Session: 17-04: Symposium Steinfeld - New solar chemical processes and cycles

Paper Number: 137359

137359 - Green Ammonia Production via Concentrating Solar Thermochemistry 

Abstract: 

Ammonia (NH₃) is an energy-dense chemical and a vital component of fertilizer. It is currently synthesized via the Haber-Bosch process, which has changed little over the last century and requires pressures of 15- 25 MPa and temperatures of 400-500 °C. Hydrocarbons provide the heat and mechanical energy required to drive the NH₃ reaction, as well as source the hydrogen (H₂) and nitrogen (N₂) feedstocks. As a result, NH₃ production accounts for almost 1.5% of global CO₂ emissions. The development of a renewable pathway to NH₃ synthesis that utilizes concentrated solar irradiation for the process heat instead of hydrocarbon combustion and operates under relatively low pressure, will result in both a decrease (or elimination) in greenhouse gas emissions and avoid the cost, complexity, and safety issues inherent in high-pressure processes.

The aim of the Solar-Thermal Ammonia Production (STAP) project is to develop a solar thermochemical looping technology to produce N₂ from air for the subsequent production of NH₃ in a coupled two-cycle process. In the first cycle, the endothermic thermal reduction of redox-active metal oxide particles is driven by concentrated solar irradiation; subsequent exposure to air re-oxidizes the particles, removing O₂ and producing relatively pure N₂ gas. In the ammonia production cycle, H₂ reacts with metal nitride particles to produce NH₃, resulting in a nitrogen deficient metal nitride. In the second step, the nitride is regenerated utilizing the N₂ produced in the previous air separation cycle. The net result is NH₃ produced from sunlight, air, and (green) H₂, while the metal oxide and nitride particles are recycled.

Nitrogen Separation: A lab-scale packed bed reduction reactor was designed and fabricated. Based on extensive screening based on redox capacity and chemical stability of singly- and doubly-substituted perovskite strontium ferrites, A_xSr_1-xFeO_3-δ (A=Ba, La; x = 0.01-0.02) were chosen as candidate materials.  Ba_0.15­Sr_0.85O_3-δ was tested for O₂ separation capacity, demonstrating >20 min of air separation at a reduction temperature of 800 °C under Ar and re-oxidation temperature of 500 °C in air.

Ammonia Synthesis: A series of mixed metal nitrides, A_xB­_yN (A=Co, Ni, Fe; B=Mo, W), was synthesized and screened for reduction and re-nitridation. Co₃Mo₃N (CMN331) was chosen for further study based on its potential to lose up to 50 mol% of its nitrogen upon reduction in H₂ to form Co₆Mo₆N (CMN661) and re-uptake that nitrogen (back to CMN331) under N₂ while remaining isostructural. Under ambient conditions the CMN331/CMN661 reaction was characterized by examining nitrogen capacities at different temperatures and N₂ partial pressures, to quantify temporal NH₃ production, and to gain insight into reaction mechanisms and limitations for informing reactor design and operation. In addition, an ammonia synthesis reactor was constructed to investigate the reaction under non-ambient pressure. NH₃ production and re-nitridation were observed over multiple cycles under isothermal conditions (650 °C) and pressures an order of magnitude below that of Haber-Bosch. Results under both ambient and pressurized conditions imply a combined bulk and catalytic element to the NH₃ synthesis reaction.

Techno-Economic Analysis: A TEA and full systems model were also performed to optimize process conditions and determine the feasilbilty and costs of process scale up. An integrated systems model of the entire STAP process was generated to determine the optimal design and operating conditions for the process. A technoeconomic analysis based on the system design, heat and mass transfer models, and empirical results reveals that the levelized cost of ammonia (LCOA) is most sensitive to the ammonia synthesis subsystem, which is also the process with the most performance uncertainty. The results show that economical production of NH₃ is via STAP is, in fact, feasible.

Presenting Author: Andrea Ambrosini Sandia National Laboratories

Presenting Author Biography: Dr. Andrea Ambrosini is a Principal Member of the Technical Staff in the Concentrating Solar Technologies department at Sandia National Laboratories. Dr. Amrbosini's research involves the synthesis, characterization, and development of functional oxide materials for renewable energy applications, particularly concentrating solar technologies, decarbonization, and solar-thermal chemistry.

Authors:

Andrea Ambrosini Sandia National Laboratories
H. Evan Bush Sandia National Laboratories
Xiang Gao Arizona State University
Nhu Nguyen Georgia Institute of Technology
Alberto De La Calle Arizona State University
Ivan Ermanoski Arizona State University
Tyler Farr Georgia Institute of Technology
Kevin Albrecht Sandia National Laboratories
Ellen Stechel Arizona State University
Peter Loutzenhiser Georgia Institute of Technology