1 Introduction
The effects of climate change emerge as a significant global challenge to be addressed. According to research (United Nations, 2023), over 80% of international cargo is transported by ships, which are responsible for about 3% of all global Greenhouse Gas (GHG) emissions. This is set to grow if no action is taken (IMO, 2020).
Specifically, ships are a substantial source of emissions released into the atmosphere, including Carbon Dioxide (CO2), Nitrogen Oxides (NOX), Carbon Monoxide (CO), Non-Methane Volatile Organic Compounds (NMVOC), Particulate Matter (PM), Sulphur Dioxide (SO2), and methane (CH4) (Shu et al., 2023; Aakko-Saksa et al., 2023).
While technologies such as Selective Catalytic Reduction (SCR), Scrubbers and Exhaust Gas Recirculation (EGR), are already established and proven to reduce NOX and SOX emissions, as reported by (Lehtoranta et al., 2015; Karatuğ et al., 2022; Andriiovych Kuropyatnyk and Victorovych Sagin, 2019), the situation differs concerning CO2 emissions.
Moreover, among these emissions, it is noteworthy that CO2 is the primary contributor to global warming (Yoro and Daramola, 2020). The escalating levels of CO2 have particularly led to a rise in global average temperatures, resulting in significant climate changes, including extreme weather events, alterations in precipitation patterns, and shifts in ecosystems (Nunes, 2023).
The consequences of CO2 emissions extend to the melting of polar ice caps and icebergs, contributing to rising sea levels that pose a direct threat to coastal communities and marine habitats (Durand et al., 2022).
Furthermore, the absorption of a substantial portion of emitted CO2 by the oceans leads to ocean acidification resulting in a decline of calcium carbonate (CaCO3) saturation state, causing a reduction in oceanic pH levels and severe repercussions for marine organisms (Kurihara, 2008; Jafari et al., 2023).
In order to address these pressing issues, regulations have been introduced to limit pollution emissions from ships. The International Maritime Organization (IMO) has implemented regulatory measures with the overarching goal of attaining net-zero GHG emissions from the shipping sector by or around 2050. Figure 1 illustrates essential regulatory measures and implementation support initiatives crucial for achieving the IMO’s emission reduction objectives (IMO, 2023).
At the European level, the European Union (EU) has integrated shipping into the EU Emissions Trading System (ETS). Within this regulatory framework, shipping companies have to make financial contributions based on the emissions of CO2 they reported in the preceding year, for voyages and port stays within the EU/European Economic Area (EEA). In 2025, shipowners’ companies will be accountable for 40% of the emissions reported in 2024; this obligation will increase to 70% of their 2025 emissions in 2026. Starting from 2027, shipping companies will bear the entire cost, covering 100% of their reported emissions (DNV, 2023; European Commission, 2023).
Compliance with regulatory measures is imperative for the shipping industry, necessitating significant investments in technologies and alternative fuels (Balcombe et al., 2019). However, shipowners show less inclination to order new ships especially due to the uncertainties related to fuel technology and bunker costs. These factors significantly impact the decision-making process regarding solutions to reduce emissions for new ships (Zhang et al., 2021) and despite an ageing fleet, shipowners have opted to sell and purchase second-hand vessels. Because of this, while the transition to alternative fuels in the shipping industry is underway, the number of vessels operating on traditional fuels remains and is projected to be significantly high in the coming years, as evidenced by an analysis of the orderbook (Figure 2) (United Nations, 2023).
It emerges that the current fleet is predominantly composed of ships running on traditional fuels, and the prevailing order book continues to prioritize traditional fuel options. Therefore, the transition to low- and zero-impact alternative fuels is not imminent, and the maritime sector needs a technological shift to mitigate emissions. Postponing the adoption of technologies and fuels leading to climate neutrality to be fully ready is not feasible, as it would result in addressing a larger volume of emissions within a shorter timeframe (Galán-Martín et al., 2021).
Shipowners are compelled to promptly invest in onboard green technologies, given their significant influence on the emission profile of the global shipping fleet and its capacity to adhere to the GHG targets set by the IMO (Piccolo, 2023). Within this framework, it is imperative to discern the optimal solution for regulatory compliance that contributes to the immediate reduction of environmental emissions.
From the analysis of current regulations and future compliance measures in the shipping sector, aimed at addressing the challenges of climate change, CCSs emerge as a promising solution to immediately mitigate a significant amount of CO2 emissions, while facilitating a gradual transition to more sustainable energy sources like renewables (Hua et al., 2023; Tavakoli et al., 2024).
This technology employs different types of chemical and physical processes for capturing and storing CO2 emissions produced by the combustion of fossil fuel sources before they are released into the atmosphere and has proven effective for various inland industries (Leeson et al., 2017; Witte, 2021).
Nevertheless, implementing CCS onboard ships presents challenges due to limited space and stringent operational requirements. Additionally, the logistics of transporting captured CO2 from ships to permanent storage sites can be complex. This process requires specialized equipment, infrastructure, and adherence to strict safety and environmental regulations to ensure the secure and effective storage of CO2 (Al Baroudi et al., 2021).
Despite these challenges, the introduction of CCS technology in the maritime sector represents a significant innovation in combating climate change, aligning with international efforts towards decarbonization.
In this framework, although there are various studies on the implementation of this technology on land (Abanades et al., 2023; Han et al., 2011), the authors aim at addressing the challenges associated with its potential implementation onboard as well as the issues of CO2 storage and transportation. To this purpose, they propose a methodology to evaluate the main features of such innovative solution never tested onboard nor yet investigated in literature. This methodology enables a preliminary sizing of the CCS, which lacks established naval applications, through stoichiometric considerations. By employing relatively quick calculations, the method facilitates a preliminary assessment of the technology’s impact on the reference vessel, providing initial dimensional estimates that allow for an understanding of whether the system can be installed onboard and, subsequently, its broader impact on the vessel’s operations. Building on this methodology, the feasibility assessment for retrofitting an existing vessel design with a CCS with Calcium Hydroxide is conducted, with particular attention given to system-level integration and operational challenges rather than the detailed design of the CO2 scrubber itself. This system, as previously examined by the authors (Bortuzzo et al., 2023), provides distinct advantages over other types of CCSs. Notably, it simplifies onboard logistics by eliminating the need for CO2 liquefaction unit and the additional energy required for its operation (Risso et al., 2023).
The objective of the system implementation is to directly reduce CO2 emissions from ships while concurrently generating a product, CaCO3. This approach aligns with ocean alkalinity enhancement strategies, such as adding lime directly to seawater, which aim at mitigating atmospheric CO2 by enhancing the ocean’s natural carbon absorption capacity (Butenschön et al., 2021; Caserini et al., 2021a, 2022; Comes et al., 2024). The proposed CCS not only reduces CO2 emissions but also transforms the captured CO2 into calcium carbonate, which can be released into seawater to increase alkalinity and counteract ocean acidification. Unlike direct lime addition, which faces logistical challenges such as transportation and distribution, this system could offer a more sustainable and practical alternative.
The most relevant outcomes obtained from the study and their direct correlation with CO2 emission reduction within the maritime transportation sector and benefits for the marine environment are properly discussed.
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