Preferential diffusion effects on flame characteristics in H 2 /CO syngas diffusion flames diluted with CO 2 Jeong Park a, *, Oh Boong Kwon a , Jin Han Yun b , Sang In Keel b , Han Chang Cho c , Sungcho Kim d a School of Mechanical Engineering, Pukyong National University, San 100, Yongdang-dong, Nam-gu, Busan 608-739, Republic of Korea b Environment & Energy Research Division, Korea Institute of Machinery and Materials, 171 Jang-dong, Yuseong-gu, Daejeon 305-343, Republic of Korea c Energy Team, Research Institute of Industrial Science and Technology, #32 Hyoja-dong, Ham-gu,Pohang, Kyungbuk 790-330, Republic of Korea d School of Mechanical & Aerospace Engineering, Sunchon National University, 315 Maegok, Suncheon, Jeonnam 540-742, Republic of Korea article info Article history: Received 9 July 2008 Received in revised form 9 September 2008 Accepted 9 September 2008 Available online 31 October 2008 Keywords: Chemical process H 2 /CO syngas Heat release rate Maximum flame temperature Preferential diffusion Scalar dissipation rate abstract Numerical study is conducted to clarify preferential diffusion effects of H 2 and H on flame characteristics in synthetic diffusion flames of the compositions of 80% H 2 /20% CO and 20% H 2 /80% CO as representatively H 2 -enriched and CO-enriched H 2 /CO flames. Impacts of CO 2 addition to the flames are also examined through the variation of added CO 2 mole fraction from 0 to 0.5. A comparison was made by employing a mixture-averaged diffusivity and the suppression of the diffusivities of H and H 2 . It is found that preferential diffusion effects on maximum flame temperature cannot be explained by the well-known behavior between maximum flame temperature and scalar dissipation rate but by chemical processes. The concrete evidence is also presented through the examination of the behavior of maximum H mole fraction and the behavior of importantly-contributing reaction steps to overall heat release rate. ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. 1. Intoduction Research efforts have been devoted to grasp fundamental flame characteristics and to advance reaction mechanism for H 2 /CO syn-gas mixture flames which are particularly prom- ising prior to a complete hydrogen-economy society [1–8]. Even if the synthetic gas fuels are mainly composed of H 2 and CO, they still contain undesirable by-products such as N 2 , CO 2 , and H 2 O, which are dependent upon the fuel sources and processing techniques. Inclusion of these by-products in H 2 / CO syn-gas mixtures may block designing H 2 /CO syn-gas combustors and reducing their pollutant emissions. These difficulties might be inferred from the previous studies that chemical effects of added H 2 O and CO 2 modified flame characteristics and NOx emissions in methane flames diluted with H 2 O and CO 2 [9–12] and furthermore light chemical species such as H and H 2 also impacted flame characteristics through preferential diffusion effects [13–18]. Furthermore it is likely to be hard for these problems to be solved easily for a while unless the drastically improved efficiency of the production through gasification reduces the contents of by-products. * Corresponding author. Tel.: þ82 51 629 6140; fax: þ82 51 629 6126. E-mail address: jeongpark@pknu.ac.kr (J. Park). Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he 0360-3199/$ – see front matter ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.09.010 international journal of hydrogen energy 33 (2008) 7286–7294