The need for ammonia
The central role of ammonia (NH3) in the global food supply chain is well-established, and its significance as a feedstock for nitrogen-release fertilizers is undeniable. In fact, 70% of global ammonia production is used as a feedstock for fertilizer production, such as urea (CO(NH2)2),1 which is the most common nitrogen-release fertilizer (N-fertilizer), accounting for 49% of global N-fertilizer consumption in 2020.2 The remaining 30% of global ammonia production utilized in various industrial applications, including the production of plastics and explosives,1 the latter being of paramount importance to the mining sector.
The problem with conventionally produced ammonia
Global ammonia production in 2020 reached 185 Mt, with China accounting for 30%, while Russia, the European Union, USA, and the Middle East each contributed 10%. More than 90% of this global production was achieved through Haber-Bosch plants,6,7 where hydrogen and nitrogen are combined in an exothermic process inside a ‘Haber-Bosch reactor’ under high pressure and temperature, with the presence of a Fe-based catalyst to produce ammonia.8 For this, 70% of the required hydrogen was obtained via steam methane reforming (SMR) in 2020, with the remaining portion derived via coal gasification (China).
These conventional pathways for ammonia production are emission and energy intensive, accounting for 2% of global energy consumption and 1.3% of global CO2 emissions linked to the energy system in 2020.1 This makes ammonia production one of the most energy-intensive chemical feedstocks worldwide.8 Most of the energy consumption in the ammonia production process is associated with the process required to conventionally produce hydrogen as the feedstock for ammonia synthesis.
Decarbonizing ammonia production
On the other hand, within the context of the recent global interest in the role of green hydrogen for decarbonizing a substantial part of the economy, ammonia has been recognized as an economically viable and carbon-free carrier for transporting and storing hydrogen. This is due to its stability under conditions close to atmospheric and its high hydrogen content (17.6 wt%), which enables low-cost long-distance hydrogen transportation.7,9 This is especially relevant given the importance that green hydrogen, or hydrogen produced with low environmental impact, has gained over the past five years as a decarbonizing energy vector for the energy transition.
Furthermore, recent disruptions in the natural gas and fertilizer supply chains to Europe, caused by the conflict in Ukraine, have heightened the interest of European countries in securing their own resources for ammonia production. As a result of these environmental and geopolitical factors, the production of ammonia using ‘green hydrogen,’ generated through the electrolysis of water and a power supply based on renewable energy sources (RES), has gained even more relevance.
Therefore, the interest in ‘green ammonia’, produced using green hydrogen (instead of being derived from natural gas via SMR), is driven both by the need to decarbonize the current ammonia production for its existing uses and by the potential of ammonia to serve as an economically viable carrier for long-distance transportation of green hydrogen.
This ever-increasing global demand, illustrated in Figure 2, shows that hydrogen production is ramping up towards economies of scale. However, this poses some challenges on the demand of electrolyser technology types to suit different requirements and applications across developments.
Transitioning from conventional ammonia to green ammonia presents several challenges. To begin with, it is not as straightforward as merely substituting the hydrogen production process with green hydrogen instead of grey hydrogen to supply the Haber-Bosch (HB) plant. The HB process has been optimized for over a century to operate in conjunction with SMR plants, relying heavily on the residual heat generated during the SMR process. Consequently, producing green ammonia requires the electrification of the HB process, which conventionally depends on the residual heat of the SMR process.
Furthermore, the HB process is known to have low flexibility, meaning that it is challenging and time-consuming to adjust the operational parameters of the process. For example, controlling the mass flow into the reactor cannot be as rapidly executed as changes in hydrogen production when derived from an electrolyzer connected to RES with an inherently variable production profile. This implies the necessity of incorporating storage and buffer stages between the green hydrogen production phases and the HB inlet itself.
While these technical and economic challenges represent obstacles to overcome, the pressing need to decarbonize ammonia production cannot be ignored. Nonetheless, there is room for optimism regarding future strategies to address these challenges.
Electrified Haber-Bosch process
One key factor driving the development of green ammonia projects is flexibility in both the production process and the energy sources used. Transitioning from the conventional natural-gas fed HB process to green ammonia production through an electrified HB introduces several new challenges as well as opportunities for increased energy efficiency.
Haber-Bosch plants are designed for 24/7 operation, and the typically sized HB+ASU (air separation unit) plant will require between 28 and 44 MW of 24/7 electricity supply.
The energy supply for green hydrogen feedstock is significantly greater than the electricity demand for the HB process. Figure 2 illustrates the energy requirements for hydrogen production using alkaline electrolyzers operating 24/7 as a reference.
The capital expenditure for a green ammonia production plant is dominated by the electrolyzer cost. In the case of 24/7 alkaline electrolyzer operations, the associated CAPEX costs are as shown in Figure 3. These costs are based on an assumed CAPEX of USD 183/kg of NH3/day for the HB plant and USD 800/kW for the alkaline electrolyzer, which can be considered as referential.
With more optimistic assumptions about CAPEX costs of HB plants, the costs can be significantly lower.
This illustrates that green ammonia production is greatly influenced by the energy demand and CAPEX costs of green hydrogen production. The cost of energy for hydrogen production will be a determining factor for overall costs. The positive news is that green hydrogen costs are decreasing significantly due to the availability of low-cost renewable energy and the rapid learning curve in the electrolyzer production industry, leading to electrolyzers with lower costs and higher efficiency.
Solar ammonia: a sustainable pathway
A particular case of green ammonia is ‘solar ammonia’, produced by integrating HB plants with solar PV-driven electrolysis.9 Solar ammonia offers a promising low-emission alternative and the potential to benefit from low-cost electricity, provided that the solar resource is robust, and the levelized cost of energy (LCOE) of the solar plant is sufficiently low. By coupling HB plants with solar PV-driven electrolysis, renewable energy sources can be utilized for ammonia production, reducing the carbon footprint and aligning with global sustainability goals. However, this integration comes with challenges.
The dynamic and intermittent nature of solar energy introduces variability, which can impact the traditionally inflexible HB cycle. By redesigning the auxiliary systems of the HB plant, including adopting electric compressors and systems to replace heat-dependent components in the conventional SMR-coupled HB, some of these challenges can be addressed.7
Operational challenges of solar ammonia
While solar ammonia presents a sustainable pathway, it poses operational challenges due to the dynamic and intermittent supply of solar energy. For instance, intraday and seasonal variations in hydrogen production can affect the adaptability of the inflexible HB process to solar PV’s variability. Here are two approaches to address these challenges:
1. Addressing solar energy variability: Strategies such as energy storage solutions, grid integration, and demand-side management can ensure continuous HB operation during nighttime and periods of reduced solar irradiation, such as winter.
2. Optimal sizing for solar hydrogen production and buffer storage: Achieving the right balance between solar hydrogen production and storage is crucial. Advanced analytics and simulation tools can aid in achieving the lowest levelized cost of ammonia (LCOA).
The primary goal of a solar ammonia plant is to overcome these challenges while maintaining a low LCOA. This necessitates optimizing the plant’s design to ensure that the configuration and size of subsystems are optimal. To achieve this, it is essential to optimize the sizing of a solar hydrogen production system and storage to enable effective coupling with an electrified HB cycle, ensuring continuous and seamless green ammonia production. One possible approach is employing a simulation-based optimization method.
Utilizing advanced software tools, mathematical techniques, and simulation-based optimization allows for the design of a cost-effective green ammonia production system. This capitalizes on solar hydrogen production and innovative control strategies for the electrified HB process. Partial adaptation of the production profile to match the seasonal availability of solar resources is one way to reduce production costs.
As an example, consider a solar hydrogen plant coupled to an HB plant following two operational scenarios: one with 100% continuous HB operation and another with a seasonally adapted profile, as illustrated in Figure 5.
Scenario 1 involves the HB plant running continuously at 100% capacity, ensuring uninterrupted ammonia production. However, this requires significant seasonal hydrogen storage and oversizing of the solar and electrolyzer plants to accommodate the surplus of hydrogen during the winter when production does not meet the required mass flow for the HB process to operate continuously. This leads to a sharp increase in CAPEX and hydrogen production overshooting during peak solar months, resulting in considerable curtailment levels in such a scenario.
In Scenario 2, a flexible approach is adopted, adjusting HB production by reducing the required inlet mass flow for the HB plant. This operational adaptation relaxes hydrogen production constraints while maintaining 24/7 plant operation. Consequently, ammonia production is lower compared to Scenario 1, which also reduces curtailment. Implementing modulated HB operation during the winter allows for a storage system with intraday capacity, reducing the reliance on large seasonal storage.
Considering these two scenarios, the significance of operational control strategies for inlet mass flow, and solar hydrogen production infrastructure and storage sizing as optimization parameters for solar ammonia techno-economic optimization becomes evident.
This can be summarized as follows:
1. Dynamic control of HB operation: Adjusting the mass flow rate in the HB reactor based on solar resource availability can enhance efficiency and reduce operational costs while maintaining 24/7 operation at a reduced mass flow compared to the nominal rate.
2. Simulation-based optimization: Leveraging advanced software tools, mathematical methods, and real-time data can help design a cost-effective system that is resilient to external variabilities.
The transition to green ammonia production systems, particularly those harnessing solar energy, is an urgent need that poses both challenges and opportunities. With the right strategies, innovations, and collaborative efforts, it is possible to design efficient and sustainable systems that not only reduce carbon emissions but also pave the way for a greener future.
About the authors
Felipe Gallardo, CEO of Southern Lights, holds degrees from Sapienza University of Rome and the University of Chile. He is also currently a PhD candidate at KTH Royal Institute of Technology.
Anton Frisk, CPO of Southern Lights, holds an MSc in nanoscience material science from Lund University.