Analysis of characteristics and mechanisms of iron-based oxygen carrier-aided oxy-fuel combustion of coal char in a bubbling fluidized bed

Oxygen carrier aided combustion (OCAC) technology, utilizing activated oxygen carriers within the fluidized bed material, aims to regulate oxygen distribution within the furnace and enhance the combustion environment through redox reactions with the macroscopic flow in the chamber. Oxy-fuel combustion is recognized as one of the mature technologies to capture CO2 from coal-fired power plants. Here, the effect of iron-based oxygen carriers (AR-Fe2O3, hematite, and steel slag) on the combustion characteristics of three coal chars (lignite, bituminous, and anthracite) are investigated in a bubbling fluidized bed operating at 850 degrees C and varying O-2 concentrations ranging from 5 % to 21 % in CO2. The results indicate that the combustion rate of coal char and CO emission level are increased and the burnout time is shortened after replacing the quartz sand bed material with AR-Fe2O3. However, the combustion of lignite chars aided by hematite and steel slag exhibits opposite characteristics compared to those observed when substituting quartz sand bed material with AR-Fe2O3. With an increase in O-2 concentration, the combustion behavior improved due to the increased mass transfer of O-2 in CO2. The promoting effect of oxygen carrier on coal char combustion is in the following order: steel slag < hematite < AR-Fe2O3. The higher the fuel rank, the more obvious promoting effect of oxygen carrier. Analysis of OCAC mechanism combined with thermodynamic calculations shows that oxygen carrier serves a dual role: it provides oxygen to the fuel and simultaneously competes with the fuel for available oxygen. When coal char exhibits high reactivity, the competition for oxygen between the oxygen carrier and the fuel becomes more significant than the oxygen provision by the carrier. This heightened competition leads to an inhibition effect, as observed in the oxygen carriers aided combustion of lignite char.