Natural gas is less environmentally harmful than conventional liquid fuels when combusted in engines due to its high hydrogen to carbon ratio and lower propensity for soot formation. However, low reactivity of the main component, methane, prevents its use in high efficiency compression ignition combustion modes. Fuel modification to increase methane reactivity can be beneficial by increasing engine efficiency while enabling the use of less carbon intensive fuel. In this study, catalytic oxidative coupling of methane (OCM) was investigated as a means for fuel pre-treatment to convert a part of the fuel into more reactive C2 species like ethane and ethylene. Multi-disciplinary research consisting of a benchtop reactor experiment, computational study of ignition delay time, and engine experiments were carried out to better understand the concept’s feasibility. The OCM benchtop experiment was conducted in the temperature range of 580 - 600°C and molar methane-to-oxygen ratio was varied from 6 to 12. Results demonstrated that 10% methane conversion and about 50% C2 selectivity can be achieved at low temperature using both Sr/La2O3 catalyst particles and a wash-coated monolith. The highest C2 yield of 10% was achieved using catalyst particles at 600°C and methane-to-oxygen ratio of 6. A computational study of combustion reactivity showed that the ignition delay was reduced significantly with the addition of C2 species and to a lesser extent when other species such as nitrogen and carbon dioxide are present. Engine experiments were conducted using a Cooperative Fuel Research (CFR) engine modified for gaseous fuel supply. The results showed that the fuel pretreatment by OCM is disadvantageous for spark-ignition combustion as the diluents in the reactor products decreased laminar flame speed, and consequently reduced thermal efficiency. However, OCM products were advantageous for operation in lean-burn homogeneous charge compression ignition (HCCI) mode due to the increased fuel reactivity. The increased fuel reactivity expanded the operating range significantly, lowering both the intake charge temperature and the stoichiometric ratio required. Integration of an OCM reactor with the engine system showed a limited but apparent advantage of the modified fuel. Indicated thermal efficiency (ITE) reached over 40% with C2 species addition and over 38% with full OCM product species at 900 rpm. Both were significantly higher than the ITE of 31% for pure methane.

Fuel modification using oxidative coupling of methane for natural gas engines

ACS Spring 2022

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