Reducing methane emissions onboard vessels
This paper is the second in the Onboard Vessel Solutions series:
Vessel Emission Reduction Technologies & Solutions
The paper series covers the impact and role of vessel greenhouse gas and air pollutant emission reduction in maturing alternative fuel pathways. Onboard impact is defined in terms of tank-to-wake global warming potential with the role of onboard emission reduction either being for regulatory compliance or as an option to reduce emissions. Fuel pathway maturity is an assessment of solution readiness across the entire value chain.
Based on identified vessel emission risks, the paper series deep dives into specific emissions that need to be addressed to increase alternative fuel pathway maturity. The objective of these deep dives is to understand current or potential emission levels, set reduction targets, and identify and map applicable technologies and solutions. Emission reduction potential is then determined, and recommendations given to mature the selected fuel pathways. Finally, areas or concepts for further research and development are identified including recommended future project topics.
Papers are based on work completed as part of Center projects and working groups consisting of Center partners and external participants and contributors. Working groups provide a collaborative framework facilitated by the Center to jointly engage partners and external experts and companies on specific topics to deliver clear and impactful results.
Onboard vessel methane emissions
Reduction technologies and solutions
The HP2st engine has such low baseline methane slip levels that an application of an after-treatment technology solely for the purpose of reducing methane slip would in most cases lead to a worse overall impact on total GHG emissions.
The most effective way to reduce methane slip on a LP2st engine would be a combination of EGR and PRS. The additional energy required to operate the PRS increases the total GHG emissions. The use of an EGR for a LP2st engine is the solution that requires less additional energy, however, only up to a 48% reduction can be achieved. Most of recent newbuilds with LP2st main engines are to be equipped with an EGR system.
LP4st engines were found to have the highest methane slip emissions. Both after-treatment technologies (MOC and PRS) that can be applied to this type of engine are in developmental stages and only lab test results are available. Depending on the baseline emissions level of a LP4st, with the use of PRS, it is claimed that 50-70% (conservative scenario) up to 78% (optimistic scenario) methane slip reduction is possible. A MOC can achieve a methane conversion from 70% (conservative scenario) up to 99% (optimistic scenario) when it is placed upstream of the turbocharger. When comparing the two solutions in terms of total GHG emissions contribution, the catalyst, having the advantage of needing no additional energy to operate, can achieve lower total GHG emissions. The performance of the catalyst is, however, dependent on the exhaust gas temperature and sulfur content.
Vessel-level methane emissions
Overall, methane slip levels are higher for the vessel configurations with an LP2st ME than an HP2st ME (7.0 gCO2eq/MJ versus 4.6 for the LR2 tanker and 8.8 versus 4.5 for the LNG carrier). This difference can mainly be attributed to the methane slip from the ME. For the LP2st ME configuration, the ME contributes more than 50% of the total methane slip while at sea. For the HPs2t ME configuration, the ME contributes less than 20% of the total methane slip.
In the vessels with an LP2st ME, methane slip is mainly emitted in sailing mode when almost all emissions are from the ME. This indicates that the baseline ME technology selection will be a primary driver in reducing total methane emissions. As sailing days represent the largest amount of time in the operational profile, in the HP2st cases, it was found that LP4st engines are the biggest methane slip contributor.
Vessel-level methane reduction potential
Based on the overall techno-economic analysis, onboard methane emission reduction technologies can provide a cost-effective way to reduce methane emissions. Onboard methane emission reduction technologies are also cost competitive with the use of alternative fuels like bio-methane in reducing overall GHG emissions of a vessel up to a certain point. For a newbuild, in addition to the selection of the baseline main engine technology, system solutions like shaft generators and preparation for shore power are recommended as they provide meaningful emission reduction at low abatement costs. Higher abatement costs should be expected for retrofits, as well as more limited options. Installation of methane catalysts on LP4st auxiliary engines present an efficient way to reduce emissions in retrofits.
Related projects and future development areas