Enhancement of Gas Desulfurization with Hydrated Lime at Low

NO2 was determined to be a promoter for SO2 uptake, because its retention can be increased up to 200%. The effect of NO2 and O2 concentration on the ...
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Ind. Eng. Chem. Res. 2005, 44, 9040-9049

Enhancement of Gas Desulfurization with Hydrated Lime at Low Temperature by the Presence of NO2 M. Bausach, M. Pera-Titus, C. Fite´ , F. Cunill,* J. F. Izquierdo, J. Tejero, and M. Iborra Chemical Engineering Department, University of Barcelona, 08028 Barcelona, Spain

The effect of NO2 on the desulfurization reaction of simulated flue gas by Ca(OH)2 was investigated in a fixed-bed reactor at 333-353 K and at relative humidities (RHs) in the range of 30%-70%. NO2 was determined to be a promoter for SO2 uptake, because its retention can be increased up to 200%. The effect of NO2 and O2 concentration on the kinetics of the desulfurization reaction was surveyed and successfully simulated by means of a deactivation model implemented with an inverse shrinking-core model (DM-ISCM). Solid analyses were also performed to identify the reaction products and provide insight into the chemistry of the process. 1. Introduction Flue gas desulfurization (FGD) processes constitute well-established scrubbing technologies for the reduction of SO2 in post-combustion stack gases emitted from coalfired industrial boilers, whose use is currently extended toward the retention of other acid gases such as NO2 and HCl. The most conventional FGD processes for postcombustion acid gas emission control are reviewed in Table 1. The principle of all these processes consists of the neutralization of the acid pollutants by the addition of an alkaline sorbent (usually CaCO3 or Ca(OH)2) with the subsequent generation of solid products. To enhance the neutralization reaction, water is often introduced to the process. Depending on the amount of water introduced and how it is added, the FGD processes can be broadly classified as (i) wet (or conventional absorption), (ii) semi-dry scrubbing, and (iii) dry scrubbing, where no water is introduced and which involve traditional in-furnace or economizer sorbent injection. In the past decades, a trend toward “no liquid effluents” has made semi-dry scrubbing technologies attractive, not only for small- and medium-sized coal combustion facilities, but also for incinerators.5,10 Among them, in-duct scrubbing processes (IDSs) constitute well-known FGD technologies, where the sorbent and water can be introduced in the ductwork downstream of the air preheater but upstream of the particulate control device, either separately (in-duct dry sorbent injection) or together as a slurry (in-duct spray drying). IDS technologies are not as widespread as wet or spraydry scrubbings, because of their lower desulfurization yield and solid utilization.6,14 In-duct dry sorbent injection, in its simplest form, offers some relevant advantages that are related to its low capital cost and low energy consumption, simple installation, and low space requirements. On the basis of these general advantages, intense research efforts have been made in the last three decades to provide insight into the reaction between SO2 and calcium-based sorbents at laboratory scale at low temperature (343573 K) and in the presence of water vapor.12-15 However, * To whom correspondence should be addressed. Tel.: +34934021304. Fax: +34-934021291. E-mail address: fcunill@ ub.edu.

fewer studies have been conducted to survey the chemistry of the HCl retention process,16 and only a few of the studies did concern the reaction between alkaline sorbents and NOx or the influence of the latter on the desulfurization reaction.17 Several researchers have shown that IDS processes are not feasible for NO emission control, because of its low aqueous solubility under standard operational conditions.21-23 Moreover, the presence of NO in simulated flue gases has been reported not to change the desulfurization ability of calcium-based sorbents under conventional IDS conditions.21 Although some studies have shown that NO uptake might be enhanced by the addition of some oxidant agents (e.g., NaClO2, H2O2, or yellow phosphorus) or quelating species that form complexes with NO (e.g., [Fe(EDTA)]2+) to wet and spray-dry scrubbers,19,22-25 the additional costs involved and the complexity of these methods have acted as a deterrent for their application. On the other hand, some authors have shown that the presence of NO2 together with SO2 in simulated flue gases might improve the desulfurization ability of calcium-based sorbents. According to O’Dowd et al.,18 despite not having found any influence in a spray dryer, the presence of NO2 seems to enhance the baghouse SO2 removal by Ca(OH)2. Nelli and Rochelle17 also reported an enhancement of both SO2 and NO2 captures in the presence of NO2 (especially at high relative humidity (RH) values) in a fixed-bed reactor using Ca(OH)2 and Fly Ash ADVACATE (advanced silicate) sorbents at 343 K and 60% RH. Therefore, a further step in the study of the desulfurization of flue gases by means of in-duct dry sorbent injection could be the simultaneous removal of SO2 and NOx, where the NO in the flue gases should be first oxidized to NO2 by the addition of methanol or hydrocarbons into the ductwork under the optimal conditions.31 The chemistry of the Ca(OH)2-NO2 system in the presence of water vapor either with or without SO2 has been studied by a few groups, who have noted that the set of reactions involved seem to be analogous to those occurring in aqueous solution (see Scheme 1).17,18 Both SO2 and NO2 can be chemically absorbed in aqueous solution (reactions R1-R3) with a disproportion of the latter. Moreover, when absorbed NO2 coexists together with SO32- ions, the set of radical chain reactions R4R6 might occur, giving SO42- as a product, according to

10.1021/ie050713w CCC: $30.25 © 2005 American Chemical Society Published on Web 10/28/2005

Table 1. State of the Art for Flue-Gas Desulfurization (FGD) Scrubbing Processes for SO2 and HCl Retention

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Scheme 1. Set of Reactions for the System SO2-NO2 in Aqueous Solutiona

a

Numbers given in parentheses are reaction numbers. Numbers given in square brackets are reference citations (see reference list).

reaction R7. Some side reactions that lead to the formation of dithionate ions (i.e., S2O62- ) from the recombination of SO3•- radicals and sulfur-nitrogen species (reactions R6 and R8, respectively) have also been proposed.28 Direct evidence of the existence of SO3•- radicals has been reported using ultraviolet (UV) spectroscopy, pulse radiolysis, and electron spin resonance (ESR) techniques.32-34 Moreover, when O2 is present (>3 vol %), SO3•- radicals might also be oxidized by O2 molecules through a mechanism involving further radical chain reactions through the radical species SO5•(reactions R9-R11).28 The solid product analyses that were performed by O’Dowd et al.,18 either with or without the injection of SO2, indicate that nitrites were the predominant nitrogen-containing species formed in the reaction of NO2 with Ca(OH)2. By increasing the NO2/SO2 ratio, they observed a decrease of sulfite ion concentration and an increase in that of the sulfate ion. On the other hand, the presence of nitrate species (but not nitrites) and some sulfur-nitrogen species formed from the reaction of nitrous acid with sulfite species were also detected, using Fly Ash ADVACATE as a solid reagent.17 The absence of nitrite species was explained by the acidification of the external surface of the sorbent, which was due to the role of reaction R3. The strong influence of water vapor in the chemistry of the SO2-NO2-Ca(OH)2 system might be related to the formation of an adsorbed water multilayer on the surface of the alkaline solid reagents at a RH of >50%.17 The increased removal of SO2 in the presence of water vapor together with NO2 might be explained by a higher water adsorption on the solid surface that is due to the deliquescent character of nitrogen-containing species on the solid surface. The kinetics of the reaction between Ca(OH)2 and SO2 at low temperature in the presence of water vapor has also been the subject of interest of many authors. In a previous work,35 we presented a deactivation model coupled with an inverse shrinking-core model

(DM-ISCM) to fit experimental kinetic data from SO2 breakthrough curves at the outlet of a fixed-bed reactor. Essentially, the model involves two contributions: (i) progressive deactivation or reduction of the active surface of the solid reagent with the progress of the reaction, and (ii) outward solid-state diffusion of hydrated Ca2+ and HO- ions through the product layer. The parameters of the model account for the sharp effect of the RH on the kinetics of the reaction. Some preliminary fittings to desulfurization kinetic data in the presence of NO2 revealed that the effect of the RH on the parameters of the model seemed to be stressed due to the action of NO2 retained by the solid sorbent. It is the aim of the present paper to provide new experimental data obtained from breakthrough curve analysis concerning the effect of NO2 both on the sulfur retention capability and on the kinetics of the reaction between Ca(OH)2 and SO2 at low temperature in the presence of water vapor. The effect of O2 on both aspects is also discussed. Solid product analyses were also conducted, to investigate the reaction pathways that might occur during the desulfurization reaction in the conditions under study. Experimental breakthrough curves have been fitted to a DM-ISCM model. The effect of both NO2 and O2 concentrations on the parameters of the model has been also evaluated. 2. Experimental Section The average sulfur retention capability of Ca(OH)2 and the kinetics of the reaction under study were determined from experimental SO2 breakthrough curves that were monitored at the outlet of an integral fixedbed reactor. Details regarding the setup and the experimental procedure used can be found elsewhere.35 Tables 2 and 3 list, respectively, the experimental conditions tested and the main properties of the reagent and inert solids used in this study. The reagent that has been used consists of a powder of Ca(OH)2 of high purity and low

Ind. Eng. Chem. Res., Vol. 44, No. 24, 2005 9043 Table 2. Experimental Conditions parameter

value

total flow SO2 inlet concentration (ppm) NO2 inlet concentration (ppm) O2 inlet concentration relative humidity, RH temperature Ca(OH)2 weight inert weight experimental times

1300 NmL/min 2000 ppm 0-275 ppm 0-18 vol % 30%-70% 333-353 K 0.87 g 26 g 1-3 h

Table 3. Main Properties of the Solids Value property composition mean particle sizec BET surface pore volume porosity mean pore size

Ca(OH)2

inert sea sand

∼99 wt % Ca(OH)2 and 1 that are indicated in Figure 5. The higher presence of nitrate species, in comparison to nitrites, can be explained by a possible role of reaction IV, which might remove the nitrite species generated

Figure 5. Effect of the RH value on the molar ratio of NO3- to NO2- retained by Ca(OH)2 for different NO2 concentrations at 333 K and reaction times of 2 h.

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Figure 6. Experimental and fitted SO2 breakthrough curves to the DM-ISCM model at 333 K, 2000 ppm of SO2, and 175 ppm of NO2 for the RH range of 30%-70%. Maximum error is e10%.

by reactions II and III, because of their disproportion to nitrate species and NO(g) in such a way analogous to reaction R3 in aqueous solution:

3Ca(NIIIO2)2(s) + 2H2O(ads) T Ca(NVO3)2(s) + 4NIIO(g) + 2Ca(OH)2(s) (IV) Compared to reaction R3 in aqueous phase, the same reaction in the solid phase (reaction IV) does not seem to require acidic conditions. Nonetheless, according to its stoichiometry, it seems to be promoted by the presence of adsorbed water. This explanation also supports the rising trend of the nitrate/nitrite ratios with the RH values that is shown in Figure 5. Regarding the positive effect of NO2 on SO2 uptake, this influence could be, in a first attempt, ascribed to reaction II, which might be shifted by the formation of sulfate species, thus enhancing sulfur retention. However, by taking into account its stoichiometry and the results of SO2 average uptake outlined in Figure 2, this reaction is not expected to explain solely the extraordinary influence in its retention. For instance, a ratio of 0.74 mol SO2/mol calcium was observed after 2 h when the reaction was conducted at 333 K, 70% RH, 2000 ppm SO2, and 175 ppm NO2 (see Figure 2a). Because the ratio obtained under the same conditions in the absence of NO2 is 0.26 mol SO2/mol calcium, the difference (0.74 - 0.26 ) 0.48 mol SO2/mol calcium) should be at least ascribed to reaction II, which would be responsible for the increase in the SO2 retention observed. However, this postulation does not seem to be feasible, because it would lead to inconsistent values of calcium conversion (e.g., 0.26 mol CaSO3/mol calcium (reaction I) + 0.48 mol CaSO4/mol calcium (reaction II) + 0.48 mol CaNO3/mol calcium (reaction II) > 1.00 Ca!!). An alternative explanation to account for the increase of SO2 uptake could focus again on the hydrophilic character of nitrite and nitrate species, whose presence might cause an effect similar to an increase of the relative humidity in the gas flow. To assess this point, an experiment using Ca(NO3)2 as an additive of Ca(OH)2 (4 wt % of Ca(NO3)2 in the mixture) and SO2 as the only gas reactant was undertaken at 333 K, 2000 ppm SO2, 70% RH, and 2 h of reaction time. An average uptake of 0.50 mol SO2/mol calcium was observed, which is greater than the value observed under the same condi-

Figure 7. Influence of RH value on the parameters of the DMISCM. Conditions are as given in Figure 6. Parameters: (a) kS, P (b) β, and (c) D h Ca 2+.

tions without the additive (0.21 mol SO2/mol calcium). Furthermore, the enhancement of water adsorption due to the presence of nitrite and nitrate species is in agreement with the results reported by Goodman et al.,36 who determined adsorption isotherms of water vapor over CaCO3 crystals reacted with HNO3 from Fourier transform infrared (FTIR) spectra in the presence of water vapor. Their results revealed that the amount of water adsorbed on the surface of reacted crystals that contain some amounts of Ca(NO3)2 was sensibly higher than that adsorbed on the surface of raw CaCO3 crystals. 4.3. Fitting of Experimental SO2 Breakthrough Curves to the DM-ISCM Model. 4.3.1. Effect of Relative Humidity. The effect of the RH value on the experimental SO2 breakthrough curves at 333 K, 2000 ppm of SO2, and 175 ppm of NO2 is shown in Figure 6. The RH value strongly affects the shape of the breakthrough curves, which leads to both a higher reaction

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Figure 8. Experimental and fitted SO2 breakthrough curves to DM-ISCM at 333 K, 2000 ppm of SO2, and 60% RH for different NO2 concentrations. Maximum error is e10%.

rate and a higher SO2 retention (see Figure 1). The slope of all the curves at the conditions tested in Figure 6 is less steep than that of the curves obtained in the absence of NO2 already shown in our previous work.35 Consequently, the presence of NO2 enhances the capability of Ca(OH)2 to retain SO2 in the RH range under study. Furthermore, it should be stressed that the ratio CSO2/C°SO2 does not achieve values of 1 in all the curves, especially in those corresponding to the experiments performed at higher RH values (70% RH), which indicates that complete conversion has not been yet attained after 2 h of reaction. As can be also observed in Figure 6, a DM-ISCM provides excellent fittings to the experimental SO2 breakthrough curves both at shorter and longer times. Moreover, according to Figure 7, the parameters kS, β, and D h PCa are dependent on the relative humidity in a similar trend as that found in the absence of NO2.35 First, the rate constant (kS) is strongly enhanced at higher RH values when the desulfurization reaction occurs in the presence of 175 ppm of NO2, which might be ascribed to a greater amount of water adsorbed on the surface of the solid reagent due to the presence of nitrite and nitrate species. Moreover, the deactivation constant (β) shows a decreasing exponential trend with the RH value, which might reveal a role of surface diffusion of product crystallites on the reagent surface to build up a more clusterlike product with increasing RH, as was already suggested in our previous work35 for the same reaction in the absence of NO2. Eventually, the solid-state diffusion coefficient (D h PCa) is strongly enhanced with the RH value, because of a better stabilization of the migrating ions across the product layer to the outer surface. 4.3.2. Effect of NO2 Concentration. Figure 8 shows the effect of the NO2 concentration on the experimental SO2 breakthrough curves, together with their fittings to the DM-ISCM at 333 K, 2000 ppm of SO2, and 70% RH. As can be seen, the shape of the curves is positively affected by the NO2 concentration, which leads to both higher rates and higher SO2 retention (see Figure 2). The evolution of the parameters of the DM-ISCM with the NO2 concentration is shown in Figure 9. Parameter kS is slightly reduced with the NO2 concentration initially and then becomes approximately constant for NO2 concentrations of >75 ppm. Although the retention of small amounts of NO2 might increase the hydrophilic

Figure 9. Influence of NO2 concentration on the parameters of the DM-ISCM. Conditions are as given in Figure 8. Parameters: P (a) kS, (b) β, and (c) D h Ca 2+.

character of the surface of the solid reagent, which might, in principle, promote the hydration of SO2, the competition between both SO2 and NO2 species might result in a reduction of the kinetics of SO2 hydration. In addition, the trend of parameter β with the NO2 concentration is also decreasing initially, which might be again ascribed to the formation of a more clusterlike product on the solid surface at the RH value tested. This trend in parameter β might partially compensate the decreasing trend in the kinetics of SO2 hydration reflected in parameter kS. As a result, the kinetics of the process would be enhanced due to the action of NO2. Finally, parameter D h PCa, which governs the kinetics of the process at long times, is apparently strongly enhanced with NO2 concentration, again because of the hygroscopic character of nitrite and nitrate species.

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an increase in the Ca2+ solid-state diffusion coefficient (D h PCa) when NO2 is present, which could be ascribed to an increase of water adsorption due to the formation of calcium nitrite and nitrate. Surprisingly, the rate constant (kS) decreases when NO2 is present, which could be explained by the competition between both SO2 and NO2 species for the adsorption on the solid surface. It should be emphasized that the interpretation of the parameters is subjective and limited to the scale at which the study has been surveyed. Direct experimental evidence to elucidate the reaction mechanism requires the use of techniques with higher resolution (e.g., atomic force microscopy). Nomenclature

Figure 10. Experimental and fitted SO2 breakthrough curves to DM-ISCM at 333 K, 2000 ppm of SO2, 175 ppm of NO2, and 60% RH for different O2 concentrations. Maximum error is e10%.

4.3.3. Effect of O2 Concentration. The effect of O2 on the shape of the SO2 breakthrough curves is depicted in Figure 10. The shape of the curves is slightly affected by the presence of 5 vol % of O2, and, beyond this value, no shape changes are observed, which accounts for the independence of SO2 retention by Ca(OH)2 reagent on O2 concentrations of >5 vol %, as already noted in Figure 3. The partial oxidation of sulfite to sulfate species on the solid surface does not seem to either increase the value of parameters kS and D h PCa significantly nor reduce the value of parameter β, which would explain the slight enhancement of both the reaction rates and SO2 retention. 5. Conclusions This study confirms that the reaction between SO2 and hydrated lime at low temperature in the presence of water vapor is greatly enhanced by NO2. Moreover, the solid analyses sustain that the surface chemistry of sulfur retention by Ca(OH)2 in the presence of NO2 is governed by the same set of reactions that occur in aqueous solution, which have been rewritten to account for their solid-state nature (reactions I-IV). According to reactions III and IV, the formation of calcium nitrite and nitrate highly hygroscopic species could explain the higher SO2 uptake when NO2 is present, because they promote water adsorption. Furthermore, the product that is obtained actually consists of a mixture of CaSO3(s) and CaSO4(s), the latter being generated from a partial oxidation of CaSO3(s), according to the redox reaction II. When both NO2 and O2 are present simultaneously, CaSO4(s) is the only product obtained by the complete oxidation of sulfites. Finally, although the presence of NO2 could improve the efficiency of the induct scrubbing (IDS) processes to retain SO2, the progress of the disproportion reaction of Ca(NO2)2(s) (reaction IV) could deter the use of these technologies for the simultaneous removal of SO2 and NOx with a prior oxidation of NO to NO2, because NO is formed as a product of the reaction. The kinetics of the desulfurization reaction in the presence of NO2 has been successfully modeled by means of a deactivation model implemented with an inverse shrinking-core model (DM-ISCM). This model predicts a reduction of the deactivation constant (β) and

Cg ) concentration of the gas in the bed [mol dm-3] CRCa ) Ca2+ concentration in the crystalline lattice of Ca(OH)2 [mol dm-3] P D h Ca2+ ) solid-state diffusion coefficient of Ca2+ ions [m2/s] DM-ISCM ) deactivation model coupled with an inverse shrinking-core model kS ) rate constant [s-1] MS ) molecular weight [g/mol] R ) mean particle radius [m] XS ) solid conversion Greek Letters β ) deactivation constant [dm3 mol-1 s-1] L ) porosity of the bed F ) density [kg/m3] ν ) speed of the gas in the reactor [m/s] Subscripts g ) gas s ) solid Superscript 0 ) inlet of the bed

Acknowledgment The authors gratefully acknowledge financial support from Spanish CICYT (Projects AMB94-0200 and QUI980361). The authors would also like to thank Commercial and Industrial Ciaries, S.A., who kindly provided the solid sorbent that was used in this study. Literature Cited (1) De Nevers, N. Ingenierı´a de control de la contaminacio´ n del aire; McGraw-Hill Interamericana Editores, S. A. de C. V.: Me´xico, 1998. (2) Brown, C. Pick the best acid-gas emission controls for your plant. Chem. Eng. Prog. 1998, 63. (3) Buonicore, A. J., Davis, W. A., Eds. Air Pollution Engineering Manual; Van Nostrand Reinhold: New York, 1992. (4) Muzio, L. J.; Offen, G. R. Dry Sorbent Emission Control Technologies. Part 1: Fundamental Processes. JAPCA 1987, 37, 642. (5) Golesworthy, T. A review of industrial flue gas cleaning: Part 3. Filtr. Sep. 1999, 36 (6), 16. (6) Dahlin, R.; Vann Busch, P.; Snyder, T. Fundamental Mechanism in flue gas conditioning. Literature review and assembly of theories on the interactions of ash and FGD sorbents. Prepared by Southern Research Institute, Birmingham, AL, for the U.S. Department of Energy of Pittsburgh Energy Technology Center, January 1992. (7) Peterson, J. R.; Durhan, M. D.; Vlachos, N. S. Fundamental Investigation of Duct/ESP Phenomena. Prepared by Radian Corporation, Austin, TX, for the U.S. Department of Energy of Pittsburgh Energy Technology Center, May 1989.

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(23) Zamansky, V. M.; Ho, L.; Maly, P. M.; Seeker, W. R. Oxidation of NO to NO2 by Hydrogen Peroxide and its Mixtures with Methanol in Natural Gas and Coal Combustion Gases. Combust. Sci. Technol. 1996, 120, 255. (24) Chien, T. W.; Chu, H. Removal of SO2 and NO from flue gas by wet scrubbing using an aqueous NaClO2 solution. J. Hazard. Mater. 2000, 80, 43. (25) Shi, Y.; Littlejohn, D.; Chang, S. G. Kinetics of NO Absorption in Aqueous Iron(II) Bis(2,3-dimercapto-1-propanesulfonate) Solutions Using a Stirred Reactor. Ind. Eng. Chem. Res. 1996, 35, 1668. (26) Shriver, D. F.; Atkins, P. W.; Langford, C. H. Inorganic Chemistry, 2nd Edition; Oxford University Press: Oxford, U.K., 1994. (27) Kobayashi, H.; Takezawa, N.; Niki, T. Removal of Nitrogen Oxides with Aqueous Solutions of Inorganic and Organic Reagents. Environ. Sci. Technol. 1977, 11, 190. (28) Littlejohn, D.; Wang, Y.; Chang, S. H. Oxidation of Aqueous Sulfite Ion by Nitrogen Dioxide. Environ. Sci. Technol. 1993, 27, 2162. (29) Takeuchi, H.; Ando, M.; Kizawa, N. Absorption of Nitrogen Oxides in Aqueous Sodium Sulfite and Bisulfite Solutions. Ind. Eng. Chem. Process Des. Dev. 1977, 16, 303. (30) Cliffton, C. L.; Alstein, N.; Hule, R. E. Rate Constant for the Reaction of NO2 with Sulfur(IV) over the pH Range 5.3-13. Environ. Sci. Technol. 1988, 22, 586. (31) Zamansky, V. M.; Ho, L.; Maly, R. M.; Seeker, W. R. Presented at the 3rd International Conference on Combustion Technologies for a Clean Environment, Lisbon, Portugal, 1995. (32) Deister, U.; Warneck, P. Photooxidation of SO32- in Aqueous Solution. J. Phys. Chem. 1990, 94, 2191. (33) Eriksen, T. E. pH Effects on the Pulse Radiolysis of Deoxygenated Aqueous Solutions of Sulfur Dioxide. J. Chem. Soc. Faraday Trans. 1 1974, 70, 208. (34) Waygood, S. J.; Mc. Elroy, W. J. Spectroscopy and Decay Kinetics of the Sulfite Radical Anion in Aqueous Solution. J. Chem. Soc., Faraday Trans. 1992, 88, 1525. (35) Bausach, M.; Pera-Titus, M.; Fite´, C.; Cunill, F.; Izquierdo, J. F.; Tejero, J.; Iborra, M. Kinetic Modeling of the Reaction between Hydrated Lime and SO2 at Low Temperature. AIChE J. 2005, 51, 1455. (36) Goodman, A. L.; Underwood, G. M.; Grassian, V. H. A laboratory study of the heterogeneous reaction of HNO3 and calcium carbonate particles. J. Geophys. Res. 2000, 105, 29053.

Received for review June 15, 2005 Revised manuscript received September 9, 2005 Accepted September 15, 2005 IE050713W