Ammonia synthesis via chromium-based nitrogen carrier looping

Abstract

Redox nitrogen fixation-nitride hydrolysis looping provides a promising pathway for ammonia synthesis; however, high N fixation temperature and low N3−lattice supplement efficiency limit its further development and application. In this regard, a two-step chemical looping ammonia synthesis route, chromium-based nitrogen carrier enabled low-temperature N fixation and nitrogen carrier hydrolysis, is proposed to intensify ammonia generation. In the N fixation step, the effects of carbon species on the carbothermal reduction performance are investigated. Results reveal that decreasing the graphitization degree of the carbon species can effectively reduce the nitrogen activation temperature from 1054 °C to 948 °C. Meanwhile, the C/Cr2O3 ratio required is correlated with the amount of oxygen in carbon species, and the appropriate active carbon/biochar-to-Cr ratios are 4.00 and 3.25, respectively. In the ammonia synthesis step, the nitrogen carrier hydrolysis rate is mainly limited by the diffusion of reactive species. The ammonia formation rate reaches 0.127 mmol·h−1·g−1 nitrogen carrier at 1100 °C. XRD retrieve refinement results reveal that the utilization of the lattice nitrogen decreases with the increase in the ammonia releasing temperature. This study provides new implications for advanced nitrogen carrier exploitation, which will substantially promote the development of chemical looping ammonia synthesis technologies with lowered energy consumption.