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Comparison of direct and indirect plasma oxidation of NO combined with oxidation by catalyst
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Comparison of direct and indirect plasma oxidation of NO combined with oxidation by catalyst
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Q1
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Indrek Jõgi a,⇑, Eugen Stamate b, Cornelia Irimiea b, Michael Schmidt c, Ronny Brandenburg c, Marcin Hołub d, Michał Bonisławski d, Tomasz Jakubowski d, Marja-Leena Kääriäinen e,1, David C. Cameron e,2 a
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University of Tartu, Tartu, Estonia Technical University of Denmark, Risø, Denmark Leibniz Institute for Plasma Science and Technology, Greifswald, Germany d West-Pomeranian Technical University, Szczecin, Poland e ASTRaL, Lappeenranta University of Technology, Mikkeli, Finland b c
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h i g h l i g h t s
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Direct and indirect plasma oxidation of NO was tested in a medium-scale test bench.
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Both methods gave similar results at NO concentrations of 200 ppm.
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The increasing DBD reactor temperature decreased the efficiency of direct oxidation.
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The increasing gas temperature improves the efficiency of indirect oxidation.
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The efficiency of indirect NO oxidation to N2O5 is further improved by catalyst.
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a r t i c l e 2 4 7 0 28 29 30 31 32
i n f o
Article history: Received 4 August 2014 Received in revised form 5 December 2014 Accepted 10 December 2014 Available online xxxx
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Keywords: Plasma Ozone Oxidation NOx Catalyst
a b s t r a c t Direct and indirect plasma oxidation of NOx was tested in a medium-scale test-bench at gas flows of 50 slm (3 m3/h). For direct plasma oxidation the synthetic flue gas was directed through a stacked DBD reactor. For indirect plasma oxidation, a DBD reactor was used to generate ozone from pure O2 and the plasma treated gas including ozone was mixed with flue gas at the entrance of a 6 m long serpentine-like reaction chamber which allowed reaction times longer than 10 s. At relatively low NOx concentrations of 200 ppm, both oxidation methods gave similar results. However, the temperature increase of the DBD reactor decreased the long-term efficiency of direct plasma oxidation. At the same time, the efficiency of indirect oxidation increased at elevated reactor temperatures. Additional experiments were carried out to investigate the improvement of indirect oxidation by the introduction of catalyst to the reaction zone. Small-scale experiments with TiO2 powder demonstrated considerable efficiency gain for NOx oxidation while in medium-scale experiments, the efficiency improvement remained negligible. Ó 2014 Published by Elsevier Ltd.
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1. Introduction
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Combustion of fuels emits large amounts of harmful NOx species [1] and the prevention of this environmental threat requires treatment of flue gas before it enters the atmosphere. The emission of NOx on land is regulated by several legislations, regarding sta⇑ Corresponding author at: University of Tartu, Institute of Physics, Ravila 14c, 50411 Tartu, Estonia. Tel.: +372 7375565. E-mail address: [email protected] (I. Jõgi). 1 Present address: University of Colorado, Boulder, Colorado, United States. 2 Present address: Dublin City University, Dublin, Ireland.
tionary combustion processes (e.g. TA Luft in Germany) or motor vehicles (e.g. EURO- or TIER-Norm). In addition to these emitters, aviation and shipping came recently into the focus as pollution sources with high impact on the environment. The transportation sector contributes to about 58% of total NOx emissions, 30% of total CO emissions and around 27% of total PM2.5 (particulate matter with an aerodynamic diameter of 2.5 micrometers or less) emis- Q3 sions at the European Union level [2]. Several emission controlled areas are established (Baltic Sea Region, North Sea, and North America) or planned (e.g. Mediterranian Sea) [3,4]. In these regions the air polluting emission, e.g. NOx, SOx, and PM, originating from vessels are subject to restrictions. The NOx emissions, for example,
http://dx.doi.org/10.1016/j.fuel.2014.12.025 0016-2361/Ó 2014 Published by Elsevier Ltd.
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must be reduced by 80% from the beginning of the year 2016 with respect to the emissions before the year 2000. In addition to these already adopted legislative means, some ports started to differentiate their dues according to the environmental impact of the visiting vessels [5]. Thus, there is urgent need for emission reduction methods in the domain of marine transportation [6]. There are several methods available for flue gas treatment e.g. selective catalytic reduction, storage by adsorption or wet absorption [7]. However, most of these methods suffer from poor absorption and adsorption of NO which usually constitutes about 90–99% of the NOx in the flue gas. In ships, the available space has to be considered as well, a fact that limits the possible size of the NOx removal devices and allowable backpressure in the exhaust system. One possibility for improving the efficiency of the available NOx removal methods is to efficiently oxidize the NO to NO2 or further to N2O5 which can be removed more easily from the gas stream. The oxidation of NO can be done at low and elevated temperatures by active oxygen species e.g. O, OH and OH2 radicals or O3 which are produced by non-thermal low-temperature plasmas [7–9]. This issue, especially regarding marine diesel exhausts, was recently investigated by several researchers [10–15]. In most of these studies, plasma technology was combined with additional reactants and/or catalysts to improve the NO oxidation and the NO2 removal performance. The catalyst can be positioned downstream the plasma reactor (post-plasma catalysis, PPC) or directly in the active plasma zone (in-plasma catalysis, IPC) [16–19]. Moreover, the NOx containing flue gas can be either directed through the plasma (direct plasma oxidation) or mixed with ozone produced by plasma (indirect oxidation). The direct plasma oxidation can be used for removal of both NOx and VOC-s [19] and requires no additional gas feeds. As a downside, in the case of pure NOx in mixtures with O2 and N2, the NO oxidation by the direct method is limited to certain NO concentrations [20–23] because O atoms are also consumed by the back-reaction of NO2 to NO [20]. The presence of unsaturated VOC-s in the gas feed may improve the oxidation but makes the removal dependent on the additives in the gas mixture [25]. The back-reaction to NO is absent for indirect oxidation by ozone and the oxidized amount of NO and oxidation efficiency to NO2 can be considerably higher without the need of additives [26]. In addition, contamination of the reactor due to the presence of water vapour and hydrocarbons [27] can be avoided when ozone is produced from pure oxygen or dry air. Furthermore, the indirect method allows oxidation of NOx to N2O5 which is desirable in some applications due to considerably better absorption of N2O5 into water [28–31]. However, the industrial scale experiments with indirect oxidation of NOx to N2O5 [28,32,33] used long residence times or surplus amounts of ozone for removal of NOx. These studies demonstrated that the efficiency of ozone oxidation remains still insufficient for practical considerations and additional improvements are necessary for industrial usage. The aim of the present study was to compare the direct and indirect plasma treatment of NOx species at a compact medium-scale test-bench [29]. There are some studies where the comparison has been carried out in laboratory scale but the semi-pilot scale studies are still missing. Furthermore, residence times more acceptable for industrial usage were used in present study. An additional goal was to investigate the possibility of improving the oxidation efficiency of indirect plasma treatment by placing a catalyst into the reaction zone. Preliminary experiments with catalysts were carried out at small scale with flow rates of 1 sl/min to demonstrate the usability of the method. Medium-scale experiments with flow rates of 50–60 sl/min were carried out to investigate the scalability of the process. As one of the steps in the up-scaling of the process, a new concept for catalyst preparation for large scale devices had to be developed and tested. The catalyst used in medium-scale
experiments had to possess large surface area and small pressure drop. For this purpose, a widely used porous glass–fiber mat was used as the template and a thin layer of catalyst was coated on the template by the atomic layer deposition method [34].
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2. Experimental setup
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2.1. Small-scale experiments
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The experimental setup for the small-scale experiments was described in several earlier publications [35,36]. The coaxial dielectric barrier discharge (DBD) reactor consisted of stainless steel tube as the inner electrode, quartz tube as the dielectric and a metal mesh wrapped around the quartz tube as the outer electrode. The inner diameter of dielectric tube was 1.4 cm, the discharge gap was 0.12 cm and the length of the reactor was 9 cm. A sinusoidal voltage of 14 kV peak-to peak and 50–1000 Hz frequency was applied to the inner electrode. The corresponding input power was measured by the Manley method [37] and was in the range of 0.1–2 W. The indirect plasma treatment was carried out by using the DBD reactor as the ozone generator and a similar reactor without outer electrode as the reaction zone. The NO in N2 carrier gas was mixed with the outlet of the ozone generator (which used pure O2 as feedstock) and directed into the reaction zone. The reaction time before measuring the concentrations of NOx and O3 was approximately 1.5 s as estimated by the flow rate and the dimensions of tubing. The NOx, N2O5 and ozone concentrations were measured by optical absorption spectroscopy [33]. The catalyst powder (TiO2, Degussa P25) was placed into the second reactor as a thin 0.2 mm thick layer and the gas flowed over this catalyst layer. The residence time was approximately 0.3 s. The reaction zone was heated to 70 °C because earlier experiments have shown that the TiO2 catalyst needs some thermal activation Q4 [23,38] (see Fig. 1).
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2.2. Medium-scale experiments
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All medium-scale experiments were carried out with a flexible and mobile system [29] with compact reaction chamber in serpentine configuration (Fig. 2). The reaction chamber had inner diameter of 9.7 cm (74 cm2) and length of more than 6 m. Five sampling ports at different positions along the length were used for the measurements of temperature, NOx and O3 concentration and for FTIR measurements. Most of the measurements were carried out at the first port which was placed at 60 cm downstream from the inlet of the reactor. Synthetic exhaust gas based on compressed air and a variable amount of NOx (NO and NO2) was mixed with the ozone in the mixing zone [29]. The total synthetic exhaust gas flow was usually 50 sl/min which resulted in the linear gas velocity of about 11.3 cm/s and the reaction time before the first sampling port was correspondingly 5.3 s. The reactor and flue gas could be heated
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Fig. 1. The small-scale experimental apparatus for indirect treatment of NOx by ozone and catalyst.
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Fig. 3. Scheme of the stack reactor.
Fig. 2. Drawing of the experimental setup [24] (a) and picture of serpentine like reaction-chamber (b).
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up to 130 °C. The highest temperature used during these experiments was 70 °C. A stacked DBD reactor was composed of mesh electrodes made of stainless steel separated by dielectric plates made of magnesium mica (phlogopite) as shown in Fig. 3 [39]. The electrodes were contacted alternately to high voltage and ground. The reactor was driven by sinusoidal voltage provided by a high-efficiency, resonant power supply [40]. The schematic power supply construction is depicted in Fig. 4. A two-stage system was proposed allowing output voltage amplitude control in a wide range. The first, full bridge stage defines the DC link voltage value, in addition the Ts1–Ts2 transistor timing allows for low loss, zero current switching output voltage control [41]. The DBD reactor was placed at the beginning of the serpentinelike reaction chamber. Pure NOx (1.2 scc/min) was diluted with
dry air (50 sl/min) and directed through the reactor. The voltage peak-to-peak value was varied in the range of 5–5.8 kV where 5 kV was the ignition voltage and 5.8 kV the maximum output voltage provided by the power supply and the frequency was approximately 1.25 kHz. The discharge power was determined by multiplication of measured voltage and current signals by means of an optical storage oscilloscope. The temperature increase in the stacked reactor due to applied power was measured by an OSENSA Innovations fiber-optic temperature sensor. The sensor was placed into the contact of the dielectric surface in the outlet side of the reactor. The experiments with indirect plasma treatment (ozone injection) of NOx were carried out with 50 sl/min of air with two concentrations of NOx (200 ppm and 400 ppm). The ozone was produced from 20 sl/min of air by a commercial ozone generator (Wedeco) in the range of 0–3000 ppm. The plug power of the ozone generator used was up to 300 W giving ozone concentration of 2300 ppm. The corresponding ozone production efficiency was approximately 19 gO3/kW h. Several tests were made where ozone was produced from pure O2 (20 sl/min) but the results were similar to the ones obtained with air except the ozone production efficiency was twice as large (38 gO3/kW h). Thus, the air was used for the production of ozone in the following experiments. The output from the ozone generator was mixed with 30 sl/min of air and NOx (either 2.1 scc/min or 1.2 scc/min). The NOx concentration in the total airstream of 50 sl/min was either 400 ppm (300 ppm NO) or 210 ppm (150 ppm NO). The catalysts were fixed on the support of ISOVER VKL glass–fibre mats with thickness of 1.4 cm and diameter of 98 mm (see Fig. 5(a) and (b)). The density of the mats was approximately 120 kg/m3. Two of such mats were fitted into a section of ISO100K nipple specially designed for holding the catalyst coated mats (Fig. 5(c)). These catalyst supports were coated with 30 nm thick ALD TiO2 or ZnO layers with N doping [34,42,43]. The nitrogen doped TiO2 was deposited at 270 °C and was partially amorphous while the ZnO layer was deposited and at 185 °C and was crystalline. The ozone concentrations in the outlet of the ozonizer and in the reaction chamber were determined by BMT ozone sensors which are based on UV absorption at 254 nm. The NO and NOx concentrations were measured by chemo-luminescence sensors CLD 62 (Eco Physics). An FTIR spectrometer (Bomem) was additionally used to determine the presence of other nitrogen-oxide species (N2O, N2O5 and HNO3). The chemo-luminescence (CLD) NOx sensor also had cross-sensitivity to N2O5 which made the measurement of NOx at higher O3 concentrations unreliable. The N2O5 cross-sensitivity was verified with the use of a bubbler which was placed before the inlet of the chemo-luminescence sensor. Without the bubbler, the NOx concentration retained a high value even at the highest O3 concentrations while the NOx concentration decreased to zero when the analysed gas was directed through the bubbler. Most of the experiments were carried out with the bubbler placed before the CLD
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Fig. 4. Schematic construction of the resonant power supply.
Fig. 5. Pure glass–fibre supports of about 10 cm diameter and 1.4 cm thickness were coated by ALD method with thin films of TiO2 (a) or ZnO (b). These catalysts were placed inside of an ISO100-K nipple (c).
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sensor to obtain precisely the O3 concentration where all NOx was converted to N2O5. Both NOx and NO concentrations obtained by chemo-luminescence sensor with bubbler were smaller compared to the case when bubbler was not used (at low O3 concentrations the N2O5 did not form). Re-calibration was carried out by comparing the results obtained with and without the bubbler. The corrected dependences of outlet NO2 concentrations on inlet O3 concentration were very similar to the dependences obtained by FTIR which confirmed the correctness of the procedure. The concentrations of outlet species as a function of inlet ozone concentrations were also calculated by a simple numerical model which included following main reactions between ozone, NO, NO2 and N2O5:
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NO þ O3 ! NO2 þ O2 ;
1:4 1012 expð1320=TÞcm3 =s
NO2 þ O3 ! NO3 þ O2 ;
1:4 1013 expð2470=TÞcm3 =s
NO þ NO3 ! 2NO2 ;
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1:8 10
14
NO2 þ NO3 þ M ! N2 O5 þ M; 4:5 10
11
NO2 þ NO3 ! NO þ NO2 þ O2 ; 2:8 10 271 272 273 274
N2 O5 þ M ! NO2 þ NO3 ;
3
expð110=TÞcm =s 3
expð1250=TÞcm =s 3:5
ðT=298Þ
2:7 1016 ðT=298Þ
3:5
cm3 =s
expð11000=TÞcm3 =s
The rate constants for the reactions were obtained from NIST Chemical Kinetics Database [44]. The calculations were carried out for the conditions used in real experiments.
3. Results and discussion
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3.1. Plasma oxidation of NOx in small-scale experiments
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The effect of TiO2 catalyst on the NO oxidation efficiency by direct plasma oxidation has been investigated in several small-scale experiments [16,23,45]. The highest amount of NO which could be oxidized by direct plasma treatment at room temperature was about 250 ppm. This maximum value further decreased at increasing reactor temperatures. The presence of TiO2 catalyst in the plasma reactor improved the oxidation of NO by about 40–50 ppm. As one conclusion, the direct plasma oxidation of NO at concentrations above 300 ppm is not feasible without the presence of hydrocarbons and the effect of TiO2 catalyst was further investigated for indirect plasma oxidation of NO. The stable oxidation of NOx by ozone in a small-scale reactor with total gas flow of 1 sl/min and at temperature of 70 °C is shown in Fig. 6(a) without catalyst and (b) with TiO2. The initial NO concentration was in the range of 220–230 ppm and the inlet ozone concentration produced from pure O2 in DBD reactor was increased up to 1200 ppm. After the injection of ozone the NO was initially oxidized to NO2. At increasing inlet ozone concentrations all NO was removed from the exhaust gas and the NO2 produced was further oxidized to N2O5. The removal of NO and production of NO2 was characterized by a linear relation with the inlet ozone
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Fig. 6. The indirect plasma oxidation of NO to NO2 and N2O5 by ozone injection at 70 °C without the catalyst (a) and in the presence of TiO2 catalyst (b) in the reaction zone. Solid curves show the concentrations calculated by the simple numerical model. The dashed curve for the results obtained for NO2 with TiO2 gives the exponential fit of the experimental results.
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concentration and was not affected by presence of a catalyst. This is explainable by the fast reaction between ozone and NO which should proceed to the end in less than 0.1 s. According to Fig. 6, the oxidation of NO2 to N2O5 was a sub-linear function of ozone concentration and was enhanced by the presence of catalyst. The NOx and N2O5 concentrations obtained by numerical calculations (solid lines) by the simple model described above had reasonable agreement with the experiments (marks) in the case when catalyst was not used. The presence of catalyst improved considerably the conversion of NO2 to N2O5. As a note, in the presence of TiO2 catalyst the outlet NOx concentration remained smaller than the inlet NOx concentration during the first 5–10 min of NOx oxidation by ozone. This effect can be explained by the adsorption of NOx at the beginning of the oxidation [36,46]. The results presented in Fig. 6 were stable values registered 20 min after the start of oxidation. The estimated amount of ozone necessary to oxidize 95% of NOx to N2O5 was 880 ppm and 635 ppm without the catalyst and with the TiO2 catalyst respectively. These results demonstrate that at short residence times the catalyst can considerably increase the oxidation efficiency of NOx (30%). Even in the presence of catalyst, the O3/NOx ratio necessary to oxidize 95% of NOx at 70 °C was 2.8 while the stoichiometric amount is 1.5 when pure NO is fed into the reactor. One conclusion of these results carried out in small-scale was the need of longer reaction times for efficient oxidation of NOx which could be obtained with the use of longer reactors.
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3.2. Direct plasma oxidation of NOx in the medium-scale test-bench
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The investigation of scalability of the oxidation process was carried out in the 6 m long serpentine-like reactor at flow rates
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of 50–60 sl/min. In the applied voltage range of 5–5.8 kV, a linear dependence of discharge power P from the applied voltage Upp was observed and the power dissipated into the plasma was 50–150 W (Fig. 7(a)). The temperature measured at the reactor outlet reached up to 100 °C during the operation of the reactor. Ozone production as a function of input power for the plasma device used in direct plasma oxidation was determined for compressed air inlet gas (Fig. 7(b)). The concentration of ozone was a linear function of input power up to the highest used power values of 125 W (SIE = 150 J/L) where the ozone concentration was about 500 ppm. This concentration corresponds to the efficiency of about 26 gO3/kW h. The obtained value was considerably larger than the efficiency of a commercial ozonizer but taking into account the power efficiency of the supply (70%) the efficiencies become quite similar. As a remark, the ozone production at low O production values of less than 500 ppm (low SIE values below 100 J/L) can be used as an estimation of O radical production [47,48]. The NOx removal/oxidation in the medium scale experiments is shown as a function of ozone production at the same power to compare the results of direct and indirect oxidation in Fig. 8. The results are shown both for increasing power (cool device) and for decreasing power (heated device). There was a clear difference in NOx removal when the reactor was allowed to heat up. In the cold device, the NO concentration decreased almost linearly with increased power (O production) until all NO was oxidized to NO2 by the reactions NO + O + M ? NO2 + M (M is the third body) and NO + O3 ? NO2 + O2 [44]. This occurred at about 200 ppm of equivalent ozone production which corresponds to SIE 60 J/L. Further increase of equivalent ozone concentration decreased the outlet NOx concentration due to the oxidation of NO2 to N2O5. After the reactor was heated up by the
Fig. 7. (a) Voltage–power-characteristic of the stacked reactor and (b) ozone output as a function of the power.
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Fig. 8. Outlet concentrations of NOx and NO as a function of equivalent ozone production in the absence of NOx in the gas.
Fig. 9. Comparison of direct (plasma) and indirect (ozone injection) removal of NOx as a function of ozone concentration. In the case of direct plasma treatment, equivalent ozone production was used.
high input power, the outlet concentration of NO did not decrease below the value of 45 ppm and total NOx concentration in the outlet slightly increased. With the increase of gas temperature, the importance of back-reaction NO2 + O ? NO + O2 increases [44]. The increased total NOx concentration at higher reactor temperatures can be attributed to the production of additional NOx inside the plasma by reactions involving various active nitrogen species [49–51]. The results of FTIR confirmed that the NO was indeed oxidized to NO2 and after the NO was totally oxidized in the cool device, N2O5 appeared in the spectra suggesting further oxidation of NO2 to N2O5.
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3.3. Indirect plasma oxidation of NOx in the medium-scale test-bench
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Similarly to the direct treatment, the increasing inlet ozone concentration resulted in the decrease of NO concentration while the NOx concentration remained constant (Fig. 9). At about 200 ppm of ozone, all NO was oxidized to NO2. At higher concentrations of inlet ozone, NOx somewhat decreased and then slightly increased. The general trends for direct and indirect removal of NOx in the cool device were quite similar at these inlet NOx concentrations suggesting that both methods are suitable for the oxidation of low concentrations of NO found for example in gas fired powerplants [28]. Subsequent indirect plasma oxidation experiments were carried out at two concentrations, 200 and 400 ppm and at two temperatures 22 and 70 °C. The use of a bubbler before the chemo-luminescence sensor and FTIR measurements were used to distinguish between NOx, N2O5 and HNO3. For inlet NOx concentrations of 200 ppm and 400 ppm, the NO was initially oxidized to NO2 and then NO2 further oxidized to N2O5 (Fig. 10). The increase of the reactor temperature did not affect the oxidation of NO to NO2 and somewhat improved the oxidation of NO2 to N2O5. This result is in contrast with the direct plasma oxidation where the
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Fig. 10. The oxidation of 200 ppm (a and b) and 400 ppm (c and d) of NOx in 50 sl/min flow at 25 (a and c) and 70 °C (b and d) by ozone injection. The concentrations of NO and NO2 determined by chemo-luminescence sensors when bubbler was used to remove N2O5 are shown by filled symbols. The concentrations of NO2 and N2O5 were also determined by FTIR and are shown as empty symbols. The absorbance is scaled in the way that NO2 concentrations determined by different methods coincide. Outlet O3 concentration is shown by black circles for 400 ppm inlet NOx. Dashed curves show the concentrations calculated by the simple model.
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oxidation efficiency decreased at higher temperatures and shows the advantage of indirect plasma oxidation at the gas composition used. At a reactor temperature of 70 °C, conversion of 95% of NOx to N2O5 was achieved when inlet ozone concentration was 360 ppm and 575 ppm at inlet NOx concentrations of 200 and 400 ppm respectively (Fig. 10). This gives the O3/NOx ratio of 1.8 for 200 ppm and 1.45 for 400 ppm of inlet NOx. From these results, it was clear that the conversion was almost finished by the time that the treated gas reached the first measurement port (6 s). The appearance of ozone in the outlet only when all NO2 was converted to N2O5 also indicated the completeness of reaction 2NO2 + O3 ? N2O5 + O2. At 25 °C there was still some ozone and NOx detectable simultaneously while at 70 °C the ozone appeared in the outlet only when practically all NOx was oxidized to N2O5. The numerical calculations using a simple model with six reactions was carried out for a reaction time of 8 s to take into account approximately 2 s transit time from the reactor to the NOx analyser or FTIR apparatus. The calculated and measured concentrations have reasonable agreement and also confirm that the residence time was sufficiently long to obtain almost stoichiometric conversion of NOx to N2O5.
3.4. Effect of catalyst during indirect plasma oxidation in the medium-scale test-bench Small-scale experiments of NOx removal by indirect plasmatreatment demonstrated the improvement of the oxidation of NO2 to N2O5 in the presence of TiO2 catalyst. The influence of the catalyst on the NOx oxidation was also tested in the medium-scale test-bench to investigate the scalability of the processes. At larger flow rates, it is not possible to use powders in similar way as in small-scale experiments. As a consequence, a new type of catalyst had to be developed for medium-scale tests. For this purpose, glass–fibre mats with high surface area were coated with thin layers of TiO2 or ZnO catalyst. The catalysts obtained in such way were inserted into the reactor directly after the mixing zone. The total thickness of catalyst zone was 3 cm and the estimated residence time in the catalyst zone was 0.26 s. This time was comparable to the residence time corresponding to the small-scale experiments (0.3 s). During the mid-scale experiments, the adsorption of NOx species was evident as the outlet NOx concentrations stabilized in timeframes of about 5–10 min, similar to observations with
Fig. 11. Outlet NO2 concentration determined by chemo-luminescence sensor with bubbler (filled symbols) and by FTIR absorbance (empty symbols) for the absence of catalyst (orange diamonds) and for the catalysts with TiO2 (blue triangles) or ZnO (green rectangles). Dashed lines indicate the stoichiometric concentration of outlet NO2 as a function of O3 when all O3 is used for the oxidation of NOx. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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small-scale experiments. According to the comparison of outlet NO2 concentrations as a function of inlet O3 concentration for different catalysts, the long-term improvement of NOx removal in the presence of catalyst was not achieved (Fig. 11). Apparently, the reaction time before the measurement of the concentration (8 s) was sufficiently long to complete the oxidation of NO2 by O3 and there was no room for improvements with the current setup.
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4. Conclusions
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Direct and indirect plasma oxidation of NOx in synthetic air was tested in a mid-scale serpentine-like reactor. At inlet NOx concentration of 200 ppm both direct and indirect oxidation gave similar results when reactions proceeded close to room temperature. The temperature increase in the DBD reactor reduced the efficiency of direct oxidation while indirect oxidation became more efficient at higher temperatures. With the indirect plasma oxidation, the NOx oxidation to N2O5 proceeds efficiently at residence times below 10 s for flow-rates of 50 sl/min (3 m3/h), especially at elevated temperatures. The possibility of further improve the oxidation efficiency by the use of a catalyst was tested both in small-scale and medium-scale experiments. For medium-scale experiments, a novel preparation route of TiO2 and ZnO based catalyst with high surface area was developed and tested. The use of the catalyst gave promising results in small-scale while in the medium-scale testbench it was not beneficial at the conditions used. Future tests with higher flow rates corresponding to shorter reaction times are necessary for the clarification of the positive effect of catalyst.
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Acknowledgements
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This study was partially financed by European Union (European Regional Development Fund) Interreg IVb Baltic Sea Region program (project PlasTEP) and Estonian Science Foundation (Project No. 9310). We are grateful to Alexander Schwock for the organisation of the experiments.
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References
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[13] Schmidt M, Basner R, Brandenburg R. Hydrocarbon assisted NO oxidation with non-thermal plasma in simulated marine diesel exhaust gases. Plasma Chem Plasma Process 2013;33:323–35. [14] Licki J, Chmielewski AG, Pawelec A, Zimek Z, Witman S. Electron beam treatment of exhaust gas with high NOx concentration. Phys Scr 2014;T161. 014067-4. [15] Kuwahara T, Yoshida K, Kuroki T, Hanamoto K, Sato K, Okubo M. Pilot-scale aftertreatment using nonthermal plasma reduction of adsorbed NOx in marine diesel-engine exhaust gas. Plasma Chem Plasma Process 2014;34: 65–81. [16] Kim HH, Tsunoda K, Katsura S, Mizuno A. A novel plasma reactor for NOx control using photocatalyst and hydrogen peroxide injection. IEEE Trans Ind Appl 1999;34:1306–10. [17] Kim HH, Ogata A. Nonthermal plasma activates catalyst: from current understanding and future prospects. Eur Phys J Appl Phys 2011;55. 13806p11. [18] Pasquiers S. Removal of pollutants by plasma catalytic processes. Eur Phys J Appl Phys 2004;28:319–24. [19] Durme JV, Dewulf J, Leys C, Langenhove HV. Combining non-thermal plasma with heterogeneous catalysis in waste gas treatment: a review. Appl Catal B: Environ 2008;78:324–33. [20] Mizuno A, Shimizu K, Chakrabarti A, Dascalescu L, Furuta S. Nox removal process using pulsed discharge plasma. IEEE Trans Ind Appl 1995;31:957–63. [21] Shimizu K, Oda T. DeNOx process in flue gas combined with nonthermal plasma and catalyst. IEEE Trans Ind Appl 1999;43:1311–7. [22] Stefanovic´ I, Bibinov NK, Deryugin AA, Vinogradov IP, Napartovich AP, Wiesemann K. Kinetics of ozone and nitric oxides in dielectric barrier dischages in O2/NOx and N2/O2/NOx mixtures. Plasma Sources Sci Technol 2001;10:406–16. [23] Jõgi I, Haljaste A, Laan M. Hybrid TiO2 based plasma-catalytic reactors for the removal of hazardous gases. Surf Coat Technol 2014;242:195–9. [24] Penetrante BM, Brusasco RM, Merritt BT, Pitz WJ, Vogtlin GE, Kung MC, et al. Plasma-assisted catalytic reduction of NOx. SAE paper 1998. p. 982508. [25] Penetrante BM, Brusasco RM, Merritt BT, Vogtlin GE. Environmental applications of low-temperature plasmas. Pure Appl Chem 1999;71:1829–35. [26] Mok YS. Direct and indirect applications of dielectric barrier discharge plasma to catalytic reduction of nitrogen oxides from exhaust gas. Plasma Sci Technol 2006;8:207–12. [27] Lopez JL, Vezzu G, Freilich A, Paolini B. Effects of hydrocarbon contamination on ozone generation with dielectric barrier discharges. Eur Phys J D 2013;67. 180-p8. [28] Stamate E, Chen W, Jørgensen L, Jensen TK, Fateev A, Michelsen PK. IR and UV gas absorption measurements during NOx reduction on an industrial natural gas fired power plant. Fuel 2010;89:978–85. [29] Stamate E, Irimiea C, Salewski M. Investigation of NOx reduction by low temperature oxidation using ozone produced by dielectric barrier discharge. Jpn J Appl Phys 2013;52. 05EE03-p5. [30] Skalska K, Miller JS, Ledakowicz S. Intensification of NOx absorption process by means of ozone injection into exhaust gas stream. Chem Eng Process 2012;61:69–74. [31] Skalska K, Miller JS, Ledakowicz S. Kinetic model of NOx ozonation and its experimental verification. Chem Eng Sci 2011;66:3386–91.
[32] Jakubiak MP, Kordylewski W. Pilot-scale studies on NOx removal from flue gas via NO ozonation and absorption into NaOH solution. Chem Process Eng 2012;33:345–58. [33] Skalska K, Miller JS, Wilk M, Ledakowicz S. Nitrogen oxides ozonation as a method for NOx emission abatement. Ozone Sci Eng 2012;34:252–8. [34] Kääriäinen ML, Kääriäinen TO, Cameron DC. Titanium dioxide thin films, their structure and its effect on their photoactivity and photocatalytic properties. Thin Solid Films 2009;517:6666–70. [35] Jõgi I, Bichevin V, Laan M, Haljaste A, Käämbre H. NO conversion by dielectric barrier discharge and TiO2 catalyst: effect of oxygen. Plasma Chem Plasma Process 2009;29:205–15. [36] Jõgi I, Erme K, Haljaste A, Laan M. Oxidation of nitrogen oxide in hybrid plasma-catalytic reactors based on DBD and Fe2O3. Eur Phys J Appl Phys 2013;61. 24305-p5. [37] Manley TC. The electric characteristics of the ozonator discharge. J Electrochem Soc 1943;84:83–96. [38] Sano T, Negishi N, Sakai E, Matsuzawa S. Contributions of photocatalytic/ catalytic activities of TiO2 and c-Al2O3 in nonthermal plasma on oxidation of acetaldehyde and CO. J Mol Catal A Chem 2006;245:235–41. [39] Müller S, Zahn RJ. Air pollution control by non-thermal plasma. Contrib Plasma Phys 2007;47:520–9. [40] Jakubowski T, Hołub M, Kalisiak S. Resonant inverter with resonance frequency tracking for DBD plasma reactor supply. Eur Phys J Appl Phys 2013;61. 24304-p5. [41] Jakubowski T, Kalisiak S, Hołub M, Pałka R, Borkowski T, Mys´ków J. New resonant inverter topology with active energy recovery in PDM mode for DBD plasma reactor supply. In: Proc 14th Eur. Conf. power electronics applications; 2011. [42] Kääriäinen ML, Cameron DC. The importance of the majority carrier polarity and p–n junction in titanium dioxide films to their photoactivity and photocatalytic properties. Surf Sci 2012;606. L22-5. [43] Kääriäinen ML, Cameron DC. Nitrogen doping in atomic layer deposition grown titanium dioxide films by using ammonium hydroxide. Thin Solid Films 2012;526:212–7. [44] NIST:. [45] Daito S, Tochikubo F, Watanabe T. NOx removal process in pulsed corona discharge combined with TiO2 photocatalyst. Jpn J Appl Phys 2001;40:2475–9. [46] Sivachandiran L, Thevenet F, Gravejat P, Rousseau A. Investigation of NO and NO2 adsorption mechanisms on TiO2 at room temperature. Appl Catal B Environ 2013;142–143:196–204. [47] Eliasson B, Hirth M, Kogelschatz U. Ozone synthesis from oxygen in dielectric barrier discharges. J Phys D Appl Phys 1987;20:1421–37. [48] Kitayama J, Kuzumoto M. Analysis of ozone generation from air in silent discharge. J Phys D Appl Phys 1999;32:3032–40. [49] Zhao GB, Janardhan Garikipati SVB, Hu X, Argyle MD, Radosz M. Effect of oxygen on nonthermal plasma reactions of nitrogen oxides in nitrogen. Environ Energy Eng 2004;51:1800–12. [50] Leipold F, Fateev A, Kusano Y, Stenum B, Bindslev H. Reduction of NO in the exhaust gas by reaction with N radicals. Fuel 2006;85:1383–8. [51] Ionikh Y, Meshchanov AV, Röpcke J, Rousseau A. A diode laser study and modelling of NO and NO2 formation in a pulsed DC air discharge. Chem Phys 2006;322:411–22.
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Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel 5 6
Comparison of direct and indirect plasma oxidation of NO combined with oxidation by catalyst
3 4 7
Q1
8 9
Indrek Jõgi a,⇑, Eugen Stamate b, Cornelia Irimiea b, Michael Schmidt c, Ronny Brandenburg c, Marcin Hołub d, Michał Bonisławski d, Tomasz Jakubowski d, Marja-Leena Kääriäinen e,1, David C. Cameron e,2 a
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University of Tartu, Tartu, Estonia Technical University of Denmark, Risø, Denmark Leibniz Institute for Plasma Science and Technology, Greifswald, Germany d West-Pomeranian Technical University, Szczecin, Poland e ASTRaL, Lappeenranta University of Technology, Mikkeli, Finland b c
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h i g h l i g h t s
1 8 19
Direct and indirect plasma oxidation of NO was tested in a medium-scale test bench.
20 21
Both methods gave similar results at NO concentrations of 200 ppm.
22
The increasing DBD reactor temperature decreased the efficiency of direct oxidation.
23
The increasing gas temperature improves the efficiency of indirect oxidation.
24
The efficiency of indirect NO oxidation to N2O5 is further improved by catalyst.
25
a r t i c l e 2 4 7 0 28 29 30 31 32
i n f o
Article history: Received 4 August 2014 Received in revised form 5 December 2014 Accepted 10 December 2014 Available online xxxx
33 34 35 36 37 38 39
Keywords: Plasma Ozone Oxidation NOx Catalyst
a b s t r a c t Direct and indirect plasma oxidation of NOx was tested in a medium-scale test-bench at gas flows of 50 slm (3 m3/h). For direct plasma oxidation the synthetic flue gas was directed through a stacked DBD reactor. For indirect plasma oxidation, a DBD reactor was used to generate ozone from pure O2 and the plasma treated gas including ozone was mixed with flue gas at the entrance of a 6 m long serpentine-like reaction chamber which allowed reaction times longer than 10 s. At relatively low NOx concentrations of 200 ppm, both oxidation methods gave similar results. However, the temperature increase of the DBD reactor decreased the long-term efficiency of direct plasma oxidation. At the same time, the efficiency of indirect oxidation increased at elevated reactor temperatures. Additional experiments were carried out to investigate the improvement of indirect oxidation by the introduction of catalyst to the reaction zone. Small-scale experiments with TiO2 powder demonstrated considerable efficiency gain for NOx oxidation while in medium-scale experiments, the efficiency improvement remained negligible. Ó 2014 Published by Elsevier Ltd.
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55
1. Introduction
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Q2
Combustion of fuels emits large amounts of harmful NOx species [1] and the prevention of this environmental threat requires treatment of flue gas before it enters the atmosphere. The emission of NOx on land is regulated by several legislations, regarding sta⇑ Corresponding author at: University of Tartu, Institute of Physics, Ravila 14c, 50411 Tartu, Estonia. Tel.: +372 7375565. E-mail address: [email protected] (I. Jõgi). 1 Present address: University of Colorado, Boulder, Colorado, United States. 2 Present address: Dublin City University, Dublin, Ireland.
tionary combustion processes (e.g. TA Luft in Germany) or motor vehicles (e.g. EURO- or TIER-Norm). In addition to these emitters, aviation and shipping came recently into the focus as pollution sources with high impact on the environment. The transportation sector contributes to about 58% of total NOx emissions, 30% of total CO emissions and around 27% of total PM2.5 (particulate matter with an aerodynamic diameter of 2.5 micrometers or less) emis- Q3 sions at the European Union level [2]. Several emission controlled areas are established (Baltic Sea Region, North Sea, and North America) or planned (e.g. Mediterranian Sea) [3,4]. In these regions the air polluting emission, e.g. NOx, SOx, and PM, originating from vessels are subject to restrictions. The NOx emissions, for example,
http://dx.doi.org/10.1016/j.fuel.2014.12.025 0016-2361/Ó 2014 Published by Elsevier Ltd.
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must be reduced by 80% from the beginning of the year 2016 with respect to the emissions before the year 2000. In addition to these already adopted legislative means, some ports started to differentiate their dues according to the environmental impact of the visiting vessels [5]. Thus, there is urgent need for emission reduction methods in the domain of marine transportation [6]. There are several methods available for flue gas treatment e.g. selective catalytic reduction, storage by adsorption or wet absorption [7]. However, most of these methods suffer from poor absorption and adsorption of NO which usually constitutes about 90–99% of the NOx in the flue gas. In ships, the available space has to be considered as well, a fact that limits the possible size of the NOx removal devices and allowable backpressure in the exhaust system. One possibility for improving the efficiency of the available NOx removal methods is to efficiently oxidize the NO to NO2 or further to N2O5 which can be removed more easily from the gas stream. The oxidation of NO can be done at low and elevated temperatures by active oxygen species e.g. O, OH and OH2 radicals or O3 which are produced by non-thermal low-temperature plasmas [7–9]. This issue, especially regarding marine diesel exhausts, was recently investigated by several researchers [10–15]. In most of these studies, plasma technology was combined with additional reactants and/or catalysts to improve the NO oxidation and the NO2 removal performance. The catalyst can be positioned downstream the plasma reactor (post-plasma catalysis, PPC) or directly in the active plasma zone (in-plasma catalysis, IPC) [16–19]. Moreover, the NOx containing flue gas can be either directed through the plasma (direct plasma oxidation) or mixed with ozone produced by plasma (indirect oxidation). The direct plasma oxidation can be used for removal of both NOx and VOC-s [19] and requires no additional gas feeds. As a downside, in the case of pure NOx in mixtures with O2 and N2, the NO oxidation by the direct method is limited to certain NO concentrations [20–23] because O atoms are also consumed by the back-reaction of NO2 to NO [20]. The presence of unsaturated VOC-s in the gas feed may improve the oxidation but makes the removal dependent on the additives in the gas mixture [25]. The back-reaction to NO is absent for indirect oxidation by ozone and the oxidized amount of NO and oxidation efficiency to NO2 can be considerably higher without the need of additives [26]. In addition, contamination of the reactor due to the presence of water vapour and hydrocarbons [27] can be avoided when ozone is produced from pure oxygen or dry air. Furthermore, the indirect method allows oxidation of NOx to N2O5 which is desirable in some applications due to considerably better absorption of N2O5 into water [28–31]. However, the industrial scale experiments with indirect oxidation of NOx to N2O5 [28,32,33] used long residence times or surplus amounts of ozone for removal of NOx. These studies demonstrated that the efficiency of ozone oxidation remains still insufficient for practical considerations and additional improvements are necessary for industrial usage. The aim of the present study was to compare the direct and indirect plasma treatment of NOx species at a compact medium-scale test-bench [29]. There are some studies where the comparison has been carried out in laboratory scale but the semi-pilot scale studies are still missing. Furthermore, residence times more acceptable for industrial usage were used in present study. An additional goal was to investigate the possibility of improving the oxidation efficiency of indirect plasma treatment by placing a catalyst into the reaction zone. Preliminary experiments with catalysts were carried out at small scale with flow rates of 1 sl/min to demonstrate the usability of the method. Medium-scale experiments with flow rates of 50–60 sl/min were carried out to investigate the scalability of the process. As one of the steps in the up-scaling of the process, a new concept for catalyst preparation for large scale devices had to be developed and tested. The catalyst used in medium-scale
experiments had to possess large surface area and small pressure drop. For this purpose, a widely used porous glass–fiber mat was used as the template and a thin layer of catalyst was coated on the template by the atomic layer deposition method [34].
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2. Experimental setup
143
2.1. Small-scale experiments
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The experimental setup for the small-scale experiments was described in several earlier publications [35,36]. The coaxial dielectric barrier discharge (DBD) reactor consisted of stainless steel tube as the inner electrode, quartz tube as the dielectric and a metal mesh wrapped around the quartz tube as the outer electrode. The inner diameter of dielectric tube was 1.4 cm, the discharge gap was 0.12 cm and the length of the reactor was 9 cm. A sinusoidal voltage of 14 kV peak-to peak and 50–1000 Hz frequency was applied to the inner electrode. The corresponding input power was measured by the Manley method [37] and was in the range of 0.1–2 W. The indirect plasma treatment was carried out by using the DBD reactor as the ozone generator and a similar reactor without outer electrode as the reaction zone. The NO in N2 carrier gas was mixed with the outlet of the ozone generator (which used pure O2 as feedstock) and directed into the reaction zone. The reaction time before measuring the concentrations of NOx and O3 was approximately 1.5 s as estimated by the flow rate and the dimensions of tubing. The NOx, N2O5 and ozone concentrations were measured by optical absorption spectroscopy [33]. The catalyst powder (TiO2, Degussa P25) was placed into the second reactor as a thin 0.2 mm thick layer and the gas flowed over this catalyst layer. The residence time was approximately 0.3 s. The reaction zone was heated to 70 °C because earlier experiments have shown that the TiO2 catalyst needs some thermal activation Q4 [23,38] (see Fig. 1).
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2.2. Medium-scale experiments
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All medium-scale experiments were carried out with a flexible and mobile system [29] with compact reaction chamber in serpentine configuration (Fig. 2). The reaction chamber had inner diameter of 9.7 cm (74 cm2) and length of more than 6 m. Five sampling ports at different positions along the length were used for the measurements of temperature, NOx and O3 concentration and for FTIR measurements. Most of the measurements were carried out at the first port which was placed at 60 cm downstream from the inlet of the reactor. Synthetic exhaust gas based on compressed air and a variable amount of NOx (NO and NO2) was mixed with the ozone in the mixing zone [29]. The total synthetic exhaust gas flow was usually 50 sl/min which resulted in the linear gas velocity of about 11.3 cm/s and the reaction time before the first sampling port was correspondingly 5.3 s. The reactor and flue gas could be heated
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Fig. 1. The small-scale experimental apparatus for indirect treatment of NOx by ozone and catalyst.
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Fig. 3. Scheme of the stack reactor.
Fig. 2. Drawing of the experimental setup [24] (a) and picture of serpentine like reaction-chamber (b).
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up to 130 °C. The highest temperature used during these experiments was 70 °C. A stacked DBD reactor was composed of mesh electrodes made of stainless steel separated by dielectric plates made of magnesium mica (phlogopite) as shown in Fig. 3 [39]. The electrodes were contacted alternately to high voltage and ground. The reactor was driven by sinusoidal voltage provided by a high-efficiency, resonant power supply [40]. The schematic power supply construction is depicted in Fig. 4. A two-stage system was proposed allowing output voltage amplitude control in a wide range. The first, full bridge stage defines the DC link voltage value, in addition the Ts1–Ts2 transistor timing allows for low loss, zero current switching output voltage control [41]. The DBD reactor was placed at the beginning of the serpentinelike reaction chamber. Pure NOx (1.2 scc/min) was diluted with
dry air (50 sl/min) and directed through the reactor. The voltage peak-to-peak value was varied in the range of 5–5.8 kV where 5 kV was the ignition voltage and 5.8 kV the maximum output voltage provided by the power supply and the frequency was approximately 1.25 kHz. The discharge power was determined by multiplication of measured voltage and current signals by means of an optical storage oscilloscope. The temperature increase in the stacked reactor due to applied power was measured by an OSENSA Innovations fiber-optic temperature sensor. The sensor was placed into the contact of the dielectric surface in the outlet side of the reactor. The experiments with indirect plasma treatment (ozone injection) of NOx were carried out with 50 sl/min of air with two concentrations of NOx (200 ppm and 400 ppm). The ozone was produced from 20 sl/min of air by a commercial ozone generator (Wedeco) in the range of 0–3000 ppm. The plug power of the ozone generator used was up to 300 W giving ozone concentration of 2300 ppm. The corresponding ozone production efficiency was approximately 19 gO3/kW h. Several tests were made where ozone was produced from pure O2 (20 sl/min) but the results were similar to the ones obtained with air except the ozone production efficiency was twice as large (38 gO3/kW h). Thus, the air was used for the production of ozone in the following experiments. The output from the ozone generator was mixed with 30 sl/min of air and NOx (either 2.1 scc/min or 1.2 scc/min). The NOx concentration in the total airstream of 50 sl/min was either 400 ppm (300 ppm NO) or 210 ppm (150 ppm NO). The catalysts were fixed on the support of ISOVER VKL glass–fibre mats with thickness of 1.4 cm and diameter of 98 mm (see Fig. 5(a) and (b)). The density of the mats was approximately 120 kg/m3. Two of such mats were fitted into a section of ISO100K nipple specially designed for holding the catalyst coated mats (Fig. 5(c)). These catalyst supports were coated with 30 nm thick ALD TiO2 or ZnO layers with N doping [34,42,43]. The nitrogen doped TiO2 was deposited at 270 °C and was partially amorphous while the ZnO layer was deposited and at 185 °C and was crystalline. The ozone concentrations in the outlet of the ozonizer and in the reaction chamber were determined by BMT ozone sensors which are based on UV absorption at 254 nm. The NO and NOx concentrations were measured by chemo-luminescence sensors CLD 62 (Eco Physics). An FTIR spectrometer (Bomem) was additionally used to determine the presence of other nitrogen-oxide species (N2O, N2O5 and HNO3). The chemo-luminescence (CLD) NOx sensor also had cross-sensitivity to N2O5 which made the measurement of NOx at higher O3 concentrations unreliable. The N2O5 cross-sensitivity was verified with the use of a bubbler which was placed before the inlet of the chemo-luminescence sensor. Without the bubbler, the NOx concentration retained a high value even at the highest O3 concentrations while the NOx concentration decreased to zero when the analysed gas was directed through the bubbler. Most of the experiments were carried out with the bubbler placed before the CLD
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Fig. 4. Schematic construction of the resonant power supply.
Fig. 5. Pure glass–fibre supports of about 10 cm diameter and 1.4 cm thickness were coated by ALD method with thin films of TiO2 (a) or ZnO (b). These catalysts were placed inside of an ISO100-K nipple (c).
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sensor to obtain precisely the O3 concentration where all NOx was converted to N2O5. Both NOx and NO concentrations obtained by chemo-luminescence sensor with bubbler were smaller compared to the case when bubbler was not used (at low O3 concentrations the N2O5 did not form). Re-calibration was carried out by comparing the results obtained with and without the bubbler. The corrected dependences of outlet NO2 concentrations on inlet O3 concentration were very similar to the dependences obtained by FTIR which confirmed the correctness of the procedure. The concentrations of outlet species as a function of inlet ozone concentrations were also calculated by a simple numerical model which included following main reactions between ozone, NO, NO2 and N2O5:
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NO þ O3 ! NO2 þ O2 ;
1:4 1012 expð1320=TÞcm3 =s
NO2 þ O3 ! NO3 þ O2 ;
1:4 1013 expð2470=TÞcm3 =s
NO þ NO3 ! 2NO2 ;
11
1:8 10
14
NO2 þ NO3 þ M ! N2 O5 þ M; 4:5 10
11
NO2 þ NO3 ! NO þ NO2 þ O2 ; 2:8 10 271 272 273 274
N2 O5 þ M ! NO2 þ NO3 ;
3
expð110=TÞcm =s 3
expð1250=TÞcm =s 3:5
ðT=298Þ
2:7 1016 ðT=298Þ
3:5
cm3 =s
expð11000=TÞcm3 =s
The rate constants for the reactions were obtained from NIST Chemical Kinetics Database [44]. The calculations were carried out for the conditions used in real experiments.
3. Results and discussion
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3.1. Plasma oxidation of NOx in small-scale experiments
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The effect of TiO2 catalyst on the NO oxidation efficiency by direct plasma oxidation has been investigated in several small-scale experiments [16,23,45]. The highest amount of NO which could be oxidized by direct plasma treatment at room temperature was about 250 ppm. This maximum value further decreased at increasing reactor temperatures. The presence of TiO2 catalyst in the plasma reactor improved the oxidation of NO by about 40–50 ppm. As one conclusion, the direct plasma oxidation of NO at concentrations above 300 ppm is not feasible without the presence of hydrocarbons and the effect of TiO2 catalyst was further investigated for indirect plasma oxidation of NO. The stable oxidation of NOx by ozone in a small-scale reactor with total gas flow of 1 sl/min and at temperature of 70 °C is shown in Fig. 6(a) without catalyst and (b) with TiO2. The initial NO concentration was in the range of 220–230 ppm and the inlet ozone concentration produced from pure O2 in DBD reactor was increased up to 1200 ppm. After the injection of ozone the NO was initially oxidized to NO2. At increasing inlet ozone concentrations all NO was removed from the exhaust gas and the NO2 produced was further oxidized to N2O5. The removal of NO and production of NO2 was characterized by a linear relation with the inlet ozone
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Fig. 6. The indirect plasma oxidation of NO to NO2 and N2O5 by ozone injection at 70 °C without the catalyst (a) and in the presence of TiO2 catalyst (b) in the reaction zone. Solid curves show the concentrations calculated by the simple numerical model. The dashed curve for the results obtained for NO2 with TiO2 gives the exponential fit of the experimental results.
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concentration and was not affected by presence of a catalyst. This is explainable by the fast reaction between ozone and NO which should proceed to the end in less than 0.1 s. According to Fig. 6, the oxidation of NO2 to N2O5 was a sub-linear function of ozone concentration and was enhanced by the presence of catalyst. The NOx and N2O5 concentrations obtained by numerical calculations (solid lines) by the simple model described above had reasonable agreement with the experiments (marks) in the case when catalyst was not used. The presence of catalyst improved considerably the conversion of NO2 to N2O5. As a note, in the presence of TiO2 catalyst the outlet NOx concentration remained smaller than the inlet NOx concentration during the first 5–10 min of NOx oxidation by ozone. This effect can be explained by the adsorption of NOx at the beginning of the oxidation [36,46]. The results presented in Fig. 6 were stable values registered 20 min after the start of oxidation. The estimated amount of ozone necessary to oxidize 95% of NOx to N2O5 was 880 ppm and 635 ppm without the catalyst and with the TiO2 catalyst respectively. These results demonstrate that at short residence times the catalyst can considerably increase the oxidation efficiency of NOx (30%). Even in the presence of catalyst, the O3/NOx ratio necessary to oxidize 95% of NOx at 70 °C was 2.8 while the stoichiometric amount is 1.5 when pure NO is fed into the reactor. One conclusion of these results carried out in small-scale was the need of longer reaction times for efficient oxidation of NOx which could be obtained with the use of longer reactors.
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3.2. Direct plasma oxidation of NOx in the medium-scale test-bench
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The investigation of scalability of the oxidation process was carried out in the 6 m long serpentine-like reactor at flow rates
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of 50–60 sl/min. In the applied voltage range of 5–5.8 kV, a linear dependence of discharge power P from the applied voltage Upp was observed and the power dissipated into the plasma was 50–150 W (Fig. 7(a)). The temperature measured at the reactor outlet reached up to 100 °C during the operation of the reactor. Ozone production as a function of input power for the plasma device used in direct plasma oxidation was determined for compressed air inlet gas (Fig. 7(b)). The concentration of ozone was a linear function of input power up to the highest used power values of 125 W (SIE = 150 J/L) where the ozone concentration was about 500 ppm. This concentration corresponds to the efficiency of about 26 gO3/kW h. The obtained value was considerably larger than the efficiency of a commercial ozonizer but taking into account the power efficiency of the supply (70%) the efficiencies become quite similar. As a remark, the ozone production at low O production values of less than 500 ppm (low SIE values below 100 J/L) can be used as an estimation of O radical production [47,48]. The NOx removal/oxidation in the medium scale experiments is shown as a function of ozone production at the same power to compare the results of direct and indirect oxidation in Fig. 8. The results are shown both for increasing power (cool device) and for decreasing power (heated device). There was a clear difference in NOx removal when the reactor was allowed to heat up. In the cold device, the NO concentration decreased almost linearly with increased power (O production) until all NO was oxidized to NO2 by the reactions NO + O + M ? NO2 + M (M is the third body) and NO + O3 ? NO2 + O2 [44]. This occurred at about 200 ppm of equivalent ozone production which corresponds to SIE 60 J/L. Further increase of equivalent ozone concentration decreased the outlet NOx concentration due to the oxidation of NO2 to N2O5. After the reactor was heated up by the
Fig. 7. (a) Voltage–power-characteristic of the stacked reactor and (b) ozone output as a function of the power.
Please cite this article in press as: Jõgi I et al. Comparison of direct and indirect plasma oxidation of NO combined with oxidation by catalyst. Fuel (2014), http://dx.doi.org/10.1016/j.fuel.2014.12.025
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Fig. 8. Outlet concentrations of NOx and NO as a function of equivalent ozone production in the absence of NOx in the gas.
Fig. 9. Comparison of direct (plasma) and indirect (ozone injection) removal of NOx as a function of ozone concentration. In the case of direct plasma treatment, equivalent ozone production was used.
high input power, the outlet concentration of NO did not decrease below the value of 45 ppm and total NOx concentration in the outlet slightly increased. With the increase of gas temperature, the importance of back-reaction NO2 + O ? NO + O2 increases [44]. The increased total NOx concentration at higher reactor temperatures can be attributed to the production of additional NOx inside the plasma by reactions involving various active nitrogen species [49–51]. The results of FTIR confirmed that the NO was indeed oxidized to NO2 and after the NO was totally oxidized in the cool device, N2O5 appeared in the spectra suggesting further oxidation of NO2 to N2O5.
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3.3. Indirect plasma oxidation of NOx in the medium-scale test-bench
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Similarly to the direct treatment, the increasing inlet ozone concentration resulted in the decrease of NO concentration while the NOx concentration remained constant (Fig. 9). At about 200 ppm of ozone, all NO was oxidized to NO2. At higher concentrations of inlet ozone, NOx somewhat decreased and then slightly increased. The general trends for direct and indirect removal of NOx in the cool device were quite similar at these inlet NOx concentrations suggesting that both methods are suitable for the oxidation of low concentrations of NO found for example in gas fired powerplants [28]. Subsequent indirect plasma oxidation experiments were carried out at two concentrations, 200 and 400 ppm and at two temperatures 22 and 70 °C. The use of a bubbler before the chemo-luminescence sensor and FTIR measurements were used to distinguish between NOx, N2O5 and HNO3. For inlet NOx concentrations of 200 ppm and 400 ppm, the NO was initially oxidized to NO2 and then NO2 further oxidized to N2O5 (Fig. 10). The increase of the reactor temperature did not affect the oxidation of NO to NO2 and somewhat improved the oxidation of NO2 to N2O5. This result is in contrast with the direct plasma oxidation where the
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Fig. 10. The oxidation of 200 ppm (a and b) and 400 ppm (c and d) of NOx in 50 sl/min flow at 25 (a and c) and 70 °C (b and d) by ozone injection. The concentrations of NO and NO2 determined by chemo-luminescence sensors when bubbler was used to remove N2O5 are shown by filled symbols. The concentrations of NO2 and N2O5 were also determined by FTIR and are shown as empty symbols. The absorbance is scaled in the way that NO2 concentrations determined by different methods coincide. Outlet O3 concentration is shown by black circles for 400 ppm inlet NOx. Dashed curves show the concentrations calculated by the simple model.
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oxidation efficiency decreased at higher temperatures and shows the advantage of indirect plasma oxidation at the gas composition used. At a reactor temperature of 70 °C, conversion of 95% of NOx to N2O5 was achieved when inlet ozone concentration was 360 ppm and 575 ppm at inlet NOx concentrations of 200 and 400 ppm respectively (Fig. 10). This gives the O3/NOx ratio of 1.8 for 200 ppm and 1.45 for 400 ppm of inlet NOx. From these results, it was clear that the conversion was almost finished by the time that the treated gas reached the first measurement port (6 s). The appearance of ozone in the outlet only when all NO2 was converted to N2O5 also indicated the completeness of reaction 2NO2 + O3 ? N2O5 + O2. At 25 °C there was still some ozone and NOx detectable simultaneously while at 70 °C the ozone appeared in the outlet only when practically all NOx was oxidized to N2O5. The numerical calculations using a simple model with six reactions was carried out for a reaction time of 8 s to take into account approximately 2 s transit time from the reactor to the NOx analyser or FTIR apparatus. The calculated and measured concentrations have reasonable agreement and also confirm that the residence time was sufficiently long to obtain almost stoichiometric conversion of NOx to N2O5.
3.4. Effect of catalyst during indirect plasma oxidation in the medium-scale test-bench Small-scale experiments of NOx removal by indirect plasmatreatment demonstrated the improvement of the oxidation of NO2 to N2O5 in the presence of TiO2 catalyst. The influence of the catalyst on the NOx oxidation was also tested in the medium-scale test-bench to investigate the scalability of the processes. At larger flow rates, it is not possible to use powders in similar way as in small-scale experiments. As a consequence, a new type of catalyst had to be developed for medium-scale tests. For this purpose, glass–fibre mats with high surface area were coated with thin layers of TiO2 or ZnO catalyst. The catalysts obtained in such way were inserted into the reactor directly after the mixing zone. The total thickness of catalyst zone was 3 cm and the estimated residence time in the catalyst zone was 0.26 s. This time was comparable to the residence time corresponding to the small-scale experiments (0.3 s). During the mid-scale experiments, the adsorption of NOx species was evident as the outlet NOx concentrations stabilized in timeframes of about 5–10 min, similar to observations with
Fig. 11. Outlet NO2 concentration determined by chemo-luminescence sensor with bubbler (filled symbols) and by FTIR absorbance (empty symbols) for the absence of catalyst (orange diamonds) and for the catalysts with TiO2 (blue triangles) or ZnO (green rectangles). Dashed lines indicate the stoichiometric concentration of outlet NO2 as a function of O3 when all O3 is used for the oxidation of NOx. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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small-scale experiments. According to the comparison of outlet NO2 concentrations as a function of inlet O3 concentration for different catalysts, the long-term improvement of NOx removal in the presence of catalyst was not achieved (Fig. 11). Apparently, the reaction time before the measurement of the concentration (8 s) was sufficiently long to complete the oxidation of NO2 by O3 and there was no room for improvements with the current setup.
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4. Conclusions
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Direct and indirect plasma oxidation of NOx in synthetic air was tested in a mid-scale serpentine-like reactor. At inlet NOx concentration of 200 ppm both direct and indirect oxidation gave similar results when reactions proceeded close to room temperature. The temperature increase in the DBD reactor reduced the efficiency of direct oxidation while indirect oxidation became more efficient at higher temperatures. With the indirect plasma oxidation, the NOx oxidation to N2O5 proceeds efficiently at residence times below 10 s for flow-rates of 50 sl/min (3 m3/h), especially at elevated temperatures. The possibility of further improve the oxidation efficiency by the use of a catalyst was tested both in small-scale and medium-scale experiments. For medium-scale experiments, a novel preparation route of TiO2 and ZnO based catalyst with high surface area was developed and tested. The use of the catalyst gave promising results in small-scale while in the medium-scale testbench it was not beneficial at the conditions used. Future tests with higher flow rates corresponding to shorter reaction times are necessary for the clarification of the positive effect of catalyst.
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Acknowledgements
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This study was partially financed by European Union (European Regional Development Fund) Interreg IVb Baltic Sea Region program (project PlasTEP) and Estonian Science Foundation (Project No. 9310). We are grateful to Alexander Schwock for the organisation of the experiments.
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Please cite this article in press as: Jõgi I et al. Comparison of direct and indirect plasma oxidation of NO combined with oxidation by catalyst. Fuel (2014), http://dx.doi.org/10.1016/j.fuel.2014.12.025