SO 2 Gas Physicochemical Removal through Pulse Streamer Discharge Technique Assisted by Vapor Additive

SO 2 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 SO 2 molecule has been physically dissociated into SO andO 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 H 2 O vapor additive, the SO 2 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 0.5μs with H 2 O/SO 2 initial concentration ratio of 0.1 : 1. OH radicals decomposed from H 2 O 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 HSO 3 and HOSO 2 to H 2 SO 4 when vapor ratio increased above 1.27 : 1.


Introduction
Sulfur dioxide (SO 2 ) 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 SO 2 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 SO 2 emissions have propelled the advancements of SO 2 removal techniques, including spray-dry or wet scrubbing and catalyst [6,7].Jin et al. reported that chlorine dioxide (ClO 2 ) gas could be utilized to clean up SO 2 , and about 100% removal efficiency was achieved under optimal conditions of passing sufficient ClO 2 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 SO 2 removal activity [10].Mn-based activated carbon catalysts were prepared, with MnO and Mn 3 O 4 coexisting in catalysts, and had exhibited SO 2 removal ability [11].Pt/CeO 2 catalysts prepared on Cu (111) had been applied to assist the transformation of SO 2 into atomic sulfur on its surface at the temperature above 300 K [12].The Mo and Co doped V 2 O 5 /AC catalyst-sorbents were also used as catalyst for SO 2 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 ClO 2 and O 3 , exampled by the chlorate-chloride process as The ClO 2 scrubber gas is usually generated on-site since ClO 2 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 SO 2 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 O 2 , H 2 O, and so on, to generate the radical agents such as O, OH, and HO 2 , for gas-phase oxidizing SO 2 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 10 2 ∼10 3 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 (∼10 1 kV) through simpler power supply, and the discharge instruments could be miniaturized.Such pulsed discharge removing SO 2 , NO  , 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 SO 2 remediation, the pulse discharging technique can inject high energy electrons to physically dissociate the SO 2 molecules and further chemically transform the SO 2 molecules into benign or easily captured species [19,20].Gas additive mixed with SO 2 can sometimes present improvement effect.Ma et al. reported that SO 2 removal was improved by adding NH 3 into the air stream through the DBD discharge nonthermal plasma technique [21].But the (NH 4 ) 2 SO 3 or NH 4 HSO 3 production after discharge is not thermally stable enough and can further decompose into SO 2 .Since NH 3 additive for discharging removal of SO 2 is unstable, the NH 3 injection is usually utilized into the terminal of the pulse discharging instrument to collect the H 2 SO 4 aerogel dust, and the cost of injected NH 3 is also expensive [22][23][24].The catalyst combined plasma technique is also noticed.For example, TiO 2 -coated glass beads had been applied for SO 2 removal.The SO 2 removal efficiency was improved by the radicals generated from plasma reactions and TiO 2 photo-catalyst [25].
Usually, hydroxyl (OH) radicals are highly active and can be derived from the H 2 O decomposition [26].The hydroxyl radical is often referred to as the "detergent" because it can react with many pollutants [27][28][29].In this paper, the SO 2 removal physical chemistry kinetics without and with vapor additive are analyzed, and the OH improvement effect on SO 2 remediation is focused on.The pulse streamer discharge technique for SO 2 removal is introduced in Section 2. The emission spectra are detected and diagnosed for analyzing the SO 2 removal mechanism, and a zero-dimensional physicochemical reaction model is established in Section 3. Numerical simulation is quantitatively achieved.Section 4 announces the conclusions.

Experimental Section
The SO 2 removal system is diagramed in Figure 1.The SO 2 is experimentally generated through the reaction between H 2 SO 4 and Na 2 SO 3 .N 2 acts as carrier gas to deliver SO 2 gas to the discharge zone.After discharge, the residual SO 2 and other gaseous productions are neutralized by NaOH solution.
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 SO 2 molecules can be physically collided.
The discharge voltage is 9.5 kV, with the pulse duration of 0.5 s.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 SO 2 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).

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.

Emission Spectra Diagnosis.
For the pulse discharging plasma, the emission spectrum is sourced from the mechanism that the SO 2 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 SO 2 molecules are excited to energy states in a wide range.Furthermore, the more important effect of such collision is that the SO 2 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.
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 N 2 transition from its C 3 Π u excited state to B 3 Π g ground state [31].The N 2 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 N 2 emission sequence, they are evaluated as the continuous emission band of SO 2 molecule and are related to the SO 2 transition paths of , respectively [32][33][34].It means that the SO 2 has been excited to the B 1 B 1 excited state through the inelastic collision by the high energy electrons.Then, the excited SO 2 relaxes to its X 1 A 1 ground state through radiation transition.For the A 1 A 2 or a 3 B 1 excited state of SO 2 , it is transferred from B 1 B 1 state through nonradiative transition process and then relaxed to the X  collision onto SO 2 molecule has induced complex excitation and energy transition processes.
There also has been an unattached emission peak around 237.17 nm in Figure 2, which is evaluated as the characteristic emission of sulfur monoxide (SO) from its excited A 3 Π state to the X 3 Σ state [35,36].SO possesses poor stability and can only be generated by dissociation of SO 2 during the electron collision process.It indicates that some part of the SO 2 molecules has been successfully removed through the pulse streamer discharge technique.
The possible SO 2 removal routines are deduced based on the emission spectra and the evaluated transition paths as In (2), the SO 2 in ground state of X 1 A 1 state can be physically collided and excited by the electrons injected from the electrode in Figure 1 and dissociated into SO in A 3 Π excited state and O in 3 P ground state.Such dissociative threshold energy is about 10.36 eV [37].The excited SO compounds further transfer to the ground state of X 3 Σ through radiation.
There also have been other possible routines such as  the removal process is investigated through establishing a zero-dimensional reaction model.In order to improve the removal efficiency, the H 2 O vapor additive is considered.

Establishment of SO
There have been two procedures for SO 2 removal.

Physical Decomposition of SO 2 and H 2 O through Inelastic
Collision by the Electrons.The electron collision dissociative cross sections are presented in Figure 3.It should be noticed that the dissociative energy of H 2 O is smaller than that of SO 2 , and cross sections of the former are higher at about 10 1 cm 2 magnitude order than that of SO 2 .H 2 O molecule is easier to be decomposed.For the electron collision onto SO 2 or H 2 O, the physical reaction kinetics are ruled by the reaction rate coefficient, denoted as the symbol of .Such rate coefficients can be calculated by solving the Boltzmann 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 In pulse streamer discharging plasma, the SO 2 or H 2 O molecules can be physically decomposed.The new byproduct "fragments" are SO, O, OH, H, and so forth.are synthesized.There also have been reverse reactions to transform the new products into SO 2 pollutant molecules.

Further Chemical Reactions between the
The main reaction routines are graph-outlined in Figure 4.
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 the Arrhenius principle that the concentration of a given th species (one species selected from the reacting ingredients in the model, such as SO 2 , SO, SO 3 , O, O 2 , H 2 O, OH, HO 2 , H, HSO 3 , HOSO 2 , and H 2 SO 4 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 th species caused by the reaction between th and th species; then, the decreased concentration in unit time, or the losing rate of concentration   /| losing , is described as −      , in which the symbols of   ,   denoted the respective concentration of th or th species and   as the rate coefficient of the reaction between th and th species.
All the concentration decreasing processes of th species in unit time should be abstracted from the  reactions about th species losing processes and linear superimposed together as Similarly, the concentration generating processes of th species in unit time, which are abstracted from all the  reactions related to the th species generating processes based on the reaction between species th and th, are denoted as  Then, the concentration varying process of th species in unit time is decided by the losing and generating process and denoted as 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 by Runge-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, SO 2 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.

SO 2 Removal Physicochemical Kinetics without H 2 O Vapor Additive.
According to the reaction model without vapor additive, SO 2 can be dissociated by electron collision during discharge.To clarify the removal kinetics, timeresolved concentration evolution of SO 2 , O, and SO and further oxidized species such as O 2 and SO 3 are presented in Figure 5.
In Figure 5(a), the SO 2 concentration is varied at a monotonic decreasing trend when discharge time increased.The SO 2 removal has obviously been achieved through the pulse streamer discharge technique.After discharge lasted for 0.5 s, the removal efficiency is about 0.296%, which is leanly low.Most of the removed SO 2 has been transformed to SO and O 2 as shown in Figure 5(b), with the former concentration accumulating to 7.163 × 10 16 cm −3 and the latter to 3.458 × 10 16 cm −3 .For the SO 3 , its final concentration is about 1.082 × 10 15 cm −3 .When it comes to the O radicals, there appears an accumulating trend during discharge and concentration of 2.506 × 10 15 cm −3 has been accumulated.After discharge, the O species have been fast consumed out to be zero to form SO 3 , SO, and O 2 .
The removal process of the SO 2 is deduced as two procedures.The first is the decomposition of SO 2 into SO and O.The second is the oxidation process, during which the O 2 is easier to be generated through the reaction between O and SO 2 with a higher reaction rate coefficient of 1.17 × 10 −12 cm 3 s −1 than that for forming SO 3 of 3.52 × 10 −14 cm 3 s −1 .The O radical decomposed from SO 2 during discharge has played the key roles in the SO 2 removal process under the hypothesis without H 2 O vapor additive.
The injected electrical energy is essential to influence the SO 2 removal efficiency.With the discharge pulse duration widened, the inputted electron concentration is increased.Under such a variance, the removal efficiency of SO 2 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 SO 2 to be physically decomposed.The further chemical reactions for forming O 2 , SO 3 , and so on are then accelerated.
Under the discharge pulse with duration of 3 s, which bears six times energy compared to the pulse with duration of 0.5 s, the removal efficiency has only heightened to 1.796%.From the viewpoint of energy consumption, such SO 2 removal through direct decomposition by electron inelastic collision has high cost and low efficiency.

Vapor Additive Effect on SO 2 Physicochemical Removal
Kinetics.Without H 2 O vapor added, the SO 2 removal efficiency is very low.To improve the removal process, the H 2 O vapor is considered, which is usually mixed in the SO 2 exhaust gases and the out-injecting H 2 O 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   H 2 O molecules by electron collision, can participate in many chemical reactions related to SO 2 or the radicals.Even the H 2 O itself can transform SO 3 into H 2 SO 4 .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 SO 2 .In Figure 7, the time-resolved concentration variance of SO 2 and all other byproducts is presented under the initial concentration ratio between H 2 O and SO 2 of 0.1 : 1.The discharge pulse duration is the same as that in Figure 5 of 0.5 s.Compared to the 0.296% removal efficiency in Figure 5, the removal efficiency is remarkably improved by H 2 O 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 H 2 O, and the produced H and OH radicals have efficiently accelerated the SO 2 removal kinetics, which can be verified from the byproduct concentration variance in Figures 7(b), 7(c), and 7(d).
In Figure 7(b), the SO 3 formation is affected by vapor additive.Its final concentration is about 3.90 × 10 15 cm −3 , which is at the same magnitude order as that without vapor additive.But a monotonic increasing trend for the SO 3 concentration appeared, which is due to the HO 2 oxidizing SO 2 and the reaction between HOSO 2 and O 2 , though O radicals have been consumed out after discharge.
The obvious increment occurred for SO concentration in Figure 7(b), which has accumulated to 1.036 × 10 18 cm −3 after 0.5 s, and is higher than that without vapor additive of 7.163 × 10 16 cm −3 at 10 2 cm −3 magnitude order.Such a remarkable increment is decided by the H radicals, which are directly decomposed from H 2 O molecules.The H radical can react with SO 2 to produce SO and OH and is formulated in (13).Vapor additive has accelerated the SO generation efficiency: ( But the O 2 concentration of only 3.049 × 10 16 cm −3 has been obtained after 0.5 s in Figure 7(b), which is slightly lower than 3.458 × 10 16 cm −3 without H 2 O vapor added.The decrement is ascribed to the consumption of OH not only by O to produce O 2 as ruled by (14), but also by other reaction paths to produce HSO 3 , HOSO 2 , H 2 SO 4 , or even SO 2 as shown in Table 1.And the consumption of O 2 by HOSO 2 and SO is another important reason for the decrement of O 2 concentration.
The concentrations of HSO 3 and HOSO 2 in Figure 7(c) have accumulated to 1.736 × 10 18 and 0.724 × 10 18 cm −3 , respectively.For the former, it has become the major production due to its highest final concentration.There also has been 0.093 × 10 18 cm −3 H 2 SO 4 produced through the reaction between SO 3 and H 2 O or between HSO 3 and OH.And the concentration of HO 2 is about 0.084 × 10 18 cm −3 in Figure 7(c).Such low concentrations imply that both H 2 SO 4 and HO 2 are not the main final productions under the initial H 2 O/SO 2 ratio of 0.1 : 1.
All such concentration variances are decided by the H 2 O physical decomposition into H and OH through electron inelastic collision, and the H 2 O has been consumed with its final concentration decrement amount of about 1.2802 × 10 18 cm −3 after discharge lasted for 0.5 s in Figure 7(d).And the OH radicals have played the major roles for SO 2 removal to transfer SO 2 into HSO 3 : When the concentration ratio between H 2 O and SO 2 is 0.1 : 1, the major productions are HSO 3 with a little HOSO 2 , and the H 2 SO 4 concentration is lower than them with 10 2 cm −3 magnitude orders.For SO 2 removal, the main production is expected to be H 2 SO 4 , since H 2 SO 4 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 H 2 SO 4 molecules have been produced.The H 2 SO 4 concentration is even higher than that of HSO 3 when the initial vapor/SO 2 concentration ratio is above 1.27 : 1.
H 2 O additive with higher ratio has generated more OH radicals and consequently accelerated the reactions between HSO 3 and OH as HSO 3 + OH → H 2 SO 4  = 9.80 × 10 −12 cm 3 s −1 (16) by which the HSO 3 has been transformed into H 2 SO 4 .Such reaction has simultaneously decreased the HSO 3 concentration and increased the H 2 SO 4 concentration, as shown in Figure 8(a).More vapor additive has effectively adjusted the SO 2 removal physicochemical kinetics to the favorite directions, and H 2 SO 4 has become the major production when the initial vapor mixing ratio is above 1.27 : 1.
For other species such as HOSO 2 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 H 2 SO 4 .The latter SO is increased at low vapor ratio and decreased at high vapor ratio.Such varying trends of  HOSO 2 and SO are ascribed to the more OH decomposed at higher vapor ratio.The HOSO 2 concentration is heightened through the reaction between OH and SO 2 , and the SO concentration is decreased by the reaction between OH and SO to reproduce SO 2 .Such reactions are formulated as follows: In conclusion, vapor additive has effectively improved the SO 2 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 H 2 O and SO 2 .

Conclusions
SO 2 removal is important for air pollution treatment.In this paper, the pulse streamer discharge technique is investigated.Emission spectra diagnosis implies that the SO 2 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 SO 2 removal without H 2 O 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 SO 2 removal efficiency is increased from 0.296% at the pulse duration of 0.5 s to only 1.80% at the pulse duration of 3 s.In order to improve the removal process, the H 2 O vapor additive is applied.Under the pulse duration of 0.5 s and the initial concentration ratio between H 2 O and SO 2 at 0.1 : 1, there appears remarkable increment of the SO 2 removal efficiency as 13.0195%.But the major productions are HSO 3 and HOSO 2 , and H 2 SO 4 concentration is lower than them with 10 2 cm −3 magnitude order.More H 2 O additive has generated more OH radicals, which effectively adjusted the SO 2 physicochemical removal process to the favorite directions.H 2 SO 4 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 H 2 O and SO 2 .
From the viewpoint of energy consumption and pollutant gas removal efficiency, the H 2 O vapor additive is verified and effective enough to be considered for commercial applications in pulse streamer discharge system for SO 2 removal.

Figure 1 :
Figure 1: Diagram of the pulse streamer discharge system for SO 2 removal.

Figure 2 :
Figure 2: The emission spectra detected from the pulse discharge SO 2 removal system in the wavelength range from 200 to 500 nm.

Figure 3 :
Figure 3: The relationship between electron collision dissociative cross sections and the collision energy.

Figure 4 :
Figure 4: Diagram of main reaction paths and species produced during discharge.

Figure 5 :
Figure 5: Without H 2 O vapor added, (a) time-resolved evolution of SO 2 concentration and the removal efficiency, and (b) time-resolved concentration evolution of SO, O 2 , O, and SO 3 .

Figure 6 :
Figure 6: Relationship between SO 2 removal efficiency and the discharge pulse duration.

Figure 8 :
Figure 8: Under different initial vapor additive ratio, (a) the final concentration of H 2 SO 4 , HSO 3 , HOSO 2 , and SO and (b) the SO 2 removal efficiency after discharge lasted for 0.5 s.