Plasma-Assisted Combustion Technology for NOx Reduction in

Aug 30, 2013 - This material is available free of charge via the Internet at http://pubs.acs.org. ... available by participants in Crossref's Cited-by...
1 downloads 0 Views 3MB Size
Download PDF
Article pubs.acs.org/est

Plasma-Assisted Combustion Technology for NOx Reduction in Industrial Burners 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

‡

S Supporting Information *

ABSTRACT: Stronger regulations on nitrogen oxide (NOx) production have recently promoted the creation of a diverse array of technologies for NOx reduction, particularly within the combustion process, where reduction is least expensive. In this paper, we discuss a new combustion technology that can reduce NOx emissions within industrial burners to single-digit parts per million levels without employing exhaust gas recirculation or other NOx reduction mechanisms. This new technology uses a simple modification of commercial burners, such that they are able to perform plasma-assisted staged combustion without altering the outer configuration of the commercial reference burner. We embedded the first-stage combustor within the head of the commercial reference burner, where it operated as a reformer that could host a partial oxidation process, producing hydrogen-rich reformate or synthesis gas product. The resulting hydrogen-rich flow then ignited and stabilized the combustion flame apart from the burner rim. Ultimately, the enhanced mixing and removal of hot spots with a widened flame area acted as the main mechanisms of NOx reduction. Because this plasma burner acted as a low NOx burner and was able to reduce NOx by more than half compared to the commercial reference burner, this methodology offers important cost-effective possibilities for NOx reduction in industrial applications.

1. INTRODUCTION

process that are also cost-effective and do not require the use of overly large, complex additional structures are in high demand. The use of plasma within the combustion process has been recommended and applied as a solution to these challenges. Plasma contains a chemically active species with a diverse array of characteristics12 and is typically used to enhance or catalyze chemical reactions.13 In addition, because it can be generated more rapidly than thermally activated species, plasma can play a significant role in the ignition of combustibles that have been exposed to harsh conditions, such as fast flows, or are beyond flammability limits.14−17 In industrial applications, plasma is often used as a reformer for flame stabilizers, a supplier of reducing agents for SCR,18,19 and a fuel processor.20 Plasma technologies applied for NOx reduction thus far have been focused on NO conversion to NO2 by plasma oxidation. Because NO conversion to NO2 is favorable for the SCR process, it has been actively studied.21,22 Previous results have demonstrated that this approach relies on the SCR process for conversion of NO2 to N2 and that electric power for NO conversion is proportional to the flow rate.23 For this reason, to convert NO in an industrial-scale combustor inevitably requires too great of an amount of electric power, ranging on the order of kJ/L. Therefore, NOx reduction in the combustion process is

Nitrogen oxide (NOx) is a representative emission produced by many types of combustors, including internal combustion engines and industrial burners. This compound, when released into the atmosphere, can cause acid rain, smog, and damage to the respiratory system of humans.1 Because of its harmful effects, a number of regulations have recently been proposed or enacted that place limits on the generation of this compound,2−4 promoting the creation of new technologies in industrial applications. Technologies for NOx reduction can be employed either within the combustion process or following after-treatment processes. After-treatment processes [e.g., selective catalytic reduction (SCR)] can be expensive and require additional systems that are unnecessarily bulky.5 Thus, methods that reduce NOx within the combustion process, examples of which include staged combustion and exhaust gas recirculation (EGR), tend to be favored.6−8 However, these reduction methods also present limitations. For example, while staged combustion is an effective tool for NOx reduction,9 it requires separate spaces for combustion as well as multiple passages for fuel and airflow, which can lead to large and complex combustors. EGR is also an effective reduction tool, but the internal form of this method is not as effective as the external form.5 In addition, the external form requires a large, complex structure for flame stabilization.10,11 For these reasons, new technologies for external NOx reduction within the combustion © 2013 American Chemical Society

Received: Revised: Accepted: Published: 10964

April 11, 2013 July 1, 2013 August 30, 2013 August 30, 2013 dx.doi.org/10.1021/es401513t | Environ. Sci. Technol. 2013, 47, 10964−10970

Environmental Science & Technology

Article

the better approach if electric power does not directly depend upon the amount of NOx reduction. In this paper, we introduce a new technology for the reduction of NOx in industrial applications that was constructed with slight modifications to a commercial industrial burner as well as plasma as a flame stabilizer. We also explore how operating parameters (e.g., excess air ratio and O2/C ratio) in the first-stage plasma burner affect the production of NOx to discuss optimum strategies for reducing NOx using this new technology.

2. MATERIALS AND METHODS 2.1. Development of the Plasma Burner. We created a new NOx reduction technology by embedding a first-stage plasma combustor in the head of a commercial reference burner without changing the outer configuration of the burner. The reference burner in this study is a commercial low NOx burner (see Figure S-1 of the Supporting Information). We controlled the amount and ratio of fuel [liquefied natural gas (LNG) with a heating value of 43 537 kJ/Nm3] and air as they were supplied into the first-stage combustor; however, we did not change their supply in the second-stage combustor from that of the reference burner (type P burner, Sookook Corporation). We installed a burner with a maximum capacity of 1130 MJ/h in the cylindrical boiler (diameter, 60 cm; length, 1.5 m) with an outer water jacket and operated that burner at 419−1047 MJ/h. We also installed an economizer containing an end-point sampling port. We applied plasma within the burner with a rotating arc-type reactor,24,25 which produced an arc that was convected downward with the fluid-dynamic rotation of reactant flow (see Figure S-2 of the Supporting Information). This plasma reactor, which had a cylindrical shape (inner diameter, 40 mm; volume, 125 cm3), was small enough to install within the reference burner head without necessitating modification of the outer configuration of the reference burner structure. LNG was supplied through the bottom hole of the reactor, where it was rapidly mixed with air and allowed to enter into the arc region. We then generated the plasma using an alternating current (AC) power supply with a high-voltage transformer (frequency of 10 kHz) and measured power consumption with a highvoltage probe and a current probe connected to an oscilloscope. Using this method, we determined that 50−150 W of power was used to generate plasma, a value less than 0.1% of the fuel heating value. In the commercially available reference burner head, fuel and air were rapidly mixed at the exit of the fuel supply tube, where they collided along a perpendicular flow path that was favorable for minimizing NOx production (Figure 1). The pilot flame was then lit by an igniter located in the vicinity of the pilot fuel exit, which, in turn, ignited the main flames at the exit of the fuel nozzle. Here, the diffusion flame was anchored by the pilot flame, where it was extinguished as soon as the flame was detached from the pilot flame. However, in the modified plasma burner integrated into the first-stage combustor, we modified the flame structure such that there was no igniter or pilot flame (Figure 2). Reformate gas produced in the first-stage combustor was supplied through the central hole, and hot, hydrogen-rich synthesis gas allowed for the autoignition of the fuel/air mixture. In addition, the main flame (or second flame) was not anchored close to the rim of the burner head but was instead detached from the burner head without blow-off and anchored by the synthesis gas (such that the flame appeared to

Figure 1. Flame structure of the reference burner.

be floating midair). The distance from the flame root to the burner head was about the diameter of the burner head and depended upon flame conditions, such as the flow rate of heat released in the flame. Because we detached the flame, flame instability and blowout could occur. However, in cases where extinguishment can be prevented, a detached flame can enhance the mixing of fuel and air, giving the two more time to mix and removing hightemperature spots, as observed close to the fuel nozzle in the reference burner (Figure 3). 2.2. Experimental Design: Factors That Impact NOx Production. To evaluate the performance of our plasma burner and to understand the optimal conditions for NOx reduction, we experimentally varied the (1) heat capacity of the burner, (2) O2/C ratio of the first-stage plasma burner, (3) total excess air ratio of the burner, (4) thermal load division between first- and second-stage burners, and (5) boiler pressure (Table 1). We examined these characteristics for the following reasons: (1) Heat capacity of the burner or total heating value of the fuel supplied to the burner determines thermal density in the boiler that is directly connected to NOx generation. Also, heat capacity is directly related to the flow rate. Increased flow velocity enhances mixing but may increase flame instability. (2) The O2/C ratio is the molar ratio of oxygen in the air to LNG in the first-stage burner and determines the concentration of hydrogen that is crucial for flame stabilization. The O2/C ratio is also related to the flow rate (a change of the O2/C ratio causes a change in the flow rate of air) and temperature of the product generated in the first-stage burner that controls the attachment of the flame root to the burner rim. (3) NOx generation differs according to the total excess air ratio (i.e., the extra air supplied over the stoichiometric ratio). The excess air ratio is the most direct and important operation parameter in a typical industrial burner. (4) Thermal load division is defined as the ratio of fuel supplied in the first-stage burner to the total fuel flow rate. This division determines the flow rate and temperature of the first-stage burner. Too much fuel may cause too high combustion intensity, resulting in too high temperature in the first-stage combustor. In addition, a flow that is too 10965

dx.doi.org/10.1021/es401513t | Environ. Sci. Technol. 2013, 47, 10964−10970

Environmental Science & Technology

Article

Figure 2. Flame structure of the modified plasma burner, where the flame was detached from the burner rim and stabilized midair.

Figure 3. Flame images of side and back views in the (a) reference and (b) modified plasma burners.

Table 1. Test Parameters for Optimal Burner Operation

Table 2. Standard Conditions for Parametric Tests

parameter

ranges

parameter

condition

burner capacity (×1000, kcal/h) excess air ratio O2/C ratio (first stage) fuel division (%, first/second) boiler pressure (mmAq)

100 160 200 225 250 several points between 1.02 and 1.45 0.7−2.0 with an interval of 0.1 4 5 8 10 10 15 20 29

burner capacity (×1000, kcal/h) excess air ratio O2/C ratio (first stage) fuel division (%, first/second) boiler pressure (mmAq)

200 1.15 1.5 5 10−13

high in the first-stage burner can force the flame out of the first combustor. (5) Finally, boiler pressure is related to the excess air ratio and flow rate. When air is supplied through the fan in the burner at a supply pressure of 80 mmAq, the capacity of the induction fan draft of the boiler system affects the boiler pressure and changes the air flow rate. We varied each of these parameters independently while keeping all others at a standard value (Table 2) to isolate the individual effect of each parameter on NOx generation.

3. RESULTS We investigated the effects of a number of characteristics on the generation of NOx in industrial applications using both our plasma burner technology and the reference burner (Figures 1 and 2). We found that NOx production slightly decreased with an increased burner capacity/thermal load in the plasma burner, while production increased with an increased thermal load in the reference burner (Figure 4). In the case of the plasma burner, a lower heating value resulted in a weakened mixing condition in the second-stage flame because of a 10966

dx.doi.org/10.1021/es401513t | Environ. Sci. Technol. 2013, 47, 10964−10970

Environmental Science & Technology

Article

(Figure 5a). We hypothesize that this pattern was caused by different mixing conditions of fuel and air. Figure 5b shows a comparison of NOx generation between the reference and plasma burners under similar CO generation conditions. On the basis of this comparison, the plasma burner was effective in the simultaneous reduction of both NOx and CO. Following experimental changes to the O2/C ratio in the first-stage combustor, which controlled the shape and character-

Figure 4. Comparison of nitrogen oxide (NOx) and carbon monoxide (CO) production in the reference and modified burners as a function of the burner operating capacity.

Figure 6. Comparison of nitrogen oxide (NOx) and carbon monoxide (CO) production in the reference and modified burners as a function of the change in the O2/C ratio.

istics of the flame used, we determined that NOx production gradually decreased as this ratio increased (Figure 6). At an O2/ C ratio below 1.1, the first-stage combustor could not maintain a flame within the first-stage combustor. The flame migrated and stabilized at the exit rim, comparable to the pilot flame in the reference burner. An O2/C ratio of 1.1−1.2 corresponded to the point where the flame was swallowed into the first-stage combustor embedded in the burner head, and the flame in the second-stage combustor detached from the burner rim. When the O2/C ratio exceeded 1.2, the second-stage flame lifted without blowing out. In general, NOx production remained low as long as the flame was compartmentalized in the first-stage combustor, producing hydrogen-rich gases. In this case, the second-stage flame root became well-formed over a wide area, which decreased the production of NOx. When we examined the impact of load division on NOx production and burner performance, we found that load division (i.e., the ratio of fuel supplied to the first- and secondstage combustors) also affected NOx generation. Too much fuel in the first-stage combustor produced too much heat, increasing the amount of thermal NOx. However, too little outflow from the first-stage combustor supplied insufficient hydrogen to the second-stage combustor (Figure 7). A positive or negative departure from a 5% fuel division, which could occur through changes in the volume of the first-stage combustor, resulted in an increase in NOx. Finally, we found that the effect of boiler pressure on the generation of NOx was not sensitive within tested conditions (Figure 8).

Figure 5. Comparison of nitrogen oxide (NOx) and carbon monoxide (CO) production in the reference and modified burners as a function of the excess air ratio under conditions for (a) lowest NOx emission and (b) CO incineration. Panels a and b both have the same operating conditions. The only difference is the modification of the fuel exit nozzle to have a more evenly distributed flow pattern in panel a. Results in panel b are obtained using the fuel exit nozzle of the reference burner. Modification for evenly distributed fuel flow out of the exit nozzle can further lower the NOx emission in panel a; however, reduction of NOx can trigger an increase of CO.

4. DISCUSSION The mechanism of flame stabilization differed according to the O2/C ratio of the first-stage combustor and can be summarized as follows (Figure 9): (i) An O2/C ratio less than 1.2 offered a strong partial oxidation condition, where the flame could exist

reduced flow rate, which also resulted in incomplete combustion of CO produced in the first-stage combustor. The plasma burner in this study did not show an increase in the production of NOx with an increased excess air ratio 10967

dx.doi.org/10.1021/es401513t | Environ. Sci. Technol. 2013, 47, 10964−10970

Environmental Science & Technology

Article

Figure 7. Comparison of nitrogen oxide (NOx) and carbon monoxide (CO) production in the reference and modified burners as a function of the fuel division ratio in the first-stage combustor.

Figure 9. Three different modes of operation according to the change in the O2/C ratio.

hydrogen. Similar results were observed in the study of an internal combustion engine, where the transition between kinetically controlled and mixing-controlled conditions resulted in a difference in NOx generation.26,27 While NOx production by the reference burner followed a typical diffusion flame pattern, where NOx increased above a stoichiometric ratio of 1, the plasma burner showed almost no change in NO x production even as the excess air ratio increased. The use of a plasma-embedded staged burner changed the flame characteristics from those of a typical diffusion flame. This suggests that the flame in the plasma burner had hybrid characteristics

Figure 8. Comparison of nitrogen oxide (NOx) and carbon monoxide (CO) production in the reference and modified burners as a function of changes in the boiler pressure that resulted from changes in the excess air ratio.

within the first-stage combustor. The flame was constructed around the exit of the first-stage combustor and functioned as a pilot flame, resulting in the reduction of the length scale for flame lift. (ii) The flame condition when the O2/C ratio was between 1.2 and 1.9 existed between partial oxidation and combustion. In this case, the flame was swallowed into the firststage combustor and hydrogen-rich product was supplied to the second-stage combustor. The hydrogen-containing hot gas anchored the flame in the air. The distance between the burner rim and the flame root was used for the enhanced mixing of fuel and air, resulting in a reduction in NOx generation. Under this condition, hot spots were removed and the widened flame area reduced the thermal density of the flame, resulting in NOx reduction. (iii) Combustion with a stoichiometric ratio in the first-stage combustor occurred when the O2/C ratio was greater than 1.9. In this case, fuel was completely consumed within the first-stage combustor, and almost no hydrogen was produced. The combustion product from the first-stage combustor did not have a flame stabilization mechanism, and the second flame blew out. We found that the optimal condition for NOx reduction occurred while operating the burner under condition ii, where the O2/C ratio was between 1.2 and 1.9. We also determined that the most important mechanism for NOx reduction was the enhanced mixing by a lifted flame structure stabilized by

Figure 10. Changes to flame characteristics and nitrogen oxide (NOx) production in the modified plasma burner with enhanced air and fuel mixing.

between those of diffusion and premixed flames that originated from the enhanced mixing of fuel and air (Figure 10).28 This change of flame characteristics can be assured also from the characteristics of CO generation. In general, NO x generation reached a maximum level around a stoichiometric ratio of 1 in the premixed flame. On the other hand, in the diffusion flame, NOx generation continued to increase over the ratio of 1, as observed in the reference burner. Although NOx 10968

dx.doi.org/10.1021/es401513t | Environ. Sci. Technol. 2013, 47, 10964−10970

Environmental Science & Technology

Article

Notes

can be decreased by operating a burner with an excess air ratio close to 1; however, reduction of an excess air ratio close to 1 inevitably leads to an increase in CO and flame instability. That is why typical industrial burners with diffusion flames operate at excess air ratios ranging from 1.2 to 1.4. However, the plasma burner showed different behavior. Dependent upon the condition, NOx and CO could be reduced simultaneously by virtue of the change of flame characteristics. Flame stabilization and resulting NOx reduction can be explained by the following sequence of events and conditions: (1) The plasma reactor inside the burner head operated under fuel-rich or partial oxidation conditions. The partial oxidation reaction of natural gas produced synthesis gas that contained hydrogen at around 10 vol %. The hydrogen concentration was confirmed in the operating condition of the plasma reactor only. (2) The hydrogen-rich, high-temperature flow moved along the central hole of the burner head, where it ignited the mixture of fuel and air. (3) The expanded, high-temperature synthesis gas detached the main flame from the burner rim. This was in contrast with the flame in the reference (unmodified) burner, where the diffusion flame was attached to the fuel exit nozzle. (4) The lifted flame did not blow out and floated from 5 to 15 cm above the burner head. The floating height depended upon the total reactant flow rate and the condition of the plasma reactor inside the burner head, which determines the amount of hydrogen provided to the main flame. Because of the high burning velocity of hydrogen contained in the main flame, the flame can avoid blow off. (5) In the reference burner, hot spots (i.e., the white zones close to the fuel nozzle in Figure 3a) formed at the flame root. However, the lifted flame in the modified burner eliminated these hot spots because the reactor and air had more time for mixing and the area of the flame root was enlarged. A hot spot was not observed for the modified burner in Figure 3b. The removal of the hot spot ultimately contributed to the reduction in thermal NOx production. (6) Because the flame was lifted and time and space for mixing was provided, the flame shifted from a diffusion flame to a (partially) premixed flame that was favorable for NOx reduction. (7) Finally, unreacted synthesis gas around the flame root acted as a reductant for NOx in the post-flame region as H2 and CO reacted with NOx to form H2O and CO2.29 These collective conditions and series of events contributed to the reduction of NOx in the modified burner. In the future, numerical simulations may be helpful in understanding individual contributions of the effects by modification to NOx reduction. This new technology improves upon existing methodologies with important industrial applications.

■

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) under the Project “NOx and CO Reduction Using 3.5 Ton/h Combustor Aided by Plasma”.

■

(1) Detels, R.; Tashkin, D. P.; Sayre, J. W.; Rokaw, S. N.; Massey, F. J.; Coulson, A. H., Jr.; Wegman, D. H. The UCLA population studies of CORD: X. A cohort study of changes in respiratory function associated with chronic exposure to SOx, NOx, and hydrocarbons. Am. J. Public Health 1991, 81 (3), 350−359. (2) Skalska, K.; Miller, J. S.; Ledakowicz, S. Trends in NOx abatement: A review. Sci. Total Environ. 2010, 408 (19), 3976−3989. (3) Emission Standards for the European Union: Cars and Light Trucks; http://www.dieselnet.com/standards/eu/ld.php. (4) Ministry of Environment, Government of Korea. Digital Library; library.me.go.kr/viewer/MediaViewer.ax?cid=141111&rid=19. (5) Adams, J. D.; Reed, S. D.; Itse, D. C. Minimize NOx emissions cost-effectively. Hydrocarbon Process. 2001, June, 51−58. (6) Adachi, T.; Aoyagi, Y.; Kobayashi, M.; Murayama, T.; Goto, Y.; Suzuki, H. Effective NOx reduction in high boost, wide range and high EGR rate in a heavy duty diesel engine. SAE [Tech. Pap.] 2009, DOI: 10.4271/2009-01-1438. (7) Tanabe, H.; Yamamoto, J.-I.; Okazaki, K. NOx formation and reduction mechanism in staged O2/CO2 combustion. Combust. Flame 2011, 158, 1255−1263. (8) Aithal, S. M. Modeling of NOx formation in diesel engines using finite-rate chemical kinetics. Appl. Energy 2010, 87, 2256−2265. (9) Li, S.; Xu, T.; Hui, S.; Wei, X. NOx emission and thermal efficiency of a 300 MWe utility boiler retrofitted by air staging. Appl. Energy 2009, 86, 1797−1803. (10) Shim, B.; Cho, Y.; Han, D.; Song, S.; Chun, K. M. Hydrogen effects on NOx emissions and brake thermal efficiency in a diesel engine under low-temperature and heavy-EGR conditions. Int. J. Hydrogen Energy 2011, 36 (10), 6281−6291. (11) Littlejohn, D.; Majeski, A. J.; Tonse, S.; Castaldini, C.; Cheng, R. K. Laboratory investigation of an ultralow NOx premixed combustion concept for industrial boilers. Proc. Combust. Inst. 2002, 29 (1), 1115− 1121. (12) Fridman, A. A.; Kennedy, L. A. Plasma Physics and Engineering; Taylor and Francis Group: New York, 2004. (13) Lee, D.; Kim, K.-T.; Song, Y.-H.; Kang, W. S.; Jo, S. Mapping plasma chemistry in hydrocarbon fuel processing processes. Plasma Chem. Plasma Process. 2013, 33, 249−269. (14) Anikin, N. B.; Mintoussov, E. I.; Pancheshnyi, S. V.; Roupassov, D. V.; Sych, V. E.; Starikovskii, A. Y. Nonequilibrium plasmas and its applications for combustion and hypersonic flow control. Proceedings of the 41st AIAA Aerospace Sciences Meeting and Exhibition; Reno, NV, Jan 6−9, 2003; AIAA-2003-1053. (15) Galley, D.; Pilla, G.; Lacoste, D.; Ducruix, S.; Laux, C. Plasmaenhanced combustion of a lean premixed air−propane turbulent flame using a nanosecond repetitively pulsed plasma. Proceedings of the 43rd AIAA Aerospace Science Meeting and Exhibition; Reno, NV, Jan 10−13, 2005; AIAA-2005-1193. (16) Singleton, D.; Pendleton, S. J.; Gundersen, M. A. The role of non-thermal transient plasma for enhanced flame ignition in C2H4−air. Appl Phys. 2011, 44, 022001. (17) Kosarev, I. N.; Aleksandrov, N. L.; Kindysheve, S. V.; Starikovskais, S. M.; Starikovskii, A. Y. Kinetics of ignition of saturated hydrocarbons by nonequilibrium plasma: CH4-containing mixtures. Combust. Flame 2008, 154, 569−586. (18) Bromberg, L.; Cohn, D. R.; Rabinovich, A. Plasma reforming of CH4. Energy Fuels 1998, 12, 11−18.

ASSOCIATED CONTENT

S Supporting Information *

Conceptual design of the proposed plasma-embedded staged burner with the modification procedure (Figure S-1) and schematic on the structure of the rotating arc reactor with simulated reactant streamline inside the reactor and electric field distribution (Figure S-2). This material is available free of charge via the Internet at http://pubs.acs.org.

■

REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Telephone: +82-42-868-7406. Fax: +82-42-868-7284. E-mail: [email protected]. 10969

dx.doi.org/10.1021/es401513t | Environ. Sci. Technol. 2013, 47, 10964−10970

Environmental Science & Technology

Article

(19) Lee, D. H.; Lee, J.-O.; Kim, K.-T.; Song, Y.-H.; Kim, E.; Han, H.-S. Characteristics of plasma-assisted hydrocarbon SCR system. Int. J. Hydrogen Energy 2011, 36, 11718−11726. (20) Petitpas, G.; Gonzalez-Aguilar, R.; Darmon, A.; Fulcheri, L. Ethanol and E85 reforming assisted by non-thermal arc discharge. Energy Fuels 2010, 24, 2607−2613. (21) Barankova, H.; Bardos, L. Conversion of nitrogen and carbon oxides by the atmospheric hollow cathode discharges. IEEE Tran. Plasma Sci. 2012, 40 (5), 1324−1328. (22) Barankova, H.; Bardos, L. Optimization and performance of atmospheric fused hollow cathodes. Vacuum 2013, 87, 128−131. (23) Lee, J. O.; Song, Y.-H.; Cha, M. S.; Kim, S. J. Effects of hydrocarbons and water vapor on NOx using V2O5−WO3/TiO2 catalyst reduction in combination with nonthermal plasma. Ind. Eng. Chem. Res. 2007, 46, 5570−5575. (24) 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. (25) 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. (26) Jia, M.; Xie, M.; Wang, T.; Peng, Z. The effect of injection timing and intake valve close timing on performance and emissions of diesel PCCI engine with a full engine cycle CFD simulation. Appl. Energy 2011, 88, 2967−2975. (27) Paster, J. V.; Garcia-Oliver, J. M.; Garcia, A.; Mico, C.; Durrett, R. A spectroscopy study of gasoline partially premixed compression ignition spark assisted combustion. Appl. Energy 2013, 104, 568−575. (28) Waibel, R. T. Ultra low NOx burners for industrial process heaters. Proceedings of the 2nd International Conference on Combustion Technologies for a Clean Environment; Lisbon, Portugal, July 19−22, 1993. (29) Roy, S.; Hegde, M. S.; Madras, G. Catalysis for NOx abatement. Appl. Energy 2009, 86, 2283−2297.

10970

dx.doi.org/10.1021/es401513t | Environ. Sci. Technol. 2013, 47, 10964−10970