NOx Reduction Strategy by Staged Combustion with Plasma-Assisted

May 31, 2012 - Sungkwon Jo , Kwan-Tae Kim , Dae Hoon Lee , Young-Hoon Song. Journal of Korean Society for Atmospheric Environment 2016 32 (5), 526- ...
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NOx Reduction Strategy by Staged Combustion with Plasma-Assisted Flame Stabilization Dae Hoon Lee,*,† Kwan-Tae Kim,† Hee Seok Kang,† Young-Hoon Song,† and Jae Eon Park‡ †

Korea Institute of Machinery and Materials, 104 Sinseong-ro, Yusong-gu, Daejeon 305-343, Korea Sookook Corporation, 3454-4, Suha-ri, Shindun-myeon, Icheon-si, Gyeongi-do 467-842, Korea

‡

ABSTRACT: A plasma-assisted staged combustor that can reduce NOx generation is introduced. This combustor is based on a commercial combustor. Only the internal structure of the commercial combustor head was modified to introduce staged combustion and plasma into the combustor. The outer frame and configuration of the combustor remain unchanged. Staged combustion adopts successive fuel-rich and fuel-lean flames to achieve low NOx combustion. However, this can cause flame instability. In the proposed combustor, plasma was adopted for stable operation of the first-stage combustor. The proposed combustor recorded lower NOx generation than a prototype combustor, and three different mechanisms that affected this NOx reduction in the staged combustor were analyzed. Plasma-stabilized fuel-rich flame in the first-stage combustor and hydrogen from the rich flame played a key role for NOx reduction. Plasma appears to be a prominent tool for flame stabilization under the harsh conditions of a combustor for low NOx applications. However, the stability of the flame is related to the amount of NOx, with a low amount of NOx producing a more unstable flame, which results in the generation of higher amounts of unburned hydrocarbon and CO.11 Typical ways to resolve this flame instability are (1) the addition of fuel with a high burning velocity, such as H2,12 (2) modifying the combustor structure to anchor or hold the flame,13 and (3) supplying a continuous heat source. In view of the above approaches, plasma can be used as a flame stabilizer. Plasma is the ionized state of a gas and has highly reactive species, such as excited molecules and high-energy electrons.14 In the case of alternating current (AC) power, successive ignition can be obtained according to the frequency of the AC. Moreover, electric power is converted to thermal energy by heat transfer from the discharge area to the reactant gas.15 With these characteristics of plasma as a background, diverse trials to apply plasma to the combustion field have been reported.16−18 Plasma can be classified as thermal and non-thermal plasma according to the relative temperature of its electrons and gas molecules. In the case of non-thermal plasma, the gas temperature might even remain at room temperature. On the other hand, in the case of thermal plasma, the gas temperature approaches 10 000 K.14 The gas temperature of plasma can be controlled according to the plasma source or method used to generate it. It is often misunderstood that the collision of a high-energy electron with a gas molecule works as a strong driving force and dominantly controls the reaction. However, the electron density in a plasma typically has a range of 1010− 1016 cm−3, depending upon the plasma source, which is much smaller than the density of atmospheric pressure gas molecules. Actually, an electric field and streamer can enhance the burning velocity,19 enabling ignition and light off in a relatively lower

1. INTRODUCTION In addition to automobile engines, industrial combustors also have to comply with harsh regulations on NOx generation. The Korean government legislated a law on the “regulation of total emissions” that has been valid since 2008.1 In addition to Korea, most nations have scheduled regulations on NOx, and various research and development efforts to reduce NOx using diverse techniques of combustion have been reported.2−4 In general, thermal NOx makes up a relatively large portion of total NOx. For this reason, the most common and effective way for reducing NOx is to reduce the temperature of the combustion zone.5 Exhaust gas recirculation (EGR) and staged combustion are representative of such techniques.6,7 The EGR technique recirculates part of the exhaust gas into the combustor. Using the exhaust gas as a reactant suppresses the increase in the temperature by increasing the thermal capacity of the total reactant because the exhaust gas contains a large amount of CO2 and H2O, which are inert. EGR also has the effect of reducing the O2 concentration, which decreases NOx.6,8 On the other hand, staged combustion divides the combustion process into two or more steps. Although an equivalent total fuel/oxidant ratio is maintained in comparison to a typical combustor, the equivalent ratios of the individual steps alternate between fuel-rich and fuel-lean mixtures.7,9 Because the heat released during the combustion process reaches a maximum at around an equivalent ratio of 1, avoiding this ratio by alternating between rich and lean flames produces flames with lower temperatures, which results in reduced NOx. In the case of EGR, an excessive amount of EGR flow brings about flame instability by incomplete combustion within the combustor.10 In the case of staged combustion, because the reaction takes place under the condition of an equivalent ratio that deviates from 1, the size of this deviation is inversely proportional to the amount of NOx that is generated, with a greater deviation producing a smaller amount of NOx. © 2012 American Chemical Society

Received: April 16, 2012 Revised: May 31, 2012 Published: May 31, 2012 4284

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temperature,20 but, in most cases, thermal activation is more dominant. The highly thermal environment produced by the heat transfer from the discharge area at a high temperature is more prone to initiate and sustain the flame.21 For this reason, a plasma source that can somehow provide a thermal environment is more favorable for plasma-assisted combustion. The rotating arc used in this study was generated by an AC power supply with a frequency of 20 kHz. The arc generated between the electrodes was convected down with a swirl by the tangential flow of the reactant, which resulted in the extension of the arc string.22 The rotational extension of an arc string is much more beneficial in heat transfer than a typical arc torch. The temperature of the arc string in a rotating arc is considered to be 2000−5000 K.23 This is much lower than that of a typical arc torch, which has a temperature of up to 10 000 K. Too high of a temperature implies lower power consumption efficiency. A rotating arc has shown its flame stabilization ability in a fuelrich condition using diesel fuel.24 This study introduced a modification of a commercial combustor to adopt staged combustion. In the course of this modification, plasma was suggested as a tool for possible flame stabilization and broadening of the flammability limit.

Table 1. Test Condition Matrix, Including Stoichiometry, Power, and Fuel Division stoichiometry (first stage) power (W) fuel division (%)

1.00 100 10

1.25 200 20

1.67 300 25

1.82 400

2.00 500

2.50

plasma generation, which is less than 0.4% of the heating value of the fuel used. 2.2. Prototype Combustor. This study modified a commercial combustor (Sookook, PG4) to realize staged combustion. To evaluate the modification, the performance of the prototype combustor was evaluated prior to the modification. Figure 2 shows a schematic drawing of the prototype combustor.

Figure 2. Schematic drawing of head part of the prototype combustor.

2. PLASMA STAGED COMBUSTOR A longitudinal air flow collided with a radial fuel flow to enhance rapid mixing, resulting in the reduction of prompt NOx generation, and the slight venting of the air flow inside produced recirculation flow outside the flame region. The design capacity of the combustor was 150 000 kcal/h, which was suitable for operation of a 0.5 ton/h scale

2.1. Experimental Apparatus. A boiler system was built to study the combustion characteristics of a combustor, as shown in Figure 1.

Figure 1. Experimental apparatus, including the boiler system, flow control system, and power control system. Figure 3. Test results for NOx, CO, and total hydrocarbon (THC) on the prototype combustor. The test condition for the combustor was 100 000 kcal/h.

The boiler was designed for use with a combustor having a heat capacity of 350 000 kcal/h. The temperature was controlled by the indirect exchange of heat between water and burned gas. Quartz windows for optical observations were installed on both the side and backside walls. The fuel flow rate was controlled using a mass flow controller (MFC), and the flow rate of the main combustion air was controlled using a fan installed in the combustor with proportional− integral−derivative (PID) control. The stack connected to the boiler had ports for gas sampling. The sampled gas passed through a chiller to remove water and possible particles. The burned gas was analyzed in situ using a gas analyzer (VarioPlus, MRU AIR) and ex situ by gas chromatography (GC) (Agilent, HP 6890) using a gas-sampling bag. The combustor was operated at a thermal capacity of 100 000 kcal/ h, and liquefied petroleum gas (LPG), of which the main component was C3H8, was used as the fuel, considering the fuel supply capacity of the laboratory. The flow rate of fuel and air in the tested condition was 4.55 and 163.64 N m3 h−1 each. The matrix of the tested condition is given in Table 1. An AC power supply with a frequency of 20 kHz (DawonSys, AP6K) was used. A total of 100−500 W of power was consumed for

boiler. Figure 3 shows NOx generation according to the stoichiometric ratio using LPG as the fuel. The combustor was operated with the capacity of 100 000 kcal/h, with the operation controlled to fix the stack temperature at around 150 °C. These data were used as a reference NOx level in the design modification for staged combustion. 2.3. Staged Combustion. In general, staged combustion divides the fuel and oxidant into two or more stages. In this division, the fuel/ oxidant ratio and stoichiometry of each stage determines the characteristics of the overall staged combustion. In this study, the combustor was modified to host two stages of burning. To prevent changing the shape and outer dimensions of the prototype combustor, the combustor for the first stage was designed to be inside the combustor head. The combustor head was located just at the edge of the boiler inlet, resulting in insufficient space for burned gas recirculation inside the boiler. Because only the effect of staged combustion for NOx reduction was to be investigated, internal flow 4285

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recirculation, which could have the effect of EGR, was suppressed. The modified design of the combustor is shown in Figure 4

Figure 4. Schematic of the modified design of the combustor. The first-stage combustor is embedded in the combustor head. Fuel and air are supplied separately to the first and second combustor stages.

Considering the volume of the first-stage combustor, the amount of fuel supplied for the first-stage combustor was designed to be within 25% of the total fuel. Because the purpose of staged combustion lies in the operation of successive fuel-rich and fuel-lean operations, the firststage combustor was designed to be fuel-rich and the second-stage combustor was designed to be fuel-lean. To maintain the same total stoichiometry as that of the prototype combustor operation, the stoichiometry of each stage was varied to determine the effect of staged combustion on NOx generation. During the course of the operation, as the stoichiometry of the first stage approached the flammability limit, the flame instability increased and the flame could not be sustained within the first-stage combustor and was generated only in the second-stage combustor. In this case, only the supply conditions for the fuel and air were different from the prototype combustor. The flow rate, where the flame could not be sustained in the first stage, was determined by the stoichiometry and flow rate of the first stage. The condition for sustaining the flame in the first-stage combustor was confirmed by a test using the first-stage combustor only. To prevent air entrainment and the burning of fuel in the second combustor zone, a 50 cm long tube was connected to the combustor head. The burned gas was sampled at the end of this tube (40 cm from the combustor head). Figure 5 shows the flame images taken from the backside for each stoichiometry with a schematic of the flame shape. The flame was swallowed or hosted in the first-stage combustor with a stoichiometry of around 1.8. After the characteristics of the first-stage combustor were determined, the combustor was installed in the boiler to operate both the first and second stages of the combustor. NOx generation in the staged combustor was measured and plotted against the stoichiometry of the first-stage combustor in Figure 6. The total stoichoimetry was set to be 0.67. NOx generation in the prototype combustor with the total stoichiometry of 0.67 was 76 ppm. As shown in the figure, under all of the conditions, the amount of NOx generated in the staged combustor was less than that generated in the prototype combustor. The difference in the NOx generation values between the prototype combustor and the staged combustor differed with the change in the stoichiometry in the first-stage combustor. The flame shape and characteristics in the staged combustor were changed by the stoichiometry in the first-stage combustor, and these changes resulted in different NOx reduction mechanisms. A detailed discussion of the mechanism is given in the Discussion. During the operation of the combustor, flame swallowing into the first-stage combustor could be obtained with a higher stoichiometry depending upon the thermal condition, such as overheating from a long period of operation, which resulted in a flame swallowing limit of 1.75−1.81 in the first stage. The uncertainty in the flame swallowing condition became a cause of instability in the combustor operation. This type of instability depends upon the cold start condition, changes in the second-stage air flow, which can increase the cooling in the firststage combustor by heat transfer, and a change in the fuel (change in the amount of heat released during combustion by the different

Figure 5. Operation of the modified combustor without plasma. Flame images were taken from the backside. The illustration of the flame is provided to define the behavior of the flame, such as flame swallowing and stabilization.

Figure 6. NOx generation in the modified combustor without plasma. NOx generation was reduced under all of the tested conditions. A definite and cascade-like behavior for the NOx generation was observed. heating value of the fuel). In this study, plasma was suggested to remove this kind of instability and uncertainty of operation. 2.4. Plasma Combustor. Rotating-arc-type plasma was applied for the modification of the combustor. In a plasma-assisted combustion process, using a rotating arc, it has been confirmed that a flame can be stabilized under harsh conditions. A plasma-assisted flame was stabilized in an extremely fuel-rich condition, in the kind of strong wind that can be experienced in the exhaust conduit of a diesel engine.16 As mentioned earlier, the stabilizing mechanism relies on the fact that plasma produces highly reactive species and repetitive ignition. In addition, the thermal energy from electric power can be provided to the reaction zone by heat transfer to the reacting gas from a discharge arc. The plasma combustor in this work was based on the 4286

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previously reported rotating arc reactor, with the only difference involving the supply of fuel into the high-voltage electrode. The fuel was mixed with tangentially flowing air from a cylindrical ground electrode. The characteristics of the modified first-stage combustor were tested without operation of the second-stage combustor before the entire combustor was installed in the boiler. The flame shape, swallowing criteria, and products, including NOx, according to the stoichiometry were estimated. An image of the flame in the first-stage

partial oxidation reaction was observed. With an increasing stoichiometry, further oxidation took place and the portion of full oxidation increased, resulting in partially sooting flame. An increased electric power will result in an enhanced partial oxidation reaction, which means an increased amount of hydrogen. Increased hydrogen will be beneficial for the flame stabilization. The flame length in the first-stage combustor will be shorter, and the sooting flame will be reduced. In addition, a greater amount of fuel can be stabilized in the first-stage combustor. That is beneficial for the increased effect of staged combustion. In contrast to Figure 5, flame swallowing can consistently be observed under all stoichiometry conditions. A stoichiometry above 2 is close to the flammability limit, making it hard to stabilize a flame; however, the plasma consistently stabilized the flame, and a reaction took place. Under this extremely fuel-rich condition, a partial oxidation process that produced synthesis gas or H2 and CO occurred. Here, H2 contributed to the flame stabilization, and the synthesis gas itself worked as a reductant for NOx reduction. Figure 8 shows the products of the first-stage combustor with and without plasma. Regardless of plasma, fuel stoichiometry determines the composition of major products, such as hydrogen and carbon monoxide, in the partial oxidation process. What is different is that we can obtain hydrogen in fuel stoichiomentry above 2, where it is impossible to produce hydrogen within the first-stage combustor if plasma is off. In addition, the difference is the source of the plasma function in flame stabilization in the first-stage combustor. As shown in Figure 5, with the plasma off, the flame could not be swallowed with a stoichiometry below 2.0 and the fuel was combusted in the connected tube rather than in the firststage combustor. As shown in the figure, the plasma clearly expanded the flammability limit. Plasma stably hosted the partial oxidation process and produced a synthesis gas that worked favorably for NOx reduction. However, once the flame was swallowed in the first-stage combustor in the plasma-off condition with a rather lower stoichiometry, the reaction approached full oxidation and the production of synthesis gas decreased. Moreover, the amount of NOx generated by the plasma itself was relatively high. The arc string generated had a small diameter (millimeter scale), but the temperature was above 3000 K23 and inevitably generated NOx. All of the above factors affected NOx generation and are reflected in Figure 8, which compares the NOx generation values from the first-stage combustor with and without the plasma operation. When the plasma was on, the flame was maintained in the first-stage combustor under all of the tested stoichiometry conditions. As the stoichiometry increased, partial oxidation took place and H2 and CO produced participated in the NOx reduction. The process was a type of selective non-catalytic reduction (SNCR). Because the plasma produced the locally high thermal environment that is required for SNCR, NOx could be reduced. As the stoichiometry approached 1, full oxidation replaced partial oxidation and the increased heat release resulted in higher NOx generation. When the plasma was off, the flame

Figure 7. Operation of the modified combustor with plasma. Flame images were taken from the backside. The illustration of the flame is provided to define the behavior of the flame, such as the flame swallowing and stabilization. Under all of the tested conditions, the flame was swallowed in the first-stage combustor. combustor, according to the stoichiometry, is shown in Figure 7. In all of the conditions of stoichiometry, flame was stabilized within the firststage combustor. In the condition of stoichiometry of 1, which is a partial oxidation condition, the blue flame that is characteristic of a

Figure 8. Operation results for the first-stage combustor only. The generation of NOx, CO, and THC are compared with and without plasma. A total of 150 W of power was used for the generation of plasma. 4287

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was swallowed with a stoichiometry below 1.5 and stayed in the tube rather than in the first-stage combustor, above a stoichiometry of 1.5. Although a partial oxidation reaction took place, the rather large reaction volume and absence of plasma could not constitute the thermal environment necessary for the occurrence of SNCR, even though the product had almost the same composition under the plasma-on condition, as shown in the figure. As a consequence, NOx generation was much higher than that of the plasma-on condition. However, once the flame was swallowed under both conditions (plasma on and off), H2 was seldom produced and the plasma worked as another heat source for NOx generation, resulting in higher NOx generation in the plasma-on condition. 2.5. Plasma Staged Combustor. Staged combustion using plasma in the first stage was tested. On the basis of the preliminary work using the plasma combustor only and staged combustion, the characteristics of the staged combustion operation, especially NOx

Figure 10. NOx and CO generation according to power used for plasma generation. The stoichiometry of the first-stage combustor was 1.82, and the total stoichiometry of the combustor was 0.67. temperature of the arc string increased. Although the diameter of the arc string was only 1 mm or so, the maximum temperature was 3000− 4000 K, and this elevated temperature resulted in higher NOx generation. In this configuration, about 200 W of power showed minimum NOx generation. An increase in the fuel division ratio in the first-stage combustor also supported this reasoning. As the amount of fuel increased, the specific energy density decreased, implying that less power was provided per unit volume of reactant. In addition, the reduced electric power per unit volume of reactant resulted in reduced activation of the partial oxidation reaction.

3. DISCUSSION In general, flame stabilization by plasma can be explained by several functions of plasma. At first, the discharge is operated with a frequency of 10 kHz. It can be understood that successive high-energy ignition is performed with the frequency of 10 kHz. Second, rich flame by plasma produces hydrogen. Hydrogen is well-known for its effect in flame stabilization by fast burning velocity. Third, arc discharge induces heat transfer from the arc string to the vicinity of the arc. The heat transfer enhances the chemical reaction thermally. All of the above synergistically contributes to flame stabilization. Plasma could be used to stabilize the flame in the first-stage combustor and was shown to be effective at NOx reduction. As discussed earlier, there were different mechanisms that affected NOx generation in the combustion process, and the plasma staged combustor in this work showed three different NOx reduction mechanisms, according to the fuel stoichiometry in the first-stage combustor. The details of these mechanisms are explained below. 3.1. Regime I (Stoichiometry of the First-Stage Combustor at 2.0−2.5). In this regime, the flame could not be hosted or swallowed in the first-stage combustor. As a result, non-combusting fuel and oxidant air exited the first-stage combustor as a fuel-rich mixture. In the second-stage combustor, a fuel-lean flame was formed inside a fuel-rich flame coaxially by this fuel-rich mixture from the first-stage combustor. A NOx reduction mechanism was obtained from this stratified flame, which had a structure similar to a radial staged combustor. However, under this condition, the lean flame and rich flame did not have independent and sufficient space for staged combustion and each flame mixed rapidly with the distance from the combustor head. 3.2. Regime II (Stoichiometry of the First-Stage Combustor at 1.5−2.0). The flame was swallowed into the

Figure 9. NOx and CO generation in the modified combustor. The shaded area indicates the range of NOx generation by the uncertainty of the flame stabilization in the first-stage combustor without plasma. The total stoichoimetry of the combustor was 0.67. The power for the plasma generation was 150 W. generation, were estimated and analyzed. Figure 9 compares the amount of NOx generated in the combustor with and without the plasma operation. The shaded zone in the figure designates the region of uncertainty. The flame swallowing and following NOx generation differed case by case within this region in the operation without plasma. However, in the case of the plasma operation, the stoichiometry of the flame swallowing and following NOx generation did not change. As mentioned earlier, the plasma could host flame swallowing in the first-stage combustor under the higher stoichiometry of the first stage and, once the plasma began operating, the value of the stoichiometry at the moment of flame swallowing did not change. This is the point of plasma application in a staged combustor. Without the plasma, the value of the stoichiometry at the flame swallowing changed case by case. Moreover, NOx generation differed, even with the same stoichiometry for the first-stage combustor. Actually, as discussed above, the use of the plasma itself can work as a source of NOx, and the parameters of a plasma operation should be evaluated and optimized to minimize NOx generation by the plasma operation itself. The parameters for the plasma operation include the electric power consumed, reactor geometry, which defines the space for arc generation, and gas composition. Among these, electric power is an active parameter, and the effect of the power on flame stabilization and NOx generation was evaluated, as shown in Figure 10. There were two competing factors with an increase in power. One was the enhancement of the partial oxidation process. H2 production in the plasma-assisted partial oxidation was almost linearly proportional to the electric power provided. The other factor was an increase in the temperature of the arc string. As the power increased, the 4288

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first-stage combustor, and staged combustion was hosted. According to the principle of staged combustion, as the stoichiometry of the first-stage combustor became richer, less NOx was generated. The richest flame in the first-stage combustor produced the lowest NOx in the overall combustor operation. The amount of NOx reduction differed with the amount of fuel in the first stage. As the amount increased, the thermal density of the first-stage combustor increased, which resulted in a slight increase in NOx, as shown in Figure 11.

Figure 12. Comparison of NOx and CO generation by plasma staged combustion using LNG as the fuel.

4. CONCLUSION The feasibility of plasma-assisted staged combustion was investigated. Staged combustion adopts successive fuel-rich and fuel-lean flames to reduce NOx generation. However, flame instability can be an issue in the course of the lean and rich flame operations. In this work, an embedded-type staged combustor was introduced. During the operation of the combustor, the flame should be swallowed in the first-stage combustor to achieve staged combustion. The value of the stoichiometry in the first-stage combustor for flame swallowing varied with the operating conditions, which shows room for plasma application. Stable flame swallowing and expansion of the flammability limit in the first-stage combustor could be obtained using the plasma. The relative role of the stabilization mechanism and the usability of plasma depend upon the fuel used. Plasma can be more prominent under harsh conditions for flame stabilization, such as a fuel with a low heating value or a combustor operated around the flammability limit. The staged combustor introduced in this work used diverse mechanisms for NOx reduction. Once the flame was stabilized, NOx could be reduced by three different mechanisms. The relative role of each mechanism depended upon the stoichiometry of the first-stage combustor. The introduced staged combustor hosted staged combustion for successive fuelrich/-lean flames. It also provided a type of EGR effect in the second-stage combustor. All of these effects contributed toward a reduction in NOx. Actually, this work did not consider the EGR effect by internal flow recirculation. If the combustor was installed in a boiler to induce the internal flow of burned gas to host EGR, NOx could be reduced further. Additional work to achieve single-digit NOx by adopting EGR using LNG as a fuel is now ongoing and will soon be reported.

Figure 11. Comparison of NOx generation according to the fuel division of first and second stages of the combustor. A total of 150 W was used for the plasma generation. The total stoichiometry was 0.67.

3.3. Regime III (Stoichiometry of the First-Stage Combustor at 1.0−1.5). The flame could be hosted in the first-stage combustor in this regime, but as the stoichiometry of the first and second stages of the combustor approached full oxidation, the effect of the staged combustion became weaker. However, although the effect of the staged combustion diminished, a much lower amount of NOx was generated than in the prototype combustor. This is because the burned gas that exited from the first-stage combustor expanded with a high temperature. The exit of the burned gas from the firststage combustor itself could work as EGR, which provided CO2 and H2O that did not participate in the reaction. Moreover, the burned gas expanded the second-stage flame radially, resulting in a lowered thermal space density in the second stage, which was favorable for NOx reduction. Regime II was found to be the most effective at NOx reduction, and the usefulness of the plasma application lies in this regime. The plasma expanded regime II and secured its boundary. In this work, where LPG was used as a fuel, the unstable condition for flame swallowing is rather narrow compared to that of liquefied natural gas (LNG). Because the heating value of LNG is much smaller than LPG, flame swallowing is more difficult in LNG than in LPG. Although the total amount of NOx generation using LNG as a fuel may be lower than that using LPG, the flame becomes more unstable in staged combustion with LNG. This implies that plasma could be used more effectively. Actually, in an earlier feasibility test with LNG, the difference in the stoichiometry at the point of flame swallowing was very large and resulted in NOx reduction over a wider stoichiometry, as shown in Figure 12.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 82-42-868-7406. Fax: 82-42-868-7284. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) with the 4289

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project title “NOx and CO Reduction Using 3.5 Ton/h Combustor Aided by Plasma”.

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(21) Lee, D. H.; Kim, K.-T.; Cha, M. S.; Song, Y.-H. Plasma controlled chemistry in plasma reforming of methane. Int. J. Hydrogen Energy 2010, 35, 10967−10976. (22) Lee, D. H.; Kim, K.-T.; Cha, M. S.; Song, Y.-H. Optimization scheme of a rotating gliding arc reactor for partial oxidation of methane. Proc. Combust. Inst. 2007, 31, 3343−3351. (23) Hwang, N.; Song, Y.-H.; Cha, M. S. Efficient use of CO2 reforming of methane with an arc-jet plasma. IEEE Trans. Plasma Sci. 2010, 38 (12), 3291−3299. (24) Lee, D. H.; Kim, K.-T.; Cha, M. S.; Song, Y.-H. Effect of excess oxygen in plasma reforming of diesel fuel. Int. J. Hydrogen Energy 2010, 35, 4668−4675.

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dx.doi.org/10.1021/ef3006367 | Energy Fuels 2012, 26, 4284−4290