Thermodynamics of Nitrogen Carriers for Chemical Looping Ammonia Synthesis

Abstract

Producing ammonia at lower pressures and temperatures can reduce energy and economic costs of the process. To accomplish this, novel materials and/or synthesis routes with higher ammonia formation rates at lower temperatures are necessary. Chemical looping ammonia synthesis (CL-NH3) is one such promising route that has shown higher ammonia synthesis rates and at milder temperatures than the current Haber-Bosch process. With CL-NH3, a solid nitrogen carrying material is used to form ammonia in steps by alternating between pure hydrogen and nitrogen gas feeds. Unlike the Haber-Bosch process, the ammonia yield of the reaction is dependent on the kinetic and thermodynamic properties of the nitrogen carrying material. This introduces additional opportunities for novel designs, though finding suitable materials becomes a challenge. Thermodynamic properties of solid nitrogen carrying materials are partially available, hence, it is unclear whether these materials produce sufficient or better ammonia yields under relevant operating conditions. Demystifying this question is at the core of this thesis and is fundamental to understanding the potential and feasibility of CL- NH3. To answer this question, ‘sufficient’ reaction conversions and material properties were defined through sensitivity studies of a simulated CL-NH3 process, and ammonia yields greater than 26% were required to achieve parity with the Haber-Bosch process. Thermodynamic models were created to calculate benchmarks for solid thermodynamic properties and equilibrium gas partial pressures (𝑃𝑁2 or 𝑃𝐻2/𝑃𝑁2) of the chemical looping reactions. With reference to these models, in silico screening of nitrogen carriers from literature and the Materials Project show that alkali/alkaline earth metal imides/hydrides though promising from a kinetic perspective, may not meet benchmarks, however, 111 out of 2515 metal nitrides, including Co3InN, LiZnN, and Zn2VN3, show viable equilibrium yields. Further work is required to validate thermodynamic properties of nitrogen carriers and should be paired with kinetic studies to appropriately assess their viability.