Pacification of high calcic residues using carbon dioxide

The development of an alternative to hydration for the treatment and/or dis posal of FBC ash was investigated. Carbonation (pacification) of the ashes was the option examined. The process is rapid at FBC operating conditions f or dry ashes, but does not proceed below 400 degrees C. Hydration of the as hes promotes the carbonation reaction below this temperature. Rapid carbona tion of hydrated ashes occurs at 200-230 degrees C. Carbonation will occur at ambient conditions for hydrated ashes, although the reaction is extremel y slow. An unexpected finding in this research was that CaO, combined in th e form of silicates, ferrites and aluminates (OCCs), also react with CO2. S pecifically, OCCs, which are formed by reaction of the CaO in the sorbent a nd 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 lime stone is an important factor in the success of the recarbonation. Dolomitic limestone, a more porous sorbent, should have greater conversion rates, ma king the process more economic. (C) 2000 Elsevier Science Ltd. All rights r eserved..