Separation of Fine Particles from Gases in Wet Flue Gas

Feb 15, 2012 - Flue gas relative humidity at the inlet of the main tower can influence the separation of fine particles significantly. The collection ...
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Separation of Fine Particles from Gases in Wet Flue Gas Desulfurization System Using a Cascade of Double Towers Jingjing Bao,† Linjun Yang,†,* Shijuan Song,‡ and Guilong Xiong† †

Key Laboratory of Energy Thermal Conversion and Control, Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, China ‡ Xuzhou Municipal Engineering Design Institute Co., LED, Xuzhou 221002, Jiangsu, China ABSTRACT: Separation of fine particles from coal-fired flue gas by vapor heterogeneous condensation was investigated experimentally in a wet flue gas desulfurization (WFGD) system using a cascade of double towers. The supersaturated vapor environment can be achieved in two ways. The influences of flue gas relative humidity and the temperature difference between the flue gas and the desulfurization liquid at the inlet of the main tower on the collection efficiency were investigated in this paper. The results show that high collection efficiencies can be obtained by this separation process and the collection efficiency is higher than that of single tower WFGD system. Flue gas relative humidity at the inlet of the main tower can influence the separation of fine particles significantly. The collection efficiency is improved significantly by injecting steam among the rotatingstream-tray shutters and optimizing the temperature difference between the inlet flue gas of the main tower and the desulfurization liquid. The influences of temperature difference on the separation of fine particles include the following: the collection efficiency decreases with increases in the temperature of the desulfurization liquid (TL) in lower temperatures than that of the flue gas at the inlet of the main tower (TG), and the influence trend is contrary while TL is higher than TG. Particle collection efficiency also increases with increasing liquid/gas ratio in the main tower.

1. INTRODUCTION Fine particles (PM2.5) emitted from coal combustion have attracted more and more attention for their interesting sizedependent physical and chemical properties. They often consist of toxic components and can cause various environmental problems.1−6 In contrast to the efficient separation of micrometer particles from flue gas by conventional separators, such as venture scrubbers, electrostatic precipitators, and filters, highly efficient collection of fine particles from exhaust gas is difficult and expensive.7 Large amounts of these fine particles are emitted into the ambient air and give a great threat to human health.8 However, increasing clean air demands and their relevance with regard to the deposition of heavy metals and toxic components require efficient removal of these particles. Separation of fine particles from gases can be considerably simplified if the particles are first enlarged to a size of some micrometers or more by a preconditioning technique. One example for this is the heterogeneous condensation of water vapor with the fine particles acting as nucleation centers.1,9 The mechanism of this technique can be explained as follows. When the degree of supersaturation exceeds a critical value Scr, the fine particles can be activated and enlarged with high growth rates to droplets with a size of some micrometers. Then, these droplets can be efficiently collected by means of conventional separators. Therefore, the supersaturated water vapor environment is the key for the separation of fine particles by this preconditioning technique. Heterogeneous condensation of water vapor as a preconditioning technique for the separation of fine particles has been investigated for decades.10−13 The results show that fine particles can be enlarged by heterogeneous condensation and © 2012 American Chemical Society

efficiently separated from saturated warm gas streams in packed or sieve plate scrubbers trickled with cold water. In these investigations, the particles with a single component, such as quartz particles, paraffin oil droplets, residual particles, etc. were used as particle source. However, the separation of coal-fired fine particles by vapor heterogeneous condensation is investigated rarely. Since there is a great difference between the above particles and the coal-fired particles in the physicochemical characteristics, the existing research results can not applied to the separation of fine particles from coalfired flue gas directly. Because of the low moisture content of coal-fired flue gas, high energy consumption is necessary to establish the supersaturated vapor phase. The technical route used in these works is not suitable for coal-fired flue gas. Moreover, except the separation of submicrometer particles, the removal of SO2 is not mentioned in these investigations. As mentioned above, the key of fine particle separation by vapor heterogeneous condensation is the establishment of the supersaturated vapor phase. However, because of the low moisture content (5−8%) and high temperature of the initial coal-fired flue gas, the necessary energy consumption would be too high to achieve a supersaturated vapor phase by injecting water vapor or being cooled simply.14 Hence, it would be more practical to apply this technique to the flue gas with higher moisture content. At present, most large coal-fired power plants are equipped with wet flue gas desulfurization (WFGD) systems downstream to an electrostatic precipitator. In the WFGD process, the hot flue gas contacts normal temperature Received: November 30, 2011 Revised: February 15, 2012 Published: February 15, 2012 2090

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Figure 1. Schematic diagram of experimental setup.

2. EXPERIMENTAL DETAILS

desulfurization liquid. Simultaneous mass and energy transfer takes place, causing the gas phase temperature to decrease and the liquid phase to evaporate, entering the gas phase. Consequently, the relative humidity of flue gas after scrubbing can be greatly raised and approaches saturation with water vapor (relative humidity ≥90−95%) at a temperature of about 45−60 °C. Thus, it is possible to produce a supersaturated vapor phase with lower energy consumption by injecting a small amount of steam or lowering the temperature of scrubbed flue gas by 2−5 °C.15,16 Separation of fine particles by adding steam in the gas inlet and outlet of the desulfurization tower in the single tower WFGD system was investigated in the previous work.14,17,18 Because of the competitive condensation between the fine particles and the droplets of desulfurization liquid in the inner of scrubber, it is found that the necessary energy consumption is higher for vapor heterogeneous condensation in the SO2 scrubbing zone. For the latter case, since the size of grown droplets is still smaller than the cutting diameter of wire mesh demister, the collection efficiency of fine particles needs to be improved. The WFGD system using a cascade of double towers has been more and more widely used in order to obtain a higher desulfurization efficiency. 19 Therefore, a novel process combining vapor heterogeneous condensation and the WFGD system, using a cascade of double towers to separate fine particles from coal-fired flue gas is presented in this paper. A spray scrubber and a rotating-stream-tray scrubber were used as the prescrubber and the main desulfurization tower, respectively. The supersaturated vapor phase was achieved by injecting water vapor among the rotating-stream-tray shutters or optimizing the temperature difference between the inlet flue gas of the main tower and the desulfurization liquid. The condensation grown droplets were effectively collected by desulfurization liquid and wire mesh demister. Three test conditions, which can be described as unsaturated (relative humidity (RH) ≈ 60%), saturated (RH ≈ 100%), and supersaturated (supersaturation (S) ≈ 1.20) inlet flue gas of the main tower, were investigated.

2.1. Experimental Setup. The experimental setup is schematically shown in Figure 1. Flue gas with volume flux of 150 Nm3·h−1 was generated by a coal-fired boiler. Anthracite was used in these experiments. A stirrer and an electric heater were installed in the buffer vessel to ensure constant particle concentration and size distribution and regulate the temperature of flue gas. In the cyclone, large particles were separated from the flue gas. Heat preservation was used for the buffer, the cyclone, and the pipelines. After passing through the cyclone, the flue gas was pressurized by a booster fan, and then, it entered the double towers in sequence, where it passed countercurrent to the scrubbing liquid and desulfurization liquid. Both towers were made of polycarbonate pipes and plates with excellent heat resistance. The diameter and height of the towers were 150 mm and 3600 mm, respectively. The spray scrubber was used as a prescrubber, which included 2250 mm of scrubbing zone and 1350 mm of phase transition chamber. Pure water was used as scrubbing liquid. There was a corrugated plate demister above the scrubbing zone to separate the entrained droplets from the scrubbed flue gas. A high-efficiency wire mesh demister was installed at the top of the phase transition chamber to collect the preliminary grown droplets in the case of S ≈ 1.20. The rotating-stream-tray scrubber was used as the main desulfurization tower, where three pieces of rotating-stream-tray shutters were installed. In addition, a high-efficiency wire mesh demister was installed at the top of this tower to separate the grown droplets. The grade removal efficiency of fine particles by this highefficiency wire mesh demister is illustrated in Figure 2.

Figure 2. Grade removal efficiency of the high-efficiency wire mesh demister. 2091

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The experiments were carried out at a initial particle number concentration (CN) of (1.0−2.0) × 107 cm−3 and a superficial gas velocity of 2.4 m/s. Na2CO3 was used as a SO2 absorbent to simulate the double-alkali desulfurization processes. Desulfurization liquid was circulated with a recycle ratio of near 100%. The flue gas temperature at the inlet of the prescrubber was in the range 110−160 °C. A supersaturated vapor phase in the main tower was achieved by injecting steam among the rotating-stream-trays shutters or optimizing the difference between the inlet temperature of the flue gas and the desulfurization liquid of the main tower. The water vapor was generated simultaneously by an electric steam generator and an oilfired steam generator and mixed in the mixer, in which the pressure and temperature of steam can be kept stable. The flue gas relative humidity (RH) or supersaturation (S) degree at the inlet of the main tower was controlled by adjusting the liquid/gas ratio (L/G) of the prescrubber and injecting appropriate steam into the phase transition chamber. The temperature difference between the inlet flue gas of the main tower and the desulfurization liquid was controlled by changing the desulfurization liquid temperature with a constant temperature of inlet flue gas. 2.2. Measurement Technique. The size distribution and concentration of fine particles were measured in real time by means of an electrical low pressure impactor (ELPI, Dekati Ltd., Finland), which has 13 stages (12 channels). The cutoff diameter of each stage

3. RESULTS AND DISCUSSION 3.1. Coal-Fired Fine Particles. Flue gas with fine particles was generated by a coal-fired boiler burning anthracite. Figure 3

Table 1. Cutting Diameter and Median Diameter of ELPI stage

cutting diam., D50(μm)

median diam., Di(μm)

1 2 3 4 5 6 7 8 9 10 11 12 13

0.023 0.030 0.050 0.098 0.211 0.317 0.576 0.891 1.505 2.243 3.758 6.285 9.314

0.026 0.039 0.070 0.144 0.259 0.427 0.716 1.158 1.837 2.903 4.860 7.651

Figure 3. Size distribution of fine particles: (a) number and mass concentration; (b) cumulative number and mass distribution.

illustrates the size distribution of these fine particles for number and mass concentration. It can be seen that these coal-fired fine particles display unimodal distribution in the number concentration, which may be attributed to the combustion condition, the type of coal, and the boiler employed. Moreover, they were mostly in the submicrometer size range, with maximum concentration at 0.07 μm. Most of these particles (a proportion of more than 99%) were smaller than 0.20 μm. A few particles larger than 1.0 μm were discovered. As can also be seen from Figure 3, the mass concentration concentrates on the particles larger than 1−2 μm, and the median value of the size distribution was 5.0 μm for the cumulative mass concentration. Since the mass of fine particles is small, number concentration would reflect the influence of these particles on atmospheric environment and human health better than mass concentration. In the paper, the initial particle number concentrations in flue gas exhausted from coal-fired boiler were in the order of (1.0− 2.0) × 107 cm−3. 3.2. Separation of Fine Particles without Steam Injection in the Phase Transition Chamber. Two test conditions, in which the inlet flue gas of the main tower is unsaturated (RH ≈ 60%) and saturated (RH ≈ 100%), were involved to investigate the influence of flue gas relative humidity on the separation of fine particles. There exists simultaneous mass and energy transfer in the gas−liquid contacting process in the prescrubber, which causes the gas phase temperature to decrease and the liquid phase to evaporate and then enter the gas phase. Consequently, the relative humidity of flue gas at the inlet of the main tower can be greatly raised, and it can be controlled by changing L/G of the prescrubber. Figure 4 gives the relative humidity of outlet

and median diameter of each channel are shown in Table 1. The Dp used in the following figures is the median diameter, which divides the cumulative number concentration distribution in half, 50% of the particle number has particles with a larger diameter, and 50% of the particle number has particles with a smaller diameter. Because of the high moisture content of the sample gas stream, a special sampling setup was necessary for this measurement.20 The sample gas was routed through an isokinetical sampling gun to a cyclone and then two-stage diluted with particle free, hot dry air (150 °C, dilution ratio 67:1) before entering the ELPI measurement system. In the cyclone, the particles with aerodynamic diameter larger than 9.314 μm were separated. Condensation of water vapor on the wall of sampling pipelines and on the impact plate of the ELPI could be avoided by diluting with hot air. To estimate the error of measured values, repeated experiments were done in this research. The test results of repeated experiments show that the relative error was mostly in the range 4∼8%. Since the thermodynamic state of the flue gas and the water vapor is important for the mass and energy transfer, their corresponding temperatures, pressures, and humidities were measured. Temperatures and relative humidities were measured by means of the HMT337 humidity transmitter (Vaisala Oyj., Finland). The volume flow velocities of flue gas were measured with pitot tube. 2092

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Table 2. Total Collection Efficiencies by the Main Tower and Double-Towers of the Cascade System for the Three Conditions (RH ≈ 60%, RH ≈ 100%, S ≈ 1.20) condition

main tower

double-towers

RH ≈ 60% RH ≈ 100% S ≈ 1.20

25.71 39.64 26.73

35.73 47.97 65.35

main tower (ηi,m) and double-towers (ηi,d) of the cascade system is defined as ηi ,m =

Figure 4. Flue gas relative humidity (RH) and temperature at the outlet of prescrubber as a function of L/G.

flue gas of the prescrubber (that is inlet flue gas of the main tower) as a function of L/G. It can be observed that the relative humidity is in the range 60−100% depending on the L/G, and the temperature of flue gas at the inlet of the main tower is in the range 50−60 °C. With an increase of L/G, the relative humidity of outlet flue gas increases and the temperature decreases. In this work, the test conditions of RH ≈ 60% and RH ≈ 100% were obtained while the liquid/gas ratios of the prescrubber were 5 L/Nm3 and 20 L/Nm3. Figure 5 illustrates

ηi ,d =

(Ni ,m − Ni ,t) Ni ,m

(Ni ,0 − Ni ,t) Ni ,0

× 100 (1)

× 100 (2)

where Ni,0, Ni,m, and Ni,t are the number concentrations of fine particles in the ith size range of the initial flue gas, inlet flue gas of the main tower, and outlet flue gas of the cascade system of double towers, respectively. As can be seen from Figure 5a, with an increase in the flue gas relative humidity, the separation of fine particles by the main tower improves. For the case of RH ≈ 60%, fine particles were mainly separated either by inertial or diffusional forces, depending on the size of the particles to be separated during the gas−liquid contacting process in the main tower. In contrast to the efficient collection of particles larger than 3.0− 5.0 μm, the fine particles can not be separated by the WFGD system. As shown in Figure 3, the coal-fired particles used in these experiments were mostly in the submicrometer size range with a maximum concentration at 0.07 μm. Hence, a lower collection efficiency of about 26% was obtained in this condition. The collection efficiency increases significantly for the condition of RH ≈ 100%. In this case, the flue gas with a higher relative humidity of about 100% countercurrent contacts desulfurization liquid in the main tower. Simultaneous mass and energy transfer took place in this scrubber, causing a further increase in moisture content of flue gas. Therefore, a supersaturation vapor phase can be achieved in the inner of the main tower. Then, some fine particles can be activated and grow to larger droplets, which then can be efficiently separated by desulfurization liquid and wire mesh demister. However, for the particles smaller than 0.01 μm, the grown droplets are still in the range 0.01−0.05 μm and cannot be collected by desulfurization liquid and wire mesh demister. Therefore, the grade collection efficiency of fine particles in the range 0.01− 0.05 μm appeared negative and lower than that of RH = 60%. In Figure 5b, since a few fine particles can be collected by the prescrubber, it appears that the collection efficiency of fine particles improve slightly in the double-towers of the cascade system. In addition, it increases with the relative humidity of flue gas at the inlet of the main tower. The higher flue gas relative humidity is beneficial to the establishment of supersaturation vapor phase, which would improve the vapor heterogeneous condensation and separation of fine particles. Figure 5 also indicates that the grade collection efficiency first increases and then tends to be constant with particle size. On the one hand, the larger particles can be collected by inertial force more easily. On the other hand, according to the classical heterogeneous condensation theory of Fletcher,21,22 the critical supersaturation (Scr) of partially wettable, insoluble spherical

Figure 5. Grade collection efficiencies by the main tower and doubletowers of the cascade system for the conditions of RH ≈ 60% and RH ≈ 100%: (a) main tower; (b) double-towers.

the number grade collection efficiency of fine particles by the main tower and double-towers of the cascade system for these two conditions. The corresponding total collection efficiencies are given in Table 2. The L/G of the main desulfurization tower is 4 L/Nm3. The temperature of flue gas at the inlet of the main tower and desulfurization liquid are 50 and 30 °C respectively. The number grade collection efficiency by the 2093

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ment of collection efficiency is more obvious for the condition of RH ≈ 60% than that of RH ≈ 100%, with an increase of more than 30% at different liquid/gas ratios. For this case, the supersaturation vapor phase necessary for the vapor heterogeneous condensation can not be achieved in the main tower without steam injection. Fine particles are collected only by the scrubbing action, such as the force of thermophoresis and diffusiophoresis. However, the supersaturation vapor phase can be achieved in the main tower by injecting steam among the rotating-stream-tray shutters. Fine particles grow to larger droplets and then are collected efficiently by desulfurization liquid and wire mesh demister. For the condition of RH ≈ 100%, the supersaturation vapor phase can be achieved in the space between the rotating-stream-tray shutters when there was no steam injection. Some fine particles can grow to droplets and then be collected by inertial force. Particle collection efficiencies for this condition are higher than those of the RH ≈ 60% condition, as shown in Figure 6a. The residual particles in the flue gas are smaller. According to the classical heterogeneous condensation theory of Fletcher,21,22 these residual particles with smaller diameter are more difficult to be activated. Fewer residual particles can be activated and grow to larger droplets by injecting the same amount of steam among the rotating-stream-tray shutters. In addition, as the results in Figure 5b indicate, particle collection efficiency increases with the relative humidity of flue gas at the inlet of the main tower for the both cases without steam injection and injecting steam among the rotating-stream-tray shutters. Optimizing the temperature difference between the inlet flue gas and the desulfurization liquid of the main tower is another alternative way to obtain the supersaturation vapor environment in the inner of the main tower. In this work, the temperature difference was controlled by changing the desulfurization liquid temperature (TL) while the temperature of inlet flue gas (TG) was kept constant. Figure 7 illustrates the

particles is inversely proportional to the particle size. Hence, the smaller particles are comparatively difficult to be activated and grow to lager droplets. In contrast, the larger particles are more easily activated, which is beneficial for separating them by vapor heterogeneous condensation. As mentioned above, a few fine particles can be separated in the double-towers cascade WFGD system. The collection efficiency is no more than 48%, as shown in Table 2. In general, the collection efficiency of particles larger than 3.0−5.0 μm can be more than 70−80% by the WFGD system. Hence, the simultaneous efficient separation of coal-fired fine particles and SO2 can be realized if the fine particles grown to larger droplets by vapor heterogeneous condensation. Although a supersaturation vapor phase can be achieved in the main tower as a result of the mass and energy transfer between the gas and desulfurization liquid for the condition of RH ≈ 100%, the supersaturation degree is not enough for activating and enlarging all the fine particles. Only a small part of particles with larger size was activated and grew to larger droplets. Therefore, it is necessary to further increase the supersaturation degree by injecting water vapor among the rotating-stream-tray shutters or optimizing the temperature difference between the inlet flue gas of the main tower and the desulfurization liquid. Figure 6 shows a comparison of the total number collection efficiency of fine particles by the double-towers of the cascade

Figure 7. Total collection efficiency as a function of the desulfurization liquid temperature for the conditions of RH ≈ 60% and RH ≈ 100%.

total collection efficiency of fine particles by the double-towers of the cascade system as a function of desulfurization liquid temperature. In these experiments, the temperature of flue gas at the inlet of the main tower is about 50 °C, and the liquid/gas ratio of the main tower is 4 L/m3. It is obvious that the separation of fine particles can be improved by increasing the temperature difference in the following two ways. When the desulfurization liquid temperature is lower than 50 °C, the decrease of desulfurization liquid temperature is beneficial to the separation of fine particles. In this temperature range, the temperature of desulfurization liquid is lower than that of flue

Figure 6. Total collection efficiencies by the double-towers of the cascade system for the conditions of RH ≈ 60% and RH ≈ 100%: (a) without steam injection; (b) steam injected among the rotatingstream-tray shutters.

system for the cases of without steam injection and with 0.04 kg/m3 steam injected among the rotating-stream-tray shutters. The temperature of flue gas at the inlet of the main tower and desulfurization liquid are 50 and 30 °C, respectively. It can be seen that particle collection efficiency improves by injecting steam among the rotating-stream-tray shutters. The improve2094

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droplets by rotational ascending flue gas in the space between two pieces of rotating-stream-tray, which would lead to an increase of gas−liquid interfacial area significantly. Hence, a better effectiveness of gas−liquid mass and energy transfer is achieved.24 Moreover, a supersaturation vapor phase can be achieved in the interior of the rotating-stream-tray scrubber with 0.04 kg/m3 steam injecting between the rotating-streamtray shutters, which cause an enlargement of fine particles due to heterogeneous condensation. These fine particles and the grown droplets in the flue gas would adhere to the atomization droplets in the gas−liquid contacting process and can be collected by the centrifugal force caused by the rotating gas stream and droplets. In addition, some fine particles and droplets can also be separated by the wire mesh demister at the top of the scrubber. Figure 8 also shows that the double-towers cascade system has advantages in the separation of fine particles in comparison to the single tower system. Taking the case of L/G = 6 L/Nm3 as an example, the collection efficiency of fine particles by the double-towers cascade system is about 79%, higher than that of the single tower system, which is 55%. On the one hand, some fine particles can be collected in the prescrubber. On the other hand, there exists simultaneous mass and energy transfer in the gas−liquid contacting process in the prescrubber, causing the gas phase temperature to decrease and the liquid phase to evaporate, entering the gas phase. Consequently, the relative humidity of flue gas can be greatly raised. In these experiments, the relative humidity of outlet flue gas of the prescrubber is in the range 60−100%, depending on the L/G, as shown in Figure 4. Therefore, the supersaturation degree of flue gas in the inner of the main desulfurization tower can be much higher than that of single tower system at the same amount of steam injection. Hence, more particles can be activated and grow to larger droplets, which can be collected efficiently by the desulfurization liquid and wire mesh demister. Moreover, the final diameter of grown droplets is much larger, which is also beneficial to the separation of fine particles. 3.3. Particle Collection Efficiency with Steam Injection in the Phase Transition Chamber. A third test condition, which can be described as supersaturation (S ≈ 1.20) of the inlet flue gas of the main desulfurization, was carried out in this work. There were two times of heterogeneous condensation and separation of fine particles in this condition. It was achieved by keeping L/G = 20 L/Nm3 in the prescrubber and injecting 0.04 kg/m3 steam in the phase transition chamber simultaneously. The number grade collection efficiencies of fine particles by the main tower and double-towers of the cascade system without steam injection among the rotating-stream-tray shutters are given in Figure 9, and the corresponding total collection efficiencies are given in Table 2. The liquid/gas ratio (L/G) of the main tower is 4 L/Nm3. The temperature of flue gas at the inlet of the main tower and desulfurization liquid are 50 and 30 °C, respectively. It is observed that the distribution of grade collection efficiency is different from that of the conditions of RH ≈ 60% and RH ≈ 100% in Figure 5. As can be seen from Figure 9, the grade efficiencies decrease with increasing the particle size for particles smaller than 0.1 μm. There is a lowest level or even a negative value in the size range 0.1−1.0 μm. This can be explained as follows: before entering in the main tower, flue gas is supersaturated with water vapor in the phase transition chamber. Some fine particles can be activated and the water vapor condenses heterogeneously on their surface, which would cause these particles to grow to lager

gas at the inlet of the main tower. With decreasing the desulfurization liquid temperature, the temperature difference between gas and liquid increases, which could then lead to the increase of the thermophoresis and diffusiophoresis force, and so improve the separation of fine particles. On the other hand, the effectiveness of heat and mass transfer enhances with decreasing desulfurization liquid temperature, which would be beneficial to decrease the temperature of scrubbed flue gas and establish a supersaturation vapor phase with higher supersaturation degree in the scrubber. According to Kelvin equation, with the same moisture content of flue gas, the supersaturation degree mainly depends on the temperature. Hence, the supersaturation degree and supersaturated region both improve with decreasing scrubbed flue gas temperature, which would cause more particles to be activated and grow to larger droplets. However, the influence trend is contrary when the temperature of desulfurization liquid is higher than 50 °C. In this case, the influences of desulfurization liquid temperature on particle collection efficiency include three aspects. First, the temperature difference increases with desulfurization liquid temperature, which would intensify the mass and energy transfer. Thus the thermophoresis and diffusiophoresis scrubbing action improve. Second, with an improvement of Tl and the mass and energy transfer, more desulfurization liquid evaporates and enters the gas phase. This is beneficial to increase the supersaturation degree and supersaturated region in the main tower. Last, the temperature of scrubbed flue gas increases with liquid temperature. According to Kelvin equation, the supersaturation degree of scrubbed flue gas decreases. As we can see from the testing results, the former two reasons predominate in this temperature range. Separation of fine particles by heterogeneous condensation of water vapor in the single rotating-stream-tray scrubber was investigated in the previous work.23 Figure 8 gives the number

Figure 8. Particle collection efficiency by the single tower and doubletowers cascade WFGD system.

collection efficiencies of fine particles by single tower and double-towers cascade WFGD system as a function of L/G of the main tower. In the single tower system, only a rotatingstream-tray scrubber was used as desulfurization tower. For the case of double-towers cascade WFGD, the liquid/gas ratio (L/ G) of the prescrubber was 20 L/Nm3. The measurements were carried out at a flue gas temperature at the inlet of the main tower of 50 °C and a desulfurization liquid temperature of 30 °C, and 0.04 kg/m3 steam was injected among the rotatingstream-tray shutters. It can be seen that some fine particles can be removed in the single tower system, with the collection efficiency in the range 40−55%. In the rotating-stream-tray scrubber, the descending liquid is repeatedly atomized to 2095

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flue gas at the inlet of the main tower and desulfurization liquid are 50 and 30 °C, respectively. It can be seen that the collection efficiency improved by injecting a small amount of steam among the rotating-stream-tray shutters. For example, it can increase from 59% to 80% by injecting 0.04 kg/m3 steam at an L/G of 4 L/m3. In the condition of S = 1.20, since some fine particles grow to larger droplets and are collected by the wire mesh demister in the phase transition chamber, the residual particles in the inlet flue gas of the main tower are smaller. These residual particles with smaller diameter are more difficult to activate. Fewer residual particles can be activated and grow to larger droplets by injecting the same amount of steam among the rotating-stream-tray shutters. Hence, the improvement of collection efficiency by adding the same amount of steam is not obvious in comparison to the conditions of RH ≈ 60% and RH ≈ 100%. However, in comparison with the initial flue gas, the total collection efficiency by the double-towers of the cascade system for the both cases (without steam injection and 0.04 kg/ m3 steam injected among the rotating-stream-tray shutters) is the highest in these three test conditions. In the WFGD system, the liquid/gas ratio (L/G) is an important operational parameter that can influence SO2 removal and gas−liquid mass and energy transfer characteristics. As can also be seen from Figures 6 and 10, particle collection efficiency increased with the liquid/gas ratio of the main tower for the three test conditions. A higher liquid/gas ratio intensifies the energy and mass transfer between the desulfurization liquid and the gas stream. For example, with an increase of liquid/gas ratio from 2.0 to 6.0 L Nm3−, the collection efficiency increased from 24% to 43% for the condition of RH ≈ 60% without steam injection. Moreover, with an improvement of gas−liquid energy and mass transfer, more liquid phase evaporates and enters the gas phase, which causes a lower temperature and a higher moisture content of flue gas. Therefore, the supersaturation degree of the vapor phase increases, and particle enlargement by heterogeneous condensation is enhanced, which is also beneficial to the separation of fine particles. However, the curve of particle collection efficiency becomes flat while the liquid/gas ratio increases from 5.0 to 6.0 L·Nm3−. It is found that a liquid/gas ratio of 5.0 L·Nm3− is an optimum value. Figure 11 gives the total collection efficiency of fine particles by the double-towers of the cascade system as a function of

Figure 9. Grade collection efficiencies by the main tower and doubletowers of the cascade system for the condition of S ≈ 1.20.

droplets of some micrometers. Then, these droplets can be collected by the high-efficiency wire mesh demister at the top of the phase transition chamber. The particles permeating the wire mesh demister are mostly in a smaller size range. Since a supersaturation vapor phase also can be achieved in the interior of the main tower, these residual submicrometer particles and droplets may grow to larger droplets by vapor heterogeneous condensation in this tower. Therefore, there are two times of vapor heterogeneous condensation, both in the phase transition chamber and in the inner of the main tower. More submicrometer particles smaller than 0.1 μm grow to larger droplets because of the two occurrences of vapor heterogeneous condensation. However, these grown droplets are mostly in the range 0.1−1.0 μm and cannot be removed efficiently by desulfurization liquid and wire mesh demister, which causes a lowest level or even a negative value of grade collection efficiency in this size range. This also can explain the interesting phenomena that the particle collection efficiencies by the main tower for this condition are obviously lower than that of the condition of RH ≈ 100%, as shown in Table 2. Nevertheless, the collection efficiencies by the double-towers for this condition are the highest. The collection efficiency of fine particles for this condition also can be improved in two ways: injecting some steam among the rotating-stream-tray shutters and optimizing the temperature difference between the inlet flue gas of the main tower and the desulfurization liquid. The curves of particle collection efficiency by the double-towers of the cascade system for the cases of without steam injection and 0.04 kg/m3 steam injected among the rotating-stream-tray shutters are illustrated in Figure 10 as a function of liquid/gas ratio (L/G). The temperature of

Figure 11. Total collection efficiency as a function of the desulfurization liquid temperature for the condition of S ≈ 1.20.

desulfurization liquid temperature for this condition. The Figure 10. Total collection efficiencies by the double-towers of the cascade system for the condition of S ≈ 1.20.

temperature of flue gas at the inlet of the main tower is about 50 °C, and the liquid/gas ratio of the main tower is 4 L/m3. 2096

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The same influence law as the conditions of RH ≈ 60% and RH ≈ 100% was found for this test condition. The separation of fine particles can be improved by increasing the temperature difference. However, it can be found that the improvement of particle collection efficiency is more significant by injecting steam than that of optimizing the temperature difference by comparing the results in Figures 10 and 11.

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4. CONCLUSIONS Separation of fine particles from flue gas by vapor heterogeneous condensation in WFGD system using a cascade of double towers is presented. The supersaturated vapor environment was achieved by injecting water vapor among the rotating-stream-tray shutters in the main tower or optimizing the temperature difference between the inlet flue gas of the main tower and the desulfurization liquid. The grown droplets are removed efficiently by desulfurization liquid and highefficiency wire mesh demister. Three test conditions, in which the inlet flue gas of the main tower is unsaturated (RH ≈ 60%), saturated (RH ≈ 100%), and supersaturated (S ≈ 1.20), were investigated. The results show that heterogeneous condensation in combination with WFGD system using a cascade of double towers is a highly efficient technique for the separation of fine particles, with higher collection efficiency than that of a single tower WFGD system. The relative humidity of inlet flue gas of the main tower can influence the separation of fine particles significantly. Particle collection efficiency can be improved significantly by injecting steam among the rotatingstream-tray shutters and optimizing the temperature difference between the inlet flue gas of the main tower and the desulfurization liquid. Because of the enhancement of energy and mass transfer between the desulfurization liquid and the gas stream, particle collection efficiency also increases with increasing liquid/gas ratio of the main tower.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86 25 83795824. Fax: +86 25 83795824. E-mail: ylj@ seu.edu.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the High-Tech Research and Development Program of China (No. 2008AA05Z306), the Natural Science Foundation of Jiangsu Province (No. BK2008283), and the Scientific Research Foundation of Graduate School of Southeast University for their financial support.

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REFERENCES

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dx.doi.org/10.1021/ef201868q | Energy Fuels 2012, 26, 2090−2097