Equilibrium analysis on sulfur material in oxyfuel combustion

Transcription

1 Advanced Materials Research Submitted: ISSN: , Vols , pp Accepted: doi: / Online: Trans Tech Publications, Switzerland Equilibrium analysis on sulfur material in oxyfuel combustion XIAO Haiping a, Han Gaoyan b, Dai Yukun c, Dong Lin d North China Electric Power University, Beijing , China a xiaohaiping@ncepu.edu.cn, b hangaoyan123@126.com, c changshoubuyu@163.com, d @qq.com Keywords: oxyfuel combustion; sulfur; SO 2 ; SO 3 ; thermodynamic equilibrium Abstract: To study migration and transformation of sulfur species in oxyfuel combustion, the study attempts to analyze distribution of sulfur compounds with thermodynamic equilibrium. Results show that sulfur-containing gases predominantly are SO 2 and SO 3, the maximum thermodynamic equilibrium concentration of those in oxyfuel combustion respectively increase by 3.4 and 4.5 times compared with the conventional combustion. Furthermore, SO 2 gas formation rate decreases while SO 3 increases under oxyfuel combustion. Sulfur-containing gases are generally more sensitive to temperature and excess air coefficient. The amount of sulfur compounds significantly increases in oxyfuel combustion. Introduction fuel combustion is one of the promising technologies for capturing CO 2 from power plant. Concentrations of SO 2 and H 2 O increase rapidly. Sulfur species change and SO 3 concentration is improved significantly because of flue circulation [1]. Hao Liu et al [2] presents that SO 2 concentration is 6 times for oxyfuel combustion as large as that for traditional combustion and content of H 2 O in flue gas varies from 10% to 40%. Daniel Fleig et al [3] indicates that SO 3 concentration in oxyfuel combustion is 4 times as large as that in traditional combustion. Existing forms of sulfur is not only related to the oxygen content in flue gas, but also to temperature and mineral composition of coals. Measurement for sulfur existing forms is comparatively complicated because of the interference between SO 2 and SO 3 [4], whereas thermodynamic equilibrium is proposed as a guidance for analysis. Through thermodynamic equilibrium calculation, the distribution of sulfur species for traditional combustion and oxyfuel combustion are predicted. Rules of sulfur s migration and transformation are revealed. Computation method Thermodynamic equilibrium calculation is based on the minimum free energy of Gibbs principle. Amount of sulfur element is set to 1mol in coal sample. Mole fractions of other components will be converted accordingly. The data for components of bituminous coal and the components of bituminous coal ash are shown in Table.1 and Table.2 respectively. Table.1 Components of bituminous coal[%] C ar H ar O ar N ar S ar M ar A ar Table.2 Components of bituminous coal ash[%] SiO2 Al 2 O 3 Fe 2 O 3 CaO MgO K 2 O Na 2 O others 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-06/03/16,20:03:21)

2 68 Energy Research and Power Engineering 2014 As for oxyfuel combustion, oxidizing agent consists of circulated flue gas and pure oxygen. In consideration of simplification, flue gas is regarded as theoretical flue gas. In terms of converted mole fractions of all components, the components of theoretical flue gas are deduced by iterative computation in oxyfuel combustion. O2 concentration in oxidizing agent required by the combustion of bituminous coal is set to 29%. Accordingly the compositions of oxidizing agent are presented. Converted mole fractions of all components for bituminous coal and the amount of oxidizing agent for different combustion methods were entered into the software for thermal equilibrium calculation. Components of theoretical flue with wet flue circulation under oxyfuel combustion are shown in Table.3. Table.3 Components of circulated flue gas[vol%] O 2 N 2 CO 2 H 2 O SO Computational result analysis Temperature. The objective of present work is to research the impact of temperature on existing forms of sulfur in power station boiler. All initial conditions are as follows: excess air coefficient is 1.2, air pressure is 0.999atm and the range of temperature is from 200 C to 1800 C. The results of calculation are presented in Figure.1 and Figure.2. For oxyfuel combustion in pulverized coal fired boiler, the sulfur-containing gases are predominantly SO 2 and SO 3. The distribution of SO 2 is shown in Figure.1(a). From 487 C, thermodynamic equilibrium concentration of SO 2 increases rapidly with the raise of temperature. For traditional combustion, the concentration of SO 2 is basically stable around 900 C, while the concentration of SO 2 is not stable for oxyfuel combustion until the temperature is about 1103 C and the highest concentration of SO 2 is 0.322%, which is 4.4 times compared with traditional combustion. SO 2 is the predominantly existing form of sulfur at high temperature SO2(g) (%) SO3(g) (%) (a)volume concentration of SO 2 (b)volume concentration of SO 3 Figure.1 The distribution of sulfur-containing gases Figure.1(b) indicates the distribution of SO 3 concentration. As the rise of temperature, thermodynamic equilibrium concentration of SO 3 first increases and then declines. SO 3 combines with steam to generate sulfuric acid steam at low temperature. However, SO 2 is more stable than SO 3 at high temperature. For traditional combustion, the concentration of SO 3 reaches a peak of 0.036% at 585 C, while for oxyfuel combustion, the concentration of SO 3 peaks at 580 C, which is 0.199% and it is 5.5 times than that of traditional combustion. Actually, the resistance time of flue gas is pretty short in pulverized coal furnace. The formation rate of SO 3 is small at low temperature while it is big at high temperature. Therefore, the actual

3 Advanced Materials Research Vols concentration of SO 3 in pulverized coal furnace is near the thermodynamic equilibrium concentration of SO 3 at high temperature, but it is far below the thermodynamic equilibrium concentration of SO 3 at low temperature. D. Fleig [5] estimates the time required by SO 3 in the flue gas to reach 90% of thermodynamic equilibrium concentration of SO 3, when temperature is higher than 1400K, required time within 1 second, and it increases steeply with the fall of temperature, for instance, it requires 20 hours to reach 90% of thermodynamic equilibrium concentration of SO 3 when temperature drops to 800K. The concentrations of sulfur-containing gases in oxyfuel combustion raise rapidly compared with traditional combustion, two main factors are as follows: one is that the concentration of oxidizing agent is higher than that of traditional combustion, and after air staging N 2 is not fed, which makes the decrease of flue gas. When oxygen concentration of oxidizing agent is 29%, the amount of flue gas decreases 27.3%, which leads to higher concentration of sulfur-containing gases; another reason is that approximately 70% flue gas recycling in oxyfuel combustion, leading to accumulation of moisture and sulfur-containing gases in flue gas, as a result, the concentrations of SO 2 and SO 3 are higher than that of traditional combustion SO2(g) (%) SO3(g) (%) (a)conversion for S to SO 2 in fuels (b)conversion rate for SO 2 to SO 3 Figure.2 Conversion rate for existing forms of sulfur. Figure.2 indicates the relation between temperature and conversion rate of sulfur-containing gases, as the raise of temperature, the formation rate of SO 2 increases with the decrease of conversion rate for SO 2 to SO 3. Compared with traditional combustion, the formation rate of SO 2 is lower and the formation rate of SO 3 is higher in oxyfuel combustion. At the temperature of 1100 C, formation rate of SO 2 for oxyfuel combustion is only 94.59%, which is 3.9% lower than that of traditional combustion, while in 1550 C the formation rate of SO 2 for oxyfuel combustion is merely 0.21% lower. At a temperature of 1100 C conversion rate for SO 2 to SO 3 in oxyfuel combustion is merely 1.63%, 0.25% higher than that of traditional combustion, and at 1550 C the conversion rate for oxyfuel combustion is only 0.02% higher than traditional combustion. With the same excess air coefficient in thermodynamic equilibrium, the content of O 2 for oxyfuel combustion is higher than that of traditional combustion, which leads to a considerable improvement of SO 3 formation rate. Hence, the ratio of sulfur in the form of sulfates increases, and it results in a decline for the formation rate of SO 2, since it is hard for sulfates existing in high temperature, the formation rate of SO 2 and SO 3 for two combustion methods are closer to each other as temperature rising. Solid sulfur compounds.figure.3 shows the solid sulfur compounds in thermodynamic equilibrium for different combustion methods. Sulfates commonly exist below 1000 C and the amount of sulfates increase as the decline of temperature. As for discussed coal in this paper, aluminum sulfate takes the maximum amount in the oxyfuel combustion, whereas for traditional combustion,

4 70 Energy Research and Power Engineering 2014 magnesium sulfate has the largest amount. Furthermore, CaSO 4 would not form until temperature drops to 1000 C or below and MgSO 4 generates only when temperature is 750 C or below. Consequently, Ca performs best in sulfur immobilization, Mg comes next. Their ability of sulfur immobilization is better than most other alkali metals. Monckert P etc [7] presents that capture of sulfur happens in convection section of boiler over the range of temperature from 450 C to 1100 C, in an agreement with the range of sulfates formation temperature. Thus, during the cooling process of flue gas, gaseous sulfur first reacts with Ca to generate CaSO 4, when the content of gaseous sulfur is large enough and also Ca is depleted completely, gaseous sulfur starts to react with Mg to generate CaSO 4 when temperature is below 750 C. Theoretically, as long as the coal contains enough alkali metals, all the sulfur can be immobilized. The concentration of gaseous sulfur increases by a large margin in the oxyfuel combustion, leading to much more powerful sulfation ability in boiler, and Al in the ash is taken and depleted to generate more Al 2 (SO 4 ) 3 [7]. (mol) MgSO4 KAl(SO4)2 Fe2(SO4)3 CaSO4 FeSO4 K2SO4 Al2(SO4)3 (mol) MgSO4 KAl(SO4)2 Fe2(SO4)3 CaSO4 FeSO4 K2SO4 Al2(SO4)3 Na2SO T( ) (a)traditional combustion (b)fuel combustion Figure.3 The distribution of solid sulfur compounds. Excess air coefficient. In order to discuss the impact of excess air coefficient on sulfur existing forms, initial parameters are set as follows: temperature is 1200 C, air pressure is 0.999atm and excess air coefficient varies from 1.1 to 1.5, and according to the 6% oxygen content, concentrations of sulfur-containing gases are converted SO2(g) (%) SO3(g) (%) α α (a)volume concentration of SO 2 (b)volume concentration of SO 3 Figure.4 The distribution of SO 2 and SO 3 under different excess air coefficient. Figure.4 indicates the distribution of SO 2 and SO 3 with different excess air coefficients. The concentrations of SO 2 and SO 3 increase correspondingly as the increase of excess air coefficient, The SO 2 and SO 3 concentrations are more sensitive to excess air coefficient in oxyfuel combustion. When the excessive air coefficient increases 0.1, SO 2 concentration increased by %. SO 3 concentration increases by % in oxyfuel combustion, while the SO 2 concentration changes only %, the SO 3 concentration changes only % in traditional combustion. The O 2 concentration increases with the increase of excess air coefficient in traditional combustion, which

5 Advanced Materials Research Vols promotes the generation of SO 2 and SO 3, however, the increase of excess air coefficient contributes to the increase of O 2 concentration as well as the amount of circulated flue gas in oxyfuel combustion. This leads to the increase of SO 2 brought by circulated flue gas in the boiler, since SO 3 formation is promoted, as the result, the concentration of SO 3 increases. Conclusion (1) The main sulfur-containing gases are SO 2 and SO 3. Compared with the traditional combustion, the maximum thermodynamic equilibrium concentration of SO 2 and SO 3 respectively increases by 3.4 and 4.5 times in oxyfuel combustion. (2)The solid sulfur compounds are primarily alkali metal sulfates in oxidative atmosphere, which exist steadily at low temperature and decompose at high temperature. Ca performs best in sulfur immobilization, Mg comes next. The alkali metal sulfation is stronger in oxyfuel combustion. (3) The formation rate of SO 2 is lower and SO 3 is higher in oxyfuel combustion than in traditional combustion. In addition, the sulfur-containing gases are generally more sensitive to temperature and excess air coefficient. Acknowledgments Project is supported by National Natural Science Foundation of China ( ) and the Fundamental Research Funds for the Central Universities, Which is grateful acknowledged. Reference [1] E.Croiset, K.V. Thambimuthu. NO x and SO x emissions from O 2 /CO 2 recycle coal combustion. fuel, 2001(8): [2] Hao Liu, Yingjuan Shao. Predictions of the impurities in the CO 2 stream of an oxy-coal combustion plant. Applied Energy. 2010(87): [3] Daniel Fleig, Fredrik Normann, Klas Andersson, etc. The fate of sulphur during oxy-fuel combustion of lignite. Energy Procedia, 2009,1: [4] D. Fleig, E.Vainio, K. Andersson, A. Brink, F. Johnsson, M. Hupa. Evaluation of SO 3 Measurement Techniques in and -Fuel. Energy Fuels, 2012, 26: [5] D. Fleig, Experimental and Modeling Studies of Sulfur-Based Reactions in -Fuel Combustion. Chalmers University of Technology, Göteborg, Sweden [6] Monckert P, Dhungel B, Kull R, Maier J. Impact of combustion conditions on emission formation (SO 2, NO x ) and Fly Ash. In: 3rd MEETING of the OXY-FUEL COMBUSTION NETWORK Yokohama Symposia, Yokohama, Japan: IEA Greenhouse Gas R&D Programme. [7] Rohan Stanger, Terry Wall. Sulphur impacts during pulverised coal combustion in oxy-fuel technology for carbon capture and storage. Progress in Progress in Energy and Combustion Science 37 (2011)

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