Energy & Fuels 2008, 22, 321–330
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Combined Homo- and Heterogeneous Model for Mercury Speciation in Pulverized Fuel Combustion Flue Gases Shishir P. Sable,* Wiebren de Jong, and Hartmut Spliethoff† Section Energy Technology, Department of Process and Energy, Faculty of Mechanical, Marine, and Material Engineering, Delft UniVersity Technology, Leegwaterstraat 44, 2628CA Delft, The Netherlands ReceiVed July 26, 2007. ReVised Manuscript ReceiVed September 29, 2007
A new model is developed to predict Hg0, Hg+, Hg2+, and Hgp in the post-combustion zone upstream of a particulate control device (PCD) in pulverized coal-fired power plants. The model incorporates reactions of mercury with chlorinating agents (HCl) and other gaseous species and simultaneous adsorption of oxidized mercury (HgCl2) on fly ash particles in the cooling of flue gases. The homogeneous kinetic model from the literature has been revised to understand the effect of the NO + OH + M T HONO + M reaction on mercury oxidation. Because it is a pressure-dependent reaction, the choice of proper reaction rates was very critical. It was found that mercury oxidation reduces from 100 to 0% while going from high- to low-pressure limit rates with 100 ppmv NO. On the basis of the revised chemistry of SO2, NOx, HCl, Cl, and C/H/O, the mercury reaction mechanisms of Niksa et al., Qiu et al., and Widmer et al. were compared to the Sliger et al. furnace data on mercury speciation. The rate constants proposed by Niksa et al. were found to be in reasonable agreement with experimental data and, therefore, chosen for further model development. The heterogeneous model describes selective in-duct Langmuir-Hinshelwood adsorption of mercury chloride on ash particles. The heterogeneous model has been built using Fortran and linked to Chemkin 4.0. This way, the simultaneous formation of HgCl2 in the gas phase and capture on ash is ensured. The final predictions of elemental, oxidized, and particulate mercury were compared to mercury speciation from power plant data. Information collection request (ICR) data were used for this comparison. The model results follow very similar trends compared to those of the plant data; however, quantitative deviation was considerable. These deviations are due to the errors in the measurement of mercury upstream of PCD, lack of adsorption kinetic data, accurate homogeneous reaction mechanisms, and certain modeling assumptions. The model definitely follows a new approach for the prediction of mercury speciation, and further refinement will improve the model significantly.
Introduction Mercury emission from a thermal power plant has been a matter of concern for more than a decade. A considerable amount of work has been carried out to understand the complex interaction of mercury with flue gas and fly ash and its capture in air-pollution control devices. Mercury, which is present in the coal matrix, is volatilized and converted into the elemental form in the high-temperature regions of a combustion furnace. As the flue gas cools, elemental mercury is partially oxidized to ionic mercury by interacting with flue gas and fly ash constituents. The rate of oxidation depends upon the temperature, flue gas composition, properties and amount of fly ash, and any entrained sorbents. Mercury speciation in the flue gas has a significant impact on the design strategies for preventing mercury emissions. Early studies on mercury oxidation in combustion systems have focused on chemical equilibrium calculations. Equilibrium calculations conducted for mercury at stack gas conditions indicate that the oxidized form is thermodynamically stable. Chlorine in flue gas is a very important constituent for mercury oxidation, and mercury chloride (HgCl2) is reported as the most * To whom correspondence should be addressed. Telephone: +31-15278-6541. Fax: +31-15-278-2460. E-mail:
[email protected] and/or
[email protected]. † Current address: Institute of Energy System, Technical University of Munich, Boltzmannstrasse 15, 85747 Garching, Germany.
prominent form of oxidized mercury. Recently, Vosteen5 suggested that bromine reacts faster than chlorine to produce HgBr2. However, this process is still immature and needs more investigations. Observations of kinetic limitations for other species in practical combustion systems and measurements of mercury speciation in coal-fired power plants suggest that thermodynamic equilibrium will not be attained for mercury in this type of flue gases.6–8 In the later research, marked strides have been made in unraveling the reaction mechanism for homogeneous mercury oxidation. Such efforts include those of Widmer et al.,3 Sliger et al.,4 Edwards et al.,9 Niksa et al.,1 Xu et al.,10 and Qiu et al.2 These gas-phase reaction mechanisms (1) Niksa, S.; Helble, J. J.; Fujiwara, N. EnViron. Sci. Technol. 2001, 35 (18), 3701–3706. (2) Qiu, J.; Sterling, R. O.; Helble, J. J. Development of an Improved Model for Determining the Effects of SO2 on Homogeneous Mercury Oxidation. 28th International Technical Conference on Coal Utilization and Fuel Systems, Clearwater, FL, March 10-13, 2003. (3) Widmer, N. C.; West, J.; Cole, J. A. Thermochemical Study of Mercury Oxidation Utility Boiler Flue Gases. Proceedings of Air and Waste Management Associations 93rd Annual Conference and Exhibition, Pittsburgh, PA, 2000. (4) Sliger, R. N.; Kramlich, J. C.; Marinov, N. M. Fuel Process. Technol. 2000, 65–66, 423–438. (5) Vosteen, B.; Lindau, L. VGB PowerTech. 2006, 86 (3), 70–75. (6) 3rd International Expert’s Workshop on Mercury Emission from Coal, Katowice, Poland, June, 2006. (7) Linak, W. P.; Wendt, J. O. L. Fuel Process. Technol. 1994, 39, 173– 198. (8) Frandsen, F.; Johansen, K. D.; Rasmussen, P. Prog. Energy Combust. Sci. 1994, 20, 115–138.
10.1021/ef700444q CCC: $40.75 2008 American Chemical Society Published on Web 12/06/2007
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include mercury reactions with chlorine, the chlorine reactions, C/H/O chemistry, and SOx and NOx chemistry. These mechanisms predict mercury oxidation within a reasonable range at higher temperatures; however, at low temperatures, the predictions were very different. The variations at lower temperatures may suggest a possible interaction of flue gases with fly ash particles. In recent years, some efforts by Chen et al.,11 Scala,12 Zhuang et al.,13 and Flora et al.14 were focused on modeling mercury capture on activated carbon or fly ash. All of these models assumed only gas-solid interactions of oxidized mercury, and no homogeneous kinetic mechanism was considered. However, in real combustion conditions, the reaction of gaseous mercury with other flue gas components can significantly affect the simultaneous adsorption on the particle surface. The main focus of this study is to include a heterogeneous model into the existing but revised homogenous model and to predict mercury speciation in a more reliable way. An attempt has been made to compare model results with information collection request (ICR) plant data on mercury speciation before the particulate matter control device. The approach and outcome of this work can provide a guideline to improve modeling efforts on mercury speciation. Model Development The model is applicable to a straight duct of constant diameter situated upstream of a particulate control device (PCD). It incorporates the homogeneous gas-phase interaction of mercury with flue gas constituents and the heterogeneous interaction in the form of physical adsorption/desorption of oxidized mercury with fly ash particles. These two interactions are discussed further in detail. Gas-Phase (Homogeneous) Mercury Speciation Model. It is very important to use an accurate homogeneous mercury oxidation mechanism to incorporate it with a heterogeneous model. There are four sets of Hg mechanisms available in the literature thus far: Sliger et al.,4 Widmer et al.,3,15 Niksa et al.,1 and Qiu et al.2 The model of Sliger et al.4 differs from others in terms of global reaction rates, which involve uncertainties and a number of different pathways for mercury oxidation. Therefore, this model is not considered further in this study. Qiu et al.2 have used Widmer’s model as a starting point and recalculated the rate constants for important mercury reactions using transition-state theory (TST), whereas Niksa et al. had chosen the rates mainly from the literature of Widmer et al.3,15 and Sliger et al.4 for most of the mercury reactions. These models are then validated either with their own experimental data or with experimental data of Sliger et al.,4,16 Ghorishi et al.,17 and Mamani-Paco et al.18 These models predict (9) Edwards, J. R.; Srivastava, R. K.; Kilgroe, J. D. J. Air Waste Manage. Assoc. 2001, 51, 869–877. (10) Xu, M.; Qiao, Y.; Zheng, C.; Li, L.; Liu, J. Combust. Flame 2003, 132, 208–218. (11) Chen, S.; Rostan-abadi, M.; Chang, R. Prepr. Pap.—Am. Chem. Soc., DiV. Fuel Chem. 1996, 41 (1), 442. (12) Scala, F. EnViron. Sci. Technol. 2001, 35, 4367. (13) Zhuang, Y.; Thompson, J. S.; Zygarlicke, C. J.; Pavlish, J. H. EnViron. Sci. Technol. 2004, 38, 5803. (14) Flora, J. R. V.; Hargis, R. A.; O’Dowd, W. J.; Pennline, H. W.; Vidic, R. D. J. Air Waste Manage. Assoc. 2003, 53, 478. (15) Widmer, N. C.; Cole, J. A.; Seeker, W. R.; Gaspar, J. A. Combust. Sci. Technol. 1998, 134, 315–326. (16) Sliger, R. N. Mercury Transformations and Capture by Alkali Aerosol in the Post-Flame Regions of Pulverized Coal Combustion. Ph.D. Thesis, University of Washington, Seattle, WA, 2001. (17) Ghorishi, S. G.; Lee, C. W.; Kilgroe, J. D. Mercury Control Research: Effects of Fly Ash and Flue Gas Parameters on Mercury Speciation. Proceedings of the 6th Annual Waste-to-Energy Conference, Miami Beach, FL, May 11-13, 1998. (18) Mamani-Paco, R. M.; Helble, J. J. Bench-Scale Examination of Mercury Oxidation under Non-isothermal Conditions. Proceedings of the Air and Waste Management Association Annual Conference, Salt Lake City, UT, June, 2000.
Sable et al. Table 1. Reaction Rate Constants for NO + OH + M T HONO + M Reaction case
kf (cm3 mol-1 s-1)
n
low-pressure limit (100 ppmv) from the power plant. On the other hand, the highpressure rate constants are able to predict reasonable mercury oxidation. These observations may imply a need for a new mercury oxidation kinetic mechanism that can predict mercury oxidation with wide variations in other species concentrations. The rate mechanism of Niksa et al.1 performs better as compared to that of Qiu et al.2 and Widmer et al.,3 when the chemistry of CO2, H2O, O2, SO2, NOx, HCl, Cl, and C/H/O is revised. The developed heterogeneous model involves selective adsorption of mercury chloride on ash particles. The model shows that higher ash loading, smaller sized particles, high residence time, and lower temperatures are favorable for adsorption parameters for simultaneous formation and capture of mercury chloride on the particle. The combined homogeneous-heterogeneous model results were compared to the ICR data of selected power plants. The results are in good agreement qualitatively; however, quantitatively, the results differ considerably. The discrepancies are attributed to inaccurate power plant data, adsorption kinetic data, and modeling assumptions. Despite these deviations, the model can serve as a starting point for more rigorous mercury speciation modeling. Nomenclature a ) Total interfacial area of adsorbent available for adsorption per unit volume of flue gas (cm2/cm3) C ) Bulk-phase concentration (g/cm3) C* ) Local gaseous mercury chloride concentration (g/cm3) dp ) Diameter of particle (cm) dpore ) Diameter of pore (Å) Dkn ) Knudsen diffusivity (cm2/s) Dm ) Molecular diffusivity (cm2/s) Deff ) Effective diffusivity (cm2/s) kg ) Mass-transfer coefficient (cm/s) kadsor ) Rate constant for adsorption (cm3 g-1 s-1) kdesor ) Rate constant for desorption (s-1) m ) Ash loading (g/cm3) MwHgCl2 ) Molecular weight of HgCl2 (g/mol) Mwflue gas ) Molecular weight of flue gas (g/mol) q ) Local capture of mercury on the ash particle surface (g/g) qmax ) Maximum adsorption capacity on the ash particle surface (g/g)
330 Energy & Fuels, Vol. 22, No. 1, 2008 r ) Particle radius (cm) t ) Time (s) T ) Temperature (K) Greek Letters ΩHgCl2 flue gas ) Collision integral p ) Particle porosity
Sable et al. τp ) Particle pore tortuosity Fp ) Particle density (g/cm3) σHgCl2 flue gas ) Average collision diameter between mercury chloride and flue gas molecules (Å) EF700444Q
