## 3 Results and Discussion
### Life-Cycle Impact Assessment Based on Hydrogen Production  
In this section, the FU under consideration was 1 kg of hydrogen with a molar purity of 99.97%. The results obtained in each scenario for each impact category are shown in Figure 3.  
Figure 3: Environmental impact results of the APG and COG processes (FU = 1 kg H2) in the categories (**a**) Abiotic Depletion Potential for elements (ADPE), (**b**) Abiotic Depletion Potential fossil fuels (ADPF), (**c**) Global Warming Potential (GWP), and (**d**) Acidification Potential (AP). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & & & \multicolumn{2}{c}{**Inputs**} & \multicolumn{2}{c}{**Outputs**} \\ \cline{3-8}
**Gas** & **Scenarios** & **Feed** & **N2** & **CH4** & **Fuel** & **Fuel Cell** & **Engine** \\
**Stream** & & **Flow** & **Separation** & **Separation** & **Cell** & **Energy** & **Energy** \\  & & **(kg/h)** & **Energy (kWh)** & **Energy (kWh)** & **(Units)** & **(kWh)** & **(kWh)** \\ \hline COG & E1/E2 & 1566 & 0 & 0 & 1 & 2927 & 6789 \\ APG & E3/E4 & 137 & 0 & 2.7 & 1 & 245 & 128 \\ COG & E3/E4 & 1566 & 0 & 0 & 1 & 2927 & 0 \\ APG & 137 & 0 & 0 & 1 & 245 & 0 \\ COG & E5/E6 & 1566 & 555 & 0 & 1 & 2927 & 6789 \\ APG & 137 & 44 & 2.7 & 1 & 245 & 128 \\ COG & E7/E8 & 1566 & 555 & 0 & 1 & 2927 & 0 \\ APG & 137 & 44 & 0 & 1 & 245 & 0 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Energy balances of the system in different scenarios. Regarding ADPE, the values obtained ranged from \(-\)1.73\(\cdot\)10\({}^{-6}\) to 7.8\(\cdot\)10\({}^{-6}\) kg Sb eq/kg H\({}_{2}\) for the APG case and from \(-\)7.87\(\cdot\)10\({}^{-6}\) to 7.95\(\cdot\)10\({}^{-6}\) kg Sb eq/kg H\({}_{2}\) for the COG case. All the emissions and avoided burdens associated with this category were due to the electricity demand and recovery in the process, resulting in zero in scenarios 3 and 4, since there was no methane energy recovery (no electricity production) and the stream was pressurized (no electricity demand). When electricity from the grid was used (scenarios 1, 5, and 7), the ADPE values were more favorable than the ones resulted from the use of renewable energy (scenarios 2, 6, and 8), because of the consumption of natural resources required to produce photovoltaic panels [52]. The results corresponding to the COG stream were more environmentally friendly than those of the APG stream in all the scenarios, except 7 and 8. This fact was a consequence of the higher energy spent in the COG case to separate the hydrogen without recovery of energy from methane, so the emissions cannot be compensated as in other cases (scenarios 1, 2, and 5). It was clear that in these three latter scenarios, the recovery stage of methane produced more energy than the required one by the rest of the system, so negative ADPE values were obtained associated with the energy credits. Consequently, the best conditions were found in scenario 1 (pressurized stream, energy recovery from methane, and electricity from the grid), while the worst conditions were associated with scenario 8 (non-pressurized stream, without energy recovery from methane and photovoltaic electricity). In relation to the ADPF, the values obtained were between \(-\)33.7 and 12.0 MJ for the APG case and \(-\)149.7 MJ/kg H\({}_{2}\) and 12.2 MJ/kg H\({}_{2}\) for the COG case. Like in the case of the ADPE category, these impacts were directly linked to the electricity. However, in this case, the use of photovoltaic electricity offered better results, and it was preferred to the use of electricity from the grid. Therefore, in this category, the best value corresponded to scenario 2 (pressurized stream, energy recovery, and photovoltaic electricity), while scenario 7 appeared as the worst option (non-pressurized current, without energy recovery and electricity from the grid). The values of GWP laid between 3.90. and 7.88 kg CO\({}_{2}\) eq/kg H\({}_{2}\) for the APG case and 1.27 and 15.24 kg CO\({}_{2}\) eq/kg H\({}_{2}\) for the COG case.