文献出处：Alam F. Oxidization of SO2 by Reactive Oxygen Species for Flue Gas Desulfurization and H2SO4 Production [J]. Biology and fertility of soils, 2015, 5(6): 981-992.
Oxidization of SO2 by Reactive Oxygen Species for Flue Gas Desulfurization and H2SO4
Production
reactive toAlam F
Abstract
A strong ionization dielectric barrier discharge was used to produce a high concentration of reactive oxygen species that were then injected into a simulated flue gas in a duct to remove SO2 by oxidation. Sulfuric acid (H2SO4) was produced through the following two reactions: (1) O3 oxidation of SO2–SO3, which then reacted with H2O to produce H2SO4; and (2) reaction of O2 + with H2O to produce ·OH radicals, which then rapidly and non-selectively oxidized SO2–H2SO4. When the molar ratio of reactive oxygen species to SO2 was 4:1, the SO2 removal efficiency was 94.6%, the energy consumption per cubic meter of flue gas was 13.3 Wh/m3, the concentration of recovered H2SO4 was 4.53 g/l, and the H2SO4 recovery efficiency was 28.8%. The H2O volume fraction in the simulated flue gas affected the S
O2 removal efficiency, whereas the O2 and CO2 volume fractions did not. These results prove that oxidation by reactive oxygen species is a feasible method for flue gas desulfurization.
Keywords：Reactive oxygen species；SO2 removal efficiency；H2SO4
1Introduction
Wet limestone-gypsum flue gas desulfurization (FGD) is widely used to treat flue gas in coal-fired power plants in the United States, Germany, Japan and other developed countries [1, 2]. However, this technology is too expensive for developing countries, requires a large area, and produces large volumes of wastewater and low quality by-products [3 4]. As an alternative to this treatment method, many studies have demonstrated that no thermal plasma can be used for efficient SO2 removal from polluted flue gas [5–12]. Saver [12] highlighted that no thermal plasma can be used for simultaneous removal of several pollutants at atmospheric pressure. Furthermore, the initial investment and operation costs for no thermal plasma are relatively low. No thermal plasmas for flue gas desulfurization are usually created by electron beam irradiation or pulsed corona discharge [13]. FGD using electron beam irradiation has been demonstrated at 200 MW power plants in Chengdu, China [14]. Techniques have been developed for recovery of ammonium salt that is generated as a byproduct
during this process, when NH3 absorbs SO2 from the flue gas. However, there are other problems with this technique, such as the large size of the plasma source and vacuum system, and X-ray radiation hazards. Bernie and Penetrate [15] suggested that the high capital cost and X-ray hazards of the electron beam method have discouraged its use in many pollution control applications.
To resolve the presented disadvantages in electron beam irradiation, many scholars have studied the treatment of flue gas using the pulse corona discharge. Encouraging results have been obtained for the FGD, but the energy efficiency obtained by electron beam irradiation is twice that obtained by the pulsed corona discharge [13]. In the pulsed corona discharge zone, a large number of electrons lose considerable energy through vibration excitation, which does little to enhance the desired reaction of FGD. High desulfurization efficiency is with injected ammonia under the pulsed corona discharge. Sandakan [16] pointed out that the pulse corona method is a thermal chemical reaction, and that the main products [(NH4)2SO3] are likely to decomposed into SO2
and NH3 again above 30°C.
To solve this problem, in this paper a strong ionization dielectric barrier discharge (DBD) method was used to ionize and dissociate O2 into a high concentration of reactive oxygen species, which were the
n injected into a duct containing a simulated flue gas to produce ·OH radicals. These radicals rapidly and non-selectively oxidized SO2 to produce sulfuric acid (H2SO4). The whole plasma chemistry reaction takes place in a duct, without the need for other conventional treatment methods. The basic principles and the plasma chemistry reactions for SO2 removal, the plasma source, and the effect of H2O, O2, and CO2 volume fractions on SO2 removal efficiency are discussed.
2 Experimental
Reactive Oxygen Species Generation and Plasma Chemistry Reactions for SO2 Removal. The basic principles for flue gas desulfurization by reactive oxygen species injected into the duct are shown in Fig. 1. High concentrations of reactive oxygen species, such as O2 +, O, O (1D), O2 −, O2 (a1∆g), and O3 [12], are produced by a strong ionization dielectric barrier discharge in a reactive oxygen species generator (1 in Fig. 1). However, among these species, only O2 +and O3 are injected into the duct (3 in Fig.1) to react with SO2 and produce H2SO4. This is because O, O (1D) O2 (a1∆g) are short-lived. The main reaction is of O2 + with H2O to produce ·OH radicals, which then rapidly and non-selectively oxidize SO2 to H2SO4 mist. This occurs because the rate coefficient and oxidization pote ntial of ·OH radicals are about 10−12 cm3/s and 2.80 V, respectively. The secondary reaction involves O3 oxidation of SO2–SO3, and subsequent reaction of SO3 with H2O to produce H2SO4. The H2SO4 mis
t passes through a high-voltage direct current electric field and is captured and collected as liquid H2SO4 [17] in an electric acid mist remover (4 in Fig. 1). The purified experimental gas is discharged from the duct by an induced-draft fan (6 in Fig. 1).
Fig. 1
Generation of reactive oxygen species for removal of SO2 from simulated flue gas in a duct with the following components:    1 reactive oxygen species generator,    2 high-voltage high-frequency power supply, 3 duct, 4 electric acid mist remover, 5 high-voltage DC power supply, 6 induced-draft fan
The rectangular (L × W × H, 280 × 50 × 220 mm) reactive oxygen species generator (Fig. 2a, b) contains discharge electrodes, ground electrodes, spacers, and dielectric layers. A high-voltage, high-frequency discharge output from the high-voltage, high-frequency power supply (2 in Fig. 1) is applied to the discharge electrodes, and the peak voltage (6 kV), current (100 am) and waveform (11.5 kHz) are measured by a oscilloscope (TDS3032, Tektronix, Beaverton, OR) (Fig. 3). The field strength E, which was calculated as the peak voltage (shown in Fig. 3) divided by the electrode gap spacing (0.2 mm), was divided by the total gas density n to acquire the reduced
electric field strength E/n. This is discussed in detail in elsewhere [18]. The mean electron energy depe
nded primarily on the reduced field strength, E/n, which is discussed in detail elsewhere [14]. When the electric field strength reached >97.2 kV/cm in the discharge channels, the mean electron energy increased to >8 eve, which is around 6 eve higher than the mean electron energy for the pulsed corona discharge and about 3–4 eve higher than that for the conventional dielectric barrier discharge. An image of the strong ionization discharge formed in the discharge channels is shown in Fig. 2c. The energy of electrons ranges from ionization energy (8.4 eves) to dissociation energy (12.5 eve). These high energy electrons are deposited on O2 to ionize or dissociate it into a high concentration of reactive oxygen species, such as O2 +, O, O (1D), O−, O2 −, O2 (a1∆g), and O3, in the discharge channel. The remaining low energy electrons do not contribute to the desired reaction because the reactive oxygen species O, O−, O (1D), O2 −, and O2 (a1∆g) are short-lived (approximately 1 ×10−8s), only O2 −, O2 + and O3 are injected into the duct (3) for oxidation of SO2. The plasma chemistry reactions and their rate coefficients are presented in Table 1. The rate coefficients of 1–6 are taken into account with values of E/n = 150 Td in the discharge channel.
Fig. 2
Structure of the reactive oxygen species generator and discharge image in the discharge channel. I a Structure of reactive oxygen species’ b Photo of the reactive oxygen species generator. C Photo of dis
charge image
Fig. 3
V oltage, current and waveform output from high-voltage high-frequency power supply
Table 1
Plasma chemistry reactions involved in reactive oxygen species generation
Electron impact reaction Rate coefficient 150 Td Reference
O2 + e → O2 + + 2e k 1  = 8.0 ×10−10 cm3/s (1) [19]
O2 + e → O + O + e k 2  = 7.9 ×10−10 cm3/s (2) [19]
O2 + e →O− + O k 3  = 3.2 ×10−11 cm3/s (3) [19]
O2 + e →O2 − k 4  = 5.6 ×10−12 cm3/s (4) [19]
O2 + e → O + O(1D) + e k 5  = 2.3 ×10−9 cm3/s (5) [19]
O2 + e →O2(a1∆g) + e k 6  = 7.2 ×10−10 cm3/s (6) [19]
Reaction Rate coefficient Reference
O− + O2(a1∆g)→ O3 + e k 7  = 3.0 ×10−10 cm3/s (7) [19]
O(1D) + O2 → O + O2(a1∆g) k 8  = 3.4 ×10−11 cm3/s (8) [19]
O(1D) + O2 → O + O2 k 9  = 6.3 ×10−12 cm3/s (9) [19]
O + O2 → O3 k 10  = 1.0 ×10−14 cm3/s (10) [19]
When O2 +, O2 − and O3 are injected into the duct, the electron energy of O2 − produced through electron deposition on O2 is so low that it cannot oxidize SO2. Only O2 + and O3 can oxidize SO2 to produce H2SO4 through the following two major reactions. The first of these is reaction of O2 + with H2O to produce ·OH radicals, and subsequent oxidation of SO2–H2SO4 mist. The second reaction involves O3 oxidation of SO2–SO3, and subsequent reaction of SO3 with H2O to produce H2SO4. The plasma reactions proceed to completion in the duct (Fig. 4, length 1 m) between where the reactive oxygen species are injected and the entrance of the electric acid mist remover. The plasma reactions are described below.
Fig. 4
Photograph of the plasma reaction duct
First O2 + reacts with H2O to form water cluster ions, O2 + ·H2O, and these then dissociate to form ·OH radicals.
O2 + + H2O + M → O2 + ·H2O + M k 11 = 2.5 ×10−28 cm6/s (11) [20]
O2 + ·H2O + H2O → H3O+ + ·OH + O2 k 12 = 1.2 ×10−9 cm3/s (12) [20]
cm3/s and
2.80 V, respectively, the ·OH radicals can rapidly and non-selectively oxidize SO2–H2SO4 mist. SO2 + ·OH → HSO3 k 13 = 7.5 ×10−12 cm3/s (13) [19]
HSO3 + ·OH → H2SO4 k 14 = 1.0 ×10−12 cm3/s (14) [19]
H2SO4 is also produced by O3 oxidization of SO2–SO3, which then reacts with H2O to produce H2SO4 mist.
SO2 + O3 → SO3 + O2 k 15 = 3.0 ×10−12 cm3/s (15) [21]
SO3 + H2O → H2SO4 k 16 = 6.0 ×10−15 cm3/s (16) [19]
This method for the SO2 removal is very different to conventional gas ionization discharge, in which the total flue gas passes through the plasma source [5]. Compared with conventional gas ionization discharge, the present method requires a smaller volume of plasma, has a simplified procedure, lower investment cost and energy consumption, and does not require additional catalysts, reluctant, oxidants or the use of other technologies. This method also produces liquid H2SO4, which is an important raw material in the chemistry industry.
3 System for Simulated Flue Gas Desulfurization
The experimental setup for simulated flue gas desulfurization is shown in Fig. 5. The experimental gas contained SO2, O2, CO2, and N2 standard gases, which were from compressed gas cylinders, and dry air. The flow rate was adjusted using mass flow meters. The experiment gases were mixed in the gas-mixing chamber (8 in Fig. 5) before adjusting the temperature and humidity (9 in Fig. 5) so that they were similar to those in flue gas from a coal-fired power plant. The simulated gas was introduced into the duct (3 in Fig.5), which was a stainless tube with an inner diameter of 20 mm. The reactive oxy
gen species (O2 +, and O3) with concentrations of 200–300 mg/l produced by DBD in the reactive oxygen species generator (1 in Fig. 5) were injected into the center of the duct. These species were transferred through a PTFE tube with an inner diameter 6 mm. In the duct, O2 + and O3 oxidized SO2 to H2SO4 mist. This mist passed through the high-voltage direct current electric field and was captured and collected as liquid H2SO4 in an electric acid mist remover (4 in Fig. 5). The electric acid mist remover was a grounded, titanium steel cylinder with the inner diameter of 200 mm, thickness of 1.5 mm and length of 1,200 mm that contained a star-shaped electrode wire with the average diameter of 3 mm. The electric field strength in the middle of the acid mist remover was 14 kV/cm. Residual reactive oxygen species in the experimental gas were removed by heat treatment, and the final purified experimental gas was discharged from the duct using an induced-draft fan (6 in Fig. 5).

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