Captured char particles on the molten slag continue to burn or transform into residual carbon in a slagging combustor or furnace, which affect the complete carbon conversion. This study applied a high temperature stage microscope to investigate the combustion behavior of captured chars on the molten slag surface. The combustion process of captured chars with air on the slag surface was observed and recorded, compared to the original char combustion. Particle size evolution calculated from the measured cross section area versus time indicated that the burnout time of chars was prolonged on the molten slag surface. Besides the heat transfer analysis also showed that the temperatures of char particles on the molten slag were decreased due to the thermal conduction between char and slag. Molten slag layer reduced the char reactivity from the analysis of combustibility index, indicating the captured char combustion on the molten slag surface was hindered. Coupled with heat transfer analysis, shrinking particle model (SPM) was applied and modified to predict the combustion time at carbon conversion of 0.9, and results showed an agreement with the experimental data. (C) 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved.