Investigation into the Influence of the Ethanol Concentration on the Flame Structure and Soot Precursor Formation of the n‑Heptane/Ethanol Premixed Laminar Flame

• Published in: Energy & fuels 2018-04, Vol.32 (4), p.4732-4746
• Main Authors: Li, Runzhao, Liu, Zhongchang, Han, Yongqiang, Tan, Manzhi, Xu, Yun, Tian, Jing, Yan, Jiayao, Yu, Xiangfeng, Liu, Jiahui, Chai, Jiahong
• Format: Article
• Language: English
• ISSN: 0887-0624; 1520-5029
• DOI: 10.1021/acs.energyfuels.7b04076

The purpose of this work is to investigate the effect of the ethanol blending ratio on the flame structure (species concentrations, heat release rate, and flame temperature profile) and soot precursor (CH3, C2H2, aC3H5, and C3H3) formation of the n-heptane/ethanol premixed laminar flame at the equivalence ratio of 2 and pressure of 3.861 MPa. The results indicate the following: First, n-heptane consumes more rapidly than ethanol for its high dehydrogenation reaction rate under the research temperature range. n-Heptane decomposes completely (over 99% n-heptane is consumed) at about 0.723–0.839 mm above the burner surface, while that of ethanol (over 99% ethanol is consumed) ranges from 1.657 to 8.021 mm. Second, the ethanol addition facilitates the reaction sequences of CH2O → HCO → CO → CO2 by providing reactive radicals, such as O, OH, and H. Third, the thickness of the reaction zone increases by 2.34% when the ethanol blending ratio grows from 0 to 25% because the dehydrogenation reaction rates of ethanol by H/OH radicals are slower than that from n-heptane at the studied flame temperature. However, it decreases by 0.585, 7.018, and 23.0021% at the ethanol blending ratios of 50, 75, and 100% compared to pure n-heptane as a result of the low-temperature heat release inhibition by ethanol. Fourth, even though the ethanol addition reduces the aC3H5/C3H3 and C2H2 formation, their principles are different. The ethanol addition has no direct impact on the reaction flux of the aC3H5 and C3H3 formation, and their reductions are caused by replacing n-heptane with ethanol. On the contrary, the C2H2 reduction with ethanol addition is attributed to the increasing active radical (such as CH2O, OH, and H) concentration. However, the existence of a high concentration of CH3 and sequential reactions of CH3 → sC3H5CO → sC3H5 → C2H2 also facilitate the acetylene formation. Therefore, the decline of acetylene is about 12–43% lower than aC3H5 and C3H3 at the same ethanol blending ratio. The CH3 concentration increases through C2H5OH → CH4 → CH3 with ethanol addition.