Total Methane and CO₂ Emissions from Liquefied Natural Gas Carrier Ships: The First Primary Measurements

Abstract

Mitigating methane emissions is vital in meeting global climate targets, but there is a lack of understanding of emissions and abatement opportunities to enable this. The natural gas supply chain is a key emission source, where methane emissions from liquefied natural gas (LNG) shipping have until now not been directly measured. This study provides the first measurement and modeling of total methane and CO 2 emissions from an LNG carrier on a round trip voyage from the USA to Belgium and back, including loading, laden voyage, unloading, and ballast voyage, measuring emissions from exhaust stacks, vents, and fugitives. Venting and fugitive emissions were extremely low, contributing less than 0.1% of total greenhouse gas emissions. CO 2 emissions from fuel usage were also lower than previous estimates due to improved efficiencies in modern engines and ship design. However, methane slip through the engines were higher than those in prior studies, averaging 3.8% across all engines: equating to 0.1% of delivered LNG. Generator engines are not typically included in emissions analyses but were the key cause of methane emissions. Engines exhibited higher methane slip rates at low loads, and optimized operation could reduce slip rates by half. More measurement studies are now needed to better understand fleet emissions and enable cost-effective mitigation strategies.

The use of alternative fuels for maritime decarbonization: Special marine environmental risks and solutions from an international law perspective

“…A report by the International Council on Clean Transportation (ICCT) noted that "the most popular LNG ship engine, particularly for cruise ships, emits between 70% and 82% more life cycle GHG emissions over the short term than engines powered by clean distillate fuels" (Pavlenko et al, 2020). While it does indeed reduce carbon dioxide emissions, as a carbon-based fuel, LNG continues to emit carbon dioxide (Balcombe et al, 2022) and can only be used as a mitigation option (Bouman et al, 2017;Hwang et al, 2020).…”

Present global warming: a justifiable and stable metric for evaluating short-lived climate pollutants

“…Among step function metrics, the 20 year GWP and the 100 year GWP for methane differ by a factor of 3 [4]. In some cases when the aggregated effects of multiple GHGs are considered, the selection of which GWP to use determines whether methane is the dominant pollutant or a minor one [28].…”

Policy approaches to mitigate in-use methane emissions from natural gas use as a marine fuel

“…The IMO estimates that methane emissions from shipping have increased by 151%-155% (59-148 thousand tonnes (vessel-based); 55-140 thousand tonnes (voyage-based)) during the period from 2012 to 2018 [3]. These changes reflect the increased uptake of LNG as a marine fuel, where the low-pressure dual-fuel (LPDF) and lean-burn spark-ignited engines, with relatively high methane slip levels (1.5%-16% of throughput), dominate the market share (47% of all LNG-fueled engines in 2018) [3,16,17]. Despite the significant increase, current regulatory frameworks are not equipped to address methane emissions from LNG as a marine fuel, particularly during the use phase.…”

“…Much research has focused on upstream components of this value chain, however, engine methane slip during fuel combustion onboard ships are a growing area of concern. Methane slip may constitute up to 99.8% of the total methane emissions of an LNG-fueled voyage [6,16]. The downstream combustion component, including methane slip has been estimated to constitute nearly 70%-91% of lifecycle GHG emissions in the LNG value chain under certain conditions [4,9,10,[18][19][20].…”

“…Compared to the air quality and economic benefits, the benefits of fossil-based LNG towards decarbonization are more limited and less clear [5,9]. Fugitive methane (CH 4 ) emissions throughout the LNG value chain (natural gas extraction, processing, liquefaction, storage, transportation, regasification, and distribution), venting emissions (intentional methane emissions during operations and maintenance when using LNG as a marine fuel), and in-use engine methane emissions threaten GHG reductions required to meet the IMO 2050 climate target-at least 50% reduction in GHG emissions from international shipping compared to 2008 levels [9,15,16]. In-use engine methane emissions or methane slip happens when unburnt methane emits from marine gas engines into the atmosphere [6].…”