Biomass combustion generates renewable energy, which is optimized by designing a biomass combustion system that controls excess air intake and evaluates the ash fouling/slagging potential. The objective of this study was to (1) investigate the effect of excess air ratio (EAR) on the combustion of switchgrass (Panicum vigratum L.) and hardwood, (2) assess their ash fouling and slagging tendencies, and (3) perform an in-depth thermogravimetric kinetic analysis to understand their combustion. Switchgrass and hardwood contained 17.5 and 17.7 MJ/kg of energy value, which was appropriate for heat generation. The greatest energy conversion efficiency and combustion completeness rate were obtained with an EAR of 20% for switchgrass and 30% for hardwood based on our combustion system with 4 mm particles of fuel. Kinetic analysis confirmed that increasing the oxygen availability resulted in superior energy conversion. In general, switchgrass ash had lower fouling and slagging tendencies than hardwood owing to its more acidic chemical composition. Heat and mass transfer delays were still observed from this combustion system, thus making the combustion request more air to even achieve a stoichiometric condition. However, rather than an ideal test (e.g. single particle combustion), the conclusions made by this study were a practical guidance for boiler operations, since the heat and mass transfer delays were a common phenomenon in real applications that should not be eliminated in our lab-scale studies.. (C) 2015 Elsevier Ltd. All rights reserved.