Effect of coal blending and ashing temperature on ash sintering and fusion characteristics during combustion of Zhundong lignite

Fuel 195 (2017) 131–142

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Full Length Article

Effect of coal blending and ashing temperature on ash sintering and fusion characteristics during combustion of Zhundong lignite Jianbo Li a, Mingming Zhu a,⇑, Zhezi Zhang a, Kai Zhang a,b, Guoqing Shen a,b, Dongke Zhang a a b

Centre for Energy (M473), The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia Beijing Key Laboratory of Emission Surveillance and Control for Thermal Power Generation, North China Electric Power University, Beijing 102206, China

h i g h l i g h t s  Effect of coal blending on ash sintering and fusion was systematically studied.  Blending ZL with AB and IL increased the ash sintering temperature.  AFTs decreased first and then increased as the AB or IL blending ratio increased.  Lower ashing temperature resulted in lower ash sintering and fusion temperatures.  Mineralogical and morphological changes during sintering and fusion explained.

a r t i c l e

i n f o

Article history: Received 16 November 2016 Received in revised form 12 January 2017 Accepted 15 January 2017 Available online 21 January 2017 Keywords: Ash fusibility Ash sintering Ashing temperature Coal blending Zhundong lignite Mineral transformation

a b s t r a c t Ash sintering and fusion characteristics of pulverised Zhundong lignite and its blends combusted at different temperatures in a muffle furnace were investigated. Zhundong lignite (ZL) was blended with an Australian bituminous (AB) and an Indonesian lignite (IL), respectively, at various blending ratios. The blends were then combusted in the muffle furnace in air at 550 °C and 815 °C. The sintering and fusion temperatures of the ashes of the blends were determined. In addition, the ashes prepared at 550 °C were further heated to 850 °C, 1050 °C and 1250 °C in air for different periods of time to study the mineralogical and morphological changes during the ash sintering and fusion. The results showed that the ZL ash at 815 °C was rich in basic oxides with Ca-bearing minerals anhydrite, calcium silicate and yeelimite being dominant, incurring relatively low ash sintering and fusion temperatures. Both AB and IL ashes, however, were rich in acidic oxides and dominant with refractory minerals such as quartz or mullite, leading to relatively high ash sintering and fusion temperatures. As the AB or IL addition ratio in the blend increased, the ash sintering temperature also increased, indicating that the addition of AB or IL into ZL decreased the ash sintering tendency. However, as the AB or IL addition ratio increased, the ash softening temperature (ST) and fluid temperature (FT) decreased first and then increased, reaching the lowest values at addition ratios of 40% for AB and 20% for IL. In addition, owing to the higher alkali and alkali earth contents in the forms of fluxing minerals, the sintering and fusion temperatures of the ash samples at 550 °C were correspondingly lower than those of the ashes at 815 °C. XRD and SEM-EDS analyses of the ash heated at 850 °C, 1050 °C and 1250 °C revealed that the sintering of the ZL ash was attributed to Na- and Cl-bearing minerals whereas the fusion of the ZL ash was attributed to the eutectics enriched with Ca, Al, Fe, and S. The addition of 20% AB into ZL resulted in the formation of hauyne, gehlenite and nepheline of low melting points and thus promoted the fusion of the ZL80AB20 ash, explaining why FT and HT of the ash initially decreased with increasing AB addition. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction Zhundong lignite deposit, with an abundant reserve of up to 3.9 Gt [1], is expected to be a significant energy source for China ⇑ Corresponding author. E-mail address: [email protected] (M. Zhu). http://dx.doi.org/10.1016/j.fuel.2017.01.064 0016-2361/Ó 2017 Elsevier Ltd. All rights reserved.

in the future. Zhundong lignite is often characterised by low contents of ash and sulphur, and is thus regarded as a high quality thermal coal for power generation [1]. However, the alkali and alkali earth contents in the ash are strikingly high [1–3], consequently resulting in severe ash fouling and slagging problems when Zhundong lignite is fired in utility boilers. To mitigate ash deposition, coal blending is currently practised in utility boilers

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[1,4,5]. However, the effect of coal blending on ash deposition is dependent on coal type and is non-linear with blending ratio [4,6]. In some cases, adding other coals (or additives) into Zhundong lignite promoted ash fusion due to the formation of low melting-point minerals or eutectics, and aggravated ash slagging during combustion [6–11]. Therefore, evaluating the ash deposition characteristics of blended coals is imperative to expand the use of Zhundong lignite. Approaches to evaluating the ash deposition include direct ash deposition experiments [12,13], and indirect tests or measurements such as viscosity, shrinkage, sintering and fusion test on ashes prepared in labs following standards [6,14–19]. The sintering and fusion characteristics of coal ashes have been considered as key parameters to evaluate ash deposition [6,14–18,20,21]. Sintering is the bonding or welding of adjacent particles under the excess of surface energy [22–25], which is a significant mechanism for deposit development, and has been regarded as one criterion to evaluate ash deposition propensity [4,12]. During sintering, complex physical-chemical changes including the reduction in porosity, the changes in particle contacts and deposit strength, the eutectic formation and the interaction between mineral phases would occur [22,23,26–29]. Ash fusibility is another characteristic to assess the propensity of ash to slag and foul during combustion, which can be achieved through the determination of four characteristic temperatures as the ashes are heated. If the fusion temperatures of an ash are low, the ash particles will be easily fused and become adhesive during combustion and thus aggregate ash deposition. The sintering and fusion characteristics of coal ashes are dependent on the chemistry and mineralogy of the ashes [18]. Vassilev et al. [30] found that lower ash fusion temperature (AFT) is related to increased proportions of the fluxing sulfate, silicate and oxide minerals such as anhydrite, acid plagioclases, K feldspars, Ca silicates, and hematite in high temperature coal ashes, while higher AFT is a result of decreased concentrations of fluxing minerals and increased concentrations of refractory minerals such as quartz, metakaolinite, mullite, and rutile in these ashes. For Zhundong lignite ashes, the abundance of alkali and alkali earth contents in the forms of fluxing minerals (hauyne, anhydrite and halite) and the absence of refractory ash minerals decreased the sintering and fusion temperatures [4,30]. Blending other coals into Zhundong lignite could increase the amount of acidic oxides in the forms of refractory minerals and therefore increased ash sintering temperature [18]. Nonetheless, this does not apply to their ash fusion temperatures as the formation of low-melting point eutectics during ash fusion process resulted in an even lower ash fusion temperatures than those of the Zhundong lignite ashes [6]. However, the ash sintering and fusion characteristics of Zhundong lignite blends have not been sufficiently investigated. In particular, the significant changes during the processes of ash sintering and fusion of blended coals have not been comprehensively understood. In addition, it is also noted that the chemistry and mineralogy of the ash are dependent on ashing temperature [31,32]. Typically for Zhundong lignite with high Na content in the ash, a large amount of Na would be released during combustion and further evaporated as ashing temperature increased [33]. This indicates that the ashing temperature effect could not be overlooked while characterising the ash sintering and fusion characteristics. Therefore, the present work as reported in this paper was aimed to investigate the effect of coal blending on the ash sintering and fusion characteristics during the combustion of Zhundong lignite, with the effect of ashing temperature investigated. The ashes prepared in a muffle furnace from a Zhundong lignite and its blends were firstly analysed for their chemistry and mineralogy, and then subjected to the sintering temperature measurement using the

pressure-drop sintering technique [18], and to the ash fusion temperatures measurement. Moreover, the mineral transformation and interactions during sintering and fusion processes were also investigated by analysing the morphology and mineralogy of the ashes being heated at temperatures of 850 °C, 1050 °C, and 1250 °C. It is expected that this contribution would provide a valuable reference for improved understanding of the ash sintering and fusion characteristics of high sodium Zhundong lignite and its blends during combustion, and provide a better comprehension for its ash deposition problem. 2. Experimental 2.1. Materials A sample of Zhundong lignite (ZL) from Xiangjiang province in northwest China was chosen in the current study. In addition, an Australian bituminous (AB) and an Indonesian lignite (IL) with high ash fusion temperatures and low alkali and alkali earth contents that contrary to ZL were selected for preparation of coal blends. The received coal samples were ground and sieved into particles less than 200 lm. The proximate and ultimate analyses of the coal samples, and the chemical composition of their ashes prepared in a muffle furnace in air at 815 °C are shown in Table 1. It is obvious that the ZL ash, with a basic oxides to acidic oxides (B/A ratio) of 4.79, was dominated by basic oxides (CaO, MgO, Na2O, Fe2O3 and K2O), but was absent of acidic oxides (SiO2, Al2O3 and TiO2), suggesting that its fusion temperatures and sintering temperature were relatively low [13,34–38]. In contrast, the AB and IL coal ashes, with B/A ratios of 0.19 and 0.34, respectively, were dominated by acidic oxides including SiO2 and Al2O3, suggesting the abundance of silica, silicates, or aluminosilicate in the ashes. This indicates that the AB and IL coal ashes had relatively high ash fusion and sintering temperatures [36,37]. Coal blends were prepared by mixing the pre-dried ZL and AB (or IL) coal samples in proportions with the weight percentage of AB (or IL) in the blends being 20%, 40%, 50%, 60% and 80%, respectively. For convenience in subsequent discussion, the blends were

Table 1 Proximate, and ultimate analysis of coal, and the chemical composition of ash prepared in muffle furnace in air at 815 °C. Zhundong lignite

Australian bituminous

Proximate analysis (wt% on dry base) Ash 3.4 9.8 Fixed carbon 59.7 55.8 Volatile 36.9 34.4 matter HHV (MJ/kg) 22.9 29.6

Indonesian lignite 8.4 42.7 48.9 22.3

Ultimate analysis (wt% on dry and ash free base) C 70.5 75.6 H 2.6 4.2 O 25.3 16.9 N 0.6 1.7 S 1.0 1.6

64.6 4.5 28.1 1.2 1.6

Ash composition SiO2 Al2O3 CaO Fe2O3 K2O MgO Na2O SO3 P2O5 TiO2 B/A ratio

35.0 32.0 12.7 3.70 0.18 5.77 0.94 4.24 1.16 2.42 0.34

(wt%) 5.42 6.39 40.7 3.06 0.55 7.62 6.08 26.9 0.048 0.30 4.79

64.9 24.1 0.38 5.24 1.46 0.69 0.67 0.22 0.065 1.44 0.19

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denoted in a general form ZLxABy (or ZLxILy), with x and y indicating the weight percentages of ZL and AB (or IL) in the blends, respectively. Ash samples of the three base coals and their blends were also prepared in a muffle furnace according to ASTM standard for evaluating ash fusibility. The coal samples were spread onto aluminium mullite crucibles and heated in a muffle furnace in air at a heating rate of 20 °C min 1 from room temperature to 550 °C for 10 h and to 815 °C for 2 h, respectively, to burnout the inorganic matter in coal. The ashing temperature of 815 °C was chosen according to the ASTM standard for measuring ash fusibility and the ashing temperature of 550 °C was used to minimise the evaporation of alkali and alkali earth metals during ashing [7]. The ash samples prepared in this way were not intended to represent real boiler ashes but to serve as a standard coal and ash testing method to predict ash deposition behaviour in boilers [17,24]. Note that a real boiler ash would be highly variable in nature.

2.2. Sintering temperature measurement A pressure-drop sintering device, as described elsewhere [36], was used to measure the sintering temperature of the ash prepared at 550 °C and 815 °C. Briefly, 0.4 g of ash was compacted into a pellet, which was then heated in a horizontal furnace from ambient temperature at a heating rate of 6.7 °C min 1 in air at a flow rate of 10 ml min 1. The gauge pressure in front of the pellet as a function of temperature was continually recorded. The sintering temperature was determined according to a new criterion based on the first-order and second-order derivatives of the pressure drop curves as a function of temperature in accordance with the literature [18]. The accuracy of this method is within ±12 °C, more accurate compared with other sintering temperature measurements [23].

2.3. Ash fusion temperature measurement The fusion temperatures of the ash samples prepared at 550 °C and 815 °C under air atmosphere were determined using an ash fusion auto-analyser according to Chinese Standard GB219-74 [39]. The ash fusion temperatures were determined by observing the shapes of the ash cones during heating. Four characteristic temperatures including deformation temperature (DT), softening temperature (ST), hemisphere temperature (HT) and flow temperature (FT) were determined [17,39]. Each measurement was repeated three times to ensure a good repeatability of the results.

2.4. Heat treatment of ash samples To study the mineralogical and morphological changes during ash sintering and fusion, the ash sample prepared in the muffle furnace at 550 °C was further subjected to heat treatment. In brief, a sample of about 2 g was compacted into one ash pellet and heated at 850 °C, 1050 °C, and 1250 °C, respectively, in the muffle furnace in air for 2 h. The sintered or fused ash samples prepared in this way were obtained for further XRD and SEM-EDS analyses.

2.5. XRD and SEM-EDS analysis The ash samples prepared under the aforementioned conditions were also subjected to mineralogical characterisation using a Panlytical Empyrean X-ray diffraction (XRD) with Cu Ka radiation at an accelerating voltage of 40 kV and a current of 40 mA. Ash samples were 2h scanned from 5° to 75° with a scanning rate of 5° min 1. The mineral phases of the deposits were identified using X’pert HighScore Plus software based on the ICDD Powder Diffraction File (PDF) 4 database. In addition, the intensity of the characteristic peak for each mineral was recorded and used to indicate the content of this mineral phase in the ash [14,15,39]. The XRD measurements on the ash samples were repeated three times, and it was found that the relative error of the XRD peaks for each mineral phase was within 15%, indicating a good repeatability of the XRD measurement. A high intensity of the characteristic peak represented a high content of this mineral phase in the ash, and vice versa.

Table 2 Ash fusion temperatures of blended coals under oxidising atmosphere. Samples

DT (°C)

ST (°C)

HT (°C)

FT (°C)

AB100 IL100 ZL100

1350 1253 1070

>1500 >1500 1273

>1500 >1500 1358

>1500 >1500 1447

ZL80AB20 ZL60AB40 ZL50AB50 ZL40AB60 ZL20AB80

1208 1216 1250 1284 1299

1245 1219 1299 1344 1406

1253 1229 1360 1386 1441

1283 1253 1388 1441 1478

ZL80IL20 ZL60IL40 ZL50IL50 ZL40IL60 ZL20IL80

1263 1269 1290 1327 1364

1337 1343 1359 1378 >1500

1349 1361 1391 1438 >1500

1383 1386 1406 1490 >1500

Fig. 1. Sintering temperatures of the ashes as a function of (a) AB ratio in the ZL/AB blends, and (b) IL ratio in the ZL/IL blends.

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Table 3 The calculated chemical composition of the blended coal ash samples. Coal blends

ZL80AB20 ZL60AB40 ZL50AB50 ZL40AB60 ZL20AB80 ZL80IL20 ZL60IL40 ZL50IL50 ZL40IL60 ZL20IL80

Ash chemical composition (wt%) SiO2

Al2O3

CaO

Fe2O3

K2O

MgO

Na2O

SO3

P2O5

TiO2

B/A ratio

33.03 46.93 51.58 55.30 60.90 17.28 24.38 26.96 29.11 32.47

14.61 18.75 20.13 21.24 22.91 16.66 22.81 25.04 26.90 29.81

21.99 12.56 9.41 6.89 3.09 29.47 22.75 20.31 18.28 15.09

4.07 4.58 4.75 4.89 5.09 3.32 3.47 3.53 3.57 3.65

0.97 1.19 1.26 1.31 1.40 0.40 0.31 0.28 0.25 0.21

4.40 2.78 2.24 1.81 1.16 6.88 6.43 6.27 6.14 5.93

3.57 2.30 1.88 1.54 1.03 4.02 2.79 2.34 1.96 1.38

14.52 8.28 6.20 4.53 2.02 17.81 12.37 10.40 8.76 6.17

0.06 0.06 0.06 0.06 0.06 0.49 0.76 0.86 0.94 1.07

0.83 1.10 1.18 1.26 1.36 1.15 1.66 1.84 2.00 2.24

0.72 0.35 0.27 0.21 0.14 1.26 0.73 0.61 0.52 0.41

Fig. 2. XRD patterns of the AB, ZL, and IL ash samples. 1 – Quartz, 2 – Hematite, 3 – Titanium Oxide, 4 – Sodium Iron Oxide (NaFeO2), 5 – Anhydrite, 6 – Periclase, 7 – Yeelimite, 8 – Lime, 9 – Calcium silicate, and 10 – Silicon oxide.

Fig. 4. XRD patterns of the ash samples of the ZL and IL blends, 1 – Anhydrite, 2 – Quartz, 3 – Nepheline 4 – Sodium aluminium silicate, 5 – Potassium aluminium silicate, and 6 – Hematite.

Fig. 3. XRD patterns of the ash samples of ZL and AB blends. 1 – Quartz, 2 – anhydrite, 3 – Nepheline, 4 – Hematite, 5 – Hauyne, 6 – Sodium Iron Oxide (NaFeO2), 7 – Titanium Oxide.

Fig. 5. Sintering temperatures of the ashes prepared at 550 °C and 815 °C respectively in muffle furnace in air.

3. Results and discussion A Tescan Vega 3 scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS) was used to observe the ash morphology and to provide semi-quantitative spot elemental composition analysis of the sintered or fused ash samples. All samples were embedded into epoxy resin, polished, and coated prior to analysis.

3.1. Effect of coal blending 3.1.1. Ash sintering temperature Fig. 1 illustrates the sintering temperatures of the ashes of various blends prepared at 815 °C as a function of the AB or IL ratio. The results showed that the sintering temperature of the ZL ash

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was the lowest (748 °C), indicating that the ZL ash had the highest sintering and fouling tendency among these ashes. As the AB or IL ratio in the blends increased, the ash sintering temperature increased. This indicates that the addition of AB or IL into the ZL ash decreased the ash sintering tendency.

3.1.2. Ash fusion temperatures Table 2 shows the measured fusion temperatures (DT, ST, HT and FT) of the ashes of various blends that prepared at 815 °C. It is obvious that the ZL ash had relatively low ash fusion temperatures with DT of 1070 °C and ST of 1273 °C and, thus, relatively high fusion propensity [14,15]. However, the AB ash, with DT of 1350 °C and ST being higher than 1500 °C, had relatively high fusion propensity. Likewise, the IL ash had DT of 1253 °C and ST of being higher than 1500 °C. These indicate that both AB and IL coal ashes were refractory and had low fusion propensity [14,15]. For the ZL/AB blends, it is clear that DT of the ashes increased as the AB addition ratio increased. However, as the AB ratio increased, ST, HT, and FT firstly decreased to the lowest, being 1219 °C, 1229 °C, and 1253 °C, respectively when the AB ratio was 40%, and then increased afterwards. This means that the addition of AB with high ash fusion temperatures was not always effective in elevating fusion temperatures of the ZL ash. It has been reported that the addition of SiO2 at low levels promoted the melting of ZL ash [8]. As seen in Table 1, AB is abundant in acidic oxides, particularly SiO2, which may explains why AB addition at low ratio reduced ST, HT and FT. Likewise, it is observed that both DT and ST of the ashes increased as the ratio of IL in the ZL/IL blends increased. The HT and FT, however, decreased first when the IL ratio increased up to 20%, and then increased afterwards. The difference in ash chemistry and mineralogy was responsible for the variations in ash fusion temperatures, which will be discussed in the following sections. 3.1.3. Ash chemistry of the blends The difference in chemistry of the ash is one aspect responsible for the variation in the ash sintering and fusion temperatures of the blends [18]. Table 3 shows the calculated chemical composition of the blended coal ash samples based on the blending ratio and the ash contents of their parent coals [14,15]. It is clear that the

Table 4 Chemical compositions of ashes prepared at 550 °C, and 815 °C in muffle furnace in air. Ash chemical composition (ppm)

Na Mg Al Si P S K Ca Ti Fe Ash yield (%)

550 °C

815 °C

84700 39300 43500 33500 210 99200 1190 252,000 1710 25900 3.7

38800 45100 40500 28400 82.4 126,000 6770 304,000 1330 29800 3.2

Table 5 Mineralogy of Zhundong lignite ashes at different ashing temperatures in muffle furnace in air. Ashing temperature (°C)

Mineral phases in the ZL ashes

550

Anhydrite, Calcite, Halite, Sodium calcium silicate, Chlorapatite, Spinel (Mg2SiO4), Potassium Chloride Oxide (KClO3), Periclase (MgO) Anhydrite, Lime, Sodium calcium silicate, Calcite, Periclase, Calcium Hydroxide Chloride Silicate Sulfate, Magnesite Anhydrite, Periclase, Yeelimite (Ca4Al6O12SO4), Lime, Cristobalite, Calcium silicate (Ca2SiO4), Magnesite

700

815 Fig. 6. Fusion temperatures of the (a) ZL100, (b) ZL80AB20 and (c) ZL20AB80 ashes prepared in muffle furnace at 550 °C and 815 °C.

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Fig. 7. XRD patterns of the ash samples of the ZL ashes at different temperatures: 1 – Anhydrite, 2 – Halite (NaCl), 3 – Calcite, 4 – Spinel (Mg2SiO4), 5 – Chlorapatite (Ca9.97(PO4)6Cl1.94), 6 – Sodium calcium silicate (Na2CaSiO4), 7 – Potassium chloride oxide (KClO3), 8 – Periclase (MgO), 9 – Wadalite (Ca6(Al,Fe,Mg)5Si2O16Cl3), 10 – Sylvine (KCl), 11 – Sodium oxide (NaO2), 12 – Calcium silicate (CaSiO3), 13 – Quartz, 14 – Potassium Magnesium Silicate (K2MgSiO4), 15 – Calcium Silicate (Ca2SiO4), 16 – Calcium Aluminium Iron Oxide Sulfate Ca4((Al0.95Fe0.05))6O12(SO4)), 17 – Brownmillerite (Ca2(Al,Fe)2O5), and 18 – Calcium iron oxide (CaFeO4).

Fig. 8. XRD patterns of the ash samples of the ZL80AB20 ashes at different temperatures: 1 – Quartz, 2 – Anhydrite, 3 – Halite, 4 – Sodium iron oxide (NaFeO2), 5 – Aluminium phosphate hydrate (Al2P6O18
9.5H2O), 6 – Hauyne ((Na, K, Ca)8(Si, Al)12O24 (SO4)2), 7 – Nepheline (KNa3Al4Si4O16), 8 – Calcium silicate (CaSiO3), 9 – Calcium Magnesium Silicate (Ca2Mg(Si2O7), 10 – Akermanite (Ca2(Mg0.75Al0.25) (Si1.75Al0.25O7)), 11 – Hematite (Fe2O3), 12 – Augite ((Mg,Fe,Al,Ti) (Ca,Na,Mg,Fe) (Si,Al2O6), and 13 – Gehlenite (Ca2Al (AlSi)O7).

chemistry of the ash vary with coal type and the blending ratio. For the ZL/AB blends, the B/A ratio decreased from 0.72 to 0.14 as the AB ratio in the blends increased from 20% to 80%. Of particular concern is that the Na content (reported as Na2O) decreased as the AB ratio in the blends increased. Similar trends were also applied to the ZL/IL blends that the B/A ratio and the Na content decreased with increasing the IL ratio in the blends. This could partially explain why the ash sintering temperatures of the blends increased as the AB or IL ratio in the blends increased.

3.1.4. Ash mineralogy of the blends The difference in the ash mineralogy is another aspect responsible for the variation in the ash sintering and fusion temperatures of the blends [18]. Fig. 2 shows the XRD patterns of the AB, ZL, and IL ash samples prepared at 815 °C. The ZL ash was rich in anhydrite, periclase, yeelimite (Ca4Al6O12SO4), lime, cristobalite, calcium silicate (Ca2SiO4), and magnesite. These mineral phases have lowmelting points and are able to form low melting-point eutectics at a lower temperature, thus increasing ash sintering and fusion

J. Li et al. / Fuel 195 (2017) 131–142

tendency [5,28]. In contrast, quartz was found to be the dominant mineral phase in the AB ash, while hematite, titania (TiO2), and sodium iron oxide (NaFeO2) were among the minor mineral phases. Quartz is one of the main refractory minerals inhibiting the formation of low-melting point eutectics and therefore increasing ash fusion temperatures and sintering temperature [8]. For the IL ash, only quartz and anhydrite were identified as minor mineral phases, implying that most of the inorganic elements existed as amorphous or glass phases in the IL ash. ZL/AB blends: The XRD patterns of the ash samples of the ZL/AB blends are shown in Fig. 3. It is obvious that coal blending had a significant effect on ash mineralogy. For individual ZL ash, anhydrite, periclase, yeelimite (Ca4Al6O12SO4), lime, cristobalite, calcium silicate (Ca2SiO4), and magnesite were identified in the ash as shown in Fig. 3. As the AB ratio in the blends increased to 20%, nepheline and hauyne were identified in the ZL80AB20 ash sample while Ca-bearing mineral phases including yeelimite, lime, and calcium silicate decreased. This is because the total Ca content in the coal blend decreased, leading to less presence of yeelimite, lime and calcium silicate in the ash. For Na, however, the presence of nepheline and hauyne indicates that Na had been captured by silica and silicate from the AB coal. At the AB addition ratio of 40%, nepheline was still identified in the ash of the blend. The presence of nepheline explains why the ash fusion temperatures (mainly ST, and FT) decreased when the AB ratio was less than 40%. As the AB ratio further increased, the remaining mineral phases in the ash were quartz, anhydrite, hematite, sodium iron oxide, and titanium oxide. This means that the formation of nepheline and hauyne were inhibited as the AB ratio in the blends was greater than 40%. Meanwhile, the peak intensity of quartz increased as the AB ratio increased, indicating that the quartz content in the ash increased. Likewise, the content of anhydrite decreased as the AB ratio increased as indicated by its peak intensity. These indicate that the ash sample became more refractory as the AB ratio increased, explaining why ash fusion temperatures increased as the AB ratio further increased. ZL/IL blends: The XRD patterns of the ash samples of the ZL and IL blends are shown in Fig. 4. It is obvious that the mineral phases in the ash varied with blending ratio. For ZL80IL20 coal ash, anhydrite, quartz, nepheline, sodium aluminium silicate, and potassium

137

aluminium silicate were identified. Among these mineral phases, nepheline, sodium aluminium silicate and potassium aluminium silicate were not identified in both the individual ZL and IL coal ash samples, indicating that mineral interactions had occurred in the ZL80IL20 ash sample. The presence of nepheline, sodium aluminium silicate explains the decreases in FT and HT of the ZL80IL20 ashes. As the IL ratio in the coal blends further increased, the formation of nepheline, sodium aluminium silicate and potassium aluminium silicate was inhibited due to increasing amount of silicates and decreasing amount of alkali in the ash (Table 3). As a result, the mineral phases in the ZL60AB40, ZL50AB50, ZL40AB60, and ZL20AB80 ash samples were identified as anhydrite, quartz, and hematite. This explains why the ash fusion temperatures increased as the IL ratio further increased.

Fig. 10. Visual observation on (a) the pellets of the ashes (prepared at 550 °C) prior to heating, and the ash pellets after heated at (b) 850 °C, (c) 1050 °C and (d) 1250 °C, respectively for 2 h in muffle furnace in air.

Fig. 9. XRD patterns of the ash samples of the ZL20AB80 ashes at different temperatures: 1 – Quartz, 2 – Anhydrite, 3 – Sodium iron oxide (NaFeO2), 4 – Sodalite (Na4Al3Si3O12Cl), 5 – Hematite, 6 – Iron hydroxide (Fe(OH)3), 7 – Mullite Al(Al.69Si1.22O4.85), and 8 – Anorthite (Al2CaO8Si2).

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(a) 4 1 3 2

(b)

4 3

1

2

(c) 4 1 3 2

Fig. 11. Representative SEM images of the ZL100 ash pellets being heated at (a) 850 °C, (b) 1050 °C, and (c) 1250 °C, and the EDS results of elemental analysis at selected spots.

3.2. Effect of ashing temperature Figs. 5 and 6 illustrate the effect of ashing temperature on the sintering and fusion temperatures of the ZL100, ZL80AB20, and ZL20AB80 ash samples prepared at 550 °C and 815 °C. It is observed that for each coal blend, the sintering and fusion temperatures of the ash prepared at 815 °C were correspondingly higher than those of the ash prepared at 550 °C. In addition, regardless of the ashing temperature, the sintering temperatures of the ashes of the ZL and AB blends increased as the AB addition ratio increased, while ST and FT decreased first at 20% AB addition ratio and then increased as the AB addition ratio reached to 80%. The difference in the ash chemistry and mineralogy was responsible for the different ash sintering and fusion temperatures observed. In particular, for Zhundong lignite, the effect of ashing temperature on the ash chemical composition and mineralogy was studied and the results are shown in Tables 4 and 5. The content of Na was observed to decrease as ashing temperature increased from 550 °C to 815 °C. In addition, the analysis of the

ash mineralogy identified halite at 550 °C but not at 700 °C or above. A similar trend was also found for the other Cl-bearing minerals. These indicated that Na and Cl were evaporated into the gas phase at temperatures above 815 °C. Moreover, yeelimite and calcium silicate were observed at 815 °C. As a result, the increase in ashing temperature promoted the evaporation of Na and Cl and the transformation of fluxing minerals. These led to the elevated ash sintering and fusion temperatures as ashing temperature increased.

3.3. Discussion The experimental work as described above clearly illustrated how coal blending and ashing temperature affected the chemistry and mineralogy of the ashes and their sintering and fusion temperatures. However, it is also noted that the significant changes in the chemistry and mineralogy during ash sintering and fusion process have not been sufficiently comprehended. To achieve this, XRD and

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(a) 4 3

1 2

(b) 1 3 4

2

(c) 4 3

1

2

Fig. 12. Representative SEM images of the ZL80AB20 ash pellets being heated at (a) 850 °C, (b) 1050 °C, and (c) 1250 °C, and the EDS results of elemental analysis at selected spots.

SEM-EDS analyses of the ZL100, ZL80AB20, and ZL20AB80 ash pellets being heated at 850 °C, 1050 °C, and 1250 °C were performed. 3.3.1. Mineralogical changes during sintering and fusion process Fig. 7 shows the XRD patterns of the ZL ashes at 550 °C, 850 °C, 1050 °C, and 1250 °C. It is clear that the ZL ash at 550 °C mainly consisted of fluxing minerals including anhydrite, halite, calcite, spinel (Mg2SiO4) and periclase. As the temperature increased to 850 °C at which sintering of the ash had occurred, wadalite (Ca6(Al,Fe,Mg)5Si2O16Cl3), sylvine, sodium oxide, and calcium silicate, which were not identified at 550 °C, were present in the ash. Moreover, the peak intensities of halite and spinel decreased while calcite disappeared as temperature increased to 850 °C. These indicate that the changes in Na, Cl, or Ca-bearing minerals were responsible for the sintering of the ZL100 ash. As temperature increased to 1050 °C, potassium magnesium silicate (K2MgSiO4) and calcium silicate (Ca2SiO4) were identified in the ash whereas Cl-bearing minerals including chlorapatite and potassium chloride oxide (KClO3) decreased. This means that the high temperature

(1050 °C) promoted the mineral transformations from Cl-bearing minerals to K- or Ca-bearing silicates. In addition, as temperature further increased to 1250 °C, calcium aluminium iron oxide sulfate Ca4((Al0.95Fe0.05)6O12(SO4)), calcium iron oxide and brownmillerite (Ca2(Al,Fe)2O5) were identified whereas Na-bearing minerals were not identified. The absence of Na-bearing minerals indicates that Na might not be retained in the ZL100 ash at 1250 °C. It follows that the eutectics between Ca, Fe, Al and S as identified were responsible for the fusion of the ZL100 ash. The mineral phases in the ZL80AB20 ashes at the heat treatment temperatures of 550 °C, 850 °C, 1050 °C, and 1250 °C are illustrated in Fig. 8. It is clear that the mineral phases in the ZL80AB20 ash at 550 °C were quartz, anhydrite, halite, sodium iron oxide, and aluminium phosphate hydrate (Al2P6O18
9.5H2O). As the heat treatment temperature increased to 850 °C, hauyne, nepheline (KNa3Al4Si4O16), calcium silicate (CaSiO3), and calcium magnesium silicate were identified whereas halite disappeared. This suggests that the formations of Na-bearing minerals (Hauyne and nepheline) and Ca-bearing silicates were responsible for the

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sintering of the ZL80AB20 ash. As the heat treatment temperature further increased to 1050 °C, the mineral interactions proceeded, leading to the formation of akermanite, hematite and augite. Meanwhile, hauyne and nepheline were still present in the ZL80AB20. At 1250 °C, the mineral phases in the ZL80AB20 ash were identified as gehlenite, hauyne, and nepheline. The presence of these fluxing minerals explained why the fusion temperatures of the ZL80AB20 ash were lower than those of the ZL100 ash. The 20% AB addition in ZL would introduce more acidic oxides (in the form of quartz) in the ash. These acidic oxides would bind with Na in the ash at 1250 °C, leading to the formation of hauyne, nepheline and gehlenite with even lower fusion temperatures and therefore promoting ash melting. This is also consistent with the literature that the addition of SiO2 at low levels promoted the melting of ZL ash [8]. The mineral phases in the ZL20AB80 ashes at the heat treatment temperatures of 550 °C, 850 °C, 1050 °C, and 1250 °C are illustrated in Fig. 9. It is obvious that the ZL20AB80 ash at 550 °C, being dominant with quartz, anhydrite, sodium iron oxide, soda-

lite, was more refractory than the ZL100 and ZL80AB20 ashes. As the heat treatment temperature increased to 850 °C, only minor changes related to iron occurred in the ash. Moreover, mullite and anorthite were formed at 1050 °C and above but quartz still was dominant in the ash. Meanwhile, anhydrite was not identified in the ZL20AB80 at 1050 °C and above, indicating that anhydrite had transformed into other Ca-bearing minerals. These evidences suggest that more refractory minerals were present in the ashes. The abundance of refractory minerals and the absence of fluxing minerals explained why the sintering and fusion temperatures further increased as the AB ratio further increased to 50% and above. 3.3.2. Morphological changes during sintering and fusion process Typical visual observations on these ash pellets are shown in Fig. 10. It is found that the ash pellets not only changed their shape and colour during heating, but also developed their strength as temperature increased from 850 °C to 1250 °C. The ash pellets were grey in colour prior to heating, but gradually became dark grey or brown as temperature increased from 850 °C to 1250 °C.

(a) 4 1

2 3

(b) 4 1

3 2

(c)

4 3

1

2

Fig. 13. Representative SEM images of the ZL20AB80 ash pellets being heated at (a) 850 °C, (b) 1050 °C, and (c) 1250 °C, and the EDS results of elemental analysis at selected spots.

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This indicates that significant chemical and mineralogical changes had occurred in the pellets. Moreover, for each blend, the strength of the pellet gradually developed as indicated by the fact that they became more difficult to crush. However, unlike ZL100 and ZL80AB20 pellets, the ZL20AB80 pellet at 850 °C was not sintered as the heat treatment temperature of 850 °C was lower than its sintering temperature (855 °C). In addition, the ZL80AB20 pellet at 1250 °C was fused on the mullite holder, indicating that a significant amount of liquid phases was formed in the ash pellet. These observations were consistent with the findings that the addition of AB into ZL would increase the ash sintering temperature of the blends but would decrease ash fusion temperature of the blend when the AB addition ratio was 20%. The morphological changes of the ash pellets being heated at different temperatures were also analysed and are shown in Figs. 11–13. It is obvious that the heat treatment temperature had significantly affected the morphology of the pellets. For ZL100 ash pellets as shown in Fig. 11, the ash particles at 850 °C were discrete with Ca, Na, S and Cl being dominant (Fig. 11a). Sintering is believed to have occurred within the pellets as the heat treatment temperature 850 °C was higher than its sintering temperature (612 °C). As temperature increased to 1050 °C and 1250 °C, the ash particles in the ZL100 ash pellet were fused into large agglomerate, indicating that significant amount of liquid phase have been formed at 1050 °C and above. EDS analysis revealed that the amount of Cl decreased as temperature increased from 850 °C to 1050 °C and disappeared at 1250 °C, indicating that Cl had vaporised during heating. This is also consistent with the XRD analysis as discussed above. Fig. 12 shows representative SEM images of the ZL80AB20 ash pellet being heated at temperatures of 850 °C, 1050 °C and 1250 °C. It is obvious that the ash particles were discrete at 850 °C and 1050 °C. The EDS analysis revealed that sodium aluminium silicates were formed at 850 °C, consistent with identification of hauyne and nepheline from XRD analysis. As temperature increased to 1250 °C, the ZL80AB20 ash pellet was fused. The EDS results showed that the ZL80AB20 ash pellets at 1250 °C were all rich in Si, Al, Na, Ca, and Mg at selected spot, indicating that the ash particles had completely melted due to the eutectic formation. This confirms that the fluxing minerals including hauyne, nepheline and gehlenite were responsible for the low softening temperature of the ZL80AB20 ash. The SEM-EDS results showing the morphology and chemistry of the ZL20AB80 ash pellet heated at different temperatures are presented in Fig. 13. It is obvious that the ash pellets at 850 °C and 1050 °C were discrete ash particles rich in Si and Al. As temperature increased to 1250 °C, the ash pellet became fused. The EDS analysis revealed that the ash pellet was rich in Si, and/or Al, confirming the presence of quartz and mullite in the ash at 1250 °C. Moreover, some areas (e.g. area 3 in Fig. 13c) of the pellet were identified to be rich in Fe, Si, Al and Na, indicating that Fe has incorporated into the silicates. The presence of the refractory mineral explains why the fusion temperatures of the ZL20AB80 ash were higher than the ZL100 and ZL80AB20 ash.

4. Conclusions The present work systematically analysed the chemistry, mineralogy and morphology of a series of ash samples of the Zhundong lignite and its blends with an Australian bituminous coal (AB) and an Indonesian lignite (IL). The result showed that the sintering temperature of the ash increased as the AB or IL blending ratio in the coal blends increased, suggesting that the addition of AB or IL decreased the sintering tendency of the ash. However, the ash fusion temperatures of the blends did not always vary in propor-

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tion to the changes in the blending ratio of AB or IL. The ash fusion temperatures of the ZL/AB blends decreased first when the AB ratio was less than 40% and increased afterwards as the AB ratio further increased. For the ZL/IL blends, FT decreased first, reaching the lowest values at the 20% IL ratio, and then increased. In addition, due to the higher contents of alkali and alkali earth metals in the forms of fluxing minerals, the ashes prepared at 550 °C showed lower sintering and fusion temperatures compared with those of the ashes prepared at 815 °C. The mechanisms of ash sintering and fusion were elucidated through the investigation into the mineralogical and morphological changes of the ash at different heat treatment temperatures. It was revealed that sintering of the ZL ash was attributed to Na- and Cl-bearing minerals whereas fusion of the ZL ash was attributed to the eutectics formed between Ca, Al, Fe, and S. The addition of 20%AB into ZL inhibited ash sintering because of the introduction of refractory minerals at 850 °C, but promoted ash fusion due to the formation of hauyen, gehlenite and nepheline at 1250 °C. In addition, the addition of 80%AB into ZL inhibited both ash sintering and fusion owing to the abundant refractory minerals including mullite and quartz in the ash. This work provided an improved understanding of the effect of coal blending and ashing temperature on ash sintering and fusion characteristics Acknowledgement This research has received partial financial support from the Australia Research Council under the ARC Linkage Projects Scheme (Project Number: LP100200135). Jianbo Li would also like to acknowledge the scholarships received from the China Scholarship Council (CSC) and Centre for Energy at The University of Western Australia. References [1] Xu J, Yu D, Fan B, Zeng X, Lv W, Chen J. Characterization of ash particles from co-combustion with a Zhundong coal for understanding ash deposition behavior. Energy Fuels 2013;28:678–84. [2] Zhou J, Zhuang X, Alastuey A, Querol X, Li J. Geochemistry and mineralogy of coal in the recently explored Zhundong large coal field in the Junggar basin, Xinjiang province, China. Int J Coal Geol 2010;82:51–67. [3] Li G, Li S, Huang Q, Yao Q. Fine particulate formation and ash deposition during pulverized coal combustion of high-sodium lignite in a down-fired furnace. Fuel 2015;143:430–7. [4] Li J, Zhu M, Zhang Z, Zhang K, Shen G, Zhang D. The mineralogy, morphology and sintering characteristics of ash deposits on a probe at different temperatures during combustion of blends of Zhundong lignite and a bituminous coal in a drop tube furnace. Fuel Process Technol 2016;149:176–86. [5] Wang X, Xu Z, Wei B, Zhang L, Tan H, Yang T, et al. The ash deposition mechanism in boilers burning Zhundong coal with high contents of sodium and calcium: a study from ash evaporating to condensing. Appl Therm Eng 2015;80:150–9. [6] Shen M, Qiu K, Zhang L, Huang Z, Wang Z, Liu J. Influence of coal blending on ash fusibility in reducing atmosphere. Energies 2015;8:4735–54. [7] Zhang X, Zhang H, Na Y. Transformation of sodium during the ashing of Zhundong coal. Procedia Eng 2015;102:305–14. [8] Zhou H, Wang J, Zhou B. Effect of five different additives on the sintering behavior of coal ash rich in sodium under an oxy-fuel combustion atmosphere. Energy Fuels 2015;29:5519–33. [9] Li G, Wang Ca, Yan Y, Jin X, Liu Y, Che D. Release and transformation of sodium during combustion of Zhundong coals. J Energy Inst 2016;89:48–56. [10] Wei B, Wang X, Tan H, Zhang L, Wang Y, Wang Z. Effect of silicon–aluminum additives on ash fusion and ash mineral conversion of Xinjiang high-sodium coal. Fuel 2016;181:1224–9. [11] Yao Y, Jin J, Liu D, Wang Y, Kou X, Lin Y. Evaluation of vermiculite in reducing ash deposition during the combustion of high-calcium and high-sodium Zhundong coal in a drop-tube furnace. Energy Fuels 2016;30:3488–94. [12] Li J, Zhu M, Zhang Z, Zhang K, Shen G, Zhang D. Characterisation of ash deposits on a probe at different temperatures during combustion of a Zhundong lignite in a drop tube furnace. Fuel Process Technol 2016;144:155–63. [13] Baxter LL. Ash deposit formation and deposit properties. A Comprehensive Summary of Research Conducted at Sandia’s Combustion Research Facility. In: Sandia National Labs., Livermore, CA (US); 2000.

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