In-situ evolution of biochar structure along with carbon/nitrogen migration during biomass oxy-fuel combustion

The integration of biomass oxy-fuel combustion with renewable energy sources presents a viable and sustainable pathway for the production of green methanol, offering a significant strategy to advance carbon neutrality goals. The high volatile content and alkali-rich nature of biomass limit the applicability of pulverized coal oxy-fuel combustion principles to biomass systems. This study investigates the oxy-fuel combustion characteristics of typical biomass fuels in a circulating fluidized bed experimental system, with particular focus on CO₂ gasification etching effects and in-situ biochar structural evolution. Results indicate that the oxy-fuel atmosphere enhances carbon enrichment by 15.8% for corn straw and 11.2% for rice husk, respectively, while improving burnout efficiency. The process preferentially attacks N-5 structures, promoting fuel-nitrogen release. High oxygen concentration and CO₂ gasification etch internal pores of in situ biochar, cleave aromatic layers, and facilitate outward migration of alkali metals that catalyze combustion. Thermal effects drive deep CO₂ etching into microporous structures, enhancing carbon ordering, whereas temperatures above 800 °C accelerate large aromatic ring decomposition and carbon matrix collapse. The molten silicate network in rice husk biochar exerts a certain inhibitory effect on oxidative etching. In the late combustion stage, the carbon framework exhibits apparent ordering and structural refinement, accompanied by pore wall collapse and channel merging. The findings offer theoretical and data-driven support for implementing carbon-negative, high-value biomass utilization strategies alongside oxy-fuel combustion technology.