Stability Assessment of Small-Scale Distributed Ammonia Production Systems

Distributed ammonia production has the potential to revolutionize the food and energy industries of the future. Small-scale ammonia production systems can convert stranded renewable resources to ammonia for long-term energy storage during times of excess energy production or fertilizers during times of resource availability. While their intermittent operation with an uncertain feedstock may be economically feasible in some applications, in this work, we explore the technical feasibility of small-scale ammonia production systems. Ammonia synthesis reactors are particularly sensitive to disturbances and can exhibit oscillatory behavior detrimental to the system when perturbed. Therefore, to determine the technically feasibility of distributed ammonia production from a variable feed, we simulated a conventional ammonia production process fed by stranded natural gas. The conventional plant model was then scaled down and subjected to disturbances in the natural gas flow and process conditions. Three ammonia production models were simulated in Aspen Plus at small-, medium-, and large-scales with a production capacity of 135, 675, and 1350 tNH3/d, respectively. It was shown that decreasing the ammonia production capacity increases the sensitivity of the production plant to disturbances. A 5% change in natural gas feed resulted in oscillations in the small-scale ammonia synthesis reactor of 21.4% of its nominal value. Additionally, while the conventional large-scale plant was able to withstand up to a 15% increase in natural gas flow rate before exhibiting instabilities, the small-scale plant exhibited instabilities at a 3% increase in flow. Findings from disturbance tests performed in this work were used to propose methods of stabilizing the ammonia synthesis reactor at the small scale via improved ammonia separation techniques and new reactor designs.