Article pubs.acs.org/IECR
Cite This: Ind. Eng. Chem. Res. 2018, 57, 14347−14354
Gas-Phase Chemistry of the NO−SO2−ClO2 System Applied to Flue Gas Cleaning Jakob Johansson,*,† Anette Heijnesson Hulteń ,‡ Sima Ajdari,† Pär Nilsson,‡ Marie Samuelsson,‡ Fredrik Normann,† and Klas Andersson† †
Department of Space, Earth and Environment, Chalmers University of Technology, 41296, Gothenburg, Sweden Akzo Nobel Pulp and Performance Chemicals, 44580, Bohus, Sweden
Ind. Eng. Chem. Res. 2018.57:14347-14354. Downloaded from pubs.acs.org by UNIV STRASBOURG on 11/04/18. For personal use only.
‡
ABSTRACT: The chemical interactions that occur between NO, SO2, and ClO2 are investigated. In focus is the oxidation of NO with gaseous ClO2 for simultaneous removal of NOx and SOx from combustion-derived flue gases. Laboratory-scale experiments were conducted to examine the conversion of NO to NO2, under the following conditions: temperature range of 100−180 °C; H2O concentrations in the range of 0%−25%; ClO2−NO molar ratios in the range of 0.2−0.6; and NO and SO2 flue gas concentrations in the ranges of 0− 250 ppm and 0−1000 ppm, respectively. The results show that NO is oxidized efficiently by ClO2, whereas the ClO2− SO2 reactions are insignificant. The water concentration had no effect on oxidation, and the temperature had a limited effect, within the investigated ranges. The outcomes favor the process economics of the studied application, since ClO2 consumption is negligible with respect to SO2 oxidation.
1. INTRODUCTION Research conducted over the last few decades has resulted in technologies that efficiently remove nitrogen and sulfur species from flue gases.1 However, the capital and operational costs associated with these processes are high, preventing implementation in those business areas and regions in which profit margins are narrow and competition is high and/or regulatory frameworks are less-developed.2 In business segments that are regionally based, such as heat and power generation, legislation functions as an instrument to force all the actors to clean their flue gases.3 In 2001, the European Union introduced a nonbinding legislation called the large combustion plant directive (LCPD), which defines the limits for air pollutants such as NOx and SOx for combustion plants with outputs larger than 50 MWth.4 An extension of the LCPD was issued in 2015 to include plants with an installed capacity in the range of 1−50 MWth.5 This thermal capacity range is heterogeneous and includes a wide variety of processes, which places higher demands on the cleaning system, for example in terms of flexibility and investment. Simultaneous control of NOx and SOx could be engineered so that is more compact and thus easier to retrofit and more affordable for the discussed applications. Simultaneous removal of NOx and SOx could be performed using wet scrubbing, which is a proven method for acid gas treatment. However, for NOx to be absorbed into an aqueous solution, NO has to be driven to a higher oxidation state, i.e., NO2 or higher.6 In this higher oxidation state, NOx can be absorbed and form HNO2 and HNO3 in the liquid phase.7 Oxidation of NO to NO2 by © 2018 American Chemical Society
oxygen is favored by low temperature and high pressure, although the reaction is relatively slow under atmospheric conditions. The correlation with pressure makes simultaneous absorption a promising technology for carbon capture and storage (CCS) systems in which the flue gas path is pressurized.8,9 However, the oxidation process may be forced to occur rapidly under atmospheric conditions through the addition of oxidation chemicals.10−13 This work focuses on ClO2 as an oxidizing agent. The chemistry of the oxides of chlorine has been scrutinized in a broad spectrum of fields. Early research was directed toward atmospheric interactions.14−18 The use of ClO2 as a bleaching/oxidizing agent also prompted research regarding interactions coupled to the pulp and paper industry19−21 and water treatment systems.22−24 In recent years, there has been interest in using ClO2 to oxidize NO to NO2 for flue gas treatment applications, often in the presence of SO2. The available data are from experiments involving gas-phase oxidation prior to an absorber13,25−27 and gas−liquid oxidation through the addition of a soluble oxidant to the scrubber solution.10,28−30 It has been shown in laboratory-scale experiments with a bubbling reactor that ClO2(aq) completely oxidizes NO to NO2 and gives a removal efficiency for NOx of at least 60%, with performance being heavily dependent upon Received: Revised: Accepted: Published: 14347
July 9, 2018 September 21, 2018 September 25, 2018 September 25, 2018 DOI: 10.1021/acs.iecr.8b03067 Ind. Eng. Chem. Res. 2018, 57, 14347−14354
Article
Industrial & Engineering Chemistry Research the process conditions.28 This study focuses on the oxidation of NO through the use of gaseous ClO2 (rather than aqueous ClO2) using an superior setup to that used in our previous study.13 Details of the gas-phase chemistry of this system are given in the next section. Using ClO2 gas offers additional possibilities, for instance in terms of the dosing position, and it might be preferable to aqueous ClO2 depending on the method of production. The use of gaseous ClO2 also has advantages over aqueous chlorine components in terms of lower consumption levels and increased rate of oxidation. Gaseous ClO2 is both hazardous and unstable if not handled properly.31 However, based on the experience from the pulp and paper industry, where ClO2 is and has been used for a long time, the hazards linked to both the generation and usage of ClO2 are manageable.32,33 Thus, the application is novel but the scale and technique for the generation and handling of ClO2 are well-established. The aims of the present work were to map the gas-phase chemistry of the NO−SO2−ClO2 system by determining its selectivity for NO oxidation, and to define in greater detail the product composition. The investigation on the viability of gaseous ClO2 for usage of NO oxidation is novel and the data obtained are intended to support process and model development in the future.
(1)
Cl + ClO2 → 2ClO
(2)
ClO + ClO → Cl 2 + O2
(3)
2Cl → Cl 2
(4)
Cl 2 + H 2O → 2HCl + O
(5)
NO + ClO → NO2 + Cl
(8)
NO2 + NO2 V N2O4
(9)
NO + NO2 V N2O3
(10)
3NO2 + H 2O(g) → 2HNO3(g) + NO
(11)
NO + NO2 + H 2O(g) → 2HNO2 (g)
(12)
The oxidation of SO2 to SO3 is favored under ambient conditions, as described by reaction 13, and in similarity to that of NO, oxidation by O2 is too slow to be of relevance for the intended application. However, unlike NO, the solubility of SO2 in H2O is relatively high and oxidation is not required for efficient absorption. Any reaction that occurs between ClO2 and SO2 is therefore otiose and undesirable in the discussed application. Demore and co-workers studied the ClO2−ClO reactions with SO2 and have shown that they are considerably slower than the corresponding reactions with NO.17 The most significant reaction between SO2 and ClO2−ClO, according to the work of Demore et al.,17 is between SO2 and ClO, as described by reaction 14. In studies of the NO−SO2−O3(g) system, the selectivity of O3 for NO was found to be high. As O3 has a higher oxidation potential than ClO2, similar or superior selectivity may be expected for ClO2.38 Previous results obtained with ClO2 gas have shown that this is the case, under dry conditions.13 In addition, it has been proposed in the literature that NO2 oxidizes SO2 to SO3 according to reaction 15. The rate of reaction 15 has been investigated by Armitage and Cullis39 for subatmospheric pressures (0.1−0.4 bar) and high temperatures (430°−920 °C). Extrapolation of the rate expression suggests that SO2 oxidation by NO2 is not significant under the conditions experienced by the flue gas trains in combustion systems, although this needs to be validated.
During combustion, the oxides of nitrogen are dominated by NO. As the temperature decreases, NO2 is thermodynamically favored over NO. Under atmospheric conditions, NO is completely oxidized, according to reaction 6, given a sufficient residence time. reaction 6, however, proceeds too slowly to have relevance for flue gas cleaning systems. The oxidation rate can be enhanced by increasing the pressure or by introducing an oxidizing agent that is more reactive than oxygen. As discussed, ClO2 is a powerful oxidizing agent that is suitable for the oxidation of NO to NO2 in a flue gas system through reactions 7 and 8. The rates of these individual reactions have been studied by several research groups.16,34,35 The reactions are all sufficiently rapid to be of relevance for the investigated application. Once NO2 is formed, it is in equilibrium with its dimer, dinitrogen tetroxide (N2O4), as in reaction 9, and it may also form dinitrogen trioxide (N2O3) in a reaction with NO, as in reaction 10. Both N2O4 and N2O3 are highly soluble in water.7 2NO + O2 V 2NO2
(7)
If water is present in the form of vapor, gaseous nitric (HNO3) and nitrous acid (HNO2) may form via reactions 11 and 12. Whether or not HNO3 is formed in the gas phase from NO2 and water vapor is a matter of debate in the literature.36,37
2. CHEMISTRY The behaviors of chlorine components in the atmosphere have been studied by several research groups.15,18 It was initially determined that ClO2 is a highly reactive molecule in redox reactions and decomposes to form chlorine gas, Cl2, through reactions 1−4 (as defined below). If Cl2 comes in contact with water it will react to form hydrochloric acid, HCl, according to reaction 5. In more recent years, the focus has changed to include flue gas cleaning conceptsan area in which the applied chemistry has not yet been fully analyzed. ClO2 + hν → Cl + O2
NO + ClO2 → NO2 + ClO
2SO2 + O2 V 2SO3
(13)
SO2 + ClO → SO3 + Cl
(14)
NO2 + SO2 V NO + SO3
(15)
In contact with a liquid phase, oxidation concomitant with absorption could occur for SO2 through reaction 16, resulting in sulfuric acid, H2SO4, which is dissolved in the liquid phase. This reaction, which has been studied by several research groups,10,28 is that comprising reaction 14 followed by absorption. If water is present as vapor, gaseous H2SO4 may form via reaction 17. 5SO2 + 2ClO2 + 6H 2O(l) → 5H 2SO4 (aq) + 2HCl(aq) (16)
SO3(g) + H 2O(g) → H 2SO4 (g)
(17)
3. EXPERIMENTAL SECTION Two experimental setups were used, one for the ClO2 experiments and one for the NO2−SO2 system.
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DOI: 10.1021/acs.iecr.8b03067 Ind. Eng. Chem. Res. 2018, 57, 14347−14354
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decreases. The total flow is maintained at a constant level by balancing the main N2 flow. The amount of chlorine dioxide is chosen so as to obtain a certain molar ratio of ClO2 to NO. The reactor, after the injection point, is composed of titanium to prevent interference by the surfaces and material deterioration resulting from exposure to ClO2. The residence time of the gas in the reactor is almost 2 s. After the reactor, the gas is transported via thermally controlled, Teflon-lined tubing to the FTIR instrument. The tubing is kept at 160 °C to avoid any acid condensation. This is a major improvement over the system used in our previous experiments13 where the gas passed a condensation trap before the gas analyzer, as that step introduced liquid-phase interactions and made the analysis of the gas-phase reactions less reliable. During the experiments, the mass balance of nitrogen was continuously monitored by measuring the levels of NO, NO2, N2O and HNO3 in the gas phase. Assuming that N2 is inert, all the tests were within ±10%, and >90% of the test runs were within ±5% with respect to the accuracy of the measurements. 3.2. Process Parameters. Experiments were performed using a wide range of flue gas compositions and operating conditions. The variables and their set values are presented in Table 1. The concentration of NO and SO2 have been set as to
3.1. NO−ClO2 System. The ClO2 experiments were carried out using a bench-scale flow reactor (1.5−5.0 NL/ min), as illustrated in Figure 1. The setup consists of a gas
Table 1. Investigated Parameters and Their Set Values
Figure 1. Experimental setup of the ClO2−NO−SO2 system. Upper panel: A photograph of the actual setup. Lower panel. A schematic of the setup. Matching colors are provided in the photography and schematic to distinguish the different process steps.
parameter
value
inlet SO2 concentration inlet NO concentration inlet O2 concentration inlet CO2 concentration humidifier H2O levels inlet N2 concentration reactor temperature ClO2/NO molar ratios
400 ppm 200 ppm 3% 10% 0%, 10%, 15%, 25% balance 160 °C 0.2, 0.4, 0.6
represent a flue gas from a coal fired power plant. SO2 should have a higher concentration than NO to favor the interactions between the two species and ClO2. The O2 concentration represents a flue gas generated with an air to fuel ratio of 1.15. The levels of H2O have been tested to evaluate the influence of H2O on the reactions as it is discussed in literature. The ClO2 to NO molar ratio has been varied to establish if the oxidation follows a linear trend or if there are any deviations. 3.3. Data Evaluation. The oxidation potential of ClO2 was evaluated at three molar ratios of ClO2 to NO: 0.2, 0.4, and 0.6. The ClO2 experiments were evaluated using four indicators: (1) NO conversion, which is the difference between inlet (i) and outlet (k) NO concentration from the ClO2 reactor:
mixing system, which simulates different flue gas compositions, a gas heating component, a ClO2 injection point, and a reactor for the oxidation process. After the reactor, the gas composition is analyzed using Fourier-transform infrared spectroscopy (FTIR). The FTIR instrument is a MKS MultiGas 2030, which is capable of measuring NO, NO2, HNO3, N2O, SO2, and HCl, as well as other less likely species for the system. Gases of interest that are not measured by the FTIR include Cl2 and O2 due to not being active in the IR spectra as well as HNO2. The gas mixing system, heating unit, injection point, and reactor are described in detail by Heijnesson Hultén et al.13 The simulated flue gas is obtained by mixing NO, SO2, CO2, N2, and O2. Each flow is set using mass flow controllers to obtain the desired composition. The CO2, O2, and N2 streams are mixed and passed through a humidifier to control the H2O content of the flue gas. After the humidifier, all the flows are mixed and transported to the heater. The heating is controlled by an oil heater, which provides a uniform reaction temperature (±1 °C) up to 300 °C. Chlorine dioxide is fed through stripping with N2 of aqueous ClO2 that is kept at constant temperature in a water bath. The concentration of ClO2 is continuously monitored by measuring the absorbance of the solution. The stripping is performed by passing a flow of N2 through the ClO2. The flow of N2 to the stripper is increased as the concentration of ClO2 solution
NOConversion =
i k − c NO c NO i c NO
(18)
(2) The formation ratio of NO2: NO2,Formation =
k c NO 2 i c NO
(19)
(3) The formation ratio of HNO3. HNO3,Formation = 14349
k c HNO 3 i c NO
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Industrial & Engineering Chemistry Research and
Table 2. Parameters and Their Corresponding Values Used during the Experiments with the SO2−NO2 System
(4) SO2 conversion, the definition of which is similar to that for NO conversion, SO2,Conversion =
k cSO 2 i cSO 2
(21)
which is used as a rough indicator of the interactions between SO2 and ClO2−ClO. 3.4. The NO2−SO2 System. The gas-phase reactions between SO2 and NO2 are much slower than the ClO2 reactions, so they are studied in a system that allows for higher inlet concentrations of NO2 and SO2 and longer residence times. The NO2−SO2 system is outlined in Figure 2.
parameter
value
inlet SO2 concentration inlet NO2 concentration reactor temperature reactor pressure gas flow residence time
10 vol % 10 vol % 100°−180 °C 1 bar 0.7 NL/min 4−8 min
4. RESULTS AND DISCUSSION 4.1. Oxidation of NO by ClO2. Figure 3 shows the measured (symbols) conversion rate of NO according to the
Figure 2. Experimental setup of the NO2−SO2 system.
A preheated mixture of 10 vol % SO2 and 10 vol % NO2 in N2 is introduced to the reactor. The reactor has a volume of 9.8 L, which gives a residence time of 4−8 min depending on reactor temperature. The relatively high concentrations and residence times were chosen to ensure detectable formation of NO and SO3. This was estimated by the extrapolation of the available kinetics for reaction 15 by Armitage and Cullis.39 The reactor temperature is controlled by an oil heater, rendering a uniform temperature (±1 °C) up to 200 °C. All the lines are heated Teflon-lined pipes of the same temperature as the reactor, so as to maintain a uniform temperature profile. The reactor is also Teflon-lined, to avoid any interactions with the gas. The inlet stream is kept dry, in order to isolate the SO2 −NO 2 interactions and avoid disruptive interactions due to condensation in the case of unknown cold spots. The inlet gas is controlled by a volumetric flow meter (Kytola Instruments) to give a flow of 0.7 NL/min. The outlet gas is analyzed for NO2, SO2, and NO with the FTIR instrument (Bomem MB 9100) and for SO3 using the KCl salt method. In the KCl salt method, the gas is led through tubes filled with KCl, which captures the SO3. The tubes are heated to 200 °C to avoid any condensation, and after the measurement the salt is dissolved in water and analyzed for sulfate ions using ion chromatography. The temperature of the gas is kept constant before and during mixing, to ensure a homogeneous reaction profile. This method used has been described in detail by Vainio et al.40 The full set of experiment parameters and conditions are shown in Table 2.
Figure 3. Conversion of NO (as defined by eq 18) after ClO2 injection at different humidity levels: (a) 0% H2O; (b) 10% H2O; (c) 15% H2O; and (d) 25% H2O. Measurements are indicated by circles and the theoretical dry conversion rate is shown with a solid line in each panel. The experimental conditions were listed in Table 1.
ClO2/NO ratio and inlet water concentration. Overall, the experiments showed good repeatability and consistency between runs. Complete oxidation of NO was achieved for a ClO2 ratio >0.6, which is comparable to the theoretical stoichiometries of 0.4 and 0.5 under wet and dry conditions, respectively [qv. reactions 7 and 8]. The oxidation essentially follows the linear theoretical conversion profile corresponding to the ClO2/NO ratio indicated by the solid lines in Figure 3. It should be noted that since the ClO2 is stripped from a water solution at 25 °C, water vapor is always present to some extent, which makes possible interference from reaction 12 for all the cases. On the basis of saturation at 25 °C, the amount of water will be between 0 and 30 ppm depending on the concentration of the ClO2 solution. The measured NO conversion rate was similar across the investigated range of inlet water concentrations, from no added H2O in Figure 3a to 25% H2O in Figure 3d. There is, thus, no apparent influence of H2O on the oxidation of NO. There are low levels of HCl measured when adding higher amounts of ClO2 (ratios >0.4). The remaining 14350
DOI: 10.1021/acs.iecr.8b03067 Ind. Eng. Chem. Res. 2018, 57, 14347−14354
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Industrial & Engineering Chemistry Research chlorine is assumed to be in form of Cl or Cl2, which cannot be measured by the FTIR. In contact with liquid water, the chlorine will form HCl (aq), adding to the acidity of the liquid. Figure 4 presents the rate of NO2 formation defined by eq 19. For rClO2 < 0.5, the amount of NO2 formed matches the
Figure 5. Outlet concentrations of HNO3 for experiments with ClO2 injection at different humidity levels: (a) 0% H2O; (b) 10% H2O; (c) 15% H2O; and (d) 25% H2O. The experimental conditions were according to Table 1.
Figure 4. NO2 formation (as defined by eq 19) after ClO2 injection at different humidity levels: (a) 0% H2O; (b) 10% H2O; (c) 15% H2O; and (d) 25% H2O. Measurements are indicated by circles and the theoretical dry conversion rate is shown with a solid line. The experimental conditions were listed in Table 1.
amount of NO converted. However, at higher ClO2/NO ratios, NO2 no longer accounts for all the converted NO. The discrepancy between the rates of NO conversion and NO2 formation is somewhat exacerbated and migrates to lower ClO2 ratios in the presence of H2O. This indicates that reactions 7 and 8 represent the main pathway of oxidation, although other interactions assume increasing importance at higher ClO2/NO ratios, especially when water is present. In the setup with a condenser,13 the outlet NO2 concentration decreased dramatically in the presence of water, which appears to be due to condensation and absorption. In Figure 5, the levels of formed HNO3 are given for different ClO2/NO inlet ratios and H2O contents. No HNO3 was detected at the two lower levels of chlorine dioxide injection without water addition, as shown in Figure 5a. HNO3 was, however, measured at higher levels of added ClO2. The higher level of formation is expected for increasing amounts of ClO2 as the oxidation potential is also increased and as more H2O is introduced via the ClO2 solution. The amount of HNO3 measured completes the nitrogen balance together with NO2 to more than 95%. Even though HNO2 is not included in the gas analysis and is a possible product from reaction 12, it can thus be concluded from the nitrogen balance that it could only comprise a minor fraction, a few ppm, of the total NO oxidized by ClO2. Figure 6 compares the levels of NO conversion for various ClO2/NO ratios at reactor temperatures of 100 and 180 °C.
Figure 6. Conversion of NO (as defined by eq 18) after ClO2 injection. H2O level = 0−30%; [SO2] = 0−1000 ppmdry; [NO] = 250 ppmdry; [CO2] = 17%dry; [O2] = 4%dry; and [N2] = balanced to achieve a flow of 3 NL/min.
Note that these experiments cover a broad range of H2O and SO2 concentrations. The results for the experiments conducted at the two temperatures were similar. Therefore, oxidation of NO by ClO2 is not sensitive to temperature in the range of 100°−180 °C under the present conditions. While these results were obtained with the original experimental setup,13 they compare well with the results shown in Figure 3. 4.2. SO2 Oxidation in a ClO2 Environment. Figure 7 presents the levels of SO2 conversion, calculated from the difference between the measured inlet and outlet SO2 concentrations. SO2 conversion is limited to a few percentage 14351
DOI: 10.1021/acs.iecr.8b03067 Ind. Eng. Chem. Res. 2018, 57, 14347−14354
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Industrial & Engineering Chemistry Research
compares them to the values predicted by the kinetics analysis of Armitage and Cullis.39 The level of oxidation of SO2 by NO2 at the studied temperatures was insignificant even at these extreme concentrations (10% inlet NO2 and SO2) and long residence time (5 min). The model ably captures the conversion phenomena despite the fact that operation was far outside the validated range for the kinetics; if anything the model overestimates the oxidation rate. Thus, it is reasonable to conclude that the gas-phase oxidation of SO2 by NO2 in the ClO2 experiments (residence times of seconds and NO2 and SO2 concentrations in the hundreds of ppm) is insignificant in terms of the current process. The interaction between NO2 and SO2 in the gas phase, reaction 15, as discussed above and confirmed by the kinetics,39 is not expected to have a significant effect on SO2 conversion. This is in accordance with the result by Murciano et al.41
5. CONCLUSIONS The gas-phase NO−SO2−ClO2 chemistry at low temperatures (
