Effect of kaolin additive on PM2.5 reduction during pulverized coal combustion: Importance of sodium and its occurrence in coal

Applied Energy 114 (2014) 434–444

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Effect of kaolin additive on PM2.5 reduction during pulverized coal combustion: Importance of sodium and its occurrence in coal Junping Si a, Xiaowei Liu a,⇑, Minghou Xu a,⇑, Lei Sheng a, Zijian Zhou a, Chao Wang a, Yang Zhang a, Yong-Chil Seo b a b

State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, China Department of Environmental Engineering, YIEST, Yonsei University, 234 Heungup, Wonju 220-710, South Korea

h i g h l i g h t s  The sodium aluminosilicate plays an important role in PM2.5 reduction by kaolin.  The capability of kaolin to reduce PM2.5 depends on the sodium occurrence in coals.  The effect of kaolin on PM2.5 reduction becomes weaker during O2/CO2 combustion.  Particle collision may be taken into consideration for PM2.5 reduction.

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Article history: Received 22 May 2013 Received in revised form 29 September 2013 Accepted 2 October 2013 Available online 24 October 2013 Keywords: PM2.5 reduction Kaolin Sodium Coal combustion

a b s t r a c t Little work has been performed on the importance of sodium and its occurrence in coal to PM2.5 (particles less than 2.5 lm in aerodynamic diameter) reduction by kaolin during O2/N2 combustion and O2/CO2 combustion at high temperatures. In this study, the combustion experiment of a treated low-sodium coal with sodium aluminosilicate additive was conducted in a lab-scale drop tube furnace (DTF) at 1500 °C to reveal the contribution of mineral melting and coalescence to PM2.5 reduction. Meanwhile, two typical Na-loaded coals (in which the sodium was loaded in the form of NaCl and sodium carboxylate, respectively) with kaolin added were also burnt under O2/N2 and O2/CO2 atmospheres to investigate the effect of interaction between kaolin and different chemical form sodium on PM2.5 reduction. The results show that sodium aluminosilicate is able to promote the migration of PM0.5–2.5 (particles in aerodynamic diameter of 0.5–2.5 lm) to form coarse particles. Due to the stronger reactivity of sodium carboxylate reacting with kaolin than that of NaCl, PM0.2–0.5 (particles in aerodynamic diameter of 0.2–0.5 lm) decreases more significantly in the combustion when adding kaolin into the NaAc-loaded coal than into NaCl-loaded coal. In addition, the PM0.2–0.5 reduction in O2/CO2 combustion is lower than that in O2/N2 combustion owing to the less vaporization of metals and the slower diffusion rate of vapors in the O2/CO2 atmosphere in comparison to those in the O2/N2 atmosphere. The mineral coalescence varied in interactions of kaolin with NaAc and NaCl. Besides, the PM0.5–2.5 emission differed as a result of differences in coal characteristic and the atmosphere, and this would cause the difference of collision frequency between particles and additive. With the joint actions of mineral coalescence and particle collision, the NaAc-loaded coal has a higher PM0.5–2.5 reduction by kaolin than NaCl-loaded coal, especially under the O2/N2 combustion. An expression describing the relationship of PM0.5–2.5 reduction, mineral coalescence and particle collision was fitted and it is found that the mineral coalescence has a stronger influence than particle collision on PM0.5–2.5 reduction by kaolin. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Coal plays a dominant role in China’s primary energy. The combustion of coal inevitably generates massive particulate matter ⇑ Corresponding authors. Tel.: +86 27 87544779x8207; fax: +86 27 87545526 (X. Liu), tel.: +86 27 87544779x8309; fax: +86 27 87545526 (M. Xu). E-mail addresses: [email protected] (X. Liu), [email protected] (M. Xu). 0306-2619/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2013.10.002

(PM). Although particle emission control devices, e.g., electrostatic precipitators and bag filters, installed in coal-fired power plants can effectively capture most of fly ash particles, a significant part of PM2.5 (particles with an aerodynamic diameter of <2.5 lm) escapes to the atmosphere due to its high penetration to these devices [1]. PM2.5 is generally rich in heavy metals and has a long-time stay in the atmosphere, therefore poses serious threats to the environment and human health [1]. Recently, PM emission

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from coal-fired power plant has become a major source of ambient PM2.5 in some Chinese cities [2,3]. In order to comply with the strengthened regulation on pollutant emissions, power plants are under increasing pressure to take measures to reduce PM2.5 emission. Besides the often-used approaches of improving the performance of the emission control devices, reducing PM2.5 formation during the combustion processes in the furnaces is also considered as a promising way. In-furnace PM2.5 reduction is closely associated with PM formation behavior. According to a trimodal particles size distribution of PM10 (particles with an aerodynamic diameter of <10 lm) [4–6], PM2.5 generated from coal combustion consists of two parts: PM0.5 (particles less than 0.5 lm in aerodynamic diameter) as ‘‘ultrafine mode particles’’ and PM0.5–2.5 (particles in aerodynamic diameter of 0.5-2.5 lm) as ‘‘central mode particles’’. The former is formed by volatile species including alkali metals (Na, K), sulfur and refractory elements due to the vaporization–condensation mechanism [4,7], while the latter is formed by char fragmentation [8,9], direct transformation of fine mineral grain [10], and among other mechanisms [5,6]. Besides, Neville et al. [11] indicated that the PM0.5–1.0 (particles in aerodynamic diameter of 0.5–1.0 lm), a part of central mode particles, also could be formed through coagulation. Previous studies [12–21] have proven that adding kaolin can capture metal vapors during coal combustion. The capture is attributed to the adsorption of the vapors by kaolin through complex processes involving the combined effects of diffusion and surface reaction. The main product of the surface reaction between kaolin and vapors is considered to be aluminosilicates, e.g., sodium aluminosilicate (Na–Al–Si). A promotion or competition phenomenon is found during the interaction between multiple-metals and kaolin. Because the metallic vapors are the preliminary predecessors of ultrafine modal particles during coal combustion, their adsorbed by kaolin is potential to reduce the ultrafine mode particles formation [22–24]. Takuwa and Naruse [22] and Zhou et al. [23] respectively reported that kaolin could adsorb Na and K, and thus inhibit the contribution of alkali metals to the formation of PM1. Moreover, adding kaolin was also found to be effective for PM1 reduction during O2/CO2 combustion [24]. Besides the adsorption mechanism, Ninomiya and his coworkers [25–27] suggested that the liquidus capture of fine particles was another effective path to reduce PM2.5 emission. Taking Mgbased additives for example [25,26], they can react with minerals in coal and transform into aluminosilicates such as Mg–Al–Si and Mg–Ca–Al–Si, which melt and form liquidus at high temperatures, enhance the coalescence of collided and captured mineral grains, and consequently reduces PM2.5 formation. Because the products of the surface reaction between kaolin and metallic vapors are also aluminosilicates and potential to be liquidus at high temperatures, it can be speculated that, similar to Mg–Al–Si, the sticky surface of the products enabling the capture of fine particles is likely to be another pathway for PM2.5 reduction by adding kaolin. Therefore, further study is required to understand the PM2.5 reduction capability of kaolin reaction products. The capability of kaolin to control PM2.5 formation is closely associated with the presence of volatile metals in coal and the in-flame behavior of the metals, particularly alkali metals. Sodium is the major alkali metal in coals. It occurs in three main forms: as soluble salts (e.g., NaCl), in an organic state (e.g., carboxylate) and as insoluble silicate or aluminosilicate (e.g., Na–Al–Si) [28,29]. The occurrences and contents of sodium are greatly dependent on coal type and combustion processes [30], which determines sodium behavior during coal combustion. Neville and Sarofim [30] observed that the sodium present in coal as organically bounded and halide would volatilize during coal combustion, while that in silicate state rarely volatilized and was mainly retained in residual ash. Van Eyk et al. [31] showed that, in the presence of H2O, only a

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part of NaCl, in contrast to overall organically bounded-Na, transformed and released into gas phase as NaOH [32]. Gallagher et al. [15] and Mwabe and Wendt [16] reported the importance of NaOH to the sodium adsorption by kaolin, because the adsorption was mainly achieved by the interaction between kaolin and NaOH. Mwabe and Wendt [16] also revealed that the presence of chlorine would undermine the formation of NaOH and thus weaken kaolin adsorbing sodium. Therefore, it can be inferred that kaolin has a higher adsorption capability for organically bounded sodium than for sodium chloride. Although extensive studies have been conducted on kaolin capturing sodium and reducing PM1 during coal combustion, the importance of sodium occurrence to PM2.5 reduction by kaolin has not acquired enough attention. Oxy-fuel (O2/CO2) combustion is regarded as one of the effective ways to reduce CO2 emissions from coal combustion. Considering the characteristics differences between CO2 and N2, a lot of investigations have been focused on the influence of the oxidant change, compared to conventional air combustion, on coal combustion and related processes [33,34], including ignition and burnout of pulverized coal [35–38], heat transfer in the furnace [39,40], formation and control of pollutant [41]. Nevertheless, few studies [22–24] have been conducted on PM2.5 reduction by kaolin during coal combustion under O2/CO2 atmosphere. The present work aims to reveal the capability of sodium aluminosilicate (Na–Al–Si) formed by kaolin adsorbing alkali metals to reduce PM2.5 formation and investigate the effect of kaolin on PM2.5 reduction during the combustion of coal loaded with different forms of Na under O2/N2 and O2/CO2 atmospheres. A combustion experiment of low-sodium coal with Na–Si–Al additive was conducted under the O2/N2 atmosphere in a lab-scale drop tube furnace (DTF) to detect the melting of Na–Al–Si and its contribution to the PM2.5 reduction. Furthermore, considering the differences in the interaction between kaolin and different forms of sodium species, coupled with the diversity of subsequently formed sodium aluminosilicate in these processes, two typical Na-loaded coals (in which the sodium was loaded in the form of NaCl and sodium carboxylate, respectively) were mixed physically with kaolin and burnt in the DTF under O2/N2 and O2/CO2 atmospheres to test these factors on PM2.5 reduction. Meanwhile, the mixture of kaolin and NaCl or NaAc was also processed in the DTF under both atmospheres in order to acquaint information about mineral melting and coalescence. This study will improve understanding of the dependence of the capability of kaolin to reduce PM2.5 on the sodium forms in the coal, which are helpful for power plants to achieve PM2.5 reduction with additives in the furnace based on their coal properties.

2. Experimental section 2.1. Coal samples A Chinese coal was pulverized and sieved into a size fraction of 45–100 lm. The sample was sequentially extracted for 24 h with deionized water, 0.5 mol/L ammonium acetate (CH3COONH4) and 0.5 mol/L hydrochloric acid (HCl) with a solid to liquid ratio of 100 g of coal/400 mL of solution, respectively, and then repeatedly flushed with deionized water, filtered, and dried at 45 °C. The sequential leaching process is able to remove most of organically-bounded and acid-soluble inorganic species in the coal [29]. As a result, the obtained coal sample, denoted as ‘‘leached coal’’, has a low content of sodium (Na) that mainly exists as acid-insoluble silicate and/or aluminosilicate. To investigate the effect of Na occurrence in coal on PM2.5 reduction by adding kaolin, two forms of Na were loaded on the leached coal by using the solutions of sodium acetate (NaAc) and sodium chloride (NaCl), respectively.

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Briefly, the leached coal was soaked in 0.5 mol/L NaAc solution with the identical solid to liquid ratio used for coal leaching and stirred for 24 h, and then washed, filtered and dried to prepare a ‘‘NaAc-loaded coal’’ sample. The content of Na in the NaAc-loaded coal was determined by digesting it with acids in a Milestone ETHOS microwave, followed by quantifying Na content with an inductively coupled plasma mass spectrometry (ICP-MS, PerkinElmer ELAN DRC-e). A ‘‘NaCl-loaded coal’’ was also prepared by soaking the leached coal in a NaCl solution with the concentration of 0.019 mol/L at the same solid-to-liquid ratio for 24 h. The slurry was stirred and dried to obtain the ‘‘NaCl-loaded coal’’ sample. The concentration of the NaCl solution was determined to ensure the resulted ‘‘NaCl-loaded coal’’ having the Na content similar to that of the NaAc-loaded coal. The properties of the prepared coal samples are presented in Table 1. Kaolin, a chemical pure standard material with a Sauter mean diameter of 5.78 lm, was used as the additive or as the raw material for preparing the additives for coal combustion experiments. Its properties are shown in Table 2. Kaolin was physically mixed at a kaolin-to-ash (in coal) ratio of 10 wt% into the leached coal, NaAc-loaded coal and NaCl-loaded coal, respectively. To investigate the influence of interaction between kaolin and sodium carboxylate or NaCl on mineral melting and coalescence, pure NaAc and NaCl were mixed with kaolin at a molar ratio of 0.6 (an approximate molar ratio of sodium in NaAc-loaded or NaCl-loaded coal to added kaolin) and referred as ‘‘kaolin + NaCl’’ and ‘‘kaolin + NaAc’’ hereafter, respectively. To study the effect of Na–Al–Si formed by kaolin adsorbing sodium on PM2.5 reduction, kaolin + NaAc and kaolin were calcined in a muffle furnace at 1100 °C for 2 h to get a Na–Al–Si mixture and the treated kaolin, denoted as ‘‘Na–Al–Si (T)’’ and ‘‘kaolin (T)’’, respectively. They were then adequately crushed to the Sauter mean diameter of 7.56 lm and 7.75 lm, respectively, and physically mixed with the leached coal with the method aforementioned. The properties of Na–Al– Si (T) and kaolin (T) are also presented in Table 2. 2.2. Coal combustion and PM sampling The prepared coal samples, with and without additives, were burnt in an electrically heated drop-tube furnace (DTF). The furnace tube is 40 mm in inner diameter and 1440 mm in length (see Fig. 1). The furnace temperature was set at 1500 °C. The atmosphere for the combustion was 21% vol O2 balanced with N2 or CO2. The total gas flow rate was 5 L/min, leading to a particle residence time of 1.2 s at 1500 °C. It is important to note that, for the coal samples with additives, the feeding rate was based on the coal mass and maintained at 0.2 g/min. The combustion of the leached

Table 1 Properties of coal samples studied. NaCl-loaded coal

NaAc-loaded coal

Proximate analysis (ad, wt%) Moisture 3.53 Volatile matter 30.26 Fixed carbon 34.94 Ash 31.27

Leached coal

4.01 30.45 33.63 31.91

3.54 31.72 33.42 31.32

Ultimate analysis (ad, wt%) C 48.08 H 3.70 N 0.76 S 0.23 Oa 12.43

47.88 3.51 0.70 0.24 11.75

47.61 3.53 0.74 0.27 12.99

Na (mg/g coal, ad)b 0.259 a b

By difference. Measured by ICP-MS.

1.687

1.738

Table 2 Properties of additives. Kaolin Compositions (wt%) Al2O3 SiO2 Na2O MgO P2O5 SO3 K2O CaO TiO2 Fe2O3

Kaolin (T)a

Na–Al–Si (T)b

42.87 52.63 0.71 0.49 0.54 1.80 0.45 0.07 0.21 0.23

45.62 51.43 0.90 0.69 0.39 0.06 0.36 0.04 0.24 0.27

42.16 48.67 6.78 0.52 0.41 0.24 0.34 0.06 0.25 0.57

Sauter mean diameter (lm) 5.78

7.75

7.56

Ash fusion analysis (°C) DT >1500 ST >1500 FT >1500

>1500 >1500 >1500

1284 1410 >1500

a

Thermal treated kaolin in the muffle furnace at 1100 °C. Thermal treated kaolin with NaAc in the muffle furnace at 1100 °C, in which the content of sodium aluminosilicate (AlNaSi2O6) is about 44.17%. b

coal with and without additives was only conducted in O2/N2 atmosphere. The coal at each case was nearly burnt out at the experimental conditions studied. The temperature of the top point of the sampling probe was about 220 °C. Then, the fly ash was collected by a cyclone and a Dekati gravimetric impactor (DGI) after quenched by N2-gas (flow rate: 5 L/min) in a sampling probe. The coarse particles with the aerodynamic diameter greater than 10 lm were removed by the cyclone at a gas flow rate of 10 L/ min. The particles smaller than 10 lm were further size-segregated by the DGI into four stages of 2.5–10 lm, 1–2.5 lm, 0.5– 1 lm and 0.2–0.5 lm at 70 L/min of gas input. Additional gas provided was filtrated air at a flow rate of 60 L/min. Aluminum foils covered with apiezon-L grease were used as the substrates for PM collection in each stage of the DGI. The sampling time was properly set so that the mass of PM collected in each stage was less than 10 mg to avoid particles bounce off from the substrates. The PM sampling was repeated at least three times for each combustion test case to ensure satisfactory reproducibility and accuracy. 2.3. Thermal treatment of kaolin mixed with NaCl or NaAc in DTF To investigate the influence of the interaction between kaolin and sodium carboxylate or NaCl on mineral melting and coalescence, kaolin and its derived mixtures (kaolin + NaCl and kaolin + NaAc) were fed into the DTF under the same conditions as those for coal combustion. The treated samples were N2-quenched at the exit of the DTF, and collected with glass fiber filters with a pore size of 0.3 lm. The collected samples were marked as ‘‘kaolin(O2/N2)’’, ‘‘kaolin(O2/CO2)’’, ‘‘kaolin + NaCl(O2/N2)’’, ‘‘kaolin + NaCl(O2/CO2)’’, ‘‘kaolin + NaAc(O2/N2)’’ and ‘‘kaolin + NaAc(O2/ CO2)’’, respectively. 2.4. Samples analysis The mass of PM collected in each stage of the DGI was determined by weighing the substrates before and after the sampling with a Sartorius M2P microbalance (accuracy: 1 lg). The mass of the PM in the size range of 0.2–0.5 lm was termed as PM0.2–0.5, representing the ultrafine mode particles, while those in the size range of 0.5–2.5 lm were summed and referred to as PM0.5–2.5, representing the central mode particles.

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Fig. 1. Schematic diagram of experimental setup.

A Digital Imaging Coal Ash Fusibility System (produced by Carbolite) was used to measure the fusion temperatures of the minerals obtained from the thermally treated samples, i.e., treated kaolin and its derived mixtures. The particle sizes of the samples were quantified using a Malvern particle-size analyzer. The main mineral phases in these samples were analyzed by a X’Pert PRO Xray diffraction (PANalytical B.V. Inc.) using Cu Ka radiation in the range of 5° < 2h < 80°. 3. Results and discussion 3.1. Importance of sodium aluminosilicate on PM0.5–2.5 reduction by kaolin Previous studies identified the ability of kaolin reducing PM2.5 because of its capturing metallic vapors (Na, K, etc.) [14,16] and consequently reducing ultrafine mode particles formation [22– 24]. However, the effect of aluminosilicates such as Na–Al–Si formed by the interaction between kaolin and metallic vapors on PM2.5 formation behavior was mostly not noted. Fig. 2 shows PM0.2–0.5 and PM0.5–2.5 emissions during the combustion of solely the leached coal and the leached coal with the additives of kaolin, kaolin (T) and Na–Al–Si (T). When compared to those from the combustion of the leached coal, both PM0.2–0.5 and PM0.5–2.5 emissions are not affected by adding kaolin or kaolin (T). It implies that the contributions of the added kaolin and kaolin (T) in the leach coal with 10 wt% ratio (additive-to-ash in coal) to the yields of PM0.2–0.5 and PM0.5–2.5 are not considerable. In contrast, although adding Na–Al–Si (T) also showed no considerable effect on PM0.2–0.5 emission if the error bars were considered in Fig. 2a, there was a significant decrease of 27.2% in PM0.5–2.5 yield during the combustion of the leached coal with Na–Al–Si (T) added (Fig. 2b). As a result of the sequential extraction, while most active alkali metals (Na, K) in coal were leached out, the alkali metals left chiefly exist in silicates or/and aluminosilicates and have low

Fig. 2. Effect of thermal treated kaolin and Na–Al–Si mixture on the reduction of (a) PM0.2–0.5 and (b) PM0.5–2.5 yields during coal combustion. Note that the unit of Yaxis is standardized as mg/g-coal, indicating the quantification of PM emission is based on the feeding rate of coal.

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volatility. It is expected that, during the combustion of the leached coal, the release of alkali metals available for kaolin adsorption is considerably low. It determines that adding kaolin, kaolin (T) and Na–Al–Si (T) had no influence on PM0.2–0.5 emission. The significant reduction of PM0.5–2.5 by Na–Al–Si (T) additive can be attributed to the softening additive particles capturing the PM0.5–2.5. Na–Al–Si (T) has a deformation temperature of 1284 °C (Table 2) and it melts and forms softening particles in the DTF. PM0.5–2.5 reduction by Na–Al–Si (T) is closely associated with particle collision, and it is necessary to reveal the factors that influences the particle collision. Brownian coagulation is a key factor influencing PM1 formation, whereas it has a slight effect on PM1.0–2.5, the collision of which is mainly caused by inertial impaction [42]. Therefore, in order to further discuss the factors that affect the particle collision, PM0.5–2.5 should be further divided into PM0.5–1.0 and PM1.0–2.5. The particles, including the particulate matter and Na–Al–Si (T) are both simplified to spheres. According to their mass, the particles size distributions and the carrier flow rate, it is estimated that the number concentrations of Na–Al–Si (T), PM0.5–1.0 and PM1.0–2.5 in the flue gas are about 106/L, 108/L and 107/L, respectively. However, the reduction of PM0.5–2.5 in the furnace caused by adding Na–Al–Si (T) mainly happens during the process of its formation, and it is necessary to get the number concentration of PM0.5–2.5 precursors, especially for PM0.5–1.0. Partial PM0.5–1.0 is formed through the coagulation or agglomeration of small particles (probably much smaller than 0.01 lm). It is estimated that the number concentration of particles potentially colliding with Na–Al–Si (T) is markedly higher than that of PM0.5–1.0 particles and could be as high as 1014/L. According to the model of Lai et al. [43], it is speculated that there will be 1012–1013 times Brownian coagulation per liter within a second. Meanwhile, the Reynolds number of the flue gas is approximately 60, leading to a laminar flow in drop tube furnace. Therefore, the velocity distribution in the radial direction is inhomogeneous resulting from the fluid viscosity. The velocity is highest along the center axis, and gradually decreases to zero near the wall. This phenomenon will induce uneven distribution of the small particles during the process of PM0.5–1.0 formation, and these small particles will accordingly diffuse in the gas, which will lead to particle collision simultaneously. Based on the theory of Edzwald [44], it can be speculated that the characteristic collision of small particles due to diffusion is about 107–108 times per liter within a second. The value is far less than Brownian coagulation times, indicating that the Brownian motion is the major factor influencing the collision between PM0.5–1.0 particles and Na–Al–Si (T). Due to char fragmentation, mineral transformation and the disturbance of larger size particles to upstream flow field [45], there is some local turbulence in the system, and this will lead to collision between particles with larger size through inertial impaction, which plays a vital role in PM1.0–2.5 particles colliding with Na–Al–Si (T). Overall, in such circumstances, the collision between PM0.5–2.5 particles and softening additive particles are highly possible [46], resulting in the capture of PM0.5–2.5 by Na–Al–Si (T) and a significant decrease in PM0.5–2.5 emission (Fig. 2b). In contrast, both kaolin and kaolin (T) have a deformation temperature higher than 1500 °C, indicating that these additives are still solid particles at the furnace temperature. Although the surface reaction of kaolin and alkali metals may form Na-containing aluminosilicates having low deformation temperatures, little release of alkali metals from the leached coal combustion and consequently the amount of the surface reaction product is not enough to result in sticky surfaces of the additive particles. It explains that kaolin and kaolin (T) having no influence on PM0.5–2.5 emission of the leached coal and also confirms that Na–Al–Si (T) has a similar ability as Mg–Al–Si forming liquidus and promoting the coalescence of particles to reduce PM0.5–2.5 formation.

Note that despite the presence of liquidus, adding Na–Al–Si (T) have no obviously effect on PM0.2–0.5 reduction during leached coal combustion. Generally, the effect of liquidus capturing on solid phase is stronger than on vapor phase. At high temperature, because of a relatively higher saturated pressure for vapors, the volatile alkali metals occurs chiefly as vapor phase and they do not considerably condense to form solid ultrafine mode particles (PM0.2–0.5). However, the high temperature is the essential factor for liquidus formation for Na–Al–Si (T). This implies that the liquidus and solid particles of PM0.2–0.5 cannot coexist at high temperature. That is to say, the liquidus capturing due to the melting of Na–Al–Si (T) is not effective for ultrafine mode particles (PM0.2–0.5) emission (Fig. 2a), but merely reduces the central mode particles (PM0.5–2.5) emission (Fig. 2b). 3.2. Effect of Na occurrence in coal on PM reduction by kaolin Fig. 3 presents PM2.5 yields from NaCl-loaded coal and NaAcloaded coal burning in O2/N2 atmosphere, in which the effects of adding kaolin on the yields of PM0.2–0.5 and PM0.5–2.5 are demonstrated. The percentages in the figure denote the reduction fraction (Qr) of PM0.2–0.5 and PM0.5–2.5 by adding kaolin. Qr is defined as Qr = [PM mass (coal)-PM mass (coal added kaolin)]/PM mass (coal). In comparison with the PM yields from the leached coal shown in Fig. 2, it can be found that much more PM0.2–0.5 was formed from the combustion of both NaCl-loaded coal and NaAc-loaded coal than that from burning the leached coal. On the other hand, the combustion of NaAc-loaded coal yielded more and the combustion of NaCl-loaded coal yielded less PM0.5–2.5 than burning the leached coal. Meanwhile, Fig. 3 also illustrates that NaAc-loaded coal had higher PM0.2–0.5 and PM0.5–2.5 yields than NaCl-loaded coal. The differences of PM0.2–0.5 and PM0.5–2.5 yields when loading different occurrences of sodium in coal are closely related with the transformation behaviors of sodium. Whether the NaCl or organically bounded sodium loaded on the leached coal, they both are active to vaporize and are expected to make a great contribution to PM0.2–0.5 formation. And it is not surprising that either NaAc-loaded coal or NaCl-loaded coal yields more PM0.2–0.5 than the leached coal (Figs. 2 and 3). However, there are significant differences between transformations of different

Fig. 3. Comparison of PM2.5 emission with and without kaolin during NaCl-loaded coal and NaAc-loaded coal combustion in O2/N2 atmosphere. Note that the X-axis and Y-axis are segmented with different scale, and the unit of Y-axis is standardized as mg/g-coal, indicating the quantification of PM emission is based on the feeding rate of coal. A small figure was inserted to show clearly the difference of PM yield under O2/N2 combustion condition from that under the experimental condition mentioned below.

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occurrence of sodium at 1500 °C. At high temperatures, the sodium occurring as carboxylate in coal was readily released in the metallic form through the reactions [28]: CMCOONa ? CMNa + CO2, and CMNa ? CM + Na, and mainly transformed into NaOH in the flue gas. In contrast, the sodium in the NaCl-loaded coal was firstly vaporized in the form of NaCl crystal, and because of the presence of equivalent chlorine, the sodium in the NaCl-loaded coal mainly transformed into NaCl in the flue gas [32]. Due to the stronger capability to be vaporized into flue gas of sodium as organically bounded than that of sodium as NaCl crystal, it is speculated that more sodium is condensed onto PM0.2–0.5 during NaAcloaded coal combustion, which results in a higher PM0.2–0.5 yield of NaAc-loaded coal than that of NaCl-loaded coal (Fig. 3). Besides vaporing into flue gas, as well as some sodium interacted with minerals in the coal to form eutectics matter, and exacerbates the mineral coalescence to inhibit the PM0.5–2.5 formation. On the other hand, some sodium, specifically the organically bounded sodium exhibits perfect catalysis on the reaction between carbon in coal and gas medium [47], which induces a more intense fragmentation of chars and promotes the formation of PM0.5–2.5. Due to a higher effect of NaCl on mineral coalescence than on char fragmentation, the combustion of NaCl-loaded coal yields less PM0.5–2.5 than that of the leached coal (Figs. 2 and 3). In contrast, although sodium in the organic state is also favor to mineral coalescence, even maybe has a more apparent effect on mineral coalescence than NaCl, the char fragmentation resulted from its catalysis obviously plays the dominant role in PM0.5–2.5 formation, and accordingly NaAc-loaded coal yields more PM0.5–2.5 than the leached coal and NaCl-loaded coal (Figs. 2 and 3). Fig. 3 clearly shows that, regardless of the forms of Na loaded on the leached coal, adding kaolin greatly decreased PM0.2–0.5 formation. It is obviously attributed to kaolin product adsorbing sodium-containing vapors released from the combustion of both coals and consequently depressing the contribution of the vapors to PM0.2–0.5 formation. Moreover, Fig. 3 shows that, for both coals, adding kaolin also significantly decreased PM0.5–2.5 yields particularly for NaAc-loaded coal although the reductions are relatively less than those for PM0.2–0.5. The resulted PM0.5–2.5 yields of both coals with kaolin added are even much lower than that from burning the leached coal and comparable to that from burning the leached coal added with Na–Al–Si (T) (see the value in Fig. 2). Practically, kaolin particles are not likely to interact with coal/char particles to affect the formation of PM0.5–2.5 during coal/char particles combustion. The only plausible reason is the interaction between kaolin particles and PM0.5–2.5 particles formed from coal/ char particles combustion, which leads to PM0.5–2.5 reduction. As discussed above, adding Na–Al–Si (T) significantly reduced PM0.5–2.5 yield through colliding with and captured by softening additive particles. Similarly, the surface reaction between kaolin and adsorbed sodium species forms sodium aluminosilicates, which can be viscous at the DTF temperature. The significant reduction of PM0.2–0.5 by adding kaolin in the two coals implies considerable amounts of sodium adsorbed on the surface of kaolin product particles, which results in sticky surfaces for capturing PM0.5–2.5 particles. The reduction of PM0.5–2.5 yield implies that sodium aluminosilicates formed from the surface reaction between kaolin product and adsorbed sodium species play an important role in PM0.5–2.5 reduction. When the yields from NaAc-loaded coal and NaCl-loaded coal are compared, Fig. 3 also shows that kaolin achieved much greater reductions in both PM0.2–0.5 and PM0.5–2.5 for NaAc-loaded coal combustion. It is apparently attributed to the difference in the occurrence of sodium in coal and consequently the behavior of sodium release and transformation. As previously mentioned, for NaAc-loaded coal, sodium is likely to occur as carboxylates in coal matrix and mainly transform into NaOH in the flue gas, while for

Fig. 4. The XRD patterns of samples after thermal treatment at 1500 °C in O2/N2 atmosphere (Q = SiO2, M = 3Al2O32SiO2, Sc = NaCl, Sa1–Sa2 = sodium aluminosilicate).

NaCl-loaded coal, sodium occurs mainly as NaCl salt in coal and is still in the form of NaCl in the flue gas. In order to compare the interaction between kaolin with NaCl and with sodium carboxylate, the mixtures of kaolin with NaCl and NaAc were treated in the DTF, as described in Section 2.3. XRD patterns of the products from the treatment are shown in Fig. 4. Fig. 4 shows that, while NaAc reacting with kaolin was completely transformed to aluminosilicate, NaCl was detected by XRD, indicating a fraction of NaCl still left in the product under the same treatment condition. It implies that the products of sodium carboxylate are much more reactive with kaolin than NaCl, consistent with the results of Mwabe and Wendt [16], Van Eyk et al. [32] and Lindner et al. [48]. It may explain that PM0.2–0.5 reduction by adding kaolin is higher for NaAc-loaded coal combustion than for NaCl-loaded coal combustion. More PM0.2–0.5 reduction during NaAc-loaded coal combustion than during NaCl-loaded coal combustion (Fig. 3) implies that more sodium was adsorbed by kaolin to form Na-containing aluminosilicates on the additive surface. As discussed in Section 3.1, the resulted sodium aluminosilicate would form liquidus to enhance the mineral coalescence and decrease the PM0.5–2.5 formation. Due to the more sodium aluminosilicate formation and the stronger mineral coalescence, kaolin resulted in more reduction of PM0.5–2.5 during NaAc-loaded coal combustion (Fig. 3). The difference in the behavior related to PM2.5 reduction between NaAcloaded coal and NaCl-loaded coal combustion indicates that the chemical form of sodium occurring in coal is an important factor affecting the extent of PM2.5 reduction by kaolin during coal combustion. The mineral coalescence is closely associated with its melting ability. Usually, the fusion temperature is used to measure the melting ability of a mineral. Kaolin with and without NaCl or NaAc was treated thermally at 1500 °C in DTF (as described in Section 2.3). Table 3 shows the fusion temperatures of these treated samples, including deformation temperature (DT), softening temperature (ST) and flow temperature (FT) [49]. Generally, mineral with lower characteristic temperatures has a stronger melting ability. Note that the evaluation method may ignore the melting of

Table 3 Fusion temperature of samples after thermal treatment at 1500 °C in the O2/N2 atmosphere. Samples

DT/°C

ST/°C

FT/°C

Kaolin(O2/N2) Kaolin + NaCl(O2/N2) Kaolin + NaAc(O2/N2)

>1500 >1500 1320

>1500 >1500 1450

>1500 >1500 >1500

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some minerals in minor amount because of their feeble influence on ash cone shape alteration. As shown in Table 3, the DT of pure kaolin product was still higher than 1500 °C after treatment. It is known that, at the temperatures higher than about 1200 °C, kaolin decomposes to form mullite with the melting point of 1810 °C and quartz with the melting point of 1750 °C. Therefore, melting hardly takes place during the transformation of kaolin. It is also found that adding NaCl to kaolin had a slight effect on the melting behavior. In contrast, the DT of ‘‘kaolin + NaAc(O2/N2)’’ dropped to about 1320 °C, indicating that the ability of the mineral to melt was significantly enhanced in the interaction between kaolin and NaAc. As discussed above, NaCl and sodium carboxylate have different transformation behaviors in the interaction with kaolin, which are the main reasons for the difference in the melting ability of minerals. Due to the difference in mineral melting, the mineral particles may have different coalescence behavior that can be represented by the change of the particles size. Usually, the volume average particle diameter D[4,3] is used for comparing the size of the particles. A coalescence factor Qa is defined as

Q a ¼ D½4;3 ðsampleÞ=D½4;3 ðkaolinðO2 =N2 ÞÞ to examine the size change of the samples generated from the reaction of kaolin and NaCl or NaAc, relative to the size of the sample ‘‘kaolin(O2/N2)’’. It is found that the mineral particles show different extents of the increase in the size after the interaction of kaolin with NaCl or NaAc. The measured Qas show that, while the particle size of kaolin + NaCl(O2/N2) sample increases 19.0% relative to kaolin (O2/N2) sample, that of kaolin + NaAc(O2/N2) increases 83.8% because the interaction of kaolin and NaAc had a stronger effect on mineral particle coalescence. Overall, PM0.5–2.5 was significantly reduced by kaolin during NaAc-loaded coal combustion. As a reason, the stronger interaction between kaolin and sodium carboxylate contributes to mineral melting promotion and coalescence enhancement, which favor the PM0.5–2.5 reduction. However, the amount of PM0.5–2.5 depends on the coal properties (Fig. 3), and its alteration will cause different collision frequency between additive and particulate matters, which also affect probably the PM0.5–2.5 reduction. The relationship

Fig. 5. Comparison of PM2.5 emission with and without kaolin during NaCl-loaded coal and NaAc-loaded coal combustion in O2/CO2 atmosphere. Note that the X-axis and Y-axis are segmented with different scale, and the unit of Y-axis is standardized as mg/g-coal, indicating the quantification of PM emission is based on the feeding rate of coal. In comparison of the small figure in the corner to that in Fig. 3, it is found that both NaCl-loaded coal and NaAc-loaded coal obviously yield less PM in O2/CO2 atmosphere than in O2/N2 atmosphere.

between mineral melting/particle collision and PM0.5–2.5 reduction by kaolin will be discussed below. 3.3. Effect of combustion atmosphere on PM reduction by kaolin Fig. 5 shows PM2.5 yields and reductions by adding kaolin when burning NaCl-loaded coal and NaAc-loaded coal in the O2/CO2 atmosphere. Compared to those in O2/N2 combustion shown in Fig. 5, it was found that both coals yields much less PM0.2–0.5 and PM0.5–2.5 during O2/CO2 combustion. The char particle temperatures during O2/CO2 combustion were lower than those during O2/N2 combustion by about 150–200 °C [50]. At lower combustion temperatures, less vaporization of metals including Na results in less PM0.2–0.5 formation while less char particle fragmentation leads to less PM0.5–2.5 yield. Different from those for O2/N2 combustion, Fig. 5 shows that the two coals yielded almost the same amounts of PM0.2–0.5 and PM0.5–2.5. According to the transforming pathway of sodium as carboxylate [28], the high concentration CO2 may abate the volatilization and transformation of sodium carboxylate to Na. Therefore, the lower volatilization quantity of sodium in NaAc-loaded coal leads to comparable PM0.2–0.5 yield of NaAc-loaded coal to that of NaCl-loaded coal during O2/CO2 combustion. Due to the lower char particles temperature, the catalysis effect of sodium as carboxylate on reaction between carbon in coal and gas medium is weakened, and correspondingly the char fragmentation of NaAc-loaded coal becomes less severe during O2/ CO2 combustion. Meanwhile, the suppression of sodium in NaAcloaded coal being released into flue gas results more sodium reacting with minerals in coal, and this enhances the mineral coalescence. Therefore, compared to during O2/N2 combustion, the difference of PM0.5–2.5 yields between NaAc-loaded coal and NaCl-loaded coal became slighter during O2/CO2 combustion (Figs. 3 and 5). When kaolin was added, Fig. 5 shows that both PM0.2–0.5 and PM0.5–2.5 yields from NaAc-loaded coal combustion were significantly reduced. In contrast, adding kaolin had marginal effects on PM0.2–0.5 and PM0.5–2.5 yields from NaCl-loaded coal combustion. As shown in Fig. 6, Table 4 and Fig. 7, the ability of NaAc transforming to sodium aluminosilicate was also stronger than that of NaCl in the O2/CO2 atmosphere. As compared with that of kaolin + NaCl(O2/CO2), the ST of kaolin + NaAc(O2/CO2) was lower, coupled with a higher coalescence potential. These phenomena are consistent with those in the O2/N2 atmosphere, indicating the O2/ CO2 atmosphere does not change the fact that the capability of kaolin to reduce PM2.5 is stronger during NaAc-loaded coal combustion than that during NaCl-loaded coal combustion. However, whether for NaCl-loaded coal or NaAc-loaded coal, PM0.2–0.5 reduction by adding kaolin is less in O2/CO2 atmosphere

Fig. 6. The XRD patterns of samples after thermal treatment at 1500 °C in O2/CO2 atmosphere (Q = SiO2, M = 3Al2O32SiO2, Sc = NaCl, Sa3–Sa4 = sodium aluminosilicate).

J. Si et al. / Applied Energy 114 (2014) 434–444 Table 4 Fusion temperature of samples after thermal treatment at 1500 °C in the O2/CO2 atmosphere. Samples

DT/°C

ST/°C

FT/°C

Kaolin(O2/CO2) Kaolin + NaCl(O2/CO2) Kaolin + NaAc(O2/CO2)

>1500 >1500 1340

>1500 >1500 >1500

>1500 >1500 >1500

Fig. 7. Comparison of mineral coalescence under the O2/N2 and O2/CO2 atmospheres.

than that in O2/N2 atmosphere (Figs. 3 and 5). It is known that kaolin captures metallic vapors through surface reaction to reduce PM0.2–0.5 formation. Because of the lower coal particle combustion temperatures leading to less vaporization of metals as well as the lower diffusivity of vapors to kaolin particle surface, the extent of kaolin reacting with metallic vapors is expected to be weaker in the O2/CO2 atmosphere than that in the O2/N2 atmosphere, leading to a lower reduction of PM0.2–0.5 during O2/CO2 combustion. Kaolin also shows a weaker ability to control PM0.5–2.5 in the O2/ CO2 atmosphere than in the O2/N2 atmosphere. In particular, when burning NaAc-loaded coal, the reduction is 18.6% during O2/CO2 combustion (Fig. 5), much less than 35.2% during O2/N2 combustion (Fig. 3). As shown in Tables 3 and 4, the ST of kaolin + NaAc (O2/CO2) is higher than 1500 °C, while that of ‘‘kaolin + NaAc(O2/ N2)’’ is only about 1450 °C. This indicates that the melting ability of the surface products of kaolin reacting with NaAc is weaker in the O2/CO2 atmosphere than that in the O2/N2 atmosphere. Meanwhile, the high concentration CO2 may not favor the transformation of sodium carboxylate into Na. Therefore, less Na vaporization and less sodium aluminosilicate formation during coal combustion further weaken the interaction between kaolin and sodium. Consequently, the mineral coalescence of ‘‘kaolin + NaAc(O2/CO2)’’ decreases (Fig. 7), and PM0.5–2.5 reduction by kaolin is lower during O2/CO2 combustion. Overall, the promotion of mineral melting favors the PM0.5–2.5 reduction. Moreover, as shown in Figs. 3 and 5, the PM0.5–2.5 emission depends on coal properties and combustion atmosphere, and the difference in the emission will cause different collision frequency between additive and particulate matters, which is also an important factor influencing PM0.5–2.5 reduction [46]. It is necessary and effective for PM0.5–2.5 reduction that PM0.5–2.5 particles collide and then coalesce with Na–Al–Si particles. Moreover, particle collision frequency is positively associated with their number concentration and sizes, meanwhile, particles coalescence is positively correlated with mineral melting. That is to say, without either particle collision or mineral melting, Na–Al–Si cannot reduce PM0.5–2.5 formation. Supposing that the density and size of particles vary within a small range, the mass concentration can

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typically be used to substitute the number concentration and size of particles to measure the collision frequency. Note that only an approximate trend is obtained using mass concentration to measure the collision frequency, because it ignores the influence of particle size may be less than that of number concentration. Overall, The above two reasons, i.e., mineral melting and coalescence due to the aluminosilicate formation, together with the particle collision, determine the difference in PM0.5–2.5 reduction with adding kaolin. Figs. 8 and 9 mainly discuss which factor playing the dominant role in PM0.5–2.5 reduction, particle collision or mineral melting. However, it is difficult to quantify these two factors. Simplistically, it is necessary to use the relative value to compare their influence on PM0.5–2.5 reduction. As shown in Fig. 8, if the minimum PM0.5–2.5 emission (generated from NaCl-loaded coal under O2/CO2 combustion) is used as the base and the ratio of PM0.5–2.5 formed under each condition to this minimum value is defined as the relative concentration (Cr). The Crs of NaCl-loaded coal(O2/N2), NaAcloaded coal(O2/N2), NaCl-loaded coal(O2/CO2) and NaAc-loaded coal(O2/CO2) are 1.95, 2.61, 1.00 and 1.05, respectively. Similarly, using the coalescence factor of kaolin + NaCl(O2/CO2) as the base and defining the ratio of the other samples to it as the relative coalescence (Ar), the Ars of kaolin + NaCl(O2/N2), kaolin + NaAc(O2/N2), kaolin + NaCl(O2/CO2) and kaolin + NaAc(O2/CO2) are equal to 1.01, 1.56, 1.00 and 1.18, respectively. Integrating the discussion about the influence of mineral melting and coalescence together with the particle collision on the PM0.5–2.5 reduction by kaolin, the relationship of Qr, Cr and Ar can be expressed in an equation of Q r ¼ a  Abr  C cr . The coefficient ‘‘a’’ is the compensation factor; Abr and C cr respectively represent the influence of coalescence resulting from mineral melting and particle collision due to various PM0.5–2.5 concentration on PM0.5–2.5 reduction, and the index ‘‘b’’ and index ‘‘c’’ are their impact factor, respectively. Fig. 9 shows the fitted results and their comparison with experimental values. It is found that the impact factor of Cr exceeds zero, which indicates the particle collision due to various PM0.5–2.5 mass concentrations maybe has some effect on PM0.5–2.5 reduction. Furthermore, the impact factor of ‘‘Ar’’ (3.606) is greater than that of ‘‘Cr’’ (1.658), implying that the mineral coalescence has a stronger influence than particle collision on PM0.5–2.5 reduction by kaolin. Generally, collision frequency has a square relationship with the number concentration of the particles [51]. In this study, the impact factor of mass concentration on particle collision is only 1.658, indicating that there is some deviation in measuring particle collision using mass concentration of particles. One possible reason is that the influence of particle size on particle collision was overestimated by taking it to be equal to that of number concentration in the fitted process described above. 3.4. Further discussion and practical implication As stated above, at high temperatures, the sodium aluminosilicate is able to produce liquidus and induce migration of fine particles (PM0.5–2.5) toward coarse particles (PM2.5+) to reduce PM0.5–2.5 formation (Fig. 2). Whether during NaAc-loaded coal combustion or during NaCl-loaded coal combustion, PM0.2–0.5 reduction by adding kaolin implies the formation of sodium aluminosilicate. Due to the presence of PM0.5–2.5 reduction behavior when adding kaolin into NaAc-loaded coal and NaCl-loaded coal (Figs. 3 and 5), it can be inferred that there are two pathways for PM2.5 reduction by kaolin associated with PM2.5 formation mechanisms. As shown in Fig. 10, kaolin is able to adsorb chemically vapor metals such as Na, and inhibit condensation of these elements to reduce ultrafine mode particles (PM0.2–0.5) formation. This has been proved by our early studies [23,24] and Takuwa and Naruse [22]. On the other hand, the aluminosilicate such as Na–Al–Si formed by the surface

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Fig. 8. The relationship of PM0.5–2.5 reduction, mineral coalescence and particle collision.

Fig. 9. The experimental and fitted value of PM2.5 reduction.

reactions between kaolin and sodium species, can cause fine particles to coarse particles (PM2.5+) migration to reduce central mode particles (PM0.5–2.5) formation (Fig. 2b). Note that ultrafine mode particles are not be effectively reduced by molten aluminosilicate because solid particles formation of PM0.2–0.5 is incompatible with the existence of liquidus. The dotted line in Fig. 10 reflects this process. PM2.5 reduction by adding kaolin is closely associated with the occurrence of sodium in coal (Fig. 3). The interaction between kaolin and sodium carboxylate is stronger than that between kaolin and NaCl (Fig. 4), and the stronger interaction enhances the mineral melting and coalescence (Table 3 and Fig. 7). As a result, both PM0.2–0.5 and PM0.5–2.5 yields are reduced significantly by adding kaolin during the NaAc-loaded coal combustion (Fig. 5), while the reductions during the NaCl-loaded coal combustion is not so considerable. It indicates that occurrence of Na in coal is important to PM2.5 reduction efficiency by adding kaolin. Therefore, the

Fig. 10. Schematic diagram of PM2.5 reduction by kaolin at high temperature. (a) The diagram of PM2.5 reduction by kaolin, (b) the flowchart of PM2.5 reduction by kaolin.

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quality and properties of coals should be taken into consideration for using kaolin to control PM2.5 formation. For example, low rank coals are generally rich in organically bounded Na, for which employing kaolin as a PM2.5 reduction additive is expected to be effective. In the O2/CO2 atmosphere, the capability of kaolin to reduce PM2.5 is also stronger for NaAc-loaded coal combustion than that for NaCl-loaded coal combustion (Fig. 5). As compared with that in the O2/N2 atmosphere, PM0.2–0.5 reduction by adding kaolin is lower in O2/CO2 atmosphere due to the less vaporization of metals and the slower diffusion rate of vapors in the O2/CO2 atmosphere. Moreover, the relative weak mineral melting and coalescence is an important factor leading to the lower PM0.5–2.5 reduction in the O2/ CO2 atmosphere (Table 4 and Fig. 7). Besides, in the laboratoryscale experiment, at the same concentration of oxygen, the concentration of the PM particles is obviously lower during O2/CO2 combustion than that during O2/N2 combustion (Fig. 8). Therefore, collision between particles may be another important factor that cannot be ignored (Fig. 9). With the joint weaker action of mineral coalescence and particle collision, the PM0.5–2.5 reduction by adding kaolin is evidently lower during the O2/CO2 combustion than that during the O2/N2 combustion. Overall, in O2/CO2 atmosphere, adding kaolin is also helpful for PM2.5 reduction; however, compared with in O2/N2 atmosphere, the effect of kaolin on PM2.5 reduction becomes weaker, especially during the NaAc-loaded coal combustion. It is worth noting that, considering the recycle of fine particles with the recycled flue gas in practical oxy-fuel combustion, the actual concentration of PM particles is not necessarily lower in large scale oxy-fuel (O2/CO2) plants, which is different from the conditions of this study. Overall, the further researches on contribution of particles concentration to PM2.5 reduction in large-scale oxy-fuel plants need to be carried out. In addition, some combustion characteristics in DTF, including the particles concentration and aerodynamics, are different from those in practice. Nevertheless, the results in the paper are still helpful for building the model of coal-derived PM2.5 reduction in the furnace. For example, due to more intense flue gas turbulence and higher combustion temperature in practical boiler than those in DTF, either particle collision or mineral melting is stronger in practice, and the effect of Na–Al–Si (T) on PM0.5–2.5 reduction becomes more significant. Therefore, 27.2% PM0.5–2.5 reduction by Na–Al–Si (T) in DTF can be considered as the lowest that in practice. 4. Conclusions In this study, combustion experiments of a treated low-sodium coal mixed with sodium aluminosilicate were conducted in a labscale DTF to reveal the effect of mineral melting and coalescence on PM2.5 reduction. Meanwhile, two typical Na-loaded coals (in which the sodium was loaded in the form of NaCl and sodium carboxylate, respectively) with kaolin added were also burnt under O2/N2 and O2/CO2 atmospheres to investigate the effect of interaction of kaolin with different chemical forms of sodium on PM2.5 reduction. The conclusions are drawn as follows: (1) The surface reaction between kaolin and vapor metals is not the only pathway for coal-derived PM2.5 reduction by adding kaolin. The aluminosilicate such as Na–Al–Si formed by the reaction is able to produce ‘‘liquidus’’ at high temperature and induce migration of fine particles (PM0.5–2.5) toward coarse particles (PM2.5+) to reduce PM2.5 formation. (2) The sodium carboxylate has a much stronger interaction with kaolin than NaCl, and the ultrafine mode particles (PM0.2–0.5) reduction by kaolin is higher during NaAc-loaded coal combustion than that during NaCl-loaded coal combus-

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tion. Moreover, the PM0.2–0.5 reduction in O2/CO2 combustion is lower than that in O2/N2 combustion, due to the less vaporization of metals and the slower diffusion rate of vapors in the O2/CO2 atmosphere. (3) The mineral coalescence resulting from the melting of sodium aluminosilicate is an important factor influencing PM0.5–2.5 reduction by kaolin. Besides, the PM0.5–2.5 emission depends on coal properties and combustion atmosphere, and the difference in emission will cause different collision frequency between particles and additive, which also influences the PM0.5–2.5 reduction by kaolin. With the joint actions of mineral coalescence and particle collision, the NaAc-loaded coal has a higher PM0.5–2.5 reduction by kaolin than the NaCl-loaded coal, especially under the O2/N2 combustion. (4) An expression that described the relationship of PM0.5–2.5 reduction, mineral coalescence and particle collision was fitted and it is found that the mineral coalescence has a stronger influence than particle collision on PM0.5–2.5 reduction by kaolin.

Acknowledgments The authors acknowledge the financial support of the National Basic Research Program of China (2013CB228501) and the National Natural Science Foundation of China (51276072, U1261204). The authors would also like to thank the support from the fund for international cooperation and exchange of the National Natural Science Foundation of China and Korea. Comments and suggestions by Professor Changdong Sheng at Southeast University of China is greatly appreciated. The support of the Analytical and Testing Center at the Huazhong University of Science and Technology is also appreciated.

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