## 1 Introduction  
Shipping is the main freight transport mode around the world, transporting about 90% of global trade. Although it is the least atmosphere-damaging means of transport, its cumulative effect contributes to the overall air quality problem [1]. The International Maritime Organization (IMO) is the United Nations' regulatory agency for safety, security, and environmental performance of the shipping industry. IMO controls the reduction of major ship exhaust emissions (SO\({}_{x}\), NO\({}_{x}\), VOCs, and ODS) through Annex VI of the International Convention for the Prevention of Pollution from ships (MARPOL) [2]. This regulation reduces the global SO\({}_{x}\) limit to 0.50% m/m outside an Emission Control Area (ECA), and to 0.10% m/m inside, and it regulates the NO\({}_{x}\) emissions through three tiers based on the ship construction date and the engine speed, as indicated in Table 1[3]. Concerning the CO\({}_{2}\) emissions from ships, IMO adopted an Initial Strategy, still pending review in 2023, whose main goal is to cut annual GHG emissions by at least half by 2050 compared with their level in 2008 [4]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Tier** & **Construction Date** & _n < 130_ & _n = 130–1999_ & _n \(\geq\) 2000_ \\ \hline I & 1 January 2000 & 17.0 & 45 \(\times\)\(n^{-0.2}\) & 9.8 \\ II & 1 January 2011 & 14.4 & 44 \(\times\)\(n^{-0.23}\) & 7.7 \\ III & 1 January 2016 & 3.4 & 9 \(\times\)\(n^{-0.2}\) & 2.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Total weighted cycle NO\({}_{x}\) emission limit [g/kWh] for ship engines [3]. Currently, there are three reduction strategies for marine engines in order to meet the SO\({}_{\rm x}\) and NO\({}_{\rm x}\) emission standards on ships: fuel optimization, pre-combustion control, and exhaust after-treatment systems. Fuel optimization consists of replacing heavy ship fuels, such as heavy fuel oil (HFO) and marine diesel oil (MDO), with clean alternative fuels to mitigate SO\({}_{\rm x}\) and PM emissions due to low sulfur content; pre-combustion control techniques, such as water addition and EGR, are adopted for controlling NO\({}_{\rm x}\) emissions by preventing its formation conditions during combustion; and the exhaust after-treatment systems, among them SCR to decrease NO\({}_{\rm x}\) and DPF to remove PM, are technologies that effectively reduce NO\({}_{\rm x}\) and PM emissions without significant penalty in the engine power and fuel economy [5]. Intending to reduce their environmental footprint, the shipping industry is switching to cleaner fuels, either by replacing fossil fuels with low-carbon and carbon-negative fuels, or by using carbon-neutral fuels [6]. However, a technology to decrease CO\({}_{2}\) emissions is still necessary to meet the global objective of the Paris Agreement; Carbon Capture and Storage is one of the key technologies for this purpose. In CCS, the CO\({}_{2}\) is captured in three ways--post-combustion capture, pre-combustion capture, and oxy-combustion--and then transported to permanent offshore storage via pipeline, ship, or both [7]. As an alternative to storing the captured CO\({}_{2}\), it is also used during the processing of synthetic and low-carbon fuels together with green H\({}_{2}\)[8]. As an alternative to NO\({}_{\rm x}\) emission-reducing technologies and complementary to CCS, oxy-fuel combustion appears as a novel option. In this combustion process, air is replaced by high-purity oxygen to eliminate NO\({}_{\rm x}\) emissions by the absence of N\({}_{2}\), obtaining as exhaust gases mainly CO\({}_{2}\) and H\({}_{2}\)O, which are used to control the high combustion temperature [9]. oxy-fuel combustion facilitates the separation of CO\({}_{2}\) from H\({}_{2}\)O after condensation, so this concept was initially implemented in power plants, both coal-fired [10] and biomass-fired [11], as an alternative to the Carbon Capture and Storage (CCS) methods of post-combustion and pre-combustion [12].