Increased stability of CuFe2O4 oxygen carriers in biomass combustion by Mg doping

Abstract

Biomass derived from agricultural waste is a promising source of renewable energy. When used in low-emission combustion technologies such as chemical looping combustion (CLC), it has the potential to achieve net negative CO₂ emissions. In CLC, the fuel is isolated from atmospheric air, resulting in flue gases that comprise mainly CO₂ and H₂O. Since the fumes are not diluted by atmospheric N₂, low-cost CO₂ capture is possible. The oxygen required for CLC is delivered entirely by an oxygen carrier (OC). Spinel-type OCs have a high oxygen-transport capacity, mechanical durability, and chemical stability. However, biomass ash is rich in alkali metals and SiO₂, which adversely affect OCs by promoting cracking and agglomeration. Herein, the effect of Mg doping on the resistance of OCs to biomass ash is explored. Five Mg_xCu_1−xFe₂O₃ −type spinels (x = 0–1) are evaluated for the combustion of four types of biomass with varying ash compositions: three agricultural waste products (pine wood, kenaf, and rice husk) and one dedicated energy crop (Miscanthus). Among the tested OCs, Cu_0.5Mg_0.5Fe₂O₄ demonstrates the highest reactivity and conversion rates, with a reaction rate of 2.70 wt.%/min for kenaf and 95.9 % conversion for Miscanthus. Following multiple reaction cycles, undoped and low-Mg OCs (x ≤ 0.5) exhibit cracking and structural degradation, whereas high-Mg OCs (x ≥ 0.75) retain their structural integrity, confirming the benefit of Mg doping on the durability of spinel-type OCs. This study provides insight into the design of more resilient OCs for biomass combustion, which will guide future research on CLC technologies.