Experimental Study on Removal Characteristics of SO3 by Wet Flue

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Environmental and Carbon Dioxide Issues

Experimental Study on Removal Characteristics of SO3 by Wet Flue Gas Desulfurization Absorber Chenghang Zheng, Yipan Hong, Zhewei Xu, Cunjie Li, Li Wang, Zhengda Yang, Yongxin Zhang, and Xiang Gao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b04057 • Publication Date (Web): 09 Apr 2018 Downloaded from http://pubs.acs.org on April 9, 2018

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Energy & Fuels

1

Experimental Study on Removal Characteristics of SO3 by Wet

2

Flue Gas Desulfurization Absorber

3

Chenghang Zheng, Yipan Hong, Zhewei Xu, Cunjie Li, Li Wang, Zhengda Yang, Yongxin Zhang, Xiang

4

Gao*

5

State Key Lab of Clean Energy Utilization, State Environmental Protection Center for

6

Coal-Fired Air Pollution Control, Zhejiang University, Hangzhou 310027, China

7

Abstract

8

This paper reports a study on the simultaneous removal characteristics of SO3 in a wet flue gas

9

desulfurization (WFGD) system. The impacts of the operating parameters, including liquid-to-gas

10

(L/G) ratio, inlet SO3 concentration, flue gas temperature, superficial velocity, and slurry

11

temperature, on the SO3 removal efficiency and SO3 aerosol characteristic were studied. Results

12

showed that the overall SO3 removal efficiency varied from 20% to 55% under the experimental

13

conditions. For the SO3 aerosol, particles measuring < 0.1 µm and > 0.5 µm had a higher removal

14

efficiency, whereas particles measuring 0.1–0.5 µm were relatively difficult to remove in the

15

WFGD system. Increases in L/G ratio and SO3 concentration exerted positive effects on the SO3

16

aerosol capture, whereas superficial velocity, flue gas temperature, and slurry temperature had

17

negative effects. After installing a sieve plate, the SO3 removal efficiency increased from 49.88%

18

and 53.46% to 51.77% and 58.05%, respectively, under two typical conditions. A multi-layer

19

perceptron artificial neural network model was applied to evaluate the relative importance of each

20

parameter in SO3 removal performance. Results showed that inlet flue gas temperature, inlet SO3

21

concentration, and slurry temperature had a relatively strong impact on SO3 removal performance.

22 23

Keywords: SO3, aerosol, removal efficiency, wet flue gas desulfurization

24 25

1. Introduction

26

SO3 emission in coal-fired power plants has attracted considerable attention from researchers

27

over the last few years because it can cause serious problems, such as plume opacity, acid

28

deposition, and damage to human health[1-3]. SO3 is mainly formed from SO2 in two ways. During

29

the combustion of coal, most of the sulfur is converted to SO2, but a small amount of SO3 is

30

always produced[4]. Additional SO3 can be formed through SO2 oxidation in subsequent processes,

31

especially during contact with the catalysts used in the commonly adopted selective catalytic

32

reaction (SCR) process for flue gas cleaning[5, 6]. The rate of primary conversion of SO2 in a

33

furnace is generally 0.1%–1.5%[7]. The values for the conversion rate in an SCR unit range from

34

0.25% to 1.25%[7, 8].

ACS Paragon Plus Environment

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1

Numerous devices in power plants can remove SO3 simultaneously. For example,

2

electrostatic precipitators (ESPs) are the main equipment to control dust emission from coal-fired

3

power plants. The particle removal efficiency in ESPs can reach and exceed 99.5%, but the SO3

4

removal efficiency in ESPs is low. When the ESP inlet temperature drops below the acid dew

5

point, the SO3 removal efficiency can reach 80%[9]. Wet electrostatic precipitators (WESPs) are

6

usually installed ahead of the stack, and they act as the ultimate emission control technology[10, 11].

7

SO3 emission can be reduced by WESPs due to H2SO4 capture via electrostatic force and SO3

8

absorption by the basic sprayed liquid[12]. As described by Huang et al.[13], the investigated WESP

9

showed more than 99% removal efficiency on the basis of the mass of the sulfuric acid mist.

10

Among all the flue gas cleaning devices in power plants, WFGD absorbers are superior due

11

to their rapidly changing temperature and supersaturation conditions. These conditions result in

12

homogeneous nucleation and heterogeneous nucleation, thereby converting the incoming gaseous

13

H2SO4 into H2SO4 aerosol[14]. Such aerosol cannot be easily precipitated in the subsequent

14

processes because its small size makes its removal challenging[15]. The formation and removal

15

processes are extremely complex. Researchers have conducted studies in different dimensions,

16

such as condensation mechanism, removal process, removal efficiency, test method, and

17

simulation. Sinanis et al.[16] presented that the mechanism of homogeneous nucleation is

18

predominant at high SO3 raw gas concentrations > 30 mg/m3 (STP) while aerosol formation can

19

be attributed to heterogeneous nucleation at low raw gas concentrations between 2 mg/m3 and 10

20

mg/m3 (STP) SO3. Wyslouzil et al.[17] investigated homogeneous nucleation from the vapor phase

21

and noted that H2SO4 plays a critical role in new particle formation. Brachert et al.[18] examined

22

particle size distribution and number of SO3 aerosols in a wet scrubber. Pan et al.[19] analyzed the

23

effects of desulfurization operating conditions on the SO3 removal efficiency, with the sulfuric

24

acid mist removal efficiency of the WFGD system ranging from 36% to 50%. Cao et al.[8]

25

conducted lab-scale tests on coal-fired utility boilers and concluded that the SO3 removal

26

efficiency of a WFGD is approximately 35% or lower. Huang et al.[20] conducted an experiment

27

on an ammonia-based WFGD and found the SO3 removal efficiencies to range from 50% to 65%,

28

which are higher than those for a limestone–gypsum WFGD system. Yu et al.[21] adopted a simple

29

and rapid method for detecting the chemical components of individual aerosol particles on a

30

Klarite substrate with surface-enhanced Raman spectroscopy. Despite the availability of such

31

studies, the removal mechanisms and characteristics of SO3 in different conditions remain unclear,

32

and the feasible ways to strengthen the removal of SO3 remain vacant. Therefore, carrying out

33

research and proposing methods to enhance the synergistic removal of SO3 in WFGD systems are

34

necessary.

35

In the present study, the factors that affect SO3/H2SO4 removal efficiency were studied.

36

Furthermore, the number concentration, particle size distribution, and transfer characteristic of

37

SO3 aerosol in different conditions were analyzed. The removal characteristics of SO3 with a sieve

38

plate scrubber were also investigated with consideration of the capability of a sieve plate tower to

39

significantly enhance the SO2 removal efficiency. The results should provide theoretical and

40

technical support for the enhancement of SO3 removal in WFGD systems.


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Energy & Fuels

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2. Experimental Setup

2

2.1. Experimental system

3

The experimental system is schematically presented in Figure 1. The system mainly

4

comprises an SO3 generator, an air heater, a temperature measurement and control device, a

5

desulfurization scrubber, and a pollutant test system. Air was fed into an air heater by the main fan.

6

The simulated hot flue gas entered the scrubber through the stainless steel flue pipe and was then

7

discharged by a drift fan. The stainless steel flue pipe had apertures to add gaseous SO3, SO2, fly

8

ash, and other components. A heat insulator was installed outside the pipe. Gaseous SO3 was

9

produced by an SO3 generator, which could stably and precisely generate gaseous SO3 at wide

10

concentration ranges[22].

11

The spray tower was 1500 mm high and had an inner diameter of 150 mm. The slurry tank,

12

which was made of organic glass, had an inner diameter of 400 mm. The system was equipped

13

with a slurry circulating pump, and a mechanical stirrer was installed for slurry mixing. The tank

14

was equipped with three electric heating tubes for heating the desulfurization slurry. The pH of the

15

slurry was measured in real time by a pH probe (FE20 type, Mettler) with a measurement error of

16

± 0.01. The preceding heating equipment temperature mentioned was controlled by an REX-C700

17

temperature controller. CaCO3 slurry with a concentration of 1.0 wt% was used as an absorbent in

18

the experiment.

19 20 21

Fig. 1 Schematic of the laboratory experimental setup

2.2 Measurement technique

22

Three types of concentrations, namely, SO3 concentration, mass concentration, and number

23

concentration of SO3 aerosols, were measured by two different apparatuses. On the basis of the


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1

selective absorption of SO3 in isopropanol (IPA), SO3 concentration and removal efficiency were

2

tested. The mass and number concentrations of SO3 (H2SO4) aerosols, as well as the aerosol size

3

distribution, were tested using an electrical low pressure impactor (ELPI+). Hence, the removal

4

characteristics of SO3 (H2SO4) in the desulfurization scrubber and the transfer characteristics of

5

the SO3 (H2SO4) aerosol particle size were analyzed.

6

2.2.1 Sampling of SO3 and concentration measurement

7

A continuous monitor developed by our group[22] on the basis of EPA method 8 was used for

8

the quantitative sampling of SO3 (Figure 2). Flue gas was extracted from the gas duct through a

9

heated sampling probe to avoid gaseous H2SO4 condensation on the inner surfaces. The quartz

10

probe liner was held at 280 °C to ensure the complete conversion of sulfuric acid to the gaseous

11

phase even in saturated atmospheres. The sampled gas flow was circulated through the wool filter

12

to the absorption unit, where gas was mixed with the absorption solution. An 80% IPA solution (in

13

water) was used as the absorption liquid because IPA could inhibit the oxidization of SO2, whereas

14

the solubility of SO3 in pure IPA was insufficiently large.

15 16

Fig. 2 Sulfur trioxide automatic sampler

17

After sampling, ion chromatography (IC-900) was performed to test the sulfate content in the

18

sample solution. SO3 concentration in flue gas can be calculated (H2SO4 in flue gas is also

19

converted to SO3) by the following equation:

CSO3 =

cVM 1 , qTM 2

(1)

20

where CSO3 is the SO3 concentration (mg/m3), c is the sulfate ion concentration (mg/L), V is the

21

volume of the sample (L), q is the pumping flow rate (m3/min), T is the pumping time (min), and

22

M1 and M2 are the molecular weights of SO3 and sulfate ion, respectively.

23 24

The SO3 (H2SO4) removal efficiency in a wet desulfurization scrubber is calculated according to Equation (2).


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Energy & Fuels

η SO =

CSO 3 ,in − CSO3 ,out

3

CSO 3 ,in

× 100% ,

(2)

1

where ηୗ୓య is the removal efficiency of SO3 (H2SO4) in the wet desulfurization absorber and

2

cୗ୓య ,୧୬ and cୗ୓య,୭୳୲ are the inlet and outlet SO3 (H2SO4) concentrations of the absorber

3

(mg/m3), respectively.

4

2.2.1 SO3 aerosol sampling and measurement

5

The size distribution and number concentration of aerosols were measured by ELPI+.

6

Brachert et al.[14] proved the viability of ELPI+ and proposed that a dilution rate of at least 10 was

7

needed to avoid condensation on impactor plates. Thus, a cyclone separator and a diluter were also

8

used in the experiment, and the dilution rate of 16 was selected in our experimental procedure.

9

The grade removal efficiency η୧ under different operation conditions was obtained by

10

comparing the inlet and outlet concentrations of SO3 (H2SO4) aerosols. The definition of η୧ is as

11

follows:

ηi =

N i ,in − N i ,out N i ,in

× 100% ,

(3)

12

where i is the particle collecting stage of ELPI+ and N୧,୧୬ and N୧,୭୳୲ represent the concentration

13

of aerosol particles at the inlet and outlet of the WFGD absorber (/Nm3), respectively. The

14

concentration adopted in the experiment is the average value of a period in a steady state.

15

3 Results and Discussion

16

3.1 Effect of Liquid-to-Gas Ratio

17

Liquid-to-gas (L/G) ratio is an important factor that affects the gas–liquid heat transfer and

18

mass transfer performance, as well as the removal efficiency of fine particles (including SO3

19

aerosols) in a WFGD system. In this section, SO3 removal efficiency, SO3 aerosol removal

20

efficiency of number and mass concentrations, and SO3 aerosol grade removal efficiency are

21

analyzed.

22

Figure 3a shows the effect of L/G ratio on SO3 removal efficiency at different SO3 inlet

23

concentrations. The experiments were conducted with a flue gas volume of 120 m3/h, scrubber

24

inlet gas temperature of 90 °C, desulfurization slurry temperature of 45 °C, and L/G ratios of 5, 10,

25

15, and 20 L/m3. Figure 3a shows that when the L/G ratio increases from 5 L/m3 to 20 L/m3, the

26

removal efficiency of SO3 increases from 37.49% and 40.62% to 50.86% and 53.77% with the

27

inlet concentrations of 53.6 and 78.6 mg/m3, respectively. The results indicated that L/G ratio had

28

a significant positive effect on the removal of SO3. Figure 3b shows the number/mass

29

concentration of the SO3 aerosol corresponding to different L/G ratios. This figure also shows that

30

a considerable amount of SO3 aerosol was produced at the inlet of the scrubber. The number and


Energy & Fuels

1

mass concentration of the SO3 aerosol gradually decreases with the increase in L/G ratio, thereby

2

indicating that increasing the L/G ratio can enhance the capture of SO3 aerosol. However, even if

3

the L/G ratio is 20 L/m3, the number concentration of SO3 aerosol remains 3.57 × 107 /cm3, which

4

is only 47.88% lower than that of the inlet. The corresponding SO3 number concentration removal

5

efficiency is only 52.12 because increasing the L/G ratio increases the slurry droplet density of the

6

contact area in the scrubber. Moreover, the mass transfer contact area is enlarged, and the contact

7

time is prolonged, thus capturing an increased aerosol probability.

Number concentration (/cm3)

3

Cin = 53.6 mg/m Cin = 78.6 mg/m3

55 50 45 40

Q = 120 m3/h TG = 90

35 30

TL = 45

Number concentration Mass concentration

8x107

100 7x107

90

6x107

80

5x107 7

4x10

70 Q = 120 m3/h TG = 90

60

Cin = 53.6 mg/m3

TL = 45 7

5

10

15

3x10

20

0

5

10

15

20

50

3

3

L/G (L/m )

L/G (L/m )

8

110

Mass concentration (mg/m3)

9x107

60

Removal efficiency(%)

9

Fig. 3 a) Effect of L/G ratio on SO3 removal efficiency; b) effect of L/G ratio on number and mass

10

concentration of SO3 aerosol

11

Figure 4 shows the comparison of three removal efficiencies. Evidently, SO3 removal

12

efficiency is higher than the number and mass removal efficiency of SO3 aerosol. This finding

13

indicates that removing SO3 in a scrubber is difficult when it transforms into aerosol. Comparing

14

the mass concentration removal efficiency with the number concentration removal efficiency is

15

interesting. That is, with the increase in L/G ratio, the increase rate of mass concentration removal

16

efficiency is slower than that of number concentration removal efficiency. This finding indicates

17

that when L/G ratio increases, the mean diameter of aerosol particles enlarges. This conclusion is

18

consistent with the simulation and experiment results of previous studies [23-26].

Removal efficiency (%)

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SO3 removal efficiency

50

Number removal efficiency of SO3 aerosol

45

Mass removal efficiency of SO3 aerosol

40 35 30 25 4

6

8

10

12

14

16

18

20

22

3

19 20

L/G (L/m )

Fig. 4 Comparison of three removal efficiencies


Page 7 of 18

1

ELPI+ was used to test the inlet and outlet SO3 aerosol number/mass concentration and

2

particle size distribution of the scrubber to verify the analysis. The test results are shown in

3

Figures 5a and 5b. The effect of L/G ratio on the particle size distribution of SO3 aerosol, where

4

Dp represents the aerosol particle size, is shown in Figure 5a. Evidently, SO3 aerosol mainly

5

comprises submicron particles with particle sizes below 0.1 µm because SO3 easily combines with

6

water vapor to form into H2SO4 vapor. A previous study showed that gaseous H2SO4 with a

7

concentration of 0.1 mg/m3 has already reached a saturated condition with a flue gas temperature

8

of 55 °C and relative humidity of 80%

9

present experiment was higher than the saturated concentration. In addition, the slurry easily

10

evaporated, and the flue gas temperature rapidly decreased due to the strong heat and mass

11

transfer between the high-temperature gas and the low-temperature slurry. Hence, the humidity in

12

the scrubber was higher than 80%. SO3 (H2SO4) rapidly formed a considerable amount of small

13

SO3 aerosols by nucleation in a short time. As shown in the figure, particle size of above 1 µm and

14

below 0.1 µm visibly decreases when the L/G ratio increases.

15

109 108 107

.) By contrast, the SO3/H2SO4 concentration in the

1.5x107 1.2x107 9.0x106 6.0x106 3.0x106 0.01

0.1 Dp (µm)

106 105

Before spary scrubber 3 L/G = 5 L/m 3 L/G = 10 L/m L/G = 15 L/m3 L/G = 20 L/m3

104 3

10

102

[26]

Grade removal efficiency(%)

1010

Number concentration (/cm3 )

Number concentration (/cm3)

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Energy & Fuels

0.01

0.1

1

10

70 3

L/G = 20 L/m

60

L/G = 15 L/m3

50

L/G = 10 L/m3

40 30 20 L/G = 5 L/m3

10 0

0.01

Dp (µm)

0.1

1

10

Dp (µm)

16

Fig. 5 a) Particle size distribution of SO3 aerosol; b) grade removal efficiency under different L/G

17

ratios

18

Figure 5b shows the grade removal efficiency of SO3 aerosols at different L/G ratios. As

19

illustrated in the figure, when the particle size of an aerosol particle is less than 0.1 µm, the

20

removal efficiency of the SO3 aerosol decreases with the increase in particle size, and the grade

21

efficiency reaches the lowest point at approximately 0.25 µm. After that point, the removal

22

efficiency rapidly increases, and the growth rate abruptly slows down to approximately 1.0 µm.

23

Two types of fine particle capture mechanism exist in the spray scrubber: Brown diffusion and

24

inertia collision[27]. Brown diffusion plays a dominant role in aerosol particles with particle sizes

25

below 0.1 µm. Moreover, diffusion strength decreases with an increase in particle size, leading to a

26

decrease in grade removal efficiency. The effect of inertial collision has become increasingly

27

significant for aerosol particles with particle sizes above 0.1 µm. Furthermore, the inertial effect

28

increases with an increase in particle size, leading to an increase in grade removal efficiency. In

29

addition, the figure shows that aerosol particle grade removal efficiency increases with the

30

increase in L/G ratio, especially at particle sizes above 1 µm and below 0.1 µm. Apart from the


Energy & Fuels

1

preceding analysis, this finding can be attributed to the strengthened heat transfer between gas and

2

liquid due to the increasing L/G ratio.

3

3.2 Effect of Inlet SO3 Concentration

4

The influence of SO3 concentration on SO3 removal efficiency in the flue gas of the

5

desulfurization tower is shown in Figure 6. The experiments were conducted with a flue gas

6

volume of 120 m3/h, flue gas temperature of 90 °C, scrubber slurry temperature of 45 °C, L/G

7

ratio of 15 L/m3, and inlet SO3 concentrations of 17.9, 36.25, 53.6, and 78.6 mg/m3. The figure

8

shows that with the increase in inlet SO3 concentration, the SO3 aerosol removal efficiency

9

increases from 40.93% to 50.41%, but the outlet concentration of SO3 also increases with the

10

increase in inlet concentration from 10.6 mg/m3 to 39.0 mg/m3. This finding may be attributed to

11

the increased SO3 concentration, which raises the partial pressure of SO3/H2SO4 vapor, thereby

12

promoting the condensation of SO3 aerosols[18] and strengthening the mutual condensation of SO3

13

aerosol particles[28]. The large aerosol particle formation is likely to be trapped by the

14

desulfurization slurry droplets. Furthermore, the increase of inlet SO3 concentration causes

15

significant concentration gradient, which leads to a high mass transfer rate between aerosols and

16

slurry. Thus, SO3 aerosol removal efficiency increases with the increase in inlet SO3

17

concentration. 55 50

70 Q = 120 m3/h TG = 90

L/G = 15 L/m3 TL = 45

60 50

45

40 40 30 35

20

30 25

Cout (mg/m3)


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Page 8 of 18

10 20

30

40

50

60

70

80

0

18

Cin (mg/m3)

19

Fig. 6 Effect of inlet SO3 concentration on SO3 removal efficiency

20

The influence of inlet flue gas SO3 concentration on the cumulative number distribution of

21

SO3 aerosols at the outlet of the scrubber is shown in Figure 7. The figure shows that with the

22

increase in SO3 concentration, the cumulative distribution curve of SO3 aerosols at the outlet of

23

the scrubber shifts to the right, and the median particle size increases from 0.025 µm to 0.053 µm.

24

The results are consistent with the results of Brachert[18], who also found that changes in SO3 mass

25

concentration in flue gas do not lead to significant changes in the number concentration of SO3

26

aerosols. Therefore, in conjunction with Figure 7, increasing the inlet SO3 concentration within a

27

certain range can promote the growth and condensation of SO3 aerosols and form aerosols with

28

large particle sizes, thus increasing the removal efficiency of SO3 aerosols.


1.0 0.8

D50=0.053µm

0.6

D50=0.033µm

0.4

D50=0.025µm

Cin= 78.6 mg/m3

Cin= 53.6 mg/m3

Cin= 17.9 mg/m3

0.2 After spray scrubber

0.0

0.01

0.1

1

10

Size (µm)

1 2 3

Fig. 7 Cumulative number distribution of outlet SO3 aerosols

3.3 Effect of Inlet Flue Gas Temperature

4

Figure 8 shows the influence of flue gas temperature at the inlet of the WFGD system on the

5

SO3 removal efficiency. The experiments were conducted with a flue gas volume of 120 m3/h,

6

inlet SO3 concentration of 53.6 mg/m3, slurry temperature of 45 °C, L/G ratio of 15 L/m3, and

7

inlet flue gas temperatures of 70 °C, 80 °C, 90 °C, 100 °C, and 110 °C. The figure shows that the

8

SO3 removal efficiency decreases with the increase in inlet flue gas temperature. When the inlet

9

flue gas temperature rises from 70 °C to 110 °C, the removal efficiency of SO3 decreases from

10

51.23% to 42.02%. This decrease can be attributed to the increase in humidity when the

11

temperature of the flue gas in the scrubber is reduced, thus promoting the condensation and

12

growth of the SO3 aerosol particles in the supersaturated environment[29].

60

50 Q = 120 m3/h TL = 45 ℃

55

L/G = 15 L/m3 Cin = 53.6 mg/m3

45

50 40

45 40

35

35

30

Cout (mg/m3)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Cumulative patical size distribution (%)

Page 9 of 18

30 25 25 70

80

90

100

110

20

13

TG (℃ )

14

Fig. 8 Effect of inlet gas temperature on SO3 removal efficiency


Energy & Fuels

Figures 9a and 9b are the number concentration and cumulative number concentration of

2

SO3 aerosols at different inlet flue gas temperatures, respectively. The figures show that with the

3

increase in the inlet flue gas temperature, the SO3 number concentration of aerosols significantly

4

increases at the inlet and outlet of the scrubber. When the inlet flue gas temperature rises from

5

70 °C to 100 °C, the number concentration of SO3 aerosols at the inlet increases from 4.7 ×

6

107/cm3 to 9.1 × 107/cm3. The cumulative distribution curve of aerosols moves to the left with the

7

increase in the inlet flue gas temperature, that is, the median particle size of SO3 aerosols

8

decreases with the increase in flue gas temperature. This finding can be attributed to the increase

9

in inlet flue gas temperature, which promotes the nucleation rate of SO3 aerosols[30], thereby

10

producing high number concentrations of SO3 aerosols in a short time. Such condition also leads

11

to a large temperature difference between the slurry and the flue gas, thereby increasing the

12

cooling rate. Under the function of nucleation mechanism, additional fine SO3 aerosols are

13

formed[31]; these aerosols are difficult to remove. 1.0x108 8.0x10

25.8%

3

L/G = 15 L/m TL = 45 CO = 53.6 mg/m3

7

6.0x10

40.7%

3

Q = 120 m /h 38.0% 37.3%

7

4.0x10

2.0x107 0.0

14

Inlet of spray scrubber Outlet of spray scrubber

7

70

80

90

100

Cumulative number distribution

1

Number concentration(/cm3)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 18

1.0 0.8

Inlet of spray scrubber TG = 70

0.6

TG = 80 TG = 90

0.4

TG = 100

0.2

Q = 120 m /h

CO = 53.6 mg/m3

0.0

3

0.01

TG ( )

0.1

1

10

Dp (µm)

15

Fig. 9 a) Effect of inlet flue gas temperature on number concentration of SO3 aerosols; b) effect of

16

inlet flue gas temperature on cumulative number distribution of SO3 aerosols

17

The inlet flue gas temperature exerts a significant effect on SO3 aerosol formation in a spray

18

scrubber. With the low inlet flue gas temperature, the relative humidity of flue gas increases in the

19

scrubber, thereby forming a supersaturated vapor environment that benefits the SO3 aerosol

20

coagulation[32]. A portion of SO3 aerosols with large particle sizes is condensed and coagulated,

21

thereby achieving high removal efficiency. Therefore, reducing flue gas temperature at the inlet of

22

the scrubber is beneficial to promote the SO3 aerosol removal efficiency in a certain range.

23

3.4 Effect of Superficial Velocity

24

Figure 10 shows the influence of superficial velocity on SO3 removal efficiency. The

25

experiments were conducted with a flue gas temperature of 90 °C, slurry temperature of 45 °C,

26

inlet SO3 concentration of 53.6 mg/m3, and superficial velocities of 1.26, 1.57, 1.89, and 2.20 m/s.

27

Evidently, the SO3 removal efficiency decreases with the increase in superficial velocity from

28

49.88% and 53.46% to 38.49% and 42.76%. According to a previous study, increasing the flow

29

rate of flue gas increases the relative velocity of aerosol particles and sprayed droplets, thereby

30

improving the collision probability. In addition, with the increase in superficial velocity, the


Page 11 of 18

1

turbulence between flue gas and liquid increases, and such change benefits the diffusion of aerosol

2

particles[33]. However, increasing the flow rate of flue gas leads to the decline of collision

3

probability between aerosol particles and slurry droplets. Another study shows that with a decrease

4

in flue gas flow rate and increase in residence time, the median particle size of SO3 aerosols at the

5

outlet of the scrubber increases; that is, in the WFGD system, prolonging the residence time is

6

beneficial for the removal of SO3 aerosols because SO3 aerosols grow under the dual function of

7

condensation and coagulation[26]. According to the test results in the current work, appropriately

8

reducing the velocity of flue gas flow is beneficial for the removal of SO3 aerosols in the WFGD

9

system.

60


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

L/G = 15 L/m3 L/G = 10 L/m3

55 50 45 40 35

Cin = 53.6 ± 3.8 mg/m3 TG = 90 TL = 45

1.2

1.4

12

1.8

2.0

2.2

UG (m/s)

10 11

1.6

Fig. 10 Effect of superficial velocity on SO3 removal efficiency

3.5 Effect of Slurry Temperature

13

Figure 11a shows the effect of desulfurization slurry temperature on SO3 removal efficiency.

14

The experiments were conducted with a flue gas volume of 120 m3/h, inlet SO3 concentration of

15

53.6 mg/m3, inlet flue gas temperature of 90 °C, L/G ratio of 15 L/m3, and slurry temperatures of

16

37 °C, 40 °C, 45 °C, 50 °C, and 55 °C. The figure shows that with the increase in slurry

17

temperature, the SO3 removal efficiency gradually decreases from 50.6% to 44.1%. The

18

temperature of the flue gas is quenched, the relative humidity is increased, and the saturation of

19

the vapor environment is large with a low slurry temperature[19,

20

promotes vapor condensation on the SO3 surface, which benefits SO3 removal.


34]

. Low slurry temperature

Energy & Fuels

38 36 34

45

32 30

40

28 35

26 40

45

TL (

50

55

24

4x107

70% N (/cm3)

L/G = 15 L/m3 Cin = 53.6 mg/m3

Cout (mg/m3)

50

35

1

Q = 120 m3/h TG = 90

Grade removal efficiency

55

efficiency(%) Removal efficiency

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 18

60%

3x107 2x10

50%

7

40

45 TL ( )

50

40% 30% 20% 10% 0%

Q = 120 m3/h L/G = 15 L/m3 TG = 90

TL = 40 TL = 45

3

CO = 53.6 mg/m

0.01

TL = 50

0.1

)

1

10

Dp (µm)

2

Fig. 11 a) Effect of slurry temperature on SO3 removal efficiency; b) SO3 aerosol grade removal

3

efficiency at different slurry temperatures

4

Figure 11b shows the grade removal efficiency of SO3 aerosols at different slurry

5

temperatures. The grade removal efficiency of SO3 aerosols with particle sizes less than 0.1 µm

6

increases with the decrease in slurry temperature. In addition, with the decrease in slurry

7

temperature, the SO3 aerosol concentration at the outlet of the scrubber decreases. Therefore,

8

reducing the slurry temperature within a certain range promotes the condensation growth of SO3

9

aerosols in the spray scrubber. With the increase in the particle size of the SO3 aerosol particles,

10

the condensation of the aerosol particles is enhanced, and large size particles are formed, thereby

11

decreasing the outlet number concentration of SO3 aerosols and increasing the removal efficiency.

12

3.6 Effect of Sieve Plate

13

Figure 12 presents the comparison of SO3 removal efficiency between the sieve plate–spray

14

tower and the spray tower at different L/G ratios. The experiments were conducted with flue gas

15

volume of 80 m3/h, inlet SO3 concentration of 53.6 mg/m3, inlet flue gas temperature of 90 °C,

16

and slurry temperature of 45 °C. The figure shows that when the L/G ratio is 10 L/m3, the SO3

17

aerosol removal efficiencies of the sieve plate–spray tower and spray tower are 51.77% and

18

49.88%, respectively. When the L/G ratio is 15 L/m3, the removal efficiencies of the sieve plate–

19

spray tower and spray tower are 58.05% and 53.46%, respectively. After adding the sieve plate,

20

the removal efficiency of SO3 aerosols increases by 1.89% and 4.59%. A layer of holding liquid is

21

formed above the sieve plate, and it increases the contact area between the aerosol particles and

22

the slurry, thereby promoting collision probability. Meanwhile, the increase in L/G ratio increases

23

the air rate of the holding liquid and the turbulence, thereby increasing the rupture and renewal

24

strength of bubbles, which is beneficial to the capture of fine particles[35].


Page 13 of 18

100 SO3 removal efficiency

Removal efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

80

60

without sieve plate SO3 removal efficiency with sieve plate SO2 removal efficiency without sieve plate SO2 removal efficiency with sieve plate

40

20

0 10

15 3

L/G ratio (L/m )

1 2

Fig. 12 Comparison of SO2 and SO3 removal efficiencies before and after the installation of a

3

sieve plate

4

3.7 Impact Value of Different Parameters

5

A multi-layer perceptron (MLP) neural network is a computational model based on the

6

structure and operation of a biological neural network. It is capable of implementing

7

approximating nonlinear functions with arbitrary accuracy by regulating variable weight

8

connections. The mean impact value was used with the MLP artificial neural network model to

9

evaluate the relative importance of each parameter on SO3 aerosol removal[36]. The relative

10

importance of each parameter is shown in Table 1, which shows that the inlet flue gas temperature,

11

inlet SO3 concentration, and slurry temperature have a relatively strong impact on SO3 aerosol

12

removal efficiency of approximately 24%. Figure 13 graphically shows the sensitivities of

13

different parameters. The base values are as follows: L/G ratio = 15 L/m3; slurry temperature =

14

45 °C; superficial velocity = 1.89 m/s; inlet SO3 concentration = 53.6 mg/m3; and inlet flue gas

15

temperature = 90 °C. When inlet SO3 concentration decreases by 20%, the SO3 aerosol removal

16

efficiency could increase by 30%. When the inlet SO3 concentration decreases by 20%, the SO3

17

aerosol removal efficiency could decrease by 25%.

18

Table 1 Relative importance (%) of parameters

Parameter

Relative importance (%)

L/G ratio (L/m3)

11.18

Slurry temperature (℃ ℃)

23.90

Superficial velocity (m/s)

15.88

Inlet SO3 concentration (mg/m3)

24.12

Inlet flue gas temperature (℃ ℃)

24.93

19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Change in removal efficiency (%)

Energy & Fuels

L/G ratio Slurry temperature Superficial velocity Inlet SO3 concentration

30 20

Inlet flue gas temperature

10 0 -10 -20 -30 -20

-10

0

10

20

Change in input parameters (%)

1 2 3

Page 14 of 18

Fig. 13 Sensitivity analysis results for the parameters

4 Conclusion

4

In this study, the mechanism of the effect of different factors was analyzed, and ways to

5

enhance removal efficiency were proposed. Furthermore, a sieve plate was adopted to observe its

6

effect on removal efficiency.

7

(1) Increasing the L/G ratio increases the saturation of the vapor environment, which is

8

beneficial to the formation of large size particles; hence, SO3 aerosol removal efficiency increases

9

with the increase in L/G ratio. When the L/G ratio increases from 5 L/m3 to 20 L/m3, SO3 aerosol

10

removal efficiency increases by 13.37%, and the maximum removal efficiency is 50.86% at the

11

inlet SO3 concentration of 53.6 mg/m3.

12

(2) The increase in inlet SO3 concentration promotes the condensation and growth of SO3

13

aerosol and strengthens the mutual coagulation of SO3 aerosol particles, thereby forming aerosol

14

particles with large particle sizes that can easily be captured by the desulfurization slurry. The

15

removal efficiency of SO3 aerosols in a certain range increases with the increase in inlet SO3

16

concentration. In addition, the cumulative particle size distribution curve of the SO3 aerosols

17

migrates to the right with the increase in inlet SO3 concentration.

18

(3) A decrease in the slurry and inlet flue gas temperatures increases humidity and promotes

19

the coagulation of aerosols. A high inlet flue gas temperature equates to a high concentration of

20

SO3 aerosol particles and leads to the production of well-proportioned fine particles. Moreover,

21

reducing the flue gas temperature improves the removal efficiency of SO3 aerosols. At a slurry

22

temperature ranging from 40 °C to 55 °C, the removal efficiency increases by approximately 3%

23

when the slurry temperature decreases by 10 °C.

24

(4) The removal efficiency of SO3 can be improved by installing a sieve plate. Under the

25

experimental conditions, the removal efficiency of SO3 increases by 1.89%–4.59% after sieve

26

plate installation.


Page 15 of 18 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1 2 3

Acknowledgment This work was financially sponsored by the National Key Research and Development Plan

4

(2016YFC0203705),

5

Environmental Welfare Project of the Ministry of Environmental Projection of China (201509012),

6

and the Key Research & Development Plan of Shandong Province (2014GJJS0501).

the National Natural Science Foundation (No. U1609212),


the

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Physical

21


Chemistry

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Experimental Study on Removal Characteristics of SO3 by Wet Flue

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