Emission Characteristics of Fine Particles from Wet Flue Gas Desulfurization System Using a Cascade of Double Towers

The removal of fine particles from coal-fired flue gas using chemical and physical reactions was investigated experimentally in a wet flue gas desulfurization (WFGD) system with a cascade of double-towers system. The flue gas particles were collected by an Andersen 8-stage impactor, and their mass concentration and particle size distribution were investigated. Based on analysis via scanning electron microscopy/energy dispersive X-ray spectrometry (SEM-EDX), X-ray diffraction (XRD) and inductively coupled plasma atomic emission spectrometry (ICP-AES), the morphological characteristics and the major and minor-element concentrations of particles were studied. The results indicate that the size distribution of fly ash particles at the inlet of the WFGD system was typically distributed bimodally. Although a bimodal distribution was still observed after the desulfurization, all the peaks had decreased. Furthermore, the content of S and Ca increased. Fine particles in the flue gas consisted of about 26.48% limestone and 41.19% gypsum particulate matter, eventually forming the Ca11.3Al14O32.3 crystal. The net removal efficiency of the double tower WFGD system reached 84.16% for the original particles, which was similar to that of the conventional single tower system. The entrainment of recirculated slurry contributed to the submicron particle emissions, and the total removal efficiency of the double tower WFGD system decreased to 51.1%.


INTRODUCTION
In recent years, the eastern part of China has suffered from severe pollution related to primary fine particulate matter (PM 2.5 ).The serious harm to human health caused by inhalable particulate matter has attracted more and more attention (Garea et al., 2005;Yue et al., 2005;Diaz-Somoano et al., 2007;Wang et al., 2008b;Jiang et al., 2015).Coal-fired power plants (CFPPs) are considered a major source of PM 2.5 and precursors of secondary particle including sulphur dioxide (SO 2 ) and nitrogen oxides (NO x ) in consideration of 44.8% of coal consumed nationwide being used for electricity generation at the end of 2015 (Li et al., 2014;NBS, 2016;EIA, 2016;Dodla et al., 2017).The Chinese government has been involved in reducing the pollutant emission from CFPPs.To meet with the ultralow and even zero emission regulation for air pollutants, the vast majority of CFPPs have been equipped with Cascade of Double Towers WFGD (CDT-WFGD) installations and hybrid ESP/BAGs to reduce the emissions of SO 2 and PM.The limestone-based WFGD and electrostatic precipitator (ESP) or fabric filter (FF) are now the most commonly used technologies for PM and SO 2 removal.Although the collection efficiency of particles by ESP or FF can be achieved as high as 99.9%, the fine particles cannot be effectively captured (Clarke, 1993;Meij, 1994;Ratafia-Brown, 1994;Huang et al., 2003;Ke, 2013).
The utilization of limestone-based wet FGD has led the trend since the 1970s, as it is the most cost-effective and reliable method, currently representing more than 90% of the installed desulfurization capacity worldwide (Kiil et al., 1998;Nolan, 2000;Taylor et al., 2005;Hrastel et al., 2007).The research and development over the last years has focused on improving removal efficiency (Chen et al., 2009).According to the study, low pH values accelerate the dissolution of limestone, as it is mass transfer controlled, at the expense of inhibiting the dissociation of SO 2 (Wallin and Bjerle, 1989;Alvarezayuso et al., 2006;Shengyu et al., 2010;Carletti et al., 2013).In contrast, high pH values reduce the solid solubility while benefitting the absorption of SO 2 (Lancia et al., 1997;Frandsen et al., 2001).The dual-pH cycle desulfurization process with a cascade of double towers was developed to obtain higher SO 2 removal efficiency (Rochelle and King, 1977;Honghe et al., 2016;Pan et al., 2016b).Due to the washing effect of desulfurization slurry, not only SO 2 can be removed from WFGD system installed in the CFPPs, but particle matter and other harmful substances can also be removed by WFGD process simultaneously (Nielsen et al., 2002;Bao et al., 2009;Pan et al., 2016a).In the study by Meij and Winkel (2004) and Cordoba (2015), the total particle removal efficiency of a WFGD system was 51.0%, but the concentration of fine particulate increased by 30%-80%, and the content of calcium increased significantly.The results from Wang et al. (2008a) and Zhou et al. (2013) showed that the entrainment of gypsum slurry was the main source of fine particles in the fly ash, and the fine particles in the flue gas consist of about 55.4% limestone and 7.9% gypsum particulate matters at the outlet of WFGD.Similar results were found by Fraboulet, Sinanis and Ma (Fraboulet et al., 2007;Sinanis et al., 2008;Ma et al., 2017), which the coarser particles were reduced by limestone-gypsum desulfurization process, while the emission of submicron particles increased.
Compared with the single tower, the double-tower system increases the number of spray layers which contribute to the SO 2 absorption and gypsum formation, and works in different space and pH value.Thus, the physical form and chemical composition of the particulate matter after desulfurization are also changed (Bao et al., 2012;Zhuang et al., 2015).However, little attention has been paid to the particle characteristics emission from CDT-WFGD process during actual operation in power plants.The purpose of this study is to investigate the particle removal efficiency morphological characteristics, element content of particulate, and formation mechanism of the fine particles from CFPPs equipped with CDT-WFGD.The difference of particle emission characteristics from a single tower WFGD system was also discussed in this work concurrently.

Experimental Conditions
A conventional pulverized coal-fired power plant (Tangshan, Hebei Province, China) was selected for PM emission characterization in this study.The boiler type is HG-2030/17.5-YM9, which is a subcritical forced-circulation coal-fired boiler with a maximum continuous evaporation of 2030 t h −1 .The low-NO x combustion burners and an ammonia-based SCR system were used for NO x removal, followed by electrostatic precipitators (ESPs) and a limestone-based Cascade of Double Towers WFGD scrubber in sequence.The technological process of the Cascade of Double Towers WFGD system was illustrated in Fig. 1.The effect of the 2 towers was significantly different for particulate removal.As the prescrubber, the 1 st absorption tower was initially used to reduce the SO 2 concentration of the flue gas and generate gypsum, and the 2 nd was mainly used to absorb the remaining SO 2 .The device can achieve very high desulfurization efficiency (99.9%) even for a high concentration of SO 2 .A two-stage flat plate demister was installed on the upper part of the absorption tower.The specific parameters of the desulfurization system are given in Table 1.

Sampling Method
An 8-stage Andersen Stack Impactor (TE-20-800-Tisch, Thermo Andersen Instruments Inc., U.S.A.) was used to   (Yong et al., 2005).At the inlet and outlet of the WFGD, a dust sampling instrument (3012H, Laoying, Qingdao, China) was used to collect the total dust, according to GB/ T16157-1996T16157- (MEP, 1996)).The sampling volume for flue gas was not less than 2.5 Nm 3 at each site to obtain enough samples for chemical analysis.Additionally, at least 3 groups of samples were collected at each site, and each sample was divided into 8 subsamples with different particle sizes by the sampling membranes.
In order to analyze the change of morphological characteristics and minor elements of particles, the testing points were located at both the inlet and outlet of the desulfurization system (Figs.1(a) and 1(b)).Additionally, the gypsum and limestone samples in the absorption tower was collected and analyzed for comparative (Figs. 1(c) and 1(d)).

Sample Processing and Analysis
Gypsum and limestone samples were grinded into particles with the diameter less than 0.2 mm.The gypsum samples were dried at 50°C for 2.5 h to remove the free water, and at 230°C for 3 h to remove the crystallization water.The sample film was dried at 105°C for 1 h and then cooled for 2 h to room temperature.According to GB/T 16157-1996 (in China) the sample film was weighed with an electronic balance with an accuracy of 1 µg.Organic impurities were removed by microwave digestion (PreeKem-Excel, Shanghai) before analyses.The contents of Fe, Cu, Mn, Mg, Al, Si, Ba, Ca and S in the samples were analyzed by ICP-AES (ICP-AES-X II, ThermoFisher, Germany).The morphology, chemical composition, and the solid phases were analyzed by SEM-EDX (S-4500, Hitachi, Japan) and XRD (Smart LAB, RIGAKU, U.S.A.), respectively.
For 9 elements analyzed using ICP-AES, the method detection limits (MDLs) were determined as the concentration equivalent of three times the standard deviation of seven replicate measurements of the analyte in reagent water.The MDLs were between 0.001 mg L −1 (Ba) to 0.58 mg L −1 (Fe).All the relative standard deviation (RSD) values for inorganic elements were lower than 5%, they were 2.93%, 3.16%, 2.96%, 2.83%, 3.37%, 2.61%, 4.64%, 3.65% and 4.21% for Al, Ba, Ca, Cu, Fe, Mg, Mn, S and Si, respectively.The accuracy of ICP-AES was determined by detection of standard soil materials as GBW07446-GBW07457 (Center for National Standard Matter, China).The recoveries for analyzed elements using analysis method adopted in this study were in the range of 80-120%.

Removal of Total Fly Ash Particles
Three groups of samples were collected at the inlet and outlet of the desulfurization tower.The concentrations of particles in each sample were determined by weight method.The average value of the test results was given in Table 2.The total mass concentration of particles decreased from 23.77 mg Nm −3 at the inlet to 11.63 mg Nm −3 at the outlet, therefore the total removal efficiency was 51.1%.The particles from coal-fired flue gas were collected and removed by inertia impaction, interception and Brownian diffusion during the WFGD process (Cordoba, 2015).Each WFGD scrubber was equipped with a vertically oriented vane pack demister, which is typically used for CFPPs in China.The spray layer was increased in the CDT-WFGD scrubbers, the liquid sprays can promote the removal of particles > 1 µm, but relatively inefficient in removing finer particles (Wang et al., 2008a;Weiguo et al., 2015;Zhuang et al., 2015).Depending on the design parameters and operating conditions, the demister can remove all particles larger than 10 µm in diameter, while the collection efficiency is much lower for PM 2.5 , resulting in a lower total removal efficiency of particles.

Fine Particle Size Distribution
The particle mass concentration distributions at the inlet and outlet of the WFGD can be expressed by dM/dlog Dp (Allen, 1997), and the result of computation was shown in Fig. 2.
where D P is the aerodynamic diameter of particles, M is the size-segregated particulate mass concentration, uup and low are the previous and next stage particle size, respectively.It was found that the particle size at the inlet of WFGD showed a typical bimodal distribution, a number of previous studies have also found similar results (Ylatalo and Hautanen, 1998;Fraboulet et al., 2007;Li et al., 2009;Niemela et al., 2009;Li et al., 2017).In the XRD spectrogram, the peak height at 2.9 µm is 2 times higher than that of B. The two peaks reflected two different mechanisms of particle  formation in the process of coal combustion.Fine particles mainly come from the gasification-condensation process of inorganic matter in coal, and coarse particles mainly consist of residual minerals from the coke (Wang et al., 2014).The size distribution markedly changed when flue gas had passed through the WFGD scrubbers.At the WFGD outlet, the particle size was still distributed in bimodal-peaks, but the particle concentration decreased.The peaks moved to the small particle size range, with the peak at 0.8 µm and 2.5 µm, and the former concentration was significantly higher than the latter.
It seems that the liquid sprays in the WFGD scrubbers can promote the removal of coarser particles, but it was difficult to remove PM 1 (Fig. 3).The particle removal efficiency of the CDT-WFGD system for PM 10 and PM 0.45 was decreased from ~89% to ~8% (Fig. 3).This was probably due to PM 1 was typically smaller than the minimum that can be removed during the slurry spraying process.Additionally, some secondary particles were generated by the entrainment of droplets.

Particles Size Accumulative Distribution
Rosin-Rammler distribution (R-R distribution) was used to describe the size accumulative distribution of fly ash particles after coal combustion, which is widely used in cumulative distribution characteristics of various dusts (Allen, 1997), and can estimate the percentage of the total where R is the particle mass cumulative fraction of screen residue, a is the particle diameter distribution coefficient, and n is the particle size distribution index.It was shown in Fig. 4 that the inosculation between inlet and inlet-fitting was synchronous.But significant deviations existed in the distribution of the outlet.This was because the particles at the inlet were directly derived from the combustion of coal, and the capture and removal of ESP did not change the chemical properties for PM 10 (Meij and Winkel, 2004).At the outlet of the WFGD system, the particles also contain the fine particles of limestone or gypsum that were produced during the desulfurization process, which contributed to the deviation from the R-R fitting.
By comparing the change of the median diameter D 50 , we could see that D 50 was 2.83 µm at the inlet, while it was significantly reduced to 0.95 µm at the outlet, and the mass ratio of w(PM 2.5 )/w(PM 10 ) and w(PM 1 )/w(PM 10 ) increased by 82% and 213%, respectively.Therefore, the proportion of fine particles in PM 10 at the outlet has risen a lot.The larger particles were removed by desulfurization slurry, but large amount of fine particles were emitted into the air and left a great threat to human health.

Particle Morphology and Minor Elements
The morphology of particles in flue gas before and after desulfurization was characterized by SEM (Fig. 5).Before desulfurization (Fig. 5(a)), the particles were irregularly spherical and relatively dispersed, but the degree of agglomeration increased after desulfurization (Fig. 5(b)) and these particles formed a dense irregular lumps or flocculent structure.
To further investigate the influence of entrainment on element contents, ICP-AES and energy dispersive spectroscopic (EDS, Fig. 6) were used to analyze the minor elements in the gypsum, limestone and fly ash before and after desulfurization (Table 3).During the desulfurization, mass concentrations of Ca and S increased from 3.77 to 8.19 mg g −1 and 89.73 to 96.96 mg g −1 .This was consistent with the observation of EDS.The total mass of Si, Al, S and Ca was more than 90% at the inlet and the outlet (Fig. 6).After desulfurization, the Si and Al decreased slightly while S and Ca increased.
The change of the elemental content was mainly due to the introduction of gypsum granules in the desulfurization tower and the absorption of SO 2 by CaO/CaCO 3 .The increase of S and Ca in particles can be explained by the entrainment of dissolution and gypsum slurry, where S and Ca in gypsum and limestone were the main constituent.Therefore, the entrainment of gypsum and slurry contributed more to coarser PM 2.5 .As for Si and Al, their high volatilization point and stable chemical property lead the mass of them changed little but the proportion decreased in flue gas (Clarke, 1993;Meij, 1994).
Based on the change of the content of Ca in the particles, the proportion of limestone and gypsum particles on the fly ash particles can be obtained.Due to the nonvolatility and stability of Ba and Fe (Clarke, 1993;Meij, 1994;Ratafia-Brown, 1994;Meij and Winkel, 2004), we used the mass change of Ba and Fe to calculate the proportion of limestone and gypsum for the reason that Ba and Fe decreased proportionally when limestone and gypsum attached to original particles.
where W is mass, s is the mass concentration of S element, m is the mass concentration of one of Ba and Fe, the footsteps fa,in, fa,out, g and l respectively, represent the inlet and outlet of the desulfurization system, fly ash, gypsum and limestone.
The results showed that the contents of limestone and gypsum in fly ash after desulfurization were 51.5% and 22.6%, respectively.After further calculation, the net removal efficiency of fly ash was 84.2%, which meant the removal efficiency of the original fly ash, not covering the entrainment.For CDT-WFGD system and single tower WFGD system (Wang et al., 2008b;Wang et al., 2014), the net removal efficiency for particulate matters was similar, but the CDT-WFGD system formed much more submicronscale particles which led to a decrease of total removal efficiency.

Mineral Composition
The mineral component of the fly ash was characterized by XRD.The pattern before desulfurization has a number of peaks that can be well-indexed to mullite (Fig. 7 This can be ascribed to the particles before WFGD from coal combustion sources.The composition of particles before and after desulfurization is roughly the same (Fig. 7(b)).After desulfurization, Ca 11.3 Al 14 O 32.3 was detected except for mullite.
Remaining CaO and CaCO 3 would be emitted into the Gas Gas Heater (GGH) and collected by the sampler formed porous structure, which adsorbed Al and other metal elements in the flue gas generated Ca 11.3 Al 14 O 32.3 and other substances.Moreover, the condensation and adsorption of As 4 O 6 , SeO 2 and other gaseous matters could also attribute    to this structure (Ghosh-Dastidar et al., 1996;Sun et al., 2004;Jianyi and Dingkai, 2006).In general, the oxidizing substances in the flue gas, such as CaO and CaCO 3 , can promote the cohesion and agglomeration of particles, which create a favorable environment for the final formation of Ca 11.3 Al 14 O 32.3 .

CONCLUSION
CDT-WFGD system improved the efficiency of desulfurization and significantly reduced the concentration of particulate in flue gas.A bimodal distribution of particle concentrations was observed at both the inlet and the outlet of the system.The results showed that the total removal efficiency of particles was 51%.During desulfurization, the mass concentrations of Ca and S increased from 3.77 to 8.19 mg g −1 and from 89.73 to 96.96 mg g −1 , respectively.Compared with the single tower WFGD, the double-tower system had a similar net removal efficiency, of 84.16%.The particle removal efficiency of the system decreased from ~89% at 10 µm to ~8% at 0.45 µm as the particle size decreased.This can be ascribed to the fact that the CDT-WFGD system is relatively incapable of removing PM 1 .The percentage of limestone and gypsum generated from entrainment of the desulfurization slurry was 51.5% and 22.6%, respectively.On the other hand, through entrainment and evaporation, a significant amount of fine particles from limestone and gypsum may be emitted into the atmosphere and form Ca 11.3 Al 14 O 32.3 through cohesion and agglomeration.

Fig. 1 .
Fig. 1.The technological process of a Cascade of Double Towers WFGD system.

Fig. 2 .
Fig. 2. Flue gas particle size distribution before and after WFGD system

Fig. 4 .
Fig. 4. Particles size accumulative distribution experimental results and R-R fitting curves before and after desulfurization.

Fig. 5 .
Fig. 5. SEM images of flue gas particles at inlet and outlet of desulfurization tower.

Fig. 6 .
Fig. 6.EDS result of flue gas particles at inlet and outlet of desulfurization tower.

Table 1 .
Main design parameters of the desulfurization system.

Table 3 .
Content of various elements in the particulate matter before and after desulfurization by ICP.