Improving the Removal of Fine Particles with an Electrostatic

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Improving the removal of fine particles with an electrostatic precipitator by chemical agglomeration Yong Liu, Bin Hu, Lei Zhou, Yezheng Jiang, and Linjun Yang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00626 • Publication Date (Web): 06 Sep 2016 Downloaded from http://pubs.acs.org on September 16, 2016

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Improving the removal of fine particles with an electrostatic precipitator by chemical agglomeration Liu Yong, Hu Bin, Zhou Lei, Jiang Yezheng, YANG Linjun (Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, Southeast University, Nanjing 210096, Jiangsu Province, China)

ABSTRACT: The emission of fine particles, especially that aerodynamic diameter less than 2.5µm named PM2.5, from coal combustion is an important source of atmospheric PM2.5. To improve the removal of PM2.5 by an electrostatic precipitator (ESP), an agglomerant solution prepared by dissolving agglomerant in process water or desulfurization wastewater was sprayed at the inlet of the ESP. The number concentration and diameter distribution of the particles were investigated before and after the agglomeration solution addition based on a coal-fired thermal system, and the effects of the operating parameters, such as the species and concentration of the agglomeration solution, flue gas temperature, pH value of the agglomeration solution, and diameter of the spray droplets, on the fine particle removal efficiency were analyzed. The results show that the average diameter of the particles could grow more than four times due to the effects of wetting, the liquid bridge force and adsorption bridging, and the PM2.5 concentration at the ESP outlet could decrease by 40% under typical flue gas conditions. The removal efficiency of the ESP on fine particles can be increased with a high concentration and low pH value in the agglomeration solution. The concentration of fine particles changes slightly at the outlet of the ESP when only spraying desulfurization wastewater, but it decreases considerably when the desulfurization wastewater contains agglomerants. KEY WORDS: fine particles; removal; improvement; chemical agglomeration; desulfurization wastewater; ESP; PM2.5

Introduction Fine particles are harmful to human health because their large surface area can be enriched with toxic heavy metals and other pollutants, which are also an important factor causing environmental problems such as low visibility and smog. 1-3 Emissions from coal-fired power

ACS Paragon Plus Environment

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plants are one of the major sources of fine particles.

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4-6

At present, more than 80% of

coal-combustion power plants are equipped with ESPs, and the removal efficiency of particles can reach up to 99%, but it is difficult to remove fine particles, 7 and the largest part of them are emitted into the atmosphere. To improve the removal efficiency of fine particles by ESPs, previous studies have focused on the following aspects: improving the charging technology and fine particle coagulation and growth technology, flow control and optimization, ion wind control to decrease re-entrainment of dust, energizing with a pulsed or new switching power source, compound dust collection technology and electrostatic enhancement filtration. 8-10The growth in the size of fine particles caused by physical or chemical pretreatment including acoustic agglomeration, electro coagulation, turbulence coalescence, and chemical agglomeration is one of the main research orientations, and the enlarged particles can then be removed efficiently by conventional dust removal equipment. 11-14 Chemical agglomeration technology can improve the removal of fine particles after a simple renovation of the existing dust removal installations，which is an important practical application prospect. Durham et al.15 conceived a spraying solution device for installation before the ESP to improve its performance. Rajniak et al.16, 17 adjusted the particle viscosity and mass resistivity by spraying a special adhesive solution that can lower the mass resistivity entering the ESP. Chen, Liu et al.18,

19

analyzed the mechanism and influence factors of chemical agglomeration

theoretically. Zhao, Wei et al20, 21 investigated the influences of the viscosity, density, and droplet size of the agglomeration solution on the agglomerative properties of the particles in a simulated flue gas system. Yan, Zhao et al.22-24 investigated the wettability of a wetting agent on fine particles and its promotive action for chemical agglomeration. However, the above studies investigate the feasibility of chemical agglomeration theoretically, and a simulated flue gas system is obviously different from a practical experimental system. Desulfurization wastewater evaporation in a chimney flue is a new technology for desulfurization wastewater treatment introduced due to the strict standards of wastewater emissions and the increased clean production consciousness. Ma25 analyzed the feasibility of desulfurization wastewater evaporation technology theoretically, but the combination of desulfurization wastewater evaporation and chemical agglomeration has not been investigated yet. The removal of fine particles by spraying an agglomeration solution prepared by dissolving


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agglomerants in process water or desulfurization wastewater into the flue gas at the inlet of the ESP with coal-fired flue gas as the particle source was investigated in this paper. The agglomeration effects and growth of fine particles as well as the particle removal efficiency were analyzed experimentally. The results can provide an experimental and theoretical basis for improving the removal of fine particles from flue gas by chemical agglomeration.

1. Experimental investigation 1.1Experimental set-up The experimental set-up is shown schematically in Fig. 1. A coal-fired boiler, buffer vessel, chemical agglomeration chamber, ESP, wet flue gas desulfurization (WFGD) system, and analysis-detection system were included in the experimental system. Flue gas having a flow rate of 350 Nm3/h was generated by the boiler, which burned anthracite. A stirrer and an electric heater were installed in the buffer vessel to ensure a constant particle concentration and size distribution, and to regulate the temperature of the flue gas. The agglomeration solution/ desulfurization wastewater was sprayed into the chemical agglomeration chamber, and the flue gas had enough time to mix with agglomeration solution in the chemical agglomeration chamber, before entering the ESP. The flue gas was pressurized by a booster fan and then passed through the desulfurization tower.

Fig. 1 Schematic diagram of the experimental system

The chemical agglomeration system consists of an agglomeration solution addition system and an agglomeration chamber. The agglomeration solution addition system comprises an


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agglomeration solution tank, metering pump, atomizing nozzle, and air compressor. The agglomeration solution is sprayed into the flue gas in front of the ESP as droplets generated from the atomizing nozzle. The dust removal system consists of an electrostatic precipitator and its control devices. The ESP adopted was a single-stage wire-plate type, and the operating voltage ranges from -30kV DC to -60kV DC. The rapping apparatus was set on the dust collecting plates, and the distance between the energized wires and earthed plates can be adjusted from 0.15m to 0.45m, and the maximum output voltage of the electrical source is -100kV DC. The atomizing nozzle adopted was made by BETE Fog Nozzle, Inc., Greenfield, MA, USA, and can produce droplets of 30µm diameter. In addition to the high polymer agglomerant, the agglomeration solution also contains a wetting agent, pH value regulator and other ingredients. Common polymer agglomerants are listed in Table 1. Twelve sodium dodecyl sulfate (C12H25SO4Na) was chosen as wetting agent, and phosphoric acid was adopted as the pH value regulator. The solution was prepared in a solution tank, and it was sprayed into the agglomeration chamber by a metering pump. The surfactant concentration in the agglomeration solution was 250ppm. Six types of agglomerant were selected for this study based on their molecular characteristics (Table 1). Table 1 Characteristics of agglomerants Agglomerant

Molecular weight

Characteristic

PG

50000~150000

Natural organic polysaccharide

CMC

2420000


Xanthan gum

XTG

1000000


Poly aluminum chloride

PAC

Pectin gum

Abbreviation

Sodium carboxymethyl cellulose

1500~ Inorganic polymer flocculant 3000 Poly ferric sulfate

PFS

Sodium alginate

SA

2000~5000

Inorganic polymer flocculant

100000~ Natural organic polysaccharide 400000

The desulfurization wastewater was taken from the WFGD set-up, in particular from the supernatant fluid after it was left to precipitate for 1 hour and had its desulfurization agent filtered


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out. The wastewater properties were as follows: the pH value is between 5.0 and 6.5; the suspended solid concentration is 200 mg/L; and the ionic concentration are Cl-, 11093mg/L; SO42-, 2230 mg/L; Fe3+, 0.946 mg/L; Mg2+, 3058 mg/L; and Ca2+, 1687 mg/L. 1.2Measurement technique The size distribution and concentration of the fine particles were measured by means of an electrical low pressure impactor (ELPI). The measurement size range is a 0.023-9.314µm aerodynamic diameter.26 Because of the high moisture content in the sample gas stream, the water vapor has a tendency to condense on the internal surface of the sampling pipelines and on the ELPI’s impact plate, which influences the experimental results. Therefore, heat preservation was necessary for the sampling pipelines, and the sample gas was diluted with particle-free hot dry air (150°C, dilution ratio 8.19:1) prior to entering the ELPI measurement system.

2. Results and Discussion 2.1 Chemical agglomeration and growth properties of fine particles XTG was adopted as the agglomerant in this experiment. The mass concentrations of the agglomerant and wetting agent were 0.05% and 0.025%, respectively. The physical properties change of the fine particles were measured at the outlet of the ESP by spraying agglomeration solution into the flue gas with a double-liquid atomizing nozzle when the ESP is off. The flue gas temperature is 150°C, the agglomeration solution flow rate is 15 kg/h, and the amount of high polymer added into flue gas accounts for 0.25% of the flue gas dust mass. The results are shown in Fig.2. The peak value of the particle number concentration is ~0.15µm in the original flue gas, which increased to ~0.4µm after adding XTG as the fine particles grow up into larger ones. Fig.2 also shows that the peak value of the particle number concentration can increase to ~0.65µm when both XTG and wetting agent are added into the flue gas. This increase in particle size can be explained by the fact that the atomized droplets more easily to adhere onto the fine particles after wetting and the adsorption reaction between fine particles causes the further growth of the particles. The principle of particle growth after spraying the chemical agglomeration solution is shown in Fig.3. A fog cloud with high viscosity is formed after spraying the agglomeration solution, and


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it will adhere on the particle surfaces, forming liquid bridges between the fine particles. Because of the high flue gas temperature (150 °C), the moisture in the fog cloud evaporates gradually, and the liquid bridge turns into a solid one. Thus, the agglomeration power between fine particles is reinforced, and the particles can grow larger by forming chains or groups. Furthermore, fine particles can adhere onto larger ones, which will lead to a decrease in the particle number concentration.

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original flue gas agglomerant plus wetting agent agglomerant

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Fig. 2 Particle diameter distribution under different conditions

Fine particles

Adsorption

Water evaporation

Fine particles Agglomerant

+ Agglomeration droplet

Large particle group

Fig. 3 Schematic diagram of agglomeration mechanism

(a) SEM without agglomeration

(b)SEM with agglomeration

Fig. 4 Agglomeration and growth properties of fine particles under SEM

Fig. 4 shows Scanning Electron Microscope (SEM) pictures of samples collected from the


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flue gas by a fiber glass filter cartridge. Fig 4(a) shows the appearance of the fine particles before agglomeration, in which most of the fine particles are accumulated with each other as groups. The appearances of fine particles after agglomeration are shown in Fig. 4(b). A great number of particles ~ 2.5µm in diameter are connected together by macromolecular chains as bridges, and the particles turn to larger ones, which can be effectively removed by the ESP. 2.2 Removal effects of fine particles by chemical agglomeration 2.2.1 Particle removal efficiency under typical conditions Experiments are conducted to test the improvement of the fine particle removal efficiency by chemical agglomeration under in the typical situation. The improvement is investigated by spraying water and the chemical agglomeration solution before the chemical agglomeration chamber when the ESP is on. The spraying rate of the agglomeration solution is 15kg/h, and the PG mass concentration is 0.05%, accounting for ~0.25% of the flue gas dust mass. The wetting agent concentration is 0.025%, and the flue gas temperature at the addition position is 150°C. The flow rate of flue gas is 350 Nm3/h, and the energization voltage of the ESP is -30kV DC. The agglomeration solution was sprayed into the flue gas by the use of two fluid atomization nozzles. The fine particle concentration at the ESP outlet spraying PG solution is shown in Fig.5. Fig. 5(a) shows that the fine particle concentration changes little when spraying water. Fig 5(b) shows that the particle number concentration decreases from ~5.72×106/cm3 to ~3.24×106/cm3 at the outlet of the ESP after spraying the PG agglomeration solution, a reduction of 43.3%. The mass concentration decreases from ~65.4 mg/m3 to ~34.4 mg/m3, and the mass concentration reduces 47.4%. This can be explained that the agglomeration solution can adhere on the fine particles, and due to the higher flue gas temperature, the moisture in the fog cloud evaporates gradually, turning the liquid bridge into a solid one. The agglomeration power between fine particles is reinforced, and the particles can grow into larger ones that can be removed effectively.


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(a) No spray agglomeration solution used

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Fig.5 Influence of spraying chemical agglomeration solution on fine particle removal

2.2.2Influence of the agglomeration solution concentration The agglomeration solution concentration was shown to have a great influence on the fine particle removal efficiency. Three types of agglomeration solutions, PG, XTG and SA, were tested at the typical conditions above, and the agglomeration concentrations were 0.025%, 0.05%, 0.75% and 0.1%.

Decrease percentage of fine particles concentration/%

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PG XTG SA 25

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0.04

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Agglomerants solution concentration/%

Fig.6 Influence of agglomeration solution concentration

The removal efficiency of fine particle improved by chemical agglomeration can be described by the decrease percentage of fine particles concentration in the outlet of ESP. The decrease in percentage is the ratio of decrease amount of fine particles concentration after spraying agglomeration solution to the original fine particles concentration in the outlet of ESP. The Fig.6 shows the decrease in the fine particle concentration with different concentration of agglomerations of agglomeration solution. The percent decrease in the fine particle concentration improves with the increasing agglomeration solution concentration, and the use of SA led to the highest reduction. The percent decrease in the fine particle concentration sharply improves with


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the increased concentration of the agglomeration solution when it is lower than 0.05%, and the increase is approximately 17% upon adding SA into the flue gas at a 0.025% concentration. Fig. 6 shows that the percent decrease changes only slightly when the concentration is 0.025%. This is because the number of condensable droplets is low at the lower concentration, which reduces the probability of contact between droplets and particles. The removal efficiency rises only gently when the concentration of the agglomeration solution is more than 0.05%. This is mainly because the adsorption sites on the fine particle surface are occupied by fine particles, making them unavailable to other particles. Therefore, the optimal concentration is 0.05% 2.2.3Influence of flue gas temperature Fig.7 illustrates the influence of the flue gas temperature on the removal of fine particles, testing temperatures of 120°C，150°C and 250°C, with all other operating conditions as described in paragraph 2.2.1. In this set of experiments, the agglomeration solutions include four different organic solutions and two inorganic solutions，and all the agglomeration solutions concentration are 0.05%. The agglomeration effect is closely associated with the flue gas temperature，and 150°C is the optimal temperature, with all six agglomeration solutions. The organic solutions have minimal effects when the temperature is 250°C, while the two kinds of inorganic aggregation agents (PAC, PFS) maintain similar effect, and only PG of the polysaccharide solutions can improve fine particles removal. This can be interpreted as the viscosity and other properties of the CMC significantly decreasing after a long time heating at 250°C, while the XTG thermal stability decreases significantly at 250°C. The viscosity of the SA will also significantly decrease in a hot environment. 20 Fig.7 also shows that the agglomeration effect weakened at 120°C, and the main reason is the agglomeration solution cannot easily evaporate completely at 120°C, which cause the liquid bridge between particles cannot transform into a solid bridge. Therefore, the spraying position and the evaporating properties of atomized droplets should be considered when selecting a flue gas temperature.


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30

150 °C 250 °C 120 °C

25

20

15

10

5

0 SA

PG

CMC

XTG

PAC

PFS

Different agglomerants

Fig. 7 Influence of flue gas temperature on fine particles removal

2.2.4Influence of the pH value of the agglomeration solution A decrease in the pH value can change the electrical properties and double electrode layer components of agglometant, which is propitious to the adsorption of particles by the formation of macromolecular chains and the agglomeration effect of particles [15]. The influence of the pH value on the fine particles removal is shown in Fig. 8. PFS, XTG and PG are tested in the experiment as the agglomeration solutions, and a phosphoric acid solution with a volume fraction 0.5% is chosen as the pH conditioner. The agglomeration solution is mixed with phosphoric acid solution and then is sprayed into flue gas. The results show that the percent decrease in the fine particles concentration is between 5-10% after adding the phosphoric acid solution compared with the original efficiency, which indicates that the pH decrease caused by the phosphoric acid addition and stretches the macromolecular chain, which is favorable to the adsorption of fine particles.


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25

No phosphoric acid Adding 0.5% phosphoric acid 20

15

10

5

0 PFS

XTG

PG

Different agglomerants

Fig. 8 Influence of pH on fine particles removal by using 0.05% agglomeration solution at a flue gas temperature of 150

2.2.5Influence of the flue gas retention


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The influence of the flue gas retention on the particles removal efficiency using PG as the agglomeration solution is shown in Fig.9. The flue gas retention refers to the time that flue gas passes through the agglomeration chamber, which can be adjusted by changing the flue gas flow rate. It can be observed that the percent decrease increases slightly when the flue gas retention time changes from 1.3 seconds to 1.0 seconds. This is because the flow turbulence is strengthened and the collision probability increases as the flue gas amount increases. However, the removal efficiency decreases sharply at only 2% when the flue gas retention time is reduced to 0.8 second. The flue gas velocity increases with the flow rate of the flue gas, and the atomized droplets cannot easily adsorb fine particles. Meanwhile, the distance between the spraying position and ESP is quite close, making the agglomeration solution hard to evaporate. Therefore, the optimal flue gas retention time is 1.0 second.

decrease percentage of fine particles concentration/%

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20

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1.2

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Fig. 9 Influence of flue gas retention time on fine particles removal

2.2.6 Influence of the atomized droplets diameter Using PG as the agglomeration solution, Fig.10 shows the influence of the diameter of the atomized droplets, controlled by using an ordinary atomized nozzle and a BETE atomizing nozzle, on the agglomeration effects of the fine particles. The diameter of the ordinary atomizing nozzle is

~50µm, while that of the BETE atomizing nozzle is ~30µm.The other operating conditions are as described in paragraph 2.2.1. As shown, the fine particles number concentration at the outlet of the ESP is ~6×105 /cm3 when using the ordinary atomizing nozzle, which is higher than that achieved by using the BETE atomizing nozzle. Due to the increase in the removal efficiency of the fine particles, the value is 5% higher upon using the BETE atomizing nozzle. This is because the large droplets generated by the ordinary atomizing nozzle are more difficult to evaporate, weakening the agglomeration effect, which leads to a low removal efficiency of fine particles by the ESP.


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BETE nozzle

d≈50µm

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Fig. 10 Influence of the diameter of the spray droplets on fine particles removal by using 0.05% agglomeration solution at a flue gas temperature of 150

2.3 Fine particles characteristics change and removal efficiency improved by chemical

agglomeration after desulfurization wastewater evaporation 2.3.1 Changes of fine particles after desulfurization wastewater evaporation when ESP is off Fig.11 shows the number concentration change of the fine particles after spraying desulfurization wastewater into the chemical agglomeration chamber before the ESP by a double-flow atomized nozzle when the ESP is off. The volumetric flow of flue gas is 350Nm3/h, its temperature is 150°C, and the flow rate of desulfurization wastewater is 10L/h. Fig. 11 shows that the number concentration of fine particles increases from ~1.0×107/cm3 to ~1.2×107/cm3 after spraying the desulfurization wastewater. This can be explained by the existence of submicron particles in the desulfurization wastewater after the precipitation that could enter the flue gas. Fig.12 shows that the diameter of the fine particles significantly increases with the number concentration when spraying is performed, which indicates that the agglomeration effect of the fine particles occurs after the addition of the desulfurization wastewater. This is because the Fe3+ and Al3+ in the desulfurization can form hydroxide flocculating agents, which have agglomerating effects of the fine particles in flue gas. 27

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Fig. 11 Change in fine particles number concentration at the outlet of the ESP by spraying desulfurization

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wastewater at a flue gas temperature of 150

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Energy & Fuels

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Fig.12 Change in diameter distribution at the outlet of the ESP by spraying desulfurization wastewater at a flue gas temperature of 150

2.3.2 Fine particles removal by chemical agglomeration coupled with desulfurization wastewater evaporation Fig.13 shows the particles concentration change after spraying desulfurization wastewater and wastewater coupled with agglomeration solution into the flue gas before the ESP by a double-flow atomizing nozzle when the ESP is on and the voltage is -30kV DC. The flow rate of the flue gas is 350Nm3/h and its temperature is 150°C, and the flow rate of the desulfurization wastewater is 10L/h. As is shown, the average fine particles concentration change at the outlet of the ESP is very small after spraying desulfurization wastewater, as the average number concentration is maintained at 3.7×106/m3 and the mass concentration is maintained at 36mg/m3. The fine particles removal efficiency increases because the particles diameter increases, though the particles number concentration in inlet of the ESP enhancing. The composition of the desulfurization is also more complex, and the saline materials in it can decrease the mass resistivity of the fine particles,

28

which can improve the particles removal efficiency by the ESP,

and the high humidity and low temperature can improve the ESP performance as well.


Energy & Fuels

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Fig. 13 Change in fine particles concentration at the outlet of the ESP by spraying desulfurization wastewater at a flue gas temperature of 150

The concentration change of the fine particles is shown in Fig.14. The agglomeration solution adopted was PG, with a concentration of 0.05% in desulfurization wastewater, and the other experimental conditions are as described in paragraph 2.2.1. Fig.14 shows that the particles number concentration decreases from ~3.9×107/cm3to ~2.5×107/cm3, and the mass concentration decreases from ~27 mg/m3 to ~18mg/m3, which indicates that the removal of fine particles can be effectively improved by chemical agglomeration coupled with desulfurization wastewater evaporation. The atomized droplets can adhere to fine particles as the desulfurization wastewater containing agglomeration solution is sprayed into the flue gas, and the macromolecular linkage of the agglomeration solution would catch fine particles in desulfurization wastewater by an adsorption bridging action, which causes a decrease in the number concentration, an increase in the diameter of the fine particles, and improved particle removal efficiency.

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Fig. 14 Change in particles concentration at the outlet of the ESP by spraying desulfurization wastewater and agglomerant mixed solution at a flue gas temperature of 150

3. Conclusions The diameter for the peak value of the fine particle number concentration increases from 0.15µm to 0.4µm after spraying XTG into coal-fired flue gas, as the fine particles grow into larger particles. The diameter can increase to 0.65µm upon adding wetting agent, which is 4 times larger


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than the original diameter. SEM shows that a great number of particles are connected together with macromolecular chains as bridges, and as the particles grow larger, they can be effectively removed by a dust collector. Under typical conditions, the particle number concentration decreases from 5.72×106/cm3 to 3.24×106/cm3 at the outlet of the ESP after adding an agglomeration solution of PG, and the mass concentration decreases from 65.4mg/m3 to 34.4mg/m3. The concentration of the agglomeration solution, flue gas temperature, agglomeration solution pH, volumtric flow of flue gas and diameter of the spray droplets can influence the agglomeration effects of the fine particles when adding agglomeration solution in front of the ESP. The removal efficiency rises slowly when the solution concentration is more than 0.05%. The agglomeration effect is be weak when the flue gas temperature is higher than 250°C or lower than 120°C; the optimal temperature is 150°C. A lower pH value is beneficial to the agglomeration of the fine particles, and the percent decrease in the fine particles concentration is between 5-10% after adding the phosphoric acid solution. The optimal flue gas retention time is 1.0 second, and the small droplets generated by BETE are more efficiency on chemical agglomeration than the large droplets generated by the ordinary atomizing nozzle. By spraying the desulfurization wastewater before the inlet of the ESP, the number concentration of fine particles increases, while the mass concentration of fine particles doesn’t change significantly when the ESP is off. The removal efficiency of fine particles is enhanced by spraying the desulfurization wastewater.

Acknowledgments This work is supported by the National Basic Research Program of China（973 Program）. The authors of this paper are sincerely grateful to all the reviewers for their insightful comments and suggestions.

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Energy & Fuels

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