Methane slip emissions from LNG vessels: review

The International Maritime Organization (IMO) regulations on emissions of nitrogen oxides (NOx) and sulphur oxides (SOx) have lead to increased utilization of liquefied natural gas (LNG) as a fuel for shipping. Due to very low sulphur content in LNG, the contribution to SOx emissions is negligible. NOx emissions depend on the engine combustion cycle and with LNG engines utilizing otto-cycle (or diesel cycle engines with post combustion treatment) also the strict IMO Tier III NOx limit can be achieved. In addition, it is shown that LNG utilization leads to significantly lower particle emissions compared to liquid marine fuels. Thus, LNG utilization has direct effects and indeed benefits on air quality and human health. Moreover, CO2 emission can be reduced with LNG use compared to diesel fuels, since LNG is mainly composed of methane with a higher H/C ratio compared to diesel. The hydrocarbon emissions, on the other hand, are higher with LNG compared to diesel fuels and mostly include the main component of LNG, methane. This ‘methane slip’ should be minimized because methane is a
strong greenhouse gas and reduces the benefit of lower CO2 emissions. While the formation of methane slip is known to result from LNG combustion, there has been a lack of knowledge of the methane slip emission’s magnitude from the LNG engines. In this review paper, methane slip values are collected from the current literature and ship owner data is utilized to complement the data with engines from recent years. This will contribute to understanding the methane slip from the current LNG fleet. High-pressure 2-stroke slow speed (diesel cycle) engines already show very little methane slip today, while higher methane slip values are reported for low-pressure dual fuel engines. Out of 614 vessels with an identified LNG engine, the low-pressure dual fuel concept (either 4-S or 2-S) is also the most popular LNG engine technology found in 78.1% of the ships, while high-pressure dual fuel technology is found in 14.8% of the ships and lean burn spark ignited engines in 1.7%. This is reflected in the amount of methane slip data found in the scientific literature which focuses on low pressure dual-fuel-engines. The engine load has a significant effect on the methane slip formation. In general, the lower loads tend to increase the methane slip formation compared to higher engine loads.