Article pubs.acs.org/EF
Impact of Surface Functional Groups, Water Vapor, and Flue Gas Components on Mercury Adsorption and Oxidation by Sulfur-Impregnated Activated Carbons Hsing-Cheng Hsi,*,† Cheng-Yen Tsai,‡ and Kuei-Ju Lin§ †
Graduate Institute of Environmental Engineering, National Taiwan University, 71 Chou-Shan Road, Taipei 106, Taiwan Graduate Institute of Engineering Science and Technology, and §Department of Safety, Health, and Environmental Engineering, National Kaohsiung First University of Science and Technology, 2 Jhuoyue Road, Nanzih, Kaohsiung 811, Taiwan
‡
ABSTRACT: Coconut-shell activated carbon and activated carbon fiber (ACF) were impregnated with sulfur at 400 and 650 °C, and then the resulting physical properties of these adsorbents, the functional groups formed, and the adsorption and oxidation of Hg0 occurring on the adsorbents were evaluated. The change in the properties of the adsorbents after sulfur impregnation was strongly affected by the inherent physical and chemical properties of the starting samples and treatment temperature. The sulfur content of the samples increased when the sulfur impregnation temperature was decreased. At 650 °C, sulfur acted as both a dopant and a chemical activator and thereby broadened the porous structures that cause a reduction in the micropore surface and increase the total surface area of ACF. Furthermore, Hg0 adsorption and oxidation were enhanced after sulfur impregnation, strongly affected by flue gas components and various surface active sites, and greatly increased by the sulfur introduced at 400 °C. The original oxygenated groups present on the carbon surface effectively interacted with Hg0 and flue gas components to form oxidized Hg, but these groups did not appear to act as strongly adsorptive sites and capture the oxidized Hg. The sulfur groups introduced at 650 °C were stronger adsorptive sites than the oxygenated groups, but the sulfur sites were not effective oxidative sites, especially in the presence of H2O.
1. INTRODUCTION Over the past few decades, limiting mercury (Hg) emissions has been a primary focus of environmental efforts because Hg is toxic, bioaccumulative, and challenging to control. The United States’ Clean Air Act Amendments of 1990 list Hg as one of the original 188 hazardous air pollutants (HAPs). The Minamata Convention on Mercury, a major environmental treaty that lists globally and legally binding agreements between countries that were developed to prevent Hg emission and release, was delivered in March 2013, and by October 2013, 92 governments had signed the agreements.1 With respect to controlling the air emissions of Hg, the new treaty focuses on emissions from large industrial facilities, such as coal-fired power plants, industrial boilers, waste incinerators, and cement clinker facilities. The countries involved have agreed to install devices featuring the most effective control technologies available at all existing and new power plants. Several countries have also drawn up regulations to reduce Hg emissions from existing facilities.2 Typically, Hg in the gaseous phase exists in three major forms: elemental (Hg0), oxidized (Hg2+), and particle-bound (Hgp) forms.3 Hg can be spread globally through its release into the environment from natural and anthropogenic sources. Coal-fired power plants are known to be the largest single anthropogenic source of Hg in most countries.4−6 Traditional devices used for controlling air pollution, such as wet flue gas desulfurization devices and electrostatic precipitators, can readily remove Hg2+ and Hgp from gas streams. However, Hg0 is highly volatile and insoluble in water and, therefore, barely removed from the flue gases of coal-fired power plants. Furthermore, lowconcentration Hg (1−10 ppbv) present in coal combustion flue © 2014 American Chemical Society
gas streams is extremely challenging to control because of masstransfer limitations. Thus, substantial attention has been devoted to designing methods to capture the Hg0 released from coalfired power plants. Several techniques have been developed to capture lowconcentration Hg from coal combustion flue gases. Adsorption using porous materials is a favorable approach employed to remove low-concentration Hg species from coal combustion flue gas streams.7 Activated carbon has been extensively investigated for its use in capturing low-concentration Hg species emitted by coal-fired power plants, and adsorbent injection and carbon fixed beds have been verified to be effective in removing both Hg2+ and Hg0. Furthermore, sulfur impregnation has been widely reported to considerably enhance the capacity of activated carbons to adsorb Hg0.8−17 Adsorption onto sulfurimpregnated activated carbon at low temperatures has also been demonstrated to be the most mature technology available for directly capturing the gaseous Hg released during the coal gasification processes.18 Our previous studies indicated that carbonaceous adsorbents featuring large Hg adsorption capacities (i.e., >5000 μg of Hg/g of adsorbent at an inlet Hg0 concentration of 50 μg Nm−3) can be developed by impregnating the adsorbents with sulfur at 200−650 °C.12,13 These studies demonstrated that an impregnation temperature of 400 °C was optimal because, at that temperature, both the porous structure and the active functional sites of the carbonaceous adsorbents can be retained. Received: January 9, 2014 Revised: April 12, 2014 Published: April 16, 2014 3300
dx.doi.org/10.1021/ef500075d | Energy Fuels 2014, 28, 3300−3309
Energy & Fuels
Article
of the N2 adsorption results obtained at 77 K (Quantachrome NOVA 2000e). SBET was calculated using the Brunauer−Emmett−Teller (BET) equation based on the ASTM D4820-96a method. Smicro and Vmicro were calculated from t-plot analyses using the Jura−Harkins equation: t = [13.99/(0.0340 − log(p/p0))]0.5.30 The range of relative pressures used for determining Smicro and Vmicro was based on the values of thickness t being between 0.45 and 0.8 nm. Micropore size distribution was determined according to the quenched solid density functional theory. The Barrett−Joyner−Halenda (BJH) method was used for determining the mesopore size distribution.31 The chemical composition of samples, including the mass concentrations of C, H, N, S, and O of the raw and sulfur-impregnated adsorbents, was determined using an elemental analyzer (Elementar Vario, model EL III) according to the NIEA R409.21C method. The surface functional groups of the samples were examined using X-ray photoelectron spectroscopy (XPS; Physical Electronics, model PHI 1600). Surface functional groups including sulfur and carbon groups were deconvoluted within 163−172 and 284−295 eV, respectively, on the basis of their characteristic binding energies. 2.3. Low-Concentration Hg0 Adsorption and Oxidation Tests. Hg0 adsorption tests were conducted using a simulated coal combustion flue gas, in which the Hg0 concentration was 10 μg Nm−3. The apparatus consisted of three major components: a portion for generating the simulated coal combustion flue gas, a fixed bed for adsorbing vapor-phase Hg0, and a data acquisition system.32 Hg0 was generated using a certificated Hg0 permeation tube (VICI Metronics) at 70 ± 0.1 °C to ensure a constant Hg0 diffusion rate. The simulated coal combustion flue gas contained 14 vol % CO2, 10 vol % H2O, 6 vol % O2, 50 ppmv HCl, 200 ppmv SO2, and 200 ppmv NO; the balance was N2. The composition of the flue gas was selected to reflect the typical conditions in coal-fired power plants of Taiwan, in which low-sulfur bituminous and sub-bituminous coal blends are typical burned. The generated gas stream passed through a temperaturecontrolled fixed-bed column (0.5 in. inner diameter) containing 10 mg of samples mixed with 5 g of quartz sand. The temperature of the fixed-bed column was controlled at 150 °C, and the flow rate of the gas through the column was approximately 1.5 L min−1. The column length of the sample−sand mixture was around 5 cm, and the gas stream passed through the mixture in approximately 0.3 s. The effluent gas from the fixed-bed column flowed through heated lines to an impinger containing SnCl2(aq), which reduced any oxidized Hg compounds to Hg0. The gas then flowed through an impinger containing Na2CO3(aq), which removed acidic components, and through a moisture trap (i.e., a nefion tube) that removed H2O; this protected the downstream detector system. The gas coming out of the impinger solution flowed through a gold amalgamation system (Brooks Rand model AC-01), in which the Hg0 present in the gas was adsorbed. The Hg0 concentrated on the gold was then thermally desorbed (>400 °C) and transferred in the form of a concentrated Hg0 stream to a cold-vapor atomic fluorescence spectrophotometer (CVAFS; Brooks Rand Lab model III) for analysis. The Hg0 adsorption capacities (grams of Hg0 per gram of adsorbent) at any given time were determined by summing the mass of the Hg0 removed from the gas stream, which was calculated on the basis of the measured breakthrough curves, and then dividing this sum by the mass of the adsorbent in the adsorption bed
In contrast, we determined that increasing the sulfur amount did not guarantee an enhanced uptake of Hg. Another report also indicated that a high sulfur content did not ensure increased Hg uptake, because the excess sulfur blocked or filled the pores of the adsorbent that are required for adsorbing Hg.16 On the basis of determining that both the sulfur content and the porous structure of the adsorbent were critical for Hg adsorption, Feng et al.16 estimated the probability of optimal Hg adsorption by adsorbents impregnated with a sulfur monolayer. Furthermore, in addition to the structure and properties of activated carbon, flue gas constituents have been shown to enhance oxidation and competitive adsorption to potently affect the capacity of carbonaceous adsorbents to capture Hg0.19−25 These effects may be produced by both the alternation of the surface of the adsorbent and the speciation of Hg, resulting from interactions with the constituents of the flue gas, such as O2, HCl, SOx, NOx, and H2O. In this study, we investigated how effectively a coconut-shell activated carbon (designated ACS) and an activated carbon fiber (ACF), which were impregnated with sulfur at 400 and 650 °C, respectively, removed low-concentration [parts per billion (ppb) level] Hg0 from a simulated coal combustion flue gas. Although several studies have evaluated the Hg0 removal efficiency of these materials, sulfur-impregnated activated carbons, especially ACF, continue to attract considerable attention because they absorb Hg0 in both the gaseous and aqueous phases more effectively than other surface-modified adsorbents.26−29 Next, we investigated the effects of sulfur impregnation on the physicochemical characteristics of the samples and the consequent Hg0 adsorption and oxidation. Our goal was to help elucidate the roles played by surface functional groups, which include inherent oxygenated groups and impregnated sulfur groups, in Hg adsorption and oxidation; these roles have remained unclear and are thus widely studied. We also focused attention on the competitive role of flue gas components, especially H2O, in the adsorption and oxidation of Hg0. We propose potential interactions that occur between Hg0 and oxidized Hg species, flue gas components, and the surface functional groups of raw and sulfur-treated activated carbons, which have not been comprehensively discussed before.
2. MATERIALS AND METHODS 2.1. Preparation of Sulfur-Impregnated Adsorbents. Two carbonaceous adsorbents, ACS and ACF, were impregnated with sulfur in an effort to control the emission of low-concentration Hg0 from a simulated coal combustion gas. We obtained the raw ACS and ACF materials, which featured total surface areas of approximately 1300 and 1400 m2 g−1, respectively, from commercial sources. The samples were initially washed with deionized (DI) water to remove impurities and then oven-dried at 105 °C for 24 h; after cooling, the samples were ground and passed through a 200-mesh sieve. To impregnate the adsorbents with sulfur, the ACS and ACF samples were mixed with elemental sulfur (1:1 sulfur/adsorbent mass ratio) in a ceramic boat and then heated in a tubular furnace at 400 and 650 °C for 1 h, respectively. Ultrahigh-purity N2 flowed continuously (
