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Article

Evolutionary Characteristics of Sulphate Ions in Condensable Particulate Matter Following Ultra-Low Emissions from Coal-Fired Power Plants During Low Winter Temperatures

by
Yun Xu
1,†,
Haixiang Lu
2,†,
Kai Zhou
1,
Ke Zhuang
1,*,
Yaoyu Zhang
2,
Chunlei Zhang
1,
Liu Yang
2 and
Zhongyi Sheng
2,*
1
State Key Laboratory of Low-Carbon Smart Coal-Fired Power Generation and Ultra-Clean Emission, China Energy Science and Technology Research Institute Co., Ltd., Nanjing 210023, China
2
School of Environment, Nanjing Normal University, Nanjing 210023, China
*
Authors to whom correspondence should be addressed.
†
These author are contributed equally to this work.
Sustainability 2025, 17(14), 6342; https://doi.org/10.3390/su17146342
Submission received: 28 March 2025 / Revised: 5 June 2025 / Accepted: 17 June 2025 / Published: 10 July 2025
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Abstract

Coal-fired power plants exacerbate hazy weather under low winter temperatures, while sulphate ions (SO42−) in condensable particulate matter (CPM) emitted from ultra-low emission coal-fired power plants accelerate sulphate formation. The transformation of gaseous precursors (SO2, NOx, NH3) is the main pathway for sulphate formation by homogeneous or non-homogeneous reactions. For the sustainability of the world, in this paper, the effects of condensation temperature, H2O, NOX and NH3 on the SO42− generation characteristics under low-temperature rapid condensation conditions are investigated. With lower temperatures, especially from 0 °C cooling to −20 °C, the concentration of SO42− was as high as 26.79 mg/m3. With a greater proportion of H2SO4 in the aerosol state, and a faster rate of sulphate formation, H2O vapour condensation can provide a reaction site for sulphuric acid aerosol generation. SO42− in CPM is mainly derived from the non-homogeneous reaction of SO2. SO3 is an important component of CPM and provides a reaction site for the formation of SO42−. SO2 and SO3, in combination with Stefan flow, jointly play a synergistic role in the generation of SO42−. The content of SO42− was as high as 36.18 mg/m3. While NOX sometimes inhibits the formation of SO42−, NH3 has a key role in the nucleation process of CPM. NH3, SO2 and NOX have been found to rapidly form sulphate with particle sizes up to 5 µm at sub-zero temperatures and promote the formation of sulphuric acid aerosols.

Graphical Abstract

1. Introduction

In upcoming years, thermal power generation will continue to be the predominant mode of power generation in China [1,2]. And the gaseous pollutants emitted from coal-fired power plants will exacerbate the generation of hazy weather, especially in winter heating situations [3]. Even though coal-fired power plants in China have already achieved compliance with the ultra-low emission standards [4], the present standards have not yet set restrictions on the concentration of condensable particulate matter (CPM) in the course of flue gas emission [5]. It is still impossible to eliminate the harm caused by non-conventional pollutants’ CPM [6,7]. To effectively tackle this issue, it is imperative to undertake a comprehensive elucidation of the distinctive characteristics of CPM originating from coal-fired power plants.
According to the definition of CPM in the U.S. Environmental Protection Agency (EPA) Method 202, CPM is a gaseous substance discharged into the atmosphere. Once discharged, it undergoes dilution, cooling and condensation within a few seconds, transforming into a liquid or solid state. At the flue gas temperature, CPM exists in a gaseous form [8]. As a gas within flue gas pollutants, it cannot be eliminated by conventional ultra-low emission facilities. This leads to an increase in the proportion of CPM in particulate matter (PM) [9,10,11]. Studies have shown that CPM is chiefly made up of inorganic and organic components. The mass concentration of the inorganic part spans from around 0.15 to 24.70 mg/Nm3. Additionally, sulphate ions (SO42−) make up about 63.09–89.75% of the inorganic component [3,10,12,13]. Notably, SO42− exerts a crucial influence on the formation of haze [14]. Through condensation, SO42− has the ability to produce fine particulate matter (fine PM) with a particle size below 0.30 μm [15]. These fine particles start to aggregate, initially resulting in fine PM with a particle size under 0.30 μm and later coalescing into larger particles. It has been firmly established that the emissions emanating from coal-fired power plants have a connection with the production of sulphate [16], an important component of atmospheric particulate matter that experiences explosive growth in hazy weather [3]. Therefore, the patterns of evolution and the traits of SO42− originating from CPM in the flue gas of coal-fired power plants merit deeper research.
During coal combustion and selective catalytic reduction (SCR), sulphur dioxide (SO2) is oxidized to sulphur trioxide (SO3). It represents one of the key contributors to the presence of SO42− in CPM [17]. When the ultra-low emission standard is implemented, it has been found that as the emitted SO2 becomes less concentrated, the share of SO3 in CPM becomes larger [18,19,20]. It has been confirmed that SO3 is converted to H2SO4 at room temperature and exists in the form of gas, aerosolized and surface-deposited forms [21,22]. The efficiency of SO3 conversion varies at different temperatures and humidities, and SO42− concentration increases with the proportion of deposited H2SO4 [23]. Moreover, both homogeneous and heterogeneous reactions enable the oxidation of SO2 to H2SO4. This is particularly the case during the winter season. These reactions are inevitably influenced by the condensation temperature and humidity [24,25,26]. Gas-phase reactions occur in winter when coal-fired flue gas exits the chimney, and gas-phase reactions of multiphase chemistry are unable to capture high concentrations of sulphate aerosols during the haze process [27]. Therefore, a higher-precision spatial resolution is required to investigate the transformation patterns of sulphur trioxide (SO3) and sulphur dioxide (SO2) into sulphate ions (SO42−) under low-temperature and rapid-condensation conditions. This can offer scientific backing for the further governance of atmospheric particulate pollution.
Aerosols formed at low temperatures contain large amounts of fine particulate matter, which may carry heavy metals and hazardous substances. Studies have shown that they can enter the bodies of plants and animals through the respiratory tract. For plants, they may block leaf stomata and hinder photosynthesis, thereby affecting plant growth and development; for animals, long-term exposure may cause respiratory diseases, affecting respiratory health, lowering animal immunity and increasing the risk of disease [28]. However, quantitative studies on the extent of the negative effects still need to be strengthened.
At this stage, plenty of scholars have endeavoured to investigate the characteristics of SO42− during fast condensation at room temperature, as well as the effects of studies using ammonia (NH3) and nitrogen oxides (NOX) [29,30], and few people have paid attention to the properties of rapid condensation at low temperatures. NH3 not only can react with SO3 at room temperature to form NH4HSO4 and (NH4)2SO4 [15,31], but it can also react at low temperatures and can also synergize with SO2 to promote SO42− generation as well as enhance the nucleation of H2SO4 aerosol in the atmosphere [32]. Meanwhile, nitrogen oxides (NOx), as a prevalent gaseous pollutant within flue gas, has the ability to enhance the oxidation of SO2. Even the rate of synergistic nucleation of NH3, NOX, SO3 and SO2 is faster at low temperatures [28]. Although some studies have been conducted on the transformation of pollutants at low temperatures, there are indeed fewer studies on the effect of NOX on the transformation of SO3 to SO42− at low temperatures. In contrast, the present study focuses on the effect of NOX on SO42−—generation under low-temperature rapid condensation conditions, filling a partial gap in this area. There have been studies that have yielded some results in pollutant transformation, but they have mostly focused on ambient temperature environments. Barely any studies have been found to address the influence of NOX on the conversion of SO3 into SO42− in low-temperature scenarios. Consequently, there is a pressing need to study the effects of NH3 and NOX on SO42− generation in CPM, particularly at these reduced temperatures.
In this study, testing without gas in the work area to determine initial characterisation was not carried out as the focus was on the effect of a variety of gaseous pollutants (e.g., SO2, NOX, NH3, etc.) on the sulphate ion generation characteristics, which were intended to simulate flue gas emissions from an actual coal-fired power plant. Research has been done to achieve some results in pollutant transformation but has mostly focused on ambient environments. Under low-temperature and swift-condensation conditions, the impacts of various factors such as condensation temperature, as well as the amount of H2O, NOX, and NH3 on the formation of SO42−, were examined. In addition, the simulations carried out in this research furnished the concentration of SO42−, the morphological features of condensable sulphate PM, and the semi-quantitative element detection of the principal components within CPM. The intention is to supply theoretical guidance for the mechanism of sulphate’s explosive growth during haze events in China and the control of SO42− in the CPM discharged by coal-fired power plants in the winter.

2. Methods

2.1. Experimental System

As depicted in Figure 1, the experimental configuration was established. A dynamic flue-gas simulation system was put together, which was made up of a flue-gas generation system, a reaction unit, a quick-condensation system, and an exhaust-gas treatment system. The flue-gas generation system contained a variety of gas cylinders and SO3 generators. Additionally, the key parameters involved in the experiment are presented in a tabular form in Table 1. A certain number of gaseous pollutants was fed into the reaction device from the gas cylinders. The different types of gaseous pollutants were regulated by their respective mass flow meters. A humidity generator was also included to obtain H2O vapour by evaporation of ultrapure H2O, and the flow rate of N2 in the evaporator was adjusted to control the humidity variation in the experiment. The reaction unit mainly consisted of a premixing furnace and a liquid evaporator. Each and every gaseous pollutant accumulated within the system was responsible for the generation of flue gas. Then, they entered the reaction unit to undergo reactions. After that, the substances were sampled via the condensation unit, and finally, the exhaust gas was processed.
The gas concentrations in Table 1 are target set values, which were precisely adjusted (accuracy ±1%) by a mass flow controller. In actual measurements, the concentrations of SO2, NOx and other components deviated from the set values within ±5%, and the total flow rate fluctuated within ±2.5%. The experiment verified the data repeatability by repeated measurements (n = 3), and the results showed that the SO42− concentration fluctuation was less than 10%, indicating that the parameter control met the experimental requirements.

2.2. The Conditions Applied in the Experimen

The overall flow rate of the flue gas ranged from 1.94 to 2.06 L/min. The sampling duration for the CPM was set at 30 min. Each sampling operation collected approximately 60 L of the sample. The molar fractions of water vapor (H2O) were 3.98%, 8.14% and 12.89%, which, respectively, corresponded to the dry, semi-dry, and wet desulfurization processes; CPM sampling followed the principles of EPA Method 202, with modifications tailored to simulate rapid condensation scenarios under sub-zero temperatures. The customized setup included a pre-mixing furnace and a liquid evaporator to replicate real-world flue gas cooling dynamics [8]. Respectively, the initial concentration of SO2 was set at 35 mg/Nm3 (according to the ultra-low emission standard), and the condensing temperature was −20~10 °C (referring to the ambient winter air temperature) and 10 °C (referring to the ambient winter air temperature). Given that the SO2 concentration stands at 35 mg/Nm3, it can still exist at −20~10 °C and participate in the formation of sulphate [32] and can form H2SO4 in different states [21,33,34]. Therefore, 35 mg/Nm3 of SO2 can be used to observe SO42− formation in the condensation temperature range of −20~10 °C. However, the measured SO42− concentration measured below zero was extremely low, which affected the assessment of SO42− properties, so 100 mg/Nm3 of SO2 was used to amplify the experimental study. In general, coal-fired power plants over-inject NH3 to improve the efficiency [35]; therefore, the NH3 concentration was set to 12 mg/Nm3 (referring to the actual coal-fired power plants). Moreover, when 50 mg/Nm3 of NOX was used, the concentrations of NO3− and NO2− measured turned out to be lower than the detectable limit. As a result, a value of 110 mg/Nm3 for NOX was specified to accentuate its influence on SO42−, which facilitated the study of the characteristics of SO42− (the rest of the parameters were kept unchanged, except for the special instructions).
After convergence, the gaseous pollutants were pre-mixed in a pre-mixing furnace (volume 0.50 L), lasting for around 20 s in the relevant environment. After reaching the full mixing, the exhaust gas was set to make its way into the flue gas heating furnace (volume 0.25 L) at 350 °C to reheat the flue gas, and finally enter the liquid evaporator (volume 0.25 L) at a steady temperature of 540 °C. At the same time, the diluted H2SO4, with a concentration of about 8.70 mmol/L, wase injected into the liquid evaporator using a microsampler with a constant speed of 0.03 mL/min, and the high temperature evaporated the diluted H2SO4 into SO3. The high temperature evaporated the diluted H2SO4 into SO3 and H2O. It has been found that the state of SO3 changes at different temperatures, and it is slowly converted to H2SO4 from 440 °C to 200 °C [21,36]. Consequently, the temperature of the liquid evaporator was maintained at 520 °C and the temperature of the flue gas was held steady at 260 °C, respectively. Eventually, the flue gas entered the condensation system, passed through the condenser tube (with a capacity of about 0.3 L) and stayed for about 1 s. It was then connected to the CPM filter and entered the exhaust gas treatment system.
The experimental planning was based on the research objectives aimed at investigating the effects of condensation temperature, water vapour, NOX and NH3 on the SO42− generation under low-temperature fast condensation conditions. We performed multiple sets of experiments by setting different experimental conditions, such as varying the condensation temperature range (−20~10 °C), water vapour molar fraction (3.98%, 8.14%, 12.89%) and adding different combinations of gaseous pollutants (SO2, SO3, NOX, NH3). In terms of mathematical methods, the main focus was to visually analyse the experimental data and draw conclusions by comparing the trends of SO42− concentration under different conditions.

2.3. Process of Analysis

The process of treating the condensate was divided into two main steps. In the first step, 10 mL of ultrapure water was used to gently rinse the condensate tube three times in a slow manner. Then, the concentration of SO42− in the collected 30 mL of rinse solution was determined using an ion chromatograph. In addition, each pooled rinse solution needed to be filtered 2~3 times with a 0.2 μm filter membrane and analysed 3~4 times.
In the subsequent stage, the membranes within the CPM filters (quartz filters having a diameter of 47 mm and a pore size of 0.2 μm) were positioned in an oven maintained at around 110 °C for a duration of 1 h. This procedure was implemented to eliminate the error caused by interfacial water. Subsequently, the membrane underwent drying within a silica-gel desiccator maintained at 20 °C with a relative humidity of 30% for a duration of 24 h. Post drying, the membrane was cut into small sections and placed into a centrifuge tube. Thereafter, 20 mL of ultrapure water was introduced into the tube, and the tube was submerged in an ultrasonic bath for a period of 10 min. Ultimately, the concentration of SO42− in the extract was obtained and measured through Ion Chromatography (IC).
In this experiment, a scanning electron microscope (SEM, Zeiss Sigma 300, Carl Zeiss, Oberkochen, Germany) was employed to capture images of the CPM on the membrane, aiming to acquire the morphological characteristics of the CPM on the quartz filter. Additionally, elemental energy-dispersive X-ray spectroscopy (EDX) was utilized to measure the quantity of the main elements in the CPM, so as to verify the hypothesis of this experiment.

3. Results and Analysis

3.1. How Condensation Conditions Make an Impact

How Condensation Temperature Affects the Situation

In this experiment, each test point was repeated three times to ensure the reliability of the data. Regarding the confidence error of the measurements, due to the limitation of the experimental conditions, no precise calculations have been made, but we have strictly controlled the experimental conditions during the experiment to minimise the error as much as possible. In our future research, we will introduce more precise error analysis methods to assess the reliability of the experimental data more accurately. Regarding the convergence of the experimental data, from the data of several repeated tests, the fluctuation of the data was small under the same experimental conditions, which indicates that the experimental results have better repeatability and convergence.
In order to investigate the evolution of the rapid condensation of SO2 and SO3 at low temperatures, three experiments were carried out in this paper: experiment A (flue gas including only SO3), experiment B (flue gas including only SO2) and experiment C (flue gas including both SO2 and SO3).
Figure 2a shows the concentration of SO42− in the CPM during the low-temperature condensation experiment A. It can be observed that the SO42− concentration of 100 mg/Nm3 SO2 transformed decreased with decreasing temperature. There was an increase at 0 °C because of the liquefaction of H2O vapour in the air, which promoted the non-homogeneous reaction of SO2. However, as the temperature decreased below zero, the SO42− concentration became lower; this implies that the generation of SO42− through the conversion of SO2 was impeded. And this phenomenon also showed that it was difficult for ice crystals to directly adsorb SO42− [37]. In addition, the findings of the experiments demonstrate that when 35 mg/Nm3 of sulphur dioxide (SO2) undergoes a chemical process, it can be turned into sulphate ions (SO42−) with a concentration ranging from 0.83 mg/Nm3 to 2.93 mg/Nm3. This range bears a greater resemblance to the actual measured values obtained from coal-fired power plants [10,12,38,39].
Additionally, it was shown that the SO42− concentration of 10 mg/Nm3 SO3 converted to SO42− reached a maximum value of 9.79 mg/Nm3 at −5 °C and a minimum value of 6.18 mg/Nm3 at 10 °C. The increase in SO42− concentration at 5 °C was attributed to the fact that most of the H2SO4 in the flue gas was deposited on the condenser tubes in the form of surface deposition at around 10 °C [22,36,40,41], and a small portion was in the form of aerosols, which could not be effectively captured by EPA Method 202 [42,43]. As the temperature decreased to the freezing point of dilute H2SO4 (around 5 °C), ice crystals began to take shape. The surface architecture of these ice crystals is not solely favourable for the attachment of fine particulate matter [33], but the quasi-liquid layer on the surface of the crystals can also promote non-homogeneous phase reactions [44]. As a result, the SO42− concentration in the condensate increased, triggering a growth in the concentration of SO42− within the CPM. As the temperature continued to decrease, the ratio of surface-deposited state H2SO4 in the flue gas decreased as the temperature within the exhaust gas stream steadily diminished, and SO3 appeared more in the form of sulphuric acid aerosol [22]. This was accompanied by a temperature drop to 0 °C and below, when H2O vapour turned into ice crystals, and the growth of ice crystals entailed a shift from the gaseous state to the solid state. This process encompassed two phase transitions: condensation and freezing [33,45]. Therefore, SO42− containing aerosols slowly adsorbed to the surface after ice crystal formation. This explains the increase in SO42− concentration after temperature decrease. According to Feng et al. [8], the errors brought about by the escape of gas and aerosol cannot be entirely eradicated by EPA Method 202. The general conclusion is that H2SO4 may be present in gaseous, aerosol and surface-deposited states on the condensate tube and CPM membrane, and the results of the study show that the pattern of SO42− change in the rinse solution and membrane is basically the same. Therefore, it also validates that the proportion of SO42− in the surface deposition state in the exhaust gas is the key factor having an impact on the SO42− content in the filtration membrane and the washing liquid. [23].
As shown in Figure 2b, the concentration of SO42− generated in Experiment 3C was much higher than that in experiments 3A and 3B combined, which proves that SO2 and SO3 can synergise to promote the generation of SO42− in CPM. From a theoretical perspective, when 10 mg/Nm3 of SO3 undergoes conversion, the upper limit of the resulting SO42− concentration is 12 mg/Nm3. However, this value pales in comparison to the concentration of SO42− that is generated through the joint action of SO2 and SO3. In this study, we put forward the hypothesis that the reason for the combined effect of SO2 and SO3 is mainly the strengthening of the non-homogeneous reaction of SO2. In particular, from 5 to −5 °C, a gas-phase reaction involving multiphase chemistry, SO3 will be converted to sulphuric acid aerosol under rapid condensation conditions, creating a brand-new locale that enables the non-uniform chemical transformation processes where SO2 takes part. The H2SO4-rich liquid particulate matter will then form larger particles by “condensation-agglomeration” and be captured by the fast condensation system, resulting in a rise in SO42− concentration in the CPM. The decrease in SO42− concentration under continuous cooling at −5 °C was presumably due to the rapid formation of ice crystals, which inhibit the non-homogeneous reaction of SO2. In addition, some other conclusions were obtained during the experiment:
(1) As the condensation temperature decreased within the range from 10 °C to −5 °C, the proportion of H2SO4 in the flue gas escalated due to an enhanced proportion of H2SO4 transitioning into a surface deposition state [21,34]. Additionally, the proportion of H2O in moles increased as the condensation temperature declined (10~0 °C), and H2O vapour in the air condensed into small droplets, at which time SO2 was highly susceptible to aqueous-phase oxidation in the low-temperature environment. Consequently, there was a notable increase in the SO42− content within CPM [46,47].
(2) As the condensation temperature dipped below 0 °C, the conversion of SO2 to SO42− diminished significantly. This was primarily attributed to the enhanced freezing of H2O vapor at lower temperatures, which limited the formation of droplets. Consequently, the non-homogeneous reaction between SO2 and droplets was hindered, ultimately resulting in a decrease in the SO42− content within CPM.
(3) The decreased trapping of SO42− on the membrane with lower temperatures indicates that the condenser tube effectively captured the majority of SO42− formed through the conversion of SO3 and SO2 in CPM. This observation suggests that sulphuric acid aerosols are more prevalent in the winter air environment, providing a compelling explanation for the higher incidence of grey haze days during this season.

3.2. Influence of H2O

To fully evaluate the impact of H2O content in flue gas on SO42− in CPM, three different humidity levels were set in this experiment, and the molar fractions of H2O in the flue gas were set to 3.98%, 8.14% and 12.89%, respectively.
As depicted in Figure 3, with the rise in the molar fraction of H2O in the flue gas, the concentration of SO42− in the CPM increased. This is mainly due to the changes in the water vapor content. Fundamentally, a higher content of H2O vapor in the flue gas facilitates condensation, causing the generation of more droplets. As a result, when a temperature gradient occurs between the flue gas and the droplets, a Stefan flow directed towards the droplets is produced, giving rise to diffusion electrophoresis. Subsequently, this process propels the SO42− in the flue gas to approach the droplets, leading to an increase in the concentration of SO42− [42,46,48,49]. Stefan flow is the macroscopic fluid motion caused by phase transitions (e.g., melting, evaporation) at the interface, driven by mass transport during phase change rather than external pressure gradients. What is more significant is that research findings indicate that droplets did not merely serve as reaction sites but also functioned as reactants or even catalysts. Consequently, with the increase in the molar fraction of H2O in the flue gas, the intensity of the Stefan flow escalated, and the non-homogeneous reaction of SO2 was also augmented. As a result, the concentration of SO42− in the CPM rose in tandem. A non-homogeneous reaction happened at the interface between different phases (e.g., solid–liquid), driven by mass transfer across phase boundaries, unlike reactions within a single phase.
As shown in Figure 3c, in experiment A, the increase of flue gas humidity, the concentration of SO42− trapped in the flushing solution and the membrane also increased. The reason is that the increase in droplets enhanced the Stefan flow effect, thus increasing the concentration of SO42−. Experiments have shown the presence of Stefan flow and non-homogeneous reactions in both the membrane and the flushing solution, with no variation due to distributional differences. In addition, the low SO42− concentration detected in the membrane in experiment B confirms that SO3 is an important source of SO42− in the CPM [23].

3.3. The Impact of NH3 and NOX

In an attempt to investigate the effects that NOX and NH3 exert on SO42− within CPM, 110 mg/Nm3 of NOX and 12 mg/Nm3 of NH3 were introduced into the flue gas. During this experimental process, the concentration arrangements of the gaseous pollutants, namely NOX, NH3, SO3 and SO2, were ascertained by taking references from and making adjustments to the ultra-low emission standards applicable to coal-fired power plants in China.

3.3.1. Features and Traits of SO2

Figure 4a clearly shows that the addition of either NOX or NH3 can promote the generation of SO42−, which is due to the fact that NOX can react with SO2 in a redox reaction under the condition of 12.89% H2O, which can promote the generation of SO42−. NH3, which is highly soluble in water, is prone to form a liquid layer that absorbs SO2, which leads to a further enhancement of the oxidation degree of SO32− and HSO3−, and subsequently promotes the generation of SO42−. In addition, given the powerful stability that acid–base clusters possess, NH3 promotes the generation of SO42− more significantly than NOX. The highest concentration of SO42− in CPM was observed in the presence of both NOX and NH3. This suggests that there is a synergistic effect between the two, with both NOX and NH3 promoting the non-homogeneous reaction of SO2 [24,25,26,50], which subsequently promotes SO42− production.

3.3.2. Features and Traits of SO3

As depicted in Figure 4b, NH3 and NOX had an impact on the conversion of SO3. The results showed that the addition of NH3 to SO3 and the increase in concentration at SO42− in the cooling interval from 10 to 0 °C was due to the liquefaction of H2O vapour and the uptake of SO42− by the droplets, which promoted the increase in SO42− concentration [47,50]. After cooling down to sub-zero temperature, due to the condensation of H2O, which inhibited the non-homogeneous reaction of SO2, the conversion of SO42− was not obvious, resulting in little change in SO42− concentration. On the contrary, SO42− concentration increased with the addition of NOX, owing to the fact that neutral nucleation serves as the crucial factor in the nucleation process of multicomponent systems. Additionally, nitrogen oxides (NOX) are capable of facilitating the generation of deposited sulphuric acid (H2SO4), and this, in turn, exerted a negative impact on the steadiness degree of the grouped particles in the dispersive system of sulphuric acid [44,51,52]. In turn, HNO3 also reacted non-homogeneously with sulphuric acid aerosols [53]. When the temperature decreased to the freezing point of dilute HNO3 (around −5 °C), ice crystal adsorption deposited H2SO4 in the condenser. So, the proportion of deposited SO42− increased, giving rise to a growth in concentration.
Furthermore, the experimental results also showed that when NH3 and NOX coexisted, the SO42− concentration showed an increase on the basis of NH3 only. The lower the accompanying temperature was, the less SO42− was captured, and an inhibitory effect occurred. This was due to the formation of particulate matter (NH4)2SO4, NH4HSO4 and NH4NO3 (Figure 5), which is one of the main components in CPM, and the diameter of this particulate matter predominantly focuses on the range of 0.1–1.0 μm [15,31,54], which is extremely difficult to capture [42]. Morphological analyses also showed that SO3, NH3 and NOX produced small amounts of particulate matter (Figure 5). This finding serves as an indirect confirmation that nitric acid, sulphuric acid and ammonia act in concert to generate particles at an accelerated rate than any two of the three components [28].

3.3.3. Features and Traits of SO2 and SO3

In the presence of all gaseous pollutants, with ammonia (NH3) included, nitrogen oxides (NOX), sulfuric dioxide (SO3) and sulfuric trioxide (SO2) were present within the reactor; Figure 4c shows that the SO42− concentration was concentrated at 16.74~27.39 mg/Nm3. Leaving aside the amplification experiments, this result was also closer to the previous year-round measured data from the coal-burning power station [14] and was even closer to northern winter haze weather pollution monitoring [26]. Therefore, by conducting this experiment, we were able to simulate the quick condensation of exhaust gas to sub-zero in winter and provide a base reference for controlling SO42− in CPM. In addition, the SO42−concentration rose to a maximum concentration of 27.39 mg/Nm3 during the process of the condensation temperature lowering from 10 °C down to −10 °C, and then decreased, a phenomenon caused by the increase in the molar fraction of H2SO4 at the cooling temperature and the process of SO2 being oxidized in the aqueous-phase at temperatures on the lower side [47], suggesting that the SO42− evolution is similarly regulated by the stack gas condensation temperature. When conducting the experiments, NH3 greatly promoted the generation of SO42− (Figure 4c). This phenomenon is due to the aqueous-phase oxidation of SO2, which is promoted by NH3 and is also present in the synergistic interaction of SO3 and SO2. NH3 promotes the non-homogeneous interaction reaction of SO2 and enhances the nucleation of atmospheric H2SO4 aerosol.
When NOX was added in the presence of NH3, the experimental results confirmed that NOX inhibited SO42− production. One contributing factor is that the acidic conditions within the exhaust gas, which was brought about by the process of H2SO4 formation from SO3 along with the dissolution of NOX, impeded the oxidation of SO3 and HSO3 [46], and thus there was a lower concentration of SO42− than that in the absence of NOX and NH3, and the inhibitory effect of NOX was also present in the synergistic effect of SO3 and SO2. The other part is due to the fact that below about 5 °C, HNO3 and NH3 can condense into new nucleated particles with diameters of a few nanometres. Moreover, when cold enough (−15 °C below), HNO3 and NH3 are capable of nucleating directly via an acid–base stabilisation mechanism to form NH4NO3 particles [13,53]. This is in accordance with the SEM outcomes presented in the figure (Figure 5c). Moreover, the nucleation process of ammonium nitrate might decrease the overall quantity of NH3. As a result, it indirectly restrained the enhancement effect of NH3 on sulphate; this led to a reduction in the concentration of the seized sulphate ions (SO42−).

3.4. Morphological Characterization

This experiment focused on analysing the morphology and energy spectrum characteristics of condensable sulphate particulate matter. The results revealed that scarcely any particulate matter was detected on the membrane at sub-zero temperature in the absence of SO3 flow. This phenomenon can be ascribed to the lack of a reaction site provided by the condensation of SO3. Inside the membrane, SO3 and NH3 were prone to generate highly aggregated particles (as depicted in Figure 5a). When SO3, SO2 and NH3 co-existed, they were likely to form droplet-like particulate matter (Figure 5b). SEM results showed that SO42− formation from SO2 in CPM was mainly from non-homogeneous phase reaction, and the aerosolised H2SO4 formed by SO3 condensation provided reaction space for sulphate generation. The cooperative effect of SO2 and SO3 strengthened the nucleation mechanism of NH4+ and SO42−, and NH3 could promote the generation of sulphate PM [32]. Therefore, SO3 and NH3 play a crucial role in atmospheric aerosol formation.
In addition, in the coexistence of SO3, NH3 and NOX, the particulate matter was significantly smaller (Figure 5d), and the EDS showed that there was an increase in elemental N and a decrease in elemental S. This is due to the fact that the lower the temperature is, the higher the likelihood is of trapping HNO3 [45]. It indicates that NOX suppressed the production of sulphate and increased the production of nitrate and ammonium salts. Moreover, under the complex flue gas conditions, SEM detected fine particles wrapped on the quartz film (Figure 5c), and EDS analysis found that the content of S, N, and O elements was also the highest, which confirms that sub-zero-temperature NH3 and NOX can form sulphate, nitrate and ammonium salts of fine particles with SO2 and SO3 rapidly and coalesce into larger particles [28].

4. Concluding Remarks

This piece of research was cantered around the pattern of change in the concentration of sulphate ions (SO42−) and the morphological traits of sulphate ions (SO42−) under conditions of rapid condensation; the subsequent conclusions were reached:
(1) In this study, we focused on the rapid condensation conditions at low temperatures for the first time and systematically investigated the effects of condensation temperature, water vapour, NOX, NH3 and other factors on SO42− generation. The mechanism of SO42− generation was found to be more complex in low-temperature environments compared with studies at high temperatures. Factors such as ice crystal formation and gas phase transition significantly affected its generation, and the synergistic promotion of SO2 and SO3 was more prominent at low temperatures. Meanwhile, the reaction properties of NOX and NH3 at low temperatures differed from those at high temperatures, and these findings provide new perspectives for understanding the formation of sulphate in hazy winter weather, which can help to come up with more impactful air pollution control plans.
(2) With the alteration of condensation temperature, the concentration of SO42− in the CPM of flue gas varied from 16.41 mg/m3 to 26.79 mg/m3, and then to 19.83 mg/m3. This was due to the gas-phase reaction of multiphase chemistry that occurs in the condensation and freezing of H2O vapour, H2SO4, HNO3, etc. The form of H2SO4 also affected the change of the concentration of SO42−. The concentration of SO42− from SO2 and SO3 reached 36.18 mg/m3, which was greater than the sum of the SO42− from single SO2, at 7.84 mg/m3, and single SO3, at 9.79 mg/m3. Commonly, SO2 and SO3 can cooperatively boost the formation of SO42− in the CPM, and the increase in H2O can promote the non-homogeneous reaction. SO42− can also promote non-homogeneous reactions, which, in turn, can also promote SO42− production.
(3) NH3 enhanced the nucleation of atmospheric H2SO4 aerosols and promoted the formation of sulphate particles, with particle sizes even exceeding 5 µm. Whereas NOX may have the opposite effect on SO42− production. At sub-zero temperatures, NH3 and NOX can react with SO2 and SO3 to rapidly form sulfate, nitrate, and ammonium salts, these fine particles are only 2 µm in size or even smaller. These are fine particles and difficult to capture, and eventually react in the atmosphere to produce atmospheric aerosols. Therefore, the burning of coal in the winter is highly susceptible to the formation of hazy weather.

Author Contributions

Y.X.: conceptualization; supervision, project administration. H.L.: data curation, conceptualization, formal analysis, research exploration, methodological approaches. C.Z. and K.Z. (Kai Zhou): resources, writing—original draft. Y.Z.: methodological approaches, verification, data sources and Materials, manuscript revision and editing, oversight, funding procurement. L.Y.: investigation. Z.S. and K.Z. (Ke Zhuang): methodology, writing—reviewing and editing, validation, supervision, funding acquisition. All authors have read and agreed to the published version of the manuscript.

Funding

This study was funded by multiple sources. It received support from the Open Project Program of the State Key Laboratory of Low-carbon Smart Coal-fired Power Generation and Ultra-clean Emission, with grant numbers D2023FK096 and D2022FK079. Additionally, it was funded by the National Natural Science Foundation of China under grant numbers 52470121, 41877469, and 41771498. The Natural Science Foundation of Jiangsu Province, under the grant number BK20240597, also provided support. Moreover, the Natural Science Research Project of Universities in Jiangsu Province of China, with the project number 24KJB610010, contributed to this work.

Institutional Review Board Statement

This research does not necessitate ethical clearance.

Informed Consent Statement

There was no need to obtain consent to participate in this research.

Data Availability Statement

The data and materials are available from the corresponding author upon reasonable request.

Conflicts of Interest

Author Yun Xu, Kai Zhou, Ke Zhuang and Chunlei Zhang is employed by the company China Energy Science and Technology Research Institute Co., Ltd. The authors explicitly confirm that there are absolutely no conflicts of interest.

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Figure 1. Dynamic flue gas simulation system.
Figure 1. Dynamic flue gas simulation system.
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Figure 2. The concentration levels of SO42− in the CPM vary under different reaction conditions. Reaction condition (a): in the presence of 10 mg/Nm3 of SO3 or 100 mg/Nm3 of SO2 at a relative humidity of 12.89% H2O. Reaction condition (b): in the presence of both 10 mg/Nm3 of SO3 and 100 mg/Nm3 of SO2 at a relative humidity of 12.89% H2O.
Figure 2. The concentration levels of SO42− in the CPM vary under different reaction conditions. Reaction condition (a): in the presence of 10 mg/Nm3 of SO3 or 100 mg/Nm3 of SO2 at a relative humidity of 12.89% H2O. Reaction condition (b): in the presence of both 10 mg/Nm3 of SO3 and 100 mg/Nm3 of SO2 at a relative humidity of 12.89% H2O.
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Figure 3. The impact of H2O vapor on SO42−. Reaction conditions (a): SO42− concentration variation in CPM when flue gas contained 10 mg/Nm3 SO3. Reaction conditions (b): SO42− concentration alteration in CPM when flue gas consisted of 100 mg/Nm3 SO2. Reaction conditions (c): SO42− concentration change in CPM when flue gas included 10 mg/Nm3 SO3.
Figure 3. The impact of H2O vapor on SO42−. Reaction conditions (a): SO42− concentration variation in CPM when flue gas contained 10 mg/Nm3 SO3. Reaction conditions (b): SO42− concentration alteration in CPM when flue gas consisted of 100 mg/Nm3 SO2. Reaction conditions (c): SO42− concentration change in CPM when flue gas included 10 mg/Nm3 SO3.
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Figure 4. The effect of NOx and NH3 the concentration of SO42−. Reaction conditions (a): the gas mixture contained 100 mg/Nm3 of SO2, 110 mg/Nm3 of NOX and 12 mg/Nm3 of NH3, with a 12.89% H2O content in the reaction atmosphere. Reaction conditions (b): composed of 10 mg/Nm3 of SO3, 110 mg/Nm3 of NOX and 12 mg/Nm3 of NH3, reacting in an environment with 12.89% H2O. Reaction conditions (c): the reaction system had 10 mg/Nm3 of SO3,100 mg/Nm3 of SO2, 110 mg/Nm3 of NOX and 12 mg/Nm3 of NH3 in the presence of 12.89% H2O.
Figure 4. The effect of NOx and NH3 the concentration of SO42−. Reaction conditions (a): the gas mixture contained 100 mg/Nm3 of SO2, 110 mg/Nm3 of NOX and 12 mg/Nm3 of NH3, with a 12.89% H2O content in the reaction atmosphere. Reaction conditions (b): composed of 10 mg/Nm3 of SO3, 110 mg/Nm3 of NOX and 12 mg/Nm3 of NH3, reacting in an environment with 12.89% H2O. Reaction conditions (c): the reaction system had 10 mg/Nm3 of SO3,100 mg/Nm3 of SO2, 110 mg/Nm3 of NOX and 12 mg/Nm3 of NH3 in the presence of 12.89% H2O.
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Figure 5. Morphological and energy spectral characterization of CPM in SO3 experiments. The reaction conditions were as follows: (a) A reaction occurred between SO3 and NH3; (b) SO3, SO2 and NH3 reacted together; (c) SO2, SO3, NH3 and NOX participated in the reaction; (d) SO3, NH3 and NOX were involved in the reaction.
Figure 5. Morphological and energy spectral characterization of CPM in SO3 experiments. The reaction conditions were as follows: (a) A reaction occurred between SO3 and NH3; (b) SO3, SO2 and NH3 reacted together; (c) SO2, SO3, NH3 and NOX participated in the reaction; (d) SO3, NH3 and NOX were involved in the reaction.
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Table 1. The parameters associated with every major component found in the flue gas.
Table 1. The parameters associated with every major component found in the flue gas.
ParametersSO3
(mg/Nm3)		SO2
(mg/Nm3)		NH3
(mg/Nm3)		NOX
(mg/Nm3)		Total Flow (L/Min)Ratio of O2 (%)
Flow value12100121101.95–2.056–9
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Xu, Y.; Lu, H.; Zhou, K.; Zhuang, K.; Zhang, Y.; Zhang, C.; Yang, L.; Sheng, Z. Evolutionary Characteristics of Sulphate Ions in Condensable Particulate Matter Following Ultra-Low Emissions from Coal-Fired Power Plants During Low Winter Temperatures. Sustainability 2025, 17, 6342. https://doi.org/10.3390/su17146342

AMA Style

Xu Y, Lu H, Zhou K, Zhuang K, Zhang Y, Zhang C, Yang L, Sheng Z. Evolutionary Characteristics of Sulphate Ions in Condensable Particulate Matter Following Ultra-Low Emissions from Coal-Fired Power Plants During Low Winter Temperatures. Sustainability. 2025; 17(14):6342. https://doi.org/10.3390/su17146342

Chicago/Turabian Style

Xu, Yun, Haixiang Lu, Kai Zhou, Ke Zhuang, Yaoyu Zhang, Chunlei Zhang, Liu Yang, and Zhongyi Sheng. 2025. "Evolutionary Characteristics of Sulphate Ions in Condensable Particulate Matter Following Ultra-Low Emissions from Coal-Fired Power Plants During Low Winter Temperatures" Sustainability 17, no. 14: 6342. https://doi.org/10.3390/su17146342

APA Style

Xu, Y., Lu, H., Zhou, K., Zhuang, K., Zhang, Y., Zhang, C., Yang, L., & Sheng, Z. (2025). Evolutionary Characteristics of Sulphate Ions in Condensable Particulate Matter Following Ultra-Low Emissions from Coal-Fired Power Plants During Low Winter Temperatures. Sustainability, 17(14), 6342. https://doi.org/10.3390/su17146342

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