Optimisation of the Autothermal NH3 Production Process for Power-to-Ammonia

Abstract

The power-to-ammonia process requires flexible operation due to intermittent renewable energy supply. In this work, we analyse three-bed autothermal reactor systems for design and off-design performance for power-to-ammonia application. The five reactor systems differ in terms of inter-stage cooling methods, i.e., direct cooling by quenching (2Q), combination of indirect and direct cooling (HQ and QH) and indirect cooling (2H) with variations. At optimum nominal operation conditions, the inter-stage indirect cooling (2H) reactor systems result in the highest NH3 production. For off-design performance analysis, NH3 production is minimised or maximised by varying one of the following process variables at a time: inert gas, feed flow rate or H2-to-N2 ratio. For each variation, the effect on H2 intake, recycle stream load and recycle-to-feed ratio is also analysed. Among the three process variables, the H2-to-N2 ratio provided ca. 70% lower NH3 production and 70% lower H2 intake than at nominal operation for all five reactor systems. Operation of autothermal reactor systems at significantly lower H2 intake makes them reliable for power-to-ammonia application; as during energy outage period, shutdown can be delayed.

Optimal Operation of a Reaction-Absorption Process for Ammonia Production at Low Pressure

“…2 Among various configurations, a three-bed reactor system with intercooling has been the preferred option in the ammonia industry in terms of production, energy savings, capital, and maintenance cost. 34 The synthesis loop considered in the present study consists of a three-bed autothermal reactor with direct inter-stage cooling by quenching streams, three heat exchangers, and an absorption column, all of which are depicted in Figure 1. This process system is a simplified version of that considered in ref 16.…”

“…Azarhoosh et al addressed the maximization of ammonia production by adjusting the inlet temperature, total feed flow rate, and operating pressure. Cheema and Krewer analyzed five different autothermal ammonia synthesis reactor systems for design and off-design performance for power-to-ammonia applications. Despite all those studies, the optimization of the synthesis reactor in a dynamic environment as well as the optimization of the overall synthesis loop (including both reaction and separation) have received little attention.…”

How sensitive is a dynamic ammonia synthesis process? Global sensitivity analysis of a dynamic Haber-Bosch process (for flexible seasonal energy storage)

“…where ∆H reaction is the heat of reaction. A recycle and purge stream are in place to recuperate the unreacted H 2 and N 2 gas in the ammonia synthesis process and remove inert gases that would accumulate in the synthesis loop (Figure 1) [17,[45][46][47]. The challenge to operate the Haber-Bosch process for its renewable purpose (i.e.…”

“…The primary concern of utilizing this Power-to-Ammonia (PtA) process is assuring the durability of the reactor against fluctuations, e.g. thermal cycling [14][15][16], and adapt the design for its implicit off-design operations [17]. Modern NH 3 synthesis plants are conventionally designed for steady-state operations [18][19][20].…”

“…As shown by these research papers, variations in the model and design parameters could significantly impact the economics and performance of the PtA process. Cheema et al considered implicit variations (H 2 /N 2 ratio, Argon concentration and mass flow rate) in the Haber-Bosch loop and ammonia reactor design to improve the flexibility on the operation and production of NH 3 in steady-state [13,17]. This work considered that this flexibility provides a stable ammonia production at its off-design performance for large-scale energy storage.…”