Iron based oxygen-carrier-aided oxy-fuel combustion of coal char: Reactivity and oxygen transfer mechanism

Adding an active oxygen carrier to the inert bed material may favor the oxygen distribution in fluidized bed (FB) combustors. This so-called method Oxygen-carrier-aided combustion, OCAC has been applied recently to air combustion in FB boilers, but could also be potentially interesting in oxy-fuel combustion. Here we analyze OCAC in oxy-fuel combustion by experimental determination of the reactivity of char from three coals (lignite, bituminous and anthracite coal) using three oxygen carriers (OC, analytical reagent Fe2O3, hematite, and steel slag). The tests were conducted in a micro thermo-gravimetric analyzer (micro-TGA) and the combustion kinetics was determined for the nine combinations of char and OC. Additional tests were conducted in a macro-TGA i.e., much sample in a big ceramic crucible, to evaluate the phase composition and crystal quality using X-ray diffraction (XRD) to the combustion residues collected at various degrees of carbon conversion (0.2, 0.5, and 0.8). The results show that the burnout temperature decreases with OCs as the combustion rate increases, while the ignition temperature hardly changes. The enhancement of the rate of char combustion by OCs is in the following order: steel slag < hematite < AR-Fe2O3. The lower the fuel rank, the stronger the effect of the oxygen carrier. The combustion kinetics of chars using OC has a higher activation energy and pre-exponential factor than pure char. The enlarged lattice constant of Fe2O3 measured by XRD during the char combustion indicates that the redox reactions of Fe2O3 accelerate the oxidization of char by enhancing the rate of oxygen transport to the carbon surface.

Automated summary

Adding active oxygen carriers to the inert bed material improves oxygen distribution in fluidized bed combustors. This method, known as Oxygen-carrier-aided combustion (OCAC), has been used in air combustion in FB boilers and shows potential for oxy-fuel combustion. Experimental analysis of OCAC in oxy-fuel combustion was conducted using different coals and oxygen carriers. The results demonstrate that the burnout temperature decreases with oxygen carriers, while ignition temperature remains stable. The effect of oxygen carriers on char combustion rate follows the sequence: steel slag < hematite < AR-Fe2O3, with stronger effects observed for lower rank fuels. Combustion kinetics of chars using oxygen carriers exhibit higher activation energy and pre-exponential factor compared to pure char, indicating enhanced oxygen transport to the carbon surface.