关键词: 二氧化碳捕集, 化学反应, 催化, 产物层扩散系数, 活化能

Abstract:

Steam is present in combustion flue gas, oxy-fuel combustor/calciner and fuel gas. Some previous works in this field have examined the reactivity of calcium oxide in the presence of steam. There is general agreement that the presence of steam increases the rate of carbonation even at low concentrations. However, there is no description about the effect of steam on the carbonation reaction of calcined limestone. The effects of temperature, concentration of steam on carbonation were investigated in a thermogravimetric analyzer. The understanding of the mechanisms participating in the carbonation reaction could be arrived at through the use of model interpretations of the rate controlling process. The experimental data were analyzed by means of the random pore model. During carbonation reaction, a very initial rapid reaction was followed by the second stage of the reaction occurring in the next slower regime. The experiment results showed that the effect of steam on carbonation could be neglected during the first stage of reaction. However, the conversion of carbonation in the presence of 1.5%(vol), 10%(vol) and 20%(vol) steam were 19.8%, 27.2% and 30.5% higher than the conversion without steam after 10 min during the second stage of reaction at 500℃. Steam was beneficial to enhancing the conversion of carbonation, but the extent of increase became not significant with increasing concentration of steam. The linearity of the second stage reaction data strongly suggested that this stage was controlled by a diffusion process occurring in a layer of calcium carbonate surrounding the calcium oxide in the pores of the solid. The parameters related to effective product layer diffusivities were plotted in Arrhenius form and the changes in activation energy at various steam concentrations were also shown. Comparison with the experimental effective diffusivities calculated for the carbonation reaction showed that the activation energies in the lower temperature range were in agreement with those obtained in the conductivity measurements. This suggested that the product layer diffusion process was proceeded by a mechanism similar to that of conduction in calcium carbonate. The activation energy of carbonation decreased from 179.9 kJ·mol^-1 to 237.7 kJ·mol^-1 when 1.5% steam was added according to the random pore model. The activation energy was 156.6 kJ·mol^-1 and 148.6 kJ·mol^-1 respectively for the atmospheres of 10% and 20% steam. There were two characteristics of product layer diffusion during carbonation. One was the slope of diffusional coefficient to increase from about 550℃, which was irrelevant to the presence of steam. The other was the effect of steam on carbonation to turn weak with increasing concentration of steam. At the higher temperature range, the possible mechanism for effective diffusion could be associated with sequential decompositions of carbonate ions in the calcium carbonate layer. A carbonate ion momentarily decomposed to generate carbon dioxide and an oxygen ion. The carbon dioxide molecule then moved to a neighboring similarly vacated site, while another carbon dioxide so generated elsewhere moved to take its place and reform the carbonate ion. In this way by a site to site random walk the carbon dioxide molecule diffused through the product layer, before reaction at the interface of calcium oxide and calcium carbonate. Such a mechanism appeared more prominent than the motion of carbonate ion at the higher temperature range. Both the above mechanisms for the diffusion stage were likely, and the true situation might involve one or a combination of both.

Key words: CO₂ capture, chemical reaction, catalysis, product layer diffusion coefficient, activation energy