Low-cost MgO-based sorbents were prepared through modification of natural dolomite and were found to be capable of capturing CO 2 from coal gas at elevated temperatures and pressures (i.e., 330°< T < 450 °C and P > 20 bar). The effect of steam on the reactivity of the MgO sorbents was investigated experimentally. The results indicate that the presence of steam significantly improves the overall rate of the carbonation reaction through two different mechanisms. The presence of steam improves the intrinsic rate of the carbonation reaction by altering the carbonation reaction pathway through the formation of more reactive transient MgO•H 2 O • /Mg(OH) 2 • intermediate compounds. The formation of the transient intermediate compounds with larger molar volume (compared with MgO) results in the expansion of the inner pores, which lowers diffusion resistance to the reactant gas (i.e., CO 2 ) during carbonation.
Flooding carbon dioxide into oil reservoirs is a promising technique for improving the pressure of a reservoir when it is depleted through primary and secondary production. In the context of global warming, it is a viable method for geological storage of CO 2 emissions. Once CO 2 is injected into a reservoir, it is forced to come into partial contact with formation water. To estimate the rate of CO 2 transfer and the total amount of CO 2 dissolved in the formation water, correct estimation of CO 2 diffusivity is required. In this study, the rate of CO 2 diffusion in water was experimentally determined in a PVT cell using the pressure depletion method at reservoir conditions (temperature: 50-75°C and pressure: 17,450 kPa). As expected, the rate of CO 2 diffusion in water increases with increasing temperatures. In addition, the impact of salinity of the water on the rate of CO 2 diffusion was investigated. A significant decrease in the rate of CO 2 diffusion was found with increasing salinity. Subsequently, a diffusion model describing the experiments was developed to predict the behavior of CO 2 diffusivity under simulated conditions. Unique correlations between CO 2 diffusion coefficients and water at different temperatures and salinities were obtained using the results of modeling.
Regenerable MgO-based sorbent, which was prepared and evaluated in the thermogravimetric analyzer (TGA) in part 1, was also evaluated in high-pressure packed-bed unit in CO 2 /N 2 /H 2 O mixture and simulated pre-combustion syngas environment. In CO 2 /N 2 /H 2 O environment, The CO 2 absorption capacity of the sorbent increases with increasing temperatures from 6.7% at 350°C to 9.5% 450°C. The sorbent is capable of achieving over 95% CO 2 capture and 40% conversion in the WGS reaction, which should be attributed to positive effect of WGS reaction in producing CO 2 during the process. The sorbent reactivity and absorption capacity toward CO 2 , as well as its WGS catalytic activity decreases with increasing temperature. The maximum prebreakthrough WGS conversion occurs at 350°C, which diminishes as the sorbent is carbonated. The variable diffusivity shrinking core reaction model coupled with the two-fluid CFD model was shown to accurately predict the breakthrough gas compositions at different operating conditions.
Partially decomposed dolomite is a promising base material for regenerable MgO-based sorbents to capture CO2 from precombustion syngas at high pressures and temperatures. An important characteristic of the sorbent affecting the economic viability of this class of sorbents is the reactivity and the capacity of the sorbent for CO2 capture. To improve the reactivity of the sorbent, the thermal behavior and the kinetics of partial decomposition of dolomite were studied in the temperature range of 520–610 °C in a dispersed-bed reactor. The microstructure and the nature of the solid products were found to be strongly dependent on the CO2 partial pressure near the reacting interface and on the decomposition temperature. A significant increase in the rate of the dolomite decomposition reaction was found in the presence of steam. Steam improves the kinetics of decomposition, modifies the radial distribution of the pores, and improves the connectivity of the pores inside the dolomite particles, which decreases the diffusion resistance of produced carbon dioxide inside the particle. A shrinking core model with variable product layer diffusivity was used to fit the experimental data and determine the kinetic parameters of the dolomite decomposition reaction. The results indicate that transport of CO2 across the reacting interface in the porous particle was the main limiting factor in the decomposition reaction at the experimental conditions investigated. The model is shown to provide an excellent fit to the experimental data on partial decomposition of dolomite in the temperature range studied.
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