9---air pollution control by electrical discharges-heckam

654 IEEE Transactions on Dielectrics and Electrical Insulation Vol. 7”o. 5, October 2000

Air Pollution Control by Electrical Discharges

R. Hackam and H. Akiyama Department of Electrical and Computer Engineering

The University of Kumamoto Kumamoto, Japan

ABSTRACT Air pollution caused by gas emission of pollutants produced from a wide range of sources including coal, oil and gas burning power plants, diesel engines, paper mills, steel and chemical production plants must be reduced drastically and urgently, as mandated by recent worldwide national legislation which recently are being reinforced increasingly by international agreements. Non-thermal plasmas in which the mean energy of the electrons is substantially higher than that of the gas offer advantages in reducing the energy required to remove the pollutants. The electrical energy supplied into the discharge is used preferentially to create energetic electrons which are then used to produce radicals by dissociation and ionization of the carrier gas in which the pollutants are present. These radicals are used to decompose the pollutants. There are two technologically promising techniques for generating non-thermal plasmas in atmospheric gas pressure containing the pollutants, namely electron beam irradiation and electrical discharge techniques. Both techniques are undergoing intensive and continuous development worldwide. This is done to reduce the energy requirement for pollutant removal, and therefore the associated cost, as well as to obtain a better understanding of the physical and chemical processes involved in reducing the pollutants. In the present paper only electrical discharge techniques are reviewed and emphasis is given to the more recent published work. The paper summarizes the chemical reactions responsible for the removal of the major polluting constituents of NO, NO2 and SO1 encountered in flue gases and exhaust emissions. The constructional features of the various types of electrical discharge reactors commonly employed in the removal of gas pollutants as well as pilot systems used in industrial plants are described briefly. The results on the removal efficiency of the various pollutants including hydrocarbons and volatile compounds and their dependency on the type of discharge reactor, the type and the magnitude of the applied voltage (dc, ac and pulsed), the polarity of the voltage (dc and pulsed), the effect of the pulse width, the initial concentration of the pollutants, the addition of ammonia, argon and other hydrocarbons, the gas flow rate, the residence time of the pollutants in the discharge reactor, the gas temperature and on the type of the gas constituents will be reviewed. The removal of pollutants using arc plasmas will be discussed. The specific energy density which is supplied into various forms of electrical discharges to reduce the pollutants will be discussed. The energy required to remove the pollutants is expected to be one of the main considerations in selecting the technology to be used to remove the pollutants and therefore it is of prime importance.

1 INTRODUCTION tional agreements. These pollutants are major causes of acid rain, planet warming, smog and also increasingly are being thought of as detrimen-

IR pollution caused by gas emission of pollutants produced by tal to human health, to vegetation growth and to fresh water lakes. A a variety of sources including coal, oil and gas burning electric Several techniques have been applied in recent years to remove pol- power generating Plants, motor vehicles, diesel engine exhausts, paper lutants from the air with various degrees of success. Non-thermal plas- mills and steel and chemical production plants [1-111 must be substan- mas in which the mean energy of the electrons is substantially higher tially reduced as mandated by recent national legislation and interna- than that of the ions and the neutrals offer considerable advantage in

1070-9878101 $3.00 0 2000 IEEE

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 7No. 5, October 2000 655

reducing the energy requirements to remove the pollutants. Under cer- tain conditions the electrical energy fed into the discharge is used preferentially to create energetic electrons instead of heating the ions and the neutral gas molecules. The energetic electrons are employed to dis- sociate and ionize the carrier gas molecules in which the pollutants are present to produce radicals. The radicals are then used to decompose the pollutants.

There are essentially two main techniques that have been developed for creating non-thermal plasmas at atmospheric pressure containing the pollutants. These methods are electron beam irradiation and electrical discharge. Both methods are undergoing continuous development at an intensive level worldwide due to the urgency of achieving mandated legislated emission reduction. The current efforts to improve the technologies of pollution reduction have the main objectives of reducing the energy requirements as well as achieving a better understanding of the collision processes involved in the removal of the toxic pollutants in the flue gases and in simulated gas mixtures containing pollutants.

Electron beam irradiation techniques have been investigated widely and were applied in pilot projects [2,10-201. However, in the present paper only electrical discharge techniques will be reviewed and partic- ular emphasis will be given to the more recent published work. Only reported results, available in the public domain, are in this review. A reference to the electron beam irradiation method will be made for comparison purpose wherever applicable.

This paper will begin with a brief review of the main reactions which are thought to be responsible for the removal of the major polluting constituents of NO, NO2 and SO2 encountered in flue gases and vehicle exhausts. A brief description of laboratory and pilot industrial systems will be given. The essential constructional features of the various types of electrical discharge reactors commonly employed in the removal of gas pollutants in flue gases will be discussed. The dependence of the removal efficiencies of the pollutants on the geometry of the reactors, the type of the discharge reactor (without dielectric barrier, with dielectric barrier and surface discharges), the type and the magnitude of the applied voltage (dc, ac and pulsed), the gas flow rate, the residence time of the pollutants in the discharge reactor, the gas temperature, the type of gas constituents which carry the pollutants, the polarity of the applied voltage (dc and pulsed), the effects of the pulse width, the initial concentration of the pollutants, the addition of hydrocarbons, argon, ammonia and other compounds will be reviewed. The removal of pollutants using arc plasmas will be discussed. The most recently reported results on the specific energy consumption to remove NO, NO2 and SO2 and the prevalent toxic pollutants also will be reviewed.

2 CHEMICAL REACTIONS FOR REMOVALS OF POLLUTANTS

A substantially large number of studies have been directed at reducing or eliminating particularly the main polluting constituents in flue gases which are the NO, and SO,. The NO, is generally referred to as the sum of the concentrations of NO and NO2 while the SO, is primarily composed of SO?. The concentration of the pollutants are

usually given in parts per million (ppm) of the pollutant of the main carrier gas. 1 ppm in l.01x105 Pa of the carrier gas such as air or other mixtures at 273 K is equal to a density of 2 . 6 9 ~ 1 0 ~ ' mol/m3. The main carrier gas is usually air with enhanced amounts of water vapor (HzO), carbon dioxide (CO?), carbon monoxide (CO) and varying amounts of NO, NO2 and SO?.

The application of HV to a pair of electrodes immersed in a gas con- tainiig the pollutants creates high energy electrons which produce ions and free radicals. The main objective of producing the plasma is to provide the radicals N, 0 and OH which interact with the pollutant molecules [21]. Depending on the constituents of the gas mixture the following reactions have been suggested widely to occur, in which the radicals N, 0, and OH are produced in the presence of water vapor 1221

e t N 2 + e t N t N (1) e t Nz + 2 e t N t t N (2) e t 02 + e + o ( ~ P ) + o ( ~ P ) (3) e t 02 --* e + O(3P) + O('D) (4) e t 02 + 2e + 0 + O+ (5)

e t H20 i e t H t OH (6) e t H 2 0 + 2 e t O H t H ' (7)

f ' " " ' " ' I " ':.;' " ' I " ' ' 1

1 0 2 0 50 100 150 200 250 300

E (kV/cm)

Figure 1. Initial yield of neutrals in an electrical discharge from electron impact processes. Gas mixture: 7% N2, 5% 0 2 , 10% H20, and 15% CO,. Electric field corresponds to gas pressure of 1 .01~10~ Pa and 300 K 1221.

Figure 1 shows the initial yields of neutral species calculated by Pen- etrante [22] which are produced in an electrical discharge by electron impact processes in a gas mixture of 70% N2, 5% 02, 10% HzO and 15% CO2 at a pressure of 1 .01~10~ Pa and 300 K. The values shown in Figure 1 which were derived from the excitation, dissociation and ionization reactions of WZ, 02 and C02, were considered reliable but those from the dissociation of HzO were uncertain by a factor of < 2 [22].

Radicals may also be produced by other reactions such as electron attachment to and dissociation of water molecules as shown in Figure 2

656 Hackam et al.: Air Pollution Control by Electrical Discharges

10'1 , , ' " ' ' I " " ' ' ' ' " " . ' ' ' ' ' ' : OH Production i

. mnuibubw tfam anachment

o 50 i o 0 150 200 250 300 E (kV/cm)

Figure 2. Calculated production of OH radicals in an electrical discharge by collision reactions with H20 of electron attachment, excited oxygen atoms, dissociation by electrons and positive ions as a function of electric field. Gas mixture and other conditions are as in Figure 1 P21.

[221 e t H2O ---t H t OH (8)

(9)

(10) (11)

0 (1D) t H20 + 2 0 H And from reaction (5) by charge transfer processes [22]

0' t Nz. + 0 t N2+ Ot t 0 2 --t 0 t 02+

Figure 2 shows the production of the radical OH as a function of the electric field at 1 . 0 1 ~ 1 0 ~ Pa in an electrical discharge from collision reactions with H20 of electron attachment, excited oxygen atoms, dissociation by electrons and positive ions [22]. It will be shown later in this paper that the addition of water to the gas mixture aids con- siderably the removal of NO, and SO,. This is partly due to the added production of the radical OH which then interacts with and reduces the pollutants.

The radical OH may remove NO according to the following reaction [21-231

OH t NO t M + HN02 t M

where M is a third molecule which can be, say N2. In the latter case the reaction rate Klz = 6 . 7 ~ 1 0 ~ ~ ~ cm6/s [5,23-261. NO2 may be reduced by the radical OH [2, 22,26,28,29]

OH t NO2 t Nz + HN03 t N2

where K13 = 2 . 6 ~ 1 0 - ~ ~ cm6/s [5,25-281. And for SO2 [2, 21,30,34, 351

OH t SO2 + HS03

with a reaction rate of K14 = 7 . 4 ~ 1 0 ~ ~ ' cm3/s [34,35].

(12)

(13)

(14)

Reaction (15) is followed with the production of the radical H02 [2], HS03 t 0 2 + HOz t so3 (15)

so3 t H20 + (16) HO2 t NO + NO2 t OH (17)

NO2 reacts with H02 to produce HN03 and the radical 0 [25] NO2 t H 0 2 t N 2 + HNOJ t N2 t 0 (18)

with a reaction rate of Kla = ~ . I X ~ O - ~ ~ cm6/s [25]. Reactions (14) to (17) oxidize SO2 and NO which generate OH that

also reacts with NO2 according to (13). NO is also removed by oxidation with the radical 0 [2,22,23,31,32]

with a reaction rate of Klg = 6.9x1OW3' cm6/s [23]. The latter coefficient is in agreement with the value of 6 . 3 ~ 1 0 ~ ~ ~ cm6/s reported by

with a rate coefficient of K15 = 4 ~ 1 0 ~ ' ~ cm3/s [34]. And [2]

O t N O t M + N 0 2 t M (19)

[301. Similarly [24,25,31,34],

O t S O z t M + S 0 3 t M (20)

(21)

(22)

with a reaction rate K20 = 8 . 2 ~ 1 0 - ~ ' cm6/s [24,25,33,34]. And [25] so3 t H2O + H2S04

And [6] SO2 t H20 + H2S03

H2SO3 is easily oxidized to H2SO4 [19]. Ammonia is added to neutralize H2S04 to form ammonium sulfate [6,21,25],

NO may be reduced directly by the reaction with N [15,21,22,28,32],

with a reaction rate of K24 = 5.9~10-l1 cm3/s [28].

in reaction (19), may decompose into NO by [23,28,31],

with a reaction rate K25 = 9.0x10-12 cm3/s [23,28].

suggested [23,28]

with a reaction rate of KZS = 5 . 2 ~ 1 0 ~ ~ ~ cm3/s [23,28].

HzS04 t 2 NH3 + (NH4)2S04 (23)

N t N O + N 2 t O (24)

NO2 which initially is present in the flue gas as well as that produced

N t N O z + 2N0 (25)

In the presence of ammonia (NH3) the following reactions have been

(26) NH3 t NO2 + HN02 t NH2

N is also removed by [23-25,27,28], NH2 t NO + H20 t N2

with a reaction rate of K27 = 2 . 1 ~ 1 0 ~ ~ ' cm3/s [23-25,27,28]. NH2 IS created by (26) and

NH3 t OH + NH2 t H20 with a reaction rate KZS = 3 ~ 1 0 - l ~ cm3/s [30].

3 ELECTRICAL DISCHARGE

The development of electrical discharge technology for the reduction of pollutants was spurred in part by the high capital cost of the equipment associated with electron beam irradiation systems. The electrical discharge techniques involve a creation of a plasma in a flowing gas within a reactor. There are different designs of discharge reactors which attempt to produce very high electric field by employing thin wires, sharp needles and narrow pipes. The HV electrodes are placed

(27)

(28)

REACTORS


between grounded plates which can be planar, in the form of a rect- angular duct, or cylindrical, either metal or dielectric wrapped with a metal foil. However, cylindrical reactors have been used very widely in simulation studies because of the relative simplicity of their construction. -

gas .-.+a? --+ gas in out

high-voltage

('I,

wire anode dielectric tube

- \ I

-w.2+ gas 7;s -L+ out

hlgh-voltage

metal sheet around dielectric tube

wire anode dielectric pellets - \

/ dielectric tube \ (C )

metal sheet around dielectric lube wire anode

Figure 3. Three types of discharge reactors. (a) concentric coaxial electrodes for pulsed corona, (b) coaxial or planar electrodes for dielectric barrier (also called silent) discharge (dielectric barrier either on one or both electrodes when plates are employed) and (c) cylindrical pellet- bed reactor (electrodes may be in the form of flat metal mesh placed on both sides of the cylinder containing the pellets) [36].

Embedded eleclrode (Surface H V. eleclrode) Suitace eleclrode

',\ / Ceotial eleclfode

(Sllenl H V eleclrode)

Figure 4. Schematic diagram of combined surface discharge and dielectric barrier discharge reactor (281.

There are essentially four types of discharge reactors three of which are shown in Figure 3 [36] and the fourth in Figure 4 [28,23]. The construction of the Dulse corona reactor (Figure 3(a)) uses a thin wire to

and typically N 15 to N 200 ns duration is applied. The low voltage electrode could be cylindrical (Figure 3(a)) or a flat plate and is grounded.

The short duration pulses produce energetic electrons which generate radicals that decompose the pollutants. Due to the limited duration of the narrow pulses, the discharge is short lived and consequently the ion and gas molecules remain effectively at the ambient temperature. Thus only a negligibly small amount of the supplied electrical energy is dissipated to heat the gas. This type of reactor (Figure 3(a)) can be adapted readily to industrial precipitators which use wire-plate geometry [10,11].

The second type of reactor is shown schematically in Figure 3@) [36] and is called 'dielectric barrier discharge' reactor or 'silent discharge' reactor. This reactor employs a thin dielectric sheet of glass, quartz or ceramic, either planar or cylindrical in shape, of - 1 to - 5 mm in thick- ness and on its outer surface a metal foil which is grounded. The central HV electrode usually is a thin wire. A high alternating voltage, 50 or 60 Hz or higher frequency of N 1 to 100 kHz is usually employed with this reactor [lo, 111 but also a pulsed voltage has been used extensively [36]. Dielectric barrier discharge reactors have been used widely in the commercial production of ozone for water purification for a long time [37,38]. In this type of reactor (Figure 3(b)) microdischarge streamers are generated in the gap which self-extinguish when the electric field is reduced by the buildup of surface charge on the dielectric. When a short pulsed voltage is used with this type of reactor, a higher electric field to pressure ratio ( E / P ) can be achieved which results in a larger production of radicals to destroy the pollutants. This leads to a more efficient operation of this dielectric barrier discharge reactor.

The third type of discharge reactors is the dielectric pellet-bed discharge reactor shown in Figure 3(c) [36]. It uses high permittivity dielectric pellets which are held between two metal electrodes to which usually HV ac is applied, but pulsed operations also have been employed. In Figure 3(c) the voltage is applied between the central wire and the outer cylinder, but in other arrangements two mesh forming flat electrodes are placed on both sides of the cylinder, holding the pellets therein. The pellets are polarized and a high field is created at each pellet upon application of the external voltage and microdischarges are created at each pellet.

The fourth type of discharge reactor is the 'surface discharge' reactor. It is composed of a series of strip electrodes placed on the surface of alumina ceramic, while a thin metal electrode is embedded inside the ceramic. The ceramic could be in a planar [39] or a cylindrical form 1281. HV ac is applied to generate the surface discharge, but pulsed operations were used also. Figure 4 shows a schematic diagram of a cylindrical surface discharge reactor where strip electrodes are placed on the surface, an electrode is embedded inside a ceramic and a central rod electrode [28]. A surface discharge is formed between the strip electrodes; in this arrangement a dielectric barrier (or silent) discharge is also formed between the central and the surface electrodes [28].

A schematic arrangement of a positive pulsed corona discharge reactor to remove the NO,, and SO2 from the flue Eases of an iron ore " sintering plant followed by an electrostatic precipitator (ESP) is shown in Figure 5 [6]. The ESP uses negative HV dc to collect the products which a very short, positive or negativek' pulse of - 10 to - 60 kV


FLUE CAS FROM

Figure 5. A schematic diagram of a pilot system for removal of NOx, SO2 and solid particulates from the flue gases of iron-ore sintering plant [6].

Figure 6. Positive pulsed discharge reactor for removal of NO, and SO1 from the flue gases of an iron-ore sintering plant followed with an electrostatic precipitator to collect ammonium sulfate, ammonium nitrate and other solid particulates. Wire diameter, 3 mm. Up to 5000 Nm3/h of the flue gas can be treated N, normalized to l.01x105 Pa and 0°C [6].

of ammonium sulfate, ammonium nitrate and other solid particulates. Both the discharge reactor and the ESP employ thin wires (3 mm in diameter) and plate electrodes combinations which are depicted in Fig- ure 6 [6]. This reactor can treat up to 5000 Nm3/h (N-normalized to 0°C and l.01x105 Pa) of the flue gases. The ammonia and other additives are inserted into the gas stream at the inlet of the corona discharge reactor (Figure 5) [6].

Another variation of the corona type discharge reactor which employs a pipe electrode and either positive or negative HV dc, is shown in Figure 7 [40,41]. In this reactor additives, such as ammonia, are injected into the gas stream via the pipe electrode (Figure 7).

A typical laboratory setup for studying the removal of pollutants using a concentric wire cylinder geometry is shown in Figure 8 [32]. The simulated gas mixture is obtained from commercially available cylin-

H.V. (Positive DC)

4- Gasnow

Plate elecimle Exhausr

------,I

u u UULJ Addiriod 8s

Figure 7. Schematic diagram of it discharge reactor employing a pipe as the HV electrode. Ammonia and other gases are fed to the gas stream via the pipe electrode [40,41]

O.S.C.

Plrmnsl rompuar

Figure 8. Schematic diagram of a laboratory apparatus for the study of pollutant removal using pulsed corona in a coaxial discharge reactor with a concentric wire [32].

ders. The percentage of added moisture can be varied and is deter- mined by adjusting the temperature of the water. The composition of the gas is analyzed at the exhaust of the reactor. The flow rate of the gas is adjustable and is measured using flow meters. The temperature of the gas can be raised by placing the reactor in an oven to simulate the actual temperatures of the flue gases in boilers and diesel exhausts.

A more effective coaxial discharge reactor for the removal of gas pollutants employs a spiral wire, usually wound on a hollow insulator rod. A typical discharge reactor that employs this principle of construction is shown in Figure 9 [42]. This reactor employs a copper wire, 1 mm in diameter, wound on a polyvinyl chloride tube 3 cm in diameter and is placed concentric in a copper cylinder having 96 mm inner diameter and is 3.5 m long.

Typical corona pulses of the voltage and current and the variation of the waveforms with pulse width are shown in Figures 10(a) and (b), respectively [43]. The pulse width (or duration) is defined in the pulsed power technology as the full-width-at-half-maximum (FWHM) of the voltage pulse. It will be shown later in this paper that a short pulse is very desirable in obtaining a higher energy efficiency for the removal of pollutants. The electrical energy supplied into the discharge reactor is calculated from the digitized signals of the applied voltage to and

IEEE Transactions on Dielectrics and Electrical Insulation

O.S.C.

351111 mm i / J . .... . ._. >

Figure 9. A laboratory setup employing a coaxial discharge reactor with spiral central electrode. Copper wire, 1 mm diameter, spiral pitch, 10 mm; central tube diameter, 30 nun; outer cylinder, 96 mm; reactor length, 3.5 m [42].

the discharge current in the reactor using J vidt, where v (in V) is the voltage, i (in A) the discharge current, and t is the time (ins). A typical dependence of the energy per pulse on the pulse width is shown in Figure 11 [43], where it can be seen that the energy pulse increases with increasing pulse width. This is of importance when determining the energy density required for the removal of pollutants from flue gases.

A typical magnetic pulse compressor (MPC), the type of which is increasingly being used to produce short pulse durations that are directly applied to discharge reactors for pollutant reductions is shown in Figure 12 [44].

4 DEVELOPMENT OF THE STREAMER DISCHARGE

It has been reported that applying short durations and fast rising positive pulses to a concentric coaxial wire-to-cylinder discharge reactor resulted in superior room-temperature performance in the removal of NO and NO2 than using negative pulses [4]. Similarly in a discharge reactor which employed a needle or rod to plate electrodes geometries, positive pulsed corona performed better than negative corona in removing SO2 [7]. This has been attributed to the production of more uniform streamers that extended further into the gap for positive pulsed streamers [7]. For example it was shown that in a positive wire-planar geometry in l.01x105 Pa air, there were more streamer channels (8 chan- nelsicm) compared to the negative wire (1 to 2 channels/cm) and the latter had a reduced branching compared to the positive pulsed corona [45]. For the same reason of more abundant and spread out streamers the positive pulsed corona led to a higher production of ozone than that for the negative polarity in the wire-cylinder configuration [46].

Figure 13 shows the abundance of positive streamer discharges in air when a single pulse of 60 kV and having 200 ns duration was applied to a wire of 1.5 mm in diameter and a plate with a gap of 20 mm [32]. The observed density of the streamers shown in Figure 13 was typically 6 streamer channels/cm. Figure 14 shows the positive pulsed streamer

I Vol. 7 No. 5, October 2000

40

k 6 m 0 m 0 > IC. -

F L --80 ns ----- 100 ns

- - - F, I , I I , , I I I I I I , I , , , , I , , , , I , , , , , 0 100 200 300

Time, ns

a 100

?! IC.” C

L s o

-40 ns . -60 ns

t- W

-100 E-,,,,,,,,,,,,,,,,,, 0 100 200 300

Time, ns IQ)

Figure 10. (a) Applied voltage to and (b) discharge current in a coaxial discharge reactor for different pulse widths. Discharge reactor: central wire, stainless steel 0.5 mm in diameter; outer cylinder, copper 76 mm in diameter; length of reactor 0.5 m. Gas mixture: 200 ppm NO, 5% Oz,4% HzO,91% Nz; gas flow rate 2.0 limin; pressure l.01x105 Pa; temperature 25°C [a].

__.._.___..... - - - 5 F

659

0 100 200 300 Time, ns

Figure 11. Input energy to a coaxial discharge reactor per pulse for different pulse widths. Curve 1,40 ns; curve 2, 60 ns; curve 3, 80 ns; curve 4,100 ns; curve 5,120 ns. Gas mixture and reactor type are as in Figure 9 [U].

in air after 3 pulses using a shorter duration of the applied voltage of 100 ns and a longer gap distance of 38 mm [47].

Figure 15 shows the development of positive streamers in a flowing nitrogen (2 l/min) filled coaxial discharge reactor (wire diameter 2 mm, inner diameter of outer cylinder 76 mm, length 500 mm) at a pressure of 1 . 0 1 ~ 1 0 ~ Pa, when successively a larger number of positive pulses (50 kV, 100 ns) were applied to the wire [48]. Figure 15 shows that the


I

Figure 12. Typical circuit diagram of an MPC used to produce short duration HV pulses for pollution control. GTO, gate turn off thyristor; C1, primary energy storage capacitor; SL1 and SL2 saturable induc- tors; PT, step-up pulse transformer; C3 peaking capacitor [44].

Figure 13. Positive pulsed streamer discharges in air between wire (top) and grounded plane electrode (bottom). Wire diameter 1.4 mm, gap distance 20 mm; applied voltage 60 kV; pulse duration 200 ns; pressure l.01x105 Pa; temperature 25T; single shot 1321.

Figure 14. Positive pulsed streamer discharges in air between wire (top) and grounded plane electrode (bottom). Wire diameter 1.0 mm, gap distance 38 nun; applied voltage 50 kV; pulse duration 100 ns; pressure l.01x105 Pa; temperature 25°C; number of pulses 3 [47].

streamers gradually extend across the gap with increasing number of pulses from 1 pulse (Figure 15(a)) to 30 pulses (Figure 15(f)).

5 DEVELOPMENT OF dc CORONA

There have been extensive studies of emission control employing HVDC to create corona discharges of either negative [5,26,27], positive

Figure 15. Development of positive pulsed streamers in nitrogen in coaxial discharge reactor. Wire diameter 2 mm, inner diameter of the outer cylinder 76 mm, length of reactor 500 nun; applied voltage 50 kV;pulse duration 100ns; pressure 1 . 0 1 ~ 1 0 ~ Pa; temperature 25°C; gas flow rate 2 I/min; number of total applied pulses: a, 1; b, 3; c, 5; d, 7; e, 10; f, 30 1481.

[4,7,25,40,41,49,50,51] or a combination of dc and pulsed corona discharges [7,26]. Generally dc corona discharges are less energy efficient than fast rising, short duration pulsed corona for the removal of pollutants from flue gases.

Figure 16 shows a typical dc corona current us. applied positive dc voltage for the treatment of a flowing (5.3 l/min) mixture of gases containing 8016 N2,20% 0 2 and 155 ppm NO, with and without additions of argon (Ar), ammonia (NH3) and Nz t 02 (401. The discharge reactor used has the electrode geometry as depicted in Figure 7. It will be observed from Figure 16 that the corona discharge current increased with increasing applied dc voltage, and at fixed voltage the current increased with increasing flow rate of the additives Ar and NH3 (Ar: NH3 = 973). The current increased further when additional N2 t 0 2 (N2: 02 = 8020) was injected into the simulated flue gas. Argon was employed to minimize the generation of negative ions in the corona discharge [5, 401. The increased corona current with increasing gas flow rate was attributed to stabilizing electrohydrodynamic flow effects [52]. At flow rates t0.4 l/min of the added NZ t 02, the corona discharge became unstable and was transformed into a spark discharge which was less effective and therefore not suitable for NO, removal.

Figure 17 shows the corona discharge current in the flue gases from


Additional gas N2+02 Ar+NH3

0.8 I/min 0.1 \ /min A 0.8 Vmin 0 Vmin 0 0.4 l/nin 0 I/min

E

U

a Applied voltage (kV)

Figure 16. Corona current vs. positive dc voltage in a simulated mixture of flue gases containing 80% N2,20% 02, and 155 ppm NO and with additives of argon and ammonia (NH,). Conditions: discharge reactor shown in Figure 7; distance between pipe electrode and duct wall 25 mm; concentration of NH3 in Ar 3%; flow rate of treated gas 5.3 l/min; gas pressure l .01x105 Pa; flow rates of additives are shown on the Figure [40].

-10 -20 -30 -40 -50 APPLIED VOLTAGE (kV)

Figure 17. Corona discharge current os. negative applied voltage in flue gas from a heavy oil-fired boiler with additives of either ammonia or ammonia and argon. Conditions: Flue gas contained 200 to 1000 ppm SO2; 50 to 200 ppm NO,; 13 to 14% 02; gas temperature 65°C; water vapor concentration not specified; flow rate 1200 m3/h. t 10 l/min of NH3; A 2 l/min of NH3 with 16 l/min of Ar [5].

a heavy oil-fired boiler vs. negative dc applied voltage with additives of either ammonia or ammonia and argon [5]. The flue gases contained concentrations of 200 to 1000 ppm of SO2 and 50 to 200 ppm of NO,. After applying water spray for cooling, the oxygen concentration was 13 to 14%. The temperature of the flue gases was reduced from an initial high value of 280 to 65°C by using a heat exchanger and atomized water injection [5]. The additives of Ar and NH3 were injected into the corona discharge reactor via a hollow electrode (5 mm outside diameter and 10 cm long) which also formed the corona electrode. It will be observed from Figure 17 that the corona discharge increased with

I Vol. 7 No. 5, October 2000 661

increasing applied voltage. Further, the addition of a mixture of ammonia and argon (NH3: Ar = 3:97) led to a larger discharge current than that of adding a pure ammonia [5].

After 10 h of operation of this reactor the currents were observed to increase at the same appliedvoltage. This was attributed to the accumu- lation of particulate contaminants from the flue gases on the electrode surface leading to an enhanced electric field therein [5]. In this pollution control test which was carried out on the Fujisawa plant in Japan using a combination of electron beam irradiation and corona discharge the particulate of ammonium sulfate and ammonium nitrate were col- lected by an electrostatic precipitator and a baghouse filter. The cleaned flue gases were released through the stack by an induced draft fan [5].

A Needle IoPlate c

0 10 20 30 + dc bias voltage ( k V )

Figure 18. Corona discharge current us. applied positive dc voltage for dc, and for pulse t dc in rod-to-plate and needle-to-plate geometries. Needle tip radius of curvature 0.1 mm; rod diameter 5 mm with 2.5 mm radius of tip curvature; electrode spacing 4 cm; gas mixture air with 1666 ppm SO2 and 2.5% H20; pressure 1 .01~10~ Pa; temperature 22°C; flow rate 1.2 I/ min; peak of pulse 45 kV; repetition rate 60 PPS [71

For rod-to-plate and needle-to-plate discharge reactor geometries the dependence of the corona current on the applied dc voltage in a mixture of air, 1666 ppm of SO2 and 2.5% HzO, an electrode gap distance of 4 cm, at a pressure of l.01x105 Pa, 22°C and a gas flow rate of 1.2 limin is shown in Figure 18 [7]. The corona current is shown for positive dc voltage and for a combined pulsed and dc voltages [7]. Fig- ure 18 shows that under the same applied voltage the corona current was higher for the positive needle-to-plate than for the positive rod-to- plate geometries. Under the same conditions the corona current was higher for the positive pulsed t positive dc voltages than for positive dc voltage (Figure 18) [7]. For positive dc cor0n.a using needle-to-plate electrodes at t30 kV, which was slightly below the sparking voltage and therefore the highest that could be used, 78% of SO2 was removed while for negative corona of even a higher voltage of -50 kV only 54% was removed. Thus positive dc voltage had a better performance than negative. Further, it was concluded that the removal efficiency of SO2 was poorer in the dc corona compared with the pulsed corona treat- ments [7].


6 REMOVAL OF POLLUTANTS USING dc VOLTAGE

The reduction of NO and NO2 in a simulated mixture of gases containing N2: 02: C02: NO = 83.96 8: 8: 0.04 as a function of applied negative dc voltage with ammonia (NH3) and methane (CHa) injections are shown in Figures 19 and 20, respectively [26]. The initial concentration of NO of 438 ppm decreased to 398 ppm when ammonia was injected in a ratio of NH3/NO = 0.5 without applying the voltage. Fig- ure 19 shows that NO continued to decrease with increasing voltage. At the highest voltage of 34.7 kV, NO decreased to 197 ppm constituting a removal ratio of 55%. Using Fourier transform infrared (FTIR) measurements, trace amounts of HN03 and N20 were detected in agreement with other studies [9,24,27,53]. Figure 20 shows the concentrations of NO, NO2 and NO, as a function of the negative dc voltage with methane injection [26]. It will be observed that the initial NO concentration was not affected by the methane injection without the corona discharge. The maximum removal ratio of NO in the latter case was 52.7% which occurred at -30 kV (Figure 20) [26].

I N W 400 500 r-7

O J I 0 10 20 30 40

dc voltage [kv

Figure 19. Concentrations of NO, NO2 and NOx as a function of applied negative dc voltage with ammonia injection. Initial concentration of NO 438 ppm; NH3/NOx = 0.5; flow rate 5 I/min; gas mixture NZ: 02: C02: NO = 83.96 8: 8: 0.04; pressure l.01x105 Pa; discharge reactor plane to plane [26].

In a simulated mixture of gases of N2: 02: CO2 = 89.4: 2.63 7.9 with - 0.5% H20 containing 200 ppm NO and 800 ppm SO2 the dependence of the NO, and SO2 removal ratios on the applied positive dc voltage are shown with addition of ammonia in Figures 21 and 22, respectively [25]. The gas flow rate was 5 l/min and the controls of the pollutants were carried out at room temperature. It will be observed from Figure 21 that the NO, removal ratio increased with increasing applied positive dc voltage. - 82% of NO, was removed at 24 kV The contaminated electrodes resulted in larger removal efficiency of NO, in agreement with other studies [54,55] and was attributed to surface reactions of aerosol particles with NH3 radicals and NO, [53].

Figure 22 shows the removal ratio of SO2 as a function of the applied positive dc voltage with the addition of ammonia at a concentration of

0 10 20 30 40

dc vokage [kq Figure 20. Concentration of NO, NO? and NO, as a function of negative dc voltage with methane (CHa) injection CH4/NO, = 1 Other conditions are as m Figure 19 [26]

P

40

E I

Voltage (kV)

Figure 21. Dependence of the removal ratio of NO, on positive dc voltage with ammonia injection for clean and contaminated electrodes. Gas mixture: N2: 0 2 : CO2 = 89.4: 2.63: 7.9 t 200 ppm NO t 800 ppm SOz with - 0.5% H20; pressure l.01x105 Pa; room temperature; gas flow rate 5 l/min; injected additives (NH3/(NO t SO2)) = 1.2; (NH3 (10%) t Ar) diluted with air [25].

[NH3/(NO t SOz)] = 1.2 [25]. It is noteworthy, and as can be seen from Figure 22, that < 85% of SO2 can be removed without applying any voltage. This was attributed to the reactions with SO2 of the aerosols generated by the adiabatic expansion cooling of air-Ar-NHs mixture which was injected into the simulated flue gas [53,56]. Figure 22 shows that the removal efficiency of SO2 increases slightly with increasing applied positive dc voltage to 23 kV, and then remains constant with further increase in the voltage when the electrodes are clean. The dom- inant reactions for SO2 removal are the formation of ammonium sulfate [(NH&S02] by particle surface reactions between NH3 and SO2 molecules [25,56]. After - l h of operation of the discharge reactor, the electrode surfaces became contaminated with aerosol particle deposition and the removal efficiency of SO2 decreased (Figure 22) [25].

The dependence of the removal ratios of NO (a) and NO, (b) on the dc input power to the corona discharge is shown in Figure 23 for a sim-

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 7 N o . 5, October 2000 663

G 8Q

- +Clean electrode 2 0 Contaminated electrode

e! 0" BO U)

I I I I I 0 4 8 12 16

(a) Corana power (W)

I I 0 4 E 12 16

@) Corona power (W)

Figure 23. Dependence of removal ratios of NO (a) and NO, @) on dc input power to the corona discharge. Discharge reactor is shown in Figure 7. Gas mixture: 80% N2 t 20% 02 t 149 ppm NO at 5 and 10 l/min t 80% NZ t 20% 02 at 0.8 I/min t Ar t NH3 (3%) at 0.1 I/min; positive dc voltage; pressure 1 .01~10~ Pa; 0 5 l/min, 10 l/min ~401.

ulated flue gas mixture of N2: 02 = 80%: 20% t 149 ppm of NO at 5 and 10 l/min and using additional gases injected into the flue gas of 80% Nz t 20% 02 at 0.8 l/min and (97% Ar t 3% NH3) at 0.1 l/min [40]. The discharge reactor used in this study [40] is shown in Figure 7 [40,41]. The dependence of the corona current on applied voltage from which

the power was calculated is shown in Fi ure 16 [40]. Fi re 23 shows

tio of NO (Figure 23(a)) and on NO, (Figure 23@)). It will be observed that both NO and NO, removal ratios decreased substantially with increasing the flow rate of the gas from 5 to 10 l/min. The gas residence time in the corona region was calculated to be 19 s for the higher flow rate of 10 l/min [40].

the effect of the flow rate of the simulate (H flue gases on t E removal ra-

100 *.*..*.-*--.

75 s B

I I I 0 1 2 3 4

Time (h)

Figure 24. Dependence of NOx reduction rate on time of application of positive dc voltage in a corona discharge reactor. Conditions: simulated flue gas: 80% NZ t 20% 02 t 190 ppm NO at flow rate 4.9 l/min t 19% 02 t 4.76% CO2 t 0.23% NH3 t 76% N? at flow rate of 0.84 l/min; pressure l.01x105 Pa; discharge reactor as per Figure 7 [41].

The dependence of the NO, removal ratio on the time of application of a positive dc voltage is shown in Figure 24 for a simulated mixture of gases containing 80% NZ t 20% 02 t 190 ppm NO at a flow rate of 4.9 l/min and seeded with 19% 02 t 4.76% CO2 t 0.23% NH3 t 76% N2 flowing at a rate of 0.84 I/min [41]. The time to reach a complete removal of NO, depended on the applied voltage level. At 28 kV it took - 1 h to reach complete removal (100%) of NO, while at higher voltage the steady state condition was reached at a much faster time (- 10 min) (Figure 24) [41].

The removal of CO2 from a simulated gas mixture of Nz: 02: CO2 = 0.744 0.158: 0.98 at l.01x105 Pa was investigated using dc corona with the addition of argon [57]. The discharge reactor employed a pair of cylindrical hollow electrodes through which the gas mixture flowed [52,58]. The removal of CO2 increased with increasing corona discharge current and with the addition of argon (Figure 25) [57]. It also increased with addition of an argon admixture to 0.4 (ratio of argon to gas mixture) and thereafter decreased with a further increase in the argon content to a ratio of 2.4 (Figure 25). The loss of CO2 was accompanied by a corresponding increase in the concentration of CO which is partly produced by collision of excited atomic oxygen with CO2 [57,59]. The production of CO in relation to CO2 depended on the discharge current and it increased with increasing current [57].

7 REMOVAL OF POLLUTANTS USING ARC PLASMAS

A few studies have been conducted on the removal of NO, and SO2 from simulated exhaust gases by creating an arc plasma using argon


I d ( m A 1

Figure 25. Reduced CO2 from simulated combustion gas as a function of dc corona discharge current for different ratios 7 of argon to simulated gas mixture. Gas mixture: N2: 02: CO2 = 74.4 15.8: 9.8; 7 = Ar/(N2+02tC02); total flow rate 0.7 I/min pressure l.01x105 Pa.

[60,61], nitrogen [62,63] and with N2 arc plasma injection into the simulated gas mixture [64,65]. A typical NO, removal ratio in a mixture of 346 to 2317 ppm of NO, to 11% 02, to 8% COX, and the balance N2 with an undetermined amount of water vapor flowing at 20 l/min and a plasma jet generated in nitrogen and injected into the mixture containing the NO at a flow rate of 13 l/min is shown in Figure 26 as a function of dc plasma power [64]. The dc arc plasma (20 to 35 A, 15 to 60 V) was generated using thoriated tungsten electrodes. The plasma generates N radicals which remove the pollutants. Figure 26 shows that when the plasma power is in the range 0.8 to 1.2 kW the NO, removal ratio is at 100%. At higher powers the temperature of the plasma was reported to increase and the removal ratio of NO, decreased (Figure 26) and this was attributed to the regeneration of NO, [64]. At low powers ( 4 . 6 kW) the plasma arc was unstable, resulting in a lower removal of NO, (Figure 26) and this was attributed to incomplete dissociation

4 " " 1 I ' " " 2 ' " Power kW

Figure 26. Dependence of NO, removal ratio on injected input power in nitrogen plasma to a simulated flue gas. Gas mixture: 346 to 2317 ppm NO, to 11% 02; to 8% Cor, balance N2 and undetermined amount of H20; flow rate of gas mixture 20 l/min; flow rate of N2 plasma jet 13 l/min; plasma arc 20 to 35 A, 15 to 60 V [64].

Figure 27 shows the NO, removal ratio as a function of power of

Power kW

Figure 27. NO, Removal ratio as a function of dc power of plasma jet in N2 and for different 0 2 concentrations. 2150 ppm of NO in the simulated gas mixture; other conditions are as in Figure 26 [64].

a dc arc plasma jet in nitrogen for different amounts of 02 content in the range 3.3 to 10.23% [64]. The initial concentration of NO was 2150 ppm while all other conditions remained as for Figure 26 [64]. It will be observed from Figure 27 that the maximum reduction of NO, occurred at a low power of arc plasma of N 600 W and the removal ratio decreased with increasing power and increasing oxygen content. With larger power, NO, increased rather than decreased, and Figure 27 shows this as negative removal ratios (64).

8 REMOVAL OF POLLUTANTS USING ac

Discharge reactors for pollution control using dielectric barriers (Figure 3@)) [23,28,67,69,70-72,74771, with dielectric pellets (Fig- ure 3(c)) [66,68,73,75] or surface discharges [23,28] often employ alternating current (ac) at the power frequency of 50 and 60 Hz or at higher frequencies. Dielectric barrier [36] and surface discharge [39] reactors also have been used with pulsed power, but these will be discussed later in this paper (Section 9).

100 200 300 400 500

Inlet NO Coocenvawn (ppmv)

Figure 28. Dependence of the removal ratio (solid line) and the absolute (dashed line) of NO on its initial concentration using ac (60 Hz) in a dielectric barrier discharge reactor. Gas mixture: N2: 02: H20 C Q = 76: 6 6 12; pressure l.01x105 Pa; temperature 130°C; discharge reactor: concentric cylindrical 2.4 mm central rod, quartz 40 mm in diameter, 2 mm thick; flow rate 2.7 I/ min; applied voltage constant at 23 and 25 kV [67].

Using a cylindrical quartz tube of inner diameter of 40 mm, and 2 mm thick with 2.4 mm diameter central rod, a dielectric barrier dis-


charge was formed with a stainless steel wire mesh screen on the outside [67]. 60 Hz was used to initiate a discharge in a simulated gas of N2: 02: H20: CO2 = 76: 6: 6 12 while the NO concentration was varied from 150 to 500 ppm. Figure 28 shows the removal ratio of NO (solid line) as well as the absolute values removed (dashed line) using ac (60 Hz) as a function of the initial concentration of NO [67]. The input power into the discharge was kept constant for two fixed applied voltages of 23 and 25 kV. A 94% removal ratio of NO was obtained at 25 kV applied voltage at the lowest initial concentration of 150 ppm of NO but this removal ratio decreased to about 58% at 500 ppm. Fig- ure 28 also shows that both the absolute number of the removed NO and the removal ratio increased slightly with increasing ac voltage at a fixed power deposition into the discharge. It was also found that the NO removal ratio decreased slightly with increasing concentration of NO at a fixed applied voltage and increased with increasing H20 [67].

800

700

600

"-7

g 500 e 3 400

U1

2 I

300 U

200

t o o

0

NO2 .......*... ...................... _ . ,.*"

0 2 4 6 8 10

power &Vl

Figure 29. Dependence of NO, NO2 and NO, on ac input power for in-phase and combined surface and dielectric barrier discharges. Dis- charge reactor is shown in Figure 4. Gas mixture: NI t CO2 t 02 (proportions were not specified) 800 ppm NO; flow rate 2 b i n ; pressure l.01x105 Pa [28].

A discharge reactor (Figure 4) containing a dielectric barrier and a surface discharge was used with ac (60 Hz) to study the removal of NO, with the two discharges combined either in-phase or out-of- phase [23, 281. Figures 29 and 30 show the dependence of the concentrations of NO, NO? and NO, on the input power for in-phase and out-of-phase applied ac voltages, respectively [28]. A simulated gas mixture containing an initial concentration of 800 ppm of NO and NP t CO2 t 02 (proportions were not specified) was used [28]. It will be observed that there are large reductions in the concentrations of NO and NO, for the out-of-phase (Figure 30) compared to the in-phase (Figure 29) discharges.

Figure 31 shows the dependence of NO and NO, removal on the input energy (J/l) to an ac dielectric barrier discharge operating at 10 to 100 kHz [70]. The discharge reactor employed a flat soda glass 2 mm

Vol. 7 No. 5, October 2000

5 500 - 1 s E

400 2 U

300

NOx

NO

----------* NO - -_ '.

665

........... .............................. IO0 I,,,,;'.''; ; N O 2

0 .' 0 2 4 8 8 10

power Iwi

Figure 30. As for Figure 29, except for out-of-phase combined surface and dielectric barrier discharges [28].

200 - 5 150 U

c 0 .- % 100 .P

0 0

U 8 50

0 10 100

Input energy density [J.L-'I

Figure 31. Dependence of NO and NO, removals on input energy density to an ac (10 to 100 kHz) dielectric barrier discharge. Initial concentration of NO 200 ppm; gas mixture 90% Nz, 10% 02; pressure l.01x105 Pa; temperature 10 to 25°C; dielectric barrier, flat soda glass 2 nun thick; each block A-type electrode (x 1) = 275 needles, 13 mm high, 1.4 mm in diameter on a holder base 84x 58 mm2; up to 3 blocks used; flow rate 5 to 20 l/min 1701.

thick as a dielectric barrier and up to three blocks of needle electrodes, each comprised of 275 needles of 13 mm in height and 1.4 mm in diameter set on a holder base of 84x 58 mm2. The flow rate was 5 to 20 l/min. Figure 31 shows that the initial concentration of NO of 200 ppm in a mixture of Nz: 02 = 9: 1 decreased with increasing input energy into the discharge and it was independent of the number of the needle electrodes [70]. Figure 31 shows that it was possible to remove NO completely when the ac input energy was >150 J/I [70].

The reduction of NO, from the exhaust gases of a diesel engine was investigated using 50 Hz power applied to a cylindrical barrier discharge reactor [71]. The reactor comprised a stainless steel rod 6 mm in diameter and was 300 mm long. The outer electrode was an aluminum


5 6 7 0 9 1011 1 2 0 1 4 1 5 -P Applied voltage (kV)

0 0324 153 2.88 Input power [kVAl

Figure 32. NOx reduction as a function of applied ac (50 Hz) voltage and input power into a coaxial Pyrex glass dielectric barrier discharge reactor. Exhaust gases from a diesel engine. Curve 1,5 discharge reactors per transformer; curve 2, lO discharge reactors per transformer [711.

foil placed on the outer surface of a Pyrex glass tube, 1.6 mm thick and 16.8 mm in inner diameter [71]. Figure 32 shows the dependence of the removal ratio of NO, on the applied ac (50 Hz) voltage and input power into the discharge in exhaust gases from a diesel engine [71]. The removal ratio of NO, increased with increased voltage and increased input power. Furthermore, a higher removal ratio was obtained when the input impedance of the transformer was smaller at 31.25 kR (Fig- ure 32, curve 1) compared to 62.5 kR (Figure 32, curve 2) [71].

The effects of SO2 concentration on SO2 removal ratio was studied using 60 Hz at 24 kV peak to peak in a coaxial dielectric barrier discharge reactor employing glass [69]. A mixture of S02, Nz, and 02 at a pressure of 8 . 7 ~ 1 0 ~ Pa at room temperature was used. The SO2 concentration was in the range 300 to 14000 ppm. The highest reduction ratio was 70% which occurred at low flow rate and this decreased to - 15% at a high flow rate. Increasing the pressure at the same applied voltage resulted in a reduction in the removal of SO2 due to the lowering of the electric field to the pressure ratio ( E / P ) causing a lowering of the electron density. Similarly, lowering the applied voltage for the same gas pressure and fixed flow rate resulted in a lowering of the SO2 removal [69].

A dielectric barrier discharge reactor employing two flat plates glass (3 mm t h i c k 38 cmx 70 cm) with a gap spacing of 3.5 mm and aluminum electrodes with 1.2 ! d z ac driven power source (3 kW) was used to remove three volatile organic compound (VOC) in dry air [72]. The voc removal of trichloroethylene (TCE), methylethylketone (MEK) and methylene chloride (MECL) were studied over a wide range of energy density input to the discharge using both ac and pulsed voltages and are shown in Figures 33 to 35 [72]. The pulse corona reactor employed a concentric coaxial stainless steel electrodes with a wire of 0.5 mm in diameter, 25 mm of inner diameter of the outer cylinder and 90 cm long. A positive pulsed voltage having a rise time of 6 ns, a duration of full-width-at-half-maximum (FWHM) of about 20 ns and a peak voltage of - 26 kV. The results from the pulsed corona are also presented

0 PukedCorons 0 ACDielecMcRanier

g 0.1 4 0.01 -

0 100 200 300 400 600 Energy Density (JA)

Figure 33. Dependence of the removal ratio of the volatile organic compound (VOC) trichloroethylene (TCE) in dry air 8s. energy density Discharge reactors: flat bed dielectric barrier (1.2 kHz) and pulsed corona discharge reactors; initial concentration of TCE 200 ppm; dielectric reactor: flat glass 3 mm thick, 38 cmx 70 cm, gap separation 3.5 mm; pulse corona reactor: coaxial, stainless steel electrodes, 0.5 mm wire diameter, 25 mm cylinder inner diameter, 90 cm long; pulse voltage: rise time 6 ns, FWHM 20 ns, peak voltage 26 kV; X final concentration of vOC, X , initial concentration [72].

1

0.1 -

- 0 ACDielecMcBanier

L 0 . 0 1 ~ " " " " ' " " " ~ ' " ~

0 1 2 3 4 Energy Density (mi)

Figure 34. As for Figure 33 except for the removal of methylethylketone (MEK). Initial concentration of MEK in air 1000 ppm [72].

in figures 33 to 35 together with the removal ratios using ac [72]. The removal ratios of all three voc shown in Figures 33 to 35 decrease with increasing energy density u/1) input into the discharge for both the ac and pulsed driven power sources [72]. Figures 33 and 34 show that the removal ratio X / X o ( X = final concentration, X, = initial concentration of VOC) for TCE and MEK, respectively in dry air mixture were similar for both types of energy density of either ac or pulsed. Figure 35 demonstrates that for MECL in air also similar energies were required from either sources up to 1 kJ/l but for higher energy density the pulsed corona discharge had significantly better removal efficiency. The difference in the removal efficiencies shown in Figure 35 was attributed to the increased temperature of the gas. This was measured and is shown for the pulsed corona reactor in Figure 35. The removal efficiency was also independent of the concentration for 100 to 200 ppm of TCE and

Figure 36 shows a comparison between the removal efficiency of TCE in air for ac (1.2 kHz) and pulsed dielectric barrier discharges [72].

for 200 to 1000 ppm Of MECL [72].

XEEE Transactions on Dielectrics and Electrical Insulation Vol. 7 No. 5, October 2000

0 ACDbkctricRanler -

0.01 ' ' ' ' ' ' ' ' I ' ' ' ' I ' ' ' ' " ' ' ' I ' ' ' ' I ' ' ' 'A 0 1 2 3 4 8 6 7

Enem Density (kJ/I) Figure 35. As for Figure 33 except for the removal of methylene chloride (MECL). Initial concentration of MECL in air, 1000 ppm; temperature in "c is shown for the pulsed corona discharge reactor [72].

0 Puked DbbcMcBnler 0 ACDbkctrlcBnler

0 100 200 300 400 600 Energy Density (J/I)

Figure 36. As for Figure 33 except for pulsed and ac driven dielectric barrier discharges. ac coaxial dielectric barrier reactor as in Figure 33; pulsed dielectric barrier reactor: a single barrier 3 mm thick flat Pyrex glass, 38 mm in diameter, gap spacing 2 mm; stainless steel electrodes: negative peak voltage 40 kV; rise time 6 ns; FWHM 20 m; initial concentration of TCE 200 ppm [72].

In this case the 3 mm thick Pyrex glass dielectric barrier covered one flat electrode 38 mm in diameter and forming a gap spacing of 2 mm. Figure 36 shows that the removal efficiency of TCE in dry air was the same for pulsed dielectric barrier and ac dielectric barrier discharges for up to 320 J/I [72]. The higher breakdown field in the pulsed dielectric barrier discharge compared to the ac dielectric barrier discharge did not affect the removal efficiency for TCE in air (Figure 36) [72].

Benzene (C6H6) in air was decomposed in a ferroelectric pellets coaxial reactor using either 50 Hz or 24 kHz power sources [66]. A cylindrical reactor was used with a central electrode 10 mm in diameter, outer electrode of 30 mm inner diameter and 50 mm long. The pellets were 1 to 3 mm in diameter and were held in position between perforated Teflon plates [66]. It was found that ferroelectric pellets of 1 to 2 mm in diameter and having a relative permittivity >1100 (BaTiOs and SrTiOs) decomposed benzene at an input power of 8 W (Figure 37) [66]. The benzene in dry air (80% N2 and 20% 02) was completely decomposed into CO and CO2 at low concentrations (t50 ppm) without formation of other hydrocarbons [66]. It also was reported that the use

100

-e : 80 2 A 3 60 5 40 2

20

n

+ 0

.- a, >

0

- 0 2 4 6 8 10 Input power I W

Figure 37. Dependence of the removal of benzene in dry air on 50 Hz inout Dower in a ferroelectric oellet discharee reartor usiw differ-

0 -~~~~~~ ( 1 ~ ~ ...... ~ ~~ .... ~ I ~~~~ ~~-~~~~ ~~~~ ~

en't peimittivities of the pellets. Gas mixture: 80% Nz, 20% 02; discharge reactor: cylindrical, rod diameter 10 mm, inner diameter of outer cylinder 30 nun, 50 mm long; pellets diameters 1 to 3 mm; pellets held between perforated Teflon plates; gas flow rate 0.2 I/min; initial concentration of benzene 200 ppm. Relative permittivities: curve 1,20 (MglTiOa); curve 2,100 (CaTiOa), curve 3,200 (CaTi03); curve 4, 330 (SrTiO3); curve 5,870 (BaTi03); curve 6,1100 to 15000 (SrTiOs and BaTi03) [66].

667

of 50 Hz resulted in a higher energy efficiency for the decomposition of benzene compared to 24 kHz, and further that the concentrations of NO, and nitrous oxide (N20) were lower with the lower frequency power source [66].

-0- 5 7 p p m l m m + m p p m . 1 m m + ni ppm. 3 mm + 6 9 P m s M n + P.ppm.5"

0 2 4 6 8 10 12 14 18

ac Voltage (kv)

Figure 38. Destruction efficiency of toluene us. ac voltage (60 Hz) using a ferroelectric pellet discharge reactor. Initial concentrations of toluene and pellet diameters are shown. BaTi03 pellets; gap separation between the flat mesh electrodes 25 nun; flow rate 0.2 l/min; gases: dry air, oxygen and nitrogen [73].

Figures 38 and 39 show the destruction efficiency of toluene (C~HB) and trichlorotrifluoroethane ( c~) -113 , respectively, using 60 Hz applied voltage for a flow in the range of 0.1 to 0.5 l/min (dry air, oxygen and nitrogen) and for varying initial concentrations from 57 to 234 ppm and using BaTiOa pellet diameters in the range 1 to 5 mm [73]. The gap separation between the flat mesh electrodes which held the pellets in position was 25 m. Figure 38 shows that the destruction rate depends


I-

*O 0 0 L

-m- 500 ppm, 0.1 Umin -+ 500 ppm, 0 2 Umin -A- 500 ppm, 0.5 Umjn o 1000 ppm, 0.2 Umm

5 10 15 20 Voltage (kv)

Figure 39. Destruction efficiency of trichlorotrifluoroethane (CFC)-113 as a function of ac voltage (60 Hz) in a pellet discharge reactor. Initial concentration and flow rates are shown in the Figure. Pellet size 3 mm; other conditions are as in Figure 38 [73].

on the pellet size. This is because the corona onset voltage decreased as the pellet diameter increased [73]. A 100% destruction rate of toluene was attained using 60 Hz applied voltage for all pellet diameters in the range 1 to 5 mm (Figure 38) and higher efficiencies were obtained at lower initial concentrations of toluene (57 and 59 ppm) than at higher concentrations (231 to 237 ppm) (Figure 38) [73].

Figure 39 shows the destruction efficiency of CFC-113 as a function of the applied ac voltage using 3 mm diameter BaTi03 pellets and for different initial concentrations in the range 500 to 1000 ppm and flow rate from 0.1 to 0.5 l/min [73]. The maximum destruction efficiency was 28% with a flow rate of 0.2 l/min and 500 ppm. The low destruction efficiency of CFC-113 was due to the stronger bonding of the chlorine and fluorine in the molecule compared to toluene [73]. When smaller pellets of 1 mm in diameter were used, a higher operating voltage of 24 kV,, could be realized with a destruction efficiency of 52% [73].

Formaldehyde and benzene also were removed from gas streams of oxygen using a 60 Hz dielectric barrier discharge reactor [75]. The initial concentrations of the pollutants were in the range 100 to 5000 ppm and the flow rate from 0.1 to 1.0 ]/min. The discharge gap was 2.5 mm and the length of the cylindrical dielectric barrier discharge reactor was varied between 10 to 100 cm [75].

9 POLLUTION CONTROL USING PULSED POWER

Pulsed power is increasingly being used for removal of pollutants such as NO, and SO2 from flue gases [3,4,7,10,11,21,29,31,39,50,78- 871 as well as volatile compounds such as toluene (C7H8) [73,87,88], dichloromethane (CHzC12) [88,89], dichlorodifloromethane (CFzC12) [88], trichloroethylene [72], methyl ethyl ketone [72], methylene chloride [72,108], sulfurhexafluoride [89,94], perfluorinated compounds (NH3, C2F6 and CFa) [89], chlorofluorocarbons (CC12F2, C7H8 and CH3CC13) [89,94], trichlorotrifluoroethane (CFC-113) [39,73], methane [90], hydrogen sulfide (HzS)[91], methylene chloride (CH2C12) [73,94], styrene [87], ethylene [87], CF4 [94], C2F6 [94], NF3 [94] and carbon tetrachloride (CCla) [108].

The main advantages of using the pulsed power method for the removal of pollutants from flue and exhaust gases compared to the electron beam irradiation method are:

1. lower cost of the equipment, 2. absence of high energy X-rays and other radiation which might affect

3. homogeneous discharges at atmospheric pressure can be produced, 4. high energy electrons are generated readily, which produce the radicals

5. because the pulsed power is very short in duration, the ions and neu-

the personnel working in the treatment area,

that are necessary for pollutant removal, and

trals are not heated significantly above the ambient [lo, 111.

The homogeneity of the pulsed discharge was confirmed recently using 25 ns pulse which generated negative corona in air containing NO in a needle to plane gap geometry with a flowing gas [92]. A uniform destruction of NO (40% removal ratio) throughout the gap was found, thus indirectly confirming that the discharge was homogeneous throughout the electrode gap.

Concentric wire-cylinder discharge reactors of the type shown in Figure 3(a) [36] and Figure 8 [32] have been used widely in pulsed power operations with some notable improvement by modifying the HV electrode from a straight wire to helical construction as shown in Figure 9 [42].

Using the three types of cylindrical discharge reactors shown in Fig- ure 3 and employing short duration pulses of 100 ns in a mixture of 100 ppm NO in nitrogen with a gas flow rate of 65 l/min, it was found that the energy density input to the discharge necessary to remove NO generally was similar in magnitude in all three reactors for 5 50 J/1

It was concluded that the production efficiency of the radicals per input energy in electrical discharge reactors could not be increased by changing the voltage pulse parameters, changing the electrode struc- tures, employing a dielectric barrier, or dielectric pellets [36]. This was attributed to the field in the plasma producing the radicals, being space charge shielded to the same value in all cases [36].

1361.

10 EFFECTS OF VOLTAGE POLARITY OF PULSED

POWER ON PERFORMANCE

It has been shown that in a mixture of 180 ppm NO in air at room temperature the application of positive polarity pulses to the wire of a coaxial discharge reactor (Figure 3(a)) produced a better performance in the removal of NO than negative pulses, although the energy efficiencies in both cases were the same [4]. However, at higher temperatures the situation was reversed and the negative polarity yielded a better performance. In a combustion gas from an incinerator boiler plant both polarities gave a similar performance while the energy efficiency was better for the positive pulse polarity [4]. The difference in the performance due to changing the voltage polarity in the flue and the simulated gases used in the laboratory was attributed to the presence of 17% H20,10% COz, 300 ppm CO and 350 ppm HC1 as well as due to the presence of fine particulates in the combustion gases.

Figure 40 shows a comparison in the reduction of NO and NO, for positive and negative polarities as a function of peak pulsed voltage using FWHM = 350 ns in a simulated gas mixture of 6% 02, 8% CO2, 7.5% HzO, 78.3% Nz, 0.2% SOz, 205 ppm NO, 35 ppm NO2 and 0.8 stoichiometric NH3 (NH3/(NOX t SO2)) [86]. The discharge reactor

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 7"o. 5, October 2000 669

Fieure 40. Deuendence of the removal of NO and NO, on the uolaritv " I ,

and magnitude of pulsed voltage in a coaxial wire-cylinder geometry Gas mixture: 6% 02,8% C02,7.5% H20,78.3% Nz, 0.2% S02,205 ppm NO, 35 ppm NO>, 0.8 stoichiometric NH3; gas flow rate 14 Vmin; room temperature; FWHM 350 ns; discharge reactor: coaxial, 3 mm wire diameter, 55 mm inner diameter of outer cylinder, length 45 cm; pressure l.01x105 Pa 1861.

used was a coaxial reactor, 3 mm wire diameter, 55 mm inner diameter of the outer cylinder and 45 cm long. It will be observed from Figure 40 that the positive polarity pulses were more effective in removing NO andNO, [86].

For a mixture of air with 2.5% H20 and 1666 ppm SO2 and a flow rate of 1.2 I/min in needle to plate or rod-plate geometries, the positive pulsed voltage performed better than the negative polarity in removing SO2 [7]. This was because the positive pulse polarity produced more uniform streamers that extended further into the gap and 90% of SO2 was thus removed with positive polarity [7].

11 EFFECTS OF MAGNITUDE OF PEAK PULSED VOLTAGE ON

PERFORMANCE

It has been found that the removal ratio of the pollutants increases with increasing peak value of the pulsed voltage when all other conditions are kept unchanged [4,81]. A typical dependence of the removal of NO and NO, in a mixture of 6% 02, 8% COz, 7.5% H20,78.3% Nz, 0.2% SO2 and 240 ppm NO, on the peaks of positive and negative voltages is shown in Figure 40 [86]. It can be seen that both NO and NO, concentrations decreased with increasing peak voltages for both polarities but the rate of decrease was more pronounced with increasing positive pulsed voltage (Figure 40) [86]. The removal of NH3 and 0.5 to 1.0 pm particulates in air, also was found to be enhanced with increasing the peak pulse voltage [93]. Masuda reported that the removal of NO in a mixture of 180 ppm NO, 600 ppm NH3 in air using negative pulsed voltage was enhanced with increasing the magnitude of the peak of the pulse [4,81].

Generally the shorter the rise time of the applied pulse voltage, the more intense and elongated are the pulsed streamers and the better the performance [4].

12 EFFECTS OF PULSE WIDTH AND REPETITION RATE ON REMOVAL OF POLLUTANTS

It was reported that the use of dc voltage produced a large ionic current which reduced the efficiency of the discharge reactor and therefore it was concluded that a very short pulse width would be advantageous [4]. A reduction in the pulse width from 165 to 81 ns and to 41 ns with corresponding shorter rise times of 25, 15 and 9 ns resulted in an increase in the destruction of NO (100 ppm) in nitrogen and toluene (C7H8) in air [94]. Figure 41 shows the percentage removal of toluene in air for different pulse lengths as a function of the specific energy input into a coaxial pulsed corona discharge reactor [94]. It will be observed from Figure 41 that a higher destruction rate was obtained at a shorter pulse width for a given energy density input, and therefore a higher energy efficiency. Figure 41 also shows that in order to attain a required destruction ratio less energy would be required when decreasing the pulse width from 165 to 45 ns 1941.

75 !-

Pulse Width FWHM

165 ns

300 400 500 600 700 EO0

Specific Energy [JA]

Figure 41. Destruction of toluene in air using different positive pulse widths in a coaxial corona discharge reactor. Pulse rise time and corresponding FWHM: 9 ns and 45 ns; 15 ns (81 ns); 25 ns (165 ns); concentration of toluene 200 ppm [94].

Figure 42 shows the removal efficiency of NO (mol/kWh) as a function of the removal ratio of NO in a mixture of 91% N2, 5% 02, 4% HzO and an initial concentration of 200 ppm NO for different pulse widths from 40 to 120 ns [43]. It will be observed that a higher removal efficiency was obtained with successively shorter pulse widths. In Fig- ure 42 the pulse rate increased from 1 to 13 pulses per second (pps) with successive points on each curve starting from lower to higher removal ratios of NO [43].

Figure 43 shows that at a fixed pulse width the final concentration of NO decreased with increasing pulse rate. For a pulse width of 120 ns the final concentration of NO decreased from the initial concentration of 200 ppm to 4 ppm at 10 pps which was 98% removal efficiency [43]. At a fixed pulse repetition rate, the final concentration decreased with increasing pulse length. Typically at 7 pps the final concentration of


4

E

9 0 20 40 60 80 100 L- NO removal ratio, NOR, %

Figure 42. Removal efficiency of nitric oxide (mol/kWh) as a function of its removal ratio using different pulse widths. Gas mixture: 91% N2, 5% 02, 4% H20, and 200 ppm NO; flow rate 2 l/min; pressure l.01x105 Pa; temperature 25T; discharge reactor of the type shown in Figure 8; 0.5 mm diameter stainless steel wire, 76 mm inner diameter cylinder, 0.5 m long; positive peak voltage 49.2 kV; pulse rate increases from 1 to 13 pps with successive points on each curve from lower to higher removal ratios; 0 40 ns; I 60 ns; + 80 ns; A 100 ns; v 120 ns [43].

b t

0 2 4 6 E 1 0 1 2 1 4 4 0

(a) Pulse repetltlon rate, pps

E

L

0 2 4 6 8 1 0 1 2 1 4 Pulse repetltlon rate, pps

Figure 43. Reduction of NO (a) and NO1 (b) as a function of pulse repetition rate using different pulse widths. Conditions and symbols are as in Figure 42 [43].

NO decreased from 49 to 24 ppm with increasing pulse length from 40 to 120 ns (Figure 43) [43].

In a mixture of 10% 02,2% H20,88% N2 and 200 ppm NO and a flow rate of 1.94 l/min, NO was removed completely at 3 pps while 96% of NO, was removed at 7 pps using a coaxial reactor having a spirally wound 1 mm wire in diameter, 96 mm inner diameter of outer cylinder and 3.5 m long (Figure 44) [42].

Figure 44. Dependence of the removal ratio of NO and NO, on the pulse repetition rate in a mixture of 10% 02, 2% H20, 88% Nz and 200 ppm. Positive voltage pulse, FWHM 120 ns; flow rate 1.94 l/min; coaxial discharge reactor: 1 mm wire diameter wound spirally at 1 mm pitch on 25 mm diameter insulating tube, 96 mm inner diameter of outer cylinder, 3.5 m long; pressure l.01x105 Pa 1421.

13 TIME TO STEADY STATE CONDITION WITH PULSED

POWER

The time to reach a steady state condition of the final concentration of the pollutant depends on many parameters including the flow rate of the gas, the concentration of the pollutants, the composition of the gas mixture, the input pulse power to the discharge and the size of the discharge reactor. There is a dearth of published results on the time to reach a steady state condition. Figure 45 shows the final concentration of NO and NO2 as a function of the time of application of a positive pulsed voltage of 49.2 kV, 120 ns pulse length and 1 pps to a gas mixture containing 200 ppm NO, 91% Nz, 5% 02 and 4% H20 at a flow rate of 2 l/min [43]. It will be observed that it took - 3 min for the concentration of NO to decrease from 200 ppm to 140 ppm and then remained constant thereafter (Figure 45) [43].

E c 100 1 al

' 0 1 2 3 4 5 6 7 Time, min

Figure 45. Dependence of NO and NO2 concentrations on time of application of positive pulsed voltage. Pulse rate 1 pps; FWHM 120 ns; other conditions as in Figure 42 [43].

Figure 46 shows the concentration of NO, as a function of time following the application of positive pulses of FWHM = 20 ns to a simulated gas mixture of 400 ppm NO, 10% 02,10% CO2 and 80% N2 at a flow

IEEE Transactions on Dielectrlcs and Electrical Insulation

L

80 - -c with discharge @eNO) - with discharge (DeNOx) . E 8 60 - E - 40-

- ,^

6 z : *\

z o 20 i- - . - -

--c Spray rcactor -t- Wet rcocior 400

Y -41- pH value

'2 3M)

0 10 20 30 Voltage application timo [min]

Figure 46. Dependence of NO, concentration on the time of application of positive pulsed voltage. Gas mixture: 400 ppm NO, 10% 0 2 ;

10% CO2; 80% N2; flow rate 2 l/min; pressure l.01x105 Pa; temperature 25°C; input power 30 W; discharge reactors: coaxial glass dielectric barrier 1 mm thick, 23 mm inside diameter, 210 mm long; wet reactor, 0.2 mm diameter stainless steel wire; spray reactor: 2.5 mm diameter tube with holes (0.15 mm diameter) forming the HV electrode [951.

rate of 2 l/min using a dielectric barrier discharge reactor [95]. It will be observed that a saturation is reached after 4 to 15 min of application of the pulsed voltage, depending on the experimental conditions. In Figure 46 the pH value of HzO due to the absorption of NO2 is also shown where it took - 30 min to reach a steady state value [95].

14 DEPENDENCE ON FLOW RATE AND RESIDENCE TIME

Figure 47 shows the NO and NO, removal efficiency as a function of the flow rate of a simulated mixture of gases (940 ppm NO, 10% 02, and 89.9% Nz) using positive square pulses which were applied to a dielectric barrier discharge reactor [96]. The coaxial discharge reactor was made of 0.2 mm diameter wire concentric in a 20 mm quartz tube 1.5 mm thick, which was wrapped on the outside with an aluminum foil [96]. It will be observed from Figure 47 that at room temperature the removal efficiencies of NO decreased from 30% to lo%, respectively with increasing flow rate from 1 to 8 I/ min [96].

Figure 48 shows the change in NO concentration as a function of residence time in a coaxial discharge reactor for different negative pulsed voltages and pulsed electric fields [U]. It will be observed that the removal ratio of NO in a mixture of 180 ppm NO, 600 ppm NH3 in air decreased with increasing residence time in the discharge reactor for all applied voltages (Figure 48) [81].

15 EFFECT OF GAS TEMPERATURE ON THE

REMOVAL OF POLLUTANTS

The temperature of the flue gases is usually reduced by heat ex- changers to - 80 to - 100°C [97-991 before they are treated either in a corona discharge or electron beam reactors. The exhaust gas temperature from a diesel engine was reported to be in the range 150 to 200°C [96]. The effect of varying the temperature of the gas on the removal of the pollutants has been reported in several studies [4,9,14,67,

Vol. 7 No. 5, October 2000 671

1M) I 1

Figure 47. Dependence of the removal efficiencies of NO and NO, on the gas flow rate using positive square pulsed voltage. Gas mixture: 940 ppm NO, 10% 02, 89.9% N2; discharge reactor: coaxial dielectric barrier 0.2 mm diameter wire, quartz 1.5 mm thick, 20 mm inner diameter; room temperature; 60 pps [96].

" > c - I c I

56 kV(11.8 kV/cm) .

2 0 30 60 G a s Rertdence Ttne Ta ( 5 )

Figure 48. Change in NO concentration as a function of residence time in a coaxial discharge reactor for different negative pulsed voltages and pulsed electric fields. Discharge reactor: coaxial, wire 3 mm diameter, cylinder 100 mm inner diameter, 416 mm long; pulsed voltage: rise time 50 ns, FWHM 300 ns, pps 50; gas mixture: air with 180 ppm NO 600 ppm NH3; pressure 1 . 0 1 ~ 1 0 ~ Pa; temperature 20°C [81].

72,95-971. Generally, higher gas temperatures lead to a lower removal efficiency of the pollutants.

The application of pulsed power for the removal of pollutants does not raise the temperature of the gas significantly above the ambient. This was confirmed using spectroscopic measurements which showed that for streamer pulses lasting 400 ns, the gas temperature in atmospheric nitrogen remained in the range 300 to 350 K [loo, 1011.

Figure 49 shows the magnitude reduction in NO concentration as a function of temperature in the range 20 to 220°C in a mixture of 400 ppm NO, 10% 02 and 90% Nz using positive pulsed voltage in a coaxial dielectric barrier discharge reactor [9]. Different input power levels into the corona discharge were employed. It will be observed that an increase in the gas temperature resulted in a decrease in the NO removal (Figure 49) [9]. For example the decrease in NO concentration at 20°C was - 200 ppm while at 220°C only 76 ppm of NO were removed [9].

672

- 80 s

Hackam et al.: Air Pollution Control by Electrical Discharges

- 150 ppm 0

- 600 p p m 0

*

4 0 0 p p m

1 100 200 01

0 Tempemture ["Cl

Figure 49. Reduction in NO concentration as a function of gas temperature for different input power into the discharge. Reactor: coaxial dielectric barrier, glass 1 mm thick, 18 mm inner diameter, 78 mm long; positive pulsed voltage, width 500 ns; gas mixture: 400 ppm NO, 10% 02,90% N2; flow rate 2 l/min; pressure 1.01x105 Pa [9].

Figure 35 shows that for a dielectric barrier discharge using pulsed power the removal of methylene chloride (MECL) in dry air deteriorated with increasing temperature from room temperature to 60°C [72]. The percentage removal of NO in a mixture of 940 ppm NO, 10% 0 2 and 89.9% N2 using positive pulsed voltage in a dielectric barrier discharge reactor increased when the temperature decreased from room temperature to -114°C (solid ethanol) and to -196°C (liquid nitrogen) [96]. This occurred over a wide range of gas flow rate from 1 to 8 ]/min. Typically at 4 I/min flow rate, the removal ratios of NO were 15.7%, 60.6%, and 98%, respectively at room temperature, -114°C and -196°C [96].

The removal of NO in air using negative pulsed voltage was less effective with increasing gas temperature from 50 to 250°C [4]. For a positive polarity pulsed voltage at 250°C not only NO was not removed, but its concentration actually increased above the initial value and likewise at 200°C for peak voltages >60 kV [4].

The removal ratios of SO2 in a mixture of 73% N2, 13% C02, 6% 02 and 8% H20,350 to 550 ppm NO,, 300 to 550 ppm SOX, a ratio of NHs/(NO, t SO*) = 0.7 to 0.8 and using pulsed power (FWHM = 1.5 /AS,

pps = 50) and either parallel plates or coaxial discharge reactors, were reported to be - 75% at 100°C and increased to 90% with decreasing gas temperature to 70°C [97].

16 EFFECT OF INITIAL CONCENTRATION ON REMOVAL EFFICIENCY

The concentration of the pollutants in the flue gases varies over a wide range depending on the type of fuel used. The concentration of NO, from coal burning plants varied from 300 to 550 ppm [97], 430 to 550 ppm [99], and 250 ppm [l, 981 while those of SO2 varied from 350 to 550 ppm [97], 500 ppm [98] and 360 to 500 ppm [99]. The flue gas from a methane burner had 40 ppm NO and 0 NO2 [97].

In general at a given input power to the discharge, the removal ratio of the pollutants decreases with increasing initial concentration of the pollutant.

100

CFC-113 Frequency: 5kHr Pc'rkVollogc:6kV Eleclrlc Poycr

.- e

0" 60

0 Cwcntntion: 0 l00ppm * K W m * W O P P m

0 2 4 6 8 1 0 1 2 1 4 Residence Time(s)

Figure 51. Decomposition of CFC-113 (freon 13) in air for different initial concentration using surface discharge reactor [39].

The destruction of CFC-113 (Freon-13) in air using a pulsed power for

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 7 N o . 5, October 2000 673

different initial concentrations in the range 100 to 104 ppm is depicted in Figure 51 [39]. It will be observed that for a given residence time in the surface discharge reactor a higher decomposition ratio was obtained with a lower initial concentration of CFC-113 [39]. In flue gases from a coal burning power plant the NO, removal efficiency was also found to depend on its initial concentration [97].

17 INFLUENCE OF ADDITIVES ON THE REMOVAL OF

POLLUTANTS

Additives are introduced into the gas stream of discharge reactors in order to enhance the removal of the pollutants and neutralize the nitric and sulfuric acids [2] which are produced by interaction of the radicals with NO, and SO2 (reactions (12), (14), (16), (18), (23) and (26)). One of the most widely used additives is ammonia [5,6,23,25,26,28, 95,97-99,1021 which converts the acids into ammonium sulfate and ammonium nitrate which can be utilized as agricultural fertilizers [21]. Other additives such as lime 1971, methane 126,271, ethylene (C2H4) [6, 9,85,90], propylene (CJH~) [6], argon with ammonia mixture [25,26, 491, CO2 with ammonia mixture [40], hydrocarbons such as Z-propane- 1-01 or 2-propanol [103], copper coated zeolite catalyst [85,103], H20 [81,95,96,102] and hydrated lime have been used in discharge reactors.

The addition of ammonia to the flue gases from a coal-burning power station resulted in increases in the removal ratios of both NO, and SO2 [97]. The flue gas contained 420 ppm NO,, 360 ppm S02,73% N2,13% C02,6% 0 2 and 8% H2O. The flow rate was 500 m3/h and the gas temperature was 90°C. The removal ratio of NO, increased from 15 to 33% and of SO2 from 14 to 7570, respectively, before and after the addition of 0.7 to 0.8 stoichiometric ammonia (NH3/(NOX t SOz)) when positive pulses (FWHM at 1 ps) were applied at an energy density of 6 Whim3 [97].

Without the application of pulsed voltage, the mere addition of ammonia resulted in the removal of 55% of SO2 [97]. The injection of hydrated lime using compressed air at a stoichiometric unity value resulted in an increase in the removal ratio of SO2 from 19 to 31%, respectively before and after the application of positive pulsed voltage [97]. The injection of hydrated lime had no effect on the removal of NO, [97].

Figure 52 shows the removal efficiency of SO2 in air containing 6% H20 at 65°C and as a function of the ratio of the added NH3/S02 where SO2 = 1000 ppm [102]. A dc applied voltage at a current density of 10 mA/cm2 and energy density of 20 J/g were injected into the discharge using multi-pin cathodes in a point-plane geometry The length of the discharge reactor was 1 m, the gas flow rate 70 to 200 m3/h and the gas flow speed was in the range 70 to 200 m/s [102]. Figure 52 shows that the removal efficiency increases with increasing NH3 until saturation is reached.

It was reported that in flue gas from a coal-burning plant containing 430 to 550 ppm NO,, 360 to 500 ppm SOz, a flow rate of 600 m3/h at 70 to 100°C and using a positive pulsed corona coaxial discharge reactor, removal efficiencies of N 60% for NO, and N 80% for SO2 were obtained with energy input of 15 Wh/m3 with injection of ammonia of 1000 ppm [99].

Figure 52. SO2 removal ratio from air as a function of addition of the ratio of NH3/S02. Conditions: initial SO2 0.1%; 6% H20; negative dc applied voltage; current 10 mA/cm2; energy density 20 J/g; temperature 65T; discharge reactor: multipin needle to plane [102].

I 8 0 0% 0.1 0.15 0.2 r"

z Additional NH, gas flow (I/nin)

Figure 53. Removal ratios of NO, NO, and surplus NH3 as a function of the added NH3. Treated gas mixture: 80% N2,20% 02, and 190 ppm NO with flow rate 4.9 1/ min; additional gases: 80% N2, 20% 0 2 with flow rate from 0.05 to 0.2 l/min; positive dc voltage, electrode gap 5 cm [40].

The added ammonia into the flue gases should be consumed within the discharge completely, because any amount left unused will con- tribute to air pollution and thus must be removed. Figure 53 shows the amount of NH3 left in the gas stream after treatment in a discharge reactor of the type shown in Figure 7 as well as the corresponding reduction ratios of NO and NO, as a function of added NH3 [40]. The treated gas mixture contained 80% N2,20% 0 2 and 190 ppm NO with a flow rate of 4.9 I/min. Additional gases of 80% N2, and 20% 02 with a flow rate of 0.8 I/min were fed into the treated gas via a pipe electrode (Figure 7). Additionally, Ar with NH3 (Ar: NH3 = 97 3) were fed into the treated gas at a rate of 0.05 l/min (Figure 53) [40].

A positive dc voltage was applied to the pipe electrode. It will be observed in Figure 53 that a complete removal of NO and NOxcould be


100

I .

-

without HzO. NHi

Y)

0

1,

H20 IO ~01%. NHI 400 pp

IO 20 30 40 Input power [W] IO 20 30 40 Input power [W] (b)

Figure 54. NO (a) and NO, (b) removal ratios as a function of input of power using positive pulsed voltage in a mixture of 400 ppm NO, 10% 02,10%0 CO2 and 80% NZ for four conditions of with and without additions of Nh and Hz0. Discharge reactor: coaxial glass dielectric barrier 1 mm thick, 23 mm inner diameter, 210 mm long, stainless steel wire 0.2 mm in diameter; flow rate 2 l/min; temperature 150°C; pressure l.01x105 Pa. In (a) the top curve is for added 10% HsO t 400 ppm NH3 [95].

Figure 54 shows the NO and NO, removal ratios as a function of input power using positive pulsed voltage in a mixture of 400 ppffl NO, 10% 02,10% CO2 and 80% N2 for four conditions of with and without addition of NH3 and H2O [95]. The discharge reactor was a coaxial glass dielectric barrier, 1 mm thick, 23 mm inner diameter and 210 nun long with 0.2 mm stainless steel wire. The flow rate was 2 l/min and the temperature of the gas 150°C. It will be seen on Figure 54 that in the presence of either NH3 (400 ppm) or water vapor (10%) the removal of both NO and NO, were enhanced [95]. When both water vapor and ammonia were added simultaneously, the removal ratio increased substantially For example at 30 W input power, without additives the removal ratios of NO and NO, were, respectively - 18% and 1% (Fig- ure 54). For NO this increased to 6O%, 75% and 89% and for NO, to 20%, 33% and 74%, respectively, when 400 ppm of NH3,10% HzO, and 400 ppm NH3 plus 10% of HzO were added to the gas mixture (Fig- ure 54) [95]. It was reported that there was no excess ammonia left in the exhaust gases due to the absorption of NH3 by H20 [95].

Figure 19 shows the concentrations of NO, NO1 and NO, with the addition of a fixed amount of NH3 of 219 ppm (NH3/(NO,), = 0.5) and Figure 20 with addition of a fixed amount of methane of 438 ppm (CH4/(NOX), = 1) using a negative dc voltage in a plane-plane geometry discharge reactor [26].

600

480

400

1% 260 k

i zoo

4 160

roo

50

0 0 10 20 30 40

dc or chaqlng vonpge bv Figure 55. Reduction of NOx for different stoichiometric concentrations of the additives methane and ammonia using negative dc or pulsed vdltagesa Gas mixture; 438 ppm NO, 83.96% Nz, 8% 02, 8% CQ; pressure 1.0ix105 Pa; flow rate 5 l/min; discharge reactor: plane-plane; N h (or CHa)/(NO,), = 1 (1 st); NH3 (or CHa)/(NO,),= 0.5 (0,5 st); (NO.& = 438 ppm NO; Voltage for pulse was charging voltage of pulsed circuit and not peak of pulsed voltage [26].

Figure 55 shows the effect of varying separately the amounts of the added NH3 and CHa on the reduction of NO, when either pulsed or negative dc voltages were applied [26]. Figure 55 shows that the addition of ammonia was more effective than methane for reducing NO, for both NHs/(NO,), = CH4/(NOX), = 1 and 0.5. Further increasing the stoichiometry (ratio of additives to the initial value of NO,) led to a larger reduction in the concentration of NO, (Figure 55) [26].

Figure 55 shows that the negative dc was more effective at the same voltage than the pulsed voltage [26]. However, Figure 3 of [26] indicates that the pulse used was very wide such that after 1.1 ps it only dropped to 67% of its peak and therefore the FWHM would be much longer than 1.1 ps. It is known that very narrow pulses ( 4 0 0 ns) are required to obtain effective denitrification and desulfurization of the flue gases [4,81]. The pulse voltage shown in Figure 3 of [26] to which the dc performance of NO, removal was compared was actually that, as stated by the authors, of the charging voltage to the pulse forming network and not the peak of the pulsed voltage. The charging voltage is usually lower than the peak of the pulsed voltage provided by the dc charged circuit.

Figures 56 and 57 show the dependence of the removal ratios of NO,

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 7No . 5, October 2000 675

G D .- ti - m

70-

&.

0" '

Lo 6 0 -

SO.,

0 3 , . I . , , , , , , , . , I 0 8 1 0 1 2 1 4 1 6 1 8 2 0

MR

' 0 100 0 '

-lean electrode

,-. o Contaminated electrode E ,8 !g 50 a

D

, , , . , . , . , , , . ,

Figure 57. SO2 removal efficiency as a function of MR. Conditions are as in Figure 56 [25]. 0 1

PlQ=l.87 Wh"' Ny-360 ppm

U SO 100 150 200 250 300


80 /

electron beam irradiation [5] the NO, removal also was enhanced with increasing the added ammonia concentration.

The addition of double bond structure hydrocarbons (2-propane-l- 01) to a mixture of 400 ppm NO, 10% C02, 80% N2 and 10% 0 2 at a flow rate of 2 l/min (residence time in the coaxial reactor 1.58 s) led to an enhanced removal of NO, [103]. A pulsed coaxial glass dielectric barrier reactor (0.5 mm diameter stainless steel wire, 13.3 mm inner diameter of the glass tube and 380 mm long) was employed at room temperature [103]. The addition of 2-propanol, which does not have a double bond did not result in a significant reduction of NO, [103].

Using a copper coated zeolite catalyst (Cu-ZSM-5) with hydrocarbons promoted an improvement in the removal of NO, using positive pulsed voltage in a glass dielectric discharge reactor [85,103].

18 POLLUTANT REMOVAL AND

A number of studies have been reported on the dependence of the removal ratio of the pollutant on the input power to the discharge [9, 64,85,93,95,96,103].

Figure 26 shows the dependence of the NO, removal ratio on the input power to the discharge in the form of an arc plasma in a simulated mixture of gases of 346 to 2317ppm NO, to 11% 02, to 8% CO:! and balance N2, including an unknown quantity of water vapor [64]. Figure 26 shows that the NO, removal ratio steadily increased with increasing power and reached 100% when the plasma power was in the range 0.8 to 1.2 kW. At high power, the temperature of the plasma increased and more NO was generated which resulted in lowering the NO, removal efficiency [64]. This occurred for all inlet values of NO [64].

INPUT POWER

0 IO 20 30 40 Input power [W]

Figure 60. Dependence of NO, removal on input power of pulsed voltage into a coaxial glass dielectric barrier discharge reactor. Gas mixture, 400 ppm NO, 10% 02, 10% COz, 80% N2; discharge reactor, gas pressure and flow rate as in Figure 54; dry reactor, 25°C 1951.

Figure 54 shows the dependence of the removal ratio of NO and NO, on the input power of positive pulsed voltage into a coaxial glass discharge reactor in 400 ppm NO, 10% 0:!,10% C02, and 80% NZ at 150°C [95]. The removal ratios of both NO and NO, steadily increased with increasing input power and reached constant removal ratios, depending on the additives used, in the range 20 to 30 W [95]. With further increase in the input power to the discharge, a reduction in the removal of both ensued (Figure 54). However, at 25°C and using the

0 Pulse (Dry) Pulse (Semi-wet)

same gas mixture, the same gas flow rate, without H20, the NO, removal steadily increased with increasing pulsed power with (200 to 800 ppm) and without addition of NH3 (Figure 60) [95].

Figure 49 shows that the decrease in the magnitude of the NO concentration using positive discharge in a glass coaxial dielectric barrier discharge was larger at 40 and 30 W power input to the discharge than at 20 W for all temperatures in the range 25 to 220°C [9].

40

3 8 .3

ii Q) l8kHz (Semi-wet) -

Discharge power [W] Figure 61. Removal efficiency of N h from air as a function of input discharge power using positive square pulses, 60 Hz and 18 kHz. Dis- charge reactor: coaxial glass dielectric barrier, 0.2 mm diameter wire, 18 mm inner diameter of glass tube, 1 mm thick, 150 mm long; semi- wet: filtered paper soaked with CaCh solution and placed on inside surface of glass tube to absorb and retain moisture from air; gas residence time 0.6 s; initial NH3 concentration 18 to 25 ppm [93].

Figure 61 shows the removal efficiency of NO, from air as a function of input power to the discharge using positive square pulses, 60 Hz and 18 kHz in a coaxial glass dielectric barrier discharge [93]. It will be observed that the pulse voltage performed better than the ac for dry air. This was attributed to more intense and uniform plasma obtained by the positive pulsed voltage [93]. For the semi wet condition (filter paper was soaked with CaC12 solution and placed on the inside surface of the glass tube to absorb moisture from the air and retain it) the maximum removal performance was at about the same level (Figure 61) [93]. In all cases the removal ratios of NO, increased with increasing discharge power (Figure 61) [93]. NH3 is dissociated by the discharge and reacts with H20 to form NH4N03 [78,106,107]. This explains the finding in Figure 61 that for the semi-wet condition the removal efficiency of NO, was higher than for dry air.

19 ENERGY DENSITY REQU I REMENTS

It is of more general interest to ascertain the energy density required to remove the pollutants from the flue gases than the power fed into the discharge. This is for the purposes of comparison of different discharge techniques employed as well as with the alternative method of electron beam irradiation and for determining the energy cost associated with the removal process. The selection of the technology for implementing the pollution control will depend largely on the energy cost.

Figure 62 shows the input energy density for removal of NO in a mixture of 100 NO in nitrogen at l.01x105 Pa and 25°C using pulse


b.. b 6 electron '..e

7 . .

A beam

I b

100

80 a.

!?

Y

C 60 -

C

!! 40

0 220

0"

0

0

. '&--- --- - - - - A - , . . . / . . . . I . . . . # . . . .

Input Energy Density (J/I) Figure 62. Energy density requirements to remove NO in NZ using positive pulsed corona and electron beam methods. Cas mixture: 100 ppm NO; pressure 1.01x105 Pa; temperature 25T; discharge reactor: coaxial, 1.5 mm wire diameter, 60 mm diameter metal tube, 300 mm long; pulse width 100 ns; electron beam system, 125 keV via titanium window 17.8 pm thick [108].

corona discharge and electron beam reactors [108]. The discharge reactor employed a 1.5 mm diameter wire in a 60 mm diameter metal tube 300 mm long. The pulsed power was obtained from a magnetic pulse compressor and had 100 ns FWHM. The electron beam reactor employed an electron beam of 125 keV which was produced from a low pressure helium plasma and fed into the reactor containing the pollutant via 17.8 pm thick titanium window [108]. Figure 62 shows that the energy density required to remove NO completely was 120 J/1(0.134 mol (NO)/kWh = 4.02 g (NO)/kWh) and 20 J/l (0.804 mol (NO)/kWh = 24.1 g (NO/kWh), respectively, using the pulsed corona and the electron beam methods in nitrogen. Therefore the removal of NO in dry nitrogen using pulsed corona is much less energy efficient compared to the electron beam process.

The energy density requirements for reducing carbon tetrachloride (CCl4) in dry air (20% 02,8O% N2) at 25°C are shown in Figure 63 [108]. It was suggested that the energy required for CC4 removal was deter- mined by the creation of electron-ion pairs from ionization of nitrogen and oxygen in the gas mixture which is followed by dissociative attachment of CCll (e t CCla + Cl- t CC13) [108].

The energy density required for the reduction of methylene chloride

Vol. 7 No. 5, October 2000 677

i 100 ppm CCI, in Dry Air 25%

0 1 . ' " ' . . . I I . . . I . . . . ]

0 50 100 150 200 Input Energy Density (JA)

Figure 63. Reduction of carbon tetrachloride (CCla) in dry air vs. energy density using pulsed corona and electron beam methods. Gas mixture: 100 ppm CCla; 20% 02,80% Nz; other conditions as in Fig- ure 62 [108].

Pulsed Corona Processing 160 ppm CH,CI, in Dry Air

3 1 '4

Figure 64. Reduction of methylene chloride (CHzC12) in dry air vs. input energy density using pulsed corona discharge for different gas temperatures. Gas mixture: 160 ppm CHzC12,20% 02,80% Nz; other conditions as in Figure 62 [108].

(CH2Cl2) in dry air using pulsed corona is shown in Figure 64 for different gas temperatures in the range 25 to 300°C [108]. It will be observed that the energy efficiency for the removal of methylene chloride in air increases substantially with increasing gas temperatures.

In a mixture of 200 ppm NO, 5% 02,4% H20, and 91% NP the energy requirement to remove NO using positive pulsed voltage in a coaxial discharge reactor is shown in Figure 42 [43]. It will be observed that a higher energy efficiency (mol NO/kWh) was found with decreasing pulse width from 120 to 40 ns. Typically, at 95% removal ratio of NO from the mixture, the energy efficiency was 0.76 and 0.3 mol/kWh (1

mol NO = 30 g NO), respectively for 40 and 120 ns (Figure 42) [43]. The removal energy efficiency depends strongly on the removal ratio of NO and it increases with decreasing removal ratio. For example, using 40 ns positive pulse width, the removal energy efficiency increased from 0.76 mol NO/kWh (22.8 g NO/kWh at 95% NO removal ratio to 2.1 mol NO/kWh (63 g NO/kWh) at 20% removal ratio [43].

A possible reason for the reduction of the removal energy efficiency (mol/kWh) with increasing removal ratio of NO shown in Figure 42 is because the increase in the removal ratio was obtained with increased energy input into the discharge (higher pulse rate in Figure 42) while


35

30

25

z f 20 P .t

E

in-phase ~ m i n ..:.* .......-..+...

,:

-9 --_ ,,> ~ --- -

I' ,<- ,: out-af-phase ,' : PUmin

e . , . ,' . - ,,'

aut-of-ptiase 1Umin

'I

. A + 'I 0

5 A

L 8

50 PPm 100 ppm 200 ppm 400 ppm 600 ppm 800 ppm

:,I,: 10

Pulse repetition rate [pps]

50ppm . 100ppm A 200ppm + 400ppm

6OOppm (c a00ppm

'I *

Figure 65. NO and NO, removal energy efficiency as a function of pulse repetition rate for different initial concentration of NO. Gas mixture: 5% 02, 4% H20, 91% N2, 50 to 500 ppm NO; pressure l.01x105 Pa; temperature 25°C; flow rate 2 limin; (reduced to OT); discharge reactor: 0.5 mm stainless steel diameter wire, 76 mm inner diameter copper cylinder, 800 mm long; positive pulse, peak voltage 53 kV, peak current 54.2 A, pulse width 100 ns [109].

,.?

the amount of NO available for destruction was steadily decreasing. This can be seen from Figure 65 where the energy efficiency decreases at a fixed initial concentration of NO with increasing pulse rate, and

i-------'-; in-phase lUmin 1 5

[HO]o/[NH3]o

also decreases at a fixed pulse rate with decreasing initial concentration 0 . of NO [109]. Figure 65 shows that the removal efficiency of both NO 0 0.2 0.4 0.6 0.8 1 1.2 1.4

and NO, increase with increasinr initial concentration of NO from 50 to 600 ppm and then decrease withy further increase in NO [109]. Similar behavior was reported for the removal efficiency of NO, (Figure 65) [109].

Masuda and Nakao reported that the removal energy efficiency of NO in air depended on the power input, the gas flow rate and the temperature [all. For a mixture of 180 ppm NO in dry air with 180 ppm NH3 and using coaxial discharge reactor of 3 mm wire in diameter, 90 mm in diameter metal cylinder and 1.3 m long the energy yield of NO removal was 20 g/kWh [NI. This was higher than that of 3 to 5 g/kWh obtained earlier by the same group using dc [U].

Using pulsed power and a coaxial discharge reactor Dinelli et al. reported a yield of 25 g NO/kWh for the removal of 50% NO, from the flue gases of a coal-burning power station when ammonia was used and the initial NO, concentration was in the range 500 to 550 ppm [97].

Shimizu et al. reported that using a positive pulse to treat the exhaust gases from diesel engine which were at 150°C required 220 J/g to attain 60% removal of NO, for dry reactor, and 60 J/g (70% of NO, removed) for wet reactor [96]. The initial concentrations of NO were 100 to 120 ppm and of NO, 110 to 160 ppm. Other constituents in the

Figure 66. Dependence of energy yield (g/kWh) of NO, reduction on the injected ammonia using combined dielectric and surface discharges. Discharge reactor is shown in Figure 4. gas mixture: N2, COz, 02 (proportion not specified), 800 ppm NO; flow rates 1 and 2 l/min; pressure 1.01~10~ Pa 1281.

Figure 66 shows the energy yield (g/kWh) for NO, reduction using ac (60 Hz) in a combined dielectric barrier and surface discharges reactor with addition of ammonia [28]. It will be observed that the energy yield increased with decreasing ammonia concentration and with increasing flow rate. Further when the ac voltage supplied to the dielectric barrier and the surface discharges were out-of-phase a larger yield was attained for the same amount of injected ammonia [28]. Figure 66 shows that the highest yield of 32 g NOJkWh was obtained at 2 l/min gas flow rate at an initial concentration of 800 ppm NO and injected ratio of NO/NH3 of 1.2 [28].

For flue gases from a boiler of a thermoelectric plant and using a coaxial discharge reactor, an energy density input to the discharge of 12 Wh/m3 was required to attain a complete removal of SO2 for initial

IEEE Transactions on Dielectrics and EJectrical Insulation Vol. 7 N o . 5, October 2000 679

concentrations from 443 to 645 ppm of NO, with ammonia injection [98]. The concentration of ammonia at the outlet of the discharge reactor (called ammonia slip) was generally t4 ppm [98]. For an inlet concentration of NO, of 250 ppm and 60% removal ratio 12 to 15 Wh/m3 was supplied to the pulsed discharge [98].

Vogtlin and Penetrante reported on the required energy density into the discharge for the removal of NO and SO, in air with a large injection of n-octane (n-octane: NO, = 8: 1) using pulse power [110]. At 224"c, N 750 ppm of the initial 800 ppm of S O were removed with an energy density input of 9.5 Whim3, and 500 ppm was removed when the initial concentration was 1100 ppm [110]. This corresponded to 18 eV per removed NO molecule (1 eV = 3.216 kJ/g NO) and 29 eV per removed SO, molecule.

The influence of the capacitance of the pulse forming circuit on the energy density consumed for the removal of NO and SO2 from simulated flue gases (air containing 3.5% H20) and initial concentrations of 200 ppm NO and 200 ppm SO2 was investigated [lll]. Generally lower energy density was required to remove S O and SO2 with smaller capacitance.

The energy density required for the removal of various volatile compounds using pulsed corona in dry air are shown in Figures 33 to 35 [72]. The energy density u/l) required to remove trichloroethylene (TCE) in dry air was the same using pulsed dielectric barrier, ac dielectric barrier and flat-bed (figure not shown) reactors. The energy density increased with decreasing the removal ratio of TCE [72].

The removal efficiency of methylene chloride (MC) and TCE from dry air were found to increase with increasing energy density u/1) into the discharge using a ceramic dielectric barrier with pulsed corona [112]. - 1400 J/l was required to remove 95% of TCE and 78% of MC [112].

Takaki et RI. reported that removing 50% of NO from a mixture of 200 ppm NO, 90% Nz, and 10% 02 using a dielectric barrier flat glass discharge reactor with ac (10 to 100 kHz) required 18 g NO/kWh [70]. Figure 31 shows the reduced NO and SO, in this mixture vs. energy density O/l) [70].

The removal efficiency of NO? from flue gases of a coal-burning plant us. the specific energy input into the discharge of pulsed corona reactor is shown in Figure 67 [99]. The removal ratio steadily increased with increasing energy density (Wh/m3) into the discharge. The reactor was in the form of a duct with multiple wires 3 mm in diameter. At a removal ratio of 50% of NO2 (initial concentration of NO, = 515 ppm), - 25 g NO/kWh was consumed 1991.

The SO2 removal ratio as a function of energy density u/g) in air without addition of NH3 is shown in Figure 68 for a mixture of 1000 ppm S02,6% H20, and using multi pin needles-plane geometry reactor with a negative dc voltage [102]. It will be observed that the removal efficiency increased linearly with increasing energy density. The addition of water was found to be beneficial for the removal of SO2. For example, when a fixed amount of 20 Jig was deposited into the discharge, 75 ppm of SO2 were removed (from an initial concentration of 300 ppm) at a water content of 1.5%, and 110 ppm were removed (initial concentration of 1000 ppm) at 6% water content [102].

It was claimed [lo21 that the energy requirement using the multi pin needle to plane geometry with negative dc voltage at high current density of 10 mA/cm2 resulted in 8 J/g per (100 ppm SO2) was comparable

5 - 1 50

40

30

0 2 4 6 8 10 12 14 16 18 20

Specif ic energy ( w / I / N ~ ~ )

Figure 67. NO2 removal ratio from flue gases of coal burning plant us. energy density (Wh/m3) input into the discharge. NO, 515 ppm; SO2 360 to 550 ppm; discharge reactor: duct with multiple wires 3 mm in diameter; solid particulates 150 mg/m3; flow rate 600 m3/h; temperature 70 to 100°C; residence time 4 to 7 s; NHJ/(NO, t SO2) = 0.7 to 0.8; pressure 1 . 0 1 ~ 1 0 ~ Pa 1991,

Figure 68. SO2 removal ratio from air vs. energy density u/g) input into the discharge without addition of ammonia. Other conditions are as in Figure 52 [102].

to the best results for the electron beam technology [113] and pulsed techniaues 1971.

I

Using 100 kV, 10 ns rise time, 200 ns wide pulses, it was possible to decompose completely in air styrene, toluene, ethylene and S O with a coaxial reactor having a wire 0.25 to 1 mm in diameter, 250 mm in diameter stainless steel cylinder, 3.5 m long [87]. The specific energy required and the initial concentrations were, respectively for NO: 15 kWh/kg, 213 ppm; styrene: 7 kWh/kg, 30 to 190 ppm and ethylene: 12 kWh/kg, 150 to 2500 ppm [87]. Pentane, 1,1,1 trichloroethylene (TCE), propane and butane were not completely decomposed in air. The required energy density was higher and varied from 24 kWh/kg for toluene to 180 kWh/kg for both 1,1,1 TCE and propane [87].

Figures 69 and 70 show the dependence of SO2 and NO reduction on


Figure 69. Dependence of SO2 a n d NO, reduction o n energy input into positive and negative d c discharge with injected ammonia. Gas mixture: 70.4% N2, 1.8% 02, 9.9% CO2, 17.9% H20; temperature 100°C; gas flow rate 0.111 g/s; discharge reactor: 3 stainless steel wires 0.3 mm in diameter to plate; gap distance 2 cm; gas residence time 12.3 s; gas flow speed 4.06 cm/s; injected NH3 diluted in NZ (1:lO); NO, 300 ppm; SO2 300 ppm [104].

a 0

A

'I L-----4-0 80 0 20 40 60

Spcilic diichmge input (119)

Figure 70. As for Figure 69 except for positive and negative pulsed voltage. Pulsed wid th 100 ns [104].

the energy density u/g) in a mixture of 70.5% Nz, 1.8% 02, 9.9% COz, 17.9% H20 and ammonia for dc and pulsed voltages, respectively [50, 1041. Both voltage polarities were employed. The ammonia was first diluted with N2 at a ratio of 1:lO and the amount injected into the gas mixture was equivalent to convert SO2 and NO to ammonium sulfate and ammonium nitrate, respectively

Figure 69 shows that negative dc corona is superior than positive dc for reducing NO and NO, [104]. A comparison between Figures 70 and 69 shows that the energy density input necessary to reduce NO, NO, and SO1 is much less for pulsed than for dc corona. To obtain - 90% reduction of NO, from an initial concentration of 300 ppm, 50 J /g was required using pulsed corona, while for dc to obtain 70% reduction 600 J/g was consumed (Figure 69) [104].

20 CONCLUSIONS HE current state of the art in using discharges to reduce pollutants T present in air that originate from a variety of sources including coal-

burning power plants, diesel engine exhausts, industrial plants, as well as simulated mixtures of gases used in laboratories, is reviewed. It appears that non-thermal discharges using very fast rise ( 4 0 ns) and short duration pulses of <50 ns are likely to offer promising prospects.

This is because the energy supplied into the discharge is predomi- nantly used to create energetic electrons which produce radicals such as N, OH and 0 that lead to the removal of the pollutants.

The energy required to remove the pollutants is of prime concern. It would serve as one of the main considerations in the selection of the technology to be used ultimately

Substantial effort is being expended worldwide to design more efficient pulsed power sources to be employed for pollutant control as well as improved discharge reactor geometries.

The requirements of recent national legislation and international agreements to reduce significantly the amounts of emissions of pollutants into the air necessitate a development of a cost effective system that can be used in electrical generation and other plants. This has be- come of some urgency of late.

It is expected that the research into new techniques to remove yol- lutants, better understanding of the chemical and physical processes involved as well as development and testing prototypes of discharge reactors in power and other plants will continue at an accelerated rate.

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Manuscript was received on 12June 2000.

9---air pollution control by electrical discharges-heckam

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