Ind. Eng. Chem. Res. 2007, 46, 927-931
927
Mercury Retention by Fly Ashes from Coal Combustion: Influence of the Unburned Carbon Content M. Antonia Lo´ pez-Anto´ n, Mercedes Dı´az-Somoano, and M. Rosa Martı´nez-Tarazona* Instituto Nacional del Carbo´ n (CSIC), C/Francisco Pintado Fe No 26, 33011, OViedo, Spain
The objective of this study was to evaluate the effect of unburned carbon particles present in fly ashes produced by coal combustion on mercury retention. To achieve this objective, the work was divided into two parts. The aim of the first part of the study was to estimate the amount of mercury captured by the fly ashes during combustion in power stations and the relationship of this retention to the unburned carbon content. The second part was a laboratory-scale study aimed at evaluating the retention of mercury concentrations greater than those produced in power stations by fly ashes of different characteristics and by unburned carbon particles. From the results obtained it can be inferred that the unburned carbon content is not the only variable that controls mercury capture in fly ashes. The textural characteristics of these unburned particles and of other components of fly ashes also influence retention. 1. Introduction Airborne mercury is a problem of major concern among the toxic metals addressed in the 1990 Clean Air Act Amendments (CAAA) because of its volatility, persistence, and bioaccumulation as methylmercury in the environment and its neurological health impact. This element is often found as a trace contaminant in coal. When coal is combusted, the combination of elevated temperatures and the volatility of mercury and its compounds enables the mercury to enter the combustion gas exhaust stream. Coal-fired power plants are cited as one of the largest sources of mercury emissions to the environment. In March 2005, the EPA (U.S. Environmental Protection Agency) issued the Clean Air Mercury Rule to permanently cap and reduce mercury emissions from coal-fired power plants. This rule makes the United States the first country in the world to regulate mercury emissions from utilities.1 Although less attention has been paid to this problem in Europe, mercury emission from coal combustion is becoming a matter of growing interest. On April 4, 2001, the European Council approved a protocol on heavy metals in order to reduce the emissions of metals that are prone to long-range transboundary atmospheric transport and are likely to have adverse effects on human health and the environment. For the Development of the EU Mercury Strategy the European Commission published in 2004 a consultation document inviting comments from stakeholders and other related persons in the field. A major statement of this document was the identification of large-scale coal combustion units as the largest emitters of mercury compounds into the air. In January 2005, the Commission adopted a mercury strategy that envisages a number of measures to protect the health of citizens and their environment.2 Extensive efforts have been made to evaluate the quantity of mercury emitted from various sources over the past few years, to understand its behavior, and to reduce mercury pollution.3-6 It has been found that elemental mercury evades capture in power plant emission control systems and remains predominantly in gaseous form even at stack temperatures. Although various control technologies have been investigated, until now no cost-effective or efficient control process has been developed for mercury removal. Recent research has shown that certain * To whom correspondence should be addressed. Tel.: +34 985119090. Fax: +34 985297662. E-mail:
[email protected].
fly ash materials have an affinity for mercury. It has been observed that, although some fly ash inorganic components exhibit low mercury retention capacities, the unburned material present in fly ash does show considerable retention capacity.7-9 In recent years the carbon content in fly ashes has increased due to modifications carried out in coal boilers to minimize NOx emissions. These modifications have produced an increment in the unburned fraction which is in some cases higher than 10% of the total weight.10 Carbonaceous particles present in fly ashes are capable of retaining mercury species in different proportions depending on their characteristics and the retention conditions.11-15 However, the exact nature of Hg-fly ash interactions is still unknown and mercury capture by carbon particles in fly ash needs to be investigated more thoroughly. In the present work the capture of mercury in fly ashes was studied in two different situations. The first part of the study was aimed at estimating mercury capture in industrial power stations that burn different types of coal. In the second part, a laboratory-scale study was carried out on the retention of high concentrations of Hg0 and HgCl2 in fly ashes of different characteristics and in fly ash fractions where unburned carbon particles are concentrated. 2. Experimental Section Four fly ash samples from power stations that burn coals of different rank were used in this study. CTA fly ash was obtained from a pulverized coal power plant in which high rank coals are burned. CTSR and CTL were taken from pulverized coal power plants which burn bituminous coals, and CTES was obtained from a power station in which coal blends including subbituminous coal are used. The raw fly ashes were fractionated by wet screening, and the mercury content was determined in each fraction. The fraction sizes were obtained from the different fly ashes as a function of their unburned carbon particle distribution. The fractions separated in CTSR were >80, 8063, 63-45, 45-20, and >20 µm; those in CTL and CTES were >500, 500-400, 400-300, 300-200, 200-100, and >100 µm, and finally in CTA the fractions were >150, 150-125, 125100, 100-80, 80-63, 63-45, 45-20, and >20 µm. The unburned carbon particle content in each fraction was estimated as loss of ignition (LOI). The mercury content of each fraction was determined in an Automatic Mercury Analyser (AMA-254). In the second part of the study, two of the above-described fly ash samples (CTA and CTSR) were used as sorbents for
10.1021/ie060772p CCC: $37.00 © 2007 American Chemical Society Published on Web 12/30/2006
928
Ind. Eng. Chem. Res., Vol. 46, No. 3, 2007 Table 1. Unburned Carbon Content (LOI), BET Surface Area, and Hg Content in Fly Ash Samples sample
LOI (%)
surf. area (m2 g-1)
Hg (µg g-1)
CTAorig CTA>150 CTA>150agl CTSRorig CTSR>80 CTESorig CTLorig
5.7 22.4 73.0 7.2 54.2 5.6 2.0
1.6 4.2 5.8 9.4 17.6 1.9 4.1
0.39 0.40 1.80 4.95 0.04 0.42
3. Results and Discussion
Figure 1. Schematic diagram of the experimental device.
mercury retention. Fractions of these samples, enriched in unburned carbon particles, were separated by size fractionation (CTA > 150 µm and CTSR > 80 µm) and by oil agglomeration (CTA>150agl). Oil agglomeration experiments were conducted in a commercial seven-speed Waring blender, using domestic waste oils as agglomerants and following a method previously developed.16 Loss of ignition was determined by the combustion of the organic matter in air at 815 °C. Sulfur content was determined by means of LECO automatic analyzers, and the elemental composition was determined by atomic absorption spectrometry (AAS). Free CaO was analyzed by the Standard method UNE-EN 451-1,17 and the BET surface area was determined by volumetric adsorption of nitrogen at 77 K. The experimental device used for the retention experiments at the laboratory scale consisted of a glass reactor fitted with an internal tube and external tube and heated by two different furnaces (Figure 1). Hg0(g) and HgCl2(g) in the gas atmosphere were obtained by the evaporation of solid Hg0 and HgCl2, respectively. The temperature of evaporation was calibrated to obtain concentrations of these species in the gas phase between 0.1 and 0.7 µg mL-1. The mercury concentrations in the gas phase for Hg0 were 0.4 µg mL-1 in the combustion and gasification atmospheres and 0.1 µg mL-1 in the inert atmosphere. The mercury concentrations in the gas phase for HgCl2 were 0.4 µg mL-1 in the combustion and inert atmospheres and 0.7 µg mL-1 in the gasification atmosphere. The evaporation temperature was 190 °C for Hg0 in the three atmospheres and for HgCl2 in the combustion and N2 atmospheres. For the evaporation of HgCl2 in the gasification atmosphere, the temperature was 300 °C. The sorbent and the element source were placed inside the same internal tube but heated separately in the two furnaces. Synthetic gas mixtures, typical of coal combustion and gasification processes, were passed through the reactor. The gasification gas mixture contained 64% CO, 3.7% CO2, 20.9% H2, 4.0% H2O, and 1.0% H2S and was balanced with N2. The combustion atmosphere contained 15% CO2, 9.2% O2, 0.2% SO2, and 6.6% H2O and was also balanced with N2. These mixtures carried the element compound in the vapor phase through the sorbent bed at a flow rate of 0.5 L min-1. The temperature of the sorbent was 120 °C. The element that could not be retained in the sorbent bed was captured in impingers containing 4% KMnO4 + 10% H2SO4 and 0.5 N HNO3. The amount of mercury retained was determined by analyzing the fly ashes post retention by means of cold vapor atomic absorption (CV-AA) after mercury extraction using 60% (v/v) HNO3.
The mercury content of the raw fly ash samples (CTAorig, CTSRorig, CTLorig, and CTESorig) varies from sample to sample and ranges from 0.04 µg g-1 (CTES) to 1.8 µg g-1 (CTSR) (Table 1). Loss of ignition (LOI), surface area, mercury content, and the composition of these fly ashes are presented in Tables 1 and 2 together with a characterization of the fractions to be discussed in the second part of the work. This composition has been calculated using two different bases: (i) the percentage of oxides in the samples (FA) and (ii) the percentage of oxides in the standard ashes of these samples obtained at 850 °C (HTA). By comparing the compositions of the HTA it is possible to balance the differences in the relative proportions of each component, while a comparison of the FA compositions makes it possible to contrast the absolute concentrations in the samples. In the case of the raw fly ashes (orig), the sample with the highest mercury content of 1.80 µg g-1 is also the sample with the highest surface area (9.4 m2 g-1) and the highest unburned content (7.2%) (Table 1). The other samples, however, do not seem to show any particular relationship between Hg and the surface area. However, when all the samples were compared, no relationship between Hg and the surface area or between Hg and the unburned content was observed (Figure 2). Likewise, no correlations between the mercury content and the concentrations of the components of the ashes were found (Table 2). These observations were confirmed when the mercury content was determined in the fly ash fractions with different unburned carbon percentages. The mercury contents in the fractions separated from the ashes were plotted versus the LOI value, which is considered to be equivalent to the unburned carbon content (Figures 3 and 4). It can be observed that the mercury concentration varies considerably in the different fractions of each sample. In CTSR mercury ranges from 1.62 to 4.95 µg g-1, and it is significantly higher in the fraction of the highest unburned carbon content (Figure 3a). However, in CTL the range of mercury concentrations is much lower (0.16-0.72 µg g-1) and the highest mercury concentration is present in the fraction that has an intermediate unburned content (Figure 3b). A similar tendency is observed for the fly ash sample obtained from the power station in which high rank coals are burned (CTA). In this sample (CTA), the mercury content increases slightly with LOI up to 10.2%, where it reaches a maximum value of 0.84 µg g-1 and decreases thereafter, as LOI increases (Figure 4a). Finally, in the size fractions of the CTES fly ash, which were taken from a power station that burns subbituminous coals, there is a very low mercury content that does not vary when the unburned carbon increases (Figure 4b). These observations regarding the distribution of mercury in the fly ash size fractions indicate that the influence of unburned carbon particles on mercury retention does not depend only on the unburned content. In order to confirm these observations, some of the fly ashes and some of their fractions enriched in unburned particles were used as a fixed bed, through which high concentrations of mercury species obtained from the evaporation
Ind. Eng. Chem. Res., Vol. 46, No. 3, 2007 929
Figure 2. Mercury content versus unburned content (LOI) and versus surface area (BET).
Figure 3. Unburned content (LOI) versus mercury content in CTSR and CTL.
Figure 4. Unburned content (LOI) versus mercury content in CTA and CTES. Table 2. Elemental Compositions of Fly Ash Samples CTAorig
CTA>150
CTA>150agl
% (db)
FA
HTA
FA
HTA
FA
SiO2 Al2O3 Fe2O3 MgO Na2O K2O TiO2 SO3 CaOtotal CaOfree
53.3 25.6 5.87 1.82 0.72 3.37 150agl). The LOI value of CTA>150 was 22.4%; this concentration was as high as 73% when CTA>150 was agglomerated with a 5% vegetable oil emulsion (Table 1).18 Although the differences in surface area are not significant compared to those of porous materials (Table 1), according to the results of this and other works,19 they are significant enough to have an influence on mercury capture. A comparison of the noncarbonaceous composition of the original fly ashes CTA and CTSR used in the experimental work (Table 2) indicates that the two fly ashes may be considered similar since the concentrations of all the components are on
930
Ind. Eng. Chem. Res., Vol. 46, No. 3, 2007
Figure 5. Examples of retention of different mercury species by the CTA fractions of CTA in a combustion atmosphere.
Figure 6. Maximum retention capacity as a function of unburned content (a-c) and as a function of BET (d-f) for different mercury species: b, CTA; 2, CTSR. Table 3. Mercury Capture in Fly Ash Samples from Hg Hg
combustion
sorbent
LOI
MRC (mg
CTAorig CTA>150 CTA>150agl CTSRorig CTSR>80
5.7 22.4 73.0 7.2 54.2
12.0 13.0 13.3 25.0 27.2
gasification
g-1)
%E 11 ( 2 15 ( 2 14 ( 3 8.1 ( 3 10 ( 2
g-1)
MRC (mg
inert %E 2.6 ( 1 3.5 ( 2 3.0 ( 1 12 ( 3
0.35 0.30 0.20 3.9
MRC (mg
g-1)
%E 2.7 ( 1 2.6 ( 1 2.6 ( 1 16 ( 2 15 ( 1
0.30 0.32 0.21 3.7 3.82
Table 4. Mercury Capture in Fly Ash Samples from HgCl2 HgCl2
combustion
sorbent
LOI
MRC (mg
CTAorig CTA>150 CTA>150agl CTSRorig CTSR>80
5.7 22.4 73.0 7.2 54.2
2.53 2.19 2.30 12.3 16.6
g-1)
gasification %E 14 ( 1 17 ( 3 16 ( 2 21 ( 1 16 ( 8
the same order. However, the mineral phases identified by X-ray diffraction in these two fly ashes were different. The only crystalline species identified in CTA was quartz, whereas in CTSR aluminosilicates (mullite) were also detected. These results indicate only that aluminosilicates are present in higher proportions in CTSR than in CTA. Since interactions between CaO particles and HgCl2(g) are possible,20,21 the CaO free content should be considered an important parameter for a satisfactory ash composition evaluation (Table 2). A difference may be observed in the case of CaO free content, which is also 2 times greater in CTSR than in CTA. The significant differences in the content of the various components of fly ashes
MRC (mg 2.24 2.15 2.55 4.15
g-1)
inert %E 14 ( 2 13 ( 5 16 ( 1 13 ( 2
MRC (mg 2.51 2.48 2.61 3.63 4.28
g-1)
%E 16 ( 4 14 ( 2 16 ( 3 14 ( 1 11 ( 2
and fractions need to be taken into account when evaluating the results of the mercury capture experiments. In the adsorption experiments the efficiency of the retention was calculated as the percentage of element captured (%E), which is an indication of the kinetics of the process, and as the maximum retention capacity (MRC), which was determined as the maximum amount of element retained in milligrams of mercury per gram of fly ash. To determine these parameters, a sequence of experiments was conducted, the results of which are plotted in Figure 5. This figure shows examples of the retention of different mercury species in experiments carried out in a typical combustion atmosphere for the CTA sample
Ind. Eng. Chem. Res., Vol. 46, No. 3, 2007 931
and its fractions enriched in unburned carbon matter. The efficiency value was the average of a series of experiments using different concentrations of element in the gas phase, whereas MRC was the saturation value. The confidence limit of the value of the efficiency was calculated as the standard deviation of the results obtained. The results of the MRC and %E obtained from the retention experiments for different mercury species at 120 °C in the fly ash samples are presented in Tables 3 and 4. The retention of all the possible mercury species in the different gas atmospheres is higher in CTSR than in CTA. In each group of samples prepared from the same fly ash, a small increase in retention is observed when the unburned carbon content rises in the combustion atmosphere (Tables 3 and 4). The fly ashes show different retention capacities and efficiencies in the combustion and gasification atmospheres. Retention capacities for Hg0 are similar to or lower than those for HgCl2, with the exception of those achieved in the combustion atmosphere. Retention capacities for the species formed from Hg0 evaporation in the combustion atmosphere attained values as high as 13 and 27 mg g-1, compared to 0.30 and 3.9 mg g-1 in the gasification and inert atmospheres. The different results for the combustion and gasification/inert atmospheres have already been discussed and explained as possibly due to the catalytic effects of the inorganic components of the fly ashes on mercury oxidation.18,22 This implies that the species retained in the fly ashes in a combustion atmosphere was not Hg0, but probably HgO formed from Hg0 oxidation. In view of these results our discussion will be based on the assumption that HgO is the species retained in the combustion atmospheres and Hg0 is retained in the gasification and inert atmospheres when Hg0 is the source of mercury in the gas phase. HgCl2(g) is assumed to be the species retained in all atmospheres when HgCl2 is the source of mercury. No relationship was found between mercury retention and the fly ash inorganic components presented in Table 2. Nor was any relation observed between the MRC and the LOI value. To clarify these data, Figure 6a-c shows the MRC against LOI for the three expected mercury species. The same trend can be observed in all cases, the difference being the concentration of the element retained: the highest amount corresponds to HgO, then HgCl2, and Hg0 shows the lowest retention values. However, when MRC is compared with the surface area of the fly ash sample, it can be observed that increasing the surface area enhances mercury capture (Figure 6d-f). This tendency is the same for the three species in the different gas atmospheres. In the samples and fractions with the highest surface area but not the greatest unburned content (CTSR and CTSR>80), the MRC for all the mercury species is the highest. However, when all the ashes and fractions are compared, the relation is not linear. This suggests that not only is the surface area or the carbon content determinant in the capture of mercury, but also the characteristics peculiar to each sample play a role. It is clear from the study of the ashes obtained at the industrial scale, where contact time and mercury concentrations are low, and also from experiments at the laboratory scale, where the contact time between the ashes and mercury was greater and the mercury concentration significantly higher, that an increase of carbon does not imply greater mercury retention. In other words, retention should be ascribed not only to unburned carbon content but also to the textural properties of these carbonaceous materials. Moreover, the influence of the inorganic constituents as reflected in the combustion conditions also must be considered.
Acknowledgment This work was carried out with financial support from the Spanish Ministerio de Ciencia y Tecnologı´a PN I+D+I (Project PPQ2001-2359-C02-02). Literature Cited (1) U.S. Environmental Protection Agency. Mercury; http:// www.epa.gov/mercury. (2) http://europa.eu.int/comm/environment/chemicals/mercury/ index.htm. (3) Senior, L. C. Behavior of Mercury in Air Pollution Control Devices on Coal-Fired Utility Boilers. Presented at the Conference of Power Production in the 21st Century: Impacts of Fuel Quality and Operations, October 28-November 2, 2001; 17 pp. (4) Ghorishi, S. B.; Sedman, C. B. Low concentration mercury sorption mechanism and control by calcium-based sorbents: application in coalfired processes. J. Air Waste Manage. 1998, 48, 1191-1198. (5) Ghorishi, S. B.; Keeney, R. M.; Serre, S. D.; Gullett, B. K.; Jozewicz, W. S. Development of a Cl-impregnated activated carbon for entrainedflow capture of elemental mercury. EnViron. Sci. Technol. 2002, 36, 44544459. (6) Sakulpitakphon, T.; Hower, J. C.; Trimble, A. S.; Schram, W. H.; Thomas, G. A. Mercury capture by fly ash: study of the combustion of a high-mercury coal at a utility boiler. Energy Fuels 2000, 14, 727-733. (7) Pavlish, J. H.; Sondreal, E. A.; Mann, M. D.; Olson, E. S.; Galbreath, K. C.; Laudal, D. L.; Benson, S. A. Status review of mercury control options for coal-fired power plants. Fuel Process. Technol. 2003, 82 (2-3), 89165. (8) Sloss, L. L. Mercury emissions and effects: The role of coal; IEAPER/19; IEA Coal Research: London, 1995; 39 pp. (9) Li, Z.; Sun, X.; Luo, J.; Hwang, J. Y.; Crittenden, J. C. Unburned carbons from fly ash for mercury adsorption II: Adsorption Isotherms and Mechanisms. J. Min. Mater. Charact. Eng. 2002, 1, 79-96. (10) Kalyoncu, R. S. Coal combustion productssProduction and Uses. In Proceedings of the 18th Annual International Pittsburgh Coal Conference, 2001; pp 1815-1830. (11) Karatza, D.; Lancia, A.; Musmarra, D. Fly ash capture of mercuric chloride vapors from exhaust combustion gas. EnViron. Sci. Technol. 1998, 32, 3999-4004. (12) Hassett, D. J.; Eylands, K. E. Mercury capture on coal combustion fly ash. Fuel 1999, 78, 243-248. (13) Serre, S. D.; Silcox, G. D. Adsorption of elemental mercury on the residual carbon in coal fly ash. Ind. Eng. Chem. Res. 2000, 39, 17231730. (14) Hower, J. C.; Maroto-Valer, M. M.; Taulbee, D. N.; Sakulpitakphon, T. Mercury capture by distinct fly ash carbon forms. Energy Fuels 2000, 14, 224-226. (15) Hower, J. C.; Finkelmen, R. B.; Rathborne, R. F.; Goodman, J. Intra- and inter-unit variation in fly ash petrography and mercury adsorption: Examples from a Western Kentucky Power Station. Energy Fuels 2000, 14, 212-216. (16) Alonso, M. I.; Valde´s, A. F.; Tarazona, R. M.; Garcı´a, A. B. Coal recovery from fines cleaning wastes by agglomeration with colza oil: a contribution to the environment and energy preservation. Fuel Process. Technol. 2002, 75, 85-95. (17) European Standard EN 451-1, 2002. Method of testing fly ash. Part 1: Determination of free calcium oxide content. (18) Lo´pez-Anto´n, M. A. Retencio´n de compuestos gaseosos de Hg, As y Se en sorbentes so´lidos: Aplicacio´n a la combustio´n y gasificacio´n de carbo´n. Thesis, Universidad de Oviedo, Spain, 2004. (19) Dunham, G. E.; DeWall, R. A.; Senior, C. L. Fixed-bed studies of the interactions between mercury and coal combustion fly ash. Fuel Process. Technol. 2003, 82, 197-213. (20) Gullett, B. K.; Ragnunathan, K. Reduction of coal-based metal emissions by furnace sorbent injection. Energy Fuels 1994, 8, 1068-1076. (21) Krishnan, S. V.; Gullett, B. K.; Jozewicz, W. Mercury control in Municipal waste combustors and coal-fired utilities. EnViron. Prog. 1997, 16, 47-53. (22) Norton, G. A.; Yang, H.; Brown, R. C.; Laudal, D. L.; Dunham, G. E.; Erjavec, J.; Okoh, J. M. Effects of fly ash on mercury oxidation during post combustion conditions; Final report DOE Award No. DE-FG2698FT40111; Sept 1, 1998-Nov 30, 2001.
ReceiVed for reView June 16, 2006 ReVised manuscript receiVed November 8, 2006 Accepted November 17, 2006 IE060772P
