Session: 19-01: Symposium to Honor Professor Jane Davidson I

Paper Number: 170029

170029 - Applying Solar Thermochemical Approaches to Ammonia Synthesis 

Abstract: 

Chemical looping is a strategy for chemical processes wherein the desired reaction is divided into at least two reactions.  Those reactions are linked to one another through an intermediate material, commonly a reactive metal oxide that is capable of effectively storing and releasing oxygen in response to its environment. This approach offers advantages such as providing an inherent air separation, providing product streams free from diluent gasses, (e.g. N₂-free CO₂ in the case of combustion), controlling or moderating the reactivity of the oxygen, etc.  Thermochemical processes are subset of chemical looping in which the desired reaction is divided into several reactions to provide a means of carrying out a challenging endothermic reaction, such as water splitting, under more favorable/less-extreme conditions, while still primarily driving the process with thermal energy. In the case of two-step metal oxide cycles for water or CO₂ splitting, the high temperature, high flux input available from concentrating solar energy is well-suited to provide the necessary thermal energy. 

Inspired by advances in solar thermochemistry, we sought to apply the solar thermochemical/chemical looping approach to other challenging reactions.  We identified ammonia synthesis as being of particular interest due to the massive scale on which ammonia is produced, its importance to the global economy and feeding the world, and its correspondingly large environmental footprint. In addition, despite being a very mature chemical technology, the industrial process for producing ammonia must still be carried out at temperatures whereat the thermodynamics severely limits the per-pass conversion, despite partial compensation by extreme pressures. We seek to circumvent this limitation by dividing the reaction into two halves. Our approach to ammonia synthesis is analogous to other cyclic or looping reaction processes, however, we have replaced the metal oxide oxygen carrier with a nitrogen-carrying metal nitride. 

In this presentation, we will discuss the thermodynamic basis for our approach, outline our strategy for identifying materials of interest and improving upon them, and also describe some of the unique challenges inherent to the nitride system. The nitride looping reaction chemistry has been convincingly demonstrated in our laboratory using a ternary CoMo formulation. This nitride can be repeatedly cycled between 661 and 331 (Co:Mo:N) compositions.  We have recently returned to this effort and will present data towards understanding the process on a qualitative and quantitative basis as well as describe our efforts towards synthesizing related materials to move the development of this approach forward. 

Presenting Author: James Miller Arizona State University

Presenting Author Biography: Dr. James E. Miller (Jim) is a chemical engineer who has directed his research efforts towards energy, materials, and chemical processing, spanning topics from heterogeneous catalysis to desalination. For almost two decades, Jim’s R&D efforts have been primarily focused on high temperature thermochemical processes, for example water and carbon dioxide splitting to produce hydrogen or synthesis gas, thermochemical energy storage, and other processes such as ammonia synthesis. His contributions to these effort span chemistry, materials, reactors, and systems. Jim retired from Sandia National Laboratories in 2017 and is currently employed by Arizona State University. At ASU he holds the titles of Senior Global Futures Scientist, Julie Ann Wrigley Global Futures Laboratory; and Professor of Practice, LightWorks®, Global Institute of Sustainability and Innovation.