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
Sulfur dioxide (SO2) has played important roles in acid rain formation [
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
The ClO2 scrubber gas is usually generated on-site since ClO2 can rapidly decompose through photo dissociation [
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 [
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,
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 [
Usually, hydroxyl (OH) radicals are highly active and can be derived from the H2O decomposition [
The SO2 removal system is diagramed in Figure
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 0.5
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).
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.
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 [
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
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
There also has been an unattached emission peak around 237.17 nm in Figure
The possible SO2 removal routines are deduced based on the emission spectra and the evaluated transition paths as
In (
There also have been other possible routines such as
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.
The electron collision dissociative cross sections are presented in Figure
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
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.
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
Main reactions and the corresponding rate coefficients.
Reactions |
|
---|---|
SO2 + O → SO3 | 3.52 × 10−14 [ |
SO2 + O → SO + O2 | 1.17 × 10−12 [ |
SO + O → SO2 | 5.10 × 10−31 [ |
SO + O2 → SO2 + O | 7.60 × 10−17 [ |
SO3 + SO → SO2 + SO2 | 1.99 × 10−15 [ |
SO2 + OH → HSO3 | 7.40 × 10−12 [ |
SO2 + OH → HOSO2 | 1.31 × 10−12 [ |
HSO3 + OH → H2SO4 | 9.80 × 10−12 [ |
SO + OH → SO2 + H | 8.60 × 10−11 [ |
SO2 + H → OH + SO | 3.06 × 10−12 [ |
SO3 + H2O → H2SO4 | 1.20 × 10−15 [ |
HOSO2 + O2 → HO2 + SO3 | 4.30 × 10−13 [ |
SO2 + HO2 → OH + SO3 | 2.01 × 10−17 [ |
H2O + H → OH + H2 | 4.20 × 10−12 [ |
OH + O → O2 + H | 3.00 × 10−11 [ |
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
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 the
All the concentration decreasing processes of
Similarly, the concentration generating processes of
Then, the concentration varying process of
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
It should be noticed that there are no spatial variables in (
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
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.
In Figure
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
Relationship between SO2 removal efficiency and the discharge pulse duration.
Under the discharge pulse with duration of 3
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
In Figure
Under H2O/SO2 initial ratio of 0.1 : 1 and discharge pulse duration of 0.5
In Figure
The obvious increment occurred for SO concentration in Figure
More OH production is beneficial to the O2 generation according to
But the O2 concentration of only 3.049 × 1016 cm−3 has been obtained after 0.5
The concentrations of HSO3 and HOSO2 in Figure
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
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
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
H2O additive with higher ratio has generated more OH radicals and consequently accelerated the reactions between HSO3 and OH as
by 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
For other species such as HOSO2 and SO in Figure
In conclusion, vapor additive has effectively improved the SO2 removal efficiency in Figure
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
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.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work is financially supported by National Nature Science Foundation of China (no. 10875036) and the Fundamental Research Funds for the Central Universities (no. 12MS146).