<p>Heterogeneous condensation has been widely explored as a pretreatment strategy to enhance the removal of fine particles in coal-fired power plants by utilizing the high temperature and humidity of flue gas. However, its high energy consumption remains a major obstacle for large-scale application. This study proposes a flue gas recirculation technique to promote the growth of PM2.5 under low supersaturation conditions. Experiments using representative low-wettability (SiO₂) and high-wettability (CaSO₄) particles were conducted to investigate the effects of particle wettability, and supersaturation on particle growth. The results show that flue gas recirculation significantly facilitates the growth of poorly wetted fine particles at low supersaturation due to the combined effects of particle collision and secondary growth. To decouple these mechanisms, two specialized configurations—a collision and secondary growth tubes were designed to isolate collision growth from secondary growth. The findings reveal that collisions mainly govern the growth of low-wettability particles, whereas for CaSO₄ particles with higher wettability, collisions play a stronger role at relatively high (1.14) and low (1.03) supersaturations, and secondary growth becomes dominant under intermediate supersaturated condition. These findings clarify the relative roles of collision and secondary condensation and provide practical guidance to optimize partial flue-gas recirculation for industrial PM2.5 control.</p>