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The impact of coal fly ash addition as in-furnace alkali capture additive on the fine particle formation is investigated. Therefore, measurements in two full-scale boilers (the tangentially fired 800 MW th boiler Avedøre Power Station Unit 2 and the boxer-fired 900 MW th Studstrup Power Station Unit 3) and a pilotscale combustion test rig at the Technical University of Munich (120 kW th ) were carried out. Combustion aerosol measurements with an electrical low-pressure impactor varying the inserted coal fly ash load were conducted and PM 1 and PM 2.5 values were determined. Detailed particle-size-dependent characterization of the chemical composition of the aerosols was performed using scanning electron microscopy coupled with energy dispersive X-ray spectroscopy. The results show that the chemistry of the submicron particles migrated from high Na, K, S, and Cl shares toward P-and Ca-rich particles when increasing the coal fly ash load. Significant PM reduction effects are observed when using coal fly ash for both, the full-scale boilers and the pilot-scale test rig. PM 1 emission reductions of 36.4% for AVV2 and 42.2% for SSV3 were reported when coal fly ash load was increased from 0.94 to 2.42 wt % (dry/dry) and 2.14 to 3.22 wt % (dry/dry), respectively. The results from the pilot-scale tests revealed drastically higher PM values compared with the full-scale measurements. The reference experiment without additives showed a PM 1 of 234.6 mg/ Nm 3 . A reduction efficiency of 31.8% was obtained using 1.05 wt % coal fly ash (dry/dry). The discrepancy of the observed PM emissions between the tests could be explained by lower temperatures in the pilot-scale reactor, enhanced PM reduction by condensation of gaseous metal vapors, impaction of PM onto cooled heat exchanger surfaces in the full-scale boilers, and increased capture via the liquidus pathway.