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PM2.5 and SO3 collaborative removal in electrostatic precipitator
PM2.5 and SO3 collaborative removal in electrostatic precipitator Hu Bin, Zhang Lin, Yi Yang, Luo Fei, Liang Cai, Yang Linjun PII: DOI: Reference:
S0032-5910(17)30466-7 doi:10.1016/j.powtec.2017.06.008 PTEC 12584
To appear in:
Powder Technology
Received date: Revised date: Accepted date:
22 December 2016 16 May 2017 1 June 2017
Please cite this article as: Hu Bin, Zhang Lin, Yi Yang, Luo Fei, Liang Cai, Yang Linjun, PM2.5 and SO3 collaborative removal in electrostatic precipitator, Powder Technology (2017), doi:10.1016/j.powtec.2017.06.008
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ACCEPTED MANUSCRIPT PM2.5 and SO3 collaborative removal in Electrostatic Precipitator
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Hu Bina, Zhang Lina, Yi Yangb, Luo Feia, Liang Caia, Yang Linjuna*
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a Key Laboratory of Energy Thermal Conversion and Control, Ministry of Education, School of Energy and Environment, Southeast University, Nanjing
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210096, China
b Advanced Materials Engineering and Modelling Group, Faculty of Chemistry, Wroclaw University of Science and Technology, Wroclaw 50-370,
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Poland
*Corresponding author. Tel.: +86 25 83795053; fax: +86 25 83795053.
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Email: [email protected]（Yang Linjun）
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ACCEPTED MANUSCRIPT ABSTRACT:
Coal-burning
pollution caused harmfully to
human and
environment, However, the efficiency of electrostatic precipitator ( ESP ) for PM2.5 is low. Reducing the ESP inlet temperature is widely concerned. This technology
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could simultaneously control of PM2.5 and SO3. In this study, a lab-scale low temperature ESP performance was designed under controlled conditions, the
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operating effects were evaluated under different temperature and SO3 concentration, SO3 and particles coagulation mechanism was discussed. Experimental results show that the operating temperature was important for ESP,
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The ESP outlet particle size showed a increasing trend that the median diameter d50 increased to 0.08 μm from 0.05 μm, and the d90 increased to 0.23 μm from 0.09
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μm with the inlet temperature reduced from 150 ℃ to 90 ℃. Particle removal efficiency increased when ESP inlet temperature reduced, especially particles with
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diameters of around 0.1-1 μm increased obviously. SO3 removal efficiency
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improved from about 20% to above 80%, when the ESP inlet temperature drops below the acid dew point. Furthermore, SO3 binary nucleation mechanism was
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discussed, the analysis results of the adsorption mechanism indicated that SO3 condensation on the fly ash was mainly controlled by internal diffusion. KEY WORDS: Low Temperature Electrostatic Precipitator; PM2.5; SO3;
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simultaneous control;
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1. Introduction Coal is mainly used for combustion as a dominant energy source in China,
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and coal-burning pollution have serious environmental problem[1]. Northern
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China suffers a long-term haze and fog cover frequently. The particles mainly
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come from the haze and fog[2, 3], and the fine particle particularly less than 2.5 μm (PM2.5) is detrimental to human healthy[4]. In order to control pollution, there are stringent emission standards for particulate matter having been issued to
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control the particles concentration below 30 mg/m3 instead of 50 mg/m3[5]. To reach the new emission standard (GB 13223-2011), many studies on the particle
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matter (PM) have been carried out. In many techniques, enhance the efficiency of the existing pollutant treatment facilities is target for us.
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Currently, electrostatic precipitators (ESPs) are mainly equipment to control
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dust emission in coal-fired power plants in China[3, 5]. ESP removal particles efficiency can reach 99.5% or even higher. However, the removal efficiency of
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PM2.5 is 95-98%, especially for 0.1-1 μm particles with a lower efficiency[6-8]. Therefore, the escape mechanism of fly ash is studied widely. Researches have promoted the ESP collection efficiency such as the acoustic agglomeration,
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chemical agglomeration, SO3 and NH3 flue gas conditioning and composite precipitators[1, 9, 10]. These technologies not only improve the collection efficiency, but reduce the emission rather than comprehensively considering the relationship between energy saving and emission reduction. Hence, install heat exchanger before the ESP and reduce the ESP inlet temperature becoming a research hotspot in numerous technology. Low temperature ESP compares with conventional electrical precipitator, the main performance advantages are the following four points: (1) The fly ash resistance reduced, the temperature decreased and SO3 condensation on the surface of the particles will reduce the resistance. (2) The flue gas volume flow is reduced, according to the formula of Deutsch, and this result will improve the particle removal efficiency. (3) The breakdown voltage increases when the temperature reduces, the gas density and 3
ACCEPTED MANUSCRIPT charged particle collisions increases, and ion migration rate reduces effectively. Correspondingly, the charge density increases near Corona, and breakdown voltage increases. (4) To Remove SO3 the flue gas drops below the acid dew point
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and SO3 exists in the form of sulfuric acid, and deposition on the particle surface are removed with particle[2, 3, 5, 11, 12].
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There are considerable work have been done in the area of PM2.5 and SO3 in the past, Chao Wang[13] measured the mass particle size distribution at the inlet and outlet of the ESP, which equipment had the low temperature economizer.
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Alireza Bahadori[14] discussed the problem that the rate of low temperature corrosion was hard when the mass ratio of dust for SO3 (D/S) was higher than 10.
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Xu[15] studied the influence factors of volume resistivity and surface resistivity such as temperature, composition, and the formula of fly ash resistivity. Qi[16]
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showed the relationship between resistivity and temperature, and the results
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showed that dust resistivity increased at the beginning and decreased with the increasing temperature with maximum value at about 150 ℃. Srivastava and Qi[6,
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16] studied the effect of SO3 on fly ash resistivity, and showed SO3 concentration having important influence on the electric performance. Naoki Noda[17] studied the influence of operating temperature on the ESP performance. Since 1997,
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Mitsubishi industries had applied the low temperature ESP technology in large-scale coal-fired thermal power unit which the outlet particle concentration was less than 30 mg·m-3 and the SO3 concentration was lower than 3.57 mg·m-3 when the ESP operating temperature keep at 90 ℃. In the testing field, Lin Xiang[18] found that the outlet particle concentration changed from 52.5 mg·m-3 to 17.25 mg·m-3, SO3 removal efficiency was 88.14%, total mercury removal efficiency was 40% and the fine particle removal efficiency was above 99.8%[18]. Most of the current researches are focused on the low temperature electric precipitation process optimization and the engineering application. However, the study of the particle emission characteristics and the SO3 removal mechanism is inadequate. 4
ACCEPTED MANUSCRIPT In this study, the ESP inlet flue gas temperature and SO3 concentration were adjusted at the laboratory scale coal-fired thermal state test system to study the removal characteristics of fine particles and SO3. In our experiment, particles and
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2. Experimental system and apparatus
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SO3 removal mechanism was analyzed.
2.1 Experimental system
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All experiments were carried out in a lab coal combustion experiment test-bed as shown in Fig.1. The system was mainly consist of a coal-fired boiler, a
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buffer vessel, an evaporation chamber, a heat exchanger, an ESP, a WFGD, and the analysis detecting system. The flue gas with approximately 350 m3·h-1 was provided by a coal- fired boiler. The buffer vessel ensures to mix the flue gas
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adequately, and adjusts the flue gas temperature. An evaporation chamber could
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adjust flue gas humidity by water evaporation, and a heat exchanger was set in the electric entrance which was finned heat exchanger with the rated flow 2000 kg·h-1
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and heat transfer 10 kV. The temperature was adjusted by waters flow in heat exchanger, and flue gas entered the ESP by booster fan. Coarse particles were
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removed by passing through the ESP, which is a barb-plate tube type ESP and the rated rectifier voltage was 50 kV. The flue gas desulfurization in the desulfurization tower, and it was discharged from the chimney by the ID fan. During the testing phase, the flue gas temperature was changed from 150 °C to 90 °C by heat exchanger. Table 1 shows the specification of the experimental ESP and the design value for the ESP. The specification of the ESP was the same as for the utility boiler. The gas velocity was kept constant with all operating temperatures in the ESP. To investigate the influence of operating temperature on discharge characteristics and collection efficiency of ESP, anthracite coal was used in the experimental process. Table 2 shows the properties of the coals, the subscript ar is as received and the ad is air dry.
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ACCEPTED MANUSCRIPT 2.2 Measuring instruments During the experiments, the total dust was measured by a microcomputer dust
change of the filter drum could be weighed. Subsequently,
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sampling, the mass
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parallel sampling meter. Filter drums were dried at 110 ℃ before and after the
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dust concentration could be calculated. The size distributions of PM10 were measured by an electrical low-pressure impactor (ELPI). The ELPI enables real-time monitor the particle number and mass size distributions, meanwhile, the
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response time of ELPI is less than 2 sec. The ELPI consists of 13 stages (12 channels), which measures aerodynamic size distribution in the size range of
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0.03-10 μm. In this work, the dilution ratio was 1:67, all of the sampling lines kept short to avoid losing large particle. According to the national standard of China
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(controlled condensation method)[19], the full automatic flue gas sampling device
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and spiral condenser was used for collecting sulfate droplets during the measurement, the sampling lines were set to 180℃ to avoid the sulfuric acid
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condensation, and spiral condenser was set to 65 ℃ and adjusted by thermostatic waterbath. The condensate was analyzed by an ion chromatograph analyzer
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(ICS-2100, American) to determine the SO42- concentrations. Combined with the sampling volume of the flue gas, the SO3 concentration in flue gas can be determined.
3. Experimental results and analysis 3.1 Results and discussion of PM 3.1.1 Mass and number distribution of PM The ESP voltage and SO3 concentration was set at 50 kV and 80 mg·m-3. Fig.2 shows the number size distributions of PM10 at the inlet and outlet of ESP. The size distributions of particle matter from coal combustion display a bimodal distribution that contained a submicron mode and a coarse mode with peaks near 6
ACCEPTED MANUSCRIPT 0.1 μm and 1 μm, respectively. The particles in the coarse mode are formed by the transformation of minerals that remain after the carbon burns away. The particle size distribution are determined by char fragmentation and coalescence of surface
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ash droplets. The particles in the submicron mode are formed by vaporization, condensation, and nucleation of inorganic constituents in the fuel[20].
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Respectively, the unimodal number size distribution can be seen with a peak around 0.15 μm with inlet temperature 150 ℃, the mean particle diameters of the
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fine mode are shifted to 0.3 μm with the inlet temperature down to 90 ℃. This is most probably because the flue gas temperature is lower than the acid dew point,
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result in sulfuric acid vapor condenses with sub-micron grade fly ash particles as condensation nuclei. At the same time, a part of water vapor condenses due to the strong water imbibition of sulfuric acid. Because of the force of liquid, the
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particles are not bounce off after the collision. Instead, they reunite to form larger
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particles, the ESP outlet particles have a tendency to increase when the inlet
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temperature reduces. Fig.3(a) shows the particle number cumulative distribution. The particles are generally fine, and the majority of them are less than 1 μm in size. The median diameter d50 of particle at the inlet is 0.13 μm, and the d90 is 0.32 μm.
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When the flue gas temperature dropped from 150 °C to 90 °C, the median diameter d50 increased to 0.08 μm, and the d90 increased to 0.23 μm. Compared to the ESP imports cumulative distribution, the proportion of export submicron particulate matter increased obviously. The main reason may be that ESP removal efficiency of coarse particle is higher and the submicron particle ratio is increased, but the number concentration is lower at the same moment. The outlet particle diameter has a tendency to increase when ESP inlet temperature is lower than acid dew point. The ESP outlet mass size distribution is in fig.3(b), the ESP outlet mass concentration is 28 mg·m-3, the particle mass mainly concentrates in more than 1 μm particles. In combination with the results in fig.3(b), the mass concentration is focused on the coarse particles which is different from the number concentration, and the number concentration of the particles is larger with diameter less 1 μm. In 7
ACCEPTED MANUSCRIPT the view of control pollutant emissions, it is more meaningful to control the fine particles.
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3.1.2 Collection efficiencies of PM
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The experiment of classification removal efficiency is discussed, particle
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number concentration ratio in a certain channel, it can be written as equation (6): N i 0 N it 100 % N i0
(6)
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Where, ηNi is the resolved efficiency, Ni0 is the initial state particle number concentration, and Nit is the removal of particle number concentration at the outlet
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of the ESP. Operating conditions with reference to 3.1.1. Fig.4 shows the grade removal efficiency under different temperature, the grade removal efficiency was above 85% except 0.1-1 μm diameter section when ESP inlet temperature at
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150 ℃. A low removal efficiency in the size range of 0.1-1 μm contrast to other sizes. This is because different mechanism of charged in different size particle, the
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field charging mechanism is primarily while particle size>1.0 μm, on the contrary, the diffusion charging mechanism is primarily while particle size<1.0 μm. The particle is easily collected by ESP when particle charged. However, as in the
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transitional region of the two charging mechanisms, the collection efficiency for particle size in the range of 0.1-1 μm is low. When the inlet temperature reduced from 150 ℃ to 90 ℃ , the removal efficiency improved above 5%. The improvements were very obvious for particles with diameters of around 0.1-1 μm. The decreases of temperature was beneficial to the improvement the ESP removal efficiency. Lower temperature could reduce the flue gas flow rate and increase the flue gas residence time in ESP[21], which improved the ESP efficiency. Meanwhile, temperature could significantly affect the electrical resistivity. Generally, the ESP is operated effectively when the fly ash resistivity is within the limit of 104 - 2×1010 Ω·cm. The fly ash resistivity reduced from 1.92×1010 Ω·cm to 6.3×109 Ω·cm when the inlet temperature reduced from 150 ℃ to 90 ℃. The 8
ACCEPTED MANUSCRIPT dust resistivity is consists of volume and surface resistivity. Volume resistivity dominates at high temperature and is inversely proportional to working temperature. Surface resistivity mainly occurs at low temperature and is
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proportional to the temperature, usually at the temperature of 120-180 ℃. The
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volume and surface resistivity are near the maximum. Furthermore, while the flue gas temperature decreases to approximately 90 ℃, some gaseous components condensed and adsorbed on the surface fine particles, such as steam, SO3 and HCl.
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The fly ash surface resistivity decreased with acidic gas condensation, and fly ash charged and removed more easily in ESP.
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3.1.3 The influence of SO3 concentration
Fig.5 shows the ESP removal efficiency and the fly resistivity at different SO3
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concentration. The ESP voltage and temperature was set at 50 kV and 90 ℃. The
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total dust removal efficiency increased from 93% to 97%, and the PM2.5 removal efficiency from 73% to 85% with the SO3 concentration changed from 40 to 200
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mg·m-3. The PM2.5 removal efficiency improved obviously. The effect of SO3 on particles could attributed to the following reasons, according to 3.1.1 analyzed, the
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increasing of SO3 concentration will promote particle size becoming bigger in the flue gas, the increase of particle size is beneficial to promote the ESP removal efficiency. In addition, SO3 concentration has a great influence on fly ash resistance. The fly ash resistance is determined by the volume resistivity and surface resistivity, and the surface conductivity is influenced by the vapor and other chemical components on the surface of the fly ash. Therefore, the higher vapor adsorption of the dust particles affects the fly ash resistivity to a large extent. On the other hand, fly ash particles acts as condensation nuclei when the flue gas temperature is lower than the acid dew point. Sulfuric acid condensation in the fly ash forms a liquid bridge force between particles, which improves the probability of
collision
coalescence
between
particles.
This
result
indicates
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conglomeration, hence, when the droplets contact with the particles the surface 9
ACCEPTED MANUSCRIPT tension of the liquid forms a liquid bridge pulling the particles together. This process results in an increased collection efficiency of ESP. SO3 can produce anion in the electric discharge environment, fine particles are more likely to be positive
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plate capture with capture the anion.
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3.2 Results and discussion of SO3 3.2.1 Influence of temperature
The flue gas acid dew point temperature has a close relationship to the SO3
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concentration. Coal-fired power plants generate SO3 according to the sulfur contains in the coal, and the SO3 concentration is in the range from 10-6 mg·m-3 to
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40×10-6 mg·m-3[22-24]. Previous works indicated that the primary mechanism for
third body (M), given by：
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flame based formation of SO3 is interaction of SO2 with O in the presence of a
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SO2 + O+ M → SO3+ M
Fig.6 shows the SO3 concentration at the ESP outlet at different inlet
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temperature. The acid dew point was 90-100 ℃ when SO3 concentration was 40 mg·m-3. It indicates that the SO3 concentration increased from 6 to 24.6 mg·m-3
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with temperature increasing from 90 ℃ to 150 ℃. Similarly, when the initial SO3 concentration was 120 mg·m-3 the SO3 concentration was 12.4 mg·m-3 at 90 ℃, which increased to 100.5 mg·m-3 at 150 ℃. Fig.7 shows SO3 removal efficiency under different temperature. It also shows that the SO3 removal efficiency can reach above 80% below acid dew point temperature, and SO3 removal efficiency is low above the acid dew point temperature. The main reason is the two mechanism of fly ash adsorption of SO3. When the temperature was higher than acid dew point, the residual carbon played a major role to adsorption gas SO3 because of the few amount of residual carbon in fly ash, and the removed of SO3 was low. Relatively, when the temperature was lower than acid dew point SO3 condensed to form H2SO4 acid fog and the dust particles enlarged specific 10
ACCEPTED MANUSCRIPT surface area, and the H2SO4 acid fog condensed and adsorbed on the particle surface which was removed together with the dust by ESP. The effect of SO3 concentration could mainly ascribe to that high SO3 concentrations form more
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sulfuric acid droplets leading to the obvious increase of the sulfuric acid droplets and particle collision rate, besides, the interaction between particles and sulfuric
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acid droplets also enhance. 3.2.2 Influence of humidity
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Experiments were operated at several levels of humidity to investigate its influence on the SO3 removal performance of ESP, at 50 kV ESP voltage and
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90 ℃. The results in Fig.8 show that the SO3 penetration rate was reduced as relative humidity (RH) increased from 6% to 36%, and the concentration of water
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vapor had close relationship with the SO3 removing efficiency. According to the
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former research, dew point temperature is depends on the vapor partial pressure and the SO3 concentration. When the flue gas temperature was below 205 ℃, all
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the SO3 existed in the form of sulfuric acid steam. When the temperature dropped below acid dew point, water vapor and the steam condensed to sulfuric acid droplets. As fig.9 shown, the fixed SO3 concentration, the water vapor content
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increased, accordingly, the acid dew point temperature was also increased. Water molecules combined with sulfuric acid to form sulfuric acid droplets and the higher humidity which could promote the condensation of sulfuric acid vapor on the fly ash and increased the SO3 removal efficiency. Generally, the water vapor and the sulfuric acid vapor exist separately and the condensation begins at the lower temperature, however, when the water vapor and the sulfuric acid vapor exist together the condensation can occur at the temperature cooling to the acid dew point. This is caused by H2O-H2SO4 binary nucleation. On the other hand, the sulfuric acid affects the performance of the ESP. The H2O-H2SO4 molecules are charged by the interaction with electrons, and the decreasing ion mobility. Meanwhile, the increase of relative humidity benefits the agglomeration of 11
ACCEPTED MANUSCRIPT particles. The particles and the sulfuric acid droplet were removed by passing though the ESP. Thus the collection efficiency was greatly improved with the
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increase of relative humidity.
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3.2.3 Removal mechanism of SO3
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SO3 exists in the SO3-H2O-H2SO4 gaseous mixture coexistence system at low ESP temperature, H2SO4 combines the water molecules easily in the humidity environment, The combination of the H2SO4 in the ESP are H2SO4·H2O,
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H2SO4·2H2O, H2SO4·3H2O, …… , H2SO4·hH2O, The equation for reaction is as follow:
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H2O + H2SO4·(n-1)H2O⇋H2SO4·hH2O That is to say, H2O-H2SO4 nucleation is not only the H2O and H2SO4, but the hydrate sulfuric acid molecules also participation as shown in fig.10. According to
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the empirical formula given by Vehkamaki papers, the SO3 molecules number
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concentration is 1×1014 1·cm-3 to 1×1015 1·cm-3 at the SO3 concentration within the scope of 40-200 mg·m-3. The nucleation rate is more than 1014 1·cm-3 with the
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critical nucleation H2SO4 molecular number is about 30. Therefore, H2O-H2SO4 binary nucleation process is fast. The sulfate droplets are easy to condense and
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adsorb on the surface of fly ash, then the dust forms the dust layer due to the wool stoma force and the electrostatic force etc. Because of H2O-H2SO4 nucleation rate is higher than the physical adsorption process, SO3 could react with water vapor to generate sulfuric acid mist, then the sulfuric acid mist reacts with the alkaline substance in the fly ash[25-28]. The condensation process of the sulfate droplets on the fly ash is divided into four stages: external diffusion, film diffusion, internal diffusion and adsorption reaction. The adsorption rate is controlled by the slowest step. Usually, the gas film diffusion and the external diffusion rate are faster, hence, the film diffusion and the external diffusion are not main factor. In order to study the adsorption kinetics characteristics of H2SO4, determine adsorption dynamics model for describing the adsorption process, the Weber - Morris[29] empirical formula is used to analyze the experimental data: 12
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qt K t 2 C
(7)
Where K is particle diffusion rate constant; qt is fly ash adsorption quantity;
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C is constant;
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Fig.11 shows the experimental data fitting with Weber-Morris equation at different initial concentrations of SO3. When the SO3 entrance concentration
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increased the slope of the fitted curve increased, it seems to indicate that the process was controlled by diffusion. The fitted curve was not though the origin, however, the internal diffusion was not the only factor for SO3 adsorption
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condensation control steps. The whole process was determined jointly by various dynamics mechanism, namely, the diffusion and the surface reaction leaded to the
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adsorption change. The fig.12 shows that adsorption of H2SO4 on the fly ash increases along with the time. Table 3 shows the results of fitting equation under
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the different concentration. The fitting curves’ correlation coefficients R2 > 0.95
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suggests that the fitting result credibility is higher, and the H2SO4 adsorption is mainly controlled by internal diffusion.
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4. Conclusion
Reducing the inlet temperature of the ESP is a promising option for
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collaborative removal of fine particles and SO3. In this paper, the low temperature removal mechanism for fine particles and SO3 is studied through a lab scale experiment platform. During the experiment. The flue gas temperature was changed from 150 to 90 °C, and the SO3 concentration was changed from 40 to 200 mg·m-3. The conclusion is as follows: 1. The ESP outlet particle size has a increasing trend that the median diameter d50 increased to 0.08 μm from 0.05 μm, and the d90 increased to 0.23 μm from 0.09 μm. The particle removal efficiency was obviously higher than general ESP, and indicated collaborative removal for SO3, which the removal efficiency was about 80%. 2. The conventional ESP has low removal efficiency in the particle diameter range of 0.1-1.0 μm, while the low temperature ESP can improve this removal 13
ACCEPTED MANUSCRIPT efficiency. Reducing the ESP inlet temperature and increasing SO3 concentration can improve the removal efficiency of particles. 3. The flue gas temperature and humidity exert significant effects on SO3
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removal efficiency, when the temperature dropped below acid dew point, the SO3 removal efficiency was higher. Fly ash adsorption SO3 was about 10%, increasing
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the humidity and SO3 concentration was beneficial for reduce the fly ash resistance.
4. SO3 and water molecules is formation SO3-H2O-H2SO4 gaseous mixture
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coexistence system by binary nucleation in the low temperature ESP. SO3
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condensation on the fly ash is mainly controlled by internal diffusion.
Acknowledgements
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This work was supported by National Basic Research Program of China (973
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Program NO.2013CB228505), the national key research and development plan of China (NO. 2016YFC0203700). The Fundamental Research Funds for the Central
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Universities and the Ordinary University Graduate Student Scientific Research
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Innovation Projects of Jiangsu province, China (No.KYLX15-0072).
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Jaworek A, Krupa A, Czech T. Modern electrostatic devices and
methods for exhaust gas cleaning: A brief review. J ELECTROSTAT.
Jaeckervoirol A, Mirabel P, Reiss H. Hydrates in supersaturated
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[25]
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2007;65(3):133-55.
binary sulfuric acid-water vapor - a reexamination. CHEM PHYS.
Jackervoirol A, Mirabel P. Heteromolecular nucleation in the
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[26]
D
1987;87(8):4849-52.
[27]
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sulfuric acid-water system. ATMOS ENVIRON. 1989;23(9):2053-7. Kulmala M, Laaksonen A. Binary nucleation of water
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sulfuric-acid system - comparison of classical-theories with different H2SO4 saturation vapor-pressures. CHEM PHYS. 1990;93(1):696-701. [28]
Meakin P, Family F. Scaling in the kinetics of droplet growth and
coalescence - heterogeneous nucleation. JOURNAL OF PHYSICS A-MATHEMATICAL AND GENERAL. 1989;22(6):L225-30. [29]
Weber W. Kinetics of Adsorption on Carbon From Solution. Asce
Sanitary
Engineering
Division
Journal.
ASCE
ENGINEERING DIVISION JOURNAL. 1963;2(1):1-2.
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1- coal-fired boiler；2-stirer；3- buffer vessel；4-heat tube；5-evaporation chamber；6-booster fan；7-metering pump；
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8- water tank；9- heat exchanger；10- ESP；11-desulfurization tower；12- metering pump;
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Fig.1 Schematic diagram of Low-Low Temperature Electrostatic Precipitator system
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ESP inlet ESP otulet at 130℃ ESP outlet at 90℃
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0.4
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Mass dM/dlogDp
0.6
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(a) particle cumulative distribution
1
10
Dp/μm
(b) particle mass distribution
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Fig.3 The ESP outlet particle size distribution
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100
ESP inlet at 90℃ ESP inlet at 150℃
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85 80 75
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Removal efficiency /%
95
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Fig.5 The ESP removal efficiency and fly resistivity at different SO3 concentration
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SO3concentration/mg·m-3
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150
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Fig. 6 SO3 concentration at ESP outlet under different temperature
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100
40 mg·m-3 80 mg·m-3 120 mg·m-3
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60
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Removal efficiency/%
80
40
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90
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0 100
110
120
130
140
150
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Temperature /℃
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Fig.7 The SO3 removal efficiency under different temperature
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SO3 penetration rate
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SO3 penetration rate/%
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12 10
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8 6 10
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Relative humidity
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Fig.8 The SO3 penetration rate under different relative humidity
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150
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Acid dew point/℃
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135 130
SO3 concentration 40 mg·m-3 80 mg·m-3 120 mg·m-3 160 mg·m-3
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125
4
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Water vapor content/%
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Fig.9 The flue gas acid dew point under different SO3 concentrations and humidity
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Fig.10 H2O-H2SO4 binary nucleation
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SO3 concentration
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40 mg·m-3 80 mg·m-3 120 mg·m-3
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qt(mg/g)
8
6
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0 0
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Fig. 11 Fit Curve for H2SO4 adsorption in fly ash
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ACCEPTED MANUSCRIPT Table 1 The specification of the ESP Parameter
Range 3
350
The voltage (kV)
10-50
Space between collection electrodes (mm)
300
The entrance dust concentration (g/m)
1-5
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Flue gas flow rate (m /h)
Operation temperature (℃)
85-150 0.86
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Gas velocity (m/s) Specific collection area (SCA)
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84.3
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ACCEPTED MANUSCRIPT Table 2 Proximate and ultimate analyses of experimental sample Proximate analysis wad/%
Ultimate analysis wad/%
Sample Mtol
Aar
Var
Caf
Car
Har
Oar
Nar
Sar
2.38
8.84
10.00
78.78
64.39
3.18
0.72
0.81
1.70
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coal
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Stone
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ACCEPTED MANUSCRIPT Table 3 The parameters of the fitted curve C
R2
SO3 concentration at 40 mg·m-3
1.00078
0.80132
0.97688
-3
1.08798
0.87044
0.97942
1.46763
0.87263
0.97531
y = a + b×x
SO3 concentration at 80 mg·m
-3
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SO3 concentration at 120 mg·m
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K
Equation
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ACCEPTED MANUSCRIPT Graphical abstract 100
ESP inlet at 90℃ ESP inlet at 150℃
90
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85 80 75
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Removal efficiency /%
95
70 65 0.01
0.1
1
10
Fig.1 Particle efficiencies
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Dp/μm
Fig.2H2O-H2SO4 nucleation
removal
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Flue gas ELPI analysis
binary
Flue gas ELPI analysis
9
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Exhaust
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Exhaust
4
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2
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Flue gas analysis
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ELPI
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11
7
1
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1- coal-fired boiler；2-stirer；3- buffer vessel；4-heat tube；5-evaporation chamber；6-booster fan；7-metering pump； 8- water tank；9- heat exchanger；10- ESP；11-desulfurization tower；12- metering pump;
Fig.1 Schematic diagram of Low-Low Temperature Electrostatic Precipitator system
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ACCEPTED MANUSCRIPT Highlights: The PM2.5 removal efficiency improved and collaborative removal SO3
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when ESP inlet temperature reduced.
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The collection efficiency of 0.1-1 μm particles was improved with temperature reduction.
SO3 and water molecules binary nucleation.
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SO3 condensation on the fly ash mechanism was mainly controlled by
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internal diffusion.
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S0032-5910(17)30466-7 doi:10.1016/j.powtec.2017.06.008 PTEC 12584
To appear in:
Powder Technology
Received date: Revised date: Accepted date:
22 December 2016 16 May 2017 1 June 2017
Please cite this article as: Hu Bin, Zhang Lin, Yi Yang, Luo Fei, Liang Cai, Yang Linjun, PM2.5 and SO3 collaborative removal in electrostatic precipitator, Powder Technology (2017), doi:10.1016/j.powtec.2017.06.008
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ACCEPTED MANUSCRIPT PM2.5 and SO3 collaborative removal in Electrostatic Precipitator
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Hu Bina, Zhang Lina, Yi Yangb, Luo Feia, Liang Caia, Yang Linjuna*
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a Key Laboratory of Energy Thermal Conversion and Control, Ministry of Education, School of Energy and Environment, Southeast University, Nanjing
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210096, China
b Advanced Materials Engineering and Modelling Group, Faculty of Chemistry, Wroclaw University of Science and Technology, Wroclaw 50-370,
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Poland
*Corresponding author. Tel.: +86 25 83795053; fax: +86 25 83795053.
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Email: [email protected]（Yang Linjun）
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ACCEPTED MANUSCRIPT ABSTRACT:
Coal-burning
pollution caused harmfully to
human and
environment, However, the efficiency of electrostatic precipitator ( ESP ) for PM2.5 is low. Reducing the ESP inlet temperature is widely concerned. This technology
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could simultaneously control of PM2.5 and SO3. In this study, a lab-scale low temperature ESP performance was designed under controlled conditions, the
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operating effects were evaluated under different temperature and SO3 concentration, SO3 and particles coagulation mechanism was discussed. Experimental results show that the operating temperature was important for ESP,
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The ESP outlet particle size showed a increasing trend that the median diameter d50 increased to 0.08 μm from 0.05 μm, and the d90 increased to 0.23 μm from 0.09
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μm with the inlet temperature reduced from 150 ℃ to 90 ℃. Particle removal efficiency increased when ESP inlet temperature reduced, especially particles with
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diameters of around 0.1-1 μm increased obviously. SO3 removal efficiency
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improved from about 20% to above 80%, when the ESP inlet temperature drops below the acid dew point. Furthermore, SO3 binary nucleation mechanism was
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discussed, the analysis results of the adsorption mechanism indicated that SO3 condensation on the fly ash was mainly controlled by internal diffusion. KEY WORDS: Low Temperature Electrostatic Precipitator; PM2.5; SO3;
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simultaneous control;
2
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1. Introduction Coal is mainly used for combustion as a dominant energy source in China,
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and coal-burning pollution have serious environmental problem[1]. Northern
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China suffers a long-term haze and fog cover frequently. The particles mainly
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come from the haze and fog[2, 3], and the fine particle particularly less than 2.5 μm (PM2.5) is detrimental to human healthy[4]. In order to control pollution, there are stringent emission standards for particulate matter having been issued to
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control the particles concentration below 30 mg/m3 instead of 50 mg/m3[5]. To reach the new emission standard (GB 13223-2011), many studies on the particle
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matter (PM) have been carried out. In many techniques, enhance the efficiency of the existing pollutant treatment facilities is target for us.
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Currently, electrostatic precipitators (ESPs) are mainly equipment to control
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dust emission in coal-fired power plants in China[3, 5]. ESP removal particles efficiency can reach 99.5% or even higher. However, the removal efficiency of
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PM2.5 is 95-98%, especially for 0.1-1 μm particles with a lower efficiency[6-8]. Therefore, the escape mechanism of fly ash is studied widely. Researches have promoted the ESP collection efficiency such as the acoustic agglomeration,
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chemical agglomeration, SO3 and NH3 flue gas conditioning and composite precipitators[1, 9, 10]. These technologies not only improve the collection efficiency, but reduce the emission rather than comprehensively considering the relationship between energy saving and emission reduction. Hence, install heat exchanger before the ESP and reduce the ESP inlet temperature becoming a research hotspot in numerous technology. Low temperature ESP compares with conventional electrical precipitator, the main performance advantages are the following four points: (1) The fly ash resistance reduced, the temperature decreased and SO3 condensation on the surface of the particles will reduce the resistance. (2) The flue gas volume flow is reduced, according to the formula of Deutsch, and this result will improve the particle removal efficiency. (3) The breakdown voltage increases when the temperature reduces, the gas density and 3
ACCEPTED MANUSCRIPT charged particle collisions increases, and ion migration rate reduces effectively. Correspondingly, the charge density increases near Corona, and breakdown voltage increases. (4) To Remove SO3 the flue gas drops below the acid dew point
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and SO3 exists in the form of sulfuric acid, and deposition on the particle surface are removed with particle[2, 3, 5, 11, 12].
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There are considerable work have been done in the area of PM2.5 and SO3 in the past, Chao Wang[13] measured the mass particle size distribution at the inlet and outlet of the ESP, which equipment had the low temperature economizer.
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Alireza Bahadori[14] discussed the problem that the rate of low temperature corrosion was hard when the mass ratio of dust for SO3 (D/S) was higher than 10.
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Xu[15] studied the influence factors of volume resistivity and surface resistivity such as temperature, composition, and the formula of fly ash resistivity. Qi[16]
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showed the relationship between resistivity and temperature, and the results
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showed that dust resistivity increased at the beginning and decreased with the increasing temperature with maximum value at about 150 ℃. Srivastava and Qi[6,
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16] studied the effect of SO3 on fly ash resistivity, and showed SO3 concentration having important influence on the electric performance. Naoki Noda[17] studied the influence of operating temperature on the ESP performance. Since 1997,
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Mitsubishi industries had applied the low temperature ESP technology in large-scale coal-fired thermal power unit which the outlet particle concentration was less than 30 mg·m-3 and the SO3 concentration was lower than 3.57 mg·m-3 when the ESP operating temperature keep at 90 ℃. In the testing field, Lin Xiang[18] found that the outlet particle concentration changed from 52.5 mg·m-3 to 17.25 mg·m-3, SO3 removal efficiency was 88.14%, total mercury removal efficiency was 40% and the fine particle removal efficiency was above 99.8%[18]. Most of the current researches are focused on the low temperature electric precipitation process optimization and the engineering application. However, the study of the particle emission characteristics and the SO3 removal mechanism is inadequate. 4
ACCEPTED MANUSCRIPT In this study, the ESP inlet flue gas temperature and SO3 concentration were adjusted at the laboratory scale coal-fired thermal state test system to study the removal characteristics of fine particles and SO3. In our experiment, particles and
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2. Experimental system and apparatus
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SO3 removal mechanism was analyzed.
2.1 Experimental system
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All experiments were carried out in a lab coal combustion experiment test-bed as shown in Fig.1. The system was mainly consist of a coal-fired boiler, a
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buffer vessel, an evaporation chamber, a heat exchanger, an ESP, a WFGD, and the analysis detecting system. The flue gas with approximately 350 m3·h-1 was provided by a coal- fired boiler. The buffer vessel ensures to mix the flue gas
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adequately, and adjusts the flue gas temperature. An evaporation chamber could
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adjust flue gas humidity by water evaporation, and a heat exchanger was set in the electric entrance which was finned heat exchanger with the rated flow 2000 kg·h-1
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and heat transfer 10 kV. The temperature was adjusted by waters flow in heat exchanger, and flue gas entered the ESP by booster fan. Coarse particles were
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removed by passing through the ESP, which is a barb-plate tube type ESP and the rated rectifier voltage was 50 kV. The flue gas desulfurization in the desulfurization tower, and it was discharged from the chimney by the ID fan. During the testing phase, the flue gas temperature was changed from 150 °C to 90 °C by heat exchanger. Table 1 shows the specification of the experimental ESP and the design value for the ESP. The specification of the ESP was the same as for the utility boiler. The gas velocity was kept constant with all operating temperatures in the ESP. To investigate the influence of operating temperature on discharge characteristics and collection efficiency of ESP, anthracite coal was used in the experimental process. Table 2 shows the properties of the coals, the subscript ar is as received and the ad is air dry.
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ACCEPTED MANUSCRIPT 2.2 Measuring instruments During the experiments, the total dust was measured by a microcomputer dust
change of the filter drum could be weighed. Subsequently,
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sampling, the mass
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parallel sampling meter. Filter drums were dried at 110 ℃ before and after the
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dust concentration could be calculated. The size distributions of PM10 were measured by an electrical low-pressure impactor (ELPI). The ELPI enables real-time monitor the particle number and mass size distributions, meanwhile, the
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response time of ELPI is less than 2 sec. The ELPI consists of 13 stages (12 channels), which measures aerodynamic size distribution in the size range of
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0.03-10 μm. In this work, the dilution ratio was 1:67, all of the sampling lines kept short to avoid losing large particle. According to the national standard of China
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(controlled condensation method)[19], the full automatic flue gas sampling device
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and spiral condenser was used for collecting sulfate droplets during the measurement, the sampling lines were set to 180℃ to avoid the sulfuric acid
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condensation, and spiral condenser was set to 65 ℃ and adjusted by thermostatic waterbath. The condensate was analyzed by an ion chromatograph analyzer
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(ICS-2100, American) to determine the SO42- concentrations. Combined with the sampling volume of the flue gas, the SO3 concentration in flue gas can be determined.
3. Experimental results and analysis 3.1 Results and discussion of PM 3.1.1 Mass and number distribution of PM The ESP voltage and SO3 concentration was set at 50 kV and 80 mg·m-3. Fig.2 shows the number size distributions of PM10 at the inlet and outlet of ESP. The size distributions of particle matter from coal combustion display a bimodal distribution that contained a submicron mode and a coarse mode with peaks near 6
ACCEPTED MANUSCRIPT 0.1 μm and 1 μm, respectively. The particles in the coarse mode are formed by the transformation of minerals that remain after the carbon burns away. The particle size distribution are determined by char fragmentation and coalescence of surface
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ash droplets. The particles in the submicron mode are formed by vaporization, condensation, and nucleation of inorganic constituents in the fuel[20].
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Respectively, the unimodal number size distribution can be seen with a peak around 0.15 μm with inlet temperature 150 ℃, the mean particle diameters of the
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fine mode are shifted to 0.3 μm with the inlet temperature down to 90 ℃. This is most probably because the flue gas temperature is lower than the acid dew point,
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result in sulfuric acid vapor condenses with sub-micron grade fly ash particles as condensation nuclei. At the same time, a part of water vapor condenses due to the strong water imbibition of sulfuric acid. Because of the force of liquid, the
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particles are not bounce off after the collision. Instead, they reunite to form larger
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particles, the ESP outlet particles have a tendency to increase when the inlet
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temperature reduces. Fig.3(a) shows the particle number cumulative distribution. The particles are generally fine, and the majority of them are less than 1 μm in size. The median diameter d50 of particle at the inlet is 0.13 μm, and the d90 is 0.32 μm.
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When the flue gas temperature dropped from 150 °C to 90 °C, the median diameter d50 increased to 0.08 μm, and the d90 increased to 0.23 μm. Compared to the ESP imports cumulative distribution, the proportion of export submicron particulate matter increased obviously. The main reason may be that ESP removal efficiency of coarse particle is higher and the submicron particle ratio is increased, but the number concentration is lower at the same moment. The outlet particle diameter has a tendency to increase when ESP inlet temperature is lower than acid dew point. The ESP outlet mass size distribution is in fig.3(b), the ESP outlet mass concentration is 28 mg·m-3, the particle mass mainly concentrates in more than 1 μm particles. In combination with the results in fig.3(b), the mass concentration is focused on the coarse particles which is different from the number concentration, and the number concentration of the particles is larger with diameter less 1 μm. In 7
ACCEPTED MANUSCRIPT the view of control pollutant emissions, it is more meaningful to control the fine particles.
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3.1.2 Collection efficiencies of PM
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The experiment of classification removal efficiency is discussed, particle
Ni
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number concentration ratio in a certain channel, it can be written as equation (6): N i 0 N it 100 % N i0
(6)
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Where, ηNi is the resolved efficiency, Ni0 is the initial state particle number concentration, and Nit is the removal of particle number concentration at the outlet
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of the ESP. Operating conditions with reference to 3.1.1. Fig.4 shows the grade removal efficiency under different temperature, the grade removal efficiency was above 85% except 0.1-1 μm diameter section when ESP inlet temperature at
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150 ℃. A low removal efficiency in the size range of 0.1-1 μm contrast to other sizes. This is because different mechanism of charged in different size particle, the
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field charging mechanism is primarily while particle size>1.0 μm, on the contrary, the diffusion charging mechanism is primarily while particle size<1.0 μm. The particle is easily collected by ESP when particle charged. However, as in the
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transitional region of the two charging mechanisms, the collection efficiency for particle size in the range of 0.1-1 μm is low. When the inlet temperature reduced from 150 ℃ to 90 ℃ , the removal efficiency improved above 5%. The improvements were very obvious for particles with diameters of around 0.1-1 μm. The decreases of temperature was beneficial to the improvement the ESP removal efficiency. Lower temperature could reduce the flue gas flow rate and increase the flue gas residence time in ESP[21], which improved the ESP efficiency. Meanwhile, temperature could significantly affect the electrical resistivity. Generally, the ESP is operated effectively when the fly ash resistivity is within the limit of 104 - 2×1010 Ω·cm. The fly ash resistivity reduced from 1.92×1010 Ω·cm to 6.3×109 Ω·cm when the inlet temperature reduced from 150 ℃ to 90 ℃. The 8
ACCEPTED MANUSCRIPT dust resistivity is consists of volume and surface resistivity. Volume resistivity dominates at high temperature and is inversely proportional to working temperature. Surface resistivity mainly occurs at low temperature and is
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proportional to the temperature, usually at the temperature of 120-180 ℃. The
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volume and surface resistivity are near the maximum. Furthermore, while the flue gas temperature decreases to approximately 90 ℃, some gaseous components condensed and adsorbed on the surface fine particles, such as steam, SO3 and HCl.
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The fly ash surface resistivity decreased with acidic gas condensation, and fly ash charged and removed more easily in ESP.
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3.1.3 The influence of SO3 concentration
Fig.5 shows the ESP removal efficiency and the fly resistivity at different SO3
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concentration. The ESP voltage and temperature was set at 50 kV and 90 ℃. The
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total dust removal efficiency increased from 93% to 97%, and the PM2.5 removal efficiency from 73% to 85% with the SO3 concentration changed from 40 to 200
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mg·m-3. The PM2.5 removal efficiency improved obviously. The effect of SO3 on particles could attributed to the following reasons, according to 3.1.1 analyzed, the
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increasing of SO3 concentration will promote particle size becoming bigger in the flue gas, the increase of particle size is beneficial to promote the ESP removal efficiency. In addition, SO3 concentration has a great influence on fly ash resistance. The fly ash resistance is determined by the volume resistivity and surface resistivity, and the surface conductivity is influenced by the vapor and other chemical components on the surface of the fly ash. Therefore, the higher vapor adsorption of the dust particles affects the fly ash resistivity to a large extent. On the other hand, fly ash particles acts as condensation nuclei when the flue gas temperature is lower than the acid dew point. Sulfuric acid condensation in the fly ash forms a liquid bridge force between particles, which improves the probability of
collision
coalescence
between
particles.
This
result
indicates
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conglomeration, hence, when the droplets contact with the particles the surface 9
ACCEPTED MANUSCRIPT tension of the liquid forms a liquid bridge pulling the particles together. This process results in an increased collection efficiency of ESP. SO3 can produce anion in the electric discharge environment, fine particles are more likely to be positive
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plate capture with capture the anion.
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3.2 Results and discussion of SO3 3.2.1 Influence of temperature
The flue gas acid dew point temperature has a close relationship to the SO3
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concentration. Coal-fired power plants generate SO3 according to the sulfur contains in the coal, and the SO3 concentration is in the range from 10-6 mg·m-3 to
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40×10-6 mg·m-3[22-24]. Previous works indicated that the primary mechanism for
third body (M), given by：
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flame based formation of SO3 is interaction of SO2 with O in the presence of a
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SO2 + O+ M → SO3+ M
Fig.6 shows the SO3 concentration at the ESP outlet at different inlet
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temperature. The acid dew point was 90-100 ℃ when SO3 concentration was 40 mg·m-3. It indicates that the SO3 concentration increased from 6 to 24.6 mg·m-3
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with temperature increasing from 90 ℃ to 150 ℃. Similarly, when the initial SO3 concentration was 120 mg·m-3 the SO3 concentration was 12.4 mg·m-3 at 90 ℃, which increased to 100.5 mg·m-3 at 150 ℃. Fig.7 shows SO3 removal efficiency under different temperature. It also shows that the SO3 removal efficiency can reach above 80% below acid dew point temperature, and SO3 removal efficiency is low above the acid dew point temperature. The main reason is the two mechanism of fly ash adsorption of SO3. When the temperature was higher than acid dew point, the residual carbon played a major role to adsorption gas SO3 because of the few amount of residual carbon in fly ash, and the removed of SO3 was low. Relatively, when the temperature was lower than acid dew point SO3 condensed to form H2SO4 acid fog and the dust particles enlarged specific 10
ACCEPTED MANUSCRIPT surface area, and the H2SO4 acid fog condensed and adsorbed on the particle surface which was removed together with the dust by ESP. The effect of SO3 concentration could mainly ascribe to that high SO3 concentrations form more
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sulfuric acid droplets leading to the obvious increase of the sulfuric acid droplets and particle collision rate, besides, the interaction between particles and sulfuric
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acid droplets also enhance. 3.2.2 Influence of humidity
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Experiments were operated at several levels of humidity to investigate its influence on the SO3 removal performance of ESP, at 50 kV ESP voltage and
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90 ℃. The results in Fig.8 show that the SO3 penetration rate was reduced as relative humidity (RH) increased from 6% to 36%, and the concentration of water
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vapor had close relationship with the SO3 removing efficiency. According to the
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former research, dew point temperature is depends on the vapor partial pressure and the SO3 concentration. When the flue gas temperature was below 205 ℃, all
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the SO3 existed in the form of sulfuric acid steam. When the temperature dropped below acid dew point, water vapor and the steam condensed to sulfuric acid droplets. As fig.9 shown, the fixed SO3 concentration, the water vapor content
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increased, accordingly, the acid dew point temperature was also increased. Water molecules combined with sulfuric acid to form sulfuric acid droplets and the higher humidity which could promote the condensation of sulfuric acid vapor on the fly ash and increased the SO3 removal efficiency. Generally, the water vapor and the sulfuric acid vapor exist separately and the condensation begins at the lower temperature, however, when the water vapor and the sulfuric acid vapor exist together the condensation can occur at the temperature cooling to the acid dew point. This is caused by H2O-H2SO4 binary nucleation. On the other hand, the sulfuric acid affects the performance of the ESP. The H2O-H2SO4 molecules are charged by the interaction with electrons, and the decreasing ion mobility. Meanwhile, the increase of relative humidity benefits the agglomeration of 11
ACCEPTED MANUSCRIPT particles. The particles and the sulfuric acid droplet were removed by passing though the ESP. Thus the collection efficiency was greatly improved with the
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increase of relative humidity.
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3.2.3 Removal mechanism of SO3
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SO3 exists in the SO3-H2O-H2SO4 gaseous mixture coexistence system at low ESP temperature, H2SO4 combines the water molecules easily in the humidity environment, The combination of the H2SO4 in the ESP are H2SO4·H2O,
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H2SO4·2H2O, H2SO4·3H2O, …… , H2SO4·hH2O, The equation for reaction is as follow:
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H2O + H2SO4·(n-1)H2O⇋H2SO4·hH2O That is to say, H2O-H2SO4 nucleation is not only the H2O and H2SO4, but the hydrate sulfuric acid molecules also participation as shown in fig.10. According to
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the empirical formula given by Vehkamaki papers, the SO3 molecules number
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concentration is 1×1014 1·cm-3 to 1×1015 1·cm-3 at the SO3 concentration within the scope of 40-200 mg·m-3. The nucleation rate is more than 1014 1·cm-3 with the
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critical nucleation H2SO4 molecular number is about 30. Therefore, H2O-H2SO4 binary nucleation process is fast. The sulfate droplets are easy to condense and
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adsorb on the surface of fly ash, then the dust forms the dust layer due to the wool stoma force and the electrostatic force etc. Because of H2O-H2SO4 nucleation rate is higher than the physical adsorption process, SO3 could react with water vapor to generate sulfuric acid mist, then the sulfuric acid mist reacts with the alkaline substance in the fly ash[25-28]. The condensation process of the sulfate droplets on the fly ash is divided into four stages: external diffusion, film diffusion, internal diffusion and adsorption reaction. The adsorption rate is controlled by the slowest step. Usually, the gas film diffusion and the external diffusion rate are faster, hence, the film diffusion and the external diffusion are not main factor. In order to study the adsorption kinetics characteristics of H2SO4, determine adsorption dynamics model for describing the adsorption process, the Weber - Morris[29] empirical formula is used to analyze the experimental data: 12
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qt K t 2 C
(7)
Where K is particle diffusion rate constant; qt is fly ash adsorption quantity;
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C is constant;
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Fig.11 shows the experimental data fitting with Weber-Morris equation at different initial concentrations of SO3. When the SO3 entrance concentration
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increased the slope of the fitted curve increased, it seems to indicate that the process was controlled by diffusion. The fitted curve was not though the origin, however, the internal diffusion was not the only factor for SO3 adsorption
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condensation control steps. The whole process was determined jointly by various dynamics mechanism, namely, the diffusion and the surface reaction leaded to the
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adsorption change. The fig.12 shows that adsorption of H2SO4 on the fly ash increases along with the time. Table 3 shows the results of fitting equation under
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the different concentration. The fitting curves’ correlation coefficients R2 > 0.95
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suggests that the fitting result credibility is higher, and the H2SO4 adsorption is mainly controlled by internal diffusion.
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4. Conclusion
Reducing the inlet temperature of the ESP is a promising option for
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collaborative removal of fine particles and SO3. In this paper, the low temperature removal mechanism for fine particles and SO3 is studied through a lab scale experiment platform. During the experiment. The flue gas temperature was changed from 150 to 90 °C, and the SO3 concentration was changed from 40 to 200 mg·m-3. The conclusion is as follows: 1. The ESP outlet particle size has a increasing trend that the median diameter d50 increased to 0.08 μm from 0.05 μm, and the d90 increased to 0.23 μm from 0.09 μm. The particle removal efficiency was obviously higher than general ESP, and indicated collaborative removal for SO3, which the removal efficiency was about 80%. 2. The conventional ESP has low removal efficiency in the particle diameter range of 0.1-1.0 μm, while the low temperature ESP can improve this removal 13
ACCEPTED MANUSCRIPT efficiency. Reducing the ESP inlet temperature and increasing SO3 concentration can improve the removal efficiency of particles. 3. The flue gas temperature and humidity exert significant effects on SO3
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removal efficiency, when the temperature dropped below acid dew point, the SO3 removal efficiency was higher. Fly ash adsorption SO3 was about 10%, increasing
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the humidity and SO3 concentration was beneficial for reduce the fly ash resistance.
4. SO3 and water molecules is formation SO3-H2O-H2SO4 gaseous mixture
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coexistence system by binary nucleation in the low temperature ESP. SO3
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condensation on the fly ash is mainly controlled by internal diffusion.
Acknowledgements
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This work was supported by National Basic Research Program of China (973
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Program NO.2013CB228505), the national key research and development plan of China (NO. 2016YFC0203700). The Fundamental Research Funds for the Central
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Universities and the Ordinary University Graduate Student Scientific Research
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Innovation Projects of Jiangsu province, China (No.KYLX15-0072).
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SANITARY
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1- coal-fired boiler；2-stirer；3- buffer vessel；4-heat tube；5-evaporation chamber；6-booster fan；7-metering pump；
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8- water tank；9- heat exchanger；10- ESP；11-desulfurization tower；12- metering pump;
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Fig.1 Schematic diagram of Low-Low Temperature Electrostatic Precipitator system
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ESP inlet ESP otulet at 130℃ ESP outlet at 90℃
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103 0.01
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Number dN/dlogDp /cm-3
107
Dp/μm
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1.2
40
1.0 30
20
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0.0 0.1
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0 0.01
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0.1
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0.01
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ESP outlet at 150℃ ESP outlet at 90℃ ESP inlet
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Mass dM/dlogDp
0.6
Dp/μm
(a) particle cumulative distribution
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10
Dp/μm
(b) particle mass distribution
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Fig.3 The ESP outlet particle size distribution
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Cumulative /%
0.8
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ESP inlet at 90℃ ESP inlet at 150℃
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90
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85 80 75
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Removal efficiency /%
95
70
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65 0.01
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1
Dp/μm
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Fig.5 The ESP removal efficiency and fly resistivity at different SO3 concentration
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120
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80
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40
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0 100
110
120
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90
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SO3concentration/mg·m-3
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40 mg·m-3 80 mg·m-3 120 mg·m-3
130
140
150
Temperature/℃
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Fig. 6 SO3 concentration at ESP outlet under different temperature
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40 mg·m-3 80 mg·m-3 120 mg·m-3
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Removal efficiency/%
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40
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90
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0 100
110
120
130
140
150
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Temperature /℃
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Fig.7 The SO3 removal efficiency under different temperature
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SO3 penetration rate
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SO3 penetration rate/%
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12 10
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8 6 10
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20
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5
25
30
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Relative humidity
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Fig.8 The SO3 penetration rate under different relative humidity
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150
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Acid dew point/℃
145
135 130
SO3 concentration 40 mg·m-3 80 mg·m-3 120 mg·m-3 160 mg·m-3
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4
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Water vapor content/%
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Fig.9 The flue gas acid dew point under different SO3 concentrations and humidity
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Fig.10 H2O-H2SO4 binary nucleation
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SO3 concentration
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40 mg·m-3 80 mg·m-3 120 mg·m-3
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qt(mg/g)
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6
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0 0
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7
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Fig. 11 Fit Curve for H2SO4 adsorption in fly ash
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Range 3
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The voltage (kV)
10-50
Space between collection electrodes (mm)
300
The entrance dust concentration (g/m)
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Flue gas flow rate (m /h)
Operation temperature (℃)
85-150 0.86
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Gas velocity (m/s) Specific collection area (SCA)
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84.3
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ACCEPTED MANUSCRIPT Table 2 Proximate and ultimate analyses of experimental sample Proximate analysis wad/%
Ultimate analysis wad/%
Sample Mtol
Aar
Var
Caf
Car
Har
Oar
Nar
Sar
2.38
8.84
10.00
78.78
64.39
3.18
0.72
0.81
1.70
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ACCEPTED MANUSCRIPT Table 3 The parameters of the fitted curve C
R2
SO3 concentration at 40 mg·m-3
1.00078
0.80132
0.97688
-3
1.08798
0.87044
0.97942
1.46763
0.87263
0.97531
y = a + b×x
SO3 concentration at 80 mg·m
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SO3 concentration at 120 mg·m
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K
Equation
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ESP inlet at 90℃ ESP inlet at 150℃
90
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85 80 75
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Removal efficiency /%
95
70 65 0.01
0.1
1
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Fig.1 Particle efficiencies
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Dp/μm
Fig.2H2O-H2SO4 nucleation
removal
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Flue gas ELPI analysis
binary
Flue gas ELPI analysis
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Exhaust
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Flue gas analysis
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ELPI
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1
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1- coal-fired boiler；2-stirer；3- buffer vessel；4-heat tube；5-evaporation chamber；6-booster fan；7-metering pump； 8- water tank；9- heat exchanger；10- ESP；11-desulfurization tower；12- metering pump;
Fig.1 Schematic diagram of Low-Low Temperature Electrostatic Precipitator system
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ACCEPTED MANUSCRIPT Highlights: The PM2.5 removal efficiency improved and collaborative removal SO3
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when ESP inlet temperature reduced.
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The collection efficiency of 0.1-1 μm particles was improved with temperature reduction.
SO3 and water molecules binary nucleation.
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SO3 condensation on the fly ash mechanism was mainly controlled by
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internal diffusion.
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