Shock-tube study of the ignition and product formation of fuel-rich CH4/air and CH4/additive/air mixtures at high pressure

Higher-value chemicals can be produced from methane with small exergy losses by partial oxidation if the chemical conversion proceeds in an internal combustion engine (ICE) as a polygeneration process (Gossler and Deutschmann, 2015). Kinetics models are not sufficiently validated for the very fuelrich and high-pressure conditions relevant for this process. Therefore, ignition delay times of fuel-rich methane/(additive)/air mixtures were measured in a shock tube at about 30 bar and temperatures between 600 and 1650 K. n-heptane and diethylether were used as additives to increase the reactivity of the fuel so that the polygeneration process can be realized in an ICE at HCCI conditions at lower compression temperatures. At phi = 2, measured ignition delay times agree well with simulations using different mechanisms from literature. Synthesis gas (CO, H-2) is the main product at these conditions (Sen et al., 2016). For the production of higher hydrocarbons, the equivalence ratio must be increased. Very fuel-rich mixtures (phi = 10) were used because the temperature increase during the reaction of these mixtures is quite low (<450 K), so that post-ignition temperatures stay below the lower limit of soot formation. Only for mixtures with n-heptane as additive, good agreement of measured and simulated ignition delay times is found. The other mixtures show strong deviations with all mechanisms. As a further parameter to improve and validate the mechanisms at phi = 10, product distributions after ignition were determined by sampling in the cooling phase with a fast-opening valve and GC/MS analysis. Besides H-2 and H2O, CO and higher hydrocarbons like C2H2, C2H4, C2H6, and C6H6 were detected as main products. About half of the carbon of the consumed methane is converted to CO, the other half to higher hydrocarbons. The product distributions are well predicted by simulations. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.