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Sorption of mercury by activated carbon in the presence of flue gas components
Fuel Processing Technology 91 (2010) 158–163
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Fuel Processing Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / f u p r o c
Sorption of mercury by activated carbon in the presence of flue gas components Ir. Diamantopoulou a,⁎, G. Skodras c, G.P. Sakellaropoulos a,b a b c
Chemical Process Engineering Laboratory, Department of Chemical Engineering, Aristotle University of Thessaloniki, Thessaloniki, Greece Laboratory of Energy and Environmental Processes, Chemical Process Engineering Research Institute, Thessaloniki, Greece Institute for Solid Fuels Technology and Applications, Ptolemais, Greece
a r t i c l e
i n f o
Article history: Received 3 February 2009 Received in revised form 11 September 2009 Accepted 14 September 2009 Keywords: Activated carbon Elemental mercury Flue gases
a b s t r a c t The purpose of the current study is to evaluate the mercury removal ability of F400 and Norit FGD activated carbons, through fixed bed adsorption tests at inert atmosphere (Hg° + N2). Additionally, adsorption tests were realized on F400 activated carbon, in the presence of HCl, O2, SO2 and CO2 in nitrogen flow. The obtained results, revealed that F400 activated carbon, with a high-developed micropore structure and increased BET area, exhibit larger Hg° adsorptive capacity compared to Norit. HCl and O2, can strongly affect mercury adsorption, owing to heterogeneous oxidation and chemisorption reactions, which is in accordance with the assumptions of some researchers. Additionally, SO2 presence enhances mercury adsorption, in contrast with the conclusions evaluated in other studies. The above result could be attributed to the possible formation of sulphur spaces on activated carbon surface and consist of a clarification for the role of SO2 on mercury adsorption. On the contrary, the mercury adsorption efficiency of F400 activated carbon showed a decrease at about 25%, with increasing CO2 concentration from 0 to 12%. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Mercury is a trace element of special concern because its high volatility allows transfer into the flue gas stream during combustion and creates a long term contamination problem. Most global mercury emissions come from burning coal in incinerators and fossil fuel combustors. A typical mercury concentration in an incineration flue gas is 0.5 mg/Nm3, significantly higher than the mercury emission limit, which in some countries is as low as 0.05 mg/Nm3 [1]. The main forms of mercury emitted in flue gases are elemental (Hgo), oxidized (Hg 2+) and particulate (Hgp) [2]. The environmental impact of mercury released during coal combustion is of great concern, since exposure to elemental, inorganic and organic mercury forms via inhalation and ingestion can present a serious health risk [3]. Particulate matter abatement technologies (ESP's, bughouses) can mostly remove mercury in particulate phase and provide limited reduction in gaseous mercury emissions. Similarly, wet and dry scrubbers are only effective at removing oxidized mercury species. Conversely, elemental form of mercury (Hg0), is difficult to capture [4]. One promising option for removing elemental mercury from gas phase is the adsorption on solid materials such as activated carbon, calcium-based sorbents, fly ash and zeolites. Among them, activated carbon, which possesses extended surface area and high surface reactivity, seem to be suitable for Hg° adsorption, used in a fixed bed operation or in an injection process. The majority of the research has
⁎ Corresponding author. Tel.: +30 2310994374; fax: +30 2310996168. E-mail address: [email protected] (I. Diamantopoulou). 0378-3820/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2009.09.005
been focused on the injection of activated carbon upstream of flue gases, for the adsorption of both divalent (Hg2+) and elemental mercury (Hg°). Alternatively, in order to increase the contact time between mercury and sorbent particles and to achieve the removal of higher mercury concentrations, activated carbons can be used in a fixed bed type system. This technique offers the advantage of column regeneration, if this is attainable [5]. While there are numerous factors that can influence mercury removal by activated carbons, sorbent properties are likely to be among the most important. Vidic et al. studied Hgo adsorption on various sorbents [6]. Herewith the above work, many researchers concluded that for best performance, adsorbents must have a suitable pore size distribution, permitting the entrance of mercury into the carbon pores, and large surface area that offers many active sites for mercury uptake [7,8]. This study, not only focuses on the impact of activated carbon surface area on mercury adsorption, but also aims at investigating the role of pore sizes in mercury adsorption. Although using pure nitrogen and a constant low adsorption temperature is helpful for many researchers to understand the baseline performance of the examined adsorbents, the impact of some individual gas components (HCl, O2, SO2, CO2) on mercury adsorption, is also examined, at temperatures almost equal to those that existed in the flue gases [9,10]. However, there are many unclear observations concerning the competitive or positive role of various gas components on mercury adsorption on carbon surface. As much as it concerns HCl and O2 that constitute important elemental mercury oxidation components, many researchers try to elucidate the possibility of gas phase elemental mercury reactions with the above molecules, at flue gases temperatures [5,9]. The problem is mainly concerned with the
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determination of heterogeneous mercury oxidation mechanism on carbon surface, in the presence of HCl and O2 [11,12]. Some studies report mercury chemisorption mechanism on acidic sites generated on carbon aromatic ring, in the presence of HCl. In this case, the acidic sites can directly accept electrons from elemental mercury, resulting in the formation of mercury–hydrogen chloride bond [13]. Additionally, the effect of SO2 and CO2 on elemental mercury adsorption is not clarified by most of the researchers. Some of them support that both SO2 and CO2 inhibit elemental mercury adsorption on activated carbon, since they occupy similar active sites on carbon surface [14]. On the contrary, some others assert that SO2 facilitates elemental mercury oxidation and enhances its adsorption on activated carbon. This could be attributed to the formation of sulphur groups on carbon surface that posses additional active sites for elemental mercury chemisorption [15,16]. The goal of this research is to investigate the mechanism of elemental mercury adsorption on activated carbon surface, as a function of the adsorbent pore structure and reactant gas composition. In order to achieve the above targets, mercury adsorption tests have been conducted in a fixed bed activated carbon column. The interaction between elemental mercury and activated carbon surface was examined by employing adsorption tests under inert atmosphere. The impact of micropore volume of the examined adsorbents on mercury retention was studied by employing two commercial activated carbons with different micropore volume contribution to their total pore volume. Additionally, in order to elucidate elemental mercury adsorption procedure under real flue gas conditions and to investigate the validity of the proposed mechanisms by other researchers, the impact of various gas constituents on mercury adsorption at a time, has been examined. The above approach was used in this study in order to evaluate the effect of one gas constituent at a time and to avoid the complicated impacts resulting from the coexistence of several gas components. 2. Experimental 2.1. Samples selection and characterization Elemental mercury adsorption experiments were performed by employing two commercial activated carbons, Calgon F400 and Norit FGD. In order to correlate activated carbons properties with the mercury removal ability, the examined adsorbents have been characterized for their pore structure by employing physical adsorption methods (N2 at 77 K and CO2 at 298 K). A conventional volumetric apparatus (Quantasorb multiple sample manifold) was used for the N2 adsorption experiments, while CO2 adsorption isotherms were obtained by using a laboratory volumetric apparatus and the static method. With these techniques, apart from the determination of specific surface area, some important information for micropore volume contribution to the total pore volume was extracted and the effect of the pore sizes on mercury adsorption capacity was also explored. BET multiple point and Dubinin-Raduchevich equations were used to calculate surface areas from N2 and CO2 adsorption respectively, total pore volume and micropore volume contribution [17,18]. The micropore volume distributions by pore size of the activated carbons were calculated by employing the Medek method [19]. 2.2. Mercury adsorption experiments Bench scale elemental mercury adsorption tests were conducted under simulated gas conditions (0.35 ng/cm3 Hg° + 99.99% N2) at adsorption temperature 50 °C and in the presence of HCl (50 ppmv), O2 (10% v/v), SO2 (200 and 500 ppmv) and CO2 (12% v/v) separately in Hg° and N2 gas stream at 150 °C. The above concentrations have been selected as a result of the typical gas mixture composition in flue gases
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derived from coal fired plants and municipal waste combustions [20]. A schematic representation of the Hg adsorption unit is shown in Fig. 1. The adsorption unit is described with more details elsewhere [21]. A mercury permeation device was used as a source of elemental mercury Hgo (VICI Metronics Inc, Santa Clara, CA), designed to produce constant release of mercury vapor per unit time at the specified temperatures. The device was secured in a temperaturecontrolled stainless steel U-tube holder, and nitrogen was fed through it, at a preadjusted constant flow. A mass flow controller kept the nitrogen flow constant, at 200 cm3/min, and the Hgo inlet concentration was 0.35 ng/cm3. The main body of the adsorption tests was conducted with active coke mass 20 mg, mixed with 1 g sand, in a differential-bed reactor (1/4 in. inner diameter stainless steel column), enclosed in a temperature-controlled oven. In order to investigate the effect of quartz sand on elemental mercury adsorption in the examined activated carbons, adsorption test was realized by employing 1 g of sand in the reactor, at 50 °C adsorption temperature. The quartz sand has an almost null Hg° adsorptive capacity (less than 0.05 ng Hg°/mg sand) and acts as an inert material that contributes to the formation of a sufficient height of adsorption column (almost 4 cm). Additionally, mercury adsorption tests were realized in the presence of HCl (50 ppmv) and O2 (10% v/v) with empty reactor, in order to examine the possibility of gas phase reactions at the experimental conditions used. In both cases, elemental mercury measured at the outlet of the reactor was unchanged, indicating that no homogeneous reaction have been performed between mercury and HCl or O2. The concentration of elemental mercury in the gas stream was continuously monitored by an Elemental Mercury Instruments Analyzer (VM 3000 Mercury Vapor Monitor) based on Cold Vapor Atomic Adsorption Spectroscopy. It was observed that SO2 affected the detection capability of VM 3000 Analyzer. Thus a 0.1 M NaOH solution was used in the end of the reactor and connected to a dryer, in order to scrub SO2 from the gas stream before entering mercury analyzer. The aim of continuously measurement of elemental mercury concentration was the construction of mercury breakthrough curves in order to determine activated carbon adsorption capacities, by integrating the area above the breakthrough curves, with the following equation: t
q=
F ∫ ðC −Cout Þ⋅dt: m 0 in
ð1Þ
Most of the breakthrough adsorption curves that were produced from experimental tests were misleading and didn't provide the equilibrium mercury adsorption capacity. Thus, due to the timeconsuming adsorption experiments up to the establishment of equilibrium (Cout = 0.95 × Cin), mercury adsorbed quantities were determined at 80% of column saturation (Cout = 0.8 × Cin). This time is higher than the half life period of the column (Cout = 0.5 × Cin), which is a representative operation time of the column. It was selected owing to the channeling effect appeared in the fixed bed operation at the initial times that makes the half life period indistinguishable and it approaches the equilibrium time at 95% saturation. 3. Results and discussion 3.1. Characterization of adsorbents The nitrogen and carbon dioxide adsorption results of the examined activated carbons were used for calculating the pore structure characteristics, Table 1. From the results included in this table, it is evident that F400 exhibits higher N2 adsorption capacity, indicating a higher BET surface area compared to Norit (33%) and total pore volume (37%). Additionally, the CO2 adsorption capacity of F400 is higher than the one of Norit activated carbon, indicating a micropore structure for F400. This is evident by the total and the micropore volume of the samples given in Table 1. In particular, micropore
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Fig. 1. Hg adsorption unit.
volume for F400 activated carbon is about 82% higher than the one calculated for Norit. Thus, in comparison to Norit, F400 has about 33% higher BET surface area and almost double micropore volume. The Medek pore size distribution curves of the F400 and Norit activated carbons are of similar shape, Fig. 2. However, almost double values were obtained for F400 due to its micropore structure that is proclaimed by its increased CO2 adsorption capacity. 3.2. Mercury adsorption results under baseline conditions The Hg° adsorbed quantities of the examined activated carbons, calculated by integrating the breakthrough curves, Fig. 3, up to 80% column saturation, which are summarized in Table 1, correlated to their pore structure characteristics. The evolved results reveal that the high BET surface area and the large pore volume of the microporous F400 activated carbon enhance Hg° retention, since inside the micropores, where the mechanism of adsorption is taking place with pore filling, the interaction potential between the solid and the Hg° is significantly higher than in wider pores [22]. On the contrary, as shown in Fig. 3, much faster breakthrough curve was obtained for Norit activated carbon, indicating that the outlet concentration reaches faster the inlet one. Norit activated carbon presented lower Hg° adsorptive capacity, as its mesoporous structure facilitates the faster filling of the limited number of micropores and, thus, the quick saturation of the sample [14,23].
3.3. One-at-a-time mercury adsorption tests with HCl, O2, SO2 and CO2 3.3.1. Impact of HCl Fig. 4 compares mercury uptake by F400 in pure nitrogen and in HCl/N2 mixture at 150 °C adsorption temperature. This test indicates that the mercury uptake capacity of F400 was strongly increased by the presence of HCl, since mercury outlet concentration approached only the 20% of the inlet one after 2500 min of adsorption experiment. This implies that the 80% of mercury entering the column is retained by activated carbon and the equilibrium time is expected to be very high. This significant impact of HCl on elemental mercury adsorption seems to accord with a heterogeneous mercury oxidation mechanism proposed in other studies, since homogeneous gas phase interactions between mercury and HCl are excluded at low temperatures such as 150 °C [10,24]. Particularly, the concept of zigzag carbene edge structures on the aromatic rings of activated carbon is employed by
Table 1 Pore structure characteristics and Hg° adsorbed amount of activated carbons. Sample
BET surface area (m2/g)
CO2 area (m2/g)
Total pore volume Vtot (cm3/g)
Micropore volume VμpN2 (cm3/g)
Hgo adsorbed amount (ng/mg)a
Time at Cout =0.8 ×Cin (min)
F400 Norit
827 620
840 740
0.52 0.38
0.40 0.22
551 287
423 246
a
Cout = 0.8 × Cin, 50 °C.
Fig. 2. Medek pore size distribution curves.
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Fig. 5. Oxidation mechanism of elemental mercury by HCl presence.
Fig. 3. Hg° adsorption on F400 and Norit activated carbons at 50 °C.
some researchers [25]. This model provides a detailed mechanism that explains the catalytic role of acids such as HCl in mercury oxidation step, Fig. 5. The current model involves the conversion of carbene to carbenium ion by HCl attack on a Lewis basic site (free pair of electrons). Thus, the carbenium ion reacts as a Lewis acid site on the aromatic ring, which can directly accept electrons from Hg° and cause its oxidation, resulting in the formation of organomercury intermediates.
[21,28]. The surface chemistry characterization of activated carbon after its contact with O2, could be tested in a future work, in order to validate the formation of lactones and carbonyls. In comparison to the role of HCl on elemental mercury adsorption, Fig. 4, oxygen amplifying impact on mercury retention is less intense, Fig. 6. This observation reveals that the chemisorption sites generated by the presence of HCl on carbon surface are more important than carbon– oxygen functional groups formed on activated carbon in the presence of oxygen.
3.3.2. Impact of O2 The resulting mercury breakthrough curves under O2 are presented if Fig. 6. As can be seen from this figure, the mercury breakthrough curve became slower under oxygen presence and the mercury adsorbed amount increased by almost 10 times compared to the one calculated under inert gas conditions, Table 2. The enhanced mercury adsorption on activated carbon in the presence of oxygen can be attributed to the formation of carbon–oxygen complexes during the column test on carbon surface and to the heterogeneous reaction between elemental mercury and oxygen catalyzed on carbon surface [9,11]. Additionally, the possibility of carbonyls and lactones formation, which represent possible chemisorption sites for elemental mercury adsorption, is high [26,27]. The positive impact of the above oxygen groups on elemental mercury adsorption has been testified in two other published studies originated by our research activities
3.3.3. Impact of SO2 Fig. 7 compares mercury breakthrough curves by F400 in pure nitrogen and SO2/N2 mixture. This test indicate that the mercury breakthrough curves were shifted at higher adsorption times under the presence of SO2. The mercury adsorbed amount that have been calculated at 80% breakthrough increased by 2 and 5 times at 200 and 500 ppmv SO2 concentration respectively, Table 2. This result suggests that SO2 interacts with activated carbon via physisorption or chemisorption. Thus, a possible explanation for enhanced mercury retention on carbon surface could be the deposition of sulphur groups on activated carbon surface. These sulphur groups constitutes chemisorption sites for elemental mercury, since sulphur atoms can accept the two electrons presented in the outer layer of elemental mercury and generates its oxidized form (Hg2+) [29,30]. Additionally, the possibility of elemental mercury and sulphur dioxide competition for similar active sites on carbon surface is minimized, since SO2 usually is adsorbed on activated carbons with a large amount of basic groups.
Fig. 4. Effect of HCl (50 ppm) on mercury adsorption on F400 at 150 °C.
Fig. 6. Effect of O2 (10% v/v) on mercury adsorption on F400 at 150 °C.
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Table 2 Hg° adsorbed amount under baseline conditions and flue gas constitutes presence. Gas mixture
Hgo adsorbed amount (ng/mg F400)a
N2 + Hgo N2 + Hgo + O2 10% v/v N2 + Hgo + 200 ppmv SO2 N2 + Hgo + 500 ppmv SO2 N2 + Hgo + 12% v/v CO2
94 1170 181 513 13
Adsorption temperature: 150 °C. a Cout = 0.8 × Cin, 150 °C.
More specifically, the presence of pyronic or pyronic-like structures greatly increases the adsorption of SO2 on activated carbon, while elemental mercury prefers lactones and carbonyls [31,32]. 3.3.4. Impact of CO2 The breakthrough curves presented in Fig. 8 show that CO2 affects the performance of F400 activated carbon and interferes with elemental mercury measurements. Mercury breakthrough curve reached faster the equilibrium establishment in the presence of CO2 and the adsorbed amount was reduced about 85% compared to the one determined at inert gas conditions, Table 2. The above result indicated that mercury adsorption was prevented by CO2, due to the fact that CO2 fills a part of activated carbon microporous structure. Since physisorption is limited at high adsorption temperatures such as 150 °C, the occupation of similar chemisorption sites between elemental mercury and CO2 is a possible explanation for the observed competitive adsorption. These active sites are represented mainly by lactones and carbonyls and they are placed at the edges of activated carbon graphitic layers [33,34]. Thus, the utilization of an activated carbon for both elemental mercury and carbon dioxide retention seems to be difficult. 4. Conclusions The results obtained in this research showed that the extended surface area and well-developed micropore structure combined with a small volume of mesopores in activated carbons, enhanced mercury physisorption. These properties characterize the commercial activated carbon F400. On the contrary, mesoporous materials with low specific surface areas, such as the commercial activated carbon Norit, presented lower mercury removal efficiency.
Fig. 8. Effect of CO2 (12% v/v) on mercury adsorption on F400 at 150 °C.
However, at competitive adsorption conditions, HCl, O2 and SO2 seem to contribute to the chemisorption of elemental mercury on activated carbon. Among them, HCl, generated the most important increase in mercury adsorbed amount. A possible explanation, which confirms the concept employed in the literature, is the heterogeneous oxidation of elemental mercury on the acidic sites created on carbon surface by HCl presence. Oxygen presence in the reactant gas caused a smaller improvement of mercury adsorption capacity in activated carbon, in comparison to HCl effect. The chemisorption of elemental mercury on the oxygen groups formed on carbon surface could be responsible for the enhanced performance of activated carbon in the presence of oxygen. A proposed future work in order to ensure the above mechanism is the surface chemistry characterization and an Xray absorption fine structure analysis (XAFS) of activated carbon after its treatment with oxygen. Through these techniques, the formation of oxygen groups and their interaction with elemental mercury will be investigated. In the same way, SO2 presented in nitrogen and mercury gas mixture had a large benefit on elemental mercury capture, probably due to the formation of sulphur groups on carbon surface, which acts as mercury chemisorption sites. This result clarifies the confused theory in literature as much as concerns the SO2 impact on mercury adsorption. On the contrary, the adsorptive capacity of activated carbon was reduced in the presence of CO2, because of the competitive adsorption of the two gases on similar adsorption sites. With the above results, the role of SO2 and CO2 on elemental mercury adsorption was elucidated. References
Fig. 7. Effect of SO2 (200–500 ppmv) on mercury adsorption on F400 at 150 °C.
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Glossary Cin: Hg° initial concentration (ng/cm3) Cout: Hg° outlet concentration (ng/cm3) F: gas flow rate (cm3/min) m: activated carbon mass (mg) q: Hg° adsorptive capacity (ng/mg) t: adsorption time (min) VμpN2: micropore volume from nitrogen adsorption results Vtot: total pore volume of adsorbent (cm3/g)
Contents lists available at ScienceDirect
Fuel Processing Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / f u p r o c
Sorption of mercury by activated carbon in the presence of flue gas components Ir. Diamantopoulou a,⁎, G. Skodras c, G.P. Sakellaropoulos a,b a b c
Chemical Process Engineering Laboratory, Department of Chemical Engineering, Aristotle University of Thessaloniki, Thessaloniki, Greece Laboratory of Energy and Environmental Processes, Chemical Process Engineering Research Institute, Thessaloniki, Greece Institute for Solid Fuels Technology and Applications, Ptolemais, Greece
a r t i c l e
i n f o
Article history: Received 3 February 2009 Received in revised form 11 September 2009 Accepted 14 September 2009 Keywords: Activated carbon Elemental mercury Flue gases
a b s t r a c t The purpose of the current study is to evaluate the mercury removal ability of F400 and Norit FGD activated carbons, through fixed bed adsorption tests at inert atmosphere (Hg° + N2). Additionally, adsorption tests were realized on F400 activated carbon, in the presence of HCl, O2, SO2 and CO2 in nitrogen flow. The obtained results, revealed that F400 activated carbon, with a high-developed micropore structure and increased BET area, exhibit larger Hg° adsorptive capacity compared to Norit. HCl and O2, can strongly affect mercury adsorption, owing to heterogeneous oxidation and chemisorption reactions, which is in accordance with the assumptions of some researchers. Additionally, SO2 presence enhances mercury adsorption, in contrast with the conclusions evaluated in other studies. The above result could be attributed to the possible formation of sulphur spaces on activated carbon surface and consist of a clarification for the role of SO2 on mercury adsorption. On the contrary, the mercury adsorption efficiency of F400 activated carbon showed a decrease at about 25%, with increasing CO2 concentration from 0 to 12%. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Mercury is a trace element of special concern because its high volatility allows transfer into the flue gas stream during combustion and creates a long term contamination problem. Most global mercury emissions come from burning coal in incinerators and fossil fuel combustors. A typical mercury concentration in an incineration flue gas is 0.5 mg/Nm3, significantly higher than the mercury emission limit, which in some countries is as low as 0.05 mg/Nm3 [1]. The main forms of mercury emitted in flue gases are elemental (Hgo), oxidized (Hg 2+) and particulate (Hgp) [2]. The environmental impact of mercury released during coal combustion is of great concern, since exposure to elemental, inorganic and organic mercury forms via inhalation and ingestion can present a serious health risk [3]. Particulate matter abatement technologies (ESP's, bughouses) can mostly remove mercury in particulate phase and provide limited reduction in gaseous mercury emissions. Similarly, wet and dry scrubbers are only effective at removing oxidized mercury species. Conversely, elemental form of mercury (Hg0), is difficult to capture [4]. One promising option for removing elemental mercury from gas phase is the adsorption on solid materials such as activated carbon, calcium-based sorbents, fly ash and zeolites. Among them, activated carbon, which possesses extended surface area and high surface reactivity, seem to be suitable for Hg° adsorption, used in a fixed bed operation or in an injection process. The majority of the research has
⁎ Corresponding author. Tel.: +30 2310994374; fax: +30 2310996168. E-mail address: [email protected] (I. Diamantopoulou). 0378-3820/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2009.09.005
been focused on the injection of activated carbon upstream of flue gases, for the adsorption of both divalent (Hg2+) and elemental mercury (Hg°). Alternatively, in order to increase the contact time between mercury and sorbent particles and to achieve the removal of higher mercury concentrations, activated carbons can be used in a fixed bed type system. This technique offers the advantage of column regeneration, if this is attainable [5]. While there are numerous factors that can influence mercury removal by activated carbons, sorbent properties are likely to be among the most important. Vidic et al. studied Hgo adsorption on various sorbents [6]. Herewith the above work, many researchers concluded that for best performance, adsorbents must have a suitable pore size distribution, permitting the entrance of mercury into the carbon pores, and large surface area that offers many active sites for mercury uptake [7,8]. This study, not only focuses on the impact of activated carbon surface area on mercury adsorption, but also aims at investigating the role of pore sizes in mercury adsorption. Although using pure nitrogen and a constant low adsorption temperature is helpful for many researchers to understand the baseline performance of the examined adsorbents, the impact of some individual gas components (HCl, O2, SO2, CO2) on mercury adsorption, is also examined, at temperatures almost equal to those that existed in the flue gases [9,10]. However, there are many unclear observations concerning the competitive or positive role of various gas components on mercury adsorption on carbon surface. As much as it concerns HCl and O2 that constitute important elemental mercury oxidation components, many researchers try to elucidate the possibility of gas phase elemental mercury reactions with the above molecules, at flue gases temperatures [5,9]. The problem is mainly concerned with the
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determination of heterogeneous mercury oxidation mechanism on carbon surface, in the presence of HCl and O2 [11,12]. Some studies report mercury chemisorption mechanism on acidic sites generated on carbon aromatic ring, in the presence of HCl. In this case, the acidic sites can directly accept electrons from elemental mercury, resulting in the formation of mercury–hydrogen chloride bond [13]. Additionally, the effect of SO2 and CO2 on elemental mercury adsorption is not clarified by most of the researchers. Some of them support that both SO2 and CO2 inhibit elemental mercury adsorption on activated carbon, since they occupy similar active sites on carbon surface [14]. On the contrary, some others assert that SO2 facilitates elemental mercury oxidation and enhances its adsorption on activated carbon. This could be attributed to the formation of sulphur groups on carbon surface that posses additional active sites for elemental mercury chemisorption [15,16]. The goal of this research is to investigate the mechanism of elemental mercury adsorption on activated carbon surface, as a function of the adsorbent pore structure and reactant gas composition. In order to achieve the above targets, mercury adsorption tests have been conducted in a fixed bed activated carbon column. The interaction between elemental mercury and activated carbon surface was examined by employing adsorption tests under inert atmosphere. The impact of micropore volume of the examined adsorbents on mercury retention was studied by employing two commercial activated carbons with different micropore volume contribution to their total pore volume. Additionally, in order to elucidate elemental mercury adsorption procedure under real flue gas conditions and to investigate the validity of the proposed mechanisms by other researchers, the impact of various gas constituents on mercury adsorption at a time, has been examined. The above approach was used in this study in order to evaluate the effect of one gas constituent at a time and to avoid the complicated impacts resulting from the coexistence of several gas components. 2. Experimental 2.1. Samples selection and characterization Elemental mercury adsorption experiments were performed by employing two commercial activated carbons, Calgon F400 and Norit FGD. In order to correlate activated carbons properties with the mercury removal ability, the examined adsorbents have been characterized for their pore structure by employing physical adsorption methods (N2 at 77 K and CO2 at 298 K). A conventional volumetric apparatus (Quantasorb multiple sample manifold) was used for the N2 adsorption experiments, while CO2 adsorption isotherms were obtained by using a laboratory volumetric apparatus and the static method. With these techniques, apart from the determination of specific surface area, some important information for micropore volume contribution to the total pore volume was extracted and the effect of the pore sizes on mercury adsorption capacity was also explored. BET multiple point and Dubinin-Raduchevich equations were used to calculate surface areas from N2 and CO2 adsorption respectively, total pore volume and micropore volume contribution [17,18]. The micropore volume distributions by pore size of the activated carbons were calculated by employing the Medek method [19]. 2.2. Mercury adsorption experiments Bench scale elemental mercury adsorption tests were conducted under simulated gas conditions (0.35 ng/cm3 Hg° + 99.99% N2) at adsorption temperature 50 °C and in the presence of HCl (50 ppmv), O2 (10% v/v), SO2 (200 and 500 ppmv) and CO2 (12% v/v) separately in Hg° and N2 gas stream at 150 °C. The above concentrations have been selected as a result of the typical gas mixture composition in flue gases
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derived from coal fired plants and municipal waste combustions [20]. A schematic representation of the Hg adsorption unit is shown in Fig. 1. The adsorption unit is described with more details elsewhere [21]. A mercury permeation device was used as a source of elemental mercury Hgo (VICI Metronics Inc, Santa Clara, CA), designed to produce constant release of mercury vapor per unit time at the specified temperatures. The device was secured in a temperaturecontrolled stainless steel U-tube holder, and nitrogen was fed through it, at a preadjusted constant flow. A mass flow controller kept the nitrogen flow constant, at 200 cm3/min, and the Hgo inlet concentration was 0.35 ng/cm3. The main body of the adsorption tests was conducted with active coke mass 20 mg, mixed with 1 g sand, in a differential-bed reactor (1/4 in. inner diameter stainless steel column), enclosed in a temperature-controlled oven. In order to investigate the effect of quartz sand on elemental mercury adsorption in the examined activated carbons, adsorption test was realized by employing 1 g of sand in the reactor, at 50 °C adsorption temperature. The quartz sand has an almost null Hg° adsorptive capacity (less than 0.05 ng Hg°/mg sand) and acts as an inert material that contributes to the formation of a sufficient height of adsorption column (almost 4 cm). Additionally, mercury adsorption tests were realized in the presence of HCl (50 ppmv) and O2 (10% v/v) with empty reactor, in order to examine the possibility of gas phase reactions at the experimental conditions used. In both cases, elemental mercury measured at the outlet of the reactor was unchanged, indicating that no homogeneous reaction have been performed between mercury and HCl or O2. The concentration of elemental mercury in the gas stream was continuously monitored by an Elemental Mercury Instruments Analyzer (VM 3000 Mercury Vapor Monitor) based on Cold Vapor Atomic Adsorption Spectroscopy. It was observed that SO2 affected the detection capability of VM 3000 Analyzer. Thus a 0.1 M NaOH solution was used in the end of the reactor and connected to a dryer, in order to scrub SO2 from the gas stream before entering mercury analyzer. The aim of continuously measurement of elemental mercury concentration was the construction of mercury breakthrough curves in order to determine activated carbon adsorption capacities, by integrating the area above the breakthrough curves, with the following equation: t
q=
F ∫ ðC −Cout Þ⋅dt: m 0 in
ð1Þ
Most of the breakthrough adsorption curves that were produced from experimental tests were misleading and didn't provide the equilibrium mercury adsorption capacity. Thus, due to the timeconsuming adsorption experiments up to the establishment of equilibrium (Cout = 0.95 × Cin), mercury adsorbed quantities were determined at 80% of column saturation (Cout = 0.8 × Cin). This time is higher than the half life period of the column (Cout = 0.5 × Cin), which is a representative operation time of the column. It was selected owing to the channeling effect appeared in the fixed bed operation at the initial times that makes the half life period indistinguishable and it approaches the equilibrium time at 95% saturation. 3. Results and discussion 3.1. Characterization of adsorbents The nitrogen and carbon dioxide adsorption results of the examined activated carbons were used for calculating the pore structure characteristics, Table 1. From the results included in this table, it is evident that F400 exhibits higher N2 adsorption capacity, indicating a higher BET surface area compared to Norit (33%) and total pore volume (37%). Additionally, the CO2 adsorption capacity of F400 is higher than the one of Norit activated carbon, indicating a micropore structure for F400. This is evident by the total and the micropore volume of the samples given in Table 1. In particular, micropore
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Fig. 1. Hg adsorption unit.
volume for F400 activated carbon is about 82% higher than the one calculated for Norit. Thus, in comparison to Norit, F400 has about 33% higher BET surface area and almost double micropore volume. The Medek pore size distribution curves of the F400 and Norit activated carbons are of similar shape, Fig. 2. However, almost double values were obtained for F400 due to its micropore structure that is proclaimed by its increased CO2 adsorption capacity. 3.2. Mercury adsorption results under baseline conditions The Hg° adsorbed quantities of the examined activated carbons, calculated by integrating the breakthrough curves, Fig. 3, up to 80% column saturation, which are summarized in Table 1, correlated to their pore structure characteristics. The evolved results reveal that the high BET surface area and the large pore volume of the microporous F400 activated carbon enhance Hg° retention, since inside the micropores, where the mechanism of adsorption is taking place with pore filling, the interaction potential between the solid and the Hg° is significantly higher than in wider pores [22]. On the contrary, as shown in Fig. 3, much faster breakthrough curve was obtained for Norit activated carbon, indicating that the outlet concentration reaches faster the inlet one. Norit activated carbon presented lower Hg° adsorptive capacity, as its mesoporous structure facilitates the faster filling of the limited number of micropores and, thus, the quick saturation of the sample [14,23].
3.3. One-at-a-time mercury adsorption tests with HCl, O2, SO2 and CO2 3.3.1. Impact of HCl Fig. 4 compares mercury uptake by F400 in pure nitrogen and in HCl/N2 mixture at 150 °C adsorption temperature. This test indicates that the mercury uptake capacity of F400 was strongly increased by the presence of HCl, since mercury outlet concentration approached only the 20% of the inlet one after 2500 min of adsorption experiment. This implies that the 80% of mercury entering the column is retained by activated carbon and the equilibrium time is expected to be very high. This significant impact of HCl on elemental mercury adsorption seems to accord with a heterogeneous mercury oxidation mechanism proposed in other studies, since homogeneous gas phase interactions between mercury and HCl are excluded at low temperatures such as 150 °C [10,24]. Particularly, the concept of zigzag carbene edge structures on the aromatic rings of activated carbon is employed by
Table 1 Pore structure characteristics and Hg° adsorbed amount of activated carbons. Sample
BET surface area (m2/g)
CO2 area (m2/g)
Total pore volume Vtot (cm3/g)
Micropore volume VμpN2 (cm3/g)
Hgo adsorbed amount (ng/mg)a
Time at Cout =0.8 ×Cin (min)
F400 Norit
827 620
840 740
0.52 0.38
0.40 0.22
551 287
423 246
a
Cout = 0.8 × Cin, 50 °C.
Fig. 2. Medek pore size distribution curves.
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Fig. 5. Oxidation mechanism of elemental mercury by HCl presence.
Fig. 3. Hg° adsorption on F400 and Norit activated carbons at 50 °C.
some researchers [25]. This model provides a detailed mechanism that explains the catalytic role of acids such as HCl in mercury oxidation step, Fig. 5. The current model involves the conversion of carbene to carbenium ion by HCl attack on a Lewis basic site (free pair of electrons). Thus, the carbenium ion reacts as a Lewis acid site on the aromatic ring, which can directly accept electrons from Hg° and cause its oxidation, resulting in the formation of organomercury intermediates.
[21,28]. The surface chemistry characterization of activated carbon after its contact with O2, could be tested in a future work, in order to validate the formation of lactones and carbonyls. In comparison to the role of HCl on elemental mercury adsorption, Fig. 4, oxygen amplifying impact on mercury retention is less intense, Fig. 6. This observation reveals that the chemisorption sites generated by the presence of HCl on carbon surface are more important than carbon– oxygen functional groups formed on activated carbon in the presence of oxygen.
3.3.2. Impact of O2 The resulting mercury breakthrough curves under O2 are presented if Fig. 6. As can be seen from this figure, the mercury breakthrough curve became slower under oxygen presence and the mercury adsorbed amount increased by almost 10 times compared to the one calculated under inert gas conditions, Table 2. The enhanced mercury adsorption on activated carbon in the presence of oxygen can be attributed to the formation of carbon–oxygen complexes during the column test on carbon surface and to the heterogeneous reaction between elemental mercury and oxygen catalyzed on carbon surface [9,11]. Additionally, the possibility of carbonyls and lactones formation, which represent possible chemisorption sites for elemental mercury adsorption, is high [26,27]. The positive impact of the above oxygen groups on elemental mercury adsorption has been testified in two other published studies originated by our research activities
3.3.3. Impact of SO2 Fig. 7 compares mercury breakthrough curves by F400 in pure nitrogen and SO2/N2 mixture. This test indicate that the mercury breakthrough curves were shifted at higher adsorption times under the presence of SO2. The mercury adsorbed amount that have been calculated at 80% breakthrough increased by 2 and 5 times at 200 and 500 ppmv SO2 concentration respectively, Table 2. This result suggests that SO2 interacts with activated carbon via physisorption or chemisorption. Thus, a possible explanation for enhanced mercury retention on carbon surface could be the deposition of sulphur groups on activated carbon surface. These sulphur groups constitutes chemisorption sites for elemental mercury, since sulphur atoms can accept the two electrons presented in the outer layer of elemental mercury and generates its oxidized form (Hg2+) [29,30]. Additionally, the possibility of elemental mercury and sulphur dioxide competition for similar active sites on carbon surface is minimized, since SO2 usually is adsorbed on activated carbons with a large amount of basic groups.
Fig. 4. Effect of HCl (50 ppm) on mercury adsorption on F400 at 150 °C.
Fig. 6. Effect of O2 (10% v/v) on mercury adsorption on F400 at 150 °C.
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Table 2 Hg° adsorbed amount under baseline conditions and flue gas constitutes presence. Gas mixture
Hgo adsorbed amount (ng/mg F400)a
N2 + Hgo N2 + Hgo + O2 10% v/v N2 + Hgo + 200 ppmv SO2 N2 + Hgo + 500 ppmv SO2 N2 + Hgo + 12% v/v CO2
94 1170 181 513 13
Adsorption temperature: 150 °C. a Cout = 0.8 × Cin, 150 °C.
More specifically, the presence of pyronic or pyronic-like structures greatly increases the adsorption of SO2 on activated carbon, while elemental mercury prefers lactones and carbonyls [31,32]. 3.3.4. Impact of CO2 The breakthrough curves presented in Fig. 8 show that CO2 affects the performance of F400 activated carbon and interferes with elemental mercury measurements. Mercury breakthrough curve reached faster the equilibrium establishment in the presence of CO2 and the adsorbed amount was reduced about 85% compared to the one determined at inert gas conditions, Table 2. The above result indicated that mercury adsorption was prevented by CO2, due to the fact that CO2 fills a part of activated carbon microporous structure. Since physisorption is limited at high adsorption temperatures such as 150 °C, the occupation of similar chemisorption sites between elemental mercury and CO2 is a possible explanation for the observed competitive adsorption. These active sites are represented mainly by lactones and carbonyls and they are placed at the edges of activated carbon graphitic layers [33,34]. Thus, the utilization of an activated carbon for both elemental mercury and carbon dioxide retention seems to be difficult. 4. Conclusions The results obtained in this research showed that the extended surface area and well-developed micropore structure combined with a small volume of mesopores in activated carbons, enhanced mercury physisorption. These properties characterize the commercial activated carbon F400. On the contrary, mesoporous materials with low specific surface areas, such as the commercial activated carbon Norit, presented lower mercury removal efficiency.
Fig. 8. Effect of CO2 (12% v/v) on mercury adsorption on F400 at 150 °C.
However, at competitive adsorption conditions, HCl, O2 and SO2 seem to contribute to the chemisorption of elemental mercury on activated carbon. Among them, HCl, generated the most important increase in mercury adsorbed amount. A possible explanation, which confirms the concept employed in the literature, is the heterogeneous oxidation of elemental mercury on the acidic sites created on carbon surface by HCl presence. Oxygen presence in the reactant gas caused a smaller improvement of mercury adsorption capacity in activated carbon, in comparison to HCl effect. The chemisorption of elemental mercury on the oxygen groups formed on carbon surface could be responsible for the enhanced performance of activated carbon in the presence of oxygen. A proposed future work in order to ensure the above mechanism is the surface chemistry characterization and an Xray absorption fine structure analysis (XAFS) of activated carbon after its treatment with oxygen. Through these techniques, the formation of oxygen groups and their interaction with elemental mercury will be investigated. In the same way, SO2 presented in nitrogen and mercury gas mixture had a large benefit on elemental mercury capture, probably due to the formation of sulphur groups on carbon surface, which acts as mercury chemisorption sites. This result clarifies the confused theory in literature as much as concerns the SO2 impact on mercury adsorption. On the contrary, the adsorptive capacity of activated carbon was reduced in the presence of CO2, because of the competitive adsorption of the two gases on similar adsorption sites. With the above results, the role of SO2 and CO2 on elemental mercury adsorption was elucidated. References
Fig. 7. Effect of SO2 (200–500 ppmv) on mercury adsorption on F400 at 150 °C.
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Glossary Cin: Hg° initial concentration (ng/cm3) Cout: Hg° outlet concentration (ng/cm3) F: gas flow rate (cm3/min) m: activated carbon mass (mg) q: Hg° adsorptive capacity (ng/mg) t: adsorption time (min) VμpN2: micropore volume from nitrogen adsorption results Vtot: total pore volume of adsorbent (cm3/g)