Alternative Fuels for Environmentally-Friendly Ships

The definition of alternative fuels is evolving from a material-based approach (focusing on what is used) to a process-based approach (emphasizing how fuels are produced). For alternative fuels to be considered suitable for environmentally friendly ships in future, it seems that they need to meet three following criteria. First, to truly estimate the impact on climate change, carbon dioxide (CO₂) equivalent greenhouse gas (GHG) emissions considering global warming potential should be evaluated, rather than just CO₂ emissions from fuel use. Second, well-to-wake GHG emissions should be evaluated considering lifecycle assessment from the production to use of a fuel. Third, specific quantitative criteria will be required to qualify a fuel as an alternative fuel, ensuring meaningful reductions in GHG emissions. The recently announced 2023 IMO GHG strategy, along with discussions at the Marine Environment Protection Committee shows these features well. This book introduces alternative fuels for environmentally friendly ships including the topics in liquefied natural gas, hydrogen, ammonia, biofuels, e-fuels and carbon capture, utilization, and storage.

Climate change and global warming are primarily driven by anthropogenic greenhouse gas (GHG) emissions from human activities. In the early days, there were debates that climate change was a conspiracy, or exaggerated. Over time, evidence supporting human-induced climate change has become overwhelming, leading to a broad scientific consensus on anthropogenic global warming. The Kyoto Protocol (1997) marked the first global effort to mitigate GHG emissions but proved limited in its effectiveness, primarily due to the absence of binding commitments from major emitters and the rapid industrial growth in developing countries. The Paris Agreement (2015) introduced a more flexible, bottom-up framework, requiring all parties to set Nationally Determined Contributions (NDCs) to reduce emissions in alignment with global temperature targets. Despite progress in international climate policy, global energy consumption remains heavily dependent on fossil fuels. Furthermore, the GHG emissions from international shipping are the eighth largest source of GHG emissions but were not included in the national GHG reduction targets. This was the starting point for the IMO to develop GHG reduction strategies for ships.

As a specialized agency of the United Nations (UN), the International Maritime Organization (IMO) plays a key role in regulating maritime activities on the high seas which are beyond the sovereignty and jurisdiction of any particular state. The IMO’s Marine Environment Protection Committee (MEPC) adopted the international convention for the prevention of MARine POLlution from ships (MARPOL) in 1978, and revised it in 2011 to include regulations to reduce CO₂ emissions reduction from international shipping in response to global climate concerns. To assess the carbon intensity of ships, the IMO introduced the energy efficiency design index (EEDI) for newbuild ships, the energy efficiency existing index (EEXI) for existing ships, and the carbon intensity indicator (CII), which assesses the annual operational performance of international shipping. The IMO has strengthened greenhouse gas reduction targets, aiming for net-zero emissions by 2050. Future regulations are expected to consider the well-to-wake (WtW) GHG emissions intensity based on lifecycle assessments, including greenhouse gases other than CO₂.

This chapter explores the role of liquefied natural gas (LNG) in global energy systems, its environmental impact, transportation issues, and potential as a marine fuel. LNG is a fossil fuel consisting primarily of methane, with smaller amounts of ethane, propane, butane, nitrogen, and carbon dioxide. A large LNG carrier is typically used because LNG's high density compared to NG makes it economical for long-distance transport, but it has the operational issues of boil-off gas (BOG) generation due to the very low storage temperature of LNG. With stricter GHG emission regulations, advanced dual-fuel gas injection engines with reliquefaction are now being used. While LNG emits less CO₂ than conventional heavy fuel oil (HFO), its GHG emissions intensity may close to that of HFO when considering engine type and methane slip. Demand for LNG as a fuel will gradually decline in the distant future because it is essentially a fossil fuel, but the use of LNG as a feedstock for alternative fuels such as hydrogen is expected to continue for a considerable period of time. In addition, alternative sources of LNG, such as biogas, can offer a more sustainable future by reducing net GHG emissions.

Hydrogen and ammonia are gaining attention as alternative fuels due to their carbon-free combustion feature, and offer a potential solution for decarbonization. However, they also face challenges to overcome. Currently, most hydrogen is produced from natural gas through steam methane reforming (SMR), which produces significant GHGs, called as “gray” hydrogen. In contrast, “green” hydrogen, produced by water electrolysis using renewable energy, generate little GHGs but not yet economically feasible due to high production costs. Liquefied hydrogen (LH₂) has logistical challenges, such as low energy density and high liquefaction costs, as well as the risk of GHG emissions from the energy-intensive liquefaction process. Ammonia is easier to transport due to its milder liquefaction conditions, and can be used directly as a fuel once the ammonia-fueled engines currently under development are commercialized. However, like hydrogen, the environmental impact of ammonia depends on how it is produced. “Gray” ammonia, synthesized from fossil fuels, has high GHG emissions, while green ammonia, produced from green hydrogen, is more sustainable but still requires careful consideration of its energy-intensive synthesis process. Additionally, ammonia is a toxic substance that poses health risks and requires stringent safety measures.

Biofuels such as biodiesel, bioethanol, biomethanol, and biomethane are fuels produced from biomass, which includes organic matter. While biofuels reduce net GHG emissions by absorbing CO₂ during their growth, their overall impact depends on the balance between CO₂ absorbed and emitted during production and combustion. First-generation biofuels from edible crops have a problem of competition with food and may lead to higher food prices. Additionally, the effects of land use change (LUC), such as deforestation, can exacerbate GHG emissions. As a result, the EU and IMO have developed guidelines for biofuel criteria, focusing on WtW GHG emissions intensity from production to use of biofuels. In the future, advanced biofuels produced from non-edible biomass, may offer a more sustainable alternative. E-fuels produced from renewable hydrogen and captured CO₂ are also gaining attention, as they can reduce WtW GHG emissions and be used without modifying conventional fossil fuel-based systems. However, the benefits of e-fuels depend on the source of CO₂, because using of non-renewable CO₂ from fossil fuels may not reduce the WtW GHG emissions sufficiently. While challenges remain, both biofuels and e-fuels are seen as key options for achieving net-zero emissions in the future.

Carbon capture, utilization, and storage (CCUS) is a combination of technologies for managing CO₂ emissions. CCUS includes technologies that capture CO₂ from sources of CO₂ emissions and either store it in sequestered locations or convert it into other useful materials to prevent the release of CO₂ into the atmosphere. CCUS has a long history, and the technologies to capture, transport, utilize and store CO₂ are relatively mature compared to other alternative fuels, so it can be applied to conventional systems without major modifications. Onboard carbon capture and storage (OCCS) is the application of CCUS technologies to a ship, to reduce the CO₂ emissions from a ship to the atmosphere by capturing and storing CO₂ onboard. OCCS also faces the challenges of a lack of regulation, high energy consumption, high costs, and insufficient infrastructure for offloading and transporting CO₂ to a final storage site. Nevertheless, to extend the lifetime of existing ships using fossil fuels and as a part of supply chains for sustainable alternative fuels, OCCS needs to be considered as a bridge technology to get through the transitional period until the era of fully sustainable alternative fuels arrives.