## 3 Results and Discussion
### Life-Cycle Impact Assessment Based on Energy Production  
In this section, the functional unit considered was 1 kWh generated by the HT PEM fuel cell. Therefore, in this system, in addition to taking into account the production of hydrogen, the manufacture and use of the fuel cell were included too. The environmental impacts assessed for the APG and COG cases are shown in Figures 4 and 5, respectively. Figure 4: Environmental impact results of the APG process (FU = 1 kWh) in the categories (**a**) ADPE, (**b**) ADFE, (**c**) GWP, and (**d**) AP. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Production Process** & **AP (kg SO2 eq./kg H2)** & **Reference** \\ \hline Steam methane reforming & 0.008 & [54] \\ Electrophysis (photovoltaic electricity) & 0.028 & [63] \\ Electrophysis (wind electricity) & 0.003 & [63] \\ Biomass gasification & 0.034 & [63] \\ Methane partial oxidation & 0.008 & [60] \\ Supercritical water reforming of glycerol & 0.012 & [62] \\ Autothermal bioethanol reforming & 0.028 & [61] \\ Supercritical water gasification (photovoltaic energy) & 0.190 & [58] \\ Electrophysis (USA mix electricity) & 0.069 & [56] \\ Coal gasification & 0.011 & [56] \\ Biomass gasification & 0.016 & [56] \\ Ethanol reforming & 0.032 & [56] \\ Methanol reforming & 0.017 & [56] \\ Methane reforming & 0.012 & [56] \\ \hline \hline \end{tabular}
\end{table} 
Table 5: AP impact, related to different technologies that produce hydrogen. In the ADPE impact category, the values were between 2.56\(\cdot\)10\({}^{-6}\) and 3.13\(\cdot\)10\({}^{-6}\) kg Sb eq./kWh for the APG case and between 2.19\(\cdot\)10\({}^{-6}\) and 3.14\(\cdot\)10\({}^{-6}\) kg Sb eq. /kWh for the COG case. Most of the impact was attributable to the fuel cell, since the production of this device required scarce elements like platinum. Although in scenarios 6, 7, and 8 positive values (environmental impacts) of the hydrogen recovery stage occurred, the contributions were more significant in scenarios 6 and 8, which worked with streams not pressurized that required the use of photovoltaic electricity. The rest of the scenarios exhibited negative values (avoided burdens) for the hydrogen recovery stage, but the impacts, due to the cell could not be compensated, since these latter were at least an order of magnitude higher in absolute terms. Regarding ADPF, the values ranged from \(-\)1.95 to 0.81 MJ/kWh for the APG system and from \(-\)8.94 to 0.82 MJ/kWh for the COG system. The negative values for the hydrogen recovery stage obtained in scenarios 1, 2, 5, and 6 as a consequence of the energy recovery from methane must be highlighted, since they clearly compensated the loads, due to the fuel cell operation. Since the recovery stage generated more energy than the requirement of the full system, the resulting negative values implied clear, environmentally friendly conditions in these scenarios. In scenarios 3 and 4, the impacts were completely caused by the cell, due to the absence of electricity consumption, while in scenarios 7 and 8, there were impacts, due to the recovery of hydrogen since energy recovery from methane was not implemented. The GWP values fell in the interval between 0.28 and 0.52 kg CO\({}_{2}\) eq./kWh for the APG system and between 0.12 kg and 0.96 kg CO\({}_{2}\) eq./kWh for the COG case. These results were in the range of values that characterize the carbon footprint of natural gas-fired power generation [66, 67]. The contribution of the hydrogen recovery stage in this category is much greater than that of the cell, being between 82% and 90% for the APG case and from 61% to 95% for the COG system. In the scenarios without methane recovery (scenarios 3, 4, 7, and 8), the impact was significantly higher, and therefore, the contribution of the hydrogen recovery stage is greater, due to the emissions produced in the burning of gases in the torch.