The mechanisms of a novel method, biomass gasification under a mixed agent of CO2 and H2O using the thermal heat in high temperature (1450-1650 degrees C) slags, were identified in this study for the purpose of biomass treatment in the agriculture and slag disposal in the steel industry. The characteristics of gasification equilibriums, with varying factors including temperature, pressure, reacted CO2 and H2O amounts, were clarified especially the roles of hot slags. Both the target syngas including CO, H-2 and CH4 and the polluting gases comprising of NH3, NO and NO2 were considered here. The results indicated that compared to the limited influence of blast furnace slags, the introduction of steel slags remarkably increased both the syngas yields and the char formed. Furthermore, it was found that there existed a transition temperature range for the H-2 production and NH3 release, providing significant ideas for syngas optimization. The present study could thus not only offer important clues for the scientific understandings of biomass gasification using the thermal heat in hot slags but also show important guidance toward utilization of this emerging method. (C) 2018 Elsevier Ltd. All rights reserved.