Environ. Sci. Technol. 1990, 2 4 , 108-111
(9) Wai, C. M.; Mok, W. M. A Chemical Speciation Approach to Evaluate Water Quality Problems in the Blackbird Mining Area, Idaho. Research Technical Report; Idaho Water Resources Research Institut,e;Moscow, ID, 1986; pp 61. (10) Mok, W. M.; Wai, C. M. Water Res. 1989, 23, 7. (11) Crecelius, E. A. Limnol. Oceanogr. 1975, 20, 441. (12) Farmer, J. G.; Cross, J. D. Radiochem. Radioanal. Lett. 1979, 39, 429. (13) Neal, C.; Elderfield, H.; Chester, R. Mar. Chem. 1979, 7, 207. (14) Mok, W. M.; Riley, J. A.; Wai, C. M. Water Res. 1988,22,
769. (15) Aggett, J.; Lybley, S. Environ. Sci. Technol. 1986,20, 183. (16) Wilson, F. H.; Hawkins, D. B. Environ. Geol. 1978,2, 195. (17) Takamatsu, T.; Kawashima, M.; Koyama, M. Water Res. 1985, 19, 1029. (18) Jackson, M. L. Soil Chemical Analysis-Advanced Course,
2nd ed.; University of Wisconsin: Madison, WI, 1969. (19) Anderson, B. J.; Jenne, E. A. Soil Sci. 1970,109, 163. (20) Olson, R. V.; Roscoe, E. In Methods of Soil Analysis, 2nd ed.; Agronomy Monograph No. 9; Page, A. L., Ed.; University of Wisconsin: Madison, WI, 1982. (21) Brannon, J. M.; Patrick, W. H., Jr. Environ. Sci. Technol. 1987, 21,
450.
Received for review June 10,1988. Revised manuscript received J u n e 19,1989. Accepted August 3,1989. This work was supported in part by the Idaho Water Resources Research Institute with funding f r o m the U.S. Geological Survey. Neutron irradiations were performed at the Washington State University Nuclear Radiation Center under the reactor sharing program supported by the Department o f Energy. However, the views and conclusions presented i n this paper may not reflect the views and conclusions o f the supporting entities.
Mercury Chemistry in Simulated Flue Gases Related to Waste Incineration Conditions Bjorn Hall, Oliver Lindqvist, and Evert Ljungstrom
Department of Inorganic Chemistry, Chalmers University of Technology and University of Goteborg, S 412 96 Goteborg, Sweden
A flue gas generator has been built in Order to mercury reactions. The generator consists of a propanefurnace and a l2 long temperature-contro11ed steel duct with a fabric filter. When vaporized (elemental) mercury was added to the propane flame in a concentration of 150 pg/m3 and with 8% of oxygen, 20-30% of the mercury was oxidized after 0.8 residence time in the furnace and the duct (T> 500 o c ) . the presence of HCl(g) in the flue gas, most of the elemental mercury was oxidized after 0.8 s. The reaction product is assumed to be mercuric chloride. A %ereduction” of the oxidized mercury occurs when the temperature has decreased below 200 “C. This reduction is probably a heterogeneous reaction at the surface of the flue gas duct. Experiments with mercury and activated carbon powder [0.5-1.0 g/m3, at NTP, specific surface 792 m2/g (BET)]in the flue gas duct have also been performed. The results indicate that activated carbon may act as a catalyst in the oxidation of mercury.
Introduction Combustion of fossil fuels and of municipal solid waste mobilizes a number of substances (1). Many of the components liberated in the combustion process are notorious pollutants; others are strongly suspected of being harmful. The rapidly growing insight into the environmental problems caused by combustion, combined with the constant need for energy production, has accelerated the development of techniques for emission control. There are, however, flue gas components for which no simple cleanup process exists today. One such component is elemental mercury. It has been observed in several cases that mercury is partly associated with the solids from the flue gas that are captured in electrostatic precipitators and bag filters, hence less being emitted into the atmosphere. The mechanism behind this retention is not well-known ( 2 ) and needs to be studied further. The actual environmental effects caused by mercury emissions depend on the deposition pattern and the levels already present. A small increase in deposition may cause levels to become unacceptable and give rise to severe en108
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vironmental damage. In Sweden, for example, it has been known for Some time that fish from a large number of lakes, especially in where acidification has proceeded, contain mercury at levels that make the fish unsuitable to eat, and thus the lakes have been sblacklistedn. Most of these lakes do not have a local source of mercury and it is reasonable to assume that deposition from the air, directly on the lake or on its surrounding watershed, is the cause of the elevated mercury levels (3). It is therefore of great importance to reduce mercury depositiorl and to study possible ways of reducing the emission of combustion-generated mercury into the atmosphere. One way of achieving this could be to find the optimal conditions for removal of mercury with the fly ash, in a stable and insoluble form. Laboratory studies of mercury’s behavior in simulated flue gases related to waste incineration conditions have been performed ( 4 ) ,and the results of these experiments are reported in this paper.
Experimental Section A small-scale,propane-fired flue gas generator was built to study chemical reactions of mercury in the flue gases. The apparatus is shown in Figure 1. The gas is produced in a furnace equipped with a low-pressure propane burner with a maximum output of 17 kW. The furnace is connected to a 12-m flue gas duct, which gives a residence time of -3 s for the gas. Particulates and trace gases, e.g., HC1, SO2, and HgO, may be introduced either through or after the flame. Following the duct is a fabric filter with a bypass where particulates may be separated from the gas. The actual dimensions of the apparatus were dictated by the need to pass particulates through the system and to be able to collect samples large enough to be analyzed for mercury. The flow of gas through the system is maintained by an induced draught ejector. The furnace and ducts are air cooled, and the cooling air may be recirculated or shut off at each duct section to produce a suitable temperature profile in the duct. A simple computer-controlled data acquisition system collects and stores temperature and combustion parameter data during the experiments (Figure 2).
0013-936X/89/0924-0108$02.50/0
0 1989 American Chemical Society
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Temperature ('C)
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Figure 1. Schematic drawing of the flue gas generator. Key: (1) furnace, (2) flue gas duct, (3) propane burner, (4) cooling air jacket, (5) trace gas inlet, (6) fabric filter, (7) sampling probes, (8) induced draught ejector. EXPANSIONUNIT
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Figure 2. Data collection system.
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Figure 3. S stem for continuous measurement of mercury in flue gas. Key: (1) Hg line, (2) Hg(tot) line (3) ice box with gas/liquid separators, (4) three-way valve, (5) mercury analyzer, (6) recorder, (7) pump for gas (1 L/min), (8) gas flow meter, (9) drain valve, (10) drain, (11) pump for liquid (2 mL/min), (12) container with acidic Sn(I1) solution.
I
Mercury is analyzed by the cold vapor atomic absorption (CVAA) technique by using an apparatus originally described by Iwasaki (5) but modified to measure both elemental and total mercury. A diagram of the analysis instrument is shown in Figure 3. To analyze elemental mercury in the flue gas, a flow of 1L/min is continuously drawn from one of the sampling points on the steel duct (cf. Figure l),through a Teflon tube to a cooled liquid/gas separator where condensed water is separated. Since elemental mercury has a low solubility in water, the mercury vapor will pass into an 80 cm long absorption cell where the light absorption at 253.6 nm is measured. The detection limit in the present setup is 10 pg/m3. A measure of total mercury is obtained by sampling through a second line where an acidic tin(I1) solution is
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20
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40
1 1 ~ x 1 0(K.') ~
Figure 4. Vapor pressure over condensed phase vs temperature for some mercury compounds (9, 77- 79).
in contact with the sampled gas and is transported with the gas to another liquid/gas separator. The mercury vapor then goes on to the absorption cell. Reactive forms of oxidized mercury, either gaseous or solid, are reduced to elemental mercury by the Sn(I1) solution. The elemental mercury measured after reduction in this line is the s u m of the original elemental mercury and the oxidized mercury at the sampling point. However, some forms of nonreactive oxidized mercury, such as mercury sulfide (3), are not reduced by this process and will escape detection if present in the flue gas. Results and Discussion The aim of the present project is to study the transformations of mercury in flue gases, with and without added trace gases and particulates. In coal, mercury is present mainly as the sulfide (6),while in domestic refuse, elemental mercury and mercuric oxide are dominant species. At combustion temperatures, in an oxidizing environment, elemental mercury will evaporate (Figure 4). Mercuric oxide is thermally unstable at temperatures above ca. 500 "C and decomposes into elemental mercury and oxygen (7). Under these conditions mercuric sulfide will also react with oxygen and produce elemental mercury and sulfur dioxide. All chemical forms of mercury in the fuel are thus expected to leave the combustion zone as gaseous, elemental mercury with a partial pressure in the gas of lo4 Pa in the coal case and an order of magnitude greater for refuse incineration (8). This is well below the saturation pressure, even at ambient temperature, as is shown in Figure 4. Thus, if no chemical or physical processes took place, the vapor would pass through the flue gas system and out into the atmosphere. The temperature before the flue gas enters the duct is >600 "C and thus probably no HgO has been formed. As the flow gas gradually cools along the duct, it reaches the temperature range of 300-500 "C, where the oxidation of elemental mercury to HgO may occur (reaction 1). At temperatures lower than 300 "C, the reaction rate is probably too slow (7). By use of total pressures over decomposing mercuric oxide as measured by Taylor and Hulett (9)and the calculated equilibrium values (IO) for the reactions + f/zOz(g) HgO(g) (1) and HgO(g) * HgOb) (2)
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Environ. Sci. Technol., Vol. 24, No. 1, 1990 109
Reaction Temperature 320 220
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Flgure 5. Theoretical calculations of the mercury and mercuric oxide concentrations in the gas phase with a total mercury concentration of 20 ppb and an oxygen concentration of 10% [the data on HgO(g) are uncertain]. HgO(g)', concentration of mercuric oxide according to reaction 1. HgO(g)', Concentration of mercuric oxide according to reaction 2.
it is possible to calculate the concentrations of Hg(g) and HgO(g) at different temperatures (Figure 5). If trace amounts of hydrogen chloride are present, which is the case in waste incineration, several other reactions are possible in the cooling flue gas, e.g.: HgO(g,s) + 2HCl(g)
Hg&lz(s)
Q
Q
HgClz(g) + HzO(g)
Hgo(g) + HgClz(g)
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HgzClz(s) + 2HCHg)
Environ. Sci. Technol., Vol. 24, No. 1, 1990
Reaction Temperature ('C) 320 220
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(7)
(8)
on the basis of HCl formation and that no elemental mercury was found in their study. The H,(g) was assumed to be hydrogen occluded in steel. Braun et al. (12) suggested that some rereduction of oxidized mercury may occur in the Teflon tubes ordinarily used for mercury sampling, especially when SO,(g) is present. Also, on the basis of thermodynamic considerations, Stevens et al. (13) proposed the possibility of a reaction between mercury oxide and sulfur dioxide in plumes from combustion as well as in ambient air. Whether any of these effects may occur to any extent in the present measurement system is not known. There were no such indications, e.g., when elemental mercury from the calibrated mercury feed system was monitored, and it is thus assumed that they do not have any major effects on the results reported here. Nevertheless, this is a question of great interest for anyone dealing with mercury sampling in flue gases and ought to be further investigated. Addition of Hgo(g). When only elemental mercury is added t o the flue gas from the propane flame, oxygen, 110
Figure 6. Mercury concentration vs reaction time/temperature. = 8%.
(3)
The sampling probes used in our investigation are made from stainless steel followed by Teflon tubing. Wang et al. (11) showed that gaseous mercuric chloride may react when it is passed through a stainless steel tube at 200 "C. They proposed the following reaction: 2HgClz(g) + Hz(g)
3
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1
Reaction Temperature (K)
2
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Reaction Time (s)
Figure 7. Mercury concentration vs reaction timeitemperature. = 250 ppm.
carbon dioxide, water vapor, and small amounts of carbon monoxide, nitrogen oxide, and organic compounds are present to react with the mercury. Figure 6 shows the amount of total and elemental mercury in the flue gas duct. It is reasonable to assume that reactions 1and 2 are mainly responsible for the results obtained. The concentrations of both elemental and total mercury are decreasing slowly along the duct, except at the last sampling point, where a slight rise of elemental mercury occurs. A possible explanation is that solid HgO deposits in the duct. This is in agreement with the rather low estimated vapor pressure of HgO (Figure 4). Addition of Hg"(g) HCl(g). As can be seen from Figure 7 , the fraction of elemental mercury drops when HCl(g) is added. In the experiments performed, HC1 has been present in substantial excess, 50-250 ppm compared with 0.02 ppm Hgo. The main reaction appears to occur in the flame zone or in the flue gas at temperatures higher than 500 "C and is almost completed when the gas temperature has fallen below 500 "C. The mechanisms for oxidation and chloride formation are not known in detail, and both reactions 1 and 3 should be regarded as overall
+
Reaction Temperature ('C) 320 220
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carbon), which corresponds to between 13 and 20% of the mercury added. This value agrees well with the difference between the total amounts of mercury, with and without activated carbon. Further Investigations. Investigations continue on the influence of sulfur dioxide, nitrogen dioxide, and fly ash particles on the chemical transformations of mercury in the flue gas simulator. The mechanisms and kinetics of important processes, such as reactions between elemental mercury and oxygen, between Hg and HC1, and between Hg(I1) compounds and activated Fe surfaces, all in the temperature range of 100-800 "C, will also be studied in greater detail, in a continuous-flow reactor. Registry No. HC1,7647-01-0; Hg, 7439-97-6; HgO, 2190853-2;
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carbon, 7440-44-0.
Literature Cited I
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Flgure 8. Mercury concentration vs reaction timeltemperature. [Activated carbon] = 0.5 g/m3.
reactions only. Other reactions, such as 4-7, may also occur. The almost constant and nearly theoretical value of the total Hg concentration shows that the product of the reaction between HC1 and mercury is easily reduced and present in the vapor phase. Mercuric chloride is the most probable product, since it is reduced by tin(I1) in the analytical procedure and has a high vapor pressure (Figure 4). This conclusion is in agreement with previous reports on solid waste incineration, where it has been proposed that nearly all mercury exists as mercury chlorides, predominantly mercury(I1) chloride (14, 15). At lower temperatures in the flue gas, around 150-200 "C at T8, a reduction of oxidized mercury (to elemental mercury) occurs in many of the experiments. We assume that steel corrosion induced by hydrochloric acid at these temperatures may give rise to an activated iron surface on which Hg(I1) compounds may be reduced: Fe
+ Hg(I1) * Hgo(g) + Fe(I1)
(9)
This rereduction process seems to be very sensitive to local variations in the steel surface of the duct and may be the explanation to the very different ratios H$/Hg(tot) measured in combustion plants of similar construction. The understanding and control of this process may therefore be of critical importance for retaining mercury in its oxidized forms in filter fly ash. Mercury and Activated Carbon. A noticeable oxidation of the metallic mercury is obtained even in the absence of HC1 when activated carbon [0.5-1.0 g/m3, at NTP, specific surface 792 m2/g (BET)]is added to the flue gas. The total mercury concentration is, on the other hand, only slightly reduced, as shown in Figure 8. P'yankov (16) showed in his work with mercury and ozone that activated carbon acts as a catalyst for the formation of mercuric oxide. It is quite possible that activated carbon can act as a catalyst even in the reaction between mercury and oxygen at elevated temperatures. The carbon that was collected in the fabric filter at 150 "C contained 40-60 ppm mercury (pg of mercury/g of
(1) Cato, G. A. Field testing: Trace element and organic emissions from industrial boilers. Pb-261 263; US. Department of Commerce, Oct 1976. (2) Lindqvist, 0. Waste Manage. Res. 1986, 4, 35. (3) Lindqvist, 0.; Jernelov, A.; Johansson, K.; Rodhe, H. Mercury in the Swedish environment: Global and local sources. Report SNV P M 1816; Distributed by the National Swedish Environmental Protection Board, Box 1302 S-171 25 Solna, Sweden, 1984. (4) Hall, B.; Ljungstrom, E.; Lindqvist, 0. Mercury retention in filter fly ash. Briinsleteknik, No. 263; Report to the Swediih Thermal Engineering Research Institute, Box 6405, S-113 82 Stockholm, Sweden (in Swedish with English abstract), 1987. (5) Iwasaki, Y. Air Pollution Control Dept., Tokyo Metropolitan Research Institute for Environmental Protection, 175 Shinsuna Kotoku, Tokyo, Japan, personal communication, 1985. (6) Royal Swedish Academy of Sciences Report 192; (in Swedish with English abstract), 1981. (7) Sidgwick, N. V. The Chemical Elements and their Compounds; Oxford University Press: London, 1950. (8) Airey, D. Sci. Total Environ. 1982, 25, 19. (9) Taylor, G. B.; Hulett, G. H. J. Phys. Chem. 1913,17, 565. (10) Chase, M. W.; Davies, C. A.; Downey, J. R.; Frurip, D. J.; McDonald, R. A,; Syverud, A. N. J. Phys. Chem. Ref. Data 1985, 14(2). (11) Wang, R. G.; Dillon, M. A.; Spence, D. J.Chem. Phys. 1983, 79(2), 1100. (12) Braun, H.; Metzger, M.; Vogg, H. Mull und Abfull 1986, 2, 62. (13) Stevens, R. D. S.; Reid, N. W.; Schroeder, W. H.; McLean, R. A. N. The chemical forms and lifetimes of mercury in the atmosphere. Second Symposium on Combustion and the Nonurban Troposphere, Williamsburg, VA, 25-28 May 1982. (14) Vogg, H.; Braun, H.; Metzger, M.; Schneider, J. Waste Manage. Res. 1986,4, 65. (15) Bergstrom, J. G. T. Waste Manage. Res. 1986, 4, 57. (16) P'yankov, V. A. J. Gen. Chem. USSR (Engl. Transl.) 1949, 19, 187. (17) Smith, A.; Menzies, A. W. C. J. Am. Chem. SOC. 1910,32, 1541. (18) Pressions de vapeur du mercure et de ses chlorures, Btude bibliographique. Inf. Chin. 1978, No. 182, 227. (19) Weast, R. C., Ed. CRC Handbook of Chemistry and Physics, 58th ed.; CRC Press: Boca Raton, FL, 1977-78.
Received for review December 30, 1988. Accepted August 25, 1989. This work was financed by the Thermal Engineering Research Institute in Sweden.
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