Pacification of high calcic residues using carbon dioxide

The development of an alternative to hydration for the treatment and/or disposal of FBC ash was investigated. Carbonation (pacification) of the ashes was the option examined. The process is rapid at FBC operating conditions for dry ashes, but does not proceed below 400°C. Hydration of the ashes promotes the carbonation reaction below this temperature. Rapid carbonation of hydrated ashes occurs at 200–230°C. Carbonation will occur at ambient conditions for hydrated ashes, although the reaction is extremely slow. An unexpected finding in this research was that CaO, combined in the form of silicates, ferrites and aluminates (OCCs), also react with CO2. Specifically, OCCs, which are formed by reaction of the CaO in the sorbent and the fuel ash components, react with CO2 if the ashes are fully hydrated and liquid water is present. This has an impact on the disposal of FBC ash, as the reaction with CO2 will continue after placement in a disposal area. Control of the reaction would be advantageous in stabilization of the FBC ash. An effective limit to conversion of the free lime to carbonate, in the range of 60–70% was observed for the ash. The work showed the type of limestone is an important factor in the success of the recarbonation. Dolomitic limestone, a more porous sorbent, should have greater conversion rates, making the process more economic.