SO3 removal efficiency and ash particle flowability of low-low-temperature flue gas systems (LLTSs)

Applied Thermal Engineering 171 (2020) 115132

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SO3 removal efficiency and ash particle flowability of low-low-temperature flue gas systems (LLTSs) Ke Suna,b, Yu Yana, Jiahao Jianga, Lei Denga, Defu Chea, a b

T

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State Key Laboratory of Multiphase Flow in Power Engineering, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China Huadian Electric Power Research Institute Co., LTD., Hangzhou 310030, China

H I GH L IG H T S

optimum temperature for SO removal efficiency (T ) of LLTSs was proposed. • The adsorption process and mechanism of H SO mist on ash particles were studied. • The adsorption depends on the metal elements, especially Al elements. • Chemical effects of various factors on the SO removal efficiency were investigated. • The • At T , particle flowability gets worse and serious blockage problem will happen. 3

opt

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4

3

opt

A R T I C LE I N FO

A B S T R A C T

Keywords: Low-low temperature flue gas system SO3 Removal Ash blockage Adsorption Agglomeration Flowability

SO3 removal efficiency and ash particle flowability of low-low-temperature flue gas systems (LLTSs) were studied. The results indicate that when the temperature is approximately 40 °C lower than acid dew point (Td), the SO3 removal efficiency reaches a maximum. This optimum temperature is affected by particle physical characteristics and elemental composition. The SO3 removal efficiency increases as ash/sulfur ratio (D/S) increases. However, when temperature is low, the effect of D/S is weak. Particle size is also an important factor for the removal efficiency. The adsorption process is a synergistic effect of physical and chemical reactions. Chemical adsorption, which depends on the metal elements, especially Al elements, helps fix the sulfur element on particle surface and increase the stickiness of it. The agglomeration of ash particles mainly happens between smaller particles or between small particles and large particles. However, when temperature drops too much, large particles will agglomerate with each other due to the effect of H2O vapor. The particle flowability is greatly reduced after the adsorption process. When temperature is 20 °C lower than Td, the repose angle increased greatly. Hence, the temperature should be properly lowered in pursuit of high sulfur oxide removal efficiency and dust removal efficiency.

1. Introduction China’s energy and environmental issues have attracted increasing attention both at home and abroad [1–4]. On one hand, as the largest energy consumer in the world, China still relies heavily on fossil fuels, and this primary energy structure will not be changed in the coming decades [5–9]. On the other hand, the massive use of coal in power plants has caused serious environmental problems [10–13]. In 2018, coal consumption in the power industry reached 2.1 billion tons (standard coal), resulting in a large number of air pollutants, including fine particles and sulfur oxides [14]. These air pollutants may give rise to serious smog and hazy weather, which is harmful to human health

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[15–17]. Hence, pollutant emission control has become the focus of power plant retrofitting and construction. To achieve the goal of ultralow emissions, many emission control methods [18–20] have been proposed. Among these methods, low-lowtemperature flue gas systems (LLTSs) are widely used in the power plants in China [21]. This kind of system was first adopted by Mitsubishi Heavy Industries (MHI) and has been widely used in Japan. It has a high removal efficiency for multiple pollutants, such as SO3 and PM2.5, and can also utilize the recovered heat to increase power generation efficiency [22–26]. The core concept of LLTSs is to lower the flue gas temperature before it enters the dry electrostatic precipitator (ESP). When the temperature drops, the gaseous SO3 will be adsorbed

Corresponding author. E-mail address: [email protected] (D. Che).

https://doi.org/10.1016/j.applthermaleng.2020.115132 Received 25 October 2019; Received in revised form 26 January 2020; Accepted 25 February 2020 Available online 27 February 2020 1359-4311/ © 2020 Elsevier Ltd. All rights reserved.

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analyse the synergetic removal mechanism. He considered both the effects of temperature and coal applicability on the charging characteristics and removal efficiency of ash particles. However, the model built in this study simplified the condensation and adsorption process. Hence, the results cannot represent the actual situation of LLTSs in power plants. The study of Wu [46] focused on the removal of H2SO4 mist during the heterogeneous condensation process. This work analysed the effects of H2SO4 mist and H2O vapor and considered the simultaneous effects of LLTSs and wet flue gas desulfurization systems. In Gao’s study [47], the effects of flue gas temperature, acid content, water content and ash particle concentration on particle agglomeration properties were studied. In Wang's research [48], the coupling mechanism between ash deposition and corrosion was studied. While, Shi focused on the effect of ash deposition on heat transfer [49]. These research results were valuable, but the SO3 removal efficiency, the particle flowability and the parameter design problems were not considered. In this study, to find the optimal temperature for the heat exchanger, the SO3 removal efficiency and particle flowability characteristics of LLTSs were investigated using an entrained flow synergistic removal system in the lab. The adsorption and agglomeration mechanism were studied. The effects of temperature, particle constituents, particle size and flue gas residence time on SO3 removal efficiency and particle agglomeration characteristics were evaluated. The changes of particle surface morphology and agglomeration state after adsorbing H2SO4 mist were analysed by scanning electron microscopy (SEM). Furthermore, the repose angle was used to represent the flowability of ash particles, and it was analysed after adsorption and agglomeration. The results of this study have practical significance for the selection of the temperature as design parameter of heat exchangers and the alleviation of corrosion and blockage problems in LLTSs.

by ash particles in the form of H2SO4 droplets in the heat exchanger before the dry ESP. Hence, SO3 can be removed with ash particles at the same time in the ESP. Moreover, the decrease of the temperature can also increase the ash particle collecting efficiency in a dry ESP, since the agglomeration of particles occurs after the adsorption reaction, and ash particle resistivity also increases with the lowered gas temperature. For LLTSs, the temperature is a key design parameter for the system efficiency and security. In Japan, the coal is imported selectively, uniform in properties and especially low in sulfur content. Therefore, the system temperature parameter can be maintained at 90 °C [23]. However, the operation of these systems in China is not as effective as it is in Japan. In China, the coal types used in the utility boilers are often from different mines and differ greatly, which makes the sulfur contents of the coal changeable [27,28]. As a result, thermal power units may burn various high-sulfur coal. In such situations, the operating temperature cannot simply be fixed at a certain value, because the choice of an inappropriate temperature parameter may cause the corrosion and blockage of the heat exchanger and ash hoppers of ESP. However, there are no complete temperature parameter design principles for the coal conditions in China. If flue gas temperature drops to much, although the SO3 removal efficiency can be maintained at a high level, the superfluous condensation of H2SO4 mist will cause the corrosion and blockage problems. The blockage problem is related to the reduction of ash particle flowability, which is caused by the adsorption of H2SO4 mist and the agglomeration of particles. Otherwise, if the selected temperature is high, although the above problems can be avoided, the SO3 removal efficiency will decrease. Therefore, to find out whether there is an optimal temperature, and determine it, the effects of temperature on SO3 removal efficiency and particle flowability in LLTSs should be studied. Unfortunately, most of the studies about corrosion and ash deposition focused on the temperature range above 400 °C [29–31]. Less attention has been paid on SO3 removal efficiency and the ash particle flowability in LLTSs. Among the existing studies on LLTSs, the ash blockage problems and the optimization measures of matching performance in ash hopper area were studied by Gao et al. [32]. The ESP and flue gas duct design problems were investigated by Guo [33] and Ye [34] using a numerical simulation method. Some suggestions regarding the parameter design problems were put forward by these studies, but the effects of temperature parameter were not taken into consideration. The abrasion mechanism of the heat exchanger in LLTSs was studied, and corresponding improvement measures were suggested in Wen's research [35]. Zhu et al. [36] analysed the consequences of heat exchanger leakage in LLTSs and tried to determine the reasons from the perspective of design, manufacture and installation. Although the above studies all focused on the problems that occurred during the operation of LLTSs and corresponding suggestions were put forward, the root causes of these problems were ignored. The root causes are the incomplete adsorption of H2SO4 mist and the excessive agglomeration of ash particles, which are due to the improper temperature design of the heat exchanger. The simultaneous removal mechanism of PM and SOx within external fields was studied by Yang and Zheng [37–40]. In their studies, a simultaneous removal experimental system of pollutants was constructed, and the formation process of H2SO4 mist was investigated from the aspects of particle collision and agglomeration. The research methods and results were meaningful. However, these studies were mainly aimed at wet electrostatic precipitator systems, and the results were not applicable for LLTSs, since the temperature ranges of these two systems were different. Chen and Li studied the fouling and corrosion characteristics of the heat exchanger by field sampling [41–44]. After analyzing the deposit samples, the fouling and corrosion mechanisms was discussed, the agglomeration process was summarized. But the temperature effects were neglected and the ash sample was specific, which made the universality of the conclusions insufficient. Zhang [45] established an interaction model of SO3 and ash particles to

2. Experimental materials and methods 2.1. Ash samples and selection of the experimental temperature One of the factors affecting the removal efficiency of SO3 is the chemical composition of ash particles. In this study, 3 types of coal ash were used for the experiments. The 3 types of coal were from Jingyuan (JY), Lingwu (LW) and Tongzi (TZ). The corresponding coal ash samples were taken from different coal-fired power plants that have finished an LLTS retrofit. The ash sampling points were at the inlet and outlet of the low-temperature heat exchanger. The flue gas temperature at the inlet was above the acid dew point (Td). Hence, the ash collected from the inlet was considered the original sample without an adsorption reaction with H2SO4 mist. Correspondingly, the ash samples from the outlet were the ash after the adsorption reaction. To guarantee that the samples were uncontaminated, all samples were stored in Ziploc bags for transportation. The ash samples were dried in a drying oven before the SO3 removal experiments. To investigate the effect of ash particle size on the SO3 removal efficiency and particle agglomeration, the ash particles were sieved to different sizes (< 68, 68–75, 75–91, and 91–125 μm). In this study, three parts of experiments were carried out: the entrained flow SO3 removal experiments, the particle agglomeration experiments and the repose angle experiments. The inlet ash samples were used in all the three parts of experiments to simulate the adsorption and agglomeration process in practical LLTSs. The outlet ash samples were used in agglomeration and repose angle experiments as the comparison groups. To determine the effects of the chemical elements of ash particles, the chemical compositions of these 3 kinds of inlet ash samples were analysed before the experiments using X-ray fluorescence (XRF) (Bruker, GER, Model S4 Pioneer). The results are shown in Fig. 1. The main metal elements contained in these 3 kinds of ash samples were K, Na, Ca, Mg, Al and Fe. Overall, the metal element content in the TZ ash sample was the highest, while the metal element content in the JY ash 2

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generate precisely under lab conditions. In previous studies, SO3 could be generated by catalytic oxidation, ozone oxidization, pyrolysis of H2SO4 or oleum [53–57]. Before the experiment, the first three methods were chosen and tested. The results showed that the H2SO4 pyrolysis method is more accurate and stable. As the figure shows, a 20% H2SO4 solution was filled in a glass syringe, which was fixed onto the microinjection pump. The H2SO4 solution was compounded by ultra-pure water and H2SO4 (98.3%, Gao Kexiang Inc., CHN). The pump speed was set to 0.15 mL min−1. To achieve continuous injection, the 8# spinal needle (YY/T1148-2009) was used and a silica sand core was placed in the inlet of the silica tube. When the H2SO4 droplet came out of the needle, it infiltrated in the silica sand core. Then it was evaporated and carried into the silica tube by the N2. Before the experiment, the furnace was heated to 550 °C. At this temperature, the H2SO4 solution would pyrolysis into gaseous SO3 and H2O vapor. The ash feeding device was designed and fabricated by our team. This was a kind of entrained flow reactor in which the flue gas blew over the surface of the ash layer and carried the ash particles into the pipeline. Using this device, the ash particles can be fed into the reaction tube continuously and precisely at a small feeding speed. The removal process of SO3 took place in the adsorption reaction tube, which was wrapped with electrical heating belts and insulated cotton. The reaction temperature (90–160 °C) was controlled by a thermocouple on the top of the tube. When the two paths of flue gas blew into the reaction tube, the SO3 vapor and the ash particles mixed together. Then, the cooling process began, and the H2SO4 mist was formed in the flue gas. After that, the H2SO4 droplets were adsorbed by the coal ash particles, and the SO3 was removed in this way. At the end of the experimental system, a sedimentation chamber was connected to the adsorption reaction tube. When the mixed gas flowed through the chamber, the ash particles could be collected on the glass plate. In addition, the mixed flue gas was treated by the saturated NaOH solution (Kemiou Chemical Reagent Co., CHN.) before entering the atmosphere. To obtain the SO3 adsorption amount, the sulfur content of the ash samples was tested by an element analyser (Elementar, GER, vario MACRO cube) and XRF before and after the experiment. By comparing the test results, the amount of adsorbed SO3 can be calculated as the SO3 removal efficiency. After the experiment, to study the agglomeration and the surface morphology of the ash particles, the ash samples were observed by scanning electron microscopy, and the element distribution of ash particles was analysed by X-ray energy spectroscopy (SEM-EDS) (Shimadzu, JPN, SSX-550). Moreover, to study the flowability of ash particles after the adsorption reaction, a repose angle tester was built in the lab. This test device consists of a funnel and an ash deposition platform. The ash sample was dropped from the funnel to the platform to form an ash pile. Then the ash pile was photographed and the pictures were analysed by the CAD software on a computer to determine the repose angle.

Fig. 1. Chemical composition of the inlet particle samples.

sample was the lowest. For each kind of ash sample, the element content of Al was the highest, followed by the Fe content. The Mg content was the lowest in all three kinds of ash samples. It is noteworthy that for different kinds of ash samples, the content of Al and Fe in the TZ ash sample was much higher than that in the other two kinds of ash samples. The content of Ca and Na in the LW ash sample was slightly higher than that in the other two kinds of ash samples. For pulverized-coal-fired boilers and wet-bottom boilers, the flue gas Td typically ranges between 120 and 150 °C according to the different sulfur contents of the coal [50–52]. In common LLTS applications in China's coal-fired power plants, the system temperature of the heat exchanger is often designated at 90–100 °C, which is 30–50 °C lower than the Td. In the experimental system of this study, the flue gas Td is 178 °C, as calculated by the Müller prediction model. The Müller model is the basis for predicting the sulfuric acid dew point temperature, and it shows the relationship between the temperature of sulfuric acid dew point and the partial pressure of sulfur trioxide vapor in flue gas. To correspond to a practical situation of lower temperatures, the experimental temperature range was selected as 90–160 °C, which is 20–90 °C lower than the experimental Td. It should be noted that since the practical flue gas Td is changeable for different types of coal and combustion situations, in this study, the focus is on the value of the difference between the experimental temperature and the experimental Td. When the selected concentration of SO3 in the flue gas is too small in this experiment, the stability and accuracy of the SO3 generation system will be affected. Hence, the flue gas Td of this experiment is designed to be 178 °C. Although this Td is higher than the flue gas Td in practice, the accuracy of the experiment can be guaranteed, as the Td is not the focus of the study.

2.3. The calibration procedure 2.2. Experimental system To ensure the feasibility and accuracy of the entrained flow synergistic removal system, several calibration experiments were taken to calibrate the experimental system. First, the stability and precision of the SO3 generation method were tested. To collect the condensed SO3, the manually controlled condensation method (CCM) [58] was chosen for its accuracy. The barium chromate spectrophotometry method was used to analyse the SO3 content. The SO3 contents of the flue gas are listed in Table 1 while the flue gas flow rate was 6 L min−1 and the pump injection speed was set to 0.1, 0.125 and 0.15 mL min−1. The calibration experiments were repeated three times. As shown in Table 1, for each injection speed, the test results of the three measurements were almost the same. The difference between each measurement was within 3% which was within the required experimental error (5%). Hence, the SO3 generation method is accurate and stable. However, when injection speed is low, the droplets from the needle of

In this study, the SO3 removal efficiency experiment was conducted on an entrained flow synergistic removal system, which was designed to simulate the cooling process of flue gas in a heat exchanger. As shown in Fig. 2, this system can be divided into five parts: the preheating of N2, the generation of SO3, the coal ash feeding device, the removal process of SO3 and the collection of ash particles. In the gas preheating part, an electric-resistance furnace (Y-feng Inc., CHN) was set to 250 °C to heat two N2 streams. One was used to carry the H2SO4 vapor (2 L min−1), while the other one was used to carry ash particles (4 L min−1). This ensures that the flue gas temperature is high enough before it enters the adsorption reaction tube; thus, the flue gas cooling process can take place. The SO3 generation part consisted of a micro-injection pump (LSP01-3A, Halma, Britain) and an electric-resistance furnace with a silica tube. SO3 is difficult to 3

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Fig. 2. The entrained flow synergistic removal system. Table 1 Test results of SO3 content of the flue gas (at a gas flow rate of 6 L min−1). Sulfuric acid injection speed (mL min−1) 0.1 0.125 0.15

SO3 content (μL L−1) Measurement 1

Measurement 2

Measurement 3

Mean value

379.41 579.73 692.51

389.16 576.33 709.46

385.15 585.08 686.93

384.57 580.38 696.30

the syringe are discontinuous. To ensure the continuity of SO3 generation, the H2SO4 injection speed was chosen to be 0.15 mL min−1. In addition, the mean value in the table was used to calculate the SO3 removal efficiency in this study. To determine the relationship between the stepper motor rotational speed and the feeding speed of different kinds of ash particles, calibration experiments of the ash feeding device were carried out. The ash feeding speeds of different particle sizes and different coal ash types are shown in the following figures. As shown in Fig. 3, the ash feeding speed was linear with the stepper motor rotational speed, and the ash feeding speed was stable. It can also be seen that at the same rotational speed, the feeding speed of particles with larger sizes was relatively slower. In Fig. 4, when the particle size was 91–125 μm, the ash feeding speeds of different types of coal ash were slightly different from each other, but they were all linear with the rotational speed and had good stability. For LLTSs, the ash/sulfur ratio (D/S), i.e., the ratio of the ash particle concentration (mg m−3) to the SO3 concentration (mg m−3) (calculated in the form of H2SO4), is a key parameter for SO3 removal efficiency. In this experiment, a different D/S can be obtained by changing the ash feeding speed, which can be controlled by selecting the corresponding motor rotational speed from the curve in Figs. 3 and 4.

Fig. 3. Test results of JY coal ash sample.

important influencing factors for SO3 removal efficiency, as well as corrosion and blockage problems in LLTSs. Experiments to investigate the effect of the operating temperature were carried out, and the results are shown in Fig. 5. The experimental ash samples were JY coal ash and the gas flow rate was 6 L min−1. At the same time, the changes of D/S were also taken into consideration. The experimental results indicated that the SO3 removal efficiency had an optimal value with the change of temperature. Although the SO3 removal efficiencies of different D/Ss were different, the trends were similar. The highest removal efficiency value appeared at approximately 140 °C. When the temperature of the adsorption reaction tube dropped to 90 °C, the SO3 removal efficiency dropped to the lowest value, which was no more than 10%. When the temperature was changed from 140 to 160 °C, the removal efficiency also dropped significantly to below 20% for all three kinds of D/S. As mentioned above, the flue gas Td was approximately 178 °C. Thus, the optimal SO3 removal efficiency appeared when the temperature was approximately 40 °C lower than the flue gas Td. The flue gas cooling process from the top of the adsorption reaction tube to the bottom was a continuous process. When the set temperature

3. Results and discussion 3.1. SO3 removal efficiency 3.1.1. The effect of the temperature parameter The temperature parameter of the heat exchanger is one of the most 4

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a cluster formation appeared, leading to capillary condensation in the micro-pores, leading to the drop of the saturated vapor pressure. Under such circumstance, the condensation of sulphuric acid vapor in the micro-pores was further prompted. As the gas temperature continued to drop, H2SO4 droplets were formed in the mixed gas. The collision of particles and H2SO4 droplets accelerated the physical adsorption rate. As the adsorption reaction proceeded, an H2SO4 liquid film appeared and covered the surface of the ash particles. Then, chemical adsorption occurred at the interface of the particle surface and acid liquid film. At this stage, chemical adsorption began and proceeded with physical adsorption simultaneously. The adsorption reactions between the particle compounds and H2SO4 helped fix the adsorbed S elements; hence, the SO3 could be removed with ash particles. After the chemical reaction, the pores and gullies on the surface of ash particles deepened. That made the specific area of the ash particles larger, leading to increased physical adsorption. The condensation and adsorption of H2SO4 mist during the cooling process can be summarized in Fig. 6. Since the flue gas flowed through the reaction tube over a short time period, the cooling rate significantly affected the physical adsorption. If the temperature of the reaction tube is high, such as 150 or 160 °C, the amount of formed H2SO4 droplets is small. Hence, the liquid film cannot be formed on particle surfaces. That is, the second stage of the physical adsorption is insufficient. Therefore, when the temperature is high, the adsorbed H2SO4 is reduced and the SO3 removal efficiency is lowered. Moreover, when reaction temperature is higher, the reaction rate is faster. However, it can be seen that the increase of the chemical reaction rate is not enough to improve the overall adsorption rate at high temperatures. Therefore, the adsorption process is mainly affected by physical adsorption. The effect of chemical adsorption is mainly to fix the S element on the particle surface, making it difficult to desorb from the ash particles. When the temperature continues to drop, the concentration of H2SO4 increases greatly. This may lead to condensation of the acid droplets on the tube surface and collisions between the droplets and the tube surface. Under these circumstances, some H2SO4 droplets will adhere to the tube surface. Hence, the SO3 removal efficiency is reduced. In practical applications, this can also cause corrosion problems in the heat exchanger.

Fig. 4. Test results of ash samples of 91–125 μm.

3.1.2. The effect of D/S From Fig. 5, different D/Ss led to different SO3 removal efficiencies. If the ash particle content is relatively higher than SO3, then most H2SO4 mist will condense on the ash particles rather than the pipe surface of the heat exchanger. Thus, the D/S has great significance not only for SO3 removal but also for corrosion prevention. To further study the effect of the D/S, the following experiments were carried out. The ash samples were from JY coal ash, the gas flow rate was 6 L min−1, the particle size was 91–125 μm, the D/S was selected to be 20–120, and three different temperatures were taken into consideration for comparison. The experimental results are shown in Fig. 7. As shown in the figure, the SO3 removal efficiency increased as the D/S changed from 20 to 120 at the higher investigated temperatures. When the D/S was 20, the removal efficiency of the three temperatures was all lower than 20%. Most H2SO4 mist condensed on the surface of the reaction tube under this scenario. The effect of the D/S was greatly affected by the temperature. When the temperature was low, such as 90 °C, the D/S had almost no influence on SO3 removal efficiency. This means that the temperature parameter is the decisive factor for the LLTSs. Thus, in the practical application of LLTSs, even for low-sulfur coal, the temperature of the heat exchanger should not be set to too low. However, when the temperature was high, the effect of D/S was significant. At 130 °C, the SO3 removal efficiency increased proportionally with the D/S. When the D/S changed from 20 to 120, the removal efficiency increased by four times. The curves also showed that the removal efficiency changing rate was higher when the D/S was low, and it dropped when the D/S was above 100. It can be inferred that, at high temperatures, when the D/S is large, physical adsorption tends to

Fig. 5. The experimental results of the effect of the adsorption temperature for JY coal ash sample (experiments performed at a gas flow rate of 6 L/min).

of the reaction tube was low, such as 90 °C, the flue gas temperature dropped sharply. In contrast, when the set temperature was high, the cooling process was moderate. The cooling rate of the flue gas affected the condensation of the H2SO4 mist. Meanwhile, the removal of SO3 was achieved by the adsorption of H2SO4 mist on the ash particles. The adsorption process of the H2SO4 mist included physical adsorption and chemical adsorption. Physical adsorption began before the temperature decreased below the Td. The first stage of physical adsorption occurs in the micro-pores on the particle surfaces. According to the research of Roedel [59], Jaecker-Voirol and Mirabel [60], Kulmala and Laaksonen [61], and Vehkamäki [62] et al., when H2O vapor and H2SO4 vapor coexist in the gas stream, the condensation of the vapor is due to the binary nucleation of H2O-H2SO4. H2SO4 is easily combined with H2O molecules to become hydrates in aqueous gas. Therefore, under this condition, H2SO4 exists in the form of H2SO4·H2O, H2SO4·2H2O, H2SO4·3H2O, H2SO4·hH2O, and the following reaction will occur: H2O + H2SO4·(h-1)H2O → H2SO4·hH2O

(1)

As the temperature decreased to near Td, H2SO4·hH2O molecules appeared in the flue gas and were adsorbed on the micro-pores of ash particles. Then, with the effect of adsorbate-adsorbate interaction [63],

5

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Fig. 6. Illustration of the condensation and adsorption process of H2SO4 mist.

also an important influencing factor for SO3 removal efficiency. For China's power plants, the coal type and combustion mode are multifarious. The particle size distributions of coal ash are related to the combustion modes and coal types [64–66]. Different ash particle sizes may lead to different adsorption characteristics for the H2SO4 mist. The agglomeration between ash particles after adsorption may also be affected by the particle size distribution. Therefore, the effect of particle size is needed to be investigated. In this way, the adsorption characteristics can be explained more clearly if the ash contains larger or smaller particles. In this part of the experiments, three ranges of particle size were studied (68–75, 75–91, 91–125 μm). The ash sample was JY coal ash, the gas flow rate was 6 L min−1, and the temperature was selected as 90 °C to make the contrast of the curves more obvious. The experimental results are shown in Fig. 8. As the figure shows, ash particle size had a significant effect on the removal efficiency. In general, the smaller the ash particle is, the higher the SO3 removal efficiency. When particle size increased, the effect of

Fig. 7. The experimental results of the effect of D/S for JY coal ash sample for ash particles of 91–125 μm (experiments performed at a gas flow rate of 6 L/ min).

be saturated, and chemical adsorption becomes the main factor of SO3 removal efficiency. In this situation, a relatively high temperature is more conducive to the removal of SO3. In addition, the removal efficiencies of the three situations were all lower than 65%. On the one hand, the particle size of the experimental ash sample was 91–125 μm, which caused the removal efficiency to be lower. The effect of ash particle size will be discussed in the following section. On the other hand, it should be pointed out that, as a simulation experiment in the laboratory, the adsorption reaction tube is thinner and shorter than in a practical heat exchanger. This makes the experimental SO3 removal efficiency relatively lower than in a practical situation. However, the conclusions and trends obtained from the experiments are valuable and can provide suggestions for the parameter design of practical LLTSs. Fig. 8. The experimental results of the effect of particle size for the JY coal ash sample (experiments performed at 90 °C and at a gas flow rate of 6 L/min).

3.1.3. The effect of coal ash particle size Except for the adsorption temperature and D/S, ash particle size is 6

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Table 2 The specific surface area of different particle sizes of JY coal ash sample. 68–75 μm 2

−1

Specific surface area/m g BJH adsorbed pore area/m2 g−1

7.68 4.344

75–91 μm 6.51 3.967

Table 3 Test results of specific surface area.

91–125 μm 2

6.49 1.427

−1

Specific surface area/m g BJH adsorbed pore area/m2 g−1

JY coal ash

LW coal ash

TZ coal ash

6.49 2.427

6.14 2.540

4.17 2.221

LLTSs does not drop too low, SO3 removal efficiency can be improved by increasing the heat exchanger size. However, the effect of this measure is not as good as the control of other factors. In addition, in practical LLTS retrofitting applications, the available space for the heat exchanger in power plants is limited. Therefore, this method is not feasible for LLTS design.

the D/S on the removal efficiency weakened. When the D/S was 120, the efficiency was no more than 10% for particles with a size of 91–125 μm. Meanwhile, under the same D/S conditions, the 68–75 μm ash particles achieved a removal efficiency of 70%. Therefore, the adsorption capacity of small ash particles is far greater than that of large particles. After the experiment, the specific surface areas of the ash particle samples were analysed. As shown in Table 2, the specific surface area of 68–75 μm particles was larger than that of 91–125 μm particles. From the test results of the BJH (acronym for the three scientists: Barret, Joyner and Halenda) adsorbed pore area, it can be observed that the smaller particles had more adsorbed pores. This increased the physical adsorption capacity of ash particles for H2SO4 mist. More physical adsorption caused more chemical adsorption, which in turn further prompted physical adsorption. Hence, even at a low temperature of 90 °C, the SO3 removal efficiency could still reach more than 70% when the D/S was 120. Furthermore, for smaller ash particles, the liquid film covering the surface of the ash particles was more easily formed. The liquid film may help increase the adhesion ability of the particles, allowing them to more easily adsorb the acid droplets and agglomerate with each other. Therefore, the influencing effect of ash particle size needs to be considered in the parameter design process of heat exchangers in LLTSs. 3.1.4. The effect of flue gas residence time Flue gas residence time is mainly determined by the size of the heat exchanger in the LLTSs. It also has an effect on the removal of SO3. In this experiment, flue gas residence time was controlled by changing the gas flow rate. The selected three gas flow rates were 6, 8 and 10 L min−1. The ash sample was JY coal ash, the D/S was 80 and the particle size was 91–125 μm. The results are shown in Fig. 9. The flue gas residence time increased with decreasing gas flow rate. As the residence time increased, the SO3 removal efficiency increased. However, when the adsorption temperature was low, the influence of flue gas residence time was not apparent. When the temperature was high, the increase in SO3 removal efficiency was proportional to the increase in residence time. For engineering applications, if the temperature of the

3.1.5. The effect of ash particle constituents The removal of SO3 relies on the adsorption of H2SO4 mist by ash particles. The adsorption process is a synergistic effect of physical and chemical adsorption. Thus, the constituents of ash particles have an effect on SO3 removal efficiency by influencing the chemical adsorption rate. Different power plants use different types of coal. Hence, the ash particle constituents should be analysed before the parameter design of LLTSs. In this part of the experiment, three types of coal ash were selected. The test results of the chemical composition of these ash samples are listed before (see Fig. 1). As shown in Table 3, the specific surface area of these ash samples (91–125 μm) was tested. The experimental temperature was set to 130 °C, and the particle size range was 91–125 μm. Fig. 10 shows the effect of particle constituents on SO3 removal efficiency. As shown in Table 3, the specific surface area of JY coal ash and LW coal ash were close, while the specific surface area of TZ coal ash was relatively smaller. The adsorbed pore areas of these three ash samples were almost the same. Hence, it can be inferred that the physical adsorption capacity of TZ coal ash was slightly weaker among these ash samples. However, the difference between them was not remarkable. From Fig. 10, it is obvious that the SO3 removal efficiency of these ash samples was different. The differences between the efficiency of TZ and LW coal ash were from 0.64% to 3.96%, the mean efficiency difference was 2.53%. For the coal ash LW and JY, the efficiency differences were between 0.42% and 3.82 %, the mean efficiency difference was 2.533%. The removal efficiency of TZ coal ash was the highest, while that of JY coal ash was the lowest. This may be caused by the different chemical adsorption capacities of these coal ash samples. As discussed above, the metal element content in the TZ ash sample is the highest, while the

Fig. 9. The experimental results of the effect of gas residence time for the JY coal ash sample (D/S = 80, ash particles size = 91–125 μm).

Fig. 10. The experimental results of the effect of particle constituents at 130 °C, ash particles size 91–125 μm. 7

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blockages in the ash hopper. Fig. 12(a) and (b) show the SEM results of the JY ash samples at the inlet and outlet of the heat exchanger. Obviously, the ash particles from the inlet were independent of each other, while the outlet ash particles agglomerated together to a large degree. The surfaces of outlet ash particles were relatively rough compared with the inlet particles. This is because of the chemical adsorption on the surface. The agglomeration mainly happened between smaller particles (particle size smaller than 75 μm) or between small particles and large particles (particle size larger than 75 μm). The agglomeration between large particles was not notable. To further study the relation between the adsorption and agglomeration process of ash particles, EDS tests were performed based on SEM tests. As shown in Fig. 13 and Table 4, the S contents of points 2, 3 and 4 were very high. Among them, test points 2 and 3 were at the surface of the smaller particles that adhered on the large particle. Test point 4 was at the interface of two particles. This indicates that after passing through the heat exchanger, a part of S was fixed on the ash particles. Because the desorption of the physical adsorption occurs easily, the S element was certainly fixed by chemical adsorption. Moreover, the liquid film and the chemical adsorption reaction increased the adhesivity of the particles, causing them to agglomerate together. Fig. 13 shows that the surfaces of the large particles were roughly smooth. Test point 6 was at the rough part, while points 1 and 5 were at the smooth part. On the other hand, the S content of point 6 was much higher than that of points 1 or 5. Hence, the chemical adsorption reaction increased the roughness of the ash particle surface, which further prompted physical adsorption. Fig. 14 shows the experimental results at 140 °C for JY coal ash samples. When the particle size was smaller than 68 μm, the agglomeration degree was very strong. In this situation, almost all individual ash particles could be distinguished. This kind of agglomeration was helpful for the removal of fine particles. Particles of 68–75 μm also agglomerated with each other tightly, but some larger particles were independent of other clustered particles. In the 75–91 μm particles, there were small particles adhering to large ones. However, the agglomeration of large particles was not obvious. When particle size is 91–125 μm, almost no agglomeration occurred. Hence, at high temperatures, agglomeration only happens between smaller particles or between small particles and large particles. Fig. 15 shows the agglomeration situation of 91–125 μm particles at 90 °C for JY coal ash samples. As the figure shows, although the particle size is 91–125 μm, agglomeration occurred between large particles. This was because under low temperature, in addition to H2SO4 mist, H2O vapor was also condensed in large amounts. The wet vapor greatly increased the adhesivity of particles. Under this condition, the SO3 removal efficiency dropped, and corrosion, ash deposition and blockage problems happened in the heat exchanger and ash hopper. Therefore, it is not advisable to blindly reduce the temperature parameter of the heat

metal element content in the JY ash sample is the lowest. During the adsorption process, when the temperature dropped to below Td, the H2SO4 acid liquid film appeared on the surface of ash particle. Then, H+, H3O+ and SO42− were ionized in the liquid film. On the other hand, during the combustion process, the minerals in coal transform into different oxidation products [67–69] in ash particles. Specifically, kaolinite (Al2O3⋅2SiO2⋅2H2O) changes to silicon spinel (3Al2O3⋅3SiO2, 925–1000 °C) and mullite (3Al2O3⋅2SiO2, greater than1000 °C) at different temperatures. In addition, corundum (Al2O3) also occurs in fly ash [66]. The iron-containing minerals are oxidized to Fe3O4, Fe2O3 and FeO. The calcium-containing minerals, such as calcite (CaCO3) and dolomite (CaCO3⋅MgCO3), react to form CaO. In addition, there are also small amounts of MgO, Na2O and K2O formed in the fly ash. To further illustrate the occurrence modes (chemical forms) of the metal compounds in ash particles, we carried out a chemical fractionation experiment. In Sugawara's study [70], a serial dissolution method was used to categorize the metal elements. The extraction solvents in that study were H2O, CH3COONH4, HCl, H2SO4, and HF solutions. Correspondingly, the metal elements were categorized to be “H2O soluble”, “CH3COONH4 soluble” and so on. This method is always chosen to study the metal occurrence forms in coal [71–74]. Using this method, the experimental JY ash samples were leached with H2O, CH3COONH4 and H2SO4 solutions to quantify the metal compounds in terms of “H2O soluble”, “CH3COONH4 soluble”, “H2SO4 soluble”, and “insoluble form”. The inductively coupled plasma mass spectrometry (ICP-MS) test results are shown in the following figures. As shown in Fig. 11, for each metal element, the proportion of “H2SO4 soluble” was relatively large, especially for the elements Al, Mg and Ca. Hence, the oxide compounds of ash particles can react with the H2SO4 solution. After the reaction, Al2(SO4)3, CaSO4, Fe2(SO4)3, FeSO4 and MgSO4 were formed. From Figs. 10 and 11, it can be inferred that within these metal elements, Al was the main factor of chemical adsorption. This was the most abundant metal element in the ash particles, and the content of the “H2SO4 soluble” form of the Al compound was very high. Therefore, TZ coal ash had the highest SO3 removal efficiency among the three types of coal ash samples. It is important to analyse the chemical composition of coal ash during the parameter design process of LLTSs. 3.2. The agglomeration and flowability of ash particles 3.2.1. The agglomeration of ash particles The adsorption of H2SO4 mist can help remove SO3 from flue gas. However, during the adsorption and cooling process, the liquid film over the ash particle surface makes it easier for ash particles to agglomerate together. On the one hand, agglomeration is propitious to the removal of fine particles in a dry ESP. On the other hand, excessive agglomeration may cause ash deposition in the heat exchanger and

Fig. 11. The occurrence modes of metal compounds in JY coal ash sample. 8

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Fig. 12. The SEM results of the JY ash samples.

repose angles were different. For the < 68 μm ash samples, the repose angle of the particles was 56.32°. With the increase of particle size, the repose angle gradually reduced. When the size increased to 91–125 μm, the repose angle changed to 51.65°. Hence, the flowability got poorer when the particle size changed smaller. There are several reasons for this phenomenon. First, the specific area of the ash particles is inversely proportional to the particle size. As the particle size reduces, the electrostatic attraction and intermolecular force between ash particles relatively increase. Hence, the flowability of particles get poorer. In addition, for ash particles with smaller particle sizes, the tendency to adsorb and agglomerate between particles is bigger. The adhesion of particles increases, causing the decreasing of the particle flowability. Moreover, when particle size is small, the accumulation layer of the particles is tighter, the air permeability decreases. Thus the repose angle increases relatively. The flue gas temperature at the outlet of the heat exchanger was lower than Td. Hence, the ash samples taken from the outlet had gone through the adsorption process. The repose angles of different particle sizes of outlet ash samples are shown in Fig. 17. It can be seen from the figures that the ash particles were obviously agglomerated with each other. The adhesion of the ash particles increased after the adsorption reaction. The repose angle of outlet ash samples also decreased with increasing particle size. For the 91–125 μm ash samples, the repose angle was 57.37°. And it increased to 72.10°, when the particle size decreased to < 68 μm. The repose angles of the outlet ash samples were much larger than that of inlet ash samples, and the top of each outlet ash heap was sharper than the corresponding inlet ash heap. Hence, after adsorbing H2SO4 mist and agglomeration with each other, the flowability of ash particles reduced greatly. It is because during the adsorption process, the acid liquid film covered on the surface of ash particles enhanced the adhesion of the particles, making them adhere tighter after the collision with each other. And the shapes of the agglomerated particles were more irregular, letting them easier to accumulate. Besides, the changes in the surface and shape of the particles enhanced their friction between them. That also made the flowability of particles decreased. Due to the large decrease of ash particle flowability, the ash deposition and blockage problems appeared in the heat exchanger and ash hopper of ESP. To deal with these problems, the large-scale agglomeration between ash particles should be avoided, which makes the temperature selection particularly critical for the system design process.

Fig. 13. The EDS results of outlet JY coal ash samples. Table 4 Element content in different positions, as indicated in Fig. 13. Test point

Point 1

Point 2

Point 3

Point 4

Point 5

Point 6

Mg Al Si S K Ca Fe

0.79 3.88 7.68 0.26 0.32 0.72 86.21

1.26 27.98 37.24 9.75 3.66 10.11 7.58

1.50 5.62 7.77 27.05 0.89 32.80 15.95

0.88 5.80 9.03 15.43 1.25 19.14 14.40

0.54 14.95 71.18 0.61 4.70 1.55 1.93

1.44 4.14 5.52 1.70 0.47 2.44 83.18

exchanger. 3.2.2. The flowability of the ash samples The flowability of ash particles is related to the ash deposition and blockage problems in heat exchangers and ash hoppers. It can be represented by the repose angle. The repose angle of the ash particles is the acute angle between the surface of the ash pile and the horizontal surface. At this angle, the particles of the ash pile will remain in place without sliding. It is related to the surface area and the shapes of ash particles. It is also affected by the coefficient of friction of ash particles, which means the adhesion of particles. In this study, the repose angles of different ash samples were studied. First, the ash samples at the heat exchanger inlet and outlet were tested. The flue gas temperature in this positon of the heat exchanger inlet was higher than Td. Hence, the adsorption reaction didn’t take place for the particles. The test results are shown in Fig. 16. As shown in Fig. 16, the inlet ash samples were dry and loose for all the particle sizes. And no obvious agglomeration between particles was appeared. The repose angles of the ash particle samples of four particle sizes were between 51.6° and 56.3°. For different particle sizes, the

3.2.3. The effect of flue gas temperature on ash particle flowability After the repose angle tests of the inlet and outlet ash samples, the experiments on the influence of the temperature on the particle flowability were carried out on the entrained flow synergistic removal system. In the experiments, the inlet ash samples of JY coal were used as the original samples without adsorption reactions. After the experiments, the ash particles were collected and the repose angle was tested. Since if lower than 140 °C, the SO3 removal efficiency drops and the 9

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Fig. 14. The agglomeration situation of experimental JY coal ash particles at 140 °C.

Fig. 15. The agglomeration situation of 91–125 μm particles at 90 °C for the JY coal ash sample. Fig. 16. The test results of the repose angle of inlet JY coal ash samples.

Fig. 17. The test results of the repose angle of outlet JY coal ash samples.

very poor. Hence, although at 140 °C the SO3 removal efficiency reaches a maximum, this temperature point should not be chosen for preventing the serious ash deposition and blockage problems. Moreover, as shown in Fig. 18, the repose angle of smaller particles is much larger than that of the large particles. And for the smaller particles, the influence of temperature on the flowability is also larger. Hence, during

agglomeration situation also gets worse, the adsorption temperatures were chosen to be 140–180 °C. Fig. 18 shows the results of the experiments. As illustrated in the figure, with the decreasing temperature, the repose angle of ash particles increased. And when the temperature decreased below 160 °C (20 °C lower than Td), the repose angle increased greatly, which means the flowability of ash particles becomes 10

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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work has been financially supported by the National Key R&D Program of China (2017YFB0602102). References [1] F. Ren, L. Xia, Analysis of China’s primary energy structure and emissions reduction targets by 2030 based on multiobjective programming, Math. Prob. Eng. 2017 (2017) 1–8. [2] Z. Mi, J. Zheng, J. Meng, Y. Shan, H. Zheng, J. Ou, D. Guan, Y. Wei, China's Energy consumption in the new normal, Earth's Future 6 (2018) 1007–1016. [3] T. Sueyoshi, Y. Yuan, China's regional sustainability and diversified resource allocation: DEA environmental assessment on economic development and air pollution, Energy Econ. 49 (2015) 239–256. [4] X. Chen, S. Shao, Z. Tian, Z. Xie, P. Yin, Impacts of air pollution and its spatial spillover effect on public health based on China's big data sample, J. Cleaner Prod. 142 (2017) 915–925. [5] H. Chen, J. Kang, H. Liao, B. Tang, Y. Wei, Costs and potentials of energy conservation in China's coal-fired power industry: a bottom-up approach considering price uncertainties, Energy Policy 104 (2017) 23–32. [6] D. Zhang, J. Wang, Y. Lin, Y. Si, C. Huang, J. Yang, B. Huang, W. Li, Present situation and future prospect of renewable energy in China, Renew. Sustain. Energy Rev. 76 (2017) 865–871. [7] J. Diao, A feasible plan for changing the direction of China’s energy structure, IOP Conf. Ser.: Earth Environ. Sci. 189 (2018) 52063. [8] J. Liu, China's renewable energy law and policy: a critical review, Renew. Sustain. Energy Rev. 99 (2019) 212–219. [9] N. Jiang, Strategic trends for future energy policy: evidence from China, Energy Sources Part B 13 (2018) 165–168. [10] Y. Gu, J. Xu, D. Chen, Z. Wang, Q. Li, Overall review of peak shaving for coal-fired power units in China, Renew. Sustain. Energy Rev. 54 (2016) 723–731. [11] W. Wei, P. Li, H. Wang, M. Song, Quantifying the effects of air pollution control policies: a case of Shanxi province in China, Atmos. Pollut. Res. 9 (2018) 429–438. [12] J. Wang, W. Qu, C. Li, C. Zhao, X. Zhong, Spatial distribution of wintertime air pollution in major cities over eastern China: relationship with the evolution of trough, ridge and synoptic system over East Asia, Atmos. Res. 212 (2018) 186–201. [13] R.B. Finkelman, H.E. Belkin, B. Zheng, Health impacts of domestic coal use in China, PNAS 96 (1999) (1999) 3427–3431. [14] R.A. Rohde, R.A. Muller, Air Pollution in China: mapping of concentrations and sources, PLoS ONE 10 (2015) e135749. [15] G. Wang, R. Zhang, M.E. Gomez, L. Yang, et al., Persistent sulfate formation from London Fog to Chinese haze, Proc. Natl. Acad. Sci. 113 (2016) (2016) 13630–13635. [16] J. Quan, X. Tie, Q. Zhang, Q. Liu, X. Li, Y. Gao, D. Zhao, Characteristics of heavy aerosol pollution during the 2012–2013 winter in Beijing, China, Atmos. Environ. 88 (2014) 83–89. [17] J. Hu, F. Duan, K. He, Y. Ma, S. Dong, X. Liu, Characteristics and mixing state of Srich particles in haze episodes in Beijing, Front. Environ. Sci. Eng. 10 (2016). [18] J. Park, J. Ahn, K. Kim, Y. Son, Historic and futuristic review of electron beam technology for the treatment of SO2 and NOx in flue gas, Chem. Eng. J. 355 (2019) 351–366. [19] J. Chen, C. Chen, Y. Cao, S. Liu, W. Jia, Review on the latest developments in modified vanadium-titanium-based SCR catalysts, Chin. J. Catal. 39 (2018) 1347–1365. [20] L. Su, Q. Du, Y. Wang, H. Dong, J. Gao, M. Wang, P. Dong, Purification characteristics of fine particulate matter treated by a self-flushing wet electrostatic precipitator equipped with a flexible electrode, J. Air Waste Manag. Assoc. 68 (2018) (1995) 725–736. [21] G. Xu, C. Xu, Y. Yang, Y. Fang, Y. Li, X. Song, A novel flue gas waste heat recovery system for coal-fired ultra-supercritical power plants, Appl. Therm. Eng. 67 (2014) 240–249. [22] T. Misaka, Y. Mochizuki, Recent Application and Running Cost of Moving Electrode Type Electrostatic Precipitator, Springer, Berlin, Heidelberg, 2009, pp. 518–522. [23] S.N. Yoshio Nakayama, M.I.S.O. Yasuhiro Takeuchi, MHI High Efficiency System Proven technology for multi pollutant removal, Research & Development Center, Nishi-ku, 2011, pp. 1-11. [24] A. Bäck, Enhancing ESP Efficiency for High Resistivity Fly Ash by Reducing the Flue Gas Temperature, Springer, Berlin, Heidelberg, 2009, pp. 406–411. [25] Y. Sasaki, Y. Tsumita, M. Torii, Y. Mori, Operation Results of IHI Flue Gas Desulfurization System for Unit 1(1,000MW) at Hitachinaka Power Station and Unit 1 at Mizue Power Station, in: Asme Power Conference, 2005. [26] J. Li, Z. Li, Y. He, H. Zhao, S. Yu, Research and application on electric precipitation technology with low-low temperature, China Environ. Prot. Ind. (2014) 28–34 in Chinese. [27] Y. Tang, X. He, A. Cheng, W. Li, X. Deng, Q. Wei, L. Li, Occurrence and sedimentary

Fig. 18. The repose angle of JY coal ash samples after agglomeration.

the temperature design process, the particle size distribution should be taken into consideration, which is related to the coal type and combustion mode of the boiler. In a word, the temperature design process of LLTSs should fully consider the influence of various factors. The temperature should not be reduced excessively, otherwise it will cause serious corrosion and blockage problems. 4. Conclusions (1) When adsorption temperature is approximately 40 °C lower than flue gas Td, the SO3 removal efficiency reaches the maximum value. The specific value of this optimal temperature is affected by the physical characteristics and element composition of ash particles. The adsorption process is a synergistic effect of physical and chemical adsorption. The SO3 removal efficiency is mainly affected by physical adsorption. The chemical adsorption depends on the metal elements, especially on the Al elements. (2) The SO3 removal efficiency increases as the D/S increases. The effect of D/S is greatly affected by the temperature. When the temperature is low, the D/S has almost no influence on removal efficiency. Hence, the temperature parameter is the decisive factor for LLTSs. Ash particle size is also an important factor for the removal of SO3. It is important to take coal type and combustion mode into consideration during the parameter design process. Gas residence time is not the main factor for LLTSs. (3) The chemical adsorption helps fix S on the ash particle surface and increase the adhesivity of ash particles. After the cooling process of LLTSs, there is an obvious agglomeration of ash particles. The agglomeration mainly happens between smaller particles or between small particles and large particles. This helps to remove fine particles. Agglomeration between large particles was not notable. However, if the temperature is too low, agglomeration will occur between large particles due to the effect of H2O vapor. This may cause ash deposition and blockage problems in heat exchangers and ash hoppers. (4) The flowability of the ash particles is greatly reduced after the adsorption and agglomeration process. The repose angle of the ash particles increases as the temperature of the flue gas decreases. The smaller the particle size is, the larger the repose angle, and the larger it is influenced by the temperature. When temperature is 20 °C lower than Td, the repose angle increased greatly. To prevent the ash deposition and blockage problems, the temperature shouldn't be reduced to the point where the SO3 removal efficiency is the highest. 11

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