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C 2003) Plasmas and Polymers, Vol. 8, No. 3, September 2003 (°
Pulsed Corona Discharges for Tar Removal from Biomass Derived Fuel Gas1 A. J. M. Pemen,2,4 S. A. Nair,2 K.Yan,2 E. J. M. van Heesch,2 K. J. Ptasinski,3 and A. A. H. Drinkenburg3 Received February 11, 2003; accepted April 17, 2003 To supply combustion engines or gasturbines with fuel gas obtained from biomass gasification, it is necessary to clean the fuel gas. Also the production of chemicals by processes such as Fisher-Tropsch requires a high gas quality. Especially heavy aromatic hydrocarbons (“tars”) must be removed. In this work, we give an overview of our investigations on tar removal by pulsed corona discharges as an alternative approach to catalytic or thermal tar cracking. Experimental results (at a gas temperature of 200◦ C) are reported for the removal of various model tar components in synthetic fuel gas. In order to identify the major reaction pathways, experiments were also done on tars in individual fuel gas components. The results show that tar removal by pulsed corona processing is possible. The process for tar removal is mainly via oxidation. Also termination reactions by CO play an important role. KEY WORDS: Biomass; fuel gas; gasification; tar removal; non-thermal plasma; pulsed corona.
1. INTRODUCTION Literature quotes the contribution of biomass to the world’s energy supply as ranging from 10 to 14 %.(1) Biomass gasification has a higher energy efficiency than biomass combustion. The produced fuel gasses possess a heating value of about 4∼5 MJ/Nm3 and mainly consist of H2 , CO, CH4 , CO2 and N2 . In order to utilize the fuel gas for electricity production or for the production of chemicals, e.g., via the Fischer-Tropsch process, a high gas quality is required. However, gasification of 1 Extended
version of a paper presented at the International Symposium on High Pressure Low Temperature Plasma Chemistry, HAKONE VIII, P¨uhaj¨arve, Estonia, July 21–25, 2002. 2 Eindhoven University of Technology, Faculty of Electrical Engineering, Group EPS, P.O. Box 513, 5600 MB Eindhoven, The Netherlands. 3 Eindhoven University of Technology, Faculty of Chemical Engineering. P.O. Box 513, 5600 MB Eindhoven, The Netherlands. 4 To whom correspondence should be addressed. E-mail:
[email protected] 209 C 2003 Plenum Publishing Corporation 1084-0184/03/0900-0209/0 °
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biomass produces significant amounts of heavy hydrocarbons (referred to as “tars”) along with particulates. The presence of tars leads to condensation in downstream equipment, thereby leading to severe operational problems such as clogging of the engine. For the Fischer-Tropsch process, tars can deposit on the active sites of catalysts, leading to loss of activity and hence the need for frequent regeneration. Thermodynamic analysis shows that the high dew point of fuel gas is due to the high concentration of polyaromatic tar compounds such as naphthalene, phenanthrene, anthracene, etc.(2) Hence it is necessary to reduce the aromatic content of the gases, as these hydrocarbons also possess a calorific value that can be utilized for power generation. Various methods have been tried, such as: (i) modifying the gasifier design; (ii) catalytic and thermal cracking; and (iii) mechanical methods such as filtration and condensation. A detailed summary can be found in reviews by Neeft et al.(3) and Milne et al.(4) Here we discuss the use of non-thermal plasma as an alternative method for catalytic and thermal treatment. In both catalytic as well as thermal treatment processes, the fundamental mechanism involves creation and stabilizing of reactive species, which thereafter initiate the necessary reactions under kinetic and thermodynamic limitations. The same can be expected to occur in a non-thermal plasma, where similar species can be created by energetic electron-molecule collisions. The resulting reactive atmosphere initiates the subsequent chemical processing. The plasma thus closely resembles a catalyst in terms of its operation to reduce severity of operating conditions. 2. PULSED CORONA DISCHARGES Non-thermal plasma has been extensively studied for pollution control and gas treatment over the past decade.(5–7) The number of applications is now steadily growing. Ozone generation by using a silent discharge reactor is a classical example and is used in industry for over a century.(8) Processes such as controlling particulate emissions, acid gas abatement,(9) VOC control,(10) odor control, flue gas cleaning and CO2 utilization(11) are increasingly being investigated. Experimental studies have been carried out to investigate the effectiveness of nonthermal plasmas to overcome the thermodynamic equilibrium conditions.(12) Investigations were also made on utilizing electrical discharges as a means to synthesize organic compounds.(13) Present day research aims at converting these ideas into actual industrial systems, where the critical aspects are costs, efficiency and reliability of operation. Different types of non-thermal plasmas are potential candidates. Pulsed corona induced non-thermal plasma seems to be very suitable for large volume gas treatment.(5,10) At Eindhoven University, research in this field covers fundamental processes of corona discharges,(14−16) corona plasma for gas(17) and water treatment,(18,19) pulsed power technology,(5,20) electrical diagnostics(6,17) and EMC
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techniques for reliable and disturbance free operation of electronic circuits.(21) Pulsed corona discharges are advantageous in that they: (i) enable combined plasma processing, aerosol formation, and particulate/aerosol removal; (ii) run effectively at high gas temperatures(22) ; (iii) run under wet conditions; and (iv) are very suitable for large volume gas cleaning. Generally speaking, in order to generate intense streamer corona discharges while avoiding their evolution into spark breakdown, voltage pulses with short duration and electrode arrangements producing highly non-uniform electric fields are used. In a wire-cylinder corona reactor, streamers cross the gap from wire to cylinder during tens of nanoseconds during a short high-voltage pulse (Fig. 1). Although a solid background exists on development of pulsed corona plasma techniques, the relations between voltage pulse parameters, discharge properties and plasma processing are not fully understood. However, rule-of-thumb values have been established to optimize energization and discharge generation(5) : (i) at higher peak voltages, the equivalent reactor impedance tends to approach the output impedance of the generator, which corresponds to maximum energy transfer efficiency (Fig. 1b); and (ii) to avoid the development of less efficient secondary streamers, the pulsed power duration must be equal to the primary streamer duration (Fig. 1b). The energy efficiency and costs of pulsed corona plasma processing depend to a great extent on the pulse generator. Prototypes of efficient high-power pulse
Fig. 1. (a) End-on CCD views (5 ns shutter time) showing the development of streamers during a voltage pulse in a wire-cylinder reactor. Indicated is the transit time after the start of the voltage pulse. (b) Effect of the peak voltage on the transit time of streamers and on the ratio of the equivalent reactor impedance with the output impedance of the generator. Note that the data has been obtained for two different reactor diameters. For details see Yan [5].
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generators are available and have been demonstrated in laboratory and in field trials. The best and most cost effective pulse generator now available employs a fast heavy-duty spark-gap switch in combination with a transmission-line transformer (TLT).(5) Output parameters reach 10 kW average power, 100 ns pulsewidth and 40–100 kV peak-voltage. Lifetime and reliability are adequate for industrial demonstrations. Gas flows of 50.000–100.000 Nm3 /hr are possible. We expect to reach the milestone of 100 kW output power at an energy efficiency of 95% within the next year.(20) The unique TLT concept provides adequate matching with a corona reactor, protection of the spark gap switch against open- or shortcircuit conditions, voltage multiplication and easy superposition of the pulse on a DC-bias voltage. 3. TAR REMOVAL BY PULSED CORONA DISCHARGES We reported(23,24) the results from field tests, where a pulsed corona reactor was actually coupled to a wood gasifier. The results showed a conversion of heavy tars into lighter ones (70% conversion at a corona energy density of 160 J/L). This is also a favorable situation since the problem for gas engines is the condensation of the heavy tars. Converting these into lighter compounds (which means reducing the aromatic content and lowering boiling points) allows utilizing the enthalpy associated with the tar materials. In addition, it was reported that more than 90% removal of tar components such as naphthalene, toluene and phenol requires a corona energy density of about 400∼600 J/L in fuel gas of 200◦ C, whereas 200∼400 J/L is required in pure N2 . These previous results demonstrate the feasibility of pulsed corona processing for tar removal. However, for commercial utilization, pulsed corona tar removal has to be achieved at energy levels less than 200 J/L. To achieve this efficiency, a deep insight into the chemical processes involved is required. Pulsed corona, when applied to fuel gas, creates radicals, ions and other excited species, which are different from those in air. For the destruction of tar by corona discharges, the following mechanisms play a role: (i) production of radicals; (ii) utilization of radicals for tar removal; and (iii) radical termination. In order to evaluate the energy requirements for tar removal without knowing all reaction schemes, the following simplified global kinetic model will be used.(25) The model regards a radical production reaction and three types of radical utilization reactions [Eqs. (1)–(4)]: M→R
(1)
Tar removal reaction, k2
X+R→ A
(2)
Linear radical termination reaction, k3
R+M → B
(3)
Non-linear radial termination reaction, k4
R+R→C
(4)
Radical production reaction, k1
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where M – background fuel gas, R – created radicals, X – tars, A and B and C intermediates, k2 and k3 and k4 are second order reaction constants and k1 is the radical yield in terms of the corona power density. Although the actual chemical reactions and the pollutant distribution inside a corona reactor are much more complicated in comparison to this model, the following relationships between created radicals, pollutant concentration and applied corona power can be derived [Eqs. (5) and (6)]: d[R] (5) = k1 · P − k2 · [X ] · [R] − k3 · [M] · [R] − 2 · k4 · [R]2 dt d[X ] = −k 2 · [X ] · [R] (6) dt where [R], [X ] and [M] are the radical, the pollutant and the bulk gas concentrations, respectively. P is the corona power in the unit of (W/m3 ). A detailed description of this model and the solutions of Eqs. (5) and (6) can be found (see Yan(5) and Yan et al.(25) ). By solving both equations using stationary state approximation,(5) three situations can be distinguished, which are shown schematically in Fig. 2. If there are no significant radical termination reactions, the removed amount of the pollutant linearly depends on the corona energy density. In most instances however, linear radical termination reactions play an important role. Consequently, tar removal becomes less efficient and depends exponentially on the applied corona energy density. If the radical concentration is significantly affected by nonlinear radical termination reactions, the removal rate depends on the square root of the energy density. It is obvious that radical destruction processes, either via linear or nonlinear termination reactions, reduce the total energy efficiency. In fuel gas processing a
Fig. 2. Effect of radical termination reactions on the energy requirements for tar removal. E is the applied corona energy density in J/L.
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variety of reactions can be expected. The most important reactions are expected to be [Eqs. (7)–(19)]: Radical production (Eqs. 7–10): e + N2 e + H2 e + CO2 e + H2 O Radical utilization (Eqs. 11–13): OH + tar O + tar H + tar Radical termination (Eqs. 14–19): OH + CO OH + H2 O + H2 O + CO + M H + OH + N2 H + H+M
→ → → →
e e e e
→ → →
Products Products Products
→ → → → → →
CO2 H2 O OH CO2 H2 O H2
+ + + +
N+N H+H CO + O H + OH
(7) (8) (9) (10) (11) (12) (13)
+ + + + + +
H H H M N2 M
(14) (15) (16) (17) (18) (19)
Thus in order to minimize energy requirements, reaction conditions are needed where the kinetics for tar removal are more advantageous as compared to termination reactions. To achieve this, insight into the chemical processes involved is necessary. This can be obtained by determining the energy needed for tar removal at various gas mixtures, in order to establish relative trends in energy requirement with respect to different radicals. 4. EXPERIMENTAL SET-UP The experimental set-up (Fig. 3) consists of a pulsed power supply, and a wire-cylinder type corona reactor. The pulsed power supply has a pulse voltage of 80 kV, a pulse rise-time of about 10 ns, a pulse-width of 150 ns, energy-perpulse of 1 J/pulse, and runs at 50 pulses-per-second. The reactor has a diameter of 25 cm and is 3 m in length. The reactor is part of a closed loop circulation system, driven by a fan with variable speed adjustment. The internal flow rate used for the measurement is 240 Nm3 /hr and the volume of the system is 300 liters. The entire set-up is heated by means of heating tapes along with adequate temperature control, and can work at a maximum gas temperature of 250◦ C. The system is filled with a synthetic fuel gas of composition 20 % CO, 12 % CO2 , 17 % H2 , 1 % CH4 , rest N2 and heated up to about 200◦ C, at a pressure of 1 bar. Apart from this, individual gases can be added via separate gas injection ports. Filling the system with each individual gas and monitoring its partial pressure can
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Fig. 3. Experimental set-up.
make the required gas composition. Tar injection is done by means of a boiler. A weighed amount of the component can be added and vaporized into the flowing gas stream. Tar measurements are done by both FTIR (Bruker Vector22) and a GC (HP5890). The GC is equipped with a FID and a Chrompack CP-Sil 5 CB fused silica WCOT (Wall Coated Open Tubular) column for measurement of heavy hydrocarbons. Additionally a condenser is connected to the corona reactor, via a heated sample line. The condensate is then analyzed by means of GC-MS for hydrocarbons, that can be intermediates formed during the corona processing. Typical tar components monitored are naphthalene (characterized as heavy tar) and phenol or toluene (characterized as light tar). The applied high-voltage pulses, the corona current and the pulse-repetition-rate are monitored on a HP-Infinium digital oscilloscope. From these electrical measurements, the peak-power and the energy-per-pulse of the discharges can be determined. An experiment starts with filling the system with the desired gas composition. After allowing sufficient time (30 min) to attain steady state (to ensure uniform gas composition and temperature distribution), tar is vaporized into the gas stream and again the system is allowed to attain uniform tar concentration (10 min). This can be monitored from the FTIR spectrum. This spectrum is taken as the background for further analysis. Pulsed corona discharges are then run for a particular time interval (e.g., for 60 s. at 50 pulses per seconds, with an energy of 1 J/pulse). After every run, both GC and FTIR analysis are done for the tar component removed.
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Table I. Experimental Conditions Gas temperature Pressure Fuel gas composition Tar components Tar concentrations
200 ∼ 210◦ C 1 bar CO (20 %), CO2 (12 %), H2 (17 %), CH4 (1 %), and balance N2 naphthalene, toluene, phenol, both naphthalene and phenol 500 ∼ 700 ppm, or 3 ∼ 4 gr/Nm3
The tar removal experiments are carried out for different corona energy densities (J/L). An overview of the experimental conditions is given in Table I. 5. RESULTS AND DISCUSSION 5.1. Identifying the Primary Mechanisms Results for the removal of naphthalene, toluene and phenol in fuel gas are shown in Fig. 4. For the open markers, only one individual tar component was present. Experiments were also carried out to determine energy density requirements for mixtures of tar compounds (closed markers). The results for mixtures show that the tar components behave independently from each other and require about the same energy density as when they are present individually. In addition, it can be concluded that aromatic ring compounds containing side chains (such as toluene and phenol) require more energy for their removal than the simple aromatic ring compound naphthalene. This may be due to a combination of factors,(26) such
Fig. 4. Corona energy density for the removal of various tar components in fuel gas. Open markers: one tar component was present. Closed markers: mixture of two tar components.
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as kinetic limitations due to differences in rate constants, or a structural dependency. The first radical attack on an aromatic compound would be, if present, on the side chain. In this case, recombination of groups can take place, leading to an equilibrium and thereby requiring more energy. The removal rates vary exponentially with the applied corona energy density. This indicates that the tar removal process is hampered by radical termination reactions (Fig. 9). In order to gain more insight into the chemical processes involved, experiments were done on a range of different gas compositions, where naphthalene was used as a model tar component.(27) Figure 5 gives the results for naphthalene removal in mixtures of N2 with respectively H2 O, CO2 and H2 . Less energy is required for naphthalene removal in N2 + H2 O. However, adding moisture to actual fuel gas show no effect on the energy requirement for naphthalene removal (Fig. 6). This may be due to termination reactions for OH radicals [as in Eq. (18)]. In the presence of CO2 , removal will mainly be governed by an oxidation process, initiated by O radicals [Eqs. (9) and (12)]. Figure 5 shows that the oxidation process is a bit more efficient than the cracking via H radicals, which will occur in the presence of hydrogen [Eqs. (8) and (13)]. This can also be seen from Fig. 7, which shows naphthalene removal in mixtures of N2 + CO2 , N2 + H2 and N2 + CO2 + H2 . As can be observed, naphthalene removal is more efficient in the presence of CO2 whereas the presence of H2 seems to play a lesser role. A more efficient removal of naphthalene is expected when both CO2 and H2 would be dissociated. Apparantly this is not the case. This can have two reasons: (i) all the H radicals can be terminated by background gas molecules
Fig. 5. Comparison of the corona energy density for naphthalene removal in a mixture of nitrogen with respectively H2 , CO2 or H2 O.
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Fig. 6. Corona energy density for the removal of naphthalene in fuel gas, and the effect of adding water or air.
or auto terminated [e.g., Eq. (19)]; or (ii) the radical density of H may be too small and most of the input energy is used for the dissociation of CO2 . The possibility that no dissociation of H2 occurs in the presence of CO2 , cannot be verified on basis of the experiment. However, experiments in mixtures of H2 + CO2 in dielectric barrier discharges, indicate that dissociation of H2 takes place, which is shown by the formation of higher hydrocarbons.(28) Termination of H radicals within the streamer channel can be an extremely fast process in comparison to tar removal by H radicals. Alternatively, it is also possible that the produced O radicals can react with H2 to produce equivalent reactive radicals [Eq. (16)]. But from Fig. 7 it seems that apparently there is no significant difference in energy density requirements between these two mechanisms. So in the presence of CO2 , the oxidation process dominates. But at the same time termination reactions as in Eq. (17) play an important role. To further analyze this, experiments were done for gas compositions with or without CO. Figure 8 gives results for the following gas compositions: N2 + CO2 , N2 + CO2 + CO, N2 + H2 + CO2 + CO and fuel gas. Again it is found that tar removal in fuel gas is primarily an oxidation process and no or little effect of H2 can be seen. The expected primary process to explain the above results can be similar as mentioned in Eqs. (7) and (9), and naphthalene removal can be expected to occur via Eq. (12). Comparing the results for mixtures with and without CO indicate the termination of O radicals because of reactions with background CO [Eq. (17)]. When CO is present, more CO2 molecules need to be dissociated for naphthalene removal, which explains the higher energy density requirement in the presence of CO. Thus,
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Fig. 7. Comparison of the corona energy density for naphthalene removal in nitrogen; effect of adding H2 or CO2 or both.
the O radical cycle via Eqs. (7) and (9) hampers further improvement in efficiency for tar removal at the conditions of the experiment. Real fuel gas also consists of relatively small amounts of methane (for the experiments in this paper, the synthetic fuel gas consists of 1% methane). This small amount of methane can either act as a sink for all radicals, or can also lead to formation of active species by direct electron impact dissociation. Results shown in Fig. 8 give a comparison of energy density requirements for naphthalene removal in fuel gas with and without methane (curves for N2 + H2 + CO2 + CO and for fuel gas). As can be observed the corona energy density for removal of naphthalene is not affected by the presence of CH4 , within the experimental conditions. 5.2. Radical Termination Reactions From the model as discussed in Section 3, it was found that tar removal depends exponentially on the applied corona energy density in the case of linear radical termination reactions. As already verified, naphthalene removal in fuel gas is hampered by such a termination reaction: in the presence of CO, the O radicals are terminated to CO2 . The experiments confirm that the remaining fraction of naphthalene varies exponentially with the applied corona energy (Fig. 9). For gas mixtures where no CO was present, non-linear termination reactions are expected. In this case, according to the model, the remaining fraction depends
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Fig. 8. Comparison of the corona energy density for naphthalene removal in fuel gas and in various gas mixtures.
on the square root of the applied corona energy. This can be seen in Fig. 10, which gives the results for naphthalene removal in N2 + H2 . Now the following nonlinear termination reaction is expected: H + H + M → H2 + M. Figure 10 also gives the data for N2 + CO2 , where CO + O + M → CO2 + M will occur.
Fig. 9. Linear termination in the case of naphthalene removal in fuel gas. The y-axis shows the ln of the remaining fraction.
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Fig. 10. Non-linear termination reactions in the case of naphthalene removal in N2 + H2 and N2 + CO2 . The x-axis shows the square root of the corona energy density E.
5.3. Effect of Adding Air These experiments were aimed at understanding the basic chemical processes in fuel gas cleaning by corona plasma. The next question is how to reduce the energy density requirement of the process. One possibility could be to induce side oxidation reactions by using additives such as air (see Fig. 11). Experiments with addition of air (10 %, i.e., 2 % O2 ), in a gas mixture of N2 + CO2 + CO resulted in a slight decrease of the energy density requirement. It might be that adding O2 leads to the oxidation of CO to CO2 and thereby decreasing the termination of O radicals. To verify this, experiments were carried out for varying CO concentrations (Fig. 11). Results show that the energy requirement when lowering the amount of CO is still higher than in the case of addition of air/O2 . This indicates, that naphthalene removal is enhanced via Eq. (12) and less effect of termination by CO is seen. However, addition of air in actual fuel gas showed no improvement in terms of the energy consumption of the process, as can be seen in Fig. 6. As already verified the oxidation-reduction cycle can be influenced by air/O2 addition, but in real fuel gas, a combustion reaction from H2 can terminate the additional O radicals that may be produced. Actual gas phase analysis for corona processing of fuel gas was still not done, and would be part of future work. 5.4. Analysis of By-Products Possible by-products formed during the process can be determined by means of the condenser attached to the reactor. Although a quantitative analysis is still not
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Fig. 11. Comparison of the corona energy density for naphthalene removal in various gas mixtures; effect of varying CO and adding air.
done, a qualitative analysis was possible by analyzing the condensate by GC-MS. Based on these results, further work would focus on establishing a more detailed mass balance. Figure 12 shows a GC-MS spectrum of the by-products obtained from a condensate sample (dissolved in acetone) obtained during naphthalene removal in fuel gas at an energy density of 245 J/L. Experiments indicate that the primary process for tar removal from fuel gas is oxidation dominated which is reflected in the by-product GC-MS spectra as well. A typical naphthalene oxidation scheme can be found in standard literature.(29) The process of oxidation leads to formation of intermediates like phthalic anhydride and naphthalenedione. These were also detected during the corona processing (Fig. 12), which indicates that the chemistry
Fig. 12. GC-MS spectrum of a condensate sample (dissolved in acetone) obtained during naphthalene removal in fuel gas at a corona energy density of 245 J/L.
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is following the same path as conventional oxidation. The presence of naphthols and nitronaphthalenes can be expected in a corona plasma due to reactions involving background molecules such as N2 , CO2 and H2 . 6. CONCLUSIONS This paper shows that tar removal by pulsed corona processing is possible. The primary processes of pulsed corona tar removal from biomass derived fuel gas have been derived for gas temperatures of 200◦ C. It was found that tar removal mainly occurs via dissociation of CO2 . The energy density requirement is influenced by termination of O radicals by background CO. Detected by-products, formed during corona processing, fit within typical naphthalene oxidation schemes. The presence of H2 , methane and moisture has no effect on the process efficiency, under the experimental conditions. Adding air to the fuel gas, in order to promote side oxidation, gives no improvements in terms of the energy consumption of the process. Further research will focus on further reducing the energy consumption of the tar removal process. Reaction conditions are required where the kinetics for tar removal are more favorable as compared to termination reactions. Future experiments will be carried out at much higher temperatures, up to 850◦ C. In addition, the effect of catalysts inside the discharge reactor will be investigated. ACKNOWLEDGMENTS The authors acknowledge the support of SDE (Dutch foundation for sustainable energy) and the Dutch Energy Research Center ECN. We appreciate the skilful technical support of H.E.M. van Leuken, F.M. van Gompel and P. Lipman. REFERENCES 1. A. V. Bridgwater, Fuel, 74, 631–653 (1995). 2. S. V. B. van Paasen, Fuel gas treatment with pulsed electric fields, Eindhoven University of Technology-SAI report, Eindhoven (April 1999). 3. J. P. A. Neeft, H. A. M. Knoef, and P. Onaji, Behavior of Tar in biomass gasification systems. Tar related problems and their solutions, Report No. 9919. Energy from Waste and Biomass (EWAB), The Netherlands (November 1999). 4. T. A. Milne and R. J. Evans, Biomass Gasification tars: their nature, formation and conversion, NREL, Golden, Colorado. Report No. NREL/TP-570-25357 (1998). 5. K. Yan, Corona plasma Generation, Ph.D. Thesis, Eindhoven University of Technology (2002). 6. E. M. van Veldhuizen, Electrical Discharges for Environmental Purposes: Fundamentals and Applications, Nova Science Publishers, New York (2000). 7. B. M. Penetrante and S. E. Schultheis (editors): Non-thermal Plasma Techniques for Pollution Control, Springer-Verlag, Berlin, NATO ASI Series, Vol.34, Part A&B (1993). 8. B. Eliasson and U. Kogelschatz, IEEE Trans. Plasma Sci. 19, 1063–1077 (1991).
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