In the HDGAS project a dual fuel HD engine is being developed that combines diesel and liquefied natural gas. Part of this effort is the development and demonstration of the corresponding exhaust gas aftertreatment system. This aftertreatment system will reduce the tailpipe emission of – primarily – particulate matter, nitrogen oxides and methane to EURO VI levels. For this, a system has been designed that combines a state-of-the-art diesel oxidation catalyst (DOC) and a particulate matter (PM) filter with newly formulated catalysts for NOx-reduction (SCR) and methane oxidation (MOC).
At the start of the project it was realized that the long term efficiency of such aftertreatment system needed attention. Current MOC have difficulty to maintain sufficient methane conversion efficiency over the vehicle lifetime. In dual fuel application, good durability is further compromised by the presence of sulphur (S) in both the diesel and natural gas fuels as well as in the lubricant oil. This sulphur poisons the MOC and special actions are needed to regenerate it. Finally, there was no experience with the impact of liquefied natural gas on the long term efficiency of an SCR catalyst. Given these concerns, it was decided at the outset of the HDGAS project to submit full scale versions of the new catalysts to accelerated ageing experiments. This work has been done by TNO with the support of DINEX.
It was decided to submit the catalytic subsystems to the exhaust gas of a HD engine that is modified such that exhaust gas temperature, methane content and space velocity mimic that of the anticipated HDGAS dual-fuel demo engine. It was also decided to separate the investigation of S-poisoning from the investigation of other (mainly thermal) deactivation processes. For this reason, during the latter investigation, only sulphur free diesel was used and diesel to gas blend ratios were high. Also, for lack of a standardized test procedure, a dedicated test method has been defined. The final method selected is similar to non-standard tests with burner test rigs or with engine test rigs that have been used before for accelerated thermal ageing tests on diesel SCR systems. In these tests, as in the present method, the aftertreatment system is subject to temperatures that considerably exceed the highest temperature levels found in real-world application.
In the present tests, ageing temperatures went up to 625⁰C. At regular intervals of (typically) 16 hours the performance of the MOC and SCR catalysts was examined over a wide temperature range. The de-greened prototype EAS was first subject to the above test. Next S-poisoning and regeneration tests were performed. For S-poisoning the engine was run in dual-fuel mode with a specially prepared diesel fuel (containing an increased levels of sulphur).
The outcome of the tests have been used to assess the validity of the results obtained in prior laboratory scale testing.
Further, the tests confirm the well-known impact of H2O but also seem to indicate the existence of other than S-linked irreversible degradation. Finally the effects of S-poisoning on MOC were found to be to a large degree reversible and no significant long term effects on SCR conversion efficiency were measured.
View the report: Ageing impact report by TNO (Public Summary)