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Lean-burn natural gas (NG) engines are used world-wide for both stationary power generation and mobile applications ranging from passenger cars to Class 8 line-haul trucks. With the recent introduction of hydraulic fracturing gas extraction technology and increasing availability of natural gas, these engines are receiving more attention. However, the reduction of unburned hydrocarbon emissions from lean-burn NG and dual-fuel (diesel and natural gas) engines is particularly challenging due to the stability of the predominant short-chain alkane species released (e.g., methane, ethane, and propane). Supported Pd-based oxidation catalysts are generally considered the most active materials for the complete oxidation of low molecular weight alkanes at temperatures typical of lean-burn NG exhaust. However, these catalysts rapidly degrade under realistic exhaust conditions with high water vapor concentrations and traces of sulfur.

Natural gas powered vehicles are attractive in certain applications due to their lower emissions in general than conventional diesel engines and the low cost of natural gas. For stoichiometric natural gas engines, the aftertreatment system typically consists only of a three-way catalyst (TWC). However, increasingly stringent NOx and methane regulations challenge current TWC technologies. In this work, a catalyst reactor system with variable lean/rich switching capability was developed for evaluating TWCs for stoichiometric natural gas engines. The effect of varying frequency and duty-cycle during lean/rich gas switching experiments was measured with a hot-wire anemometer (HWA) due to its high sensitivity to gas thermal properties. A theoretical reactor gas dispersion model was then developed and validated with the HWA measurements. The model is capable of predicting the actual lean/rich gas exposure to the TWC under different testing conditions.

Regulations on methane emissions from lean-burn natural gas (NG) and lean-burn dual fuel (natural gas and diesel) engines are becoming more stringent due to methane’s strong greenhouse effect. Palladium-based oxidation catalysts are typically used for methane reduction due to their relative high reactivity under lean conditions. However, the catalytic activity of these catalysts is inhibited by the water vapor in exhaust and decreases over time from exposure to trace amounts of sulfur. The reduction of deactivated catalysts in a net rich environment is known to be able to regenerate the catalyst. In this work, a multicycle methane light-off & extinction test protocol was first developed to probe the catalyst reactivity and stability under simulated exhaust conditions. Then, the effect of two different regeneration gas compositions, denoted as regen-A and regen-B, was evaluated on a degreened catalyst and a catalyst previously tested on a natural gas engine.

Natural gas-powered engines are widely used due to their low fuel cost and in general their lower emissions than conventional diesel engines. In order to comply with emissions regulations, an aftertreatment system is utilized to treat exhaust from natural gas engines. Stoichiometric burn natural gas engines use three-way catalyst (TWC) technology to simultaneously remove NOx, CO, and hydrocarbon (HC). Removal of methane, one of the major HC emissions from natural gas engines, is difficult due to its high stability, posing a challenge for existing TWC technologies. In this work, degreened (DG), standard bench cycle (SBC)-aged TWC catalysts and a DG Pd-based oxidation catalyst (OC) were evaluated and compared under a variety of lean/rich gas cycling conditions, simulating stoichiometric natural gas engine emissions.