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This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institution
and sharing with colleagues.
Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third party
websites are prohibited.
In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further information
regarding Elsevier’s archiving and manuscript policies areencouraged to visit:
Chemical Engineering and Processing 57– 58 (2012) 65– 74
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Chemical Engineering and Processing:Process Intensification
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Removal characteristics of tar benzene using the externally oscillated plasmareformer
Young Nam Chuna,∗, Seong Cheon Kima, Kunio Yoshikawab
a BK21 Team for Hydrogen Production, Department of Environmental Engineering, Chosun University, 375, Seosuk-dong, Dong-gu, Gwangju 501-759, Republic of Koreab Frontier Research Center, Tokyo Institute of Technology G5-8, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan
a r t i c l e i n f o
Article history:Received 16 November 2011Received in revised form 17 March 2012Accepted 19 March 2012Available online 28 March 2012
Keywords:Tar destructionGliding arc plasmaExternal oscillationPlasma reformerBenzene tar
a b s t r a c t
To destruct tar from pyrolysis and/or gasification, an externally oscillated gliding arc plasma reformer(EOPR) was designed and verified its decomposition performance. The external oscillation device can giveto expand the discharge area of the gliding arc plasma reformer. An experiment was conducted usingsurrogate benzene, which is generated during the pyrolysis and/or gasification, as the representative tarsubstance.
To identify the characteristics of the influential parameters of tar decomposition, tests were performedon the oscillation frequency and amplitude, steam feed rate, and total gas feed rate.
The optimal operating conditions of the EOPR were 267 Hz in oscillation frequency, 3 Vpp in oscillationamplitude, 0.66 L/min in steam feed rate, 0.12% in benzene concentration, 16.7 L/min in total gas feedrate, and 0.17 kWh/m3 in SEI (specific energy input). The benzene decomposition efficiency was 90.7%,and the energy yield was 22.95 g/kWh. Without oscillation, the decomposition efficiency was 82.6%, andthe energy yield was 20.9 g/kWh, both of which were 8.9% lower than those with the external oscillation.Tar benzene was decomposed into light gases (H2, CO, and CO2), hydrocarbons (CH4, C2H4, and C2H6) andcarbon block.
It is known that pyrolysis and gasification can be used to recoverenergy from organic biomass or waste resources. The gas producedfrom the pyrolysis and gasification can be applied to diverse areas,including gas turbines, engines, fuel cells, methanol and hydro-carbon productions, etc. The producer gas contains tar that is amixture of compounds containing polyaromatic hydrocarbons andoxygenates [1].
The tar is the major problem that has not been completely solvedso far related to utilization of producer gas in downstream applica-tions. The producer gas will require compression before it is usedin gas turbines and internal combustion engines (ICE) using tur-bocharger, as well as the air if the gasifier operates at atmosphericpressure. It becomes saturated when the tar vapor pressure exceedsthe saturation pressure of the tar and leads to condensation ofthe saturated vapor. Upon condensation, tar blocks downstreampipelines and fouls engines and turbines [2,3].
Therefore, various studies are under way for the tar removal inthe producer gas. Among these, the studies on the tar destruction
are mostly about thermal cracking [4,5] and catalytic cracking [6,7].For the thermal cracking, however, high temperature and sufficientretention time are required. In the catalytic cracking, the efficiencyof the reformer worsens due to the reduced catalyst activation, andthe costs of the catalyst and the operation are high.
To ensure a high tar removal rate and high energy yield, studiesare being conducted on diverse plasma tar decomposition tech-niques, including corona and arc discharge [8,9]. The gliding arcplasma has fast (several seconds) starting and responding charac-teristics, and ensures optimal operation for diverse gas states and acompact device design. To increase the discharge area and electrondensity, however, it requires high electric energy [10,11].
Therefore, methods which develop high local energies such asdischarge plasma fields have been investigated [12]. Sound waveirradiation of various gas flows has shown that the combination ofplasma, which involves conversion of medium to charged particles,and sound, which is the propagation of the vibration motion ofthe medium, causes expansion of the discharge space, and that thedegree of the expansion depends on the magnitude of the sound[12,13].
In this study, an EOPR was designed and verified performanceto destruct tar, using the external-oscillation plasma technology.With benzene as the representative tar substance, the oscillationfrequency and amplitude of the external-oscillation generator were
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Fig. 1. Schematic diagram of a test setup for an EOPR system.
tested to examine tar decomposition efficiency and energy yield,including the parameters of steam feed rate and total gas feed rate.
2. Experimental equipment and method
2.1. Experimental setup
Fig. 1 shows the test setup for the EOPR performance for thetar removal. The setup consisted of an EOPR, an oscillation con-trol device, a steam feeding line, a tar feeding line, a power supplyequipment, a measurement and analysis line, and a control andmonitoring system.
The EOPR had three knife-type electrodes mounted at 120◦
intervals on a ceramic cylindrical insulator (Al2O3, 96 wt.%) in aquartz tube (55 mm in diameter, 200 mm in length) reactor. Theeach electrode was 2 mm in wide, 95 mm in long and 2 mm in thick-ness. The gas injection nozzle of 3 mm in a diameter was fixed atthe top of the electrode.
The oscillation control device included a loud speaker (BT40,Speaker Mall, South Korea), an amplifier (PA-4000A, INTER-M,South Korea), and a function generator (Agilent 33250A, AgilentTechnology, USA). At the rear of the EOPR, a sound pressure levelmeter (DSL-330, TECPEL, Taiwan) was installed to measure thesound pressure in the EOPR.
The steam feeding line consisted of a steam generator, a waterpump (STEPDOS 03, KNF, Switzerland) and a water tank. A setamount of distilled water in the water tank was supplied by thewater pump to the steam generator. The steam generator carriedby dilution gas (N2) controlling an MFC (Model F201AC-FAC-22-V,BRONKHOST, Netherlands).
The tar feeding line consisted of a tar generator for tar gas gener-ation and an MFC (M3030V, Linetech, South Korea) for controllingthe carrier gas (N2). The tar generator consisted of a tar containerand a mantle heater.
The power supply equipment consisted of a power supply(UAP-15K1A, Unicon Tech, South Korea), a high-voltage probe(P6015, Tektronix, USA), a low-current probe (A6303, Tektronix,USA), and an oscilloscope (TDS-3052, Tektronix, USA) for electrical-characteristic measurement. The power supply provided up to15 kW three-phase AC power (voltage: 15 kV; AC current: 1 A) tothe EOPR.
The measuring and analysis line consisted of a carbon-black fil-ter (LS-25, Advantec, Japan) for carbon black sampling, a GC-FID(GC-14B, SHIMADZU, Japan) for tar analysis, and a GC-TCD (CP-4900, Varian, Netherlands) for gas analysis.
The control and monitoring system consisted of a LabVIEW(National Instruments, LabVIEW 8.6, USA) and a computer. This sys-tem was used for controlling each set value for MFCs, a water pump,an electric heaters for a tar generator, a soot filter, and tar feedingand sampling lines. Also, the system continuously monitored thetemperature, steam flow rate, and nitrogen gas flow rate.
2.2. Experimental method
Fig. 2 shows the initial operating conditions and final stabilizedtemperatures of each device. The temperature of the steam gen-erator (1) was kept constant at 300 ◦C. The temperature of the targenerator (2) was kept at 25 ◦C because the boiling point of ben-zene is 80.1 ◦C and it can vaporize at the atmosphere temperature.The heating line (3) was heated as 100 ◦C using a tape heater toprevent condensation. The temperature of an EOPR (4) maintainedas about 290 ◦C. The temperature of a carbon-black filter (5) waskept constant at 120 ◦C.
Water in the water tank was fed into the steam generator, andsteam was generated. The dilution gas was supplied, and the gaswas delivered to the EOPR. The carrier gas was fed into the tar gen-erator that had liquid benzene at the atmosphere temperature. Thebenzene in the tar generator vaporized, and stable tar-containinggas was produced. The produced steam in the steam generator and
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Fig. 2. Initial operating and stable conditions at each device.
the tar gas in the tar generator were mixed in an orifice mixer, andthe mixture was fed into the EOPR. The test was conducted at thestable plasma discharge state.
To measure the input tar concentration, the input gas was sam-pled from the inlet of the EOPR, and the benzene in the gas wasanalyzed. And benzene, decomposed gas, and carbon black weresampled from the outlet of the EOPR, and were analyzed.
To measure the carbon-black concentration, the carbon-blackwas collected from the glass filter paper (GA-100, Advantec, Japan)on the carbon-black filter. The difference between the sampledcarbon-black weights before and after the sampling was mea-sured using an electronic balance (HS-250D, Shenyang Longteng,Taiwan). The gas was sampled as the flow rate of 2.5 L/min for20 min, and then the sample gas volume was calculated.
For tar sampling, the gases from the sampling ports at the inletand outlet part of the EOPR were removed residual soot and waterin the impinge train. And they were collected from the tar sam-pling port using a syringe (22265, Supelco, USA). The collected tarwas injected into the gas chromatograph flame ionization detector(GC-FID) port for analysis. As the FID analysis conditions, the tem-peratures of the injector and detector were kept constant at 200and 280 ◦C, respectively. The oven temperature increased at a rateof 10 ◦C/min within the range of 40–80 ◦C and at 20 ◦C/min withinthe range of 80–300 ◦C. And then the oven was left for 5 min at thefinal temperature of 300 ◦C.
To protect the GC column from the remaining tar and VOCs, thedecomposed gas was passed through backup adsorbent (cotton andactive carbon filters), consecutively. The GC-TCD (CP-4900, Varian,Netherlands) was used for gas analysis. The Molecularsieve-5A col-umn was used for H2, CO, O2 and N2, and the PoraPlot-Q columnwas used for CO2, C2H4, and C2H6.
The instantaneous voltage, current and discharged electricpower observed via the digital oscilloscope (see Fig. 3) showsalmost random feature of the history of each gliding breakdownpowered by a alternating current supplier. The electric powershould be measured by calculation of root mean square voltageand root mean square current wave.
At the steady-state plasma condition after breakdown point, thevoltage got lower than the adjusted original voltage. On the otherhand, the current value increased higher than that before break-down. This phenomenon was caused by arcs production in the
Fig. 3. Applied voltage and current waveform.
plasma, which typically occurred at low voltages and high currentscondition [14,15].
The test was performed with the parameters that influence thetar decomposition and energy yield, such as the oscillation fre-quency, oscillation amplitude, steam feed rate, and total gas feedrate. Table 1 shows the test range for the parameters.
Table 2 shows the calibrated range, accuracy and relative errorof equipments. Errors in experiments can arise from instrumentconditions, calibration, observation and test planning.
2.3. Data analysis
2.3.1. Decomposition efficiencyThe decomposition efficiency, which represents the degree of
benzene decomposition in the gas, was calculated using Eq. (1), asfollows:
DE(%) = [VC]inlet − [VC]outlet
[VC]inlet× 100 (1)
where [VC]inlet is the input benzene concentration (%) and [VC]outletis the output benzene concentration (%).
2.3.2. Energy yieldEq. (2) shows the energy yield needed to process the input gas
[9]:
�e(g kW−1 h−1) = ([MC]inlet − [MC]outlet)QIP
(2)
where [MC]inlet is the input benzene concentration (g/m3) and[MC]outlet is the output benzene concentration (g/m3). Q is the gasfeed rate (m3/h) for the reformer and IP is the plasma input power(kW).
2.3.3. Specific energy inputThe specific energy input (SEI), which is the ratio of the input
energy to the gas feed rate, was calculated using Eq. (3).
SEI (kWh m3) = IPQ
(3)
2.3.4. Carbon balanceEq. (4) shows the carbon balance, which represents the carbon
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Table 1Test condition and range by parameter.
Experimental conditions Oscillation frequency (Hz) Oscillation amplitude (Vpp) Steam feed rate (L/min) Total gas feed rate (L/min)
Range 0–1000 0.5–3 0–0.85 12.4–24.5
Table 2The calibrated range, accuracy and relative error of measurement.
Measurement Equipment Calibrated range Accuracy Relative error (%)
Gas chromatography (Tar; Benzene) Shimadzu, GC-14B 100 ppm ∼ 100% ±2.5% <0.5Gas chromatography (Gases) Varian, MicroGC-4900 1 ppm ∼ 100% ±2% <0.5Temperature K-type (D: ∅ 0.3) 273–1643 K ±1 K ±0.25Mass flow meter (Dilution N2) Bronkhost, F201AC-FAC-22-V 0–20 L/min ±1% ±0.25Mass flow meter (Tar gas generation N2) Line Tech, M3030V 0–1 L/min ±1% ±0.25Gas meter Sinagawa, DC-2A 10-2000 L/h ±1% <0.5Electronic balance Shenyang Longteng, HS-250D 0–180 g ±0.00001 g ±0.1High voltage Tektronix, P6015A 1.5–20 kV ±1 V ±0.005Current Tektronix, A6303 0–40 A ±5 mA ±0.01
where [CO2], [CH4], [C2H4], and [C2H6] are the concentrationsof each ingredient (g/m3), ST is the carbon-black concentration(g/m3), and A is the carbon constant which is 6 for benzene.
2.3.5. Sound pressureEq. (5) shows the sound pressure.
Ps (Pa) = P0 × 10Lp(dB)/20 (5)
where, ps is the sound pressure (Pa), p0 is the reference values(threshold of hearing) (=20 �Pa = 2 × 10−5 Pa) and Lp is expressedas sound pressure level (dB).
3. Tar destruction in an externally oscillated plasmadischarge
3.1. Acoustic wave interaction with plasma discharge
An increase in the acoustic wave frequency implies that a per-turbation producing rarefaction and compression in the plasma hasa small wave length, thus causing the plasma particles to collidewith increasing rate. The acoustic wave when passing through suf-ficiently dense plasma may travel with supersonic speed as a shockwave.
The variation of electron density (˛) with acoustic wave fre-quency (ω) may be explained as follows. When acoustic wavefrequency (ω) < electron-atom elastic collision frequency (�), therate of variation of pressure in the plasma is low enough such thatthe change in the electron density can easily follow the pressureperturbation. When acoustic wave frequency (ω) ≥ electron-atomelastic collision frequency (�), the electron velocity is no longer ableto follow the variations of the pressure distributions, and hence theelectron density decreases. Collision frequency (ˇ) depends on thepressure variations; an increase in the ω causes the plasma particleto collide rapidly, thus increasing ̌ [16] (Fig. 4).
3.2. Tar decomposition mechanism
The benzene removal mechanism in the EOPR can be explainedusing the following equations.
The main reactions are tar cracking (Eq. (6)) and carbon forma-tion (Eq. (7)) [17].
- Tar cracking
pCnHx → qCmHy + rH2 (6)
- Carbon formation
CnHx → nC + (x/2)H2 (7)
where, CnHx represents tars, such as the large molecular com-pounds, and CmHy represents a hydrocarbons with a smaller carbonnumber compared to that of CnHx.
Eqs. (8)–(12) show the mechanisms of the production, utiliza-tion, and termination of the radical, and the carbon reaction whenthe steam is fed into the plasma reformer [8,18].
- Radical production
e + H2O → e + H + OH (8)
- Radical utilization
OH + tar → Products (9)
- Radical termination
OH + CO → CO2 + H (10)
- Carbon decomposition
Cx + OH → Cx−1 + CO + (1/2)H2 (11)
Cx + 2OH → Cx−1 + CO2 + H2 (12)
Fig. 4. The variation of ̨ and ̌ with the acoustic wave frequency ω.
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Table 3Reference conditions and their results.
Experimental conditions for each parameter
Conditions Oscillationfrequency (Hz)
Oscillationamplitude (Vpp)
Steam feed rate(L/min)
Total gas feedrate (L/min)
Specific energyinput (kWh m−3)
Input benzeneconc. (%)
Value 267 3 0.66 16.4 0.17 0.12
Experiment results
Result Gas composition after the reformer (%,N2 excluded)
The reaction of the gas produced after tar decomposition andsteam reforming can be explained by the water-gas shift reaction(Eq. (13)) and steam reforming (Eq. (14)) [19,20].
- Water-gas shift reaction
CO + H2O → CO2 + H2 (13)
- Steam reforming
CnHm + nH2O = nCO + (n + m/2)H2 (14)
where, CnHm is the light hydrocarbons.
4. Results and discussion
4.1. Acoustic wave interaction with plasma discharge
The EOPR was developed to destruct tar from the pyrolysisand/or gasification of organic waste resources and biomass. Ben-zene was selected as the representative tar, and the test wasperformed according to the various parameters. Table 3 shows thetest results under the reference conditions, showing the maximumtar decomposition and energy yield.
With external oscillation, the benzene decomposition efficiencywas 90.7%, and the energy yield was 22.95 g/kWh. The light gasesthat were produced from the benzene decomposition included H2,CO, and CO2. The higher heating value was 9718 kJ/N m3. The carbonbalance was 88.8%. It seems that the value of the carbon balance didnot reach 100% because the heavy hydrocarbon, nitric tar products(HCN and CN), the carbon black, etc. from the benzene conversionproducts, were not considered [15]. Without oscillation, the decom-position efficiency was 82.6%, the energy yield was 20.9 g/kWh, andthe higher heating value was 9209 kJ/N m3, which were smallerthan those with external oscillation.
Fig. 5 shows the plasma discharge with and without oscillationunder the reference conditions. Sound wave gives spatio-temporalvariations of gas pressure due to the expansion and compressions ofgas. So, gas discharge should be influenced by sound wave irradia-tion. Irradiating the sound wave and increasing the sound pressure,the luminous part spreads wider due to the vibration of the gaseousmedium. The expansion of the streamer was probably caused by acyclic change in the discharge field due to the violent vibration ofmedium particles in the neighborhood of the electrodes.
On the left, the plasma discharge was not forced acoustically;on the right, its instability was forced by means of periodic soundwaves introduced through a loudspeaker near the plasma dischargeat its natural frequency. The forced acoustic waves reduce thelength of the laminar boundary layer on the periphery of the plasmadischarge and cause more regular formation of vortex rings thanunder the unforced [21].
Fig. 6 shows the sound pressure (Ps) according to different oscil-lation frequency at fixed oscillation amplitude of 3 Vpp under thereference conditions. The sound pressure was calculated with Eq.(5) by using the sound pressure level (Lp) measured from the EOPRoutlet with external oscillation.
External oscillation to the plasma discharge causes expansion ofthe discharge space, and the degree of expansion depends on themagnitude of the irradiated sound pressure. Standing sound wavesare formed due to the interference of the incident and reflectedsound waves inside the EOPR. Under the standing sound wave field,a sound pressure and particle velocity, which is defined as the vibra-tion velocity of the gaseous medium due to the sound wave, isdistributed in the acoustic tube [22].
Resonance states were observed with the external oscillationfrequency at 267, 560, and 810 Hz. The sound pressure (Ps) at theend of the EOPR’s tube is proportional to the particle velocity at theloop of the distribution.
4.2. Effects of the oscillation frequency
The factors governing the tar destruction rate in the plasmadischarge can be described by Eq. (15) [12].
r = kVdNeNn� (15)
where k is the reaction rate constant, Vd is the volume of the spacewhere discharge takes place, Ne is the density of discharged elec-trons, Nn is the density of the gas in the space, and � is the formationfrequency of active species.
Fig. 7 shows the test results at the oscillation frequencies rangingfrom 0 to 1000 Hz, with the parameters fixed as in the referenceconditions (Table 3).
Fig. 7(a) shows the benzene decomposition and energy efficien-cies as well as the benzene concentrations at the inlet and outlet ofthe EOPR. Decomposition efficiency increased, and after having apeak value it decreased. The major factor for tar benzene destruc-tion rate is acoustic wave frequency (�) in the EOPR as shown inEq. (15). Resonance states were observed with the external oscil-lation frequency at 267, 560, and 810 Hz as already explained inFig. 6. Especially, the oscillation frequency at 267 Hz might imposefewer constraints which means greatest effect although the fre-quencies at 560 and 810 Hz have higher sound pressures. Therefore,the increase of the external oscillation frequency increased thedestruction efficiency up to 267 Hz which has maximum benzenedecomposition efficiency of 91%. This is because the sound pressurecauses expansion of the discharge space in the plasma discharge.But when acoustic wave frequency (ω) is higher than the electron-atom elastic collision frequency (�), the electron velocity is nolonger able to follow the variations of the pressure distributions,and hence the electron density (˛) decreases as already explainedin Fig. 4. Therefore, the decrease in the destruction rate due to
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Fig. 5. Photo of plasma discharge with and without oscillation.
sound-wave irradiation can be attributed to the negative effectsof the decrease in the electron density, although having the posi-tive effects of the increase in collision frequency. So, after havingpeak value, the decomposition efficiency was decreased.
Energy yield had similar tendency to the decomposition effi-ciency. This is because the main factor affected in the efficiency
Fig. 6. Sound pressure according to the oscillation frequency.
(calculated by Eq. (2)) is the tar removal which is the differencebetween input and output concentration.
Fig. 7(b) shows the concentrations of light gas and carbon black.The light gases produced were H2, CO, and CO2, having low con-centration due to low tar input. Particularly, H2 and CO had slightlyhighest value at the 267 Hz. But CH4, C2H4, C2H6 and carbon blackwere not almost detected.
Fig. 8 shows the plasma discharge according to the oscillationfrequency change. At a frequency of 267 Hz, when the benzenedecomposition and energy efficiencies were highest, the plasmadischarge was most active, although it was difficult to accuratelyidentify.
4.3. Effects of the sound pressure
Fig. 9 shows the test results with the oscillation amplitude variedwithin the 0.5–3 Vpp range.
Fig. 9(a) shows the benzene decomposition and energy efficien-cies as well as the benzene concentrations at the inlet and outletof the EOPR. Decomposition efficiency gradually increased withincreasing the oscillation amplitude. The change in the oscillationamplitude influences the gas pressure. The concentration of heavyparticles (neutral atoms, positive or negative ions) increases withincreasing the oscillation amplitude [23]. So, the heavy particlesreact with each other to destruct benzene. A number of papershave reported the observation of an increase of the pressure ampli-tude A of sound waves in gas discharge plasmas in comparisonwith un-ionized air at identical values of the static gas pressure P0,
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amplitude � of the displacement of the gas particles in the soundwave, and sound frequency � [24]:
A = 2�vP0��
W(16)
where W is the sound velocity and � is the ratio of the specific heats.The energy yield showed a trend similar to that of the benzene
decomposition efficiency. This was because the energy yield, whichwas calculated using Eq. (2), was influenced by the tar decompo-sition efficiency when the gas flow rate in the reformer (Q) andplasma input energy (IP) were constant.
Fig. 9(b) shows the concentrations of light gas and carbon black,which were produced from the tar benzene decomposition. Thelight gases produced were H2, CO, and CO2, having low concentra-tion due to low tar input. The concentrations of H2 and CO2 werealmost constant (0.82 and 0.48% on average, respectively) regard-less of the change in the sound pressure. The CO concentrationslightly increased from 0.65 to 0.82% with the increase in soundpressure. But hydrocarbons (CH4, C2H4, and C2H6) and carbon blackwere not detected.
4.4. Effects of the steam feed rate
Fig. 10 shows the test results with the steam feed rate variedwithin the 0–0.85 L/min range.
Fig. 10(a) shows the benzene decomposition and energy effi-ciencies as well as the benzene concentrations at the inlet and outletof the EOPR, with and without oscillation. With the increase in thesteam feed rate, the benzene decomposition efficiency graduallyincreased and reached 90.7% at a steam feed rate of 0.66 L/min.Then it started to decrease. When the steam feed rate was 0 L/min,that is, when no steam was supplied, the decomposition efficiencywas 77.4. This was because the tar was decomposed according tothe plasma tar cracking (Eq. (6)) and external-oscillation effect (Eq.(15)), without the effect of steam. As steam was supplied, OH rad-icals, electrons, and active chemical species were created due tothe water excitation (Eq. (8)). Then the tar was decomposed dueto the utilization (Eq. (9)), and then the decomposition efficiencyincreased.
However, water also has an adverse effect on tar removal dueto its electronegative characteristics. Too many water moleculeslimit the electron density in the system and quench the activatedchemical species [25]. Therefore, after having maximum value, thebenzene decomposition efficiency decreased due to too many feed-ing of the steam. In addition, with the increase in the steam feedrate, the total gas flow rate in the EOPR increased. Accordingly,sufficient retention time was not ensured, and the benzene decom-position efficiency decreased.
Without oscillation, the decomposition efficiency according tothe change in the steam feed rate showed almost the same patternas that with oscillation. The decomposition efficiency was higher,however, with external oscillation. This was because the OH radi-cals, electrons, and active chemical species that were produced byplasma discharge in the presence of steam (Eq. (17)) had higherdensities due to the external oscillation [8,16,23].
H2O → H, e−, OH, H2, H2O2, H3O+, OH− (17)
The energy yield had a pattern that was similar to that of the ben-zene decomposition efficiency. With the increase in the steam feedrate, the energy yield increased and reached 22.9 g/kWh at a steamfeed rate of 0.66 L/min. Then it gradually decreased. As known inEq. (2), the energy yield increased due to the increase in the tarremoval and input feed rate with the increase in the steam feed,while supplied to a specific quantity of plasma input power. Afterthe maximum was reached, however, the energy yield decreased
because of the effect of the decreased tar removal, despite theincrease in the input feed rate.
The amount of carbon black was relatively large at 0.053 g/N m3,when no steam was fed (0 L/min). This was because the tar benzenewas decomposed into carbon (C) due to the carbon formation (Eq.(7)) without the oxidation caused by OH radical. As the steam feedrate increased, the carbon black gradually decreased and was hardlyproduced when the steam feed rate was 0.38 L/min or higher. Thiswas because the produced carbon black was converted into light gasaccording to the carbon-black decomposition (Eqs. (11) and (12)),due to the OH radicals that were produced according to the waterexcitation (Eq. (8)) [17].
Fig. 10(b) shows the concentration of light gas, which was pro-duced when the tar benzene was decomposed.
With the increase in the steam feed rate, H2 and CO2 continuedto increase, and CO increased to the maximum and then decreased.H2, CO2, and CO mostly increased due to the tar cracking (Eq.(6)), carbon formation (Eq. (7)), carbon-black decomposition (Eqs.(11) and (12)), and steam reforming (Eq. (14)). In the case of CO,however, it decreased later as it was converted into CO2 due tothe radical termination (Eq. (10)) and water-gas shift reaction (Eq.(13)).
When no steam was fed, light hydrocarbon gases (CH4, C2H4,and C2H6) were produced at 0.1, 0.09, and 0.11%, respectively. Theydecreased with the increase in the steam feed rate and were hardlyproduced at a steam feed rate of 0.47 L/min or higher. The hydro-carbons were produced as the benzene was decomposed due to thetar cracking (Eq. (6)), and disappeared due to the steam reforming(Eq. (14)) as steam was fed.
4.5. Effects of the total gas feed rate
Fig. 11 shows the effects of the change in the total gas feed rate.The total gas feed rate was set within the 12.6–24.7 L/min range,which can be stably operated for the EOPR.
Fig. 11(a) shows the benzene decomposition and energy effi-ciencies as well as the benzene concentrations at the inlet and outletof the EOPR, with and without oscillation.
With the increase in the total gas feed rate, the decompositionefficiency slightly decreased. This was because the gas flow rateincreased in the EOPR with the increase in the total gas feed rate,and the retention time of the benzene-containing gas decreasedwithin the EOPR. Therefore, the reaction time between the elec-trons, ions, and radicals that were produced in the plasma dischargezone and the benzene decreased [26]. Without oscillation, thedecomposition efficiency pattern for the change in the total gas feedrate was almost the same pattern as that with external oscillation.The decomposition efficiency, however, was higher by 8.91% withthe external oscillation. This indicates that the external-oscillationeffect is almost constant regardless of the total gas feed rate.
Energy yield significantly increased with increase of the totalgas feed rate. This is because the gas flow rate (Q) relativelyincreased according to Eq. (2), although the benzene removalslightly decreased according to increase of steam feed.
Fig. 11(b) shows the concentrations of the carbon black and lightgases, which were produced when the tar benzene was decom-posed.
Carbon black was hardly generated with the change in the totalgas feed rate. This was because even though the benzene concen-tration slightly increased with the increase in the total gas feed rate,the carbon black was destructed into light gases (Eqs. (11) and (12))at a constant and sufficient steam flow rate of 0.66 L/min.
With the increase in the total gas feed rate, H2 and CO gradu-ally increased. H2 increased due to the tar cracking (Eq. (6)) andcarbon formation (Eq. (7)), and H2 and CO increased due to thecarbon-black decomposition (Eq. (11)) and steam reforming (Eq.
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Fig. 7. Effects of the oscillation frequency.
Fig. 8. Photos of the plasma discharge on various oscillation frequencies.
Fig. 9. Effects of the oscillation amplitude.
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Fig. 10. Effects of the steam feed rate.
Fig. 11. Effects of the total gas feed rate.
(14)), when steam existed. The increase in CO2 was less than thoseof the two gases above. This was because the reaction did not lastsufficiently as the steam flow rate due to the radical termination(Eq. (10)), carbon-black decomposition (Eq. (12)), and water-gasshift reaction (Eq. (13)) was not high enough. Light hydrocarbongases (CH4, C2H4, and C2H6) were hardly detected. This was becausethe hydrocarbons were converted into H2 and CO due to the steamreforming (Eq. (14)).
Eventually, the increase in the total gas feed rate reduced theretention time, and then the decomposition efficiency decreased,but because of the characteristics of the test equipment, the inputbenzene concentration slightly increased, and the light gases otherthan hydrocarbon also increased.
5. Conclusion
An EOPR was developed to destruct the tar generated frompyrolysis and/or gasification, and benzene as representative tar gaswas experimentally studied.
The oscillation frequency, oscillation amplitude, steam feed rate,and total gas feed rate were used as parameters for the test. Theoscillation frequency, oscillation amplitude, and steam feed rateconditions existed for the maximum benzene decomposition andenergy efficiencies. That is, when the oscillation frequency was267 Hz, the oscillation amplitude was 3 Vpp, and the steam flow rate
was 0.66 L/min, the tar removal and energy efficiencies were 90.7%and 22.95 g/kWh, respectively. With the increase in the total gasflow rate, the benzene decomposition efficiency slightly decreased,and the energy yield increased.
The tar benzene was decomposed into light gases (H2, CO, andCO2), hydrocarbons (CH4, C2H4, and C2H6), and carbon block.
Without oscillation, the decomposition efficiency was 82.6%,and the energy yield was 20.9%, both of which were 8.9% lowerthan those with the external oscillation.
The test results showed that the EOPR can efficiently destruct tarusing less energy than that used in the gliding arc plasma reformer.
Acknowledgments
This research was supported by Basic Science Research Pro-gram through the National Research Foundation of Korea (NRF)funded by the Ministry of Education, Science and Technology(2010-0004156).
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