Experimental investigation on recycled flue gas inhibiting Na release in coal combustion

The sodium (Na) during high-alkali coal combustion induces severe slagging and fouling, which significantly constrains boiler operation. Controlling the combustion atmosphere via recycled flue gas (RFG) is a promising strategy for inhibiting Na release. This study simulated RFG by adjusting CO₂ concentration. Using coupled flame emission spectroscopy (FES) and laser-induced breakdown spectroscopy, simultaneously detected Na release in both flame and flue gas under different atmospheres. The results indicated rapid migration of gaseous Na from flame to flue gas, though with incomplete transfer (about 50%). CO₂ substantially suppressed Na release through its high thermal sink effect, with enhanced suppression at higher concentrations. This suppression delayed the peak Na release timing and reduced devolatilization and char burnout rates. The consistency between LIBS and FES results validates the efficacy of FES for monitoring Na release. Furthermore, RFG strategy was implemented on a 5 MW pilot-scale facility to quantify Na release across different RFG conditions. Increased inner secondary flue gas ratio reduced facility temperature and corresponding Na release. These industrial measurements aligned with lab-scale observations. RFG technology suppresses Na release through oxygen/temperature regulation. This study provides a theoretical basis for fouling and slagging research under widely adjustable oxygen/temperature conditions and offers a cost-effective, highly adaptable engineering strategy to enable clean and efficient combustion of high-alkali coal. • Recycled flue gas method was proposed to inhibit Na release. • A combined FES-LIBS system was developed for lab-scale simultaneous Na detection. • The effectiveness of the RFG strategy was further validated in a 5 MW pilot-scale facility.