Energy Fuels 2009, 23, 5934–5941 Published on Web 10/28/2009
: DOI:10.1021/ef900687m
Simultaneous Removal of SO2 and Small Particles in a Multistage Spouted Fluidized Tower Jiaxun Liu,† Jihui Gao,*,† Xiumin Jiang,‡ Jianmin Gao,† Qian Du,† and Shaohua Wu† †
School of Energy Science and Engineering, Harbin Institute of Technology, West Straight Street, Harbin 150001, China, and ‡ School of Mechanical Engineering, Shanghai Jiao Tong University, Minhang District, Shanghai 200240, China Received July 3, 2009. Revised Manuscript Received October 12, 2009
A novel particle agglomeration technology combined with a circulating fluidized bed-flue gas desulfurization (CFB-FGD) technique proposed in this paper can realize the simultaneous removal of multipollutions from flue gas. A pilot-scale experimental system for SO2 removal and a lab-scale experimental platform of spray agglomeration for small-scale particle removal have been designed and built. Systematic experiments about different kinds of semi-dry processes and small-scale particle removal processes were conducted in these systems. Scanning electron microscopy (SEM) was used to analyze the microstructure of the aggregation, and fractal dimension analysis on the effect of agglomeration is adopted in this work. Furthermore, the preliminary study of a modified calcium-based desulfurizing agent to realize the simultaneous removal of multi-pollutions from flue gas was discussed. Final results indicate that this technique is a useful and promising method to control the emission of small-scale particles and SO2 from coal combustion simultaneously. The findings from this work will be helpful to form the basis and provide guidance for further studies on the simultaneous removal of SO2 and sub-micrometer particles.
Zhuang et al.9 investigated the formation mechanisms of sub-micrometer particles during combustion of a pulverized coal. A vapor-phase sorbent injection method was used in the coal combustion system. Linak et al.10 investigated the in situ capture of toxic metals by sorbents. The study of Shanthakumar et al.11 reveals that, after the conditioning of the ash with ammonia, the agglomeration of the ash particles can be observed very clearly, which results in increased collection efficiency of the ESP. Weifeng et al.12 developed a novel submicrometer particle agglomeration technique to control the emissions of sub-micrometer particles. The sub-micrometer particles in the flue gas were agglomerated through physical and chemical processes by an agglomerant solution, which was sprayed before an electrostatic precipitator (ESP). A process and apparatus for treating PM and acid gas in one unit has been patented.13 The exhaust gas is first dedusted in a cyclone stage, then after carefully controlled moisture addition, it is passed to an ESP stage where the PM is collected. During the moisture addition process, some kinds of alkaline matter can be applied to capture acid gas. A small industrial module has been developed that combines centrifugation and wet scrubbing for PM removal, followed by passing the gas stream through activated carbon sponge bags for demisting. The demisting step can include acid gas fixation. Meng et al. used the pierced cylindrical gas inlet device for jet bubble reactors for purification and treatment of waste gas streams. Results show that the tower achieved 95% SO2
1. Introduction Coal is the major energy resource in China.1 Coal accounts for more than 70% of the total energy consumption, and emissions from coal combustion are the major anthropogenic contributors to air pollution in China. The emission of SO2, a major air pollutant from coal combustion, was 2.55 107 tons in 2005.2 This acid gas leads to acid rain and fog, which has harmed the natural environment and human life. In the worldwide applied flue gas desulfurization (FGD) systems, semi-dry processes have second place behind wet scrubbers. In comparison to the wet scrubbers, they have the advantages of smaller space requirement, lower water consumption, and fewer investment and operation costs. In addition, the method does not require reheating energy and wastewater treatment. In addition, this technology is very effective in reducing SO2 emissions from medium- and small-scale coal-fired power stations.3-5 Airborne fine particulate matter (PM), especially from coal combustion sources in China,6 has recently become the subject of considerable environmental concerns7,8 because of its association with increased mortality, morbidity, and weakened lung functions. Sub-micrometer particles (PM2.5) are main pollutants to the atmosphere in current China. *To whom correspondence should be addressed. Telephone: þ86-213420-6052. Fax: þ86-21-3420-5521. E-mail:
[email protected]. (1) Wang, H.; Jiang, X. M.; Liu, J. G.; Lin, W. G. Energy Fuels 2007, 21, 1924–1930. (2) Chan, C. K.; Yao, X. H. Atmos. Environ. 2008, 42, 1–42. (3) Zhou, Y. G.; Zhu, X.; Peng, J.; Liu, Y. B.; Zhang, D. W.; Zhang, M. C. J. Hazard. Mater. 2009, 170, 436–442. (4) Zhang, Q.; Gui, K. J. Hazard. Mater. 2009, 168, 1341–1345. (5) Mohanty, C. R.; Adapala, S.; Meikap, B. C. J. Hazard. Mater. 2009, 165, 427–434. (6) You, C. F.; Zhao, H. L.; Huang, B.; Qi, H. Y.; Xu, X. C. Powder Technol. 2008, 185, 267–273. (7) Neas, L. M. Fuel Process. Technol. 2000, 65-66, 55–67. (8) Senior, C. L.; Helble, J. J.; Sarofim, A. F. Fuel Process. Technol. 2000, 65-66, 263–288. r 2009 American Chemical Society
(9) Zhuang, Y.; Biswas, P. Energy Fuels 2001, 27, 510–516. (10) Linak, W. P.; Srivastava, R. K.; Wendt, J. O. L. Combust. Flame 1995, 100, 241–250. (11) Shanthakumar, S.; Singh, D. N.; Phadke, R. C. Fuel 2008, 87, 3216–3222. (12) Wei, F.; Zhang, J. Y.; Wang, C. M.; Zheng, C. G. Coal Convers. 2003, 26, 27–31 (in Chinese). (13) Darconich, K.; Jonasson, K. A.; Capes, C. E. Adv. Powder Technol. 1997, 8, 179–215.
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: DOI:10.1021/ef900687m
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removal and over 90% dust removal at a pressure drop of about 3 kPa for the entire tower.14 The moving granular bed for simultaneous removal of particulates and hydrogen sulfide has been developed and considered as a promising combined process, in which granular sorbents can not only remove sulfur gases by chemical reactions but also act as the filtering media to remove particulates from fuel gas.15 One promising concept for multi-pollutant control is the circulating fluidizedbed adsorber (CFBA),16 which may use injection of different sorbents to remove multiple pollutants. A CFBA could scrub SO2 through adsorption onto calcined lime, and a potential side benefit is the capture of fine PM through agglomeration onto sorbent particles.17 On the basis of previous research, we proposed a novel compounding spouted fluidized technique. A multistage spouted fluidized FGD technique is a new type of CFB-FGD technique. In the study of this technique, we found that desulfurization ashes in the fluidized tower have notable effects on sub-micrometer particle agglomeration. Large amounts of sub-micrometer particles were found in either surface layering or agglomeration of particles. A multistage spouted fluidized tower has stable internal circulating characteristics. The special shape of a multistage spouted fluidized tower has significant influences on the effect of agglomeration. Systematic experiments about desulfurization and spray agglomeration of fine particles have been performed in this paper. The effect of sub-micrometer particle agglomeration by the processing of desulfurization has been discussed using scanning electron microscopy (SEM). The methods of preparing agglomerant solutions and a modified calcium-based desulfurizer were also discussed, aiming to discover more economical and effective solutions for simultaneous removal of SO2 and sub-micrometer particles. The novel particle agglomeration technology combined with the CFB-FGD technique, whose cost is cheaper, can control the emission of sub-micrometer particles and decrease the opacity of flue gas obviously without changing the normal operation parameters of the combustion and dedusting equipment. Furthermore, this technology is feasible and easy to realize.18 Most importantly, along with the study of modified agglomerant solutions, it can be used to realize the simultaneous removal of multi-pollutions from flue gas. Therefore, this technique is a useful and promising method to control the emissions of sub-micrometer particles from coal combustion. The findings from this work will be very helpful to form the basis and provide guidance for further studies on the simultaneous removal of SO2 and sub-micrometer particles.
Figure 1. Sketch map of the pilot-scale experimental system.
2.5 MPa, while the nominal steam temperature is 400 °C. The MS-FGD system was designed according to the following technique requirements. The designed flow rate of the flue gas treated by the multistage spouting tower is 60 000 m3/h (175 °C), while the sulfur content of the coal is taken as 1.5-2%. The sketch map of the pilot-scale experimental system is shown in Figure 1. There are mainly seven subsystems: compressed air system, process water system, slurry storage system, solution preparing and spraying system, MS-FGD tower, dust removing, and induced draft system, and electrical and automatic control system (OS-DCS system). The MS-FGD tower consists of the flue gas distributor, Venturi gas fluidized device, the first-stage tower, the second-stage tower, and internal circulating devices. The height of the entire tower is 17.88 m, and the diameter of the first-stage tower is 1.45 m, while the diameter of the second-stage tower is 2.3 m. We can see that the variable cross-section area is adopted in the absorber. The special shape of the multistage spouted fluidized tower has significant influence on the concentration distribution of the particles. The compounding spouted fluidized FGD is an innovative flue gas purification technology with a multistage spouted tower. Systematic experiments had been carried out to study the desulfurization characteristics in different reaction zones by an online SO2 spectral measuring system.19,20 There are mainly two zones: gas-particle main reaction zone in the second stage of the tower and gas-liquid main reaction zone in the first stage. In the first stage, the velocity of the gas is high, which intensifies the transverse distribution of the airflow. In addition, because of the high velocity, a large amount of small particles are carried upward to the second stage. On the other hand, the desulfurizing agent is sprayed from the bottom of the tower and there is not enough time for the droplets to vaporize completely in the first stage. Therefore, it is called the gas-liquid main reaction zone in the first stage. With the decrease of the velocity in the second-stage tower, some large-scale particles will fall along the wall, where there are flow guide devices. The particles that reach the top of the tower can reflow along the devices to the bottom of the second stage. Again, they are carried upward by the flow from the first stage of the tower and form the internal circulation. Because of the continuous circulation, the particle concentration increases in the second stage of the tower. The droplets have vaporized completely, except for some left around the surfaces of the particles. Therefore, in the second stage, it is called the
2. Experimental Section 2.1. Pilot-Scale Experimental System. The multistage spouted fluidized FGD technique (MS-FGD) is a new style technique. The demo plant was conducted on a 20 tons/h grate-fired boiler of Long Gang paper mill in Weihai, China. The boiler was made by Nantong Wanda Boiler Co., Ltd., of which the boiler-rated evaporating capacity is 20 tons/h. The rated working pressure is (14) Meng, L.; Yang, C. P.; Gan, H. M.; Wu, T.; Zeng, G. M.; Chen, H.; Guo, S. X. Sep. Purif. Technol. 2008, 63, 86–91. (15) Zhao, J. T.; Huang, J. J.; Wu, J. H.; Fang, Y. T.; Wang, Y. Powder Technol. 2008, 180, 2–8. (16) Jiang, M. X.; Keener, T. C.; Khang, S. J. Powder Technol. 1995, 85, 115–126. (17) Mao, D. M.; Edwards, J. R.; Kuznetsov, A. V.; Srivastava, R. K. Chem. Eng. Sci. 2004, 59, 4279–4289. (18) Zhao, Y. C.; Zhang, J. Y.; Wei, F.; Chen, J.; Zheng, C. G. J. Chem. Ind. Eng. 2007, 58, 2876–2881 (in Chinese).
(19) Gao, J. L.; Gao, J. H.; Chen, X. L.; Gao, J. M. J. Eng. Therm. Energy Power 2008, 23, 655–660 (in Chinese). (20) Gao, J. H.; Gao, J. L.; Chen, X. L.; Gong, Z. R.; Gao, J. M.; Liu, J. X. J. Chem. Ind. Eng. 2008, 59, 461–466 (in Chinese).
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: DOI:10.1021/ef900687m
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Figure 2. Sketch map of the pilot-lab experimental system.
Figure 3. PSD of the powder coal ashes.
Table 1. Components of the Desulfurizing Agent CaO (%)
MgO (%)
SiO2 (%)
CO2 (%)
output of slurry (L/kg)
>80
