the effect of CO2 on the kinetics and extent of calcination of limestone and dolomite particles in fluidised beds

Measurements have been made of the rates of calcination of limestone particles (diam. 0.4–2.0 mm) in a fluidised bed, electrically heated to a well-defined temperature. Experiments were conducted at atmospheric pressure and also at pressures of 3, 6, 12 and 18 bar, for bed temperatures varying from 1073 to 1248 K. The fluidising gases were air, or occasionally nitrogen, containing up to 20 vol. % CO₂. The results indicate that under these conditions the rate of calcination of such limestone particles is controlled by chemical reaction at a sharp interface between CaCO₃ and CaO. The temperature of this reaction zone is only a few degrees (< 15 K) below that of the fluidised bed. The rate of calcination is found to be of the form: k̄(p^eCO₂ - pⁱCO₂ - Py_I) kmol/m² s, where p^eCO₂ and pⁱCO₂ are, respectively, the partial pressures of CO₂ for equilibrium at the temperature of the reaction interface and in the fluidising gas, k̄ is the rate constant associated with the reverse carbonation reaction (CO₂ + CaO → CaCO₃), P is the total pressure, and y_I is a constant, which depends on the temperature of the bed. Values of k̄ were measured. They appear to be independent of temperature, indicating that carbonation proceeds without an associated activation energy. It is hard to explain the appearance of y_I (an effective mole fraction for CO₂) in the above rate expression. The calcination of one dolomite has been briefly studied and calcination times etc. measured. In general, this appears to be a two-stage process, with the calcination of the MgCO₃ component being insensitive to pressure, unlike the CaCO₃. The value of k̄ for CO₂ + MgO → MgCO₃ is similar to that for the calcium case.