## 3 Results and Discussion
### Life-Cycle Impact Assessment Based on Hydrogen Production
The impact in this category was associated with both the electricity used and the emissions from the torch and the combustion engine. The alternatives in which there was no energy recovery of methane (scenarios 3, 4, 7, and 8) resulted in values considerably higher due to the emissions derived from the burning of gases in the torch. The influence of the pressurization of the input stream can be analyzed comparing scenarios 1 (pressurized stream, with energy recovery and electricity from the grid) and 5 (non-pressurized stream, with energy recovery and electricity from the grid), which only differ on the pressurization stage. A slightly higher value was obtained in scenario 5, due to the emissions associated with the electrical consumption of the compressor. In all cases, the use of photovoltaic energy generated fewer greenhouse gas emissions than the grid. Comparing the APG and COG streams, it can be observed that for scenarios 1, 2, 5, and 6 the GWP values were lower for the COG case, while the opposite situation occurred in the rest of scenarios. This was because the emissions, due to the torch handicapped, were much more the COG stream than the APG one, since there was a lower content in CH\({}_{4}\), CO, and CO\({}_{2}\) in this latter stream. Once again, like in the case of the ADPF category, the best environmental conditions were associated with scenario 2 and the worst ones with scenario 7. Lastly, regarding AP, the values ranged from \(-\)7.82\(\cdot\)10\({}^{-3}\) to 2.79\(\cdot\)10\({}^{-3}\) kg SO\({}_{2}\) eq/kg H\({}_{2}\) for the APG case and from \(-\)3.48\(\cdot\)10\({}^{-2}\) kg SO\({}_{2}\) eq. to 2.84\(\cdot\)10\({}^{-3}\) kg SO\({}_{2}\) eq/kg H\({}_{2}\) for the COG case. A similar trend to that observed for ADPF was identified, which can be justified because of the direct relationship between the emissions and avoided loads and the consumption and generation of electricity. Consequently, once again, scenario 2 resulted in the best option, while scenario 7 was the worst one. To evaluate the potential environmental benefits derived from hydrogen recovery systems proposed in this work, it was necessary to compare these options with the values corresponding to other conventional hydrogen production systems. Bibliographic references that have applied LCA for different processes ways to obtain hydrogen and the assessed environmental impacts are compiled in Tables 4 and 5 for the GWP and AP impacts, respectively. As observed, the GWP values obtained in this study were generally lower than the results published for steam methane reforming, which are above 10 kg CO\({}_{2}\) eq/kg H\({}_{2}\)[53; 54; 55], so hydrogen recovery from waste gas streams appeared as a competitive alternative, even when the reforming of alternative chemicals like methanol or ethanol was considered [56]. In the cases where the process conditions were not favorable, such as scenarios 3, 4, 7, and 8 for the COG stream, the emissions calculated were slightly higher, so the design and operation of the gas recovery system must be studied in detail and optimized to assure the competitiveness of the process versus alternative options. In the same way, coal gasification resulted in GWP values similar to the ones obtained by steam methane reforming, so the proposed recovery systems can compete with this process too [53; 56]. However, regarding electrolysis, the source of electricity clearly defined the competitiveness of the process. When renewable wind or photovoltaic energy was employed, the impact was lower than in the case of the hydrogen recovery process, since electrolysis under these conditions must be considered a clean and environmentally friendly process [53; 57]. Instead, they use of electricity from the grid resulted in a considerably higher GWP impact and must be avoided when compared to gas recovery.