Comparatively experimental investigation of micro-mixed and macro-mixed Fe-Ni oxygen carrier for mid-temperature chemical looping hydrogen production

Abstract

Chemical looping hydrogen production utilizing iron-based oxygen carriers is a promising solution for efficient hydrogen production and carbon dioxide emission reduction, but most studies focus on high temperature processes (>800 °C). Mid-temperature chemical looping hydrogen production (<650 °C) is achieved using bimetallic Fe-Ni oxygen carrier. Two strategies (microscopic mixing and macroscopic mixing) are experimentally compared. Contrary to the prevailing belief that tightly mixing Fe and Ni species enhances C-H bond cracking and hydrogen generation synergistically, it is revealed that the macroscopic mixing improves the mid-temperature reactivity with CH₄. The CH₄ conversion for the macroscopic mixing exceeds 90 %, alongside high purity hydrogen production from steam oxidation. Additionally, the experimental results about varying NiO percentages indicate that while a higher NiO percentage increases the CH₄ conversion, it inversely affects fuel (CO and H₂) selectivity. We propose and verify the synergistic mechanism of physical mixing, where the micron-level separation between NiO and Fe₂O₃ powders prevents active phase interaction. Therefore, H₂ from nickel-catalyzed CH₄ cracking can deeply reduce Fe₂O₃, and the resulting H₂O can gasify the deposited carbon, thus dynamically regenerating nickel and generating H₂. Our study consolidates the potential of the chemical looping hydrogen production method with mid-temperature operation and provides novel perspectives on the design of the industrially tailored Fe-Ni oxygen carriers for the processes.