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Ind. Eng. Chem. Res. 1999, 38, 1844-1855
Chemical Reaction Kinetics and Reactor Modeling of NOx Removal in a Pulsed Streamer Corona Discharge Reactor G. Sathiamoorthy, S. Kalyana, W. C. Finney, R. J. Clark,† and B. R. Locke* Department of Chemical Engineering, FAMU-FSU College of Engineering, Florida State University and Florida A & M University, 2525 Pottsdamer Street, Tallahassee, Florida 32310-6046
The removal of nitrogen oxides (NO and NO2) from dry nitrogen gas and dry air with and without ethylene using a pulsed streamer corona discharge reactor (PSCDR) was investigated. A kinetic model was developed to characterize the chemical reactions taking place in a PSCDR using a combination of the CHEMKIN and KINEMA programs. Concurrently, an experimental program to determine NO removal from a gas stream was conducted. The electron density for the reactions of N2 and O2 by electron-molecule collisions was obtained by fitting the model to experimental data on NO removal as functions of the applied electric field and the reactor residence times. The model was then used to predict the concentrations of various other species, including O3, NO2, N2O, and byproducts of ethylene decomposition, to test the model and to guide the experimental analysis of byproduct characterization. Introduction Although considerable progress has been achieved in the reduction of nitrogen oxide emissions, the release of nitrogen oxides into the atmosphere from the combustion of fossil fuels in both stationary and mobile sources continues to be a major environmental problem. Among the many processes that have been studied for the removal of nitrogen oxides from combustion gases is nonthermal plasma oxidation using a pulsed streamer corona discharge. A pulsed-streamer corona reactor utilizes a high-voltage electrical discharge between electrodes of nonuniform geometry to produce ionization waves (streamers) through the growth of electron avalanches formed by electron impact ionization events in the gas.1 A streamer is a region of highly ionized gas where a wide range of very reactive radicals and chemical species are formed through collisions among electrons, molecules, and ions. These reactive species in turn initiate bulk phase reactions that lead to the removal of various pollutants. A number of experimental and theoretical studies have been conducted to assess the applicability of pulsed streamer corona discharge for the removal of nitrogen oxides,2,3 sulfur dioxide,4 volatile organic carbons,5-7 chlorofluorocarbons, and Halon.8 The chemical reaction mechanisms for the removal of nitrogen oxides (NO and NO2) by gas phase nonthermal plasma reactions have been studied extensively.2,3,9-12 In a gas consisting of predominantly molecular nitrogen in the absence of oxygen, NO is reduced to N2.12,13 In the presence of oxygen, however, NO is generally converted to NO2 through a variety of reactions that may include direct reaction with dissociated oxygen or reactions with ozone. NO2 may be removed through subsequent reactions with hydroxyl radicals created from organic species or water vapor. These reactions then lead to the formation of nitric acid aerosols (or salts with ammonia injection) * Corresponding author. Telephone: 850-410-6165. Fax: 850-410-6150. E-mail:
[email protected]. † Present address. Department of Chemistry, Florida State University, Tallahassee, FL.
which, in turn, may be removed by scrubbers, particle filtration devices, or electrostatic precipitators. Heterogeneous reactions with aerosol particles may also play a role in nitrogen oxide removal from waste gases especially when water vapor is present.14,15 Modeling of plasma discharges is a necessary complement for experimental investigation not only because it helps in understanding the fundamental chemistry and physics governing the discharge but also because it is necessary for the design and analysis of the performance of plasma devices. Some of the major difficulties that arise in modeling the performance of pulsed corona discharge reactors are as follows: (1) estimation or measurement of the unknown parameters of the streamer structure and streamer propagation, (2) coupling streamer propagation models or pulse characteristics to subsequent bulk phase chemical reactions, (3) obtaining accurate values of bulk phase chemical reaction rate constants, and (4) determining which of the numerous bulk phase chemical reactions are most important for the analysis. Modeling of corona discharges have therefore consisted of two general areas: streamer propagation analysis and bulk chemical reaction analysis. Due to the complexity, there have been only a few attempts to connect the two aspects. One study, performed at IVTAN in Moscow,16 uses a reaction code with over 900 individual chemical reactions that is coupled to solutions of the one-dimensional streamer simulations. The streamer simulation results provide initial conditions on the concentrations of primary radicals and ions used in the subsequent bulk kinetic simulations. The chemical reactions used in this model were adapted from earlier work on electron beam technology.17 The extensive nature of these chemical reactions arises from the many molecular, ionic, and radical species that are formed from an exhaust gas that may include SO2, NO, NO2, CO, CO2, O2, N2, and H2O. Other simulations have been developed that use hundreds of reactions11 and have attempted to connect the streamer discharge characteristics to the bulk chemical reactions through methods similar to that of the Moscow group. Recent work has shown, however, that a system of approximately 50 chemical reactions can describe the
10.1021/ie980544y CCC: $18.00 © 1999 American Chemical Society Published on Web 03/27/1999
Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999 1845
Figure 1. Diagram of the power supply and pulse forming spark gap.
reaction system for nitrogen oxides in air fairly well.10,18 The availability of software such as KINEMA19 for easy solution of the electron distribution in electrical fields provides the motivation to develop alternative modeling approaches for electrical discharge reactors. Some quantitative and qualitative comparisons between various experimental measurements and theoretical model predictions for gas-phase pulsed corona discharges have been conducted;12 however, quantitative prediction of reactor performance has not yet been fully developed due to some of the difficulties mentioned above. It is clear that in order to more fully characterize the performance of a pulsed corona reactor, to improve the design of such reactors, and to estimate potential byproducts, a quantitative comparison of reactor models to experimental data is necessary. In the present study, the removal of NO from dry nitrogen, from dry air, and from dry air containing ethylene by a positive pulsed streamer corona discharge is investigated. The experimental data are used to assist in the development of the reactor model through fitting of the data on NO removal in order to determine one unknown constant, namely the average electron concentration in the reactor. The data are also used to compare the predictions of the model for other reactants and products, namely, NO2, O3, ethylene, and ethylene byproducts. The reactor model, in turn, is used to assist in the performance of experimental measurements through prediction of expected reactor byproducts, especially in the case of ethylene degradation. Experimental Apparatus and Procedures The pulsed power supply used in this investigation is shown in Figure 1. It consists of two major units: (1) a high voltage transformer/rectifier (T-R) set and (2) a pulse-generating apparatus. The power supply used was similar to the configuration used by Clements et al.4 The high voltage DC T-R set was modified by eliminating the internal rectifier, thereby producing 0-100 kV AC at 0-28 mA. Output from the T-R set was connected to the pulse generation circuit, where the current passes through a high voltage resistor (R1 ) 333 kΩ) and then through a diode array which acts as a half-wave rectifier. The output from the rectifier charged a bank of capacitors (C1 ) 2700 pF) during the charging cycle. A mechanical rotating spark gap having two fixed brass spherical electrodes and a rotating rod electrode was employed to discharge the capacitor bank twice per shaft revolution, synchronized with the ac current (60 Hz). From the output of the spark gap the circuit had two parallel pathways. One passed through a load resistor (R2 ) 300 kΩ) which enabled the gap to fire. The other was connected to the central wire within the cylindrical
reactor, from which the discharge emanated. The generation of pulsed corona discharge by a rotating spark gap was a source of electromagnetic interference; therefore, the pulse generating apparatus was encased in aluminum Faraday cage to reduce RF noise. The pulsed streamer corona reactor used in this study was a 10 cm i.d. and 45 cm long cylindrical stainless steel tube. The high voltage electrode (0.27 cm in diameter and 30.5 cm in length) was suspended concentric to the grounded outer cylinder resulting in an electrode separation of 5 cm. The active pulsed corona treatment volume of the reactor was 2.45 L. The typical gas flow rate and residence time ranges in the reactor were from 5.1 to 10.2 SLM and from 14 to 30 s, respectively. Analysis of the outlet gases from the reactor included the measurement of NO, NO2, O3, and hydrocarbon breakdown products. Nitrogen oxides (NO and NO2) were measured using a Thermo-Environmental Instruments, Inc. (Franklin, MA) chemiluminescence NOx analyzer. A Perkin-Elmer (Norwalk, CT) Autosystem XL gas chromatograph equipped with a flame ionization detector and with a J & W Scientific (Folsum, CA) alumina DS 30 m by 0.53 cm column was used for the analysis of hydrocarbons. Ozone concentration was determined by using the potassium iodide method.20 The pulse waveform characteristics were measured using a Tektronix (Beaverton, OR) TDX 460 fourchannel digitizing oscilloscope with a Tecktronix P6015A 1000x high voltage probe and a Tektronix TCP 202 AC/ DC current probe. The current probe was sensitive to 7 ns rise time and 50 A peak current. A Cole-Parmer TriSense velocity/temperature/humidity measurement system was used to measure the gas temperature and humidity in the gas system. The analytical instrumentation, the reactor configuration, and the gas feed system are shown schematically in Figure 2. The feed gas system consisted of a set of carrier gases and a set of trace gases (added in ppm range). The carrier gases used in the study were compressed air and compressed nitrogen. Cylinders of compressed nitrogen were obtained from either Air Products or Holox (min purity 99.998%). Building air supply was passed through two laboratory gas drying units (calcium sulfate packed columns) and an air purifying system to remove any oil residue and moisture, before reaching the reactor. NO in dry air at 1% and pure C2H4 (Air Products Inc., Allentown, PA) were used as the trace gases. The concentration of these components in the feed gas was in the range of 100-500 ppm. The gas flows were monitored and controlled by flow meters (Dwyer Inc., Marietta, GA) and mass flow controllers (MKS Instruments Inc., Andover, MA). The trace gases were controlled using a MKS 1259C type of controller and the carrier gas was regulated using a MKS 1559A mass flow controller. The trace gases were mixed with the carrier gas before entering the reactor. This was accomplished by sending all the gases into a 60 cm long stainless steel mixing chamber that had a diameter of 6.25 cm. Experiments were designed to determine the extent of NO removal from several gases with different compositions in order to study the reaction mechanisms for NO removal. The experiments were conducted at four different residence times (14, 20, 24, and 30 s) and at three different peak voltages (40, 44, and 49 kV). The voltage values were chosen so as to span the range from
1846 Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999
Figure 2. Diagram of the reactor and feed system.
the inception (ca. 10-15 kV) to sparkover (ca. 60 kV) for the given reactor. Since no observable breakdown occurred much below 40 kV the practical operating range was 40-50 kV. The residence times were chosen so that the NO removal from a 100 ppm feed gas spanned the range from 0 to 100%. All of the experiments were performed three times to ensure the reproducibility of the results. The average gas concentrations determined in repeat runs were used to plot NO and NO2 concentrations as functions of residence time and peak voltages. Experiments were conducted on NO removal in an atmosphere of nitrogen followed by experiments in dry air with and without ethylene. It is known that significant amounts of ozone are produced by corona discharge in dry air.21 To have an independent check on the kinetic model, experiments were carried out at 40, 48, and 58 kV for the three residence times 29, 35, and 46 s where the amount of ozone produced was measured. In these experiments, the feed gas did not contain any NO. Mathematical Model Formulation In a pulsed corona discharge in a gaseous medium, free electrons gain energy from an imposed electric field and lose energy through collisions with neutral gas molecules. This transfer of energy to the gas molecules leads to the formation of a variety of species including ions, metastables, atoms, and free radicals. These products are chemically active and react with other gas molecules to form stable compounds. The kinetic rate constants for the electron-gas reactions depend on the imposed electric field and on the gas composition. The reaction rate constants for the electron impact collisions with species i can be given by19
ki )
x
∫0∞xeσi()f0() d
2 me
(1)
where σi() is the collisional cross section for species i, f0() is the Boltzmann distribution obtained from the
local equilibrium approximation to the Boltzmann equation, and me is the mass of an electron. Note that in this equation ki is the second-order rate constant for electron collision with other gas-phase species. To connect the chemical reactions to the physical aspects of the streamer propagation, it is important to consider the various time scales involved in the process. Generally streamers propagate very fast across the gap space between electrodes, and the initial electronmolecule collisions occur rapidly with respect to subsequent radical/molecule and radical/radical reactions. Under these conditions, it has generally been assumed that the subsequent reactions of radicals and molecular species are independent of the characteristics of the pulse; only initial reactions of electrons depend on the electric field.16 Although efforts have been made to couple streamer propagation models to subsequent bulk phase chemical reactions, the difficulty and computational effort are very high, and these predictions generally do not lead to accurate evaluation of actual reactor data and performance. In addition, these approaches require severe limiting assumptions on the nature of the streamer structure (i.e., ideal uniform disk structures). It is therefore necessary to further develop the streamer/chemistry models to allow quantitative comparison of experimentally measured reactor output with predictive models. The modeling effort here accounts for the chemistry in pulsed streamer corona discharge by considering two chemical reaction sets. During the time frame of pulsing (pulse-on period), the electron concentration was assumed to be fixed, and the reaction set included the electron-molecule collision reactions in order to calculate the concentration profile of different species. During the time between pulsing (pulse-off period), the electron concentration was assumed to be zero, and so the reaction set which did not have the electron-molecule collision reactions was used to calculate the concentration profile. These two reaction sets were interchanged
Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999 1847 Table 1. List of Chemical Reactions Considered for Modeling chemical reaction
rate constant (cm3 mol-1 s-1)
source
number:
N2 f 2N O2 f O + O O f O(1D) N + N f N2 N + O f NO N + O2 f NO + O N + NO f N2 + O N + NO2 f 2NO N + NO2 f N2O + O O + O f O2 O + O2 f O3 O + O3 f 2O2 O(1D) + O3 f 2O2 O(1D) + O3 f 2O + O2 O(1D) + N2O f N2 + O2 O(1D) + N2O f 2NO 2NO + O2 f 2NO2 NO + O3 f NO2 + O2 NO + O f N + O2 NO + O + N2 f NO2 + N2 NO + NO3 f 2NO2 NO + NO3 f 2NO + O2 NO2 + O3 f NO3 + O2 NO2 + O f NO + O2 NO2 + NO3 f NO2 + NO + O2 N2O4 + M f 2NO2 + M 2NO3 f 2NO2 + O2 NO3 + O f NO2 + O2 O3 + N f NO + O2 2NO2 + M f N2O4 + M NO2 + O f NO3 O(1D) f O O(1D) + O3 f O + O3 O(1D) + NO2 f NO + O2
k1[e] k2[e] k3[e] 6.38E+10 1.45E+11 5.36E+07 2.04E+13 1.81E+12 1.81E+12 1.02E+11 1.69E+12 4.81E+09 1.45E+14 1.45E+14 2.95E+13 4.03E+13 7.09E+09 1.09E+10 2.77E+09 3.27E+16 1.74E+13 1.83E+11 1.92E+07 5.84E+12 6.45E+08 2.06E+09 1.92E+08 1.02E+13 6.02E+07 5.08E+14 1.29E+12 6.95E+08 1.45E+14 2.95E+13
exptl data exptl data exptl data Willis & Boyd, 1976 Willis & Boyd, 1976 Atkinson et al., 1989 Willis & Boyd, 1976 Atkinson et al., 1989 Atkinson et al., 1989 Willis & Boyd, 1976 Atkinson et al., 1989 Willis & Boyd, 1976 Atkinson et al., 1992 Atkinson et al., 1992 DeMore et al., 1987 DeMore et al., 1987 Atkinson et al., 1992 Atkinson et al., 1992 Demore et al., 1987 DeMore et al., 1987 DeMore et al., 1987 NIST Atkinson et al., 1989 Atkinson et al., 1989 Baulch et al., 1983 Baulch et al., 1983 Baulch et al., 1983 DeMore et al., 1987 Baulch et al., 1983 NIST Atkinson et al., 1989 DeMore et al., 1987 NIST NIST
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34
repeatedly to simulate the pulse-on, pulse-off aspect of the discharge through use of a shell program on a Unix operating system with CHEMKIN22 for solving the basic ordinary differential equations. The experimentally determined pulse width of the voltage pulse, which was the time at half-maximum, varied between 500 and 700 ns. The total pulse duration from the inception of the rise to the complete decay of the voltage peak was approximately 1 µs. In the present modeling, the pulse width was assumed to be 1 µs and the time between the two pulses was assumed to be 16.667 ms, corresponding to a pulse frequency of 60 Hz. Hence the reaction set with electron collision reactions was used to calculate the concentration profile for 1 µs, and the reaction set which did not include the collision reactions was used to calculate the concentration profiles for the next 16.667 ms repeatedly, until the final total time equaled the residence time in the reactor. This pulsing method is similar to the approach used by Peyrous et al.18 for computing ozone production in a cylindrical oxygen-fed ozonizer. It is assumed that there is no appreciable heating of the gas during the residence time in the reactor, and therefore the reactor is isothermal. The flow is ideal plug-flow; i.e., the model does not account for axial and radial dispersion in the reactor. It is also assumed that there is no variation either in space or in time of the concentration of electrons during the pulse-on period. Although ionization takes place in the corona discharge, it was assumed in the present model that the ions do not play a major role in the chemistry of the neutral gas molecules. Since the field at the streamer tip is generally much higher than the average field (20-45 Td [Townsend, where Td ) 10-21 V m2]) used in the
present calculations, this is only a rough approximation and a more detailed sensitivity analysis of the reactions, including ions, is necessary. In addition, the model does not account for the variation in electric field strength from the anode wire to the cathode cylinder. The molar species continuity equations for a plug flow reactor model without axial dispersion or molecular diffusion are given by
dci dτ
)
∑j vijRij
(2)
where ci is the concentration of species i, τ ) x/ux is the reactor residence time, ux is the molar average velocity down the axis of the reactor, vij is the stoichiometric coefficient of the ith species in the jth reaction and Rij is the reaction rate of the ith species in the jth reaction. The above system of ordinary differential equations was solved using the CHEMKIN software 22 on a Dec Alpha workstation. The set of reactions used for modeling NO removal from carrier gases of either dry nitrogen or dry air is shown in Table 1. The rate constants for these reactions were obtained primarily from the atmospheric chemistry literature23-30 and were crosschecked with the NIST Database.31 The necessary reaction rate constants have not been measured in a pulsed corona reactor. Since the pulsed corona discharge occurs at atmospheric temperature and pressure and since the time and length scales associated with the streamer propagation are very small in comparison the chemical reactions, it is therefore assumed that most reactions between longer lived species can be estimated
1848 Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999
by known rate constants from the literature. The criterion for choosing the reactions was to consider reactions of neutral species having rate constants >1.0 × 107 cm3 mol-1 s-1. A preliminary parameter analysis was performed by including several reactions with rate constants below this range and the calculated results were found to not vary. A more detailed sensitivity analysis is proposed for future work. The dissociation reactions considered which were dependent upon the electric field were
N2 + e- f N + N + e- r ) - k1[e][N2]
(3)
O2 + e- f O + O + e- r ) - k2[e][O2]
(4)
O + e- f O(1D) + e- r ) - k3[e][O]
(5)
Reaction 3 represents the production of nitrogen radicals by all possible mechanisms including dissociation, ionization, and excitation. Similarly, the rate constants for reactions 4 and 5 represent the global kinetic rate constants for the production of oxygen radicals. The kinetic rate constants, k1, k2, and k3, are dependent upon the applied electric field. The KINEMA19 program solves the Boltzmann equation using a two term Legendre polynomial approximation and calculates the second-order reaction rate constants k1, k2, and k3 for reactions 3-5. Since the electron concentration in a given reactor is not known without solution of the detailed streamer model equations and since each of the above reaction rates depend on the electron concentration, the present study uses electron concentration as a single field dependent empirically determined constant through fitting experimental data to the model. The KINEMA program was used to determine the relative rates of reactions 4 and 5 to the rate of reaction 3 through input of overall gas composition and electric field conditions. Therefore, only one adjustable constant, namely the effective reactor electron concentration, is needed to describe the experimental data and to compare the data to the model. The results from the computations are presented in terms of concentrations as functions of residence time for various voltages because of the nature of the computations performed using the CHEMKIN/KINEMA approach. Since CHEMKIN solves the set of ordinary differential equations for the concentrations of individual species as functions of residence time for various voltages inserted into the KINEMA program it is more convenient to plot the data in this way. To make the program produce plots in terms of specific energy density, as often used in nonthermal plasma processes, it would be necessary to continuously adjust the reaction rate constants in the model for the variable energy density. This would require a very substantial programming effort and does not add new information in the model/data comparison which is the major objective of the present study. Results and Discussion Physical Characteristics of the Discharge. Pulsed corona onset (corona initiation voltage) in the reactor was found to occur between 13 and 14 kV in an atmosphere of dry air and at about 10 kV in a dry nitrogen atmosphere. Below the onset of corona, the reactor voltage and current readings measured using the oscilloscope were very small. Above the corona onset
Figure 3. Voltage, current, and power waveforms for a feed gas of dry air.
the peak voltage and the current increased dramatically with voltage. The rise time of the voltage pulses was found to vary between 15 and 30 ns. Plots of typical voltage, current, and power waveforms for air are shown in Figure 3. As seen in the figure, there is a sharp increase from zero in the voltage (14-20 ns rise time), followed by an exponential decay. The pulse width at half-maximum varied from 500 to 700 ns. The current pulses were shorter in width (
