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Experimental and Mechanism Study of Elemental Mercury Removal from Coal Combustion Flue Gases by Solid Sorbents

Author: ZhaoPengFei
Tutor: ZhengChuGuang；GuoXin
School: Huazhong University of Science and Technology
Course: Thermal Power Engineering
Keywords: CaO Adsorbent Mercury Removal Activated carbon Density functional theory
CLC: X773
Type: PhD thesis
Year: 2010
Downloads: 278
Quote: 0
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Abstract


The coal-fired power plant is the main source of anthropogenic mercury emissions pollution has caused great harm to the ecological environment and human health. Solid adsorbent mercury removal technology has become a hot research adsorbent addition to the exploration and understanding of the reaction mechanism of mercury is not enough depth, this article is more systematic study of calcium based sorbents, seepage halogen infiltration of of sulfur activated charcoal and precious metals adsorbent in a single mass the reaction mechanism of the oxidation and adsorption of mercury (Hg0), carry out experimental studies of its removal from flue gas mercury, mercury pollution control technology in the development of the coal combustion process. CaO solid surface of the cluster model can accurately describe the quantum chemistry methods used in coal-fired flue gas in gas-solid adsorption mechanism, structure; compare the cluster model of adsorption by theoretical calculations, found the shell model embedded cluster (SM) The model is both effective to eliminate the effect of the boundary of the cluster model, and the calculation of the amount to be effectively controlled. Explore different forms of mercury (Hg0, HgCl2 HgCl, HgO) in the the CaO surface adsorption mechanism, the study found Hg0 when CaO adsorbed, the binding energy is -21.47kJ/mol smaller binding energy adsorption distance larger number of electrons transferred mainly physical adsorption and a smaller number of key layout; by HgCl2, HgCl and HgO, the CaO surface adsorption, are the most stable parallel adsorption bond energy and bond length data indicated that the adsorption to chemical adsorption. Were calculated in the range of 150 ℃ -1200 ℃ CaO adsorption the HgCl2 AH (reaction enthalpy), AG (Gibbs free energy), and discuss the variation with temperature. The study found that the adsorption reaction is exothermic, desorbed when the temperature is higher than 280 ° C showed a major trend, the temperature rise on the adsorption reaction has certain inhibition trends. Smoke constituents further consideration of calcium-based sorbent removal mechanism of the impact of mercury found by theoretical calculations, flue gas and acid gas (SO2, HCl, and NO2) can be stable adsorption in alkaline bit of CaO, these acidic The gas will be, and HgCl2 competitive the CaO surface active bit. SO2 reacts with CaO, will generate a pre-oxidized surface. Hg0 in the pre-oxidized surface stability generated Hg2O, while not stable formation HgO, the oxidation state of mercury generated and then adsorbed to CaO alkaline bit, the SO2 able to promote Hg0 oxidation; while coal-fired flue gas SO2 concentration is much greater than Hg0 concentration, excess of SO2 and Hg0 competitive adsorption on the surface of the pre-oxidation, and ultimately to inhibit the oxidation of Hg0 that SO2 promoting effect is limited. HCl, NO2, and CaO surface binding energy of his strongly adsorbed on the CaO surface adsorption distance and electronic changes, limiting the ability of the catalytic oxidation of Hg0. The theoretical results consistent with the experimental results, that the theory of quantum chemical calculations are an effective method of screening mercury and other trace elements in an effective adsorbent and the study of reaction mechanisms. Based on the theoretical calculation results, select and synthetic oxygen-rich type calcium base the the composite rice husk ash of adsorbent (KMnO4/CaO/RHA and I2/CaO/RHA) removal of Hg0 experimental research and to explore its mechanism. Study found that highly active calcium based adsorbents load of KMnO4 Hg0 removal efficiency is higher than 98%, the pore distribution in the modified matrix to affect the removal of calcium-based sorbent a Hg0 important factors to enhance the specific surface of the modified substrate modified adsorbent can reach a higher adsorption rate and increasing imports of mercury concentration and a suitable adsorption temperature and oxidant content; load of KMnO4, the high activity of calcium-based sorbent is different, the content of the modified matrix I2 impact calcium based adsorbents of the load I2 Removal a Hg0 important factors, the modified substrate surface area, pore distribution, and the manner of loading is less impact on the removal effect, to improve the adsorption temperature and oxidant content enables modified adsorbents achieve more high removal rate. According to the different experimental results, proposed a different reaction mechanism can better explain the phenomenon of adsorption experiments. Characterization of activated carbon surface saturated cluster model, based on density functional theory in the activated carbon and halogens, sulfur-modified carbon surface catalytic reaction mechanism of Hg0 studied and discussed in detail the intermediate state into key characteristics of the product as well as the adsorption path may exist . The calculations show that: when the Hg0 adsorption unmodified carbon surface, mainly to weak physical adsorption, the charge density and the key layout results further demonstrate the adsorption process with only a few electron transfer and the formation of weak chemical bonds. When the HCl and Cl2 activated carbon adsorption, cause fracture of the H-Cl and Cl-C, the formation of active Cl adsorption sites. The the Hg0 adsorption in activated Cl bit, the reaction path energy diagram shows that the activated Cl will Hg0 spontaneous reaction the HgCl, adsorbed on the activated carbon surface and the mercury end; charge density and the key layout results prove HgCl stable existence. Other halogen-modified activated carbon surface will catalytic Hg0, and formation of the Hg-Br and the Hg-I; catalytic strength of the order of: the I-AC GT; Br-AC GT; Cl-to-AC. This experiment observed the same order of halogen-modified activated carbon capture Hg0 effect. Sulphurized activated carbon, when the sulfur embedded into activated carbon edge bit is captured the Hg0 effective active sites; sulfur embedded into the center position configuration and the form of organic sulfur is not adsorbed to Hg0 effective activity bit. Sulfur compared with long-chain and short-chain sulfur activity is stronger, more conducive to capture Hg0. This is because the short sulfur chains in the activated carbon surface is more easily broken, making it easier to form a single sulfur configuration, this configuration is true capture Hg0 active sites. When the adsorption of the mercury atoms in the active site, the major reaction pathway, the mercury atoms close to the sulfur atom is directly formed a catalytic reaction in HgS, to stabilize the adsorbed on the activated carbon surface. 3 ways (direct calcination, boiled and pickling) made husk based adsorbent carrier, the use of the adsorption phase techniques the nano silver loading on the carrier, off Hg0 experiment carried out in a fixed bed bench. Adsorbent was characterized by N2 adsorption / desorption, X-ray fluorescent probe (XRF), transmission electron microscopy (TEM), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) analyzer. Analysis found pickling rice husk ash amorphous Si02 having a high purity, high activity, high surface area, high-OH, characterized in, is a silver loaded best carrier; adsorption phase can be prepared by the smaller silver single mass particles (approximately 8nnm is ), and the silver particles are uniformly dispersed in the carrier surface. Mercury adsorption experiments show that the adsorbent load nano silver adsorption efficiency of 90% at 150 ℃ and simulated flue gas conditions, the adsorption mechanism is nano silver particles amalgam mercury vapor reaction. The adsorbent can be regenerated at 350 ℃, with the prospects of practical application.

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