May-2023

Journey toward decarbonisation of the shipping industry

Alternative fuels for the shipping industry are explored along with methods to increase fuel efficiencies and reduce emissions.

Nathan D Wood, Robert Moorcroft and Torill Bigg
Tunley Engineering

Article Summary

The maritime transport industry is vital to our present-day globalised world, with approximately 90% of traded commodities reliant on shipping (OECD, 2023), (UKRI, 2023). The shipping industry is estimated to be responsible for approximately 2.5% of global CO2 emissions or 940 MtCO2 per year (Balcombe, et al., 2019).

An increasing number of regulations and targets are being implemented to reduce the greenhouse gas (GHG) emissions from shipping. The International Maritime Organization (IMO) has set stringent targets to reduce the carbon footprint of international shipping by at least 40% by 2030, compared to 2008 levels (EIT InnoEnergy, 2022). The 2021 EU Green Deal sets targets for a 90% reduction in port city GHG emissions by 2050 (EIT InnoEnergy, 2022). The EU has also publicly declared the need to bring shipping under its Emissions Trading System (ETS), which sets caps on the emissions of companies per annum, with financial implications for companies that breach this limit (EIT InnoEnergy, 2022).

The shipping industry is actively moving towards alternative fuels to reduce its carbon footprint, with many prospective fuel types (Mallouppas & Yfantis, 2021), (Department for Transport, 2022). Both liquid natural gas (LNG) and hydrogen are gaining traction as fuels for the future (Mallouppas & Yfantis, 2021). However, questions arise from these choices surrounding sustainability and ongoing reliance on fossil fuels.

Ports also play a dual role in the decarbonisation of shipping by reducing in-port GHG emissions and facilitating the reduction of at-sea GHG emissions (Psaraftis & Zis, 2022). Rotterdam is Europe’s most polluting port, with an estimated 13.7 million tonnes of CO2e per year, more than twice the footprint of an average coal-fired power station (T&E, 2022). With regards to sea-based emissions, as of April 2018, only 28 out of the 100 largest ports (by cargo) offered incentives for environmentally friendly ships (International Transport Forum, 2023).

Current maritime fuels
The shipping industry is heavily reliant on the use of fossil fuels, for example dock-side cargo handling, on-ship electricity generation, and ship propulsion. It relies on an assortment of fuels, such as heavy fuel oil (HFO), low sulphur fuel oil (LSFO), marine gas oil (MGO), marine diesel oil (MDO), and LNG (Mallouppas & Yfantis, 2021). Figure 1 shows a percentage breakdown of fuel use in the maritime shipping industry in 2015.

HFO, a thick viscous oil with a high sulphur content, is the primary fuel used within the shipping industry (72%) (Mallouppas & Yfantis, 2021).

When combusted, HFO releases pollutants such as COx, NOx, and SOx, and PM (Mallouppas & Yfantis, 2021), (Wu, et al., 2018), (Yeh, Shen, Cheruiyot, Nguyen, & Chang, 2022). PM2.5 particles are of particular concern to human health, as they are small enough to lead to illnesses such as asthma, chronic obstructive pulmonary disease (COPD), coronary heart disease, stroke, and lung cancers (Taskforce for Lung Health, 2022), (Akhbarizadeh, et al., 2021). Emissions of PM2.5 from typical container ships are particularly high at 3.15 ± 0.39 g per kg of combusted HFO (Wu, et al., 2018), which consume approximately 1,120 kg of fuel per hour at 25% engine power, emitting an estimated 3.53 kg of PM2.5 particles into the atmosphere per hour (Aijjou, Bahatti, & Raihani, 2019).

The shipping industry is estimated to be the source of 13% of the world’s SOx emissions (Balcombe, et al., 2019). The use of LSFO can mitigate these emissions; however, this comes at an increased financial cost. Compounding the emissions of PM2.5 are nitrous oxides, NOx, which are GHGs with a global warming potential (GWP) of GWP20 = 30 – 33 when emitted from ground sources (Lasek & Lajnert, 2022). Furthermore, exposure to NOx has been linked to cardiovascular problems and higher rates of respiratory issues (Anenberg, et al., 2017), (Faustini, Rapp, & Forastiere, 2014).

In 2015, the International Maritime Organization (IMO) implemented stricter restrictions of a maximum of 0.10% m/m (mass by mass) sulphur content for ships operating in the Emission Control Areas (ECAs) as designated under Regulation 14 of MARPOL Annex VI (IMO, 2015), (IMO, 2023). This includes areas such as the North Sea, the Baltic Sea, the US/Canadian coast, and the US Caribbean area. Furthermore, controls on SOx emissions were expanded outside ECAs in 2020, with a new regulation limiting the sulphur content in fuel oil to 0.50% m/m (IMO, 2020). This regulation aims to reduce the overall SOx emissions from ships by 77%, leading to an annual reduction of approximately 8.5 million metric tonnes of SOx (IMO, 2020b).

The industry’s response to these regulations has been varied (Cuong & van Hung, 2020), (Saez Alvarez, 2021). Many companies transitioned to low-sulphur fuel oils, such as MGO and ultra-low-sulphur fuel oil (ULSFO), while others adopted alternative fuels like LNG, which produce fewer SOx emissions. Some chose to install exhaust gas cleaning systems, or scrubbers, on their vessels, enabling them to continue using high-sulphur fuel oil (HSFO). Additionally, several companies implemented slow steaming to lower fuel consumption and emissions (Faber, 2012), while others optimised their fleets by replacing older, less efficient vessels with newer, more environmentally friendly alternatives. These combined efforts have enabled the shipping industry to respond effectively to the IMO’s stringent SOx regulations.

Alternative shipping fuels
LNG makes up approximately 2% of fuel use in the shipping industry (see Figure 1). It is proposed as one of the fuels of the future for shipping, as it produces significantly lower pollutants than fuel oils such as HFO. When burnt, it has substantially reduced emissions: PM by more than 95%, NOx by more than 80%, and SOx by more than 90% (T&E, 2016). However, LNG is liquified methane with a GWP 28 times larger than CO2 (T&E, 2016). A key issue with LNG ships is ‘methane slip’, the escape of unburned methane from marine engines. One study places methane slip at approximately 7 g per kg LNG burnt at higher engine loads, with a range of 23-36 g per kg at lower engine loads (Ahmadi Ghadikolaei, Cheung, Yung, & Shun Cheung, 2016). Transport and Environment (T&E), an NGO, highlighted that LNG might not effectively reduce GHG emissions due to methane slip, (Ricardo, 2016). One way to reduce methane slip is to use marine engines specifically designed for LNG, along with fuel storage tanks and pipelines tailored to handle this type of fuel. This is prohibitively expensive as a retrofit option, and even new ships have a significantly higher capital cost than HFO-fuelled ships (T&E, 2016). LNG-fuelled ships can be further decarbonised by using bio-LNG or biomethane (SEA-LNG, 2022).