Ind. Eng. Chem. Res. 2008, 47, 3827–3833
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KINETICS, CATALYSIS, AND REACTION ENGINEERING Nitrogen Oxides Absorption on Calcium Hydroxide at Low Temperature Xiaowen Zhang,† Huiling Tong,* Hu Zhang, and Changhe Chen Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Tsinghua UniVersity, Beijing 100084, China
Solid Ca(OH)2 was used to absorb NO and NO2 with water vapor present in the flue gas. Nitrogen oxides were captured as nitrite and nitrate, and part of NO could be released into the gas as the HNO2 decomposed, which was produced in the absorption process. A mathematical model was founded to predict the process of NOx absorption and nitrite and nitrate production. With the overall kinetic parameters (OKPs) evaluated by the typical experimental result at 70 °C and 60% relative humidity, the model can simulate the experimental results at various conditions quantitatively. 1. Introduction Sulfur dioxide and nitrogen oxides, released from coal-fired power plants, could result in serious environmental problems, so the emission has been legislated to be reduced by most countries. There are many approaches to control the SO2 and NOx emission, and the method of wet flue gas desulfurization (WFGD) to remove SO2 plus selective catalytic reduction (SCR) to remove NOx is the most common solution. In the case of old existing power plants, to meet the emission requirements, a semidry scrubbing process with calcium-based absorbents at low temperature is considered as a possible route to control SO2 emission because of its lower cost. NOx in the flue gas from a typical coal-fired power plant consists of about 90% NO, which is relatively insoluble and hardly reacts with alkaline absorbents. Lyon et al.1 proposed a possible strategy to oxidize the NO into NO2 by addition of methanol and other hydrocarbons in the flue gas at an optimum temperature range. Zhang and co-workers4,5 found that NO could be oxidized by additives of KMnO4 and removed by calcium-based absorbents and indicated that NO can be removed only in the form of NO2 at low temperature. Some researchers7,8,12,14 found that SO2 removal in the semidry scrubbing process with calcium-based absorbents would be enhanced by NO2 in the flue gas, and Bausach et al.8 thought that this enhancement was due to the fact that NO2 could inhibit the SO2 absorption rate decrease. Nelli and Rochelle7 found that the higher SO2 concentration in the flue gas could result in higher NO2 removal and lower NO secondary release. To study the NOx removal mechanism or the influence of NO2 on the semidry SO2 removal process, the absorption process of NO and NO2 on the alkaline surface must be investigated first as an important step. The reaction of NO and NO2 with alkaline solution was studied in the nitric acid manufacturing industry for decades. The reaction and control steps for the NO2 absorption process were presented in the literature.2,3 At present, there are few studies about the NO2 absorption by the alkaline solids under low-temperature conditions. Nelli and Rochelle6 investigated the effect of factors, such as temperature, relative humidity, etc., on the NO2 absorption * Corresponding author. Tel.: +86-10-62788668. Fax: +86-1062770209. E-mail:
[email protected]. † E-mail:
[email protected].
process by the sorbents and set up a model to describe it. An assumption was made in the model that accumulation of HNO3 on the sand surface led to the reaction rate decrease and was the key factor to influence the NO2 absorption process. However, a constant absorption rate on the alkaline surface was predicted by their model even though the absorption rate changed considerably in some of their experiments. Furthermore, their model did not predict the secondary release of NO in their experiments. Bausach et al.8 showed that the pore structure change and plugging was the crucial factor limiting calcium utilization and led to the reaction rate decreasing rapidly at low temperature in the SO2 removal process, while Nelli and Rochelle’s6 work indicated that pore structure change was much less at similar conditions in the NO2 removal process. This meant that the pore structure did not play such an important role for NO2 removal as that for SO2 removal. Furthermore, the NO2 removal reaction consists of more complicated steps than the SO2 removal. The reaction between NO2 and water can produce HNO2 or HNO3; however, HNO2 is unstable and can decompose and release NO or NO2 again. There are multiple products in the NO2 absorption, which represent the corresponding steps in the reaction. So the model developed in the SO2 removal process cannot be applied directly in the NO2 removal process. It is necessary to establish a model that can explain the NO2 capture mechanism. In this research, NO2 diluted with nitrogen was absorbed by hydrated lime in a packed-bed reactor, varying the reaction conditions, such as NO2 concentration, relative humidity, and mass of absorbent. Some conclusions from NO2 absorption in the aqueous solution were applied to help set up the model of the NO2 absorption by the solid alkaline under the semidry condition. 2. Experimental Method The experiments were conducted in a packed-bed reactor under atmospheric pressure (shown in Figure 1). The detailed experimental methods were described in the literature.4,5 The NO2 composition was supplied by high-pressure cylinders with N2 as balance, and the flow rate was regulated by mass flowmeter controller. Part of N2 flowed through a water humidifier (shown in Figure 1) to achieve the required relative humidity (RH) of the simulated flue gas by controlling the temperature of the humidifier. The experiments were carried
10.1021/ie070660d CCC: $40.75 2008 American Chemical Society Published on Web 04/29/2008
3828 Ind. Eng. Chem. Res., Vol. 47, No. 11, 2008
3.2. Reaction Mechanism Analysis. Suchak and Joshi3 and Patwardhan and Joshi11 concluded the main reaction steps of NO2 absorption in the aqueous solution are as follows:
Figure 1. Schematic chart of NO2 removal experiments. Table 1. Characteristics of the Absorbent
quartz sand Ca(OH)2
average diameter/µm
specific surface area/m2/g
3.6 × 102 3.2
0 6.5
out at a constant total volume flow rate of 2.167 × 10-5 m3/s. Over 20 min of prehumidification proceeded before the removal experiments were started to ensure a stable humidity in the reactor. The outflow gas from the reactor was divided into two streams, one of which flowed into the gas analyzer and the other of which went into the atmosphere after a wet-scrubber. A bypass line across the reactor allowed the synthetic flue gas to stabilize before flowing into the reactor. All the tubes for water vapor transport were wrapped with heating tapes (temperature ) 110 °C) to prevent condensation. Water in the flue gas out of the reactor was removed by a cold trap before going into the gas analyzer (shown in Figure 1). After the experiments were finished, the reaction product was dissolved into the deionized water and an ion chromatography (IC) analyzer was used to measure the amounts of nitrite and nitrate in the products. The sorbents physical characteristics are listed in Table 1. A Thermal Electron model 42C chemiluminescent NO-NO2-NOx analyzer was used to detect the NO and NO2 concentration in the flue gas, and a Dionex DX-120 IC analyzer was applied to measure the amount of NO2- and NO3- anion in the product. The particle size of the absorbent was measured by the Malvern Mastersizer 2000 analyzer and specific surface area by the Micrometritics ASAP 2010 Brunauer–Emmett–Teller (BET) analyzer. The errors of gas flowmeters, gas analyzer, and IC analyzer were less than 2%, 3%, and 5% in the experiments, respectively. 3. Experimental Results and Analysis 3.1. Typical Experimental Results. A typical experiment was carried out at 70 °C and 60% relative humidity with 20 g of quartz sand and 2.0 g of Ca(OH)2 in the reactor and 214 ppm of NO2 and 23 ppm of NO in the inlet gas. Figures 2 and 3 show the outlet NO and NO2 concentrations and the amounts of calcium nitrite and nitrate in the products at the typical experiment. The rapid decrease of NO2 in the beginning was due to the analyzer response switching from bypass line to the reactor. The NO decrease at the first 3 min showed that a little NO could be absorbed by the calcium alkaline. Then both NO2 and NO increased with time; this meant that the NO2 absorption rate decreased and more NO was released at the same time. The results in Figure 3 showed that the amount of nitrite and nitrate in the products were almost equal during the initial 11 min, and then nitrate in the products increased more rapidly than nitrite. This meant that most of the NO2 was absorbed as nitrate by Ca(OH)2 at the end of the experiment.
2NO2 T N2O4
(1)
NO + NO2 T N2O3
(2)
N2O4 + H2O T HNO2 + HNO3
(3)
2HNO2 T N2O3 + H2O
(4)
The reactions, shown in eqs 1-4, can take place both in the gas and in the liquid. In alkaline or even dilute acidic solutions, most of the nitric acid is present in the dissociated form, and the undissociated HNO3 in the aqueous phase can be negligible.11 So the HNO3 produced by eq 3 can be considered being totally dissociated as in eq 5. HNO3 f H+ + NO3
(5)
Most researchers believed that there was no undissociated HNO3 existing in the solution, so the reverse reaction shown in eq 3 will not take place in the NO2 absorption process. The accumulating and decomposing of undissociated HNO2 shown in eq 4 is the key step affecting the NO2 absorption rate and leading to the release of NO.10,11 In this research, NO2 was absorbed by Ca(OH)2 with water vapor present in the flue gas. It was considered that water vapor absorbed on the solid surface would form a “liquidlike” thin film9 and some reaction and mass transfer steps in the solution could be applied to this research. The possible reaction paths are shown in Figure 4 according to eqs 1-4. Since the solubilities of NO and NO2 were low (shown in Table 2), the concentrations of NO and NO2 in the liquid phase were low, so the reaction rates of NO and NO2 to produce N2O3 and N2O4 in the liquid phase are lower than those in the gas phase, and this means that the reactions producing N2O3 and N2O4 mainly happen in the gas phase. Because the partial pressure of water vapor was relatively low (1.87 × 104 Pa at typical conditions) and the reaction was second order in the gas phase, only a small proportion of N2O3 and N2O4 could react with water vapor to produce HNO2 and HNO3 in the gas phase. Most N2O4 and N2O3 would be absorbed in the liquid film and produce HNO2 and HNO3 through the reaction with water, which was first order in liquid, with HNO3 immediately dissociating to H+ and NO3-. If the alkaline absorbent was dissolved in the liquid film fast enough, all of the HNO3 and
Figure 2. Original data of a typical experiment from flue gas analyzer.
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Figure 3. Product composition variation with time for NO2 for removal at typical conditions.
HNO2 would react with OH-. In the case that the liquid film was converted to be acidic, most of the HNO2 produced would decompose and then release NO and NO2. The results shown in Figures 2 and 3 could be well-explained according to the above description of the absorption steps. Although it was proved in the experiments that NO could hardly be absorbed by alkaline absorbents,4 NO could be absorbed together with excess NO2 according to eq 2 and reverse eq 4, so the reason was clear that the NO concentration decreased at the first 3 min. If HNO2 absorbed in the liquid film decomposed completely, it corresponded to every 3 mol of NO2 being absorbed with 1 mol of NO being released according to eqs 1-4, but the NO produced did not approach one-third of the amount of NO2 reduced until the experiment was finished. This meant that only part of the HNO2 in the liquid film was decomposed during the experiment, and the rest was absorbed by the absorbents. The analysis above was verified by the products detection results, which showed that nitrite in the products increased slowly with time. Therefore, it could be inferred that the HNO2 accumulation and decomposition in the liquid film were the key steps in the NO2 removal process. 4. Mathematical Model The liquid-phase hydrolysis reaction rate of N2O3 and N2O4 is much higher than the mass transfer rate of the N2O3 and N2O4 into the liquid film.10,11 When the mass transfer of the species J is accompanied by an irreversible, first-order chemical reaction that goes to completion within the liquid film, the overall rate of absorption of these species is enhanced by chemical reaction and can be expressed by the following:13 r ) HJ√(kJDJ)pJ ) OKPJ pJ
(6)
In eq 6, J stands for N2O3, N2O4 species. An overall kinetic parameter (OKP) is used as the substitute of HkD. Assuming that the equilibrium of the reactions shown in eqs 1 and 2 is rapid, the partial pressures of N2O3 and N2O4 are expressed by the following: pN2O4 ) K1pNO22
(7)
pN2O3 ) K2pNO2pNO
(8)
Because the HNO2 produced at the beginning of the experiment did not decompose, and it would decompose completely after the liquid film converted to be acidic, the process of NO2 absorbed on the local alkaline surface could be divided into two stages as follows. I. The local liquid film is alkaline at the beginning, so all HNO2 could be dissociated and react with absorbent into nitrite without NO release. The overall reaction equation is
Figure 4. Absorption paths of NO and NO2. 2NO2 + 2OH- f NO2 + NO3 + H2O
(9)
If the NO component exists in the gas phase, it can also be absorbed by the alkaline solid. The NO absorption reaction equation can be expressed by eq 10: NO2 + NO + 2OH- f 2NO2 + H2O
(10)
The NO2 absorption rate is expressed by eq 11: rNO2 ) 2OKPN2O4K1pNO22 + OKPN2O3K2 pNO2 pNO
(11)
The corresponding NO absorption rate expression is shown in eq 12: rNO ) OKPN2O3K2 pNO2 pNO
(12)
II. If the local liquid film is converted to be acidic, it is assumed that all the HNO2 produced will decompose. The overall reaction equation here is eq 13: 4NO2 + H2O f 2H+ + 2NO3 + NO + NO2
(13)
NO2 absorption rate is expressed by eq 14: 3 · 2OKPN2O4K1pNO22 (14) 4 NO will not be absorbed under this condition, but the HNO2 will decompose and release NO; the NO production rate is expressed by eq 15: rNO2 )
1 · 2OKPN2O4K1pNO22 (15) 4 The minus sign here means that NO is released. To apply these expressions into a packed-bed reactor, the bed is divided into N parts along the height. When the simulated flue gas flow passes through the ith part, the NO2 and NO compositions will vary. The scheme is shown in Figure 5. In Figure 5, V stands for volume flow rate of the gas. Xi and ∆S stand for conversion and total effective surface area of the absorbent in the ith part, respectively. In this paper, N was set as 200. The change of NO2 and NO concentration through the ith part sorbents is expressed in eqs 16 and 17: rNO ) -
3830 Ind. Eng. Chem. Res., Vol. 47, No. 11, 2008 Table 3. Model Parameters
Table 2. Chemical Constants solubility-Henry’s constants/kmol/m3 · Pa
parameters
-8
1.84 × 10 1.18 × 10-7 5.92 × 10-6 1.38 × 10-5 equilibrium constants 10(2993/T-9.226)/1.013 × 105 Pa-1 10(2072/T-7.234)/1.013 × 105 Pa-1
NO NO2 N2O3 N2O4 K1 K2
cNO,i+1 - cNO,i ) -rNO
Table 4. Coefficients Corresponding to Various Relative Humidities relative humidity
(16)
∆S V
(17)
∆S is calculated by ∆S )
1 1 · S ) · msorbssorb N N
(18)
S stands for the total surface area of the absorbent and is calculated by the product of the absorbent mass (msorb) multiplied by the absorbent specific surface area (ssorb). The conversion Xi for the ith part of absorbent at time t is defined by eq 19: Xi )
0.5 × total moles of NOx absorbed in ith part
)
initial moles of Ca(OH)2in ith part T
∑ (c t)0
NO2,i+1 + cNO,i+1 - cNO2,i - cNO,i)V∆t
2msorb ⁄ 74
1.5 × 10 mol/(m2 · s · Pa) 1.5 × 10-5 mol/(m2 · s · Pa) 0.08%
OKPN2O4 OKPN2O3 Xcrit
coefficients
∆S cNO2,i+1 - cNO2,i ) -rNO2 V
values -5
(19)
∆t is time step and is set as 60 s in the model. The molar mass (g/mol) of Ca(OH)2 is 74. To distinguish the two stages described above for every local part of absorbent in the reactor, a critical conversion Xcrit is defined. When the conversion of local absorbent is less than the critical value, the local liquid film is regarded as alkaline and is calculated according to that at stage I, and the reaction rate can be expressed by eqs 11 and 12. While the conversion increases beyond the critical point, the local liquid film is considered as acidic and is calculated as that at stage II, and the rate expressions of eqs 14 and 15 are adopted.
very low
30%
60%
70%
0.8
0.5
1
1.9
quartz sand and 0.5 g of Ca(OH)2 in the reactor. The purpose was to investigate the effect of NO2 concentration on the process. The kinetic parameters, OKPN2O4 and OKPN2O3, used in the model (shown in Tables 3 and 4) were determined by the initial absorption rate of the typical experiment (shown in Figure 2), in which the mass of Ca(OH)2 is 2.0 g. The Xcrit was determined according to NO emission. These parameters above were also used to calculate all the rest of the cases at 70 °C and 60% relative humidity in the paper. It is shown in Figure 6 that the simulation results can match the experiment at various NO2 concentrations with one set of parameters; the NO2 absorption rate is also second order with respect to NO2 as expressed in eqs 11 and 15 with low NO concentration under this condition, so the assumptions in the mathematic model above are considered to be reasonable. The lower prediction of the NO release in the beginning may be due to the effect of the sand mixed with Ca(OH)2 in the reactor, which is relatively remarkable when the mass of absorbents is reduced from 2.0 g in the typical experiment to 0.5 g in the experiment shown in Figure 6. A fraction of NO2 will be absorbed in the liquid film on the sand, and then NO is
5. Comparison of Simulated and Experimental Results 5.1. Dependence on NO2 Concentration. Experiments were carried out at 70 °C and 60% relative humidity with 20 g of
Figure 5. Modeling of reaction in a fixed-bed reactor.
Figure 6. Dependence of NO2 and NO variation at reactor outlet with time on NO2 concentration of the coming gas.
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Figure 7. Products prediction by model at typical conditions.
released immediately since the liquid film on the sand does not have any alkaline matter and cannot capture it. A similar phenomenon was also observed in Nelli and Rochelle’s research.6 While NO2 is absorbed into the liquid film on the sand to saturation and nothing can be absorbed any more, no more NO will release from the sand surface. So the prediction of NO and NO2 concentrations at the outlet are more accurate in the latter stage without sand’s effect. 5.2. Nitrite and Nitrate in Products. The model established can predict not only the changes in the gas phase but also the amounts of nitrite and nitrate in the products. The nitrite amount is calculated by the total amount of N2O3 absorbed plus the half-of N2O4 absorbed, and the nitrate amount equals the other half-of N2O4 absorbed according to the chemical stoichiometry. The products prediction at the typical conditions (shown in Figure 3) by the model is shown in Figure 7. By comparing Figure 3 and Figure 7, it can be found that the model can predict the products composition qualitatively. The percentage of calcium nitrite decreases with reaction time just as the experimental results revealed. But the percentage of nitrite predicted is less than the IC detected, and the possible reason is that the ideal assumption for stages I and II is used to simulate the real gradual reaction process. Even if the reaction proceeds for a relative long time, there will be still a little HNO2 undecomposed, which can form calcium nitrite and accumulate in the products. 5.3. Physical Mechanism of Critical Conversion. In this research, a critical value of conversion is defined to describe the liquid film on the alkaline surface changing from alkaline to acidic. This parameter characterizes the capability to keep up the alkalinity of the liquid film on the absorbent. Using the same model parameters, even the same value of Xcrit, we simulated the processes with different masses of Ca(OH)2 (shown in Figure 8) in the reactor. When the absorbent includes more Ca(OH)2, the experiment and the model gave the same result with a longer transition period. Nelli and Rochelle6 gave results of much longer transition time because of the Ca(OH)2 with a larger specific area that was adopted. So the transition time can be influenced by the total alkaline surface area provided by the absorbent. 5.4. Dependence of NO2 and NO Variation on Relative Humidity. The dependence of relative humidity on the reaction process is shown in Figure 9, and “very low” means that the N2 flow does not pass the humidifier in the experiment, but because of the high water absorptive capability of Ca(OH)2, on the surface of Ca(OH)2, there is a possible thin water film; the vapor partial pressure in the gas phase is approximately 0 under this “very low” condition. Nelli and Rochelle’s experiments6 showed that the reaction rate decreased with relative humidity
Figure 8. Comparison of calculated and experimental results with different amounts of Ca(OH)2 absorbent.
Figure 9. Dependence of NO2 and NO variation on relative humidity.
rise from “very low” to 48% at 25 °C. However, a different result at 70 °C is gained in our work such that the NO2
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Figure 10. Calculated results at different temperatures.
absorption rate decreases with relative humidity increasing from “very low” to 30% and then increases with relative humidity rising from 30% to 90%. The results can be explained by the reaction mechanism. On the one hand, the thickness of the liquid film on the absorbent is determined by relative humidity; if the thickness of the liquid film increases, more pores of the absorbent will be filled with water film and the effective surface area for reaction will decrease because of liquid bridge force. On the other hand, the higher relative humidity, especially at higher temperature, corresponds to higher water vapor partial pressure in the gas. For example, 28% water vapor by volume in the gas phase at 70 °C means 90% relative humidity. The higher humidity will lead to more HNO2 and HNO3 being produced in the gas phase, and HNO2 and HNO3 with larger solubility will be absorbed more quickly than the N2O3 and N2O4, the main species through which NO and NO2 were absorbed in the liquid film. So the overall absorption rate also goes up with humidity rising. In the meantime, the decrease of surface area will have a negative effect on the reaction rate according to eqs 16 and 17 in the model, so these two effects above can result in the optimal point. It can be calculated by varying the OKP values in the model. The experiments at various relative humidities are simulated with OKP multiplied by different coefficients. The calculated results can simulate the experimental results qualitatively. Considering the assumptions applied in the model, the results are acceptable. 5.5. Dependence of NO2 Absorption on Temperature. In previous research,2 the effects of temperature on the reaction process are concluded as follows: (1) The decrease of gas solubility will reduce the NOx absorption rate with temperature rise. (2) The overall absorption reaction is enhanced with temperature rise because the elementary reaction rate coefficients increase. (3) The equilibrium of eqs 1 and 2 can be influenced by temperature. Higher temperature leads to lower concentration of N2O3 and N2O4 in the liquid film and lower absorption rate. As described above, the three parameters of relative humidity, volume fraction of water vapor in the gas phase, and temperature all influence the absorption rate, but the three parameters are combined, so we can not vary one of them with the other two unchanged. So the experiments at various temperatures were not conducted. In the model, the influence of temperature on reaction equilibrium is considered by equilibrium constant expressions including temperature effect (shown in Table 2). When temperature was changed from 70 to 25 °C, the calculated absorption rate was doubled in Figure 10. These
Figure 11. Initial absorption rate variation with temperature predicted by model.
calculated values match Nelli and Rochelle’s experimental results6 measured at 70 and 25 °C with similar relative humidity. Therefore, the effect of temperature on the reaction equilibrium may be the main factor to the absorption process. The reaction rate variation with the temperature can be predicted by the model, as shown in Figure 11. Because of the difficulty of the experiment mentioned above, only one experimental result is shown in this graph. The initial absorption rate is calculated by the amount of the absorbed NO2 at the beginning of the experiment multiplied by the volume flow rate set as 2.167 × 10-5 m3/s. It is shown that the initial absorption rate will decrease with the temperature rise from 25 to 70 °C. 6. Conclusion The mechanism of NO and NO2 reacting with alkaline solid is studied with water vapor present in the flue gas at low temperature in the paper. The reaction rate was second order with respect to NO2 under the condition. The product consists of both calcium nitrite and nitrate, and the percentage of nitrite decreases with reaction time. More Ca(OH)2 content in the absorbent provides more reactive surface area, enhancing the reaction rate and delaying NO release. At 70 °C, the reaction rate decreases with relative humidity increasing from “very low” to 30% but increases with relative humidity rise from 30% to 90%. On the basis of the mechanism analysis, a model was set up to predict the NOx absorption process. The model can predict the experimental results quantitatively by one set of parameters with various Ca(OH)2 masses and NO2 concentrations at the same temperature and relative humidity. The prediction results at various relative humidities match the experimental results qualitatively by introducing an alterable coefficient, and the calculated results at different temperatures are consistent qualitatively with those in the literature. The key steps of the NO2 absorption are the accumulation and decomposition of the HNO2 produced on the pore surface of solids. The effective means to improve the absorption process are to keep up the alkalinity on the absorbent surface to reduce the decomposition of HNO2 and prevent NO from release. Acknowledgment This research was funded by the National Basic Research Program of China (No. 2006CB200301). Supporting Information Available: Details of the RH control method (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.
Ind. Eng. Chem. Res., Vol. 47, No. 11, 2008 3833
Nomenclature c ) gas concentration, mol/m K ) equilibrium constant, Pa-1 msorb ) mass of Ca(OH)2 in the absorbent, g N ) number of parts divided OKP ) overall kinetic parameter, mol/m2 · s · Pa p ) pressure, Pa r ) reaction rate, mol/m2 · s ssorb ) specific surface area of Ca(OH)2, m2/g S ) surface area, m2 V ) volume flow rate, m3/s X ) conversion Xcrit ) critical conversion 3
Literature Cited (1) Lyon, R. K.; Cole, J. A.; Kramlich, J. C.; Chen, S. L. The Selective Reduction of SO3 to SO2 and the Oxidation of NO to NO2 by Methanol. Combust. Flame 1990, 81, 30. (2) 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. (3) Suchak, N. J.; Joshi, J. B. Simulation and Optimization of NOx Absorption System in Nitric Acid Manufacture. AIChE J. 1994, 40, 944. (4) Zhang, H.; Tong, H.; Wang, S. Simultaneous removal of SO2 and NO from flue gas with calcium-based sorbent at low temperature. Ind. Eng. Chem. Res. 2006, 45, 6099.
(5) Zhang, H. Experimental Research on Simultaneous Removal of NOx and SO2 by Calcium Sorbent. Ph. D. Dissertation, Tsinghua University, Beijing, China, 2007. (6) Nelli, C. H.; Rochelle, G. T. Nitrogen Dioxide Reaction with Alkaline Solid. Ind. Eng. Chem. Res. 1996, 35, 999. (7) 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. (8) Bausach, M.; Pera-Titus, M.; Fité, 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. (9) Bausach, M.; Pera-Titus, M.; Fité, C. Water-Induced Rearrangement of Ca(OH)2 (0001) Surfaces Reacted with SO2. AIChE J. 2006, 52, 2876. (10) Thomas, D.; Vanderschuren, J. Analysis and prediction of the liquid phase composition for the aborption of nitrogen oxides into aqueous solutions. Sep. Purif. Technol. 2002, 18, 37. (11) Patwardhan, J. A.; Joshi, J. B. Unified Model for NOx Absorption in Aqueous Alkaline and Dilute Acide Solutions. AIChE J. 2003, 49, 2728. (12) O’Dowd, W. J.; Markussen, J. M.; Pennline, H. W. Characterization of NO2 and SO2 Removals in a Spray Dryer/Baghouse System. Ind. Eng. Chem. Res. 1994, 33, 2749. (13) Cussler, E. L. Diffusion: Mass Transfer in Fluid Systems,2nd ed.; Cambridge University Press: New York, 2000. (14) 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.
ReceiVed for reView May 9, 2007 ReVised manuscript receiVed February 19, 2008 Accepted February 26, 2008 IE070660D
