Fuel
Green ammonia production: Process technologies and challenges
Introduction
Ammonia (NH3), a pivotal compound, garners widespread recognition for its multifaceted importance across diverse industrial and agricultural domains. Its versatile applications span a spectrum, embracing the realms of synthetic fertilizers, refrigeration, mining, pharmaceuticals, and water treatment, etc. [1]. Exploring cutting-edge frontiers includes utilizing renewable ammonia (RA) as a sustainable fuel with no carbon emissions. This holds particular promise within the maritime sector and as a source for stationary power production·NH3, in this context, is proposed as a viable medium for efficiently conveying hydrogen over substantial distances [2]. RA emerges as a byproduct of renewable hydrogen, synthesized by utilizing renewable electricity. Although the commercial initiation of RA production dating back to 1921, the collective output in 2021 barely exceeded 0.02 million metric tonnes (MMT), a mere fraction compared to the overall NH3 production of 182 MMT. Forecasts indicate that RA is set to assume a crucial role in all freshly established NH3 production capacities after 2025. Moreover, with the implementation of strategic procedures, RA production holds the potentiality for widespread cost competitiveness, envisaged to gain prominence from 2030 [3]. Looking ahead to 2030, the anticipated production capability of announced renewable ammonia facilities is poised to reach 15 MMT. Currently, 8 % (approx.) of current NH3 market allocated across 54 projects, with significant allocations in Australia, Mauritania, and Oman. A projected pipeline of 71 MMT extends through 2040, though it's crucial to acknowledge that a significant portion of these projects awaits investment decisions. According to estimates of International Renewable Energy Agency (IRENA), the generation capability of RA is forecasted to hit 566 MMT by 2050, with an increase of global temperature by 1.5 °C. Aggregate capacity of the disclosed projects, totalling 71 MMT, represents just over 10 % of the essential operational capacity needed for zero-carbon NH3 production by 2050 [4]. An increasing acknowledgment prevails regarding the abundant reservoirs of renewable energy in various regions, even in remote areas, coupled with the progressive advancement of associated technologies at a diminishing cost. As a result, the current challenge revolves around formulating strategies for the efficient storage and transportation of energy, emphasizing both optimal effectiveness and cost-efficiency. This underscores the imperative for technologies capable of generating high-energy–density liquid fuels, exemplified by RA, in an economic and sustainable way [5]. They can serve as fuels directly at the location or their carrier can be recovered and recycled. As a result, the term “energy carrier” is frequently preferred, reflecting its broad range of applications. Liquid-state energy carriers hold significant appeal because of their capacity to deliver energy density commonly linked with condensed phases. Additionally, they hold benefits of facile generation and dispatch, which may be absent in solids and gases [6]·NH3 synthesized through a process devoid of carbon emissions and entirely reliant on renewable resources is referred to as “green” ammonia, excluding emissions from embodied systems. The predominant method for producing green NH3 entails generating hydrogen (H2) through water splitting and extracting nitrogen (N2) from the air, with both processes executed using renewable energy (RE). The resultant renewable H2 and isolated N2 are subsequently fed into the Haber-Bosch process, wherein NH3 is produced through the reaction of N2 and H2 under elevated pressure and temperature, facilitated by catalyst’s availability.
NH3 production plants, traditionally relying on natural gas reforming, are undergoing a transformative shift by incorporating Carbon Capture, Utilization, and Storage (CCUS) systems. These systems aim in eliminating process emissions associated with the reforming process. The prevalent NH3 generation method, which employs the steam methane reforming (SMR) method for H2 generation followed by Haber-Bosch process, is not considered “green.” Approximately 90 % of emitting CO2 from commercial NH3 generation originate from the SMR method, recognized for its high energy intensity and significant contribution of 1.8 % to global CO2 emissions [7]. While numerous green NH3 producing technologies exist, this review has been designed to explore and identify emerging NH3 production methods that could rival the present Haber-Bosch technology in the future. The evaluation focuses on different metrics, chiefly production cost, CO2 emissions cost, and energy efficiency. Presently, NH3 production highly depends on fossil fuels. Within the global energy consumption landscape, NH3 generation constitutes approximately 2 % (8.6 EJ), where around 40 % of it is contributed feedstock. This includes raw materials supplying H2 to NH3 production and process energy·NH3 producing sector stands out for its emissions intensity, primarily due to the utilization of fossil fuels, contributing approximately 450 Mt CO2/year in direct emissions [8]. Approximately 170 Mt CO2/year in indirect emissions are attributed to two main sources, as indicated by [8], reactions occurring while urea-based fertilizer is introduced to soil and electricity generation. Consequently, it becomes imperative to undertake a thorough exploration of the current scholarly literature addressing recent advancements in various methods for sustainable green NH3 synthesis. Various studies have highlighted the perilous emissions stemming from conventional NH3 synthesis plants [9].
The emission of greenhouse gases from ammonia plants was approximated to fall within the range of 1.25–––2.16 kg CO2-eq./kg NH3 [10]. Consequently, guidelines pertaining to CO2 and other detrimental emissions, including NOx and Sox, are imperative and have the potential to instigate substantial technological transformations in the ammonia industry. Substantial endeavours have been undertaken to address the perilous emissions originating from traditional ammonia production plants. In 2020, a significant 70 % of 184 Mt of NH3 produced were utilized either directly or indirectly as fertilizer [11]. NH3 market is projected to experience a growth trajectory exceeding 4 % Compound Annual Growth Rate (CAGR) from 2021 to 2031 [12]. Amidst the sectors where ammonia is presently utilized, the global emphasis on decarbonization has positioned NH3 as low-carbon substitute. Its applications extend to direct usage in transport, power generation, and serving as carrier for H2 [13], leading to an anticipated surge in demand. It is crucial to note that ammonia production is characterized by its energy-intensive nature. The Ammonia Technology Roadmap report of 2021 by the International Energy Agency (IEA) [14] brought attention to the fact that, in 2020, ammonia generation contributed to 2 % (8.6 EJ) of the global energy consumption. The predominant wide-scale approach to ammonia production involves the Haber-Bosch process, where H2 is derived from fossil fuels, and N2 is extracted from air. According to IEA report, 70 % of H2 is derived from natural gas, therefore, considered as primary source, while coal makes up the remaining portion. Additionally, the report underscores the significant impact of fossil fuels on NH3 production technique, accounting for approximately 450 Mt of CO2 emissions in 2020 [15]. Presently, fossil fuels, particularly natural gas, play a pivotal role, contributing to 90 % of the existing ammonia production. Further, through comprehensive examination of various technologies such as electrochemical synthesis, biomass feedstock utilization, and carbon capture and utilization (CCU), the study elucidates potential alternatives to traditional Haber-Bosch synthesis. However, despite the detailed exploration of these innovative approaches, a notable research gap persists regarding the scalability and commercial viability of green ammonia production methods in real-world settings. The primary factors for green ammonia production are self-sustenance, flexibility, low electric power cost, avoidance of high cost of supply chain accompanied by custom duties and carbon taxes with extremely high logistics price and safety [5]. While the study provides valuable insights into the technical aspects and potential economic benefits of green ammonia production, further research is warranted to bridge the gap between laboratory-scale experiments and large-scale industrial implementation. This research endeavor was undertaken to address this gap and provide a comprehensive overview that enriches the knowledge base of readers in the journal and scientific communities at large. By elucidating the challenges, opportunities, and advancements in green ammonia production, this study aims to stimulate further research and facilitate informed decision-making towards achieving a sustainable and carbon–neutral future. In addition, this article provides a meticulous examination of diverse methods across several sections, encompassing Electrolytic Ammonia Synthesis, Photocatalytic Ammonia Synthesis (including strategies, metal-based, and metal-free photocatalysts), Photo-electrocatalytic Ammonia Synthesis, and Biocatalytic Ammonia Synthesis. Additionally, it delves into an exploration of the strengths and limitations inherent in various contemporary technologies and practices. The discourse extends to the imperative requirement for further advancements to elevate these technologies to a level of maturity conducive to the efficacious integration of green ammonia across diverse sectors.
Major ammonia producer countries at global level are shown in Fig. 1. China is the world’s top producer of ammonia, with a production of 42 million metric tons (MMT), followed by Russia with 16 MMT, the United States with 13 MMT, and India with 13 MMT in the year 2022 [16]. The ammonia industry is crucial for various applications. The success of the synthetic ammonia industry has significantly impacted world food production, addressing the food requirements for the growing population [17]. However, the production of ammonia has also led to environmental challenges, such as water contamination, especially in countries like China and India [18]. Green Ammonia is now a key component of the H2-based circular economy. It is being promoted as a green fuel due to its potential as a carbon-free energy carrier [19].
It is predicted that its demand may increase many folds. As per Statista (2023), the anticipated growth in global ammonia production capacity is projected to reach nearly 290 million metric tons by the year 2030. This expansion is foreseen due to the development of around 107 planned and announced ammonia plants, predominantly situated in Asia and the Middle East. These facilities are expected to become operational by the year 2030. That will boost the global ammonia market, which was valued at 69 billion US dollars in 2021, and it will reach 224 US dollars in 2050 (Fig. 2).
Section snippets
Green ammonia
Ammonia (NH3) is a compound composed of nitrogen and hydrogen and is commonly used in fertilizers, industrial processes, and as a potential energy carrier. Traditional ammonia production involves the Haber-Bosch process, which utilizes natural gas as a feedstock and emits substantial carbon dioxide (CO2) [20]. Green ammonia is a carbon–neutral alternative to traditional ammonia, made using renewable energy sources such as solar or wind power which can breakdown water into hydrogen and oxygen
Green ammonia production process
Today, almost half of H2 is consumed in NH3 plants, and H2 is mostly extracted from coal and natural gas, which emit CO2 approximately 420 million tons yearly [25]. In the future, NH3 production will be entirely renewable by using Bio-H2 from H2-electrolysis and N2-separated from the atmosphere. The process of green-NH3 production is shown in Fig. 3·NH3 is used as an H2 carrier or directly in a dual-fuel compression ignition engine [26] or fuel cells [27]. It has high energy density as it
Green ammonia production: Process technologies and challenges
According to Chehade and Dincer [15], the primary factors for green ammonia production are self-sustenance, flexibility, low electric power cost, avoidance of high cost of supply chain accompanied by custom duties and carbon taxes with extremely high logistics price and safety. Study of Sanchez and Martin [46] conducted a scale up/ scale down study applying modular design for green ammonia production in which hydrogen is produced by water splitting using renewable energy sources and nitrogen is
Challenges
Ammonia (NH3) is a carbon-free substitute for energy and received increasing interest in the international trade of renewable power [69]. Large-scale operations were conducted during 1990 s to produce ammonia through Haber-Bosch process using the electrolysis-based hydrogen production, proves the readiness of the technology for implementation [70], [71]. Bouaboula et al. [72] in a study identified the energy intermittency and unpredictability of alternative energy sources as the key obstacles
Economic significance of ammonia
Ammonia plays an important role in the global economy by contributing to economic growth and as a feedstock for the chemical and fertilizer industry. Ammonia as a fertilizer helped to increase global agricultural production. It is a valuable input for chemical manufacturing industries like solvents, fertilizers, etc. It is economically significant due to its unique characteristics and potential to decarbonize the energy and transportation sectors. It is primarily produced using the conventional
Conclusion
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Ammonia serves as a cornerstone of the global economy, driving growth and agricultural productivity. Recent strides in sustainable pathways like green ammonia offer avenues for mitigating environmental impact and achieving carbon neutrality, vital for long-term sustainability.
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Innovative approaches from electrochemical synthesis to biomass utilization and carbon capture advance green ammonia production, offering solutions to reduce emissions and ensure economic competitiveness.
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Despite challenges
CRediT authorship contribution statement
Neelam Bora: Writing – original draft. Akhilesh Kumar Singh: Writing – original draft. Priti Pal: Writing – original draft. Uttam Kumar Sahoo: Writing – original draft. Dibyakanta Seth: Writing – original draft. Dheeraj Rathore: Writing – original draft. Sudipa Bhadra: Writing – original draft. Surajbhan Sevda: Writing – original draft. Veluswamy Venkatramanan: Writing – original draft. Shiv Prasad: Writing – original draft. Anoop Singh: Writing – original draft. Rupam Kataki: Writing – review
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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