Shifting towards dynamic and load-flexible ammonia synthesis via polytropic fixed-bed reactors

The Power-to-Ammonia (P2A) concept is a promising solution for global, chemical-based storage and transportation of renewable energies. Nevertheless, establishing technically and economically viable P2A processes requires suitable reactor concepts capable of load-flexible operation. Conventional adiabatic reactors (according to the Haber-Bosch -principle) are severely constrained by thermodynamic, kinetic, and stability limitations, making them unsuited for load-flexible operation. However, the markedly improved activity of modern ammonia synthesis catalysts paves the way for the development of alternative reactor designs. In this study, we present a novel approach for load-flexible ammonia synthesis by utilizing External Direct-Cooled fixed-bed Reactors (EDCR) for polytropic operation. Based on detailed numerical simulation studies employing a dynamic 2D pseudo-homogeneous reactor model, we show that our proposed reactor concept can be operated over a remarkably wide load range and is much less sensitive to disturbances compared to conventional reactors. Even at instantaneous load changes (7 to 200 % load), the EDCR exhibits smooth and stable transitions between different operating points within 20 min. In addition, the polytropic EDCR fully exploits the potential of more active catalysts by operating at lower reactor temperatures, achieving single-pass conversions of up to 55 %, thereby doubling the conversion rates of state-of-the-art reactors. Since no universally applicable methodology for assessing load flexibility exists, we further introduce a systematic perturbation analysis as a new approach to quantify load-flexibility. Our results demonstrate the feasibility of load-flexible ammonia synthesis under highly dynamic conditions using the EDCR concept, while simultaneously outperforming state-of-the-art reactor concepts in terms of single-pass performance. Consequently, the EDCR represents a highly promising approach within P2A applications. • EDCR outperforms state-of-the art reactors in terms of conversion and flexibility. • Proposed EDCR concept achieves single-pass conversions of up to 55%. • EDCR shows quick and stable transitions even for drastic load-changes (7 to 200%). • Introduction of a new approach to systematically quantify load-flexibility.