Maritime sector needs zero-carbon fuels, not LNG

Jonathan Lewis,
Director, Transportation Decarbonization,
Clean Air Task Force,
USA

The global maritime sector puts more than a billion tonnes of climate-warming pollutants into the atmosphere every year, accounting for 2-3 percent of total anthropogenic greenhouse gas emissions. The United Nations’ International Maritime Organization (IMO) took a halting first step toward reducing the sector’s emissions in 2018 when it announced a commitment to reduce greenhouse gas emissions from marine vessels by at least 50% by 2050.

The ambition gap between the IMO target and the 2015 Paris Agreement, which in effect requires every major sector of the economy to eliminate its greenhouse gas emissions by midcentury, is stark. Bringing the IMO target into alignment with broader climate change mitigation obligations has emerged as a major priority within civil society, and dozens of countries, including those in the EU-27 bloc, United States, and Japan, have called on the shipping sector to target either zero or net-zero greenhouse gas emissions by 2050.

Multiple options for reducing emissions are available to the shipping industry, from optimized route planning to reductions in cruising speed to the use of modern wind-assist technologies, but the strategy that offers the largest impact is a switch from heavy fuel oil (HFO) and marine diesel oil (MDO) to fuels with lower carbon intensity.

Until recently, many in the shipping industry assumed that the successor fuel would be natural gas compressed and cooled until liquefied (LNG). Natural gas emits about 30% less carbon dioxide than fuel oils when burned. LNG is mainly comprised of methane, however, a greenhouse gas that is 84 to 86 times more potent than carbon dioxide over a 20-year period, and the natural gas supply chain as currently operated is leaky. The climate impact of LNG depends strongly on the amount of methane released during extraction, processing, and transport of the natural gas feedstock as well as the energy and emissions from the liquefaction stage.

Numerous studies have shown these emissions to be substantial. When the upstream methane and CO2 emissions that occur along the LNG supply chain are added to the methane and carbon dioxide released directly from LNG-fuelled ships to calculate the lifecycle well-to-prop emissions, the case for LNG as a climate-friendly alternative to conventional marine fuels breaks down. Depending on the leak level for upstream methane, shifting marine vessels from MGO or HFO to LNG delivers between a 10% reduction and a 9% increase in lifecycle GHG emissions. Plainly, the maritime sector cannot achieve Paris Agreement-aligned reductions in greenhouse gas emissions over the next three decades by shifting to LNG.

Eliminating greenhouse gas emissions and conventional air pollution from the maritime sector instead requires a shift to zero-carbon fuels, namely hydrogen and ammonia (which is made by combining hydrogen and nitrogen atoms). Reciprocating engines and fuel cells can convert hydrogen and ammonia into propeller-spinning energy without emitting carbon dioxide. Zero-carbon fuels can be produced at mass-scale in ways that minimize lifecycle greenhouse gas emissions: one such process uses electricity generated by renewable or nuclear power stations to electrolytically decompose water into oxygen and hydrogen; another extracts hydrogen from methane molecules in machines called reformers, and manages the associated greenhouse gas releases with methane leak controls and carbon capture equipment.

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