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With the world’s major shift towards renewable energy, the need of chemicals-based energy storage has drastically increased, as renewable energy is intermittent and energy storage medium is required. Among several chemical energy storage options, ammonia is promising for renewable energy on utility-scale. The Haber-Bosch ammonia synthesis was the first heterogeneous catalytic system employed in the chemical industry and developed over a period of century. However, the conventional ammonia process has been designed and optimized for steady state operation and high capacity. Power-to-ammonia requires a more flexible operation, small size reactors and decentralized production.
The impact of adjustable parameters, such as, total mass feed flow rate, feed temperature, reactor pressure and feed composition (N2:H2 ratio, ammonia & inert concentration) alone and together are quantified on four autothermic three-bed ammonia synthesis reactor systems for flexible operation. The variants differ in terms of inter-stage cooling techniques: direct cooling (cold-shot) by injecting fresh feed, indirect cooling i.e. heat exchange between process streams, or a combination of them (cold-shot after cooling or vice versa). The reactor volume, cold-shot flows and process streams flow in heat exchangers are optimized in a way that reaction occurs for maximum conversion and product gas leaves the reactor at the maximum operational temperature. The reactor systems with indirect cooling and cold-shot before cooling yield ammonia at high production rates. Among six parameters, N2:H2 ratio and total mass feed flow rate provide higher operational flexibility, high variations in hydrogen intake and ammonia production. Whereas, by applying multi-parametric optimization for all six parameters together, the operation of ammonia synthesis reactor for flexible load ranges enhances.
From this work it is concluded that the identified two systems are the most feasible reactor systems among four variants, as they allow operation with large flexibility for hydrogen intake, i.e. up to 15 % more and 80 % less hydrogen mass feed rate compared to nominal. Further, with multi-parametric optimization, they allow operation for a large load range span of 10 – 100 %.