Role of chlorine in ultrafine particulate matter formation during the combustion of a blend of high-Cl coal and low-Cl coal

Role of chlorine in ultrafine particulate matter formation during the combustion of a blend of high-Cl coal and low-Cl coal

Fuel 184 (2016) 185–191 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel Full Length Article Role of ...

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Fuel 184 (2016) 185–191

Contents lists available at ScienceDirect

Fuel journal homepage: www.elsevier.com/locate/fuel

Full Length Article

Role of chlorine in ultrafine particulate matter formation during the combustion of a blend of high-Cl coal and low-Cl coal Yishu Xu, Xiaowei Liu ⇑, Penghui Zhang, Junzhe Guo, Jinke Han, Zijian Zhou, Minghou Xu ⇑ State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Luoyu Road 1037, Wuhan 430074, China

a r t i c l e

i n f o

Article history: Received 1 March 2016 Received in revised form 3 July 2016 Accepted 5 July 2016 Available online 11 July 2016 Keywords: Chlorine Ultrafine particulate matter Coal combustion High-Cl coal Industrial boiler

a b s t r a c t Particulate matter (PM) sampling was performed on an industrial boiler (3 MW) that burned various blends of high-Cl coal and low-Cl coal. Blends with high-Cl coal mass fractions of 30%, 50% and 70% were fired during the sampling. The results showed that both the ultrafine PM yield and its Cl content increased when blends with a higher fraction of high-Cl coal were burned. To clarify the effects of increased Cl content on the formation of ultrafine PM during the combustion of coal, combustion of every single coal with several concentrations of extra added HCl was further conducted in a well-controlled drop tube furnace. Based on the field and laboratory study, it was concluded that increasing the concentration of HCl in the combustion atmosphere alone would not significantly promote the yield of ultrafine PM from the combustion of coal. Further characterization revealed that during coal combustion, Cl in coal migrated into ultrafine PM as chlorides, and the presence of alkali and alkaline earth metals was required for the conversion of HCl to chlorides for cations. The combined effects of Cl and Na in coal blends resulted in the increase of ultrafine PM formation and its contents of Cl and Na. Moreover, partitioning of S into the ultrafine PM was reduced due to the competition of cations with the increased content of Cl in the blends. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction China is faced with severe particulate matter (PM) pollution at present and coal combustion in power plants is one of the major emission sources, especially for ultrafine PM [1,2]. The emission of ultrafine PM should be considered when developing new combustion technology and exploiting various available fuels, and this consideration necessitates a good understanding of the various factors influencing PM formation. The formation of ultrafine PM is controlled by the vaporization and nucleation of minerals, which are affected by both the fuel properties and the combustion conditions [1–5]. The influences of ash composition and content [2,6,7], occurrence of mineral matter [6,8,9], coal rank [2,10,11], and coal particle size [3,12] on ultrafine PM formation have been extensively studied. As previous studies have shown, coal with a higher content of volatile species (e.g., Na and S) and smaller particle sizes is apt to produce more ultrafine PM [3,5,11]. Additional factors, such as combustion temperature [3,10,13], air-to-fuel ratio [3] and flue gas constituents [11,14–18] – all of which may vary strikingly with operational conditions – would also affect the characteristics of PM generation. ⇑ Corresponding authors. E-mail addresses: [email protected] (X. Liu), [email protected] (M. Xu). http://dx.doi.org/10.1016/j.fuel.2016.07.015 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.

With the increasing use of the low-quality coal (e.g., rich in moisture, ash, alkali and alkaline earth metals, sulfur, chlorine), increasing attention has been paid to the effects of the gas species (e.g., CO, H2O, SO2) released during coal combustion on the generation of ultrafine PM [11,14–18]. High CO content in the burning coal particles can enhance the transformation of refractory minerals into volatile elements or suboxides and promote their vaporization [11,13]. Previous studies by our group [14,18], as well as by others [16,17], showed that H2O in the combustion atmosphere would also enhance mineral vaporization and increase ultrafine PM emission. We have also studied the role of SOx in PM formation [14]. It was shown that SOx reacts with gaseous mineral matter and strongly affects its nucleation behavior, which controls the initial formation of ultrafine PM [17,19]. In addition to the abovementioned species, gaseous HCl is also produced during coal combustion, especially during the combustion of high-Cl coal [17,20] and the co-combustion of coal and high-Cl biomass [21–24]. Concerns and studies regarding HCl are generally focused on its influence on high-temperature corrosion and trace element speciation [17,25], but previous studies on PM formation during the co-combustion of coal and biomass or the combustion of biomass showed that Cl was an important factor in the partitioning of mineral matter and the formation of PM [19,21,24,26]. On one hand, Cl was generally a major component in ultrafine PM from

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2. Experimental

coal and a low-Cl coal was collected to show the emission characteristics of ultrafine PM during the combustion of high-Cl coal. In total, three blends – Coal AB73 (coal A and coal B mixture with mass ratio of 7/3, same below), AB55 and AB37 – were burned in the industrial boiler during the field experiments. As shown in Table 1, coal A was a low-Cl coal (0.51 mg-Cl/g-coal) while coal B was a high-Cl coal (9.70 mg-Cl/g-coal). Raw coals were blended, crushed and dried in the mill and the particles less than 100 lm in size were used. Combustion of the single high-Cl coal was not performed to avoid Cl-induced high-temperature corrosion in the boiler. The boiler (sketched in Fig. 1) has a thermal capacity of up to 3 MW and is similar to pulverized-coal-fired utility boilers: it consists of a fuel system, air system, water system, furnace body, and an ash and sludge handling system. During its operation, coal was pulverized in the fuel system and then fed into the furnace entrained by the primary air through 8 burners installed at the four corners. The coal was fired tangentially with additional secondary air in the furnace (10.5 m in height and 1.2 m in width). Flue gas out of the furnace successively passed through the water wall and air preheater in the flue duct. During the field experiments, coal was fed at a rate of approximately 220 kg/h, and an air-to-fuel ratio of 1.35 was maintained.

2.1. Test conditions in field study

2.2. Experimental conditions in the laboratory

In the first experiment, PM emitted from an industrial boiler which burned coal blends with various mixing ratios of a high-Cl

In the second experiment, well-controlled single coal combustion tests were conducted on a laboratory drop tube fur-

the combustion high-Cl biomass [19,21,27,28]. On the other hand, Cl promoted the partitioning of alkali metal and other ash constituents into ultrafine PM [29–31]. And the release of Cl and Na into PM was reported to be affected by S and clay minerals [19,28,31,32]. Compared with biomass, coal has different combustion behavior and distinctive ash compositions (e.g., Na, K, S, clay minerals), and the possible interactions of mineral elements during the combustion of coal blends with different contents of Cl may be different from those in the co-firing of coal and biomass. So, during the combustion of coal blends with high contents of Cl and Na, it was still unknown whether the Cl or the derived HCl would increase the yield of the ultrafine PM. To answer this question, two experiments were carried out in this study. First, PM sampling was conducted in an industrial boiler (3 MW) which burned coal blends with different mixing ratios of high-Cl coal, and the yield, size distribution and composition of the ultrafine PM were characterized. Then, additional laboratory experiments with every single coal in the above-used blends were conducted on a well-controlled drop tube furnace with the addition of HCl. The results from both the field and laboratory experiments are discussed.

Table 1 Properties of coal used in the experiments (air dry basis, %). Coal

M

A

C

H

N

S

Oa

Cl, mg/g

LHVb, MJ/kg

A B AB73 AB55 AB37

3.92 10.75 4.40 6.78 7.00

14.44 6.05 12.49 11.20 9.87

63.07 59.63 63.02 62.22 61.59

4.23 4.16 4.26 4.29 4.34

0.71 0.54 0.69 0.66 0.63

0.56 0.15 0.50 0.33 0.26

13.09 18.72 14.64 14.53 16.32

0.51 9.70 2.70 5.63 7.18

18.82 20.75 19.21 19.67 20.24

A B a b

Na2O

MgO

Al2O3

SiO2

P2 O5

SO3

Cl2O

K2O

CaO

TiO2

Fe2O3

1.25 8.03

5.09 2.58

20.90 8.54

38.72 11.56

0.59 0.53

6.60 6.34

0.49 18.16

1.12 0.16

18.05 39.50

0.61 0.51

6.58 4.12

Calculated by balance. Low heating value, as received basis.

Fig. 1. Sketch of the industrial boiler (3 MW), PM and chlorine sampling systems.

Y. Xu et al. / Fuel 184 (2016) 185–191

nace (DTF). The DTF consists of a vibrating fuel feeder (SANKI Inc.), an alundum tube reactor (1440 mm long) and a gas supply system; it is further described in references [14,18]. In these experiments, pulverized coal (coal A or coal B) with particle size in the 45–100 lm range was burned in simulated air (O2/N2 = 21 vol. %/79 vol.%) with the addition of gaseous HCl (0, 50 and 350 ppm, abbreviated as ppm below). In all conditions, coal was fed at 0.2 g/min and entrained by a 5 L/min simulated air flow, and the wall temperature of the reactor was set at 1773 K. Combustion of the same three coal blends burned in the industrial boiler was repeated on the DTF in the simulated air. The coal blends used in the laboratory were obtained by mechanically mixing pulverized coal A and coal B with particle sizes of 45–100 lm. Higher air-tofuel ratios (4) were set in the lab experiments to ensure burnout of the fuel. Comparison of the properties of each coal and coal blend showed that the coal blends were well-mixed.

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passed through a quartz filter to separate out the solid particles and evaporate the droplets into vapor. The filtered gas was introduced into absorbing bottles with saturated Na2CO3-NaHCO3 solution to collect HCl and Cl2. Sodium thiosulfate was added in the Na2CO3-NaHCO3 solution to convert possible Cl2 in the flue gas into Cl. Total concentration of HCl and Cl2 in the flue gas were calculated considering the total sampled gas volume. To measure Cl2 content, flue gas was first passed through absorbing bottles with saturated NaCl solution to separate HCl and then passed into absorbing bottles with Na2CO3-NaHCO3 solution. Cl in the Na2CO3-NaHCO3 solution was determined by ion chromatography (IC, Metrohm 881Compact IC pro). The concentration of HCl was further calculated by difference. Additionally, the Cl content in the coal was also determined by standard GB/T3558-2014.

3. Results and discussion 2.3. PM sampling and analysis methods The mass yields and size distributions of PM were determined with a LPI sampling system. As described in Refs. [14,18], the LPI sampling system consists of a Dekati low pressure impactor (LPI), a cyclone (LPI-10), a pressure gauge, a vacuum pump and a gas supply system. In the field experiments, PM sampling was carried out at the horizontal flue duct downstream from the air preheater (site 1#, shown in Fig. 1), where the gas temperature was approximately 473 K [33]. During sampling, a stream of 10 L/min of flue gas was extracted from the center of the duct (0.35 m in diameter) via a high-temperature probe (473 K). The flue gas was then introduced into the LPI sampling system. In the cyclone, PM with aerodynamic diameter larger than 10 lm was separated out. Then, in the LPI downstream from the cyclone, PM10 (PM less than 10 lm) was classified into 13 stages according to the particle size. Both aluminum foils and polycarbonate membranes were placed in the LPI to collect the size-fractionated PM. PM mass was obtained by weighing the foil with a microbalance (Sartorius MSA6.6S-0CE-DF) before and after sampling. To avoid PM bouncing on the foils, the aluminum foils used were coated with Apiezon grease (H). PM collected on the polycarbonate membrane were further analyzed with an X-ray fluorescence probe (XRF) to determine their mineral composition. During the sampling, the LPI, cyclone and the pipes connecting them were heated to 403 K to avoid the condensation of SOx, HCl or H2O in the flue gas [18]. More than 3 parallel runs were conducted at each condition, and the average values and error bars are both shown in the results below. In the laboratory experiments, flue gas was extracted from the bottom of the tube reactor where the temperature was about 135 °C during the sampling. However, different from the field sampling above, the extracted flue gas was first mixed with another 5 L/min stream of clean N2 to form a total stream of 10 L/min before flowing into the cyclone and LPI. PM entrained within it was collected and analyzed by the same method described above [14,18].

3.1. Particle size distributions (PSDs) and yields of ultrafine PM from industrial boiler During the PM sampling in the industrial boiler, three coal blends with different mixing ratios (coal AB73, AB55 and AB37) were burned; the Cl content increased as the fuel changed from coal AB73 to coal AB37. The yields and PSDs of PM less than 1 lm are shown in Fig. 2. With reference to the PSDs above, PM less than 0.7 lm in size is defined as ultrafine PM. The field sampling results show that when the mass fraction of the high-Cl coal (coal B) in the blends increases from 30% to 70% the peak size of ultrafine PM increases from 0.15 lm to 0.41 lm. This particle size variation is inferred to be caused by changes in the ash composition in coal which will be discussed below. Furthermore, the yields of ultrafine PM was calculated and shown in Fig. 3. As can be seen, as the coal blend changed from

Fig. 2. PSDs of PM less than 1 lm derived from the combustion of high-Cl coal blends in the industrial boiler.

2.4. Chlorine sampling and analysis methods In the field study on the industrial boiler, the contents of HCl and Cl2 in the flue gas were also measured by a modified method based on EPA Method 26A and the study of Ma et al. [34]. In the modified method, saturated NaCl solution and Na2CO3-NaHCO3 solution (2.52 g Na2CO3, 2.54 g NaHCO3 and 25 mL 30% H2O2 in 1 L deionized water) were used instead of H2SO4 solution and NaOH solution respectively in consideration of convenience and safety in the field sampling. Flue gas was first extracted from sampling site 2# (shown in Fig. 1) via a high-temperature quartz probe (453 K) and then

Fig. 3. Yields of ultrafine PM and HCl, Cl2 derived from the combustion of high-Cl coal blends in the industrial boiler.

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AB73 to AB37, the ultrafine PM yield increased from 106.9 mg/Nm3 to 151.9 mg/Nm3, corresponding to a yield improvement of over 42%. The field experiments showed that significantly more ultrafine PM was generated when blends with a larger fraction of high-Cl coal were burned. During coal blend combustion in the pilot facility, concentrations of HCl and Cl2 in the flue gas were measured; these values are also shown in Fig. 3. When the blends changed from coal blend AB73 to coal blend AB37, the concentration of HCl increased from 120.1 ppm to 351.5 ppm. The gaseous Cl-containing species account for 57.3–78.6% of the total Cl in the fed coal and HCl is the main Cl-containing species. As expected, significantly more gaseous HCl was released into the atmosphere when coal blends with higher Cl content were burned. It is clearly shown that the ultrafine PM yield and the gas-phase chlorine species increase when more of the high-Cl coal is included in the coal blend. The consistency between the increase of gaseous HCl in the combustion atmosphere and the ultrafine PM yield attracted our concerns. As revealed in previous studies on the combustion of biomass and co-firing biomass with coal, Cl promoted the vaporization of alkali metals (e.g., Na and K) and promoted their partitioning into the fine PM [31,35]. During the combustion of high-Cl coal or high-Cl coal blends, there were high contents of S and clay minerals in addition to Cl and Na, which were reported to reduce the release of alkali metals and PM formation [36–38]. In this case, the formation characteristics of ultrafine PM during the combustion of coal blends may be rather complicated and different from that during biomass combustion. In Section 3.2, the composition of the ultrafine PM collected from the industrial boiler is characterized for further discussion. 3.2. Composition of ultrafine PM and HCl in flue gas from industrial boiler The compositions of ultrafine PM collected in the experiments in the industrial boiler are shown in Fig. 4. As can be seen, Cl and Na are the primary components of the ultrafine PM. The content of Cl in the ultrafine PM increased notably when blends with a higher mixing ratio of high-Cl coal were burned, in accordance with the observed growth of the peak size and the yield of the ultrafine PM shown above. Moreover, along with Cl, the content of Na in the ultrafine PM also increased to some extent, which also promoted the formation of ultrafine PM. The volatile Cl and Na are enriched in the ultrafine PM, in accordance with the vaporizationnucleation formation mechanism of ultrafine PM [2,21,39]. The increase in HCl seems to bring about a higher content of Cl in ultrafine PM, and it seems that increased HCl in the atmosphere also promoted the yield of the ultrafine PM. However, as the PM emission characteristics of the single coals were not obtained, we could not distinguish whether the observed increase in ultrafine PM yield was caused by an enhancement of HCl on PM formation or simply a linear sum of the two blended coals. To elucidate the effects of HCl on ultrafine PM formation, further well-controlled

Fig. 4. Elemental composition of ultrafine PM from the industrial boiler.

laboratory experiments were performed, in which every single coal in the above blends was burned in atmospheres with the addition of gaseous HCl. 3.3. PM from single coal combustion with extra HCl addition in laboratory To confirm the reliability of the experiments on the industrial boiler, combustion of coal blends AB73, AB55 and AB37 in simulated air was repeated on the laboratory DTF. Then, PM sampling experiments were conducted during the combustion of individual coals with the addition of HCl. Particle size distributions (PSDs) of PM less than 1 lm in size are shown in Fig. 5. As shown in Fig. 5(a), the mass concentrations of PM from the combustion of coal blends in the laboratory DTF were lower than those in the industrial boiler, possibly due to the disparities in the combustion conditions. As mentioned above, coal particles in the lab DTF were burned in the atmosphere with a much higher air-to-fuel ratio than those in the industrial boiler. The larger air-to-fuel ratio in DTF is supposed to reduce particle temperature and partial pressure of reducing species (e.g., CO, H2) surrounding the coal particle, thereby less mineral matter vaporized and less ultrafine PM was produced [1,11,18]. Moreover, coal particles burned in the industrial boiler were heated in a higher heating rate and collided between each other more severely, which facilitated the fragmentation of coal or char particle. Thereby, mineral matter vapor transferred into the combustion atmosphere more easily which also facilitated the formation of ultrafine PM [1,12,40]. As can be seen in Fig. 5(a), the peak size and yield of ultrafine PM from the combustion of coal blends in the experimental DTF also increased as the content of Cl in the blends increased, consistent with results from the industrial boiler above. However, it is interesting that, in the combustion of single coal A and single coal B with extra gaseous HCl addition [shown in Fig. 5(b) and (c)], the peak sizes of the ultrafine PM did not appear to change even when HCl of concentration up to 350 ppm was added. The yield of ultrafine PM (shown in Fig. 6) increased by 4–8% and 11–15% when 50 ppm and 350 ppm gaseous HCl was added into the combustion atmosphere, respectively. Nevertheless, the yield change is small compared to the size of the error bars. These results indicate that the addition of HCl would only lead to a slight, if any, increase in the emission of ultrafine PM during single coal combustion. Combining the field and laboratory experiments, the effects of HCl on the combustion of blended and single coals combustion were further investigated from another aspect. The relationship between the measured ultrafine PM yields, the linear calculated values of the ultrafine PM yield based on the yield from single coal combustion and the coal mixing ratio versus the fraction of coal B in the blends are shown in Fig. 7. It is clearly shown that the amount of ultrafine PM from the combustion of coal blends is equal to the linear sum of the amounts from each single coal, which indicates that the HCl released from high-Cl coal did not notably promote the emission of ultrafine PM from the low-Cl coal [9]. The linear relationship between the coal blend combustion and single coal combustion, which is consistent with the results of single coal combustion with added HCl, confirms that HCl does not promote the generation of ultrafine PM. Results in Figs. 4–6 further suggested that the changed ultrafine PM formation was mainly caused by the increased Na content in coal blends. As shown in Table 1, there was a higher Na content in the coal blends with a higher mixing ratio of the high-Cl coal besides Cl content. The increased Na was supposed to vaporize during coal combustion and nucleated as ultrafine PM [1,21], which led to an increase in the yield of ultrafine PM and in its Na content (see Fig. 4). In the following Section 3.4, the partitioning of Cl into the ultrafine PM as well as the impacts of Cl on the par-

Y. Xu et al. / Fuel 184 (2016) 185–191

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Fig. 5. PSDs of PM less than 1 lm in size derived from (a) the coal blends and (b and c) individual coals during the combustion on the DTF in the simulated air with extra HCl addition.

Fig. 6. Yields of ultrafine PM from (a) the coal blends and (b) individual coals during the combustion in the DTF in simulated air with extra HCl addition. Fig. 9. Fractions of Na and S contained in the ultrafine PM derived from single coal combustion in simulated air with added gaseous HCl.

Fig. 7. Relationship of the measured and calculated ultrafine PM yield versus the fraction of coal B in coal blends.

titioning of other mineral matter are discussed with respect to both the field experiments and the laboratory experiments. 3.4. Partitioning of Cl and Na into ultrafine PM The compositions of ultrafine PM from the combustion of single coal (A and B) with extra HCl addition were shown in Fig. 8. Fractions of Na and S contained in the ultrafine PM were further calculated and shown in Fig. 9.

As with the field sampling results (see Fig. 4), Cl and Na are the predominant elements in the ultrafine PM from single coal combustion and S is also appeared in the PM from coal A (see Fig. 8). Additionally, it is worth noting that the content of minor mineral matter in the ultrafine PM did not vary significantly with the amount of extra HCl, except for Cl and S. Fig. 8 showed that Na content in the ultrafine PM generated from single coal combustion with HCl addition did not change notably. Furthermore, results in Fig. 9 revealed that partitioning of Na into ultrafine PM was not significantly promoted by the increased HCl in the combustion atmosphere, which was different from the results observed in the combustion of high-Cl biomass [19,31,35]. Studies on biomass combustion showed that increasing HCl content would inhibited the reaction between alkali metal and clay minerals and thereby promote their vaporization [19,28,31,32]. In this study, there were certain contents of S and Cl in coal A and coal B respectively. It was supposed that the inherent S or Cl in coal particle already inhibited the reactions between alkali metals and clay minerals; thereby, increasing HCl in the atmosphere did not have obvious effects. Particular attention was paid to the partitioning of Cl, S and alkali and alkaline earth metals (e.g., Na, Ca). Based on the mole content of related components, x ratios were defined and calculated via Eq. (1) [19,21] and the results were exhibited in Fig. 10, where x stands for Cl + 2S or Cl.

x ratio ¼ x=ðNa þ 2CaÞ

Fig. 8. Compositions of ultrafine PM derived from single coal combustion in simulated air with added gaseous HCl.

ð1Þ

As shown in Fig. 10, all values of the (Cl + 2S)/(Na + 2Ca) ratio were approximately 1, indicating that Cl and S in the ultrafine PM mainly occurred as chlorides and sulfates [9,19,21]. Moreover, the (Cl + 2S)/(Na + 2Ca) ratio varied only slightly even though HCl was added, which indicated that the added HCl did not transfer into ultrafine PM and the formation of ultrafine PM was limited by the inherent alkali and alkaline earth metals (e.g., Na, Ca) in coal. Results in Fig. 8 also showed that S content reduced from 25% to 11% when Cl content increased from 12% to 20%. Considering the

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1. Increasing the concentration of HCl in the combustion atmosphere alone does not significantly promote the formation and emission of ultrafine PM during combustion of single coals. 2. Ultrafine PM yield changes significantly when coal blends with different contents of Cl and Na are burned. The combined effects of Cl and Na in coal blends resulted in the increase of ultrafine PM formation and its contents of Cl and Na. 3. Chlorine in coal migrates into ultrafine PM as chlorides. Increased Cl content inhibits the partitioning of S into ultrafine PM because of the competition for cations to form chlorides and sulfates.

Fig. 10. Mole ratios of (Cl + 2S)/(Na + 2Ca) and (Cl)/(Na + 2Ca) in the ultrafine PM derived from single coal combustion in simulated air with added gaseous HCl.

constant (Cl + 2S)/(Na + 2Ca) ratios, these results indicated that a portion of the sulfates in ultrafine PM were substituted by the chlorides when HCl was added. When coal particle was burning, HCl and SOx were released from the inherent Cl and S [19]. And HCl and SOx would react with the vaporized alkaline species (e.g., Na, Na2O, etc.) via Eqs. 2 and 3 and further nucleated and formed ultrafine PM as chlorides or sulfates [27,28,41,42].

ðM; M2 O; MOHÞ þ SOx ! M 2 SO4 þ H2 O

ð2Þ

ðM; M2 O; MOHÞ þ HCl ! MCl þ H2 O

ð3Þ

MCl þ SO2 þ O2 þ H2 O ! M2 SO4 þ HCl

ð4Þ

where M stands for the volatile species, including Na, Ca, etc. During the combustion of single coal with extra HCl addition in the study, the extra HCl added into the combustion atmosphere did not transform into chlorides because of the lack of vaporized alkali and alkaline earth metals which provided the cations. Hence, no significant change in the particle size and yield of ultrafine PM was induced by the added HCl. Moreover, during the combustion of coal A which has some S, the added HCl would compete with SOx for those initial species (Na2O, NaOH, Ca) via Eqs. 2 and 3 or their interaction [Eq. (4), a global reaction] [43]. As SO3 was required in the sulfation [Eq. (4)], the formation of sulfates were much slower than chlorides due to the kinetic limitation [27,44]. Thereby, more initial species reacted with HCl/Cl2 and transformed into chlorides, which inhibited the formation of sulfates and resulted in the decreased S content in the ultrafine PM. No visible variation of S content in the ultrafine PM from the combustion of coal B were observed possibly due to its low S content. However, in the experiments involving combustion of coal blends, other ash constituents (e.g., Na, Ca) besides Cl species also varied with the mixing ratio of high-Cl coal. More alkaline gaseous species were generated when the fraction of high-Cl coal was increased; therefore, more HCl were transformed into chlorides and nucleated as PM. Consequently, the yield of ultrafine PM was increased. This result indicated that increased HCl would promote the formation and emission of ultrafine PM only in the presence of sufficient gaseous alkaline species. 4. Conclusions Two experiments were performed to elucidate the impacts of Cl on the formation of ultrafine PM during the combustion of blends of a high-Cl coal and a low-Cl coal. Based on field experiments in an industrial boiler burning blends of high-Cl coal and low-Cl coal and laboratory experiments in a DTF combusting single coals with added gaseous HCl, the following conclusions were made.

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