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Characterization of a regenerable sorbent for high temperature elemental mercury capture from flue gas
Fuel xxx (2011) xxx–xxx
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Characterization of a regenerable sorbent for high temperature elemental mercury capture from flue gas Fabrizio Scala ⇑, Concetta Anacleria, Stefano Cimino Istituto di Ricerche sulla Combustione – CNR, Piazzale Tecchio 80, 80125 Napoli, Italy
a r t i c l e
i n f o
Article history: Received 30 September 2010 Received in revised form 22 December 2010 Accepted 24 December 2010 Available online 8 January 2011 Keywords: Mercury capture Flue gas Regenerable sorbent Manganese oxides Adsorption
a b s t r a c t In this work we have developed a regenerable synthetic sorbent based on manganese oxides (12% w/w) impregnated on high surface area c-alumina and supported as a thin layer (40 lm) onto cordierite honeycomb monoliths (400 cpsi). Such structured sorbents are well suited for flow-through exhaust gas treatment associated with very low pressure drop. Elemental mercury capture experiments were carried out in a lab-scale quartz reactor in air at temperatures ranging from 50 to 350 °C, Hg concentration in the range 50–250 lg/m3, GHSV = 3.6 105 h1. A kinetic and capacity characterization of the sorbent was conducted, giving insight on the controlling mechanisms of the mercury capture process. Structured MnOx sorbent performed satisfactorily up to 300 °C, the performance decaying dramatically above this temperature when desorption of elemental Hg became predominant. TPD experiments after mercury uptake on the sorbent, showed that the sorbent could be completely regenerated at a temperature as low as 500 °C. Repeated cycles of mercury adsorption/desorption did not lead to any significant reduction of the sorbent capacity towards mercury uptake. No significant mercury oxidation in the gas phase was observed under the operating conditions used in the experiments. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Public concern has recently risen over the potential risk of toxic elements emitted from anthropogenic sources. Among these, mercury has drawn special attention, because of its increasing level of bioaccumulation in the environment and in the food-chain, with potential risks for human health [1]. Recent studies recognized that about 70–85% of the total anthropogenic Hg emissions are caused by combustion/gasification sources, mainly coal-fired utilities and waste incinerators [2]. Measurements in coal-fired combustors and gasifiers showed that a large fraction of emitted mercury is found in the elemental form, which is less effectively captured by pollution control devices [3]. As a consequence, there is a need to develop specific cost-effective technologies for mercury capture in the light of possible forthcoming Hg emission limits for coal-fired plants. Activated carbon injection in flue gas upstream of a particulate matter control device has been indicated as the most mature technology for mercury capture [4]. However, the use of activated carbons has several drawbacks: once-through operation must be carried out since activated carbon cannot be regenerated; low temperatures (<150 °C) and high carbon-to-mercury ratios must be used to have acceptable removals. These limits lead to a costly operation under combustion conditions and to an unacceptable ⇑ Corresponding author. Tel.: +39 081 7682969; fax: +39 081 5936936. E-mail address: [email protected] (F. Scala).
reduction of thermal efficiency under gasification conditions [5]. Noble metals and transition metal oxides have been proposed as possible alternatives to activated carbon sorbents, which can overcome the above limitations [6–8]. In particular, metal oxides appear to be promising candidates due to their much lower cost with respect to noble metals. Remarkable features of these sorbents are the possibility to be regenerated in order to achieve a cost-effective cyclic operation, and to capture mercury at higher temperatures (200–400 °C). This last point is very attractive for gasification processes where it would be more advantageous to remove mercury from the syngas before its combustion in a gas turbine [5,8]. Among the transition metal oxides, manganese oxides have shown a good mercury capture potential [6,9,10]. Mn, due to its multiple possible oxidation states, is also well known to display a high activity for the partial or total oxidation of various hydrocarbons [6]. Mn can be expected to oxidize Hg0 in a similar way, and this feature may be beneficial since oxidized mercury species can be captured more effectively than elemental mercury. In order to increase the exposed surface area of the sorbent to enhance gas solid contact, the main active phase has to be deposited on a high surface area support, which in turn has to guarantee optimal mechanical properties and thermal stability to the catalytic system. In fact, exposure to relatively high temperatures during a regeneration cycle, strongly suggests the use of a thermally stable support onto which the active phase can be anchored in order to
0016-2361/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2010.12.028
Please cite this article in press as: Scala F et al. Characterization of a regenerable sorbent for high temperature elemental mercury capture from flue gas. Fuel (2011), doi:10.1016/j.fuel.2010.12.028
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avoid sintering phenomena. In this work we have synthesized a sorbent based on manganese oxides impregnated on high surface area c-alumina and supported as a thin layer onto commercial cordierite honeycomb monoliths. Honeycomb supported sorbents are well suited for flow-through exhaust gas treatment associated with a very low pressure drop. Elemental Hg capture experiments were carried out in a lab-scale quartz reactor in air at temperatures ranging from 50 to 350 °C to characterize kinetics and capacity of the sorbent, and to shed light on the controlling mechanisms of the mercury capture process. 2. Experimental 2.1. Sorbent preparation Sorbents based on MnOx supported on high surface area c-Al2O3 were prepared in the form of honeycomb monoliths. Commercial cordierite honeycomb monoliths (Corning) with a cell density of 400 cpsi (square channel l = 1.09 mm, wall thickness 127 lm) were selected due to their high specific exposed surface (2700 m2/m3) and extremely low pressure drops (5 mbar/m for air flowing at 1 m/s and 20 °C). Honeycomb samples were washcoated using a modified dip-coating procedure with a c-Al2O3 submicronic powder of roughly 200 m2/g [11] and finally calcined in air at 550 °C. MnOx precursor was deposited on the porous alumina washcoat (nominal thickness 40 lm) through impregnation of monoliths with an aqueous solution of (CH3CO2)2Mn4H2O. Impregnated samples were dried in microwave (MW) oven at 120 °C and finally calcined at 550 °C for 3 h under flowing air. The impregnation process was repeated two times in order to achieve the target loading (12% w/w MnOx with respect to the active material, monolithic substrate excluded); total sorbent loading on the honeycomb was 184 kg/m3. 2.2. Analytic characterizations Elemental analysis was performed on fresh sorbents by inductively coupled plasma spectrometry on an Agilent 7500 ICP-MS instrument, after microwave-assisted digestion of samples in nitric/hydrochloric acid solution. Hg content in used or regenerated sorbents was checked by flame atomic absorption spectrometry on a Varian SpectrAA 220 after dissolution in aqua regia. SEM analysis was performed using a FEI Inspect S microscope equipped with an EDAX detector for EDS microanalysis. BET specific surface area of sorbent samples, evaluated by N2 adsorption at 77 K using a Quantachromm Autosorb 1-C after degassing under vacuum at 150 °C, was assigned only to the active washcoat layer (SSA of cordierite honeycomb substrate 61 m2/g). TPR experiments were carried out with Micromeritics TPD/TPR 2900 apparatus equipped with a TCD: the sample was pretreated at 400 °C under air flow before the experiment, and then reduced with a 2% H2/Ar mixture (25 cm3 min1) by heating at 10 °C min1 from room temperature up to 800 °C.
was preliminarily tested for possible Hg uptake, and was found not to capture any significant amount of Hg. Inlet Hg concentrations variable in the range 50–250 lg/m3 were obtained passing a small flow of N2 (61 Nl/h) over a mercury saturator placed in a thermostatic bath and then mixing with the main stream before entering the reactor at atmospheric pressure. The gaseous flow rates were regulated by two Bronkhorst mass flow controllers and the total flow was set to 61 Nl/h corresponding to a GHSV 3.6 105 h1 based on monolith empty volume, which gives a contact time of only 10 ms at standard conditions. Outlet elemental mercury concentration was measured continuously with a CVAA elemental mercury analyzer (Mercury Instruments VM3000). A reactor by-pass line was also present to measure the baseline inlet Hg concentration when necessary. Selected experiments were carried out by inserting in the gas line before the analyzer a bubbler containing a 2% water solution of SnCl2 for the complete reduction of any possible oxidized mercury species to elemental mercury. Operation of the system with and without the bubbler gave information on the possible Hg oxidation activity of the sorbent. It should be remarked that honeycomb supported sorbents are very well suited for a kinetic study of mercury capture onto MnOx/ c-Al2O3 due to the controlled and homogeneous fluid-dynamic conditions in each channel, the reduced internal mass transfer resistance in the thin active layer, the very short contact times achievable with relatively small flow rates and pressure drops across the monolith. Mercury TPD experiments were carried out in the same quartz apparatus after a series of mercury capture experiments. Desorption was carried out under air (or N2) flow at 60 Nl/h, ramping up to 900 °C at 10 °C min1. No difference in the results was found when using N2 instead of air as carrier gas. In case of sorbent regeneration, the temperature was limited to a maximum of 500 °C in order to preserve initial properties of the material by avoiding solid state reactions which might lead to the undesired formation of MnAlO4. All of the plumbing and valves which come into contact with mercury were made of either Pyrex glass or Teflon. These materials have been demonstrated to have good chemical resistance and inertness toward mercury [6].
3. Results and discussion 3.1. Sorbent characterization SEM inspection of sectioned honeycomb samples (Fig. 1, left) revealed the presence of a thin (average thickness 20–50 lm)
2.3. Experimental set-up and test procedures Elemental mercury capture experiments were carried out isothermally in a lab-scale vertical down-flow quartz reactor, at temperatures ranging from 50 to 350 °C and in air flow. The 400 cpsi monolith (with 21 open channels on the section, L = 8 mm) was wrapped with fiberglass wool and positioned inside the quartz reactor (ID = 10 mm) which was externally heated by an electrical tubular furnace. In a separate set of experiments at different nominal oven temperatures the central channel of the monolith was blocked to flow and used for the measurement of the actual wall temperature using a K-type thermocouple. The fiberglass wool
Fig. 1. SEM micrograph showing the longitudinal section of one channel of the cordierite honeycomb coated with the MnOx/c-Al2O3 sorbent (left) together with the corresponding EDX elemental mappings of Si and Mn (right).
Please cite this article in press as: Scala F et al. Characterization of a regenerable sorbent for high temperature elemental mercury capture from flue gas. Fuel (2011), doi:10.1016/j.fuel.2010.12.028
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3.2. Initial kinetics analysis
inlet Hg concentrations (in the range 50–250 lg/m3). A typical outlet Hg trace measured by the analyzer during a test is reported in Fig. 3. Tests repeated with a SnCl2 solution bubbler did not show any difference, indicating that no gaseous oxidized mercury species were released from the sorbent during the tests. Analysis of the results showed that at each temperature level the mercury capture rate (ri) was directly proportional to the mercury concentration (CHg). As a consequence the initial reactivity analysis was carried out by assuming a first order kinetic expression:
ri ¼ ki C Hg
ð1Þ
It was assumed that the monolith channels could be modeled as ideal plug flow reactors. Under stationary conditions and using Eq. (1), the kinetic constant (ki) is given by: OUT C IN Q Hg C Hg ki ¼ ln 1 OUT waf C Hg
Inlet concentration level
140 130 120 110 100 90 80 70 Switch to reactor
60
Mn3O4 MnO2
Mn2O3
0 0
100 200 300 400 500 600 700
Temperature, °C Fig. 2. H2-TPR of the MnOx/c-Al2O3 sorbent in comparison with the corresponding reference bulk Mn oxides.
Switch to by-pass
50 50
100
150
200
250
300
Time, s Fig. 3. Typical plot of measured outlet Hg concentration during a short-term kinetic test of elemental mercury capture in air: T = 80 °C, GHSV = 3.6 105 h1.
Initial kinetic rate constant (ki), m3/kg s
H2 consuption [mol/(gMnOx K)] 10
5
MnOx/γAl2O3
ð2Þ
150
0
The first experimental campaign of Hg capture on the structured sorbent was directed to study the initial reactivity. Short-term Hg capture experiments (2–10 min) were carried out at different nominal temperatures in the range 50–350 °C and four
!
where Q is the gas flow rate at reactor conditions, and waf is the active phase (MnOx/c-Al2O3) total mass in the monolith. Fig. 4 presents the resulting Arrhenius plot where all the data collected at different temperatures are reported (the measured real monolith temperatures were used in the calculations, rather than the nominal oven ones). Each data point consists of the average of measurements at the four different inlet Hg concentrations. The Arrhenius
Measured Hg0 concentration, µg/m3
and homogeneous washcoat layer of active phase well anchored to the squared cordierite channels. Some accumulation of material was responsible for the typical roundening of the channel section, and also caused the formation of visible cracks in the washcoat close to the corners during cutting of the samples. Deposition of Mn by the impregnation route performed on c-Al2O3 coated monoliths ensured a uniform distribution of the transition metal in the thin porous overlayer and along the channel length, whereas Mn did not penetrate in the underlying cordierite walls. This is clearly demonstrated by false colour EDS maps (Fig. 1, right) collected on the longitudinal section of a typical channel for Mn and for Si. This latter element can be used as a tracer to locate areas with exposed cordierite surface since it is only contained in its structure. Fig. 2 presents the H2 TPR pattern of the fresh supported MnOx/c-Al2O3 sorbent in comparison with those relevant to bulk Mn3O4, Mn2O3, MnO2. c-Al2O3 and MnO are not reduced up to 800 °C under the conditions explored. The reduction temperature of the manganese oxide reference phases was, as expected, in the order: MnO2 (two steps) < Mn2O3 < Mn3O4. The supported MnOx/ c-Al2O3 sorbent showed a broader profile with a maximum around 400 °C and two shoulders at 240 °C and 545 °C, which, by comparison with reference materials, suggest the presence of both MnO2 and Mn2O3 highly dispersed on the surface of the alumina support. The estimated mean oxidation state of Mn in the fresh sorbent was 3.2, corresponding to 20% of Mn4+ and 80% Mn3+. H2-TPR was also repeated on a sorbent sample after being used for Hg adsorption at 100 °C and regenerated in air at 500 °C: the characteristic reduction profile was almost superimposed with that of fresh MnOx/c-Al2O3 and the overall mean oxidation state of Mn was unchanged, therefore excluding any significant modification of the sorbent during operation or any interaction of the active phase with the alumina support during thermal regeneration (i.e. irreversible migration of Mn and formation of aluminate spinel phase). This was also confirmed by the unchanged value of the BET specific surface area (165 m2/g, referred to the mass of MnOx/c-Al2O3 active layer only, and does not include the cordierite honeycomb support mass) measured for both the fresh and regenerated sorbent.
10
1
0.1
0.01
0.001 1.5
Experimental data Intrinsic kinetics regression line
2.0
2.5
3.0
3.5
1000/T, K-1 Fig. 4. Arrhenius plot of the average kinetic constant values at different temperatures.
Please cite this article in press as: Scala F et al. Characterization of a regenerable sorbent for high temperature elemental mercury capture from flue gas. Fuel (2011), doi:10.1016/j.fuel.2010.12.028
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F. Scala et al. / Fuel xxx (2011) xxx–xxx
Eai ki ¼ ki0 exp RT
Long-term kinetic rate constant (klt), m3/kg s
plot clearly shows that at low temperatures (<100 °C) the capture rate is controlled by intrinsic kinetics, while at high temperatures (>200 °C) the capture rate is controlled by boundary layer mass transfer and is almost independent of temperature. At four nominal temperature levels (60, 70, 80 and 90 °C) the experiments were repeated for three times so that each point in the figure represents the average of twelve measurements. These data have been used to find the intrinsic kinetics parameters following the typical Arrhenius analysis (represented by the straight line shown in the figure):
ð3Þ
The analysis gave the following results (with a correlation R = 0.999): ki0 = 2.2 1011 m3/kg s; Eai /R = 1 104 K. The activation energy value obtained shows that the Hg–MnOx bond is relatively strong, indicating that chemisorption (or chemical reaction) is the most likely binding mechanism. 2
3.3. Long-term kinetics and sorbent capacity analysis
Hg concentration, µg/m3
The second Hg capture experimental campaign was devoted to study the long-term reactivity behavior and the total capacity of the sorbent. Long Hg capture experiments were carried out at different nominal temperatures (in the range 80–350 °C), and at the highest inlet Hg concentration (250 lg/m3). Since the experiments could not be conducted overnight, each test was stopped at late afternoon and started again the morning after (but leaving the reactor without flow at the test temperature during the night). This procedure was repeated for the number of periods necessary to reach the total scheduled reaction time (40 to 50 h). Also during some long-term experiments a SnCl2 solution bubbler was inserted in the outlet gas line and the results did not show any significant difference. After each series of experiments, a Hg desorption test was carried out (see next section) to regenerate the sorbent. Integration of the measured Hg profiles during the desorption tests gave the total mercury captured by the sorbent during the test. For selected tests, independent measurements of the final sorbent Hg loading were obtained by atomic absorption analysis after dissolution of the sorbent in aqua regia, and the results compared excellently with the values obtained from TPD tests. Fig. 5 shows the typical Hg profile measured in a long-term test. It is interesting to note that the initial Hg capture rate decays in a relatively short time (a few hours) to a lower asymptotic rate. This indicates that a second limiting mechanism is active at long sorption times. However, after the test was stopped overnight, the initial capture rate in the next period was higher that the asymptotic rate reached at the end of the previous one. This behavior was found after each period of the test. The same qualitative results were observed at all temperature levels.
1
0.1
0.01
Experimental data Regression line
0.001 1.5
2.0
2.5
3.0
3.5
1000/T, K-1 Fig. 6. Arrhenius plot of the long-term kinetic constant values at different temperatures.
At the highest reaction temperatures (P200 °C), and at long reaction times a different phenomenon was also observed. Suddenly the outlet Hg concentration increased to a value above the inlet Hg concentration, indicating the mercury was desorbed from the monolith (Fig. 5). After some minutes the concentration gradually decreased below the inlet Hg concentration and adsorption started again. If the test was continued, a new desorption cycle was observed. This behavior was interpreted as an evidence that sorbent saturation conditions were approached. In order to understand the nature of the long-term capture controlling rate the following analysis was performed. The asymptotic Hg capture rate at the end of the first period of the experiment was analyzed with the aid of Eqs. (1) and (2) to extract the value of a long-term kinetic constant. The assumption underlying this procedure is that the Hg loading on the sorbent at the end of the first period is much lower than the maximum capacity at that temperature. This procedure was repeated for the four lower nominal temperatures tested (80, 100, 160 and 200 °C). In fact, at T > 200 °C sorbent saturation was so rapid that the above assumption did not hold anymore. Fig. 6 reports the Arrhenius plot for the data obtained at the four temperatures. These data have been used to find kinetic parameters of an expression similar to that reported in Eq. (3), following the Arrhenius analysis (again, represented by the straight line shown in the figure). In this case, the analysis gave the following results (with a correlation R2 = 0.991): klt0 = 1.4 104 m3/kg s; Ealt /R = 4.9 103 K. In this case the correlation was not as good as for the initial reactivity analysis, mainly due to the lower number of measurements for each data point and to the approximations of the method. Nevertheless, the analysis appears to be consistent, and the apparent activation energy value obtained indicates that also the long-term limiting mechanism
400
desorption 300
200
100
1st period
2nd period
3rd period 4th period
5th period
6th period
0 0
10
20
30
40
Progressive time, h Fig. 5. Typical plot of measured outlet Hg concentration during a long-term elemental mercury capture test in air: T = 200 °C, GHSV = 3.6 105 h1.
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F. Scala et al. / Fuel xxx (2011) xxx–xxx
carried out at temperatures up to 900 °C for comparison, in order to confirm that all of the captured mercury was completely desorbed at 500 °C. Independent measurements of Hg loading after desorption were also obtained by atomic absorption analysis, and no residual Hg was found in the sorbent regenerated at 500 °C. Repeated Hg capture-desorption cycles were carried out at different capture temperatures, and results showed that both the sorbent reactivity and its capacity did not vary appreciably with respect to the fresh sorbent after regeneration.
5000
Sorbent capacity, µgHg/gaf
Saturation approached
4000
3000
2000
1000
3.5. Conclusions
0 0
100
200
300
400
Monolith temperature, °C Fig. 7. Total MnOx/c-Al2O3 sorbent capacity as a function of sorption temperature (subscript ‘‘af’’ represents ‘‘active phase’’).
has a chemical rather than diffusive nature, and could be possibly attributed to the formation of a MnHgOx mixed phase as suggested by Qiao et al. [10] after XPS analysis of similar sorbents. Fig. 7 reports the total sorbent capacity measured at the different adsorption temperatures as described before. It is important to note that Hg desorption events (like those shown in Fig. 5) were observed only at the four highest nominal temperatures, indicating that sorbent saturation was likely approached at these temperatures. On the other hand, after 50 h of operation sorbent saturation was still not approached for T < 200 °C, because of the slower kinetics of Hg capture and the larger sorbent capacities. The sorbent capacity values obtained in this work are consistent with those reported in the literature [6]. On the whole, examination of Fig. 7 shows a strong decreasing mercury capacity for T > 180 °C, which becomes eventually negligible for T > 300 °C. This temperature represents the upper limit for practical operation with the MnOx/c-Al2O3 sorbent. 3.4. Mercury desorption analysis Fig. 8 reports the measured Hg plot of a typical desorption (TPD) experiment; the maximum temperature of the reactor was kept below 500 °C in order not to deactivate the sorbent. In line with the results on Hg capacity reported in the previous section, it appears that the MnOx sorbent started to significantly desorb elemental mercury roughly at 300 °C, and peaked at 420 °C, thus allowing complete regeneration. Some desorption tests were
Measured Hg0 concentration, µg/m3
5
The structured MnOx sorbent developed in this work showed a very promising Hg capture performance up to 300 °C. The sorbent could be completely regenerated at a temperature as low as 500 °C and repeated cycles of mercury adsorption/desorption did not lead to any significant reduction of the sorbent capacity/reactivity towards mercury uptake. The high temperature removal of mercury by the manganese oxide sorbent stands in significant contrast to the lower temperature capture by carbon-based sorbents such as activated carbons, carbon present in fly ash, and thief carbons [4–6,12,13]. The initial sorbent reactivity tests indicated a first order mechanism with respect to Hg and the formation of a relatively strong Hg-MnOx bond, probably connected to a chemisorption (or chemical reaction). A second long-term mechanism was also noticed, which is possibly attributed to the slower formation of a MnHgOx mixed phase. No significant activity for mercury oxidation in the gas phase was observed for the supported MnOx sorbent when operated in air up to 350 °C. Future tests will be directed to investigate the performance of the sorbent under a realistic flue gas environment. In fact, some gaseous species typically present in combustion flue gas are well known to significantly influence both the homogeneous and the heterogeneous mercury chemistry at the temperatures of interest. In a real flue gas containing SO2 and NO, there is the possibility of sorbent poisoning through formation of manganese sulfates or nitrates [6], and this will be examined in future work. It has been previously shown that many oxides, carbons, and noble metals show activity for the catalytic oxidation of elemental mercury in the presence of flue gas halogen species such as HCl, and this will also be investigated in future work [14–16]. Acknowledgements The support of S. Russo, V. Stanzione, F. Di Natale and A. Erto for sorbent characterization is gratefully acknowledged.
600
References
500 400 300 200 100 0 100 150 200 250 300 350 400 450 500 550 600
Sorbent temperature, °C Fig. 8. Typical plot of measured outlet Hg concentration during a temperature programmed desorption in air up to 500 °C (Hg capture temperature = 80 °C).
[1] EPA. Mercury study report to congress. Report EPA-452/R-97-003; 1997. [2] Pacyna JM, Muench J. Anthropogenic Hg emission in Europe. Water Air Soil Pollut 1991;56:51–61. [3] Galbreath KC, Zygarlicke CJ. Mercury speciation in coal combustion and gasification flue gases. Environ Sci Technol 1996;30:2421–6. [4] Scala F. Modeling mercury capture in coal-fired power plant flue gas. Ind Eng Chem Res 2004;43:2575–89. [5] Portzer JW, Albritton JR, Allen CC, Gupta RP. Development of novel sorbents for mercury control at elevated temperatures in coal-derived syngas: results of initial screening of candidate materials. Fuel Process Technol 2004;85:621–30. [6] Granite EJ, Pennline HW, Hargis RA. Novel sorbents for mercury removal from flue gas. Ind Eng Chem Res 2000;39:1020–9. [7] Granite EJ, Myers CR, King WP, Stanko DC, Pennline HW. Sorbents for mercury capture from flue gas with application to gasification systems. Ind Eng Chem Res 2006;45:4844–8. [8] Poulston S, Granite EJ, Pennline HW, Myers CR, Stanko DP, Hamilton H, et al. Metal sorbents for high temperature mercury capture from fuel gas. Fuel 2007;86:2201–3.
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[9] Cavallaro S, Bertuccio N, Antonucci P, Giordano N. Mercury removal from waste gases by manganese oxide acceptors. J Catal 1982;73:337–48. [10] Qiao S, Chen J, Li J, Qu Z, Liu P, Yan N, et al. Adsorption and catalytic oxidation of gaseous elemental mercury in flue gas over MnOx/alumina. Ind Eng Chem Res 2009;48:3317–22. [11] Cimino S, Pirone R, Russo G. Thermal stability of perovskite-based monolithic reactors in the catalytic combustion of methane. Ind Eng Chem Res 2001;40:80–5. [12] O’Dowd WJ, Pennline HW, Freeman MC, Granite EJ, Hargis RA, Lacher CJ, et al. A technique to control mercury from flue gas: the thief process. Fuel Process Technol 2006;87:1071–84.
[13] Granite EJ, Freeman MC, Hargis RA, O’Dowd WJ, Pennline HW. The thief process for mercury removal from flue gas. J Environ Manage 2007;84:628–34. [14] Presto AA, Granite EJ, Karash A, Hargis RA, O’Dowd WJ, Pennline HW. A kinetic approach to the catalytic oxidation of mercury in flue gas. Energy Fuels 2006;20:1941–5. [15] Presto AA, Granite EJ. Noble metal catalysts for mercury oxidation in utility flue gas: gold, palladium and platinum formulations. Platinum Metals Rev 2008;52:144–54. [16] Presto AA, Granite EJ. Survey of catalysts for oxidation of mercury in flue gas. Environ Sci Technol 2006;40:5601–9.
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Fuel journal homepage: www.elsevier.com/locate/fuel
Characterization of a regenerable sorbent for high temperature elemental mercury capture from flue gas Fabrizio Scala ⇑, Concetta Anacleria, Stefano Cimino Istituto di Ricerche sulla Combustione – CNR, Piazzale Tecchio 80, 80125 Napoli, Italy
a r t i c l e
i n f o
Article history: Received 30 September 2010 Received in revised form 22 December 2010 Accepted 24 December 2010 Available online 8 January 2011 Keywords: Mercury capture Flue gas Regenerable sorbent Manganese oxides Adsorption
a b s t r a c t In this work we have developed a regenerable synthetic sorbent based on manganese oxides (12% w/w) impregnated on high surface area c-alumina and supported as a thin layer (40 lm) onto cordierite honeycomb monoliths (400 cpsi). Such structured sorbents are well suited for flow-through exhaust gas treatment associated with very low pressure drop. Elemental mercury capture experiments were carried out in a lab-scale quartz reactor in air at temperatures ranging from 50 to 350 °C, Hg concentration in the range 50–250 lg/m3, GHSV = 3.6 105 h1. A kinetic and capacity characterization of the sorbent was conducted, giving insight on the controlling mechanisms of the mercury capture process. Structured MnOx sorbent performed satisfactorily up to 300 °C, the performance decaying dramatically above this temperature when desorption of elemental Hg became predominant. TPD experiments after mercury uptake on the sorbent, showed that the sorbent could be completely regenerated at a temperature as low as 500 °C. Repeated cycles of mercury adsorption/desorption did not lead to any significant reduction of the sorbent capacity towards mercury uptake. No significant mercury oxidation in the gas phase was observed under the operating conditions used in the experiments. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Public concern has recently risen over the potential risk of toxic elements emitted from anthropogenic sources. Among these, mercury has drawn special attention, because of its increasing level of bioaccumulation in the environment and in the food-chain, with potential risks for human health [1]. Recent studies recognized that about 70–85% of the total anthropogenic Hg emissions are caused by combustion/gasification sources, mainly coal-fired utilities and waste incinerators [2]. Measurements in coal-fired combustors and gasifiers showed that a large fraction of emitted mercury is found in the elemental form, which is less effectively captured by pollution control devices [3]. As a consequence, there is a need to develop specific cost-effective technologies for mercury capture in the light of possible forthcoming Hg emission limits for coal-fired plants. Activated carbon injection in flue gas upstream of a particulate matter control device has been indicated as the most mature technology for mercury capture [4]. However, the use of activated carbons has several drawbacks: once-through operation must be carried out since activated carbon cannot be regenerated; low temperatures (<150 °C) and high carbon-to-mercury ratios must be used to have acceptable removals. These limits lead to a costly operation under combustion conditions and to an unacceptable ⇑ Corresponding author. Tel.: +39 081 7682969; fax: +39 081 5936936. E-mail address: [email protected] (F. Scala).
reduction of thermal efficiency under gasification conditions [5]. Noble metals and transition metal oxides have been proposed as possible alternatives to activated carbon sorbents, which can overcome the above limitations [6–8]. In particular, metal oxides appear to be promising candidates due to their much lower cost with respect to noble metals. Remarkable features of these sorbents are the possibility to be regenerated in order to achieve a cost-effective cyclic operation, and to capture mercury at higher temperatures (200–400 °C). This last point is very attractive for gasification processes where it would be more advantageous to remove mercury from the syngas before its combustion in a gas turbine [5,8]. Among the transition metal oxides, manganese oxides have shown a good mercury capture potential [6,9,10]. Mn, due to its multiple possible oxidation states, is also well known to display a high activity for the partial or total oxidation of various hydrocarbons [6]. Mn can be expected to oxidize Hg0 in a similar way, and this feature may be beneficial since oxidized mercury species can be captured more effectively than elemental mercury. In order to increase the exposed surface area of the sorbent to enhance gas solid contact, the main active phase has to be deposited on a high surface area support, which in turn has to guarantee optimal mechanical properties and thermal stability to the catalytic system. In fact, exposure to relatively high temperatures during a regeneration cycle, strongly suggests the use of a thermally stable support onto which the active phase can be anchored in order to
0016-2361/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2010.12.028
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avoid sintering phenomena. In this work we have synthesized a sorbent based on manganese oxides impregnated on high surface area c-alumina and supported as a thin layer onto commercial cordierite honeycomb monoliths. Honeycomb supported sorbents are well suited for flow-through exhaust gas treatment associated with a very low pressure drop. Elemental Hg capture experiments were carried out in a lab-scale quartz reactor in air at temperatures ranging from 50 to 350 °C to characterize kinetics and capacity of the sorbent, and to shed light on the controlling mechanisms of the mercury capture process. 2. Experimental 2.1. Sorbent preparation Sorbents based on MnOx supported on high surface area c-Al2O3 were prepared in the form of honeycomb monoliths. Commercial cordierite honeycomb monoliths (Corning) with a cell density of 400 cpsi (square channel l = 1.09 mm, wall thickness 127 lm) were selected due to their high specific exposed surface (2700 m2/m3) and extremely low pressure drops (5 mbar/m for air flowing at 1 m/s and 20 °C). Honeycomb samples were washcoated using a modified dip-coating procedure with a c-Al2O3 submicronic powder of roughly 200 m2/g [11] and finally calcined in air at 550 °C. MnOx precursor was deposited on the porous alumina washcoat (nominal thickness 40 lm) through impregnation of monoliths with an aqueous solution of (CH3CO2)2Mn4H2O. Impregnated samples were dried in microwave (MW) oven at 120 °C and finally calcined at 550 °C for 3 h under flowing air. The impregnation process was repeated two times in order to achieve the target loading (12% w/w MnOx with respect to the active material, monolithic substrate excluded); total sorbent loading on the honeycomb was 184 kg/m3. 2.2. Analytic characterizations Elemental analysis was performed on fresh sorbents by inductively coupled plasma spectrometry on an Agilent 7500 ICP-MS instrument, after microwave-assisted digestion of samples in nitric/hydrochloric acid solution. Hg content in used or regenerated sorbents was checked by flame atomic absorption spectrometry on a Varian SpectrAA 220 after dissolution in aqua regia. SEM analysis was performed using a FEI Inspect S microscope equipped with an EDAX detector for EDS microanalysis. BET specific surface area of sorbent samples, evaluated by N2 adsorption at 77 K using a Quantachromm Autosorb 1-C after degassing under vacuum at 150 °C, was assigned only to the active washcoat layer (SSA of cordierite honeycomb substrate 61 m2/g). TPR experiments were carried out with Micromeritics TPD/TPR 2900 apparatus equipped with a TCD: the sample was pretreated at 400 °C under air flow before the experiment, and then reduced with a 2% H2/Ar mixture (25 cm3 min1) by heating at 10 °C min1 from room temperature up to 800 °C.
was preliminarily tested for possible Hg uptake, and was found not to capture any significant amount of Hg. Inlet Hg concentrations variable in the range 50–250 lg/m3 were obtained passing a small flow of N2 (61 Nl/h) over a mercury saturator placed in a thermostatic bath and then mixing with the main stream before entering the reactor at atmospheric pressure. The gaseous flow rates were regulated by two Bronkhorst mass flow controllers and the total flow was set to 61 Nl/h corresponding to a GHSV 3.6 105 h1 based on monolith empty volume, which gives a contact time of only 10 ms at standard conditions. Outlet elemental mercury concentration was measured continuously with a CVAA elemental mercury analyzer (Mercury Instruments VM3000). A reactor by-pass line was also present to measure the baseline inlet Hg concentration when necessary. Selected experiments were carried out by inserting in the gas line before the analyzer a bubbler containing a 2% water solution of SnCl2 for the complete reduction of any possible oxidized mercury species to elemental mercury. Operation of the system with and without the bubbler gave information on the possible Hg oxidation activity of the sorbent. It should be remarked that honeycomb supported sorbents are very well suited for a kinetic study of mercury capture onto MnOx/ c-Al2O3 due to the controlled and homogeneous fluid-dynamic conditions in each channel, the reduced internal mass transfer resistance in the thin active layer, the very short contact times achievable with relatively small flow rates and pressure drops across the monolith. Mercury TPD experiments were carried out in the same quartz apparatus after a series of mercury capture experiments. Desorption was carried out under air (or N2) flow at 60 Nl/h, ramping up to 900 °C at 10 °C min1. No difference in the results was found when using N2 instead of air as carrier gas. In case of sorbent regeneration, the temperature was limited to a maximum of 500 °C in order to preserve initial properties of the material by avoiding solid state reactions which might lead to the undesired formation of MnAlO4. All of the plumbing and valves which come into contact with mercury were made of either Pyrex glass or Teflon. These materials have been demonstrated to have good chemical resistance and inertness toward mercury [6].
3. Results and discussion 3.1. Sorbent characterization SEM inspection of sectioned honeycomb samples (Fig. 1, left) revealed the presence of a thin (average thickness 20–50 lm)
2.3. Experimental set-up and test procedures Elemental mercury capture experiments were carried out isothermally in a lab-scale vertical down-flow quartz reactor, at temperatures ranging from 50 to 350 °C and in air flow. The 400 cpsi monolith (with 21 open channels on the section, L = 8 mm) was wrapped with fiberglass wool and positioned inside the quartz reactor (ID = 10 mm) which was externally heated by an electrical tubular furnace. In a separate set of experiments at different nominal oven temperatures the central channel of the monolith was blocked to flow and used for the measurement of the actual wall temperature using a K-type thermocouple. The fiberglass wool
Fig. 1. SEM micrograph showing the longitudinal section of one channel of the cordierite honeycomb coated with the MnOx/c-Al2O3 sorbent (left) together with the corresponding EDX elemental mappings of Si and Mn (right).
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3.2. Initial kinetics analysis
inlet Hg concentrations (in the range 50–250 lg/m3). A typical outlet Hg trace measured by the analyzer during a test is reported in Fig. 3. Tests repeated with a SnCl2 solution bubbler did not show any difference, indicating that no gaseous oxidized mercury species were released from the sorbent during the tests. Analysis of the results showed that at each temperature level the mercury capture rate (ri) was directly proportional to the mercury concentration (CHg). As a consequence the initial reactivity analysis was carried out by assuming a first order kinetic expression:
ri ¼ ki C Hg
ð1Þ
It was assumed that the monolith channels could be modeled as ideal plug flow reactors. Under stationary conditions and using Eq. (1), the kinetic constant (ki) is given by: OUT C IN Q Hg C Hg ki ¼ ln 1 OUT waf C Hg
Inlet concentration level
140 130 120 110 100 90 80 70 Switch to reactor
60
Mn3O4 MnO2
Mn2O3
0 0
100 200 300 400 500 600 700
Temperature, °C Fig. 2. H2-TPR of the MnOx/c-Al2O3 sorbent in comparison with the corresponding reference bulk Mn oxides.
Switch to by-pass
50 50
100
150
200
250
300
Time, s Fig. 3. Typical plot of measured outlet Hg concentration during a short-term kinetic test of elemental mercury capture in air: T = 80 °C, GHSV = 3.6 105 h1.
Initial kinetic rate constant (ki), m3/kg s
H2 consuption [mol/(gMnOx K)] 10
5
MnOx/γAl2O3
ð2Þ
150
0
The first experimental campaign of Hg capture on the structured sorbent was directed to study the initial reactivity. Short-term Hg capture experiments (2–10 min) were carried out at different nominal temperatures in the range 50–350 °C and four
!
where Q is the gas flow rate at reactor conditions, and waf is the active phase (MnOx/c-Al2O3) total mass in the monolith. Fig. 4 presents the resulting Arrhenius plot where all the data collected at different temperatures are reported (the measured real monolith temperatures were used in the calculations, rather than the nominal oven ones). Each data point consists of the average of measurements at the four different inlet Hg concentrations. The Arrhenius
Measured Hg0 concentration, µg/m3
and homogeneous washcoat layer of active phase well anchored to the squared cordierite channels. Some accumulation of material was responsible for the typical roundening of the channel section, and also caused the formation of visible cracks in the washcoat close to the corners during cutting of the samples. Deposition of Mn by the impregnation route performed on c-Al2O3 coated monoliths ensured a uniform distribution of the transition metal in the thin porous overlayer and along the channel length, whereas Mn did not penetrate in the underlying cordierite walls. This is clearly demonstrated by false colour EDS maps (Fig. 1, right) collected on the longitudinal section of a typical channel for Mn and for Si. This latter element can be used as a tracer to locate areas with exposed cordierite surface since it is only contained in its structure. Fig. 2 presents the H2 TPR pattern of the fresh supported MnOx/c-Al2O3 sorbent in comparison with those relevant to bulk Mn3O4, Mn2O3, MnO2. c-Al2O3 and MnO are not reduced up to 800 °C under the conditions explored. The reduction temperature of the manganese oxide reference phases was, as expected, in the order: MnO2 (two steps) < Mn2O3 < Mn3O4. The supported MnOx/ c-Al2O3 sorbent showed a broader profile with a maximum around 400 °C and two shoulders at 240 °C and 545 °C, which, by comparison with reference materials, suggest the presence of both MnO2 and Mn2O3 highly dispersed on the surface of the alumina support. The estimated mean oxidation state of Mn in the fresh sorbent was 3.2, corresponding to 20% of Mn4+ and 80% Mn3+. H2-TPR was also repeated on a sorbent sample after being used for Hg adsorption at 100 °C and regenerated in air at 500 °C: the characteristic reduction profile was almost superimposed with that of fresh MnOx/c-Al2O3 and the overall mean oxidation state of Mn was unchanged, therefore excluding any significant modification of the sorbent during operation or any interaction of the active phase with the alumina support during thermal regeneration (i.e. irreversible migration of Mn and formation of aluminate spinel phase). This was also confirmed by the unchanged value of the BET specific surface area (165 m2/g, referred to the mass of MnOx/c-Al2O3 active layer only, and does not include the cordierite honeycomb support mass) measured for both the fresh and regenerated sorbent.
10
1
0.1
0.01
0.001 1.5
Experimental data Intrinsic kinetics regression line
2.0
2.5
3.0
3.5
1000/T, K-1 Fig. 4. Arrhenius plot of the average kinetic constant values at different temperatures.
Please cite this article in press as: Scala F et al. Characterization of a regenerable sorbent for high temperature elemental mercury capture from flue gas. Fuel (2011), doi:10.1016/j.fuel.2010.12.028
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Eai ki ¼ ki0 exp RT
Long-term kinetic rate constant (klt), m3/kg s
plot clearly shows that at low temperatures (<100 °C) the capture rate is controlled by intrinsic kinetics, while at high temperatures (>200 °C) the capture rate is controlled by boundary layer mass transfer and is almost independent of temperature. At four nominal temperature levels (60, 70, 80 and 90 °C) the experiments were repeated for three times so that each point in the figure represents the average of twelve measurements. These data have been used to find the intrinsic kinetics parameters following the typical Arrhenius analysis (represented by the straight line shown in the figure):
ð3Þ
The analysis gave the following results (with a correlation R = 0.999): ki0 = 2.2 1011 m3/kg s; Eai /R = 1 104 K. The activation energy value obtained shows that the Hg–MnOx bond is relatively strong, indicating that chemisorption (or chemical reaction) is the most likely binding mechanism. 2
3.3. Long-term kinetics and sorbent capacity analysis
Hg concentration, µg/m3
The second Hg capture experimental campaign was devoted to study the long-term reactivity behavior and the total capacity of the sorbent. Long Hg capture experiments were carried out at different nominal temperatures (in the range 80–350 °C), and at the highest inlet Hg concentration (250 lg/m3). Since the experiments could not be conducted overnight, each test was stopped at late afternoon and started again the morning after (but leaving the reactor without flow at the test temperature during the night). This procedure was repeated for the number of periods necessary to reach the total scheduled reaction time (40 to 50 h). Also during some long-term experiments a SnCl2 solution bubbler was inserted in the outlet gas line and the results did not show any significant difference. After each series of experiments, a Hg desorption test was carried out (see next section) to regenerate the sorbent. Integration of the measured Hg profiles during the desorption tests gave the total mercury captured by the sorbent during the test. For selected tests, independent measurements of the final sorbent Hg loading were obtained by atomic absorption analysis after dissolution of the sorbent in aqua regia, and the results compared excellently with the values obtained from TPD tests. Fig. 5 shows the typical Hg profile measured in a long-term test. It is interesting to note that the initial Hg capture rate decays in a relatively short time (a few hours) to a lower asymptotic rate. This indicates that a second limiting mechanism is active at long sorption times. However, after the test was stopped overnight, the initial capture rate in the next period was higher that the asymptotic rate reached at the end of the previous one. This behavior was found after each period of the test. The same qualitative results were observed at all temperature levels.
1
0.1
0.01
Experimental data Regression line
0.001 1.5
2.0
2.5
3.0
3.5
1000/T, K-1 Fig. 6. Arrhenius plot of the long-term kinetic constant values at different temperatures.
At the highest reaction temperatures (P200 °C), and at long reaction times a different phenomenon was also observed. Suddenly the outlet Hg concentration increased to a value above the inlet Hg concentration, indicating the mercury was desorbed from the monolith (Fig. 5). After some minutes the concentration gradually decreased below the inlet Hg concentration and adsorption started again. If the test was continued, a new desorption cycle was observed. This behavior was interpreted as an evidence that sorbent saturation conditions were approached. In order to understand the nature of the long-term capture controlling rate the following analysis was performed. The asymptotic Hg capture rate at the end of the first period of the experiment was analyzed with the aid of Eqs. (1) and (2) to extract the value of a long-term kinetic constant. The assumption underlying this procedure is that the Hg loading on the sorbent at the end of the first period is much lower than the maximum capacity at that temperature. This procedure was repeated for the four lower nominal temperatures tested (80, 100, 160 and 200 °C). In fact, at T > 200 °C sorbent saturation was so rapid that the above assumption did not hold anymore. Fig. 6 reports the Arrhenius plot for the data obtained at the four temperatures. These data have been used to find kinetic parameters of an expression similar to that reported in Eq. (3), following the Arrhenius analysis (again, represented by the straight line shown in the figure). In this case, the analysis gave the following results (with a correlation R2 = 0.991): klt0 = 1.4 104 m3/kg s; Ealt /R = 4.9 103 K. In this case the correlation was not as good as for the initial reactivity analysis, mainly due to the lower number of measurements for each data point and to the approximations of the method. Nevertheless, the analysis appears to be consistent, and the apparent activation energy value obtained indicates that also the long-term limiting mechanism
400
desorption 300
200
100
1st period
2nd period
3rd period 4th period
5th period
6th period
0 0
10
20
30
40
Progressive time, h Fig. 5. Typical plot of measured outlet Hg concentration during a long-term elemental mercury capture test in air: T = 200 °C, GHSV = 3.6 105 h1.
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F. Scala et al. / Fuel xxx (2011) xxx–xxx
carried out at temperatures up to 900 °C for comparison, in order to confirm that all of the captured mercury was completely desorbed at 500 °C. Independent measurements of Hg loading after desorption were also obtained by atomic absorption analysis, and no residual Hg was found in the sorbent regenerated at 500 °C. Repeated Hg capture-desorption cycles were carried out at different capture temperatures, and results showed that both the sorbent reactivity and its capacity did not vary appreciably with respect to the fresh sorbent after regeneration.
5000
Sorbent capacity, µgHg/gaf
Saturation approached
4000
3000
2000
1000
3.5. Conclusions
0 0
100
200
300
400
Monolith temperature, °C Fig. 7. Total MnOx/c-Al2O3 sorbent capacity as a function of sorption temperature (subscript ‘‘af’’ represents ‘‘active phase’’).
has a chemical rather than diffusive nature, and could be possibly attributed to the formation of a MnHgOx mixed phase as suggested by Qiao et al. [10] after XPS analysis of similar sorbents. Fig. 7 reports the total sorbent capacity measured at the different adsorption temperatures as described before. It is important to note that Hg desorption events (like those shown in Fig. 5) were observed only at the four highest nominal temperatures, indicating that sorbent saturation was likely approached at these temperatures. On the other hand, after 50 h of operation sorbent saturation was still not approached for T < 200 °C, because of the slower kinetics of Hg capture and the larger sorbent capacities. The sorbent capacity values obtained in this work are consistent with those reported in the literature [6]. On the whole, examination of Fig. 7 shows a strong decreasing mercury capacity for T > 180 °C, which becomes eventually negligible for T > 300 °C. This temperature represents the upper limit for practical operation with the MnOx/c-Al2O3 sorbent. 3.4. Mercury desorption analysis Fig. 8 reports the measured Hg plot of a typical desorption (TPD) experiment; the maximum temperature of the reactor was kept below 500 °C in order not to deactivate the sorbent. In line with the results on Hg capacity reported in the previous section, it appears that the MnOx sorbent started to significantly desorb elemental mercury roughly at 300 °C, and peaked at 420 °C, thus allowing complete regeneration. Some desorption tests were
Measured Hg0 concentration, µg/m3
5
The structured MnOx sorbent developed in this work showed a very promising Hg capture performance up to 300 °C. The sorbent could be completely regenerated at a temperature as low as 500 °C and repeated cycles of mercury adsorption/desorption did not lead to any significant reduction of the sorbent capacity/reactivity towards mercury uptake. The high temperature removal of mercury by the manganese oxide sorbent stands in significant contrast to the lower temperature capture by carbon-based sorbents such as activated carbons, carbon present in fly ash, and thief carbons [4–6,12,13]. The initial sorbent reactivity tests indicated a first order mechanism with respect to Hg and the formation of a relatively strong Hg-MnOx bond, probably connected to a chemisorption (or chemical reaction). A second long-term mechanism was also noticed, which is possibly attributed to the slower formation of a MnHgOx mixed phase. No significant activity for mercury oxidation in the gas phase was observed for the supported MnOx sorbent when operated in air up to 350 °C. Future tests will be directed to investigate the performance of the sorbent under a realistic flue gas environment. In fact, some gaseous species typically present in combustion flue gas are well known to significantly influence both the homogeneous and the heterogeneous mercury chemistry at the temperatures of interest. In a real flue gas containing SO2 and NO, there is the possibility of sorbent poisoning through formation of manganese sulfates or nitrates [6], and this will be examined in future work. It has been previously shown that many oxides, carbons, and noble metals show activity for the catalytic oxidation of elemental mercury in the presence of flue gas halogen species such as HCl, and this will also be investigated in future work [14–16]. Acknowledgements The support of S. Russo, V. Stanzione, F. Di Natale and A. Erto for sorbent characterization is gratefully acknowledged.
600
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500 400 300 200 100 0 100 150 200 250 300 350 400 450 500 550 600
Sorbent temperature, °C Fig. 8. Typical plot of measured outlet Hg concentration during a temperature programmed desorption in air up to 500 °C (Hg capture temperature = 80 °C).
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Please cite this article in press as: Scala F et al. Characterization of a regenerable sorbent for high temperature elemental mercury capture from flue gas. Fuel (2011), doi:10.1016/j.fuel.2010.12.028