Aerosol Formation Characteristics during Ammonia-Based WFGD

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Aerosol Formation Characteristics During Ammonia-based WFGD Processes Rongting Huang, Yajuan Shi, Linjun Yang, Hao Wu, and Danping Pan Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00528 • Publication Date (Web): 21 Sep 2016 Downloaded from http://pubs.acs.org on September 26, 2016

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Aerosol Formation Characteristics During Ammonia-based WFGD Processes Rongting Huang, Yajuan Shi, Linjun Yang1*, Hao Wu, Danping Pan (Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, Southeast University, Nanjing 210096, Jiangsu, P. R. China)

ABSTRACT: The aerosol emission from the ammonia-based wet flue gas desulfurization (WFGD) system was investigated in an industrial device and in the lab-scale experimental system. The aerosol formation processes in WFGD system were simplified into the entrainment mechanism and the heterogeneous reaction mechanism. The aerosol formation properties were studied individually under the two mechanisms. The influences of some desulfurization parameters were also explored. The results indicated that the aerosol emission increased significantly after the ammonia-based WFGD system. The aerosols were mainly in the submicron range in terms of the numerical concentration. For the mass concentration, however, the particles were mostly micron ones. The micron particles were mainly generated by the entrainment mechanism, in which condition the aerosol formation increased with the elevation of the flue gas superficial velocity and the desulfurization solution concentration. In contrast, the heterogeneous reaction mechanism was not only the primary source of the submicron particles, but also the major cause of the aerosol increase after the WFGD system. When the NH3 concentration was changed to alter the NH3-to-SO2 molar ratio (N/S), the heterogeneous reactions were boosted by a greater N/S and a lower reaction temperature. When the temperature increased or the N/S decreased, the

*Corresponding author: Linjun Yang Tel: +86 25 83795824; Fax: +86 25 83795824; E-mail address: [email protected] 1

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submicron part of the generated aerosols declined obviously, but the micron part changed little. Accordingly, increasing appropriately the ambient temperature in the scrubber and reducing the ammonia escaping from the desulfurization solution would effectively lessen the aerosol emission from the ammonia-based WFGD system. Key words: ammonia-based WFGD; scrubber; aerosol formation; heterogeneous reaction; entrainment.

1. Introduction In recent years, the technology of the ammonia-based wet flue gas desulfurization (WFGD) has received much attention in China because of its advantages of high desulfurization efficiency and low investment.1,2 However, the aerosol problems have been reported frequently, being the major challenge in the development of this technology.3 Researchers reported that numerous aerosols were formed during the processes of the ammonia-based WFGD, which would affect the operation safety of corresponding devices and impact on the climate and the human health.4-8 The aerosol formation during flue gas cleaning processes are thus of particular interest in the atmospheric sciences with the formation mechanisms in the ammonia-based WFGD system paid great concern. The aerosol emission from WFGD systems has been reported frequently, but only a few researchers focused on the formation mechanisms. Bao et al. investigated the aerosol formation properties in the ammonia-based WFGD processes and proposed two mechanisms for the aerosol generation. One mechanism was the entrainment of the desulfurization solution droplets by the hot flue gas, and the other was the heterogeneous reactions that occurred in the scrubber.3,9,10 The aerosol emission may also be aggravated by the sulfuric acid droplets that occurred in the WFGD 2


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scrubber according to many researches.11 Jones et al. reported the sulfuric acid mist formation in a quench cooler.12 Brown et al. described the formation of the sulfuric acid aerosol in wet caustic scrubbers.13 The results indicated the sulfuric acid aerosols to be submicron and difficult to participate. Buckley et al. appraised that 30–60% of the sulfuric acid aerosols may exit the stack.14 The heterogeneous reactions of NH3-H2O-SO2 made great contributions to the aerosol generation during ammonia-based WFGD processes. Bai et al. studied the reaction of NH3 and SO2 and stated the possible equilibrium concentrations of the reactants.15,16 James L. Vance et al. conducted experiments concerning the reaction of ammonia and sulfur dioxide and found that the NH3 to SO2 reactant ratio had significant impacts on the composition of the reaction product.17 Besides, a possible route in the formation of atmospheric sulfate aerosols was also suggested in the research.18 Klaus Hjuler et al. investigated the NH3-SO2 reaction and the products, proposed an overall mechanism for the reaction, and developed a three-parameter expression for the reaction rate.19 Guo Y et al. studied experimentally the reactions of SO2-NH3-H2O in the absence of O2 and reported the influences of the temperature, the NH3/SO2 ratio, and the water vapor content on the SO2 conversing to sulfites.20 In order to avoid the damages of aerosols, many studies have been devoted to this filed. J Yan et al. studied the aerosol emission properties in the ammonia-based WFGD system and proposed a method of the vapor condensing on the surfaces of PM2.5 to improve the particle removal in the WFGD scrubber.4,21,22 The installation of a wet electrostatic precipitator (WESP) was also suggested to promote the aerosol emission control.23,24 However, the installation and operation of new devices resulted in cost growth. On the contrary, controlling the aerosol emission through the optimization of WFGD processes showed the advantages of low investment and easy 3


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installation and thus had great potentials. On the basis of the wet ammonia desulfurization technology, the aerosol emissions were tested after an industrial WFGD device and the simulation WFGD set-up. Experiments were designed to investigate the aerosol formation properties under the entrainment mechanism and under the heterogeneous reaction mechanism, respectively. Besides, the impacts of the desulfurization parameters on the aerosol formation were explored separately under these two mechanisms. This work might provide theoretical and experimental bases for the aerosol emission control through desulfurization parameter optimization.

2. Experimental 2.1. The industrial device. In an industrial power plant where the ammonia-based WFGD system was employed to deal with the SO2 contaminant, the aerosol concentrations before and after the WFGD device have been measured to investigate the aerosol growth. In the industrial system, the flue gas volume flux was 5.0×105 Nm3/h. The superficial velocity in the WFGD scrubber was ~ 5.0 m/s. The flue gas temperature before and after the WFGD system were ~ 143℃ and ~ 55℃, respectively. The temperature of the desulfurization solution was ~ 57℃. The SO2 concentration was about 1000 mg/Nm3. The L/G was 12 L/m3. There were two spray levels in the WFGD scrubber and a baffle plate mist eliminator equipped at the outlet. The pH of the desulfurization solution was 5.2-6.0. The mass concentration of the desulfurization solution was ~ 23%. 2.2. The desulfurization set-up. As shown in Figure 1, the ammonia-based desulfurization experimental set-up consists mainly of the flue gas simulating system, the desulfurization scrubber, and the desulfurization solution compounding system. To generate the simulated flue gas, the SO2 4


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measured precisely with a mass flow meter was added into the heated air through the buffer tank. The flue gas then went into the desulfurization scrubber to countercurrent contact with the desulfurization solution. The desulfurization scrubber was about 2.8 m in height with two solution spray levels set inside and a mist eliminator equipped at the outlet. The body of the scrubber was made of the polycarbonate material. After desulfurization, the clean flue gas was emitted into the atmospheric environment and the desulfurization solution went back to the crystallizer tank, in which the aqueous ammonia was added to adjust the pH value to 5.5. The desulfurization solution used in experiments was taken from an industrial desulfurization device. The elemental composition of the desulfurization solution was shown in Table 1. In the experiments, the rated volume flux of the flue gas was ~ 20 Nm3/h, which can be changed to adjust the superficial velocity in the WFGD scrubber. The flue gas temperature before WFGD system was about 100℃. The desulfurization solution was heated to 50℃. The SO2 concentration before WFGD was maintained at around 2140 mg/Nm3. The liquid-to-gas ratio (L/G) was 15 L/m3. And the mass concentration of the desulfurization solution was originally 15%. discharge

（2）

demister 2 （1）

buffer tank heater air MFC

（2）

flue gas analyzer swirl pump

SO2

desulfurization tower

（1）

ELPI

PC

desulfurization solution configuration & convey unit T desulfurization solution T

demister 1 T

flue gas analyzer

ammonia agitator M

flue gas simulation unit

sample point

solution tank thermostatic water bath buffer tank

Figure 1.

Schematic diagram of the WFGD experimental system.

Table 1. Elemental Composition of the Industrial Desulfurizing Agent 5


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Element

CK

NK

OK

Wt /%

15.36

6.33

At /%

23.60

8.34

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Na K

Mg K

Al K

Si K

40.38

1.03

0.48

4.70

6.72

46.57

0.83

0.37

3.21

4.42

SK

Ca K

Fe K

Total

16.18

3.30

5.52

100

9.31

1.52

1.83

100

In order to study separately the aerosol formation properties under the entrainment mechanism and the heterogeneous reaction mechanism, experiments were designed as follows: (1) when conducting the simulation WFGD set-up, the simulated flue gas was compounded without adding SO2 so that the heterogeneous reactions concerning SO2 were eliminated, leaving the aerosols formed only through the entrainment mechanism; (2) as shown in Figure 1, the flue gas entrance was diverted to the upper side of the spray levels, and meanwhile the scrubber was heightened to keep the residence time of the flue gas constant in the scrubber, in which case the entrainment mechanism was depressed and the aerosols were generated mainly through the heterogeneous reaction mechanism. The

2.3. The heterogeneous reaction set-up.

heterogeneous

reactions

in

the

ammonia-based WFGD processes involve the reactions between NH3 and sulfuric acid mist and the reactions of NH3-H2O-SO2 in which the H2O could exist in gaseous state and liquid state. The major part of the heterogeneous reactions is the NH3-H2O-SO2 reactions. Thus in this paper, the study of the heterogeneous reaction mechanism focused on the NH3-H2O-SO2 reactions. The experiments were conducted on the heterogeneous reaction experimental set-up. As shown in Figure 2, the simulated flue gas was compounded on basis of high purity nitrogen. The N2, O2, SO2, NH3, and moisture were added into the static mixer and mixed together. To avoid the NH3 and the SO2 reacting in the pipeline, the NH3 was added separately into the reactor via another pipeline. Each of the gas flows was precisely controlled with the rotameter or the mass flow controller. The rated volume flux of the simulated flue gas was 1 Nm3/h. The water vapor was added into the system with the micro-injection pump and the pipe oven. Before entering the 6


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reactor, the flue gas was preheated to ensure that the reaction temperature could be maintained steady in the constant temperature oven. In the heterogeneous reaction experiments, the O2 concentration was 6 vol %. The moisture content was maintained at 55 g/Nm3. The molar ratio of the NH3-to-SO2 (N/S) was set originally 0.1, in which the SO2 concentration and the NH3 concentration were 2860 mg/Nm3 and 76 mg/Nm3, respectively. The N/S could be adjusted by changing the NH3 concentration. testing & sampling point emission constant temperature oven heating & mixing device static mixer MFC

D

pipe oven

D

MFC

D NH3 micro-injection N2

pump

O2

SO2

Figure 2. Schematic diagram of the heterogeneous reaction set-up. 2.4. Testing instruments.

The electrical low pressure impactor (ELPI, Dekati L td.) was

used for the real-time measurement of aerosol concentration and size distribution. The SO2 concentration was tested online with a flue gas analyzer (RBR ECOM-J2KN, Germany). The aerosols were sampled with the pollutant PM sampler (Dekati Ltd.). The morphology of the aerosol samples was observed under a field emission scanning electron microscope (FESEM). The moisture content and the reaction temperature were measured with the temperature and humidity transmitter (HMT337, Vaisala Ltd., Finland). The elemental composition of the desulfurization solution was analyzed with an energy dispersive spectrometer (EDS). 7


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3. Results and discussion 3.1. Aerosol formation in ammonia-based WFGD processes. The aerosol concentrations before and after the ammonia-based WFGD system were tested in the industrial device and in the simulation set-up, respectively. In the industrial device, the testing results showed that the aerosol concentration increased greatly after the ammonia-based WFGD system (Figure 3). 800 8

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3

numerical concentration (1/cm )

10

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600 500 7

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mass concentration (mg/m )

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100

6

10

0 before WFGD

Figure 3.

after WFGD

Aerosol concentrations in the industrial WFGD system.

The desulfurization solution was taken back to laboratory to conduct experiments on the simulation experimental set-up. The experimental parameters were presented in section 2.2. As shown in Figure 4(a), the aerosol concentration before the WFGD device was extremely low, because the simulated flue gas was compounded on the basis of room air without adding aerosols. But the aerosol concentration raised greatly after the WFGD system, which inferred that additional aerosols were generated during the WFGD processes. Figure 4(b) and 4(c) showed separately the numerical size distribution and the mass size distribution of the aerosols emitted from the WFGD system. The distribution suggested that the aerosols consisted mainly of submicron particles in terms of the numerical concentration. For the mass concentration, however, the aerosols were mostly micron ones.

8


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3

10 numerical concentration mass concentration

8

3


10

2

10 7

10

6

1

10

10

5

10

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10 before WFGD

after WFGD

(a) Aerosol concentrations before and after simulation WFGD system. 8

10

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number dN/dlogDp / cm

-3

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(c) Mass size distribution after WFGD system. Figure 4. Aerosol emission properties in experimental WFGD system. 3.2. Aerosol formation under different mechanisms.

In

9


ammonia-based

WFGD

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system, the hot flue gas entered the scrubber from the lower part, countercurrent contacting with the desulfurization solution droplets. During this process, partial of the droplets might be evaporated to become smaller and even to generate solid particles. The particles and droplets were entrained by the flue gas to form aerosols. When passing through the mist eliminator, larger aerosols were trapped, leaving the smaller ones escaping from the WFGD system. The remaining droplets and particles were considered as the aerosols that were generated under the entrainment mechanism. The entrainment mechanism could be influenced by many factors, such as the superficial velocity of the flue gas and the concentration of the desulfurization solution. In the heat and mass transfer processes that occurred between the flue gas and the desulfurization solution in the WFGD scrubber, large quantities of gaseous ammonia escaped from the sprayed solution. When the gaseous NH3, the SO2 and the water vapor mixed together, following reactions might occur,15 2NH3(g)+SO2(g)+H2O(g) ⇔ (NH4)2SO3(s)

(1)

NH3(g)+SO2(g)+H2O(g) ⇔ NH4HSO3(s)

(2)

2NH3(g)+2SO2(g)+H2O(g) ⇔ (NH4)2S2O5(s)

(3)

(NH4)2SO3(s)+H2O(g) ⇔ (NH4)2SO3·H2O(s)

(4)

The (NH4)2SO3 and the NH4HSO3 were partially oxidized due to the existence of O2 in the flue gas,15,25-27 2(NH4)2SO3(s)+O2 ⇔ 2(NH4)2SO4(s)

(5)

2NH4HSO3(s)+O2 ⇔ 2NH4HSO4(s)

(6)

Accordingly, aerosols were formed through the heterogeneous reaction of NH3-H2O-SO2. And the chemical composition might be (NH4)2SO3, NH4HSO3, (NH4)2SO3·H2O, (NH4)2SO4, 10


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NH4HSO4, and (NH4)2S2O5.20 Previous studies showed that these reactions were largely influenced by the reaction temperature.15,20,28 Also the molar ratio of the NH3 and the SO2 would have significant impacts on the extent of the reaction and the product composition.

The aerosol formation properties were studied under the entrainment mechanism and the heterogeneous reaction mechanism, respectively. The experimental parameters were the same as in section 2.2. The concentrations of the aerosols formed under the two mechanisms were compared in Figure 5. The results showed that under the heterogeneous reaction condition, both the numerical concentration and the mass concentration were higher than those under the entrainment condition, which suggested that the heterogeneous reaction mechanism made the primary contribution to the aerosol formation during the ammonia-based WFGD processes. Figure 6 showed the numerical size distributions of the aerosols formed under the normal desulfurization condition, the entrainment condition, and the heterogeneous reaction condition, respectively. As is shown, the aerosols generated under the normal desulfurization condition consisted mainly of submicron particles, whose distribution was unimodal with the peak size around 0.07 µm. Under the heterogeneous reaction condition, the aerosols were mainly submicron particles, the unimodal distribution of which shared the same peak size of 0.07 µm. Under the entrainment condition, however, the aerosols consisted of far fewer submicron particles, while the micron particle concentration was much higher than that under the heterogeneous reaction condition. It could be concluded that the entrainment mechanism was the main source of the micron particles, while the heterogeneous reaction mechanism was the primary source of not only the submicron particles, but also the total aerosol emission from the ammonia-based WFGD system.

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200 7

3


2.5x10

numerical concentration mass concentration 150

7

2.0x10

7

1.5x10

100

7

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50


6

5.0x10

0 entrainment

heterogeneous reaction

Figure 5. Aerosol concentration under different formation conditions. 10

8

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-3

number dN/dlogDp / cm

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normal desulfurization heterogeneous reaction entrainment

2

10 0.01

0.1

1

10

Dp / µm

Figure 6.

Aerosol size distribution under different conditions.

The aerosol samples were gathered under the entrainment mechanism and the heterogeneous reaction mechanism, respectively. The morphologies were shown in Figure 7. As shown in Figure 7(a), the aerosols formed under the entrainment mechanism were mostly ball-shaped with the diameter about 1.0 µm. Figure 7(b) showed the aerosols generated under the heterogeneous reaction mechanism, which were extremely tiny and consisted mainly of submicron particles. The results were in good coincidence with Figure 6.

12


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(a) Aerosols formed in entrainment mechanism.

(b) Aerosols formed in inhomogeneous reaction mechanism. Figure 7. SEM images of aerosols formed under different mechanisms.

3.3. Influences of desulfurization parameters on the entrainment mechanism 3.3.1. Impacts of the superficial velocity. The superficial velocity of the flue gas was the primary factor influencing the entrainment of the desulfurization solution droplets. The effect of the superficial velocity on the entrainment mechanism was investigated on the simulation desulfurization set-up. The superficial velocity was adjusted separately to 1.8 m/s, 2.7 m/s, and 3.6 m/s, which were generally within the normal range of industrial WFGD system. The other parameters were the same as shown in section 2.2. The testing results showed that the aerosol emission increased with the elevation of the flue gas superficial velocity (Figure 8). Probably 13


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because the increase of the superficial velocity enhanced the entrainment of solution droplets and thus improved the aerosol formation. Besides, it was notable that when the superficial velocity was 1.8 m/s, the aerosol concentration was quite low, which inferred that in the lab-scale WFGD devices, reducing the superficial velocity to below 1.8 m/s would effectively eliminate the aerosol formation under the entrainment mechanism. 6

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6

1x10


0 1.5

2.0

2.5 3.0 superficial velocity ( m/s)

3.5

20

0 4.0

Figure 8. Aerosol concentration at different superficial velocities. 3.3.2. Impact of the desulfurization solution concentration.

The aerosol emission was

studied when the desulfurization solution concentration was 10%, 15%, and 20%, respectively. The other parameters were the same as section 2.2. As shown in Figure 9, the aerosol formation under the entrainment mechanism increased with the solution concentration rising. When the solution concentration was higher, the solid content of the desulfurization solution droplets increased, in which case more aerosols would be generated from the evaporation of the same amount of solution droplets. As a result, the aerosol formation was enhanced under the entrainment mechanism.

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30 25

Figure 9. Aerosol concentration at different desulfurization solution concentrations. 3.4. Effect of desulfurization parameters on the heterogeneous reaction mechanism 3.4.1. Aerosol formation at different reaction temperatures. Experiments

were

conducted on the heterogeneous reaction set-up. The reaction temperature was set at 40℃, 50℃, 55℃, 60℃, 65℃, 70℃, 75℃, 80℃, and 90℃, respectively. The other operating parameters were shown in section 2.3. The aerosol formation properties were studied in each condition. As shown in Figure 10, with the elevation of the reaction temperature, the numerical concentration and the mass concentration showed the similar decreasing trend. The decline was relatively slow when the temperature was below 60 ℃ . Within the temperature range of 60-65 ℃ , the concentration decreased rapidly. When the temperatures were higher than around 70 ℃ , the aerosol concentrations stayed low and steady. Such phenomenon was probably because the aerosol formation reaction (1)-(3) were exothermic, whose positive reactions were hindered at high reaction temperatures. As a result, the aerosol formation from the heterogeneous reactions was inhibited. The similar results have been reported by Bai and Tock.15,25 The differences were that in this research, the NH3-H2O-SO2 reactions were inhibited at temperatures higher than 65℃. But in their reports, the reactions were eliminated completely when the temperature exceeded about 55℃. 15


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6 5


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Figure 10.


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50

60 70 o reaction temperature ( C)

80

90

Aerosol concentrations at different temperatures.

The numerical size distributions of the aerosols formed at 40℃, 60℃, and 80℃ were presented in Figure 11. As was shown, the aerosols generated under each condition contained mainly submicron particles. The three distribution lines showed the similar decreasing trend within the submicron size range, but were different in the micron size range. When the size was smaller than around 1.0 µm, the aerosol concentration generally decreased with the temperature rising. When the size exceeded about 1.0 µm, the aerosol concentration at each temperature were close to one another. Consequently, with the increase of the temperature, the portion of the submicron particles declined obviously, but the part of the micron particles changed little. This was probably because when the temperature was higher, the negative reactions of the heterogeneous reactions were more active, while the Brownian movement of submicron particles was boosted. As a result, the submicron particles either decomposed or collided with each other to form larger particles, which led to the sharp decrease of the submicron particles.

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6

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40 C o 60 C o 80 C

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number dN/dlogDp (1/cm )

10

4

10

3

10

2

10

1

10

0

10 0.01

0.1

1

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aerosol size (µm)

Figure 11. Numerical size distributions at different temperatures. 3.4.2. Aerosol formation at different N/S conditions.

As an important impact factor in

the aerosol formation under the heterogeneous reaction mechanism, the molar ratio of the NH3 and the SO2 was adjusted to 0.1, 0.2, 1.0, 2.0, 4.0, respectively, while the reaction temperature was maintained at 70℃. The other parameters remained the same as in section 2.3. The aerosol concentrations were presented in Figure 12. The results indicated that the numerical concentration increased with the rising of the N/S, and the increase was faster when the N/S＞1.0. However, with the elevation of the N/S, the mass concentration firstly declined and then turned rising. Generally, the aerosol formation under the heterogeneous reaction mechanism was enhanced by high N/S, probably because the increase of the reactant NH3 improved the heterogeneous reaction (1)-(3), to which the similar results were reported in previous researches.20,28 5

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5

1.2x10

numerical concentration mass concentration o reaction temperature = 70 C

1.5

5

1.0x10

1.0

4

8.0x10

0.5 4

6.0x10

4

4.0x10

0.0

3


1.4x10


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4

2.0x10

-0.5 0.1

1 NH3/ SO2 ratio

Figure 12.

Aerosol concentrations in different N/S conditions. 17


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The numerical size distributions of the aerosols formed under the N/S = 0.2, 1.0, and 4.0 were shown in Figure 13. These distribution trends were similar, with the aerosols being mostly submicron particles. The numerical concentrations generally decreased with the size rising. With the elevation of the N/S, the portion of the submicron particles rose greatly, but the part of the micron ones increased little.

3

number dN/dlogDp (1/cm )

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10

6

10

5

10

4

10

3

10

2

10

1

10

0

10

N/S = 0.2 N/S = 1.0 N/S = 4.0

-1

0.01

0.1

1

10

aerosol size (µm)

Figure 13. Numerical size distributions in different N/S conditions . 3.4.3. Influences of the temperature and the N/S in WFGD devices.

According to the

discussion, high temperature would depress the heterogeneous reactions, but would also increase the actual flue gas superficial velocity in the scrubber to improve the aerosol formation under the entrainment mechanism. In addition, more gaseous ammonia would escape from the desulfurization solution when the temperature was higher, which would increase the reactant concentration and thus enhance the aerosol formation under the heterogeneous reaction mechanism. Consequently, there should be an optimal temperature that could balance the two aerosol formation mechanisms and minimize the aerosol emission from the ammonia-based WFGD system. Concluded from the current experimental results, the possible range of the optimal temperatures for the aerosol emission control was 75-95℃. The temperatures were measured at the entrance of the WFGD scrubber. In respect of the N/S, the results showed that high N/S would 18


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improve the aerosol formation through the heterogeneous reaction. Accordingly, reducing the ammonia escaping from the desulfurization solution would effectively lessen the aerosol emission. But the temperatures inside the WFGD scrubber are not uniform. In an industrial desulfurization scrubber, the temperature would change from 120-140℃ to around 55℃. The distributions of the reactants, such as NH3 and SO2, also have significant influences on the aerosol formation. All the above issues should be taken into account to develop a reasonable solution to the aerosol emission problems. Further explorations would still be needed.

4. Conclusion The ELPI was employed to investigate the aerosol formation properties in the ammonia-based WFGD system. Experiments were designed to study the aerosol formation under the entrainment mechanism and the heterogeneous reaction mechanism, respectively. The influences of some desulfurization parameters were also explored. Based on the results obtained from this study, the following main conclusions can be drawn: (1) The aerosol concentration in the flue gas increased obviously after the ammonia-based WFGD system. Additional aerosols were proved to be generated during the desulfurization processes. (2) The aerosols emitted from the ammonia-based WFGD system consisted mainly of submicron particles in terms of the numerical concentration. For the mass concentration, they were mostly micron particles. (3) The entrainment mechanism was the main source of the micron particles. While the heterogeneous reaction mechanism was the primary source of not only the submicron particles, but also the total aerosol emission from the ammonia-based WFGD system. (4) The aerosol formation under the entrainment mechanism was positively related to the 19


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superficial velocity of the flue gas in the WFGD scrubber and the concentration of the desulfurization solution. When the superficial velocity was below 1.8 m/s, the aerosol emission from the lab-scale WFGD system decreased significantly. (5) The aerosol formation under the heterogeneous reaction mechanism was inhibited at high reaction temperature and low N/S. When the reaction temperature exceeded about 65℃, the aerosol formation reactions were inhibited intensely. With the elevation of the temperature and the decline of the N/S, the concentration of the submicron particles decreased greatly, but the micron particles changed little. (6) Adjusting the temperature of the inlet flue gas at 75-95℃ and reducing the ammonia escaping from the desulfurization solution would benefit the aerosol emission control in the ammonia-based WFGD system.

Acknowledgement We appreciate the financial support from the National Natural Science Foundation of China (21276049), the National Basic Research Program of China (2013CB228505), the National High Technology Research and Development Program of China (2013AA065004), the Jiangsu Science and Technology Support Program (NO. BE2014856), and the Scientific Research Foundation of Graduate School of Southeast University (YBJJ1610).

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Aerosol Formation Characteristics during Ammonia-Based WFGD

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