Effects of ozone addition on the kinetics and efficiencies of methane conversion at fuel-rich conditions

Abstract

Compression–expansion processes have the potential of converting mechanical work to chemical energy at fuel-rich conditions, allowing for the storage of fluctuating renewable energies. In this work, the conversion of methane and natural gas (NG) is investigated for this purpose. A focus is on using ozone as a reaction promoter for the otherwise slow reaction. The kinetics of fuel-rich methane/NG oxidation with ozone addition is investigated experimentally and numerically. To this end, ignition delay times (IDTs) for CH4/O2/O3/Ar and NG/O2/O3/Ar mixtures are measured in a rapid compression machine (RCM). It is shown that a reaction mechanism obtained by simply combining a previously developed mechanism for methane conversion (PolyMech2.0) with an ozone sub-mechanism does not accurately predict IDTs. Sensitivity analyses identify reactions in the methane submechanism that become more important for ignition delay time when ozone is added in comparison to mixtures without O3. The rate coefficients of these reactions are modified within their uncertainty ranges to better match the experimentally obtained IDTs. The resulting kinetic model, named PolyMech 3.0, predicts the IDTs obtained in RCM-experiments well. Analysis reveals a two-fold promoting effect of ozone addition on methane/air ignition: Ozone causes a temperature rise by the reactions associated with its decomposition. Ozone also forms reactive products such as hydrogen and oxygen radicals, which can then promote reactions of the hydrocarbons. Quantitative analysis shows that the latter effect is more pronounced. Using PolyMech 3.0, parametric simulation studies for methane conversion in four-stroke engine cycles are carried out to explore the effects of ozone addition on chemical energy storage and efficiencies of engine-based polygeneration processes. Results show that with ozone addition, methane conversion can take place at high engine speeds, while without ozone, there is nearly zero conversion of fuel rich methane mixtures because of the low reactivity. Therefore, ozone addition allows for reasonable efficiencies across a wider range of operating conditions.