## 4 Conclusions  
Nowadays, the emissions from stationary sources are based on the use of fossil fuels. However, in the short-to-medium-term, the use of more sustainable resources will open the niche to new fuels, such as hydrogen. The study of the environmental advantages and disadvantages of the obtention and use of this product is essential to determine the best configuration. In this work, the environmental assessment of the recovery of hydrogen from waste streams to use to feed HT PEM fuel cells was carried out, applying the LCA methodology according to ISO 14040 and 14044. The stages of the complete system have been individually studied to analyze the environmental impact of each one and study the viability and competitiveness of the process. This LCA defined several functions: The generation of hydrogen and electrical energy and the treatment of residual gas streams (COG or APG). Based on these functions, we define several functional units to provide a more accurate and comparable study. Thus, the results were referred to as 1 kg of hydrogen at the requested purity of 99.97%, 1 kWh of energy obtained from the fuel cell, and 1 kg of COG or APG. The hydrogen recovery from waste streams resulted in environmental impacts between \(-7.87\cdot 10^{-6}\) and \(7.95\cdot 10^{-6}\) kg Sb eq/kg H\({}_{2}\) for the ADPE category, from \(-149.7\) to \(12.2\) MJ/kg H\({}_{2}\) for ADPF, from \(1.27\) to \(15.24\) kg CO\({}_{2}\) eq/kg H\({}_{2}\) for GWP, and from \(-3.48\cdot 10^{-2}\) to \(2.84\cdot 10^{-3}\) kg SO\({}_{2}\) eq/kg H\({}_{2}\) for AP. These impacts, specifically the GWP values, resulted lower than the impacts, due to steam reforming of methane, coal gasification, or more complex hydrogen production alternatives like supercritical water gasification, chemical looping, advanced methane cracking, or tri-reforming. Only electrolysis of water by means of renewable energies, such as wind or solar, showed values that could improve the environmental impact of the proposed recovery process, since electrolysis by grid electricity based on fossil fuels appeared disadvantageous from an environmental perspective. For the production of 1 kWh, the complete system that comprised both the hydrogen production stage and the manufacture and use of the PEM cell showed low values in the ADPE, ADPF, and AP categories. In the case of GWP, values between 0.28 and 0.52 kg CO\({}_{2}\) eq/kWh and 0.12 to 0.96 kg CO\({}_{2}\) eq/kWh were calculated for the APG and COG streams, respectively. These impacts were comparable to the range of values of the carbon footprint of energy generation by means of natural gas. Furthermore, it was demonstrated that in the GWP and ADPF categories, the greatest impacts were caused by the recovery stage, while in the other two categories, the cell implied higher burdens. These latter burdens were mainly caused by the platinum used in its composition, so an alternative process in which this component could be reduced to a lower level could decrease GWP impacts by up to 84%. Regarding the stream used, it can be concluded that depending on the impact category studied, the advantages of using one type or another of flow varied. To clearly identify the most convenient option, a more exhaustive study should be carried out, taking into account a greater number of impact categories, so that it can be observed in detail which option can provide more environmental benefits. The analysis of the alternative scenarios found that having a pressurized waste stream meant considerable energy savings, since no compressor was required, which translated into lower environmental impacts in all the categories studied. The recovery of methane supposed a relevant improvement in the complete process--since, on the one hand, it avoided to burn the residual gas flow in a torch producing harmful emissions, while, on the other hand, it allowed the recovery of additional energy.