## 1 Introduction
To avoid conversion, decrease (due to methane accumulation, a fraction of unconverted synthesis gas) is purged. These ammonia purge gases (APG) consist mainly of H\({}_{2}\) (54-58%), N\({}_{2}\) (23-26%), CH\({}_{4}\) (14-16%) and Ar (<5%) [18; 19]. The hydrogen is mostly recovered and recycled to the synthesis loop via membrane contactors or cryogenic systems, but some part of the cleaned purge gas is usually added to the reformer, or even directly released to the atmosphere [20]. On the other hand, coke oven gas (COG) is a typical by-product in the coking process of coal and can be considered as a potential feedstock for hydrogen recovery. During the coking process, the volatile coal matter is transferred to form COG, leaving carbon-intensive coke behind [21]. The obtained COG consists mainly of H\({}_{2}\) (58-60%), CH\({}_{4}\) (23-27%), CO (5-8%), and CO\({}_{2}\) (<3%) [22; 23]. Through this recovery method, these waste streams can be upgraded instead of burnt off in torches or, in the worst, directly emitted into the atmosphere [24]. Although sometimes COG is employed as fuel in the actual coke ovens or in alternative processes, this practice represents seriously ineffective use of energy when compared to the recovery of hydrogen. However, the recovery of hydrogen from these waste streams requires additional separation stages, which may imply significant amounts of energy and additional economic costs. Although hydrogen is a clean carbon-free fuel, the environmental performance of the production processes depends on the primary sources (fossil fuels or renewable energy) and the specific process [25; 26]. The primary energy inputs and the raw materials required in each process determine the environmental impact and sustainability of hydrogen production. Therefore, comprehensive analyses are needed to evaluate the suitability of each process to contribute to the hydrogen economy scenario. Life-cycle assessment (LCA) is a well-defined methodology to assess the environmental aspects and potential impacts associated with energy production [27; 28; 29], which has been extensively applied to hydrogen production options, such as steam methane reforming, gasification or ethanol reforming, so a relatively high number of LCA studies of hydrogen energy systems can be found in the scientific literature [30]. However, from our understanding, there is no study focused on the environmental evaluation of the upgrading of industrial waste gas streams to obtain high-quality hydrogen. Therefore, the main objective of this work is to realize an LCA of the recovery of hydrogen from residual gas streams from two industrial processes to use it as feed in a high-temperature PEM fuel cell (HT PEMFC). HT PEMFC typically operates at \(\geq\)160 \({}^{\circ}\)C and atmospheric pressure, with dry feed gases, and they have the advantage that they are tolerant of higher concentrations of carbon monoxide (ca. 2%) in hydrogen than low-temperature PEMFC. Although reducing costs and improving durability are two well-known challenges to implement fuel cells, a challenge that should never be neglected is precisely the origin of the need to change the energy production system: The production of energy with minimal environmental impact. Consequently, evaluating the environmental impact of innovative energy technologies is required, and LCA is the best tool to achieve the goal. The assessment of the environmental impact of emerging technology, such as the proposed recovery process, is not an easy task [31]. Therefore, this purpose implies the development of the LCA methodology, including the definition of the objective and scope, description of the limits of the systems, and consideration of different functional units. The inputs and outputs of the system were compiled to obtain the corresponding life-cycle inventory, based on material and energy balances to develop the modeling of the system.