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Energy & Fuels 2006, 20, 1946-1950
Mercury Emissions and Species during Combustion of Coal and Waste Hong Yao,* Guangqian Luo, and Minghou Xu State Key Laboratory of Coal Combustion, Huazhong UniVersity of Science and Technology, Wuhan 430074, China
Tamotsu Kameshima and Ichiro Naruse Department of Ecological Engineering, Toyohashi UniVersity of Technology, Tempaku-cho, Toyohashi 441-8580, Japan ReceiVed March 6, 2006. ReVised Manuscript ReceiVed July 4, 2006
The behaviors of mercury evolution for three types of coal and three types of dried sewage sludge are studied using a thermogravimetric (TG) analyzer. The mercury speciations in the flue gas from coal and sludge combustion are also analyzed by implementing a horizontal electrically heated tube furnace. Furthermore, the kinetic calculations of mercury oxidizing processes are carried out using the software package CHEMKIN in order to interpret the homogeneous mechanism of mercury oxidization. The results obtained show that the sulfur content in the sludge inhibits the evolution of mercury at low temperature if the Cl concentration is high enough. Chlorine enhances mercury evolution in the coal combustion, whereas there is no relationship when the Cl concentration is high. Fixed carbon content plays a role in depression of the mercury evolution. Formation of oxidized mercury (HgCl2) does not relate to the chlorine concentration in the raw coal and sludge. Whereas the ash and sulfur content in the sludge affects the Hg oxidization, kinetic calculations show that HgCl, Cl2, and HOCl formation is important in producing the oxidized mercury during combustion of coal and sludge at 873 K. A suitable temperature for Hg oxidization when Cl2 is the oxidization resource is 700-1200 K.
Introduction Recently, mercury has been recognized once again as one of global environmental pollutants worldwide because of its health risks in the middle and/or long term. The chemistry of mercury during combustion is of particular importance for the development of control technologies. The two main forms of mercury, which are oxidized and elemental mercuries, behave differently. The oxidized mercury is soluble in aqueous solutions and therefore is easily captured in pollution-control equipment such as a flue gas desulfurization (FGD) system. However, elemental mercury passes through these systems, which are released from the coal and waste combustion.1-6 On the basis of these phenomena, researchers have a variety of Hg control technologies under intense development. Some researchers focus on understanding the mechanisms of the mercury oxidization by injection of elemental mercury into solid or gas fuel flame or simulated flue gas.7-14 Other researchers reported gas-phase mercury speciation using chemical kinetics15-19 or the fate of mercury in the flue gas during combustion * Corresponding author. Phone: 86-27-87542417. Fax: 86-27-87545526. E-mail:
[email protected]. (1) Sloss, L. L. IEA Report; IEA: Paris, 2002. (2) Sloss, L. L. IEA Report; IEA: Paris, 2004. (3) Wang, Q.; Shen, W.; Ma, Z. EnViron. Sci. Technol. 2000, 34 (13), 2711-2713. (4) Center for Air Toxic Metals Newsletter; Energy & Envionmental Research Center, University of North Dakota: Grand Forks, ND, 2001; 7 (2), 1-10. (5) Yokoyama, T.; Asakura, K.; Matsuda, H.; Ito, S.; Noda, N. Sci. Total EnViron. 2000, 259, 97-103. (6) Pacyna, E. G.; Pacyna, J. M.; Pirrone, N. Atmos. EnViron. 2001, 35 (17), 2987-2996.
processes.20-23 The use of sorbents to capture Hg was carried out by many researchers.24-28 Soon, looking for a low-cost and (7) Lee, C. W.; Kilgroe, J. D.; Ghorishi, S. G. Proceedings of the 6th Annual Waste to Energy Conference, Miami Beach, FL, 1998; pp 221238. (8) Hall, B.; Schager, P.; Lindqvist, O. Water, Air, Soil Pollut. 1991, 56, 3-14. (9) Gaspar, J. A.; Widmer, N. C.; Cole, J. A.; Seeker, W. R. Proceedings of the International Conference on Incineration and Thermal Treatment Technologies, 1997; 661-665. (10) Norton, G. A.; Yang, H.; Brown, R. C.; Laudal, D. L.; Dunham, G. E.; Erjavec, J. Fuel 2003, 82, 107-116. (11) Laudal, D. L.; Brown, T. D.; Nott, B. R. Fuel Process. Technol. 2000, 65-66, 157-165. (12) Hall, B.; Lindqvist, O.; Ljungstrom, E. EnViron. Sci. Technol. 1990, 24 (1), 108-111. (13) Fujiwara, N.; Fujita, Y.; Tomura, K.; Moritomi, H.; Tuji, T.; Takasu, S.; Nikasa, S. Fuel 2002, 81, 2045-2052. (14) Mamani-Paco, R. M.; Helble, J. J. Proceedings of the Air & Waste Management Association Annual Conference, Salt Lake City, UT, June 2000. (15) Xu, M.; Qiao, Y.; Zheng, C.; Li, L.; Liu, J. Combust. Flame 2003, 132, 208-218. (16) Edwards, J. R.; Srivastava, R. K.; Kilgroe, J. D. J. Air Waste Manage. Assoc. 2001, 51, 869-877. (17) Helble, J. J.; Qiu, J.; Sterling, R. 4th Trace Element Workshop, Minamata, Japan, July 16-18, 2003; p 65. (18) Widmer, N. C.; West, J.; Cole, J. A. Proceedings of the Air & Waste Management Association Annual Conference, Salt Lake City, UT, June 2000. (19) Niksa, S.; Helble, J. J.; Fujiwara, N. EnViron. Sci. Technol. 2001, 35, 3701-3706. (20) Sjostrom, S.; Bustard, J.; Durham, M.; Chang, R. AWMA Mega Symposium: Specialty Conference on Mercury Emissions: Fate, Effects and Control, Chicago, Aug 20-23, 2001; p 16. (21) Rubin, E.; Berkenpas, M. B.; Farrell, A.; Gibbon, G. A.; Smith, D. N. AWMA Mega Symposium: Specialty Conference on Mercury Emissions: Fate, Effects and Control, Chicago, Aug 20-23, 2001; p 14.
10.1021/ef060100b CCC: $33.50 © 2006 American Chemical Society Published on Web 08/03/2006
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Table 1. Experimental Conditions for TG Analysis sample mass (mg) heating rate (K/min) holding temperature (K) holding time (min) atmosphere
coal, sludge 20 20 473, 573, 673, 773, 873 60 air
Table 2. Properties of Sludges and Coals Employed composition VM FC ash C H N O S (% fuel) Cl (mg/kg fuel) Hg (mg/kg fuel) S/Hg (mol/mol) Cl/Hg (mol/mol)
sludge A sludge B sludge C coal A
coal B
coal C
proximate analysis (wt %, dry) 67.4 61.0 43.4 27.5 10.3 8.9 1.8 55.2 22.4 30.1 54.8 17.3
47.9 39.7 12.4
35.1 55.3 9.6
ultimate analysis (wt %, d.a.f) 54.7 50.7 47.3 84.6 7.0 7.8 7.3 4.9 7.0 7.0 5.4 1.8 29.5 33.2 38.6 8.5 1.31 0.8 0.63 0.25 3258 1509 1947 170 1.145 1.150 2.421 0.052 72225 45724 16385 29909 16079 7415 4544 18473
77.8 6.2 1.1 14.8 0.09 312 0.032 171607 55094
82.8 5.3 1.8 9.9 0.27 235 0.148 114871 8972
highly effective sorbent has been recognized as one of the important tasks in controlling Hg emissions. Generally, mercury compounds easily vaporize and convert to elemental mercury at high temperature during combustion. When the flue gas is cooled, on the other hand, the elemental Hg tends to be oxidized under the appropriate conditions. The rate of the Hg oxidization depends on the temperature, flue gas composition, existence of fly ash and/or char particles, and so forth. However, the emission behaviors of mercury and the mechanisms of mercury oxidization have not been wellelucidated. In light of the need to elucidate, this study focuses on fundamental emission behaviors of Hg and Hg speciation in the flue gas during combustion of coal and sludge, whose properties are different. The kinetic simulation is also conducted, using the CHEMKIN, to understand the mechanisms of mercury oxidization. Experimental Procedures and Simulation Conditions Experimental Procedures. The experiments of Hg emission were carried out using a thermogravimetric (TG) analyzer. Three types of sludge and three types of coal were tested in the TG analyzer in order to understand the fundamental characteristics of mercury evolution. The experimental conditions are shown in Table 1. The properties of the sludges and coals employed are shown in Table 2. The mercury concentration in the fuel and residual sample were analyzed by a mercury analyzer (MA-2000, Japan Instruments Corp.). Next, a 1 m long and 40 mm inner diameter horizontal electrically heated tube furnace was used to burn the same sludge and coal as mentioned above. A schematic diagram of this furnace is shown in Figure 1. In this experiment, the temperature was kept at 873 K. (22) Gibb, W. H.; Clarke, F.; Mehta, A. K. Fuel Process. Technol. 2000, 65-66, 365-377. (23) Yao, H.; Naruse, I. 21st Annual International Pittsburgh Coal Conference, Osaka, Japan, Sept 23-27, 2004. (24) Serre, S. D.; Gullett, B. K.; Ghorishi, S. B. J. Air Waste Manage. Assoc. 2001, 51, 733-741. (25) Butz, J. R.; Turchi, C.; Broderick, T. E.; Albiston, J. 17th Annual International Pittsburgh Coal Conference, Pittsburgh, PA, 2000. (26) Hargis, R. A.; O′Dowd, W. J.; Pennline, H. W. 17th Annual International Pittsburgh Coal Conference, Pittsburgh, PA, 2000. (27) Dunham, G. E.; DeWall, R. A.; Senior, C. L. AWMA Mega Symposium: Specialty Conference on Mercury Emissions: Fate, Effects and Control, Chicago, Aug 20-23 2001; p 21. (28) Ho, T. C.; Ghai, A. R.; Guo, F.; Wang, K. S.; Hopper, J. R. Combust. Sci. Technol. 1998, 134, 263-289.
Figure 1. Schematic diagram of a horizontal electrically heated tube furnace.
Figure 2. Hg evolution from sludge and coal in air atmosphere. Table 3. Elementary Reactions of Mercury Oxidation and Their Kinetic Parameters reaction
A (cm3/mol sec)
n
E (cal/mol)
reference
1. Hg + Cl + M ) HgCl + M 2. Hg + Cl2 ) HgCl + Cl 3. Hg + HOCl ) HgCl + OH 4. Hg + HCl ) HgCl + H 5. HgCl + Cl2 ) HgCl2 + H 6. HgCl + HCl ) HgCl2 + H 7. HgCl + Cl + M ) HgCl2 + M 8. HgCl + HOCl ) HgCl2 + OH
9.49 × 1014 2.30 × 1012 1.43 × 1013 2.20 × 1008 1.48 × 1012 4.94 × 1014 1.16 × 1014 4.27 × 1013
0.5 0.0 0.0 0.0 0.0 0.0 0.5 0.0
0 1599 12790 1756 37250 21500 0 1000
Niksa (19) Helbel (17)] Helbel (17) Gaspar (9) Helbel (17) Widmer (18) Niksa (19) Widmer (18)
Table 4. Basic Calculation Conditions application V (cm3) T (K) P (atm) atmosphere (molar ratio) CO2 H2O Hg Cl2 N2
transient well-mixed reactor 1000 793 1 0.13 0.26 1.6 × 10-8 50, 100, 300 ppm balance
One gram of sludge or coal was burned for 1 h. The air was fed at a rate of 1 L/min. The Hg speciation in the flue gas was determined by the Ontrario Hydro method. The cooling section of the furnace and the pipe connection with the sampling train was heated at temperature of 423 K by a ribbon heater. The residence time in the tube was kept at 8-10 s. Simulation Procedures. The kinetic calculations were conducted using the software package CHEMKIN 4.0. The elements considered in this simulation were C, H, O, Cl, N, and Hg. The key elementary reactions relating to the Hg are shown in Table 3. On the basis of previous experimental work, which was carried out by injection of elemental mercury into CH4 flame,14 we summarized the basic calculation conditions in Table 4.23
Results and Discussions Mercury Evolution from Sludge and Coal. Figure 2 shows the mercury evolution for three types of sludge and three types of coal at different temperatures in air atmosphere obtained by
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Figure 3. Relationship between Hg evolution and fixed content in the sludge and coal.
Figure 4. Relationship between Hg evolution and the S/Hg molar ratio in the sludge.
Figure 5. Relationship between Hg evolution and the Cl/Hg molar ratio in the coal.
the TG tests. From Figure 2, it is noted that Hg evolution increases with an increase in temperature. When the temperature reaches 873 K, about 90% of the Hg evolves into the atmosphere. When the temperature is lower than 673 K, however, more than half the amount of Hg remains in the residue, except for sludge C and coal B. Temperature is a very important index for mercury evolution, although the properties of fuel and concentration of mercury are different in the coal and sludge. In Figure 2, the difference of evolution percent at 673 K for the sludge and coal may be due to the difference of fixed carbon, Cl, and/or S contents in the fuel. The fixed carbon inhibits Hg evolution, whereas Cl and S content play a role in the species of Hg existence in the raw fuel. Panel a and b of Figure 3 show the relationship between the percentage of Hg evolution and fixed carbon content in the sludge and coal, respectively. For both the sludge and coal, a decrease in the fixed carbon content causes an increase in Hg evolution, because carbon has the ability to adsorb/absorb Hg.24-26 Comparing the properties of coal and sludge shown in Table 2, although the fixed carbon content in the coal and sludge are much different, both of them are much higher than the Hg concentration, which is enough to inhibit Hg evolution. For coals A and C in Figure 3b, the evolution of Hg shows little difference, although the fixed carbon content is similar, which may be due to a higher concentration of Hg in the coal C. For the sludge, the S/Hg molar ratio also contributes to Hg evolution, because the concentration of chlorine in each sludge is sufficient for Hg evolution and Cl can react with other metals, especially alkali metals. Figure 4 shows the relationship between
Table 5. Hg Species in Flue Gas and the Mass Balance (%)
sludge A sludge B sludge C coal A coal B coal C
Hg2+ (%)
Hg (%)
mass balance (%)
0-4.5 11.3-27.6 60.7-61.6 18.6-22.4 15.5-19.0 6.1-7.3
106.3-110.4 47.9-54.0 39.2-41.9 56.7-60.5 55.9-73.8 77.6-89.2
110.4-110.9 59.2-81.9 99.9-103.5 86.6-86.9 79.6-105.1 85.8-95.3
Hg evolution and the S/Hg molar ratio in the sludge. Mercury compounds for the sludge with higher S/Hg molar ratios are hard to evolve at 673 K. This may be because HgSO4, which does not evolve at relatively low temperatures,23 exists as a solid species in the raw sludge. For the coal, the Cl/Hg molar ratio plays an important role for the Hg evolution because of the relatively low concentration of chlorine in the coal. As shown in Figure 5, the coal with a higher Cl/Hg molar ratio evolves a greater amount of Hg. This is because Hg can react with Cl easily to form HgCl2 or exists as HgCl2 species, which is easier to transition to the gas phase at low temperature.23 Mercury Speciation in Flue Gas. Because most of the Hg evolved at 873 K, as shown in Figure 2, the experiment of Hg speciation in flue gas during combustion of three types of coal and sludge was carried out at 873 K, with the results shown in Table 5. From the table, we can see there is no relationship between chlorine concentration in raw sludge and Hg2+ formation, although the mass balance of sludge B shows a little lower. For example, sludge C has the lowest chlorine concentration, but the highest fraction of Hg2+ is formed. Additionally, HgCl2 was not detected in the three raw sludges when the solubility experiments were conducted. This result suggests that the Hg2+
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Figure 6. HgCl2 formation with Cl2 concentration increase. Figure 8. Hg and HgCl2 formation at different temperature.
Figure 7. Comparison of the experiment in ref 14 and calculated values.
is formed during combustion at 873 K in the downstream. The high content of ash in sludge C, which provides sites for the oxidation of Hg even under conditions of low unburned carbon,10,11,29 plays a role in the oxidization of Hg. For sludge A, on the other hand, the sulfur, which inhibits the Hg oxidization by indirect reaction, is the highest of the three; hence, the formation of the Cl radical may be depressed.30-32 For the coal, there is a similar phenomenon observed from Table 5. The higher the ash content in coal A, the higher the oxidized fraction of Hg formation. The fraction of Hg formation has no relationship with S, Cl, and fixed carbon in the raw coal. Kinetics Simulations. From the experimental results obtained, the Hg oxidization process is complicated. To understand the fundamental mechanisms of mercury oxidization, a simple system of C-H-O-Cl- N-Hg is first considered, for which the main reactions of Hg oxidization are shown in Table 3, along with 38 other reactions for the system of Cl-O-H.15 The basic calculation conditions are summarized in Table 4 according to the experimental conditions from ref 14, which is a (29) Niksa, S.; Fujiwara, N. Combined Power Plant Air Pollutant Control Mega Symposium, Washington, DC, Aug 30-Sept 2, 2004. (30) Laudal, D. L.; Brown, T.; Nott, B. Conference on Air Quality: Mercury, Trace Elements, and Particulate Matter, McLean, VA, Dec 1-4, 1998. (31) Ghorishi, S. B.; Lee, C. W.; Kilgroe, J. D. Air and Waste Management Association 92nd Annual Meeting and Exhibition Proceedings, St. Louis, MO, June 20-24, 1999. (32) Lee, C. W.; Kilgroe, J. D.; Ghorishi, S. B. 6th Annual Waste to Energy Conference, Miami Beach, FL, May 11-13, 1998.
Figure 9. HgCl2 formation with HCl addition.
homogeneous reaction of Hg oxidization during CH4 flame by injection of elemental Hg and Cl2 and HCl gases. Figure 6 shows HgCl2 formation when the Cl2 concentration increases. From the figure, we note the 100% of HgCl2 is formed at 1 s when the Cl2 concentration reaches 300 ppm, whereas the reaction time increases to 5 s when the Cl2 concentration is 50 ppm. Figure 7 shows a comparison of the experimental results from ref 14 with the calculated results from this study. The calculated results are higher than the experimental results, although the trends are similar. This is attributed to the experiment being performed with a CH4 flame with nonisothermal experimental conditions, whereas the temperature was kept constant in the kinetic calculation. Figure 8 shows the change in the Hg and HgCl2 fraction with the temperature increase. There is a peak at 900 K, which implies that the oxidization reaction at this temperature is the fastest. When the temperature is higher or lower than 900 K, the oxidization reaction becomes slower. In temperatures higher than 1200 K or lower than 700 K, no homogeneous oxidization reaction occurs. Even if the residence time increases, the similar phenomenon occurs, although the extent of HgCl2 formation increases. When Cl2 is changed to HCl, however, only 20% of the fraction of HgCl2 is formed, even under a HCl concentration of 30 000 ppm, which is shown in Figure 9. In addition, the temperature range of oxidization is changed from 700 to 1200 K to 900-1500 K when Cl2 is replaced by HCl. That means Cl2 is very important for Hg oxidization besides temperature. Compared with the experimental results during combustion of
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Figure 10. Sensitivity analysis for the HgCl2 formation.
coal and sludge in Table 5, Cl2 formation in the flue gas is very important, rather than Cl concentration in the raw fuels. Figure 10a-c shows the sensitivity analysis of HgCl2 formation at 793, 900, and 1100 K, respectively. At lower temperatures, sensitivity analysis indicates that HgCl2 is mainly produced by reaction 8, HgCl + HOCl ) HgCl2 + OH, located in Table 3. With the temperature increase to 900 K, reaction 2, Hg + Cl2 ) HgCl + Cl, also becomes sensitive, whereas the inverse reaction of reaction 1, Hg + Cl + M ) HgCl + M, inhibits HgCl2 formation because HgCl decomposes at this temperature. At a temperature of 1100 K, reaction 2 becomes more sensitive than reaction 8 for HgCl2 formation, whereas the inverse reaction of reaction 1 and inverse reaction of reaction 6 of HgCl + HCl ) HgCl2 + H become sensitive for inhibiting HgCl2 produced. These simulation results suggest that the decomposition of HgCl and HgCl2 at high temperature plays a key role in Hg formation. Compared with the experimental results during combustion of coal and sludge in Table 5, the results of sensitivity analysis show that HOCl and Cl2 formation and HgCl produced at 873 K are very important for HgCl2 formation. Conclusions The following conclusions were obtained by this study: (1) Ninety percent of mercury in the sludge and coal evolves at a temperature of 873 K.
(2) Fuel with a higher fixed carbon content shows a lower evolution percent of mercury at lower temperature. (3) Cl enhances mercury evolution in the coal, whereas it has no relationship when the Cl concentration is much higher, for example, in the sludge. The sulfur content in sludge with a high Cl concentration inhibits the mercury evolution at a lower temperature because of the existence of HgSO4 in the raw sludge. (4) The mercury oxidation process depends on ash and sulfur content rather than Cl concentration in the raw fuel. (5) Cl2 can oxidize Hg at temperatures of 700-1200 K, whereas HCl has little effect on the Hg oxidization at much higher concentrations. (6) HgCl, Cl2, and HOCl formation is important in producing the oxidized mercury during the combustion of coal and sludge at 873 K. A suitable temperature for Hg oxidization when Cl2 is the oxidization resource is 700-1200 K. Acknowledgment. This work has been partly supported, as the 21st Century COE Program of “Ecological Engineering for Homeostatic Human Activities”, by the ministry of Education, Culture, Sports, Science and Technology, Japan, and the National Natural Science Foundation of China (Grant 50576026). EF060100B
