Session: 04-13: Kinetics

Paper Number: 82305

82305 - The Ignition of C1–C7 Natural Gas Blends and the Effect of Hydrogen Addition in the Low and High Temperature Regimes 

New ignition delay time (IDT) measurements for two natural gas (NG) blends composed of C₁ – C₇ n-alkanes, NG6 (60.625%, 20%, 10%, 5%, 2.5%, 1.25%, 0.625%) and NG7 (72.635%, 10%, 6.667%, 4.444%, 2.965%, 1.976%, 1.317%) by volume with methane as the major component are presented. The measurements were recorded using a high-pressure shock tube (HPST) for stoichiometric fuel in air mixtures at reflected shock pressures (p₅) of 20 – 30 bar and at temperatures (T₅) of 987 – 1420 K. The current results together with the rapid compression machine (RCM) measurements in the literature show that higher concentrations of the higher n-alkanes (C₄ – C₇) ~1.327% in the NG7 blend compared to the NG6 blend result in the ignition for NG7 being almost two times faster than NG6 at compressed temperatures (T_C) ≤ 1000 K due to the low-temperature chain branching reactions that occur for higher alkanes oxidation kinetics in this temperature range. On the contrary, at T_C > 1000 K, NG6 exhibits ~20% faster ignition than NG7 primarily because about 12% of the methane in the NG7 blend is primarily replaced by ethane (~10%) in NG6, which is significantly more reactive than methane at these higher temperatures. The performance of NUIGMech1.2 in simulating these data is assessed and it can reproduce the experiments within 20% for all the conditions considered in the study. We also investigate the effect of hydrogen addition to the autoignition of these NG blends using NUIGMech1.2 which has been validated against the existing literature for natural gas/hydrogen blends. The results demonstrate that hydrogen addition has both an inhibiting and promoting effect in the low- and high-temperatures regime, respectively. Sensitivity analyses of the hydrogen/NG mixtures are performed to understand the underlying kinetics controlling these opposite ignition effects. At low temperatures, H-atom abstraction by ȮH radicals from C₃ and larger fuels are the key reactions consuming the fuel and providing the necessary fuel radicals which undergo low-temperature chemistry (LTC) leading to ignition. However, with the addition of hydrogen to the fuel mixture, the competition for ȮH radicals by H₂ via the reaction H₂+ȮH↔Ḣ+H₂O reduces the progress of the LTC of the higher hydrocarbon fuels thereby slowing down ignition. At higher temperatures, since Ḣ+O₂↔Ö+ȮH is the most sensitive reaction promoting reactivity, the higher concentrations of H₂ in the fuel mixture leads to higher Ḣ atom concentrations leading to faster ignition due to an enhanced rate of the Ḣ+O₂↔Ö+ȮH reaction.

Presenting Author: Ahmed Mohamed National University of Ireland Galway

Presenting Author Biography: Ahmed is a Ph.D. student at the Combustion Chemistry Centre, NUI Galway. He received the BSc in Mechanical Engineering from Assiut University Egypt in 2011, and a master’s degree in Mechanical Engineering from Assiut University in 2017. Ahmed was working as a research and teaching assistant at Assiut university from March 2013 to May 2017 and a lecturer assistant after getting his master&#39;s from June 2017 to June 2018.<br/>His primary field of study at Assiut University was improving air motion in Internal Combustion Engines cylinder.<br/>His primary field of study at NUI Galway is to study the auto-ignition characteristics of the C1–C7 n-alkane mixture with methane-based fuel as Natural Gas (NG) as well as the effect of NOx addition to n-alkane and C1–C7 n-alkane using a rapid compression machine (RCM) and shock tube (ST). Moreover, using these data for developing chemical kinetic mechanisms.

Authors:

Ahmed Mohamed National University of Ireland Galway
Amrit Bikram Sahu National University of Ireland Galway
Snehasish Panigrahy National University of Ireland Galway
Gilles Bourque Siemens Energy Canada Ltd.,
Henry Curran National University of Ireland Galway