(573i) Advancing Green Ammonia Synthesis in Dynamic Offshore Wind Power-to-X Systems

In this study, the feasibility and dynamic operability of a Power-to-Ammonia process chain, in particular its integration into offshore wind farms, was analyzed in detail. Using the dynamic process simulation software AVEVA Process Simulation® (APS), simulation-based analyses of the process were carried out under different conditions, such as full and partial load, in order to gain a deeper understanding of the transient behavior of the synthesis and the entire offshore process chain [1].

In the context of global warming, this approach also represents an important step towards reducing greenhouse gas emissions, particularly in chemical production. The central role of ammonia since its synthetic production in 1913 by Fritz Haber and its lasting impact on agriculture and human civilization underline the importance of this study [2]. Since 72% of hydrogen in the conventional process chain is produced by steam methane reforming and 26% by coal-fired power plants, ammonia synthesis contributes about 1-2% of global carbon dioxide emissions [2], [3]. Despite the century-long dominance of the Haber-Bosch process in global ammonia production, the need for alternative, emission-free synthesis routes are becoming apparent, especially in view of the growing world population and the associated increase in demand for food and ammonia-based fertilizers. Coupling ammonia synthesis with electrolyzers to produce green hydrogen therefore offers a major lever for reducing emissions in this important industrial sector.

The implementation of an offshore production facility is an innovative solution that avoids the need for conventional infrastructure to transport electrical energy to shore and can use existing material transport routes such as pipelines or tankers. In the German hydrogen flagship project H2Mare, the environmentally friendly offshore production of hydrogen and hydrogen derivatives is being investigated by an interdisciplinary consortium [4]. Despite its many advantages, offshore integration poses a number of challenges, particularly in the development of chemical processes that are confronted with unstable energy and raw material sources, as well as unusual weather and site conditions [5]. A major hurdle is the costly connection of wind farms to the mainland via submarine cables and offshore substations. An island solution that avoids direct grid connection therefore offers potential cost advantages and the opportunity to develop remote offshore sites. This option is particularly attractive due to the above-average and consistent wind speeds at sea, allowing wind farms to operate for longer periods compared to onshore alternatives [5].

The design process was as follows: the individual sub-systems consisting of the wind turbine, electrolyzer and synthesis plant were modelled and simulated individually, optimized and then the overall performance was evaluated in a coupled manner (see figure). The system was designed for a nominal output of 100 MW electrolysis nominal load.

The initial focus of the design process was on the development and modelling of the sub-system of the dynamic ammonia synthesis process without coupling with the wind turbine and electrolyzer, and the subsequent comparison with conventional stationary plant concepts. This design includes two reactor bed arrangements with an intermediate heat exchanger to efficiently model an intercooled reactor. The adiabatic reactors were modelled using the empirical Temkin-Pyzhev rate equation for a wustite iron catalyst [6]. A key aspect was the separation of the gaseous product from the unreacted reactants by two-phase separation using cooling and condensation. A recycle stream was provided to increase the overall efficiency. The design includes control loops and buffer tanks to stabilize operation under fluctuating conditions. A dual compressor pressure stabilization strategy was also designed and implemented. The compressors and control valves were validated using commercially available components. In addition to controller tuning, the ammonia synthesis sub-system was then optimized under steady-state conditions to maximize overall efficiency.

The next step was to integrate the previously developed model into the dynamic Power-to-Ammonia process chain, which simulates the direct coupling of wind turbines with electrolyzers and ammonia synthesis plants in off-grid systems. The APS software was used to enable comprehensive simulation of the entire periphery, including pumps, compressors, heat exchangers, kinetic reactions, and electrical components. This detailed analysis provided insight into the transient behavior of the overall system. In particular, the adaptability of the system to volatile H2 injection flows and the ability to operate over a 24-hour cycle under real wind conditions were analyzed. The hydrogen flow was reduced from 18,000 Nm3/h to 4,000 Nm3/h, corresponding to a reduction in ammonia production from approximately 235 t/d to 47 t/d. The nitrogen flow was matched to the volatile hydrogen flow to ensure a constant feed ratio of 3:1. In a further step, a hydrogen storage tank was implemented and evaluated as a function of the H2 storage pressure to counteract fluctuations in the volatile feed flow and ensure stable operation. For the isolated ammonia synthesis sub-system, an efficiency of 53% at full load and 60% at partial load (50% load) was calculated, as less cooling capacity is required. In coupled operation of the entire power-to-ammonia process chain with the electrolyzer, the efficiency of the entire process chain is 37%, as the peripheral energy consumption and efficiencies of the electrolyzer also play a role.

The final step was an optimization loop in which a bi-directional heat exchanger was incorporated into the reactor configuration to increase efficiency. This heat exchanger uses the hot gas flow from the first reactor, heated by the exothermic reaction, to preheat the inlet flow of the same reactor. This strategy allows efficient heat recovery and contributes to the energy efficiency of the overall process. An electric heat exchanger was also used to precisely control and regulate the reactor temperature. In addition to ensuring the desired operating parameters, this also allows temperature control during the start-up process after a plant shutdown. The comparative analysis of this arrangement with the originally proposed design showed a significant improvement in efficiency. Efficiencies of approximately 43% at full load and 35% at partial load were achieved for the overall process.

In summary, this study investigated the integration of a Power-to-Ammonia process chain with offshore wind parks and electrolyzers. Using dynamic flow sheet simulations, the individual sub-systems were modelled and optimized for efficient and dynamic operation. The overall efficiency of the power-to-ammonia process chain was 43% at full load and 35% at partial load. This emphasizes the transient operation of ammonia synthesis under fluctuating conditions and demonstrates an innovative approach to future-proof, sustainable and dynamic energy and chemical production that adapts to the dynamic requirements of renewable energy sources.

[1] “AVEVA Process Simulation.” Accessed: Apr. 01, 2023. [Online]. Available: https://www.aveva.com/en/products/process-simulation/

[2] S. Ghavam, M. Vahdati, I. A. G. Wilson, and P. Styring, “Sustainable Ammonia Production Processes,” Front. Energy Res., vol. 9, p. 580808, Mar. 2021, doi: 10.3389/fenrg.2021.580808.

[3] J. Humphreys, R. Lan, and S. Tao, “Development and Recent Progress on Ammonia Synthesis Catalysts for Haber–Bosch Process,” Adv Energy and Sustain Res, vol. 2, no. 1, p. 2000043, Jan. 2021, doi: 10.1002/aesr.202000043.

[4] Federal Ministry of Education and Research, “How partners in the H2Mare flagship project intend to produce hydrogen on the high seas.” [Online]. Available: https://www.wasserstoff-leitprojekte.de/projects/h2mare

[5] P. Rentschler, C. Klahn, and R. Dittmeyer, “The Need for Dynamic Process Simulation: A Review of Offshore Power‐to‐X Systems,” Chemie Ingenieur Technik, vol. 96, no. 1–2, pp. 114–125, Jan. 2024, doi: 10.1002/cite.202300156.

[6] M. I. Temkin and V. Pyzhev, “Kinetics of Ammonia Synthesis on Promoted Iron Catalysts,” Acta Physicochimica U.R.S.S., vol. 12, pp. 327–356, 1940.