## 2 Methodology
### Goal and Scope
#### 2.1.3 Assumptions, Hypotheses, and Limitations  
Several assumptions have been proposed to define alternative scenarios that considered modifications of the main process configuration. The main inputs of the systems are the energy  
Figure 1: System boundaries overview of the ammonia purge gases (APG)-based process.  
Figure 2: System boundaries overview of the coke oven gas (COG)-based process.  
requirements, and thus, the main limitations and assumptions are related to this resource. These hypotheses are indicated in Figures 1 and 2 by the symbol \"?\".  
The main energy demand for hydrogen comes from the hydrogen compressor. In this unit, when the inlet flow pressure was high enough, no additional energy was needed by the hydrogen compressor. If the pressure was too low, the energy was required for hydrogen compression. Energy requirements were calculated in Section S1 of the Supplementary Materials.  
Regarding the electricity source, the use of renewable or non-renewable energy can be considered. In the case of using non-renewable energy, the electricity required for the separation processes could be obtained from the grid. In this type of analysis, the composition of the grid mix is essential, providing a source of uncertainty. The Spanish grid mix comprises 55% of non-renewable energy, 39.6% from nuclear, and 38.9% from combined cycle power plants. In renewable energy highlights the use of wind energy (17.8%), photovoltaic (17.8%), and hydropower (15.1%). To achieve the Paris Agreement targets of limiting global warming to well below 2 \({}^{\circ}\)C above pre-industrial levels and pursuing efforts to limit it to 1.5 \({}^{\circ}\)C [1], a more sustainable and low-carbon economy and energy must be contemplated. The use of renewable energy sources in the grid mix will help to achieve these goals. However, this change to a more sustainable grid mix will have a strong influence on our study. Concerning renewable sources, photovoltaic energy has been widely used as an alternative energy source at the laboratory, pilot plant, and industrial scale, providing excellent results, especially in Spain, due the climatological conditions. Therefore, this renewable source was proposed as a more sustainable scenario. Nevertheless, if we consider this study at the European level, other renewable energies could be studied, providing different results.  
Finally, with regard to methane, the system could implement methane recovery after the hydrogen separation. This recovered methane was employed to produce energy. If methane recovery was not considered, then this gas stream was burnt in a torch. Therefore, the combination of the different alternatives resulted in eight different scenarios (Table 1) to be applied for each gas stream (APG or COG) that must be compared. The defined scenarios constitute a sensitivity analysis that contributed to determine the critical points of the process and propose more sustainable alternatives to feed PEM fuel cells, in particular, HT PEMFC.  
There are several limitations that must be considered in this work. One of the main limitations is the assessment of the energy requirements for the PSA separation, which is based on the minimum thermodynamic energy consumption for the separation of a gas mixture [48]. As a result, the real energy consumption may fluctuate around the proposed levels. Another limitation is that the infrastructure for the separation and combustion is not accounted for, which is typical for machinery that can last periods over 20 years. Finally, it is important to note that this LCA is based on experimental data, since this technology is being developed. The scale-up of the process and the use of PEMFC at the industrial level could vary the obtained results.