# SO2 Gas Physicochemical Removal through Pulse Streamer Discharge Technique Assisted by Vapor Additive

Authors: Xiaojun Wang; Lianshui Zhang
Journal: Journal of Chemistry
 (2015)

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## Abstract

SO2 removal has drawn extensive attentions for air pollution treatment. In this paper, the pulse streamer discharge technique is investigated. Emission spectra diagnosis experimentally indicates that the SO2 molecule has been physically dissociated into SO and O radicals by electron collision and can be remediated through further chemical reactions during and after discharge. In order to quantitatively analyze the removal physical chemistry kinetics, a zero-dimensional physicochemical reaction model is established. Without H2O vapor additive, the SO2 removal efficiency is leanly low and only 0.296% has been achieved under pulse discharge duration of 0.5 μs. Through increasing the electrical concentration six times, the removal efficiency has been slightly heightened to 1.796% at pulse duration of 3 μs. Contrarily, vapor additive can effectively improve the removal kinetics, and removal efficiency has been remarkably heightened to 13.0195% at pulse duration of 0.5 μs with H2O/SO2 initial concentration ratio of 0.1 : 1. OH radicals decomposed from H2O through electron collision are the essential factor to achieve such improvement, which have effectively adjusted the chemical removal process to the favorite directions. The major productions have been transformed from HSO3 and HOSO2 to H2SO4 when vapor ratio increased above 1.27 : 1.


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## Body

## 1. Introduction

Sulfur dioxide (SO2) has played important roles in acid rain formation [1]. There has been natural source of this environment-polluting compound, such as the exhaust gas emitted from volcanoes [2]. But most of the SO2 ingredients produced nowadays should be ascribed to the fuel and coal combustion [3]. The exhaust gas emission from automobiles or power plants has become deteriorating social problems for generating acid rain. The acid rain can pollute the earth soil, the water, the building surface, and the metal coatings and has drawn extensive attentions from the viewpoint of government, law legislation, and the power plants, as well as the internal-combustion engine designers [4, 5]. Severe regulations on SO2 emissions have propelled the advancements of SO2 removal techniques, including spray-dry or wet scrubbing and catalyst [6, 7]. Jin et al. reported that chlorine dioxide (ClO2) gas could be utilized to clean up SO2, and about 100% removal efficiency was achieved under optimal conditions of passing sufficient ClO2 gas into the scrubbing solution [8]. Wang et al. found that the ozone injection plus a glass made alkaline washing tower could efficiently achieve desulfurization [9]. Rayon-based activated carbon fibers (ACFs) at temperatures of 313–348 K had exhibited high SO2 removal activity [10]. Mn-based activated carbon catalysts were prepared, with MnO and Mn3O4 coexisting in catalysts, and had exhibited SO2 removal ability [11]. Pt/CeO2 catalysts prepared on Cu (111) had been applied to assist the transformation of SO2 into atomic sulfur on its surface at the temperature above 300 K [12]. The Mo and Co doped V2O5/AC catalyst-sorbents were also used as catalyst for SO2 removal [13].The wet scrubbing method is effective and has the utilizing prospect for flue gas desulfurization. But it should be noticed that the wet scrubbing process should be operated in relatively large reactors and some complex chemical reactions should be precisely controlled to generate gas-phase oxidant such as ClO2 and O3, exampled by the chlorate-chloride process as(1)2H2SO4+2NaClO3+2NaCl⟶2ClO2↑⁡​+Cl2+2H2O+2Na2SO4The ClO2 scrubber gas is usually generated on-site since ClO2 can rapidly decompose through photo dissociation [14]. Despite possessing high efficiency, such wet scrubbing method has relatively high cost and should carefully dispose the end liquid waste. In addition, the design of wet scrubbing technology is highly dependent on the characteristics of the treated flue gas. Furthermore, the catalyst removing SO2 reaction is operated usually under relatively high temperature.As alternative method, the high energy e-beam (EB, electron beams) technology has also been utilized in power plants based on the mechanism of high energy electron collision on the O2, H2O, and so on, to generate the radical agents such as O, OH, and HO2, for gas-phase oxidizing SO2 in the exhaust gas [15]. There have been no or fewer amounts of wet end products, benign gas emission or easily captured aerogel dusts. The 70–98% removal efficiency had been reported through such EB method, though its disadvantage is the requirement of large space and high energy consumption, for which the injected electrons should be accelerated to several MeV magnitudes (the input power of the electron accelerator usually in the range of 102~103 kV, and the accelerator is large in space). The X-ray exposure due to the emissions sourced from the deep excited radicals and molecules is another environmental risk. Based on its inherent characters, the EB technique had been successfully applied in the thermal power plants in many countries [16].Compared to the wet scrubbing, catalyzing, or e-beam technique, the pulsed corona discharges, pulsed streamer discharges, or dielectric barrier discharges (DBD) demonstrate the advantage of low cost, for which these pulsed discharges are generated under lower voltages (~101 kV) through simpler power supply, and the discharge instruments could be miniaturized. Such pulsed discharge removing SO2, NOx, or other volatile organic compounds (VOC) has attracted the interests from the academic to industrial community, and successful application has been obtained in China and other countries [17, 18].As important candidate for high-efficient SO2 remediation, the pulse discharging technique can inject high energy electrons to physically dissociate the SO2 molecules and further chemically transform the SO2 molecules into benign or easily captured species [19, 20]. Gas additive mixed with SO2 can sometimes present improvement effect. Ma et al. reported that SO2 removal was improved by adding NH3 into the air stream through the DBD discharge nonthermal plasma technique [21]. But the (NH4)2SO3 or NH4HSO3 production after discharge is not thermally stable enough and can further decompose into SO2. Since NH3 additive for discharging removal of SO2 is unstable, the NH3 injection is usually utilized into the terminal of the pulse discharging instrument to collect the H2SO4 aerogel dust, and the cost of injected NH3 is also expensive [22–24]. The catalyst combined plasma technique is also noticed. For example, TiO2-coated glass beads had been applied for SO2 removal. The SO2 removal efficiency was improved by the radicals generated from plasma reactions and TiO2 photo-catalyst [25].Usually, hydroxyl (OH) radicals are highly active and can be derived from the H2O decomposition [26]. The hydroxyl radical is often referred to as the “detergent” because it can react with many pollutants [27–29]. In this paper, the SO2 removal physical chemistry kinetics without and with vapor additive are analyzed, and the OH improvement effect on SO2 remediation is focused on. The pulse streamer discharge technique for SO2 removal is introduced in Section 2. The emission spectra are detected and diagnosed for analyzing the SO2 removal mechanism, and a zero-dimensional physicochemical reaction model is established in Section 3. Numerical simulation is quantitatively achieved. Section 4 announces the conclusions.


## 2. Experimental Section

The SO2 removal system is diagramed in Figure 1. The SO2 is experimentally generated through the reaction between H2SO4 and Na2SO3. N2 acts as carrier gas to deliver SO2 gas to the discharge zone. After discharge, the residual SO2 and other gaseous productions are neutralized by NaOH solution.Figure 1
Diagram of the pulse streamer discharge system for SO2 removal.The pulse streamer discharge reactor is consisting of two electrodes, which are oppositely placed and encapsulated in a glass tube. High energy electrons are injected from one electrode driven by the pulse electric field and then streamed to the other electrode. During the electron streaming process, the SO2 molecules can be physically collided.The discharge voltage is 9.5 kV, with the pulse duration of 0.5μs. The discharge frequency is 50 Hz, which is the power frequency of China. Gas pressure in the tube is controlled at 1 atm.In order to monitor the SO2 removal process by untouched technique, the emission spectra are collected through a quartz window on the surface of the discharge tube by monochromator (ACR, AM-566). The collected photons are transformed into electrical signal by multiplier phototube (PMT, HAMAMATSU, and CR184) and denoised and amplified by Boxcar (SRS, SRS 280/255).


## 3. Results and Discussion

The emission spectra are collected and diagnosed to evaluate the species categories that appeared during discharge. In order to clarify the physical chemistry reaction kinetics, a zero-dimensional physicochemical reaction model is established and numerically simulated.


### 3.1. Emission Spectra Diagnosis

For the pulse discharging plasma, the emission spectrum is sourced from the mechanism that the SO2 gas molecules are excited through inelastic collision by the high energy electron. Since the kinetic energy of the electrons is ruled by statistical distribution principle, the SO2 molecules are excited to energy states in a wide range. Furthermore, the more important effect of such collision is that the SO2 would be decomposed into radicals. Such radicals also can be excited [30]. Then, the irradiation emitted from the wide-range energy upstates of the excited molecules and radicals can be observed and collected. The emission spectra are presented in Figure 2. There have been complicated emission bands at the wavelength range from 200 to 500 nm.Figure 2
The emission spectra detected from the pulse discharge SO2 removal system in the wavelength range from 200 to 500 nm.The emission bands are evaluated. There appears the emission sequence at 337.13, 358.36, 376.94, 423.84, 440.48, and 469.24 nm, which is discriminated as N2 transition from its C3Πu excited state to B3Πg ground state [31]. The N2 appeared at the discharge zone as carrier gas as shown in Figure 1.For the slow-varying peaks around 333.89, 373.55, and 440.12 nm, which are superposed onto the N2 emission sequence, they are evaluated as the continuous emission band of SO2 molecule and are related to the SO2 transition paths of (B1B1→X1A1), (A1A2→X1A1), and (a3B1→X1A1), respectively [32–34]. It means that the SO2 has been excited to the B1B1 excited state through the inelastic collision by the high energy electrons. Then, the excited SO2 relaxes to its X1A1 ground state through radiation transition. For the A1A2 or a3B1 excited state of SO2, it is transferred from B1B1 state through nonradiative transition process and then relaxed to the X1A1 state by radiative transition. The electron collision onto SO2 molecule has induced complex excitation and energy transition processes.There also has been an unattached emission peak around 237.17 nm in Figure2, which is evaluated as the characteristic emission of sulfur monoxide (SO) from its excited A3Π state to the X3Σ state [35, 36]. SO possesses poor stability and can only be generated by dissociation of SO2 during the electron collision process. It indicates that some part of the SO2 molecules has been successfully removed through the pulse streamer discharge technique.The possible SO2 removal routines are deduced based on the emission spectra and the evaluated transition paths as(2)e∗+SO2X1A1⟶SOA3Π+OP3+e(3)SOA3Π⟶SOX3Σ+hνIn (2), the SO2 in ground state of X1A1 state can be physically collided and excited by the electrons injected from the electrode in Figure 1 and dissociated into SO in A3Π excited state and O in 3P ground state. Such dissociative threshold energy is about 10.36 eV [37]. The excited SO compounds further transfer to the ground state of X3Σ through radiation.There also have been other possible routines such as(4)e∗+SO2X1A1⟶SOX3Σ+OP3+e(5)e∗+SO2X1A1⟶SO2A1A2/B1B1+e(6)SO2A1A2/B1B1⟶SOA3Π/X3Σ+OP3(7)SOA3Π⟶SOX3Σ+hν


### 3.2. Establishment of SO2 Physicochemical Removal Dynamic Model

Due to many complex physical chemistry reactions involved, it is difficult to quantitatively analyze the SO2 removal process by experimental method. In this section, the removal process is investigated through establishing a zero-dimensional reaction model. In order to improve the removal efficiency, the H2O vapor additive is considered.There have been two procedures for SO2 removal.


#### 3.2.1. Physical Decomposition of SO2 and H2O through Inelastic Collision by the Electrons

The electron collision dissociative cross sections are presented in Figure3. It should be noticed that the dissociative energy of H2O is smaller than that of SO2, and cross sections of the former are higher at about 101 cm2 magnitude order than that of SO2. H2O molecule is easier to be decomposed.Figure 3
The relationship between electron collision dissociative cross sections and the collision energy.For the electron collision onto SO2 or H2O, the physical reaction kinetics are ruled by the reaction rate coefficient, denoted as the symbol of k. Such rate coefficients can be calculated by solving theBoltzmann Equation of electron collision dissociative cross sections [38]. According to the cross sections in Figure 3, the rate coefficients are calculated in this paper as (8)e∗+SO2⟶SO+O+ek=1.16×10-12cm3s-1(9)e∗+H2O⟶OH+H+ek=8.87×10-10cm3s-1.In pulse streamer discharging plasma, the SO2 or H2O molecules can be physically decomposed. The new byproduct “fragments” are SO, O, OH, H, and so forth.


#### 3.2.2. Further Chemical Reactions between the Byproducts and SO2

The produced SO, O, OH, and H are active radicals and can further chemically react with SO2 or H2O. There also have been other reactions. The main reaction paths are analyzed and outlined in Table 1.Table 1
Main reactions and the corresponding rate coefficients.
Reactions
k/cm3s−1
SO2 + O → SO3
3.52 × 10−14 [39]
SO2 + O → SO + O2
1.17 × 10−12 [40]
SO + O → SO2
5.10 × 10−31 [40]
SO + O2 → SO2 + O
7.60 × 10−17 [41]
SO3 + SO → SO2 + SO2
1.99 × 10−15 [42]
SO2 + OH → HSO3
7.40 × 10−12  [17]
SO2 + OH → HOSO2
1.31 × 10−12 [41]
HSO3 + OH → H2SO4
9.80 × 10−12  [17]
SO + OH → SO2 + H
8.60 × 10−11 [43]
SO2 + H → OH + SO
3.06 × 10−12 [44]
SO3 + H2O → H2SO4
1.20 × 10−15 [45]
HOSO2 + O2 → HO2 + SO3
4.30 × 10−13 [41]
SO2 + HO2 → OH + SO3
2.01 × 10−17 [46]
H2O + H → OH + H2
4.20 × 10−12 [47]
OH + O → O2 + H
3.00 × 10−11  [17]After being dissociated by electron collision, the produced O radical can chemically participate in reaction for SO2 removal by forming SO3, or forming SO and O2. The OH radicals have played important roles in the removal process, and new molecules, such as HSO3, HOSO2, and H2SO4, are synthesized. There also have been reverse reactions to transform the new products into SO2 pollutant molecules. The main reaction routines are graph-outlined in Figure 4.Figure 4
Diagram of main reaction paths and species produced during discharge.Based on the reaction graph, the reaction kinetics are numerically modeled as time-varying differential equation set. Every differential equation in the set is proposed based on theArrhenius principle that the concentration of a given ith species (one species selected from the reacting ingredients in the model, such as SO2, SO, SO3, O, O2, H2O, OH, HO2, H, HSO3, HOSO2, and H2SO4 in Table 1 and Figure 4) is changing according to the law of conservation of matter [48]. Among the reactions, there has been losing process of ith species caused by the reaction between ith and jth species; then, the decreased concentration in unit time, or the losing rate of concentration dni/dtlosing, is described as -kijninj, in which the symbols of ni, nj denoted the respective concentration of ith or jth species and kij as the rate coefficient of the reaction between ith and jth species.All the concentration decreasing processes ofith species in unit time should be abstracted from the N reactions about ith species losing processes and linear superimposed together as (10)dnidtlosing=-∑j=1Nkijninj.Similarly, the concentration generating processes ofith species in unit time, which are abstracted from all the M reactions related to the ith species generating processes based on the reaction between species pth and qth, are denoted as(11)dnidtgenerating=+∑p=1,q=1Mkpqnpnq.Then, the concentration varying process ofith species in unit time is decided by the losing and generating process and denoted as(12)dnidt=dnidtlosing+dnidtgenerating=-∑j=1Nkijninj+∑p=1,q=1Mkpqnpnq.Through the same procedures, every kind of species in the model is corresponding to a given differential equation. Consequently, an equation set including 13 equations is established in this paper to describe the varying concentration of 13 kinds of different species. The time-resolved concentration evolutions of all species are obtained by solving this differential equation set byRunge-Kutta algorithm [49].It should be noticed that there are no spatial variables in (12). This means that the concentrations of all the species are uniformly hypothesized. The diffusion of electrons, SO2 molecules, and the byproducts has been ignored. Since there only has been concentration evolution of every species in time scale, a zero-dimensional physicochemical reaction model is established in this paper. During the simulation based on Table 1 and Figure 4, the discharge energy is set as 120 Td. The plasma temperature is 5000 K. The gas pressure is 1 atm, and the gaseous reactions are carried out at room temperature.


### 3.3. SO2 Removal Kinetic Simulation

#### 3.3.1. SO2 Removal Physicochemical Kinetics without H2O Vapor Additive

According to the reaction model without vapor additive, SO2 can be dissociated by electron collision during discharge. To clarify the removal kinetics, time-resolved concentration evolution of SO2, O, and SO and further oxidized species such as O2 and SO3 are presented in Figure 5.Figure 5
Without H2O vapor added, (a) time-resolved evolution of SO2 concentration and the removal efficiency, and (b) time-resolved concentration evolution of SO, O2, O, and SO3.
(a)
(b)In Figure5(a), the SO2 concentration is varied at a monotonic decreasing trend when discharge time increased. The SO2 removal has obviously been achieved through the pulse streamer discharge technique. After discharge lasted for 0.5 μs, the removal efficiency is about 0.296%, which is leanly low. Most of the removed SO2 has been transformed to SO and O2 as shown in Figure 5(b), with the former concentration accumulating to 7.163 × 1016 cm−3 and the latter to 3.458 × 1016 cm−3. For the SO3, its final concentration is about 1.082 × 1015 cm−3. When it comes to the O radicals, there appears an accumulating trend during discharge and concentration of 2.506 × 1015 cm−3 has been accumulated. After discharge, the O species have been fast consumed out to be zero to form SO3, SO, and O2.The removal process of the SO2 is deduced as two procedures. The first is the decomposition of SO2 into SO and O. The second is the oxidation process, during which the O2 is easier to be generated through the reaction between O and SO2 with a higher reaction rate coefficient of 1.17 × 10−12 cm3s−1 than that for forming SO3 of 3.52 × 10−14 cm3s−1. The O radical decomposed from SO2 during discharge has played the key roles in the SO2 removal process under the hypothesis without H2O vapor additive.The injected electrical energy is essential to influence the SO2 removal efficiency. With the discharge pulse duration widened, the inputted electron concentration is increased. Under such a variance, the removal efficiency of SO2 is presented in Figure 6. There appears an increasing trend of the removal efficiency with the pulse duration heightened. In the same reaction model, more electrons injection induces more SO2 to be physically decomposed. The further chemical reactions for forming O2, SO3, and so on are then accelerated.Figure 6
Relationship between SO2 removal efficiency and the discharge pulse duration.Under the discharge pulse with duration of 3μs, which bears six times energy compared to the pulse with duration of 0.5 μs, the removal efficiency has only heightened to 1.796%. From the viewpoint of energy consumption, such SO2 removal through direct decomposition by electron inelastic collision has high cost and low efficiency.


#### 3.3.2. Vapor Additive Effect on SO2 Physicochemical Removal Kinetics

Without H2O vapor added, the SO2 removal efficiency is very low. To improve the removal process, the H2O vapor is considered, which is usually mixed in the SO2 exhaust gases and the out-injecting H2O vapor is also very easy and cheap. According to the reaction model in Table 1 and Figure 4, the OH and H radicals, decomposed from H2O molecules by electron collision, can participate in many chemical reactions related to SO2 or the radicals. Even the H2O itself can transform SO3 into H2SO4. More effective removal is expected. But the attenuation effect of the OH radical should be noticed, by which the SO can be reversely transformed into SO2.In Figure7, the time-resolved concentration variance of SO2 and all other byproducts is presented under the initial concentration ratio between H2O and SO2 of 0.1 : 1. The discharge pulse duration is the same as that in Figure 5 of 0.5 μs. Compared to the 0.296% removal efficiency in Figure 5, the removal efficiency is remarkably improved by H2O vapor additive, and higher removal efficiency of 13.0195% has been finally achieved in Figure 7(a). Such variance is ascribed to the reason that the injected electrons are effectively utilized by H2O, and the produced H and OH radicals have efficiently accelerated the SO2 removal kinetics, which can be verified from the byproduct concentration variance in Figures 7(b), 7(c), and 7(d).Figure 7
Under H2O/SO2 initial ratio of 0.1 : 1 and discharge pulse duration of 0.5 μs, time-resolved evolution of (a) SO2 concentration and the corresponding removal efficiency, (b) SO, O2, SO3, and O concentration, (c) HSO3, HOSO2, H2SO4, H, OH, and HO2 concentration, and (d) H2O concentration.
(a)
(b)
(c)
(d)In Figure7(b), the SO3 formation is affected by vapor additive. Its final concentration is about 3.90 × 1015 cm−3, which is at the same magnitude order as that without vapor additive. But a monotonic increasing trend for the SO3 concentration appeared, which is due to the HO2 oxidizing SO2 and the reaction between HOSO2 and O2, though O radicals have been consumed out after discharge.The obvious increment occurred for SO concentration in Figure7(b), which has accumulated to 1.036 × 1018 cm−3 after 0.5 μs, and is higher than that without vapor additive of 7.163 × 1016 cm−3 at 102 cm−3 magnitude order. Such a remarkable increment is decided by the H radicals, which are directly decomposed from H2O molecules. The H radical can react with SO2 to produce SO and OH and is formulated in (13). Vapor additive has accelerated the SO generation efficiency:(13)SO2+H⟶OH+SOk=3.06×10-12cm3s-1.More OH production is beneficial to the O2 generation according to(14)OH+O⟶O2+Hk=3.00×10-11cm3s-1.But the O2 concentration of only 3.049 × 1016 cm−3 has been obtained after 0.5 μs in Figure 7(b), which is slightly lower than 3.458 × 1016 cm−3 without H2O vapor added. The decrement is ascribed to the consumption of OH not only by O to produce O2 as ruled by (14), but also by other reaction paths to produce HSO3, HOSO2, H2SO4, or even SO2 as shown in Table 1. And the consumption of O2 by HOSO2 and SO is another important reason for the decrement of O2 concentration.The concentrations of HSO3 and HOSO2 in Figure 7(c) have accumulated to 1.736 × 1018 and 0.724 × 1018 cm−3, respectively. For the former, it has become the major production due to its highest final concentration. There also has been 0.093 × 1018 cm−3 H2SO4 produced through the reaction between SO3 and H2O or between HSO3 and OH. And the concentration of HO2 is about 0.084 × 1018 cm−3 in Figure 7(c). Such low concentrations imply that both H2SO4 and HO2 are not the main final productions under the initial H2O/SO2 ratio of 0.1 : 1.All such concentration variances are decided by the H2O physical decomposition into H and OH through electron inelastic collision, and the H2O has been consumed with its final concentration decrement amount of about 1.2802 × 1018 cm−3 after discharge lasted for 0.5 μs in Figure 7(d). And the OH radicals have played the major roles for SO2 removal to transfer SO2 into HSO3:(15)SO2+OH⟶HSO3k=7.40×10-12cm3s-1.When the concentration ratio between H2O and SO2 is 0.1 : 1, the major productions are HSO3 with a little HOSO2, and the H2SO4 concentration is lower than them with 102 cm−3 magnitude orders. For SO2 removal, the main production is expected to be H2SO4, since H2SO4 is chemically stable and can be easily neutralized by alkali or captured by fabric filter or electrostatic precipitator (ESP). In order to adjust the final productions, the vapor ratio is varied in Figure 8. It presents that the higher the vapor ratio is, the more the H2SO4 molecules have been produced. The H2SO4 concentration is even higher than that of HSO3 when the initial vapor/SO2 concentration ratio is above 1.27 : 1.Figure 8
Under different initial vapor additive ratio, (a) the final concentration of H2SO4, HSO3, HOSO2, and SO and (b) the SO2 removal efficiency after discharge lasted for 0.5 μs.
(a)
(b)H2O additive with higher ratio has generated more OH radicals and consequently accelerated the reactions between HSO3 and OH as(16)HSO3+OH⟶H2SO4k=9.80×10-12cm3s-1by which the HSO3 has been transformed into H2SO4. Such reaction has simultaneously decreased the HSO3 concentration and increased the H2SO4 concentration, as shown in Figure 8(a). More vapor additive has effectively adjusted the SO2 removal physicochemical kinetics to the favorite directions, and H2SO4 has become the major production when the initial vapor mixing ratio is above 1.27 : 1.For other species such as HOSO2 and SO in Figure 8(a), the former is increased at a monotonic trend, but its highest final concentration at vapor ratio of 2 : 1 is obviously lower than that of H2SO4. The latter SO is increased at low vapor ratio and decreased at high vapor ratio. Such varying trends of HOSO2 and SO are ascribed to the more OH decomposed at higher vapor ratio. The HOSO2 concentration is heightened through the reaction between OH and SO2, and the SO concentration is decreased by the reaction between OH and SO to reproduce SO2. Such reactions are formulated as follows:(17)SO2+OH⟶HOSO2k=1.31×10-12cm3s-1(18)SO+OH⟶SO2+Hk=8.60×10-11cm3s-1.In conclusion, vapor additive has effectively improved the SO2 removal efficiency in Figure 8(b). In the simulation, even 89.1% removal efficiency has been achieved at the initial concentration ratio of 2 : 1 between H2O and SO2.


## 3.1. Emission Spectra Diagnosis

For the pulse discharging plasma, the emission spectrum is sourced from the mechanism that the SO2 gas molecules are excited through inelastic collision by the high energy electron. Since the kinetic energy of the electrons is ruled by statistical distribution principle, the SO2 molecules are excited to energy states in a wide range. Furthermore, the more important effect of such collision is that the SO2 would be decomposed into radicals. Such radicals also can be excited [30]. Then, the irradiation emitted from the wide-range energy upstates of the excited molecules and radicals can be observed and collected. The emission spectra are presented in Figure 2. There have been complicated emission bands at the wavelength range from 200 to 500 nm.Figure 2
The emission spectra detected from the pulse discharge SO2 removal system in the wavelength range from 200 to 500 nm.The emission bands are evaluated. There appears the emission sequence at 337.13, 358.36, 376.94, 423.84, 440.48, and 469.24 nm, which is discriminated as N2 transition from its C3Πu excited state to B3Πg ground state [31]. The N2 appeared at the discharge zone as carrier gas as shown in Figure 1.For the slow-varying peaks around 333.89, 373.55, and 440.12 nm, which are superposed onto the N2 emission sequence, they are evaluated as the continuous emission band of SO2 molecule and are related to the SO2 transition paths of (B1B1→X1A1), (A1A2→X1A1), and (a3B1→X1A1), respectively [32–34]. It means that the SO2 has been excited to the B1B1 excited state through the inelastic collision by the high energy electrons. Then, the excited SO2 relaxes to its X1A1 ground state through radiation transition. For the A1A2 or a3B1 excited state of SO2, it is transferred from B1B1 state through nonradiative transition process and then relaxed to the X1A1 state by radiative transition. The electron collision onto SO2 molecule has induced complex excitation and energy transition processes.There also has been an unattached emission peak around 237.17 nm in Figure2, which is evaluated as the characteristic emission of sulfur monoxide (SO) from its excited A3Π state to the X3Σ state [35, 36]. SO possesses poor stability and can only be generated by dissociation of SO2 during the electron collision process. It indicates that some part of the SO2 molecules has been successfully removed through the pulse streamer discharge technique.The possible SO2 removal routines are deduced based on the emission spectra and the evaluated transition paths as(2)e∗+SO2X1A1⟶SOA3Π+OP3+e(3)SOA3Π⟶SOX3Σ+hνIn (2), the SO2 in ground state of X1A1 state can be physically collided and excited by the electrons injected from the electrode in Figure 1 and dissociated into SO in A3Π excited state and O in 3P ground state. Such dissociative threshold energy is about 10.36 eV [37]. The excited SO compounds further transfer to the ground state of X3Σ through radiation.There also have been other possible routines such as(4)e∗+SO2X1A1⟶SOX3Σ+OP3+e(5)e∗+SO2X1A1⟶SO2A1A2/B1B1+e(6)SO2A1A2/B1B1⟶SOA3Π/X3Σ+OP3(7)SOA3Π⟶SOX3Σ+hν


## 3.2. Establishment of SO2 Physicochemical Removal Dynamic Model

Due to many complex physical chemistry reactions involved, it is difficult to quantitatively analyze the SO2 removal process by experimental method. In this section, the removal process is investigated through establishing a zero-dimensional reaction model. In order to improve the removal efficiency, the H2O vapor additive is considered.There have been two procedures for SO2 removal.


### 3.2.1. Physical Decomposition of SO2 and H2O through Inelastic Collision by the Electrons

The electron collision dissociative cross sections are presented in Figure3. It should be noticed that the dissociative energy of H2O is smaller than that of SO2, and cross sections of the former are higher at about 101 cm2 magnitude order than that of SO2. H2O molecule is easier to be decomposed.Figure 3
The relationship between electron collision dissociative cross sections and the collision energy.For the electron collision onto SO2 or H2O, the physical reaction kinetics are ruled by the reaction rate coefficient, denoted as the symbol of k. Such rate coefficients can be calculated by solving theBoltzmann Equation of electron collision dissociative cross sections [38]. According to the cross sections in Figure 3, the rate coefficients are calculated in this paper as (8)e∗+SO2⟶SO+O+ek=1.16×10-12cm3s-1(9)e∗+H2O⟶OH+H+ek=8.87×10-10cm3s-1.In pulse streamer discharging plasma, the SO2 or H2O molecules can be physically decomposed. The new byproduct “fragments” are SO, O, OH, H, and so forth.


### 3.2.2. Further Chemical Reactions between the Byproducts and SO2

The produced SO, O, OH, and H are active radicals and can further chemically react with SO2 or H2O. There also have been other reactions. The main reaction paths are analyzed and outlined in Table 1.Table 1
Main reactions and the corresponding rate coefficients.
Reactions
k/cm3s−1
SO2 + O → SO3
3.52 × 10−14 [39]
SO2 + O → SO + O2
1.17 × 10−12 [40]
SO + O → SO2
5.10 × 10−31 [40]
SO + O2 → SO2 + O
7.60 × 10−17 [41]
SO3 + SO → SO2 + SO2
1.99 × 10−15 [42]
SO2 + OH → HSO3
7.40 × 10−12  [17]
SO2 + OH → HOSO2
1.31 × 10−12 [41]
HSO3 + OH → H2SO4
9.80 × 10−12  [17]
SO + OH → SO2 + H
8.60 × 10−11 [43]
SO2 + H → OH + SO
3.06 × 10−12 [44]
SO3 + H2O → H2SO4
1.20 × 10−15 [45]
HOSO2 + O2 → HO2 + SO3
4.30 × 10−13 [41]
SO2 + HO2 → OH + SO3
2.01 × 10−17 [46]
H2O + H → OH + H2
4.20 × 10−12 [47]
OH + O → O2 + H
3.00 × 10−11  [17]After being dissociated by electron collision, the produced O radical can chemically participate in reaction for SO2 removal by forming SO3, or forming SO and O2. The OH radicals have played important roles in the removal process, and new molecules, such as HSO3, HOSO2, and H2SO4, are synthesized. There also have been reverse reactions to transform the new products into SO2 pollutant molecules. The main reaction routines are graph-outlined in Figure 4.Figure 4
Diagram of main reaction paths and species produced during discharge.Based on the reaction graph, the reaction kinetics are numerically modeled as time-varying differential equation set. Every differential equation in the set is proposed based on theArrhenius principle that the concentration of a given ith species (one species selected from the reacting ingredients in the model, such as SO2, SO, SO3, O, O2, H2O, OH, HO2, H, HSO3, HOSO2, and H2SO4 in Table 1 and Figure 4) is changing according to the law of conservation of matter [48]. Among the reactions, there has been losing process of ith species caused by the reaction between ith and jth species; then, the decreased concentration in unit time, or the losing rate of concentration dni/dtlosing, is described as -kijninj, in which the symbols of ni, nj denoted the respective concentration of ith or jth species and kij as the rate coefficient of the reaction between ith and jth species.All the concentration decreasing processes ofith species in unit time should be abstracted from the N reactions about ith species losing processes and linear superimposed together as (10)dnidtlosing=-∑j=1Nkijninj.Similarly, the concentration generating processes ofith species in unit time, which are abstracted from all the M reactions related to the ith species generating processes based on the reaction between species pth and qth, are denoted as(11)dnidtgenerating=+∑p=1,q=1Mkpqnpnq.Then, the concentration varying process ofith species in unit time is decided by the losing and generating process and denoted as(12)dnidt=dnidtlosing+dnidtgenerating=-∑j=1Nkijninj+∑p=1,q=1Mkpqnpnq.Through the same procedures, every kind of species in the model is corresponding to a given differential equation. Consequently, an equation set including 13 equations is established in this paper to describe the varying concentration of 13 kinds of different species. The time-resolved concentration evolutions of all species are obtained by solving this differential equation set byRunge-Kutta algorithm [49].It should be noticed that there are no spatial variables in (12). This means that the concentrations of all the species are uniformly hypothesized. The diffusion of electrons, SO2 molecules, and the byproducts has been ignored. Since there only has been concentration evolution of every species in time scale, a zero-dimensional physicochemical reaction model is established in this paper. During the simulation based on Table 1 and Figure 4, the discharge energy is set as 120 Td. The plasma temperature is 5000 K. The gas pressure is 1 atm, and the gaseous reactions are carried out at room temperature.


## 3.2.1. Physical Decomposition of SO2 and H2O through Inelastic Collision by the Electrons

The electron collision dissociative cross sections are presented in Figure3. It should be noticed that the dissociative energy of H2O is smaller than that of SO2, and cross sections of the former are higher at about 101 cm2 magnitude order than that of SO2. H2O molecule is easier to be decomposed.Figure 3
The relationship between electron collision dissociative cross sections and the collision energy.For the electron collision onto SO2 or H2O, the physical reaction kinetics are ruled by the reaction rate coefficient, denoted as the symbol of k. Such rate coefficients can be calculated by solving theBoltzmann Equation of electron collision dissociative cross sections [38]. According to the cross sections in Figure 3, the rate coefficients are calculated in this paper as (8)e∗+SO2⟶SO+O+ek=1.16×10-12cm3s-1(9)e∗+H2O⟶OH+H+ek=8.87×10-10cm3s-1.In pulse streamer discharging plasma, the SO2 or H2O molecules can be physically decomposed. The new byproduct “fragments” are SO, O, OH, H, and so forth.


## 3.2.2. Further Chemical Reactions between the Byproducts and SO2

The produced SO, O, OH, and H are active radicals and can further chemically react with SO2 or H2O. There also have been other reactions. The main reaction paths are analyzed and outlined in Table 1.Table 1
Main reactions and the corresponding rate coefficients.
Reactions
k/cm3s−1
SO2 + O → SO3
3.52 × 10−14 [39]
SO2 + O → SO + O2
1.17 × 10−12 [40]
SO + O → SO2
5.10 × 10−31 [40]
SO + O2 → SO2 + O
7.60 × 10−17 [41]
SO3 + SO → SO2 + SO2
1.99 × 10−15 [42]
SO2 + OH → HSO3
7.40 × 10−12  [17]
SO2 + OH → HOSO2
1.31 × 10−12 [41]
HSO3 + OH → H2SO4
9.80 × 10−12  [17]
SO + OH → SO2 + H
8.60 × 10−11 [43]
SO2 + H → OH + SO
3.06 × 10−12 [44]
SO3 + H2O → H2SO4
1.20 × 10−15 [45]
HOSO2 + O2 → HO2 + SO3
4.30 × 10−13 [41]
SO2 + HO2 → OH + SO3
2.01 × 10−17 [46]
H2O + H → OH + H2
4.20 × 10−12 [47]
OH + O → O2 + H
3.00 × 10−11  [17]After being dissociated by electron collision, the produced O radical can chemically participate in reaction for SO2 removal by forming SO3, or forming SO and O2. The OH radicals have played important roles in the removal process, and new molecules, such as HSO3, HOSO2, and H2SO4, are synthesized. There also have been reverse reactions to transform the new products into SO2 pollutant molecules. The main reaction routines are graph-outlined in Figure 4.Figure 4
Diagram of main reaction paths and species produced during discharge.Based on the reaction graph, the reaction kinetics are numerically modeled as time-varying differential equation set. Every differential equation in the set is proposed based on theArrhenius principle that the concentration of a given ith species (one species selected from the reacting ingredients in the model, such as SO2, SO, SO3, O, O2, H2O, OH, HO2, H, HSO3, HOSO2, and H2SO4 in Table 1 and Figure 4) is changing according to the law of conservation of matter [48]. Among the reactions, there has been losing process of ith species caused by the reaction between ith and jth species; then, the decreased concentration in unit time, or the losing rate of concentration dni/dtlosing, is described as -kijninj, in which the symbols of ni, nj denoted the respective concentration of ith or jth species and kij as the rate coefficient of the reaction between ith and jth species.All the concentration decreasing processes ofith species in unit time should be abstracted from the N reactions about ith species losing processes and linear superimposed together as (10)dnidtlosing=-∑j=1Nkijninj.Similarly, the concentration generating processes ofith species in unit time, which are abstracted from all the M reactions related to the ith species generating processes based on the reaction between species pth and qth, are denoted as(11)dnidtgenerating=+∑p=1,q=1Mkpqnpnq.Then, the concentration varying process ofith species in unit time is decided by the losing and generating process and denoted as(12)dnidt=dnidtlosing+dnidtgenerating=-∑j=1Nkijninj+∑p=1,q=1Mkpqnpnq.Through the same procedures, every kind of species in the model is corresponding to a given differential equation. Consequently, an equation set including 13 equations is established in this paper to describe the varying concentration of 13 kinds of different species. The time-resolved concentration evolutions of all species are obtained by solving this differential equation set byRunge-Kutta algorithm [49].It should be noticed that there are no spatial variables in (12). This means that the concentrations of all the species are uniformly hypothesized. The diffusion of electrons, SO2 molecules, and the byproducts has been ignored. Since there only has been concentration evolution of every species in time scale, a zero-dimensional physicochemical reaction model is established in this paper. During the simulation based on Table 1 and Figure 4, the discharge energy is set as 120 Td. The plasma temperature is 5000 K. The gas pressure is 1 atm, and the gaseous reactions are carried out at room temperature.


## 3.3. SO2 Removal Kinetic Simulation

### 3.3.1. SO2 Removal Physicochemical Kinetics without H2O Vapor Additive

According to the reaction model without vapor additive, SO2 can be dissociated by electron collision during discharge. To clarify the removal kinetics, time-resolved concentration evolution of SO2, O, and SO and further oxidized species such as O2 and SO3 are presented in Figure 5.Figure 5
Without H2O vapor added, (a) time-resolved evolution of SO2 concentration and the removal efficiency, and (b) time-resolved concentration evolution of SO, O2, O, and SO3.
(a)
(b)In Figure5(a), the SO2 concentration is varied at a monotonic decreasing trend when discharge time increased. The SO2 removal has obviously been achieved through the pulse streamer discharge technique. After discharge lasted for 0.5 μs, the removal efficiency is about 0.296%, which is leanly low. Most of the removed SO2 has been transformed to SO and O2 as shown in Figure 5(b), with the former concentration accumulating to 7.163 × 1016 cm−3 and the latter to 3.458 × 1016 cm−3. For the SO3, its final concentration is about 1.082 × 1015 cm−3. When it comes to the O radicals, there appears an accumulating trend during discharge and concentration of 2.506 × 1015 cm−3 has been accumulated. After discharge, the O species have been fast consumed out to be zero to form SO3, SO, and O2.The removal process of the SO2 is deduced as two procedures. The first is the decomposition of SO2 into SO and O. The second is the oxidation process, during which the O2 is easier to be generated through the reaction between O and SO2 with a higher reaction rate coefficient of 1.17 × 10−12 cm3s−1 than that for forming SO3 of 3.52 × 10−14 cm3s−1. The O radical decomposed from SO2 during discharge has played the key roles in the SO2 removal process under the hypothesis without H2O vapor additive.The injected electrical energy is essential to influence the SO2 removal efficiency. With the discharge pulse duration widened, the inputted electron concentration is increased. Under such a variance, the removal efficiency of SO2 is presented in Figure 6. There appears an increasing trend of the removal efficiency with the pulse duration heightened. In the same reaction model, more electrons injection induces more SO2 to be physically decomposed. The further chemical reactions for forming O2, SO3, and so on are then accelerated.Figure 6
Relationship between SO2 removal efficiency and the discharge pulse duration.Under the discharge pulse with duration of 3μs, which bears six times energy compared to the pulse with duration of 0.5 μs, the removal efficiency has only heightened to 1.796%. From the viewpoint of energy consumption, such SO2 removal through direct decomposition by electron inelastic collision has high cost and low efficiency.


### 3.3.2. Vapor Additive Effect on SO2 Physicochemical Removal Kinetics

Without H2O vapor added, the SO2 removal efficiency is very low. To improve the removal process, the H2O vapor is considered, which is usually mixed in the SO2 exhaust gases and the out-injecting H2O vapor is also very easy and cheap. According to the reaction model in Table 1 and Figure 4, the OH and H radicals, decomposed from H2O molecules by electron collision, can participate in many chemical reactions related to SO2 or the radicals. Even the H2O itself can transform SO3 into H2SO4. More effective removal is expected. But the attenuation effect of the OH radical should be noticed, by which the SO can be reversely transformed into SO2.In Figure7, the time-resolved concentration variance of SO2 and all other byproducts is presented under the initial concentration ratio between H2O and SO2 of 0.1 : 1. The discharge pulse duration is the same as that in Figure 5 of 0.5 μs. Compared to the 0.296% removal efficiency in Figure 5, the removal efficiency is remarkably improved by H2O vapor additive, and higher removal efficiency of 13.0195% has been finally achieved in Figure 7(a). Such variance is ascribed to the reason that the injected electrons are effectively utilized by H2O, and the produced H and OH radicals have efficiently accelerated the SO2 removal kinetics, which can be verified from the byproduct concentration variance in Figures 7(b), 7(c), and 7(d).Figure 7
Under H2O/SO2 initial ratio of 0.1 : 1 and discharge pulse duration of 0.5 μs, time-resolved evolution of (a) SO2 concentration and the corresponding removal efficiency, (b) SO, O2, SO3, and O concentration, (c) HSO3, HOSO2, H2SO4, H, OH, and HO2 concentration, and (d) H2O concentration.
(a)
(b)
(c)
(d)In Figure7(b), the SO3 formation is affected by vapor additive. Its final concentration is about 3.90 × 1015 cm−3, which is at the same magnitude order as that without vapor additive. But a monotonic increasing trend for the SO3 concentration appeared, which is due to the HO2 oxidizing SO2 and the reaction between HOSO2 and O2, though O radicals have been consumed out after discharge.The obvious increment occurred for SO concentration in Figure7(b), which has accumulated to 1.036 × 1018 cm−3 after 0.5 μs, and is higher than that without vapor additive of 7.163 × 1016 cm−3 at 102 cm−3 magnitude order. Such a remarkable increment is decided by the H radicals, which are directly decomposed from H2O molecules. The H radical can react with SO2 to produce SO and OH and is formulated in (13). Vapor additive has accelerated the SO generation efficiency:(13)SO2+H⟶OH+SOk=3.06×10-12cm3s-1.More OH production is beneficial to the O2 generation according to(14)OH+O⟶O2+Hk=3.00×10-11cm3s-1.But the O2 concentration of only 3.049 × 1016 cm−3 has been obtained after 0.5 μs in Figure 7(b), which is slightly lower than 3.458 × 1016 cm−3 without H2O vapor added. The decrement is ascribed to the consumption of OH not only by O to produce O2 as ruled by (14), but also by other reaction paths to produce HSO3, HOSO2, H2SO4, or even SO2 as shown in Table 1. And the consumption of O2 by HOSO2 and SO is another important reason for the decrement of O2 concentration.The concentrations of HSO3 and HOSO2 in Figure 7(c) have accumulated to 1.736 × 1018 and 0.724 × 1018 cm−3, respectively. For the former, it has become the major production due to its highest final concentration. There also has been 0.093 × 1018 cm−3 H2SO4 produced through the reaction between SO3 and H2O or between HSO3 and OH. And the concentration of HO2 is about 0.084 × 1018 cm−3 in Figure 7(c). Such low concentrations imply that both H2SO4 and HO2 are not the main final productions under the initial H2O/SO2 ratio of 0.1 : 1.All such concentration variances are decided by the H2O physical decomposition into H and OH through electron inelastic collision, and the H2O has been consumed with its final concentration decrement amount of about 1.2802 × 1018 cm−3 after discharge lasted for 0.5 μs in Figure 7(d). And the OH radicals have played the major roles for SO2 removal to transfer SO2 into HSO3:(15)SO2+OH⟶HSO3k=7.40×10-12cm3s-1.When the concentration ratio between H2O and SO2 is 0.1 : 1, the major productions are HSO3 with a little HOSO2, and the H2SO4 concentration is lower than them with 102 cm−3 magnitude orders. For SO2 removal, the main production is expected to be H2SO4, since H2SO4 is chemically stable and can be easily neutralized by alkali or captured by fabric filter or electrostatic precipitator (ESP). In order to adjust the final productions, the vapor ratio is varied in Figure 8. It presents that the higher the vapor ratio is, the more the H2SO4 molecules have been produced. The H2SO4 concentration is even higher than that of HSO3 when the initial vapor/SO2 concentration ratio is above 1.27 : 1.Figure 8
Under different initial vapor additive ratio, (a) the final concentration of H2SO4, HSO3, HOSO2, and SO and (b) the SO2 removal efficiency after discharge lasted for 0.5 μs.
(a)
(b)H2O additive with higher ratio has generated more OH radicals and consequently accelerated the reactions between HSO3 and OH as(16)HSO3+OH⟶H2SO4k=9.80×10-12cm3s-1by which the HSO3 has been transformed into H2SO4. Such reaction has simultaneously decreased the HSO3 concentration and increased the H2SO4 concentration, as shown in Figure 8(a). More vapor additive has effectively adjusted the SO2 removal physicochemical kinetics to the favorite directions, and H2SO4 has become the major production when the initial vapor mixing ratio is above 1.27 : 1.For other species such as HOSO2 and SO in Figure 8(a), the former is increased at a monotonic trend, but its highest final concentration at vapor ratio of 2 : 1 is obviously lower than that of H2SO4. The latter SO is increased at low vapor ratio and decreased at high vapor ratio. Such varying trends of HOSO2 and SO are ascribed to the more OH decomposed at higher vapor ratio. The HOSO2 concentration is heightened through the reaction between OH and SO2, and the SO concentration is decreased by the reaction between OH and SO to reproduce SO2. Such reactions are formulated as follows:(17)SO2+OH⟶HOSO2k=1.31×10-12cm3s-1(18)SO+OH⟶SO2+Hk=8.60×10-11cm3s-1.In conclusion, vapor additive has effectively improved the SO2 removal efficiency in Figure 8(b). In the simulation, even 89.1% removal efficiency has been achieved at the initial concentration ratio of 2 : 1 between H2O and SO2.


## 3.3.1. SO2 Removal Physicochemical Kinetics without H2O Vapor Additive

According to the reaction model without vapor additive, SO2 can be dissociated by electron collision during discharge. To clarify the removal kinetics, time-resolved concentration evolution of SO2, O, and SO and further oxidized species such as O2 and SO3 are presented in Figure 5.Figure 5
Without H2O vapor added, (a) time-resolved evolution of SO2 concentration and the removal efficiency, and (b) time-resolved concentration evolution of SO, O2, O, and SO3.
(a)
(b)In Figure5(a), the SO2 concentration is varied at a monotonic decreasing trend when discharge time increased. The SO2 removal has obviously been achieved through the pulse streamer discharge technique. After discharge lasted for 0.5 μs, the removal efficiency is about 0.296%, which is leanly low. Most of the removed SO2 has been transformed to SO and O2 as shown in Figure 5(b), with the former concentration accumulating to 7.163 × 1016 cm−3 and the latter to 3.458 × 1016 cm−3. For the SO3, its final concentration is about 1.082 × 1015 cm−3. When it comes to the O radicals, there appears an accumulating trend during discharge and concentration of 2.506 × 1015 cm−3 has been accumulated. After discharge, the O species have been fast consumed out to be zero to form SO3, SO, and O2.The removal process of the SO2 is deduced as two procedures. The first is the decomposition of SO2 into SO and O. The second is the oxidation process, during which the O2 is easier to be generated through the reaction between O and SO2 with a higher reaction rate coefficient of 1.17 × 10−12 cm3s−1 than that for forming SO3 of 3.52 × 10−14 cm3s−1. The O radical decomposed from SO2 during discharge has played the key roles in the SO2 removal process under the hypothesis without H2O vapor additive.The injected electrical energy is essential to influence the SO2 removal efficiency. With the discharge pulse duration widened, the inputted electron concentration is increased. Under such a variance, the removal efficiency of SO2 is presented in Figure 6. There appears an increasing trend of the removal efficiency with the pulse duration heightened. In the same reaction model, more electrons injection induces more SO2 to be physically decomposed. The further chemical reactions for forming O2, SO3, and so on are then accelerated.Figure 6
Relationship between SO2 removal efficiency and the discharge pulse duration.Under the discharge pulse with duration of 3μs, which bears six times energy compared to the pulse with duration of 0.5 μs, the removal efficiency has only heightened to 1.796%. From the viewpoint of energy consumption, such SO2 removal through direct decomposition by electron inelastic collision has high cost and low efficiency.


## 3.3.2. Vapor Additive Effect on SO2 Physicochemical Removal Kinetics

Without H2O vapor added, the SO2 removal efficiency is very low. To improve the removal process, the H2O vapor is considered, which is usually mixed in the SO2 exhaust gases and the out-injecting H2O vapor is also very easy and cheap. According to the reaction model in Table 1 and Figure 4, the OH and H radicals, decomposed from H2O molecules by electron collision, can participate in many chemical reactions related to SO2 or the radicals. Even the H2O itself can transform SO3 into H2SO4. More effective removal is expected. But the attenuation effect of the OH radical should be noticed, by which the SO can be reversely transformed into SO2.In Figure7, the time-resolved concentration variance of SO2 and all other byproducts is presented under the initial concentration ratio between H2O and SO2 of 0.1 : 1. The discharge pulse duration is the same as that in Figure 5 of 0.5 μs. Compared to the 0.296% removal efficiency in Figure 5, the removal efficiency is remarkably improved by H2O vapor additive, and higher removal efficiency of 13.0195% has been finally achieved in Figure 7(a). Such variance is ascribed to the reason that the injected electrons are effectively utilized by H2O, and the produced H and OH radicals have efficiently accelerated the SO2 removal kinetics, which can be verified from the byproduct concentration variance in Figures 7(b), 7(c), and 7(d).Figure 7
Under H2O/SO2 initial ratio of 0.1 : 1 and discharge pulse duration of 0.5 μs, time-resolved evolution of (a) SO2 concentration and the corresponding removal efficiency, (b) SO, O2, SO3, and O concentration, (c) HSO3, HOSO2, H2SO4, H, OH, and HO2 concentration, and (d) H2O concentration.
(a)
(b)
(c)
(d)In Figure7(b), the SO3 formation is affected by vapor additive. Its final concentration is about 3.90 × 1015 cm−3, which is at the same magnitude order as that without vapor additive. But a monotonic increasing trend for the SO3 concentration appeared, which is due to the HO2 oxidizing SO2 and the reaction between HOSO2 and O2, though O radicals have been consumed out after discharge.The obvious increment occurred for SO concentration in Figure7(b), which has accumulated to 1.036 × 1018 cm−3 after 0.5 μs, and is higher than that without vapor additive of 7.163 × 1016 cm−3 at 102 cm−3 magnitude order. Such a remarkable increment is decided by the H radicals, which are directly decomposed from H2O molecules. The H radical can react with SO2 to produce SO and OH and is formulated in (13). Vapor additive has accelerated the SO generation efficiency:(13)SO2+H⟶OH+SOk=3.06×10-12cm3s-1.More OH production is beneficial to the O2 generation according to(14)OH+O⟶O2+Hk=3.00×10-11cm3s-1.But the O2 concentration of only 3.049 × 1016 cm−3 has been obtained after 0.5 μs in Figure 7(b), which is slightly lower than 3.458 × 1016 cm−3 without H2O vapor added. The decrement is ascribed to the consumption of OH not only by O to produce O2 as ruled by (14), but also by other reaction paths to produce HSO3, HOSO2, H2SO4, or even SO2 as shown in Table 1. And the consumption of O2 by HOSO2 and SO is another important reason for the decrement of O2 concentration.The concentrations of HSO3 and HOSO2 in Figure 7(c) have accumulated to 1.736 × 1018 and 0.724 × 1018 cm−3, respectively. For the former, it has become the major production due to its highest final concentration. There also has been 0.093 × 1018 cm−3 H2SO4 produced through the reaction between SO3 and H2O or between HSO3 and OH. And the concentration of HO2 is about 0.084 × 1018 cm−3 in Figure 7(c). Such low concentrations imply that both H2SO4 and HO2 are not the main final productions under the initial H2O/SO2 ratio of 0.1 : 1.All such concentration variances are decided by the H2O physical decomposition into H and OH through electron inelastic collision, and the H2O has been consumed with its final concentration decrement amount of about 1.2802 × 1018 cm−3 after discharge lasted for 0.5 μs in Figure 7(d). And the OH radicals have played the major roles for SO2 removal to transfer SO2 into HSO3:(15)SO2+OH⟶HSO3k=7.40×10-12cm3s-1.When the concentration ratio between H2O and SO2 is 0.1 : 1, the major productions are HSO3 with a little HOSO2, and the H2SO4 concentration is lower than them with 102 cm−3 magnitude orders. For SO2 removal, the main production is expected to be H2SO4, since H2SO4 is chemically stable and can be easily neutralized by alkali or captured by fabric filter or electrostatic precipitator (ESP). In order to adjust the final productions, the vapor ratio is varied in Figure 8. It presents that the higher the vapor ratio is, the more the H2SO4 molecules have been produced. The H2SO4 concentration is even higher than that of HSO3 when the initial vapor/SO2 concentration ratio is above 1.27 : 1.Figure 8
Under different initial vapor additive ratio, (a) the final concentration of H2SO4, HSO3, HOSO2, and SO and (b) the SO2 removal efficiency after discharge lasted for 0.5 μs.
(a)
(b)H2O additive with higher ratio has generated more OH radicals and consequently accelerated the reactions between HSO3 and OH as(16)HSO3+OH⟶H2SO4k=9.80×10-12cm3s-1by which the HSO3 has been transformed into H2SO4. Such reaction has simultaneously decreased the HSO3 concentration and increased the H2SO4 concentration, as shown in Figure 8(a). More vapor additive has effectively adjusted the SO2 removal physicochemical kinetics to the favorite directions, and H2SO4 has become the major production when the initial vapor mixing ratio is above 1.27 : 1.For other species such as HOSO2 and SO in Figure 8(a), the former is increased at a monotonic trend, but its highest final concentration at vapor ratio of 2 : 1 is obviously lower than that of H2SO4. The latter SO is increased at low vapor ratio and decreased at high vapor ratio. Such varying trends of HOSO2 and SO are ascribed to the more OH decomposed at higher vapor ratio. The HOSO2 concentration is heightened through the reaction between OH and SO2, and the SO concentration is decreased by the reaction between OH and SO to reproduce SO2. Such reactions are formulated as follows:(17)SO2+OH⟶HOSO2k=1.31×10-12cm3s-1(18)SO+OH⟶SO2+Hk=8.60×10-11cm3s-1.In conclusion, vapor additive has effectively improved the SO2 removal efficiency in Figure 8(b). In the simulation, even 89.1% removal efficiency has been achieved at the initial concentration ratio of 2 : 1 between H2O and SO2.


## 4. Conclusions

SO2 removal is important for air pollution treatment. In this paper, the pulse streamer discharge technique is investigated. Emission spectra diagnosis implies that the SO2 molecules have been physically dissociated by the injected electrons and transformed into SO and O. In order to quantitatively clarify the complex removal kinetics, a zero-dimensional physicochemical simulating model is established. Simulation indicates that the SO2 removal without H2O vapor additive is leanly achieved with the final efficiency of only 0.296%. The injected electrical energy can improve the removal efficiency, and an increment trend is presented with the pulse duration increased. But the improvement is not very notable. After six times concentration of electrons injected, the SO2 removal efficiency is increased from 0.296% at the pulse duration of 0.5 μs to only 1.80% at the pulse duration of 3 μs. In order to improve the removal process, the H2O vapor additive is applied. Under the pulse duration of 0.5 μs and the initial concentration ratio between H2O and SO2 at 0.1 : 1, there appears remarkable increment of the SO2 removal efficiency as 13.0195%. But the major productions are HSO3 and HOSO2, and H2SO4 concentration is lower than them with 102 cm−3 magnitude order. More H2O additive has generated more OH radicals, which effectively adjusted the SO2 physicochemical removal process to the favorite directions. H2SO4 has become major production when initial vapor ratio is above 1.27 : 1. Even 89.1% removal efficiency has been achieved at the concentration ratio of 2 : 1 between H2O and SO2.From the viewpoint of energy consumption and pollutant gas removal efficiency, the H2O vapor additive is verified and effective enough to be considered for commercial applications in pulse streamer discharge system for SO2 removal.



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*Source: 872964-2015-12-07.xml*