Influence of desulfurization strategies for methane gaseous direct injection engine on carbon dioxide emissions

The use of fuels produced with renewable electricity from wind and solar energy and with CO${}_2$ from unavoidable sources or directly captured form the air (so called e-Fuels) is of great interest as a proposition for further limiting the climate impact of road transportation. One of the most efficiently producible e-fuels is e-methane. Feeding methane from renewable sources into the gas grid is one of the most promising pathways to achieve carbon neutral road transportation on a well-to-wheel (WTW) basis. Currently, the use of odorants is mandatory in the gas grid. It is common that sulfur compounds are used as odorants, which can lead to sulfur poisoning of the catalytic converters if an internal combustion engine is operated with it. Consequently, desulfurization will be necessary to maintain high catalyst efficiency over lifetime, which will increase the tank-to-wheel (TTW) CO${}_2$ emissions through increased fuel consumption. For desulfurization, it is necessary to increase the catalyst brick temperature to levels above 800 °C. This paper investigates how such high temperatures can be realized and derive implications on engine operation and gas grid regulation. To this end, experimental studies were conducted with a 1-liter 3-cylinder prototype engine from Ford-Werke GmbH featuring variable intake valve timing, a compression ratio of 14 and a turbocharger with variable turbine geometry (VTG). The engine was operated with gas direct injection at up to 16 bar pressure. The ECU software allowed to apply deliberate oscillations of the lambda signal (“wobbling” of the air/fuel ratio) and cylinder individual air/fuel ratios to achieve a sufficient exhaust aftertreatment. The three-way-catalyst for the investigations were particularly suitable for methane operation due to a high palladium loading and increased oxygen storage capacity of the washcoat. Different load points were used for the investigations, ranging from near idle to medium engine speed and load. The catalyst brick temperature was increased considerably by splitting the mean air/fuel ratio between lean and rich operation on different cylinders (so called “lambda spli”), which is limited by the ignition limits of air/methane charges. Furthermore, too extreme lambda split leads to unstable engine operation. Sufficient hydrocarbon reduction can be achieved at a catalyst brick temperature above 500 °C, which cannot be achieved for near idle load points without additional measures (e.g. electrically heated catalyst). Desulfurization of the catalyst requires brick temperatures above 800 °C and is accordingly not achievable with stable engine operation in a significantly large area of the low load operation conditions. In this case additional heating measures (as e.g. electrically heated catalysts or exhaust burner) or vehicle hybridization are required to avoid low load operating conditions and to comply with the emission targets. Furthermore, desulfurization causes 6 % additional CO${}_2$ emissions in the WLTP cycle for C-segment passenger cars.