Effect of Near-Wall Air Curtain on the Wall Deposition of Droplets in a

Tsinghua University, Beijing 100084, China. The wall deposition of droplets is an important issue affecting the desulfurization efficiency and operati...
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Environ. Sci. Technol. 2007, 41, 4415-4421

Effect of Near-Wall Air Curtain on the Wall Deposition of Droplets in a Semidry Flue Gas Desulfurization Reactor JIE ZHANG, CHANGFU YOU,* CHANGHE CHEN, HAIYING QI, AND XUCHANG XU Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084, China

The wall deposition of droplets is an important issue affecting the desulfurization efficiency and operating stability of semidry flue gas desulfurization (FGD) reactors. Various near-wall air velocities, near-wall air flow inlet heights, and spray characteristics were analyzed numerically to investigate their effect on the gas-liquid flow and droplet deposition characteristics. The analytical results show that the near-wall air curtain effectively reduces the wall deposition of droplets in the semidry FGD reactor. The droplet deposition ratio decreased rapidly with increasing near-wall air velocity due to the increased gas flow rates and the altered gas velocity distribution. The nearwall air flow inlet height had an optimum value due to the rapid decline of the near-wall air momentum along the reactor height. The optimum distance between the nearwall air inlet height and the droplet injection height was 1.2 times that of the droplet vertical movement distance before deposition based on the linear droplet movement. For commonly used spray characteristics in the semidry FGD process, i.e., droplet diameters of 50-150 µm, spray angles of 10-70° and droplet initial velocities of 20-100 m/s, the droplet deposition ratio with the addition of the nearwall air curtain varied slightly with the droplet diameter and the spray angle but increased rapidly with the initial droplet velocity. Therefore, for the semidry FGD processes, the near-wall air curtain is an effective method to reduce the wall deposition of droplets for various droplet diameters and spray angles while the initial droplet velocity should be carefully controlled to reduce the wall deposition of droplets and improve the operating stability.

1. Introduction The SO2 emissions from coal-fired power plants have caused significant environmental and human health effects. Various FGD processes have been developed to reduce SO2 emissions. The semidry FGD process can achieve high desulfurization efficiency with low water consumption, small space requirements, and low capital cost, which makes SO2 removal very attractive (1). China is one of the largest countries using the semidry FGD process, with up to eleven 300 MW semidry FGD projects and a large number of smaller semidry FGD units. However, the process has several problems, such as * Corresponding author [email protected]. 10.1021/es062001i CCC: $37.00 Published on Web 05/15/2007

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FIGURE 1. Schematic diagram of the reactor: 1, primary air inlet; 2, droplet injection inlet; 3, near-wall air inlet; 4, reactor wall; 5, outlet. low desulfurization efficiency and poor operating stability, in engineering applications. The main reason is that the injected spray droplets deposit on the reactor wall. The spray droplets are very important in the semidry FGD process, since they enable the chemical reaction between the sorbent and the SO2 to change from a gas-solid reaction to an ionic reaction. In general, ionic reactions are instantaneous in the aqueous phase, but extremely slow in the absence of water (2). Therefore, the desulfurization efficiency depends heavily on the utilization efficiency of the spray droplets. The wall deposition of droplets reduces the spray droplet utilization efficiency by first causing scaling and corrosion of the reactor wall which affects the operating stability (3) and second by reducing the inertial impacts between the spray droplets and the sorbent particles and the mass transfer between the spray droplet and the SO2 in the flue gas, which affect the desulfurization performance (4). Research on the wall deposition of droplets has focused on the drying field (5-8). Chen et al. (5) suggested that modified near-wall airflow patterns could reduce the wall deposition in industrial-scale spray drying of milk. There are few published studies about the wall deposition of droplets in the semidry FGD process (3, 9-11). The previous studies were mainly focused on the spray characteristics of the sprayer itself (9, 10). Wang (10) experimentally studied the spray characteristics of the twin-fluid nozzle. The effect of parameters such as the air liquid ratio, slurry concentration and nozzle structure on the Sauter mean diameter (SMD) and the spray angle was analyzed. A desulfurization nozzle with good spray characteristics is important to improve the desulfurization efficiency and the operating stability. However, after the injected droplets from a good spray nozzle enter into the reactor, high droplet utilization efficiency must be achieved by proper organization of the flow field in the reactor. Therefore, the flow field organization in the reactor is also a very important factor for reducing the wall deposition of droplets and improving the desulfurization performance. Some researchers have proVOL. 41, NO. 12, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Effect of various near-wall air velocities with droplet injection at the bottom.

FIGURE 3. Axial gas velocity distribution at various cross sections for various near-wall air velocities.

FIGURE 4. Radial gas velocity distribution at various cross sections for various near-wall air velocities. posed near-wall air flows with higher velocities than the primary air flow to reduce the wall deposition of droplets (3, 11). However, those analyses were only limited to simple qualitative analyses. The effect of the near-wall air flow on the spray characteristics and the wall deposition of droplets is still not well understood. Therefore, studies on the effects of the near-wall air curtain on the wall deposition of droplets in the semidry FGD processes are essential to improve desulfurization efficiencies and the operating stability. Various near-wall air velocities, near-wall air flow inlet heights and spray characteristics were analyzed numerically to investigate their effect on the gas-liquid flow and droplet deposition characteristics. The analytical results provide guidance for improved spray droplet utilization optimization to improve the desulfurization efficiency and operating stability in semidry FGD reactors. 4416

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2. Numerical Analysis The effect of the near-wall air injection on the wall deposition of droplets in the semidry FGD reactor was analyzed with a cold gas-liquid two phase flow model without droplet evaporation. The flow in the reactor was viscous, incompressible, steady, isothermal, turbulent gas-particle flow with a low droplet concentration. Since the flow is relatively simple, numerical simulations can achieve the accuracy for the engineering analyses. The standard k- model was used to simulate the turbulent gas flow in the relatively simple gas flow field, with the Lagrange method used to calculate the droplet particle motion. The solid sorbent particles in the actual semidry FGD reactors were not considered in the current study. The particles may affect the quantitative accuracy, but the qualitative results can be used to analyze the effect of the near-wall air on the wall deposition of droplets. The solid sorbent particles in the actual semidry

3. Results and Discussion

FIGURE 5. Effect of various gas velocities on the droplet deposition ratio for Uw ) U and Uw * U cases. FGD reactors reduced the droplet deposition on the wall a small amount in the main droplet deposition region. The solid sorbent particles collide with the droplets which accelerates the droplet evaporation and reduces the droplet mass flow toward the wall (12). However, the impacts between the solid sorbent particles and the droplets cannot markedly change the droplet movement direction due to the relatively small sorbent particle diameters below 10 µm compared to the droplet diameters of about 75 µm for droplet inertial catching of the sorbent particles (4). Therefore, the simplification of not considering the solid sorbent particles in the current analysis was a reasonable assumption to isolate the analysis of the droplet deposition on the wall to determine the detailed characteristics. The studies provide useful guidance for further studies on the organization of the reactor flow field and for improving the desulfurization performance in semidry FGD reactors. The spray droplets were assumed to be uniform diameter droplets. Droplet break-up and coalescence were also neglected. In the calculations, the maximum inlet droplet concentration was 0.087 kg/m3 and the droplet phase volume fraction was only 0.01%. The average distance between droplet particles was 1302 µm and the Stokes number was 0.232 (13). Therefore, the gasliquid flow was taken to be dilute gas-liquid twophase flow with the interactions between droplets neglected. The stochastic particle trajectory model was used to simulate the droplet particle motion, taking into account turbulent dispersion of the particles. The effect of the random gas velocity fluctuations on the droplet particle motion was considered by integrating the trajectory equations for individual particles, using the instantaneous gas velocity along the particle path. The computational domain was a three-dimensional reactor, 3 m high with a diameter of 0.3 m. A schematic diagram of the reactor is shown in Figure 1. The primary air flow inlet was at the reactor bottom plane with U ) 4 m/s. The near-wall air flow inlet was 10 mm wide close to the reactor wall. The near-wall air flow velocity (UW) and inlet height (ZW) were varied in the various numerical cases. 100 000 droplets were injected from the center of the reactor bottom plane into the reactor. When the droplet particle reached the reactor wall, the droplet particle trajectory calculation was stopped and marked as droplet deposition on the wall. The calculation considered the interactions between the gas and droplet phases with coupled two-phase calculations to ensure the converged numerical results for the gas and droplet phases.

3.1. Effect of Near-Wall Air Curtain. The effects of various near-wall air velocities and inlet heights on the wall deposition of droplets were investigated. The droplet spray characteristic parameters were selected according to the spray characteristics of the two-fluid nozzle commonly used in semidry FGD reactors (4). The droplet diameter was 75 µm, the initial velocity was 40 m/s and the spray angle was 20°. The liquid density was 998.2 kg/m3 and the droplet mass flow rate was 0.03 kg/s. 3.1.1. Effect of Near-Wall Air Velocity. The droplet deposition ratio (η) and the area-weighted average droplet concentration (c) were defined to quantitatively express the droplet deposition extent on the reactor wall. The droplet deposition ratio was defined as the percentage of droplets deposited on the reactor wall relative to the total number of injected droplets. The area-weighted average droplet concentration was defined as the sum of the droplet concentration in each grid multiplied by the corresponding grid area divided by the sum of the grid area on the reactor cross section. The droplet deposition ratio and the area-weighted average droplet concentration along the reactor height for the various near-wall air velocities are shown in Figure 2 for the near-wall air flow injection location at the reactor bottom (Zw ) 0 m). Figure 2(a) shows that the droplet deposition ratio declined rapidly with increasing near-wall air velocity. As the near-wall air velocity increased from 0 to 16 m/s, the droplet deposition ratio decreased from 57.7 to 14.7%, for a reduction of 74.5%. The spray droplets in the semidry FGD process reduce the flue gas temperature to a favorable temperature window for the desulfurization reaction and increase the flue gas humidity to humidify and activate the sorbent particles, which changes the slow gas-solid reaction into an instantaneous ionic reaction (4). The spray droplets were not consumed in the sulfate reaction but were carried out by the sulfated flue gas and the droplet deposition on the reactor wall. The near-wall air curtain not only improves the operating stability caused by the wall deposition of droplets, but also improves the droplet utilization efficiency and thus the desulfurization efficiency. Figure 2(b) shows that the average droplet concentration first increases and then decreases with increasing cross sectional height (Z) for various near-wall air velocities. The maximum average droplet concentration occurs at approximately Z ) 1.5 m. At cross sections below Z ) 1.5 m, the droplets concentrate in the reactor center rather than spreading across the cross section. The droplet concentration near the reactor wall is low so the average droplet concentrations for these cross sections were also low. At cross sections above Z ) 1.5 m, the droplets are fully diffused across the cross section. The droplets continue to move toward the reactor wall due to residual momentum. The number of droplets deposited on the wall increases with height so the average droplet concentration decreases gradually along the reactor height. The average outlet droplet concentration increases with increasing near-wall air velocity. The increased near-wall air velocity possesses greater momentum which deflects the droplets away from the reactor wall to reduce the wall deposition of droplets and increase the average outlet droplet concentration. 3.1.2. Gas Velocity Distribution. The gas velocities in the axial and radial directions at various cross sections are shown in Figures 3 and 4 for the droplet inlet at the reactor bottom with near-wall air velocities of 0 and 16 m/s. Two processes may contribute to the improved effect of the near-wall air on the droplet deposition ratio. One process is that the addition of the near-wall air increased the total VOL. 41, NO. 12, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 6. Effect of various near-wall air flow inlet heights for UW ) 12 m/s.

FIGURE 7. Effect of various droplet diameters. gas flow rate in the reactor. Figure 3 shows that the average gas velocity in the reactor was about 3.5 m/s for Uw ) 0 and 5.5 m/s for Uw ) 16 m/s. The increased average gas velocity led to shorter droplet residence times which reduced the droplet deposition ratio. The second process is that the addition of the near-wall air changed the gas velocity distribution in the reactor. Figure 3 shows that the axial gas velocities in the central part of the reactor were larger than near the reactor wall at all cross sections for Uw ) 0 m/s. However, the gas velocities near the wall were greater than between the center and the wall at Z ) 0.5 and 1.0 m for Uw ) 16 m/s. At cross sections above Z ) 1.0 m, the gas velocity distribution was almost uniform. In addition, Figure 4 shows that the radial gas velocities were toward the reactor wall at cross sections above Z ) 0.5 m for Uw ) 0 m/s, which resulted in the droplets moving toward the reactor wall due to the droplet inertia and the gas carrying the droplets toward the wall. However, the radial gas velocities were almost zero at cross sections above Z ) 1.0 m for Uw ) 16 m/s, which greatly reduced the gas-phase carrying effect and the droplet deposition ratio. The effects of various uniform axial gas velocities (Uw ) U) with near-wall air flow injection at the reactor bottom on the droplet deposition ratio are compared with near-wall air injection with nonuniform axial velocities (Uw * U) in Figure 5. All cases have the same droplet spray characteristics with each pair of uniform and nonuniform flow cases that are vertically adjacent having the same total gas flow rate. Figure 5 shows that the droplet deposition ratio rapidly decreased with increasing air velocity for the uniform flow cases due to the reduced droplet residence time. As the air velocity increased from 3.5 to 5.5 m/s, the droplet deposition ratio decreased from 56.2 to 23.6%. Figure 5 also shows that the effect of the increased total gas flow rate, which increases the near-wall air flow rate and reduces the droplet deposition, 4418

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was more important at low near-wall velocities. Increasing the near-wall air velocity gradually increased the gas velocity distribution effect of the near-wall air. Although the total gas flow rates were the same for the U ) Uw ) 5.5 m/s case and the U ) 4 m/s /Uw ) 16 m/s case, the droplet deposition ratio was higher for the uniform flow case, 23.6%, than for the nonuniform flow case, 14.7%, due to the altered gas velocity distribution in the reactor. In addition, Figure 3(b) shows that the near-wall air velocity rapidly decreased along the reactor height for the near-wall air inlet at the reactor bottom (Zw ) 0 m). The near-wall air velocity rapidly decreased due to the large friction resistance along the wall. The increased near-wall air velocity resulted in increased near-wall air momentum, which caused the near-wall air velocities to decrease over a longer distance. Therefore, the effect of various near-wall air flow inlet heights on the droplet deposition characteristics must be studied in more detail. 3.1.3. Effect of Near-Wall Air Flow Inlet Height. The area of the near-wall air inlet is about 12.1% of the reactor cross section. Although the droplet deposition ratio was lowest for the near-wall air velocity of UW ) 16 m/s in Figure 2, the high flow resistance is not good for commercial applications since the near-wall air flow rate was about 35.5% of the total air flow rate. Therefore, the remaining numerical cases used the lower near-wall air velocity of UW ) 12 m/s. The droplet deposition ratio and the average droplet concentration along the reactor height for the various nearwall air flow inlet heights are shown in Figure 6 for the nearwall air velocity UW ) 12 m/s. Figure 6 shows that the droplet deposition ratio decreased slowly and then increased rapidly with increasing cross sectional height, while the average outlet droplet concentration increased and then decreased with increasing cross sectional height. For the near-wall air flow inlet height, ZW

FIGURE 8. Droplet concentration distribution in the center plane at various heights.

FIGURE 9. Effect of various spray angles. ) 1.0 m, the droplet deposition ratio reached a minimum of 16.8% while the average outlet droplet concentration reached a maximum of 0.064 kg/m3. For near-wall air flow inlet heights below 1.0 m, the droplet deposition ratio and the average outlet droplet concentration varied only slightly. However, for near-wall air flow inlet heights above 1.0 m, they varied greatly compared with the values for the near-wall air flow inlet height of 1.0 m. Therefore, the near-wall air flow inlet height has an optimum value, ZW ) 1.0 m due to the relatively quick decline of the near-wall air momentum along the reactor height. For the specified spray characteristics (Sauter mean diameter 75 µm, spray angle 20° and initial droplet velocity 40 m/s) and the reactor diameter of 0.3 mm in the numerical simulation, the average droplet vertical movement before deposition on the reactor wall was 0.85 m from the droplet injection position. Therefore, the optimum vertical distance between the near-wall air flow inlet height and the droplet injection

position was 1.2 times that of the droplet vertical movement distance before deposition based on the linear droplet movement. In addition, to further reduce the droplet deposition ratio, several near-wall air flow inlets can be added at various reactor heights, especially for very tall semidry FGD reactors or those having several layers of spraying. 3.2. Effect of Spray Characteristics. After discovering that the near-wall air curtain can effectively reduce the wall deposition of droplets in the semidry FGD reactor, the effects of various spray characteristics, such as the droplet diameter, the spray angle, and the initial droplet velocity, on the wall deposition of droplets, were investigated to relate the applicability of the near-wall air to the various spray characteristics. The gas flow parameters were a primary air velocity U ) 4 m/s, a near-wall air velocity UW ) 12 m/s, and a near-wall air flow inlet height ZW ) 0 m. The spray characteristics were VOL. 41, NO. 12, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 10. Effect of various initial droplet velocities. a droplet diameter Dp ) 75 µm, an initial velocity Up ) 40 m/s and a spray angle θ ) 20°. The liquid density was 998.2 kg/m3 and the droplet mass flow rate was 0.03 kg/s. The gas flow parameters and two of the spray parameters were held constant while the other spray parameter was varied to investigate its effect on the droplet deposition ratio and the average droplet concentration along the reactor height. 3.2.1. Effect of Droplet Diameter. The variation of the droplet deposition ratio and the average droplet concentration along the reactor height for the various droplet diameters are shown in Figure 7. Figure 7 shows that the droplet deposition ratio increased rapidly and the average outlet droplet concentration decreased rapidly with increasing droplet diameter from 25 to 50 µm. The droplet deposition ratio and the average outlet droplet concentration then varied only a small amount in the droplet diameter range from 50 to 150 µm. The droplet deposition ratio then again increased rapidly and the average outlet droplet concentration decreased rapidly with increasing droplet diameter from 150 to 200 µm. The droplet concentration distributions in the center plane Y ) 0 m at various heights are shown in Figure 8 for four droplet diameters. Figure 8 shows that for droplet diameters of 25 and 50 µm, the droplet concentrations in the central region (near X ) 0 m) were higher than in the near-wall region (about X ) (0.16 m) at the various cross sections. The nonuniformity of the droplet concentration distribution gradually decreased with increasing height. However, the radial velocities of the smaller droplets decayed rapidly so that they favorably concentrated in the central region due to their small droplet diameters. Therefore, the droplet deposition ratio was lower and the average outlet droplet concentration was higher for the 25 µm droplets than for the 50 µm droplets. The droplet momentum increased and the radial velocity slowly decreased with increasing droplet diameter. The larger droplets diffused rapidly toward the reactor wall, which caused the maximum droplet concentration which was originally close to the center to move toward the near-wall region at higher cross sections. Figure 8 shows that the droplet concentration was almost zero between X ) (0.04 m for the 150 µm droplets. The maximum droplet concentration was located at X ) (0.06 m for Z ) 0.5 m. However, the droplet concentration in the near-wall region remained low due to the large reactor diameter of 0.3 m, which prevented a rapid increase of the droplet deposition ratio. With further increases in the droplet diameter to 200 µm, the maximum droplet concentration would be at about X ) (0.075 m for Z ) 0.5 m. The maximum droplet concentration for Z ) 1.0 and 1.5 m gradually moved toward the near-wall region, which rapidly induced droplet deposition. 4420

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The results show that the droplet deposition ratio varied little for droplet diameters between 50 and 150 µm with the near-wall air flow. Therefore, the near-wall air curtain can effectively reduce the wall deposition of droplets for a relatively wide range of droplet diameters in the semidry FGD processes. 3.2.2. Effect of Spray Angle. The variation of the droplet deposition ratio and the average droplet concentration along the reactor height for various spray angles are shown in Figure 9. Figure 9 shows that the droplet deposition ratio decreased slowly and then increased rapidly with increasing spray angle, while the average outlet droplet concentration increased and then decreased with increasing spray angle. For spray angles increasing from 10 to 60°, the droplet deposition ratio gradually decreased from 22 to 17.8%. Then for spray angles increasing from 60 to 90°, the droplet deposition ratio quickly increased from 17.8 to 36.5%. For spray angles less than 60°, the droplets were fully diffused across the cross section in a shorter vertical distance with increasing spray angle. The near-wall air flow momentum was still relatively strong to prevent the droplets from moving toward the reactor wall. For spray angles greater than 60°, the droplets’ inertial movement toward the reactor wall was enhanced by the increased spray angle. The near-wall air flow momentum was not strong enough to divert the droplets’ movement toward the reactor wall so the wall deposition of droplets increased rapidly. The results show that the droplet deposition ratio varied little for spray angles between 10 and 70°. Therefore, the near-wall air curtain can effectively reduce the wall deposition of droplets for a relatively wide range of spray angles in the semidry FGD process. 3.2.3. Effect of Initial Droplet Velocity. The droplet deposition ratio and the average droplet concentration along the reactor height for the various droplet initial velocities are shown in Figure 10. The results show that the droplet deposition ratio increased linearly with increasing initial droplet velocity, while the average droplet concentration decreased rapidly with increasing initial droplet velocity, even with the near-wall air flow. As the initial droplet velocity increased from 20 to 100 m/s, the droplet deposition ratio increased from 4.5 to 59%. The higher velocity droplets possessed larger momentum toward the reactor wall, which greatly increased the wall deposition of droplets and reduced the average outlet droplet concentration. Therefore, for semidry FGD processes even with the near-wall air flow, the initial droplet velocity should be carefully controlled to reduce the wall deposition of droplets and improve the operating stability.

Acknowledgments This research was supported by the Special Funds for Major State Basic Research Projects (no. 2006CB200305).

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Received for review August 19, 2006. Revised manuscript received April 2, 2007. Accepted April 10, 2007. ES062001I

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