Energy & Fuels 2007, 21, 3259–3263
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Adsorption of NaCl Vapor at Elevated Temperature on Mineral Adsorbents Yili Li,* Jian Li, Shuiyuan Cheng, Wenjun Liang, Yuquan Jin, Youqing Wu,† and Jinsheng Gao† College of EnVironmental and Energy Engineering, Beijing UniVersity of Technology, Beijing 100022, China ReceiVed March 27, 2007. ReVised Manuscript ReceiVed August 23, 2007
Some mineral adsorbents were selected to remove NaCl vapor at elevated temperature. Results indicate that the activated alumina is the best adsorbent with the highest sodium compound content, under conditions of 1123 K, a gas velocity of 16 L/min, an adsorption time of 5 h, and a NaCl concentration of 0.13 mg/L. Results show that sodium compound content is dependent on the microporous structure and the chemical constitutes of the adsorbent, but the extent of the effects varies with each adsorbent. Results of XRD and SEM indicate that physical adsorption exists in the NaCl vapor-capture mechanism of the activated alumina. Its kinetics results indicate that both chemical adsorption and physical adsorption exist, and the adsorption process of the activated alumina is controlled by both the chemical adsorption control and the product-layer-diffusion control of physical adsorption.
1. Introduction Alkali metal (sodium and potassium) exists in coal in various chemical and physical forms. Of the many alkali compounds released into the gas phase, the chloride form (NaCl and KCl) has been identified as the major component that is present after combustion or gasification of coal. The release of alkali species into the gas phase is a natural consequence of coal combustion or gasification. The released alkali-metal vapor is a precursor of hot condensate that causes corrosion of various parts of the combustor, gasifier, and the downstream system for secondary energy recovery. To protect the gas turbine from erosion and hot corrosion, alkalis such as chlorides and sulfates of sodium and potassium in the PFBC flue gas must be reduced to acceptable levels. The current industrial gas-turbine specification limit for total alkalis in the combustion gas entering a gas turbine is equivalent to 24 ppb at an air/fuel (petroleum distillate) ratio of 50.1 However, alkali-metal vapor concentration in hightemperature coal gas is many times greater than the allowable alkali limit of 24 ppb. Therefore, a method to control alkalimetal vapor is required. One of the promising techniques involves the use of nonvolatile inorganic adsorbents to remove alkali-metal vapor from the hot coal gases.2,3 This approach can be implemented through the use of a granular bed filter that is composed of * To whom correspondence should be addressed: Tel +86-10-67392080; Fax +86-10-67391983; e-mail
[email protected]. † Present address: Department of Chemical Engineering for Energy Resources, East China University of Science and Technology, Shanghai 200237, China. (1) Lee, S. H. D.; Teats, F. G.; Swift, W. M. Alkali-vapor emission from PFBC of Illinois coals. Combust. Sci. Technol. 1992, 86, 327–336. (2) Tran, K.-Q.; Steenari, B.-M.; Lindqvist, O.; Hagstrom, M.; Pettersson, J. B. C.; Kristiina Lisa, M. Capture of alkali metals by kaolin. Proc. Int. Conf. Fluidized Bed Combust. 2003, 403–409. (3) Tran, K.-Q.; Iisa, K.; Hagstrom, M.; Steenari, B.-M.; Lindqvist, O.; Pettersson, J. B. C. On the application of surface ionization detector for the study of alkali capture by kaolin in a fixed bed reactor. Fuel 2004, 83, 807–812.
Table 1. Main Ingredients and Surface Areas of Adsorbents adsorbent acidic argil alumina kaolinite extra grade kaolinite meerschaum diatomaceous earth activated alumina
Al2O3 SiO2 MgO surface area content (%) content (%) content (%) (m2/g) 15 81.5 38 45 0.05 5 99.9
64 10 45 54 54.37 95
30.22
1.76 1.00 17.30 9.26 4.64 1.00 197.45
adsorbent or by injection of dry adsorbent powder into the hot gas stream before the final filter. Various studies have considered the feasibility of passing the flue gases through a fixed-bed filter of appropriate adsorbents. This requires an adsorbent with high efficiency and also requires an understanding of the fundamental kinetics of adsorption. In this study, NaCl vapor was selected as the representative of alkali-metal vapor. The purpose of this study is to select a highly efficient adsorbent from some minerals to be used in high temperature for the removal of NaCl vapor and to elucidate related mechanisms. 2. Experimental Section 2.1. Preparation of NaCl Vapor Adsorbents. Some mineral adsorbents (acidic argil, alumina, kaolinite, extra-grade kaolinite, meerschaum, diatomaceous earth, and activated alumina) were selected for the removal of NaCl vapor. The main components of these minerals are SiO2 and Al2O3. A small quantity of clay and carboxylic cellulose were added as a texturizing agent to increase stickiness and porosity. Adsorbents were molded in strip and then air-dried, desiccated at the temperature of 373 K for 4 h, and calcined at 1123 K in air to a stable weight. All adsorbents were crunched and sieved to ∼1.00–1.25 mm adsorbent granularity used in the experiments. The main ingredients and surface areas of the adsorbents are listed in Table 1. 2.2. Experimental Apparatus and Conditions. The laboratory reactor equipment is schematically shown in Figure 1, which mainly consists of NaCl vapor generating equipment, a fixed-bed reactor, isothermal water bubbling, and a gas absorption sector.
10.1021/ef070157d CCC: $37.00 2007 American Chemical Society Published on Web 09/21/2007
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Figure 1. Schematic diagram of the experimental equipment: 1, adsorption section; 2, NaCl vaporization section; 3, gas absorption sector; 4, temperature control; 5, crucible; 6, rotameter; 7, desiccator; 8, N2 cylinder; 9, platinum basket. Table 2. Sodium Compound Content of Different Adsorbents adsorbent
sodium compound content (%)
acidic argil alumina kaolinite extra grade kaolinite meerschaum diatomaceous earth activated alumina
1.5 1.37 4.65 3.13 2.44 2.55 6.66
Adsorption tests were performed on a self-made vertical hightemperature reactor with two temperature controllers. The reactor consists of a warm-up section, an alkali-metal vapor section, an adsorption section, and a condensation section, based on different temperature areas. The most important characteristic of the reactor is its ability to accurately control the temperature of the alkalimetal vapor section and the temperature of the adsorption sector. The source of alkali-metal vapor was the vaporization of table salt at 1123 K, and N2 was used as a NaCl vapor carrier. The concentration of NaCl vapor was determined to be stable when experimental conditions such as crucible type, gas flow, table salt quantity, and temperature were unchanged. Simulating the sample system of thermal balance, the adsorbent comes into contact with the flowing gas via a nacelle that is composed of platinum, and ∼2.5 g of adsorbent was used for each test. The concentration of NaCl vapor was determined to be 0.13 mg/L. Gas absorption sector was used to absorb surplus NaCl vapor. Under the conditions of a temperature 1123 K, a gas velocity of 16 L/min, and a reaction time of 5 h, selective experiments with different adsorbents were performed to remove NaCl vapor, and the adsorbent with the greatest efficiency would be determined. The adsorbent mechanism and kinetics were then investigated. 2.3. Evaluation Guidelines. The adsorbent weight change after each test was determined by a balance with a tolerance of 0.01 mg. The sodium compound content was used to evaluate the efficiency of the adsorbents in removing NaCl vapor: sodium compound content (%) ) sorbent weight change after adsorption (g) × 100% sorbent weight before adsorption (g) The parallel experimental error is 10, it is safe to assume that the adsorption is under product-layer-diffusion control of physical adsorption. (External mass-transfer control might also play a secondary role unless adequate experimental steps are taken to minimize this contribution.) Intermediate values of σ2 (1 < σ2 < 10) suggest that the adsorption is controlled by both chemical adsorption and product-layerdiffusion of physical adsorption. Adsorption kinetics5,6 of the activated alumina was studied under conditions of 1123 K and 16 L/min gas velocity. The value of X was obtained by the ratio of sodium compound content and the saturation sodium compound content of the adsorbent (28.2%). 100% conversion is assumed when adsorbent gets saturated. The g(X) and p(X) rate expressions were calculated in eqs 1 and 3, respectively. Equation 7 is then minimized with respect to either τg or τp with the other set to zero, and together with eq 5, again minimizing eq 7 with respect to both τg and τp simultaneously, to determine the expression for X–t data presented in Figures 4–6; the resulting parameters are reported in Table 3. The minimum value of eq 7 denotes the sum of the squared errors (SSE) and measures the quality of fit of the model to the experiment data. Results of Figures 4–6 show that the experimental data are better modeled by neither of the chemical adsorption control models and the product-layer-diffusion control model alone (Figures 4 and 5). However, when the combination of the (6) Binlin, D.; Jinsheng, G.; Xingzhong, S. A Study on the Reaction Kinetics of HCl Removal from High-temperature Coal Gas. Fuel Process. Technol. 2001, 72, 23–33.
Six mineral adsorbents were used to remove NaCl vapor at elevated temperature in this work. Adsorption tests were performed on a self-made vertical elevated temperature reactor with two temperature controllers. The following conclusions can be drawn from the tests and analysis described in this work: (1) The sodium compound content of the activated alumina is the highest under conditions of 1123 K, a gas velocity of 16 L/min, an adsorption time of 5 h, and a NaCl concentration of 0.13 mg/L. The sodium compound content is 6.66%. The sodium compound content is dependent on the porous structure and the chemical constitutes of the adsorbent. (2) Results of XRD and SEM indicate that physical adsorption exists in the NaCl vapor-capture mechanism of the activated alumina, and kinetics results prove that both chemical adsorption and physical adsorption exist. The adsorption process of the activated alumina is controlled by both the chemical adsorption control and the product-layer-diffusion control of physical adsorption. Acknowledgment. This work is funded by the National Basic Research Program of China (973, 2005CB724201), and it is also funded by State Key Fundamental R&D Project of China (G199022104).
Nomenclature X g(X) t τg kg b ks Cs M
conversion (%) conversion function under chemical control (unitless) time (h) characteristic time for chemical adsorption control (h) chemical adsorption rate constant at surface (s-1) stoichiometric ratio of adsorbent to gaseous reactant (unitless) chemical adsorption coefficient (unitless) concentration of reactant NaCl vapor species at the surface (ppm) molecular weight of reactant (g/mol)
Adsorption of NaCl Vapor on Mineral Adsorbents F R p(X) τp kp
reactant density (kg/m3) radius of particle (m) conversion function under product-layer-diffusion control (unitless) characteristic time for diffusion control (h) product-layer-diffusion rate constant at surface (s-1)
Energy & Fuels, Vol. 21, No. 6, 2007 3263 De σ2 SSE EF070157D
effective diffusion coefficient in a porous structure (m2/s) ratio of diffusion to chemical adsorption resistance (unitless) sum of the squares of the errors
