Identification of combustion intermediates in a low-pressure premixed laminar 2,5-dimethylfuran/oxygen/argon flame with tunable synchrotron photoionization Xuesong Wu a , Zuohua Huang a, * , Tao Yuan b , Kuiwen Zhang b , Lixia Wei a, * a State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China b National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, People’s Republic of China article info Article history: Received 17 September 2008 Received in revised form 9 January 2009 Accepted 2 April 2009 Available online 8 May 2009 Keywords: 2,5-Dimethylfuran Low-pressure premixed laminar flame Combustion intermediates abstract Low-pressure (4.0 kPa) premixed laminar 2,5-dimethylfuran (DMF)/oxygen/argon flame with an equiva- lence ratio of 2.0 was studied with tunable vacuum ultraviolet (VUV) synchrotron radiation photoioniza- tion and molecular-beam mass spectrometry. Photoionization mass spectra of DMF/O 2 /Ar flame were recorded and the photoionization efficiency curves of the combustion intermediates were measured. Flame species, including isomeric intermediates, are identified by comparing the measured ionization energies with those reported in literatures or those calculated with Gaussian-3 procedure. More than 70 species have been detected, including furan and its derivatives, aromatics, and free radicals. Possible reaction pathways of DMF, 2-methylfuran, and furan are proposed based on the intermediates identified. DMF can be consumed by H-abstraction and pyrolysis reactions. 2-Methylfuran and furan can be con- sumed by H-abstraction, H-addition and pyrolysis reactions. Ó 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved. 1. Introduction The world is presently confronted with the twin crises of fos- sil fuel depletion and environmental degradation. Investigation on biofuel is important from the perspective of both the energy sources and the environment. Biofuel is a renewable resource and is thought to hold a promising future [1–4]. Ethanol, the only renewable liquid fuel currently produced in large quantities, suffers from several limitations, including low energy density, high volatility, and high energy consumption for production. Seeking for the substitute for ethanol has become the focus of the world research activities. Recently, Zhao et al. have studied the production of 5-hydroxymethylfurfural (HMF) from sugars [5]. They converted glucose to fructose, and then converted fruc- tose to HMF with metal chloride as catalyst in ionic liquid sol- vents. Román–Leshkov et al. have developed a two-stage process to convert the biomass-derived sugar into 2,5-dimethyl- furan (DMF) [6]. In the first stage, sugar was converted to HMF with an acid catalyst in the solution of a low-boiling-point sol- ute. Then HMF was converted to DMF over a copper-based cat- alyst in the second stage. Mascal et al. have converted cellulose to 5-(chloromethyl)furfural, which can be hydrogenised to DMF [7]. These researches make large scale production of DMF possible. Comparing with ethanol, DMF has a higher energy density (by 40 per cent), a higher boiling point (by 20 K), and a higher research octane number (RON). DMF is insoluble in water, thus it will not pollute the groundwater as methyl t-butyl ether (MTBE) does. The above advantages make DMF an attractive alternative for ethanol and octane improver. While before the commercial application of DMF as a fuel, its combustion charac- teristics need to be investigated. Up to now, most researches were focused on the thermal decomposition of DMF. Grela et al. studied the very low pressure pyrolysis of furan, 2-methylfuran, and DMF over the temperature range of 1050–1270 K [8]. The reactant molecules were heated in a steady flow reactor and the products were analyzed by an on- line mass spectrometer. The overall pyrolysis was determined by the decay of the parent ion intensity at M/Z = 68 (furan), 82 (2- methylfuran), and 96 (DMF). They found that loss of carbon monoxide was either the exclusive process or the major one in the decompositions of furan and 2-methylfuran or DMF. Thermal decomposition kinetics of furan (1060–1260 K), 2-methylfuran (1100–1400 K) and DMF (1070–1370 K) have been studied by Lifshitz et al. [9–11]. For DMF, they have established a reaction scheme composed of 50 species and some 180 elementary reac- tions to account for the product distribution over the tempera- ture range. They concluded that a large number of products were formed from the rupture of the furan ring and the consec- utive free radical reactions. They inferred that a methyl migra- tion from C(2) to C(3) and the consecutive elimination of CO would produce four isomers of C 5 H 8 in the unimolecular pro- cesses. An additional unimolecular decomposition process of 2,5-dimethylfuran to CH 3 CO and C 4 H 5 was also proposed. 0010-2180/$ - see front matter Ó 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.combustflame.2009.04.002 * Corresponding authors. Fax: +86 029 82668789. E-mail addresses: zhhuang@mail.xjtu.edu.cn (Z. Huang), feilix@ustc.edu (L. Wei). Combustion and Flame 156 (2009) 1365–1376 Contents lists available at ScienceDirect Combustion and Flame journal homepage: www.elsevier.com/locate/combustflame