IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 37, NO. 12, DECEMBER 2009 2303 Nitric Oxide Formation in a Premixed Flame With High-Level Plasma Energy Coupling Xing Rao, Igor B. Matveev, and Tonghun Lee Abstract—This paper presents quantitative planar laser- induced fluorescence (PLIF) imaging of nitric oxide (NO) in a transient-arc direct-current plasmatron igniter using premixed air/fuel mixtures. Quantitative measurements of NO are reported as a function of gas flow rate (20–50 standard cubic feet per hour), plasma power (100–900 mA, 150–750 W), and equivalence ratio (0.7–1.3). Images were corrected for temperature effects by using 2-D temperature field measurements obtained with infrared thermometry and calibrated by a more accurate multiline fitting technique. The signals were then quantified using an NO addition method and spectroscopic laser-induced fluorescence modeling of NO. NO PLIF images and single-point NO concentrations are presented for both plasma-discharge-only and methane/air plasma-enhanced combustion cases. NO formation occurs pre- dominantly through N 2 (v)+ O → NO + N for the plasma- discharge-only case without combustion. The NO concentration for the plasma-enhanced combustion case (500–3500 ppm) was an order of magnitude less than the plasma-discharge-only case (8000–15 000 ppm) due to the reduction of plasma reactions by the methane. Experiments show the linear decay of NO from equiv- alence ratio 0.8–1.2 under the same flow condition and discharge current. Index Terms—Nitric oxide (NO), plasma torch, plasma-assisted combustion. I. I NTRODUCTION D EVELOPMENT and investigation of nonequilibrium plasma discharge for enhancing combustion is receiving increased attention due to its potential application to a variety of problems such as finding a more efficient method of fossil fuel combustion, conversion of low-grade fuels into higher grade fuels, and reduction of pollution through ultralean burn combustion [1], [2]. Advantages of combining plasma dis- charge with thermal oxidation include faster and more intense chemical energy conversion, increased stability in the lean flammability limit, reduction of pollution by altering oxidation byproducts, improved fuel efficiency through more complete combustion, more reliable and rapid ignition, and stable fuel oxidation across a broader range of pressures and temperatures [3]–[8]. Manuscript received February 11, 2009; revised July 28, 2009 and September 30, 2009. Current version published December 11, 2009. This work was supported by Michigan State University through the Intramural Research Grants Program’s New Faculty Award. X. Rao and T. Lee are with the Department of Mechanical Engineering, Michigan State University, East Lansing, MI 48823 USA (e-mail: tonghun@ msu.edu). I. B. Matveev is with Applied Plasma Technologies, McLean, VA 22101 USA. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPS.2009.2034007 Addition of electromagnetic energy can alter the reaction mechanisms by the following: 1) decomposition of the fuel gas from larger to smaller hydrocarbon molecules and radicals via the electron gas with temperature T e ; 2) radiation-induced electron excitation; 3) increased flame temperature by ohmic heating, which increases the rates of reaction and transport; and 4) increased ion/electrons on key radical initiation and propagation reactions. Recently, Kim et al. [9], [10] have shown that fuel reforming into syngas plays a dominant role in stabilization of flames in the presence of a nonequilibrium discharge. A number of different plasma discharges, including thermal plasma discharge [11], dielectric barrier discharge [5], nanosecond pulsed discharge [12], pulsed corona discharge [13], radio frequency discharge [14], dc or low-frequency al- ternating current discharge [15], and plasmatron [3], gliding arc [16], and microwave discharge [17], have been investigated for enhancing high-temperature thermal oxidation. A more extensive review is available in [1]. Coupling high-temperature reactive flows with a nonequilib- rium plasma discharge can lead to significant changes in the formation of nitric oxide (NO), one of the most problematic combustion effluents and a critical design parameter for all practical combustion systems. The key mechanisms involved in the NO chemistry for plasma discharges and methane flames are shown in Table I. Generally, NO formation in conventional combustion is due to four main processes [18], [19]: 1) thermal (Zeldovich) mechanism, which occurs at high temperatures because of the high energy required for dissociation of the ni- trogen molecule (reactions 1, 2, and 3); 2) prompt mechanisms with important intermediate species HCN and NH (reactions 4 and 5); 3) N 2 O mechanism, which involves third body collision (reactions 6, 7, and 9); and 4) fuel-bound nitrogen mechanism, where nitrogen is supplied from the fuel itself. NO can also be removed by reacting with hydrocarbon radicals in rich flame through a process known as “NO reburn” [20], of which some of the key reactions are also listed in Table I. A slight increase of hydrocarbons in a rich flame can lead to a drastic increase in the reduction of NO. Atomic and molecular excited states reactions also play an important role in formation and reduction of NO [21], [22], and related reactions are shown in Table I. In the case where no combustion is involved and there is only a plasma discharge, formation of NO from air varies, depending on the discharge system. For example, reaction 14 can be the principal NO formation pathway in pulsed dis- charge with shorter time scales (10 −8 −10 −6 s) [23], while reaction 16 is shown to be dominant in corona discharges [24]. For a variety of reduced electric fields, different percentages 0093-3813/$26.00 © 2009 IEEE Authorized licensed use limited to: Michigan State University. Downloaded on December 8, 2009 at 14:23 from IEEE Xplore. Restrictions apply.