Ind. Eng. Chem. Res. 2010, 49, 8821–8825
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Analysis of SO2 Removal and Ammonium Sulfate Particle Growth in Dielectric Barrier Discharge-Photocatalyst Hybrid Process Anna Nasonova, Dong-Joo Kim, and Kyo-Seon Kim* Department of Chemical Engineering, Kangwon National UniVersit, Chuncheon, Kangwon-Do 200-701, Korea
We analyzed SO2 removal and ammonium sulfate particle growth in the dielectric barrier discharge-photocatalyst hybrid (DBD-PH) process. The DBD-PH reactor consists of two zones: the first for plasma generation and SO2 conversion and the second for ammonium sulfate particle formation and growth. In this work, the first zone was packed with TiO2-coated glass beads used as a dielectric material. The UV light generated from the plasma discharge activates the TiO2 photocatalyst, and the SO2 removal efficiency increases as a result of the reactive radicals generated by plasma reactions and TiO2 photocatalyst. In this work, the SO2 removal efficiency was found to increase as the applied peak voltage, residence time, and pulsed frequency increased and as the initial SO2 concentration decreased. In the DBD-PH process, gaseous SO2 is converted into H2SO4 and, upon addition of NH3, into solid (NH4)2SO4 particles, which can be separated by particle collectors. Using the second zone of the reactor, we examined (NH4)2SO4 particle growth as a function of reactor length for various process conditions. We found that the (NH4)2SO4 particles grow by particle coagulation and surface reaction along the reactor and that larger particles are produced as the residence time or initial SO2 concentration increases. This study can be a basis for the design of more efficient particle collectors in the DBD-PH process for SO2 and NO removal. Introduction Environmental problems caused by increased world energy demands have become a serious issue in many countries. Air pollutants such as SOx and NOx generated mainly by the combustion of fossil fuels are ultimately transformed into sulfuric acid and nitric acid in the atmosphere. Acid rain containing these acids makes lakes, marshes, rivers, soil, and forests more acidic. The emission limits for SOx and NOx from fossil-fuel combustion become progressively more stringent to reduce air pollutants in the atmosphere.1-5 There are several kinds of gas-phase processes for SOx and NOx removal from flue gas streams, such as corona discharge [alternating current (ac), direct current (dc), and pulsed], dielectric barrier discharge, and electron-beam excitation. The electron-beam process is economically impracticable for the removal of SOx and NOx in flue gases, because it requires high capital costs for construction and maintenance and also more techniques for the reduction of X-ray exposure. Pulsed corona discharge has also been used for the removal of SOx and NOx.5-10 Corona discharge processes can be used on a large or small scale and require lower capital costs and no X-ray radiation shielding compared with electron-beam techniques. Chang et al.9,10 tested the corona discharge-induced ammonia radical injection technique on the simultaneous removal of NOx and SO2 and found that this technique could enhance the removal efficiencies of NOx and SO2. On the other hand, Filimonova et al.11 found that the fractional active volume and average electric field in dielectric barrier discharge were 3-4 and 100 times, respectively, higher than those in corona discharge. In the dielectric barrier discharge process for SO2 removal, the highly energized electrons are generated by a highvoltage electric power supply and create various free radicals, such as OH, HO2, H, O, N, O(D1), and so on, through collisions with the gas-stream molecules. SO2 is oxidized to SO3 by reaction with those radicals, and it is converted into H2SO4 by * To whom correspondence should be addressed. E-mail: kkyoseon@ kangwon.ac.kr. Tel.: (+82 33) 250-6334. Fax: (+82 33) 251-3658.
reaction with OH radicals. The H2SO4 molecules can react with ammonia (NH3) to form neutral ammonium sulfate [(NH4)2SO4] particles, which can be separated by particle collector to be used as a fertilizer.1-5,12,13 Kim et al.14 and Kim et al.15 proposed a low-temperature plasma process combined with photocatalysis to remove SO2 and NO from gas streams efficiently. In the dielectric barrier discharge-photocatalyst hybrid (DBD-PH) process, the UV light generated from the plasma discharge activates the TiO2 photocatalyst. The reactive radicals involved in NO and SO2 removal are produced by the plasma reactions and also by UV irradiation impinging onto the TiO2 photocatalyst. Electron-hole recombination on the surface of the TiO2 photocatalyst is suppressed by an applied voltage, which leads to the enhancement of the energy yield.6-10 They showed that the combination of plasma discharge and TiO2 photocatalyst is ∼30% more effective in removing NO and SO2 than use of plasma discharge only.14,15 In the SO2 removal process by plasmas, ammonium sulfate particles are formed by the neutralization reaction between H2SO4 and NH3 and grow inside the reactor.16,17 Onda et al.18 conducted experiments in a simulated flue gas with NO2, SO2, NH3, and H2O in a dry pulsed corona reactor. They found that the particles produced in the pulsed corona reactor had diameters of about 61.5 µm and consisted of many smaller particles and also that the composition of these particles was 49 mol % (NH4)2SO4 and 47 mol % 2NH4NO3 · (NH4)2SO4.18 Kanazawa et al.19 reported that, in a corona discharge process for NOx removal, aerosol particles (mainly NH4NO3) in the range from submicrometer to a few micrometers were produced by the neutralization reaction between HNO3 and NH3. These particles are soluble in water and can be easily removed from a gas stream. Chang20 reported that the shapes of the NH4NO3 and (NH4)2SO4 particles produced in the corona discharge reactor became more crystal-like downstream of the reactor. To collect the ammonium sulfate particles produced in the SO2 removal process more efficiently, more information on particle charac-
10.1021/ie100567v 2010 American Chemical Society Published on Web 08/13/2010
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Ind. Eng. Chem. Res., Vol. 49, No. 18, 2010
Figure 1. Schematic of the experimental setup for the DBD-PH process.
teristics such as morphology, particle size, and particle concentration is needed. Systematic analysis of particle growth in SO2 removal by the DBD-PH process is required to obtain information on the particle characteristics, but has not yet been done sufficiently. In this study, we systematically analyzed the SO2 removal and the growth of ammonium sulfate particles in the DBD-PH process, changing the process conditions such as the applied voltage, pulsed frequency of the applied voltage, initial SO2 concentration, and residence time. Experimental Apparatus Figure 1 shows the experimental setup used to analyze SO2 removal and ammonium sulfate particle growth in the DBD-PH process. The reactor has two zones: the first one is for SO2 removal, and the second one is for particle formation and growth. The first zone of the reactor was a Pyrex cylinder with the inner and outer diameters of 27 and 30 mm, respectively. A copper rod (discharge electrode) with a diameter of 5 mm was located at the center of this cylinder. The outside wall of the first zone was wrapped with a stainless steel mesh that acted as a ground electrode. The first zone was packed with glass beads of 3-mm diameter as dielectric materials. The glass beads were coated with TiO2 photocatalyst (Degussa P-25 TiO2 powder) by the dip-coating method. The second zone was just the hollow Pyrex cylinder with the same inner and outer diameters as the first zone. If a high voltage is applied to the power electrode in the first zone of the reactor, highly energized electrons are generated and produce reactive radicals such as O, OH, N, H, O3, and so on. These reactive radicals are also generated by photoactivation of the TiO2 photocatalyst. In the first zone of the reactor, SO2 is converted into H2SO4 by two pathways through plasma reactions with these radicals (SO2 + OH f HSO3, HSO3 + OH f H2SO4, SO2 + O f SO3, SO3 + H2O f H2SO4). Ammonia is supplied to the middle part of the reactor connecting the first and second zones and reacts with H2SO4 to produce (NH4)2SO4 particles in the beginning of the second zone. All gas flow rates were controlled by mass flow controllers (MFCs) (model FC-280S, Tylan), and all gases were passed through a moisture trap (silica gel). The H2O concentration was
Figure 2. SO2 removal efficiencies for various initial SO2 concentrations as a function of applied peak voltage.
controlled by adjusting the bubbler temperature and flow rate of O2 supplied to the H2O bubbler. The concentration of SO2 at the outlet was measured with an electrochemical gas analyzer (Eurotron, GreenLine MK II). We divided the second zone of the reactor into four equal parts and placed transmission electron microscopy (TEM) grids at the bottom of each part to collect (NH4)2SO4 particles. The sizes and morphologies of the particles collected on the TEM grids were examined for various process conditions. Results and Discussion SO2 Removal. The standard conditions for the initial SO2 and H2O concentrations ([SO2]0 and [H2O]0), pulse frequency (f ) and residence time (τr) were 400 ppm, 0.4 mol %, 900 Hz, and 1 s [corresponding to a total gas flow rate of 5 L/min at standard temperature and pressure (STP)], respectively. The O2 concentration was controlled at 21 mol %, and N2 was the balance gas. The initial SO2 concentration, pulse frequency, applied voltage, input power and total gas flow rate were varied in the ranges 200-600 ppm, 100-900 Hz, 3-13 kV, 3-50 W, and 2.5-10 L/min (corresponding to residence times of 1-0.32 s), respectively.
Ind. Eng. Chem. Res., Vol. 49, No. 18, 2010
Figure 3. SO2 removal efficiencies for various pulse frequencies as a function of applied peak voltage.
Figure 4. SO2 removal efficiencies for various residence times as a function of applied peak voltage.
Figure 2 shows the removal efficiencies of SO2 for various initial SO2 concentrations as a function of applied peak voltage. As the applied voltage increases, the electrons become more energetic, the concentrations of reactive radicals (O, N, O3, etc.) increase as a result of faster electron-impact dissociation reactions, and SO2 is removed more quickly because of the faster oxidation reactions of SO2 with reactive radicals. As the initial SO2 concentration increases, the energy required to remove SO2 increases, and the removal efficiency decreases at a given applied peak voltage. For [SO2]0 ) 200 ppm, the removal efficiency was found to reach 100% because the energy supplied to the DBD-PH reactor was sufficient to convert all SO2 molecules. This DBD-PH system thus allowed 100% removal
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of SO2 at [SO2]0 ) 100 and 200 ppm upon application of peak voltages of 7 and 8 kV, respectively. Applying peak voltage of 9 kV (the limit for this system under the current conditions) enabled the removal of ∼80% and ∼50% of the SO2 at [SO2]0 ) 400 and 600 ppm, respectively. The specific energy consumption to remove SO2 was found to be 190-530 eV/molecule depending on the experimental conditions, which is quite energy efficient comparing to electron-beam or high-temperature plasma processes. Figure 3 shows the SO2 removal efficiencies for various pulse frequencies as a function of applied peak voltage. It was observed that increases in applied peak voltage and pulse frequency enhanced the SO2 removal efficiencies significantly. The reaction rate constants to produce the reactive radicals [H, N, O, O3, OH, HO2, O(1D), etc.] can be expressed as functions of the electron energy and also depend on the electric field imposed on the reactor. The range of electron energies of 1-10 eV is ideal for exciting atoms and molecules and breaking chemical bonds. Dielectric barrier discharge provides an electron energy of ∼5 eV, which is higher than those of high-voltage corona (
