3.1. Midpoint Environmental Performance
Life cycle inventory flows of each production technology (
Table 1) were used to calculate 18 life cycle midpoint impact category indicators (
Table 2) to identify the key midpoint environmental performance indicators and subsequently identify the main processes responsible for the potential impacts of the evaluated hydrogen production processes. For water-related effect, two LCA based methods were used to analyze the environmental performance of hydrogen production methods.
The electrolytic processes (mainly proton exchange membrane fuel cell-PEM) with grid electricity has the worst performance in most of the impact categories. The highest score of impact category in terms of absolute values is observed for global warming potential (GWP), human non-carcinogenic toxicity (HTPnc), water consumption potential (WCP) and ionizing radiation (IRP). Generating hydrogen with a SOEC system has important potential advantages over PEM electrolysis creating less environmental impacts since 28% of energy requirement is provided in form of heat (
Table 2). Theoretically, up to 40% of the energy required to produce H
2 via SOEC electrolysis can be supplied as heat [
10]. The electricity is identified as a major contributor in electrolytic production pathways (
Figure 3). Irrespective of electrolyzer technology, electrolysis is an energy-intensive method of H
2 production, where the environmental footprint is limited to the electricity supply chain [
49]. The manufacturing phase will become the prevailing life cycle impact phase when renewable resources are used, keeping in mind that the absolute emission values go down [
50]. As demonstrated in
Table 2, electrolytic technologies are competitive with other technologies only if renewable electricity is used [
10]. It should be noted that that the scope of ReSOC/SOEC systems is not just to produce H
2, but to avoid the capping/interrupting of generated fluctuating renewable electricity, thus, reducing impacts on power system’s reliability, costs and creating a competitive framework for renewable energy sources deployment. In this case, the fossil fuel-based hydrogen production methods (CG and SMR) are seen to be most environmentally harmful methods [
9]. Similar observations were reported from Bhandari et al. [
10] showing that GWP of electrolysis with grid electricity from the union for the coordination of transmission of electricity (UCTE) showed the worst performance, followed by conventional pathways and ranking of alternatives changed upon the change of electricity source. The environmental values might vary in literature depending on geographical location, fuel choices for electricity generation, and system boundary assumptions. Electrolysis with renewable energy sources can produce relatively low levels of global warming potential (GWP), fossil fuel scarcity (FFP), and toxicity-related impacts.
For the fossil-based system (SMR and CR) the environmental impacts are mainly determined by the raw material used in production processes (
Figure 3). Although it is a slightly less efficient process, the SMR process has better slightly better performance than CG due to the lower life-cycle emissions released from processing the natural gas as opposed to coal [
49]. For SMR the GWP was estimated 12.13 kg CO
2-eq/kg H
2 being in the range from 8.9 to 12.9 kg CO
2-eq/kg H
2 [
10]. For the same process, Cetinkaya et al. [
8] report a GWP value of 11.893 while Spath and Mann [
5] report a value 11.8 kg CO
2-eq/kg H
2. Even, in the fossil-based system can offer promising improvement of the environmental performance when integrated with carbon capture and storage [
51]. Coupling SMR with carbon capture and storage (CCS) can produce a GWP of 3.4 kg CO
2-eq/kg H
2 which is significantly lower than SMR stand-alone system. Verma and Kumar [
51] highlighted that H
2 production from integrated coal-CCS is more environmentally benign than SMR–CCS. The authors estimated that the net life cycle GHG emissions are 0.91 and 18.00 kg CO
2-eq/kg H
2 in H
2 production from coal gasification with and without CCS, respectively. The GREET model [
22] reports the values of 4.08 and 21.39 kg CO
2-eq/kg H
2 for coal gasification pathway with and without CCS, respectively. For SMR these values are 3.07 and 11.3 kg CO
2-eq/kg H
2 with and without CCS. But while CCS offers a great advantage to reduce the GWP, it requires additional electricity and water which will lead to benefits and trade-offs for air pollution. Electricity and water usage for CCS are 0.8 kWh/kg H
2 and around 1.8 kg of water [
24]. This underlines the adequacy of multi-criterion approaches to LCA studies to account the trade-offs between impact categories and avoid burden shifting.
From
Table 2 is shown that BMG technology performs better in most of the evaluated impact categories with respect to fossil-based systems of SMR and gasification of coal. This confirms the results of other studies that biomass-derived H
2 has great potential to reduce environmental footprint [
52]. Hydrogen production via gasification of corn stover is characterized by a GWP potential of 2.66 kg CO
2-eq/kg H
2 which is significantly lower than fossil-based H
2 production and competitive with electrolysis under renewable energy supply. For the same process, the GREET model [
22] report the values of 2.68 kg CO
2-eq/kg H
2. Susmozas et al. [
52] reported a GWP of 0.405 kg CO
2-eq/kg H
2 while Dincer and Acar [
9] around 5 kgCO
2-eq/kg H
2. Muresan [
53] compared biomass and coal gasification technologies demonstrating the superiority of biomass versus coal gasification in terms of GWP, human toxicity, and abiotic depletion potential, however, the acidification and eutrophication potentials were lower in case of the coal-to-H
2 pathway. Similar observations were detected from Acar and Dincer [
9] which found that the BMG gives considerably high acidification potential compared to other selected methods. Biomass feedstock and electricity are identified as the major contributor to the life cycle impact indicators of hydrogen produced from biomass gasification (
Figure 3). For non-fossil H
2 pathways (i.e., biomass-based systems) the impacts will depend on the type, quality, and origin of feedstock. Biomass to hydrogen is a complex process, not only because of the technical details of the conversion processes but also because of the many process types that could be employed. The yield of hydrogen from biomass varies according to the technology used, the operating parameters, and the composition of fuel used. Hydrogen production from biomass often faces technical and economic challenges especially in the small size required for the decentralized hydrogen production [
54]. Moreover, biomass-based H
2 production faces some major inter-connected challenges due to a more complicated supply chain water consumed is related to agricultural production processes.
Environmental analysis of ethanol reforming system with corn stover shows the significant impact in terms of stratospheric ozone depletion (ODP), terrestrial ecotoxicity (TETP), and land use (LOP). The ODP impact category influenced by NO
x and NMVOC tend to be higher in hydrogen pathways where biomass is involved. The impacts will depend on the origin of the feedstock of ethanol. The sensitivity analysis on ethanol production pathway (
Table 2), shows that ethanol originating from wheat is a better choice for 13 out 17 impact categories. Ethanol from wheat distillation shows higher impacts with respect to corn stover for stratospheric ozone depletion (ODP), terrestrial acidification (TAP), human non-carcinogenic toxicity (HTPnc) and land use (LOP).
The process of hydrogen production from lignocellulosic biomass via dark fermentation combined with MEC shows a relatively good environmental performance compared to electrolysis with electricity from grid mix and coal gasification, but higher in respect to SMR and biomass gasification and ethanol reforming. Electricity requirement by the production process is identified as a major contributor to 15 impact categories hydrogen produced from dark fermentation (
Figure 3). A similar conclusion was drawn from Elgoiwany et al. [
55] analyzing greenhouse gas emissions in a well-to-wheel analysis. For the impact categories of stratospheric ozone depletion (ODP) and terrestrial acidification (TAP), the highest impact is attributed to biomass feedstock (i.e., corn stover). For this pathway, the yield and the energy efficiency of hydrogen production can be increased by adopting energy recovery measures where heat requirement is completely eliminated and electricity requirement is reduced from 21.6 kWh/kg H
2 to 6.03 kg kWh/kg H
2. This offers a great potential for improving environmental performance with a significant reduction of impacts (
Table 2), thus being a competitive advantage with respect to other production processes. Elgoiwany et al. [
55] estimated that GWP of H
2 produced from the dark fermentation pathway with and without energy recovery and the values are 9.8 and 19 kg CO
2-eq/kg H
2, respectively.
At midpoint level, comparison of H
2 methods using Water Scarcity Footprint (WSF) with AWARE method [
19] share the same trends as the ReCiPe 2016, indicating that technologies with a high WSF can cause a high impact on the environment both from a water consumption and overall environmental impact. The water consumption and associated damage impacts are reduced in high efficient technologies which use less or do not require water. The highest contributor to the impacts associated with water scarcity in the majority of pathways is electricity consumption (
Figure 3). Consequently, the mix of technologies deployed to produce fuels and electricity determines the associated burden on regional water resources [
56]. As competition and conflicts among agriculture, industry, and cities for limited water supplies are already escalating further analysis would consider the particular water resources used and investigate the sustainability of using the water. Because water is consumed throughout the production supply chain and various production processes are heavily interdependent, assessment of water consumption throughout the life cycle of a fuel is necessary to understand water-related impacts.
3.2. Endpoint Environmental Performance
The LCIA-ReCiPe 2016 endpoint (damage-oriented) method is next applied to translate midpoint environmental impacts of different system configuration and technologies (
Table 2) into damage impact categories of human health, ecosystem quality, and resource scarcity.
Table 3 present the quantified total endpoint indicators. Calculation of performance at both midpoint and endpoint levels simplifies the interpretation of the LCIA results and complement the conclusions of a study given the trade-off between their respective robustness and environmental relevance [
57]. The endpoint analysis shows similar results and trends to those observed in the ReCiPe 2016 midpoint (problem-oriented), highlighting that technologies like electrolysis, biomass gasification, and reforming of renewable bio-derived liquids may be environmentally viable approaches using optimized and renewable-based electricity supply chains.
Category indicator results on the endpoint level are useful to the decision makers to interpret the midpoint indicator results and their relevance to the areas of protection (Human Health, Ecosystem Quality, and Resources impacts) which generally are the objective of the policymakers [
45]. The total contribution to the different endpoint categories might be useful to guide decision makers to select relevant midpoint categories for further examination. But which midpoint environmental impact category is more important than other?
Figure 4 depicts the contribution of each midpoint impact category to endpoint score. The results analysis shows that in the majority of technologies the damage on human health is mainly driven by water consumption (WCP), followed by global warming potential (GWP) and fine particulate matter formation (PMFP). Life-cycle water consumption among different pathways is dominated by electricity use.
For both SMR and CG the application of CCS would reduce their GWP score, thus, the human health and ecosystem quality impact, in that case, is intimately linked with water consumption. During the operation phase, the water footprint is the highest belongs to biomass-based systems (
Table 1). For conventional pathways (SMR, CG) the quantity of water consumed in production process per unit hydrogen generated does not vary substantially amongst these technologies. Nevertheless, the water consumption factors for hydrogen production via biomass gasification, SMR and electrolysis vary by feedstock source and conversion processes [
58]. In the production process, both electrolysis and steam methane reforming will tend to have higher damage impacts scores for one unit of water used since they require high-quality water (low dissolved-solids concentrations) as a feedstock for the production process, which necessitates a pretreatment of water, and thereby energy and materials [
24].
The analysis indicates that water-related impacts tend to be higher in production pathways where GWP and PMPF score is relatively low. This leads to an increased relevancy of the water impacts in these pathways. Extraction of 1 m
3 of water has a higher impact than GWP in human health when converted from midpoint to endpoint (2.22 × 10
−6 DALY/m
3 consumed vs. 9.28 × 10
−7 DALY/kg CO
2-eq). The same applies also to the damage to terrestrial and aquatic ecosystems. While this assessment was done on average end-point characterization factors, with respect to life cycle perspective for each country, the outlook is even more consequential. Furthermore, the choice of cultural perspective that are using different time horizons might have a significant influence on the results [
12]. Hence, proper assessment of end-point impact will require an appraisal using site-specific characterization factors.
For the damage to ecosystem quality, the impacts are mainly attributed to water consumption (WCP), followed by global warming potential (GWP) and terrestrial acidification (TAP). Only for the biomass-derived renewable liquid pathway, a large share of damage in ecosystems (ED) is attributed to the land use (LOP).
For all pathways, the environmental impacts on natural resource scarcity are largely attributed to the fossil fuel scarcity impact category.