Journal of Environmental Management
Research articleEnvironmental impact assessment of green ammonia coupled with urea and ammonium nitrate production
Introduction
Ammonia (NH3) is one of the most widely produced chemicals worldwide (Bicer and Dincer, 2018), with a broad range of applications in industries such as fertilizer production, explosives, plastics and pharmaceutics (Khasani et al., 2021). In 2021, global production of NH3 reached 168 million tons, and further growth by 2% annually is expected (Klaas et al., 2021) due to the increasing world population and food production, which will drive up demand for fertilizer (Ghavam et al., 2021a). China is the largest ammonia producer, accounting for 31.4% of global production, followed by Russia (i.e., 10%), the United States (i.e., 8.9%) and India (i.e., 7.8%) (Zhang et al., 2020). Recently, ammonia gas gained attention as a potential energy carrier or H2 storage medium because of its three times higher volumetric energy density than H2 (Pawar et al., 2021), as well as its advantageous storage and transportation properties (Ghavam et al., 2021b). However, the main use of ammonia (i.e., more than 80% of total ammonia production) consists of fertilizer production (Lim et al., 2021), particularly in the production of urea, ammonium nitrate and ammonium phosphate (Khademi and Lotfi-Varnoosfaderani, 2021). According to Walling and Vaneeckhaute (2020), approximately 75% of the nitrogen fertilizers consumed worldwide are supplied through urea and ammonium nitrate.
Urea is recognized as one of the most widespread chemicals, with substantial use in the agricultural sector (i.e., around 80% of total production) (Antonetti et al., 2017) due to its high nitrogen content and low production cost (Nunes et al., 2023). Urea is also utilized in other areas such as flue gas conditioning to reduce NOx concentrations in selective catalytic reduction systems (SCR) (Cinti and Desideri, 2015) or as a raw material in the plastics industry (Zhang et al., 2021). As a result, it is expected that the global production of urea will increase by 1.8% annually, reaching a total of 226 million tons by 2023 (IFA - International Fertilizer Association, 2019).
Unlike urea, which is synthesized from ammonia and CO2, ammonium nitrate is produced by the neutralization reaction between nitric acid and ammonia. Consequently, during the use phase, urea produces around 40% more greenhouse gas emissions due to its high carbon content. Although ammonium nitrate is used as a constituent in explosives, its primary application is in agriculture, where it is the second most commonly used fertilizer (Speight, 2017). Despite the fact that over half of total N2O emissions are released during fertilizer use, both ammonium nitrate and calcium ammonium nitrate are expected to be widely adopted in sustainable future scenarios due to low or even zero CO2 emissions during the use stage (IEA - International Energy Agency, 2019).
Several different routes can be undertaken to synthesize ammonia, such as the Haber-Bosch process, electrochemical synthesis (Soloveichik, 2019), photo-catalysis (Liu et al., 2022) or chemical-looping (Fang et al., 2022). However, the Haber-Bosch process accounts for more than 90% of total ammonia production (Fúnez Guerra et al., 2020). The H2 required in the process is mainly produced from fossil fuels, natural gas being considered the most common feedstock (Morales Mora et al., 2016), with coal and fuel oil also used to a lower extent (i.e., 22% for coal, and 4% respectively) (Giddey et al., 2017). Water electrolysis and biomass gasification are considered more sustainable alternatives for H2 production (Mohamed et al., 2021). Cryogenic air separation is the main technology employed for N2 production. It is well-known that the Haber-Bosch process requires high energy consumption (i.e., between 1 and 2% of the global energy demand) due to increased temperature and pressure requirements (i.e., 350–550 °C and 100–250 bar) (IEA - International Energy Agency, 2019). In addition, both H2 production by steam methane reforming (SMR) and N2 supply via ASU contribute significantly to the overall energy consumption and carbon footprint of ammonia production (i.e., roughly 2% to the anthropogenic CO2 emissions) (Klaas et al., 2021). The SMR process, together with ammonia production, generate approximately 2.15 tons of CO2 for each ton of ammonia (Kurien and Mittal, 2022).
Based on the above, natural gas is perceived as the most conventional feedstock for fertilizer production, since both urea and ammonium nitrate are commercially obtained from ammonia (Devkota et al., 2021). Despite the fact that 0.73 tons of CO2 are required to produce one ton of urea (i.e., roughly 130 Mt of CO2 consumption annually) (Zhang et al., 2021), urea contributes 9% to the total amount of CO2 released by the chemical sector (IEA - International Energy Agency, 2020). Therefore, in order to meet the ambitious goals of the Paris Agreement, which aims to limit the global temperature increase to below 2 °C compared to the pre-industrial level, much greener and sustainable ways of producing ammonia and its subsequent conversion to urea and ammonium nitrate need to be explored. Smith et al. (2020) propose several measures such as removing SMR and finding better alternatives for H2 supply, employing and using electric compressors, or discovering new methods for ammonia separation, which would result in near-zero CO2 emissions and lessen the environmental burden of the current approach.
Numerous alternatives to conventional ammonia production have been investigated, but both blue and green production routes are gaining popularity. Blue ammonia involves the integration of Carbon Capture, Utilization and Storage (CCUS) systems for CO2 capture and subsequent use (Arnaiz del Pozo and Cloete, 2022), while the green pathway involves using clean and sustainable processes, such as water electrolysis for H2 production with renewable electricity sources (Osorio-Tejada et al., 2022). Another environmentally friendly alternative concerns the use of rich nitrogen sources (e.g., microalgae) and their subsequent conversion into ammonia (Wang et al., 2022).
Several studies have analysed the environmental impact of green ammonia. Bicer et al. (2016) evaluated four different energy sources for water electrolysis in green ammonia production, finding that using municipal solid waste was the most viable alternative for electricity generation, outperforming hydro power, nuclear and biomass. Another study by Bicer and Dincer (2017) compared different nuclear routes for H2 generation coupled with the Haber-Bosch ammonia production process. The analysis indicated that using nuclear power to supply the energy demand for water electrolysis resulted in the lowest global warming potential (GWP) and climate change impact. Gomez et al. (2020) assessed the electrochemical pathway for ammonia synthesis and suggested that combining ASU with water electrolysis for the generation of N2 and H2, respectively, could enable sustainable ammonia production if renewable sources are used.
Chisalita et al. (2020) studied ammonia production from an environmental perspective, considering various routes for H2 and N2 production. Their analysis showed that replacing the SMR process with a chemical looping alternative resulted in significantly lower greenhouse gas emissions. They also found that using renewable resources for water electrolysis led to lower air emissions, but increased water and soil burden. Ghavam et al. (2021a) studied the integration of CO2 capture methods within waste-to-green ammonia and urea production, evaluating several process configurations. The authors showed that the alternative design could reduce waste and greenhouse gas emissions compared to available processes. Mohamed et al. (2021) carried out an investigation on H2 generation through thermal solar cracking of liquefied natural gas, and its utilization in sustainable ammonia synthesis. The environmental analysis revealed that the evaluated system exhibited high potential in cutting greenhouse gas emissions (i.e., approximately 69% compared to the traditional process). Osorio-Tejada et al. (2022) evaluated ammonia production in Australia and identified transportation as one of the highest contributors towards global warming. In response, the authors proposed the use of small-scale distributed plants powered by renewable energy as an alternative to conventional plants. The analysis concluded that implementation of small-scale plants leads to a significant decrease in CO2 emissions, primarily because of eliminating the need for transportation.
As noted previously, ammonia provides an alternative to fossil-based fuels, thus Bicer and Dincer (2018) examined the environmental impact of utilizing ammonia as a clean fuel in the transportation and power generation sectors. In both cases, better environmental performance was observed when using ammonia compared to diesel or gasoline for the transport sector, or compared to natural gas when assessing power generation.
Fertilizer production is of paramount importance to modern society and, with the increasing adoption of renewable resources, it has the potential to play a crucial role in mitigating climate change. The current study aims to evaluate and determine the potential environmental benefits of utilizing green ammonia coupled with the production of both urea and ammonium nitrate compared to conventional process technologies.
As outlined above, there have been limited investigations of fertilizers from an LCA perspective. The key novelty aspects brought by the present study consist of a technical and environmental performance comparison between green and blue ammonia production, both paths coupled with urea and ammonium nitrate manufacture.
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Section snippets
Process modelling and simulation
The modelling and simulation aspects of the investigated cases were carried out using ChemCAD software version 7 (Chemstations, 2022), with the following scenarios being examined in the current work:
Case 1: Conventional ammonia production coupled with either urea or ammonium nitrate production processes;
Case 2: Wind-powered H2 generation for green ammonia production coupled with either urea or ammonium nitrate production processes;
Case 3: Hydro-powered H2 generation for green ammonia production
Results and discussion on process modelling and simulation
The main technical key performance indicators for urea and ammonium nitrate production under the investigated scenarios are reported in Table 3.
Table 3 demonstrates that green urea production in all scenarios results in lower specific consumption of raw materials compared to the conventional pathway involving SMR for H2 generation (i.e., 1.71 times less water and 5.43 times lower amount of air). It should be noted that an additional 517.76 kg of natural gas are required in Case 1 to produce the
Conclusions
The current study evaluates and compares the conventional and green production processes for urea and ammonium nitrate, from both technical and environmental perspectives. The green production scenarios included various sources of renewable energy to generate electrolytic hydrogen, such as wind, hydro, photovoltaic and nuclear sources. The main technical and environmental findings of this investigation are outlined below:
- 1.
From a technical perspective, the green sustainable fertilizer production
Credit author statement
Stefan Cristian Galusnyak: Conceptualization; Formal analysis; Investigation; Methodology; Software; Visualization; Writing - original draft.; Letitia Petrescu: Conceptualization; Formal analysis; Investigation; Methodology; Resources; Software; Supervision; Validation; Writing review & editing.; Vlad-Cristian Sandu: Investigation; Methodology; Visualization; Writing review & editing.; Calin-Cristian Cormos: Resources; Software; Supervision; Validation; Writing review & editing.
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.
Acknowledgments
This work was supported by the Romanian Ministry of Education and Research, CCCDI – UEFISCDI, project number PN-III-P4-ID-PCE-2020-0032 within PNCDI III.
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