Viability of Storage Options of CO2 in Ca Silicates

Abstract

Satisfying continuously growing world energy demand by use of fossil fuels has increased carbon dioxide (CO2) concentration in the atmosphere. Dramatically higher concentration of the CO2 has a negative environmental impact, for that reason a need for analyzing possible mitigation options has arisen. The research framework on the mitigation options includes conducting studies on capturing the CO2 and its permanent and safe disposal via mineral carbonation. Mineral carbonation of wollastonite (CaSiO3) appears to be the most promising sequestration option for CO2 fixation on silicate minerals: its reaction rate is considered faster than for the other minerals such as magnesium silicates or basalts. This report investigates a direct aqueous route of mineral carbonation of wollastonite under elevated pressure and temperature. Choice of the wollastonite was made based on its high reactivity rate and the method was determined by a catalytic effect of water on the adsorption kinetics. To improve the process and limit duration of the step where CO2 diffuses in solution before attaining the solid surface, most of the experiments were dedicated to carry in a moistened sample. As a result it could benefit from the catalytic effect of water and transport of CO2 was fast enough due to pressure equilibration. To investigate the influence of the water content of the sample, the experiments for this research were carried out with different water fractions. The idea of the direct aqueous wollastonite carbonation was studied experimentally. For this purpose a set-up was designed and built. It comprised of a closed system with a reference cell, a sample cell and tubing. The whole set-up was placed in a thermal reactor to achieve elevated temperature conditions. Sample represented by mixture of water and CaSiO3 was stirred until obtaining homogeneous slurry. The slurry contained 50 v/v% of water for the samples with the highest saturation. The unsaturated sample contained 20% of water. After introducing of CO2 to the reference cell and opening a valve connecting with sample cell, pressure started to decrease until reaching an equilibrium pressure. The pressure decrease in the set-up was monitored by a pressure device. In addition a dedicated reactive diffusion model was built to interpret the experimental results. The reaction rate for the adsorbed phase and the free phase is proportional to the deviation from equilibrium, which is given by the Langmuir isotherm. A comparison between the experiment and the model leads to the determination of the reaction rate parameters. The research on the direct aqueous mineral carbonation led to the following results: the maximum adsorption is 493 kg/m3; amount of water and particle sizes have the strongest influence on the carbonation process; water behaves as a catalyst, however it may limit the conversion rate in time; the direct aqueous carbonation is a complex heterogeneous reaction that involves dissolution, nucleation, interface reaction and mass transfer.