A method of inexpensively and reliably separating CO2 from flue gases by means of using magnesium hydroxide (Mg(OH)2) has been studied. Mg(OH)2 may be easily and economically reclaimed from power plants using magnesium enhanced flue gas desulfurization systems (ME-FGD). The CO2 scrubbing system may be operated as either a once-through system which produces magnesium carbonate for sequestration of carbon, or as a regenerable system where a concentrated CO2 gas stream is created for further processing.
The results of experimental investigations and energy considerations are given. The experimental results indicate that CO2 is easily absorbed into solutions containing reclaimed Mg(OH)2. These experiments were performed in a bubble reactor with simulated flue gas containing 15%V CO2 in contact with a solution of Mg(OH)2. Experiments have shown that up to 70 % of CO2 separation may be achieved in this system. From a material balance for a system based on a typical 500 MW power plant and reclaiming the magnesium hydroxide from a ME-FGD, experiments have shown that from 7 - 17 % of the CO2 from the gas stream may be continuously removed through the regenerable system.
A series of CO2 absorption experiments was conducted with reclaimed Mg(OH)2 in a batch reactor. The resulting data validated a first order reaction, where the activation energy of this reaction was measured to be 7.7 Kcal/mol.
In addition, a study was undertaken to determine the mass transfer characteristics of the bubble column reactor. A model describing CO2 absorption into clear solutions from a bubble was developed assuming a known bubble size, solution equilibrium chemistry and overall mass transfer coefficients from the gas phase to the liquid. The overall mass transfer coefficients were found to vary from 6.05x10-6 ~ 6.63x10-7 cm/s for the temperature range of 22°C ~ 60°C. Absorption experiments were also conducted with sodium hydroxide solutions and the value of KAG with NaOH solution was found to be 8.88x10-6 cm/s.
Next, this research also include the results of a study using a Turbulent Contact Reactor (TCA) to study the absorption characteristics of CO2 from a simulated flue gas using sodium hydroxide (NaOH) and magnesium hydroxide (Mg(OH)2) slurries. The results indicated that a lower fluidized velocity, more CO2 absorption using a NaOH solution, but optimum fluidized velocity was required with Mg(OH)2 slurry using this system.
The initial pH on CO2 absorption was examined by adding sulfuric acid to the magnesium hydroxide slurry. A liquid-phase equilibrium package (MINEQL+) was used to find the optimum operational absorption pH which was found to be between pH 6 to 9, where most of the carbonate is present in the form of bicarbonate ion. This allows for CO2 regeneration after the absorption step. The gas stream is contacted with the scrubbing liquor containing the magnesium compounds at a pH low enough to allow for optimum amounts of carbonate compounds to be in solution, but high enough to allow for rapid absorption of CO2. Data showed that up to 3.75 times more absorption was produced by adjusting the initial pH compared to baseline conditions. The results indicated that the use of dilute solutions of magnesium hydroxide resulted in enhanced CO2 absorption when integrated with a thermal regeneration system. The results have shown that approximately 9.2 mole of CO2 per mole of Mg is absorbed continuously at a 0.2 m/s TCA gas velocity.
Finally, the energy requirements for CO2 separation were also evaluated for a regenerable system based on equilibrium data in the liquid phase. A liquid solution equilibrium solver, MINEQL+, was used to determine the equilibrium values. The economic evaluation is based on a 500-MW power plant burning a high sulfur coal. These calculations show that approximately 40 to 68 MW of energy are required to separate 7% of the CO2 from the flue gas stream.