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Removal of fine particles in WFGD system using the simultaneous acoustic agglomeration and supersaturated vapor condensation
Removal of Fine Particles in WFGD System Using the Simultaneous Acoustic Agglomeration and Supersaturated Vapor Condensation Jinpei Yan, Liqi Chen, Qi Lin PII: DOI: Reference:
S0032-5910(17)30273-5 doi:10.1016/j.powtec.2017.03.056 PTEC 12461
To appear in:
Powder Technology
Received date: Revised date: Accepted date:
12 January 2017 8 March 2017 22 March 2017
Please cite this article as: Jinpei Yan, Liqi Chen, Qi Lin, Removal of Fine Particles in WFGD System Using the Simultaneous Acoustic Agglomeration and Supersaturated Vapor Condensation, Powder Technology (2017), doi:10.1016/j.powtec.2017.03.056
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ACCEPTED MANUSCRIPT
Removal of Fine Particles in WFGD System Using the Simultaneous Acoustic
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Jinpei Yan *, Liqi Chen, Qi Lin
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Agglomeration and Supersaturated Vapor Condensation
Key Laboratory of Global Change and Marine-atmospheric Chemistry, Third Institute of Oceanography, SOA, Xiamen
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361005, P R China
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Abstract: The characteristics of fine particle removal from wet flue gas desulfurization (WFGD) using the simultaneous
acoustic agglomeration and supersaturated vapor condensation were investigated experimentally. The impacts of
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supersaturation degree, residence time and acoustic intensity on the fine particle removal efficiency were demonstrated.
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High moisture and tiny droplets were contained in the flue gas after WFGD, which benefited the use of coupling external
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fields in WFGD system. The results showed that the removal of fine particles can be significantly improved in the
simultaneous acoustic field and supersaturated vapor condensation. High value of sound pressure level (SPL >150 dB)
was needed to achieve a considerable removal efficiency using acoustic agglomeration only. While low SPL of 130 - 150
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dB can be used to remove fine particle efficiently in WFGD system using simultaneous external fields. Removal
efficiency higher than 70 % was obtained with S = 1.15 and SPL = 151 dB. The residence time of the simultaneous
acoustic agglomeration and condensational growth was determined by the time required by acoustic agglomeration.
Particle removal efficiency increased with the residence time, but it tended to be constant as the residence time over 3 s.
Multiple horn numbers with low acoustic intensity benefited the improvement of fine particle removal efficiency and the
reduction of energy consumption using the simultaneous external fields in WFGD system.
Keywords: Fine particles; Acoustic agglomeration; Condensational growth; Supersaturation; WFGD; Removal
*
Corresponding author. E-mail address: [email protected] 1
ACCEPTED MANUSCRIPT 1. Introduction Fine particles (PM2.5) have been regarded as the major air pollutants in urban areas, which
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constituted a major health hazard because of their containing of toxic components and ability to
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penetrate into the respiratory system [1, 2]. Coal-fired power plants are considered to be one of the major pollution sources of fine particles in ambient atmosphere [3]. Removal of fine particles from flue gas with high efficiency has become important. But conventional separation technologies, such as
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electrostatic precipitators and cyclone separators were hard to separate these particles efficiently [4, 5].
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In order to improve the removal efficiency of fine particles, a preconditioning technique is required to increase the particle size before entering conventional separation devices. Such preconditioning
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of different external field [13-15].
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technique can be acoustic agglomeration [6-9], heterogeneous condensation [10-12] or coupling effect
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Generally, fly ash and SO2 can be removed efficiently by desulfurization scrubbing installed in power plants, but it was inefficient in separating PM2.5 [16-18]. Previous studies have found that fine
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particle removal efficiency declined with particle size in wet limestone-based FGD from a coal-fired power plant. Most of the particulate matter can be ascribed to PM2.5 after wet limestone-based FGD [18]. However, rare investigations were present on the removal of fine particles in WFGD process. Hot flue gas contacted with cold desulfurization slurry counter-currently in the WFGD process. High humidity of flue gas was achieved after desulfurization scrubbing, causing by the intense heat and mass transfer of scrubbing liquid and flue gas. Removal of fine particles from flue gas in WFGD system using heterogeneous condensational growth have been investigated in previous studies [4, 19]. Supersaturated surrounding was obtained by steam addition after desulfurization scrubber. Condensational growth of water vapor occurred on the surfaces of fine particles to enlarge their sizes. In this case, grown droplets
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ACCEPTED MANUSCRIPT can be separated efficiently by the demister [19]. However, high vapor consumption inhibited the practical application of vapor condensational growth. Combination of acoustic wave with vapor
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condensational growth was considered to be a novel preconditioning process for particle separation.
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High particle removal efficiency can be obtained in the simultaneous acoustic agglomeration and supersaturated vapor condensation with low acoustic intensity [20].
High relative humidity, large quantity of tiny droplets and high concentration of fine particles were
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present in the flue gas after WFGD, which was considered to be the primary problem of flue gas exhaust
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after WFGD. Therefore, the flue gas after WFGD was quite different from the flue gas without WFGD system. In this study, according to the flue gas characteristics after WFGD, the simultaneous effect of
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acoustic field and vapor condensation was deployed to the WFGD system to improve the fine particle
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removal efficiency and reduce the droplet concentration in WFGD flue gas. Supersaturation degree
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required by the condensational growth was achieved by changing the moisture content and flue gas temperature, while standing - wave was introduced into the flue gas after WFGD to form the
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simultaneous effect of acoustic field and supersaturated vapor condensation. The influences of supersaturation degree, acoustic intensity and residence time, etc. on fine particle removal efficiency in WFGD system were investigated using the simultaneous effect of acoustic agglomeration and supersaturated vapor condensational growth. 2 Experimental details 2.1 Experimental set-up Flue gas with 80 Nm3.h-1 was generated by a fluidized bed coal - fired boiler, as seen in Fig. 1. A buffer vessel with an electric heater and stirrer were located after the boiler. Particles larger than 10 m were separated by a cyclone before passing a heat exchanger, where the flue gas was cooled by cold air.
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ACCEPTED MANUSCRIPT A desulfurization scrubber with diameter of 150 mm and 1500 mm height was used in this experiment. Supersaturated vapor injector was deployed at the top of the desulfurization scrubber to increase the
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moisture content in flue gas. Flue gas with high humidity after desulfurization scrubber then passed
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through a growth and removal chamber. A high efficiency wire mesh was employed at the outlet of removal chamber to remove the droplets enlarged by the simultaneous external fields. At the exit of the growth and removal chamber, fine particle size distribution and number concentration was measured in
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real time using an Electrical Low Pressure Impactor (ELPI, Dekati Co. Ltd., Finland) before and after
previous studies [20].
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2.2 The growth and removal chamber
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different external field. The measurement of fine particles by ELPI was descried carefully in the
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The growth and removal chamber was made of a polymethyl methacrylate tube with length of 1500
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mm and inner width of 200X200 mm. The simultaneous effect of acoustic field and supersaturated vapor surrounding was formed in the chamber, where fine particles were enlarged by the combined effect of
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acoustic agglomeration and vapor condensational growth. Multiple standing-wave fields were used in this study. To form the multiple standing-wave fields, a rectangular tube was used. Four horns (KTD 250) were employed in the same side of the growth chamber with a reflecting plate equipped in the opposite side of the growth chamber. The distance between the original acoustic wave and the reflected wave was well defined as the integer multiple of the half wavelength. Acoustic wave was generated by the horn, driving by a signal generator (DF1027B, Zhongce Co. Ltd., China), which was powered by a power amplifier (DF5883, Zhongce Co. Ltd., China). The range of the acoustic frequency was 180-7000 Hz. The supersaturated vapor surrounding required by condensational growth was achieved by cooling
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ACCEPTED MANUSCRIPT the flue gas and increasing the moisture content in the flue gas. A vapor addition device was set at the out let of the scrubber to increase the humidity of flue gas. The outside of the growth chamber was
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covered by an isothermal interlayer to achieve the requirement temperature of flue gas in the chamber.
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In this case, the supersaturation degree in the growth chamber can be well identified. Droplets grown in the growth chamber with the simultaneous external fields was captured with a high efficiency wire mesh demister installed at the outlet of the chamber.
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3 Results and discussion
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3.1 Contribution to fine particles removal in WFGD using simultaneous external fields Supersaturated vapor surrounding was required to generate the simultaneous effect of acoustic field
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and vapor condensation. In the WFGD scrubber, hot flue gas contacted with low temperature
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desulfurrization liquid counter-currently with intense mass and heat transfer, resulting in the solution
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liquid evaporation. Therefore, flue gas with high relative humidity (85 - 90 %) was present after desulfurrization scrubbing. Consequently, one can expect that supersaturated vapor required by the
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heterogeneous condensational growth can be formed easily by regulating the temperature or moisture content in the flue gas after WFGD system. Fig. 2 shows the influence of vapor addition amount on supersaturation degree with and without WFGD system. Vapor addition amount of 0.06 kg. Nm-3 was needed to generate the supersaturated vapor surroundings without WFGD, while supersaturation degree of 1.0 was achieved with steam addition amount of 0.01 kg. Nm-3 after WFGD. Standing - acoustic field was also needed to generate the simultaneous external fields. Generally, according to acoustic entrainment theory, the motion of fine particles was driven by the entrainment force. Entrainment factor neglecting inertia forces can be simplified as follow [21]:
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Where
P
Up 1 Uo 1 ( )2
is the particle entrainment factor,
represents the relaxation time,
is the air velocity amplitude,
Up
is the particle
is the angular frequency of acoustic wave.
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velocity amplitude,
Uo
(1)
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P
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Fig. 3 illustrated the entrainment factor of fine particles and droplets in acoustic field with sound pressure level of 151 dB and frequency of 1400 Hz. The value of entrainment factor of droplets was
is a
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larger than that of coal - fired particles at the size range of 1 - 20 µm. The relaxation time
function of the particle density. Since the density of water is lower than coal-fired particles, entrainment
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factor of droplets was larger than that of coal-fired particles. Submicron particles (< 1 µm) oscillated
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fully with flue gas, as the entrainment factor was close to one. However particles larger than 20 µm were
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almost at rest with the value of entrainment factor of zero [13]. As seen in Fig. 4, most of the particles from WFGD were in submicron range. Note that the particles and droplets from WFGD were mainly in
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the submicron size range. That means both particles and droplets would oscillate fully with the flue gas. Previous study have found that fine particles could not collide with each other or with droplets easily to
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form large aggregates in the acoustic field due to the little difference of entrainment factors between particles and droplets [22]. That was the reason low particle removal efficiency was present, while individual acoustic agglomeration was used to remove fine particles from flue gas after WFGD. However, when fine particles were enlarged firstly by vapor condensational growth, the entrainment factor of the grown droplets was much higher than fine particles. The difference of entrainment factors between fine particles and droplets increased the collision probability dramatically, knowing as bimodal acoustic agglomeration [7]. 3.2 Fine particle size distribution and stage removal efficiency During the desulfurization processing, fine particles were captured by the scrubbing solution due to 6
ACCEPTED MANUSCRIPT the diffusion and inertial impact in WFGD scrubbing. On the other hand, part of the desulfurization solution and products were also carried out from the desulfurization scrubber which increased the
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particle concentration of flue gas after WFGD [16]. Fig. 4 shows the particle size distribution and
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particle number concentration with different external field. In this study, the initial particle size distribution, measuring without desulfurization scrubbing and any external field, was a unimodal distribution peaking at 0.08 μm. Most of fine particles were in submicron range, accounting for more
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than 95% of the total particles. However, fine particle number concentration at the size range of 0.08 -
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0.3 μm increased after desulfurization scrubbing, indicating that new particles were generated in the desulfurization process. Previous studies have found that sulfate products and un-reacted limestone
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particulate matters were present in the flue gas after WFGD [4, 16]. However, fine particle number
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concentration decreased using individual vapor condensational growth (S = 1.15) and acoustic
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agglomeration (I = 151 dB). Comparing to the individual external field, particle number concentration decreased dramatically by the simultaneous acoustic field and supersaturated vapor condensation after
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WFGD. Large quantity of grown droplets was present in the flue gas after the simultaneous effects, which acted as the seed particles in the standing-wave field. Therefore, fine particles were captured by the grown droplets in the simultaneous external fields and separated by the demister easily. The particle stage removal efficiency was useful to provide an insight into the particle removal characteristics. The stage removal efficiencies of fine particles using acoustic agglomeration, vapor condensation, the simultaneous external fields and without any external field after WFGD were shown in Fig. 5. The variation of particle number concentration in a certain diameter range was determined by the effect of collision, agglomeration or condensational growth [20]. Fine particle removal efficiency of desulfurization scrubbing was extremely low without any external field, meaning that fine particles can
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ACCEPTED MANUSCRIPT not be captured efficiently by the desulfurization scrubber. Additional removal technique was needed to reduce the particle concentration in flue gas after WFGD. Note that a negative value of particle removal
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efficiency was present at the size range of 0.07 - 1.0 μm, indicating that the particle number
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concentration increased in this size range after desulfurization. As seen in Fig. 4, high concentration of fine particle with size smaller than 0.1 μm was present after WFGD. Fine particles in the size range smaller than 0.1 μm tended to collide and adhere into large particles by diffusion and Brownian
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coagulation, which increased the particle number concentration in the size range larger than 0.1 μm [23].
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Though the Brownian coagulation could not be neglected in this case, the increase of particle number concentration at this size range was caused by the new particles formation in the desulfurization process
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in this study. Similar results of new particles generation in WFGD system were obtained in the previous
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studies [18].
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Fine particle removal efficiency increased with the particle size in the effect of vapor condensational growth (S = 1.15), acoustic wave (I = 151 dB) and the simultaneous acoustic agglomeration and vapor
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condensation (S = 1.15, I = 151 dB). The stage removal efficiency was about 10 % - 40 % with vapor condensational growth, which was lower than the value using acoustic agglomeration. As mentioned above, flue gas after WFGD contained large quantity of fine droplets and with high relative humidity. Though high moisture content in the flue gas after WFGD benefited the reduction of vapor consumption, large number of tiny droplets would impede the condensation of supersaturated vapor on the surfaces of fine particles. Competitive condensation between fine particles and droplets occurred in the supersaturated vapor surrounding. Supersaturated vapor pressure required by the nucleation growth on the droplet surfaces was lower than the value for fine particles. That means supersaturated vapor tended to condense on the droplet surfaces but not fine particles. The condensable vapor amount of
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ACCEPTED MANUSCRIPT supersaturated vapor on the surfaces of fine particles reduced. Removal of fine particles in WFGD system can not be highly improved by the vapor condensational growth in this case, especially with low
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supersaturation degree. While acoustic agglomeration was used in the WFGD system, the collision and
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coagulation of fine particles can be enhanced by the difference of entrainment factor between fine particles and droplets in the WFGD flue gas. Moreover, high relative humidity of flue gas after WFGD benefited the formation of firm aggregates in the acoustic field [24]. Both effects contributed the
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coagulation of fine particles. However, since fine particles and droplets contained in the flue gas after
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WFGD were in small size and fully oscillating with the gas. The collision between fine particles and droplets was also inhibited in the acoustic field. High stage removal efficiency of 53 % - 80 % was
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obtained in the simultaneous acoustic agglomeration and vapor condensational growth, indicating that
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fine particles in flue gas after WFGD can be removed efficiently with the simultaneous external fields.
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3.3 The micro characteristics of fine particles with different external field The removal efficiency of fine particles in WFGD process was determined by the final aggregate
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size enlarged by the simultaneous acoustic field and vapor condensational growth. The micro morphology of fine particles can provide the further insight into the particle agglomeration properties with or without external fields. Fig. 6 shows the scanning electron micrographs of the initial particles and particles after WFGD with acoustic field only and the coupling fields. The majority initial particles were independent spherical particles, as seen in Fig. 6 a. Most of fine particles were in submicron range, which was in good agreement with the measuring results given in Fig. 4. Individual particles and aggregates with different size were both present after the effect of acoustic wave, as seen in Fig. 6 b. Different fine particles with one or several coarse nuclei constituted the aggregates. This confirmed that small particles collided with coarse particles in acoustic field due to the discrepancy of entrainment
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ACCEPTED MANUSCRIPT factor, seen in Fig. 3. Large aggregates were present after WFGD using the combined effect of acoustic agglomeration and
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condensational growth, as seen in Fig. 6 c. Comparing to Fig. 6 b, the micrographs of particles in the
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coupling external fields were much more complex and irregular. In the simultaneous acoustic field and vapor condensation, droplets and fine particles in the flue gas were grown into large droplets, which were easy to collide with fine particles to form large aggregates. The size of aggregates in Fig. 6 c was
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larger than the aggregates in Fig. 6 b. In fact, droplets usually carried with desulfurization products in
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the flue gas after WFGD. When the droplets collided with fine particles, the desulfurization products may adhere on the particle surfaces to form complex aggregates. However, as mentioned above, fine
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particles were hard to collide with fine droplets in the flue gas after WFGD with acoustic field only. In
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this case, the aggregates were smaller in size and more regular. This can be further demonstrated by the
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particle compositions with different external fields. The percentage of Ca and S elements showed a significant increase using the simultaneous external fields after WFGD, seen in table 1, suggesting that
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desulfurization products was included in the aggregates in this case. The SEM images and particle components also substantiated the formation of new particles in WFGD process, mentioned in Section 3.2. Note that both single particles and aggregates were present in Fig. 6 b, while large aggregates with rare single particles were shown in Fig. 6 c, indicating that most of fine particles were conducted to form large aggregates using the simultaneous external fields in flue gas after WFGD. Particle number concentration can be significantly reduced when large quantity of fine particles formed the aggregates. SEM images also demonstrated the beneficial effect of fine particles removal using the simultaneous acoustic agglomeration and vapor condensational growth. 3.4 Influence of supersaturation degree on fine particles removal with simultaneous fields
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ACCEPTED MANUSCRIPT Fig. 7 shows the fine particle removal efficiency varies with supersaturation degree using simultaneous external fields in WFGD system. Fine particle removal efficiencies were extremely low
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and independence with the increase of supersaturation degree, while the supersaturation degree was
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below 1.0. Supersaturation degree higher than the critical value was required by the condensational growth of droplets [25]. In this case, when the supersaturation degree was lower than 1.0, fine particles could not be enlarged by vapor condensational growth. The simultaneous effect of acoustic field and
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vapor condensational growth could not be achieved in this case, and the particles were agglomerated by
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acoustic wave only. Contrarily, when the supersaturation degree was higher than 1.0, fine particle removal efficiency can be improved significantly. Removal efficiency higher than 70 % was obtained
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using the simultaneous external fields with supersaturation degree of 1.15 and SPL = 151dB, which was
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much higher than the value (about 20 %) with supersaturation degree of 1.15 but without acoustic field.
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As seen in Fig. 7, fine particle removal efficiency without acoustic field was much lower than the value with the simultaneous external fields in WFGD for the same supersaturation degree. As known
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that the heterogeneous condensational growth of fine particles was governed by the heat and mass transfer between particles and vapor. According to the heterogeneous nucleation theory [26], if one assumed that the condensation nuclei were completely covered by a liquid layer, they can be regarded as pure droplets [10]. In this case, the critical supersaturation and the size of the nuclei in equilibrium can be described by the Kelvin equation:
Scr exp( Where,
is the surface tension, M and
2 M ) RT r
are
（2）
the molar mass and density of the liquid,
respectively. r is the radius of the nucleus, T is the temperature of liquid surface and R is the ideal gas constant. 11
ACCEPTED MANUSCRIPT While the particle surfaces were not covered by liquid layer, higher supersaturation degree was needed to activate those solid particles. Moreover, the shape and the wetability properties of the nucleus’
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surface can influence the critical supersaturation. As seen in Fig. 6 a, most of fine particles after WFGD
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were insoluble spherical particles. The critical supersaturation degree Scr needed to activate fine particles from WFGD system can be described by the following equation [27]:
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8Vw2eg3 Scr exp f (mf , x) 3kT ln(4 r 2 K ) C
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Where, Vw is the embryo volume, k is the Boltzmann constant, embryo-gas, Kc is a kinetic coefficient,
f (mf , x)
1
2
eg
(3)
is the interfacial tensions of the
is a function of relative contact angle.
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Supersaturation degree higher than the critical value was the first requirement for the condensational
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growth of water vapor. Fig. 8 illustrates the critical supersaturation degree varies with radius of droplets
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and fine particles from WFGD. The critical supersaturation degree decreased rapidly, but it trended to be constant for particles with size larger than 0.3 µm. The critical supersaturation degree required to
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activate fine particles from WFGD was much higher than the value to activate droplets with the same size. That means supersaturated vapor would firstly condense on the surfaces of water droplets but not on particle surfaces in the same supersaturated vapor surrounding, which was known as the competitive condensation, especially in a low supersaturation degree. As seen in Fig. 7, the removal efficiency was about 27 % and 70 % using vapor condensation (S = 1.15, I = 0 dB) and the simultaneous external fields (S = 1.15, I = 151 dB) individually, while the value was 87 % and 94 % for the vapor condensational growth (S = 1.43, I = 0 dB) and the coupling external fields (S = 1.43, I = 151 dB). Large quantity of fine droplets was present in flue gas after WFGD, resulting in large amount of supersaturated vapor condensed on the surfaces of droplets, which has a detrimental impact on the condensational growth of
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ACCEPTED MANUSCRIPT fine particles. In this case, the particle removal efficiency was affected directly in WFGD system using vapor condensational growth. However, with the increase of supersaturation degree, sufficient vapor
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was provided to condense on both the surfaces of particles and droplets. Fine particles can be grown
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large enough to be separated. Hence, high removal efficiency was obtained with high supersaturation degree, event without acoustic field.
In the simultaneous acoustic agglomeration and vapor condensational growth, the collision of
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particle - particle, droplet - droplet and particle-droplet occurred in the acoustic field to form large
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aggregates. Firstly, the critical supersaturation degree required by vapor condensational growth decreased when fine particles were enlarged by acoustic agglomeration. That benefited the vapor
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condensation on the surfaces of particles. Moreover, the coagulation of droplets and particles reduced
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the droplet number concentration in flue gas, which decreased the competitive condensation between
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particles and droplets. Both contributed to the vapor condensational growth on the surfaces of fine particles. Meanwhile, droplets enlarged by heterogeneous condensation acting as seed particles in the
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acoustic field, which further promoted the agglomeration of fine particles. In fact, the enhancement of agglomeration and condensational growth by the coupling effects took full advantage of the high moisture content in flue gas after WFGD to decrease the vapor consumption. Moreover, the agglomeration of fine particles and droplets in acoustic field reduced the particle and droplet concentration simultaneously in flue gas after WFGD. 3.5 Influence of residence time on fine particle removal efficiency with the simultaneous fields Residence time was a key parameter for the practical use of the simultaneous external fields in WFGD system. Fig. 9 shows the particle removal efficiency varies with the residence time under different external fields. Fine particle removal efficiency increased rapidly with the residence time. For
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ACCEPTED MANUSCRIPT vapor condensational growth, when the residence time was over 1 s, particle removal efficiency did not increase with the residence time anymore. Generally, the condensational growth of droplets was
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completed within tens of milliseconds [10]. The surrounding supersaturation degree decreased as the
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vapor condensed on the particle surfaces. Moreover, the latent heat of vapor condensation would increase the surrounding temperature, which further decreased the supersaturation degree. The droplet growth rate descended with the decrease of supersaturation degree. Droplets were no longer enlarged,
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when the supersaturation surrounding disappeared. Hence the removal efficiency remained steady in this
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case. Comparing with the vapor condensation, longer residence time was needed for the acoustic agglomeration. Fine particle removal efficiency tended to be constant when the residence time was over
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3 s. Previous studies also indicated that the acoustic agglomeration of fine particles usually completed in
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about 3 s [28]. Note that a slight decrease of removal efficiency was present, when the residence time
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was over 5 s. In fact, the formation and breakage of particle aggregates occurred stochastically in the acoustic field. The breakage of particle aggregates would be enhanced for a long residence time in the
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acoustic field, which may result in the decrease of particle removal efficiency. In the simultaneous effect of acoustic field and vapor condensation, fine particle removal efficiency increased with the residence time, but it tended to be constant as the residence time was over 3 s, which was similar with the variation of particle removal efficiency using acoustic agglomeration only. Fine particle removal efficiency increased from 40 % to 80 %, while the residence time was extended from 0.5 s to 3 s. It indicated that residence time for the simultaneous external fields was determined by the time required by the acoustic agglomeration. Hence, the considerable residence time for the coupling external fields was about 3 s. It is useful to find that the time needed by vapor condensational growth was much less than the time required by the acoustic agglomeration. Hence, in the simultaneous effect
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ACCEPTED MANUSCRIPT of acoustic field and vapor condensation, fine particles and droplets in flue gas after WFGD were first enlarged by the vapor condensational growth. The grown droplets then acted as the seed particles in the
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acoustic field to capture fine particles. This can be certified by the micrographs of fine particles, as seen
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in Fig. 6 c, aggregates formed by fine particles and large proportion of desulfurization products were present after the simultaneous effect of acoustic field and vapor condensation. It is worth to note that fine particle removal efficiency using the simultaneous fields did not decrease as the residence time
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increase. That was determined by the adhesion forces between particles and droplets [29]. The
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characteristics of particle surfaces were changed when they were covered with water. Large forces like liquid bridge were formed between particles and droplets, forming firmly aggregates. In other words, the
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particle removal efficiency.
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breakage of aggregates occurred rarely in the acoustic field, which benefited the improvement of fine
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3.6 Influence of Acoustic property on fine particle removal with the simultaneous fields As mentioned above, high acoustic intensity resulted in high particle removal efficiency, but also
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meaning high energy consumption. Fig. 10 depicts the influence of horn numbers on electric power and fine particle removal efficiency with different sound pressure level. Fine particle removal efficiency increased with the SPL as well as the electric power. The results showed that the SPL at the outlet of horn rang from 130 - 158 dB for input powers of 0.1 - 15 W of one horn with particle removal efficiency from 50% to 89 %. The input electric power increased rapidly with the sound pressure level, especially in high value of SPL. Electric power increased only 2 W when the sound pressure level increased from 130 to 151 dB, while the value increased about 13 W as the sound pressure level increased from 151 to 158 dB. Low acoustic intensity benefited the reduction of the energy consumption. As seen in Fig. 10, fine particle removal efficiency increased evidently with the horn numbers, especially for low value of
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ACCEPTED MANUSCRIPT SPL. The removal efficiency increased about 25 % with the horn numbers increasing from 1 to 4 at a SPL of 130 dB. However, the removal efficiency increased only about 5 % at a SPL of 158 dB.
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Contrarily, the input electric power increased about 36 W with the horn numbers increasing from 1 to 4
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at a SPL of 158 dB, while the value was about 8 W at a SPL of 151 dB. Multiple horn numbers with low sound pressure level would benefit the practical used of the simultaneous effect of acoustic
acoustic intensity with low energy consumption.
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agglomeration and vapor condensation. In this case, fine particles can be removed efficiently in low
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To further clarify the influence of acoustic wave on the removal efficiency, the influence of sound pressure level on fine particle removal efficiency is illustrated in Fig. 11. A considerable removal
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efficiency of about 40 % was obtained only when the sound pressure level was higher than 151 dB with
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supersaturation degree of 0.92. The acoustically induced turbulence appeared in high value of SPL (>
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158 dB), which enhanced the particle agglomeration in the acoustic field [30]. As seen in Fig. 10, high value of SPL implied high electric power and energy consumption, which was not suitable for
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commercial industrial applications. But fine particle removal efficiency was significantly improved using the simultaneous external fields (S = 1.15) in the WFGD system, even in low value of sound pressure level. Fine particle removal efficiency of 10 % and 60 % was obtained individually for S = 0.92 and S = 1.15 with the sound pressure level of 130 dB, indicating that fine particle removal efficiency can be efficiently improved by the simultaneous external fields. The results illustrated that the value of SPL higher than 150 dB was needed to achieve a considerable removal efficiency using acoustic agglomeration only. While low sound pressure level of 130 - 150 dB was a considerable acoustic intensity using the simultaneous effect of acoustic field and supersaturated vapor condensation to remove fine particle efficiently from flue gas in WFGD system.
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ACCEPTED MANUSCRIPT 4 Conclusions The simultaneous effect of acoustic agglomeration and supersaturated vapor condensation was
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employed in the WFGD system to improve fine particles removal. High moisture and tiny droplets were
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present in flue gas after WFGD, which benefited the use of coupling external fields in WFGD system. The consumption of vapor required by the simultaneous fields can be reduced efficiently based on the high moisture content in flue gas after WFGD. Droplets in flue gas benefited the agglomeration of fine
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particles in the acoustic field due to the differences of entrainment factor between particles and droplets.
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The influence of supersaturation degree, sound pressure level and residence time, etc. on fine particle removal efficiencies from WFGD were demonstrated in this study.
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Fine particle removal efficiency can be significantly improved by the simultaneous acoustic
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agglomeration and vapor condensation with stage removal efficiency up to 80 %. The removal
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efficiency increased with the sound pressure level and supersaturation degree in the simultaneous external fields. High value of SPL (> 150 dB) was required to achieve a considerable removal efficiency
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using acoustic agglomeration in WFGD system. But low sound pressure level of 130 - 150 dB was a considerable acoustic intensity using the simultaneous external fields to remove fine particle efficiently from flue gas in WFGD system. The residence time of the simultaneous acoustic agglomeration and vapor condensation was determined by the time needed for the acoustic agglomeration. Residence time larger than 3 s was needed to achieve high removal efficiency using the simultaneous fields in WFGD system. High acoustic intensity resulted in high particle removal efficiency, but also meaning high energy consumption. Multiple horn numbers with low acoustic intensity would benefit the improvement of fine particle removal efficiency and the reduction of energy consumption using simultaneous external fields
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ACCEPTED MANUSCRIPT in WFGD system. It indicates that high removal efficiency of fine particles in WFGD system can be achieved by the simultaneous effect of acoustic agglomeration and vapor condensation with low vapor
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consumption and acoustic intensity.
Acknowledgement
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This study was supported by the National Natural Science Foundation of China (No. 21106018); Natural Science Foundation of Fujian Province, China (No. 2015J05024); supported by the Scientific
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Research Foundation of Third Institute of Oceanography, SOA. (No. 2014027).
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ACCEPTED MANUSCRIPT [10] S. Heidenreich, F. Ebert, Condensational droplet growth as a preconditioning technique for the separation of submicron particles from gases, Chem. Eng. Process 34 (1995) 235-244. [11] K. Schabe, J. Korber, O. Ofenloch, R. Ehrig, P. Deuflhard, Aerosol formation in gas-liquid contact devices-
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removal, Chem. Eng. J. 290 (2016) 319-327. [21] S. Temkin, Elements of Acoustic, Wiley, New York, 1981. [22] F. Fan, X. Yang, C. N. Kim. Direct simulation of inhalable particle motion and collision in a standing wave field, J. Mech. Sci. Tech., 27 (2013) 1707-1712. [23] D. S. Kim, S. H. Park, Y. M. Song, D. H. Kim, K. W. Lee, Brownian coagulation of polydisperse aerosols in the transition regime, J. Aerosol Sci. 34 (2003) 859-868. [24] Z. Yang, Q. Guo, J. Li, Effect of atmosphere and relative humidity on particle agglomeration of fly ash in acoustic wave, J. Chem. Ind. Eng. 62 (2011) 1055-1061. [25] F. Fan, L. Yang, J. Yan, Z. Yuan, Numerical analysis of water vapor nucleation on PM2.5 from municipal solid waste incineration, Chem. Eng. J. 146 (2009) 259-265. [26] N. H. Fletcher, Size effect in heterogeneous nucleation, J. Chem. Phy. 29 (1958) 572 - 576． [27] J. Yan, Study on fine particles removal from coal combustion improved by vapor condensational growth. Ph. D. Thesis. 2009, School of Energy and Environment, Southeast Univeristy. 19
ACCEPTED MANUSCRIPT [28] S. Dong, B. Lipken, T. M. Cameron, The effects of orthokinetic collision, acoustic wave, and gravity on acoustic agglomeration of polydisperse aerosols, J. Aerosol Sci. 37 (2006) 540-553. [29] E. F. Mikhailov, S. S. Vlasenko, L. Kramer, R. Niessner, Interaction of soot aerosol particles with water droplets:
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influence of surface hydrophilicity, J. Aerosol Sci. 32 (2001) 697-711.
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[30] H. Chen, J. Cao, R. Zhang, X. Shen, Study on vibrating velocity in high-intensity standing wave field, J. Southeast
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Elements
O
Mg
Al
Si
S
K
Ca
Initial
54.81
0.59
14.82
24.49
0.76
1.55
0.98
Acoustic field
49.91
0.88
13.15
18.76
8.56
1.87
5.27
45.19
0.43
5.21
6.96
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Table 1 The percentage of particle compositions given by EDS / %
21.35
17.87
Coupling external
1.69
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ACCEPTED MANUSCRIPT The list of figure captions: Fig. 1 Schematic diagram of experimental set-up.
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Fig. 2 Influence of vapor consumption on supersaturation degree with and without WFGD.
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Fig. 3 The entrainment factor of fine particles and water droplets in acoustic field. Fig. 4 Particle size distribution with and without external field.
Fig. 5 Particle stage removal efficiency in different external field.
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Fig. 6 Scanning electron micrographs of fine particles from coal combustion.
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Fig. 7 Influence of supersaturation degree on fine particle removal efficiency with the coupling external fields.
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particles from WFGD.
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Fig. 8 Calculation of critical supersaturation degree varies with radius for droplets and fine
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Fig. 9 The influence of residence time on fine particle removal efficiency. Fig. 10 Influence of horn numbers on electric power and fine particle removal efficiency.
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Fig. 11 Fine particle removal efficiency varies with the sound pressure level.
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Cyclone Cyclone
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Blower
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Fig. 6 Scanning electron micrographs of fine particles from coal combustion. (a) Initial particles,
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(b) Particles after WFGD with acoustic field and (c) Particles after WFGD with the combined effect of acoustic agglomeration and vapor condensational growth.
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Power 60
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Turbulence coagulation
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Fig. 11 Fine particle removal efficiency varies with the sound pressure level.
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ACCEPTED MANUSCRIPT Abstract The characteristics of fine particle removal from wet flue gas desulfurization (WFGD) using the simultaneous acoustic agglomeration and supersaturated vapor condensation were investigated experimentally. The impacts of supersaturation degree, residence time and acoustic intensity on the fine particle removal efficiency were
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demonstrated. High moisture and tiny droplets were contained in the flue gas after WFGD, which benefited the use of coupling external fields in WFGD system. The results showed that the removal of fine particles can be
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significantly improved in the simultaneous acoustic field and supersaturated vapor condensation. High value of sound pressure level (SPL >150 dB) was needed to achieve a considerable removal efficiency using acoustic
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agglomeration only. While low SPL of 130 - 150 dB can be used to remove fine particle efficiently in WFGD system using simultaneous external fields. Removal efficiency higher than 70 % was obtained with S = 1.15 and SPL = 151 dB. The residence time of the simultaneous acoustic agglomeration and condensational growth was determined by the time required by acoustic agglomeration. Particle removal efficiency increased with the
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residence time, but it tended to be constant as the residence time over 3 s. Multiple horn numbers with low acoustic intensity benefited the improvement of fine particle removal efficiency and the reduction of energy
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consumption using the simultaneous external fields in WFGD system.
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ACCEPTED MANUSCRIPT Vapor
Droplets
Growth droplets
Capture
Clean flue gas
Aggregates
Particles
T/H
T/H
Mesh
Horn
Flue gas
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Lage aggregates were formed and captured by the mesh to achieved clean flue gas.
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Power amplifier PC
Desulfurization scrubber
Signal generator
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Exhaust
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Graphical abstract
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ACCEPTED MANUSCRIPT Highlights
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>Simultaneous acoustic field and vapor condensation was employed in WFGD. > High moisture and droplets contain benefited the use of simultaneous fields. > High removal efficiency is obtained with a low intensity acoustic. > The residence time of the coupling fields was determined by acoustic agglomeration. > Multiple horn numbers with low SPL contributes the use of simultaneous fields.
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S0032-5910(17)30273-5 doi:10.1016/j.powtec.2017.03.056 PTEC 12461
To appear in:
Powder Technology
Received date: Revised date: Accepted date:
12 January 2017 8 March 2017 22 March 2017
Please cite this article as: Jinpei Yan, Liqi Chen, Qi Lin, Removal of Fine Particles in WFGD System Using the Simultaneous Acoustic Agglomeration and Supersaturated Vapor Condensation, Powder Technology (2017), doi:10.1016/j.powtec.2017.03.056
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ACCEPTED MANUSCRIPT
Removal of Fine Particles in WFGD System Using the Simultaneous Acoustic
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Jinpei Yan *, Liqi Chen, Qi Lin
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Agglomeration and Supersaturated Vapor Condensation
Key Laboratory of Global Change and Marine-atmospheric Chemistry, Third Institute of Oceanography, SOA, Xiamen
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361005, P R China
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Abstract: The characteristics of fine particle removal from wet flue gas desulfurization (WFGD) using the simultaneous
acoustic agglomeration and supersaturated vapor condensation were investigated experimentally. The impacts of
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supersaturation degree, residence time and acoustic intensity on the fine particle removal efficiency were demonstrated.
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High moisture and tiny droplets were contained in the flue gas after WFGD, which benefited the use of coupling external
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fields in WFGD system. The results showed that the removal of fine particles can be significantly improved in the
simultaneous acoustic field and supersaturated vapor condensation. High value of sound pressure level (SPL >150 dB)
was needed to achieve a considerable removal efficiency using acoustic agglomeration only. While low SPL of 130 - 150
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dB can be used to remove fine particle efficiently in WFGD system using simultaneous external fields. Removal
efficiency higher than 70 % was obtained with S = 1.15 and SPL = 151 dB. The residence time of the simultaneous
acoustic agglomeration and condensational growth was determined by the time required by acoustic agglomeration.
Particle removal efficiency increased with the residence time, but it tended to be constant as the residence time over 3 s.
Multiple horn numbers with low acoustic intensity benefited the improvement of fine particle removal efficiency and the
reduction of energy consumption using the simultaneous external fields in WFGD system.
Keywords: Fine particles; Acoustic agglomeration; Condensational growth; Supersaturation; WFGD; Removal
*
Corresponding author. E-mail address: [email protected] 1
ACCEPTED MANUSCRIPT 1. Introduction Fine particles (PM2.5) have been regarded as the major air pollutants in urban areas, which
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constituted a major health hazard because of their containing of toxic components and ability to
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penetrate into the respiratory system [1, 2]. Coal-fired power plants are considered to be one of the major pollution sources of fine particles in ambient atmosphere [3]. Removal of fine particles from flue gas with high efficiency has become important. But conventional separation technologies, such as
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electrostatic precipitators and cyclone separators were hard to separate these particles efficiently [4, 5].
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In order to improve the removal efficiency of fine particles, a preconditioning technique is required to increase the particle size before entering conventional separation devices. Such preconditioning
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of different external field [13-15].
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technique can be acoustic agglomeration [6-9], heterogeneous condensation [10-12] or coupling effect
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Generally, fly ash and SO2 can be removed efficiently by desulfurization scrubbing installed in power plants, but it was inefficient in separating PM2.5 [16-18]. Previous studies have found that fine
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particle removal efficiency declined with particle size in wet limestone-based FGD from a coal-fired power plant. Most of the particulate matter can be ascribed to PM2.5 after wet limestone-based FGD [18]. However, rare investigations were present on the removal of fine particles in WFGD process. Hot flue gas contacted with cold desulfurization slurry counter-currently in the WFGD process. High humidity of flue gas was achieved after desulfurization scrubbing, causing by the intense heat and mass transfer of scrubbing liquid and flue gas. Removal of fine particles from flue gas in WFGD system using heterogeneous condensational growth have been investigated in previous studies [4, 19]. Supersaturated surrounding was obtained by steam addition after desulfurization scrubber. Condensational growth of water vapor occurred on the surfaces of fine particles to enlarge their sizes. In this case, grown droplets
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ACCEPTED MANUSCRIPT can be separated efficiently by the demister [19]. However, high vapor consumption inhibited the practical application of vapor condensational growth. Combination of acoustic wave with vapor
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condensational growth was considered to be a novel preconditioning process for particle separation.
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High particle removal efficiency can be obtained in the simultaneous acoustic agglomeration and supersaturated vapor condensation with low acoustic intensity [20].
High relative humidity, large quantity of tiny droplets and high concentration of fine particles were
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present in the flue gas after WFGD, which was considered to be the primary problem of flue gas exhaust
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after WFGD. Therefore, the flue gas after WFGD was quite different from the flue gas without WFGD system. In this study, according to the flue gas characteristics after WFGD, the simultaneous effect of
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acoustic field and vapor condensation was deployed to the WFGD system to improve the fine particle
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removal efficiency and reduce the droplet concentration in WFGD flue gas. Supersaturation degree
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required by the condensational growth was achieved by changing the moisture content and flue gas temperature, while standing - wave was introduced into the flue gas after WFGD to form the
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simultaneous effect of acoustic field and supersaturated vapor condensation. The influences of supersaturation degree, acoustic intensity and residence time, etc. on fine particle removal efficiency in WFGD system were investigated using the simultaneous effect of acoustic agglomeration and supersaturated vapor condensational growth. 2 Experimental details 2.1 Experimental set-up Flue gas with 80 Nm3.h-1 was generated by a fluidized bed coal - fired boiler, as seen in Fig. 1. A buffer vessel with an electric heater and stirrer were located after the boiler. Particles larger than 10 m were separated by a cyclone before passing a heat exchanger, where the flue gas was cooled by cold air.
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ACCEPTED MANUSCRIPT A desulfurization scrubber with diameter of 150 mm and 1500 mm height was used in this experiment. Supersaturated vapor injector was deployed at the top of the desulfurization scrubber to increase the
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moisture content in flue gas. Flue gas with high humidity after desulfurization scrubber then passed
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through a growth and removal chamber. A high efficiency wire mesh was employed at the outlet of removal chamber to remove the droplets enlarged by the simultaneous external fields. At the exit of the growth and removal chamber, fine particle size distribution and number concentration was measured in
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real time using an Electrical Low Pressure Impactor (ELPI, Dekati Co. Ltd., Finland) before and after
previous studies [20].
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2.2 The growth and removal chamber
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different external field. The measurement of fine particles by ELPI was descried carefully in the
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The growth and removal chamber was made of a polymethyl methacrylate tube with length of 1500
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mm and inner width of 200X200 mm. The simultaneous effect of acoustic field and supersaturated vapor surrounding was formed in the chamber, where fine particles were enlarged by the combined effect of
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acoustic agglomeration and vapor condensational growth. Multiple standing-wave fields were used in this study. To form the multiple standing-wave fields, a rectangular tube was used. Four horns (KTD 250) were employed in the same side of the growth chamber with a reflecting plate equipped in the opposite side of the growth chamber. The distance between the original acoustic wave and the reflected wave was well defined as the integer multiple of the half wavelength. Acoustic wave was generated by the horn, driving by a signal generator (DF1027B, Zhongce Co. Ltd., China), which was powered by a power amplifier (DF5883, Zhongce Co. Ltd., China). The range of the acoustic frequency was 180-7000 Hz. The supersaturated vapor surrounding required by condensational growth was achieved by cooling
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ACCEPTED MANUSCRIPT the flue gas and increasing the moisture content in the flue gas. A vapor addition device was set at the out let of the scrubber to increase the humidity of flue gas. The outside of the growth chamber was
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covered by an isothermal interlayer to achieve the requirement temperature of flue gas in the chamber.
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In this case, the supersaturation degree in the growth chamber can be well identified. Droplets grown in the growth chamber with the simultaneous external fields was captured with a high efficiency wire mesh demister installed at the outlet of the chamber.
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3 Results and discussion
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3.1 Contribution to fine particles removal in WFGD using simultaneous external fields Supersaturated vapor surrounding was required to generate the simultaneous effect of acoustic field
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and vapor condensation. In the WFGD scrubber, hot flue gas contacted with low temperature
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desulfurrization liquid counter-currently with intense mass and heat transfer, resulting in the solution
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liquid evaporation. Therefore, flue gas with high relative humidity (85 - 90 %) was present after desulfurrization scrubbing. Consequently, one can expect that supersaturated vapor required by the
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heterogeneous condensational growth can be formed easily by regulating the temperature or moisture content in the flue gas after WFGD system. Fig. 2 shows the influence of vapor addition amount on supersaturation degree with and without WFGD system. Vapor addition amount of 0.06 kg. Nm-3 was needed to generate the supersaturated vapor surroundings without WFGD, while supersaturation degree of 1.0 was achieved with steam addition amount of 0.01 kg. Nm-3 after WFGD. Standing - acoustic field was also needed to generate the simultaneous external fields. Generally, according to acoustic entrainment theory, the motion of fine particles was driven by the entrainment force. Entrainment factor neglecting inertia forces can be simplified as follow [21]:
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Where
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Up 1 Uo 1 ( )2
is the particle entrainment factor,
represents the relaxation time,
is the air velocity amplitude,
Up
is the particle
is the angular frequency of acoustic wave.
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velocity amplitude,
Uo
(1)
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Fig. 3 illustrated the entrainment factor of fine particles and droplets in acoustic field with sound pressure level of 151 dB and frequency of 1400 Hz. The value of entrainment factor of droplets was
is a
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larger than that of coal - fired particles at the size range of 1 - 20 µm. The relaxation time
function of the particle density. Since the density of water is lower than coal-fired particles, entrainment
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factor of droplets was larger than that of coal-fired particles. Submicron particles (< 1 µm) oscillated
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fully with flue gas, as the entrainment factor was close to one. However particles larger than 20 µm were
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almost at rest with the value of entrainment factor of zero [13]. As seen in Fig. 4, most of the particles from WFGD were in submicron range. Note that the particles and droplets from WFGD were mainly in
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the submicron size range. That means both particles and droplets would oscillate fully with the flue gas. Previous study have found that fine particles could not collide with each other or with droplets easily to
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form large aggregates in the acoustic field due to the little difference of entrainment factors between particles and droplets [22]. That was the reason low particle removal efficiency was present, while individual acoustic agglomeration was used to remove fine particles from flue gas after WFGD. However, when fine particles were enlarged firstly by vapor condensational growth, the entrainment factor of the grown droplets was much higher than fine particles. The difference of entrainment factors between fine particles and droplets increased the collision probability dramatically, knowing as bimodal acoustic agglomeration [7]. 3.2 Fine particle size distribution and stage removal efficiency During the desulfurization processing, fine particles were captured by the scrubbing solution due to 6
ACCEPTED MANUSCRIPT the diffusion and inertial impact in WFGD scrubbing. On the other hand, part of the desulfurization solution and products were also carried out from the desulfurization scrubber which increased the
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particle concentration of flue gas after WFGD [16]. Fig. 4 shows the particle size distribution and
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particle number concentration with different external field. In this study, the initial particle size distribution, measuring without desulfurization scrubbing and any external field, was a unimodal distribution peaking at 0.08 μm. Most of fine particles were in submicron range, accounting for more
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than 95% of the total particles. However, fine particle number concentration at the size range of 0.08 -
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0.3 μm increased after desulfurization scrubbing, indicating that new particles were generated in the desulfurization process. Previous studies have found that sulfate products and un-reacted limestone
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particulate matters were present in the flue gas after WFGD [4, 16]. However, fine particle number
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concentration decreased using individual vapor condensational growth (S = 1.15) and acoustic
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agglomeration (I = 151 dB). Comparing to the individual external field, particle number concentration decreased dramatically by the simultaneous acoustic field and supersaturated vapor condensation after
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WFGD. Large quantity of grown droplets was present in the flue gas after the simultaneous effects, which acted as the seed particles in the standing-wave field. Therefore, fine particles were captured by the grown droplets in the simultaneous external fields and separated by the demister easily. The particle stage removal efficiency was useful to provide an insight into the particle removal characteristics. The stage removal efficiencies of fine particles using acoustic agglomeration, vapor condensation, the simultaneous external fields and without any external field after WFGD were shown in Fig. 5. The variation of particle number concentration in a certain diameter range was determined by the effect of collision, agglomeration or condensational growth [20]. Fine particle removal efficiency of desulfurization scrubbing was extremely low without any external field, meaning that fine particles can
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ACCEPTED MANUSCRIPT not be captured efficiently by the desulfurization scrubber. Additional removal technique was needed to reduce the particle concentration in flue gas after WFGD. Note that a negative value of particle removal
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efficiency was present at the size range of 0.07 - 1.0 μm, indicating that the particle number
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concentration increased in this size range after desulfurization. As seen in Fig. 4, high concentration of fine particle with size smaller than 0.1 μm was present after WFGD. Fine particles in the size range smaller than 0.1 μm tended to collide and adhere into large particles by diffusion and Brownian
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coagulation, which increased the particle number concentration in the size range larger than 0.1 μm [23].
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Though the Brownian coagulation could not be neglected in this case, the increase of particle number concentration at this size range was caused by the new particles formation in the desulfurization process
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in this study. Similar results of new particles generation in WFGD system were obtained in the previous
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studies [18].
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Fine particle removal efficiency increased with the particle size in the effect of vapor condensational growth (S = 1.15), acoustic wave (I = 151 dB) and the simultaneous acoustic agglomeration and vapor
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condensation (S = 1.15, I = 151 dB). The stage removal efficiency was about 10 % - 40 % with vapor condensational growth, which was lower than the value using acoustic agglomeration. As mentioned above, flue gas after WFGD contained large quantity of fine droplets and with high relative humidity. Though high moisture content in the flue gas after WFGD benefited the reduction of vapor consumption, large number of tiny droplets would impede the condensation of supersaturated vapor on the surfaces of fine particles. Competitive condensation between fine particles and droplets occurred in the supersaturated vapor surrounding. Supersaturated vapor pressure required by the nucleation growth on the droplet surfaces was lower than the value for fine particles. That means supersaturated vapor tended to condense on the droplet surfaces but not fine particles. The condensable vapor amount of
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ACCEPTED MANUSCRIPT supersaturated vapor on the surfaces of fine particles reduced. Removal of fine particles in WFGD system can not be highly improved by the vapor condensational growth in this case, especially with low
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supersaturation degree. While acoustic agglomeration was used in the WFGD system, the collision and
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coagulation of fine particles can be enhanced by the difference of entrainment factor between fine particles and droplets in the WFGD flue gas. Moreover, high relative humidity of flue gas after WFGD benefited the formation of firm aggregates in the acoustic field [24]. Both effects contributed the
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coagulation of fine particles. However, since fine particles and droplets contained in the flue gas after
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WFGD were in small size and fully oscillating with the gas. The collision between fine particles and droplets was also inhibited in the acoustic field. High stage removal efficiency of 53 % - 80 % was
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obtained in the simultaneous acoustic agglomeration and vapor condensational growth, indicating that
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fine particles in flue gas after WFGD can be removed efficiently with the simultaneous external fields.
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3.3 The micro characteristics of fine particles with different external field The removal efficiency of fine particles in WFGD process was determined by the final aggregate
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size enlarged by the simultaneous acoustic field and vapor condensational growth. The micro morphology of fine particles can provide the further insight into the particle agglomeration properties with or without external fields. Fig. 6 shows the scanning electron micrographs of the initial particles and particles after WFGD with acoustic field only and the coupling fields. The majority initial particles were independent spherical particles, as seen in Fig. 6 a. Most of fine particles were in submicron range, which was in good agreement with the measuring results given in Fig. 4. Individual particles and aggregates with different size were both present after the effect of acoustic wave, as seen in Fig. 6 b. Different fine particles with one or several coarse nuclei constituted the aggregates. This confirmed that small particles collided with coarse particles in acoustic field due to the discrepancy of entrainment
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ACCEPTED MANUSCRIPT factor, seen in Fig. 3. Large aggregates were present after WFGD using the combined effect of acoustic agglomeration and
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condensational growth, as seen in Fig. 6 c. Comparing to Fig. 6 b, the micrographs of particles in the
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coupling external fields were much more complex and irregular. In the simultaneous acoustic field and vapor condensation, droplets and fine particles in the flue gas were grown into large droplets, which were easy to collide with fine particles to form large aggregates. The size of aggregates in Fig. 6 c was
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larger than the aggregates in Fig. 6 b. In fact, droplets usually carried with desulfurization products in
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the flue gas after WFGD. When the droplets collided with fine particles, the desulfurization products may adhere on the particle surfaces to form complex aggregates. However, as mentioned above, fine
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particles were hard to collide with fine droplets in the flue gas after WFGD with acoustic field only. In
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this case, the aggregates were smaller in size and more regular. This can be further demonstrated by the
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particle compositions with different external fields. The percentage of Ca and S elements showed a significant increase using the simultaneous external fields after WFGD, seen in table 1, suggesting that
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desulfurization products was included in the aggregates in this case. The SEM images and particle components also substantiated the formation of new particles in WFGD process, mentioned in Section 3.2. Note that both single particles and aggregates were present in Fig. 6 b, while large aggregates with rare single particles were shown in Fig. 6 c, indicating that most of fine particles were conducted to form large aggregates using the simultaneous external fields in flue gas after WFGD. Particle number concentration can be significantly reduced when large quantity of fine particles formed the aggregates. SEM images also demonstrated the beneficial effect of fine particles removal using the simultaneous acoustic agglomeration and vapor condensational growth. 3.4 Influence of supersaturation degree on fine particles removal with simultaneous fields
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ACCEPTED MANUSCRIPT Fig. 7 shows the fine particle removal efficiency varies with supersaturation degree using simultaneous external fields in WFGD system. Fine particle removal efficiencies were extremely low
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and independence with the increase of supersaturation degree, while the supersaturation degree was
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below 1.0. Supersaturation degree higher than the critical value was required by the condensational growth of droplets [25]. In this case, when the supersaturation degree was lower than 1.0, fine particles could not be enlarged by vapor condensational growth. The simultaneous effect of acoustic field and
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vapor condensational growth could not be achieved in this case, and the particles were agglomerated by
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acoustic wave only. Contrarily, when the supersaturation degree was higher than 1.0, fine particle removal efficiency can be improved significantly. Removal efficiency higher than 70 % was obtained
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using the simultaneous external fields with supersaturation degree of 1.15 and SPL = 151dB, which was
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much higher than the value (about 20 %) with supersaturation degree of 1.15 but without acoustic field.
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As seen in Fig. 7, fine particle removal efficiency without acoustic field was much lower than the value with the simultaneous external fields in WFGD for the same supersaturation degree. As known
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that the heterogeneous condensational growth of fine particles was governed by the heat and mass transfer between particles and vapor. According to the heterogeneous nucleation theory [26], if one assumed that the condensation nuclei were completely covered by a liquid layer, they can be regarded as pure droplets [10]. In this case, the critical supersaturation and the size of the nuclei in equilibrium can be described by the Kelvin equation:
Scr exp( Where,
is the surface tension, M and
2 M ) RT r
are
（2）
the molar mass and density of the liquid,
respectively. r is the radius of the nucleus, T is the temperature of liquid surface and R is the ideal gas constant. 11
ACCEPTED MANUSCRIPT While the particle surfaces were not covered by liquid layer, higher supersaturation degree was needed to activate those solid particles. Moreover, the shape and the wetability properties of the nucleus’
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surface can influence the critical supersaturation. As seen in Fig. 6 a, most of fine particles after WFGD
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were insoluble spherical particles. The critical supersaturation degree Scr needed to activate fine particles from WFGD system can be described by the following equation [27]:
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8Vw2eg3 Scr exp f (mf , x) 3kT ln(4 r 2 K ) C
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Where, Vw is the embryo volume, k is the Boltzmann constant, embryo-gas, Kc is a kinetic coefficient,
f (mf , x)
1
2
eg
(3)
is the interfacial tensions of the
is a function of relative contact angle.
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Supersaturation degree higher than the critical value was the first requirement for the condensational
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growth of water vapor. Fig. 8 illustrates the critical supersaturation degree varies with radius of droplets
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and fine particles from WFGD. The critical supersaturation degree decreased rapidly, but it trended to be constant for particles with size larger than 0.3 µm. The critical supersaturation degree required to
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activate fine particles from WFGD was much higher than the value to activate droplets with the same size. That means supersaturated vapor would firstly condense on the surfaces of water droplets but not on particle surfaces in the same supersaturated vapor surrounding, which was known as the competitive condensation, especially in a low supersaturation degree. As seen in Fig. 7, the removal efficiency was about 27 % and 70 % using vapor condensation (S = 1.15, I = 0 dB) and the simultaneous external fields (S = 1.15, I = 151 dB) individually, while the value was 87 % and 94 % for the vapor condensational growth (S = 1.43, I = 0 dB) and the coupling external fields (S = 1.43, I = 151 dB). Large quantity of fine droplets was present in flue gas after WFGD, resulting in large amount of supersaturated vapor condensed on the surfaces of droplets, which has a detrimental impact on the condensational growth of
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ACCEPTED MANUSCRIPT fine particles. In this case, the particle removal efficiency was affected directly in WFGD system using vapor condensational growth. However, with the increase of supersaturation degree, sufficient vapor
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was provided to condense on both the surfaces of particles and droplets. Fine particles can be grown
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large enough to be separated. Hence, high removal efficiency was obtained with high supersaturation degree, event without acoustic field.
In the simultaneous acoustic agglomeration and vapor condensational growth, the collision of
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particle - particle, droplet - droplet and particle-droplet occurred in the acoustic field to form large
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aggregates. Firstly, the critical supersaturation degree required by vapor condensational growth decreased when fine particles were enlarged by acoustic agglomeration. That benefited the vapor
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condensation on the surfaces of particles. Moreover, the coagulation of droplets and particles reduced
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the droplet number concentration in flue gas, which decreased the competitive condensation between
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particles and droplets. Both contributed to the vapor condensational growth on the surfaces of fine particles. Meanwhile, droplets enlarged by heterogeneous condensation acting as seed particles in the
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acoustic field, which further promoted the agglomeration of fine particles. In fact, the enhancement of agglomeration and condensational growth by the coupling effects took full advantage of the high moisture content in flue gas after WFGD to decrease the vapor consumption. Moreover, the agglomeration of fine particles and droplets in acoustic field reduced the particle and droplet concentration simultaneously in flue gas after WFGD. 3.5 Influence of residence time on fine particle removal efficiency with the simultaneous fields Residence time was a key parameter for the practical use of the simultaneous external fields in WFGD system. Fig. 9 shows the particle removal efficiency varies with the residence time under different external fields. Fine particle removal efficiency increased rapidly with the residence time. For
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ACCEPTED MANUSCRIPT vapor condensational growth, when the residence time was over 1 s, particle removal efficiency did not increase with the residence time anymore. Generally, the condensational growth of droplets was
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completed within tens of milliseconds [10]. The surrounding supersaturation degree decreased as the
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vapor condensed on the particle surfaces. Moreover, the latent heat of vapor condensation would increase the surrounding temperature, which further decreased the supersaturation degree. The droplet growth rate descended with the decrease of supersaturation degree. Droplets were no longer enlarged,
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when the supersaturation surrounding disappeared. Hence the removal efficiency remained steady in this
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case. Comparing with the vapor condensation, longer residence time was needed for the acoustic agglomeration. Fine particle removal efficiency tended to be constant when the residence time was over
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3 s. Previous studies also indicated that the acoustic agglomeration of fine particles usually completed in
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about 3 s [28]. Note that a slight decrease of removal efficiency was present, when the residence time
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was over 5 s. In fact, the formation and breakage of particle aggregates occurred stochastically in the acoustic field. The breakage of particle aggregates would be enhanced for a long residence time in the
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acoustic field, which may result in the decrease of particle removal efficiency. In the simultaneous effect of acoustic field and vapor condensation, fine particle removal efficiency increased with the residence time, but it tended to be constant as the residence time was over 3 s, which was similar with the variation of particle removal efficiency using acoustic agglomeration only. Fine particle removal efficiency increased from 40 % to 80 %, while the residence time was extended from 0.5 s to 3 s. It indicated that residence time for the simultaneous external fields was determined by the time required by the acoustic agglomeration. Hence, the considerable residence time for the coupling external fields was about 3 s. It is useful to find that the time needed by vapor condensational growth was much less than the time required by the acoustic agglomeration. Hence, in the simultaneous effect
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ACCEPTED MANUSCRIPT of acoustic field and vapor condensation, fine particles and droplets in flue gas after WFGD were first enlarged by the vapor condensational growth. The grown droplets then acted as the seed particles in the
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acoustic field to capture fine particles. This can be certified by the micrographs of fine particles, as seen
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in Fig. 6 c, aggregates formed by fine particles and large proportion of desulfurization products were present after the simultaneous effect of acoustic field and vapor condensation. It is worth to note that fine particle removal efficiency using the simultaneous fields did not decrease as the residence time
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increase. That was determined by the adhesion forces between particles and droplets [29]. The
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characteristics of particle surfaces were changed when they were covered with water. Large forces like liquid bridge were formed between particles and droplets, forming firmly aggregates. In other words, the
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particle removal efficiency.
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breakage of aggregates occurred rarely in the acoustic field, which benefited the improvement of fine
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3.6 Influence of Acoustic property on fine particle removal with the simultaneous fields As mentioned above, high acoustic intensity resulted in high particle removal efficiency, but also
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meaning high energy consumption. Fig. 10 depicts the influence of horn numbers on electric power and fine particle removal efficiency with different sound pressure level. Fine particle removal efficiency increased with the SPL as well as the electric power. The results showed that the SPL at the outlet of horn rang from 130 - 158 dB for input powers of 0.1 - 15 W of one horn with particle removal efficiency from 50% to 89 %. The input electric power increased rapidly with the sound pressure level, especially in high value of SPL. Electric power increased only 2 W when the sound pressure level increased from 130 to 151 dB, while the value increased about 13 W as the sound pressure level increased from 151 to 158 dB. Low acoustic intensity benefited the reduction of the energy consumption. As seen in Fig. 10, fine particle removal efficiency increased evidently with the horn numbers, especially for low value of
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ACCEPTED MANUSCRIPT SPL. The removal efficiency increased about 25 % with the horn numbers increasing from 1 to 4 at a SPL of 130 dB. However, the removal efficiency increased only about 5 % at a SPL of 158 dB.
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Contrarily, the input electric power increased about 36 W with the horn numbers increasing from 1 to 4
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at a SPL of 158 dB, while the value was about 8 W at a SPL of 151 dB. Multiple horn numbers with low sound pressure level would benefit the practical used of the simultaneous effect of acoustic
acoustic intensity with low energy consumption.
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agglomeration and vapor condensation. In this case, fine particles can be removed efficiently in low
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To further clarify the influence of acoustic wave on the removal efficiency, the influence of sound pressure level on fine particle removal efficiency is illustrated in Fig. 11. A considerable removal
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efficiency of about 40 % was obtained only when the sound pressure level was higher than 151 dB with
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supersaturation degree of 0.92. The acoustically induced turbulence appeared in high value of SPL (>
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158 dB), which enhanced the particle agglomeration in the acoustic field [30]. As seen in Fig. 10, high value of SPL implied high electric power and energy consumption, which was not suitable for
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commercial industrial applications. But fine particle removal efficiency was significantly improved using the simultaneous external fields (S = 1.15) in the WFGD system, even in low value of sound pressure level. Fine particle removal efficiency of 10 % and 60 % was obtained individually for S = 0.92 and S = 1.15 with the sound pressure level of 130 dB, indicating that fine particle removal efficiency can be efficiently improved by the simultaneous external fields. The results illustrated that the value of SPL higher than 150 dB was needed to achieve a considerable removal efficiency using acoustic agglomeration only. While low sound pressure level of 130 - 150 dB was a considerable acoustic intensity using the simultaneous effect of acoustic field and supersaturated vapor condensation to remove fine particle efficiently from flue gas in WFGD system.
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ACCEPTED MANUSCRIPT 4 Conclusions The simultaneous effect of acoustic agglomeration and supersaturated vapor condensation was
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employed in the WFGD system to improve fine particles removal. High moisture and tiny droplets were
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present in flue gas after WFGD, which benefited the use of coupling external fields in WFGD system. The consumption of vapor required by the simultaneous fields can be reduced efficiently based on the high moisture content in flue gas after WFGD. Droplets in flue gas benefited the agglomeration of fine
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particles in the acoustic field due to the differences of entrainment factor between particles and droplets.
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The influence of supersaturation degree, sound pressure level and residence time, etc. on fine particle removal efficiencies from WFGD were demonstrated in this study.
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Fine particle removal efficiency can be significantly improved by the simultaneous acoustic
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agglomeration and vapor condensation with stage removal efficiency up to 80 %. The removal
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efficiency increased with the sound pressure level and supersaturation degree in the simultaneous external fields. High value of SPL (> 150 dB) was required to achieve a considerable removal efficiency
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using acoustic agglomeration in WFGD system. But low sound pressure level of 130 - 150 dB was a considerable acoustic intensity using the simultaneous external fields to remove fine particle efficiently from flue gas in WFGD system. The residence time of the simultaneous acoustic agglomeration and vapor condensation was determined by the time needed for the acoustic agglomeration. Residence time larger than 3 s was needed to achieve high removal efficiency using the simultaneous fields in WFGD system. High acoustic intensity resulted in high particle removal efficiency, but also meaning high energy consumption. Multiple horn numbers with low acoustic intensity would benefit the improvement of fine particle removal efficiency and the reduction of energy consumption using simultaneous external fields
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ACCEPTED MANUSCRIPT in WFGD system. It indicates that high removal efficiency of fine particles in WFGD system can be achieved by the simultaneous effect of acoustic agglomeration and vapor condensation with low vapor
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consumption and acoustic intensity.
Acknowledgement
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This study was supported by the National Natural Science Foundation of China (No. 21106018); Natural Science Foundation of Fujian Province, China (No. 2015J05024); supported by the Scientific
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Research Foundation of Third Institute of Oceanography, SOA. (No. 2014027).
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ACCEPTED MANUSCRIPT [10] S. Heidenreich, F. Ebert, Condensational droplet growth as a preconditioning technique for the separation of submicron particles from gases, Chem. Eng. Process 34 (1995) 235-244. [11] K. Schabe, J. Korber, O. Ofenloch, R. Ehrig, P. Deuflhard, Aerosol formation in gas-liquid contact devices-
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desulfurated scrubbed flue gas, Fuel 108 (2013) 73-79. [19] H. Wu, L. Yang, J. Yan, G. Hong, B. Yang, Improving the removal of fine particles by heterogeneous condensation during WFGD processes, Fuel Process. Tech. 145 (2016) 116-122. [20] J. Yan, L. Chen, L. Yang, Combined effect of acoustic agglomeration and vapor condensation on fine particles
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removal, Chem. Eng. J. 290 (2016) 319-327. [21] S. Temkin, Elements of Acoustic, Wiley, New York, 1981. [22] F. Fan, X. Yang, C. N. Kim. Direct simulation of inhalable particle motion and collision in a standing wave field, J. Mech. Sci. Tech., 27 (2013) 1707-1712. [23] D. S. Kim, S. H. Park, Y. M. Song, D. H. Kim, K. W. Lee, Brownian coagulation of polydisperse aerosols in the transition regime, J. Aerosol Sci. 34 (2003) 859-868. [24] Z. Yang, Q. Guo, J. Li, Effect of atmosphere and relative humidity on particle agglomeration of fly ash in acoustic wave, J. Chem. Ind. Eng. 62 (2011) 1055-1061. [25] F. Fan, L. Yang, J. Yan, Z. Yuan, Numerical analysis of water vapor nucleation on PM2.5 from municipal solid waste incineration, Chem. Eng. J. 146 (2009) 259-265. [26] N. H. Fletcher, Size effect in heterogeneous nucleation, J. Chem. Phy. 29 (1958) 572 - 576． [27] J. Yan, Study on fine particles removal from coal combustion improved by vapor condensational growth. Ph. D. Thesis. 2009, School of Energy and Environment, Southeast Univeristy. 19
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[30] H. Chen, J. Cao, R. Zhang, X. Shen, Study on vibrating velocity in high-intensity standing wave field, J. Southeast
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Elements
O
Mg
Al
Si
S
K
Ca
Initial
54.81
0.59
14.82
24.49
0.76
1.55
0.98
Acoustic field
49.91
0.88
13.15
18.76
8.56
1.87
5.27
45.19
0.43
5.21
6.96
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Table 1 The percentage of particle compositions given by EDS / %
21.35
17.87
Coupling external
1.69
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fields
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ACCEPTED MANUSCRIPT The list of figure captions: Fig. 1 Schematic diagram of experimental set-up.
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Fig. 2 Influence of vapor consumption on supersaturation degree with and without WFGD.
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Fig. 3 The entrainment factor of fine particles and water droplets in acoustic field. Fig. 4 Particle size distribution with and without external field.
Fig. 5 Particle stage removal efficiency in different external field.
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Fig. 6 Scanning electron micrographs of fine particles from coal combustion.
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Fig. 7 Influence of supersaturation degree on fine particle removal efficiency with the coupling external fields.
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particles from WFGD.
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Fig. 8 Calculation of critical supersaturation degree varies with radius for droplets and fine
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Fig. 9 The influence of residence time on fine particle removal efficiency. Fig. 10 Influence of horn numbers on electric power and fine particle removal efficiency.
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Fig. 11 Fine particle removal efficiency varies with the sound pressure level.
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ACCEPTED MANUSCRIPT Fig. 1 T/H
T/H
Growth and removal chamber Mesh Horn Power amplifier
Exhaust
PC
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Heater Buffer Stirrer
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Signal generator Air compressor
Heat exchanger
Computer
Stirrer
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Mesh Vapor generator
ELPI
Gas analysis
Exhaust
Desulfurization scrubber
Fluid bed coal-fired Desulfurization slurry tank Exhaust
Fluid gas
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Cyclone Cyclone
Slurry pump
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Fig. 1 Schematic diagram of experimental set-up.
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Blower
ACCEPTED MANUSCRIPT Fig. 2
2.0
Flue gas without WFGD Flue gas with WFGD
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1.5
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S
Supersaturation 1.0
0.0 0.00
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0.5
0.05
0.10
0.15
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Steam consumption (kg.Nm-3)
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Fig. 2 Influence of vapor consumption on supersaturation degree with and without WFGD.
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ACCEPTED MANUSCRIPT Fig. 3 1.0
Particles Droplets
f=1400 Hz,I=151 dB
0.8
P
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0.6
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0.4
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0.2
0.0 0.1
1
10
100
Particle diameter (m)
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Fig. 3 The entrainment factor of fine particles and water droplets in acoustic field.
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ACCEPTED MANUSCRIPT Fig. 4 6x106
3x106
f=1400Hz
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dNdlogDp(1.cm-3)
4x106
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initial I=0dB, S=0.92 I=0dB, S=1.15 I=151dB, S=0.92 I=151dB, S=1.15
5x106
2x106
0 0.1
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1x106
1
Dp (m)
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Fig. 4 Particle size distribution with and without external field.
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ACCEPTED MANUSCRIPT Fig. 5 100
I=0dB， S=0.92 I=0dB， S=1.15 I=151dB， S=0.92 I=151dB， S=1.15
f = 1400Hz
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1
Dp (m)
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Fig. 5 Particle stage removal efficiency in different external field.
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Fig. 6
(b)
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Fig. 6 Scanning electron micrographs of fine particles from coal combustion. (a) Initial particles,
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(b) Particles after WFGD with acoustic field and (c) Particles after WFGD with the combined effect of acoustic agglomeration and vapor condensational growth.
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ACCEPTED MANUSCRIPT Fig. 7 100
I=0dB I=151dB,f=1400Hz
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1.0
1.1
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Removal efficiency (%)
80
1.2
S
1.3
1.4
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Fig. 7 Influence of supersaturation degree on fine particle removal efficiency with the coupling external fields.
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ACCEPTED MANUSCRIPT Fig. 8 1.6
Fine particles Droplets
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1.0
0.0
0.5
1.0
1.5
2.0
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r (m)
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Scr
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1.4
2.5
3.0
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Fig. 8 Calculation of critical supersaturation degree varies with radius for droplets and fine particles from WFGD.
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ACCEPTED MANUSCRIPT Fig. 9 100
I=0dB, S=1.15 I=1.15dB, S=0.92 I=1.15dB, S=1.15
f=1400Hz
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1
2
3
t (s)
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Removal efficiency (%)
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4
5
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Fig. 9 The influence of residence time on fine particle removal efficiency.
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ACCEPTED MANUSCRIPT Fig. 10 I=130,S=1.15 I=151,S=1.15 I=158,S=1.15
Power 60
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60
Power (W)
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80
40
20
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0 1
2
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Removal efficiency (%)
100
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Horn numbers
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4
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Fig. 10 Influence of horn numbers on electric power and fine particle removal efficiency.
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ACCEPTED MANUSCRIPT Fig. 11
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0 120
130
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Particle removal efficiency (%)
Turbulence coagulation
Considerable acoustic agglomeration area
S=0.92 S=1.15
100
150
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Sound pressure level (dB)
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Fig. 11 Fine particle removal efficiency varies with the sound pressure level.
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ACCEPTED MANUSCRIPT Abstract The characteristics of fine particle removal from wet flue gas desulfurization (WFGD) using the simultaneous acoustic agglomeration and supersaturated vapor condensation were investigated experimentally. The impacts of supersaturation degree, residence time and acoustic intensity on the fine particle removal efficiency were
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demonstrated. High moisture and tiny droplets were contained in the flue gas after WFGD, which benefited the use of coupling external fields in WFGD system. The results showed that the removal of fine particles can be
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significantly improved in the simultaneous acoustic field and supersaturated vapor condensation. High value of sound pressure level (SPL >150 dB) was needed to achieve a considerable removal efficiency using acoustic
SC R
agglomeration only. While low SPL of 130 - 150 dB can be used to remove fine particle efficiently in WFGD system using simultaneous external fields. Removal efficiency higher than 70 % was obtained with S = 1.15 and SPL = 151 dB. The residence time of the simultaneous acoustic agglomeration and condensational growth was determined by the time required by acoustic agglomeration. Particle removal efficiency increased with the
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residence time, but it tended to be constant as the residence time over 3 s. Multiple horn numbers with low acoustic intensity benefited the improvement of fine particle removal efficiency and the reduction of energy
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consumption using the simultaneous external fields in WFGD system.
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ACCEPTED MANUSCRIPT Vapor
Droplets
Growth droplets
Capture
Clean flue gas
Aggregates
Particles
T/H
T/H
Mesh
Horn
Flue gas
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Lage aggregates were formed and captured by the mesh to achieved clean flue gas.
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Power amplifier PC
Desulfurization scrubber
Signal generator
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Exhaust
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Graphical abstract
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Mesh
Vapor
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ACCEPTED MANUSCRIPT Highlights
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>Simultaneous acoustic field and vapor condensation was employed in WFGD. > High moisture and droplets contain benefited the use of simultaneous fields. > High removal efficiency is obtained with a low intensity acoustic. > The residence time of the coupling fields was determined by acoustic agglomeration. > Multiple horn numbers with low SPL contributes the use of simultaneous fields.
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