Methane slip from gas fuelled ships: a comprehensive summary based on measurement data

Abstract

Strict NO x emission regulations set for marine vessels by Tier III standard make ship owners/operators finding new efficient methods fulfilling these requirements. Utilization of LNG as main fuel at the moment is one of the most promising solutions with lean burn spark ignited (LBSI) engines and low pressure dual fuel (LPDF) ones being of primary choice. Technology provides not only low NO x levels, but also allows to reduce operational costs due to LNG currently being a cheaper fuel. The main drawback of low-pressure gas engines is rather high levels of methane slip, especially at low loads, as a result of poor fuel utilization due to low operational fuel-air ratios. Nevertheless, there are no standards that directly regulate methane slip for marine gas engines, but the topic starts to receive more and more attention due to the concerns associated with environmental effect of methane as well as due to ship operators analyzing ship data more thoroughly revealing substantial increase in gas fuel consumption at low loads. Presented study summarizes all gas engine technologies that are available for the maritime sector considering their current status and maturity and present a comprehensive measurement data summary for the main groups, namely LBSI and LPDF engines. The measurement data pool consists of both on-board and test-bed emission data revealing an interesting moments such as possible "overtuning" of engines for low NO x resulting in excessive levels of methane slip, importance of on-board measurements due to their more realistic nature, utilization of non-perfections, like

LNG and Cruise Ships, an Easy Way to Fulfil Regulations—Versus the Need for Reducing GHG Emissions

“…In addition, the thermal efficiency tends to be lower for the low-pressure (Otto) dual-fuel engines compared to the diesel option, due to the nature of the Otto process and the methane slip. For more insight into thermal efficiency of dual-fuel engines, see [44].…”

“…The weakness of this approach is that the NO x test assumes that the engine operates at 75-100% power, 70% of the time, and at 25-50% the remaining 30% of the time. The explanation is that, when power decreases below 50%, the methane slip in percent of fuel consumption increases [44,48], which is not representative for typical deep sea vessels nowadays operating at around 50% power load [30,46].…”

“…However, even this value might underestimate the real methane slip, since most of the global fleet today operates at around 50% power [30,46] and not at around 70% given by the NO x test cycle. The explanation is that, when power decreases below 50%, the methane slip in % of fuel consumption increases [44,48]. Table 3 shows the WTW emissions for the seven assessed fuel options based on the WTT emissions, the engine performance and methane slip as described in the two previous sections.…”

Dynamic simulation of fuel tank aging for LNG‐fueled ship apparatus in an X‐DF Otto cycle engine

“…Despite the X‐DF engine has limitations such as the greenhouse gas (GHG) that caused by methane slip, methane number (MN) dependency, it is still valuable in practice . Recently, the propulsion systems of the LNG‐fueled ships and LNG carriers are increasingly using the X‐DF engine supplied by WinGD as a substitute for high‐pressure DF engines because the X‐DF engine complies with the NO x emissions standard of the IMO Tier III without any additional exhaust gas treatment, and it also operates at low pressure .…”

“…When using an X‐DF engine, the methane number (MN) of the fuel gas (FG) is determined by the composition of the LNG, and the MN must be maintained at an appropriate level or a higher level for the gas mode operation. At present, the MN given by the engine manufacturers is 80 or higher to prevent a reduction in the normal engine output …”