Energy & Fuels 2004, 18, 1855-1861
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Fundamental Study on Mercury Release Characteristics during Thermal Upgrading of an Alberta Sub-bituminous Coal Zhenghe Xu,* Guoqing Lu, and On Yi Chan Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, T6G 2G6, Canada Received May 26, 2004. Revised Manuscript Received August 12, 2004
An Alberta sub-bituminous coal was tested for the level of mercury removal obtained by a low-temperature thermal upgrading. The mercury removal characteristics, relative to increasing temperature and upgrading time, were determined. Rapid thermal upgrading at 400 °C released ∼72% of the mercury in the original coal, with a negligible total thermal energy loss. A corresponding increase in the calorific value of the upgraded coal was observed, from ∼20 900 kJ/kg to 25 900 kJ/kg. A further increase in the upgrading temperature, from 400 °C to 600 °C, produced only a minimal further increase in mercury removal. Rapid thermal upgrading at 400 °C resulted in a higher mercury removal efficiency than the temperature-programmable thermal upgrading. Thermal upgrading of sub-bituminous coal could be considered to be a viable option for mercury emission control. A volume reaction model of a first-order process was applied to simulate mercury removal from coal during the thermal upgrading. A comprehensive time constant was introduced in this model to describe the mercury release kinetics. Both the activation energy (22.6 kJ/mol) and the pre-exponential factor (8.65/min) for mercury release were determined by fitting the experimental isothermal data into this model. The determined parameters were used to predict the nonisothermic release of mercury during thermal upgrading experiments.
1. Introduction The combustion of fossil fuels currently dominates the world’s energy production, and it will continue to do so in the foreseeable future. Coal has the highest potential to become a significant energy resource among the fossil fuels.1 The release of mercury during coal combustion has been targeted for potential emission control, because of its high volatility and bioaccumulative nature. Mercury is known to have detrimental effects on human health. The results of the 1990 Clean Air Act Amendments and the review by the United States Environmental Protection Agency (USEPA) of hazardous air pollutants has prompted the development of technologies to remove mercury from coal or capture mercury from flue gases. In December 2000, the USEPA announced its decision to regulate mercury emission from coal-fired power plants.2 To respond to the need for mercury emission control, several initiatives have been taken to study the fate of mercury in coal and its removal from flue gases. Some examples include the injection of various types of sorbents such as activated carbons, fly ash, limestones, * Author to whom correspondence should be addressed. E-mail address:
[email protected]. (1) Lu, G. Q.; Toyama, T.; Kim, H. J.; Naruse, I.; Ohtake, K.; Kamide, M. Kagaku Kogaku Ronbunshu 1997, 23, 404-412. (2) Afonso, R. F.; Constance, L. Assessment of Mercury Removal by Existing Air Pollution Control Device in Full Scale Power Plants. Presented at the Mega Symposium and Air & Waste Management Specialty Conference on Mercury Emission, Chicago, IL, 2001. (Available on CD-ROM.)
and in situ-generated agglomerates into flue gases.3 To improve mercury capture efficiency, activated carbons are often impregnated with sulfur and potassium iodide. These sorbents are used either independently or in conjunction with existing desulfurization systems. A major challenge encountered during the removal of mercury from flue gases is the capture of a trace amount of mercury from a vast volume of the flue gas with complex compositions. For this reason, controlling the mercury emission via sorbent injection is generally costintensive.4-6 Mercury is known to volatilize at temperatures above its relatively low boiling point. This opens a window for mercury emission control by the thermal upgrading of sub-bituminous and lignite coals prior to coal combustion. In a thermal upgrading process, coal is heated, to remove the associated moisture. During this process, the (3) Sloss, L. MercurysEmissions and Control; IEA Coal Research: London, U.K., 2002. (4) Jurng, J. S.; Lee, T. G.; Lee, G. W.; Lee, S. J.; Kim, B. H.; Seier, J. C. Mercury Removal from Incineration Flue Gas by Organic and Inorganic Adsorbents. Presented at the Mega Symposium and Air & Waste Management Specialty Conference on Mercury Emission, Chicago, IL, 2001. (Available on CD-ROM.) (5) Felsvang, K.; Lund, C.; Kumar, S.; Bechoux, E.; Hoffmann, D. Mercury Reduction and Control Option. Presented at the Mega Symposium and Air & Waste Management Specialty Conference on Mercury Emission, Chicago, IL, 2001. (Available on CD-ROM.) (6) Takaoka, M.; Takeda, N.; Fujiwara, F.; Kurata, M.; Kimura, T.; Control of Mercury Emission from Municipal Solid Waste Incinerator in Japan. Presented at the Mega Symposium and Air & Waste Management Specialty Conference on Mercury Emission, Chicago, IL, 2001. (Available on CD-ROM.)
10.1021/ef049870i CCC: $27.50 © 2004 American Chemical Society Published on Web 09/29/2004
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mercury and volatile components contained therein may also be released if the temperature is increased to a desired value, either directly or in a separate zone.7 Compared to the release into the flue gases during combustion, the release of mercury in this case can be confined to a relatively small volume of vapor phase. The capture of mercury can then be made more effective through the use of a physicochemical adsorption process with conventional mercury capture sorbents.8 Generally, raw coal that has been crushed to a suitable size enters the moisture removal zone, where it is heated to a temperature that does not exceed 150 °C. In this zone, the free water and some of the more tightly bound water are volatilized and removed from the zone in the sweep gas. The coal is then transferred to a mercury removal zone, where it is heated to a suitably higher temperature. Coal thermal upgrading has been verified as an economically and technically viable method for coal desulfurization and denitrification.9 To make thermal upgrading a viable avenue for mercury emission control from coal-fired power plants in practice,10 the mercury release kinetics during thermal upgrading of subbituminous and lignite coals, relative to particle size, need to be studied and corresponding predictive tools need to be established. These tools would be invaluable for predicting mercury release in coal combustion and gasification and for providing further understanding of the occurrence of mercury in coal. In this paper, we present our recent study on the mercury release kinetics during the thermal upgrading of an Alberta subbituminous coal. 2. Experimental Section 2.1. Sample Characteristics. An Alberta sub-bituminous coal was used in this study. The run of mine (ROM) coal sample was pulverized to a size of 380 °C, a further increase in the rate of weight loss with increasing temperature was observed (this feature is denoted as region A2). The weight loss over this temperature range is caused by the evolution of volatiles and tar, accompanied by the formation of char. Mercury removal during thermal upgrading may occur over this temperature range. 3.2. Mercury Removal by Thermal Upgrading. 3.2.1. Rapid Thermal Upgrading. Rapid thermal upgrading has been applied in various clean coal technologies for desulfurization and denitrification, because of its easy implementation and upgrading. For this reason, mercury removal characteristics during rapid coal thermal upgrading were investigated first in this study. The results in Figure 3 show negligible mercury removal at thermal upgrading temperatures of 330 °C, reaching 72% at 400 °C. Above this temperature, a slow increase in mercury removal with increasing upgrading temperature was evident, reaching a maximum of 80% at 650 °C. The mercury removal displayed a statistical decrease at temperatures of >650 °C. Generally, only hydrogen-bonded light molecules leave the coal at temperatures approaching 400 °C. Higher temperature facilitates the breaking of molecular bonds, leading to the evolution of tar and the formation of char.15 The experimental results suggest that most of the mercury in the sub-bituminous coal is weakly bound with coal. The transition from a rapid increase in mercury removal at 330 °C via rapid coal thermal upgrading further suggests the departure of two distinct mercury species: (i) mercury that is associated with weakly bound light species in the sub-bituminous coal and (ii) strongly bound volatiles such as tar, aliphatic gases, aliphatic hydrogen, aromatic hydrogen, and char. A thermal treatment for 30 min was sufficient to release all the volatiles at temperatures of >400 °C. 3.2.2. Thermal Energy Loss. The aforementioned results demonstrated that rapid thermal upgrading at 400 °C for 30 min removes 72% of the mercury present in the original coal. Figure 2 shows a 15% reduction in mass during heating to 400 °C in TGA. The majority of weight loss over this temperature range was attributed to the loss of moisture from the coal. The removal of noncombustible moisture is known to upgrade the calorific value of sub-bituminous coal. As shown in Figure 4, a significant increase in calorific values, from 20 900 kJ/kg in raw coal to 25 900 kJ/kg for the thermally upgraded coal at 400 °C (which represents an increase of 24%), was obtained. However, at temperatures >400 °C, a further increase in the thermal upgrading temperature resulted in vaporization of highheating-value volatiles and tar while retaining noncombustible mineral matter. As a result, the thermal energy of the coal, on a mass basis, was reduced. It is instructive to examine the overall energy loss during thermal upgrading. In this case, the thermal energy contained in raw coal (ROM coal) was used as a basis. The thermal energy loss due to the thermal
upgrading is shown in Figure 5. At thermal upgrading temperatures of
