Emission and Species Distribution of Mercury during Thermal Treatment of Coal Fly

The thermal treatment of coal fly ash (FA) utilized by industries will inevitably lead to the emission of mercury, potentially causing atmospheric pollution. The present study is aimed at understanding the emission amount and species distribution of mercury during FA thermal treatment, and attempts were made to further provide basic data of mercury emission for environmental utilization of the FA by industries as raw materials. The physio-chemical properties of FA were analyzed and FA samples were heated at 200–1200°C in a tubular furnace in an oxidizing atmosphere. Thermogravimetric analysis (TGA) was carried out to determine the content of residual carbon in FA. The mercury species distribution was determined using United States EPA method 3200. The results indicated that semi-mobile mercury was the main mercury species in FA. Owing to the adsorption of residual carbon on Hg, more residual carbon which lead to higher proportion of semi-mobile obtained in coarser size grades. 20% of total mercury emitted at 206°C, followed by a rapid growth of the emission rate of mercury at above 206°C, and the emission of total mercury achieved to be 50% and 90% corresponding to 291°C and 515°C, respectively. A distinct transformation from semi-mobile mercury to extractable mercury occurred at 200°C due to catalytic oxidation of Hg and enhance the mobility and potential toxicity of mercury. Both extractable and semi-mobile mercury decreased sharply at 300–400°C. Finally, the non-mobile mercury was emitted with combustion of residual carbon in FA at 600–800°C. It was thus concluded that measures would be taken to control emission of mercury for environmentally friendly utilization of coal FA by industries when the FA was heated higher than 300°C.


INTRODUCTION
Mercury is one of the most environmentally mobile hazardous heavy metal elements, of which compounds often exhibit biological toxicity, persistence and bioaccumulation (Clarke and Sloss, 1992;Yudovich and Ketris, 2005;Jen et al., 2014).Long-term exposure of high concentration of inorganic mercury poses adverse effects on the gastrointestinal tract, nervous system and kidneys of human beings (Goyer et al., 2000).In addition, mercury can also be transformed to methylmercury or ethylmercury by microbial activity which tend to be accumulated in fish resulting in harm to the fish and its predators (Wheatley and Wheatley, 2000), and eventually enter the human body via the food chain (Pavlish, 2009).
Coal fly ash (FA), one type of coal combustion by-products (CCBs), with finer particles in size (i.e., 0.01-100 µm) is considered as a hazardous material (Ram and Masto, 2010).In China, the installed capacity of coal-fired power plants increased significantly since 2002 and the coal consumption reached up 2.8 billion tons in 2010 (China, 2010).China is the largest coal consumer and accounted for 50.2% of the world coal consumption in 2012 (Yao et al., 2015) and coal generates about 70% of the primary energy in China (Hu et al., 2015;Qin et al., 2015).Annual generation of FA still increases and is anticipated to reach 580 million tons by 2015 (Yao et al., 2014).It is noted that the coal FA has been successfully utilized in various applications, such as an adsorbent for CO 2 capture (Pan et al., 2012) and/or a supplementary cementitious material (Pan et al., 2015).However, the growing awareness of the environmental concerns requests a better understanding of the physicochemical properties of FA.Therefore, there would be an urgent need for addressing this issue.
In China, the average concentration of mercury in raw coal is about 0.22 mg kg -1 (Wang et al., 2000), which is higher than the Clarke value of mercury in the crust (0.08 mg kg -1 ) (Wang and Wei, 1995) and global average concentration of mercury in raw coal (0.12 mg kg -1 ) (Wang et al., 2002).The combustion condition of coal, residual carbon content, coal type, presence of magnetite, chlorine concentration, and composition of the FA, are the main factors affecting the emission concentration of mercury in flue gas (Mardon and Hower, 2004;Maroto-Valer et al., 2005;Chiu et al., 2015).In general, the mercury content in FA is approximately 1-4 times (Yao et al., 2004) higher than that in raw coal, but in some cases are lower (Wang et al., 1999).In China, the emission standard for mercury from thermal power plants is 0.03 mg m -3 (China, 2011).
In China, the utilization ratio of FA has remained at approximately 67% in recent years, and is expected to reach 70% by 2015 (Yao et al., 2015).At present, FA is widely used in construction materials, agriculture, absorbents, smelting industries, and geotechnical engineering (Kamon et al., 2000;Cheng et al., 2004).In these industries, heat treatment such as drying, roasting, and calcining is required before the using of FA or its mixtures.As suggested by the literature (Xu et al., 2005;Cultrone and Sebastián, 2009), the preparation of the FA brick required to heated at 800-1000°C, the FA molecular sieve needed to be heated to 850°C with drying at 450-550°C, and the recovery alumina of FA require a calcining temperature in the range of 1300-1400°C.Owing to volatility and high content of the mercury in FA, it is inevitably released into the atmosphere during thermal treatment.
The residual carbon plays an important role in adsorption of mercury in FA (Li and Hwang, 1997;Serre and Silcox, 2000;Senior and Johnson, 2005;Wang and Wu, 2006) As reported, there is a positive correlation between mercury concentration and residual carbon content in FA (Hower et al., 2005).FA can also be employed as a mercury adsorbent for removing gaseous mercury from flue gas (Hassett and Eylands, 1999;Serre and Silcox, 2000;Hutson, 2008).Recently, several studies have shown that Hg 0 and Hg 2+ are the dominate chemical form of mercury in FA (Galbreath and Zygarlicke, 2000;Lopez-Anton et al., 2011).The present form of Hg 2+ was found to be HgCl 2 and HgO primarily, which could be adsorbed on the residual carbon surface in FA (Galbreath and Zygarlicke, 2000;Lopez-Anton et al., 2011).
Based on the literature, previous studies which focused on the mercury mobility of flue gas desulphurization byproducts (FGDBs) during the thermal treatment (Heebink and Hassett, 2005;Kairies et al., 2006), has shown that Hg 2+ is reduced to Hg 0 and emitted into the atmosphere at a temperature range of 150-180°C in FGDBs (Heebink and Hassett, 2005).The emission and species distribution of mercury in FA are different from FGDBs in the heat treatment process.In addition, there is a large amount of residual carbon existing in FA.However, little is known about the mercury emissions combining with transformation of mercury species during the FA thermal treatment.
The present study attempts to evaluate the emission amount of mercury and discuss species distribution of mercury in FA during the thermal treatment, and provide fundamental hazardous data of mercury emission for utilization industries of FA as raw materials.Thus, the aims of this study are to (1) analyze physico-chemical properties of FA with different particle size grades, especially to determine the residual carbon content in FA with a thermogravimetric analysis (TGA); (2) determine the total concentration and species distribution of mercury in raw/sorted FA; and (3) evaluate the emission amount and species distribution of mercury in raw/sorted FA in a thermal treatment process.

Sample Collection and Size Segregation
The Zhujiang Co-combustion Power Plant (4 × 300 MW), equipped with a cycle power generation unit and highefficiency electrostatic precipitators, is located in the Nansha Economic Development Zone (Guangzhou, P R China).Approximately 50 kg of FA sample was collected from the electrostatic precipitator outlets.The raw FA samples (0.1-180 µm in size) were screened into four particle size grades as sorted FA samples: > 150 µm, 75-150 µm, 54-75 µm, and < 54 µm with 100, 200, and 300 mesh sieves.The raw/sorted FA samples were heated in a tubular furnace at rate of 10 °C min -1 to the different final temperatures ranged from 200 to1200°C at a 2 L min -1 flow rate with air and held for 30 min, after which the sample was allowed to cool down to room temperature in the air.

Analysis of Physico-Chemical Characteristics of the Raw/Sorted FA
The chemical compositions of the raw/sorted FA samples were determined by X-Ray fluorescence (XRF, Axios, PANalytical Company, Netherlands).The X-Ray diffraction (XRD, D/Max-IIIA, Rigaku, Japan) was used to determine the mineral species of FA.The (Brunauer, Emmett and Teller) BET surface area of the FA samples was determined using a Micromeritics ASAP2020M apparatus.A scanning electron microscope (SEM, S-3700N, Hitachi, Japan) was used to determine the superficial morphology of the samples.A size distribution analyzer (LA-920, Horiba, Japan) was used to measure the particle sizes of FA.
The TGA was carried out using a TGA/STDA851e thermal analyzer from 50°C to 1000°C with a heating rate of 10°C min -1 under an air atmosphere.The residual carbon content in raw/sorted FA samples was calculated with ΔTG value, which was the weight differences from 500°C to 800°C in TGA curve.In addition, the differential scanning calorimetry (DSC) of raw FA was determined using NETZSCH STA449C thermogravimetric analyzer (50-950°C; 10 °C min -1 ; air atmosphere).It is noted that, in the case of interference, a modified presentation on TG-DTG curves should be introduced to accurately and precisely determine the weight loss due to thermal decomposition and/or oxidation combustion (Pan et al., 2016).

Sequential Extraction Experiments of Mercury Species
The raw/sorted FA sample and the FA samples with thermal treatment at different temperature were used for sequential extraction experiments, respectively.The sequential extraction procedure of United States Environmental Protection Agency (USEPA) Method 3200 was used to extract mercury species in FA samples, as shown in Fig. 1.This method provides information on both total mercury and various mercury species.According to Fig. 1, the mercury in FA samples was divided into three species, including extractable (mobile) mercury, semi-mobile mercury, and non-mobile mercury.The majority of potential mercury toxicity to the environment often derived from the extractable and semi-mobile fractions.But the non-mobile mercury fraction, such as mercury sulfide, was chemically stable in the environment.

Determination of Mercury Concentration in FA
Samples extracted by sequential extraction experiments were diluted by 5% W/V K 2 Cr 2 O 7 to 100 mL.Samples were stored at less than 4°C and analyzed completely within 5 days (Jian and McLeod, 1992).The mercury concentration was determined by an atomic fluorescence spectrometer (AFS-9130, Beijing Titan Instruments) and calculated according to Eq. ( 1).
where ρ represents the mercury concentration (ppm); C represents the mercury concentration in the digestion liquid (µg L -1 ); and M represents the quantity of the samples digested (g).The V and A are assumed to be 100 mL and 1000 mL L -1 , respectively.

Characterization of Raw and Sorted FA Physico-Chemical Properties
The raw FA mainly consisted of SiO 2 (34.0%) and Al 2 O 3 (51.1%).The minor compositions were Fe 2 O 3 and CaO of which the contents were 4.6% and 6.2%, with trace compositions of K 2 O (1.2%), MgO (1.1%), TiO 2 (1.0%) and Na 2 O (0.7%).According to the XRD results, the main mineral species in FA were quartz (SiO 2 ) and mullite (Al 2.8 Si 1.2 O 9.6 ).Rapid cooling in high temperature easily generates amorphous-phase aluminosilicate in coal combustion fly ash (Li et al., 2011), and thus, the aluminosilicate glass, quartz and mullite become the major phases in FA.No significant change in mineral species with the different particle size fractions was found similar to the previous study (He et al., 2015).
The raw FA was gray in color and had a cement-like appearance and the particle size ranged from 0.52 to 74.51 µm (average = 26.85µm).Table 1 presents the specific surface area of the raw/sorted FA samples, which indicated that the BET surface area of the raw FA was found to be 3.35 m 2 g -1 .For the sorted FA, the BET surface areas increased with the increase of FA particle size (e.g., 1.61, 2.58, 2.91, and 4.36 m 2 g -1 for < 54, 54-75, 75-150, and > 150 µm size grades, respectively).In general, the surface 306.9 ± 4.1 308.9 ± 5.9 area increases with a decrease in particle size, but for the coal fly ash, the surface area mainly depends on the carbon content (Schure et al., 1985).The loss of ignition (LOI) of FA with different particle sizes exhibits a similar trend as the BET surface area, which could be ascribed to the evidence that the coarser particles of coal are less likely to burn completely when compared to finer particles.

Residual Carbon Content and Surface Morphology
The residual carbon content in FA was determined by using the traditional standard method with LOI.In general, the carbon content was larger than the actual residual carbon content because the measurement of LOI included volatilization of other compounds in FA, such as CaSO 4 • 2H 2 O, Ca(OH) 2 and CaSO 3 •0.5H 2 O (Finkelman, 1995), which contained bound water or constitutional water.The bound water or constitutional water in calcium compounds can volatilize completely below 500°C (Jin et al., 2005).Furthermore, a method (Ba et al., 2012) of calculation by weight difference (ΔTG) before and after residual carbon combustion (500-800°C) in the TGA curve was employed to obtain a more precise residual carbon content in FA which compared with the LOI.Fig. 2 shows the relevant TG-DSC results, indicating that there is no endothermic decomposition of CaCO 3 in FA to affect the determination of residual carbon content obtained from the ΔTG value because of the exothermic peak at 500-800°C in DSC curve.
The weight of FA samples decreased sharply at the temperature ranging from 600 to 800°C due to the combustion of residual carbon in FA.The residual carbon content was found to be 2.8% in raw FA.For the sorted FA, a higher residual carbon content was observed than that of the raw FA with 8.9% and 5.0% in the > 150 µm and 150-75 µm size grades, respectively.The residual carbon contents of sorted FA in the 75-54 µm and < 54 µm size grades were 2.7% and 1.4%, respectively, indicating that the residual carbon content decreased with the decrease of FA particle size, which were similar to the results of previous studies (Guedes et al., 2008;He et al., 2015).The result represents actual residual carbon content in raw/sorted FA, which was in good agreement with the LOI results.In addition, more residual carbon existed in coarser size grades, thereby contributing to a higher BET surface area than that of FA in the fine particle size.
According to the superficial morphology determined by the SEM [Figs.3(a)-3(d)], the FA particle can be generally divided into three categories: irregular porous particle, irregular fused structural particle, and spherical smooth particle.The FA was found to be fine powder particles and predominantly spherical in shape, either solid or hollow (Fig. 3), which were similar to the results of previous studies (Ahmaruzzaman, 2010;Van der Merwe et al., 2014).The FA in > 150 µm size grade exhibited irregular spherical shape with rough edges [Fig.3(a)].Thus, the irregular particles combined with potential residual carbon with dark-colored porous structures were the main morphology in > 150 µm size grade.It was because that the residual carbon particles from pulverized coal combustion are irregular in shapes (He et al., 2015).Similar observations were also found in previous studies (Ahmaruzzaman, 2010;Ram and Masto, 2010) that the carbonaceous material in the FA is composed of angular particles.For the FA in 75-150 and 54-75 µm size grades, as shown in Figs.3(b) and Fig. 3(c), respectively, the fused structural particles were observed, where the particles remain predominantly irregular.The portion existence of spherical smooth glass particles was found to be <54 µm size grade, as shown in Fig. 3(d). Figs.
3(e)-3(h) show the SEM images of the sorted FA with different size grades after heating at 800°C.Significant change in morphology of coarser size grades with more residual carbon was observed.For the > 150 µm size grade, some of the irregular particles with potential residual carbon with dark-colored porous structures nearly disappeared [Fig.3(e)], compared to the FA without thermal treatment [Fig.3(a)].The edges of irregular particles became sleek, where the fused structures on the surface of particles and a  small amount of cohered glass microspheres were observed in the 75-150 and 54-75 µm size grades [Figs.3(f) and 3(g)].For the FA with < 54 µm size grade, no significant change was found between spherical particles with least residual carbon and FA which was shown in Fig. 3(h).A binding phenomenon showed that most coherent glass spherical particles with different sizes compared to the other sorted FA was also shown in Fig. 3(h).Therefore, the SEM results suggested that the residual carbon content decreased with decreasing FA particle size.

Total Concentration and Species Distribution of Mercury in FA
Table 1 presents the total mercury concentrations of the raw/sorted FA.The concentration of total mercury in raw FA was 308.92 ng g -1 , and increased from 292.37 and 284.09 ng g -1 in the >150 and 75-150 µm size grades to 332.63 and 306.92 ng g -1 in the 54-75 and < 54 µm size grades, respectively.The mercury species of the FA samples, followed by EPA Method 3200 (U.S., 2005), including the extractable mercury, the semi-mobile mercury, and the non-mobile mercury, were determined and shown in Fig. 4. Since the extractable and semi-mobile mercury in FA were mainly derived from coal-fired flue gas, most of the mercury in raw coal was gasified and emitted in the process of coal combustion, and then the gaseous mercury was catalytically oxidized and condensed to the FA particle surface.The extractable mercury concentration in raw FA was 45.6 ng g -1 , exhibiting slight change in the range of 43.4-48.7 ng g -1 with different FA particle size grades.The semi-mobile mercury concentrations of the FA in > 150, 75-150, 54-75, and < 54 µm size grades were 228.9, 218.3, 236.2, and 205.6 ng g -1 , respectively, accounted for 78.3%, 76.8%, 71.0%, and 67.0% of total mercury, respectively.In general, the semi-mobile mercury concentration in raw FA was 216.0 ng g -1 (69.9% of the total mercury).It was noted that the semi-mobile mercury was mainly derived from Hg 0 absorbed in FA residual carbon due to its stronger adsorption capacity for Hg 0 (Ba et al., 2012).As a result, the coarser size grades (> 150 and 75-150 µm size grades) having the higher residual carbon content absorbed more Hg 0 (semimobile mercury) than the finer size grades (54-75 and < 54 µm size grades) containing less residual carbon.
The non-mobile mercury concentration in raw FA was 47.2 ng g -1 (15.3% of the total mercury).For the sorted FA, the non-mobile mercury concentration was approximately 18 ng g -1 (about 6% of total mercury) in the > 150 and 75-150 µm size grades, and increased to 53.0 and 55.4 ng g -1 (16.0% and 18.0% of the total mercury) in the 54-75 and < 54 µm size grades, respectively.Non-mobile mercury in FA may be derived from minerals and other non-combustible ash in raw coal.This mercury fraction cannot be gasified and emitted in the process of coal combustion (Bhardwaj et al., 2009), which indicates that the extractable and semi-mobile mercury were mainly enriched in the coarser size grades and the non-mobile mercury was mainly enriched in the finer size grades (Fig. 4).A higher total mercury concentration was found in the finer size grades (Table 1) and attributed to the more non-mobile mercury that existed in the finer size grade.

Emission and Species Distribution of Mercury in FA during the Thermal Treatment Total Emission of Mercury in Raw FA
The relationship between total emission of mercury and temperature is presented in Fig. 5, and the result indicates that the total mercury concentrations decreased with an increase in temperature from 200°C to 1200°C.The variation in mercury emissions could be depicted by four stages: (1) the total mercury had a slight decrease of 16.3% around 200°C; (2) total mercury reduced dramatically with a decrement of 55.1% to 78.1% at 300 and 400°C, respectively; (3) the total mercury concentration further decreased by 85.1% and 96.5% at 600°C and 800°C, respectively; and (4) the mercury was not observed between 1000°C and 1200°C.
Considering the transformation of mercury species in FA, the lower emission rate of mercury at low temperature (< 200°C) was obtained, as shown in Fig. 5. Afterwards, most of mercury would emit with higher emission rate at middle temperature (200-600°C), and the emission rate appears to be slow at higher temperature (600-1200°C), which indicates that the process of mercury emission during thermal treatment was fitted by sigmoidal curves (Spiess et  al., 2008), as shown in Eq. ( 2) and Eq. ( 3).The relationship between total emission of mercury and temperatures can be expressed by Eq. ( 2), in which the parameters a, b, c, d and e represent as the slope, ground asymptote (initial total emission of mercury), maximum asymptote (maximum total emission of mercury), the inflection point and additional asymmetry, respectively (Spiess et al., 2008).It was assumed that the initial total emission of mercury was zero, as shown in Eq. (3).
The distribution of mercury emission was achieved by the fitted equation and constants shown in Table 2.The total emission rate of mercury from the FA samples increased sharply in a range of 20-90%.The corresponding emitting temperatures can be calculated by equation 2. For the raw FA, 20% and 50% of total mercury emitted at 206°C and 291°C, respectively.The total emission rate reached 90% when the temperature rose to 515°C.Similar observations were also found in previous study (Fernández-Miranda et  al., 2015), which indicates that the mercury in different FA is released at the temperature ranged from about 150°C to 500°C due to the emission of Hg 2+ complexes in FA, such as Hg-OM, HgCl 2 and HgS, of which desorption temperatures are between 138°C and 500°C (Biester and Scholz, 1996;Zhang, 2012;Fernández-Miranda et al., 2015).

Species Distribution of Mercury in FA
Fig. 6 illustrates the variation of species distribution of mercury in FA during the thermal treatment.At the temperature of 200°C, the semi-mobile mercury in raw/sorted FA decreased significantly (64.1-70.2%);however, the extractable mercury increased greatly by 2.0-2.7 folds compared with the FA sample without thermal treatment.Part of the semi-mobile mercury in raw/sorted FA has been transformed into the extractable mercury at 200°C.The transformation amounts of semi-mobile mercury in the > 150, 75-150, 54-75, and < 54 µm size grades and raw FA were 118.4 ± 3.5, 115.5 ± 4.3, 110.5 ± 2.1, 91.8 ± 0.9, and 101.1 ± 2.9 ng g -1 , respectively, corresponding to the conversion ratio of 51.7%, 52.9%, 46.8%, 44.6%, and 46.8%, respectively.The above mercury transformation process could be explained by the Mars-Maessen model (Granite et al., 2000), as shown in Eqs.(4-7).Some of the Hg 0 as a major species of semi-mobile mercury was oxidized into Hg 2+ at 200°C in an oxidizing atmosphere causing the increasing dramatically of extractable mercury.In an oxidizing atmosphere, Hg 0 is easily oxidized by direct oxidation with gas-phase oxygen to HgO (Eq.( 4)) and catalytic oxidation with a catalyst, such as inorganic constituents (López-Antón et  al., 2007; Bartoňová et al., 2012) and residual carbon (Sakulpitakphon et al., 2000; Dunham et al., 2003), and then leading to the formation of Hg 2+ complexes (Granite et al., 2000;Pena et al., 2001) and HgO (extractable mercury) (Galbreath and Zygarlicke, 2000).
In the heating process, the non-mobile mercury concentration remained constant at temperatures ranging from 200°C to 400°C.As the heating temperature increased to 600°C, the non-mobile mercury in the coarser size grade was reduced by approximately 45%, which was higher than the finer size grade and raw materials by 20-30%.At a temperature of 800°C, the non-mobile mercury in the coarser size grade was reduced dramatically by approximately 80%, which was lower than the finer size grade and raw materials by approximately 90%.Thus, the emission of non-mobile mercury with combustion of residual carbon in FA sample occurred mainly at 600°C, and almost completely disappeared at 800°C (Fig. 2).
According to the above evidence, the mechanism of desorption process of mercury on the residual carbon in FA could be proposed, as shown in Fig. 7.In the thermal treatment process, the emission characteristic of mercury could be briefly divided into three stages: (1) transformation of semi-mobile mercury (Hg 0 ) into extractable mercury (Hg 2+ ) occurred at 200°C; (2) both extractable and semimobile mercury emitted dramatically from 300°C to 400°C; and (3) non-mobile mercury emitted from 600°C to 800°C due to combustion of residual carbon, which also decreased significantly from 600°C to 800°C, according to the TGA curve (Fig. 2).

Strategies on Controlling the Emission of Mercury in FA
Since the utilization of coal FA by industries generally requires the thermal treatment, emission of mercury in FA occurred inevitably during the thermal treatment due to the high mercury content in FA.At the heating temperature of approximate 200°C or slightly higher, only a small fraction of the total mercury is emitted without control, but the species transformation of mercury in FA enhance the mobility and potential toxicity of mercury.It suggests that FA should be pretreated by physical washing for removal of mercury before utilization (Senior et al., 2000;Pavlish et al., 2003), and physical separation and hydrothermal treatment for removal of residual carbon in FA (Timpe et al., 2001).Furthermore, in the drying and dewatering process of coal FA, the heating temperature should be less than 150°C to reduce the mobility and potential toxicity of mercury.However, measures should be taken to control mercury emissions for environmentally concerned, when the FA was heated higher than 300°C, in the course of roasting and.et al., 1992), solvent extraction (Yoshiie et al., 2012), froth flotation (Smit et al., 1996) and heavy-media cyclone separator (Ferris et al., 1992), so called green technology, are the preventive solution to eliminate the mercury content in coal prior to combustion Significant control of mercury emissions from coal fly ash during the thermal treatment can be achieved by limiting the amount of mercury entering into the coal fly ash in the coal combustion.The air pollution control technologies for mercury removal, such as sorbent injection (Pavlish et al., 2003), wet scrubbing (Pavlish et al., 2003), spray dryer absorbers (Chu, 2000;Senior et al., 2000), and carbon capture (Na et al., 2014), also should also be applied.In addition, the relevant environmental laws, standards and regulations should be imminently established and enforced to limit the mercury emissions during the thermal treatment process, and to restrict mercury content of FA in the utilization of coal FA by industries via an integrated waste management plan (IWMP) throughout the life cycle assessment (LCA).

CONCLUSIONS
A proxy measurement for residual carbon content of FA was determined between 500°C and 800°C in TGA curves, which indicates that the residual carbon increased significantly as the FA particle size increased.The SEM images also indicated more residual carbon content was observed in a coarser FA particle size, while the glass microsphere existed in a finer FA particle size.There was no significant difference in total mercury contents in different particle size grades.Considering the different speciation of mercury in the FA, the dominant fraction in all particle size grades was the semi-mobile fraction, which accounted for at least 65% of total mercury.The content of non-mobile mercury in the coarser FA particle size grades is three times less than the finer particles.
The mercury began to emit at approximately 200°C and almost completely emitted at 1200°C in the raw FA sample.20% of the total mercury could emit at 206°C, and the emission rate of mercury, increasing sharply at temperatures higher than 206°C, reached 50% and 90% at 291°C and 515°C, respectively.A distinct transformation from semimobile mercury to extractable mercury was observed in the FA at 200°C.Both extractable and semi-mobile mercury decreased dramatically from 300°C to 400°C.The nonmobile mercury decreased significantly with combustion of residual carbon from 600°C to 800°C.Therefore, measures should be taken to control emission of mercury for the environmentally friendly utilization of coal FA by industries when the FA is heated higher than 300°C.

Fig. 2 .
Fig. 2. TG-DSC curve and TG of the raw and sorted FA samples.

Fig. 6 .
Fig. 6.Species distribution of mercury with the error bar (standard deviation) in raw/sorted FA samples during thermal treatment.

Table 1 .
BET surface area, LOI and total concentration of mercury in raw and sorted FA samples

Table 2 .
Constants and correlation coefficients of the fitted curve for mercury emission from the raw FA samples during thermal treatment.
Mechanism of desorption process of mercury on the residual carbon in FA. calcining.For instance, cleaning measures such as physical cleaning with reagents (Jacobsen