2746
Energy & Fuels 2007, 21, 2746-2750
Screening of Low Cost Sorbents for Arsenic and Mercury Capture in Gasification Systems Cedric Charpenteau,* Revata Seneviratne, Anthe George, Marcos Millan, Denis R. Dugwell, and Rafael Kandiyoti Department of Chemical Engineering, Imperial College London, Prince Consort Road, London SW7 2AZ, UK ReceiVed January 18, 2007. ReVised Manuscript ReceiVed May 16, 2007
A novel laboratory-scale fixed-bed reactor has been developed to investigate trace metal capture on selected sorbents for cleaning the hot raw gas in Integrated Gasification Combined Cycle (IGCC) power plants. The new reactor design is presented, together with initial results for mercury and arsenic capture on five sorbents. It was expected that the capture efficiency of sorbents would decrease with increasing temperature. However, a commercial activated carbon, Norit Darco “Hg”, and a pyrolysis char prepared from scrap tire rubber exhibit similar efficiencies for arsenic at 200 and at 400 °C (70% and 50%, respectively). Meta-kaolinite and fly ash both exhibit an efficiency of around 50% at 200 °C, which then dropped as the test temperature was increased to 400 °C. Activated scrap tire char performed better at 200 °C than the pyrolysis char showing an arsenic capture capacity similar to that of commercial Norit Darco “Hg”; however, efficiency dropped to below 40% at 400 °C. These results suggest that the capture mechanism of arsenic (As4) is more complex than purely physical adsorption onto the sorbents. Certain elements within the sorbents may have significant importance for chemical adsorption, in addition to the effect of surface area, as determined by the BET method. This was indeed the case for the mercury capture efficiency for all four sorbents tested. Three of the sorbents tested retained 90% of the mercury when operated at 100 °C. As the temperature increased, the efficiency of activated carbon and pyrolysis char reduced significantly. Curiously, despite having the smallest Brunauer-EmmetTeller (BET) surface area, a pf-combustion ash was the most effective in capturing mercury over the temperature range studied. These observations suggest that the observed mercury capture was not purely physical adsorption but a combination of physical and chemical processes.
Introduction Integrated Gasification Combined Cycle (IGCC) promises to provide a major advance in clean-coal power generation technology. In addition to achieving thermodynamic efficiency greater than traditional pf fired plants, IGCC is amenable to a higher level of gas cleaning and, hence, a lower environmental impact. A wide range of gas treatment processes, commercially proven for other applications, are available for IGCC gas cleaning. Such processes are capable of delivering fuel gas with extremely low levels of undesirable constituents, such as sulfur compounds. At present, however, the majority of proven gas purification processes employ either aqueous solutions or low boiling organic reagents for gas scrubbing and, thus, deliver the cleaned fuel gas at temperatures close to ambient. Consequently, until alternative high temperature, dry gas processing techniques can be introduced commercially, there are significant economic and cycle efficiency penalties in terms of plant complexity and overall thermal efficiency loss. Most of the trace elements present in the raw fuel are trapped in the glassy slag from the gasifier.1 In addition, the low temperature of the current cold gas cleanup ensures that alkali metals and the major part of the heavy metals are condensed and scrubbed out. However, as systems evolve toward the desired high-temperature cleanup, the control of the more volatile trace elements will become a problem. In this paper, the discussion is focused on the fate of * Corresponding author. E-mail:
[email protected]. (1) Sloss, L. L.; Smith, I. M. Trace elements emissions; CCC/34, IEA Coal Research: London, 2000.
mercury and arsenic. These studies form part of a project which also involves the removal of other toxic trace elements from the fuel gas, such as cadmium, zinc, and lead. When introduced into an IGCC system, the more volatile trace elements may escape to the environment as part of the combustion product gas or, in a condensed phase, as part of the emitted particulates. This latter emission pathway results from the inability of air pollution control equipment, such as candle filters, to capture all of the smallest particles. The most volatile element, mercury, is most likely to leave the gasifier in the gaseous phase. Although arsenic hydride, AsH3, shows volatile behavior, emission of arsenic is predominantly associated with the particulate phase where there is significant enrichment on the smallest particles.2 A recent analysis of samples physically deposited on the gas cooling system from the Puertolano IGCC plant showed that a major part of the arsenic condensed at temperatures above 750 °C, with nickeline (NiAs) as the main mode of occurrence.3 To compensate for the lack of experimental data, several thermodynamic equilibrium modeling studies have been carried out.4-8 The modeling results indicate that, in a coal gasification atmosphere, only Hg0(2) Clarke, L. B. Management of by-products from IGCC power generation; IEACR/38, IEA Coal Research: London, 1991. (3) Font, O. Condensing species from flue gas in Puertollano gasification power plant, Spain. Fuel 2006, 85 (14), 2229-2242. (4) Helble, J. J.; et al. Trace element partitioning during coal gasification. Fuel 1996, 75 (8), 931-939. (5) Lyyranen, J.; et al. Equilibrium modelling of trace element behaviour in fluidized bed combustion and gasification of coal. J. Aerosol Sci. 1995, 26 (Supplement 1), S687-S688.
10.1021/ef070026c CCC: $37.00 © 2007 American Chemical Society Published on Web 07/20/2007
Screening of Low Cost Sorbents
(g) is stable.7,8 It has also been predicted that, at reducing conditions, arsenic may be present in the gas phase between 200 and 1600 °C. As4(g) is the most stable form of arsenic between 300 and 500 °C with As2(g) and AsH3(g) also present. AsO(g) is formed between 500 and 800 °C while AsO(g) is the major stable form above 1000 °C. An important advantage of IGCC systems, over conventional combustion systems, is that they offer the capability of removing pollutants from the compressed gas upstream of the gas turbine. If the trace element capture unit is incorporated into the existing control process, this will minimize the volume of gas that must be treated and thus the cost of removal. The elevated pressure of the fuel gas is an advantage, as there is less volumetric flow, allowing the use of a packed bed sorbent. The capacity of solid sorbents for retaining arsenic and mercury compounds has already been studied and some experience has been gained from coal combustion. Experiments on activated carbons conducted to date have been specially focused on mercury and oxygenated organic species. Functional groups containing polar sites with chlorine or sulfur were identified as the most likely to provide the active sites for Hg0 bonding.9-11 Activated carbon was also effective in capturing arsenic oxide vapor.12 Depending on temperature, hydrated lime (Ca(OH)2), limestone (CaCO3), and fly ashes have been found to be effective in capturing arsenic vapor13-17 where the arsenic reacts with the Ca sites at the surface of the solid forming a stable calcium arsenate. Fly ash has also been tested as a potential mercury sorbent.10,18,19 Although the carbon content, as well as the surface area of sorbents, appears to play a major role in mercury retention, it was emphasized that the oxygen functionality and the presence of halogen species on the surface of the fly ash carbon may also promote mercury adsorption.20 (6) Diaz-Somoano, M.; Martinez-Tarazona, M. R. Trace element evaporation during coal gasification based on a thermodynamic equilibrium calculation approach. Fuel 2003, 82 (2), 137-145. (7) Frandsen, F.; Dam-Johansen, K.; Rasmussen, P. Trace elements from combustion and gasification of coal-An equilibrium approach. Prog. Energy Combust. Sci. 1994, 20 (2), 115-138. (8) Mojtahedi, W.; Wahab. Trace metals volatilization in fluidized-bed combustion and gasification of coal. Combust. Sci. Technol. 1989, 63 (4), 209-227. (9) Ghorishi, S. B.; et al. Development of a Cl-impregnated activated carbon for entrained-flow capture of elemental mercury. EnViron. Sci. Technol. 2002, 36 (20), 4454-4459. (10) Granite, E. J.; Pennline, H. W.; Hargis, R. A. Novel sorbents for mercury removal from flue gas. Ind. Eng. Chem. Res. 2000, 39 (4), 10201029. (11) Li, Y. H.; Lee, C. W.; Gullett, B. K. The effect of activated carbon surface moisture on low temperature mercury adsorption. Carbon 2002, 40, 65-72. (12) Wouterlood, H.; Huibert. Removal and recovery of arsenious oxide from flue gases. EnViron. Sci. Technol. 1979, 13 (1), 93-97. (13) Mahuli, S. S. Mechanism of Arsenic Sorption by Hydrated Lime. EnViron. Sci. Technol. 1997, 31 (11), 3226-3231. (14) Gullett, B. K.; Raghunathan, K. Reduction of Coal-Based Metal Emissions by Furnace Sorbent Injection. Energy Fuels 1994, 8 (5), 10681076. (15) Jadhav, R. A. Capture of gas-phase arsenic oxide by lime: kinetic and mechanistic studies. EnViron. Sci. Technol. 2001, 35 (4), 794-799. (16) Sterling, R. O.; Helble, J. J. Reaction of arsenic vapor species with fly ash compounds: kinetics and speciation of the reaction with calcium silicates. Chemosphere 2003, 51 (10), 1111-1119. (17) Diaz-Somoano, M.; Martinez-Tarazona, M. R. Retention of trace elements using fly ash in a coal gasification flue gas. J. Chem. Technol. Biotechnol. 2002, 77 (3), 396-402. (18) Serre, S. D.; Silcox, G. D. Adsorption of elemental mercury on the residual carbon in coal fly ash. Ind. Eng. Chem. Res. 2000, 39 (6), 17231730. (19) Hassett, D. J.; Eylands, K. E. Mercury capture on coal combustion fly ash. Fuel 1999, 78 (2), 243-248.
Energy & Fuels, Vol. 21, No. 5, 2007 2747
The aim of the present study is to examine the ability of different sorbents to capture and retain arsenic and mercury from the gas phase, as a function of temperature. A novel sorbent reactor has been designed, constructed, and commissioned for this purpose. The results of the preliminary investigations with the new reactor are presented here. These have been conducted for arsenic, using the dominant stable species, tetra-arsenic As4(g), between 200 and 400 °C, and elemental mercury Hg0(g), between 100 and 200 °C. The effectiveness of three inexpensive sorbents, i.e., combustion fly ash and char, plus activated char, from scrap tire rubber, has been measured and compared to that of a commercial activated carbon and an aluminum-silicate sorbent. In particular, it is desirable to identify sorbents that are less costly than high-quality commercial activated carbons (priced ca. 1200 $/t, currently21) in order to limit operating costs of the gas cleaning plant. Experimental Methods and Conditions Novel Sorbent Test Reactor. A schematic diagram of the sorbent-bed reactor system is presented in Figure 1, with the mercury generation section presented in Figure 2. The gas cleanup reactor comprises two main stages. The first stage is designed to generate vaporized species in a hot dry gas. The trace element vapor is produced by placing a few milligrams of solid powder in a thermogravimetric analysis (TGA) platinum pan that is introduced into the first section of the reactor and then heated, by means of an external electric furnace. The pan is weighed by the TGA balance before and after an experiment, to assess the amount of trace element vaporized to within (3 µg. The first section is designed to allow a simulated fuel gas mixture to pass downward through the reactor and entrain the volatilized element through a diffuser. The local temperature is measured by a thermocouple introduced in a sleeve to avoid contamination by contact with the sample. The second stage of the reactor and the furnace configuration are designed so that all the trace element-bearing gas coming from the first stage passes through the test sorbent bed, without the possibility of bypassing or deposit formation on the reactor walls. As in the first stage, the local temperature in the sorbent bed is measured and controlled using a thermocouple well. In order for the bench-scale adsorption experiments to be comparable with an actual IGCC plant, it is important to send through the sorbent bed a synthetic flue gas of representative composition. However, in the preliminary experiments reported here, the carrier gas has been pure nitrogen. Thermodynamic equilibrium modeling has shown that, under reducing conditions, over the temperature range between 200 and 500 °C, tetra-arsenic As4 and Hg are the predominant species.6 Fortunately, tetra-arsenic As4 is also produced from arsenic powder under the nitrogen used here. The experiments have been conducted in the absence of nickel. In the real plant situation, the amount of arsenic remaining in the vapor phase will be reduced by interaction with nickel, to form solid nickel arsenide species. The data presented here thus represent a worse case scenario for volatile arsenic species. The arsenic powder supplied from Sigma Aldrich had a purity of 99.997%. A concentration of arsenic in the gas phase (As4) in the region of 0.3 ppm was obtained from a low evaporation rate of the arsenic source at 160 °C, using a nitrogen sweep gas of 40 mL/ min (NTP). Arsenic passing through the sorbent bed was allowed to condense on a cool region of the reactor wall, a few centimeters beneath the sorbent bed exit. The condensed arsenic formed a small annulus, which built up gradually during the experiment. This condensed arsenic was recovered by washing the reactor overnight in 10% nitric acid. The acid wash solution was then analyzed for (20) Maroto-Valer, M. M.; et al. Effect of porous structure and surface functionality on the mercury capacity of a fly ash carbon and its activated sample. Fuel 2005, 84 (1), 105-108. (21) Jones, A. P.; et al. DOE/NETL’s phase II mercury control technology field test program; U.S. Department of Energy: Washington, DC, 2006.
2748 Energy & Fuels, Vol. 21, No. 5, 2007
Charpenteau et al. Table 1. Density and Porosity of the Sorbents
Norit Darco “Hg” pyrolized scrap tire activated carbon from scrap tire coal fly ash meta-kaolinite
Figure 1. Sectional view of the trace element quartz reactor.
Figure 2. Schematic diagram of the PSA 10.534 mercury generator.
arsenic content, along with the sorbent bed, and an arsenic mass balance was attempted. Due to its volatile nature, mercury vapor Hg could be produced precisely using the PSA (Cavkit) 10.534 mercury generator, made by PS Analytical (UK) and shown in Figure 2. The
absolute density (g/cm3)
envelope density (g/cm3)
porosity
2.118 2.094 2.285
0.698 0.549 0.475
67% 74% 79%
2.198 2.734
1.016 0.726
54% 73%
National Institute of Standards and Technology (NIST) have used a similar PSA 10.534 for the generation of Hg vapor mixtures in a flowing gas stream to establish a gaseous mercury standard.22 The unit uses an oven arrangement to generate elemental mercury from a mercury-impregnated inert substrate. Nitrogen is employed to transport the elemental mercury so generated to the reactor. This unit has two mass flow controllers to enable accurate control of gas flow rates from 0 to 20 L/min and to also manage the mercury concentration from 0 to 1300 ng/L. In the present study, 40 mL/ min (NTP) of gas with a mercury concentration of 100 ng/L has been produced and sent through the reactor. In order to achieve a mass mercury balance, a gold amalgamator was connected to the sorbent reactor gas exit to capture any residual mercury in the gas stream. Sorbents Tested. The sorbents tested in this preliminary study were Norit Darco “Hg” activated carbon, meta-kaolinite, pulverized fly ash (PFA), a carbon black produced by the pyrolysis of scrap tire and an activated carbon prepared from the pyrolyzed scrap tire. Norit Darco “Hg” is a commercial carbon commonly used in industry for gas cleaning, it offers very high retention capacity for Hg (2460-2590 µg/g).23 The meta-kaolinite Metastar 501 sorbent was provided by Imerys Minerals Ltd (England). The Metastar 501 powder is produced by high-temperature treatment of highly refined kaolin (china clay). The fly ash was collected from the electrostatic precipitator outlet at a UK power station. The carbon black and the activated carbon from scrap tires have been produced using a Carbolite HTR 11/150 laboratory scale rotary furnace. This unit has the facility to conduct pyrolysis and activation consecutively. Cut pieces of scrap tire rubber (under 20 mm) were heated up to 700 °C at 10 °C/min and pyrolyzed for 30 min under nitrogen (500 mL/min), at constant temperature. A pyrolysis char yield of 34% by weight was obtained. In a similar manner, the activation of the char from scrap tire rubber was conducted by using the same operating conditions as those used in the work of San Miguel.24 The cut pieces of scrap tire rubber were heated at 5 °C/min with a nitrogen gas flow of 500 mL/min. Once the temperature reached 700 °C, nitrogen was rapidly substituted by a flow of 500 mL/min of steam/nitrogen (80: 20, v/v). This was generated by having a water flow of 0.3 mL/ min and a nitrogen gas flow of 100 mL/min. An activation temperature of 925 °C and a holding time of 80 min were used. An activated carbon yield of 29% by weight was obtained. The Norit Darco “Hg” has a mean particle size of 9-15 µm as supplied. The pyrolysis chars and activated carbons from scrap tire rubber were crushed prior to use, and the smallest recoverable size fraction [38-75 µm] was used for the capture experiments. A minimum of 50 wt % of the meta-kaolinite has a particle size less than 2 µm. The porosity and specific pore volume of each sorbent have been measured using a AccuPyc 1330 and a GeoPyc 1360 envelope density analyzer by Micromeritics Instruments Corporation. Further information regarding the surface area and the microporosity of (22) Mitchell, G. D.; WD, D. Mercury in nitrogen gas research gas mixture; National Institute of Standards and Technology, EPA: Gaithersburg, MD, 2004, . (23) Pavlish, J. H.; et al. Status review of mercury control options for coal-fired power plants. Fuel Process. Technol. 2003, 82 (2-3), 89-165. (24) San Miguel, G.; Guillermo. Pyrolysis of Tire Rubber: Porosity and Adsorption Characteristics of the Pyrolytic Chars. Ind. Eng. Chem. Res. 1998, 37 (6), 2430-2435.
Screening of Low Cost Sorbents
Energy & Fuels, Vol. 21, No. 5, 2007 2749 Table 2. Surface Areas and Micropore Volumes of the Sorbents
Norit Darco “Hg” pyrolized scrap tire activated carbon from scrap tire coal fly ash meta-kaolinite
BET surface area (m2/g)
micropore area (m2/g)
micropore volume (mm3/g)
mesopore volume (mm3/g)
macropore volume (mm3/g)
660 72 219 15 15
367 4.6 128.0 5.1 4.0
169.30 1.29 59.50 2.30 17.51
214.2 194.2 205.1 7.6 34.5
59.0 492.9 537.2 3.9 61.7
Sorbent Characterization. Data on the porosity and specific pore volume of the five test sorbents are listed in Table 1. Further information regarding the surface area and the microporosity of each sorbent were obtained using BET analysis (Table 2). As expected, the commercial activated carbon Norit Darco “Hg” exhibits much larger BET surface areas and micropore volumes than the other sorbents, with 660 m2/g and 169.3 mm3/ g, respectively. Most of the surface area exhibited by commercial activated carbon is internal. Values for the activated tire pyrolysis carbon also show good surface area and appreciable microporosity. However, the results in Table 2 show that most of the surface area (94%) is external for the pyrolized tire carbon, with little surface area attributable to micropores. The same observation can be made for the coal ash and meta-kaolinite, which have even lower surface areas and little porosity. Thus, in terms of morphologically, these sorbents are radically different from commercial activated carbon. The initial concentrations of arsenic and mercury present in the test sorbents are listed in Table 3; these have been determined by ICP-MS and by the Leco instrument, respectively. The mass of sorbent used in each experiment was around 500 mg, which means that the initial amount of arsenic in the sorbent would vary from 0.5 µg for Norit Darco “Hg” and the pyrolized tire carbon up to about 30 µg for the coal ash. The amount of arsenic vaporized from the generation section during the experiment (1 h) was in the region of 1000 µg. The highest initial content of arsenic was therefore less than 3% of this figure.
Arsenic Capture by Sorbents. The capture efficiencies [The capture efficiency is defined as the ratio of the amount of trace element capture by the sorbent at the temperature studied compared to the amount of trace element vaporized from the generation section.] of the sorbents at 200 and 400 °C are shown in Figure 3. The commercially available activated carbon, Norit Darco “Hg”, appears to have a similar high efficiency of 70% at both temperatures; this is surprising as the capture efficiency would have been expected to decrease with increasing temperature. This observation suggests that the capture mechanism is more complex than just physical adsorption alone. Past studies12,26 have shown that activated carbon was effective in trapping arsenic oxide vapor at temperatures below 200 °C in an adsorption bed, where the interaction was of a physical nature. In the present experiments, it is possible that some particular elemental constituent of the Norit Darco “Hg” sorbent could be significant in intensifying chemical adsorption of As4 at 400 °C, over and above the benefits of the large surface area and micropore area of that sorbent. Meta-kaolinite and fly ash exhibit a comparable efficiency of around 50% at 200 °C, which then declined as the test temperature was increased to 400 °C. Although the surface areas of these sorbents were notably smaller than the Norit Darco “Hg” active carbon (Table 2), this does not seem to have had a major effect on their capture efficiencies (Figure 3). These findings support the idea that the capture process of As4 vapor may be a combination of physical and chemical adsorption. The pyrolyzed char from scrap tire showed similar trends to the Norit Darco “Hg” active carbon. The capture efficiencies were broadly similar at 200 and 400 °C. Although its BET surface area is much smaller than that of Norit Darco “Hg” (72 m2/g), the pyrolyzed tire carbon performed quite well in retaining arsenic (50% efficiency). The activated tire carbon performed better at 200 °C than the pyrolized char and showed an arsenic capture ability similar to the commercial Norit Darco “Hg”. However, its efficiency dropped below 40% at 400 °C. These observations again suggest that a chemical adsorption process is involved in arsenic capture. Several studies have been carried out to determine the effectiveness of lime and hydrated lime for the removal of arsenic from simulated flue gas streams at high temperature. Hydrated lime was found to be very effective for capturing arsenic oxide13 by forming a stable calcium arsenate product [Ca3(AsO4)2] at 600 and 1000 °C. Lime also effectively captured arsenic oxide vapor over a wider temperature range of 3001000 °C.15 It has also been suggested that CaO and Fe2O3 embedded in fly ash could be responsible, at least partially, for the retention of arsenic species in the gas phase.16,17 Interpretation of results is complicated by the effect of sorbent density and bed porosity (Table 1). Density ranges from 2.734 g/cm3 for meta-kaolin down to 2.094 g/cm3 for Norit Darco “Hg”. Bed porosity varies from 79% for activated tire rubber down to 54% for fly ash. Experiments have been conducted
(25) Richaud, R.; et al. Trace mercury concentrations in coals and coalderived material determined by atomic absorption spectrophotometry. Fuel 1998, 77 (5), 359-368.
(26) Jadhav, R. R. ActiVated carbon for gas phase arsenic capture. Proceedings 16th Annual International Pittsburgh Coal Conference, Pittsburgh, PA, Oct 11-15, 1999; pp 535-432.
Table 3. Initial Concentration of Arsenic and Mercury in the Sorbents initial As conc (ppm)
initial Hg conc (ppm)
1 1 0 58 14
nda 0.06 0.01 0.94
Norit Darco Hg pyrolized scrap tire activated carbon from scrap tire coal flyash meta-kaolinite a
Below the detection limit of the Leco.
each sorbent has been obtained using Brunauer-Emmet-Teller (BET) analysis. Arsenic and Mercury Quantification. The arsenic content in the sorbent and reactor wash solution (post experiment) has been determined using inductively coupled plasma-atomic emission spectroscopy (ICP-AES), after an initial sample preparation using microwave digestion with nitric acid (for the sorbents). The mercury content of the sorbent and the gold amalgamator has been determined using a Leco AMA 254 analyzer, an atomic-absorption based instrument designed for the rapid quantification of mercury in liquids and solid samples.25
Results and Discussion
2750 Energy & Fuels, Vol. 21, No. 5, 2007
Charpenteau et al. Table 4. Variation of Sulfur and Carbon Contents through the Pyrolysis and Activation Stage of the Rubber Tire
Figure 3. Arsenic capture efficiency of the different sorbents tested in this study.
Figure 4. Mercury capture efficiency of the different sorbents tested in this study.
with a fixed mass of sorbent (500 mg) throughout; hence, bed height, gas velocity, and residence time all change when the sorbent is changed; e.g., the residence time for activated tire rubber is calculated to be four times higher than that for fly ash (0.2 s). Furthermore, increasing temperature also has the effect of reducing the residence time of gas in a bed of fixed height. Potential mass transfer effects have not been allowed for in presenting these preliminary results. Mercury Capture by Sorbents. Investigation of the mercury retention of the test sorbents has been carried out at three different temperatures, 100, 150, and 200 °C, for a 1 h period. The results are presented in Figure 4. The capture efficiency of all the sorbents is seen to decrease as the test temperature is increased, suggesting that physical sorption might be the initial mechanism for binding mercury to these sorbents.23 Coal fly ash, Norit Darco “Hg”, and tire pyrolysis char all retained 90% of the mercury when operated at 100 °C. As the temperature increased, the efficiency of activated carbon and pyrolysis char was reduced significantly. However, the ash only lost its capture effectiveness when it was operating above 150 °C. Norit Darco “Hg” and the tire pyrolysis char exhibit similar porosity (Table 1); however, the value for the fly ash sample is significantly lower. It is also apparent (Table 2) that Norit Darco “Hg” has the greatest BET and micropore surface areas, 660 and 367 m2/g, respectively. However, the coal ash proved to be the most effective in capturing mercury across the temperature range tested, despite having the smallest BET surface area of the five sorbents. This suggests that the mercury capture process is not only physical adsorption but a combination of
stage
sample
S (%)
yield (wt %)
% of S lost from stage 1
% of S lost from stage 2
1 2 3
tire rubber tire pyrolysis tire activated
1.88 2.91 2.95
100 34 29
na -47 -54
na na -71
stage
sample
Cl (%)
yield (wt %)
% of Cl lost from stage 1
% of Cl lost from stage 2
1 2 3
tire rubber tire pyrolysis tire activated
0.04 0.03 0.01
100 34 29
na -75 -93
na na -90
physical and chemical processes. It is possible that the ash contains some particular element, or elements, that allow mercury capture by chemical adsorption as the temperature is increased. In addition, it is interesting to note that the initial mercury content in this ash was substantially higher than in the other sorbents, at 0.94 ppm (Table 3), due to its prior exposure to mercury in the power station flue gas. It is perhaps possible that the initial mercury present in the ash may have had a positive effect on subsequent mercury capture. As the activated carbon produced from tire rubber has a higher surface area than the char produced by simple pyrolysis, it would be expected to be superior in mercury capture. However, Figure 4 shows that the pyrolysis char actually performs better than the activated variety. These results confirm the hypothesis that something other than physical adsorption is involved in the mercury capture process. The activation stage of the pyrolysis char, which is conducted at 900 °C, may have led to the loss from the sorbent of an element capable of promoting the chemical adsorption of mercury. Analyses of the sulfur and chlorine content of the chars derived from scrap tire rubber show that 71% and 90% of the sulfur and chlorine content, respectively, have been lost between the activation and pyrolysis stages (Table 4). Sulfur impregnation of active alumina and zeolite has been shown to greatly enhance the removal of elemental mercury Hg0.27 Similarly, sorption capacities of various treated activated carbons for Hg0 were found to correlate well with an increase in the surface concentration of chlorine.9,10 Conclusions Preliminary experimental investigations have shown that potentially cheap sorbents made from scrap tire rubber offer good capabilities for retaining both arsenic and mercury. The results suggest that the capture mechanism of both arsenic (As4) and mercury (Hg) is more likely to be a combination of physical and chemical processes, rather than purely physical adsorption onto the sorbents. In addition to the effects of BET surface area and micropore area, certain [as yet] unidentified elements within the sorbents could be significant in intensifying chemical adsorption. Further studies on the sorbent properties are underway to gain better understanding of the capture mechanism; these include X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis of the sorbent surface before and after capture. Acknowledgment. The authors gratefully acknowledge the financial support of the European Union under contract number RFC-CR-04006 (AGAPUTE) and the British Coal Utilisation Research Association under contract B.73. EF070026C (27) Otani, Y.; Yoshio. Removal of mercury vapor from air with sulfurimpregnated adsorbents. EnViron. Sci. Technol. 1988, 22 (6), 708-711.
