Supported Manganese Dioxide Catalyst for Seawater Flue Gas Desulfurization Application

Transcription

1 Advanced Materials Research Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland Supported Manganese Dioxide Catalyst for Seawater Flue Gas Desulfurization Application Junjie Bian a, Xin Min b Shu Zhang c and Chunhu Li d Key Laboratory of Marine Chemistry Theory and Technology of Ministry of Education, Ocean University of China a junjiebian@ouc.edu.cnmail, b minxin2009@163.com, c onlytime219@yahoo.cn, d lichunhu@ouc.edu.cn Keywords: Manganese dioxide; Preparation; Seawater; Flue gas desulfurizatio. Abstract. Ceramic-supported manganese dioxide catalyst, MnO2-ceramic, prepared by impregnation- precipitation method and calcined at 350 degree C, was exposed to flue gases to investigate the catalytic oxidation of sulphur dioxide in flue gas at seawater buffering system. The results showed that compared with acid treated ceramic, MnO 2 -ceramic gained 2%-46% higher Desulfurization efficiency in temperature range of degree C. MnO 2 -ceramic exhibited 57% desulfurization efficiency at its favorable temperature (60 degree C) while ceramic showed 11% Desulfurization efficiency at 60 degree C. MnO 2 -ceramic was found to catalyze S(IV)oxidation even at low ph (ph<4) while the reaction rate of noncatalytic S(IV) oxidation is negligible under the identical conditions. No catalytic activity decay was observed after 40 hours run. XPS reveals that the Mn was in the oxidation state of +4 for the MnO x -ceramic. SEM-EDS and XRD indicated that the good catalytic activity of MnO 2 -ceramic at low temperature could be attributed to high dispersion of MnO 2 on ceramic support. The high performance of MnO 2 -ceramic on the SO 2 removal appeared to be promising in the seawater flue gas desulfurization industrial application when high sulfur coals were burned. Introduction Flue gas desulphurization (commonly known as FGD) is the technology applied for eliminating SO 2 from the exhaust flue gases in power plants that burn coal or oil. Alkaline sorbent slurries (e.g. limestone) or other scrubbing reagents (e. g. sodium hydroxide) are used to remove the SO 2 from the flue gases. FGD by scrubbing with seawater is an economical option for many industrial facilities at coastal sites because of its simple operating principle and high reliability. The process uses the natural alkalinity of seawater to absorb and neutralize sulfur dioxide in exhaust gases [1]. The two main advantages of seawater FGD are as follows: (1) an alkaline absorbent (e.g. Ca(OH) 2, CaCO 3 ) is not necessary since the seawater is a natural alkaline solution (ph = ) and (2) the water treatment plant needed is much simpler (mainly an aeration basin). The main backwards of this process are lower total SO 2 removal efficiency (compared with limestone-gypsum FGD process) and a larger amount of seawater required when high sulphur flue gas is treated [2,3]. Both factors are partially related to the low S(IV) oxidation rate at the low ph values of the absorption tower effluent. A novel concept was proposed to adopt a catalyst to increase SO 2 oxidation rate at low ph, and active carbon was chosen as the catalyst [4]. Little literal reports concerned on metal oxide(s) catalysts for seawater FGD because metal oxide(s) catalysts were, although more active than carbon materials for FGD, seldom stable in the acidic seawater in which a large amount of SO 2 had been adsorbed. Transition metal oxide(s) catalysts were able to be used under low ph conditions at the bottom of adsorption tower if these metal oxide(s) were stablized by being anchored on some inert support. The preferred catalyst support is a type of ceramic which includes a multiplicity of unobstructed channels sized to permit gas flow and bounded the active phases to ceramic walls. The walls of these channels provide the surface for transition metal catalysts supporting that facilitate catalytic oxidation of S(IV) into S(VI) in seawater buffering system. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-09/05/16,03:59:08)

2 Advanced Materials Research Vols In this work, we prepared a stable, high active ceramic supporting manganese dioxide catalyst and evaluated its suitability for SO 2 removal in an adsorption tower of seawater FGD processall manuscripts must be in English, also the table and figure texts, otherwise we cannot publish your paper. Experiments Catalysts Preparation. Ceramic support was obtained from Anyuan Industrial Caramics Co., Jiangxi, and washed by 15% v/v nitric acid at 40 degree C for 6 h. The preparation of MnO 2 -ceramic catalyst has been described in brief. All chemicals used in preparation were analytical reagents. In this study, MnO2-ceramic was prepared by impregnating ceramic in 0.60 mol/l MnSO 4 for 6 h, dope-adding 1.0 mol/l (NH 4 ) 2 CO 3 until the ph of the liquid phase reached 8.0, followed by being dried at 120 degree C for 10 h and calcined at 350 degree C for 4 h. The ceramic gained 5% more weight for MnO 2 supporting, and its surface color was turned into dark brown. Catalysts Characterization. Phase identification of MnO 2 -ceramic was conducted by X-ray diffraction analysis (XRD) using a D/Max-rB diffractometer with a CuKα radiation in the 2θ range from 10 to 70 at 0.02 steps. Scanning electron microscopy (SEM) was performed on a JSM-6700F with a detector INCA Energy for EDS. X-ray Electron Spectroscopy (XPS) was performed on a PHI-1600X ESCA. MgKα (Eb ev) source, energy resolution of 0.2 ev, and analysis chamber vacuum of 2.9 *10-5 Pa were used. Binding energy was calibrated by the C1s (Eb = ev). Catalysts Evaluation. The simulated flue gas was prepared by balancing of SO 2 (diluted with pure nitrogen) with air (from air compressor), and its inlet flow rate was 0.5 L/min. Seawater was obtained from the seaside of Laoshan Dist, Qingdao, and 3.0 ml/min seawater was injected for each run. The loaded catalyst was 2.5 g. In an adsorption tower, flue gas was contacted with the injected seawater in a countercurrent flow. The SO 2 concentration of inlet and outlet simulate flue gas were determined by Flue Gas Analyzer (Laoying 3022) with a measuring range % (mass fraction), The desulfurization efficiency η of absorption tower is defined as:(c i -C o )/C i *100%, where C i and C o refer to the inlet and outlet SO 2 concentration. Rusults and Discussion Changes of surface morphology of ceramic with and without MnO 2 layer can be readily observed in SEM images (Fig 1). These two samples were without conductive coating before imaging. The scale bar denotes 100nm. Compared with acid treated ceramic which has a rough surface, the image of MnO 2 -ceramic catalyst shows that the surface is covered with uniform clusters. The EDS spectrum for MnO 2 -ceramic catalyst reveals there is Mn present as well as Si and Al on the surface, as illustrated in Table 1. Fig.1 SEM micrographs of (left) acid treated ceramic and (right) MnO 2-ceramic.

3 966 Application of Chemical Engineering Table 1 Elemental analysis by EDS in area scanning Element Atomic Percent [%] Acid treated ceramic MnO 2-ceramic O Na Al Si S Cl K Mn Fe Intensity (a. u.) θ (degree) Fig 2. XRD patterns of (a) acid treated ceramic, and (b) MnO 2 -ceramic. The refractions are characteristic of ceramic. (b) (a) Of the manganese oxides which are known, the most important phases and their transition temperatures in air are given in ref.[5]. In this study, manganese oxides that were supported on the ceramic and calcined at 350 degree C for 4 h, consist of mainly MnO 2. XRD patterns (Fig 2) of acid treated ceramic and MnO 2 -ceramic indicate that no new phases appeared after MnO 2 supporting. Considering SEM-EDS results, this can be deduced that the MnO 2 was formed an ultra-thin layer on the surface and loaded in the pores of ceramic. Further attempts to analyze the supported MnO x structure were carried out by XPS. XPS is a technique in which the top atomic layers of a material are investigated and chemical information is obtained about the surface composition. Fig3 illustrated high resolution XPS spectra corresponding to Mn2p. In the Mn2p region, XPS detected a Mn 2p1/2 and Mn 2p3/2 peak at the binding energy centered at ev and ev, in a good accordance with the binding energy listed in Ref. [6]. It could be confirmed on the surface of MnO x -ceramic, Mn is in an oxidation state of 4+. Intensity 2p3/2 2p1/ Binding Energy(eV) Fig 3 High resolution XPS spectra of MnO 2 -ceramic Desulfurization efficiency (%) Temperature (degree C) Fig 4. Effect of temperature on SO 2 removal efficiencies, with respect to (a) acid treated ceramic, and (b) MnO 2 -ceramic (b) (a) The effects of temperature on the SO 2 removal efficiencies were shown in Fig 4. For the desulphurization efficiency of seawater is decreasing with temperature increasing [7], we studies the catalytic oxidation activity of MnO 2 -ceramic in the temperature range degree C. In this study, it was found that the acid treated ceramic showed no catalytic activity for S(IV) oxidation, and acted as the packing to prolong the contact time of SO 2 with seawater. The catalyst consists of MnO 2 bound to ceramic by impregnation- precipitation method. The ceramic supports ultra-thin MnO 2 layer with the proven ability to remove SO 2 through a mechanism of no sorption but catalytic oxidation SEM

4 Advanced Materials Research Vols micrographs of (left) acid treated ceramic and (right) MnO 2 -ceramic. [8]. The MnO 2 -ceramic demonstrated to catalyze S(IV) oxidation (SO 2 oxidation and/or SO 3 2- oxidation) down to 15 degree C and significantly increased Desulphurization efficiency in the temperature range we concerned. The maximum desulphurization efficiency of MnO 2 -ceramic could reach 57% at 60 degree C whereas the desulphurization efficiency of acid treated ceramic was only 11% at the same temperature. The oxygen in flue gases was used as oxidant and S(IV) was converted at the temperature lower than 80 degree C on the MnO 2 -ceramic catalyst mainly via solid phase catalysis [9]. It is known at the ph values below 4.0 the reaction rate of non-catalytic S(IV) oxidation is negligible in seawater. The ph value of outlet seawater was down to 3.6 when the MnO 2 -ceramic catalyst was loaded in the adsorption tower. It can be deduced that the rate of S(IV) oxidation was accelerated by the MnO 2 -ceramic catalyst. The combination of an active specie (MnO 2 ) and a hydrophilic support (ceramic) with a suitable open structure leads to a high SO 2 removal efficiency for flue gas. It was noted that the desulphurization efficiency of the catalyst decreased as the temperature increased above 60 degree C. It is possible that evaporation of seawater at higher temperature (>60 degree C) might negatively impact the S(IV) catalytic oxidation. Dark brown MnO 2 layer was stable to acid leaching at (ph =3.5, 60degree C for 3h), and no detectable mass loss of catalyst was found after 40hs run. Conclusion In our laboratory, a high desulphurization efficiency for flue gas can be obtained at about 60degree C by using the easily made inexpensive MnO 2 -ceramic catalyst in seawater adsorption tower. The combination of hydrophilic property of ceramic, and a high dispersion of MnO 2 on the ceramic support, allows that MnO 2 -ceramic present higher activity for SO 2 oxidation in seawater. Which is the dominant reaction on the MnO 2 -ceramic, SO 2 oxidation or SO 3 2- oxidation, needs to be further studied. Acknowledgements We would like to express our gratitude for support from the National Natural Science Foundation of China ( ) and National Hi-tech Research and Development Program (863 program) of China (2007AA061800). References [1] M. Tokumura, M. Baba, H. T. Znad, K. Kawase, C. Yongsiri and Takeda K: Ind Eng Chem Res Vol. 45 (2006), p [2] K. Oikawa, C. Yongsiri, K. Takeda and T. Harimoto: Environ. Prog Vol. 22 (2003), p. 67 [3] F. V. Barrero, P. Ollero, and A. L. V. Perales A: Environ Sc. Technol Vol.24 (2009), p [4] F. V. Barrero, P. Ollero, F. G. Ortiz and R. Arjona: Environ. Sci. Technol Vol.13 (2005), p [5] E. R. Stobbe, A. A. Boer, and J. W. Geus: Catal Today Vol. 47 (1999), p.161 [6] X. Tan, J. Li, L. Sun, and J. Hao: Appl. Catal. B Vol. 99 (2010), p. 156 [7] J. R. Sevilla ; M. Alvarez ; M. C. Diaz, and M. C. Marrero: J chem Eng Data Vol. 49 (2004), p [8] C. Brandt, and R. Eldik: Chemical Reviews Vol. 95 (1995), p.119. [9] J. Liu, W. Jiang, X. Tan, Z. Ye, and Y. Yang: Chin J Environ Eng Vol. 1 (2007), p. 72

5 Application of Chemical Engineering / Supported Manganese Dioxide Catalyst for Seawater Flue Gas Desulfurization Application /