Environmental evaluation of european ammonia production considering various hydrogen supply chains
Introduction
Ammonia (NH3) is one of the most synthesised chemicals worldwide, having widespread use in energy concepts and production of explosives, pharmaceutics, fibres, plastics, paper and other essential chemicals and products. Fertilizer production is the major consumer of ammonia, accounting for about 85% of the total ammonia production [1]. Additional applications of ammonia are in the field of flue gas conditioning by removing and absorbing fly ashes and components like nitrogen oxides (NOX) and carbon dioxide (CO2), as well as in refrigeration applications. Recently, ammonia has received considerable attention as a promising fuel and energy carrier (i.e. indirect hydrogen storage material) due to its characteristics of carbon-free, high energy density, and convenience in transportation and storage [2]. Ammonia for power applications was investigated by several authors. Extensive reviews were prepared by Valera-Medina et al. [3] and Yapicioglu and Dincer [4] assessing from a techno-economic and environmental perspective the advancement and research directions made in this field.
With the continuous growth of the world's population and increasing interest in energy-related ammonia applications, global ammonia production is expected to increase in the following decades. The most common ammonia production methods available in the world are Haber-Bosch process and solid state ammonia synthesis (SSAS) [5]. Currently, around 90% of the global ammonia production is obtained through the Haber-Bosch process, which combines nitrogen (N2) and hydrogen (H2) at high pressures and temperatures over an iron-based catalyst [6]. Nitrogen is usually obtained from air by a cryogenic process taking place in an air separation unit (ASU); this method is the most mature and developed technology. Hydrogen, on the other hand, can be obtained from various feedstocks (e.g. coal, natural gas, biomass, naphtha, heavy fuel oil, coke oven gas, refinery gas, water) and processes (e.g. reforming, pyrolysis, gasification, electrolysis, photocatalysis, biological fermentation). Steam methane reforming (SMR) is the most mature and generally applied hydrogen production technology for ammonia synthesis [5]. Even though electrochemical production of hydrogen is a mature and commercially applied technology, the fluctuating nature of renewable energy sources and the higher production cost make fossil-based hydrogen production the dominant production route [7]. SSAS, still a developing technology, is an electrochemical process that can operate in a large temperature range (i.e. 100 - 400 °C) at atmospheric pressure using solid state hydrogen ions (H+) cells. Gaseous hydrogen and nitrogen are introduced into the system, with H2 being converted into protons at the anode and transferred electrochemically to the cathode where they react with N2 to obtain ammonia [8]. Fig. 1 illustrates the above-mentioned ammonia production routes.
Globally, ammonia production is accountable for over 1% of the total energy-related CO2 emissions together with about 420 million tonnes of CO2 being emitted into the atmosphere during its synthesis [9]. In order to limit the impact on the environment caused by the current ammonia production routes, which rely greatly on fossil fuels both as an energy source and as feedstock, other greener and sustainable production pathways need to be implemented. In the scientific literature, there are several studies that focus on the various aspects of ammonia production from a technological, economic, or environmental perspective. In the last decade, with the advancements made in the field of renewable and sustainable energy production, electrochemical synthesis of ammonia has received a lot of attention as a more sustainable production alternative. Kyriakou et al. [10] reviewed the progress made in this field, highlighting the advantages of SSAS compared to the Haber-Bosch process, such as the energy and cost savings attributed to hydrogen purification as well as no greenhouse gas emissions, making it a green ammonia synthesis route. On the other hand, the electrical consumption of the electrolyser is much higher leading to poorer economics. Still, the development of this technology relies greatly on the development of solid state material that presents good mechanical and chemical strengths as well as high protonic activity at moderate temperatures (e.g. 250–450 °C). Yapicioglu and Dincer [4] reviewed the recent advances made in renewable ammonia production methods, focusing on electrolyte-based systems such as liquid electrolytes, molten salt, composite membranes and solid state electrolytes. Electrochemical ammonia synthesis, at close to atmospheric pressure, was investigated by Gomez et al. [11] from a techno-economic and environmental perspective and compared with the Haber-Bosch process. The results showed that the economics are greatly susceptible to electricity prices and technological advancements. From an environmental point of view, resource consumption (e.g. energy and water) was evaluated in comparison with conventional ammonia production, concluding that 90% of the total resources consumption can be attributed to fossil and natural gas fuels used for hydrogen production and electricity generation while water consumption was higher for the electrochemical ammonia production route.
Other green ammonia production routes can be achieved by providing hydrogen from water electrolysis to the Haber-Bosch process (but only if the electricity is obtained from renewable energy sources) or from other low-carbon feedstocks such as biomass [12], bioethanol [13], bio-glycerol [14,15] or biogas [16]. Xu et al. [17] developed a mathematical approach for evaluating the sustainability of low-carbon ammonia production routes in China, in which hydropower electrolysis-based ammonia production ranked first, followed by biomass, wind, solar and nuclear high-temperature electrolysis-based systems. Hydrogen production pathways, either fossil or renewable-based, have been studied to a greater extent in the recent scientific literature. Orhan et al. [18] investigated the production of green hydrogen through thermochemical water splitting coupled to nuclear and renewable energy. Hydrogen production through water electrolysis based on solar [19] and wind energy [20,21] was also investigated, the later discussing the economic viability of renewable hydrogen production as well. An overview of renewable and sustainable hydrogen production pathways considering state-of-the-art technologies is presented from a techno-economic perspective in the work of Hosseini et al. [22]. Nikolaidis and Poullikkas [23] presented in their work a comparative overview of the techno-economic performance of 14 different hydrogen production methods from fossil and renewable sources alike. It was concluded that despite the negative environmental consequences of fossil-based hydrogen production, in the medium term it will remain the preferred technological route, mainly due to reduced cost, technological advancement, and availability of resources compared to electrolytic and other sustainable hydrogen production routes. A more recent review of economic and environmental aspects of hydrogen production from various sources and technologies was carried out by Baykara [24], reaching the conclusion that biomass conversion and electrolytic hydrogen production technologies using solar-based energy will represent the most sustainable options in the long term.
From a cost perspective, the integration of carbon capture and storage (CCS) technologies to fossil-based hydrogen production technologies is more economical than the coupling of renewable energy sources with electrolysis [25]. Thus, fossil-based hydrogen production integrated with CCS represents a viable solution for low-carbon hydrogen production and, respectively a sustainable ammonia production route. The most mature and currently applied CCS technologies are based on gas-liquid absorption using amines. However, the main drawback of these technologies is the high energy intensity of solvent regeneration. Research and development activities form the last decade lead to the identification of new solvents with improved performances. As this paper addresses the performance of ammonia production, aqueous ammonia solution as solvent was taken into consideration. The major advantages of using aqueous ammonia solutions as solvents for CO2 removal over amine-based solvents are the lower energy demand for regeneration and higher absorption rate [26]. However, there are certain drawbacks as well, in particular the higher risk of fugitive emissions due to its high volatility. Wang et al. [27] reviewed the status of ammonia escape abatement technologies such as water wash, additives, new process conditions. Chilled ammonia process seems to be a good solution to lower fugitive ammonia emissions by lowering the gas temperature; however, the additional energy consumption could affect the economic competitiveness of this technology with the amine-based systems. Literature review considering applications of ammonia-based CO2 capture systems focuses mainly on power generation systems [28]. Kim et al. [29] performed a life cycle assessment (LCA) to pinpoint the hot-spots of an ammonia-based CCS system capturing CO2 from a coal-fired power plant. It was observed that the energy (i.e. coal combustion) for regeneration is the major contributor to CO2 emission followed by electricity for CO2 transport. Possible solutions were suggested to further reduce emissions such as better heat integration for potential waste-heat recovery to use for solvent regeneration, replacing the transport and storage section with CO2 utilization to avoid related emissions.
Chemical looping is a promising technology for potential hydrogen generation with in-situ CO2 removal with low energy penalty. It is based on the circulation in a closed loop of an oxygen carrier (OC) in the form of metal oxides, which are able to undergo partial oxidation by steam with the production of hydrogen. Iron-based OCs, in particular, are promising materials for hydrogen production due to the application of the steam-iron reaction. At The Ohio State University, the chemical looping technology for hydrogen production using iron-based OC was successfully demonstrated in a 25 kWth sub-pilot reaching hydrogen purities higher than 99% [30]. Luo et al. [31] reported in their work the recent advances made considering two looping technologies designed for hydrogen production (i.e. chemical looping reforming (CLR) and chemical looping hydrogen production (CLH)). Techno-economic evaluations of chemical looping technologies intended for hydrogen production show competitive costs of production with conventional SMR at CO2 avoidance costs between 20 and 80 €/tCO2 [32,33]. Integration with ammonia production was investigated by Martinez et al. [34] who assessed from a technical point of view the integration of calcium (Ca) - copper (Cu) chemical loop in ammonia production plants and Edrisi et al. [35] who proposed the use of three-reactor CLH for the production of both pure H2 and N2, ready to be used for NH3 synthesis. Ajiwibowo et al. [36] evaluated the performance of an integrated system for ammonia production, associating supercritical water gasification of palm mill waste material with syngas chemical looping for hydrogen production observing a hydrogen conversion efficiency of 76.2% and maximum biomass to ammonia conversion greater than 15%.
Techno-economic assessments of ammonia production from non-fossil sources were performed by several authors. Zhang et al. [37] performed a comparative techno-economic evaluation of the conventional ammonia production path with hydrogen obtained by SMR with two green ammonia production processes, namely hydrogen obtained from biomass gasification and water electrolysis based on renewable energy. The results of the study showed that the solid oxide electrolyser achieved the highest efficiencies but economically is not feasible due to high electricity prices, while the biomass system presents the lowest efficiencies as well as higher prices compared to the conventional ammonia production route. Andersson and Lundgren [38] reached similar conclusion with regard to the economics of ammonia production via biomass gasification. Techno-economic evaluations of other non-fossil ammonia production routes lead to similar results [39,40].
The environmental aspects of ammonia production are investigated less. Ammonia production coupled with biomass gasification was investigated from a techno-economic and environmental perspective by Gilbert et al. [41], showing that a 65% reduction in greenhouse gas emissions can be achieved compared to the natural gas-based conventional process. LCA studies of ammonia production considering various carbon-free energy sources (i.e. hydropower, nuclear, biomass, municipal waste) for hydrogen production through electrolysis were conducted by Bicer and his collaborators [42,43]. Singh et al. [44] performed an extensive LCA of various ammonia production methods based on fossil fuels, biomass gasification, and water electrolysis combined with wind and solar energy. The results showed that biomass-based ammonia production seems to be the more environmentally friendly out of the investigated methods, followed by ammonia production integrated with water electrolysis based on wind energy. Site-specific environmental evaluations considering ammonia production via SMR were performed for Algeria [45] and Mexico [46], the later considering also CO2 utilization for enhanced oil recovery (EOR). Life cycle inventory (LCI) analyses of power generation from ammonia as a hydrogen energy carrier was carried out by Ozawa et al. [47]. Taking into account also the CO2 emissions associated with ammonia supply chain, the results showed that a CO2 emission's reduction of 36% can be achieved for ammonia-fired power generation with respect to a natural gas combined cycle with CCS. Arora et al. [48] performed a techno-enviro-economic assessment of a small scale ammonia production facility based on biomass, reporting CO2 emission reductions between 54% and 68% compared to the conventional ammonia production route. The potential environmental benefits of integrating chemical looping technologies into ammonia production were not yet investigated to the authors' best knowledge. Likewise, LCA of chemical looping for hydrogen production is scarcely investigated. Thermal integration with SMR was assessed by Petrescu et al. [49] and by Wang et al. [50], showing that the integration with chemical looping is a promising alternative to reduce CO2 emissions. Other authors evaluated the environmental impact of hydrogen production by chemical looping technologies using biomass [51] and coal [52] as feedstock.
In the reviewed literature, environmental aspects of ammonia production cover mainly renewable production routes. However, considering the economic criteria, fossil-based ammonia production will remain a significant production pathway in the following decades. Thus, the present work aims to evaluate and compare the environmental footprint of various ammonia production routes considering sustainable/non-sustainable hydrogen production from natural gas. The integration of CLH with the Haber-Bosch process for ammonia synthesis is proposed as a more sustainable production pathway. CLH can be regarded as an integrated process, being capable of producing both hydrogen and nitrogen suitable for ammonia production, while inherently separating CO2. Moreover, the integration of chilled ammonia as an alternative option for acid gas removal (AGR) in the conventional synthesis path is also examined. For comparison reasons, the conventional ammonia production path (i.e. Haber-Bosch process with H2 obtained from SMR and N2 from cryogenic air separation) is used as benchmark, representing the state-of-the-art for ammonia synthesis. In addition, ammonia production integrated with hydrogen obtained from water electrolysis is evaluated as a reference for a sustainable ammonia production route. A cradle-to-gate environmental impact evaluation of the processes considered, using GaBi V8 software, is performed. Mass and energy balances derived from modelling and simulations are used as inputs in the LCA. Various impact categories, according to the ReCIPe impact assessment method, were calculated and compared, with discussions concerning the environmental impact categories affecting water, air, and soil being presented in detail. Sensitivity analysis is carried out for the electricity supply, evaluating the significance of using electricity mix in contrast to all carbon-free electricity supply.
Section snippets
Methods
In the following sections, the methods used in preparing the present study are described. Process flow modelling using commercial software was used for the development of the investigated case scenarios. The obtained data, in terms of mass and energy balances, were used as input for the environmental evaluation.
Results and discussions
Interpretation of the LCA is discussed in this section. In order to validate the present LCA methodology, the obtained results were compared with a built-in GaBi V8 [66] black-box for ammonia production, which considers nitrogen to be obtained from air by fractionation and hydrogen from natural gas by steam–methane reforming. Electricity and thermal energy (i.e. process steam) are modelled according to country-specific situations concerning energy carriers and emissions standards. Downstream
Conclusions
Environmental evaluation of ammonia synthesis via conventional and greener production routes was investigated, focusing on the hydrogen supply chain. Hydrogen production by SMR, in combination with two capture technologies based on liquid absorption using MDEA and chilled ammonia were evaluated and compared from an environmental point of view with hydrogen production through electrolysis and iron-based chemical looping, the latter being able to produce high purity streams of nitrogen as well,
CRediT authorship contribution statement
Dora-Andreea Chisalita: Conceptualization, Methodology, Formal analysis, Writing - original draft. Letitia Petrescu: Conceptualization, Writing - original draft, Investigation. Calin-Cristian Cormos: Conceptualization, Supervision, Funding acquisition.
Acknowledgements
This work was supported by a grant of Ministry of Research and Innovation, CNCS – UEFISCDI, Romania, project ID PN–III–P4-ID-PCE-2016-0031: “Developing innovative low carbon solutions for energy-intensive industrial applications by Carbon Capture, Utilization and Storage (CCUS) technologies”, within PNCDI III.
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