Hot Defluorination of Reducing Gases with Lime Pellets

Abstract

In IGCC and MCFC power generation systems the coal gas has to be purified from dust and deleterious gaseous species. Hot gas cleaning processes have advantages compared to conventional wet cleaning technologies. Losses of energy occurring during cooling and reheating of the gas can be avoided, and there is no formation of a wet slurry. In the present study the defluorination of reducing gases with dry absorbers such as calcium carbonate (CaCO3) and calcium oxide (CaO) has been investigated in the temperature range 300−700 °C. Two types of experiments were carried out to clarify the reaction kinetics: thermogravimetric experiments on single (or a few) pellets in which the weight change of the absorber due to its conversion to calcium fluoride was determined, and gas cleaning experiments using a laboratory scale fixed bed reactor in which the off-gas was analyzed. The results indicate that H2−H2O−HF and CO−H2−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 reaction rate is mainly controlled by gaseous pore diffusion. This is so also with respect to calcium oxide at high temperatures (500 °C). But at low temperatures (300 °C) the reaction occurs within the bulk of the pellet.