Impact of Siloxanes in Biomethane on the Performance of a CNG Vehicle

In this article, the results of experiments to determine the effects of silicon-containing compounds in biogas on the performance of spark-ignited gas engines for use in CNG vehicles are presented. Initial research was performed on micro-CHP units, which have many features common with automotive engines, to identify engine components sensitive for silica deposition prior to investigating a practical CNG engine. The experiments on the micro-CHP units revealed that the catalyst was the most sensitive part for silica fouling, with strong impact on the reduction of NOx. With the insight gained from these experiments, an 9-week endurance test was performed on a light-duty CNG vehicle. While the wideband-type lambda sensor originally installed upstream of the catalyst did not fail during the test, an additional switching-type lambda sensor positioned upstream of the catalyst was found to be most sensitive to silica deposition, causing a false signal regarding the oxygen content in the exhaust gas (“failure”). In contrast to the micro-CHP test, the catalyst used in the CNG vehicle was not affected by silica deposition under the experimental conditions used. Micro-CHP experiments revealed that different catalyst types can show different sensitivity towards silica deposition, which explain the relative robustness of the vehicle catalyst regarding silica deposition. Investigation of the performance of the different catalysts used in the market for CNG vehicles is required to quantify the potential impact of siloxanes in biomethane on the installed fleet.