# Comparative Analysis of Marine Alternative Fuels for Offshore Supply Vessels
## 2 Methodology
### Environmental Impacts (Step 4)
#### 2.4.1 Environmental Data Collection  
Considering the fuel production stage (WTT stage) and fuel consumption stage (TTW stage), the results of the comprehensive environmental impact on various alternative fuels were estimated, as shown in Table 9 and Figure 2. This study estimated comprehensive environmental impacts of various alternative fuels, considering both the WTT and TTW stages, based on several research works [17; 18; 19]. The results were indexed relative to HFO (set at 100). The findings show that all alternative fuels have a significant impact when produced as grey fuels, emphasising the importance of using blue or green fuels. Biodiesel shows decreasing climate change impact with each generation. Alternative fuels vary in local air pollutant emissions; ammonia and methanol emit less SOx and PM but still produce NOx, while hydrogen and electricity are cleanest. Fuel cells represent the most environmentally friendly propulsion system. However, these relative values are not absolute, as actual impact depends on factors like production method and region. A comprehensive evaluation should consider the physical characteristics of each fuel and the situation of the specific ship. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Fuel**} & \multirow{2}{*}{**Case**} & \multirow{2}{*}{**Fuel Type**} & \multicolumn{4}{c}{**Technical Impact**} \\ \cline{3-6}  & & & **Fuel Storage** & **Technological** & **Total** \\ \hline \multirow{8}{*}{LNG} & \multirow{2}{*}{Mechanical propulsion} & Grey & **4** & 5 & 9 \\ \cline{3-6}  & & Bio & **4** & 5 & 9 \\ \cline{3-6}  & & E & **4** & 5 & 9 \\ \cline{2-6}  & \multirow{2}{*}{Electric propulsion} & Grey & **4** & 5 & 9 \\ \cline{3-6}  & & Bio & **4** & 5 & 9 \\ \cline{3-6}  & & E & **4** & 5 & 9 \\ \cline{3-6}  & \multirow{2}{*}{Electric propulsion + battery} & Grey & **4** & 5 & 9 \\ \cline{3-6}  & & Bio & **4** & 5 & 9 \\ \cline{3-6}  & & E & **4** & 5 & 9 \\ \hline \multirow{4}{*}{Electricity} & \multirow{2}{*}{Battery} & Grey & **4** & 5 & 6 \\ \cline{3-6}  & & Blue & & 5 & 6 \\ \cline{1-1} \cline{3-6}  & & Green & & 5 & 6 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: _Cont._ \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & & \multicolumn{4}{c}{**(\%/Emissions of HFO)**} \\ \cline{3-6}  & & **Relative GHG** & **Relative SOx** & **Relative NOx** & **Relative PM** \\ \hline \multirow{3}{*}{Ammonia} & Grey [19] & 139 & 0 & 100 & 0 \\ \cline{2-6}  & Blue & 34 & 0 & 100 & 0 \\ \cline{2-6}  & Green & 0 & 0 & 0 & 0 \\ \hline \multirow{3}{*}{Hydrogen} & Grey [19] & 166 & 0 & 0 & 0 \\ \cline{2-6}  & Blue & 14 & 0 & 0 & 0 \\ \cline{1-1} \cline{2-6}  & Green & 0 & 0 & 0 & 0 \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Estimations for emissions of each fuel [18]. However, traditional ammonia production through the Haber-Bosch process is associated with significant CO2 emissions. In contrast, green ammonia produced through electrolysis using renewable energy is a more environmentally friendly alternative, offering a path towards reduced carbon footprints in shipping [22]. Quantitative assessments indicate that green ammonia derived from renewable sources is the most promising carbon-free fuel for maritime use, whereas grey ammonia produced from fossil fuels actually generates higher greenhouse gas (GHG) emissions compared to heavy fuel oil (HFO) [23]. Despite its potential, a major challenge in utilising ammonia as a green fuel is the emission of nitrogen oxides (NOx) during combustion. Nitrox oxide (N2O), in particular, has a global warming potential (GWP) roughly 270 times that of CO2, which underscores the need for advanced combustion technologies and effective emissions control systems when considering ammonia as a marine fuel [23]. Hydrogen is emerging as a promising marine fuel due to its potential to significantly reduce greenhouse gas emissions, as its only combustion byproduct is water vapor.