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Ind. Eng. Chem. Res. 2007, 46, 1390-1395
Removal of Vapor-Phase Elemental Mercury by Oil-Fired Fly Ashes Jeom-In Baek,† Ji-Ho Yoon,*,‡ Si-Hyun Lee,§ and Chong Kul Ryu† Korea Electric Power Research Institute (KEPRI), Korea Electric Power Corporation, Daejeon 305-380, Department of Energy & Resource Engineering, Korea Maritime UniVersity, Busan 606-791, and Clean Energy Research Department, Korea Institute of Energy Research (KIER), Daejeon 305-600, Korea
The vapor-phase elemental mercury removal efficiency of the heavy-oil-fired fly ash (HOFA) discharged from heavy-oil-fired power plants was first tested to evaluate its suitability as a base material for the development of a low-cost novel sorbent to capture vapor-phase mercury in coal combustion flue gases. Raw, CO2-activated, and sulfur-impregnated HOFAs were prepared and tested. The morphology, specific surface area, particle size, and chemical composition were analyzed for the tested samples. A bench-scale fixed-bed reactor system was used to determine the mercury removal efficiencies of the HOFAs and commercially available activated carbons for comparison. The CO2-activated HOFA showed slightly higher mercury removal efficiency than the raw HOFA, resulting from the increase of active sorption sites by the enlarged surface area. The mercury removal efficiencies of the HOFAs modified by the sulfur impregnation process significantly increased with increasing sulfur content and were comparable to those of the commercially available sulfur-impregnated activated carbons, despite their much smaller surface area. These results suggest that the sulfur sites formed on the surface of the HOFAs during the impregnation process are highly active in capturing vapor-phase elemental mercury. Introduction Various mercury control technologies to reduce mercury emission from coal-fired utilities are being developed in a manner of finding the most appropriate method to combine with air pollutant control devices.1,2 Activated carbon (AC) has intensively been studied as a promising sorbent with large adsorption capacities for vapor-phase mercury released from coal-fired utility boilers. However, direct injection of powdered activated carbon (PAC) into the flue gases upstream of the particulate control units in coal-fired utilities has some drawbacks which have yet to be resolved. The short residence time of the AC upstream of the particulate collection device, the existence of competing species in the flue gases that adsorb to the active sites of the carbon, and the low concentrations of mercury in the flue gases require a high ratio of carbon to mercury to increase the mercury removal efficiency. Chemically impregnated ACs have been studied to enhance the mercury capture efficiency and reactivity and to reduce the amount of carbon injected for mercury removal in the flue gas streams.3-10 Iodine, chlorine, sulfur, and bromine are normally used as the impregnation chemicals. Recent studies have shown that AC adsorbents with larger adsorption capacities for mercury can be developed by impregnating the samples with elemental sulfur.6-10 In parallel with the efforts to improve the mercury capture efficiency of ACs, there have also been a variety of studies on the development of novel sorbents to reduce the operation cost of sorbent injection technology.11-14 Coal-fired fly ash (CFA) has been extensively studied for its possible reuse as a mercury sorbent.15-18 The unburned carbon contained in the coal-fired fly ash is of particular interest, due to its capability to capture vapor-phase mercury, with the amount of carbon in the CFA * To whom correspondence should be addressed. Tel.: +82-51-4104684. Fax: +82-51-404-3986. E-mail:
[email protected]. † Korea Electric Power Corp. ‡ Korea Maritime University. § KIER.
considered to be a key parameter affecting the removal efficiency of vapor-phase mercury. It is known that mercury adsorption usually increases with increasing carbon content in the CFA. Heavy-oil-fired fly ash (HOFA), oil-fired fly ash discharged from heavy-oil-fired power plants, is industrial waste, and it has usually been disposed in landfills or incinerated with coal. Some studies on the transformation of HOFAs into useful porous materials have been reported in the literature.19-23 HOFAs are mainly composed of carbon, metal oxides, and water-soluble sulfate compounds.19,22,24 We note that the carbon and sulfur contents of HOFAs are over 50 and 1 wt %, respectively, much higher than those of CFAs. This indicates that HOFAs could also be used for mercury removal as a potential absorbent with high surface area and active adsorption sites. In this paper, we present the first use of the HOFAs for capturing vapor-phase elemental mercury. Both virgin and chemically modified HOFAs were tested to examine their physicochemical properties and removal efficiency of vapor-phase elemental mercury. Commercially available ACs were also tested for comparison. Experimental Section Sample Preparation. Two kinds of HOFAs were received from two different power plants. HOFA-A was obtained from a power plant which is equipped with an electrostatic precipitator and a wet flue gas desulfurization (FGD) process as an air pollutant control device and burns heavy oil with a high sulfur content of about 2.5 wt %. HOFA-B was obtained from another power plant which is equipped only with an electrostatic precipitator and burns heavy oil with a low sulfur content of about 0.3 wt %. Both HOFAs were sampled at the ash hopper of the electrostatic precipitator of each power plant because the majority of the HOFA from a boiler is captured at the electrostatic precipitator in the oil-fired power plants. HOFAA-CO2 and HOFA-B-CO2 were prepared by activating HOFA-A and HOFA-B, respectively, with CO2 to increase the surface area. A sample consisting of about 40 g of HOFA spread on a boat-type pan (dimensions (W × D) 50 × 150 mm) was
10.1021/ie060813h CCC: $37.00 © 2007 American Chemical Society Published on Web 01/13/2007
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activated with CO2 at a constant flow of 1 L/min (STP) at 900 °C for 5 h after the furnace temperature was raised to 900 °C with continuously flowing CO2. CO2-activated HOFAs are physically mixed with 1 wt % (HOFA-A-CO2-S1 and HOFAB-CO2-S1) and 10 wt % (HOFA-A-CO2-S10 and HOFAB-CO2-S10) elemental sulfur, and then sulfur impregnation is carried out at a temperature of 550 °C for 1 h. Calgon BPL 4/10, Norit GAC 12/40, and Calgon HGR are commercially available granular-type ACs derived from bituminous coal. Calgon BPL 4/10 and Norit GAC are virgin ACs on which no chemicals are impregnated, whereas Calgon HGR is a sulfurimpregnated AC. Norit Darco 4/12, which is a granular-type AC based on lignite coal, was also used in this work. These commercial ACs were crushed to obtain particles within the desired size range (45-150 µm). All samples were dried in an oven at 110 °C overnight before the mercury adsorption tests were conducted. Sample Characterization. A scanning electron microscope (JEOL JSM-6360) was used in conjunction with an energy dispersive spectrometer to observe the morphology of the samples and to characterize the external surface. The particle size distribution of the samples was determined using a laser diffraction system (Mastersize 2000, Malvern Instruments). The proximate analysis designed to obtain the composition of the volatile materials, ash, and fixed carbon was conducted using a thermogravimeter (TGA-501, LECO). In the ultimate analysis, the carbon, hydrogen, and nitrogen contents were determined by an elemental analyzer (CHN-1000, LECO). The total sulfur content was examined by a sulfur analyzer (SC-432DR, LECO) with an accuracy of (1%, and the concentration of sulfate ions (SO42-) was measured for the HOFA-immersed aqueous solution. A 1 g sample of HOFA was immersed in 100 mL of pure water at room temperature, and the solution was stirred at a speed of 500 rpm for 3 h. The stirred solution was filtered to obtain a particle-free solution. The concentration of SO42- of the filtered solution was measured by an ion chromatograph (DX-500, DIONEX). The specific surface area of the samples were determined by N2 adsorption at 77 K using an automatic multipoint volumetric apparatus (ASAP 2010, Micromeritics). Each sample was outgassed to 0.002 mmHg at 120 °C before N2 adsorption. The specific surface area was calculated from the application of the Brunauer-Emmett-Teller (BET) equation to the adsorption isotherm in the relative pressure range of 0.050.2. Experimental Apparatus and Procedures. A schematic diagram of the experimental setup used in this study is shown in Figure 1. It mainly consists of a vapor-phase elemental mercury generator, a fixed-bed reactor surrounded by a temperature-controlled air oven, an on-line mercury analyzer, and a data acquisition system. A certified Dynacal mercury permeation tube (VICI Metronics) is used as the source of vaporphase elemental mercury. The mercury permeation tube is located inside a Teflon tube immersed in a temperaturecontrolled water bath kept within (0.1 °C of the set point temperature (MC-31, Jeio Tech). The front part of the Teflon tube is charged with glass beads with a diameter of 2 mm to preheat the gas to the set point temperature. An ultra-high-purity grade (99.999%) of nitrogen gas is supplied as a carrier gas to transport the vapor-phase elemental mercury out of the permeation tube holder. The nitrogen gas laden with vapor-phase elemental mercury is diluted with nitrogen gas of the same grade input from a separate source. Each gas flow rate is kept constant during the experiments by thermal conductivity mass flow controllers (5850E, Brooks). On the basis of the standard
Figure 1. Schematic diagram of the fixed-bed mercury adsorption apparatus.
temperature and pressure conditions, the total gas flow rate of the simulated gas passing through the fixed-bed reactor is 1.65 L/min, which is in the range of residence times (about 0.5-3 s) of the actual processes used for the removal of vapor-phase mercury in the flue gas. The inlet mercury concentration (C0) in the range of 26.1-31.3 µg/m3 is used for mercury adsorption tests. The fixed-bed reactor is made of a quartz tube 30 cm in length with an outer diameter of 12.8 mm and inner diameter of 8.8 mm. A quartz tube with the same size, charged with 2 mm glass beads, is placed ahead of the reactor to preheat the gas to the reaction temperature. All of the plumbing and valves through which the mercury is passed are constructed of Teflon or quartz. These materials are known to be inert toward mercury. The fixed-bed reactor and preheater are surrounded by a temperaturecontrolled convection oven kept within (0.1 °C of the set point temperature (OF-12G, Jeio Tech). After passing through the reactor, the gas is cooled using a gas cooler maintained at a constant temperature of 15 °C to prevent the breakdown of the mercury analyzer, which requires the sample gas temperature to be under 65 °C. An ultraviolet (UV) mercury analyzer (mercury vapor monitor VM-3000, Mercury Instruments), based on cold vapor atom absorption spectrometry (CVAAS), is used to continuously measure the concentration of elemental mercury vapor within a resolution of (0.1 µg/m3. In each experiment, 60 mg of sorbent is charged into the fixed-bed reactor. The sorbent bed is placed between 0.5 g of quartz sand beds to prevent the sample entrainment in the gas flow. The sorbent and quartz sand beds are supported by quartz wool. Glass beads are packed in the inlet of the fixed-bed reactor. In the preliminary test, it is confirmed that quartz wool, quartz sand, and glass beads are inert toward mercury. Each test conducted in this work is carried out at a fixed temperature of 130 °C. Usually the powdered mercury sorbent is injected in the temperature range of 100-160 °C between an air preheater and a particulate control device. Prior to each adsorption experiment, the reactor is fully equilibrated at the desired temperature. Each sorption test was repeated at least three times and was reproducible to provide results within 3%. Results and Discussion Physicochemical Characteristics of HOFAs. Scanning electron microscopy (SEM) micrographs of the HOFAs (Figure 2) show that the HOFAs consist of spherical and crumbled
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Ind. Eng. Chem. Res., Vol. 46, No. 4, 2007 Table 1. Properties of Raw, CO2-Activated, and Sulfur-Impregnated HOFAs and ACs sulfur average particle specific surface content (wt %) size (µm) area (m2/g)
sorbent HOFA-A HOFA-A-CO2 HOFA-A-CO2-S1 HOFA-A-CO2-S10 HOFA-B HOFA-B-CO2 HOFA-B-CO2-S1 HOFA-B-CO2-S10 Calgon BPL 4/10 Norit GAC 12/40 Calgon HGR Norit Darco 4/12
75 81 56 61 125 127 112 96
1.1 ( 0.0 42.4 ( 0.3 26.3 ( 0.1 26.0 ( 0.1 3.4 ( 0.0 38.2 ( 0.2 30.0 ( 0.1 26.9 ( 0.1 1004 ( 11 919 ( 8 632 ( 7 438 ( 3
8.0 5.2 5.1 7.8 1.1 1.6 1.8 2.7 0.7 0.6 9.7 0.7
Table 2. Proximate and Ultimate Analysis (wt %) of HOFAs (Dry Basis) proximate
Figure 2. SEM images of the HOFAs: (a) HOFA-A; (b) HOFA-B; (c) HOFA-A-CO2.
particles, and the spherical particles have two different surface structures of porous and foamed types. It is clear that the macroscopic morphology of HOFAs does not change with the gasification process to obtain the samples HOFA-A-CO2 and HOFA-B-CO2. The results obtained in this study concerning the morphology of the HOFAs are very similar to those of other researchers.19,22,23 Energy-dispersive spectrometry (EDS) analysis (Figure 1 in the Supporting Information) reveals that the carbon, oxygen, and sulfur contents on the surface of the HOFAs are relatively high, whereas other metallic elements such as V, Ni, Fe, Cu, Zn, Mg, and Na are present at lower concentrations. The sulfur contents on the surface of HOFA-A and HOFA-ACO2 by EDS analysis were higher than those of HOFA-B and HOFA-B-CO2, respectively, indicating that the combustion of heavy oil with a higher sulfur content results in a higher sulfur content on the surface of the HOFAs. The SO42- concentration for the HOFA-A-immersed solution was 1286 ppm, which is approximately 6-fold higher than that for the HOFA-B-immersed solution. The SO42- concentration for each HOFA-A-CO2- and HOFA-B-CO2-immersed solution was in the range of 40-80 ppm and much lower than those for raw HOFAs. This indicates that the HOFA derived from the heavy oil may have a significant amount of sulfur compounds which are easily decomposed during the CO2 activation process. From the analysis of the total sulfur content of HOFAs (Table 1), the sulfur contents of HOFA-A and HOFA-A-CO2 are higher than those of HOFA-B and HOFA-B-CO2, respectively, which is identical to the result of the EDS analysis. The average particle sizes of HOFA-A and HOFA-B are 75 and 56 µm, respectively. As shown in Table 1, the BET surface area of the raw oil-fired fly ashes, HOFA-A and HOFA-B, is very low. However, when the HOFAs are activated with CO2, the specific surface area is markedly increased and close to that of general industrial carbon blacks, but still much smaller than that of the ACs. It is known that the physicochemical property of HOFA is significantly different from that of CFA.25-30 The major components of CFA are SiO2, Al2O3, Fe2O3, CaO, and MgO, whereas the HOFAs mainly consist of carbon and sulfur, as shown in the proximate and ultimate analysis (Table 2). We note that the carbon and sulfur contents in HOFAs are usually
sample
volatile matter
HOFA-A HOFA-A-CO2 HOFA-B HOFA-B-CO2
26.2 18.8 17.2 22.0
ultimate
ash
fixed carbon
C
H
N
S
7.2 7.8 1.4 11.4
66.6 73.4 81.4 66.6
80.7 85.3 94.7 83.5
0.16 0.12 0.14 0.14
1.2 0.9 1.9 1.5
8.0 5.2 1.1 1.6
over 50 and 1 wt %, respectively, which allows us to consider HOFA as a novel sorbent with large active adsorption sites for mercury removal, especially from the viewpoint of industrial application. The analysis of the total sulfur content for the sulfurimpregnated HOFAs represents that only a portion of sulfur mixed with CO2-activated HOFA was impregnated (Table 1). We also found that there was no apparent variation in the total sulfur content before and after HOFA-A-CO2 and HOFA-BCO2 were impregnated with 1 wt % sulfur, resulting from decomposition of the inherent sulfurs loosely bound in the raw HOFAs for the impregnation process. In the case of CO2activated HOFA mixed with 10 wt % sulfur, the total sulfur content of HOFA-A-CO2 and HOFA-B-CO2 was increased to 7.8 and 2.7 wt %, respectively. However, it must be noted that the sulfur-impregnated HOFA may have large active adsorption sites to drastically increase the mercury removal efficiency. Mercury Removal Efficiencies. In this work, our particular interest is focused on the initial maximum mercury removal efficiency which appears in the beginning stage of the adsorption reaction. The initial mercury removal efficiency is a crucial factor in sorbent injection technology because of the short contact time between the sorbents and the flue gases in the commercial plants. The experimental results of the vapor-phase elemental mercury removal are depicted in Figures 3-5, and comparison of our samples with commercial sorbents is given in Figure 6. The initial mercury removal efficiency of HOFA-A was higher than that of HOFA-B (Figures 3 and 4). This was caused by the fact that the higher sulfur content on the surface of HOFA-A enabled it to adsorb more mercury. We note that there was a slight difference in the mercury removal efficiency between the raw HOFAs and CO2-activated HOFAs, although the surface area was highly increased during the CO2 activation process, which was expected to make a lot of active sorption sites. The lower mercury removal efficiency of HOFA-B-CO2 compared to HOFA-A-CO2, in spite of their similar surface areas, might be the result of the lower sulfur content of HOFAB-CO2 and fewer active adsorption sites existing on the sorbent surface.
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Figure 3. Vapor-phase mercury adsorption breakthrough curves of HOFAA.
Figure 4. Vapor-phase mercury adsorption breakthrough curves of HOFAB.
Figure 5. Vapor-phase mercury adsorption of sulfur-impregnated HOFAs and commercially available ACs.
The initial mercury removal efficiency of sulfur-impregnated HOFAs significantly increased with increasing sulfur content. As indicated previously, we could not find any change in the total sulfur content when the CO2-activated HOFAs were impregnated with 1 wt % sulfur. Nevertheless, the maximum mercury removal efficiency of HOFA-A-CO2-S1 was increased from 24.4% to 66.3% and for HOFA-B-CO2-S1 from
Figure 6. Initial maximum mercury removal efficiencies of the sorbents.
5.7% to 10.1%. The mercury removal efficiency of HOFA-A was substantially decreased to 3.5% when it was fully washed with pure water. This indicates that the majority of active mercury sorption sites like sulfur compounds could be washed from the surface. It should be noted, here, that the elemental sulfur or sulfur complexes formed on the surface of the HOFAs during the impregnation process may provide more stable and active adsorption sites for mercury bonding, resulting in increased removal efficiency. Of particular interest is that it was possible to achieve a mercury removal efficiency of over 95% when the CO2-activated HOFAs were impregnated with 10 wt % sulfur. Although the total sulfur content of HOFA-A was higher than that of HOFA-A-CO2-S1 and HOFA-A-CO2S10, its mercury removal efficiency was lower than that of sulfur-impregnated HOFAs. This is due to the fact that only a small portion of sulfur compound in HOFA-A was exposed on the surface as active mercury sorption sites, while the majority of the impregnated sulfur was exposed on the surface and made active mercury sorption sites. Although the surface area of HOFA-A-CO2 is much smaller than that of commercial ACs such as Calgon BPL 4/10 and Norit GAC 12/40, the maximum mercury removal efficiencies of our sample and the commercial ACs were very similar and comparable. Our preliminary measurements indicate that the total sulfur contents of Calgon BPL 4/10 and Norit GAC 12/40 were 0.78 and 0.60 wt %, respectively. Note that the sulfur content of HOFA-A-CO2 is 5.2 wt %. Thus, these results suggest that the presence of active adsorption sites, such as active sulfur sites, strongly influences the mercury removal efficiencies of the HOFAs. The sulfur-impregnated commercial AC Calgon HGR has revealed high mercury removal efficiencies of over 90% (Figure 6) and is thus comparable to our sulfurimpregnated samples. The sulfur content of Calgon HGR was 9.7 wt %. However, sulfur in Calgon HGR may be predominantly in the form of S8 rings, which are less reactive and are weakly bonded to the carbon surface in the macroporous region of Calgon HGR.31 The impregnated HOFAs are supposed to be in the form of shorter chains. When sulfur is impregnated above 400 °C, the proportion of the smaller and shorter chain sulfur molecules, S2 and S6, among several sulfur allotropes increases and sulfur is bound on the surface of the porous sorbent more uniformly.8,31 This form of sulfur is more reactive with gaseous mercury. The lignite-based AC Norit Darco 4/12 also showed a high mercury removal efficiency of over 95% in spite of its small sulfur content, 0.67 wt %. It is well-known
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that lignite-based ACs have well-developed oxygen functional groups on their surface, which is one of the important active sorption sites for mercury capture. The high mercury removal efficiency of Norit Darco 4/12 is due to this sorption site even though it contains a small amount of sulfur. It is interesting to note that both Calgon HGR and Norit Darco 4/12 have approximately 10 times larger surface areas than the sulfurimpregnated HOFAs used in this work. These results support the potential applicability of the sulfur-impregnated HOFAs in capturing vapor-phase elemental mercury in the flue gas stream. In our work, only vapor-phase elemental mercury has been studied for testing the mercury removal performance for the newly developed mercury sorbents, as wet FGD technologies used to remove sulfur dioxide (SO2) from combustion gases can be effective in capturing oxidized mercury. However, the potential interferences from oxidized mercury or other contaminants such as SO2, CO2, and NOx in the flue gas streams should also be considered for practical application of the sulfurimpregnated HOFAs. In addition, future work is needed to examine how the oxygen functional groups on the surface of HOFAs developed by sorbent modification treatments may influence the mercury removal efficiency. Conclusion In this study, we presented the physicochemical properties and mercury removal efficiencies of raw, CO2-activated, and sulfur-impregnated HOFAs. Even though the HOFAs have smaller surface areas, the mercury removal efficiencies of the CO2-activated and sulfur-impregnated HOFAs were comparable to or slightly higher than those of the commercial ACs and sulfur-impregnated commercial ACs, respectively. This suggests that the sulfur sites formed on the surface of the HOFAs are highly active in capturing elemental mercury. More detailed studies are required for better understanding the role and bonding structure of sulfur impregnated on the surface of the HOFAs and the effect of modifying the mercury sulfur on the surface on the adsorption properties of the HOFAs. We believe that innovative modifications of the HOFAs including structure and morphology control and chemical treatments would definitely lead to a low-cost mercury removal process, capturing the elemental mercury at lower rates competitive with those of PAC technologies. Acknowledgment We thank Electric Power Industry Technology Evaluation & Planning (ETEP) and the Korea Electric Power Corp. (KEPCO) for funding this research. This research was also supported by the Faculty Support Program of the Korea Maritime University and the Korea Research Foundation Grant funded by the Korean Government (MOEHRD, Basic Research Promotion Fund) (KRF-2006-003-D00102). Supporting Information Available: EDS analyses of raw and CO2-activated HOFAs. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Brown, T. D.; Smith, D. N.; Hargis, R. A.; O’Dowd, W. J. Mercury measurement and its control: what we know, have learned, and need to further investigate. J. Air Waste Manage. Assoc. 1999, June, 1. (2) Pavlish, J. H.; Sondreal, E. A.; Mann, M. D.; Olson, E. S.; Galbreath, K. C.; Laudal, D. L.; Benson, S. A. Status review of mercury control options for coal-fired power plants. Fuel Process. Technol. 2003, 82, 89.
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ReceiVed for reView June 26, 2006 ReVised manuscript receiVed November 27, 2006 Accepted December 8, 2006 IE060813H
