Assessing the impact of siloxanes on emissions from lean-burn heavy-duty trucks

Fueling heavy-duty vehicles with renewable biomethane presents considerable environmental benefits over conventional diesel-powered vehicles. However, to ensure the environmental viability of this technology, it is crucial to minimize tailpipe methane slip. Therefore, high performance methane oxidation catalysts (MOCs) are required to control methane emissions, reducing its release into the atmosphere. The performance of MOC is challenged by the highly transient operating conditions encountered during normal vehicle use, as well as the presence of new trace contaminants like siloxanes, which originate from biomethane. These contaminants can poison the catalyst, significantly degrading its effectiveness over time. This paper examines the methane oxidation performance of several laboratory-scale catalysts with varying amounts of siloxane deposition and combines them with a transient engine cycle emission data to evaluate catalyst performance under realistic, transient operating conditions. Building on these findings, the results were scaled to represent siloxane deposition in a real-world heavy-duty truck, and an approximate relationship between siloxane buildup and the mileage required for this deposition was established. Finally, relative increases in methane emissions were calculated as a function of mileage at several different scenarios with varying silica and siloxane deposition efficiencies. The results indicate that siloxane deposition in the MOC may significantly increase methane emissions during the vehicle lifetime. This increase ranged from a few percents to 3-fold depending on the considered scenario of MOC poisoning, indicating a strong need to investigate real-life accumulation of siloxane compounds in catalysts when using biomethane as a heavy-duty fuel. The motorway segment of the driving cycle was the most sensitive to poisoning. Furthermore, siloxane and silica deposition exhibited distinct poisoning dynamics, with siloxane having a more pronounced impact at lower vehicle mileage, while the effect of silica intensified later in the vehicle's lifetime. This study shows that siloxane impurities may substantially increase greenhouse gas emissions of heavy-duty vehicles fueled with biomethane over the vehicle life cycle. • Effects of siloxane and silica poisons on methane emissions from heavy duty engines were evaluated • Measured methane conversions of poisoned MOCs were combined with engine dynamometer data to estimate methane emissions in transient conditions. • Results were scaled to reflect real heavy-duty applications, establishing a rough relationship between poison load and traveled mileage. • Nine scenarios with varying siloxane/silicon deposition efficiencies were constructed. • Both siloxane and silica poisons caused significant increases in methane emissions over the vehicle lifetime in most scenarios. • Despite increased methane emissions, the vehicle would still meet in-service conformity test requirements in the majority of scenarios.