Autoignition enhancement of methane by admixture of low fraction of acetaldehyde: Simulations and RCM experiments in stoichiometric and rich mixtures

The effect of small fractions of acetaldehyde (CH₃CHO) on the ignition delay time of methane (CH₄) was examined at high pressure. Measurements are reported for the ignition delay time obtained in a rapid compression machine (RCM) at a compression pressure (P_c) of ∼60 bar and temperatures after compression (T_c) in the range 750–900 K for fuel-air equivalence ratios ϕ in the range 1–4. The results show that mixtures of 2%–5% CH₃CHO in CH₄ ignite under conditions at which pure methane does not ignite experimentally. The efficiency of acetaldehyde as a promoter seems to be comparable to that of other oxygenated fuels like alcohols and ethers. For comparison with the experimental results, ignition delay times are computed using an updated reaction mechanism and two mechanisms from the literature for CH₃CHO oxidation. For most conditions, the simulations using the current mechanism agree with the measurements to within a factor of two. The ignition profile shows a pre-ignition temperature rise and two-stage ignition similar to that previously observed in low fractions of dimethyl ether in ammonia; both phenomena are captured by the simulations. Analysis of simulations at constant volume indicates that CH₃CHO is oxidized much more rapidly than CH₄, producing reactive species that initiate the oxidation of CH₄ and generates heat that accelerates oxidation toward ignition. The low-temperature chain-branching reactions of CH₃CHO are important in the early oxidation of the fuel mixture. Additional simulations were performed for equivalence ratios of ϕ = 1 and 4, at a compression pressure (P_c) of 100 bar and T_c = 750–1000 K. The simulations indicate that CH₃CHO has a strong ignition-enhancing effect on CH₄, with small fractions reducing the ignition delay time by up to a factor of 100, depending on the temperature, as compared to pure CH₄.