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Ind. Eng. Chem. Res. 2009, 48, 5808–5815
Study on Multiphase Flow and Mixing in Semidry Flue Gas Desulfurization with a Multifluid Alkaline Spray Generator Using Particle Image Velocimetry Yuegui Zhou,*,† Dongfu Wang,‡ and Mingchuan Zhang† Institute of Thermal Energy Engineering, School of Mechanical Engineering, Shanghai Jiao Tong UniVersity, 800 Dong Chuan Road, Minhang District, Shanghai, 200240, China, and Shanghai Boiler Works, Ltd., 250 Hua Ning Road, Minhang District, Shanghai, 200240, China
Particle image velocimetry (PIV) technique was used to measure the velocity fields of gas-droplet-solid multiphase flow in the experimental setup of a novel semidry flue gas desulfurization process with a multifluid alkaline spray generator. The flow structure, mixing characteristic, and interphase interaction of gas-droplet-solid multiphase flow were investigated both in the confined alkaline spray generator and in the duct bent pipe section. The results show that sorbent particles in the confined alkaline spray generator are entrained into the spray core zone by a high-speed spray jet and most of the sorbent particles can be effectively humidified by spray water fine droplets to form aqueous lime slurry droplets. Moreover, a minimum amount of air stream in the generator is necessary to achieve higher collision humidification efficiency between sorbent particles and spray water droplets and to prevent the possible deposition of fine droplets on the wall. The appropriate penetration length of the slurry droplets from the generator can make uniform mixing between the formed slurry droplets and main air stream in the duct bent pipe section, which is beneficial to improving sulfur dioxide removal efficiency and to preventing the deposition of droplets on the wall. 1. Introduction Sulfur dioxide emitted during the coal combustion process is one of the main pollutants in the atmosphere. It is urgent to control sulfur dioxide emission in coal-fired power stations with the demands of increasingly strict pollutant emission standards of recent years. Wet flue gas desulfurization (FGD) processes have been widely utilized in large scale coal-fired power stations with high SO2 removal efficiencies of over 90%. The spray dryer absorption semidry FGD process is restricted to be used widely in existing power plants for various reasons such as space limitations for complicated equipment, large spray dryer absorption towers, and heavy abrasion of atomizing nozzles due to spraying lime slurry.1 However, nearly one-third of medium and small capacity units were constructed before this strict standard, and no sufficient spaces have been left for the installation of wet and spray dryer semidry flue gas desulfurization equipment. Some simple flue gas desulfurization processes such as Duct Sorbent Injection and the Coolside process2-4 have not been widely used before now due to low removal efficiency and sorbent utilization. Therefore, a novel compact semidry flue gas desulfurization process with a multifluid alkaline spray generator was first proposed and investigated to obtain a high collision humidification efficiency of 75% between sorbent particles and spray water droplets and to achieve higher desulfurization efficiency.1,5 Recently this flue gas desulfurization process has been successfully utilized for the flue gas cleaning system in a 75 tons/h incinerator. The novel semidry flue gas desulfurization process involves complicatedphysicalandchemicalprocessessuchasgas-droplet-solid multiphase flow and mixing, evaporating spray jet flow and entrainment, effective collision humidification between sorbent particles and spray water droplets of high concentrations in the confined multifluid alkaline spray generator, and chemical * To whom correspondence should be addressed. Tel.: +86-2134206769. Fax: +86-21-34206115. E-mail:
[email protected]. † Shanghai Jiao Tong University. ‡ Shanghai Boiler Works.
reaction between aqueous lime slurry droplets and sulfur dioxide in flue gas. The collision humidification process of sorbent particles caught by spray water droplets is the key to forming most of the aqueous lime slurry droplets and to achieving high sulfur dioxide removal efficiency. Thus, it is necessary to adopt advanced measurement technology to study gas-droplet-particle multiphase flow structure, mixing characteristic, and interphase interaction between sorbent particles and spray droplets. During past two decades, the measurement techniques have been greatly improved to obtain the knowledge of the physical mechanisms and dynamics of such flows. The most commonly used measurement techniques, such as laser Doppler anemometry and phase Doppler anemometry, are single-point measurements, which can provide useful statistical information on fluid velocity, particle velocity, particle size, and concentration. One limitation of single-point measurements, however, is the difficulty associated with interpreting the data into meaningful physical mechanisms that control the dynamics between the phases.6 Thus, the microscale spatial information on the interphase dynamics and its structure relative to the carrier fluid is essential for a good understanding of multiphase flows. Particle image velocimetry (PIV) has been rapidly developed to measure the instantaneous velocity vector field from slow flows to supersonic flows.7 In contrast to single-point measurements, PIV can carry out two-dimensional and three-dimensional instantaneous velocity measurements and represent an instantaneous whole field technique which makes it possible to detect spatial flow structure and provide a direct indication of the interphase coupling. Many researchers have applied the PIV technique to investigating single-phase flow structure,8,9 fuel spray jets in diesel engines,10-13 spiral gas-solid two-phase flow in a horizontal tube,14 and gas-liquid-solid three-phase flows through a curved pipe or a hydration desulfurization reactor.15,16 The objective of the present paper is to use PIV technique to investigate the flow structure, mixing characteristic, and interphase interaction of gas-droplet-solid multiphase flow in the novel semidry flue gas desulfurization process with a multifluid alkaline spray generator, and to elucidate the physical mecha-
10.1021/ie8019714 CCC: $40.75 2009 American Chemical Society Published on Web 05/13/2009
Ind. Eng. Chem. Res., Vol. 48, No. 12, 2009
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Figure 1. Schematic of experimental setup: 1, air from I.D. fan; 2, control valve; 3, particle feeding device; 4, particle-laden air stream; 5, near wall air stream; 6, Y-jet nozzle; 7, air distributor; 8, multifluid alkaline spray generator; 9, horizontal square duct; 10, water drain; 11, duct bent pipe; 12, vertical desulfurization duct; 13, air to filter baghouse.
nism of highly efficient collision humidification between sorbent particles and spray water droplets. 2. Experimental Section 2.1. Experimental Setup. The experimental setup of the multiphase flow PIV measurement system was designed as shown in Figure 1 based on the prototype of semi-industrial scale flue gas desulfurization apparatus1 according to two-phase flow similarity modeling theory.17 The geometric similarity ratio was 0.57, and the tracer solid diameters and spray droplet diameters were calculated as the Froude number and the Stokes number were identical. The multifluid alkaline spray generator was a cylindrical pipe made of optically transparent Plexiglas with an internal diameter of 0.20 m and a height of 0.57 m, which was connected to the duct bent pipe section. The bulk air stream flowed in a long horizontal duct to maintain uniform velocity at the test section inlet with 0.225 m × 0.225 m cross section. A small amount of the air stream was divided into two parts to enter the multifluid alkaline spray generator. One entrained sorbent tracer particles from a variable-speed screw feeder into the alkaline spray generator through the internal annulus of the top air distributor. The other directly entered it through the external annulus of the air distributor, which was used as the near wall stream to prevent from the possible deposition of the wetted particles on the wall. A Y-jet twinfluid nozzle was located at the center line of the spray generator to produce fine water droplets to humidify sorbent particles. Thus, the gas-droplet-solid multiphase flow in the spray generator flowed downward, mixed with the bulk air stream at the duct bent pipe and then entered the vertical desulfurization duct and filter baghouse. The main experimental parameters in the present paper are listed in Table 1. Different air flow rates were measured by three calibrated pitot tubes and an electronic manometer. The air stream flow rate in the main duct was 0.278 m3/s, and two air stream flow rates in the multifluid alkaline spray generator varied from 0 to 0.0556 m3/s. The mass flow rate of spray water in the Y-jet nozzle was measured by a
Table 1. Main Experimental Parameters parameters main air stream flow rate air stream flow rate in the generator spray water flow rate spray water droplet mean diameter sorbent particle flow rate sorbent tracer particles gas tracer particles particle density
values 3
0.278 m /s 0, 0.0278, 0.0417, 0.0556 m3/s 4.167 g/s 40 µm, 80 µm 0.833 g/s glass beads, 25 ( 5 µm glass beads,
