also benefits the conversion of gas-phase mercury to particle-bound mercury that was subsequently captured on the filter bags. The additional chlorine in the flue gas was most likely not the only reason for the significantly enhanced mercury oxidation in the TDF cofiring tests. The reactive chlorine species generated in the TDF combustion also improved the reactivity of the fly ash and/or unburned carbon with mercury, which, in turn, benefited mercury oxidation in the flue gas. TDF is a supplemental fuel used in some coal-fired utility plants because of its high energy density and low cost and does not significantly affect plant operation. The enhanced mercury (15) Gale, T. K.; Merritt, R. L.; Cushing, K. M.; Offen, G. R. The Combined Power Plant Air Pollutant Control Symposium: Mega Symposium, Washington, DC, 2003; Paper 28.
Impact of TDF Firing on Mercury Species and Capture
oxidation from cofiring of TDF has great potential in mercuryemission control for power plants equipped with wet or dry FGD, because oxidized mercury is easily captured in these pollutant control devices. Conclusions Cofiring of TDF with subbituminous coal had a significant effect on mercury speciation in the flue gas. When 100% coal was fired, there was only 16.8% oxidized mercury in the flue gas compared to 47.7% when 5% TDF (mass basis) was cofired and 84.8% when 10% TDF was cofired. The significantly enhanced mercury oxidation may be the result of additional reactive chlorine generated in the TDF-cofiring flue gas and the in situ improved reactivity of fly ash and/or unburned carbon
Energy & Fuels, Vol. 20, No. 3, 2006 1043
in ash by the reactive chlorine species. Although the cofiring of TDF demonstrated limited improvement on mercury-emission control with the Advanced Hybrid filter, it showed potential as a very cost-effective mercury-control approach for power plants equipped with wet or dry FGD systems because of the enhanced mercury oxidation. Acknowledgment. This projected is sponsored by the U.S. Department of Energy (DOE) under DOE Cooperative Agreement DE-FC26-01NT41184. The content of this paper does not necessarily reflect the views of DOE, and no official endorsement should be inferred. EF050317Q
