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Trace Element Removal from Hot Gases: Screening Sorbents for Performance and Product Leachability H. Lachas, A. A. Herod, G. P. Reed,* D. R. Dugwell, and R. Kandiyoti Department of Chemical Engineering and Chemical Technology, Imperial College London SW7 2BY, U.K. Received July 19, 2002
High-temperature removal of toxic trace elements (TE) from gasification product streams is a requirement for exploiting the thermodynamic advantages of hot gas cleanup over conventional cold gas scrubbing. A novel bench scale reactor simulating hot gas cleanup using dry sorbents has been designed, constructed, and operated. The design enables vaporization of model sample compounds at up to 1000 °C and fixed-bed sorption of vapor phase species between 300 and 600 °C. Model compounds of As, Cd, Pb, Se, and Hg have been used in experiments examining the capture capabilities of sorbent kaolin and activated carbon; the subsequent leachability of sorbed trace elements has also been studied, to provide guidance regarding disposal as landfill. Kaolin was found to be the better sorbent for Pb and Cd, achieving high retention performance at 600 °C and low subsequent leachability. Activated carbon appears to be a better sorbent for Hg and Se. The optimum combination for high As retention and low leachability was found with kaolin at 600 °C. A possible explanation for this finding in terms of the preferred speciation at thermodynamic equilibrium has been presented. A TEM-EDXRF study of the two sorbents before and after use has indicated differences in the mode of trace element capture by the two sorbents. On kaolin, the captured species tend to be retained as an even layer of low concentration over most of the surface, while on activated carbon, the captured species appear mostly to be concentrated in discrete locations.
Introduction Gasification of Solid Fuels and Wastes. In response to tightening emissions regulations, gasification of solid fuels to produce a fuel gas for firing a gas turbine for power generation1,2 or process heat has been widely investigated. If a waste with a significant heating value can be substituted for part of the solid fuel, the economics of energy conversion by this technology may be greatly improved. However the use of wastes usually implies that the scale of the process must be modest (ca. 10 MWt), because of waste availability restrictions and transport costs. At such a scale infrastucture requirements favor air-blown gasification, the preferred technology for fuels with high ash and moderate sulfur contents.3 Contaminants that are harmful to the gas turbine or the environment must be removed from the fuel gas before it is combusted. There is a thermodynamic advantage to hot gas cleanup, rather than the more conventional approach of gas cooling and wet scrubbing; air-blown systems, such as the air-blown gasification cycle (ABGC),4 gain much of their efficiency advantage through the use of hot gas cleaning.5 Mechanical particle * Corresponding author. E-mail:
[email protected]. (1) Takematsu, T.; Maude, C. Coal Gasification for IGCC Power Generation. Report IEACR/37; IEA Coal Research: London, UK, 1991. (2) EPRI. Gasification Technologies Conference, San Francisco; 1997. EPRI: Palo Alto, CA. (3) Wheeldon, J. A. Review of PFBC Power Plant Designs. In Proceedings of the 13th International Pittsburgh Coal Conference, University of Pittsburgh, Pittsburgh, PA, 1996; pp 247-260.
separation in hot gas filters is an essential part of any hot gas clean-up system, but their effectiveness for the removal of vapors may not be sufficient to remove all of the contaminant species present. Alkali species, such as Na or K, are present as minor components in most coals; the risk of these passing through the hot gas filter as vapor and causing deposition/corrosion problems in the gas turbine or other downstream components is avoided by restricting the hot gas clean-up temperature to 600 °C. However, this may be insufficient to control the more volatile elements such as Pb. In the context of coal, the normally accepted definition of a trace element is presence in the dry coal at a level below 1000 ppm(wt), as suggested by Swaine.6 Their pathway through a gasification process into the ash or fuel gas is linked to their mode of association within the coal (also known as “mode of occurrence”7,8) and the operating conditions, as well as the physical properties of the chemical species their reactions may produce. Classifications, such as those of Clarke and Sloss7 and Kristiansen,9 can be used to group trace elements into (4) Welford, G. B. Gasification and Mitsui Babcock Energy Ltd. In Proceedings of the International IchemE Conference on Gasification, Dresden, Germany, 1998. IChemE: Rugby, UK. (5) McMullan, J. T.; Williams B. C.; Sloan E. P. Clean Coal Technologies. Proc. Inst. Mech. Eng., Part A 1997, 211, 95-107. (6) Swaine, D. J. Trace Elements in Coal; Butterworth: Oxford, UK, 1990. (7) Clarke, L. B.; Sloss, L. L. Trace Elements: Emissions from Coal Combustion and Gasification. Report IEACR/38; IEA Coal Research: London, UK, 1992. (8) Finkelman, R. B. Modes of Occurrence of Potentially Hazardous Elements in Coal: Levels of Confidence. Fuel Process. Technol. 1994, 39 (1/3), 21-34.
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three groups according to the volatility of their most common species. Some of the most volatile elements, such as As, Cd, Pb, Se, and Hg, are also potentially toxic or phytotoxic, leading to environmental concerns about their dispersal from coal consumption.10 Control of Trace Elements. Hot gas dust filter (e.g., candle filter) temperatures may be controlled to condense trace elements, so that they may be removed from the gas by filtration. The use of a sorbent (such as activated carbon or kaolin) to form a nonvolatile species is one potential method of supplementing the filter as a means of removing trace elements from hot gases. Other workers have investigated enhancing trace element removal in existing particulate removal systems by sorbent addition at various upstream locations, including cofeeding with the fuel,11 direct injection to the gasifier,12 and injection at the furnace exit.13 However, this approach may compromise efficiency, duration, and temperature of contact more than a technology specifically designed for trace elements. A fixed-bed system located after the hot gas filter as shown in Figure 1 has some advantages in these respects, as well as requiring less attention to prevent plugging by dust. Uberoi and Shadman14,15 have investigated this approach for the removal of Pb and Cd from flue gases, and showed Kaolinite to be a suitable sorbent for their oxidizing conditions at 700 °C; the application of this result to lower temperatures and the nonoxidizing conditions of a gasifier requires experimental investigation. The potential for removal of other trace elements such as As and Hg is not known. The leachability of the captured trace element from the spent sorbent would be an important factor in determining environmental acceptability during disposal, and provides some indication of the product speciation. This paper describes a novel bench-scale hot gas clean-up reactor. The apparatus enables vaporization of sample model compounds at temperatures up to 1000 °C. Trace element capture capabilities of potential sorbents is examined by passing the gas stream through fixed-beds in the 300-600 °C temperature range. The ability of kaolin and activated carbon to remove trace elements from the gas phase and to produce a nonleachable waste have been investigated, using model compounds of a selection of trace elements including As, Cd, Pb, Se, and Hg. Where the relevant thermodynamic data for product species was available, a thermodynamic equilibrium model using free energy minimization procedures was applied to aid interpretation of the results. (9) Kristiansen, A. Understanding Coal Gasification. Report IEACR/ 86; IEA Coal Research: London, UK, 1996. (10) Bowen, H. J. M. Environmental Chemistry of the Elements; Academic Press: London, UK, 1979. (11) Venkatesh, S.; Fournier, D. J., Jr.; Waterland L. R.; Carroll, G. J. Evaluation of Mineral-Based Additives as Sorbents for Hazardous Trace Metal Capture and Immobilisation in Incineration Processes. Hazard. Waste Hazard Manage. 1996, 13 (1), 73-94. (12) Scotto, M. V.; Uberoi, M.; Peterson, T. W.; Shadman, F.; Wendt, J. O. L. Metal Capture by Sorbents in Combustion Processes. Fuel Process. Technol. 1994, 39 (1/3), 357-372. (13) Gullett, B. K.; Ragnunathan, K. Reduction of Coal-Based Metal Emissions by Furnace Sorbent Injection. Energy Fuels 1994, 8 (5), 1068-1076. (14) Uberoi, M.; Shadman, F. Sorbent for Removal of Lead Compounds from Hot Flue Gases. AIChE J. 1990, 36 (2), 307-309. (15) Uberoi, M.; Shadman, F. High-Temperature Removal of Cadmium Compounds using Solid Sorbents. Environ. Sci. Technol. 1991, 25 (7), 1285-1289.
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Figure 1. The integrated hot gas clean-up system.
Experimental Section The Hot Gas Cleanup Reactor. The reactor was a benchscale unit designed for studying the removal of trace element vapors by solid sorbents at high temperature,16 and subsequently modified by Reed17 for application to lower concentrations of Hg. As shown in Figure 2, it consisted of four stages: 1. a volatilization stage (first bed) at up to 1000 °C, where trace element vapors were generated in a gas stream, 2. a capture stage (second bed) containing the sorbent under study at the test temperature of 300-600 °C, 3. an ambient temperature bed to collect trace elements that penetrated the second bed, 4. a scrubber containing an absorbing solution to remove residual trace element vapors. Measurements of the mass and concentration of trace elements in each stage enabled a mass balance to be determined. Quartz was selected as the most suitable temperatureresistant material of construction for the reactor; this enabled the reactor to be cleaned between experiments with mixtures of acids that could not be considered with a reactor of metal construction. In this way errors in the mass balance closure due to contamination were minimized. The four reactor stages were separate units, joined at flat ground flanges held together by metal clips. Graphite foil gaskets were placed between each pair of flanges, and compression was maintained by nuts, bolts, and spring washers between the metal clips. Inside stages 1-3, a molybdenum mesh (0.002in. wire diameter; aperture size: 100 µm) was positioned on an internal (16) Lachas, H. Trace Element Partitioning and Emission Control During Coal Gasification. Ph.D. Thesis, Imperial College of Science, Technology and Medicine, University of London, UK, 1999 (October 1999). (17) Reed, G. P.; Ergu¨denler, A.; Grace, J. R.; Watkinson, A. P.; Herod, A. A.; Dugwell, D.; Kandiyoti, R. Control of Gasifier Mercury Emissions in a Hot Gas Filter: The Effect of Temperature. Fuel 2001, 80, 179-194.
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Energy & Fuels, Vol. 17, No. 3, 2003 523 Table 1. Analyses of Kaolin and Activated Carbon (a) Analysis of Kaolin component
content (wt %)
SiO2 Al2O3 Fe2O3 TiO2 CaO
56 40.5 2.5 0.8 0.2
(b) Analysis of Activated Carbon proximate analysis (wt %) ultimate analysis (wt %) moisture 3.1 C 96.1 volatile 3.3 H 0.4 ash 3.0 N 0.3 fixed C 90.6 O 3.2
Figure 2. Schematic diagram of the hot gas clean-up reactor. ledge to support the solids (trace element compound as a powder in stage 1, sorbent in stages 2 and 3). A stream of nitrogen was used to transport the trace element vapor through the sorbent beds at a superficial velocity of 0.035 mm s-1. The reactor was prepared for the test by placing a weighed amount (15-50 mg) of the trace element containing model compound in dry powdered form on the mesh. The reactor was operated in batch mode; the second bed, containing the sorbent under test, was preheated to a test temperature between 300 and 600 °C. Once the temperature of the second bed had been stable for 10 min, the first bed was heated to 1000 °C to vaporize the trace element. After a hold time of approximately 1 h at the test temperature, the reactor heaters were turned off and the reactor allowed to cool while maintaining the flow of nitrogen. The Compounds used as Sources of Trace Element. One model compound was selected as source for each “trace” element studied. All were available commercially and were chosen on the basis of critical properties, such as the temperature of volatilisation, the nature of the volatile species formed, and the requirement of solubility in solvents compatible with the analytical methods used (Table 2). The Sorbents. The sorbents were ground and sieved to provide a 106-150 µm fraction for use in the reactor. The size fraction was selected to provide adequate gas permeability through the sorbent bed, within the design constraints of the quartz reactor. Kaolin. The sorbent was of Cornish origin, calcined at 750 °C to aggregate the original material into a size more suitable for use in a fixed-bed reactor. An analysis of the kaolin is given in Table 1a, showing the significant Fe impurity found in the mineral from this particular source. Activated Carbon. The sorbent was activated in steam from a commercial coconut-shell based carbon using methods described by Jia et al.;18 a proximate and ultimate analysis of the sorbent is given in Table 1b.
Sorbent Leaching. The exposed sorbent samples (except those exposed to Hg) were washed in de-ionized water at 40 °C for 1 h with stirring. Tests showed that the pH of the aqueous phase was mildly acidic (about 5) with the kaolin, and mildly basic (about 10) with the activated carbon. The solids were then recovered from the solutions by filtration on filter paper and allowed to dry at room temperature before being stored until analysis. The aqueous solutions were recovered and the trace element concentrations of the solution subsequently quantified by ICP-AES. As the risk of losing the Hg by vaporization during the leaching procedure was anticipated, leaching studies were not performed on the samples exposed to Hg. Trace Element Analysis. The measurements of As, Cd, Pb, and Se were performed by using the most relevant combination of ICP-MS, ICP-AES, and INAA techniques for each element and sorbent (Lachas13). For ICP-MS and ICPAES, the exposed solid sorbents were prepared for analysis by microwave digestion or open crucible acid digestion. Full details of these methods and their validation have been given in papers by Lachas et al.19 and Richaud et al.20 The Hg analyses were carried out using a LECO AMA-254, a dedicated Hg analyzer which has been used in earlier work.21 A major advantage of the instrument is that it avoids the need for sample digestion, and the inherent risk of losing volatile elements, such as Hg. The sample is burnt in oxygen and the Hg collected from the combustion products by a gold amalgamator. The Hg is desorbed by heating the amalgamator, which concentrates the Hg for measurement by atomic absorption spectrometry (AAS). Morphological Information and Discrete Chemical Composition of the Spent Sorbents. Electron microscopy was used to gather information regarding the sorbents: experiments performed in scanning mode (SEM) gave access to the morphology of the sorbent particles while analyses carried out in transmission mode (TEM) recorded contrasts at the surface of particle fragments. Coupled with energy dispersive X-ray fluorescence (EDXRF), these TEM shots were then used to discern variations in the chemical composition. SEM. The scanning electron microscope used for this work was a JEOL model JSM-220A. Kaolin and activated carbon samples were mounted on cylindrical aluminum stubs using (18) Jia, Y. F.; Steele C. J.; Hayward I. P.; Thomas K. M. Adsorption of Gold and Silver on Activated Carbons. Carbon 1998, 36, 1299-1308. (19) Lachas, H.; Richaud, R.; Jarvis, K. E.; Herod, A. A.; Dugwell, D. R.; Kandiyoti, R. Determination of 17 Trace Elements in Coal and Ash Reference Materials by ICP-MS Applied to Milligram Sample Sizes. Analyst 1999, 124, 177-184. (20) Richaud, R.; Lachas, H.; Healey, A. E.; Reed, G. P.; Haines, J.; Jarvis, K. E.; Herod, A. A.; Dugwell, D. R.; Kandiyoti, R. Trace Element Analysis of Gasification Plant Samples by ICP-MS: Validation by Comparison of Results from Two Laboratories. Fuel 2000, 79, 10771087. (21) Richaud, R.; Lachas, H.; Collot, A.-G.; Mannerings, A. G.; Herod, A. A.; Dugwell, D. R.; Kandiyoti, R. Trace Mercury Concentrations in Coals and Coal-derived Material Determined by Atomic Absorption Spectrophotometry. Fuel 1998, 77 (5), 359-368.
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Table 2. Retention of Trace Elements by Sorbents at Various Temperatures trace element
sorbent kaolin
As Cd Pb Se Hg As Se Hg
activated carbon
a
source of trace elements As2O3 CdCl2 PbCl2 SeO2 HgO As2O3 SeO2 HgO
As4O6 CdCl2 PbCl2 SeO2 Hg° As4O6 SeO2 Hg°
percentage retention at sorbent temperatures, °C 300 450 600 100 100 100 37 29 100 100 60
96 100 100 2 32 100 94 45
81 100 100 1 32 60 99 28
Predicted using thermodynamic equilibrium model.22 Table 3. Leachability of Trace Elements from used Sorbents
trace element
sorbent kaolin
activated carbon
a
predicteda speciation in gas
As Cd Pb Se Hg As Se Hg
percentage insolubility in sorbent used at indicated temperatures, °C 300 450 600 34 10 47 41 N/Aa 23 92 N/A
47 11 41 42 N/A 34 72 N/A
88 67 99 36 N/A 58 78 N/A
N/A: not available.
commercially available setting glue. Mounted stubs were then dried in a circulation fan oven at 50 °C to completely dry the glue and remove any moisture. Gold coating of the specimen was required to enhance the surface conductivity (the samples characterized here were nonconductive). TEM-EDXRF. The TEM analyses were performed with a JEOL model JEM 100-CX-II. The sample preparation was different as the sample had to be thin enough to transmit electrons. After grinding to the right particle size (typically 1-10 µm), the fine particles were ultrasonically suspended in ethanol and placed on a copper microgrid coated with a carbon polymer to ensure electrical conductivity. A silicon-lithium semiconductor detector was used to collect the X-rays emitted by fluorescence to produce spectra (plots of X-ray counts versus X-ray energy) characteristic of the composition of the surface hit by the electron beam. Quantitative analysis can only be achieved with good accuracy by EDXRF spectrometry when the element is present in excess of 2-3 wt %. Although the amounts retained on the sorbent for the various elements studied here were often below this level, the signal observed was sufficiently above the background noise to allow the qualitative analysis of the surface.
Results and Discussion The retention and leachability of trace elements at various temperatures by the two sorbents are summarized in Tables 2 and 3, respectively, and discussed in the following paragraphs. The imaging and characterization of the local chemical composition of specimens of the spent sorbents collected from the As, Se, and Hg runs are presented in Figures 6-13, and discussed in the last part of this section. Retention and Leachability with Kaolin as Sorbent. Arsenic. The speciation of As at the conditions before the sorbent bed has been predicted using a thermodynamic equilibrium model based on free energy minimization22 and is shown in Figure 3. The predomi-
nant form is gas-phase As4O6, with a condensed phase As species (As2O3〈Claudetite〉) only predicted to form when the temperature fell below 220 °C. The experimental data in Table 2 shows the capture of As to fall moderately with increasing temperature, from 100% at 300 °C to a little over 80% at 600 °C. However, the leachability of the captured species can be seen in Table 3 to vary more markedly with increasing fixed-bed temperature. The water-insoluble fraction of captured arsenic rises from 34%, for sorption at 300 °C, to 88%, for sorption at 600 °C. A similar trend was observed11 when heating As-spiked kaolin samples (from As2O3 solution); the leachability showed a clear decrease with increasing treatment temperature from 80% of As fraction leachable at room temperature to 40% at 300 °C and less than 10% above 500 °C. If the captured species were simply physically adsorbed As4O6 or As2O3〈Claudetite〉, it would be expected to be readily watersoluble. However, As is predicted to form a new condensed phase in the presence of an aluminosilicate with the composition of kaolin, as shown in Figure 4. The new condensed phase is AlAsO4, a species that is insoluble in water.23 The reduction in water solubility with increasing temperature is therefore taken to indicate a transition from capture by a predominantly physical process to capture mainly by chemical reaction, as the sorbent temperature increases from 300 to 600 °C. Cadmium. As seen in Table 2, 100% retention of Cd (present in the gas phase as CdCl2) was observed over the whole range of temperature. However, as indicated in Table 3, the leachability of the captured Cd remained high from 300 to 450 °C and did not fall until the sorbent temperature reached 600 °C. Although thermodynamic data for aluminosilicates was not available, it is thought that the insoluble form of Cd is likely to be a Cdaluminosilicate, formed by chemical reaction. Uberoi and Shadman15 conducted similar tests at a higher temperature of 800 °C, and found evidence for the formation of the species CdO‚Al2O3‚2SiO2. At the lower temperatures the captured species is believed to be physically adsorbed CdCl2, captured by a condensation mechanism and readily water soluble. Lead. The retention of Pb (present in the gas phase as PbCl2) by kaolin showed many similarities to that of Cd (see above), with 100% retention across the range of temperature (Table 2). The water-soluble fraction (22) Reed, G. P.; Dugwell, D. R.; Kandiyoti, R. Control of Trace Elements in a Gasifier Hot Gas Filter: A Comparison with Predictions from a Thermodynamic Equilibrium Model. Energy Fuels 2001, 15 (6), 1480-1487. (23) Handbook of Chemistry and Physics, 76th ed.; CRC Press: Boca Raton, 1995.
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Figure 3. Predicted arsenic speciation before sorbent bed.
Figure 4. Predicted arsenic speciation for kaolin system with excess of oxygen.
increased with increasing sorbent-bed temperature (Table 3). However, Pb is somewhat less leachable than Cd across the temperature range, and at 600 °C the water-soluble fraction of captured Pb falls to only 1% compared with 33% for Cd. Studies on the leachability of Pb-spiked kaolin samples, carried out by Venkatesh et al.11 to look at the effect of the thermal treatment on the interaction between Pb and kaolin revealed comparable behavior. The Lead fraction leached by water was found to drop from 50% for sorbent contacted at 300 °C to less than 5% for all the samples prepared with a contact temperature in excess of 500 °C. The water-insoluble Pb species observed in our work is thought to be a Pb aluminosilicate, formed by chemical reaction; Uberoi and Shadman14 have reported the formation of hexagonal and monoclinic forms of the species PbAl2Si2O8 (or PbO‚Al2O3‚2SiO2) by the reaction of PbCl2 vapor in a synthetic flue gas with kaolin at 700 °C. Thermodynamic data for this species was not available in the database to enable the potential for their formation under our experimental conditions to be predicted.
Selenium. The retention of Se by kaolin is quite modest (Table 2), but becomes more significant as the temperature falls to 300 °C. The proportion of insoluble Se remains quite constant across the range of temperature, suggesting that capture by physical adsorption may be the dominant mechanism at the lowest temperature (300 °C) of the range studied here. Mercury. The data in Table 2 indicates only modest retention of Hg (present in the gas phase as Hg°) by kaolin (about 30%), with little variation across the temperature range. Retention in this temperature range would not be expected from thermodynamic equilibrium predictions of the speciation,17 which indicated only gaseous species. The temperature is also higher than would normally be associated with a physical condensation mechanism for such a volatile species. For reasons outlined above, no Hg leaching data is available. Owens and Biswas24 have predicted from estimated thermodynamic data that HgSiO3 may be a stable (24) Owens, T. M.; Wu, C.-Y.; Biswas, P. An Equilibrium Analysis for Reaction of Metal Compounds with Sorbents in High-Temperature Systems. Chem. Eng. Commun. 1995, 133, 31-52.
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Figure 5. Predicted arsenic speciation of activated carbon system.
species in the temperature range 500-600 K (i.e., 227327 °C) in the absence of Cl or S, under oxidizing conditions. It is not possible to check whether the current system is sufficiently oxidizing, without access to their original data. However the present hot gas clean-up reactor is a batch process; it does not work with a steady continuous stream of trace element. If the Hg were vaporized at the same rate throughout the duration of the test, the concentration would be about 50 g m-3. The rate of vaporization must actually reach a peak value as the temperature of the first stage increases, so the peak concentration must exceed 50 g m-3. It is possible that with these relatively high concentrations, thermodynamic equilibrium between the gas and sorbent compositions is not established by the end of the test. It is also speculated that the presence of kaolin impurities shown in Table 1a (such as Fe) may play a role, which we are unable to identify with the available thermodynamic data. Retention and Leachability with Activated Carbon as Sorbent. Arsenic. As seen in Table 2, 100% retention of As in the sorbent bed was found at the lower temperatures (300 and 450 °C), but the retention fell to 60% at 600 °C. The leachability of the As retained by the sorbent bed decreased steadily from 300 through 450 to 600 °C, as shown by the steady increase in the percentage of water-insoluble As (Table 3). The effect of temperature on As speciation can be predicted by thermodynamic equilibrium modeling, using methods described in an earlier paper.18 The presence of carbon in the system has a marked effect on the predicted speciation, as shown in Figures 3 and 5. In the absence of carbon, Figure 3 shows that the preferred species is gas-phase As4O6, with condensed phase As2O3〈Claudetite〉 becoming the preferred species below 220 °C. However, in the presence of carbon, Figure 5 shows that condensed phase elemental As may form at temperatures up to 350 °C. Elemental As would be less water soluble than As2O3〈Claudetite〉, and reduction of the As4O6 to As would only be possible in the sorbent bed. The results could thus be explained in qualitative terms by the kinetics of the reduction reaction becoming too slow at temperatures below 600 °C.
In quantitative terms, this reaction should not be thermodynamically favorable at temperatures above 350 °C, but the apparent capability of active carbon for extending the upper limit for the formation of condensed species has been noted in our earlier work.17,22 The reduction of As2O3(g) into elemental As in the presence of charcoal at 700-800 °C has also been reported.25 Selenium. The data in Table 2 shows the retention of Se by activated carbon to be high at all temperatures. The majority of the Se retained is seen in Table 3 to be insoluble, rising to 92% at 300 °C. The markedly lower solubility of the Se retained by activated carbon compared to that retained on kaolin suggests that a different Se species may be present. It is possible that in the presence of activated carbon, the relatively water-soluble SeO2 has been reduced to less soluble elemental Se. Although literature on this remains scarce, the behavior of SeO2 observed here in the presence of activated carbon is confirmed by studies carried out by Greenwood and Earnshaw25 in which the thermodynamic instability of SeO2 and its propensity to be easily reduced to elemental Se in the presence of various reducing agents were underlined. Mercury. The retention of Hg by activated carbon is seen in Table 2 to decline with increasing temperature, from 60% at 300 °C to 28% at 600 °C. The temperature dependence of Hg removal from gases by activated carbon has been investigated at the bench and process scales; Krishnan et al.,26 Vidic and McLaughlin,27 and Reed et al.17 all report a reduction in Hg retention at temperatures above 140 °C, with no useful retention at above 200 °C. However, the work reported here shows that, under the conditions we have investigated, useful retention performance can be obtained at higher temperatures. The critical difference between our work and the cited literature is thought to be the Hg concentration in the gas, which at about 50 g m-3 was 4 or 5 orders of (25) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements, 2nd ed.; Butterworth-Heineman: Oxford, UK, 1997; p 548. (26) Krishnan, S.; Gullett, B. K.; Jozewicz, W. Sorption of Elemental Mercury by Activated Carbons. Environ. Sci. Technol. 1994, 28 (8), 1506-1512. (27) Vidic, R. D.; McLaughlin, J. B. Uptake of Elemental Mercury Vapours by Activated Carbons. J. Air Waste Manage. Assoc. 1996, 46, 241-250.
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Figure 6. (a) SEM micrograph of fresh kaolin (×500; the white bar at the foot of the micrograph indicates a distance of 50 µm). (b) EDXRF spectrum of fresh kaolin.
magnitude greater than the levels normally released by most coals and biomass materials used as fuel. For example, Vidic and McLaughlin27 used a concentration of 110 µg m-3. The loading of Hg on activated carbon that is attainable is also known to be dependent on the Hg concentration in the gas. This may help explain the much greater loading of Hg on active carbon observed in this work at around 5 mg Hg per g activated carbon. This value may be compared with the equilibrium loading of 10 µg Hg g-1 activated carbon, observed by Vidic and McLaughlin27 at a Hg concentration of 110 µg m-3 in the gas. It is possible that, with the present relatively high concentrations and the batch-wise nature of our tests, equilibrium between the gas and sorbent compositions was not established; this would lead to the attainable equilibrium loading being underestimated. Characterization of Sorbent Morphology and Composition: Kaolin as Sorbent for As, Se, and Hg Retention. Fresh Kaolin. An SEM micrograph of kaolin as used for the experiments is given in Figure 6a, together with its EDXRF spectrum (Figure 6b; the TEM view is not given here). The size and shape of the particle are typical of a 106-150 µm diameter sized nonporous solid, formed by aggregation of smaller particles (kaolin is naturally occurring in tiny particles smaller than 10 µm). The BET surface area reported elsewhere was estimated at 17 m2 g-1 and mostly of mesoporous nature (Lachas13). The EDXRF spectrum also revealed a chemical composition expected for an alumino-silicate (K-lines for Al, Si and O, primarily) containing impurities such as Fe, Ca, K and Ti (also see Table 1a). The detection of a large amount of Cu is explained by the copper microgrid used to support the solid powder. Arsenic. TEM/EDXRF analysis was carried out on several fragments of the kaolin recovered from the bed, exposed to 600 °C and washed with water for the leachability tests. The EDXRF spectra indicated either the presence of As in small amounts (typical K-lines at 10.54 and 11.72 keV, as shown in Figure 7) or its complete absence (giving a spectrum similar to Figure 6b). When As was detected in one region of the particle, the area around it was also explored. In all such cases, As was also found around the initial spot (incident beam of 30 nm diameter) and in quantities that were shown
Figure 7. EDXRF spectrum of kaolin fragment exposed to arsenic vapors at 600 °C.
to be similar to those displayed in Figure 7. This would suggest that, when As is found on the kaolin surface, it is always in small quantities and over an extended area. Previous work12,28 analyzing for Cd and Pb, respectively, showed that the formation of a molten layer (lead or cadmium aluminosilicate) causes pore plugging of the voids between the sorbent grains. The elemental mapping of the cross sections of sorbent particles illustrated a distinct drop in the cadmium or lead concentration from the particle edges to their centers. Results found here for As on kaolin are consistent with these findings. One may view the 106-150 µm kaolin particles used in this study as pellets composed of agglomerated smaller particles. As reacts with surface aluminosilicates to form a thin layer of the condensed phase AlAsO4, equally spread over the nonporous surface, and tending to cause blockages of interstitial volumes between the (original) smaller particles. As a result of the grinding process prior to the TEMEDXRF analysis, the particles recovered from the hightemperature reactor bed either show a complete absence of As if the fragment originates from a particle shielded from the As, or a limited (but constant As) presence over an extended region if the fragment had been in direct contact with the As vapors. Selenium. Figure 8 shows the TEM micrograph and the EDXRF spectrum recorded for two different regions of the same kaolin grain (indicated as 1 and 2 in Figure 8a) after it had been exposed to 300 °C for the Se sorption tests. In line with observations on arsenic, Se is either not detected on the surface of the fragment or it is detected through its K-lines at 11.22 and 12.5 keV (see Figure 8b; the Se L-lines are interfered by the (28) Wu, B.; Jannu, K. K. J.; Shadman, F. Multi-Functional Sorbents for the Removal of Sulfur and Metallic Contaminants for HighTemperature Gases. Environ. Sci. Technol. 1995, 29 (6), 1660-1665.
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Figure 8. (a) TEM micrograph of kaolin fragment exposed to selenium vapors at 300 °C. (b) EDXRF spectrum registered for spots 1 and 2.
Figure 9. (a) TEM micrograph of kaolin fragment exposed to mercury vapors at 300 °C. (b) EDXRF spectrum registered for spots 3.
Al L-line at lower energy). Again, the amounts detected remain very small, but appear equally spread over a large area. This could be explained by two very distinctive mechanisms: a physical adsorption leading to an even (but relatively fine) layer of Se over the external surface of the kaolin grain, or a more developed interaction with the surface leading to the formation of a new condensed species as in the case of As on kaolin. However, the invariability of the proportion of insoluble Se (around 40%) over the 300-600 °C temperature range suggested that most of the captured Se was not involved in a chemical reaction with the sorbent. Physical adsorption is therefore probably the predominant mechanism in the capture observed. Mercury. The TEM and EDXRF (parts a and b of Figure 9, respectively) indicate two distinct modes of capture as mechanisms responsible for the retention of around 30% of Hg exposed to kaolin for all the sorbent bed temperatures studied (Table 2). First, mercury was found in small amounts at the surface of some of the grains that were being examined (analysis spots 1 and 2 in Figure 9a; EDXRF spectrum not shown here, but very similar to Figure 9b with the Hg L-lines at 9.98 and 11.82 keV present in low intensities). Some mercury was also observed as discrete metallic spheres of ∼1 µm diameter. Located in 3 in Figure 9a, their EDXRF spectrum was characteristic of Hg alone with the unambiguous presence of the M-line at 2.20 keV emerging from the low intensity K-lines of the kaolin matrix components (Figure 9b). More evidence about the nature
of these spheres was also gathered when they were exposed to the focused electron beam for an extended period (for spectra replication): this resulted in a clear decrease of the Hg characteristic rays until no signal was left at all. The heat generated in the vicinity of the focused electron beam was enough to vaporize the mercury. Although the leachability tests were not performed for fear of losing easily volatilized Hg, these results emphasize the existence of Hg both in interaction with the surface (possibly in the form of HgSiO3 as predicted by Owens and Biswas24 and discussed above) and on its own (in the form of metallic spheres). They also suggest that kaolin is not a very suitable sorbent for mercury retention since a large fraction of the limited quantity of mercury retained was found free on the kaolin surface. Spot 4 in Figure 9b was found to be almost pure Fe, probably in the form of ferrite (Fe2O3); there was no evidence of captured Hg in this location, suggesting that such impurities are not directly involved in Hg capture. Activated Carbon as Sorbent for As, Hg, and Se Retention. Fresh Activated Carbon. The fresh activated carbon was also characterized by SEM and EDXRF (parts a and b of Figure 10, respectively); the TEM view has not been shown. Using appropriate magnification (×750), microscopy reveals a typical particle size of about 160-180 µm diameter, and a multitude of tiny channels accounting for the large surface area, estimated at 920 m2 g-1 by BET (Lachas13), with an almost total predominance of micropores. As expected, the
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Figure 10. (a) SEM micrograph of fresh activated carbon (×750; the white bar at the foot of the micrograph indicates a distance of 10 µm). (b) EDXRF spectrum of fresh activated carbon.
Figure 11. (a) TEM micrograph of activated carbon fragment exposed to arsenic vapors at 450 °C. (b and c) EDXRF spectra registered for spots 1, 3, and 4 and for spots 2and 5, respectively.
EDXRF spectrum shown in Figure 10b displays the signals characteristic of the main components of the carbon matrix (K-lines for C and O at 0.3 and 0.5 keV, respectively). The presence of S and Cl (K-lines at 2.32 and 2.6 keV, respectively) was also detected. Again, the copper detected refers to the Cu microgrid used to support the samples. Arsenic. Several fragments of activated carbon recovered after being exposed to arsenic oxide vapors at 450 °C in the reactor and washed for the leachability tests were examined by electron microscopy. In the example given in Figure 11a, typical of what was observed across the range of fragments studied, zones with various contrasts were analyzed by EDXRF. All the results could be put in one of the two following groups: presence of large amount of As, as illustrated in Figure 11b by three of the As-characteristic X-rays lines (at 0.95 keV for the L-line and 10.54 and 11.7 keV for its K-lines), or complete absence as shown in Figure 11c. Spots 1, 3, and 4 on one hand, and 2 and 5 on the other, were found to fall into the former and the latter categories, respectively. In contrast to what was observed with kaolin, the regions where As was detected were more localized but contained arsenic in significantly higher concentrations. Clearly, if only physisorption had been the dominant As-capture mechanism, the formation of a finely spread deposit over most of the surface would have been expected, particularly in the case of a highly porous
solid. The evidence seems to suggest, however, that the retention process is taking place in localized sites of high reactivity toward arsenic. It is not yet possible to discuss the nature of the species detected in the As-enriched regions. The signal recorded may correspond to elemental arsenic, reduced by contact with the activated carbon and in weak association with the surface, or to chemically bound arsenic at specific active surface sites, apparently forming an insoluble species. Selenium. In the case of selenium retained at 300 °C, the conclusions drawn from the TEM/EDRXF characterization of water-washed activated carbon fragments are very similar to what was observed in the case of arsenic (Figure 12). Once again, the random screening of various fragments led to the identification of two categories of activated carbon surface based upon the presence (Se K-lines at 11.22 and 12.5 keV and Se L-lines at 1.38 and 1.42 keV in Figure 12c) or absence (Figure 12b) of selenium. Looking across one particle, the same distinction was observed between regions highly enriched in Se (spots 1, 2, and 3 in Figure 12a) and completely Se-free regions (spot 4, Figure 12a). The presence of Se-enriched spots, next to Se-free activated carbon, would seem to indicate a strong interaction of Se with some areas of the surface. This tends to make less likely the possibility that Se simply sits on the surface. More characterization work is required to be able to distinguish which one of the physical (porosity)
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Figure 12. (a) TEM micrograph of activated carbon fragment exposed to selenium vapors at 300 °C. (b and c) EDXRF spectra registered for spots 1, 2, and 3 and for spot 4, respectively.
Figure 13. (a) TEM micrograph of activated carbon fragment exposed to mercury vapors at 300 °C. (b and c) EDXRF spectra registered for spots 2 and 4 and for spots 1 and 3, respectively.
or the chemical (3.2 wt % oxygen) features is responsible for the high Se retention and low leachability observed for all of the fixed-bed temperatures studied. Mercury. Figure 13 presents TEM/EDXRF data for activated carbon particles exposed to mercury vapors at 300 °C, showing the occurrence of areas either rich in mercury (spots marked 2 and 4) or entirely free of it (spots marked 1 and 3). On every spectrum (Figure 13a,b), the presence of S (K-line at 2.32 keV) and Cl (K-line at 2.6 keV) was noted; in fact, no Hg was ever detected without the presence of S and/or Cl. Since the leaching tests were not performed on the sorbent exposed to Hg for reasons outlined above, one had no indication with respect to the nature of the sorption process of what was responsible for the high Hg retention (60 wt % at 300 °C; Table 2) observed. A combination of adsorption, condensation, and chemical reaction is often presented to explain the retention of mercury on the surface of activated carbon. Krishnan et al.21 concluded from their results that physisorption and chemisorption combine at temperatures below 140 °C. On the basis of desorption studies, they took chemisorption alone to explain the retention observed at above 140 °C. Leaching experiments with H2SO4 suggested the stronger adherence of elemental mercury captured at 150 °C, as only 1% of it could be leached out of the activated carbon. Oxygen, a well-known surface component present in carbons, has been proposed as a possible candidate active site causing the Hg° sorption.
The EDXRF spectra of the fresh activated carbon (Figure 10b) and of the active carbon exposed to Hg (Figure 13b) show the presence of oxygen in significant amounts following activation in steam at 800 °C. A review by Sloss29 notes that chemically impregnated active carbon had Hg° retention several times higher than thermally activated carbon. Furthermore, impregnation with chlorine and sulfur appears to stabilize retained mercury. Sulfur impregnated active carbon retains useful capacity for Hg° sorption at temperatures above 140 °C, suggesting a chemical interaction between Hg° and S.30 Although our experiments were carried out at significantly higher temperatures (300-600 °C) and without any impregnation, the EDXRF spectra obtained for all of our activated carbon samples showed the presence of S and Cl. The chemical composition of the surface where Hg was detected was always found to contain S and Cl. It seems possible therefore to contemplate S- and Cl-bearing sites (occurring naturally in the present case) as active sites for Hg capture. Finally, the surface area of the activated carbons was also found to significantly influence the amount of Hg° captured, with an exponential increase in sorption ability for surface areas ranging from 550 to 1000 m2 g-1.24 Whether this is because the surface area available (29) Sloss, L. L. Mercury Emissions and Effects: The Role of Coal. Report IEPER/19; IEA Coal Research: London, UK, 1995. (30) Vidic, R. D.; Kwon S-K.; Siler D. P. Impregnated Activated Carbons for Elemental Mercury Adsorption. In Proceedings of the 17th Annual International Pittsburgh Coal Conference, University of Pittsburgh, Pittsburgh, PA, 11-14 Sept 2000.
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for physisorption increases, or the number of active sites of various natures (O, S, and Cl) available for Hg° chemisorption was multiplied remained unclear. Although direct evidence of the nature of the bonding with the surface is still lacking, the concomitant detection by TEM/EDXRF of mercury and chemical constituents described in various works as active sites on the surface of activated carbon tends to suggest the retention by chemisorption, even at the temperatures used here. Summary and Conclusions The sorption of five toxic trace elements (Pb, Cd, As, Se, and Hg) by kaolin and activated carbon has been studied over the hot gas clean-up temperature range (300 -600 °C). The investigation has been carried out using a purpose built reactor capable of multi stage sorption. 1. Kaolin was found to be a suitable sorbent for Pb and Cd at 600 °C. It achieved high retention performance, while the retained elements were found to be not easily leachable. The higher leachability of Pb and Cd from samples prepared during lower temperature sorption experiments suggests their greater bio-availability following disposal as spent sorbents. 2. Both kaolin and activated carbon were found to achieve good retention of As at temperatures up to 450 °C. However, the leachability of the As captured at these lower temperatures is also significant. At 600 °C, the capture of As on kaolin is higher and the leachability of the captured As notably lower than that for activated carbon. This makes kaolin more suitable for As capture. A possible explanation for the differences in terms of the preferred speciation at thermodynamic equilibrium has been put forward. The TEM-EDXRF analysis suggests the existence of strong interactions between As and these two sorbents. The data help explain the retention observed at 450 °C (activated carbon) and 600 °C (kaolin). 3. Activated carbon appears to be the better sorbent for Se. Retention was higher and the leachability lower than for kaolin. This trend was confirmed by the TEM-
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EDXRF analysis, which showed stronger interactions between Se and activated carbon compared to Se with kaolin. 4. The observed significant retention of Hg by kaolin and activated carbon over the 300-600 °C range in this study may be attributed in part to the high concentration of Hg in the sample gas. Activated carbon achieved the better performance and there is scope for improving Hg capture by reducing the sorbent temperature below 300 °C. Retention by chemisorption is strongly suggested by the TEM-EDXRF characterization of activated carbon samples exposed to mercury vapors at 300 °C. The presence of S and Cl near surface formations appears to enhance sorption. Less interaction appears to take place between Hg and kaolin, where the occurrence of free elemental Hg spheroids on kaolin surfaces has been observed. 5. TEM-EDXRF data tends to identify two different capture mechanisms for the two sorbents tested in this study. On kaolin, the trace elements examined tend to be retained as an even layer of low concentration spread over a large area. However, the retention on activated carbon seems to be more localized and concentrated. Acknowledgment. The financial support of the ECSC under research contract No. 7220-ED/069 is gratefully acknowledged. The assistance of Dr Kym Jarvis and the NERC supported ICP-MS facility at Silwood Park (now at Kingston University) under NERC support application ICP/93/0696 is also acknowledged. The authors also express their gratitude to Dr. Susan Parry and Mrs. Diana Thompson of the Neutron Reactor Facility at Silwood Park for help and guidance in performing the Neutron Activation analyses, and to Mrs. Patricia Beaunier from the Electron Microscopy Facility in the Laboratoire de Reactivite de Surface at University P. and M. Curie (Paris, France) where the TEM-EDXRF work was conducted. Our thanks also go to Mr Barry Coles from the Geology Department at Imperial College for fruitful discussions. EF020160R
