Ash deposition and sodium migration behaviors during combustion of Zhundong coals in a drop tube furnace

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Journal of the Energy Institute xxx (2016) 1e11

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Ash deposition and sodium migration behaviors during combustion of Zhundong coals in a drop tube furnace Q1

Chang'an Wang*, Guangyu Li, Yongbo Du, Yu Yan, Hao Li, Defu Che State Key Laboratory of Multiphase Flow in Power Engineering, School of Energy and Power Engineering, Xi'an Jiaotong University, Xi'an 710049, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 August 2016 Received in revised form 25 November 2016 Accepted 29 November 2016 Available online xxx

Zhundong coalfield is one super-large coalfield recently discovered in China. However, the utilization of Zhundong coal in power plants has caused serious ash-related issues mainly due to its high-sodium feature. The ash deposition problem on convection heat exchanger surfaces is still particularly difficult to resolve and its mechanism has yet to be fully understood. This study deals with the ash deposition and alkali metal migration behaviors on convection heat exchanger surfaces between 400 and 800
C during combustion of Zhundong coal using a lab-scale drop tube reactor. Experimental results show that the sodium content in ash deposit of Zhundong coals increases obviously as the deposition temperature decreases from 800 to 600
C, while it is almost unchanged below 600
C. The contents of iron and calcium in ash deposits exhibit nonmonotonic variations as the deposit probe temperature varies between 400 and 800
C. Quartz and calcium sulfate are main crystalline phases in ash deposit of Zhundong coals. Calcium is inclined to present as calcite and lime at low deposition temperature, while high temperature facilitates calcium sulfation. Sodium of crystalline phase is found as albite and sodium sulfate at low deposition temperature. Both condensation of gaseous alkali metals and formation of lowmelting minerals were responsible for the ash deposition phenomenon on convection heat exchanger surfaces involved in combustion of Zhundong coal. The sodium content in ash deposit decreases considerably with the increasing combustion temperature while the case of iron variation is opposite due to its low-volatility. In addition, the Na content in ash deposits increases obviously with the access air ratio reduced from 1.2 to 1.05, but the local weakly reducing atmosphere leads to less iron within ash deposits. Clarification of sodium migration and evaluation of ash deposition behaviors during combustion of Zhundong coal is helpful for a better exploration of the functional mechanism of ash deposit and then large-scale utilization of high-sodium coals. © 2016 Energy Institute. Published by Elsevier Ltd. All rights reserved.

Keywords: Zhundong coalfield High-sodium coal Ash deposit Transition behavior Deposition temperature

1. Introduction Coal has an important share of energy resources worldwide, particularly in China [1e4]. Coal has occupied a predominant place in primary energy of China and will continue to play a dominant role in next decades [5,6]. Zhundong coalfield, located in Junggar Basin of Xinjiang Autonomous Region, was newly discovered with a super-large reserve amount of ~1.64  1011 t [7,8]. Some scholars believed that the coals in Xinjiang region contributed to approximate 40% of the whole Chinese coal reserves nowadays [9,10]. Since Zhundong coal has features of super-large reserve, low ash content [11,12], low-cost exploitation and high quality [13,14], Zhundong coal attracts a rising concern of utilization and will hold an ever-increasing position in power generation and chemical industry of China in future. Unfortunately, the utilization of Zhundong coal in power plants has caused some serious ash-related issues, such as fouling, deposition, and slagging [15e19]. These issues could give rise to heat transfer losses, corrosion in the tubes, even unscheduled shut-down and then increasing operation cost [20]. The majority of researchers believed that the high-sodium feature of Zhundong coal ash were mainly responsible for these ash-related issues [6,13,21]. However, the sodium migration behaviors and ash deposition mechanisms have yet to be fully understood. Hence, the efficient utilization of Zhundong coal to a large extent depends on our understanding of transition behaviors of sodium and further specific study on Zhundong coal is of crucial necessity. * Corresponding author. Fax: þ86 29 82668703. E-mail address: [email protected] (C. Wang). http://dx.doi.org/10.1016/j.joei.2016.11.010 1743-9671/© 2016 Energy Institute. Published by Elsevier Ltd. All rights reserved.

Please cite this article in press as: C. Wang, et al., Ash deposition and sodium migration behaviors during combustion of Zhundong coals in a drop tube furnace, Journal of the Energy Institute (2016), http://dx.doi.org/10.1016/j.joei.2016.11.010

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C. Wang et al. / Journal of the Energy Institute xxx (2016) 1e11

Sodium (Na) usually contributes to a primary share of alkali metals in coal, while it exhibits an intense sublimation propensity among the metal minerals within macromolecular structure of coal [22]. The release of Na-containing species into gaseous phase is one of the most important inducements to ash-related problems in coal-fired boilers. Hence, many studies have been conducted on the functional mechanisms of alkali metals on ash-related issues during combustion or co-combustion of coals [23,24]. High fraction of alkali metals in coal/ biomass could lead to the formation of compounds with low-melting points and then caused ash-related problems [25]. Higher content of sodium in ash deposit was found to result in higher degree of adherence to heat-exchange tube surfaces during co-combustion of coal and agricultural residues [26]. Wang et al. [27] found that the deposition rate of fly ash was reduced with the increasing surface temperature of sample probe because of the strong influence from thermophoresis. Bartolome et al. [28] studied ash deposition behaviors of coal-biomass blends and found that aluminosilicates from coals acted as protective ash compounds preventing chlorine deposition. Wu et al. [29] found that ash deposits at high flue gas temperature was characterized by a slow and continuous growth while the deposits at low temperature started with a slow build-up and then kept constant. Shao et al. [30] explored the fly ash deposits of co-combustion in bubbling fluidizedbed combustor and indicated that the great decrease of ash deposition tendency during co-firing accounted for the formation of more minerals with high-melting points and high crystallinity. Influence of oxy-fuel combustion on ash deposition was also investigated with the increasing concern on CO2 emission and climate change [31e33]. Zhan et al. [31] found that deposits gathered on vertical surface under oxy-fuel condition were similar to the inside layer of the horizontal deposits but different from the bulk horizontal deposits reported previously. Fryda et al. [33] believed that oxy-fuel combustion did not change the ash chemistry or mechanisms of ash behaviors, while the differences in physical properties of flue gas led to changes in the flow field and ash particle size [32]. Zhou et al. [34] believed that oxy-fuel combustion had an insignificant influence on the kinds of mineral phases but affected the relative content of crystalline phases within ash. Mineral additions were also generally employed to investigate the ash deposition and mineral functional mechanism. Wigley et al. [35] examined effects of six mineral additions on coal ash deposition and found that calcite and albite led to the most obvious change in deposit characteristics. Furthermore, MgO addition can decrease the molten slag fraction and then reduce the ash deposition [36]. Yao et al. [13] indicated that vermiculite additive could efficiently reduce ash deposition phenomenon during combustion of Zhundong coal. Song et al. [21] found out that less sodium was captured in fly ash and bottom ash during combustion than gasification using a circulating fluidized bed. Li et al. [37] investigated the ash deposition and fine particulate formation using a 25 kW down-fired furnace and revealed that Zhundong lignite exhibited larger deposition tendency than contrast fuels due to its high content of AAEM species. Li et al. [11] accessed the ash deposits during combustion of Zhundong coal at various temperatures and believed that the fine ash rich in Na, Ca, S and Mg due to condensation and thermophoresis was responsible for the ash deposition. In addition, the evaluation indices of slagging behaviors [38,39], mechanistic model of ash deposit growth [40], and gravity shedding of ash deposition [41] were also conducted previously. Despite past research efforts on coal, there are still some unresolved issues with regards to the functional mechanisms of sodium on ash deposition during coal combustion partly due to the complex physical-chemical compositions and structures of various coals. Many studies were conducted on ash deposition and potassium migration behaviors during thermal conversion of biomass. Nevertheless, the physicalchemical properties of coals are significantly different from those of biomass, while Zhundong coals could be remarkably different from biomass or other common coals in the amount, occurrence mode and transition mechanism of sodium. Furthermore, the ash deposition mechanisms during combustion of Zhundong coals are still unclear, while most previous studies were conducted mainly emphasizing on slagging/fouling behaviors within furnace [14,37]. The present study focused on ash deposition behaviors on convection heat exchanger surfaces where alkali metals might easily condense or deposit on tube surface. Previous studies mainly emphasized specific deposition temperature, while the exploration on sodium migration variation with deposition temperature is yet insufficient. Except for high-sodium, Zhundong coals are characterized by high-calcium and high-iron as well. Notwithstanding, both calcium and iron have important contributions to ash deposition behaviors, less work has been carried out to reveal the effects of Ca and Fe on ash deposit and sodium migration during combustion of Zhundong coal. The objective of present study is to clarify the ash deposition features and migration behaviors of sodium at medium flue gas temperature range of 400e800
C during combustion of two Zhundong coals using a lab-scale drop tube furnace. The ash deposition and migration behaviors of main ash-related elements (Na, K, Ca, Fe, S, and Cl) in deposited samples were subjected to several off-line experimental approaches to explore the chemical composition and mineral transformation undergone in ash of Zhundong coal. The temperature dependence of major crystalline phases in ash deposits was also further investigated. The effects of deposition temperature, combustion temperature, and access air ratio on ash deposition and element transition were specifically emphasized here. The fundamentals on behaviors of Na/Ca/Fe between 400 and 800
C during combustion of Zhundong coal were elucidated, which are helpful for a better exploration of the functional mechanism of ash deposition during combustion of high-sodium coals.

2. Experimental 2.1. Sample preparation In the present study, two Zhundong bituminous coals were mainly employed, including Zijin coal (abbreviated as ZJ, hereinafter) and Wucaiwan coal (WCW). These two coals were ground in a mill, air dried at room temperature, and then sieved to the particle size of 75e125 mm. The coal samples were prepared following the ISO Standard (ISO 18283:2006(E)) Table 1 shows the proximate and ultimate analyses of coal samples on air dry basis with notation ‘ad’. The comparison of main mineral contents between Zhundong coals and some typical Chinese coals is illustrated in Fig. 1, where solid symbols are corresponding to Zhundong coals, hollow ones are other contrastive Chinese coals, and the dash lines represent the average values of these contrastive Chinese coals. Fig. 1 shows that obviously high content of sodium is present in ash of Zhundong coal. The Na2O fraction in Zhundong coal ashes is sometimes more than ten times that in other typical Chinese coal ashes [42]. The content sum of silicon dioxide (SiO2) and aluminum oxide (Al2O3) is obviously lower in Zhundong coals than other coals. In addition, Zhundong coal ashes have high contents of iron (Fe) and calcium (Ca) as well, which could lead to comprehensive effects on ash deposition. The ash compositions of Please cite this article in press as: C. Wang, et al., Ash deposition and sodium migration behaviors during combustion of Zhundong coals in a drop tube furnace, Journal of the Energy Institute (2016), http://dx.doi.org/10.1016/j.joei.2016.11.010

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Table 1 Proximate and ultimate analyses of coal samples (wt %, ad). Coal code

ZJ WCW a

Proximate analysis

Ultimate analysis

w(FC)

w(V)

w(A)

w(M)

w(C)

w(H)

w(O)a

w(N)

w(S)

53.76 54.26

28.42 26.39

3.52 6.30

14.30 13.05

64.77 65.77

3.72 2.77

12.34 10.96

0.77 0.49

0.58 0.66

w(O) ¼ 100  w(C)  w(H)  w(N)  w(S)  w(A)  w(M).

100

Content of mineral matter (%)

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80 60

SiO2+Al2O3 Fe2O3+CaO

40

Na2O

20 0 ZJ WCW JZ SM YB TC WS HL QC YCW ZQ HX

Zhundong coals

Coal type

Fig. 1. Comparison of ash composition between Zhundong coals and typical Chinese coals.

Zhundong coals are considerably different from those coals already large-scale utilized in China. Consequently, specific exploration on ash deposition behaviors of Zhundong coals has important implications. 2.2. Experimental setup The present experiments were conducted in a lab-scale drop tube furnace heated by electricity. Fig. 2 shows the schematic diagram of experimental system. The tube reactor had an inner diameter of 40 mm and a total heating length of 1600 mm. The inside alundum reactor was heated by an electrical furnace with a maximum temperature of 1400
C, while the temperature inside the furnace was read by a thermocouple with an accuracy of 1.5
C. Here, four sections of the furnace were separately heated and temperature controlled, so as to maintain a desired constant temperature region. Gas mass flow meter with an accuracy of 2% was used to control gas flow during experiments. A thermostatic water bath was used to import water vapor with heating pipe to avoid vapor condensation. A screw coal feeder together with an ejector was designed for continuous and stable sample feeding. The steel deposit probe with a diameter of 20 mm was placed at the furnace tail. The surface temperature of deposit probe was read by a K-type thermocouple connected with data acquisition equipment. The thermocouple was located on the back of deposit probe surface and the deposition temperature could be adjusted through vertically varying the probe position. A typical experimental run was conducted with the following steps. First, the reactor was heated to the pre-set temperature at a heating rate of 5
C min1. Then, the gas mixture was purged into the reactor and the coal feeder worked. After that, the deposit probe was placed at certain location and kept for a period of time (usually for 2 h). At last, the probe was taken out and then the resultant solid sample was collected and analyzed. Another probe would be used for the next experimental run. Under present experimental conditions, the deposition of airborne ash onto the deposit probe surface could conduct by mechanisms of both diffusion and inertial impaction, similar to the horizontal deposit probe surface designed by Zhan et al. [31]. 2.3. Analysis method The inductively-coupled plasma optical emission spectroscopy (ICP-OES) analysis, X-ray diffraction (XRD) analysis, and X-ray fluorescence spectroscopy (XRF) analysis coupled with physical property analysis were employed to obtain further information about ash deposit and element transition behaviors. Metal elements (Na, K, Ca, and Fe) in ash deposit were quantified by an inductively coupled plasma optical emission spectrometry (ICP-OES) e Optima 7000DV (PerkinElmer) coupled with a microwave-assisted digestion system e Multiwave 3000 (Anton Paar). The ash sample of 0.1 g was first dissolved with guaranteed reagent of 1 mL hydrofluoric acid (HF), 2.5 mL hydrochloric acid (HCl), 5 mL nitric acid (HNO3) and 0.3 mL hydrogen peroxide (H2O2), and then digested at a power of 1200 W using Multiwave 3000. After the digestion process, the contents of metal elements in digestion liquid were analyzed by ICP-OES e Optima 7000DV. The calibration curve from standard and blank solutions was first obtained and then the contents of sample solutions were measured. Chlorine (Cl) and sulfur (S) in ash deposit were further quantified by an X-ray fluorescence spectrometry S4 PIONEER (Brochure). The main mineral phases in ash deposit were analyzed by an X'Pert PRO X-ray diffraction equipment (PANalytical B. V. Inc.) using CuKa radiation in the range of 10
< 2q < 80
. Please cite this article in press as: C. Wang, et al., Ash deposition and sodium migration behaviors during combustion of Zhundong coals in a drop tube furnace, Journal of the Energy Institute (2016), http://dx.doi.org/10.1016/j.joei.2016.11.010

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Fig. 2. The schematic diagram of experimental system in present study.

3. Results and discussion 3.1. Mineral migration with deposition temperature The deposition temperature has a considerable influence on mineral migration, which can further act on ash deposition behaviors during combustion of high-sodium coals. Here, the temperature of deposit probe surface was controlled to 400, 500, 600, 700, and 800
C in experiments to investigate the effect of heat exchanger surface conditions on ash deposition in utility coal-fired boilers. Fig. 3 shows the physical appearances of ash deposit at 700
C after Zhundong coals combusting at 1200
C. Under the same condition, the amount of WCW ash deposit is more than that of ZJ due to their different ash contents. The particle size of WCW ash is obviously smaller than ZJ ash. Sodium and iron are possibly beneficial for the agglomeration of ash particle, while ZJ coal ash has higher contents of Fe and Na. The fouling tendency varies with the temperature of deposit probe. Namkung et al. [43] believed that sintering would occur with the temperature higher than a certain limit. The physical differences of ash deposit are largely related to ash compositions, which have profound influences on alkali metal migration and ash cohesiveness.

Fig. 3. Image of the ash deposit shape of Zhundong coals at 700
C: a) ash deposit of WCW coal; b) ash deposit of ZJ coal.

Please cite this article in press as: C. Wang, et al., Ash deposition and sodium migration behaviors during combustion of Zhundong coals in a drop tube furnace, Journal of the Energy Institute (2016), http://dx.doi.org/10.1016/j.joei.2016.11.010

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4

2.0

3

1.6

2

1.2

WCW ZJ

0.8

1

0

400

500

600

700

800

Content of K2O (%)

Alkali metals were usually regarded as harmful components in coal-fired boilers which are largely related to ash deposition phenomenon on heat exchanger surfaces. Sodium was usually regarded as one principal cause of fouling and slagging during utilization of Zhundong coals [14,21,42,44]. Fig. 4 depicts the content variation of alkali metals with the deposition temperature. The Na2O content in ash deposit increases obviously with the temperature decreasing from 800 to 600
C, while it is almost unchanged as the temperature is below 600
C. Sodium in Zhundong coals is predominated by water-soluble form [42,44], which is easily volatilized into gaseous phase. The present results implies that parts of gaseous Na-containing substances were condensed or captured on surfaces of fly and deposited ash at temperature range of 600e800
C. When the temperature was below 600
C, the condensation process of sodium species was almost completed and few Nacontaining species were further deposited on the sampling probe. A similar conclusion was obtained by Dai et al. [45] that gaseous Na or Na2O released from high alkali coal combustion at 1200e1400
C was inclined to condense at flue gas temperature range of 600e900
C. Lynch et al. [46] studied ash agglomeration and deposition of biomass combustion in a bubbling fluidized-bed combustor and indicated that the deposition downstream in low temperature regions occurred largely through the vaporization-condensation mechanism, which also occupies main contribution at the temperature region here. However, Zhan et al. [31] investigated the ash deposition characteristics during combustion of low-alkali coal under oxy-fuel condition and observed that alkali metal contents of the inside deposits obtained from different flue gas temperatures showed insignificant variations. The possible reason for the different results is that the temperatures selected (400e800
C) in all tests performed under present investigation were lower than those of Zhan et al. [31] (828, 926, and 1019
C). Furthermore, the content and occurrence mode of alkali metals in various coals probably present certain influence on their transition behaviors. As illustrated in Fig. 4, the variation of K2O differs from the case of Na2O. The content of K2O in ash deposit presents almost independence with deposition temperature. The differences are insignificant compared with measurement errors. The occurrence mode of potassium in Zhundong coals is obviously different from that of sodium [42,44]. Potassium is mainly present as insoluble form and very little percentage will be released into gaseous phase during combustion. The ratio of potassium deposited in subsequent cooling flue gas to potassium left in ash during combustion of Zhundong coals is quite small. Therefore, the variation of potassium in ash deposit at varying temperatures is insignificant. In addition, the Na2O content in ash from deposit probe here is lower than that from standard ash analysis method. Zhundong coal shows a decrease of sodium and sulfur in ash deposits relative to fuel ash, while Robinson et al. [47] only observed a decrease in deposit sulfur concentration for two low-sodium coals. Much higher temperature (1200
C) that the coal particles were exposed to during present experiments than the temperature (815
C) employed in coal ash analysis. Under condition of ash analysis, a moderate amount of sulfur (volatile species) and sodium (semi-volatile specie) remains in the coal ash. Whereas, the sulfur-containing inorganic species further decompose to form SO2 at the higher combustion temperature here (1200
C) [47]. In addition, more water-soluble sodium is volatiled into gaseous but not all is deposited on probe surface. ZJ and WCW coals exhibit similar behaviors in terms of reactivity, deposition, and alkali metal migration. The sodium content in ash deposit of ZJ coal is considerably lower than that of WCW coal, which corresponds to their respective contents in raw coals. Whereas, Li et al. [48] investigated biomass ash deposition at three sampling probe temperatures (550, 600, and 650
C) with the flue gas temperature near the sampling position being kept ~800
C. The differences of elemental compositions were insignificant among the chosen three sampling temperatures under their experimental conditions. Differences between coal and biomass should be taken into account. Except for alkali metals, iron (Fe), calcium (Ca), sulfur (S), and chlorine (Cl) are also largely related to ash deposition behaviors and exhibit influence on sodium migration. These elements could be classified into three groups: volatile species (S and Cl), semi-volatile species (Na and K), and nonvolatile species (Fe, and Ca) [47]. Compared with ordinary coals in China, Zhundong coals generally contain high level of iron, calcium, and sulfur. Hence, we further presented the variations of metallic elements Fe and Ca, inorganic elements S and Cl with deposition temperature in Fig. 5. Iron present in form of pyrite is likely to selectively deposit and concentrate, thus it could contribute to fouling and slagging. Difference in fuel property gives rise to different functions of Fe on ash deposit. As shown in Fig. 5a, with the deposition temperature decreasing from 800 to 400
C, the content of Fe in ash deposit initially decreases and then increases with the minimum at 600e700
C. Robinson et al. [47] observed that ash deposits from coal combustion indicated an increase in iron concentration relative to the fuel ash, which is consistent with present results. Calcium also presents slightly nonmonotonic variation but first increase and then decrease tendency with the deposition temperature. As illustrated in Fig. 5b, the content of Cl in ash deposit is reduced remarkably with the increasing deposition temperature. There is almost none Cl in ash deposit as the temperature is 600
C. Chlorine in coals is usually present

Content of Na2O (%)

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0.4

o

Deposition temperature ( C) Fig. 4. Content variations of alkali metal oxides with deposition temperature for two Zhundong coals.

Please cite this article in press as: C. Wang, et al., Ash deposition and sodium migration behaviors during combustion of Zhundong coals in a drop tube furnace, Journal of the Energy Institute (2016), http://dx.doi.org/10.1016/j.joei.2016.11.010

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15

20

10

15

5 0

10

400

500 600 700 o Temperature ( C)

800

10

25

5

WCW ZJ

8

1.0 0.8

6

0.6

4

0.4

2

0.2

0

400

500 600 700 o Temperature ( C)

800

Content of Cl (%)

WCW ZJ

Content of S (%)

20

Content of Fe (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

C. Wang et al. / Journal of the Energy Institute xxx (2016) 1e11

Content of Ca (%)

6

0.0

Fig. 5. Variations of deposition-related elements with evolution of deposition temperature: a) variations of Fe and Ca; b) variations of S and Cl.

as mobile form and considered as volatile species, which is released fairly early during combustion process [47]. Therefore, chlorine was released during combustion at 1200
C but not condensed and/or deposited at temperature 600
C. Fryda et al. [32] and Bartolome et al. [28] obtained similar results and showed practically none chlorine in ash samples when the deposit probe was placed at ~900
C with probe surface temperature maintained at ~550
C. It is interesting that the sulfur evolutions within two Zhundong coals differ distinctly. The content of sulfur in WCW ash deposit increases with the deposition temperature, whereas the case of ZJ coal is opposite. The divergence of occurrence mode of sulfur could be partly responsible for the obtained results. Wang et al. [27] investigated the oxy-coal combustion with limestone addition and observed the sulfur (SO3) content increased with raising probe surface temperature. They believed the sulfation of CaO in ash deposits at higher temperature possibly occurred. Further elucidation is necessary for exploring the detailed mechanisms of sulfur migration involved in high-sodium coals. X-ray diffraction (XRD) analysis was conducted to further evaluate the effect of deposition temperature on the evolution of mineral crystalline phases. Fig. 6 describes XRD analysis results of ash deposit at various temperatures displaying the evolutions of main crystalline phases in ashes of Zhundong coals. As clearly shown in Fig. 6a, quartz (SiO2) is the major crystalline phase in ash deposit of WCW coal. Mullite (Al6Si2O13) and hematite (Fe2O3) are always found at temperature range of 400e800
C, while other minerals are not. Calcium sulfate (CaSO4) is usually observed at deposition temperature of 500e800
C, while there is more lime (CaO) at low deposition temperature (400e700
C). High temperature is favorable for sulfation of calcium. Furthermore, anorthite (CaAl2Si2O8) is present at 700 and 800
C. Sodium of crystalline phase exists as albite (NaAlSi3O8) at deposition temperature of 600
C. It is possible that the replacement between sodium and calcium within aluminosilicate occurs at certain temperature. Sodium in flue gas was captured by silicoaluminate and deposited on probe with the decrease of flue gas temperature. Fig. 6b shows that calcium sulfate (CaSO4) is the major crystalline phase for the case of ZJ ash deposit. The gaseous phase SO2 could react with alkaline earth to form sulfates at the present deposit temperatures, similar to the conclusion obtained by Robinson et al. [47] Mullite (Al6Si2O13) also exists in ZJ ash between 400 and 800
C. According to Fig. 6b, calcium could be present as calcite (CaCO3) at low temperature (500
C). Calcite tends to be unstable at high temperature and will decompose at temperature of >800
C, while CaSO4 usually decomposes at ~1100
C [13]. Here, crystalline sodium is usually in form of sodium aluminum oxide (Na2O$xAl2O3) in ash deposit of ZJ coal. Sodium sulfate (Na2SO4) could be observed at 400
C but disappeared with the increasing temperature. Similar to potassium migration during biomass combustion [29], sodium in gaseous or condensed phase can react with SO2/SO3 to form Na2SO4 with the decrease of temperature. Therefore, sodium in ZJ coal will be captured by aluminum oxide and sulfur oxide with the reduced temperature of flue gas. The condensation of gaseous alkali metals and formation of minerals with low-melting points (such as Na2SO4 and NaAlSi3O8) should partly account for the serious ash deposition phenomenon during combustion of Zhundong coal. More variations among the mineral crystalline phases of the ash samples under various deposition temperatures were probably present under present experiments. However, the prevailing phases might be intensive enough to cover the peak variations of other crystalline phases. In addition, almost no crystalline phase of NaCl was observed from the ash deposit in present XRD analysis, while NaCl could be observed in the bottom ash obtained at ashing temperature 600
C [44]. It is possible that NaCl is in amorphous phase that cannot be detected by XRD technique because chlorine was measured from XRF analysis at low deposition temperature (see Fig. 5b). Some more phases are possible present in ash deposit but cannot be observed just because they are amorphous or less in quantity than the major phases (such as SiO2 and CaSO4 here) [32].

3.2. Effect of combustion temperature Combustion temperature has a remarkable impact on coal reactivity. The reactivity rate is generally increased with temperature and combustion temperature could play a non-ignorable role on mineral migration. As shown in Fig. 7, three different furnace temperatures on sodium transition were compared, with access air ratio of 1.2, holding time of 120 min, and deposition temperature of 700
C. Experimental results show that the amount of ash deposits decreases with an increase in combustion temperature. Zhou et al. [49] evaluated the influence of furnace temperature on slagging deposit behaviors and also indicated that low temperature could facilitate an increase in deposit thickness, which is consistent with the present conclusion. As shown in Fig. 7, the sodium content in ash deposits with the increasing probe temperature exhibits similar variation at different combustion temperatures, showing only slight changes at temperature of 600
C and an obvious decrease above 600
C. The sodium content in ash deposits increases remarkably when the combustion temperature decreases from 1200 to 1000
C. The sodium content in ash deposits obtained at combustion temperature of 1300
C is decreased compared with the case of 1200
C, while the Please cite this article in press as: C. Wang, et al., Ash deposition and sodium migration behaviors during combustion of Zhundong coals in a drop tube furnace, Journal of the Energy Institute (2016), http://dx.doi.org/10.1016/j.joei.2016.11.010

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Fig. 6. XRD analyses of deposited ashes at various temperatures: a) WCW ash deposit; b) ZJ ash deposit.

variation extent is less significant than the condition of 1000
C. Yao et al. [13] obtained similar temperature dependence of sodium content in collected ash deposit of Zhundong coal in the temperature range of 800e1100
C. The sodium in WCW coal is mainly water-soluble and CH3COONH4-soluble forms, which are easily released during combustion. On the other hand, it is possible that a part of Na can be react with aluminosilicate or aluminum oxide to form sodium aluminosilicate/aluminate and left in coal ash during the initial combustion period. The Please cite this article in press as: C. Wang, et al., Ash deposition and sodium migration behaviors during combustion of Zhundong coals in a drop tube furnace, Journal of the Energy Institute (2016), http://dx.doi.org/10.1016/j.joei.2016.11.010

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-1

Content of Na (mg·g )

16

o

1000 C o 1200 C o 1300 C

14 12 10 8 6 4 400

500

600

700

800

o

Deposition temperature ( C) Fig. 7. Influence of combustion temperature on sodium migration of WCW coal ash.

combustion temperature has a remarkable influence on these reactions [50]. The reactions between sodium and aluminosilicate are inclined to occur before the release of sodium into gaseous phase at low temperature, while it tends to proceed after sodium volatilization at high temperature. In addition, a solid-gaseous dynamic equilibrium between sodium-compounds on char surfaces and gaseous sodium in flue gas is possible to conduct during combustion of Zhundong coal. The decrease of temperature is favorable for the equilibrium proceeding to solid direction. Consequently, more sodium will be left in ash at lower combustion temperature and result in the increase of Na content in ash deposit. Nutalapati et al. [51] used chemical fractionation to distinguish the inorganics into reactive and non-reactive fractions. Na, K, Cl, S and a part of Ca were primary reactive composition of coal, while the non-reactive part comprised mainly of Si, Al, and Fe. Notwithstanding iron is not as reactive as sodium, iron can affect the formation of eutectic compounds with low-melting points and then exhibit impact on ash deposition process. Here, the evolution of iron (Fe) with temperature was also investigated. Fig. 8 illustrates the influence of combustion temperature on iron transition in ash of WCW coal under the same experimental condition as Fig. 7. As shown in Fig. 8, the variation of Fe with combustion temperature is opposite to that of sodium. The relative content of iron in ash deposit increases with the combustion temperature. The enrichment effect of iron in ash deposits is enhanced with the increase in combustion temperature due to the lowvolatility of Fe. 3.3. Influence of access air ratio on Na/Fe migration The access air ratio in practical boiler can influence the temperature distribution, gas distribution, burnout, pollutant emission and so on. Here, the influence of access air ratio (usually represented by symbol a) on release behaviors of sodium andiron was further investigated. Fig. 9 shows the ash deposit image of WCW coal combusted at various access air ratios, with combustion temperature of 1200
C, deposition probe temperature of 700
C, and coal consumption of 0.75 g min1. The amount and viscosity of ash deposit rise with the access air ratio

180 o

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1000 C o 1200 C o 1300 C

160 140 120 100 80

400

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Deposition temperature ( C) Fig. 8. Influence of combustion temperature on iron transition of WCW coal ash.

Please cite this article in press as: C. Wang, et al., Ash deposition and sodium migration behaviors during combustion of Zhundong coals in a drop tube furnace, Journal of the Energy Institute (2016), http://dx.doi.org/10.1016/j.joei.2016.11.010

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Fig. 9. Image of the ash deposit shape of WCW coal at 700
C with different access air ratios: a) access air ratio of 1.2; b) access air ratio of 1.05.

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Fig. 10. Influence of access air ratio on sodium/iron migration of WCW coal ash: (a) Na migration; (b) Fe migration.

decreased from 1.2 to 1.05. In addition, the ash color for the case of a ¼ 1.05 is slightly deepened compared with the condition of a ¼ 1.2. Subsequent TGA analysis confirms that unburnt carbon is present within the ash deposit obtained at a ¼ 1.05. Fig. 10 shows the Influence of access air ratio on release behaviors of sodium and iron. As shown in Fig. 10a, the Na content in ash deposits increases obviously with the access air ratio reduced from 1.2 to 1.05. In the present lab-scale experiments, the furnace temperature was controlled by electrical power output and the change of access air ratio mainly exhibited impact on atmosphere in furnace. With the access air ratio decreased from 1.2 to 1.05, the combustion intensity is reduced, the particle temperature decreases, and then the volatilization of sodium is weakened. All these contributions lead to less Na released into gaseous phase and more solid phase left in ash particles. Furthermore, these two Zhundong coals are also characterized by high contents of sulfur and iron. Under the condition of low access air ratio, more CO is present in furnace, which could facilitate the reactions among gaseous Na, Fe, and S to form compounds with low-melting points. These sodium-containing compounds are easy to adhere to ash particle and/or condense on deposit probe surface. Fig. 10b shows the influence of access air ratio on iron migration of WCW coal ash with combustion temperature of 1200
C. Under the condition of low access air ratio, FeO might form with melting point of only 1030
C, which is also easy to react with CaO, SiO2, Al2O3, and MgO to form eutectic compounds with low-melting points, which are inclined to discharge into gaseous phase. Hence, local weakly reducing atmosphere resulting from low access air ratio leads to less iron content within ash deposits under present experimental conditions. 4. Conclusions The ash deposition features and sodium migration behaviors between 400 and 800
C during combustion of two Zhundong coals were investigated in a drop tube furnace. Experimental results show that fouling tendency varies with the deposit probe temperature. The relative content of sodium in ash deposit increases significantly with the probe temperature decreasing from 800 to 600
C, while it is nearly unchanged below 600
C. Nevertheless, the potassium content in ash deposit presents only slight variation with temperature due to its mainly insoluble occurrence in Zhundong coals. The contents of iron and calcium in ash deposits exhibit nonmonotonic variations with deposition temperature varying between 400 and 800
C. Chlorine content in ash deposit decreases remarkably with the increase in deposition temperature, and almost none Cl exists in ash deposit above 600
C. Release and deposit behaviors of sulfur within two Zhundong coals are a bit complicated. Quartz (SiO2) and calcium sulfate (CaSO4) are major crystalline phases in ash deposits from WCW and ZJ coals, respectively. Calcium sulfate (CaSO4) was usually observed at high deposition temperature, while calcium was inclined to be as calcite (CaCO3) and lime (CaO) at low temperature. High temperature benefits calcium sulfation during combustion of Zhundong coal. Sodium of crystalline phase was found Please cite this article in press as: C. Wang, et al., Ash deposition and sodium migration behaviors during combustion of Zhundong coals in a drop tube furnace, Journal of the Energy Institute (2016), http://dx.doi.org/10.1016/j.joei.2016.11.010

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as albite (NaAlSi3O8) and sodium sulfate (Na2SO4) at low deposition temperature. Sodium in Zhundong coals was captured by silicoaluminate or sulfur oxide with the decreasing temperature of flue gas. Both the condensation of gaseous alkali metals and formation of lowmelting point compounds were responsible for the serious ash deposition phenomenon on convection heat exchanger surfaces during combustion of Zhundong coal. With the increase of combustion temperature, the amount of ash deposits decreases and the sodium content in ash deposit is significantly reduced. The decrease of combustion temperature is beneficial for sodium left in ash, while the iron variation with combustion temperature is opposite to sodium due to the low-volatility of Fe. In addition, the Na content in ash deposits increases obviously with the access air ratio decreasing from 1.2 to 1.05, but the local weakly reducing atmosphere resulting from low access air ratio leads to less iron within ash deposits at deposition temperature between 400 and 800
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Please cite this article in press as: C. Wang, et al., Ash deposition and sodium migration behaviors during combustion of Zhundong coals in a drop tube furnace, Journal of the Energy Institute (2016), http://dx.doi.org/10.1016/j.joei.2016.11.010