Environ. Sci. Technol. 2006, 40, 4300-4305
Effect of Operating Parameters and Reactor Structure on Moderate Temperature Dry Desulfurization JIE ZHANG, CHANGFU YOU,* HAIYING QI, BO HOU, CHANGHE CHEN, AND XUCHANG XU Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084, China
A moderate temperature dry desulfurization process at 600-800 °C was studied in a pilot-scale circulating fluidized bed flue gas desulfurization (CFB-FGD) experimental facility. The desulfurization efficiency was investigated for various operating parameters, such as bed temperature, CO2 concentration, and solids concentration. In addition, structural improvements in key parts of the CFB-FGD system, i.e., the cyclone separator and the distributor, were made to improve the desulfurization efficiency and flow resistance. The experimental results show that the desulfurization efficiency increased rapidly with increasing temperature above 600 °C due to enhanced gas diffusion and the shift of the equilibrium for the carbonate reaction. The sorbent sulfated gradually after quick carbonation of the sorbent with a long particle residence time necessary to realize a high desulfurization ratio. A reduced solids concentration in the bed reduced the particle residence time and the desulfurization efficiency. A single-stage cyclone separator produced no improvement in the desulfurization efficiency compared with a two-stage cyclone separator. Compared with a wind cap distributor, a large hole distributor reduced the flow resistance which reduced the desulfurization efficiency due to the reduced bed pressure drop and worsened bed fluidization. The desulfurization efficiency can be improved by increasing the collection efficiency of fine particles to prolong their residence time and by improving the solids concentration distribution to increase the gas-solid contact surface area.
1. Introduction Moderate temperature dry desulfurization has the advantages of low capital expense, low operating costs, no water consumption, and high desulfurization efficiency. Thus, it is a feasible technology for SO2 removal in developing countries, especially very arid regions. The reactions between the sorbent and the flue gas are gas-solid reactions due to the lack of water. Therefore, the ultimate reaction extent depends on the diffusion of reaction gases into the sorbent particles. However, real flue gases contain not only SO2 but also a much larger concentration of CO2. CO2 diffusion into the pores and the product layer of the sorbent is much better than SO2 diffusion due to its high concentration gradient and small molecular diameter (1-3). Moreover, the kinetic rate for carbonation is also much * Corresponding author
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 13, 2006
faster than that for sulfation (1, 4). Therefore, techniques must be invented to reduce the sorbent consumption while maximizing the SO2 absorption and minimizing the CO2 absorption. Various researchers have studied the moderate temperature dry desulfurization process (1-9). Most studies have been conducted in thermogravimetric analysis (TGA) reactors, mostly with kinetic studies of the carbonate and sulfate reactions (4-6). TGA was used to study the SO2 reaction with Ca(OH)2 at moderate temperatures (300-400 °C) (6), but the study only considered the sulfate reaction, while neglecting the carbonate reaction and the Ca(OH)2 dehydration reaction. The decomposition mechanism of calcium carbonate was also studied using TGA (10, 11). The CaCO3 decomposition reaction is a reversible reaction controlled by temperature and CO2 equilibrium concentration (11). A pilot-scale CFB system was used to study the effect of operating parameters such as temperature, Ca/S ratio, and CO2 concentration at 200-400 °C (2, 3). The CO2 consumed much of the Ca(OH)2 to form CaCO3 due to the competitive reaction mechanisms of the carbonate and sulfate reactions in this temperature range (3). The results revealed that when the CO2 concentration was increased from 3.5% to 13%, the desulfurization efficiency decreased 10-15% (2). A drydesulfurization process using Ca(OH)2/fly ash sorbent was also studied in a TGA reactor and a small CFB reactor at 250-380 °C (7, 8). It was found that CO2 had a negative effect on the desulfurization activity of the sorbent and that the presence of NO or NO2 reduced this negative effect due to the chain reaction mechanism with the NOx as gaseous catalyst and NOx also increased the amount of CaSO4 in the final product. The CaSO4 formation mechanism was investigated for temperatures from 300 to 600 °C (9). The reaction products included both CaSO3 and CaSO4. However, at temperatures above 700 °C, the reaction products were almost all CaSO4 (12). Experimental studies in experimental CFB-FGD facilities at 600-800 °C have not yet been reported. Increasing bed temperatures enhance the gas diffusion which increases the reacting surface area in the sorbent and reduces the carbonate reaction beyond 600 °C (1,3). The sulfate reaction will then become dominant and the desulfurization efficiency will increase rapidly beyond 600 °C which will lead to good commercial applications. In addition, operating parameters such as bed temperature and solids concentration are also important for a CFB reactor to realize high efficiency with low sorbent consumption. Reactor structures such as the cyclone separator and distributor can also be modified to increase desulfurization efficiency in CFB-FGD systems. However, these structures also affect the system flow resistance, which can increase costs. Therefore, the influence of these structures on the desulfurization efficiency and flow resistance must be carefully studied to design a system with low capital and operating costs. The dry desulfurization process at temperatures of 600800 °C was investigated in a pilot-scale CFB experimental facility. The CFB reactor has a height of 6 m and an inner diameter of 0.305 m. The desulfurization efficiency was measured for various operating parameters, such as the bed temperature, the CO2 concentration, and the solids concentration. In addition, structural improvements in the cyclone separator and distributor were also made to study their effects on the desulfurization efficiency and flow resistance, so as to reduce the energy cost of the dry 10.1021/es052168w CCC: $33.50
2006 American Chemical Society Published on Web 05/20/2006
FIGURE 1. Pilot-scale CFB reactor experimental system diagram. desulfurization system as much as possible without reducing the desulfurization efficiency.
2. Experimental System The pilot-scale CFB reactor system is shown in Figure 1. The main subsystems were the sorbent preparation system, the flue gas generation system, and the CFB reactor. The sorbent preparation subsystem included a mixing container, a vacuum filter, and an infrared dryer. The flue gas generation subsystem included a fan, an oil burner, an SO2 mixing chamber, and an air cooler. The CFB reactor included the main bed, a two-stage cyclone separator, a bed material circulating facility, a bed material feeder and drain, a bag filter, and a compressor. Flue gas generated by the oil burner was mixed with a small amount of cool air to produce 600-800 °C simulated flue gas. SO2 was added to the flue gas before the CFB reactor. The flue gas passed through the CFB reactor and reacted with the sorbent, then passed through a two-stage cyclone separator and bag filter before being emitted from the stack. The sorbent particles collected in the cyclone separator and bag filter were fed back into the reactor for further circulation or drained out of the system. The flue gas temperatures and static pressures at various positions along the riser height were measured automatically with thermocouples and pressure sensors. The O2, CO2, and SO2 concentrations in the flue gas were measured on-line at the CFB reactor inlet and outlet using a PS3400 type gas analyzer. The desulfurization efficiency was directly calculated from the inlet and outlet SO2 concentrations. The sorbent was produced by quick hydration of lime and coal fly ash, using hydration at an ambient temperature for
