Modeling of Three Way Catalyst Behavior Under Steady and Transient Operations in a Stoichiometric Natural Gas Fueled Engine 2021-24-0074

Methane abatement in the exhaust gas of natural gas engines is much more challenging in respect to the oxidation of other higher order hydrocarbons. Under steady state λ sweep, the methane conversion efficiency is high at exact stoichiometric, and decreases steeply under both slightly rich and slightly lean conditions. Transient lean to rich transitions can improve methane conversion at the rich side. Previous experimental work has attributed the enhanced methane conversion to activation of methane steam reforming. The steam reforming rate, however, attenuates over time and the methane conversion rate gradually converges to the low steady state values. In this work, a reactor model is established to predict steady state and transient transition characteristics of a three-way catalyst (TWC) mounted in the exhaust of a natural gas heavy-duty engine. In total 17 global reactions are selected and implemented in the reactor model, including oxidation reactions, NO_x related reactions, water gas shift and steam reforming, and ceria oxygen storage related reactions. Inhibition factors are introduced to model the competition of different species for active sites. To model the transient enhancement of methane conversion, an activation and deactivation mechanism of methane steam reforming based on active ceria sites is proposed and developed. Both lean to rich and rich to lean λ transitions are simulated and compared with engine experiment results. The proposed model captures key characteristic behaviors of the TWC in both steady state λ sweep and transient λ transitions. The results provide insights in TWC reaction mechanisms at near stoichiometric operation and can be used as a starting point to predict the effect of dedicated control strategies.