Feasibility study on concurrent capture of mercury and carbon dioxide from coal-fired power plant flue gas using amine-based absorption process.

摘要

Standards for the control of mercury (Hg) emissions from coal-combustion flue gas will be implemented in 2010 according to the Canada-Wide Standards for Mercury Emissions from Coal-fired Electric Power Generation Plants (2006). Accordingly, in order to satisfy the required standards in the very near future, study of the best available mercury (Hg) emission control technologies for coal-fired power plants is urgently needed. Gas absorption into a chemical absorbent has been proven the most suitable technology for post-combustion carbon dioxide (CO2) removal. However, there is little knowledge and experimental data on Hg absorption from flue gas using this absorption process. The objective of this thesis is, therefore, to study the feasibility of using the amine absorption-based CO2 capture process for the concurrent capture of Hg and CO2.;The feasibility study was performed by carrying out over 120 Hg absorption experiments using a laboratory-scale spray column with three different types of absorption solutions: mixtures of sodium chloride (NaCl) and sodium hypochlorite (NaOCl), monoethanolamine (MEA), and blended MEA and NaCl-NaOCl. The Hg absorption performance was evaluated in terms of Hg removal rate, Hg removal efficiency, and the volumetric overall mass transfer coefficient (KGa e) as a function of various process variables, i.e., gaseous-phase Hg inlet concentration (350, 500, 750, 1000, and 1400 ng/m3), gaseous-phase Hg velocity (22.92 and 45.84 m3/m2-h), solution velocity (3.50, 4.81, and 6.88 m3/m2-h), CO2 loading of the solution (0.00, 0.15, 0.25, and 0.35 mol/mol), MEA concentration (1.0, 3.0, and 5.0 kmol/m3) and NaCl concentration (0.01, 0.05, 0.10, 0.50, 0.60, 0.80, and 1.00 kmol/m3) and NaOCl concentration (1.0 X 10-4, 3.0X10-4, 5.0 X 10-4, and 1.0X10-3 kmol/m3). Results show that the Hg absorption performance of the aqueous NaCl-NaOCl solution is much higher than that offered by the aqueous MEA solution and the blend of MEA and NaCl-NaOCl. Hg absorption into a MEA solution is controlled by mass-transfer in the liquid phase, not in the gas phase, and is affected by MEA concentration, CO2 loading, and solution flow rate but not by gas-phase Hg partial pressure and gas velocity. The Hg absorption performance of the aqueous NaOCl-NaCl solution is affected by the mixing concentration of NaCl in the solutions but not by the NaOCl concentration. Mixtures of NaOCl-NaCl do not perform as rate enhancers for Hg removal in the presence of MEA. The presence of Hg in the MEA solutions does not affect the CO2 absorption performance of MEA. To capture both Hg and CO2, the two-step capture process that employs the aqueous NaCl-NaOCl solution as the absorption solvent for Hg removal, prior to CO2 capture in the amine unit, is technically feasible. However, if the Hg-loaded NaCl-NaOCl solution is sent to the amine absorber, two possible operational problems may arise: 1) the desorption of gaseous Hg from the blended MEA and NaOCl-NaCl solutions during CO2absorption and 2) the dilution of the MEA solution in the CO2 capture unit.