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. 2023 May 5;8(19):16656–16672. doi: 10.1021/acsomega.2c07476

Experimental Investigation of Removal of SO3 from Flue Gas with Modified Fly Ash Adsorbents

Guiling Xu 1,*, Feihan Chen 1, Yangjie Qian 1, Ping Lu 1, Miaomin Hong 1, Qi Zhang 1, Qiang Zhou 1
PMCID: PMC10193403  PMID: 37214666

Abstract

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The removal of nonconventional pollutants in coal-fired power plants, such as SO3, has been receiving more and more attention. However, due to its unique nature, it is difficult to remove SO3 effectively with the widely used wet flue gas desulfurization systems. Nowadays, dry-sorbent injection technology has become a promising method for SO3 emission control in coal-fired power plants. The removal characteristics of SO3 from flue gas with modified fly ash adsorbents were investigated in a fixed-bed reactor. Factors affecting the adsorption efficiency of SO3 were studied, including modification method, modified fly ash adsorbent particle size, reaction temperature, and flue gas component. Combined with adsorbent characterization analysis, the adsorption kinetics of SO3 by modified fly ash adsorbents were carried out with four different adsorption kinetics models. The results show that the SO3 adsorption efficiency of the fly ash samples increases after modification; the best SO3 removal performance of fly ash was achieved when 1.5 mol/L NaOH solution was used, with the highest SO3 adsorption efficiency of up to 98.3%. The modified fly ash adsorbent particle size, water vapor content, and the addition of NO have little effect on the adsorption efficiency of SO3. As the reaction temperature increases from 250 to 450 °C, the SO3 adsorption efficiency first increases and then decreases, with an optimal reaction temperature of 350 °C. The addition of SO2 would compete with SO3 for adsorption and inhibit the uptake of SO3 by the adsorbent. Adsorption kinetics data show that external mass transfer and chemical adsorption are the main critical mechanisms affecting the adsorption efficiency of the modified fly ash adsorbent in the SO3 removal process compared to internal diffusion.

1. Introduction

With the implementation of increasingly strict environmental standards, ultraclean emission in coal-fired power plants has been extensively advocated and gradually realized.1,2 In the past several years, studies related to mainstream pollutant emission from coal-fired power plants were mainly focused on nitrogen oxides (NOx), sulfur dioxide (SO2), particulate matter, Hg, etc.3 Nowadays, controlling nonconventional pollutant sulfur trioxide (SO3), which is in trace amount in the flue gas has been receiving more and more attention.4−6 With the combustion of high-sulfur coal and the application of oxy-fuel combustion for CO2 capture and storage in power plants, the formation of SO3 is enhanced.4,7 Although the amount of SO3 in flue gas is far less than that of SO2, its toxicity is ten times that of SO2. SO3 is highly reactive and it can easily react with ammonia (NH3) in a selective catalytic reduction (SCR) reactor to produce sticky ammonium bisulfate (NH4HSO4), causing SCR catalyst deactivation, downstream air preheater plugging.8 In coal-fired power plant systems, when the flue gas temperature decreases below the acid dew point, sulfuric acid vapor generated by SO3 would condense on the metal surfaces and lead to low-temperature corrosion.9 Furthermore, when SO3 is released through the stack as submicron acid aerosol, it would cause visible blue plumes. SO3 is also harmful to the atmospheric environment and human health.10 Therefore, it is urgent to control the emission of SO3 and develop high-efficiency SO3 removal technologies.

Due to its unique nature, SO3 is difficult to remove effectively in widely used wet flue gas desulfurization (WFGD) systems.11,12 Recently, dry-sorbent injection technology has become a promising method for SO3 emission control.13,14 The sorbent could be injected before the air preheater, at the economizer outlet or the SCR outlet, and then all SO3 formed in the boiler furnace and SCR can be reduced.14 The SO3 removal characteristics is mainly influenced by sorbent properties, flue gas parameters, reactor structure parameters, etc.15,16 Different sorbents have been studied to absorb SO3 in the literature, which are mainly sodium-, calcium-, and magnesium-based substance, such as NaHCO3,17 Na2CO3,15 Na2SO3,18 Ca(OH)2,2,13−15,17,19,20 CaCO3,19 and Mg(OH)2.15,17 Meanwhile, some researches focused on metal oxides,21−23 like CaO, MgO, Na2O, and K2O, which are fly ash components, especially CaO,4,19,22,24 which is one of the main components of fly ash. For the wide industrial applications of dry-sorbent injection technology, selecting a suitable adsorbent material is of great importance.25 The calcium-based adsorbents have the disadvantage of a low utilization rate, and the sodium- and magnesium-based adsorbents are relatively expensive, leading to high cost.19,23 Therefore, it is critical to choose high-efficiency and low-cost adsorbents for this SO3 emission control technique.

Fly ash is the main by-product of the coal combustion process in coal-fired power plants, is predominantly composed of SiO2, Al2O3, and Fe2O3 at weight percentages of 50–60, 20–30, and 5–15% respectively, and alkali and alkaline metal oxides, CaO, MgO, Na2O, and K2O at weight percentages of 1–40, 0–10, 0–6, and 0–4%, respectively.23 With increasing coal-fired power generation, a million of tonnes of fly ash could be produced each year, its utilization ratio is less than 30% and the environmentally friendly resource utilization of fly ash is also an attractive method for waste management in coal-fired power plants.26 Fly ash could be used as low-cost adsorbents for removing various gaseous pollutants in flue gas, such as NOx, SOx, Hg, and CO2, due to its chemical and physical properties.27,28 Spörl et al.29 found that higher ash contents are beneficial for capturing SO3 in a baghouse filter system. Romero et al.30 reported that depletion of SO3 concentration in coal-fired boilers was because that part of the generated SO3 in the boiler had reacted with the alkali oxides in suspended fly ash. Many studies indicated that fly ash could capture acid gaseous sulfur oxides by both adsorption and reaction in a wide temperature range from 30 to 600 °C.26,31 Developing a novel modified fly ash sorbent for SO3 removal in the flue gas with high removal capability and low production cost has become a promising topic.32,33

The reaction rates of heterogeneous reactions between fly ash and SO3 are dependent on the chemical and physical characteristics of fly ash particle surface, and the surface characteristics could be modified by the reaction or by adding other chemical species.34 However, there are rather rare literature reports relating to the modification of fly ash for SO3 adsorption. Wang et al.35 studied the adsorption characteristics of ammonium bromide-modified fly ash to SO3. Some researchers found that alkali modification and alkali activation on fly ash could improve the adsorption capacity of fly ash.36 Dindi et al.37 developed a NaOH-activated fly ash material to investigate the CO2 capture performance. Gao et al.38 used novel low-cost NaOH-modified fly ash to prepare dye adsorbent, and analyzed its adsorption mechanism with adsorption kinetics. Tian et al.39 used the mixture of NaOH and NaHCO3 to modify fly ash to treat the adsorption property of oil-bearing wastewater, and when the optimum volume ratio is 3:1 and the modification temperature is 600 °C, the treatment effect of oily wastewater is the best, reaching 82%. Apart from adsorbent properties, other critical factors that significantly affect SO3 removal of fly ash are reaction temperature and flue gas component. Meanwhile, the effects of reaction temperature and flue gas component on SO3 removal characteristics by modified fly ash adsorbents have not been systematically investigated, meanwhile, minimal work has been conducted to reveal the adsorption mechanism of fly ash-based adsorbents for SO3 removal.

This state of art strategy has motivated this study. In this paper, the removal characteristics of SO3 from flue gas with modified fly ash adsorbents were investigated on a fixed-bed reactor. Three modified fly ash adsorbents (FA-NH4Br, FA-NaHCO3, and FA-NaOH) were prepared and characterized. The effects of the modification method, modified fly ash adsorbent particle size, reaction temperature, and flue gas component (SO3, water vapor, NO, and SO2) on the adsorption efficiency of SO3 were discussed. Meanwhile, combined with adsorbent characterization analysis, the adsorption kinetics of SO3 by the modified fly ash adsorbent was determined with different adsorption kinetics models in order to explore the underlying adsorption removal mechanism of SO3 by the modified fly ash adsorbent. This work would be helpful for providing a certain reference for further research on the efficient removal of SO3 from flue gas with low-cost fly ash-based adsorbents and achieving the goal of fly ash resource utilization for pollutant control in coal-fired power plants.

2. Experimental Section

2.1. Experimental System

The SO3 removal experiments were carried out with a fixed-bed reactor apparatus, as shown in Figure 1, which included the SO3 generation system, the adsorption system, and the SO3 sampling system. The SO3 generation system was composed of a peristaltic pump, evaporation chamber, nitrogen (N2) preheating pipe, etc. A dilute sulfuric acid solution was injected into the high-temperature zone (400 °C) of the evaporation chamber through the peristaltic pump to generate SO3. The adsorption system mainly included a heat tracing belt, a vertical tubular furnace, and a quartz tube reactor. The reactor was a quartz tube (10 mm i.d.) with thermocouples placed at the external wall to control the temperature. During each test, the adsorbent sample with a mass of 0.2 g was packed into the adsorption reactor. The generated simulated flue gas entered the reactor from the upper part of the fixed bed and fully reacted with the adsorbent. The connecting tube was heated to above 260 °C and covered with the heat insulation material to prevent SO3 from condensation. The space velocity is 4780 h–1. The experimental parameters are listed in Table 1.

Figure 1.

Figure 1

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Schematic diagram of SO3 removal experimental system

Table 1. Experimental Parameters.

parameters values unit
reaction temperature 250, 300, 350, 400, 450 °C
SO3 concentration 10, 20, 50, 100 μL/L
water vapor content 6, 8, 10, 12 %
NO concentration 0, 100 μL/L
SO2 concentration 0, 1000, 2000 μL/L
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The SO3 sampling system was built based on the controlled condensation method according to EPA method 8A, as shown in Figure 2. The inner diameter of the spiral condensing tube was 3 mm, and its total length was 2 m. The water bath temperature was 70 °C, and the sampling gas flow rate was 1 L/min. The flue gas leaving the spiral condensing tube passed through four Greenburg–Smith impingers in the ice bath. The first two impingers were filled with 100 mL of 3% hydrogen peroxide solution for the capture of SO2. The third impinger was filled with 100 mL of deionized water. The last impinger was filled with silica gel to absorb the residual moisture before the flue gas entered the dry gas meter. After the sampling procedure, the spiral condensing tube was rinsed repeatedly with deionized water, and the solution in the impingers was collected. The amount of SO42– in the solution was analyzed with an ion chromatograph analyzer (ICS-900, DIONEX). Then, the concentration of SO3/H2SO4 in flue gas could be obtained.

Figure 2.

Figure 2

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Schematic diagram of the SO3 sampling system.

The capacity of the adsorbent for removing SO3 is measured and the adsorption efficiency η can be calculated as follows

2.1. 1

where Cinlet is the SO3 concentration at the inlet and Coutlet is the SO3 concentration at the outlet with units of mmol.

2.2. Materials and Methods

Raw fly ash samples were collected from the electrostatic precipitators of three different pulverized coal power plants in Inter Mongolia, Anhui, and Shanxi in China, which were named FA1, FA2, and FA3, respectively. The mean particle sizes of FA1, FA2, and FA3 are 14.5, 74.1, and 27.9 μm, respectively. The particle size distribution for raw fly ash samples is shown in Figure 3. The main compositions of fly ash samples are listed in Table 2. NH4Br, NaHCO3, and NaOH were used as the modification agents for fly ash. The modified fly ash adsorbents were prepared by chemical impregnation by referring to the research of Wang et al.35 Raw fly ash samples with a mass of 10 g were impregnated into 10 ml of NH4Br, NaHCO3, and NaOH solutions with modification reagent molar concentration of n mol/L at room temperature, respectively. Here, n can be varied to investigate the effect of the molar concentration of the modification reagent on the SO3 removal characteristics of modified fly ash. The solutions were stirred at 800 rpm with a magnetic stirrer for 2 h, and then were placed in a fume hood for 12 h. After pouring out the supernatant liquid, the samples were dried in a thermostatic drying oven at 105 °C for 24 h. The dried samples were ground and sieved with standard sieves into different particle sizes. The modified fly ash adsorbents were labeled according to the modification reagent type and its molar concentration. For instance, when FA1 was modified with 1.5 mol/L NaOH solution, the modified fly ash was labeled as FA1-1.5 NaOH.

Figure 3.

Figure 3

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Particle size distribution for raw fly ash samples.

Table 2. Main Compositions of Fly Ash Samples.

  content (%)
sample SiO2 Al2O3 Fe2O3 CaO K2O
FA1 32.929 14.464 16.794 24.148 3.038
FA2 51.36 30.34 8.461 3.033 1.909
FA3 41.916 28.327 12.749 9.013 2.428
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The specific surface area and pore structure parameters of modified fly ash adsorbents were measured using a specific surface area and porosity analyzer (NOVA1000e, Quanta chrome). The apparent morphology and microstructure of adsorbent samples were tested by high-resolution scanning electron microscopy (JSM-5610 LV, JEOL, Japan). The particle size and particle size distribution of adsorbent samples were measured using a laser particle size analyzer (ST-1076, Yishite Instrument Ltd., China). In order to further study the mechanism of SO3 removal by modified fly ash adsorbent, an X-ray powder diffractometer (D/max 2500/PC, Nippon Science Company, Japan) was used to determine the crystallinity contained in the sample. The transformation of the functional group was measured using a Fourier Transform Infrared Spectrometer (Vertex 70, Bruker, Germany), and the chemical composition was detected using an X-ray Fluorescence Spectrometer (AXIOS-MAX, PANalytical B.V., Netherlands).

3. Results and Discussion

3.1. Effect of the Modification Method on SO3 Adsorption Efficiency

3.1.1. Effect of the Modification Solution Type

Figure 4 shows the effect of the modification solution type on SO3 adsorption efficiency. SO3 adsorption efficiency of the modified fly ash samples increases after modification. Meanwhile, the fly ash samples modified with NaOH basically show the greatest adsorption ability of SO3, while fly ash samples modified with NH4Br have the poorest SO3 adsorption ability. NaOH has the strongest alkalinity among the three modification solutions. After being modified, the vitreous structure in fly ash particles could be destroyed and the internal metal oxides are released and become active.32 The most stable form of SO3 binding with these oxides was the combination of the S atom of SO3 and O of oxide to form stable sulfate.23 The porosity, pore volume, and specific surface area of the fly ash particles increase, improving the physical adsorption ability, and the functional group on the particle surface changes, enhancing the chemical adsorption ability.40 NaHCO3 solution is weakly alkaline, its enhancement effect is not as obvious as that of NaOH. The fly ash modified with NH4Br would decompose gaseous NH3 at high temperatures, which can react with SO3 and H2O in the flue gas. However, the promoting effect on SO3 adsorption is not so good.

Figure 4.

Figure 4

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Effect of modification of the solution type on SO3 adsorption efficiency.

Figure 5 compares the SEM micrographs of fly ash samples before and after modification. As shown in Figure 5a,c,e, the raw fly ash samples are mainly in granular morphology, smooth spherical particles are dominant, with fine porous material adhering to the particle surface. As shown in Figure 5b,d,f, the particle surface is loosened, the porosity of the structure increases, and the specific surface area increases correspondingly. During the modification process, activation of raw fly ash begins with the digestion of vitreous-phase silica and alumina by the effect of modification solutions. The physical and chemical adsorption ability of fly ash particles can both be improved after being modified.41,42

Figure 5.

Figure 5

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SEM photos of fly ash samples before and after modification.

3.1.2. Effect of NaOH Modification Solution Concentration

Figure 6 shows the effects of NaOH modification solution concentration on the SO3 adsorption capacity and efficiency, respectively. As for FA1, when NaOH modification solution concentration increases from 0.5 to 3 mol/L, the SO3 adsorption capacity and efficiency of modified fly ash both first increase and then decrease. When the modification solution concentration is 1.5 mol/L, the SO3 adsorption efficiency of modified fly ash reaches the maximum value, which is 98.3%, and the SO3 adsorption capacity is 62.3 mg/g. However, for FA2 and FA3, as NaOH modification solution concentration increases from 0.5 to 3 mol/L, the SO3 adsorption capacity and efficiency of modified fly ash show an increasing trend. When the NaOH modification solution concentration exceeds 2 mol/L, the increasing trend for SO3 adsorption capacity and efficiency tends to flatten. It may be because after being modified with NaOH solution, a desilication reaction will occur in the fly ash particles. The amorphous glass phase and quartz component of fly ash are partially dissolved in NaOH solution, leading to decreasing Si/Al ratio and increasing micropores.43 With the increase in the NaOH solution concentration, the pore volume and specific surface area of fly ash particles increase, the alkalinity of modified fly ash increases, and the adsorption efficiency of SO3 increases gradually. However, when the NaOH modification solution exceeds a certain value, e.g., 1.5 mol/L for FA1, the crystal structure inside fly ash particles becomes loose and the internal structure would collapse,44 leading to the decreasing SO3 adsorption ability. Then, the SO3 adsorption capacity and efficiency of modified fly ash will show a variation tendency of decrease.

Figure 6.

Figure 6

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Effect of the NaOH-modified solution concentration.

3.1.3. Characterization Analysis of Modified Fly Ash

3.1.3.1. SEM Analysis

Figure 7 shows the SEM micrographs for FA1-1.5 NaOH before and after adsorption. It can be seen from Figure 7a that the adsorbent particle before adsorption was relatively complete, and the particle surface was rough, which provides a large specific surface area for SO3 adsorption. After adsorption, as shown in Figure 7b, the overall morphology of the adsorbent particle changed greatly, and a serious adhesion phenomenon occurred on the surface of the particles, which may be caused by the chemical reaction between SO3 and the surface-active substance of the adsorbent. Meanwhile, the reaction products covered the surface of the adsorbent particles, reducing the specific surface area, and the formed agglomeration substance prevented the internal diffusion of SO3 in the adsorbent.45

Figure 7.

Figure 7

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SEM micrographs.

3.1.3.2. XRF Analysis

Table 3 lists the changes of the S and Si elemental content before and after SO3 adsorption by FA1-1.5 NaOH. It can be seen that the Si content increases slightly because the adsorbent was mixed with high-purity quartz sand, and a small part of quartz sand remained in the reacted samples after the experiment.46 Before adsorption, the content of the S element in the adsorbent was very low, only 0.153%. After adsorption, the content of the S element in the sample increased to 3.452%.

Table 3. Changes of the S and Si Elemental Content before and after SO3 Adsorption.
  content (%)
element Si S
before adsorption 30.276 0.153
after adsorption 31.024 3.452
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3.1.3.3. XPS Analysis

Figure 8 illustrates the XPS spectra of S element for FA1-1.5 NaOH before and after adsorption. There was no obvious characteristic peak before adsorption, indicating that the modified fly ash adsorbent did not contain the S element before adsorbing SO3. Furthermore, the peak intensity of the S element changed obviously after the modified fly ash adsorbed SO3. There was an obvious characteristic peak in the S 2p spectrum after adsorption, which was located near the electron binding energy of 169 eV. The characteristic peak may come from Al2(SO4)3, CaSO4, Na2SO4, and K2SO4.47,48 It was indicated that there was metal sulfate in the products after adsorption, which was formed by the chemical reaction between SO3 in flue gas and alkali metal oxides in the modified fly ash adsorbent.

Figure 8.

Figure 8

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XPS spectra.

3.1.3.4. FTIR Analysis

Figure 9 presents the Fourier transform infrared (FTIR) spectra of FAl-1.5 NaOH before and after adsorption. There is no obvious change in FTIR spectra of the modified fly ash adsorbent before and after adsorption, indicating that the structure of the modified fly ash adsorbent did not change after adsorption. However, the positions and contents of some functional groups changed. The wide peak at 3200–3550 cm–1 is alcohol O–H stretching vibration.49 After adsorption, the peak intensity is significantly reduced, which may be because the O–H bond was involved in the adsorption process of SO3 by the modified fly ash adsorbent. Alcohol O–H bending vibration occurs at 1330–1420 and 750–800 cm–1, which may be due to the reaction of water vapor in flue gas during the adsorption process, forming a small amount of O–H bonds on the surface of the adsorbent.50 830–1110 cm–1 is Si–O stretching vibration, and the characteristic peak shifted after the reaction, indicating that the Si–O bond also plays a certain role in the adsorption of SO3.51 It should be noted that at 608–620 cm–1 a new characteristic peak appeared after adsorption, which was the sulfate ion absorption peak. As the concentration of SO3 in flue gas is extremely low, resulting in the low intensity of the characteristic peak for sulfate ions. The appearance of sulfate ion shows that the modified fly ash adsorbent has a chemical adsorption effect on SO3.

Figure 9.

Figure 9

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FTIR spectra.

3.1.4.5. XRD Analysis

Figure 10 shows the XRD patterns of SO3 adsorbed by FA1-1.5 NaOH before and after adsorption. After adsorption, the diffraction peak of SiO2 was enhanced. It was because in the fixed-bed adsorption experiments, the modified fly ash adsorbent and high-purity quartz sand were mixed together and fixed with quartz cotton in the reactor. After the experiments, a small portion of quartz sand and quartz cotton remained in the used adsorbent sample. The new diffraction peaks of KFe(SO4)2(H2O)2, CaSO4, and Na2SO4 appeared in the adsorbent after the adsorption reaction, which is due to the chemical reaction among alkali metal oxide in fly ash, sulfur trioxide, and water vapor. At a diffraction angle 2θ of 27°, there was a substance both before and after adsorption, which was Na2SiO3. It was due to the chemical reaction between sodium hydroxide and silica during the modification process that α-phase sodium disilicate was formed.52

Figure 10.

Figure 10

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XRD patterns: K:KFe(SO4)2(H2O)2 C:CaSO4 N:Na2SO4 A:Al2(SO4)3•8H2O S:SiO2 H:Na2SiO3.

3.2. Effect of Modified Fly Ash Adsorbent Particle Size on SO3 Adsorption Efficiency

Figure 11 illustrates the effects of modified fly ash adsorbent particle size on SO3 adsorption capacity and efficiency, respectively. When the modified fly ash adsorbent particle size is in the range of 0.065–0.154 mm, the SO3 adsorption capacity varies between 56.5 and 60.5 mg/g, and the SO3 adsorption efficiency is in the range of 91.4–95.5% with the maximum difference of SO3 adsorption of only 4.1%. Within the modified fly ash adsorbent particle size range investigated in this study, modified fly ash adsorbent particle size has little influence on the SO3 adsorption efficiency. The reactions between adsorbent particles and SO3 are composed of two parts, first, the surface reaction between SO3 and particle surface which is not covered by the product layer, and second, the diffusing of SO3 through the product layer and the reaction between SO3 and the unreacted adsorbent core.20 Therefore, the absorption of SO3 can be divided into the chemical kinetics-controlled stage and product layer diffusion-controlled stage.13 The reduction in the particle size cannot achieve obvious improvement in SO3 adsorption efficiency because the diffusion resistance of individual particles is dominant. Thibault et al.22 reported the results of adsorption of SO3 by CaO and MgO particles, and found that for efficient capture of SO3, small particle size and open macropore structure are both of great importance. Their results showed that the adsorption rates are independent of the particle size in the range of 0.06–0.33 mm. Kocaefe et al.53 compared the sulfation rates of calcium, magnesium, and zinc oxides with SO2 and SO3, experiments were conducted with various oxide particle sizes (0.071–0.151 mm), and their data indicated that particle size in their experimental range has an insignificant effect on the reaction rates for all the three different kinds of oxide particles. Here, the particle size of modified fly ash was 0.065–0.074 mm in subsequent experiments of this study.

Figure 11.

Figure 11

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Effect of the modified fly ash adsorbent particle size.

3.3. Effect of the Reaction Temperature on SO3 Adsorption Efficiency

Figure 12 presents the effects of reaction temperature on SO3 adsorption capacity and efficiency, respectively. Considering that the flue gas outlet temperature of the SCR reactor is commonly about 350 °C, 250/300/350/400/450 °C is selected as the reaction temperature. As the reaction temperature increases, the SO3 adsorption capacity and efficiency both first increase and then decrease. The SO3 adsorption of modified fly ash includes physical adsorption and chemical adsorption. Physical adsorption is the adsorption of SO3 on the surface and the pore structure of the adsorbent particles due to the action of van der Waals force. The heat required for the physical adsorption process is very low, so low temperature is favorable for physical adsorption.54 Chemical adsorption depends on the reaction of active components in the adsorbent particles with SO3 in the flue gas, which involves the breaking and formation of chemical bonds. The changes of chemical bonds require a certain amount of adsorption heat and activation energy. As the reaction temperature increases, the chemical reaction rate increases correspondingly, the SO3 adsorption capacity and efficiency both increase. When the reaction temperature increases to 350 °C, the SO3 adsorption capacity and efficiency both reach their maximum values, corresponding to the optimal reaction temperature. With a further increase in the reaction temperature, the active components in the adsorbent particles may be inactivated as the temperature becomes too high, the chemical reaction rate decreases, and the chemical adsorption ability decreases. Meanwhile, the SO3 adsorbed in the modified fly ash particles may be easy to desorb at high reaction temperatures, and the physical adsorption ability decreases. Therefore, when the reaction temperature exceeds 350 °C, the SO3 adsorption capacity and efficiency both show a decreasing tendency. Wang et al.19 reported SO3 adsorption experimental results with Ca(OH)2, CaO, and CaCO3 under medium temperatures (300, 350, and 400 °C), and results show that as the reaction temperature increases, the final sulfuration rate of the sorbents increases, and the breakthrough curves formed for different alkaline substances are different. Zheng et al.15 found that the SO3 adsorption efficiency for Na2CO3 increases as the reaction temperature increases from 150 to 300 °C, while when the reaction temperature is above 300 °C, the temperature has little influence on the SO3 adsorption efficiency. Their results show that the SO3 adsorption efficiency at 350 °C is lower than that at 300 °C, which is because when the reaction temperature exceeds 300 °C, the gas diffusion becomes the controlling step for SO3 adsorption compared to the gas–solid interface reaction.

Figure 12.

Figure 12

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Effect of the reaction temperature.

3.4. Effect of Flue Gas Components on SO3 Adsorption Efficiency

3.4.1. Effect of the SO3 Concentration

In order to explore the effect of SO3 concentration on the SO3 adsorption performance of modified fly ash adsorbents, the initial SO3 concentration in flue gas was 10 to 100 μL/L. The particle size of modified fly ash was 0.065–0.074 mm. The reaction temperature was 350 °C, and the water vapor content was 10%. Figure 13 shows the effects of SO3 concentration on SO3 adsorption capacity and efficiency, respectively. As the SO3 concentration increases from 10 to 100 μL/L, the SO3 adsorption capacity and efficiency for three different modified fly ash adsorbents show a variation tendency of decrease. When the SO3 concentration is less than 50 μL/L, the decreasing tendency is not so obvious, while when the SO3 concentration exceeds 50 μL/L, the SO3 adsorption capacity and efficiency for different modified fly ash samples both decrease significantly, except for the FA1 modified with 1.5 mol/L NaOH solution. This may be because with the increase in SO3 concentration, part of the SO3 passes through the modified fly ash adsorbent layer directly without reacting with the adsorbent, resulting in decreasing SO3 adsorption efficiency. Meanwhile, as the active chemical bonds on the surface of the modified fly ash particles continue to react with SO3, the number of active chemical bonds decreases and the probability of the active adsorption sites of the modified fly ash particles in contact with SO3 is reduced, leading to a decrease in the SO3 adsorption efficiency.

Figure 13.

Figure 13

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Effect of SO3 concentration.

3.4.2. Effect of Water Vapor Content

Figure 14 represents the effect of water vapor content on SO3 adsorption capacity and efficiency, respectively. The SO3 concentration in the flue gas was 50 μL/L. The particle size of modified fly ash was 0.065–0.074 mm. The reaction temperature was 350 °C. As the research background of this study is to control the SO3 emission in the flue gas of coal-fired power plants, and the water vapor content was chosen from 6 to 12% in order to simulate the real water vapor content of coal flue gas under the actual engineering conditions. As shown in Figure 14, when the water vapor content is in the range of 6–12%, the SO3 adsorption capacity differences for all three differently modified fly ash adsorbents are small, and the water vapor content has little influence on the SO3 adsorption efficiency. Zheng et al.15 carried out SO3 removal experiments by alkaline sorbents, they used catalytic oxidation of SO2 to generate SO3, they compared the results under the condition without water vapor and under the condition of 1% water vapor volume fraction, and found that the presence of water vapor could promote the conversion of SO3 into H2SO4, enhancing the SO3 adsorption performance. Different from their experiments, the SO3 in this paper is generated based on the dilute sulfuric acid heating method, water vapor is always present in simulated flue gas. Under the reaction temperature of 350 °C, the amount of water vapor in the flue gas is far more than that needed to form H2SO4. According the calculation of flue gas thermodynamic acid dew point,9 as the water vapor content increases from 6 to 12%, the acid dew point of flue gas increases gradually, but it is lower than the reaction temperature in the adsorption bed in these experiments. After the SO3 is absorbed by the modified fly ash adsorbents, the sulfuric acid vapor entering the air preheater is greatly reduced, which can effectively avoid the corrosion and ash blocking of the air preheater.

Figure 14.

Figure 14

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Effect of water vapor content.

3.4.3. Effect of the NO Concentration

Figure 15 represents the effect of NO concentration on SO3 adsorption capacity and efficiency, respectively. The SO3 concentration in the flue gas was 50 μL/L. The particle size of modified fly ash was 0.065–0.074 mm. The reaction temperature was 350 °C. The water vapor content was 10%. The modified fly ash adsorbent was FA1-1.5 NaOH. When 100 μL/L NO was added to the flue gas, the SO3 adsorption capacity and efficiency were not much different from those without NO in the flue gas. In the literature, there is rare research on the effect of NO concentration on SO3 adsorption performance. Yang et al.32 carried out simultaneous removal experiments of NO and SO2 by modified fly ash, and found that the NO concentration variation had little effect on SO2 removal efficiency. Based on the results of this paper, it can be inferred that the adsorption processes of SO3 and NO for modified fly ash adsorbents do not affect each other, and there seems to be no competitive adsorption between SO3 and NO.

Figure 15.

Figure 15

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Effect of NO concentration.

3.4.4. Effect of the SO2 Concentration

Figure 16 represents the effect of SO2 concentration on SO3 adsorption capacity and efficiency, respectively. The SO3 concentration in the flue gas was 50 μL/L. The particle size of modified fly ash was 0.065–.074 mm. The reaction temperature was 350 °C. The water vapor content was 10%. The modified fly ash adsorbent was FA1-1.5 NaOH. As shown in Figure 16, when there is no SO2 in the flue gas, the SO3 adsorption capacity is larger than 60 mg/g within 60 min, and the SO3 adsorption efficiency remains higher than 95%. After 1000 μL/L SO2 is added to the flue gas, the SO3 adsorption capacity and efficiency both decrease significantly. Meanwhile, the SO3 adsorption efficiency decreases gradually with the increasing reaction time. When the SO2 concentration was further increased to 2000 μL/L, the SO3 adsorption capacity and efficiency become lower. The increase in SO2 concentration of the flue gas would inhibit SO3 adsorption and removal by the modified fly ash adsorbents. This may be because there exists competitive adsorption between SO2 and SO3,13 and the increasing SO2 concentration decreases the SO3 adsorption efficiency of modified fly ash adsorbents.

Figure 16.

Figure 16

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Effect of the SO2 concentration.

The concentration of SO2 in the flue gas is dozens of times that of SO3, which reduces the probability of SO3 contact with the active sites, and the SO3 adsorption capacity for the modified fly ash adsorbents decreases. The number of active sites on the surface of the adsorbent is limited. The reaction between SO2 and the active sites will lead to irreversible loss of active sites due to the formation of sulfites/sulfates.10 As the reaction time increases, the active sites decrease concomitantly, and the pore structures are blocked by the substance formed, and the SO3 adsorption efficiency decreases. He et al.13 investigated competitive adsorption between SO3 and SO2 by calcium hydroxide, and they pointed out that the SO3 and SO2 selectivity of the adsorbent influences its SO3 removal efficiency. The SO3 and SO2 adsorption processes can be divided into a chemical kinetics-controlled stage and the product layer diffusion-controlled stage. The SO3 reactivity is much higher than that of SO2, the SO3 molecule is relatively larger than the SO2 molecule,20 and the SO3 concentration in the flue gas is much lower than that of SO2. In the chemical kinetics-controlled stage, the increasing SO2 concentration decreases the SO3 adsorption efficiency because of the competitive consumption of the active sites between SO2 and SO3. In the product layer diffusion-controlled stage, the effective diffusion coefficient for SO3 was lower than that for SO2, the slower diffusion of SO3 through the product layer decreases the SO3 adsorption efficiency. Liu et al.4 also pointed out that although SO3 has strong interactions with alkaline adsorbents than SO2, the increase in the SO2 concentration could decrease SO3 adsorption. Some researchers have shown that SO2 may be oxidized to SO3 by the metal oxide on the adsorbent, which inhibits the adsorption efficiency of SO3 by the adsorbent. The consumption of SO2 on the surface functional group of the adsorbent will reduce the removal of SO3 by the adsorbent.

SO3 in the flue gas of coal-fired power plant mainly comes from the combustion process and the SCR catalytic process. Under the influence of many factors, no more than 2% of SO2 produced by combustion is oxidized to SO3. In addition, about 1–2% of SO2 is oxidized to SO3 under the action of high temperature and catalyst during the SCR catalysis process. The concentration of SO2 in the flue gas is much higher than that of SO3. Most of the injected adsorbents would also react with SO2, leading to the rapid consumption of fresh adsorbents. Pang et al.55 suggested to screen the adsorbents with high selectivity toward SO3, providing a reference for the selection of modification agent for raw fly ash. He et al.18 also found that the increase in SO2 concentration would weaken the SO3 adsorption and decrease the SO3 selectivity, they pointed out that the strong competitive reactions between SO3 and SO2 would reduce the effective utilization of SO3 adsorbents, which needs to be considered during the industrial application. Meanwhile, there is little research on the competitive adsorption between SO2 and SO3, especially for fly ash-based adsorbents. Meanwhile, as the composition of fly ash is relatively complex, the competitive reactions between SO2 and SO3 for modified fly ash adsorbents are very complex, which still need further experimental and theoretical investigations.

3.5. Adsorption Kinetics Analysis

The adsorption kinetics of SO3 by the modified fly ash adsorbent was carried out with four different adsorption kinetics models to explore the underlying adsorption removal mechanism of SO3 by the modified fly ash adsorbent. In this section, modified fly ash samples prepared by the best modification methods were used to carry out experimental investigation of the change in the SO3 removal rate with time, which were FA1-1.5 NaOH, FA2-3 NaOH, and FA3-3 NaOH. The relationship between SO3 accumulated adsorption amount and adsorption time within 60 min (interval of 10 min) was obtained. The particle size of modified fly ash was 0.065–0.074 mm, the concentration of SO3 was 50 μL/L, the water vapor content was 10%, and N2 was used as the balance gas. The experimental results were analyzed using four adsorption kinetic models (pseudo-first-order kinetic model, pseudo-second-order kinetic model, internal diffusion model, and Elovich adsorption model). The error between the fitting result and the experimental data is represented by correlation coefficient R2. The larger the R2 value, the better the model matches the adsorption process.

3.5.1. Pseudo-First-Order Kinetic Model56

The pseudo-first-order kinetic model is a new model obtained by modifying the ideal kinetic model, which is mainly used to describe the process of external mass transfer in the adsorption process. The integral expression formula for the adsorption rate is as follows.

3.5.1. 2

where qt is the amount of SO3 adsorbed per unit mass adsorbent, mg/g, qe is the adsorption capacity of SO3 at equilibrium, mg/g, and k1 is the adsorption rate constant of the pseudo-first-order equation.

3.5.2. Pseudo-Second-Order Kinetic Model57

The quasi-second-order kinetic model shows a quadratic relationship between the reaction rate and the reactant concentration in the reaction process, which is mainly used to describe the chemical adsorption, and its integral expression formula for the adsorption rate is as follows.

3.5.2. 3

where k2 is the adsorption rate constant of the pseudo-second-order equation.

3.5.3. Internal Diffusion Model58

The internal diffusion model is the one in which the reaction rate is controlled by internal pore diffusion, and its integral expression for the adsorption rate is as follows.

3.5.3. 4

where C is a constant and k3 is the adsorption rate constant of the internal diffusion model.

3.5.4. Elovich Adsorption Model59

The Elovich adsorption model is widely used in the chemisorption kinetics of pollutants on the surface of solid sorbents, and its integral expression for adsorption rate is as follows.

3.5.4. 5

where t0 = 1/(ab), a is the initial adsorption rate, mg/(g·min) and b is a constant related to surface coverage and activation energy, g/mg.

Figures 17–19 show the fitting curves of experimental data and the adsorption model for modified fly ash adsorbents at 350 and 400 °C, respectively. The pseudo-first-order kinetic model, pseudo-second-order kinetic model, and Elovich adsorption model have a high fitting degree, while the internal diffusion model has a poor fitting degree. This indicates that internal diffusion is not the main mechanism in the removal process of SO3 by modified fly ash sorbents. The adsorption of SO3 with modified fly ash adsorbents is significantly dependent on the external mass transfer and chemical adsorption process.

Figure 17.

Figure 17

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Fitting curve of SO3 adsorption by FA1-1.5 NaOH adsorbent.

Figure 19.

Figure 19

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Fitting curve of SO3 adsorption by the FA3-3 NaOH adsorbent.

Figure 18.

Figure 18

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Fitting curve of SO3 adsorption by the FA2-3 NaOH adsorbent.

Tables 4–6 list the kinetic data and correlation coefficients R2. For pseudo-first-order kinetic model and pseudo-second-order kinetic model, the adsorption capacity of SO3 at equilibrium (qe) for the three modified fly ash sorbents at 350 °C is larger than that at 400 °C; however, the adsorption rate constants (k1) at 350 °C are smaller than those at 400 °C, and the correlation coefficient R2 is larger than 0.99, indicating that the external mass transfer described by the pseudo-first-order model and the chemisorption described by the pseudo-second-order model are the main steps affecting the adsorption efficiency of the modified fly ash sorbent.60 For internal diffusion model, the adsorption rate constants (k3) at 350 °C are larger than those at 400 °C, and the correlation coefficient R2 was larger than 0.9. Compared with the first two kinetic models, the correlation coefficient R2 was a little smaller, from which it may be inferred that internal diffusion is not the most critical adsorption mechanism in the process of SO3 adsorption. For the Elovich adsorption model, the correlation coefficient R2 was larger than 0.99, indicating that adsorption of SO3 by the modified fly ash adsorbents conforms to the principle of chemical adsorption kinetics on solid adsorbents. The initial adsorption rate (a) obtained by the Elovich adsorption model was very close to the initial adsorption rate (k2qe2) obtained by the Pseudo-second order kinetic equation, which indicates the reliability of the fitting results.61 It also shows that the chemical adsorption process is the main critical process of SO3 adsorption by modified fly ash adsorbents.

Table 4. Kinetic Data and Correlation Coefficient R2 Obtained from Different Models for FA1-1.5 NaOH.
model T (°C) qe (mg/g) k1 R2 model T (°C) qe (mg/g) k2 R2
Pseudo-first-order 400 454.26 0.0024 0.99954 Pseudo-second-order 400 878.45 1.37 × 10–6 0.99963
350 1271.16 0.00083 0.99628 350 2497.12 1.69 × 10–7 0.99672
model T °C C k3 R2 model T °C a (mg/(g·min)) b (g/mg) R2
internal diffusion 400 –26.28 10.88 0.98653 Elovich 400 1.08 0.0024 0.99954
350 –26.91 11.06 0.97716 350 1.05 0.00082 0.99672
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Table 6. Kinetic Data and Correlation Coefficient R2 Obtained from Different Models for FA3-3 NaOH.
model T (°C) qe (mg/g) k1 R2 model T (°C) qe (mg/g) k2 R2
Pseudo-first-order 400 336.49 0.0031 0.99954 Pseudo-second-order 400 646.74 2.5 × 10–6 0.99953
350 3733.32 0.0018 0.99692 350 1212.68 7.1 × 10–7 0.99679
model T (°C) C k3 R2 model T (°C) a (mg/(g·min)) b (g/mg) R2
internal diffusion 400 –24.64 10.36 0.98603 Elovich 400 1.06 0.0032 0.99962
350 –27.32 11.01 0.97139 350 1.01 0.0010 0.99684
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Table 5. Kinetic Data and Correlation Coefficient R2 Obtained from Different Models for FA2-3 NaOH.
model T (°C) qe (mg/g) k1 R2 model T (°C) qe (mg/g) k2 R2
Pseudo-first-order 400 382.98 0.0027 0.99943 Pseudo-second-order 400 734.71 1.9 × 10–6 0.99943
350 687.02 0.0015 0.99871 350 1344.69 5.8 × 10–7 0.99892
model T (°C) C k3 R2 model T (°C) a (mg/(g·min)) b (g/mg) R2
internal diffusion 400 –24.72 10.43 0.98415 Elovich 400 1.06 0.0028 0.99943
350 –26.86 10.96 0.98364 350 1.06 0.00015 0.99875
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4. Conclusions

In this paper, the adsorption performance of SO3 with modified fly ash adsorbents was investigated using a fixed-bed adsorption experimental system within the temperature range of 250–450 °C. Combined with adsorbent characterization analysis, the adsorption removal mechanism of SO3 by modified fly ash adsorbents was discussed with different adsorption kinetics models. The results are summarized as follows.

  • (1)

    The SO3 adsorption efficiency of the fly ash samples increases after modification. The fly ash samples modified with NaOH show the greatest adsorption ability of SO3, while fly ash samples modified with NH4Br have the poorest SO3 adsorption ability. The best adsorption performance of fly ash was achieved when 1.5 mol/L NaOH solution was used, with the highest SO3 adsorption efficiency of up to 98.3%.

  • (2)

    When the modified fly ash adsorbent particle size is in the range of 0.065–0.154 mm, the SO3 adsorption capacity varies between 56.5 and 60.5 mg/g, and the SO3 adsorption efficiency is in the range of 91.4–95.5%. The modified fly ash adsorbent particle size has little influence on the SO3 adsorption efficiency. As the reaction temperature increases from 250 to 450 °C, the SO3 adsorption efficiency first increases and then decreases, with an optimal reaction temperature of 350 °C.

  • (3)

    As the SO3 concentration increases from 10 to 100 μL/L, the SO3 adsorption capacity and efficiency decrease. When the water vapor content is in the range of 6–12%, it has little influence on the SO3 adsorption efficiency. There is no competitive adsorption between SO3 and NO, however, increasing SO2 concentration decreases the SO3 adsorption efficiency of the modified fly ash adsorbents.

  • (4)

    The adsorption kinetic data show that external mass transfer and chemical adsorption are the main critical mechanisms affecting the adsorption efficiency of the modified fly ash adsorbent in the SO3 removal process compared to internal diffusion.

Acknowledgments

This work was funded by the National Natural Science Foundation of China (51606105).

The authors declare no competing financial interest.

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