Study On CO₂ Sequestration Through Carbonate Precipitate Formation In Sea Water

With the increasing concern about human-induced global climate change, C02 becomes the primary focus because its concentration increace is believed to be the most important factor for the greenhouse effect. It is necessary and urgent to develop carbon capture and sequestration. The climate change caused by CO2 emission is a global and multi-tiered crisis. A variety of methods and technologies should be studied to solve the problem, because no single one can settle the problem alone.This paper reported a novel method to carbon sequestration through carbonation process between gaseous CO2 and brine solution or other Ca2+/Mg2+-rich aqueous solutions. Mass transfer and reaction enhance the conversion of CO2 to CO32-, which is finally fixed in carbonate precipitation by Ca2+/Mg2+. The solid carbonate sequestration stands for a simple, permanent and environmentally friendly method of trapping CO2, which makes our method a promising complementary solution to other methods of CO2 sequestration.First, the theory of CO2 sequestration by seawater is analyzed. The integrated carbonate system, which involves gas-liquid-solid three phases, plays a key role in carbon capture and sequestration using Ca2+/Mg2+-rich aqueous solutions. Driven by the C02 partial pressure and alkalinity of the brine solution, the gaseous CO2 dissolves in solution and fractionally transfers to HCO3- and CO32-, when the concentration of CO32- rises to saturated level, it will precipitate from solution in the formation of CaCO3/MgCO3. Effect of two important factors on carbonation process-CO2 partial pressure and pH of the brine solution are analyzed emphatically. The estimation of carbon fixation capacity for per unit volume of brine water shows our method is a potential and promising way to carbon sequestration. The likelihood of using rejected brines from desalination plants, produced water, underground brine and waste water from salt pans makes our method an even more attractive solution.In order to investigate characteristics of carbonation process and solid precipitation product, experiments are carried out based on theory analysis. The reaction between brine solution and C02 flux happens in a bubbling bed reactor, and it is essential to raise the pH of brine solution and keep it at an acceptable variety during the carbonation reaction. The carbonation process between brine solution and C02 flux occurs in a pH range 8-10. Ammonia-ammonium chloride buffer is applied to regulate brine solution in a suitable pH range, and it is a specified agent in the nation standard which could resist dramatic changes of pH and maintain a relatively constant alkaline condition. With the bubbling of CO2, the pH of the solution system was declining slowly without any sudden changes and more and more CO32- ions were generated. When the concentration of CO32- rises to saturated, it will precipitate from solution in the formation of CaCO3/MgCO3. Because the solubility product constant of CaCO3 is lower than that of MgCO3, phenomenon of fractional precipitation happens:CaCO3 precipitate from solution system first. The initial concentration of Ca2+ and Mg2+ were about 420mg/L and 1350mg/L respectively. The final concentrations of Ca2+ and Mg2+ were 12 mg/L and 120 mg/L respectively, which were about 3% and 9% of their initial concentrations. This means over 90% of Ca/Mg ions in the seawater solution were migrated into the precipitation, fixing gaseous CO2 in the formation of solid carbonate. Carbon fixation capacity for per unit volume of brine water is about 61.45mmol/L. The carbonate salts crystallize to a stable-shaped crystal form, and their thermodynamic and chemical stability insure that they won't decompose in environmental conditions, which means CO2 is fixed stably in them. Variable condition experiments show that higher concentration of brine solution, higher alkalinity of the solution system, higher reaction temperature and larger quantity of air flow are stimulative factors for carbonation process.In the end, the chemical analysis software ASPEN PLUS was employed to investigate the carbonation process-the removing rate of CO2 is taken as objective functions, and sensitivity analysis of several factors are researched. The simulations fill up a deficiency under laboratory conditions and improve our schemes.