Product Layer Diffusion during the Reaction of Calcium Oxide with Carbon Dioxide

Abstract

The carbonation rate of 15−20 μm, nonporous, calcium oxide crystals has been studied over a temperature range of 550−1100 °C and a CO 2 pressure range of 1−11.7 atm. At temperatures greater than 600 °C, the carbonation rate decreases more rapidly with time than would be expected from diffusion through a uniform product layer and the activation energy is initially low but increases with conversion. The product layer consists of crystalline grains, and these product layer grains grow by coalescence from less than one μm diameter to the approximate dimension of the particle. The carbonation rate can be described by a model where CO 2 pressure-independent grain boundary diffusion and diffusion through the carbonate crystals act in parallel. The relative importance of bulk diffusion through the product layer crystals increases with time relative to transport through the grain boundaries and has an effective activation energy of 57 kcal/mol. Diffusion through the crystal boundaries has low activation energy and low dependence on CO 2 pressure. At temperatures greater than 900 °C and times greater than 600 min, bulk diffusion through the product layer becomes dominant and the rate approaches first order in CO 2 pressure.