(501g) Process Design and Optimization for Green Ammonia Production

Large scale industrial ammonia production is essential to support the world population as approximately 85% of global production is converted to fertilizers and applied in the agricultural sector. The conventional Haber–Bosch process is based on hydrogen production from steam methane reforming, which results in approximately 235 million tons of CO₂ emissions per year [1]. In this context, a crucial challenge is to reduce the environmental impact of ammonia production, while keeping up with the production demand. One of the most promising opportunities in this field is green ammonia, whose production is based on green hydrogen [2]. Water electrolysis generates H₂ and O₂ from deionized water taking advantage of renewable electricity sources such as solar, wind, and hydropower. Even if shifting from fossil-based to renewable-based H₂ could enable substantial decarbonization of ammonia synthesis, green ammonia is not yet competitive at the current costs of green hydrogen production [3]. This work, developed within the FME Hydrogeni project funded by the Resource Council of Norway, aims at reducing the Levelized Cost of Ammonia (LCOA) through energy integration and optimal process design. More specifically, this work includes:

• Modelling of the green ammonia production process in a steady-state process simulator;
• Quantification of mass and energy balances and assessment of Key Performance Indicators (KPIs);
• Sensitivity analysis to identify optimal process configuration (electrolyser type, operating pressure);
• LCOA for the optimized process.

An industrial-relevant electrolyser scale of 500 MW is considered as the process size.

Model structure

Submodels have been developed to characterize the building blocks involved in green ammonia synthesis, namely Air Separation Unit (ASU), Proton Exchange Membrane (PEM) and Solid Oxide Electrolyser Cell (SOEC) electrolysers, and the ammonia synthesis reactor. A rigorous model approach is followed for both the ammonia synthesis reactor and ASU. The conventional Haber-Bosch model is largely based on the process description reported in Ullmann’s encyclopaedia [4]. The main assumptions are:

• operating pressure is 210 bar;
• three packed beds in series with intermediate cooling. Inlet temperature to each bed is 380°C;
• kinetics of forward and reverse reactions retrieved from Burrows and Bollas [5];
• ammonia recovery by liquefaction in two steps operating at 20°C and 0°C, respectively.

For the low-pressure process, a pressure range between 50 and 100 bar is investigated. The kinetics for the ruthenium catalyst is retrieved from Yoshida et al. [6]. At reduced pressure, lower temperatures are needed to condense ammonia. The major implication is the need to replace the ammonia refrigeration cycle considered for Haber-Bosch with alternative refrigerants. A list of the adopted cycles for different operating pressures is available in Table 1.The electrolyser is represented with a simplified soft model accounting only for the global water splitting reaction, and the corresponding efficiencies and performance indicators are directly retrieved from a previously published work [7].Flowsheet is developed in COCO-COFE (CAPE-OPEN to CAPE-OPEN – CAPE-OPEN Simulation Environment), a simulation package available free of charge. The Peng-Robinson Equation of State is used as the thermodynamic model. The flowsheet of the integrated process is drawn in Figure 1 (for the PEM case-study).

Costs assessment

Costs will be calculated through a class III estimate. Mass and energy balances from the COFE model are used for unit sizing and to quantify raw materials and utilities consumption. CAPEX and OPEX will be determined using published correlations [8,9].

Sensitivity analysis

1. Performances of green ammonia production at conventional pressure using PEM and SOEC are compared in terms of KPIs and LCOA.
2. A sensitivity analysis is carried out to investigate how energy requirements and reactor size change when operating with Ru-catalyst at different pressures between 50 and 100 bar. The target is to find the optimal pressure and compare low-pressure process with conventional Haber-Bosch.

Results

Table 2 compares the KPIs for the conventional Haber-Bosch process integrated with PEM and SOEC electrolysers, respectively. Results show that SOEC and PEM provide the same ammonia yield. However, SOEC allows generating a higher ammonia flow (+19%) thanks to the higher hydrogen production at given electrolyser power (500 MW). On the other hand, the increase in the total electricity demand observed for SOEC is limited to 1%. Noteworthily, more than 70% of the steam needed for SOEC can be produced through heat integration using the residual heat available from the hot streams leaving each packed bed in ammonia synthesis (T > 450°C). Table 3 reports a summary of the main KPIs for the low-pressure process using PEM electrolysers.

The following considerations can be drawn:

• At lower pressure, lower temperatures are required to condense ammonia. This results in a considerable increase in the electricity requirement of the refrigeration cycle.
• The lower the pressure, the higher the flow of unconverted reactants to be recycled. Indeed, the thermodynamic equilibrium of NH₃ production becomes less favorable, so the conversion per pass decreases.
• The reaction kinetics is slower at lower pressure.
• Circulating flows inside the reactor are higher, thus higher packed bed heights are needed to achieve the same yield.
• Working at lower pressures considerably drops the cost for make-up gas compression.
• The optimal pressure range for the low-pressure ammonia synthesis is 90 bar. The total electricity demand is only +6.5% compared to conventional Haber-Bosch.

Similar considerations hold for the SOEC-based low-temperature process. Calculations of the LCOA for PEM and SOEC-based processes at both 210 bar (conventional) and 90 bar (optimized low-pressure) are in progress and will be presented at the meeting. This will be crucial to assess the economic feasibility of the proposed integrated ammonia production process.

Acknowledgement

The present work received financial support from the Norwegian Research Council’s Centres for Environment-friendly Energy Research program.

References

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[2] A.G. Olabi, M.A. Abdelkareem, M. Al-Murisi, N. Shehata, A.H. Alami, A. Radwan, T. Wilberforce, K.J. Chae, E.T. Sayed, Recent progress in Green Ammonia: Production, applications, assessment; barriers, and its role in achieving the sustainable development goals, Energy Convers Manag 277 (2023) 116594. https://doi.org/10.1016/J.ENCONMAN.2022.116594.

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[6] M. Yoshida, T. Ogawa, Y. Imamura, K.N. Ishihara, Economies of scale in ammonia synthesis loops embedded with iron- and ruthenium-based catalysts, Int J Hydrogen Energy 46 (2021) 28840–28854. https://doi.org/10.1016/J.IJHYDENE.2020.12.081.

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[8] K.M. Guthrie, Process Plant Estimating, Evaluation and Control, Solana Ed., 1974.

[9] R. Turton, J.A. Shaewitz, D. Bhattacharya, W.B. Whiting, Analysis, Synthesis, and Design of Chemical Processes, Fifth Edition, Pearson, 2018.