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Simultaneous Capture of Mercury and CO2 in Amine-Based CO2 Absorption Process Zheng Cui, Adisorn Aroonwilas, and Amornvadee Veawab* Energy Technology Laboratory, Faculty of Engineering and Applied Science, UniVersity of Regina, Saskatchewan, Canada S4S 0A2
The feasibility of using an amine-based carbon dioxide (CO2) capture unit for the simultaneous capture of mercury (Hg) and CO2 was studied by carrying out Hg absorption experiments 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 results show that it is not effective to use the amine-based CO2 unit for the concurrent capture of Hg and CO2 because the Hg absorption performance of aqueous solutions of MEA and blended MEA/NaCl/NaOCl is low. Mixtures of NaOCl and NaCl do not perform as a rate enhancer 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, a two-step capture process that employs an aqueous NaCl/NaOCl solution for Hg removal, prior to CO2 capture in the amine unit, might be an option. 1. Introduction Coal-fired power plants combust coal to generate electricity and, at the same time, produce flue gas containing a number of air pollutants. These include hazardous pollutants such as mercury (Hg) and criteria pollutants such as particulate matter (PM), sulfur oxides (SOx), and nitrogen dioxide (NO2). Coalfired power plants also produce and release carbon dioxide (CO2) to the atmosphere, a major greenhouse gas contributing to global climate change. These CO2 emissions are of great concern because of the large quantity released and the implications such release has for the environment. In Canada, emissions of PM, SOx, and NO2 are currently being regulated under environmental laws and regulations, whereas no regulations are imposed on the emissions of CO2. The regulation on Hg emissions from coal-fired power plants was endorsed by the Canadian Council of Ministers of the Environment (CCME) in October of 2006.1 Because of their significant impacts on human health and the environment, Hg and CO2 must be removed during postcombustion treatments of the flue gas from coal-fired power plants. Several technologies are being evaluated for these treatments. Examples of Hg removal technologies include the sorbent injection and K-fuel processes.2-6 Examples of CO2 capture technologies include absorption into chemical solvents, permeation through membranes, and adsorption onto solid sorbents.7 However, use of these technologies for the removal of Hg and CO2 in two separate processes requires a great deal of expenditure because of both initial capital investments and operating costs. As such, capturing Hg and CO2 in one single process would help reduce the treatment costs substantially. This work explores the feasibility of using a CO2 capture unit for the concurrent capture of Hg and CO2. Although CO2 capture is not currently required of power plants, it is anticipated that such unit will be widely installed in the near future for environmental reasons. The CO2 capture unit in this study relies on the principle of gas absorption into an aqueous solution of alkanolamine. The CO2 gaseous component is removed from the flue gas by the alkanolamine solution in the absorption column, and a nearly pure CO2 gas stream is released from the * To whom correspondence should be addressed. Tel.: (306) 5855665. Fax: (306) 585-4855. E-mail:
[email protected].
alkanolamine solution in the solvent regeneration column.7 Because no previous works on the concurrent capture of Hg and CO2 are available in the literature, this feasibility study involves the following tasks: (1) studying the absorption of Hg into an aqueous solution of alkanolamine to examine whether and to what extent Hg can be absorbed into an aqueous solution of alkanolamine used in the CO2 capture unit under service conditions, (2) studying CO2 absorption performance in the presence of Hg to evaluate the effect of Hg in a solution on the CO2 absorption performance, (3) studying Hg absorption into water and into aqueous solutions of alkanolamine containing sodium chloride (NaCl) and sodium hypochlorite (NaOCl) to evaluate the performance of these two chemicals as kinetic enhancers for Hg absorption in the solution, and (4) modifying the absorption-based CO2 capture unit to extend its capability with respect to capturing CO2 and Hg from the flue gas. To implement these task, over 120 experimental gas absorption runs were carried out under the operating conditions listed in Table 1. 2. Experiments 2.1. Experimental Setup. The feasibility of the concurrent capture of CO2 and Hg in an absorption-based CO2 capture unit for coal-fired flue gas treatment was evaluated by carrying out a series of gas absorption experiments in a bench-scale gas absorption unit specially designed and built to accommodate both CO2 and Hg absorption modes of operation. As illustrated Table 1. Summary of Experimental Parameters and Their Ranges of Operation parameter
experimental values
elemental Hg inlet concentration (ng/m3) MEA concentration (kmol/m3) gas velocity [m3/(m2 · h)] liquid velocity [m3/(m2 · h)] CO2 loading (mol of CO2/mol of MEA) NaOCl concentration (kmol/m3)
350-1400 0.0, 1.0, 3.0, 5.0 22.92, 45.84 3.50, 4.81, 6.88 0.00, 0.15, 0.25, 0.35 1.0 × 10-4, 3.0 × 10-4, 5.0 × 10-4, 1.0 × 10-3 0.01, 0.05, 0.1, 0.5, 0.6, 0.8, 1.0 5, 10, 15 23.0
NaCl concentration (kmol/m3) CO2 concentration in gas stream (%) temperature (°C)
10.1021/ie100687a 2010 American Chemical Society Published on Web 10/29/2010
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Figure 1. Schematic diagram of the experimental setup of the gas absorption unit.
in Figure 1, the setup was designed to facilitate three sequential modes of operation, namely, generation of Hg vapor, absorption of gas, and the treatment of waste gas. By purging nitrogen (N2) over liquid Hg at a controlled temperature, a vapor mixture of elemental Hg and N2 was generated and then diluted by a stream of N2 to obtain the desired Hg concentration. The setup for Hg vapor generation consisted of a 25 cm3 impinger with a plain stopper (Chemglass, Vineland, NJ), an ice-water bath, and two gas flow meters (Cole-Parmer Instrument Company, Vernon Hills, IL) calibrated with a Humonics Outflows 650 soap flow meter (Sigma-Aldrich, St. Louis, MO). The gas absorption took place in an absorption system consisting of (1) an absorption column made of acrylic plastic with a 0.552-m height and a 0.10-m internal diameter, (2) a
spray nozzle in the absorption column (model P-40, BETE Industrial Spray Nozzle, Greenfield, MA), (3) a gear pump with a 0.1 hp electric motor (Cole-Parmer) for pumping the liquid solution to the top of the column, (4) a solution feed reservoir, (5) a gas flow meter for CO2 (Cole-Parmer) calibrated with a Humonics Outflows 650 soap flow meter (Sigma-Aldrich), (6) a CO2 analyzer (model 302WP, Nova Analytical Systems Inc., Hamilton, ON, Canada) with a testing range of 0.0-20.0 vol. % CO2, and (7) a Hg vapor analyzer (model 2537A) from Tekran Instruments Cooperation (Toronto, Canada) used for gaseous Hg concentration measurement with a detection limit of 0.1 ng/m3 and a range of 0.1-2000 ng/m3.
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The treatment of waste gas was carried out in a 250 cm3 acidic permanganate scrubber containing 0.1 kmol/m3 potassium permanganate (KMnO4) and 10% sulfuric acid (H2SO4). The entire experimental setup was placed in a fume hood for safety purposes. 2.2. Materials. A number of chemicals were purchased and used for three different purposes. Monoethanolamine (MEA), elemental Hg, NaCl, and NaOCl were used directly for gas absorption experiments. Hydrochloric acid (HCl) and methyl orange were used for solution analysis, and sulfuric acid (H2SO4) and potassium permanganate (KMnO4) were used for the treatment of waste gas. Aqueous solutions of these chemicals were prepared from deionized water. In addition to chemicals, gases, including argon (Ar), N2, and CO2, were required during the experiments. Ar was used as a carrier gas for the Hg analyzer. N2 and CO2 were used for the simulation of flue gas from coal-fired power plants. 2.3. Solution Analysis. The MEA concentration was determined through titration with a standard 1.0 N hydrochloric acid (HCl) solution using methyl orange as an indicator. A standard method from the Association of Official Analytical Chemists (AOAC) was followed to analyze the CO2 loadings in MEA solutions.8 The CO2 loadings were assessed by adding an excess volume of HCl solution into the MEA solution to release the entire amount of CO2 absorbed in the MEA solution. The released CO2 was then collected in a precision gas buret. The total amount of CO2 released was used in the following equation to calculate the CO2 loading R)
VCO2 22.414VsampleCsol
(1)
where R is the CO2 loading in MEA solution (mol of CO2/mol of MEA), VCO2 is the volume of CO2 collected in the precision gas buret (cm3), Vsample is the volume of MEA solution used for titration (cm3), and Csol is the concentration of MEA solution (kmol/m3). 2.4. Experimental Procedure. Each experiment began with the generation of a gaseous Hg mixture. This was done by immersing an impinger containing liquid elemental Hg in an ice-water bath to maintain the impinger’s temperature at 0 °C in order to naturally produce Hg vapor. A stream of nitrogen (N2) with a flow rate of less than 0.02 m3/(m2 · h) was then introduced into the impinger and allowed to flow over the liquid Hg, capture the Hg vapor, and leave the impinger. This gaseous mixture was subsequently diluted with a stream of N2 to achieve the desired Hg concentration and the desired total gas flow rate. To facilitate a countercurrent gas absorption operation, the prepared Hg was introduced into the bottom of the spray column through a gas distributor, where it flowed upward and left the top of the column. The Hg concentrations at the inlet and outlet were measured over time using a Hg analyzer that was calibrated prior to each experiment. Once the inlet Hg concentration had become steady, the prepared absorption solution was pumped from the solution feed reservoir to the top of the spray column and distributed uniformly through a spray nozzle over the cross section of the column. The solution traveled downward, was collected at the bottom of the column, and was then circulated back to the top of the column for further absorption. The inlet and outlet Hg concentrations recorded throughout the experiment were taken only when the gas absorption system reached a steady state in which changes in both inlet and outlet Hg concentrations were negligible (e.g.,
