Davide Carrara, CASALE, and Robert Kender, Linde GmbH, analyse process optimisation and dynamic evaluation of ammonia plants operating under fluctuating conditions.
Ammonia synthesis is a well established process traditionally based on fossil fuel feedstocks, primarily natural gas and coal. When hydrogen is instead produced from renewable power, green ammonia becomes a versatile energy carrier, capable of efficiently storing and transporting clean energy. Its higher energy density and easier handling compared to hydrogen make it particularly suitable for long distance transport and large scale energy integration.
Conventional ammonia plants benefit from a stable and continuous energy supply, allowing operation at high load factors with limited variability and mature, optimised process infrastructures. In contrast, green ammonia production is intrinsically subject to fluctuations in renewable energy availability. As a result, irregular operation, lower capacity factors, and challenges in cost effective sizing of plant components become inherent aspects of the technology.
Design and control of a green plant
The design and control of a green plant based on renewable energy principles are supremely important for avoiding an uncontrollable surge of capital investment, and to provide continuous and stable operation of the plant without affecting the reliability of the relevant components.
Tools such as levelised cost optimisers and dynamic operation simulators have become indispensable for green ammonia projects to be convenient and reliable. Specifically:
- Process optimisers, like the one developed by CASALE, are of utmost importance for the analysis of the power input profile, and eventually defining the best design to minimise levelised cost of the product.
- Dynamic analysis of the plant under fluctuating operation is mandatory to assess the reliability of the equipment.
Optimising process scheme and design
The predominant contribution to the levelised cost of ammonia (LCOA) is the levelised cost of hydrogen (LCOH), being that more than 90% of operating expenses (OPEX) and capital expenditure (CAPEX) are tied to the production of hydrogen. For this reason, the wise use of produced hydrogen is the one factor that can determine whether a project will see the light of day. Process optimisers define the minimum LCOA achievable on the basis of a specific power profile, i.e., the best way to make use of the available power (Figure 1). The dynamic analysis includes:
- Renewable power profile analysis.
- Availability and cost of grid power.
- Electrolysers, specifying their optimal size.
- Hydrogen storage, including its sizing, control philosophy, and integration in the plant.
- Ammonia plant, with its unique and independent controls.
- Nitrogen generation, suggesting the optimal N2 profile (consequent on hydrogen profile) .
All of a plant’s sections act as a whole, with the design being closely interconnected and heavily contributing to the project’s economic feasibility.
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