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Energy & Fuels 2009, 23, 774–778
Mercury Removal from Coal by Leaching with SO2 Eung Ha Cho,* Poornima Chateker, Ravinder Garlapalli, and Ray Y. K. Yang Chemical Engineering Department, West Virginia UniVersity, Morgantown, West Virginia 26506 ReceiVed September 16, 2008. ReVised Manuscript ReceiVed NoVember 18, 2008
This paper is about the pretreatment of coal to remove mercury content prior to coal combustion. The minute amount of mercury in Pittsburgh No. 8 coal (0.177 ppm) was removed by flowing a gas stream containing 10% oxygen and 1000 ppm SO2 into a coal-water slurry at 30 mL/s. A total of 50 g of 35-65 mesh coal was leached each time in 500 mL of solution. The temperature was varied from 50 to 80 °C, and the initial solution pH varied at 5.7, 1.8, and 1.5. It was found that the mercury removal increased from 44.2% at 50 °C to 88.6% at 75 °C at the natural pH (5.7) and after 3 h of reaction time. The pyrite conversions were much lower than those of mercury. Both the mercury and pyrite removals increased as the pH decreased at 71 °C. Mercury removal was high enough to consider the application of this technology to a commercial process.
Introduction Mercury from coal-fired utilities has been identified as one of the most hazardous air pollutants and a great potential public health concern. Furthermore, it has a tendency to bioaccumulate in the food chain. Generally, mercury is present in coal in concentrations well below 1 ppm; however, the large tonnages of coal consumed for electric power generation represent a significant source of mercury entering the environment. It is estimated that about 48 tons of mercury is accumulated in the environment annually in the U.S.A. alone from coal-fired power plants.1 Under Title III of the 1990 Clean Air Act Amendments, the Environmental Protection Agency (EPA) is empowered to set emission standards for 189 pollutants, including mercury. There are various technologies available to control the emission of mercury from coal-fired power plants. Among them is activated carbon injection into the flue gas stream. This technology has been studied for many years and is considered to be the “standard technology” at this time to control the mercury emissions from the flue gas. It can remove mercury by as much as 90% if activated carbon at a level of 4000 times its stoichiometric amount is injected. The requirement of this excessive amount of activated carbon may be due to the fact that the adsorption of mercury species on activated carbon is not thermodynamically very favorable at the high flue gas temperatures (i.e., 150 °C), and thus, only the activated carbon that has high adsorption capability can be involved in the removal process. This makes the technology very expensive. For example, it is reported that the cost of mercury control is estimated to range from $33 000 to $131 000/lb of mercury removed for the bituminous coal-fired power plant unit.2 The upper end of the cost includes a 90% mercury capture case via a very high activated carbon injection rate. When fly ash sales and added disposal costs are included, the cost of mercury * To whom correspondence should be addressed. Fax: (304) 293-4139. E-mail:
[email protected]. (1) http://www.epa.gov/camr/basic.htm. (2) Hoffman, J.; Brown, J. R. Preliminary cost estimate of activated carbon injection for controlling mercury emissions from an un-scrubbed 500 MW coal-fired power plant. Final Report to U.S. Department of Energy/ National Energy Technology Laboratory (DOE/NETL) (Innovations for Existing Plants Programs) by Science Applications International Corporation, Nov 2003.
control is estimated to range from $49 000 to $246 000/lb of mercury removed. Very small amounts of mercury are contained in coal. Its typical concentration in coal is in the vicinity of 0.1 ppm. It is reported that 58% of the mercury in Pittsburgh No. 8 coal exists in association with coal pyrite, 16% as oxides, and 26% as organic mercury, which may be associated with the coal structure.3 Coal contains toxic trace elements, such as arsenic, cadmium, chromium, mercury, lead, and selenium in ppm ranges. The average values for arsenic in raw Pittsburgh No. 8 coal are 22 ppm; lead, 19 ppm; selenium, 1.9 ppm; cadmium, 0.15 ppm; and mercury, 0.13 ppm.4 These trace elements are subject to removal by conventional coal-cleaning processes. In general, the removal of mercury is poor, with a value less than 30%, while those of the other elements, except for selenium, reach approximately 50%.4 This poor removal of mercury from coal by the conventional coal-cleaning process may necessitate exploration of an alternative method to remove mercury more effectively. Cho5 found that coal pyrite could be leached effectively by the combination of SO2 and oxygen and that this coal scrubbing can capture incoming SO2 effectively. Because the largest fraction of the mercury exists in association with coal pyrite, this gas mixture of SO2 and oxygen can liberate mercury sulfide from the pyrite matrix, which can subsequently be leached by the gas mixture. One advantage of this method is that the leaching reagents are the waste gas mixture of the flue gas stream, thus making this method economically competitive. The other advantage is that this method serves two purposes of removing mercury from coal and, at the same time, capturing SO2 from the stack gas stream. The objective of this study is to (3) Luttrell, G. H.; Yoon, R. H.; Adel, G. T. Precombustion removal of hazardous air pollutant precursors. Coal Liquefaction and Solid Fuels Contractors Review ConferencesHazardous Air Pollutant Precursor Control, Federal Energy Technology Center/Department of Energy (FETC/DOE), Pittsburgh, PA, 1997. (4) Rosandale, L. W.; Devito, M. S.; Conrad, V. B.; Meenan, G. F. The effect of coal cleaning on trace element concentrations. An Internal Document, CONSOL, Inc., Research and Development, Library, PA, 1993. (5) Sundaram, H. P.; Cho, E. H.; Miller, A. SO2 removal by leaching coal pyrite. Energy Fuels 2001, 15 (2), 470–476.
10.1021/ef800776a CCC: $40.75 2009 American Chemical Society Published on Web 01/07/2009
Mercury RemoVal from Coal by Leaching with SO2
Energy & Fuels, Vol. 23, 2009 775
study this new technology and determine its technical feasibility for the pretreatment of coal for mercury removal. Theory
Experimental Methods
The removal of mercury from coal is based on the chemistry in which mercury is oxidized by a strong oxidizing reagent, which is a combination of SO2 and oxygen, which are both selfsupplied from the flue gas. SO2 is a reducing reagent, but when it combines with oxygen in an acid solution, it becomes a strong oxidizing reagent. The cathode reaction for the oxidizing reagent can be written as H2SO3 + O2 + 2e ) SO42- + H2O E° ) 2.29 eV
However, the study of adsorption/desorption of mercuric ions on the chitosan beads is beyond the scope of this research.
(1)
where H2SO3 represents the aqueous SO2. It can be seen that the magnitude of the E° value of reaction 1 is very high. This value is much higher than those of strong oxidizing reagents, such as permanganate (1.51 eV) and dichromate (1.33 eV). When SO2 is dissolved in an aqueous medium, it forms H2SO3 in an acid solution, which is then dissociated into HSO3- as pH is increased as shown SO2 + H2O ) H2SO3 K ) 1.204
(2)
H2SO3 ) H+ + HSO3- K ) 10-1.88
(3)
The equilibrium constants of reactions 2 and 3 are those at room temperature. In an acid solution, dissolved SO2 forms H2SO3. When the pH is increased, H2SO3 turns to HSO3-. The concentrations of H2SO3 and HSO3- are the same at pH 1.88. Thus, to increase the H2SO3 concentration in the dissolved SO2, the solution pH should be below 1.88. The dissolution of coal pyrite in the presence of SO2 and oxygen may be based on an electrochemical reaction. The anode reaction may be given as FeS2 + 8H2O ) Fe2+ + 2SO42- + 16H+ + 14e E° (for cathode reaction) ) 0.355 eV (4) The cathode reaction is reaction 1. The difference in E° between cathode and anode reactions is large, suggesting that the leaching reaction is thermodynamically very much spontaneous under normal conditions. The overall leaching reaction can then be written as FeS2 + H2O + 7H2SO3 + 7O2 ) Fe2+ + 9SO42- + 16H+ (5) The mercury sulfide upon liberation may be leached on the basis of an electrochemical reaction. The anodic reaction may be written as HgS + 4H2O ) Hg2+ + SO42- + 8H+ + 8e E° (for cathode reaction) ) 0.54 eV (6) It is seen that the difference in E° between cathode and anode reactions is large, suggesting that the leaching of HgS is thermodynamically spontaneous under normal conditions. The leaching product may be mercuric ion, Hg2+. This ion should be recovered from the leach solution in a conceptual process. It is reported that aminated chitosan beads formed through the chemical reaction using ethylenediamine and carbodiimide show the highest uptake capacity for mercuric ions.6 The uptake capacity reaches about 0.8 mmol of Hg2+/g of dry mass. It is possible then to adsorb the mercuric ions from the leach solution of filtrate and to recover them by desorption. (6) Jeon, C.; Holl, W. H. Chemical modification of chitosan and equilibrium study for mercury ion removal. Water Res. 2003, 37, 4770– 4780.
Sample Preparation. Coal samples of Pittsburgh No. 8 coal were collected from the Blacksville Mine #2 mine (Consol), Wadestown, West Virginia. The coal was crushed and screened to give a fraction of 35-65 mesh, which was used in this study. This size fraction of the coal sample was analyzed for ash, nonpyritic iron, pyritic sulfur, and total sulfur using American Society for Testing and Materials (ASTM) methods. The results showed that the nonpyritic iron was 0.11%, the pyritic sulfur was 1.11%, the total sulfur was 2.9%, and the ash content was 12.3%. This size fraction was submitted to a commercial analytical laboratory for mercury analysis. The mercury content was found to be 0.177 ppm. The raw coal was used because cleaning with froth flotation would decrease the pyrite content and would result in difficulty particularly in analyzing for leaching of a smaller amount of pyrite. Leaching Experiments. The apparatus used in this study is similar to the one that was previously used by the author.5 The apparatus consisted of three gas cylinders (nitrogen, oxygen, and SO2) and a four-necked 1 L reactor. The reactor was immersed into a thermostated constant-temperature bath. It has four necks: the central neck is equipped with an overhead stirrer connected to a motor operating at a speed of 500 rpm. One of the side necks is fitted with a condenser; another is fitted with a bubbling dispersion tube; and the last is fitted with a sample-taking device. The gases, after metered for their gas flow rates, were combined and introduced into the reactor and bubbled through the coal slurry added to the reactor. During the leaching, the SO2 and oxygen were dissolved and oxidized mercury content and coal pyrite. The undissolved fraction of SO2 exited the reactor and flowed through the condenser to knock out its moisture content. This undissolved SO2 concentration was expected to be very low (e.g.,
