Investigation on Formation Characteristics of Aerosol Particles

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Investigation on Formation Characteristics of Aerosol Particles during Wet Ammonia Desulfurization Process Jingjing Bao, Licheng Sun, Zhengyu Mo, Guo Xie, Jiguo Tang, and Hongmin Yang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00672 • Publication Date (Web): 11 Jul 2017 Downloaded from http://pubs.acs.org on July 12, 2017

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Investigation on Formation Characteristics of Aerosol Particles during Wet Ammonia Desulfurization Process Jingjing Bao1, Licheng Sun1*, Zhengyu Mo1*, Guo Xie1, Jiguo Tang1, Hongmin Yang2 (1. State Key Laboratory of Hydraulics and Mountain River Engineering, College of Water Resource & Hydropower, Sichuan University, Chengdu, 610065, China; 2. Engineering Laboratory of Energy System Process Conversion and Emission Reduction Technology of Jiangsu Province, School of Energy and Mechanical Engineering, Nanjing Normal University, Nanjing, 210042, China) ABSTRACT: An experimental research was carried out on formation characteristics of aerosol particles during Wet Flue Gas Desulfurization (WFGD) process with different ammonia-based desulfurizers. Three desulfurizers of aqueous ammonia, ammonium sulfate and ammonium sulfite were compared and influences of desulfurization operating conditions were investigated. The results revealed that numerous aerosol particles were produced during the desulfurization process for all the three desulfurizers. In terms of the quantity of aerosol particles produced, the aqueous ammonia and ammonium sulfite took the first and second place, respectively. Ammonium sulfate produced the least aerosol particles, with submicron sizes for most cases. For all the three desulfurizers, the production rate of aerosol particles increased with increasing the liquid-to-gas ratio, pH value as well as the desulfurization liquid temperature; a higher concentration of SO2 in the flue gas resulted in more aerosol particles being generated at a lower pH value of the desulfurization liquid, while an opposite effect was realized for the situation with a higher pH value; a small amount of SO3 in the flue gas intensified the formation of aerosol particles, and with its increase the number concentration of aerosols increased continuously. Optimum operating conditions for WFGD process were finally achieved, which are 50℃ of the desulfurization liquid temperature, 10% of the mass concentration of the desulfurizing agent, 10~15L/Nm3 of the *Corresponding author. Tel: +86028 85405633; Fax: +86028 85405633; E-mail: [email protected] (L. Sun), [email protected] (Z. Mo). 1

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liquid-to-gas ratio, 11, 7.0 and 8.0 of pH value for aqueous ammonia, ammonium sulfate and ammonium sulfite, respectively. Keywords: Aerosol; Particle; Formation characteristics; Wet Ammonia Desulfurization; desulfurizer 1、、Introduction Environmental pollution caused by energy consumption is a global issue [1, 2]. Numerous pollutants are generated from fossil fuel combustion, forming the primary source of SO2 and particles in the atmosphere [3, 4]. Various techniques for flue gas desulfurization (FGD) have been developed and applied to control SO2 emission, among which the wet flue gas desulfurization (WFGD) is the most widely used[5]. In recent years, a wet flue gas desulfurization using ammonia as the absorbent has drawn increasing attention since its main production is ammonium sulfate, a good fertilizer for crops, containing no any other pollutant by-products [6-10]. In the actual operation process, (NH4)2SO3 and small amounts of NH4HSO3 coexist in the desulfurization liquid. The former plays a key role in the removal of SO2 from flue gas, obeying the following reaction[9]: (NH4)2SO3 + SO2 + H2O = 2NH4HSO3

(1)

Since NH4HSO3 cannot absorb SO2 effectively, aqueous ammonia has to be added to revitalize the absorption liquid: NH4HSO3 + NH3 = (NH4)2SO3

(2)

Moreover, aqueous ammonia can also adjust the pH value of the absorption liquid[7]. (NH4)2SO3 and NH4HSO3 can be oxidized to (NH4)2SO4 and NH4HSO4 in the presence of O2.

2


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However, WFGD with ammonia-based desulfurizers cannot avoid producing a large number of aerosols, which are mainly of the size in submicron range and cannot be scrubbed off by the desulfurization liquid[10]. They can easily escape out of the WFGD system to the atmosphere, forming secondary pollutants by reactions between aerosol particles and some chemical matters in the atmosphere. Having large specific surface area, aerosol particles can carry a mass of toxic matters, causing problems threatening environments and human health[11]. Efficient ways should have to be developed to restrict the emission of aerosol particles during WFGD process using ammonia-based desulfurizers. With a high efficiency of removing SO2, WFGD using ammonia-based desulfurizers has been extensively studied by many scholars [7, 12, 13]. However, they mainly focused on the influence of operation parameters on SO2 removal efficiency and recycling utilization of the by-products and desulfurization agents. Only a few studies reported the phenomenon of aerosol formation during the WFGD process, whereas rarely involve the formation of aerosol particles and relevant control technologies during the process. In fact, it is a rather complicated process of the formation of aerosol particles during the WFGD process using ammonia-based desulfurizers. Yan et al. [10, 14, 15] experimentally and numerically studied the formation and removal of aerosol particles with this method. They proposed that heterogeneous condensation of water vapor on the surface of aerosol particles can augment these particles in sizes, leading to them being removed more easily. Bao et al.[16,17] investigated features of the formation and emission of aerosol particles during this process and suggested that aerosols were probably formed from the two following processes: 1) the evaporation and entrainment of desulfurization solution droplets; 2) the heterogeneous reactions between SO2, H2O, and gaseous NH3 volatilized out from the desulfurization solution. 3


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Huang et al. [18] found that aerosols formed by the first way were mostly larger particles in micron range, while those by the second way were mainly submicron particles, taking up a great proportion in the total aerosol emission. Most of the research results were obtained with aqueous ammonia as the desulfurizer, but actually, more regular desulfurizers in industry are ammonium sulfate and ammonium sulfite. It is necessary to take a further study on aerosol formation for cases with ammonium sulfate and ammonium sulfite as the desulfurizers. In this work, aqueous ammonia, ammonium sulfate and ammonium sulfite were used as the desulfurizers in WFGD process. Fresh aqueous ammonia was used to adjust the pH value of the desulfurization liquid to maintain high desulfurization efficiency. Main objective of current work is to make a comparative study between the three desulfurizers in terms of aerosol formation during desulfurization process. 2、、Experimental system 2.1 Experimental Apparatus A schematic of the experimental system is shown in Fig. 1. It mainly consists of an automatic coal-fired boiler, a buffer vessel, a cyclone, a WFGD system and a measurement system. Flue gas with a volumetric flux of 200 Nm3·h-1 is provided by the boiler burning anthracite. The coal fired boiler is a self-sustained stoker feed boiler, which burns coal briquette with the average size of about 20 mm. Detailed information of the boiler is shown in Table 1. The flue gas discharged from the boiler is similar to that from a coal fired power plant [19]. In the buffer vessel, an electric heater and a stirrer are employed to ensure constant particle concentration and distribution in sizes, and to regulate the temperature of the flue gas. SO2 and SO3 can be injected into the coal-fired flue gas in the buffer vessel. Before entering the WFGD system, large particles are separated from the 4


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flue gas in the cyclone. Thermal insulation measures are taken for the buffer, the cyclone and the pipelines. During the WFGD process, the flue gas was passed through the desulfurization liquid counter-currently and then was released into the atmosphere after desulfurization with the help of an exhaust fan. The desulfurization tower could be used as either a spray scrubber or a packed tower or even a rotating-stream-tray scrubber provided that only the internals are replaced. In this work, a spray scrubber was used as the desulfurization tower. The tower is made of polycarbonate pipes and plates with excellent thermal insulating properties. Its height and diameter are 3600 mm and 150 mm, respectively. A corrugated plate demister is installed at its upside for separating the entrained droplets from the desulfurized flue gas. An electric heater was set in the open storage tank of desulfurization liquid in order to control the temperature of the desulfurization liquid. 2.2 Experimental parameters In the experiments, aqueous ammonia, ammonium sulfate and ammonium sulfite were used as the desulfurizing agents. Fresh aqueous ammonia was utilized for adjusting the pH value of desulfurization liquid. Desulfurization liquid was circulated with a recycle ratio of about 100%. In practical coal-fired power plant, the temperature of flue gas at the inlet of WFGD system and the desulfurization liquid are in the range of 100~150℃ and 40~60℃, respectively. In order to approximating the actual conditions, the flue gas temperature (TG) at the inlet of the desulfurization tower was controlled in the range of 100~110 ℃. The temperature of the desulfurization liquid (TL) was in the range of 40~60℃. All the experiments were carried out at an initial particle number concentration (CN) of (4.0-7.0)×106 cm-3. pH values of the desulfurization liquid could be adjusted to insure the desulfurization efficiencies exceeding 90%. 2.3 Test and Analysis Methods 5


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The concentration and size distribution of aerosol particles were measured in real time by an electrical low pressure impactor (ELPI, Dekati Ltd., Finland), which has been described in details in[20]. A gas analyzer (ECOM J2KN, RBR Ltd., Germany) was used to measure the concentration of SO2 in the flue gas. Humidity and temperature of the flue gas were measured by a humidity transmitter (HMT337, Vaisala Ltd., Finland). Measurement point was set at the outlet of the desulfurization tower. The phase composition of the particles was characterized by an X-ray diffraction (SmartLab™ XRD). Particle samples in measurements were taken by a PM10/PM2.5 sampler (Dekati Ltd., Finland). The overall production rate (P) and grade production rate (Pi) of the aerosol particles measured in the experiment are defined as: P=

Pi =

No − Ni × 100% Ni

（3）

N j , o − N j ,i

（4）

N j ,i

×100%

Ni, No are the overall number concentration of the particles in the flue gas at the inlet and outlet of the desulfurization tower, wherein the number concentration of those in a specific size range of sequence jth are represented by Nj,i and Nj,o. 3、、Results and Discussion 3.1 Effect of WFGD process on particle concentration With the three desulfurizers of aqueous ammonia, ammonium sulfate and ammonium sulfite, the number concentrations of aerosol particles in the desulfurized flue gas at the outlet of WFGD system were measured online by ELPI. In experiments, concentration of SO2 (c0(SO2)) in the flue gas at the inlet of the desulfurization tower was 2850mg/m3. The flue gas temperature at the inlet (TG) and the desulfurization liquid temperature (TL) were 110℃ and 50℃, respectively. The mass 6


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concentrations of ammonium sulfate (w[(NH4)2SO4]) and ammonium sulfite (w[(NH4)2SO3]) in the desulfurization liquid were the same of 10%. To ensure the desulfurization efficiency higher than 90%, pH values of the desulfurization liquids with the three desulfurizers of aqueous ammonia, ammonium sulfate and ammonium sulfite must be kept at 10.0, 7.0 and 8.0, respectively. The liquid-to-gas ratio (L/G) was 15L/Nm3. Fig. 2 presents the temporal variation of the number concentration and size distribution of aerosol particles in the flue gas at the outlet of the WFGD system. As indicated in Fig. 2(a), section A (0~170s) showed the shutoff state of the system, during which the number concentration of particles in the flue gas remained around 4.8×106 cm-3. The system was actuated at 170s (Section B). An obvious increase of the number concentration of particles in the desulfurized flue gas was indicated during WFGD process for all the desulfurizing agents. For the case with aqueous ammonia as the desulfurizer, the number concentration in the desulfurized flue gas was even increased to 5.6×107cm-3, much higher than that of the other two desulfurizers. The desulfurizer of ammonium sulfate resulted in the smallest increase of aerosol particles, with the number concentration of 8.1×106cm-3. The system was shut off after 365s (Section C), the number concentration recovered to the same level with section A. In general, a large amount of aerosol particles were produced during WFGD processes with the three ammonia-based desulfurizers, causing remarkable increase of their level at the outlet of the system. Meanwhile, it can be seen from the results that good stability and repeatability of the experiments were realized in the experimental system. The fluctuation of the particle number concentration is less than 3~5% in each section. Fig. 2(b) illustrated size distributions of aerosol particles in the coal-fired flue gas at the inlet and outlet of the desulfurization tower. The number concentration of fine particles in the flue gas 7


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at the inlet of the tower displays a unimodal distribution within the detectable range of ELPI. They were mostly in the submicron size range with a maximum number concentration at the size of 0.07 µm. However, the number concentrations in each size grades at the outlet increased remarkably after the WFGD process, especially in the range of 0.07~0.70µm. The peak values of the number concentration were present at the size of 0.15µm, much higher than that before the desulfurization. This indicated that the aerosol particles generated during WFGD process with the three ammonia-based desulfurizers were mostly submicron fine particulates. 3.2 Influence of desulfurization operating conditions on aerosol formation Previous researches [16-18] showed that the main components of the aerosol particles generated during ammonia-based WFGD process are (NH4)2SO3, NH4HSO3, (NH4)2SO4 and NH4HSO4, etc. These aerosol particles were generated by two possible mechanisms. One is heterogeneous reactions between SO2, H2O, and gaseous NH3 volatilized from the desulfurization solution. Submicron particles from this way account for a very large proportion of the aerosol particles generated. The heterogeneous reaction is related to the concentrations of NH3 and SO2 in the flue gas, and the concentration of NH3 depends on the concentration of aqueous ammonia (or the pH value of desulfurization liquid) and the temperature of the desulfurization liquid. Higher pH value and temperature of the desulfurization liquid intensify the volatilization of gaseous NH3, leading to the increase of NH3 concentration in the flue gas and much more aerosol particles being generated from the heterogeneous reactions. The other mechanism contributing to the formation of aerosol particles is the evaporation and entrainment of desulfurization solution droplets in the flue gas. It is influenced by temperatures of the flue gas and the desulfurization liquid as well as superficial gas velocity, etc. The particles from the second process are mostly larger ones with 8


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micron size[11, 21]. In order to verify the above mechanisms and explore an optimal operating condition for restricting the formation of aerosol particles, influences of the operating conditions on aerosol formation during the desulfurization process were investigated. 3.2.1 pH value of desulfurization liquid Fig. 3 illustrates the influence of pH value of the desulfurization liquid on the aerosol production rate with the three desulfurizers. SO2 concentration in the flue gas at the inlet of the desulfurization tower was 2850mg/m3. The flue gas temperature at the inlet and the temperature of the desulfurization liquid were 110℃ and 50℃, respectively. Mass concentrations of ammonium sulfate and ammonium sulfite in the desulfurization liquid were the same of 10%. The liquid-to-gas ratio (L/G) was 10L/Nm3. As can be seen from the figure, the production rates of aerosols increased with increasing pH value of the desulfurization liquid for the three desulfurizers. Since the pH value of the desulfurization liquid was adjusted by adding fresh aqueous ammonia, a higher concentration of aqueous ammonia means a higher pH value of the liquid. The gaseous NH3 was much prone to escape from the desulfurization liquid, resulting in more intensified reactions between gaseous SO2, H2O, and NH3. Hence, aerosol particles resulted from the heterogeneous reactions increase remarkably. With the submicron size as presented in Fig. 2(b), most of them cannot be separated efficiently by the desulfurization liquid and the corrugated plate demister installed at the upper of the desulfurization tower. They escaped with the flue gas, causing the aerosol production rate increasing significantly. For the case with aqueous ammonia as the desulfurizer, the production rate of aerosol increased from 70% to 910% with pH value of the desulfurization liquid increasing from 9.0 to 11.0. In order to obtain a higher removal efficiency of SO2 and less aerosol production rate, a pH value in the range of 9.5~10.5 is appropriate for 9


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aqueous ammonia. Similarly, optimal pH values for ammonium sulfate and ammonium sulfite were in the range of 6.5~7.5 and 7.5~8.5, respectively. 3.2.2 Liquid-to-gas ratio (L/G) The influence of liquid-to-gas ratio on the formation of aerosol particles are shown in Fig. 4 for the three ammonia-based desulfurizers at different pH values of the desulfurization liquid. In the experiments, SO2 concentration and temperature of the flue gas at the inlet of the desulfurization tower were 2850mg/m3 and 110℃,respectively. Temperature of the desulfurization liquid was 50℃. Mass concentrations of ammonium sulfate and ammonium sulfite were the same of 10%. It was observed that the aerosol production rate has a slight increase with the liquid-to-gas ratio at a lower pH value for all the three desulfurizers. As indicated in Fig.3, only a small number of aerosol particles were generated at a lower pH value, therefore the change of liquid-to-gas ratio exhibits little effect on the formation of aerosol particles in this case. Taken the aqueous ammonia in Fig. 4(a) as an example, the aerosol production rate slightly increased from 45% to 82% as the ratio increasing from 5L/Nm3 to 20L/Nm3 with a pH value of 9.0. However, a higher pH value resulted in intensified formation of aerosol particles, accordingly the number concentration of aerosol particles increased obviously with liquid-to-gas ratio increasing. As can be seen from Fig. 4(c), for the case with ammonium sulfite as the desulfurizer, the aerosol production rate increased from 160% to 715% with the liquid-to-gas ratio varied from 5L/Nm3 to 20L/Nm3 at pH value of 8.0. This can be explained from the following two aspects. Firstly, for the case of a higher liquid-to-gas ratio, more gaseous NH3 evaporated from the desulfurization liquid, and more aerosol particles were released from the SO2-NH3-H2O heterogeneous reactions; secondly, a higher liquid-to-gas ratio would also give rise to more desulfurization liquid droplets reacting with 10


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SO2 in the flue gas, leading to higher concentrations of (NH4)2SO3, NH4HSO3, (NH4)2SO4 and NH4HSO4, etc. in the droplets of the desulfurization liquid. Therefore, the aerosol particles generated from the evaporation and entrainment of desulfurization solution droplets increased remarkably. In order to inhibit the formation of aerosol particles, a lower liquid-to-gas ratio is required, whereas an excessive low value would lower the desulfurization efficiency. In practice, an optimal liquid-to-gas ratio in the range of 10~15L/Nm3 can ensure the desulfurization efficiency higher than 90% and less aerosol particles being formed during the desulfurization process. Fig. 5 illustrates the production rates of aerosol particles within different size grades for the three desulfurizers, with pH values of the desulfurization liquids of 10.0, 7.0 and 8.0, respectively. It is clearly shown that the production rates of aerosol particles increased with the liquid-to-gas ratio increasing. The maximum production rate appeared around the particle size in the range of 0.07~0.70µm. This also means that aerosols generated from the desulfurization process using ammonia-based desulfurizers are mostly in the submicron size. 3.2.3 Temperature of the desulfurization liquid Fig. 6(a) ~6(c) represent the overall production rate of aerosol particles as a function of L/G during the desulfurization process at different desulfurization liquid temperature. pH values of the three desulfurization liquids of aqueous ammonia, ammonium sulfate and ammonium sulfite were specified at 10.0, 7.0 and 8.0 respectively. SO2 concentration and temperature of the flue gas at the inlet were kept 2850mg/m3 and 110℃, respectively. Mass concentrations of ammonium sulfate and ammonium sulfite in the desulfurization liquid were the same of 10%. It can be seen that the production rate of aerosols increased remarkably with the desulfurization liquid temperature 11


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increasing. For ammonium sulfite, as indicated in Fig. 6(c), the rate rose up from 233% to 679% with the desulfurization liquid temperature increasing from 40℃ to 60℃ at a liquid-to-gas ratio of 15 L/Nm3. This is due to that an increase in the desulfurization liquid temperature intensifies the volatilization of gaseous NH3 from the solution, causing the increase of the containment of gaseous NH3 in the flue gas. With that, heterogeneous reactions between gaseous NH3 and SO2 as well as H2O significantly enrich the concentration of aerosol particles ((NH4)2SO3、NH4HSO3, etc.) in the flue gas. Meanwhile, the higher liquid temperature is also in favor of the evaporation of the desulfurization solution and the entrainment of the solution droplets by the flue gas, resulting in more aerosol particles being generated. Therefore, for restricting generating aerosol particles, an excessively high temperature of the desulfurization liquid should be avoided in practical desulfurization process on the premise of without sacrificing desulfurization efficiency and increasing the operation cost. 3.2.4 SO2 concentration in the flue gas Investigation was also conducted on the effect of the concentration of SO2 in the flue gas on the formation of aerosol particles during WFGD process. The temperatures of the flue gas at the inlet of the desulfurization tower and the desulfurization liquid were 110℃ and 50℃, respectively. The production rates of aerosol particles using aqueous ammonia, ammonium sulfate (10%) and ammonium sulfite (10%) as the desulfurizers are depicted in Fig.7(a)~7(c). It could be seen that the SO2 concentration has significant influence on the formation of aerosol particles. At a lower pH value of the desulfurization liquid, a higher SO2 concentration led to larger production rate of aerosol particles. However, the influence of SO2 concentration on aerosol production rate was exactly the opposite at a higher pH value of the desulfurization liquid. The number concentration 12


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of aerosols decreased with the concentration of SO2 increasing. This can be explained by the formation mechanism of aerosol particles. The specific reaction mechanism of heterogeneous reactions between SO2, H2O, and gaseous NH3 can be summarized as follows[22]. 2NH3(g) + SO2(g) + H2O(g) ↔ (NH4)2SO3(s)

(3)

NH3(g) + SO2(g) + H2O(g) ↔ NH4HSO3(s)

(4)

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

(5)

(NH4)2SO3(s) + SO2(g) + H2O(g) ↔ 2NH4HSO3(s)

(6)

In the presence of O2 in the flue gas, (NH4)2SO4 and NH4HSO4 would be generated via the following reactions: 2(NH4)2SO3(s) + O2(g) ↔ 2(NH4)2SO4(s)

(7)

2NH4HSO3(s) + O2(g) ↔ 2NH4HSO4(s)

(8)

2NH4HSO3(s,aq) + O2(g) + 2NH3 ↔ 2(NH4)2SO4(s,aq)

(9)

The heterogeneous reaction between SO2, H2O, and gaseous NH3 experiences three stages[23]. First, a gaseous adduct of NH3·SO2 is generated by the gas-phase reaction of NH3 and SO2 (Reaction (10)). After that, a competitive reaction occurs between NH3·SO2 and NH3 or with itself to produce the adducts of (NH3)2·SO2 and (NH3·SO2)n (Reaction (11),(12), where K1 and K2 are the equilibrium constants.)[24]. Reaction (10)~(12) can finish instantaneously and H2O plays a role of catalyst. When the partial pressure of the adducts in the flue gas exceeds a critical value, the gaseous NH3·SO2 and (NH3)2·SO2 would condense on the surface of solid aerosol particles until the supersaturation environment disappears. Since the intermediates of NH3·SO2 and (NH3)2·SO2 are extremely unstable, the condensate on the aerosol particles can react with H2O in the flue gas to produce NH4HSO3 and (NH4)2SO3 particulates( Reaction (13), (14)). 13


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NH g + SO g ⇔ NH ∙ SO g

(10)

NH ∙ SO g +NH g ⇔ NH ∙ SO s

(11)

nNH3·SO2 (g) → (NH3·SO2)n (s)

(12)

NH3·SO2 (s) + H2O (g) → NH4HSO3 (s)

(13)

(NH3)2·SO2 (s) + H2O (g) → (NH4)2SO3 (s)

(14)

An equilibrium supersaturation degree (S1, S2) of NH3·SO2 and (NH3)2·SO2 can be defined as[23]:

S1 =

S2 = 0

Where P1

0

and P2

K1PNH3 PSO2

（15）

P10 2 K1 K 2 PNH P 3 SO2

（16）

P20

are the equilibrium partial pressures of NH3·SO2 and (NH3)2·SO2,

respectively. PNH 3 and PSO2 are the partial pressure of NH3 and SO2 in the flue gas. Since the 0

0

equilibrium partial pressure of (NH3)2·SO2 is far less than that of NH3·SO2 ( P2 « P1 ), it can be speculated that the supersaturation degree of (NH3)2·SO2 is much greater than that of NH3·SO2 ( S2 » S1 ) on the basis of equation (15) and (16). According to the classical nucleation theory, smaller initial nucleation size ( rcrit 2 ) is necessary for the nucleation of (NH3)2·SO2 particulate in comparison to NH3·SO2 particulate. The nucleation rate is also fast for the former and its diameter is smaller than the latter. The partial pressure of gaseous (NH3)2·SO2 decreases rapidly due to its fast nucleation, which is in favor of the forward reaction (11). As a result, (NH4)2SO3 particles become the main components of the generated aerosol particles. The percentage of NH4HSO3 particles would increase provided that the flue gas contains excessive SO2. Reactions (7)~(9) intensify remarkably for the flue gas with a higher O2 containment correspondingly, the percentage of (NH4)2SO4 in the generated aerosol particles increases significantly. As can be seen 14


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from the above analysis, the composition of aerosol particles depends on the ratio of SO2 to NH3, temperature and the concentration of O2 and H2O in the flue gas. When the desulfurization liquid has a lower pH value, higher SO2 concentration in the flue gas is favorable to the forward reaction of the reactions (3), (4), (6) and (10). Thus more submicron aerosol particles of (NH4)2SO3, NH4HSO3 and (NH4)2SO4 are formed via these heterogeneous reactions. Meanwhile, high SO2 concentration can also intensify the absorption of SO2 by the desulfurization liquid, causing an increase of (NH4)2SO3 and NH4HSO3 droplets. Therefore, the aerosol particles resulted from the evaporation and entrainment of desulfurization solution droplets increases significantly. On the contrary, the concentration of aerosol particles in the flue gas at the outlet of the desulfurization tower decreases with SO2 concentration increasing at a higher pH value of the desulfurization liquid. Analyzing the components of aerosol particles might account for this phenomenon. Higher pH value would lead to more gaseous NH3 volatilizing from desulfurization solution at a lower SO2 concentration. Excessive NH3 can make the reaction between NH3·SO2 and NH3 (Reaction (11)) predominate over the reaction between NH3·SO2 and itself (Reaction (12)). Thus more adducts of (NH3)2·SO2 are generated, which can react with H2O to form (NH4)2SO3 in high-humidity flue gas (Reaction (14)). With increasing SO2 concentration, situation with excessive NH3 in the flue gas will not continue. The reaction between NH3·SO2 and NH3 will not prevail in the competition any longer. More NH4HSO3 are generated from reactions (12) and (13). Since the solubility of (NH4)2SO3 is smaller than that of NH4HSO3, it’s more easily for the formation of (NH4)2SO3 aerosol particle. Thus, more aerosol particles will be generated under a lower SO2 concentration condition. In addition, it is indicated that the main component might be (NH4)2SO3 in the aerosol particles during WFGD processes with ammonia-based 15


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desulfurizers. This can be verified by the XRD analysis results shown in Fig. 8. Only (NH4)2SO4 was detected in the aerosol particles. For the restrictions under experimental conditions, the particle samples cannot be analyzed immediately after collection. (NH4)2SO3 would be oxidized to form (NH4)2SO4 with exposure to air. Hence, only (NH4)2SO4 was found in the results of XRD analysis. 3.2.5 SO3 concentration in the flue gas In view of the presence of SO3 in practical coal-fired flue gas, influence of SO3 on the formation of aerosol particles should be considered. SO3 was added to the flue gas to investigate its effect. Fig. 9 illustrates the effect of SO3 concentration on the variation of concentration of aerosol particles in the flue gas at the outlet of the desulfurization tower. The desulfurization solution with 10% ammonium sulfite was used in the test. SO2 concentration in the flue gas at the inlet was 900mg/m3. The temperatures of the flue gas at the inlet and desulfurization liquid were 110℃ and 50℃, respectively. pH value of the desulfurization solution was 8.0. The liquid-to-gas ratio (L/G) was 15L/Nm3. SO3 was obtained from heating 50% oleum in a water bath with temperature of 50℃and was added into the flue gas at the inlet of the desulfurization tower with N2 as the carrier gas. The concentration of SO3 can be adjusted by changing the flow rate of N2. In Fig. 9, section A presented the number concentration of fine particles in initial coal-fired flue gas. In section B, the desulfurization system was kept running without adding SO3. Compared to the flue gas in section A, only a small amount of aerosol particles were generated during the desulfurization process due to the presence of SO2 in the flue gas. For sections C~F, concentration of SO3 were kept at 35mg/m3, 70mg/m3, 105mg/m3 and 140 mg/m3, respectively. Since SO3 would react with gaseous NH3 volatilizing from the desulfurization solution to produce (NH4)2SO4 and 16


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Energy & Fuels

NH4HSO4 aerosol particles, the number concentration of aerosol particles in the flue gas at the outlet of the desulfurization tower increased remarkably. And it was intensified continually with increasing SO3 concentration. Particle number concentration of aerosols recovered to the level of section A once the desulfurization tower stopped running in section G. Compared to the production rates of aerosol particles at different SO2 concentration in Fig. 7, it was observed that a small amount of SO3 could result in the formation of a large number of aerosol particles. Taken SO3 concentration of 105 mg/m3 as an example, the concentration of aerosol particles in the flue gas at the outlet increased from 8.3×106cm-3 to 5.5×107cm-3. It increased by 560%, much higher than that of 2850mg/m3 SO2 concentration under the same conditions. Therefore, controlling SO3 concentration in the flue gas is more practical and applicable for inhibiting the formation and emission of aerosol particles during WFGD processes using ammonia-based desulfurizers. 4、、Conclusions The formation characteristics of aerosol particles during WFGD process were comparatively studied by using aqueous ammonia, ammonium sulfate and ammonium sulfite as the desulfurizers. A large amount of aerosol particles were produced during WFGD processes for all the three ammonia-based desulfurizers. WFGD with aqueous ammonia as the desulfurizer generated highest number concentration of aerosol of about 5.6×107 cm-3 in the flue gas. The minimum one was that with ammonium sulfate as the desulfurizer. These aerosol particles were mostly in submicron size range and their main components were (NH4)2SO3, NH4HSO3, (NH4)2SO4 and NH4HSO4, etc. Two possible mechanisms forming these aerosol particles are: 1) heterogeneous reactions between SO2, H2O, and gaseous NH3 volatilizing from the desulfurization solution; 2) 17


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evaporation and entrainment of desulfurization solution droplets in the flue gas. The increase of liquid-to-gas ratio, pH value and temperature of the desulfurization liquid would intensify the formation of aerosol particles. The overall production rate of aerosol particles increases with SO2 concentration in the coal-fired flue gas at a lower pH value of the desulfurization liquid. However, the influence of SO2 concentration on the production rate is exactly the opposite at a higher pH value. A small amount of SO3 could result in the formation of a large number of aerosol particles, and with its increase the number concentration of aerosol particles increased continuously. The formation of aerosol particles during the WFGD process using ammonia-based desulfurizers could be inhibited by optimizing the operating conditions. The optimum operating conditions were as follows: the temperature of desulfurization liquid 50℃, the mass concentration of desulfurizing agent 10%, the liquid-to-gas ratio of 10~15L/Nm3, pH value of 11, 7.0 and 8.0 for aqueous ammonia, ammonium sulfate and ammonium sulfite, respectively.

Acknowledgments The authors thank the National Natural Science Foundations of China (51506099, 51676101, 51606130) and the Natural Science Foundation of Jiangsu Province (BK20161558) for their financial support.

References [1] Nurrohim, A.; Sakugawa, H. A fuel-based inventory of NOx and SO2 emissions from manufacturing industries in Hiroshima prefecture. Applied Energy. 2008, 78(4), 355-369. [2] Chung, W. S.; Tohno, S.; Shim, S. Y. An estimation of energy and GHG emission intensity caused by energy consumption in Korea: an energy IO approach. Applied Energy. 2009, 86(10), 1902-1914. 18


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[3] Roy, S.; Hegde, M. S.; Madras, G. Catalysis for NOx abatement. Applied Energy. 2009, 86(11), 2283-2297. [4] Wang, J. S.; Anthon, E. J. Clean combustion of solid fuels. Applied Energy. 2008, 85(2-3), 73-79. [5] Kaminski, J. Technologies and costs of SO2-emissions reduction for the energy sector. Applied Energy. 2003, 75(3-4), 165-172.

[6] Gao, X.; Du, Z.; Ding, H. L.; Wu, Z. L.; Lu, H.; Luo, Z. Y.; Cen, K. F. Kinetics of NOx Absorption into (NH4)2SO3 Solution in an Ammonia-Based Wet Flue Gas Desulfurization Process. Energy & Fuels. 2010, 24(11), 5876-5882. [7] Gao, X.; Ding, H. L.; Du, Z.; Wu, Z. L.; Fang, M. X.; Luo, Z. Y.; Cen, K. F. Gas-liquid absorption reaction between (NH4)2SO3 solution and SO2 for ammonia-based wet flue gas desulfurization. Applied Energy. 2010, 87(8), 2647-2651. [8] Jia, Y.; Zhong, Q.; Fan, X. Y.; Wang, X. R. Kinetics of oxidation of total sulfite in the ammonia-based wet flue gas desulfurization process. Chemical Engineering Journal. 2010, 164(1), 132-138. [9] Jia, Y.; Zhong, Q.; Fan, X. Y.; Chen, Q. Q.; Sun, H. B. Modeling of ammonia-based wet flue gas desulfurization in the spray scrubber. Korean Journal of Chemical Engineering. 2011, 28(4), 1058-1064. [10] Yan, J. P.; Bao, J. J.; Yang, L. J.; Fan, F. X.; Shen, X. L. The formation and removal characteristics of aerosols in ammonia-based wet flue gas desulfurization. Journal of Aerosol Science. 2011, 42(9), 604-614.

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[11] Pan, D. P.; Yu, R.; Bao, J. J.; Wu, H.; Huang, R. T.; Yang, L. J. Emission and Formation Characteristics of Aerosols from Ammonia-Based Wet Flue Gas Desulfurization. Energy & Fuels. 2016, 30(1), 666-673.

[12] Liu, G. R.; Wang, Z. W.; Wei, Y. L.; Ji, Q. L. Experimental study and modeling for ammonia desulphurization in spray tower. CIESC Journal. 2010, 61(9), 2463-2467. [13] Antonia, G.; Norberto, F.; Alfredo, T. Detailed modeling of a flue-gas desulphurization plant. Computer and Chemical Engineering. 2007, 31, 1419-1431.

[14] Yan, J. P.; Yang, L. J.; Bao, J. J. Impact property on fine particles from coal combustion in wet flue gas desulfurization process. Journal of Southeast University. 2011, 41(2), 387-392. [15] Yan, J. P.; Yang, L. J.; Shen, X. L. Dynamics of Aerosol Condensational Removal in Ammonia Desulfurization Process. Proceeding of the CSEE. 2011, 31(29), 41-47. [16] Bao, J. J.; Yin, H. B.; Yang, L. J.; Yan, J. P. Formation Characteristics of Aerosols in Wet Ammonia-Based Desulphurization Process. Journal Chemical Engineering Chinese University. 2010, 24(2), 325-330.

[17] Bao, J. J.; Yang, L. J.; Yan, J. P.; Liu, J. H.; Song, S. J. Performance of removal of fine particles by WFGD system. CIESC Journal. 2009, 60(5), 1260-1267. [18] Huang, R. T.; Pan, D. P.; Sheng, Y.; Yang, L. J. Properties of aerosol formation during wet ammonia-based desulfurization. CIESC Journal. 2015, 66(11), 4366-4372. [19] Wu, H.; Yang, L. J.; Yan, J. P.; Hong, G. X.; Yang, B. Improving the removal of fine particles by heterogeneous condensation during WFGD processes. Fuel Process. Technol. 2016, 145, 116-122.

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[20] Bao, J. J.; Mao, L.; Zhang, Y. H.; Fang, H. M.; Shi, Y. J.; Yang, L. J.; Yang, H. M. Effect of Selective Catalytic Reduction System on Fine Particle Emission Characteristics. Energy & Fuels. 2016, 30(2), 1325-1334.

[21] Huang, R. T.; Shi, Y. J.; Yang, L. J.; Pan, D. P. Aerosol Formation Characteristics During Ammonia-based WFGD Processes. Energy & Fuels. 2016, 30(11): 9914–9921. [22] Chen, M. Q.; He, B. S.; Chen, G. H.; Fan, L. J.; Liu, S. M. Chemical kinetics based analyses on SO2 removal reactions by ammonia scrubbing. Acta Scientiae Circumstantiae. 2005, 25(7), 886-890. [23] Vance, J. L.; Peters, L. K. Aerosol Formation Resulting from the Reaction of Ammonia and Sulfur Dioxide. Industrial & Engineering Chemistry Fundamentals. 1976, 15(3), 202-206. [24] Edwin, M.; Hartley, J. R.; Michael, J. Sulfur Dioxide Reaction with Ammonia in Humid Air. Industrial & Engineering Chemistry Fundamentals. 1975, 14(1), 67-72.

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Table 1 Detailed information of stoker feed boiler Boiler

Working

Thermal

SR number rating

pressure

efficiency

Atmospheric CZML-0.12

Coal

Coal feeding

consumption

methods

22-24 kg/h

Self-sustained

Fuel

Coal ≥75%

0.12MW pressure

briquette

22


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Energy & Fuels

Fig.1 Schematic diagram of the experimental system Fig.2 Number concentrations of aerosol particles in flue gas at the outlet of the WFGD system (c0(SO2)=2850mg/m3, TG=110℃, TL=50℃, w[(NH4)2SO4] =10%, w[(NH4)2SO3]=10%, L/G=15L/Nm3) (a) Particle number concentration

(b) Particle size distributions

Fig.3 Overall production rate of aerosol particles as a function of desulfurization liquid pH (TL=50℃，TG=110℃，c0(SO2) =2850mg/m3，L/G=10L/Nm3) Fig.4 Overall production rate of aerosol particles as a function of liquid-to-gas ratio at different desulfurization liquid pH (TL=50℃，TG=110℃，c0(SO2) =2850mg/m3) (a) Aqueous ammonia

(b) Ammonium sulfate

(c) Ammonium sulfite

Fig.5 Effect of liquid-to-gas ratio on production rate of aerosol particles at different grades (TL=50℃，TG=110℃，c0(SO2) =2850mg/m3) (a) Aqueous ammonia, pH=10.0

(b)Ammonium sulfate, pH=7.0

(c)Ammonium sulfite, pH=8.0 Fig.6 Influence of desulfurization liquid temperature on overall production rate of aerosol particles (TG=110℃，c0(SO2) =2850mg/m3) (a) Aqueous ammonia, pH=10.0

(b) Ammonium sulfate, pH=7.0

(c) Ammonium sulfite, pH=8.0 Fig.7 Influence of SO2 concentration of the flue gas on formation of aerosol particles (TL=50℃，TG=110℃) (a) Aqueous ammonia

(b)Ammonium sulfate

(c)Ammonium sulfite

Fig.8 XRD analysis of fine particles in purified flue gas by ammonia-based WFGD 23


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Fig.9 Influence of SO3 concentration in flue gas on formation of aerosol particles (TL=50℃，TG=110℃，c0(SO2) =900mg/m3，pH=8.0，L/G=15L/Nm3)

24


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Energy & Fuels

Fig.1

25


Energy & Fuels

Fig.2 7

NH3·H2O

6x10

(NH4)2SO3

dN/dlogDp / 1.cm-3

7

5x10

(NH4)2SO4

7

4x10

7

3x10

B

7

2x10

A 7

C

1x10

0 0

100

200

300

400

500

t/s (a) 8

10

inlet flue gas NH3· H2O

7

10

(NH4)2SO3

-3

dN/dlogDp/1.cm

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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(NH4)2SO4

6

10

5

10

4

10

3

10

2

10 0.01

0.1

Dp / µm

1

(b)

26


10

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Fig. 3 1000

NH3· H2O Overall production rate / %

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

(NH4)2SO4

800

(NH4)2SO3

600 400 200 0 4

5

6

7

8

9

10

pH

27


11

Energy & Fuels

Fig. 4

Overall production rate / %

1000 800 600

pH=8.0 pH=9.0 pH=10.0 pH=11.0

400 200 0 4

8

12

16

20

Liquid-to-gas ratio / L· N m-3

(a)


200

pH=4.0 pH=5.0 pH=6.0 pH=7.0

150

100

50

0

-50

4

8

12

16

20


(b) 750


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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pH=5.0 pH=6.0 pH=7.0 pH=8.0

600 450 300 150 0 4

8

12

16


(c) 28


20

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Fig 5 3200

5.0L/Nm3 3 10.0L/Nm 15.0L/Nm3 20.0L/Nm3

Grade production rate / %

2800 2400 2000 1600 1200 800 400 0 0.01

0.1

Dp / µm

1

10

(a) 800

5.0L/Nm3 10.0L/Nm3 15.0L/Nm3 20.0L/Nm3


700 600 500 400 300 200 100 0 0.01

0.1

1

D p / µm

10

(b) 1400

3

5.0L/Nm 3 10.0L/Nm 3 15.0L/Nm 20.0L/Nm3

1200


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

1000 800 600 400 200 0 0.01

0.1

D p / µm

1

(c) 29


10

Energy & Fuels

Fig. 6 1000


TL=40℃ TL=50℃

800

TL=60℃

600

400

200

0

5

10

15

20


(a) 280


TL=40℃ 240

TL=50℃

200

TL=60℃

160 120 80 40 5

10

15

20


(b) 900

TL=40 ℃ Overall production rate / %

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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TL=50 ℃

750

TL=60 ℃ 600 450 300 150 5

10

15


(c) 30


20

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Fig. 7 7500


6000

pH=11.0: 3 SO2 2850mg/m

4500

SO2 1550mg/m

3

3000 1500

120

pH=9.0: 3 SO2 2850mg/m

80

SO2 1550mg/m

3

40 0

4

6

8

10

12

14

16

18

Liquid-to-gas ratio / L· N m

20

22

-3

(a)


600 450

pH=11.0: 3 SO2 2850mg/m SO2 1550mg/m

300

3

150 30

pH=9.0: 3 SO2 2850mg/m SO2 1550mg/m

20

3

10 4

6

8

10

12

14

16


18

20

22

-3

(b) 1000 800


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

pH=11.0: 3 SO2 2850mg/m SO2 1550mg/m

600

3

400 200 120 90

pH=9.0: 3 SO2 2850mg/m

60 30

SO2 1550mg/m 4

6

8

10

12

14

16


18 -3

(c) 31


20

3

22

Energy & Fuels

Fig. 8 18000 M

16000 14000 12000 10000

CPS

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 32 of 33

M

8000

M

6000

M M

4000

M MF

2000

M M

F M M

M

0 10

20

30

40

50

60

2θ ( ) °

M—(NH4)2SO4，F—Na6Fe2(CO3) 4(SO4)

32


70

80

Page 33 of 33

Fig. 9 7

8x10

7

7x10

A

B

D

C

E

F

G

7

dN/dlogDp / 1.cm-3

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

Energy & Fuels

6x10

7

5x10

7

4x10

7

3x10

7

2x10

7

1x10

0 15:42:00 15:45:20 15:48:40 15:52:00 15:55:20

Time t / s

33


Investigation on Formation Characteristics of Aerosol Particles

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