Kinetics of the Reaction of Hydrated Lime with SO2 at Low

Nov 13, 2008 - The effects of the presence of CO2, O2, and NOx in the flue gas on the ... When both O2 and NOx were present, sulfation of hydrated lim...
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Ind. Eng. Chem. Res. 2008, 47, 9878–9881

Kinetics of the Reaction of Hydrated Lime with SO2 at Low Temperatures: Effects of the Presence of CO2, O2, and NOx Chiung-Fang Liu† and Shin-Min Shih*,‡ Energy and EnVironment Research Laboratories, Industrial Technology Research Institute, Hsinchu, Taiwan, 310 and Department of Chemical Engineering, National Taiwan UniVersity, Taipei, Taiwan 106

The effects of the presence of CO2, O2, and NOx in the flue gas on the kinetics of the sulfation of hydrated lime at low temperatures were studied using a differential fixed-bed reactor. When O2 and NOx were not present together the reaction kinetics was about the same as that under gas mixtures containing SO2, H2O, and N2 only. When both O2 and NOx were present, sulfation of hydrated lime was greatly enhanced, forming a large amount of calcium sulfate in addition to calcium sulfite. Sulfation of hydrated lime was well described by the surface coverage model, despite the gas-phase conditions being different. Relative humidity is the major factor affecting the reaction, and its effect was more marked when both O2 and NOx were present. The kinetic model equations obtained in this work can be used to describe the sulfation of hydrated lime in the low-temperature dry and semidry flue gas desulfurization processes with or without an upstream NOx removal unit. Introduction The low-temperature spraying (semidry) and dry flue gas desulfurization (FGD) processes are effective means of reducing SO2 from coal-fired power plants. Hydrated lime is commonly used as the sorbent in these processes. Kinetic analysis of the reaction of hydrated lime with SO2 in the gas mixtures composed of SO2, H2O, and N2 at low temperatures has been reported in our previous work.1,2 However, in practice, besides SO2, H2O, and N2, the flue gas from a coal-fired boiler contains CO2, O2, and NOx,3 which may affect the reaction of hydrated lime with SO2.4 The influences of the latter gaseous species on the sulfation of hydrated lime at low temperatures have been studied by many investigators. Some investigators focused on the dry scrubbing technology for combined SO2 and NOx removal,5,6 while some focused on the effects of individual gas species. 7-14 Liu and Shih4 reviewed the results reported in the literature and performed experiments to study the reactions of hydrated lime with gas mixtures containing these gaseous species and SO2 at their typical concentrations in the flue gas. They found that the ultimate extent of sulfation of hydrated lime increased only a little when O2 and NOx were not present together in the gas mixture and increased markedly when O2 and NOx or CO2, O2, and NOx were present together in the gas mixture. Therefore, the kinetics of the sulfation of hydrated lime with the presence of O2 and NOx would be different from that without their presence. However, kinetic analysis of the sulfation of hydrated lime has been conducted mostly without the presence of O2/NOx1 and scarcely with the presence of O2/NOx.6 Because of the lack of prior research and the importance to the design and operation of FGD processes, it is worthwhile to study the kinetics of the sulfation of hydrated lime with CO2, O2, and NOx present. In this work, the effects of the presence of CO2, O2, and NOx in different combinations on the kinetics of the sulfation of hydrated lime at low temperatures were studied. The * To whom correspondence should be addressed. Tel.: 886-223633974. Fax: 886-2-23623040. E-mail: [email protected]. † Industrial Technology Research Institute. ‡ National Taiwan University.

kinetic model equations appropriate to describe the sulfation of hydrated lime under different flue gas conditions were derived. Experimental Section The hydrated lime used was reagent-grade Ca(OH)2 (purity > 95%, Hayashi Pure Chemical Industries). Its mean particle diameter, specific surface area (Sg0), and solid weight per mole of Ca (M) were determined to be 6.0 µm, 10.0 m2/g, and 75 g/mol, respectively. The same differential fixed-bed reactor and experimental procedure as those employed in our previous work4 were used for the sulfation tests. About 40 mg of hydrated lime was used for each run. The CO2, O2, NOx, SO2, and N2 gases supplied from cylinders and the H2O vapor from a water evaporator were mixed to form the simulated flue gas. The NOx gas consisted of NO (>97%) and NO2. The composition of the gas mixture was adjusted by controlling the flow rate of each component. The CO2, O2, NOx, and SO2 concentrations were controlled at 12.6%, 5%, 600 ppm, and 1000-3000 ppm, respectively, which are their typical concentrations in the flue gas evolved from a coal-fired boiler; the relative humidity (30-70%) and temperature (60-80 °C) of the gas mixture were in the ranges typical in the bag filters of a semidry FGD system.3 The conversion, X, due to sulfation for a reacted sample was determined by the molar ratio of the amount of total sulfur to that of Ca contained in the reacted sample. When the sample was sulfated without O2 and/or NOx present, the amount of total sulfur was that of sulfite determined by iodimetric titration of the sample dissolved in acid. When the sample was sulfated with O2 and/or NOx present, the amounts of total sulfur and sulfite were determined by ion chromatography (IC) and iodimetric titration, respectively. H2O2 was added to the sample solution to oxidize any S(IV) to S(VI) before injecting it into the IC. The amount of sulfate was obtained by subtracting the amount of sulfite from that of total sulfur. The amount of Ca was determined by EDTA titration. From the above measure-

10.1021/ie801105s CCC: $40.75  2008 American Chemical Society Published on Web 11/13/2008

Ind. Eng. Chem. Res., Vol. 47, No. 24, 2008 9879

Figure 1. Conversion versus time for reaction of Ca(OH)2 at 60 °C, 70% RH, 1000 ppm SO2, and various CO2, NOx, and O2 concentrations.

Figure 3. Conversion versus time for reaction of Ca(OH)2: (a) at 60 °C, 1000 ppm SO2, 12.6% CO2, 5% O2, 600 ppm NOx, and various relative humidities; (b) at 80 °C and 1000 ppm SO2 and 60 °C and 3000 ppm SO2 with 70% RH, 12.6% CO2, 5% O2, and 600 ppm NOx. Table 1. Results of Hydrated Lime Reacted at 60 °C, 70% RH, 1000 ppm SO2, and Various CO2, NOx, and O2 Concentrations for 1 h

Figure 2. Conversion versus time for reaction of Ca(OH)2 at 12.6% CO2, 5% O2, and various other reaction conditions.

ments the fractions of calcium sulfite (XS1) and sulfate (XS2) in the reacted sample were calculated. Results and Discussion Effect of Gas Composition. Figures 1, 2, and 3 show the conversion (X) versus reaction time (t) data for hydrated lime reacted under various experimental conditions. In spite of the experimental conditions being different, the experimental results for each case exhibit the same reaction pattern that the reaction is rapid in the initial period but stops after about 15 min, leaving the sorbent incompletely converted. This reaction behavior is typical for reactions of solids which lead to formation of an impervious products layer.15 The effect of gas composition on the sulfation reaction of hydrated lime was assessed by comparing the experimental results with those obtained without the presence of CO2, O2, and NOx and taking into account the experimental errors (about 0.02 in X). As can be seen from Figure 1, the reaction was unaffected by the presence of CO2 and slightly enhanced by the presence of O2 or NOx or CO2/O2 in the gas mixture when O2 and NOx were not present together, whereas the reaction was greatly enhanced when CO2, O2, and NOx were present together.

NOx, ppm

CO2, %

O 2, %

XS1

XS2

X

0 0 0 0 600 600 600

0 12.6 0 12.6 0 0 12.6

0 0 5 5 0 5 5

0.22 0.22 0.20 0.22 0.20 0.28 0.30

0 0 0.02 0.04 0.04 0.24 0.32

0.22 0.22 0.22 0.26 0.24 0.52 0.62

The enhancement effect due to the simultaneous presence of O2 and NOx at different relative humidities, temperatures, and SO2 concentrations can be readily observed by comparing Figure 2 with Figure 3. Moreover, Figures 2 and 3 show that the reaction was significantly enhanced by increasing the relative humidity but affected little by the reaction temperature and SO2 concentration. Similar effects have been reported for the case without CO2, O2, and NOx.1,16 The conversions and the mole fractions of sulfite and sulfate for samples which had reacted with various gas mixtures for 1 h are listed in Table 1. In experiments with O2 and/or NOx present, sulfate was formed in addition to sulfite. When O2 and NOx were not present together, the fractions of sulfate (0.02-0.04) were very small and fractions of sulfite (0.20-0.22) were about the same as that obtained without O2 and NOx. However, when both O2 and NOx were present, both fractions of sulfite and sulfate were large and even larger when CO2 was also present. For instance, the fractions of sulfite and sulfate were 0.30 and 0.32, respectively, in the sample which had reacted at 60 °C,

9880 Ind. Eng. Chem. Res., Vol. 47, No. 24, 2008 Table 2. Comparison of the Experimental and Calculated 1 h Conversions for Sulfation of Ca(OH)2 with 12.6% CO2 and 5% O2 Present X (calcd) RH (%)

T (K)

SO2 (ppm)

X (exp)

ref 1

ref 4

ref 19

70 50 30 70 70

333 333 333 333 353

1000 1000 1000 3000 1000

0.26 0.18 0.12 0.27 0.25

0.21 0.14 0.05 0.25 0.26

0.20 0.14 0.08 0.20 0.20

0.27 0.20 0.12 0.27 0.27

70% RH, and 1000 ppm SO2 for 1 h with the presence of 12.6% CO2, 5% O2, and 600 ppm NOx. Kinetic Analysis. The reaction behavior observed in Figures 1, 2, and 3 is the same as that predicted by the surface coverage model proposed by Shih et al.15 X ) [1-exp(-k1k2t)] ⁄ k2

(1)

where k1 and k2 are functions of sorbent properties and reaction conditions. This model has been reported to describe the sulfation of hydrated lime,1,2 fly ash/hydrated lime sorbents,17,18 and iron blast furnace slag/hydrated lime sorbents19 in gas mixtures containing SO2, H2O, and N2 only. The results of sulfation tests indicate that whether O2 and NOx are present together or not in the gas phase would significantly affect the values of k1 and k2 in eq 1. Therefore, the equations representing k1 and k2 as functions of reaction conditions were obtained by analyzing the results for the following two cases: sulfation with CO2 and O2 present and sulfation with CO2, O2, and NOx present. Sulfation with CO2 and O2 Present. As mentioned above, the presence of one or two gaseous species of CO2, O2, and NOx affected the sulfation of hydrated lime slightly as long as O2 and NOx were not present together in the simulated flue gas. Therefore, the equations for k1 and k2-1 obtained in our previous studies1,18,19 for reaction of calcium-based sorbents under gas mixtures of SO2, H2O, and N2 were tested for validity in this case. As shown in Table 2, the equations proposed in our previous reports all gave 1 h conversion values close to the experimental measurements, but the values predicted by the equations in ref 19 agree better with the experimental values. The equations in ref 19 are k1 ) 0.00779Sg0e0.0124RHe-9500⁄RTy0.31

(2)

-1 0.96 k-1 2 ) 0.0344Sg0M RH

(3)

where RH is the relative humidity (%), R is the gas constant (J/mol K), T is the reaction temperature (K), and y is the SO2 concentration (ppm). The curves in Figure 2 were plotted using eqs 1-3. One can see that the model predictions agree satisfactorily with the experimental data; the average deviation of the experimental X value from the prediction was about 0.03, which is very close to the experimental error in X, 0.02. The comparisons between the predictions and the data shown in Figure 2 indicate that the effect of relative humidity on the sulfation reaction did not change due to the presence of CO2 and O2. However, the slight effects of SO2 concentration and temperature measured when CO2 and O2 were absent were not observed in the presence of CO2 and O2. Nevertheless, eqs 1-3 are valid for describing the kinetics of the sulfation of hydrated lime with CO2 and O2 present together in the simulated flue gas.

Figure 4. Comparison of the experimental conversions and values calculated using eqs 1, 4, and 5 for sulfation of Ca(OH)2 with 12.6% CO2, 5% O2, and 600 ppm NOx present.

Sulfation with CO2, O2, and NOx Present. The X-t plots for hydrated lime sulfated under the simulated flue gases containing CO2, O2, and NOx in addition to SO2, H2O, and N2 are shown in Figure 3. The values of k1 and k2 were estimated by fitting eq 1 to the X-t data; the curves in Figure 3 are the best fitting curves. From the values of k1 and k2 obtained at different reaction conditions and the values of Sg0 and M for hydrated lime, the best equations representing k1 and k2 as functions of sorbent properties and reaction conditions were searched and were found to be k1 ) 0.00112Sg0e3.91RH

(4)

k2-1 ) 8.28Sg0M-1RH1.62

(5)

The conversions calculated using eqs 1, 4, and 5 are compared with the experimental values in Figure 4. The model predictions are in good agreement with the experimental values; the average deviation of the experimental value from the prediction was about 0.034. The effects of the concentrations of the gas components on the ultimate sulfation extents of hydrated lime were also studied by varying the SO2 concentration from 100 to 1000 ppm, NOx from 300 to 600 ppm, CO2 from 3.2% to 12.6%, and O2 from 1.0% to 5.4%, and negligible effects were found. Comparing eqs 4 and 5 with the model equations obtained without the presence of CO2, O2, and NOx, eqs 2 and 3, one can see that the effect of RH became more marked. It is well known that reactions of hydrated lime with the reactive gases at low temperatures proceed through reactions involving water adsorbed on the solid surface and relative humidity should be high enough for reaction of hydrated lime to be appreciable.1,16 The thickness of the water layer adsorbed on the solid surface increases with increasing relative humidity. As the water layer becomes thicker, more molecules of gas reactants can be absorbed into the water layer and greater quantities of acids, which can attack hydrated lime, will be produced. Therefore, the rate and extent of reaction of hydrated lime increase with increasing relative humidity. Furthermore, if a reaction product is a deliquescent salt, more water than that adsorbed may be collected and the reaction may be greatly enhanced. Liu and Shih4 pointed out that the great enhancement of the sulfation of Ca(OH)2 by the presence of O2/NOx in the gas mixture results from the rise in the NO2 concentration in the gas phase. When NOx (mostly NO) is mixed with O2, NO is oxidized to NO2 and the concentration of NO2 is thus raised. As NO2 is a much stronger oxidant and more soluble in water

Ind. Eng. Chem. Res., Vol. 47, No. 24, 2008 9881

than NO and O2, there are more NO2 molecules absorbed into the water layer adsorbed on the solid surface and oxidation of bisulfite and sulfite ions to sulfate ions is hence enhanced, which induces more SO2 molecules to be captured into the water layer. It has been reported that every mole of NO2 absorbed can lead to the oxidation of several moles of SO32- or HSO3- if O2 is present.6,20 Meanwhile, when NO and NO2 react with HSO3-, SO32-, and water, H+, NO2-, and NO3- are formed, leading to formation of Ca(NO2)2 and Ca(NO3)2. These salts are deliquescent; their deliquescence will collect a great quantity of water and thus enhance reaction of the sorbent with the reactive gases. Furthermore, the marked difference in X between 50 and 70% RH (Figure 3) may indicate that Ca(NO2)2 or Ca(NO3)2 started to deliquesce in that range of RH at 60 °C. Also, the greater effect of RH on the reaction is thought to be due to the deliquescence of these salts at RH > 50%. Practical Implications. The above results reveal the fact that the presence of H2O, CO2, O2, and NOx with SO2 in the flue gas has a positive effect on the SO2 removal efficiencies and calcium utilization for the low-temperature dry and semidry FGD processes which use hydrated lime as the sorbent. Because SO2 and NOx are air pollutants they have to be removed from the flue gas in order to reduce their concentrations to meet the emission standards. According to the findings of this work, removing SO2 before removing NOx can take the advantage of the enhancement effect of O2/NOx on sulfation of the sorbent. In practice, depending on the design of the air pollution control system the flue gas entering an FGD unit may or may not have been treated with a NOx removal unit. For practical application of the kinetic model proposed, eqs 1, 2, and 3 and eqs 1, 4, and 5 are applicable to describe the sulfation of hydrated lime in cases with and without an upstream NOx removal unit, respectively. Conclusion The presence of CO2, O2, and NOx with SO2 in the gas phase at their typical concentrations in the flue gas had slight effects on the sulfation of hydrated lime if O2 and NOx were not present together. The reaction was mainly affected by the RH. The reaction kinetics can be well described by the surface coverage model and model equations derived previously for the reaction under the gas mixtures containing SO2, H2O, and N2 only. When both O2 and NOx were present, sulfation of hydrated lime was greatly enhanced and a large amount of sulfate was formed in addition to sulfite. The surface coverage model is still valid in this case, but the model equations obtained show a more marked effect of RH and negligible effects of SO2 concentration and temperature. The presence of H2O, CO2, O2, and NOx in the flue gas is beneficial to the SO2 capture in the low-temperature dry and semidry FGD processes which use hydrated lime as the sorbent. The model equations obtained in this work can be used to describe the sulfation of hydrated lime in the FGD processes either with or without an upstream NOx removal unit. Abbreviations k1 ) parameter defined by eq 1, min-1 k2 ) parameter defined by eq 2, dimensionless M ) initial weight of sorbent per mole of Ca, g/mol Ca R ) gas constant, 8.314 J/mol/K RH ) relative humidity, % Sg0 ) initial specific surface area, m2/g T ) reaction temperature, K

T ) reaction temperature, K X ) conversion, dimensionless XS1 ) mole fraction of calcium sulfite, dimensionless XS2 ) mole fraction of calcium sulfate, dimensionless y ) SO2 concentration, ppm

Acknowledgment This research was supported by the National Science Council, Taiwan. Literature Cited (1) Ho, C. S.; Shih, S. M.; Liu, C. F.; Chu, H. M.; Lee, C. D. Kinetics of the Sulfation of Ca(OH)2 at Low Temperatures. Ind. Eng. Chem. Res. 2002, 41 (14), 3357–3364. (2) Liu, C. F.; Shih, S. M. A Surface Coverage Model for the Reaction of Ca(OH)2 with SO2 at Low Temperatures. J. Chin. Inst. Chem. Eng. 2002, 33 (4), 407–413. (3) Slack, A. V.; Hollinden, G. A. Sulfur Dioxide RemoVal from Waste Gases; Noyes Data Corp.: Park Ridge, NJ, 1975. (4) Liu, C. F.; Shih, S. M. Effects of Flue Gas Components on the Reaction of Ca(OH) 2 with SO2. Ind. Eng. Chem. Res. 2006, 45, 8765– 8769. (5) Chu, P.; Rochelle, G. T. Removal of SO2 and NOX from Stack Gas by Reaction with Calcium Hydroxide Solids. JAPCA 1989, 39 (2), 175– 179. (6) Nelli, C. H.; Rochelle, G. T. Simultaneous Sulfur Dioxide and Nitrogen Dioxide Removal by Calcium Hydroxide and Calcium Silicate Solids. J. Air Waste Manage. Assoc. 1998, 48, 819–828. (7) Ishizuka, T.; Kabashima, H.; Yamaguchi, T.; Tanabe, K.; Hattori, H. Initial Step of Flue Gas Desulfurizations-An IR Study of the Reaction of SO2 with NOX on CaO. EnViron. Sci. Technol. 2000, 34, 2799–2803. (8) Klingspor, J.; Stromberg, A.; Karlsson, H. T.; Bjerle, I. Similarities between Lime and Limestone in Wet-dry Scrubbing. Chem. Eng. Process. 1984, 18, 239–247. (9) Seeker, W. R.; Chen, S. L.; Kramlich, J. C.; Greene, S. B.; Overmoe, B. J. Fundamental Studies of Low-Temperature Sulfur Capture by Dry Calcite Sorbent Injection. Proceedings of the Joint Symposium on Dry SO2 and Simultaneous SO2/NOX Control Technology, Raleigh, NC, 1986. (10) Moyeda, D. K.; Newton, G. H.; La Fond, J. F.; Payne, R. ; Kramlich, J. C. Rate Controlling Processes and Enhancement Strategies in Humidification for Duct SO2 Capture; EPA/600/2.88/0.47, Order PB882459615; U.S. Government Printing Office: Washington, D.C., 1988. (11) Irabien, A.; Cortabitarte, F.; Viguri, J.; Ortiz, M. I. Kinetic Model for Desulfurization at Low-Temperature Using Calcium Hydroxide. Chem. Eng. Sci. 1990, 45, 3427–3433. (12) Ho, C. S.; Shih, S. M. Effect of O2 on the Reaction of Ca(OH) 2 with SO2. J. Chin. Inst. Chem. Eng. 1992, 24, 405–411. (13) Ho, C. S.; Shih, S. M.; Lee, C. D. Influence of CO2 and O2 on the Reaction of Ca(OH) 2 under Spraying-Drying Flue Gas Desulfurization Conditions. Ind. Eng. Chem. Res. 1996, 35 (11), 3915–3919. (14) Liu, C. F.; Shih, S. M. Study on the Absorption of CO2 from Flue Gas by Hydrated Lime. Proceedings Symposium on Transport Phenomena and Applications, Taipei, Taiwan, 2000. 627–630. (15) Shih, S. M.; Ho, C. S.; Song, Y. S.; Lin, J. P. Kinetics of the Reaction of Ca(OH)2 with CO2 at Low Temperatures. Ind. Eng. Chem. Res. 1999, 38, 1316–1322. (16) Ho, C. S.; Shih, S. M. Factors Influencing the Reaction of Ca(OH)2 with SO2. J. Chin. Inst. Chem. Eng. 1993, 24, 187–195. (17) Liu, C. F.; Shih, S. M.; Lin, R. B. Kinetics of the Reactions of Ca(OH)2/Fly Ash Sorbents with SO2 at Low Temperatures. Chem. Eng. Sci. 2002, 57, 93–104. (18) Liu, C. F.; Shih, S. M.; Lin, R. B. Kinetic Model for the Reaction of Ca(OH)2/Fly Ash with SO2 at Low Temperatures. Ind. Eng. Chem. Res. 2004, 43, 4112–4117. (19) Liu, C. F.; Shih, S. M. Kinetics of the Reaction of Iron Blast Furnace Slag/Hydrated Lime Sorbents with SO2 at Low Temperatures: effects of sorbent preparation conditions. Chem. Eng. Sci. 2004, 59, 1001–1008. (20) Littlejohn, D.; Wang, Y.; Chang, S. G. Oxidation of Aqueous Sulfite Ion by Nitrogen Dioxide. EnViron. Sci. Technol. 1993, 27, 2162–2167.

ReceiVed for reView July 17, 2008 ReVised manuscript receiVed September 21, 2008 Accepted September 22, 2008 IE801105S