Combined effect of acoustic agglomeration and vapor condensation on fine particles removal

Chemical Engineering Journal 290 (2016) 319–327

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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Combined effect of acoustic agglomeration and vapor condensation on fine particles removal Jinpei Yan a,⇑, Liqi Chen a, Linjun Yang b a b

Key Laboratory of Global Change and Marine-Atmospheric Chemistry, Third Institute of Oceanography, SOA, Xiamen 361005, PR China School of Energy and Environment, Southeast University, Nanjing 210096, PR China

h i g h l i g h t s
The combined effect of acoustic agglomeration and vapor condensation.
Fine particles are effectively captured in the coupling fields.
High removal efficiency is obtained in a low intensity acoustic.
Sound pressure level has a significant affect on the efficiency.
Removal effects depend on supersaturation degree in acoustic field.

a r t i c l e

i n f o

Article history: Received 27 September 2015 Received in revised form 8 December 2015 Accepted 22 January 2016 Available online 28 January 2016 Keywords: Fine particle Vapor condensation Acoustic wave Agglomeration Removal

a b s t r a c t A novel preconditioning process using the combined effect of acoustic agglomeration and vapor condensation for fine particles removal with high efficiency was presented. The effect of operation parameters on the enlargement and removal of fine particles were investigated experimentally. Particle size distribution and number concentration with and without external fields were measured by Electrical Low Pressure Impactor (ELPI). The results showed that the stage removal efficiency of fine particles was about 10–23% by acoustic agglomeration only with sound pressure level (SPL) of 150 dB. However, it was significantly improved by the combined effect of acoustic agglomeration and vapor condensation, reaching up to 53–80% with a SPL of 150 dB and supersaturation degree (S) of 1.2. Fine particle entrainment factor in acoustic field increased with the supersaturation degree, as well as the removal efficiency. While the supersaturation degree was lower than 1.0, the removal efficiency was extremely low, and increased slightly with the supersaturation degree. However, removal efficiency increased with the supersaturation degree rapidly when the supersaturation degree was higher than 1, which was improved by about 50% as the supersaturation degree increased from 1.0 to 1.4. The coupling external fields cannot be formed when the supersaturation degree was lower than the critical one, resulting in low removal efficiency. Removal efficiency was increased substantially to 63% in the supersaturation degree of 1.2, even in a low SPL of 130 dB. It indicates that high removal efficiency can be obtained in the combined effect of acoustic agglomeration and vapor condensational growth even in low intensity acoustic field. Ó 2016 Elsevier B.V. All rights reserved.

1. Introduction Fine particles have recently gained much attention because of their relevance with the deposition of toxic components and serious public health concern [1,2]. Continuous increasing consumption of energy leads to more and more inhalable particulate matter emissions [3,4]. Consequently, large quantity of fine particles was emitted into the ambient air, and they have been regarded ⇑ Corresponding author. Tel.: +86 592 2195370. E-mail address: [email protected] (J. Yan). http://dx.doi.org/10.1016/j.cej.2016.01.075 1385-8947/Ó 2016 Elsevier B.V. All rights reserved.

as the major air pollutant in cities. The separation of fine particle has become important because of increasing clean air demands. However, the efficient separation of fine particles is difficult and expensive. They can be efficiently separated with conventional devices, if they are first enlarged by means of a preconditioning technique. Such preconditioning process can be the agglomeration effect or heterogeneous condensational growth of vapor on the surfaces of fine particles. Acoustic agglomeration is considered to be a useful technique for fine particles removal. High-intensity sound wave enforces the relative motion among fine particles to produce efficient

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Nomenclature A C da dp F Fp Fv f I k Kc p r S Scr

acoustic wave amplitude Cunningham correction coefficient average particle diameter, lm particle diameter, lm resultant force of particles, N pressure gradient force, N Stokes viscous force, N acoustic frequency, Hz sound pressure level, dB Boltzmann constant, JK1 kinetic coefficient sound pressure, Pa particle radius, lm supersaturation degree critical supersaturation degree

collisions. Several theoretical descriptions for aerosol particle dynamics in acoustic fields have been investigated [5–8]. Experimental studies and numerous computational simulations have indicated that acoustic agglomeration significantly shifted the particle size distribution, from smaller to larger sizes in an acoustic field and reduced the particle number concentration [9–16]. But high value of sound pressure level (SPL) was needed to achieve a considerable removal efficiency of fine particles. Recently, the combined effect of acoustic wave with other external field was investigated [17,18], which showed that the removal efficiency of fine particles could be improved by the coupling effect of gas jet and acoustic wave or adding seed particles in acoustic field [19,20]. Therefore, the combination of acoustic wave with other external field is considered to be a novel preconditioning process for particle separation. High particle agglomeration efficiency with low sound pressure level was required for acoustic agglomeration application. Investigations showed that heterogeneous condensation of water vapor with fine particles acting as nucleation centers was an effective method for particle growth [21–27]. The condensational growth and removal of submicron particles were confirmed, but high water vapor consumption to achieve the high supersaturation degree (defined as the ratio of the partial pressure of water vapor to its equilibrium vapor pressure) was the key problem [28,29]. Particles agglomeration and condensational growth could be achieved when supersaturation water vapor was introduced into an acoustic wave field. Under the combined effect of acoustic wave and vapor condensation, fine particles agglomeration and condensational growth occurred simultaneously. The agglomeration of fine particles by acoustic wave was useful for the condensational growth of supersaturation vapor on the surfaces of fine particles. On the other hand, the difference of aerosol size between fine particles and new born droplets promoted the particles collision and agglomeration in the acoustic field. Although the acoustic agglomeration and condensational growth have been extensively investigated, the combined effect of acoustic agglomeration and vapor condensational growth has not been elucidated. The aim of this study is to contribute to the fundamental knowledge of the coupling effect of acoustic agglomeration and vapor condensation. Water vapor was introduced into the acoustic agglomeration chamber to form the coupling external fields. The effects of operation parameters on fine particle removal were investigated experimentally in the coupling intensification effect of acoustic agglomeration and vapor condensation.

T u up Uo Up Vw

lg

ka k

x s qp reg

gas temperature, K air amplitude velocity, ms1 particle velocity, ms1 air velocity amplitude particle velocity amplitude embryo volume, m3 gas dynamic viscosity, Pas molecular mean free path, m acoustic wave length, m angular frequency of acoustic wave, rads1 relaxation time of the particle, s particle density, kgm3 interfacial tensions of the embryo-gas, Nm1

2. Experiment sections 2.1. Experimental setup The schematic experimental design is shown in Fig. 1. The experimental setup comprised a fluidized bed aerosol generator, a buffer chamber, an acoustic agglomeration chamber and a settling chamber. Fine particles from coal fired were generated used a fluidized bed aerosol generator with a gas flow rate of 50 L/ min. The flue gas then passed through a buffer chamber, in which an electric heater and water vapor adding device were located. Water vapor was generated by an electric water vapor generator (LB-7.5D, Lanbao Co. Ltd., China). Fine particles were enlarged in the acoustic agglomeration chamber. A settling chamber with a length of 350 mm, width of 200 mm and height of 150 mm was used to separate the grown particles. At the exit of the settling chamber, an Electrical Low Pressure Impactor (ELPI, Dekati Co. Ltd., Finland) was employed to measure particle size distribution in real time with and without external fields. The ELPI was described in detail previously (www.dekati.fi). A brief description about the ELPI is given here. ELPI is suitable for application where a wide size rang and fast response times are required. The gas sample containing the particles is first sampled through a unipolar corona charger. The charged particles then pass into a low pressure impactor with electrically isolated collection stages. The impactor operating principle is based on particle size classification according to the aerodynamic diameter of the particles. The sampling flow rate is 10 LPM and the measuring size range is 0.023–9.314 lm of aerodynamic diameter. The supersaturation degree achieved in the acoustic agglomeration chamber was controlled by the flue gas temperature and water vapor addition. According to the flue gas temperature and humidity detected at the outlet of buffer chamber, the supersaturation degree and temperature needed in the acoustic agglomeration chamber, the amount of water vapor required in the experiment was first calculated. Then the mass flow rates of vapor and gas temperature were regulated by the vapor mass flow meter and heater. A sampling gas stream was withdrawn from main gas stream and diluted when it was routed into the ELPI. Since the sampling gas stream contained high moisture, vapor might condense on the sampling pipe and the impact plates of ELPI. To avoid the condensation of water vapor, the sampling gas was heated and diluted with particle free and hot dry air (dilution ratio was well defined of 8.18:1) prior to entering the ELPI measurement system. Heat

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Fig. 1. Schematic diagram of experimental system. (1) fluidized gas, (2) fluidized bed aerosol generator, (3) heater, (4) buffer chamber, (5) water vapor generator, (6) mass flow meter, (7) agglomeration chamber, (8) separation chamber, (9) horn, (10) power amplifier, (11) signal generator, (12) reflecting plate, (13) ELPI, (14) cool/hot water inlet, (15) water outlet, (16) gas exhaust.

preservation measures were also taken for the entire pipeline to prevent vapor condensation.

160

120dB

2.2. The acoustic agglomeration chamber

2.3. Distribution of sound pressure level in the agglomeration chamber Fig. 2 shows the variation of sound pressure level at the axial height with acoustic frequency of 1800 Hz. The measurement point was located at every half of wave length (94.4 mm) from acoustic

140dB

150dB

f= 1800Hz

Sound pressure level (dB)

The agglomeration chamber was the place where the combined effect of acoustic agglomeration and vapor condensational growth took place. Stable standing-wave field should be formed in the agglomeration chamber to generate the combination fields. In this study, we use the superposition of the reflected wave and the original acoustic wave to generate the stable standing-wave field. In this case, a sound generating system and reflecting plate were required in the agglomeration chamber. In order to generate the standing-wave field, the distance between the original acoustic wave and the reflected wave should be the integer multiple of the half wavelength. Hence, the position of the reflecting plate along the axis of agglomeration chamber was moveable to adjust the distance between original and reflected wave in this study. The acoustic was generated by a horn with a frequency range of 180–7000 Hz, which was powered by a signal generator (DF1027B, Zhongce Co. Ltd., China) combined with a power amplifier (DF5883, Zhongce Co. Ltd., China). Additionally, the supersaturation degree achieved in the acoustic agglomeration chamber was controlled by the flue gas temperature and water vapor addition. Hence, the temperature in the agglomeration chamber should be well defined. In this study, an isothermal interlayer was employed at the outside of the acoustic agglomeration chamber to keep the requirement temperature in the acoustic agglomeration chamber. Another important parameter for the agglomeration chamber was the residence time. As known that the condensational growth time for fine particle is very short (typically less than 1 s) [23] and the suitable time for acoustic agglomeration is less than 3 s [20]. Hence, in this study the residence time of about 2.5 s in the agglomeration chamber was given. A length of 1200 mm and inner width of 40  40 mm square tube made with polymethyl methacrylate was used, based on the residence time and the flue gas flow rate.

130dB

150

140

130

120

200

400

600

800

Axial length (mm) Fig. 2. Variations of sound pressure level on the axial length in agglomeration chamber.

Table 1 The major forces for fine particles in the acoustic fields. Forces

Value range (N)

References

Viscous force (Fv) Pressure gradient force (Fp) Inertial forces (Fi)

105–103 1011–1012 1015–1018

Temkin [31] and Yao [32] Temkin [33] and Fan [30] Yao [32]

source in the axial direction of the agglomeration chamber. The fluctuation of SPL was less than 0.6%. Thus, the attenuation of SPL in agglomeration chamber can be neglected. In this case, steady standing-wave field can be formed by the source acoustic wave and the reflected wave in the acoustic agglomeration chamber. 3. Results and discussion 3.1. Coupling effect of acoustic agglomeration and vapor condensation on particles enlargement Fine particles are driven by the relative motion between particles and the gas. Particles suspended in the acoustic field are gov-

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erned by different forces, such as the viscous force, pressure gradient and the inertial forces, which are summarized in Table 1. Comparing to the viscous force and pressure gradient, the inertial forces can be neglected. In this case, the motion equation of fine particle in standing-wave field can be expressed as followed [30]:

1.0

0.8

2

d x dt

2

¼ Fp þ Fv

ð1Þ

0.6

where F is resultant force of particles, Fp is pressure gradient force. There is a discrepancy of pressure at the spherical surface along the acoustic wave. The pressure gradient force of fine particle at x position can be obtained as followed:


2 dp F p ¼ ½pðx  xo ; tÞ  pðx þ xo ; tÞp 2

f=1800Hz,I=150dB

ηP

F¼

Particles Droplets

0.4

0.2

ð2Þ

0.0 0.1

1

where dp is particle diameter, p is sound pressure, and xo can be calculated as followed:

xo ¼

pffiffiffi 2 dp 4

Fig. 4. The entrainment factor of fine particles and water droplets in acoustic field.

F v ¼ 3plg dp ðup  uÞ=C

ð4Þ

where lg is gas dynamic viscosity, up is particle velocity, C is Cunningham correction coefficient, which can be obtained as followed:

2:514 þ 0:08 expð0:55dp =ka Þ dp =ka

ð5Þ

where ka is the molecular mean free path, u represents the air amplitude velocity, which can be derived from wave equation:

@n x uðx; tÞ ¼ ¼ 4pfA cosð2p Þ sinð2pftÞ @t k

ð6Þ

where f is frequency, A is amplitude, and k is acoustic wave length. The calculation method of the motion Eq. (1) was described in detail [30]. Fig. 3 shows the motion characteristics of fine particles in acoustic field with wave frequency of 1800 Hz, according to the motion equation. Fine particles moved toward the wave node and converged rapidly in a very short time under the effect of standing-wave acoustic field. Fine particles converged to the wave node continuously as residence time increased. Strong convergence of fine particles was presented at residence time of 0.15 s. The convergence of fine particles at the wave node in acoustic field not only increased the collision probability of particles, but also made vapor

condense intensively in some special regions of the agglomeration chamber. The increase of particle concentration provided more condensable surfaces in the wave node, promoting the condensation of water vapor on the surfaces of fine particles and decreasing the condensation on the wall, which enhanced the condensational growth of water vapor on the surfaces of fine particle. In addition, acoustic waves acting on small particles produce oscillatory among them, according to acoustic entrainment theory, entrainment factor of fine particle in acoustic field neglecting inertia forces can be simplified as followed [31]:



ð7Þ

where gP is the particle entrainment factor, Up is the particle velocity amplitude, Uo is the air velocity amplitude, x = 2pf is the angular frequency of acoustic wave, f is the acoustic frequency, s represents the relaxation time of a particle having a diameter dp given by:

s¼

qp d2p 18lg

ð8Þ

where qp is the particle density. The entrainment factor variations of coal-fired fine particles and droplets are illustrated in Fig. 4. A great difference of entrainment factor between coal-fired particles and droplets was presented. For the same size, entrainment factor

100

t=0.05s t=0.10s t=0.15s

1500



U  1 gP ¼  p  ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Uo 1 þ ðxsÞ2

Cumulative percentage(%)

Particle number concentration change rate(%)

100

ð3Þ

Fv is Stokes viscous force, which can be calculated by Stokes law:

C ¼1þ

10

Partilce diameter (μm)

I=150dB, f= 1800Hz 1000

500

80

Number concentration Mass concentrtion

60

40

20

0 0

0.00

0.05

0.10

0.15

Position (m) Fig. 3. Particle aggregation characteristics in acoustic field.

0.1

1

Dp(μm) Fig. 5. Initial particle cumulative size distribution.

10

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of droplets was larger than that of coal-fired particles. The entrainment factor was close to one for both particles and droplets smaller than 1 lm, which meant that the particles oscillated fully with the gas. While the particle size was greater than 20 lm, the entrainment factor was close to zero, which meant that the particles were almost at rest [20]. Fine particles used in the experiment were polydispersed, seen in Fig. 5, but most particles used in this study were in the submicron range. In this case, most of the fine particles have almost the same entrainment factor and oscillated fully. Hence, fine particles oscillating in the acoustic field could not collide with each other easily to form large aggregates. However, when the particles were enlarged by condensational growth of water vapor, the entrainment factors of those particles were changed. In this case, fine particles were hard to collide with each other, but they collided with droplets enlarged by the condensational growth easily in acoustic field. The collision probability was improved significantly caused by the difference of entrainment factors between fine particles and droplets was known as bimodal acoustic agglomeration [14], which enhanced the collision and agglomeration of fine particles. 3.2. Particle removal characteristics in different external fields Fig. 5 shows the initial size distribution of number concentration and mass concentration of coal-fired fine particles measured at the outlet of the separator. Generally, it is the case that the initial particles should be measured before entering the agglomeration chamber. However, considering that the chambers may also collect particles without any external fields, though the collection of submicron particles is very low. In order to eliminate this influence on the final removal efficiency, in this study the initial size distribution and number concentration of fine particles were measured at the outlet of the separator chamber as well as the size distribution and number concentration after the external fields. The majority of fine particles used in the experiment were in the submicron range. The value of PM1/PM10 for number concentration was larger than 90%. However, mass concentration for those particles diameter larger than 1 lm took account over 80% of the total mass concentration. It indicates that the particle number concentration is more important than the mass concentration for the separation of fine particles. Since high concentration of particle number may only with very small mass concentration. Therefore, the following discussions are conducted based on the particle number concentration. The particle size distribution with and without the external field is demonstrated in Fig. 6. A peak value of particle number con-

Ni0  Nit  100 Ni0

ð9Þ

100 Da=0.185μm Da=0.183μm

9.0x105

DNDlogDp(1.cm-3)

gNi ¼

Initial S=1.2, I=0dB S=0.3, I=150dB, f=1800Hz S=1.2, I=150dB, f=1800Hz

Da=0.179μm Da=0.175μm 5

6.0x10

3.0x105

0.0

80

Removal efficiency (%)

1.2x106

centration was presented at the size of 0.3 lm for initial particles. The particle number concentration decreased in different scales with different external fields. Comparing to the individual effect of acoustic field or vapor condensation, particle number concentration has an evidence drop in the combined effect of acoustic agglomeration and vapor condensation. In the coupling external fields, the particle agglomeration enlargement and condensational growth occurred simultaneously. Supersaturation vapor condensed on the surfaces of fine particles to form droplets, which enlarged the particle size. Although, the change of entrainment factor between fine particles and growth droplets caused by the condensational growth improved the collision probability, which was known as the bimodal acoustic agglomeration [14], the difference of particle diameters and entrainment factors enhanced the collision and agglomeration of fine particles. On the other hand, critical supersaturation degree for nucleation decreased with the increasing of the particle diameter. Acoustic agglomeration increased the particle size, which was useful for the condensation of supersaturation vapor on the surfaces of particles. Fine particles enlargements were enhanced by both acoustic agglomeration and vapor condensational growth. Therefore, the removal efficiency of fine particles in the combined external field was significantly improved. To further clarify the particle size distribution change, the particle stage removal curves with different external fields are given in Fig. 7. The variation of particle number concentration in a certain diameter range is determined by the following effects. For a certain size range of d ± Dd, particles with size smaller than d fall into this size range due to collision, agglomeration or condensational growth. This increases the particle number concentration in the size range of d ± Dd. On the contrary, particles in the size range of d ± Dd also enlarge into the larger size range than d due to agglomeration or condensational growth, which descends the particle number concentration in the size range of d ± Dd. Additionally, particles in the size range of d ± Dd are captured by the effects of inertia and diffusion, which also decreases the particle number concentration in the size range of d ± Dd. Therefore, the stage removal efficiency for the size range of d ± Dd is conducted by the three effects mentioned above. Based on the variation of the particle number concentration for the size range of d ± Dd, the stage removal efficiency of fine particles is defined as followed:

S=1.2, I=0dB S=0.3, I=150dB, f=1800Hz S=1.2, I=150dB, f=1800Hz

60

40

20

0 0.1

1

Dp(μm) Fig. 6. Particle size distribution with and without external fields.

10

0.1

1

Dp (μm) Fig. 7. Particle stage removal efficiency in different external fields.

10

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3.3. Effect of supersaturation degree on fine particle removal efficiency in acoustic field The separation of fine particles in settling chamber was caused mainly by inertial forces, depending on the final sizes enlarged by the external fields. Under the acoustic field, particles agglomeration depended on the collision between fine particles. Acoustic wave encouraged the fine particles to collide and form large agglomerates. But not all the collisions of fine particles can form large aggregates, which were determined by the adhesion forces, including Vander Waals force, capillary force and acoustic entrainment. Therefore, improve the collision probability of fine particles and enhance the adhesion forces were the effective way to form large aggregates. Fig. 8 illustrates the influence of the supersaturation degree on fine particle removal efficiency with or without the acoustic wave. Fine particle removal efficiencies were extremely low at about 10% in the acoustic field for the supersaturation degree lower than 1.0. The degree of supersaturation greater than the critical one was required for the condensational growth of droplets [23,27]. Fine particles from coal-fired used in the experiment were hydrophobic silica–alumina minerals. High supersaturation degree was needed to activate these particles. In fact, when the supersaturation degree was lower than 1.0, fine particle could not be enlarged by condensational growth. That meant the coupling effects could not be formed in this case, and the particles were agglomerated by acoustic wave only. Removal efficiency of fine particle was very low in acoustic field with sound pressure level lower than 140 dB [16,20,34]. Since the sound pressure level used in the experiment was only 130 dB, an extremely low removal efficiency was presented when the supersaturation lower than 1.0.

100

80

Removal efficiency (%)

where, Ni0 and Nit are the particle number concentration in the ith size range of measurement before and after external fields. Under the effect of acoustic field with SPL of 150 dB and supersaturation degree of 0.3, the stage removal efficiencies were quite low at about 10–23%. Supersaturation degree larger than 1 was needed for condensational growth of water vapor on the surfaces of fine particles. Hence, under the supersaturation degree of 0.3, particles cannot be enlarged by the condensational growth. That means there is not external effect under the supersaturation degree of 0.3 solely. In fact, under the effect of acoustic field with SPL of 150 dB and supersaturation degree of 0.3, particles were enlarged by the acoustic agglomeration only. In this case, the collision and agglomeration only caused by acoustic wave. As mentioned above, the majority of particles used in the experiment were in submicron range, which had almost the same entrainment factor, seen in Fig. 4, and fine particles were hard to collide with each other in the acoustic field. Hence, the agglomeration enlargement of fine particles was week and resulted in low removal efficiency. Stage removal efficiencies were improved to about 20–40% in the effect of vapor condensational growth with a supersaturation degree of 1.2. According to the Fletcher’s heterogeneous nucleation theory [34], the grown droplet size was determined by the supersaturation degree. If high enough supersaturation degree was given, droplets grown by heterogeneous condensation could be large enough to be separated by conventional separators [24,28]. The separation of fine particles can be promoted efficiently under the combined effect of acoustic agglomeration and vapor condensation. The stage removal efficiencies of fine particles were promoted to 53–80% with a sound pressure level of 150 dB and supersaturation degree of 1.2. Particles agglomeration in acoustic field enlarged the particle size, which benefited the condensational growth of water vapor. Furthermore, since the growth droplets of the entrainment factor was different from fine particles, which enhanced the collision and agglomeration between droplets and fine particles.

I=130dB, f=1800Hz I=0dB

60

40

20

0 0.4

0.8

1.2

S Fig. 8. Influence of supersaturation degree on fine particle removal efficiency with and without acoustic field.

Although, condensational growth cannot occur under the supersaturation degree lower than 1.0, a slight increase of removal efficiency was presented with the increase of supersaturation degree. Some studies have showed that the increase of gas humidity could change the adhesion forces, which benefited the formation of large aggregates [17,18]. In fact, the entrainment factor was also influenced by the supersaturation degree. Fig. 9 depicts the fine particle entrainment factors in different supersaturation conditions, which were calculated by Eq. (7). Entrainment factor increased with supersaturation degree for particles for the same particle size, which indicated that flue gas humidity could also affect the agglomeration of fine particles. When the supersaturation degree was lower than 1.0, the removal efficiency of fine particles was mainly determined by the change of adhesion forces and entrainment factors caused by the humidity variation. However, the promotion of fine particle removal efficiency was extremely low, which only increased from 10% to 16% with S increasing from 0.32 to 0.80. The critical supersaturation degree Scr required to activate fine particles from coal-fired can be expressed as follow [35]:

!12 8pV 2w r3eg Scr ¼ exp f ðmf ; xÞ 3kTlnð4pr 2 K C Þ

ð10Þ

1.0 S=0, f=1800HZ, I=130dB S=0.5, f=1800HZ, I=130dB S=1, f=1800HZ, I=130dB S=1.6, f=1800HZ, I=130dB

0.8

0.6

ηP

324

0.4

0.2

0.0 10

100

Particle diameter (μm) Fig. 9. The entrainment factor of fine particles varies with particle size in different supersaturation ambient.

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1.6

Scr

1.4

1.2

1.0

the practical application use. If the flue gases are warm and high moisture contents, such as flue gases after wet flue gas desulfurization (WFGD) or wet scrubber, supersaturation of these flue gases can be easily achieved by cooling or mixing another saturated gas of different temperature or adding water vapor. In this condition, it can dramatically reduce the consumption of water vapor, which is benefit for reducing the energy consumption and cost in practical use. Therefore, the combined effect of acoustic agglomeration and vapor condensation has a good application for those flue gases with warm and high humidity, where exhaust gas streams cleaning are demanded. 3.4. Effect of SPL on fine particles removal with or without vapor condensation According to the theoretical analysis, increasing the sound pressure level will intensify the agglomeration of fine particles. The dependence of particle removal efficiency on the SPL at a given supersaturation degree of S = 0.3 and S = 1.2 is given in Fig. 11. When the value of S was 0.3, the removal efficiency was very low at about 18% and increased slowly with increasing sound pressure level in the acoustic field. As mentioned above, at the supersaturation degree of S = 0.3, the coupling effects could not be formed in this condition. The particle removal was caused by the acoustic agglomeration only. Hence, the particle removal efficiency was low in this case. Preview investigations illustrated that optimal sound pressure level was higher than 140 dB [16,20,36]. High removal efficiency can be obtained when the value of SPL was higher than 158 dB [37]. The acoustically induced turbulence appeared, and played an important role in the particles agglomeration, which promoted the removal efficiency rapidly. However, high value of SPL was not suitable for commercial industrial applications because of high cost and energy consumption. Meanwhile, the acoustic system became unstable to putout a high SPL wave signal. While the value of the supersaturation degree was up to 1.2, the removal efficiency was greatly improved even in a low intensity acoustic field. In this case, the coupling external fields were achieved and the particle removal was caused by the combined effect of acoustic agglomeration and vapor condensational growth. Such as at the sound pressure level of 130 dB, fine particle removal efficiency of about 63% was obtained for S = 1.2, but this value was only about 12% for S = 0.3. It indicates that the particle removal efficiency can be effectively improved by the coupling external effects. Once the supersaturation degree was higher than the crit-

100

Partilce removal efficiency (%)

where reg is the interfacial tensions of the embryo-gas, Vw is the embryo volume, Kc is a kinetic coefficient, k is the Boltzmann constant, T is the temperature, r is the particle radius, f (mf, x) is a function of relative contact angle. The calculation of Scr was described in detail previously [35]. The critical supersaturation degree decreased rapidly with increasing particle size for those particles with diameter smaller than 0.5 lm, seen in Fig. 10. If the particle size was larger than 1.0 lm, the value of Scr was close to 1.13, indicating that the supersaturation degree larger than 1.13 was required for heterogeneous condensation of water vapor on the surfaces of these particles. In fact, coupling effects of acoustic agglomeration and vapor condensation could not be formed, when the supersaturation degree was lower than the critical supersaturation degree. In this case, the removal efficiency of fine particles was low, seen in Fig. 8. The removal efficiency was improved significantly while the supersaturation degree was over 1.2. It improved by 50% as the supersaturation degree increased from 1.0 to 1.2. Obviously, the substantial improvement of fine particles removal efficiency was not the result of entrainment factors and adhesion forces change. While the supersaturation degree was over 1.2, fine particle removal efficiencies were determined by the combined effect of acoustic wave and vapor condensation. As seen in Fig. 10, high value of Scr was required for smaller particle size, especially in the submicron range. However, when fine particles were grown to micron ones by acoustic agglomeration firstly, they were easy to be activated and enlarged by heterogeneous condensational growth. In this case, particles which cannot be activated in this supersaturation degree would be enlarged now by the condensational growth of water vapor after large agglomerates were formed in acoustic field. Moreover, the removal efficiency of fine particles increased substantially with the supersaturation degree. As the supersaturation degree increased, more condensable vapor was provided, which led to a larger size of final grown droplets and consequently high particle removal efficiency was obtained. Fine particle removal efficiency without acoustic wave field was therefore much lower than that in the combined effect of acoustic agglomeration and vapor condensation. At the supersaturation degree of 1.2, removal efficiencies of 76% and 38% were obtained with and without acoustic field, seen in Fig. 8. Without acoustic field, fine particles were enlarged by condensational growth only. While in the coupling effects, the oscillations of fine particles made them easy to be captured by the droplets or coarse particles. These droplets or agglomerates were large enough to be separated by the inertial settling device. Hence, high removal efficiency can be obtained in the coupling external fields. Generally, the combined effect of acoustic agglomeration and vapor condensation can be used for any application where fine particles separated from flue gases are needed. However, high supersaturation may be required in the combination use of acoustic agglomeration and vapor condensation. So the steam consumption should be considered in

1

2

3

4

5

Rp (μm)

80

S=0.3, f=1800Hz S=1.2, f=1800Hz

Slope=1.267, R2=0.933

60

40

20

Slope=0.473, R2=0.936

0 120

130

140

150

Sound pressure level (dB) Fig. 10. Calculation of critical supersaturation degree varies with fine particle radius.

Fig. 11. Particle removal efficiency varies with sound pressure level.

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ical supersaturation degree, particles with a size down to a few nanometers can be enlarged with high growth rates to droplets with diameters of several microns [23]. Moreover, under the combined effect of acoustic field and vapor condensational growth, fine particles oscillated and were easily captured by the new born droplets acting as seed particles in acoustic field. These droplets were separated by means of classical inertial separator easily. That was the reason high particle removal efficiency could be achieved in the coupling external effects. Note that the removal efficiency increased quickly by approximately 40% when the value of SPL rose from 120 dB to 153 dB for the supersaturation degree of 1.2, but it only about 10% for the supersaturation degree of 0.3. That means the influence of acoustic intensity on removal efficiency under the combined effect of acoustic agglomeration and vapor condensation was more significant than that of in the acoustic field only. In fact, the formation and breakage of particle aggregates occurred stochastically in the acoustic field, depending on the adhesion forces. The characteristics of particle surfaces were changed when they were covered with water. If particles with surface moisture merged, the system free energy of the liquid–solid surface reduced. The contraction of the surface tension of water produced attraction between particles and droplets, causing the particles to aggregate. The large forces like a liquid bridge were formed between particles and droplets, forming firmly aggregates. In other words, the breakage of aggregates occurred rarely. In additional, when fine particles collided with droplets, they may deep into the droplets. The particle agglomeration can also be promoted, and this effect cannot be negligible [38]. However, in a relatively low supersaturation degree (S = 0.3), particles cannot be enlarged by the condensational growth. Consequently, the influence of water on the particle agglomeration was negligible. The formation of agglomerates increased with the value of SPL in acoustic field, but the breakage of agglomerates should be considered in this case. And the particle removal efficiency would be affected by the breakage of agglomerates.

4. Conclusions A novel technique to improve fine particles removal efficiently in the combined effect of acoustic agglomeration and vapor condensational growth was presented. In the combined effect of acoustic agglomeration and vapor condensation, fine particles agglomeration enlargement and condensational growth occurred simultaneously. The stage removal efficiencies of 53–80% were obtained in the coupling external fields, which were much higher than those caused by acoustic agglomeration or vapor condensational growth only. The coupling external fields could not be formed when the supersaturation degree was lower than the critical supersaturation degree. The total removal efficiencies of fine particles were extremely low, and increased slightly with the increases of acoustic intensity and supersaturation degree. Such as, removal efficiency of about 10% was presented in the sound pressure level of 130 dB for the supersaturation degree lower than 1.0. And the removal efficiency improved by only about 10% when the value of SPL increased from 120 dB to 153 dB for supersaturation degree of 0.3. Supersaturation degree as well as acoustic intensity has a significant effect on the particle removal efficiency in the coupling external fields. Fine particle removal efficiency increased with the supersaturation degree rapidly when the supersaturation degree was higher than 1, which was improved by about 50% as the supersaturation degree increased from 1.0 to 1.4. And the removal efficiency increased quickly by approximately 40% when the value of SPL rose from 120 dB to 153 dB in the coupling exter-

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