Journal of Environmental Management
Volume 342
, 15 September 2023, 118348
Review
Green ammonia production technologies: A review of practical progress
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Opeyemi A. Ojelade a
Highlights
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Conventional H–B process with process optimization and CCSU technology is a near-term option.
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Sustainable N-fertilizer that relies on air and water could revolutionize the agriculture business.
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Data from demonstration plants would accelerate scale-up and market trend analysis.
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Cost of electrolysis-driven HB ammonia is competitive with traditional technology in some regions.
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Simultaneously achieving DOE target ammonia yield and energy efficiency remains elusive.
Graphical abstract
Introduction
Due to the prevalence of nitrogen (N2) in the air, it is critical for the efficient functioning of living creatures (plants and animals). Although naturally occurring N2 is inert, the N2 fixation process converts it to reactive species (i.e., ammonia, nitrate, nitrites, nitric acid, fertilizer, and so on) (Marcarelli et al., 2022). The transformation of atmospheric N2 to reactive species has immensely contributed to the agricultural and energy industries, however, it has also affected our environment. One of the notable contributions of reactive N-species is in the agricultural domain via the consequential discovery of the Haber-Bosch (H–B) ammonia production process (Humphreys et al., 2021). Before this discovery, the availability of fixed nitrogen was very scarce which limited the global production of food needed for survival (Sutton et al., 2011). At the dawn of the 20th century, the development of the H–B process, which allows large-scale ammonia production, is unarguably regarded as one of the best industrial discoveries that effectively accelerated the nitrogen cycle and enabled humankind to nobly increase global food production (Gruber and Galloway, 2008). The large quantity production of ammonia (∼180 MT/yr, with anticipation to increase by 2.3% through 2024) (Pattabathula and Richardson, 2016), has revolutionized the agricultural industry to date. In addition, the conversion of N-species has contributed to the global energy landscape. Although, roughly 80% of ammonia production is used for producing fertilizer, it also finds applications in direct ammonia fuel cells (DAFC) and hydrogen energy storage vector, owing to its ability to allow long-term chemical energy storage regardless of location (Morlanés et al., 2021; Valera-Medina et al., 2018; Wang et al., 2022). Besides ammonia being COx-free, it possesses favorable chemical and technical characteristics including high energy density (corresponding to a low heating value of 18.6 MJ/kg), easy storage (−33 °C at 1 atm), non-explosive and excellent stability without significant energy loss (Tilman et al., 2011). H2 energy has been exploited from N-species such as ammonia via catalytic decomposition at elevated temperatures (Jolaoso and Zaman, 2020; Pinzón et al., 2022).
The accumulation of reactive N2 in the environment can be mainly attributed to those from intentional use (for instance, N-fertilizer from Haber-Bosch), although, others can arise from unintentional N-fixation (for example, combustion of fossil fuels in industrial plants) (Suddick et al., 2013). In the absence of N-fertilizer, the world would lose ∼30% of the crops we rely on for food (Bernhard, 2010), however, its increasing production and use have led to several environmental concerns. The increasing accumulation of reactive N2 in our environment is the main cause of several universal problems such as global acidification, ozone depletion, eutrophication, and climate change (Gruber and Galloway, 2008). Ideally, not all reactive N2 intentionally introduced into the soil for crop growth is used, some escape to the atmosphere as nitrous oxide (N2O), while some are released as ammonia and NOx into the air, consequently leading to climate change and poor air quality (Suddick et al., 2013).
Beyond these challenges are concerns with the energy being used as feedstock and electricity in the production of ammonia and in extension, N-based fertilizers. As the world's population grows, so will the energy demand (70 percent from fossil fuels) (BP Review, 2018), as well as agricultural practices for abundant food production (Sutton et al., 2011). As a result, this will contribute to rising CO2 emissions in the atmosphere, which are thought to be the primary blame for driving the global climate to an enigmatic level (Tilman et al., 2011). While ammonia has many industrial advantages, 96% of its production is through the conventional H–B process using fossil fuels (as feedstock and energy), and posing an environmental threat via greenhouse gas (GHG) emissions. Ammonia via the H–B process route appears to be the highest emitter of CO2 (∼1.8% of global CO2 emission). In addition, the H–B process is highly energy demanding, utilizing close to 2% of the global energy production, which amounts to wasting too much energy (MacFarlane et al., 2020). These challenges coupled with the highly geographical centralization of ammonia plants have driven the need to develop sustainable and energy-efficient small-to-medium-scale distributed ammonia production alternatives.
In several attempts to transition into environmentally sustainable and energy-efficient ammonia production alternatives, research attention has been focused mostly on electrochemical and photochemical technologies. In extension, many review articles on these technologies are centered on the design of catalysts with a comprehensive exploration of catalytic performance, reaction mechanisms, and defect engineering (Foster et al., 2018; Hao et al., 2020; Li et al., 2019; Suryanto et al., 2019). Some previous reviews have also reported advances in ammonia production technologies. A review by Ghavam and co-workers focused mainly on H2 production technologies but also touched on emerging ammonia production technologies and environmental impacts (Ghavam et al., 2021). In another study, the concept of a solar ammonia refinery, with a detailed analysis of energy efficiency was reviewed (D. Wang et al., 2018). The review mainly focused on advances in ammonia production from N2 and H2O using solar energy without utilizing heat or electricity (i.e. relying only on light). While these reviews attempted to describe important parameters such as energy efficiency, emissions, and renewable energy sources as they affect the “greening ammonia” concept, a comprehensive overview of commercialization efforts of green ammonia synthesis is missing. Hence, this study seeks to fill the knowledge gap. This is in addition to describing the thermodynamic energy and exergy efficiencies of distributed ammonia production in a bid to improve system efficiency which will eventually impact commercialization success. Beyond these, the study attempts to give an overview of advances in the H–B system's improvement (in terms of thermodynamic limitations and separation efficiency). These can be considered desirable while the research community continues to work towards achieving the realization of commercialized green ammonia production.
The success of relatively less expensive green ammonia production across various regions will advance the use of ammonia as a flexible long-term renewable energy carrier, feedstock for green fertilizer, as well as zero-carbon fuel. Although there are many green ammonia production technologies, the review reported herewith has been conceptualized to ascertain which of the emerging ammonia production methods would compete with the current Haber-Bosch technology in the near- and mid-term future, in terms of various metrics (particularly, production cost, cost CO2 emission, and energy efficiency). In addition, the rising interest by different countries in the large-scale commercialization of green ammonia production is worth discussing. Thus, this study aims to discuss such green ammonia plant demonstration research efforts, in a bid to clearly understand what progress has been made in the realization of commercially available green ammonia, the challenges, and opportunities available to be explored. Specifically, the goal of the review includes: (1) comparing different ammonia production systems in terms of energy efficiency, cost, and direct CO2 emissions (2) addressing the current state of large-scale industrial green ammonia production projects (3) putting research problems, possibilities, and innovative directions into perspective as to what the future holds for green ammonia production. This review is deemed desirable in guiding the readership of this journal in related future research.
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Section snippets
Ammonia production technologies
Broadly, the potential technologies for the synthesis of ammonia can be categorized into (1) centralized and modified methane-based Haber-Bosch, (2) electrolysis-driven Haber Bosch, where the H2 is produced from renewable feedstocks and energy, and (3) distributed ammonia production technologies, which completely excludes the conventional Haber-Bosch process.
Commercialization progress: towards green ammonia production
Despite efforts towards actualizing commercial green ammonia production either to completely replace or augment the traditional H–B process, the cost of electricity/energy remains the main obstructing factor(s). However, the U.S. Department of Energy (DOE), through their Advanced Research Projects Agency-Energy (ARPA-E) is investing a huge amount of research funds to achieve practical demonstration of green ammonia production in the near future (Wolden, 2019). Through their renewable energy to
Summary and outlook
The hunt for alternative eco-friendly, efficient, and cost-effective technologies for converting N2 into ammonia continues to be driven by the energy and carbon-intensive Haber-Bosch ammonia manufacturing system. Based on these goals, an innovative permanent alternative is the direct ammonia production from water and air, which is currently a subject of extensive basic research (especially, catalysts development). However, challenges are still being faced owing to the inertness of nitrogen
Conclusion
Ammonia manufacturing via Haber-Bosch technology has enabled humans to boost global food production for more than a century. However, it consumes a lot of energy and produces a lot of CO2. As a result, developing decarbonization plans is critical to meet the goal of net-zero carbon emissions by 2050. At this point, humanity cannot continue to jeopardize the environment's preservation to develop a sustainable civilization in which key chemical processes and energy productions are based on
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.
Acknowledgement
Georgia Institute of Technology in Atlanta, Georgia, USA, King Abdulaziz University, Jeddah, Saudi Arabia, and the University of Technology Sydney, NSW, Australia collaborated on this project. As a result, the authors express their gratitude to the institutions for their assistance.
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