Hot defluorination of reducing gases with lime pellets

In IGCC and MCFC power generation systems the coal gas has to be purified f rom dust and deleterious gaseous species. Hot gas cleaning processes have a dvantages compared to conventional wet cleaning technologies. Losses of ene rgy occurring during cooling and reheating of the gas can be avoided, and t here is no formation of a wet slurry. In the present study the defluorinati on of reducing gases with dry absorbers such as calcium carbonate (CaCO3) a nd calcium oxide (CaO) has been investigated in the temperature range 300-7 00 degrees C. Two types of experiments were carried out to clarify the reac tion kinetics: thermogravimetric experiments on single (or a few) pellets i n which the weight change of the absorber due to its conversion to calcium fluoride was determined, and gas cleaning experiments using a laboratory sc ale fixed bed reactor in which the off-gas was analyzed. The results indica te that H-2-H2O-HF and CO-H-2-CO2-H2O-HF gas mixtures can be defluorinated both with CaO and with CaCO3. The calcium fluoride (CaF2) forms as a shell around the unreacted core of calcium carbonate of the pellet, and the react ion rate is mainly controlled by gaseous pore diffusion. This is so also wi th respect to calcium oxide at high temperatures (500 degrees C). But at lo w temperatures (300 degrees C) the reaction occurs within the bulk of the p ellet.