(5g) Process Alternatives to Stabilize Small-Scale Ammonia Production.

Small-scale ammonia, produced with renewable energy, has the potential to serve as carrier in a future carbon-free energy landscape. However, the intermittency of stranded resources and the instability of traditional ammonia production jeopardizes this carbon-free future. A recent study showed that at small scales, traditional ammonia synthesis processes are too unstable to be fed by stranded resources. Previous attempts at stabilizing ammonia production have been done largely using advanced controls, which may prove difficult to implement at small scale. Therefore, in this work we propose two process alternatives to stabilize small-scale ammonia production. The first process alternative uses low-temperature catalysts for ammonia synthesis and the second uses a new reactor design called chemical looping ammonia synthesis. The two process alternatives and the conventional ammonia synthesis reactor were modeled and subjected to disturbances in reactor feed flow rate and feed temperature in Aspen Dynamics. Each reactor system was subjected to a +/- 20% change in reactor flow and a +/- 20 °C change in reactor feed temperature, and their responses were measured.

It was found that both process alternatives significantly improved the stability of small-scale ammonia production. While a 20 °C drop in reactor feed temperature initiated limit cycle behavior in the conventional reactor system, both alternatives remained stable. The alternative processes improved stability however, each was implemented with varying degrees of difficulty. The use of low-temperature catalyst is a simple solution to implement because it only entails a change of catalyst. The chemical looping reactor system is attractive because it offers the potential to operate with a wide array of carriers. However, it requires separate streams to feed the ammonia synthesis step, which entails further infrastructure modifications. The presentation will report our findings and conclude with a discussion on the practical implications of each process alternative towards distributed ammonia production.