A combination effect of reburn, post-flame air and acoustic excitation on NO x reduction Mohamed I. Hassan a,⇑ , T.W. Wu b , Kozo Saito b a Masdar Institute of Science and Technology, Abu Dhabi, United Arab Emirates b Mechanical Engineering Department, University of Kentucky, Lexington, KY, USA highlights " We studied the effect of acoustic on NO x generation in premixed combustion. " We studied the combination effect of reburn, post-flame air, and acoustic. " Using reburn with post air and acoustic reduced the NO x production significantly. " Acoustic wave itself was found to have insignificant effects on NO x below 1500 ppm. " The 50 Hz acoustic wave had the most significant effect on NO x and CO reduction. article info Article history: Received 23 February 2012 Received in revised form 13 November 2012 Accepted 13 February 2013 Available online 13 March 2013 Keywords: Post-flame air with acoustic NO x reduction Reburn with acoustic abstract The effect of applying acoustic excitation on generated NO x and CO within a combustion tube burning a premixed propane flame was investigated during reburn with and without post air injection. The labora- tory-scale, acoustically-enhanced combustion apparatus consisted of a vertically oriented combustion tube with a horizontally oriented acoustic excitation tube. A propane–air premixed burner was located at the bottom of the combustion tube. In order to study the formation of fuel NO x , the propane fuel line was injected with a small amount of ammonia prior to the mixing with combustion air. Methane and sec- ondary air were injected into the combustion tube for reburn and post air, respectively. Gas sampling was made at 20 cm above the center of the acoustic exciter tube by using a vacuum sampling probe; concen- trations of NO x , CO, CO 2 ,O 2 and total hydrocarbons were analyzed by using an on-line gas analyzer. A series of combustion experiments were conducted to investigate the effects of the following three param- eters on NO x concentration: (1) frequency of the acoustic wave, (2) methane reburn with post air injec- tion, and (3) the amount of injected post-flame air. The largest reduction in the NO x concentration was 95%, and was achieved by a combination of acoustic wave excitation with reburn and post air injection. The second largest NO x reduction, 90%, was obtained with acoustic wave excitation and post air injection. The third largest NO x concentration reduction, 65%, was achieved by using methane reburn regardless of acoustic wave excitation. The combination of the acoustic excitation with the post-flame air injection was found to produce the best result in this study. Ó 2013 Elsevier Ltd. All rights reserved. 1. Background There is a rich literature in research and technology development on NO x reduction using reburn of hydrocarbon fuels downstream of the combustion zone to establish a fuel-rich zone to convert nitric oxides to HCN [1]. Reburn technology has been ap- plied commercially using reburn fuels in the range of 10–30% of the combustion fuel and is known to reduce emission of nitric oxi- des by 30–65% depending on the boiler size [1]. In this study, we focused on acoustic excitation effects on NO x reduction with and without reburn. Refs. [2–7] are relevant to our current study. Among them, the three studies [2–4] are the most relevant to our research; thereby we summarize their results. McQuay et al. [2] and Carvalho et al. [3] studied diffusion flame- produced NO x using a Rijke tube and found that the temperature fluctuation was significantly decreased due to acoustic pulses ap- plied at two different levels within the tube. Poppe et al. [4] showed that, by increasing the pressure fluctuation, NO x emission decreased by 20–30% as a function of the pressure root mean square value, rms. The pressure fluctuation was created by an acoustic wave in a horizontally-oriented, long tube with a 0016-2361/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2013.02.032 ⇑ Corresponding author. Tel.: +971 2 810 9332. E-mail address: miali@masdar.ac.ae (M.I. Hassan). Fuel 108 (2013) 231–237 Contents lists available at SciVerse ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel