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Promoting the removal of particulate matters by heterogeneous vapor condensation in double-loop wet flue gas desulfurization system Hao Wu, Qianwen Wang, and Hongmin Yang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00759 • Publication Date (Web): 16 Apr 2019 Downloaded from http://pubs.acs.org on April 17, 2019
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Energy & Fuels
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Promoting the removal of particulate matters by heterogeneous vapor
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condensation in double-loop wet flue gas desulfurization system
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Hao Wu a,*, Qianwen Wang a, Hongmin Yang b
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a
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Reduction Technology, School of Energy & Mechanical Engineering, Nanjing Normal University,
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210023 Nanjing, China
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b
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China
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ABSTRACT
Jiangsu Provincial Engineering Laboratory of Energy System Process Conversion and Emission
Key Laboratory of Materials Cycling & Pollution Control of Jiangsu Province, 210023 Nanjing,
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According to the theory of heterogeneous vapor condensation, an innovative process to promote
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particulate matter removal in a double-loop wet flue gas desulfurization system was proposed. The
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essential supersaturated vapor field for heterogeneous vapor condensation was obtained by reducing the
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absorption slurry temperature in the absorption zone. The particulate matters could be nucleated and
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then enlarged to larger condensational-grown droplets, which would be effectively separated by the
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scrubbing effect of spray layers and intercepting effect of demister. Meanwhile, the establishment of
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supersaturated vapor field could also inhibit the generation of secondary particulate matters in the wet
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desulfurization process. The feasibility of this process was investigated by numerical calculations, and
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the results indicated that the necessary supersaturated vapor field could be established in the absorption
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zone. The promotion of particulate matter removal by heterogeneous vapor condensation was studied in
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typical operation conditions. Furthermore, the key parameters, such as absorption slurry temperature
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and liquid-to-gas ratio in absorption zone, were also investigated. The experimental results showed that *Corresponding author: Hao Wu; E-mail address:
[email protected].
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the lower absorption slurry temperature and higher liquid-to-gas ratio in absorption zone were
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beneficial for the promotion of particulate matter removal. The promotion of removal efficiency was
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more significant in number concentration than that in mass concentration. In typical operation
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conditions, the removal efficiency could be promoted by about 20% in number concentration and about
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5% in mass concentration.
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Key words: Heterogeneous vapor condensation; Particulate matters; Removal; Double-loop wet flue
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gas desulfurization
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1. Introduction
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Particulate matter emission has been regarded as one of major sources of air pollution [1]. Due to the
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strong surface activity and large surface area, the particulate matter is much higher in enrichment of
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toxic heavy metals, bacteria and viruses
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atmosphere can not only cause many environmental problems, but also bring many risks to human
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health
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anthropogenic emission sources of particulate matters, thus reducing the emission of particulate matters
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in coal-fired flue gas has drawn more and more attentions
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technologies are less effective in particulate matter removal because of the existence of Greenfield Gap
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[5].
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too small for inertia force to be significant and too large for diffusion force to be effective. That is, the
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effects of diffusive force and inertia force are both very limited, resulting in an inefficient removal
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performance [5]. However, it is believed that if the size of particulate matter would be enlarged by some
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pretreatment technologies, the removal performance of conventional dust removal technologies for
[3].
[2].
It has been already proven that the particulate matters in
At present, coal-fired power plants are considered as one of the most important
[4].
The conventional dust removal
Especially for the particulate matters with sizes in the range of 0.1-1 μm, the particulate matters are
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Energy & Fuels
these particulate matters can be considerably promoted.
Heterogeneous vapor condensation as one of pretreatment technologies is proposed and developed [6-9].
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due to its potential application value
The particulate matters will nucleate in the supersaturated
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vapor field and then begin to grow up to larger droplets spontaneously, which are more easily to be
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separated by subsequent conventional dust removal devices [10]. The key of the pretreatment technology
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that based on heterogeneous vapor condensation is the establishment of essential supersaturated vapor
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field, which is closely related to the relative humidity of flue gas. Generally, taking into account of the
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characteristic of coal-fired flue gas, it is usually applied with wet flue gas desulfurization (WFGD)
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process, in which the flue gas humidity is relatively high
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vapor field can be achieved by increasing the flue gas relative humidity at WFGD inlet via adding
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steam into flue gas or decreasing flue gas temperature
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applicable for the original coal-fired flue gas with high humidity (60-100 gNm-3)
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added steam or temperature drop is unrealistic for the flue gas with low humidity (40-60 gNm-3).
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However, the humidity of most coal-fired flue gas is in the range of 40-60 gNm-3 [16], meaning that the
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applicability of the above processes is limited. Thus, it is necessary to further seek a more appropriate
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process to establish supersaturated vapor field for heterogeneous vapor condensation.
[11,12].
[13,14].
It is reported that the supersaturated
Nevertheless, these methods are only [15].
The quantity of
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In recent years, with the increasingly stringent environmental requirements, the emission standard of
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SO2 is further enhanced, which lead to a transformation and upgrading in traditional WFGD system.
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The double-loop WFGD system has been gradually applied in many coal-fired power plants [17]. There
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are two circulation loops in double-loop WFGD system, which can be divided into oxidation
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circulation loop and absorption circulation loop
[18,19].
In oxidation circulation loop, the pH value of
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slurry is in the range of 4.0-5.0, which is beneficial for the dissolution of limestone, thus the oxidation
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circulation loop is mainly used for SO2 absorbing and flue gas cooling. In absorption circulation loop,
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the pH value of slurry is in the range of 5.8-6.2, thus the absorption circulation loop is mainly used to
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further absorb SO2 in the flue gas. Usually, the flue gas firstly enters into oxidation zone to
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countercurrent contact with oxidation slurry, leading an increase in flue gas humidity and decrease in
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flue gas temperature. After that, the flue gas leaves oxidation zone for absorption zone to contact with
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absorption slurry countercurrently. As has been reported [20-22], the heat and mass transfer between gas
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phase and liquid phase mainly occurs in oxidation zone, the flue gas temperature can be decreased from
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about 120 C to 50-60 C and relative humidity can be raised to about 90%. However, when the flue
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gas enters into absorption zone, only a slight decrease and a slight increase in the flue gas temperature
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and relative humidity can be observed [23].
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In order to promoting the removal of particulate matters in coal-fired flue gas, an innovative process
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was proposed in a double-loop WFGD system based on heterogeneous vapor condensation. The
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essential supersaturated vapor field for heterogeneous vapor condensation was established by
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decreasing the absorption slurry temperature in absorption zone. The particulate matters could enlarge
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to larger condensational-grown droplets by heterogeneous vapor condensation, and then be efficiently
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separated by the effects of slurry spraying and demister intercepting. Meanwhile, the formation of
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supersaturated vapor field would also inhibit the evaporation of slurry droplets, causing a reduction in
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secondary particulate matter emission from wet desulfurization process [24].
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In this paper, the theoretical supersaturated degrees formed in the absorption zone at different
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absorption slurry temperatures were calculated numerically. The typical performance of this innovative ACS Paragon Plus Environment
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process for particulate matter removal was studied. In addition, the influences of key parameters, such
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as absorption slurry temperature and liquid-to-gas ratio in absorption zone, on particulate matter
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removal were also investigated.
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2. Experimental Section
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2.1 Experimental facility
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The experimental facility is outlined in Fig.1, including a coal-fired boiler, buffer tank, SCR reactor,
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electrostatic precipitator and WFGD system. The flue gas with volume flux of about 450 Nm3h-1 was
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generated by coal-fired boiler, and then successively passed through the buffer tank, SCR reactor and
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electrostatic precipitator, which was in accordance with that in actual coal-fired power plant. Finally,
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the flue gas entered into a double-loop WFGD system. Fig.2 shows the schematic diagram of
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double-loop WFGD system. The flue gas entered into scrubber from bottom, contacted
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countercurrently with slurry in scrubber that adopted 2 spray levels in the oxidation zone and 2 spray
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levels in the absorption zone. The slurry in the absorption zone was collected into absorption slurry
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tank, in which a fluoroplastics heat exchanger was equipped to control the slurry temperature. A
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demister was installed above the absorption zone to intercept the droplets and enlarged particulate
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matters.
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During the experiments, the WFGD inlet flue gas was maintained about 120 C, the SO2
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concentration was approximately 2500 mgm-3. The number and mass concentration of particulate
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matters stabilized around 3.61 × 106 1cm-3 and 45.0 mgNm-3. In addition, the superficial gas velocity
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in scrubber was set as 3.5 ms-1. The pH values of the slurry in the oxidation zone and absorption zone
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were set as 4.5 and 6.0, respectively. The slurry temperature and liquid-to-gas ratio in the oxidation
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zone were set as 50 C and 10 LNm3, respectively.
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2.2 Measuring method
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The concentration and size distribution of particulate matters in flue gas were online measured at the
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scrubber inlet and outlet by electrical low pressure impactor (ELPI, Dekati, Finland). The measurement
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range of ELPI is from 0.023 m to 9.314 m. Since the humidity of desulfurized flue gas was
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relatively high, the diluter (DI-1000, Dekati, Finland) was also applied to ensure measurement accuracy
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and avoid vapor interference
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°C) in diluter and the dilution ratio of diluter was 67.
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3. Results and discussion
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3.1 Feasibility analysis
[25-26].
The sampling gas was diluted by the clean hot dry dilute gas (120
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The key to the occurrence of heterogeneous vapor condensation is the formation of supersaturated
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vapor field. Since the supersaturated degree is difficult to measure, numerical calculations are often
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used. The supersaturated phase is an unsteady phase, in which the partial pressure of actual vapor
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exceeds that of saturated vapor. Thus, the supersaturated degree can be defined as follows [27]:
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S
Pa (T , x) Ps (T , x)
(1)
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where Pa T , x is the partial pressure of actual vapor and Ps T , x is the partial pressure of saturated
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vapor.
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The heat and mass transfer model in desulfurization scrubber was established based on following
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assumptions: 1) scrubber was adiabatic; 2) flue gas was ideal gas; 3) vertical distributions of flue gas
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temperature, droplets temperature and flue gas humidity were continuous; 4) impact of desulfurization
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slurry on equilibrium vapor pressure was ignored. Besides, the major focus was on the distribution of ACS Paragon Plus Environment
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supersaturated degree formed in the scrubber, thus the desulfurizing agent in slurry and pollutants in
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flue gas were not considered. The modeling approaches can be found in various publications
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Furthermore, the model was meshed by the exterior node method and the equations were solved by the
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finite difference method.
[14, 28, 29].
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Fig.3 shows theoretical supersaturated degrees formed in the absorption zone at different absorption
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slurry temperatures, where the liquid-to-gas ratio in the absorption zone was assumed as 10 LNm3. The
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results indicated that the supersaturated vapor field could be established in the absorption zone by
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decreasing the absorption slurry temperature, in which the particulate matters might be nucleated and
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enlarged by heterogeneous vapor condensation. In the regular case, the temperature of absorption slurry
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was about 50 C. With the increase of relative height of scrubber, the relative humidity increased
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correspondingly. However, it can be seen from Fig.3 that the relative humidity of flue gas only
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increased from 95% to 98% with the relative height increasing from 0.5 to 1, indicating that the
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supersaturated vapor field could not be established in the regular case. When the temperature of
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absorption slurry was decreased from 50 C to 40 C, the situation was different. The maximum
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theoretical supersaturated degree of 1.11 could be obtained and thus many particulate matters would be
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enlarged. Then, with the absorption slurry temperature consistently decreasing, the maximum
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theoretical supersaturated degree increased correspondingly, but the increase trend tended to slow
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down.
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The other key to achieving heterogeneous vapor condensation is the nucleation of particulate matter,
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which should satisfy a condition of that: the supersaturated degree of flue gas should be equal to or
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greater than the critical supersaturated degree of particulate matter
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degree of particulate matter is relevant to many parameters, which can be expressed as following [31]: ACS Paragon Plus Environment
[30].
The critical supersaturated
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2 8 3VL S cr exp 3 3 f m , r 2 3k T ln 4R p K C
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where is the liquid surface tension, VL is the vapor molecular volume, k is the Boltzmann
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constant, T is the vapor temperature, R p is the particulate matter radius, K C is the kinetic constant,
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2
(2)
f m , r is the function of relative contact angle.
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It can been seen in Eq.(2) that with the increase of particulate matter radius, the critical
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supersaturated degree of particulate matter decreases. That is, the critical supersaturated degree for
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smaller particulate matters is higher than that of larger ones. Many people investigated the critical
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supersaturated degree of particulate matter and found that most particulate matters in coal-fired flue gas
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will be nucleated and begin to enlarge to larger condensational-grown droplets at the supersaturated
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degree of 1.10-1.20 [14, 32, 33].
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According to the calculation results and literature data, it can be inferred that decreasing the
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absorption slurry temperature could establish supersaturated vapor field and the formed supersaturated
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degree might excess the critical supersaturated degrees of most particulate matters. Hence, the
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particulate matters would be nucleated and then enlarged to larger condensational-grown droplets by
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this innovative process.
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3.2 Concentration and size distribution of particulate matter in coal-fired flue gas before
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desulfurization scrubber
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The concentration and size distribution of particulate matter in coal-fired flue gas before
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desulfurization scrubber is an important parameter, which has a major impact on the experimental
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results. Fig.4 gives the concentration and size distribution of particulate matter before scrubber. It can
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be seen from Fig.4a that the number and mass concentrations were about 3.61 × 106 1cm-3 and 45.0
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mgNm-3, respectively. In Fig.4b and Fig.4c, it can be noted that most particulate matter sizes
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concentrated in the sub-micron range in number and most particulate matter sizes concentrated in the
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micron range in mass. The results were consistent with actual situation
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matters were in the sub-micron range in number, many researchers believed that the number emission
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control is more meaningful than mass emission control. In other words, the promotion of particulate
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matter removal by heterogeneous vapor condensation should not only be evaluated by mass
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concentration, but also be evaluated by number concentration. In this paper, the number concentration
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and mass concentration were all provided to evaluate the promotion of removal performance for
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particulate matters by heterogeneous vapor condensation.
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3.3 The promotion of removal performance for particulate matters by heterogeneous vapor
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condensation
[34, 35].
Since most particulate
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The promotion of removal performance for particulate matters by heterogeneous vapor condensation
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was investigated in the following typical conditions: the liquid-to-gas ratio in absorption zone was 10
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LNm3, the temperatures of absorption slurry were set as 50 C in the case without heterogeneous vapor
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condensation and 30 C in the case with heterogeneous vapor condensation, which could be determined
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by Fig.3.
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Fig.5 shows the particulate matter concentration in flue gas with and without heterogeneous vapor
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condensation. It can be seen from Fig.5 that the number and mass concentration of particulate matters
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before desulfurization scrubber was about 3.61 × 106 1cm-3 and 45.0 mgNm-3. After the flue gas
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passing through the scrubber, the particulate matter number concentration decreased to about 2.71 ×
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106 1cm-3 without heterogeneous vapor condensation and 2.01 × 106 1cm-3 with heterogeneous vapor
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condensation. Correspondingly, the mass concentration decreased to about 17.2 mgNm-3 and 19.6 ACS Paragon Plus Environment
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mgNm-3 with and without heterogeneous vapor condensation.
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It can be noted from Fig.5 that some particulate matters could be separated from flue gas by WFGD
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system without heterogeneous vapor condensation, and the removal efficiency could be 25.3% in
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number concentration and 56.5% in mass concentration, as shown in Fig.6. On one hand, some
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particulate matters could be captured by slurry droplets via inertial impaction; on the other hand, the
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demister in the top of scrubber could not only intercept the droplets, but also separate some larger
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particulate matters from flue gas. Also, it can be found that the number concentration significantly
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decreased with heterogeneous vapor condensation, while the change of mass concentration was
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inconspicuous. As presented in Fig.6, the number removal efficiency was increased from 25.3% to
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44.5%, while the mass removal efficiency was increased from 56.5% to 61.8% in the case of
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heterogeneous vapor condensation. It is because that the maximum supersaturated degree in absorption
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zone could reach 1.20 in this case, causing plenty of particulate matters to be nucleated and then
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spontaneously enlarged to condensational-grown droplets with higher mass, larger size and greater
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inertia force. These condensational-grown droplets were easy to collide with slurry droplets,
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condensational-grown droplets, or unnucleated particulate matters to form bigger droplets that might
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drop down. Besides, some condensational-grown droplets that could still pass through the spraying
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layer in scrubber would also be intercepted by the demister due to the enhancement of inertia force and
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size. However, it is worth noting that the promotion of mass removal efficiency of this process for
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particulate matters was limited. This phenomenon can be explained by Fig.7 and Fig.8. As shown in
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Fig.7a, it can be seen that the particulate matters in flue gas were mainly concentrated in sub-micron
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range in number. However, in Fig.7b, it can be seen that the proportion of these sub-micron particulate
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matters was very limited in mass. According to the Fig.8, it is also found that some sub-micron ACS Paragon Plus Environment
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particulate matters that might not be nucleated in this supersaturated degree were still removed
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significantly. As has been reported that the removal of these sub-micron particulate matters was mainly
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due to the effects of diffusiophoresis force and thermophoresis force
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difference between gas-liquid two phases and the establishment of supersaturated vapor field could
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lead an increase in temperature gradient and vapor concentration gradient, which lead to an
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enhancement in thermophoresis force and diffusiophoresis force, then causing a promotion in the
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removal of these sub-micron particulate matters. However, the proportion of these particulate matters
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was little in mass, thus the decrease of these particulate matters in number was more significant than
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that in mass.
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3.4 Influence of the operation parameters on particulate matter removal
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3.4.1 Influence of absorption slurry temperature
[14].
The increase of temperature
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Fig.9 reveals the influence of absorption slurry temperature on the emission and removal
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performance of particulate matters. In the experiments, liquid-to-gas ratio in absorption zone was kept
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as 10 LNm3.
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It can be obtained from Fig.9a that with the absorption slurry temperature decreasing, the number
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concentration of particulate matters in desulfurized flue gas decreased significantly, while the mass
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concentration decreased slightly. As shown in Fig.9b, it can be found that with the absorption slurry
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temperature decreased from 50 C to 10 C, the total number removal efficiency increased from 23.6%
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to 53.1% and the total mass removal efficiency increased from 56.7% to 63.8%. It has been calculated
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that when the absorption slurry temperature was 50 C, the supersaturated vapor field was not formed
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in the scrubber; the removal of particulate matters was mainly due to the scrubbing effect of slurry and
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intercepting effect of demister. When the absorption slurry temperature was decreased to 40 C, the ACS Paragon Plus Environment
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supersaturated vapor field was formed, some larger particulate matters began to be nucleated and then
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enlarged to larger ones. Thus the stage removal efficiency of these larger particulate matters was
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promoted, as shown in Fig.9c. However, the established supersaturated degree was about 1.11, which
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was not high enough to make all the particulate matters be nucleated, only larger ones were enlarged to
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condensational-grown droplets and then be removed
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particulate matters in number was more likely to reflect to the variation of mass concentration, because
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of the proportion of larger particulate matter was higher in mass. Many smaller ones would not be
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nucleated and remained unchanged. Nevertheless, the formation of supersaturated vapor field in
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scrubber would also lead to an enhancement in thermophoresis force and diffusiophoresis force, which
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was beneficial for the collision and coalescence of these particulate matters. The smaller particulate
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matters could also be enlarged on this occasion, and thus the removal efficiency for these smaller
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particulate matters was promoted (see Fig.9c). As illustrated in Fig.9c, it can be also found that the for
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the particulate matters with size between 0.1 m to 1 m, the promotion of stage removal efficiency
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was limited. The reason for this phenomenon is that in this size range, the particulate matters were too
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small for inertia force to be significant and too large for diffusion force to be effective. With the
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absorption slurry temperature continuously decreasing from 40 C to 30 C, the supersaturated vapor
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field with a higher degree of about 1.19 would be achieved, and the removal efficiency of particulate
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matters was further increased. On one hand, the higher supersaturated degree would exceed critical
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supersaturated degree of more smaller particulate matters, leading more particulate matters to be
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nucleated and enlarged. On the other hand, lower absorption slurry temperature would also lead to a
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further decrease in flue gas temperature. Hence, the amount of condensable vapor was increased,
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causing the larger final condensational-grown droplets to be formed. However, when the absorption
[30].
Meanwhile, the removal of these larger
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slurry temperature was decreased from 30 C to 20 C, the removal of particulate matter was further
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promoted, but the amplification was limited. It can be explained by the relationship between particulate
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matter radius and its critical supersaturated degree. As expressed in Eq.(2), although the critical
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supersaturated degree was inversely proportional to the radius of particulate matters, but the
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relationship was not linear. That is, with the radius of particulate matter decreasing, the critical
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supersaturated degree increased sharply. The increase of supersaturated degree of flue gas still could
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not satisfy the requirement of nucleation of some smaller particulate matters. In consideration of the
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practical operation of WFGD system and the removal performance of particulate matters, it is
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considered that the 30 C was a suitable value for the absorption slurry temperature in this process.
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3.4.2 Influence of liquid-to-gas ratio in absorption zone
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Fig.10 and Fig.11 depict the concentration and total removal efficiency of particulate matter at
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different liquid-to-gas ratios in the absorption zone with and without heterogeneous vapor condensation.
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In the experiment, for the case without heterogeneous vapor condensation, the absorption slurry
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temperature was set as 50 C; for the case with heterogeneous vapor condensation, the absorption
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slurry temperature was set as 30 C.
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Fig.10 presents the concentration and total removal efficiency of particulate matter at different
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liquid-to-gas ratios in absorption zone without heterogeneous vapor condensation. It can be found from
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Fig.10a that when the liquid-to-gas ratio increased from 5 to 15 LNm3, the number concentration of
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particulate matters in desulfurized flue gas only had a slightly decreased, while the mass concentration
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had a comparatively obvious decrease. In Fig.10b, the similar variation tendency in total removal
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efficiency was also presented. With the liquid-to-gas ratio increased from 5 to 15 LNm3, the total
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number removal efficiency only increased from 23.8% to 26.7%, while the total mass removal ACS Paragon Plus Environment
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efficiency increased from 50.7% to 62.1%. The reasons for this variation tendency were attributed to
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the removal and generation characteristics of particulate matters in wet desulfurization process. The
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original particulate matters in inlet flue gas were removed mainly due to the scrubbing effect and
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intercepting effect of spraying layer and demister. When the liquid-to-gas ratio was increased, the
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scrubbing effect was enhanced. Hence, the removal of original particulate matters was increased.
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Especially for particulate matters with larger sizes, since the inertia force of these particulate matters
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was larger, these larger particulate matters were more easily to be removal by the increasing of
287
spraying volume (liquid-to-gas ratio). However, recent study also shows that many secondary
288
particulate matters could be generated from WFGD system and these secondary particulate matters
289
mainly consisted of sulfate, sulfite and unreacted limestone, which escaped from desulfurization
290
scrubber by the entrainment of flue gas and evaporation of slurry droplets
291
majority of these secondary particulate matters were mainly concentration in the smaller size range [36].
292
With the increase of liquid-to-gas ratio, the spraying volume increased, causing more slurry droplets to
293
be generated, resulting in an increase of secondary particulate matter emission [24]. That is, the removal
294
of larger original particulate matters was enhanced and the emission of smaller secondary particulate
295
matters was also increased in this process. Thus, with the liquid-to-gas ratio increasing, the increase of
296
particulate matter removal was obvious in mass concentration, but inconspicuous in number
297
concentration.
[7,24].
It is proven that the
298
For the case with heterogeneous vapor condensation, the situation was different. As shown in
299
Fig.11a, it can be seen that with the liquid-to-gas ratio increasing, the concentration of particulate
300
matters increased both in mass and in number. Also, the total number and mass removal efficiency
301
were both increased significantly. It can be seen from Fig.11b that with the liquid-to-gas ratio ACS Paragon Plus Environment
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increasing from 5 to 15 LNm3, the total number removal efficiency increased from 37.5% to 49.6%,
303
and the total mass removal efficiency increased from 55.1% to 67.5%. When the heterogeneous vapor
304
condensation occurred in desulfurization scrubber, the particulate matters were nucleated and grew up
305
to larger ones, which was beneficial for the removal. Besides, some smaller slurry droplets could be
306
also enlarged and removed, leading to a decrease in the generation of secondary particulate matter.
307
Furthermore, since the supersaturated vapor field was formed in scrubber, the evaporation of some
308
slurry droplets was also inhibited, which was also adverse to the generation of secondary particulate
309
matters. Besides, with the liquid-to-gas ratio increasing, the number of slurry droplets increased,
310
enhancing the collision probability between droplets and particulate matters, resulting in an increase in
311
total removal efficiency. In addition, higher liquid-to-gas ratio also meant stronger heat and mass
312
transfer between the liquid phase and gas phase. The flue gas temperature would be further decreased
313
and more vapors would enter into the gas phase from the liquid phase. In other words, the
314
supersaturated degree and the amount of condensable vapor would increase with liquid-to-gas ratio
315
increasing. More smaller particulate matters could be nucleated and more larger condensational-grown
316
droplets might be formed, thus the removal of particulate matters was increased. Therefore, for the case
317
with heterogeneous vapor condensation, the increase of liquid-to-gas ratio not only improved the
318
removal of original particulate matters in flue gas, but also inhibited the generation of secondary
319
particulate matter in WFGD process, which was advantageous for the decrease of particulate matter
320
emission in desulfurized flue gas.
321
4. Conclusion
322
To promoting the removal of particulate matters in coal-fired flue gas, an innovative process is
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proposed in a double-loop wet flue gas desulfurization system based on the heterogeneous vapor
324
condensation. The essential supersaturated vapor field for heterogeneous vapor condensation was
325
achieved by reducing the slurry temperature in the absorption zone. The process was theoretically
326
proven to be feasible by numerical calculation. The experimental results indicated that the removal of
327
particulate matters in flue gas would be promoted by heterogeneous vapor condensation in double-loop
328
wet desulfurization process. In typical operation parameters, the total number removal efficiency was
329
increased from 25.3% to 44.5%, while the total mass removal efficiency was increased from 56.5% to
330
61.8%. The occurrence of heterogeneous vapor condensation could not only make particulate matters
331
be enlarged to larger condensational-droplets, but also inhibit the generation of secondary particulate
332
matters in wet desulfurization process. The parameters, such as absorption slurry temperature and
333
liquid-to-gas ratio in absorption zone, had a great influence on the removal of particulate matters in this
334
process. The lower absorption slurry temperature was beneficial for the establishment of supersaturated
335
vapor field and thus was positive in particulate matters removal. The higher liquid-to-gas ratio could
336
enhance the spraying effect of slurry, which would promote the particulate matter removal.
337
Additionally, further investigations about this innovative process of heterogeneous vapor condensation
338
in double-loop wet flue gas desulfurization system are necessary for industrial application.
339
Acknowledgments
340
This work was supported by the National Natural Science Foundation (51806107).
341
References
342
[1] Englert N. Fine particles and human health - a review of epidemiological studies. Toxicol. Lett. 2004, 149 (S1-3),
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235-242.
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[2] Buonanno, G.; Stabile, L.; Morawska, L.; Russia, A. Children exposure assessment to ultrafine particles and black carbon: The role of transport and cooking activities. Atmos. Environ. 2013, 79, 53-58.
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[3] Pope, C. A.; Burnett, R. T.; Thurston, G. D.; Thun, M. J.; Calle, E. E.; Krewski, D.; Godleski, J. J.
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Cardiovascular mortality and long-term exposure to particulate air pollution epidemiological evidence of general
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pathophysiological pathways of disease. Circulation 2004, 109 (1), 71-77.
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[4] Li, G. D.; Li, S. Q.; Huang, Q.; Yao, Q. Fine particulate formation and ash deposition during pulverized coal combustion of high-sodium lignite in a down-fired furnace. Fuel 2015, 143, 430-437.
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[5] Seinfeld, J. H.; Pandis, S. N. Atmospheric Chemistry and Physics; John Wiley: Hoboken, NJ, 1998; p 1326.
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[6] Tammaro, M.; Natale, F. D.; Salluzzo, A.; Lancia, A. Heterogeneous condensation of submicron particles in a
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growth tube. Chem. Eng. Sci. 2012, 74 (22), 124-134. [7] Yang, L. J.; Bao, J. J.; Yan, J. P.; Liu, J. H.; Song, S. J.; Fan, F. X. Removal of fine particles in wet flue gas desulfurization system by heterogeneous condensation. Chem. Eng. J. 2010, 156 (1), 25-32. [8] Heidenreich, S.; Vogt, U.; Buttner, H.; Ebert, F. A novel process to separate submicron particles from gases - a cascade of packed columns. Chem. Eng. Sci. 2000, 55 (15), 2895-2905. [9] Zhang, R.; Wu, H.; Si, X. D.; Zhao, L. L.; Yang, L. J. Improving the removal of fine particulate matter based on heterogeneous condensation in desulfurized flue gas. Fuel Process. Technol. 2018, 174, 9-16. [10] Vogt, U.; Heidenreich, S.; Buttner, H.; Ebert, F. An extensive study of droplet growth and separation of submicron particles in packed columns. J. Aerosol Sci. 1997, 28 (S1), S395-S396. [11] Xiong, Y. Y.; Niu, Y. Q.; Tan, H. Z.; Liu, Y. Y.; Wang, X. B. Experimental study of a zero water consumption wet FGD system. Appl. Therm. Eng. 2014, 63 (1), 272-277. [12] Xu, J. C.; Yu, Y.; Yin, Y. S.; Zhang, J.; Zhong, H. Heterogeneous condensation coupled with partial gas circulation for fine particles abatement. Chem. Eng. J. 2017, 330, 979-986. ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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[13] Bao, J. J.; Yang, L. J.; Yan, J. P.; Xiong, G. L.; Lu, B.; Xin, C. Y.
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Experimental study of fine particles removal
[14] Wu, H.; Yang, L. J.; Yan, J. P.; Hong, G. X.; Yang, B. Improving the removal of fine particles by heterogeneous condensation during WFGD processes. Fuel Process. Technol. 2016, 145, 116-122.
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[15] Wu, H.; Pan, D. P.; Bao, J. J.; Jiang, Y. Z.; Hong, G. X.; Yang, B.; Yang, L. J. Improving the removal efficiency
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of sulfuric acid droplets from flue gas using heterogeneous vapor condensation in a limestone-gypsum
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desulfurization process. J. Chem. Technol. Biotechnol. 2017, 92, 230-237.
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[16] Wang, D. X.; Bao, A. N.; Kunc, W.; William, L. Coal power plant flue gas waste heat and water recovery. Appl. Energ. 2012, 91 (1), 341-348. [17] Deuster, E. V.; Mensing, A.; Jiang, M. X.; Majdeski, H. Cleaning of flue gas from solid waste incinerator plants by wet/semi-dry process. Environ. Prog. 1994, 13 (2), 149-153. [18] Srivastava, R. K.; Jozewicz, W. Flue gas desulfurization: the state of the art. J. Air Waste Manag. Assoc. 2001, 51 (12), 1676-1688. [19] Tian, L. J.; Wang, L. P.; Wang, L. L.; Zhou, X. Flue gas desulfurization enhanced by inorganic salts using double circulation and multi-stage water film tower. J. Cent. South Univ. (Sci. Technol.) 2011, 42 (2), 555-560. [20] Marocco, L.; Inzoli, F. Multiphase Euler-Lagrange CFD simulation applied to wet flue gas desulphurisation technology. Int. J. Multiphase Flow 2009, 35 (2), 185-194.
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[21] Lin, Y.; Chen, D. Z. Numerical simulation and verification of gas-liquid two phases flow and heat transfer in
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spraying zone of large-scale desulphurization absorption tower. J. Combust. Sci. Technol. 2016, 22 (1), 1-8.
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[22] Xu, Z.; Xiao, Y. H,; Wang Y. Experimental and theoretical studies on air humification by a water spray at
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elevated pressure. Appl. Therm. Eng. 2007, 27 (14-15), 2549-2558. [23] Soren, K.; Michelsen, M. L.; Dam-Johansen, K. Experimental investigation and modeling of a wet flue gas ACS Paragon Plus Environment
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desulphurisation pilot plant. Ind. Eng. Chem. Res. 1998, 37 (7), 2792-2806. [24] Pan, D. P.; Wu, H.; Yang, L. J. Fine particle transformation during the limestone gypsum desulfurization process. Energy Fuels 2016, 30 (11), 9737-9744.
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[25] Zhou, L.; Yang, S. Y.; Chen, W.; Wang, X. B.; Yuan, Z. L.; Yang L. J.; Wu, H. Chemical agglomeration
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properties of fine particles immersed in solutions and the reduction in fine particle emission by adding emulsion
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polymers. Fuel Process. Technol. 2018, 175, 44-53.
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[26] Pan, D. P., Wu, H.; Yang, L. J. Investigation of the relationship between droplets and fine particle emission during the limestone-gypsum WFGD process. Energy Fuels 2017, 31 (6), 6472-6477. [27] Reiss, H.; Koper, G. J. M. The Kelvin Relation: stability, fluctuation, and factors involved in measurement. J. Phys. Chem. 1995, 99 (19), 7837-7844. [28] Yan, J. P.; Study on fine particles removal from coal combustion improved by vapor condensational growth Ph.D. Dissertation, Southeast University, Nanjing, China, 2009. [29] Fan, F. X.; Yang, L. J.; Yuan, Z. L. Properties of water vapor supersaturation under spray scrubbing conditions. CIESC J. 2009, 60 (7), 1644-1650. [30] Heidenreich, S.; Ebert, F. Condensational droplet growth as a preconditioning technique for the separation of submicron particles from gases. Chem. Eng. Process. 1995, 34 (3), 235-244. [31] Yan, J. P.; Chen, L. Q.; Yang, L. J. Combined effect of acoustic agglomeration and vapor condensation on fine particles removal. Chem. Eng. J. 2016, 290, 319-327. [32] Chen, C. C.; Tao, C. J. Condensation of supersaturated water vapor on submicrometer particles of SiO2 and TiO2. J. Chem. Phys. 2000, 112 (22), 9967-9977. [33] Fan, F. X.; Yang, L. J.; Yuan, Z. L.; Yan, J. P. Numerical prediction of water vapor nucleation behavior on PM2.5 from coal combustion. CIESC J. 2007, 58 (10), 2561-2566. ACS Paragon Plus Environment
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[34] Quann, R. J.; Neville, M.; Sarofim, A. F. A Laboratory Study of the Effect of Coal Selection on the Amount and
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Composition of Combustion Generated Submicron Particles. Combust. Sci. Technol. 1990, 74 (1-6), 245-265.
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[35] Yan, L.; Gupta, R.; Wall, T. Fragmentation behavior of pyrite and calcite during high-temperature processing
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and mathematical simulation. Energy Fuels 2001, 15 (2), 389-394.
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[36] Wu, H.; Pan, D. P.; Huang, R. T.; Hong, G. X.; Yang, B.; Peng, Z. M.; Yang, L. J. Abatement of fine particle
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emission by heterogeneous vapor condensation during wet limestone-gypsum flue gas desulfurization. Energy
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Fuels 2016, 30 (7), 6103-6109.
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The list of figure captions:
418
Fig.1 Experimental facility.
419
Fig.2 Double-loop wet flue gas desulfurization system.
420
Fig.3 Theoretical supersaturated degrees formed in absorption zone at different absorption slurry
421 422
temperature. Fig.4 Concentration and size distribution of particulate matters in coal-fired flue gas before
423
desulfurization scrubber. (a) Concentration of particulate matters, (b) number size distribution, (c)
424
mass size distribution.
425 426 427 428
Fig.5 Concentration of particulate matters in flue gas with and without heterogeneous vapor condensation. Fig.6 Total removal efficiency of particulate matters with and without heterogeneous vapor condensation.
429
Fig.7 Size distribution of particulate matters in flue gas with and without heterogeneous vapor
430
condensation. (a) number concentration of particulate matters, (b) mass concentration of
431
particulate matters.
432 433
Fig.8 Stage removal efficiency of particulate matters with and without heterogeneous vapor condensation
434
Fig.9 Concentration and removal efficiency of particulate matters in flue gas at different absorption
435
slurry temperatures. (a) concentration of particulate matters, (b) total removal efficiency, (c)
436
stage removal efficiency.
437
Fig.10 Concentration and total removal efficiency of particulate matters at different liquid-to-gas ratios
438
in absorption zone without heterogeneous vapor condensation. (a) concentration of particulate ACS Paragon Plus Environment
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Page 22 of 35
matters, (b) total removal efficiency
440
Fig.11 Concentration and total removal efficiency of particulate matters at different liquid-to-gas ratios
441
in absorption zone with heterogeneous vapor condensation. (a) concentration of particulate
442
matters, (b) total removal efficiency
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Fig.1 Experimental facility.
Buffer tank
SCR Coal-fired boiler
Electrostatic precipitator
Double-loop WFGD
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Heat exchanger
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Fig.2 Double-loop wet flue gas desulfurization system.
Exhaust Measuring point
T
Demister Absorption Absorption zone circulation loop
T Oxidation circulation loop Oxidation zone T
Slurry tank
T Flue gas Pump Measuring point Double-loop wet flue gas desulfurization
Pump
Heat exchanger
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Fig.3 Theoretical supersaturated degrees formed in absorption zone at different absorption slurry temperature. 1.4
Supersaturation degree
1 2 3 447 4 5 448 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 449 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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Oxidation zone
Absorption zone
1.2 1.0 0.8 0.6
50 oC 40 oC 30 oC 20 oC
0.4 0.2 0.0 0.0
0.2
0.4
0.6
0.8
Relative height
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1.0
Energy & Fuels
Fig.4 Concentration and size distribution of particulate matters in coal-fired flue gas before
451
desulfurization scrubber. (a) Concentration of particulate matters, (b) number size distribution, (c) mass
452
size distribution. 6.0x106
(a)
Number concentration Mass concentration
5.0x106
80
4.0x106 3.0x10
60
6
40
2.0x106
20
1.0x106 0.0
0
60
120
180
240
0 300
2.5x106 2.0x106
120 Number concentration (b) Cumulative number concentration 100 80
1.5x106
60
1.0x106
40
5.0x105 0.0 0.01
Cumulative number concentration / %
Time / s
453
454
100
Mass concentration / mgNm-3
Number concentration / 1cm-3
450
dN/dlogDp /1cm-3
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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20 0.1
Dp / m
1
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0 10
50
dM/dlogDp /mgNm-3
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 455 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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40
120
Mass concentration (c) Cumulative mass concentration 100 80
30
60 20
40
10 0 0.01
Cumulative mass concentration / %
Page 27 of 35
20 0.1
Dp / m
1
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0 10
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Fig.5 Concentration of particulate matters in flue gas with and without heterogeneous vapor condensation. 80
Number concentration Mass concentration
4x106
Mass concentration / mgNm-3
5x106
Number concentration / 1cm-3
1 2 3 456 4 5 457 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 458 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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With heterogeneous Without heterogeneous 60 vapor condensation vapor condensation
3x106
40
2x10
6
20
1x106 0
Before WFGD 0
300
600
After WFGD 900
1200
Time / s
1500
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0 1800
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Fig.6 Total removal efficiency of particulate matters with and without heterogeneous vapor condensation. 100
Total emoval efficiency / %
1 2 3 459 4 5 460 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 461 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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80
Number removal efficiency Mass removal efficiency
60 40 20 0
Without heterogenous vapor condensation
With heterogenous vapor condensation
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Energy & Fuels
462
Fig.7 Size distribution of particulate matters in flue gas with and without heterogeneous vapor
463
condensation. (a) number concentration of particulate matters, (b) mass concentration of particulate
464
matters.
dN/dlogDp /1cm-3
10
WFGD inlet (a) WFGD outlet (without heterogenous condensation) WFGD outlet (with heterogenous condensation)
7
106 105 104 106
10
3 105
102
104 0.01
101 0.01
0.1
0.1
465 25
1
Dp / m
1
10
(b) WFGD inlet WFGD outlet (without heterogenous condensation) WFGD outlet (with heterogenous condensation)
20
dM/dlogDp /mgNm-3
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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2.0
15
1.5 1.0
10
0.5 0.0 0.01
0.1
1
5 0 0.01
0.1
Dp / m
1
466
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10
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Fig.8 Stage removal efficiency of particulate matters with and without heterogeneous vapor condensation. 100
Stage removal efficiency / %
1 2 3 467 4 5 468 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 469 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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80
without heterogenous condensation with heterogenous condensation
60 40 20 0 0.01
0.1
Dp / m
1
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470
Fig.9 Concentration and removal efficiency of particulate matters in flue gas at different absorption
471
slurry temperatures. (a) concentration of particulate matters, (b) total removal efficiency, (c) stage
472
removal efficiency. 3.5x106
Number concentration Mass concentration
3.0x106
(a)
25
2.0x106
20
1.5x106 1.0x10 5.0x10
5
0.0
15 40 oC
50 oC
30 oC
20 oC
10
10 oC
5 0
500
1000
1500
2000
Time / s
2500
3000
473
Total removal efficiency / %
100
(b)
Number removal efficiency Mass removal efficiency
80 60 40 20 0
35 30
2.5x106
6
40
10
20
30
40
50
Slurry temperature of absorption zone / oC 474
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0
Mass concentration / mgNm-3
Number concentration / 1cm-3
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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100
Stage removal efficiency / %
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 475 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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80 60
50 oC 40 oC 30 oC 20 oC 10 oC
(c)
40 20 0 0.01
0.1
Dp / m
1
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476
Fig.10 Concentration and total removal efficiency of particulate matters at different liquid-to-gas ratio
477
in absorption zone without heterogeneous vapor condensation. (a) concentration of particulate matters,
478
(b) total removal efficiency 4x106 5 LNm-3
10 LNm-3
(a)
60
15 LNm-3
6
45
2x106
30
3x10
1x106
15 Number concentration Mass concentration
0
0
300
600
900
1200
Time / s
1500
0 1800
479
Total removal efficiency / %
80
60
40
20
0
480
(b)
Number removal efficiency Mass removal efficiency
5
10
15
Liquid-to-gas ratio in absorption zone / LNm-3
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Mass concentration / mgNm-3
Number concentration / 1cm-3
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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481
Fig.11 Concentration and total removal efficiency of particulate matters at different liquid-to-gas ratio
482
in absorption zone with heterogeneous vapor condensation. (a) concentration of particulate matters, (b)
483
total removal efficiency
(a) 5 LNm-3
10 LNm-3
60
15 LNm-3
Mass concentration / mgNm-3
4x106
Number concentration / 1cm-3
6
45
2x106
30
3x10
1x106 0
15 Number concentration Mass concentration 0
300
600
900
1200
Time / s
1500
0 1800
484
Total mass removal efficiency / %
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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80
(b)
Number removal efficiency Mass removal efficiency
60
40
20
0
5
10
15
Liquid-to-gas ratio in absorption zone / LNm-3 485
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