Methane Conversion in Stoichiometric Natural Gas Engine Exhaust

Stoichiometric natural gas (CNG) engines are an attractive solution for heavy-duty vehicles considering their inherent advantage in emitting lower CO₂ emissions compared to their Diesel counterparts. Additionally, their aftertreatment system can be simpler and less costly as NOx reduction is handled simultaneously with CO/HC oxidation by a Three-Way Catalyst (TWC). The conversion of methane over a TWC shows a complex behavior, significantly different than non-methane hydrocarbons in stoichiometric gasoline engines. Its performance is maximized in a narrow A/F window and is strongly affected by the lean/rich cycling frequency. Experimental and simulation results indicate that lean-mode efficiency is governed by the palladium’s oxidation state while rich conversion is governed by the gradual formation of carbonaceous compounds which temporarily deactivate the active materials. Lean/rich cycling around stoichiometry enables a higher CH₄ oxidation as the oxygen storage seems to balance the individual effects of Pd oxidation and rich deactivation. In this work, the catalytic reaction mechanisms involved in CH₄, CO and NOx conversion were studied by means of a multi-scale experimental campaign and mathematical modeling. Initially, a detailed kinetic study was performed on the synthetic-gas bench to understand the underlying phenomena and formulate the appropriate reaction mechanisms. The model was then evaluated under transient reactor experiments while final validation was performed against driving cycle measurements on the engine bench.