(228c) The Influence of Technical, Economic, and Site-Specific Factors on Optimal Ammonia Production from Renewables

The chemical industry is still heavily dependent on fossil raw materials and consequently responsible for large quantities of anthropogenic emissions. Ammonia is one of the most commonly synthesized bulk chemicals globally, with reactant production primarily relying on steam methane reforming and coal gasification. Sustainable ammonia production using only water, air, and electrical energy from renewables is a way to reduce greenhouse gas emissions.

Large-scale chemical conversion processes, such as ammonia synthesis, are typically operated continuously throughout the year. Wind and solar resources, however, are inherently intermittent in their availability. Coupling fluctuating energy supply with the operation of chemical plants presents a significant challenge for sustainable systems. To investigate this issue, we formulate a large-scale superstructure-based mathematical mixed-integer linear program within this work. While the majority of existing studies consider either an optimal design and scheduling [1–3] or a supply chain problem [4], classical facility location constraints are added herein to solve a combined design, scheduling, and facility location problem, as shown in Figure 1. Thus, in addition to the optimal process route and the optimal operation over time, the best allocation of five potential production locations with respect to a demand center within an exemplary German scenario is determined. In addition to the availability of renewable resources, other location factors such as the land potential for the construction of wind power and photovoltaic plants, taxes, and transportation costs are also taken into account.

According to the optimization results, the construction of a single large plant is economically more favorable compared to several distributed small plants. In particular, decreasing marginal costs for increasing plant capacities due to economies of scale for chemical conversion processes are the reason for this. It is shown that neglecting economies of scale and assuming linear relationships between plant capacity and costs when solving such superstructure optimization problems can provide misleading results. However, the construction of one central ammonia plant requires the maximum installable capacity of the electricity-generating processes at one location to be sufficient to satisfy the specified ammonia demand. In particular, high wind speeds throughout the year and available space for the installation of wind turbines are identified as decisive location factors for the optimal site.

Although the optimal process design and scheduling suggests the technical feasibility of green ammonia production, negative after-tax net-present values show that it is currently not economically viable. Sensitivity analyses demonstrate that the main decisive factors for the selection of the production site in Germany are the installable wind turbine capacity and the prevailing wind speeds throughout the entire year. Especially neglecting land availability can be misleading and result in an overestimation of a location’s potential. Solar irradiance, corporate income taxes and transport costs (at least over short and medium distances within Germany), on the other hand, are less crucial.

Our work underscores the location dependency of the production of chemical compounds utilizing renewable energy sources. For this reason, it is necessary to determine the process design and scheduling coupled with the optimal site allocation. A superstructure-based MILP formulation combined with real-world weather data and site-specific parameters is proven to be suitable for analyzing the coupling of wind and solar energy with chemical conversion processes.

References:

[1] Ganzer, C. and Mac Dowell, N. 2020. A comparative assessment framework for sustainable production of fuels and chemicals explicitly accounting for intermittency. Sustainable Energy Fuels 4, 8, 3888–3903.

[2] Palys, M. J., Mitrai, I., and Daoutidis, P. 2021. Renewable hydrogen and ammonia for combined heat and power systems in remote locations: Optimal design and scheduling. Optim Control Appl Meth, 1–20.

[3] Wang, H., Daoutidis, P., and Zhang, Q. 2021. Harnessing the Wind Power of the Ocean with Green Offshore Ammonia. ACS Sustainable Chem. Eng. 9, 43, 14605–14617.

[4] Wang, H., Daoutidis, P., and Zhang, Q. 2023. Ammonia-based green corridors for sustainable maritime transportation. Digital Chemical Engineering 6, 100082.