Reducing methane emissions onboard vessels

Liquefied electro- and bio-methane have been identified by the Mærsk Mc-Kinney Møller Center for Zero Carbon Shipping (MMMCZCS) as potential low-emission alternative fuel pathways. In addition, given the rapid expansion of the liquified natural gas (LNG)-fueled vessel fleet and industry projections, the use of LNG as a fuel for the maritime industry will continue well into the future. The use of these methane-based fuels, however, present both a regulatory compliance and climate risk related to onboard vessel methane emissions, in particular methane slip from internal combustion engines, that increases a vessel’s overall CO2-equalivent (CO2-eq) emissions.

Potential and upcoming regulation of onboard vessel methane emissions presents a risk for methane-based fueled vessel owners, operators, and charterers. Currently, there are no international regulations on methane emissions from vessels, however, ongoing initiatives and regional guidelines indicate that regulations are highly likely to appear soon. The FuelEU for Maritime regulation, for example, will include methane slip in its CO2-eq methodology.

While CO2 is the main source of shipping’s climate impact with over 90% of total greenhouse gas (GHG) emissions, methane has a higher climate impact in terms of global warming potential (GWP). As a result, methane emission reduction can be an efficient way to reduce a vessel’s overall CO2-eq emissions, allowing compliance with upcoming regulations and increasing the viability and competitiveness of methane-based alternative fuel pathways.

A dedicated MMMCZCS working group was established to study reducing methane emissions onboard vessels, which is one vessel specific emission-related consideration for methane-based alternative fuel pathways. Based on its results, the following conclusions have been made:

• A vessel’s total methane emissions should be considered: While the main source of onboard vessel methane emissions is methane slip from main and auxiliary internal combustion engines, total methane emissions of a vessel is highly dependent on a vessel’s operations, system dimensioning, machinery configurations and connected technologies. In addition to selecting baseline engine and potential after-treatment technologies, system solutions can significantly reduce onboard vessel methane emissions.
• Cost-efficient onboard vessel methane emission reduction is possible but limited for existing vessels: For the vessels studied, onboard methane emissions can be cost-efficiently reduced by 40-80% for a newbuild and 20-50% for an existing vessel through the selection of baseline engine technologies and the use of after-treatment technologies and system solutions. These reductions translate to onboard methane emissions being reduced from 7-14% of total tank-to-wake (TTW) GHG emissions to 2-8% for a newbuild and 4-12% for an existing vessel. Cost efficiency is considered as being less than the forecasted cost of bio-methane and is associated with CO₂-eq abatement cost of less than about $200/tonCO₂-eq. Ship owners should carefully consider onboard methane emission reduction at the newbuild phase to avoid potential costly modifications later in the vessel’s lifetime. While it is technically feasible to further reduce onboard vessel methane emissions beyond these levels, utilizing other options like the use of low-emission fuels could be more cost-efficient if further GHG emission reductions are required.
• Reducing onboard vessel methane emissions are needed to increase viability of electro- and bio-methane fuel pathways: Reducing onboard vessel methane emissions to these cost-efficient levels increases the longer-term viability of the electro- and bio-methane fuel pathways, however, it is still unclear if upstream well-to-tank fugitive emissions can be reduced to acceptable levels. Using the FuelEU methodology and cost-efficient onboard methane emission reduction measures, GHG WTW emissions can be reduced to 5-9 gCO₂eq/MJ using 100% electro-methane and hydrothermal liquefaction (HTL) Oil as a pilot fuel (a 90-95% decrease relative to heavy fuel oil).
• Proposed FuelEU for Maritime limits are not strict enough to activate onboard vessel methane emission reduction: For the vessels studied, GHG emission levels are already compliant with the 2025 and 2030 FuelEU GHG intensity index limits without introducing any onboard vessel methane emission reduction measures. This is due to LNG’s lower CO₂ emission factor used within its 100-year GWP methodology. If a CO₂-eq regulation with the proposed FuelEU limits is introduced, no emission reduction actions would be needed until 2035.
• Regulation is required for widespread adoption of onboard vessel methane emission reduction technologies and solutions: Without strong incentives or regulatory requirements to reduce methane emissions, there is limited commitment from ship owners to adopt methane emission reduction technologies and solutions. There are ongoing discussions at the IMO to include methane into its LCA methodology, a CO₂-eq approach like FuelEU. There is also the possibility that methane is regulated in a more direct way using a vessel’s Technical File like NO_X emissions. This type of regulation could more directly target methane slip levels and the need to reduce them onboard the vessel either for newbuilds or existing vessels if retroactive.

To properly assess the viability of methane-based alternative fuel pathways like electro- and bio-methane, the ability to reduce upstream well-to-tank fugitive emissions needs to be fully understood. Upstream fugitive emissions are not covered in this paper but are currently being studied at the MMMCZCS to enable a complete viability assessment of the methane-based fuel pathways. The MMMCZCS also plans to study onboard vessel emissions in operation where factors like dynamics engine loads and sea states can influence methane emission levels.

Despite the slow progress to incentivize or require LNG-fuelled vessels to limit their methane emissions, there is significant international social pressure to reduce emissions of GHGs, particularly methane. From the Global Methane Pledge (COP26) to the US’ Inflation Reduction Act of 2022, growing worldwide concern is strongly pushing for GHG reductions to limit the increase in the global average temperature to well below 2°C above pre-industrial levels. It is expected that this social pressure will lead to definitive action by stakeholders across all industries.

A Mærsk Mc-Kinney Møller Center for Zero Carbon Shipping (MMMCZCS) assessment of vessel emissions from the main alternative fuel pathways has found that onboard vessel methane slip when using methane-based fuels including liquified natural gas (LNG), liquified electro- and bio-methane presents a risk of increased CO2-equivalent emissions. The assessment stated that methane slip reduction solutions have been identified, but not fully developed, tested, or demonstrated. In addition, given the rapid expansion of the LNG-fueled vessel fleet and industry projections, the use of LNG as a fuel for the maritime industry will continue well into the future.

Potential and upcoming regulation of onboard vessel methane emissions presents a risk to LNG-fueled vessel owners, operators, and charterers. Currently, there are no international regulations on methane emissions from vessels, however, ongoing initiatives and regional guidelines indicate that regulations are highly likely to appear soon. A global methane pledge to cut emissions of methane by 2030 was announced at the 2021 United Nations Climate Change Conference (COP 26) and has been signed by both the United States and the European Union (EU). While this can only be considered as a manifestation of intent by these countries and covers all methane emission sources, it highlights the importance of methane emissions and could accelerate the implementation of regulatory measures within shipping. Such measures may include national regulations, measures to reduce methane slip, or international regulations accounting for methane in addition to CO2 such as inclusion in the next EEDI phases, market-based measures (MBMs), and lifecycle assessment (LCA) guidelines.

The risk of increased CO2-equivalent emissions and upcoming and potential regulation coupled with the rapid expansion of the LNG-fueled vessel fleet led to the MMMCZCS establishing a dedicated working group to complete a deep dive on the onboard vessel emission risks. This paper presents the results from the working group, including an overview of regulatory drivers, onboard methane emission sources and expected emission levels, reduction technologies and solutions, and techno-economics.

While CO2 is the main source of shipping’s climate impact with over 90% of total greenhouse gas (GHG) emissions, methane has a higher climate impact in terms of global warming potential (GWP). As a result, methane emission reduction can be an efficient way to reduce a vessel’s overall CO2-equalivent emissions, allowing compliance with upcoming regulations and increasing the viability and competitiveness of methane-based alternative fuel pathways. Reducing methane emissions onboard can be a cost-efficient way to make meaningful emission reductions in the short-term while alternative fuel availability increases, and prices stabilize.

The dual fuel (DF) methane-based engine market is mainly comprised of three baseline engine technologies:

• A low pressure two-stroke engine (LP2st) based on the Otto cycle combustion principle, a main engine (ME) option.
• A high pressure two-stroke engine (HP2st) based on the Diesel cycle combustion principle, a ME option.
• A low pressure four-stroke engine (LP4st) based on the Otto cycle combustion principle, a ME and auxiliary engine option.

Based on current ME market share analysis, methane emission reduction solutions are needed for all three baseline engine technologies as the two-stroke engine market share grows, while four-stroke MEs continue to hold a large market share due to their use as auxiliary engines.

The most referenced source of methane is ‘methane slip’, where a specific methane quantity evades combustion and is emitted via engine exhaust. The amount of methane slip can vary with the selected engine and fuel gas supply system technologies as well as operations.

The baseline engine technology with the highest methane slip is the LP4st, which is typically used as an auxiliary engine onboard LNG-fueled vessels or as part of a diesel-electric arrangement. There is also a significant difference between the ME options, as the HP2st engine has very low amount of methane slip levels (0.20-0.28 g/kWh), whereas the LP2st engine has higher levels of methane slip.

While methane slip is the predominant methane emission while operating DF methane-based engines, there are other potential sources of methane emissions that are not related to the engine or combustion directly. These non-engine methane emission sources can be categorized into three main emission types: fugitive emissions, operational releases, and emergency releases.

While quantification of the non-engine methane emissions is desired, at this point there is not a substantial amount of information on the frequency and amounts of these emissions. The task of quantifying the effect of the fugitive emissions and operational and emergency releases needs to be investigated further.

For fugitive emissions, high pressure systems generally have a higher number of emission sources (valves, flanges, PSVs) than low pressure systems, thus a higher level of fugitive emissions should be expected. Higher power output and machinery loads can also result in higher fugitive emissions as larger supporting equipment is required.

Operational Releases​ are vastly dependent on operating procedures and the equipment installed/setup for non-standard operations (DF boilers, GCU, etc.).​ Operational releases are generally higher for high pressure systems compared to low pressure systems as more methane is released to the atmosphere per engine vent due to the pressure and the density of the gas compared to low-pressure Otto engines. ​

Emergency releases are a direct result of the operating and maintenance procedures and hence in direct relation to the crew’s training, knowledge and operating practices​. Frequency of emergency releases are expected to be rare when operating the vessel properly. However, any opening of the tank safety relief valve(s) would change this, but such an event is not expected during a vessel’s lifetime.

When focusing on onboard methane emission reduction, technologies will play an important role, however, how these technologies and other systems are integrated together is just as critical and should be considered during design and development. Solutions include engine-related and after-treatment technologies as well as their integration together into power and propulsion concepts or system solutions.

The three main solution categories include:

1. Engine technology: fully integrated with the engine,
2. After-treatment technologies: separate from the engine, but integrated, and
3. System solutions: system dimensioning, configuration and connected technologies.

Some solutions span multiple categories based on how they are integrated. System solutions that could be considered as primarily energy efficiency technologies can also play an important role in reducing methane emissions.

The two main engine technologies include high pressure (direct) injection and exhaust gas recirculation (EGR). High pressure (direct) injection is used on the HP2st engines based on the Diesel cycle, which have the lowest methane slip values across the engine load range compared to other baseline engine technologies. The low methane slip values are mainly achieved due to the Diesel cycle combustion principle. EGR recirculates a portion of the exhaust gas back into the engine. In combination with other design elements, engine makers have used EGR technology to achieve significant reduction of methane slip in LP2st engines. Second generation engines have EGR technology incorporated as an integral part of engine tuning, and according to engine makers these second generation LP2st engines with EGR technology have 50% lower methane slip than first generation LP2st engines.

The working group identified two main after-treatment solutions the below after-treatment options: methane oxidation catalyst (MOC) that is only applicable to four-stroke engines and plasma reduction system (PRS). A MOC reduces methane emissions in DF methane-fueled engines with the use of a precious (noble) metal-based coated catalyst. The tetrahedral structure of the methane molecule is very stable and very difficult to break, however, this can be effectively achieved through oxidation by utilizing a catalyst. A PRS consists of a catalyst and absorbent free after-treatment technology that utilizes electric power to convert methane to carbon monoxide and water.

System solutions include system dimensioning, configurations and connected technologies. System solutions that could be considered as primarily energy efficiency technologies can also play an important role in reducing specific emissions. Methane slip from some internal combustion engines vary based on engine load with typically higher slip at lower loads. A shaft generator or batteries can be used to reduce consumption and emissions from auxiliary engines. Reduction potential of these solutions are highly vessel-dependent and general reduction levels cannot be determined.

An overview of methane slip emission reduction including applicable engine, after-treatment and combined technologies is provided in Figure 1 for each baseline engine technology. The left graphs show reduction potential in g/kWh of methane slip and the right graphs show the total tank-to-wake (TTW) GHG emissions in gCO₂eq/kWh. In some cases, reduction technologies can either decrease or increase CO₂ emissions while reducing methane emissions. Both emission changes are captured in the total TTW emissions. Expected reduction ranges are shown with solid lines and the hatched lines to the right indicate the reduction amount relative to the baseline engine technology. Hatched lines to the left indicate for some cases the maximum potential of the technology for some applications.

The EGR and the two after-treatment solutions (MOC and PDS) are considered for each baseline engine technology based on applicability. Other engine-specific reductions are considered part of the baseline engine technology emissions. System solutions cannot be addressed at a general level as shown but are considered later when looking holistically at the total methane emissions of a vessel.

Looking at the baseline engine technologies and emission reduction technologies and solutions independently does not provide a holistic view of the total methane emissions from a vessel. To determine the GHG emissions of methane DF engines at a vessel level, we developed a complete vessel emission calculation model using the draft FuelEU methodology. The model can help quantify the total methane emissions of a vessel and determine if the vessel complies with a CO2eq regulation, such as the regulations proposed in FuelEU.

Onboard methane emissions were found to be in the range of 6-11% of the total WTW GHG emissions with the LP2st ME and LP4st AEs (for both ships) associated with the largest methane emission contribution. While total GHG emissions are less, the HP2st ME and LP4st AEs produce slighter higher GHG TTW (CO2) emissions mainly due to additional energy consumption of the high-pressure equipment needed. In addition to the selected baseline engine technologies, methane emissions are highly dependent on the operational profile and the onboard power and propulsion concept. Figure 2 shows the methane emission levels associated with different types of operations.

The baseline main engine technology can be a major contributor to the vessel's total emissions and, as a result, should be the initial focus for emission reduction efforts. With an HP2st ME configuration, methane emission reduction efforts should start with reducing methane emissions associated with the LP4st AEs. With an LP2st ME configuration, investigating methane emission reduction associated with the main engine should be considered first. Applicable emission reduction technologies and solutions will depend on if the vessel is a newbuild or if retrofitting an existing vessel is being considered. For example, retrofitting a shaft generator is unlikely to be feasible due to the vessel's existing hull form design; however, one can be integrated into a newbuild.

This analysis allows for both an assessment of regulatory compliance and a comparison between methane emission reduction technologies and solutions that can be used to reduce emissions and achieve compliance. For the two vessels studied and both ME configurations, the emission levels before emission reduction measures are implemented are already below the 2025 and 2030 FuelEU GHG intensity index limits. However, they are very close to or exceed the 2035 limits. For reference, the FuelEU level for HFO is 91.6 gCO₂eq/MJ.

As this is a CO₂eq methodology, there are multiple ways to reduce emissions to comply with the regulation, including using alternative fuels or onboard emission reduction technologies and solutions. However, it is important to highlight that energy efficiency initiatives to reduce the vessel's total energy demand do not impact the FuelEU index levels as it is a GHG intensity metric that measures emissions per unit of energy used by the ship. As a result, energy efficiency initiatives would result in lower absolute emissions, but also lower energy consumed, so the GHG intensity index would remain unchanged.

For the LR2 tanker with an HP2st ME, PRS with only the LP4st AEs could reduce emissions in line with 2035 FuelEU limits. However, for the LP2st ME configuration, PRS must be applied to both the ME and AEs for compliance to be achieved. Shore power also effectively eliminated auxiliary loads and thus the methane slip produced by LP4st engines.

For LNG carriers, using a shaft generator can significantly reduce methane slip onboard by reducing LP4st engine use. 2035 FuelEU compliance can be achieved by applying a shaft generator on an LNG carrier powered by an HP2st engine. A similar result can also be expected with a catalyst system.

EGR can reduce methane slip onboard vessels fitted with an LP2st engine and is set to become a standard piece of equipment with the latest LP2st engines post-2021. However, this solution alone cannot achieve 2035 Fuel EU compliance – a combination of methods is needed, such as a shaft generator with an LP2st fitted with an EGR and using shore power.

When determining the best way to achieve regulatory compliance or reach a specified emission level, the economics are equally important as the emission reduction potential. For each of the emission reduction technologies and solutions as well as applicable combinations, a CAPEX and OPEX was estimated including fuel costs. A GHG abatement cost in terms of $/ton CO₂eq was calculated for a 10- and 20-year period.

The abatement cost of onboard technologies should not be evaluated in isolation but together with all options for achieving an emission reduction target so that the most cost-effective solution can be identified. As a result, we also included the GHG abatement cost of using a 20% blend of bio-methane for comparative purposes and an indicator of relative cost-efficiency. However, the availability and price of bio-methane are not clear or established. Therefore, relying on this option only presents its own risks and uncertainties that would need to be considered separately.

The GHG abatement cost based on the estimated CAPEX and OPEX can be used with the expected emission reduction potential to assess which solutions should be applied based on a specific vessel, newbuild or retrofit, baseline engine technology, and emission reduction targets. The most optimal solution varies by scenario, particularly for reducing emissions of newbuilds vs. retrofitting existing vessels. Based on our integration studies, we concluded that shaft generators and EGR are not retrofittable and are only suitable for newbuilds due to engine room space limitations. MOC and PRS, on the other hand, can be retrofitted. It is also possible to retrofit shore power in most cases; however, this was not directly studied for this paper. It's important to note that shore power may not be readily available at all ports resulting in unrealized emission reduction even if the capability is installed on the vessel.

GHG cost abatement graphs provide a simple tool to evaluate your emission reduction options given a particular target to balance the need to achieve a certain emission level while also considering the cost-efficiency of emission reduction measures. GHG abatement cost graphs were generated for the two case study vessels and baseline engine technologies. Note that the LR2 tanker with the LP2st engine has an EGR included in the baseline, while the LNGC LP2st engine does not have an EGR.

Figure 3 shows the GHG cost abatement graphs for HP2st ME configurations, which provides both the emission reduction potential and GHG abatement cost of various emission reduction technologies, solutions, and combinations. Two main objectives can be evaluated using such figures: 1) maximize emission reduction by minimizing GHG emissions or achieving a desired reduction target, and 2) minimize the associated GHG abatement cost. The ideal or utopia point on the graphs is in the bottom left side (zero GHG emissions at zero GHG abatement cost), a point that is not achievable. When comparing solutions, one is considered dominated if another solution has both lower GHG emissions and GHG abatement cost. For example, in Figure 19b, the plasma for D/G solution is dominated by all others and should not be considered a valid solution. The red horizontal lines indicate the original vessel’s emissions without and methane emission reduction.

With the HP2st main engine baseline, the most efficient solutions (see Figure 3) are related to reducing the use of the LP4st auxiliary engines, including the installation of a shaft generator and the use of shore power. For a retrofit scenario, using a catalyst or a shore power connection are efficient options. For the LNGC vessel with increased auxiliary power demands compared to the LR2 tanker, the shaft generator becomes more efficient with lower GHG emissions and GHG abatement costs. This is reflected in the latest LNGC newbuild design trend of incorporating shaft generator systems.

Methane emission reduction can be an efficient way to reduce a vessel’s overall CO₂-eq emissions, allowing compliance with upcoming regulations and increasing the viability and competitiveness of methane-based alternative fuel pathways.

To properly assess the viability of methane-based alternative fuel pathways like electro- and bio-methane, the ability to reduce upstream well-to-tank fugitive emissions needs to be fully understood. Upstream fugitive emissions are not covered in this paper but are currently being studied at the MMMCZCS to enable a complete viability assessment of the methane-based fuel pathways. The MMMCZCS also plans to study onboard vessel emissions in operation where factors like dynamics engine loads and sea states can influence methane emission levels.

Despite the slow progress to incentivize or require LNG-fuelled vessels to limit their methane emissions, there is significant international social pressure to reduce emissions of GHGs, particularly methane. From the Global Methane Pledge (COP26) to the US’ Inflation Reduction Act of 2022, growing worldwide concern is strongly pushing for GHG reductions to limit the increase in the global average temperature to well below 2°C above pre-industrial levels. It is expected that this social pressure will lead to definitive action by stakeholders across all industries.

In addition to the assessment of onboard methane emission reduction, the MMMCZCS is investigating biogenic routes to produce methane. Liquified methane of biogenic origin can be produced with the same specifications as LNG. The two products are indistinguishable and liquified bio-methane has a 100% “drop-in” potential. The MMMCZCS is currently studying whether biogas (renewable gas) is one of the contenders to supply net zero carbon fuels to shipping. In a project running in the second half of 2022, and partnering with numerous third parties representing technology, energy producers, consulting firms, we will clarify biomass availability, LCA aspects, cost of manufacturing and strategies to boost the industry relevant for shipping.

Onboard emission measurement is a general topic that is currently being studied further to better understand actual onboard emissions including operational factors like dynamic engine loads and heavy sea states. Onboard emission monitoring is also being considered as a potential tool for emission regulation compliance assurance and enforcement.