While geologic sequestration method may be used to store large volumes of CO2, it involves two distinct steps, i.e., capture and storage, with the former being a more costly step. Also, finding suitable storage sites, pumping supercritical CO2, safekeeping it underground, and handling of saline water discharges could impede its deployment. CO2 mineralization, on the other hand, can be a single-step process and the product can be stored permanently. In some cases, the product can be utilized as construction materials. Most of the CO2 mineralization studies conducted to date involve reacting the greenhouse gas with basic minerals. It is necessary, however, that the minerals be activated either through fine grinding or heating, so that it can more readily react with CO2.
In the present work, we have explored the possibility of reacting CO2 with the species that are already present in waters such as produced water from oil and gas industries and sea water. Many of the species present in these waters (e.g., Ca2+ and Mg2+ ions) are reactive without or with minimal activation energies, thereby reducing both capital and operating costs. Further, the process can be used to clean-up the produced water while achieving both capture and storage of CO2, all simultaneously. Recognizing that sea water represents the single largest reservoir of the reactive species, we have conducted mineralization experiments using 1,400 mg/L of Mg2+ and 400 mg/L of Ca2+ ions which represent average concentrations in sea water. The amounts of these species present in sea water alone are such that we can use these two reactive species for the next 100,000 years at the present level of global CO2 emission (35 Gt/year).
It was found that Ca2+ ions react more readily than Mg2+ ions. A series of mineralization experiments conducted at temperatures in the range of 10 to 40oC showed that the activation energy for the formation of nesquehonite (MgCO3.3H2O) is 64.6 kJ/mol. It was found that the activation energy barrier can be readily overcome by simple agitation and heating at slightly elevated temperatures, e.g., 40oC. The kinetics of mineralization and the %Mg2+ ion utilization varies depending on energy dissipation rate, temperature, pH, and NaCl concentration. The maximum Mg2+ ion utilization we achieved was 86%. We also conducted thermodynamic calculations to construct the species distribution diagrams, predict the pH of CO2 mineralization, and to predict %Mg ion utilization (or extraction) from sea water. The CO2 mineralization method developed in the present work may lead to a process of extracting magnesium, which is listed as a “near-critical” material in the 2011 DOE Critical Materials Strategy Report.
One of the concerns in CO2 mineralization with reactive species may be pH lowering. We have addressed this issue by contacting spent solutions with basic minerals such as limestone and olivine. It was found that in the presence of these minerals the pH rises to the pH of minimum solubility of the buffering mineral. The pH of minimum solubility of limestone is 8.3 and that of olivine is 8.6. Other means of pH neutralization will be discussed.
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