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A Study of the Effect of Nitrogen Dioxide on the Absorption of Sulfur Dioxide in Wet Flue Gas Cleaning Processes M. A. Siddiqi* and J. Petersen Department of Thermodynamics, Gerhard-Mercator-University Duisburg, Lotharstrasse 1, D-47048 Duisburg, Germany
K. Lucas Chair of Technical Thermodynamics, RWTH Aachen, Schinkelstrasse 8, D-52062 Aachen, Germany
The effect of nitrogen dioxide on the physicochemical processes which take place during the absorption of sulfur dioxide in aqueous solution under conditions similar to those of a wet flue gas cleaning process has been studied. The study is done at 298.65, 318.45, and 333.35 K and ambient pressure. The gas phase and the liquid phase could be specified completely. The results show that after a few hours NO2 vanishes from the gas phase. This leads to a reproducible frozen (stable) state in which tetravalent sulfur exists in phase and chemical equilibrium. The simultaneous absorption of SO2 and NO2 in water depends on their initial concentrations and the temperature and is governed by the kinetics of a number of concurrent and consecutive reactions in the liquid phase. The gas phase contains SO2, N2O, and NO. The liquid phase contains a number of nitrogen-sulfur compounds besides tetravalent and hexavalent sulfur and nitrate. A scheme for the reactions that can take place in aqueous solution has been proposed. The analysis results together with the known thermodynamic and kinetic data from the literature have been used to develop a model for the description of the SO2/NO2/N2/H2O system. The model has been used to predict not only the stable state composition at various temperatures but also the time dependence of the system composition. The agreement between the experimental results and model calculations is quite good. The effects of temperature and the initial concentration of the gas mixture on the absorption behavior of SO2 are discussed. Introduction Oxides of sulfur and nitrogen are emitted in large quantities from fossil-fueled electric power plants and incineration plants. The removal of these oxides mostly takes place via a wet scrubbing process. For the design and the optimization of the flue gas scrubbers, a proper description of the physical and chemical processes responsible for the absorption of the components involved is needed. To describe these processes in a thermodynamically consistent way, a knowledge of the phase behavior of the components and the specification of the liquid phase are the prerequisite. We have undertaken a systematic study of sulfur dioxide absorption. Recent studies from this laboratory reported the absorption of sulfur dioxide (SO2) in aqueous solutions under different conditions, e.g., in the presence of hydrogen chloride (HCl), sulfuric acid1 (H2SO4), calcium chloride2 (CaCl2), or mercury3 (Hg). For this purpose an apparatus has been developed. The phase and chemical equilibrium is determined by analyzing spectrophotometrically the gaseous phase and the liquid phase. The use of the fiber-optic technique allows the in situ analysis of the gas and liquid phases. The direct interaction of SO2 and NOx leads to the formation of a series of mixed nitrogen-sulfur compounds that can have a significant influence on the absorption of sulfur dioxide.4 The oxides of S(IV) and * To whom correspondence should be addressed. Telephone: +49-203-379-3353. Fax: +49-203-379-1594. E-mail:
[email protected].
N(II-IV) undergo a series of concurrent and consecutive chemical reactions in aqueous solution. This leads to the formation of a series of mixed nitrogen-sulfur compounds which are believed to inhibit the oxidation of sulfite in aqueous solutions.5 The experimental work was directed to analyze and quantify all possible compounds which may be formed during the process to obtain a better understanding of the physicochemical processes which accompany the wet flue gas desulfurization. The initial concentrations of SO2 in the gas mixture are pertinent to its concentration in flue gases and varied from 2000 mg/mN3 ()volume fraction of 700 ppm) to 10 000 mg/mN3 ()volume fraction of 3500 ppm), and the concentration of NO2 varied from 100 mg/mN3 ()volume fraction of 49 ppm) to 3000 mg/mN3 ()volume fraction of 1462 ppm). The volume of the gas phase was 22.25 dm3 and the mass of liquid water 0.4 kg. The absorption process has been studied at 298.65, 318.45, and 333.35 K. A scheme for the reactions that can take place in aqueous solution has been proposed and used to develop a model for the SO2/NO2/N2/H2O system. The results of model calculations that give the concentration profile of species produced in this system as a function of time are presented. The effects of temperature and the initial concentration of the gas mixture are discussed. Experimental Section The measurements have been carried out using the apparatus described in previous papers.6,7 The whole apparatus was placed in an air thermostat to accomplish
10.1021/ie000815g CCC: $20.00 © 2001 American Chemical Society Published on Web 04/10/2001
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the measurements at elevated temperatures. The temperature within the thermostat did not vary more than (0.5 K. This was checked up to 333.35 K by measuring the temperature at different reference points within the thermostat with the help of precalibrated thermocouples. An ancillary part of the apparatus, not shown in the diagram, consisted of a Fourier transform infrared (FTIR) spectrometer from Bio-Rad Laboratories (Richmond, CA) equipped with a long-path gas cell. This assembly was used to measure the concentration of nitrous oxide (N2O) in the gas phase. Materials. The certified standard mixtures (sulfur dioxide + nitrogen and nitrogen dioxide + nitrogen) were supplied by Messer-Griesheim (Krefeld, Germany). The standard mixtures having the volume fractions of 10 000 ppm SO2, 1000 mg m-3 NO2, and 10 000 mg m-3 NO2 were used to prepare the desired gas mixtures. The sulfur dioxide in the standard mixtures had a mole purity x(SO2) g 0.9998 {impurities: x(CO2) e 30 × 10-6; x(H2O) e 50 × 10-6}. The nitrogen dioxide standard mixtures had a mole purity x(NO2) g 0.98 {impurities: x(CO2) e 30 × 10-6; x(H2O) e 50 × 10-6}. Pure nitrogen and the nitrogen in the standard mixtures had a purity x(N2) g 0.999 99 {impurities: x(O2) e 0.5 × 10-6; x(H2O) e 0.5 × 10-6}. Purified and deionized water, which had a conductivity 97%. The other nitrogen-sulfur compounds, viz., hydroxylaminetrisulfonic acid (HATS), hydroxylaminedisulfonic acid (HADS), hydroxylaminemonosulfonic acid (HAMS), aminetrisulfonic acid [nitrilotrisulfonic acid (NTS)], aminedisulfonic acid [imidodisulfonic acid (IDS)], and hydroxylamine-NO-disulfonic acid (HAODS), used for calibration purposes in the ion chromatography, were not available commercially. These were synthesized according to the methods given in the literature.8-11 All of the other chemicals used for the ion chromatography measurements were “pro-analysis”. All of the chemicals were dried before use. Procedure. The experimental procedure has already been described in previous papers,6,7 and so only a brief description of the modifications will be given here. The basic procedure for filling the gas mixtures is the same except that first SO2 standard gas mixture was filled to a precalculated pressure in the gas vessel, then the NO2 standard gas mixture was pressurized up to its precalculated pressure, and then the mixture was diluted with pure nitrogen up to the experimental pressure to obtain the sulfur dioxide + nitrogen dioxide + nitrogen mixture of the desired composition. The initial pressure in the system depended on the desired experimental temperature. It was chosen to furnish a total pressure of nearly 104 kPa after making allowance for the saturated vapor pressure of the liquid phase. It was kept at 101, 95, or 84 kPa respectively for studies at 298.15, 318.45, or 333.35 K. The initial concentrations of SO2 in the gas mixture are pertinent to its concentration in flue gases. The volume of the gas phase was 22.25 dm3 and the mass of liquid water 0.4 kg. After attainment of the experimental pressure and temperature, the mixture was brought into contact with water (about 0.4 kg; accurately weighed) in the reaction cell by opening the appropriate valves and starting a circulatory pump. The spectra of the gaseous phase and
of the liquid were taken by scanning through the wavelength range of 220-450 nm with the help of the respective spectrophotometer at different time intervals. It was found that after about 4 h all of the nitrogen dioxide disappeared from the gas phase (no absorption band in the 390-410 nm region). This stable state was considered as the state of quasi-equilibrium. After this state the gaseous phase and the liquid phase were analyzed in 30 min intervals. The gas-phase and liquidphase spectra were taken in the UV-vis region and evaluated6 to determine the composition of the gas phase. Only sulfur dioxide could be detected in the gas phase. The liquid-phase spectra were evaluated by taking the molar absorption coefficient for SO2(aq) from the literature6 at 260 and 276 nm. In this way the concentration of molecularly dissolved sulfur dioxide SO2(aq) could be determined. The samples of the liquid phase were taken from the bottom of the reaction cell and analyzed by ion chromatography for NO3-, NO2-, SO32-, and SO42- as well as for HAMS, HADS, HAODS, HAOMS, HATS, IDS, AS, and NTS separately. At the end of each experiment, the gas phase was sent through the long path gas cell in the FTIR spectrometer and the IR spectra were taken in the region 2180-2260 cm-1 for the analysis of nitrous oxide. The spectra were evaluated in the same manner as UV spectra using the multivariate analysis method described in detail in a previous publication. Before spectroscopic measurements were performed, a calibration was made by taking spectra of nitrous oxide + nitrogen gas mixtures of known compositions. The initial concentrations used were 2000 and 100, 200, and 500; 4000 and 200, 500, 1000, and 2000; 6000 and 500, 1000, 2000, and 3000; 8000 and 500, 1000, 2000, and 3000; 10 000 and 500, 1000, 2000, and 3000 mg/mN3 respectively for SO2 and NO2. Ion Chromatography. For the ion chromatographic determination, a Dionex DX-100 ion chromatograph equipped with a membrane suppressor and a conductivity detector was used. The sample injection loop had a volume of 10 µL. A standard anion-exchanger column Dionex IonPac AS9-SC was used for the determination of sulfate, sulfite, nitrite, and nitrate. The flow rate was kept constant at 2 mL/min. The eluent consisted of an aqueous solution of 1.6 mM sodium carbonate and 2.0 mM sodium bicarbonate. A standard cation-exchanger column Dionex IonPac CS12 was used for the determination of the ammonium ion. A 20.0 mM methanesulfonic acid solution was used as the eluent. The flow rate was 1.0 mL/min. For the determination of nitrogen-sulfur compounds (HAMS, HADS, HAODS, HAOMS, HATS, IDS, AS, and NTS), ion-pair chromatography was employed. The standard anion-exchanger column used was a Dionex IonPac NS1 (10 µm). Nitrate could also be determined with this column. The flow rate was kept constant at 1 mL/min. The eluent consisted of an aqueous solution of 1.6 mM sodium carbonate and 2.0 mM tetrabutylammonium hydroxide, to which different amounts of acetonitrile were added. The amount of acetonitrile was optimized for the separation and detection of various compounds. The most suitable concentration ratios of acetonitrile for the separation and analysis of various nitrogen-sulfur compounds were found to be 10 vol % acetonitrile for HAOMS, 15 vol % acetonitrile for HAMS, NO3-, and HADS, 22 vol % acetonitrile for HAODS, and 30 vol % acetonitrile for HATS, IDS, and NTS.
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The calibration curve for the conductivity detector was established by running the standard solutions through the chromatograph. Description of the System. The dissolution of SO2 in aqueous solution is a combined phase and chemical equilibrium and may be described in the low pH range (between 1 and 3) by the following reactions:6
SO2(g) T SO2(aq)
(1)
SO2(aq) + H2O(l) T HSO3- + H+
(2)
The absorption of SO2 in the presence of NO2 is a much more complex process. After the transfer of NO2 into the liquid phase, the following concurrent reactions may take place:12-15
2NO2(aq) + H2O(l) f HNO2(aq) + NO3- + H+ (3) 2NO2(aq) + HSO3- + H2O(l) f 2NO2- + SO42- + 3H+ (4) 2NO2(aq) + 2HSO3- f 2NO2- + S2O62- + 2H+ (5) 2NO2(aq) + 2HSO3- f 2NO2- + HON(SO3)22- + 1/2O2 + H+ (6) HADS Which of these reactions is responsible for the consumption of NO2 depends on the concentrations of the components, the temperature, and the pH of the solution. For example, reaction (4) is the main reaction in the case of very small NO2 concentrations (0.01 Pa) and at pH 614 whose rate increases with increasing pH.16 At high pH values and high temperatures, the production of dithionate according to reaction (5) is favored.13,15 In acidic solution, which is the case in the present investigation, SO2 exists mainly as HSO3- and interacts with NO2- produced in the above-mentioned reactions to form HADS according to the overall reaction17
NO2- + H+ + 2HSO3- f HON(SO3)22- + H2O(l) (7) HADS via the intermediate product nitrososulfonic acid (NSS)
NO2- + H+ + HSO3- f ONSO3- + H2O(l) NSS
(8)
ONSO3- + HSO3- f HON(SO3)22HADS
(9)
The experiments show that the nitrogen-sulfur compounds are found as stable species. This is due to kinetic restraints. HADS is the initial product which can undergo different reactions to produce diverse nitrogensulfur compounds under different conditions. A summary of the reactions that can take place in aqueous solution is shown in the reaction scheme5 given in Figure 1. Experimental Results Studies at 298.65 K. It is found that after 4 h practically all NO2 vanishes from the gas phase. The
Figure 1. Scheme for the formation of various nitrogen-sulfur compounds.
specification of the system has been undertaken after attaining this reproducible state. This state is marked by the fact that the mass transfer of NO2 from the gaseous phase to the liquid phase is completed. After the two phases are mixed for 4 h, the spectra in the liquid phase and in the gas phase were taken at half hour intervals. The liquid phase was analyzed additionally with the help of an ion chromatograph. The gas phase consists of SO2, N2O, and N2 besides a small amount of NO. The liquid phase consists of molecularly dissolved SO2, NO3-, HADS, and HAMS as well as tetravalent sulfur [S(IV)]{HSO3-} and hexavalent sulfur [S(VI)]{HSO4- + SO42-}. It may be remarked at this point that a small proportion (2-5%) of sulfur measured as S(VI) in solution results from the oxidation of HSO3through atmospheric oxygen during sampling and analysis with an ion chromatograph. A study of the concentration of these components during the time period between 4 and 6 h shows that the concentrations of SO2(g), NO3-, and S(IV) do not change during this period. The concentration of HADS decreases, whereas the concentrations of HAMS and S(VI) increase. A typical time dependence is shown in Figure 2. The concentrations of SO42-, NO3-, HADS, and HAMS were determined in two separate runs because different separation columns have to be used for their analysis. This was considered to be legitimate because the reproducibility of the measurements is very good. The variation of concentration with time may be interpreted by considering reaction (7), through which
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Figure 2. Time dependence of the concentrations of the components SO2(g), HAMS, HADS, S(IV), S(VI), and NO3- at 298.65 K for a gas mixture [cSO2(ini) ) 4000 mg/mN3 and cNO2(ini) ) 1000 mg/mN3].
HADS is produced. Because no NO2- was detected in the system, it is concluded that this reaction is completed within 4 h. The following hydrolysis of HADS to HAMS and HSO4- seems to be a slow process and could be measured.
HON(SO3)22- + H2O(l) f HONHSO3- + HSO4HADS HAMS (10) The observed concentration course of HADS, HAMS, and S(VI) is in accordance with the stoichiometry of the reaction. This quantitative agreement and the fact that an additional detailed chromatographic analysis of the liquid phase did not indicate the presence of S2O62- or any other nitrogen-sulfur compound except HADS and HAMS leads to the conclusion that the sulfonation of HADS and HAMS
HON(SO3)22- + HSO3- f N(SO3)33- + H2O(l) (11) HADS NTS HONHSO3- + HSO3- f HN(SO3)22- + H2O(l) (12) HAMS IDS and the hydrolysis of HAMS
HONHSO3- + H2O(l) f NH2OH + HSO4- (13) HAMS do not play any significant role for the description of the system at the conditions of the present study. This scheme is confirmed by performing a comprehensive ion chromatographic analysis. For this the retention times for all possible nitrogen-sulfur compounds and S2O62- were determined. The analysis of the liquid phase showed the presence of HADS, HAMS, NO3-, SO32-, and SO42- only. No other components could be detected [detection limit 2-3 mg L-1]. This is also consistent with the overall mass balance for sulfur. A total of 97-98% of the sulfur introduced in the system as SO2 gas could be found as SO2(g), S(IV), S(VI), HADS, and HAMS. The mass balance is satisfactory because the difference lies within experimental uncertainties or detection limits. In contrast to this, the mass balance
for nitrogen was not fulfilled. Only 65-75% of nitrogen introduced as NO2 was found in the solution. The other possible nitrogen compounds are NH2OH, NH4+, NO, N2, and N2O. The presence of NH2OH in any appreciable amount may be excluded from a study of the reactions for its formation and consumption given in the reaction scheme and the following considerations: first, that the components HAMS and HAOMS whose hydrolysis may lead to the formation of NH2OH are themselves very stable18 and, second, that the NH2OH formed should be further converted to AS, which could not be detected [even 1 mg L-1] in a long time experiment of 2 days. The ion chromatographic analysis of the liquid phase for cations yielded only negligible amounts of NH4+ [on the order of micrograms per liter]. N2O was measured in the gas phase with the help of FTIR spectroscopy. NO was also found in the gas phase. However, its measurement is associated with greater experimental uncertainties. Because of the overlapping of the sharp peak of NO in the UV region by a broader peak of SO2, the evaluation of small concentrations of NO (
