Article pubs.acs.org/est
Sequestration of Flue Gas CO2 by Direct Gas−Solid Carbonation of Air Pollution Control System Residues Sicong Tian† and Jianguo Jiang†,‡,* †
School of Environment, Tsinghua University, 100084 Beijing, PR China Key Laboratory for Solid Waste Management and Environment Safety, Ministry of Education of China, PR China
‡
S Supporting Information *
ABSTRACT: Direct gas−solid carbonation reactions of residues from an air pollution control system (APCr) were conducted using different combinations of simulated flue gas to study the impact on CO2 sequestration. X-ray diffraction analysis of APCr determined the existence of CaClOH, whose maximum theoretical CO2 sequestration potential of 58.13 g CO2/kg APCr was calculated by the reference intensity ratio method. The reaction mechanism obeyed a model of a fast kineticscontrolled process followed by a slow product layer diffusion-controlled process. Temperature is the key factor in direct gas−solid carbonation and had a notable influence on both the carbonation conversion and the CO2 sequestration rate. The optimal CO2 sequestrating temperature of 395 °C was easily obtained for APCr using a continuous heating experiment. CO2 content in the flue gas had a definite influence on the CO2 sequestration rate of the kinetics-controlled process, but almost no influence on the final carbonation conversion. Typical concentrations of SO2 in the flue gas could not only accelerate the carbonation reaction rate of the product layer diffusion-controlled process, but also could improve the final carbonation conversion. Maximum carbonation conversions of between 68.6% and 77.1% were achieved in a typical flue gas. Features of rapid CO2 sequestration rate, strong impurities resistance, and high capture conversion for direct gas−solid carbonation were proved in this study, which presents a theoretical foundation for the applied use of this encouraging technology on carbon capture and storage.
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INTRODUCTION It is generally acknowledged that anthropogenic CO2 emission is the primary driver of global warming. As one of the six greenhouse gases regulated in the Kyoto Protocol, CO2 concentrations in the atmosphere have rapidly increased from the preindustrial values of 280 ppm to the current (2010) values of 388 ppm. If levels eventually exceed 550 ppm, the natural environment on Earth is likely to suffer great damage.1 Thus, it is of great importance to study and develop effective techniques to reduce CO2 emissions. Besides the state-of-the-art amine-based systems (MEA), many emerging technologies, such as the use of membranes, metal organic frameworks, and ionic liquids, have been proposed for carbon capture and storage (CCS).2 Among these different options, mineral carbonation, which was originally proposed by Seifritz,3 holds great potential for CO2 sequestration. In this process, certain alkaline minerals that are rich in calcium or magnesium react with CO2 and sequester it by forming the corresponding thermodynamically stable carbonates. The products are thermodynamically stable, ensuring the permanent and inherently safe storage of CO2. Mineral carbonation has other advantages including the extensive and cheap sources of materials (large amount of natural minerals and industrial residues) and the exothermic nature of carbonation reactions, which both save costs and energy consumption.4 Moreover, studies have found that the © 2012 American Chemical Society
natural carbonation process can be accelerated by using different sources of CO2 and various methods.5−7 An alternative source of raw materials is provided by alkaline wastes, which are available in relatively large amounts and are generally rich in Ca or Mg. These residues mainly contain steel slag, residues from air pollution control systems (APCr), municipal solid waste incinerator (MSWI) fly ash and bottom ash, waste building materials, and tailings from the smelting processes of certain metals.8 Gunning et al. investigated the CO2 sequestration capacity of 17 industrial wastes, including cement kiln/bypass dusts, paper sludge incineration ash, and MSWI fly ash, by accelerated carbonation; treatment conditions consisted of adding various amounts of water, 75% humidity, an atmosphere of 2 bar, and 100% CO2. Results showed that most residues were carbonated to different degrees, their CO2 uptake was correlated with calcium content, and their reactivity with CO2 was influenced by the mineral composition.9 Thus, many industrial residues have the potential to sequestrate CO2. As a sort of major and increasing waste streams, APCr has been considered to have economic value as it has potential for CO2 Received: Revised: Accepted: Published: 13545
September 13, 2012 November 9, 2012 November 26, 2012 November 26, 2012 dx.doi.org/10.1021/es303713a | Environ. Sci. Technol. 2012, 46, 13545−13551
Environmental Science & Technology
Article
X-ray fluorescence was performed to analyze the composition and content of elements in the APCr using an XRF-1800 analyzer (SHIMADZU Co., Japan). For accuracy, the carbon content was initially analyzed using a CE-440 elemental analyzer (Electronic Associates, Inc.), and the contents of other elements were calibrated using this value. X-ray diffraction analysis was performed on a Smartlab X-ray diffractometer (Rigaku Co., Japan) with the following operating parameters: Cu Kα radiation (λ = 1.5418 Å), Fe Kb filter, 40 kV, 200 mA power generator. An angular range of 10−60° 2θ was measured with a step size of 0.02° and 2 s counting time per step. Each sample was prepared and then measured three times. The identification of all crystalline phases was undertaken with JADE5 software (Materials Data, Inc., Livermore, CA) and the PDF-2 2004 database (International Centre for Diffraction Data, Newton Square, PA). The reference intensity ratio (RIR) method, which is also known as the “matrix flushing” method,19 was used for mineral phase quantification of the sample, with corundum (Alfa Aesar α-Al2O3 powder ≥99.95%) acting as an internal standard. The content of any phase i in the sample, added with a known amount of corundum, is given by eq 1:
sequestration because of its high content of available calcium and fine particles.10,11 Research on accelerated carbonation has generally examined two routes: direct gas−solid carbonation and aqueous carbonation. Aqueous carbonation, which generally involves the activation of calcium or magnesium in the moist conditions followed by carbonation in a subsequent step in the aqueous environment, has been well researched in recent years,12−14 and it has been shown that the aqueous conditions may limit the increase in reaction temperature and complicate the operating procedure, restricting, to some extent, the application of this technology. Direct gas−solid carbonation can be an alternative. Baciocchi et al. performed gas−solid carbonation experiments using APCr and pure CO2 at 200−500 °C, obtaining the maximum calcium conversion of 57% and also a storage capacity of 0.12 kg of CO2/kg of dry solid.10 Prigiobbe et al. found that all carbonation kinetics were characterized by a rapid chemically controlled reaction followed by a slower product layer diffusion-controlled process. In their study, maximum conversions ranging between 60% and 80% were achieved, depending on the operating temperature and CO2 concentration.15 APCr must be stringently stabilized before their final disposal (landfill or reuse) as they are a hazardous waste. Thus, accelerated carbonation of APCr using flue gas may not only reduce CO2 emissions but also improve waste stabilization, thereby achieving the goal of “waste control by waste”. Although the total CO2 sequestration capacity of industrial residues such as APCr is limited, this option has the potential for CO2 capture from the initial source at sites where both large quantities of CO2 and suitable solid residues are produced (e.g., steel production facilities and MSWIs). However, research on the carbonation of industrial residues published so far has focused mainly on the improvement of their environmental quality.16−18 Few existing studies, especially for the direct gas− solid carbonation of APCr, lacked further discussion for CO2 sequestration in the condition of typical flue gas compositions. This paper presents the characterization of APCr. The influence of process variables during the gas−solid carbonation reaction is studied, the feature of direct gas−solid carbonation is analyzed, and the CO2 sequestration capacity of this type of residue is determined.
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xi =
Ii Icor
·
x cor 1 · k i,cor 1 − xcor
(1)
where x denotes the weight fraction, I denotes the intensity of the most intense line, and the subscripts i and cor indicate phase i and the standard phase corundum, respectively. The value ki,cor represents the reference intensity ratio of phase i to corundum, which is determined according to convention from the most intense line of phase i and corundum in a 50:50 mixture by weight. Experimental Procedure. Thermo-gravimetric analysis (TGA) was used to investigate the direct gas−solid carbonation reactions between CO2 and APCr using a TGA/DSC 1 STARe thermogravimetric analyzer (Mettler-Toledo, Inc., Columbus, OH). The fresh APCr was heated continuously from 150 to 550 °C in 1 bar of 100% CO2 atmosphere at a rate of 10 °C/ min. Carbonation experiments were then conducted separately at 300 °C, 350 °C, 400 °C, 450 °C, and 500 °C under conditions of 100% CO2 atmosphere at atmospheric pressure for one hour. Finally six different simulated reaction gases were prepared according to the typical CO2 and SO2 contents in actual incineration flue gas. These reaction gases were used to study the influence of CO2 and SO2 on the CO2 sequestration ability of APCr (Table 1). All carbonation experiments were conducted in duplicate. Analytical Methods. The sequestered CO2 using the APCr sample was determined with the method of thermal gravimetric analysis. The uptake of CO2 measured by thermal gravimetric
MATERIALS AND METHODS
Air Pollution Control System Residues. APCr used in this experiment were collected from the cloth-bag dedusting system of a MSWI power plant in Shenzhen, China. The flue gas purification treatment in this plant is a three-class disposal system consisting of a lime half-dry absorption reaction tower, active carbon insufflation absorption, and a cloth-bag dedusting system. The fresh APCr were ground into smaller particles that could pass through a 200-mesh screen (particle size
