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

Paper Number: 136927

136927 - Two-step chemical looping cycle for renewable NH3 production based on non-catalytic Co3Mo3N/Co6Mo6N reactions 

Abstract: 

A two-step solar thermochemical looping cycle based on Co3Mo3N/Co6Mo6N (CMN331/CMN661) reduction/nitridation reactions offers a pathway for green NH3 production that utilizes concentrated solar irradiation, H­2 from H2O splitting, and N2 derived from air separation as feedstocks. The NH3 production cycle steps derive process heat from concentrated solar irradiation and encompass (1) the chemical reduction of CMN331 with H2 to CMN661 and NH3; and (2) nitridation of CMN661 to CMN331 with N2. CMN331 reduction/nitridation reactions were examined at different H2 and N2 partial pressures and temperatures. Limited knowledge is available pertaining to optimal conditions to maximize reaction rates and NH3 yield via CMN331/CMN661 reduction/nitridation. The focus of this work was to characterize the CMN331/CMN661 reaction 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.

CMN331 was synthesized by nitridizing CoMoO4 powder in 10% H2/N2 at 805°C following a gradual stepping temperature schedule. Powder x-ray diffraction (PXRD) was used to verify sample crystallographic structure before and after reduction/re-nitridation experiments. Elemental analysis was performed with inductively coupled plasma optical emission spectrometry (ICP-OES) and combustion analysis. An unreduced and a fully reduced sample were identified as Co3.00Mo3.10N1.13 and Co6.00Mo6.17N0.95, respectively, matching the expected stoichiometry for CMN331 and CMN661. Scanning electron microscopy (SEM), x-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and energy-dispersive x-ray spectroscopy (EDS) were used to investigate particle morphologies, to determine elemental distributions, and to identify a surface oxygen layer that necessitated the addition of H2 during cycling to prevent surface oxidation by trace amounts of O2.

Sample reduction/re-nitridation cycling with 5% H2 was conducted using thermogravimetric analysis (TGA), and a horizontal tube furnace reactor was used with gas flows containing H2 concentrations between 25 and 75%. NH3 was measured using mass spectrometry (MS) coupled with liquid conductivity meter.  Results from the cycling suggested that temperatures >500°C and H2 concentrations >5% were needed to reduce CMN331. Nitridation conditions of 700°C under 25-75% H2/N2 were sufficient to fully regenerate CMN331 from CMN661.

CMN331 reduction rates were examined through H2 pressure-swing experiments under 25 – 75% H2/Ar at temperatures of 700°C and 650°C and total pressure of 1 bar. The onset of sample reduction was immediately observed after gas switches from 100% Ar to 25-75% H2/Ar at both temperatures, evidenced by significant increases in N2 and NH3. Isothermal H2 pressure-swings resulted in measurable NH3 in 25-75% H2/Ar, compared to no detectable NH3 when samples were gradually heated from temperatures <200°C to 700°C under 25-75% H2/Ar. The molar ratios of NH3 produced to the initial CMN331 were n_NH3/n_CMN331=0.0307 and 0.0161 for H2 pressure-swings to 75% H2/Ar at 650 and 700°C, respectively. H2 pressure-swings for CMN331 led to more NH3. An elevated temperature of 700°C was needed to fully reduce CMN331 within 70 min under 75% H2/Ar and 140 min under 25% H2/Ar, indicating that the reduction reaction rates were strongly correlated to H2 partial pressure and temperature. Five consecutive cycles were performed with a CMN sample undergoing H2 pressure-swing reduction to produce NH3 at 700°C under 75% H2/Ar and nitridation at 700°C under 75% H2/N2. Consistent NH3 production during H2 pressure-swing was observed between cycles.

H2 pressure-swing results demonstrated successful NH3 production from CMN331 reduction at 1 bar and suggest higher total pressures are needed to minimize NH3 dissociation and improve NH3 production relative to N2 in an NH3 reactor coupled to concentrated solar irradiation. The required elevated temperatures correlate with lower NH3 at chemical equilibrium, but this is addressable by altering the reaction mechanism of NH3 formation. These results represent the first comprehensive characterization and definitive non-catalytic production of NH3 via chemical looping with metal nitrides and provide insights for technology development.

Presenting Author: Nhu P. Nguyen Georgia Institute of Technology

Presenting Author Biography: Nhu “Ty” Nguyen is a PhD candidate and Graduate Research Assistant in the Woodruff School of Mechanical Engineering at Georgia Institute of Technology. Ty works with Prof. Peter Loutzenhiser at the Solar Fuels and Technology lab, where her work focuses on investigating and characterizing materials for Thermochemical Energy Conversion and Storage.

Authors:

Nhu P. Nguyen Georgia Institute of Technology
Shaspreet Kaur Georgia Institute of Technology
H. Evan Bush Sandia National Laboratories
James E. Miller Arizona State University
Andrea Ambrosini Sandia National Laboratories
Peter G. Loutzenhiser Georgia Institute of Technology