Methane Slip Summarized: Lab vs. Field Data

ABSTRACT
The interest in gas-fueled marine engines from shipowners’ side has increased tremendously during the last decade. This is not only because of concerns with possible shortage of distillate marine fuels due to enforcement of much stricter global sulphur cap in 2020, but also because of rapid development in gas-engine technology currently being able to prove versatile and efficient propulsion solutions for the global maritime industry.
As more and more experience is gained by the operators using different marine gas engines, more and more understanding of what that can and need to be improved is also acquired. Not considering purely technical issues often related to durability or non-optimal operation of a certain engine component(s), the major concern is often poor efficiency of lean burn spark ignited (LBSI) and lowpressure dual-fuel (LPDF) engines at low loads. These types of gas engines comprise the majority of
currently operated gas engines and the shipowners are often concerned with the increased fuel consumption at low loads resulting in higher operational costs of the vessel. At the same time, engine (environmental) researchers are often much more concerned with high emissions of unburned methane (i.e. unburned fuel), which typically are called methane slip. Emissions of methane are not regulated in international maritime transport, so from regulatory perspective can be ignored by vessel operators. It should be noted that it is widely known that methane has a rather strong greenhouse gas effect and normally is controlled in all on-shore applications. 

Norway is one of the leading nations in terms of acceptance of gas-engine technology for shipping with dozens of gas-driven ships operated by Norwegian companies. At the same time, Norway has one of the strictest emission regulations set for shipping and is constantly investing in development and testing of different emission reduction technologies allowing even further reduction of harmful emissions. Methane slip is not an exception to this rule.
Current article deals with the analysis and comparison of gaseous emission data measured from ship engines in field (on board) with similar data from the laboratory measurements. The main emphasis is paid to methane slip and the data is also compared with that available from the measurements done by the engine manufacturer. Both LBSI and LPDF engines are covered by the study. The reasons for the agreement and (or) disagreement between the collected data sets are transparently discussed, providing the reader with ideas of how the methane slip emissions from marine diesel engines can be further reduced not affecting compliance of the engines with Tier III emission standards.