Natural Gas Partial Oxidation and Reforming for Lean NOx Trap Catalysis Regeneration

Program goals for the Advanced Reciprocating Engine Systems (ARES) program of the Department of Energy include efficiency and environmental goals. Lean-burn natural gas engines offer higher efficiency than engines that operate with Stoichiometric air-to-fuel mixtures; however, the excess oxygen in the exhaust of lean engines makes NOx reduction with catalytic aftertreatment difficult. Thus, advancing efficiency via lean combustion results in challenges to meet environmental goals. The lean NOx trap catalyst technology is capable of reducing NOx in lean exhaust and, thereby, enables the potential for lean combustion to meet both efficiency and environmental goals. During lean NOx trap catalysis, NOx in oxygen-rich exhaust is trapped on the catalyst by alkali or alkaline earth-based sorbate materials; then, upon exposure to oxygen-depleted exhaust, the NOx is released and reduced to nitrogen in a process called regeneration. The regeneration process renews the catalyst for more NOx trapping; the cyclic process repeats at periods on the order of a minute. Oxygen depletion during regeneration is accomplished by temporarily operating the catalyst at rich air-to-fuel ratios; traditionally, a variety of methods have been utilized to achieve rich conditions for the catalyst. In this presentation, research of a lean NOx trap on a lean natural gas engine will be presented. Natural gas from the engine supply was used to provide the reductant for the lean NOx trap regeneration process. The natural gas is injected into the exhaust system where oxidation and reforming catalysts partially oxidize and/or reform the natural gas into reductants suitable for lean NOx trap regeneration. Studies of the natural gas oxidation and reforming processes and their relation to NOx reduction performance will be presented.