An experimental investigation into the effect of flue gas recirculation on ash deposition and Na migration behaviour in circulating fluidized bed during combustion of high sodium Zhundong lignite

An experimental investigation into the effect of flue gas recirculation on ash deposition and Na migration behaviour in circulating fluidized bed during combustion of high sodium Zhundong lignite

Fuel Processing Technology 199 (2020) 106300 Contents lists available at ScienceDirect Fuel Processing Technology journal homepage: www.elsevier.com...

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Fuel Processing Technology 199 (2020) 106300

Contents lists available at ScienceDirect

Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc

Research article

An experimental investigation into the effect of flue gas recirculation on ash deposition and Na migration behaviour in circulating fluidized bed during combustion of high sodium Zhundong lignite

T



Zhuo Liua, Jianbo Lia, , Mingming Zhub, Fangqin Chengc, Xiaofeng Lua, Zhezi Zhangb, Dongke Zhangb a

Key Laboratory of Low-grade Energy Utilization Technologies and Systems of Ministry of Education of PRC, Chongqing University, 174 Shazheng Street, Shapingba, Chongqing 400044, PR China Centre for Energy (M473), The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia c State Environmental Protection Key Laboratory of Efficient Utilization Technology of Coal Waste Resources, Shanxi University, Wucheng Road, Taiyuan 030006, PR China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Ash deposition Bed agglomeration Circulating fluidized bed combustion Flue gas recirculation Sodium migration

The effect of flue gas recirculation (FGR) on ash deposition and Na migration in a laboratory-scale circulating fluidized bed (CFB) burning Zhundong lignite was investigated. The CFB dense phase was maintained at temperature 950 °C, whereas that of the furnace exit was adjusted to 730 and 680 °C respectively by manipulating the FGR flow rate. Two ash deposition probes maintained at 550 °C were installed in the furnace chamber (P1) and cyclone outlet (P2). The deposited ashes along with the bottom and fly ash were collected and analysed by using XRD, SEM-EDX and ICP-OES. Results showed that the P1 deposit consisted of sintered Ca/Mg/Na sulphates and aluminosilicates at 730 °C, but became granular Na2SO4/CaSO4 at 680 °C. The P2 deposit and fly ash were sintered and rich in CaSO4 and Na2SO4 at 730 °C, but did not show signs of sintering at 680 °C. The bottom ash was enriched in CaSiO3 and NaAlSi3O8 at 730 °C, but were dominated by Na6(AlSiO4)6 at 680 °C. Na in the bottom ash increased from 31.7 to 35.2 mg g−1 as FGR flow rate increased, leading to severe bed particle agglomeration consequently. The FGR thus exerted a trade-off effect on the sintering and deposition propensity of fly ash and agglomeration of bed materials.

1. Introduction Zhundong coalfield in Northwest China has an estimated reserve of 390 Gt and will be crucial to China's energy supply [1]. The coal in this coalfield has low ash and sulphur contents and high reactivity [2]. However, alkali and alkali earth metal (AAEM) contents in the Zhundong lignite were extremely high [3] and would cause severe ash slagging and fouling during its utilisation [4,5]. These ash-related issues include increased heat transfer resistance, high temperature corrosion, decreased thermal efficiency, and even unexpected outage of power plants [2,6,7]. Understanding the ash deposition mechanisms and developing relevant mitigation strategies are therefore essential. Ash deposition behaviour and its underlying mechanisms during combustion of high sodium Zhundong lignite have been extensively investigated [6,8–13]. It is accepted that the severe ash deposition was mainly attributed to its high Na and Ca contents [14]. When Zhundong



lignite was burned in pulverized coal-fired boilers at temperatures higher than 1000 °C, Na would be released and subsequently deposit on heat transfer surfaces or ash particles, increasing the stickiness of the deposits and the ash particles [5,6,8,15–17]. Severe ash sintering and deposition would be induced as a consequence. Burning Zhundong lignite in circulating fluidized bed (CFB) combustion at 850–950 °C would reduce the release of Na [18,19]. The slagging and fouling of ash are expected to be decreased [9,10,20,21]. However, agglomeration of bed materials and ash deposition become operational issues of concern during CFB combustion of Zhundong lignite, due to the abundance of Na/Ca/Fe-bearing minerals and eutectics [19,22,23]. Mitigation strategies for Zhundong lignite ash deposition have been proposed in literature. Beyond those strategies focused on fuel and its treatment (water washing [24], the use of additives such as kaolin, SiO2, Al2O3, oil shale and its semi-coke etc. [19,25,26], and coal blending [27,28]), the optimization of operating condition and boiler

Corresponding author. E-mail address: [email protected] (J. Li).

https://doi.org/10.1016/j.fuproc.2019.106300 Received 16 September 2019; Received in revised form 28 October 2019; Accepted 22 November 2019 0378-3820/ © 2019 Elsevier B.V. All rights reserved.

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respectively, were used to simulate ash deposition on heat transfer surfaces. For the FGR system, the flue gas after the bag filter was pumped out by a recirculating fan, which was sent into a flue gas scrubber by water spray and a drying filter to obtain clean flue gas without SO2, moisture, and dust particles. The flue gas with a temperature of 40 °C was then fed into the middle zone (1.52 m above the distributor) of the CFB furnace in order to manipulate the furnace exit temperature by adjusting the FGR flow rate through a rotor flowmeter. During the experimentation, the temperature of the CFB dense phase was always maintained at 950 °C. The furnace exit temperature was typically 900 °C when without FGR, but decreased to 730 (FGR730) and 680 °C (FGR680) when the FGR flow rate was 15 and 18 Nm3 h−1, respectively. It was hypothesised that a decrease in the furnace exit temperature would mitigate ash deposition. The detailed operating parameters can be found and summarised in Table 2. It is noted that the coal feeding rate was 7, 6.6, and 6.4 kg h−1 for maintaining combustion at 950 °C, respectively. This would bring 5.7% or 8.6% variation in the fuel throughput, ash yield and Na content, and to some degree, would influence ash deposition during 6 hour experimentation. However, this would not influence the XRD and SEM-EDX analysis which are qualitative or semi-quantitative. For quantitative ICP-OES analysis in Section 3.3, Na in the bottom ash in fact increased as FGR increased, contrary to the decrease in the fuel throughput. This enabled discussion on the effect of FGR on ash deposition, Na migration and bed agglomeration, without considering the effect of fuel throughput. The variation in fuel throughput was thus not considered as a significant factor influencing discussion and interpretation of the experimental results. To comprehend the effect of FGR on flue gas atmosphere and subsequently on ash sintering and deposition, the concentrations of O2, CO, and CO2 around the probes and at the flue path outlet were measured by using an “ECOM-J2KN” gas analyser and a Gasboard 3000 infrared gas analyser. As shown in Table 3, when ZD was burned without FGR, the O2 concentration in the gas around P1, P2 and at flue path outlet were measured to be 4.6%, 3.9%, and 3.7%, respectively, showing a decrease in O2 along with the gas flow passage. CO was also decreased from 8750 to 648, and 360 mg m−3, while CO2 correspondingly increased from 12.9% to 15.1%, and 15.3%. These indicate that oxidation of C and CO and further consumption of O2 occurred significantly in the flue gas after P1 but almost ceased after P2. When FGR was applied, the O2 concentration in the flue gas at the abovementioned positions, being 4.3%, 3%, and 2.7% at FGR730 and 4.2%, 3.1%, and 2.6% at FGR680, were slightly lower than those without FGR. Similar trend was also applied for CO concentration in the flue gas. The CO2 concentration was however increased as more CO2 was incorporated due to FGR. The variation in the O2, CO, and CO2 concentrations as analysed would affect the flue gas atmosphere around the probes, which to some degree would further affect the sintering and deposition propensity of the ash [17,20,34,40]. However, this is outside the scope of this work and will not be discussed further. The temperature profile of the CFB system is shown in Fig. 2. It can be seen that incorporation of FGR indeed decreased the temperature of flue gas at furnace exit and later in the flue path. While the probe temperatures were controlled at 550 °C constantly, the flue gas temperatures around P1 were measured to ca. 900, 730, and 680 °C, and those of P2 were 800, 570, and 530 °C, respectively. The velocities of flue gas around P1 were determined as 3.4, 3.3, and 3.2 m s−1, and those around P2 were 15.6, 14.2, and 13.7 m s−1, respectively, suggesting that the velocities of flue gas were slightly decreased upon the application of low temperature FGR. This would to some degree affect the inertial impaction of particles on the probe surface. Moreover, the temperature difference between the flue gas and the probe decreased, which might further weaken the role of condensation and thermophoresis [17]. During stable operation of 6 h, the deposited ash on both the top and side surfaces of these probes, along with the bottom and fly ash were collected. These deposits were denoted as PxT and PxS, where Px

design are also regarded as alternative and effective means for mitigating ash deposition [16]. An increase in the size and capacity of the furnace scale and manipulation of the temperature at furnace exit are believed to be effective [8,16,17]. Our recent work [29] also proved that burning Zhundong lignite in CFB at furnace exit temperature of 750–680 °C would mitigate the sintering and deposition propensity of ash particles. The use of flue gas recirculation (FGR) is considered as another means for the control of furnace exit temperature and subsequently alter ash deposition and sintering. However, to the author's best knowledge, this has not been verified experimentally and the effect of FGR on Na migration and the mineral interactions has not been understood comprehensively. The present contribution reports a preliminary investigation into the effect of FGR on furnace exit temperature control and subsequently the ash deposition and Na migration. The CFB dense phase was maintained at 950 °C whereas the furnace exit temperature was adjusted to 730 and 680 °C by manipulating the flow rate of FGR. The mineralogy, morphology, chemistry and fusion characteristics of the deposit along with the bottom and fly ash were analysed in detail. The present work would validate this method for deposition mitigation during Zhundong lignite combustion in circulating fluidized bed. 2. Experimental 2.1. Materials A representative Zhundong lignite (ZD) from Zhundong coalfield was used during experimentation. It was grounded and sieved into particles within 3 mm in size. The proximate and ultimate analysis of ZD and its ash chemistry are as shown in Table 1. It is seen that the ZD has an ash content of 4.3 wt%, among which Na and Ca accounted for 4.9 wt% and 32.51 wt%, respectively. This would bring severe ash sintering and deposition during combustion [15,30,31]. In addition, quartz sand with cut-size between 0.15 and 1 mm and a purity of 95% was used as the bed material. The remaining 5% impurities in the quartz sand were Al, Ca, Cr, and S, and would not contribute to bed material agglomeration significantly [7,20,23,40,41]. 2.2. CFB system A schematic diagram of the CFB combustion system with FGR is illustrated in Fig. 1. The main structure of the CFB system and the structure of the air-cooled ash sampling probe have been detailed in our published work [9,10] and are not described herein. Two probes, installed within the furnace (P1) and at the cyclone outlet (P2), Table 1 Proximate and ultimate analysis of ZD and the chemical composition of its ash. Proximate analysis (wt%, dry basis) Moisture

15.6

Volatile matter

Fixed carbon

25.38

Ash

54.72

Low heating value (MJ kg−1)

4.3

21.66

Ultimate analysis (wt%, dry basis) C

H

75.45

O

3.51

N

14.69

S

0.69

0.57

Ash chemistry (wt%, dry basis) SiO2

Al2O3

Fe2O3

11.71

6.69

5.93

CaO

SO3

MgO

Na2O

32.51

27.93

7.56

4.9

2

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Fig. 1. Schematic diagram of the experimental CFB system with FGR. Table 2 Operating parameters during experimentation. Parameters

Without FGR

FGR730

FGR680

Bed temperature (°C, T2) Furnace exit temperature (°C, T5) Coal feeding rate (kg h−1) Superficial fluidization velocity (m s−1) Flue gas recirculation flow rate (Nm3 h−1)

950 ± 10 900 ± 10 7 3.68

950 ± 10 730 ± 10 6.6 3.2

950 ± 10 680 ± 10 6.4 3.1

0

15

18

Table 3 The flue gas composition during experimentation. Experimentation

Position

O2 (%)

CO2 (%)

CO (mg m−3)

Without FGR

Probe 1 Probe 2 Flue path outlet Probe 1 Probe 2 Flue path outlet Probe 1 Probe 2 Flue path outlet

4.6 3.9 3.7 4.3 3 2.7 4.2 3.1 2.6

12.9 15.1 15.3 14.2 16.1 16.7 15.2 16.4 16.8

8750 648 360 1033 341 305 892 330 291

FGR730

FGR680

Fig. 2. Temperature profiles of the CFB system when without FGR, and with FGR at furnace exit temperature 730 and 680 °C.

3

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represent the probe P1 and P2, and T and S are the top and side surface deposits for short. 2.3. Analysis A Bruker X-ray diffraction (XRD) with copper Kα radiation was used to identify the crystalline phases in the collected ashes and deposits. The accelerating voltage and current were 40 kV and 40 mA, respectively, with a scanning speed of 0.0833°·s−1 between 10° and 80°/2θ. The main mineral phases were analysed by using the X'Pert HighScore Plus software. Meanwhile, a TM3030 scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDX) was used to observe the morphological features, and semi-quantitatively analyse the elemental composition of the ash and deposits. Furthermore, an inductively coupled plasma optical emission spectrometry (ICP-OES) coupled with a microwave-assisted HF/HNO3 digestion system was used to quantitatively determine the contents of Na in these samples. In addition, an ash fusion analyser (HR-8000B) was used to determine the fusion temperatures of collected ash samples, following Chinese Standard GB/T 219-2008.

Fig. 4. XRD patterns of the fly ash during combustion of ZD at FGR730 and FGR680. 1 SiO2, 2 Fe2O3, 3 Ca2Al2SiO7, 4 CaCl2, 5 MgO, 6 CaO, 7 CaSO4, 8 CaCO3, 9 Ca3Mg(SiO4)2, 10 MgFe2O4, and 11 Na2SO4.

3. Results and discussion 3.1. Mineralogical characterisation

further decreased, the mineral phases in the bottom ash became SiO2, CaSiO3, Na6(AlSiO4)6, Ca2Mg(Si2O7), NaAlSi3O8, and MgFe2O4. In particular, Na6(AlSiO4)6 with high XRD peak intensity was exclusively identified at FGR680, indicating that more Na-bearing minerals presented in the bottom ash. This is consistent with those reported in [34–36]. Moreover, Ca2Mg(Si2O7) and MgFe2O4 with higher XRD intensities were observed, indicating the presence of more Ca/Mg/Febearing minerals. This further implies that the sintering and fusion characteristics of the bottom ash might be changed.

3.1.1. Bottom ash Fig. 3 presents the mineral phases in the bottom ash during CFB combustion burning ZD at FGR730 and FGR680. At FGR730, the main mineral phases in the bottom ash were SiO2, CaSiO3, Ca2Mg(Si2O7), NaAlSi3O8, and MgFe2O4. Among these mineral phases, the peak intensity of SiO2 was the highest, suggesting that SiO2 was the dominant mineral phase [32]. This is because quartz sand was used as the bed material. Na-bearing minerals were found in the form of NaAlSi3O8, instead of being (Ca,Na)(Si,Al)4O8 when without FGR [33]. This indicates that (Ca,Na)(Si,Al)4O8 was not formed in the presence of FGR when the furnace exit temperature decreased to 730 °C. In addition, Ca2Mg(Si2O7) and MgFe2O4 were identified, which were however absent when without FGR [33] This suggests that the application of FGR would allow the presence of Ca/Mg/Fe/Na-bearing minerals in the bottom ash. As the flow rate of FGR increased and the furnace exit temperature

3.1.2. Fly ash Fig. 4 shows the XRD patterns of the fly ash during ZD combustion in CFB at FGR730 and FGR680. When ZD was burned without FGR [33], the fly ash consisted of SiO2, Fe2O3, Na2SiO3, Na2Si2O5, Ca2Al2SiO7, Ca3Mg(SiO4)2, Ca2SiO4, MgO, CaO, NaAlSi3O8, and NaAlSiO4. These Na-bearing minerals in the fly ash would stimulate ash sintering and deposition. With the application of FGR, the fly ash at FGR730 consisted of SiO2, Fe2O3, Ca2Al2SiO7, CaCl2, MgO, CaO, CaSO4, Ca3Mg(SiO4)2, MgFe2O4, and Na2SO4. In comparison, Na-bearing minerals including Na2SiO3 (melting points (MP) of 1089 °C), Na2Si2O5 (MP of 874 °C), NaAlSi3O8 (MP of 1100 °C), and NaAlSiO4 (MP of 1520 °C) were not identified at FGR730. Instead, Na2SO4 (MP of 884 °C) of low melting-points was exclusively identified at FGR730. This suggests that the mineral interactions between Na/Na2O/NaCl and the aluminosilicate decreased as the furnace temperature decreased because of FGR [19]. Moreover, CaCl2 (MP of 782 °C) was also identified, indicating the presence of Ca chloride. These Ca/Na-bearing minerals of low melting-points would thus decrease the fusion temperatures of fly ash. At FGR680 as the flow rate of FGR further increased, the mineral phases in fly ash became SiO2, Fe2O3, Ca2Al2SiO7, CaCl2, MgO, CaO, CaSO4, CaCO3, Ca3Mg(SiO4)2, MgFe2O4, and Na2SO4. Compared with those minerals at FGR730, CaCO3 (MP of 1339 °C) was identified in the fly ash, indicating that Ca carbonates might not be decomposed completely when the furnace exit temperature decreased to 680 °C at FGR680 [37]. In addition, CaCl2 and Na2SO4 of low melting-points had minor peak intensities, suggesting that the contents of these minerals decreased. The decrease in CaCl2 and Na2SO4 might decrease the sintering and deposition propensity of the fly ash at FGR680.

Fig. 3. XRD patterns of the bottom ash during combustion of ZD at FGR730 and FGR680. 1 SiO2, 2 Ca2Mg(Si2O7), 3 NaAlSi3O8, 4 CaSiO3, 5 MgFe2O4, and 6. Na6(AlSiO4)6. 4

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particles (Fig. 6(b)) had sintered into coarse-grained agglomerates, indicating the occurrence of bed agglomeration and sintering. EDX analysis revealed that particles (e.g., area 1) rich in Ca, Si, Mg, Fe, and Al were identified, indicating the presence of Ca aluminosilicates or Mg/ Fe-bearing minerals. Particles (e.g., areas 2 and 3) rich in Ca, Si, Al, Mg, Fe, and Na were identified, indicating the presence of Ca/Mg/Na aluminosilicates or Fe-bearing minerals. Compared with the EDX analysis of the bottom ash without FGR [33], more Na, Mg and Fe were identified in the bottom ash. This suggests that these Na/Mg/Fe-bearing minerals or eutectics were remained in the bottom ash particles, leading to an increase in particle stickiness and ash sintering. This explains why the bed material had agglomerated. At FRG680 as the flow rate of FGR further increased, the bottom ash mainly consisted of agglomerated particles, indicating the occurrence of severe bed agglomeration and ash sintering. These agglomerates had even larger sizes compared with those at FGR730, as observed from the SEM images (Fig. 6(e–f)). This suggests that the sintering and agglomeration of bottom ash particles were further aggravated at FGR 680. In terms of ash chemistry, particles (e.g., area 4) rich in Na, Si, Al, and Fe were identified, indicating the presence of Na aluminosilicates or NaeFe eutectics [19]. Particles (e.g., area 5) rich in Ca, Si, Mg, Al, and Na were identified, indicating the presence of Ca/Na/Mg aluminosilicates. In addition, Ca and Si were of high contents in these particles (e.g., area 6), indicating the presence of Ca silicates. This is consistent with the XRD analysis of the bottom ash. In comparison with those at FGR730, more Na was presented in the bottom ash, implying a retention of Na. The abundance of Na and Fe would lead to severe bed agglomeration and sintering.

Fig. 5. XRD patterns of the P2S deposits during combustion of ZD at FGR730 and FGR680. 1 SiO2, 2 Fe2O3, 3 Ca2Al2SiO7, 4 MgO, 5 CaO, 6 CaCO3, 7 CaSO4, 8 MgFe2O4, 9 Na2SO4, and 10 Na7Al3O8.

3.1.3. Probe 2 Fig. 5 presents the XRD patterns of the P2S deposits at experimental conditions selected. When ZD was burned but without FGR, CaSO4, Fe2O3, SiO2, Ca2Al2SiO7, MgO, CaO, Ca3Mg(SiO4)2, MgFe2O4, NaCl, Na2Si2O5, Na2SO4, and NaAlSiO4 were identified in the P2S deposit. Among these minerals, CaSO4 was the dominant mineral phase in P2S deposits where NaCl (MP of 801 °C), Na2SO4, and Na2Si2O5 also presented, incurring severe ash sintering and deposition [33]. Once FGR was applied, the mineral phases in P2S became SiO2, Fe2O3, Ca2Al2SiO7, MgO, CaO, CaCO3, CaSO4, MgFe2O4, Na2SO4, and Na7Al3O8. In comparison, Na-bearing minerals including NaCl and Na2Si2O5 were not identified at FGR730, indicating a decrease in Na-bearing minerals. In addition, Na7Al3O8 was identified at FGR730, indicating that mineral interactions between Na and Al occurred with the application of FGR. Moreover, CaCO3 was also identified, again probably due to incomplete decomposition [37]. The decrease in Na-bearing minerals and flue gas temperature would mitigate the sintering and agglomeration of the deposits. At FGR680 as the flow rate of FGR further increased, SiO2, Fe2O3, Ca2Al2SiO7, MgO, CaO, CaCO3, CaSO4, MgFe2O4, and Na2SO4 were identified in the P2S. In particular, SiO2 became the dominant mineral phase, rather than being CaSO4 at FGR730. This indicates an enrichment of SiO2 at FGR680. Moreover, Na7Al3O8 was not identified, indicating a decrease in Na-bearing mineral. This might be attributed to the decrease in furnace exit temperature at FGR680, decreasing the mineral interaction between Na and Al2O3. The abundance of SiO2 and absence of Na-bearing minerals would increase the fusion temperatures of deposited ash, and mitigate ash sintering and deposition propensity.

3.2.2. Fly ash Fig. 7 shows the morphology of the fly ash during CFB combustion burning ZD at FGR730 and FGR680. As has been reported in our previous work [33], the fly ash when without FGR consisted of agglomerates rich in AAEM sulphates. Obvious sintering and coalescence of fly ash particles had also occurred [33]. With the application of FGR, the fly ash at FGR730 consisted of both discrete sub-micron particles and their agglomerates, suggesting that the sintering degree of fly ash lessened. In terms of ash chemistry, particles (e.g., particle 1 and area 2) rich in Ca and S were identified, indicating the presence of Ca sulphate. Particles (e.g., particle 3) rich in Ca, S, and Cl were identified, indicating the presence of Ca sulphates or chlorides. This is consistent with the XRD analysis. Compared with those without FGR, Na was less identified in the fly ash. The fly ash might therefore be less sticky. At FGR680 as the flow rate of FGR further increased, the fly ash mainly consisted of discrete sub-micron particles, with its agglomerates occasionally observed. This indicates a further decrease in ash sintering. EDX analysis reveals that particles (e.g., areas 4 and 5) rich in Ca and S were identified, indicating the presence of Ca sulphate. Moreover, particles (e.g., area 6) rich in Ca, S, Mg, and Cl were also identified, indicating the presence of Ca/Mg sulphates or chlorides. This is consistent with the XRD analysis of the fly ash as above. Compared with that at FGR730, the content of Na was negligible, indicating that less Na presented in the fly ash. Na might have been retained in the bottom ash, as have been discussed in the previous section. The absence of Na at low flue gas temperature would increase the stickiness of fly ash particles, consequently alleviating the deposition and sintering propensity.

3.2. Morphological characterisation 3.2.1. Bottom ash Fig. 6 illustrates the morphological features and EDX elemental analysis of the bottom ash at FGR730 and FGR680. When ZD was burned without FGR, as have been well characterised in our previous work [33], the surfaces of these bottom ash particles were coated by a layer of sintered sub-micron particles enriched in Ca/Si but absent of Na/S. With the application of FGR, the bottom ash at FGR730 consisted of both discrete bed material particles and its agglomerates. A larger number of sub-micron particles had adhered on the surfaces of these particles, indicating the presence of ash sintering. In particular, some

3.2.3. Probe 1 top Fig. 8 presents the morphological features and EDX elemental analysis of the P1T deposits at FGR730 and FGR680. With the application of FGR, the P1T deposit at FGR730 consisted of sub-micron or micron particles and agglomerates with less than 4 μm in size. In particular, the deposited particles with irregular shape (Fig. 8(c)) were observed, which differed from those without FGR where melted and sintered ash particles were observed [33]. This suggests a decrease in ash sintering or melting at FGR730. For the chemistry of the deposit, 5

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Fig. 6. Representative SEM images and EDX elemental analysis of (a–d) the FGR730 bottom ash and (e–h) the FGR680 bottom ash.

indicating the presence of Ca sulphate. Despite these particles were enriched in Na sulphate in P1T deposit, Na sulphate had not sintered or agglomerated with other particles at furnace exit temperature as low as 680 °C. The increase in the flow rate of FGR would thus mitigate the sintering propensity of deposited ash.

particles (e.g., particle 1) rich in Ca, S, Al, Si, Mg, and Na were identified, indicating the presence of AAEM sulphates and aluminosilicates. Some zones (e.g., area 2) were rich in Fe, which were believed to be the probe materials. This suggests that less ash particles deposited on the probe surface at FGR730. In addition, particles (e.g., area 3) rich in Ca, S, Si, Al, and Na were identified, indicating the presence of Ca/Na sulphates and aluminosilicates. Compared with those without FGR [33], Na content was significantly decreased whereas more Si/Al/Mg were presented in the P1T deposit. This suggests that these deposited particles were less sticky. At FGR680 as the flow rate of FGR further increased, the P1T deposit at FGR680 composed of granular sub-micron particles, indicating that the deposited particles were less sintered. In particular, coarse-grained particles (e.g., particle 4) had deposited on the surface, indicating that the deposit chemistry might vary as the furnace exit temperature further decreased. EDX analysis revealed that particles (e.g., particle 4) rich in Na and S were identified, indicating the presence of Na sulphate. This implies that these coarse-grained particles were Na2SO4. In addition, particles (e.g., areas 5 and 6) rich in Ca and S were identified,

3.2.4. Probe 1 side Fig. 9 illustrates the morphological features and EDX elemental analysis of the P1S deposits at FGR730 and FGR680. When ZD was burned without FGR, the P1S deposit consisted of melted and sintered agglomerates enriched in Ca/Na sulphates [33]. With the application of FGR, the deposit at FGR730 consisted of sub-micron particles as well as agglomerates with irregular shape, indicating a decrease in sintering and melting degree of the deposit. In terms of ash chemistry, particles (e.g., particle 1) rich in Na and S were identified, indicating the presence of Na sulphate. Agglomerates (e.g., particle 2) enriched in Na, Al, Si, and S were identified, indicating the presence of Na sulphate and aluminosilicates. In addition, particles (e.g., particle 3) rich in Ca, S, Si, Al, Mg, and Na were identified, indicating the presence of Ca/Na/Mg

Fig. 7. Morphological features and EDX elemental analysis of the obtained fly ashes at (a–d) FGR730 and (e–h) FGR680. 6

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Fig. 8. Morphological features and EDX elemental analysis of the P1T deposits of (a–d) FGR730 and (e–h) FGR680.

Fig. 9. Morphological features and EDX elemental analysis of the P1S deposits of (a–d) FGR730 and (e–h) FGR680.

3.2.5. Probe 2 top Fig. 10 shows the morphological features and EDX elemental analysis of the P2T deposits at FGR730 and FGR680. As a reference, the deposit without FGR [33] consisted of coarse-grained particles and agglomerates enriched in Na/Ca/K sulphates that had been sintered or melted. On the contrast, with the application of FGR, the deposit at FGR730 consisted of sub-micron particles and agglomerates with irregular shape, indicating that these deposited particles were less sintered. In addition, the deposit was thin where bare surfaces could be observed (Fig. 10(b)), indicating that less ash particles were deposited on the probe surface. For the ash chemistry of the P2T deposit, particles (e.g., particle 1) rich in Fe, Ca, and S were identified, indicating the presence of Ca/Fe sulphate. Particles (e.g., particles in areas 2 and 3) rich in Ca, S, Mg, and Na were also presented, indicating the presence of Ca/Mg/ Na sulphates. Na content in P2T deposit decreased from ca. 30 mol% to ca. 10 mol% once FGR was applied, indicating that less Na was

sulphates and aluminosilicates. It is noted that more Si/Al/Mg and less Na were identified in the P1S deposited ash when compared with those without FGR [33]. The deposited particles were thus less sintered [38,39]. At FGR680 as the flow rate of FGR further increased, the deposit at FGR680 consisted of sub-micron particles with granular and irregular shapes. This indicates that sintering among the deposited ash decreased. EDX analysis revealed that particles (e.g., particle 4) rich in Na and S were identified, indicating the presence of Na sulphate. Moreover, granular particles (e.g., areas 5 and 6) rich in Ca, S, Fe, K, and Na were identified, indicating those Ca/Na/K/Fe sulphates or eutectics did not undergo significant sintering or melting. Particles rich in Na sulphate also presented as the furnace exit temperature further decreased. Despite this, these particles would retain their shape and be less sticky at flue gas temperature 680 °C. The application of FGR thus decreased the sintering degree of the ash. 7

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Fig. 10. Morphological features and EDX elemental analysis of the P2T deposits of (a–d) FGR730 and (e–h) FGR680.

FGR730 and FGR680. When FGR was applied, the P2S deposits at FGR730 and FGR680 consisted of a cone shape deposit where sintering was incurred. However, when without FGR, the deposit consisted of both a sintered inner layer as well as an outer layer of discrete ash particles [33]. This implies that ash particles were less sticky, and the propensity of fly ash deposition decreased. This is consistent with those reported in [19,29] where these discrete outer deposits were absent at combustion temperature 750–870 °C. As denoted in the literature [33], the dimensions of the sintered layers at FGR730 and FGR680 were measured to be ca. 1.6 and 1.5 mm in height, with a centre of angle γ of ca. 25.68° and 23.10°, respectively, all of which were less than those without FGR being 2.2 mm in height and a centre angle of 40.26°. This suggests that the application of FGR would decrease the sintering and ash deposition propensity on P2S significantly. For the FGR730, the deposit consisted of sub-micron particles and agglomerates with irregular shapes, which differed from that when without FGR among which the deposit had melted and sintered to

presented in the deposit. This explains that the P2S deposit was less sintered. At FGR680 as the flow rate of FGR further increased, the deposit at FGR680 consisted of sub-micron particles with granular shapes (Fig. 10(g)), indicating a further decrease in ash sintering. In particular, a larger number of bare surfaces (Fig. 10(f)) were observed, implying that less ash particles were able to deposit on the probe surfaces. In terms of its ash chemistry, particles (e.g., particles in area 5) rich in Ca, S, and Na were identified, indicating the presence of Ca/Na sulphates. Particles (e.g., area 6) rich in Ca, Si, Al, S, and Na were also presented, indicating the presence of Ca/Na sulphates and aluminosilicates. Compared with that at FGR730, Na content in deposit was apparently decreased. This suggests that the ash particles were less sticky, contributing to a decrease in sintering and deposition propensity of it.

3.2.6. Probe 2 side Fig. 11 shows the SEM-EDX analysis of the deposits on P2S at

Fig. 11. Morphological features and EDX elemental analysis of the P2S deposits of (a–d) FGR730 and (e–h) FGR680. 8

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Fig. 12. Variation of the ash chemistry of (a) Na contents in the fly ash, bottom ash, and the P2S, and (b) the ash fusion temperatures of the fly ash at FGR730 and FGR680.

remained refractory, despite that more Na was retained in the bottom ash. For the fly ash at FGR730, its DT, ST, HT, and FT were 1243, 1247, 1249, and 1252 °C, which were lower than those when without FGR being 1296, 1303, 1305, and 1314 °C, respectively. This is attributed to presence of low melting-point mineral phases such as Na2SO4 and CaCl2 at FGR730. At FGR680 as the flow rate of FGR further increased, DT, ST, HT, and FT of the fly ash became 1252, 1270, 1275, and 1278 °C, which were slightly higher than those at FGR730. The decrease in Na content and the variation in the mineral phases as analysed from XRD, SEM-EDX, and ICP-OES analysis were responsible for such a difference in fusion temperatures.

agglomerates [33]. This suggests that ash sintering was lessened at FGR730. EDX analysis showed that particles (e.g., particle 1) rich in Ca and S were identified, indicating the presence of Ca sulphates. Particles (e.g., particles in area 2) rich in Si, Al, Ca, and S were identified, indicating the presence of Ca sulphates and aluminosilicates. Moreover, particles (e.g., particles in area 3) rich in Ca, Mg, and S were identified, indicating the presence of Ca/Mg sulphates. Compared with the EDX analysis when without FGR, a decrease in Na content was identified, indicating a decrease in Na in the deposited ash on the surface. At FGR680 as the flow rate of FGR further increased, the deposit at FGR680 consisted of discrete sub-micron particles and its agglomerates with irregular shape, indicating a further decrease in ash sintering degree. In terms of ash chemistry, particles (e.g., area 4) rich in Ca, Si, and Al were identified, indicating the presence of Ca aluminosilicates. Moreover, particles (e.g., particles in areas 5 and 6) rich in Ca and S were identified, indicating the presence of Ca sulphate. In particular, Na content in the deposit was negligible, suggesting that Na was seldom presented in the deposit. This explains why less ash particles deposited on P2S.

3.4. Effect of FGR on ash deposition and Na migration Fig. 13 illustrates the effect of FGR on ash deposition and Na migration behaviour during CFB combustion burning ZD. During ZD combustion when without FGR, the released Na species would deposit on fly ash particles or heat transfer surfaces [21,31,40], incurring severe ash deposition [41,42]. At combustion 950 °C, Na would seldom remain in the bottom ash when quartz was the bed material. A sintered coating layer rich in CaSiO3 and (Ca,Na)(Si,Al)4O8 were formed on the surfaces of bed materials [33]. Despite this, bed agglomeration and sintering had not been incurred significantly during experimentation. At FGR730 while FGR was applied, based on the above analysis, more Na was retained in the bottom ash in the form of NaAlSi3O8, resulting in the formation of coarse-grained agglomerates. This is because the upper part of the CFB furnace had decreased its temperature to ca. 730 °C, allowing for the condensation, separation and further retention of Na species. However, as the temperature of the CFB dense phase was maintained at 950 °C, those Na would react with bed material and the ash particles, consequently leading to severe ash sintering and agglomeration. At FGR680 as the furnace exit temperature decreased to 680 °C, more Na/Mg/Fe-bearing minerals including Na6(AlSiO4)6 and MgFe2O4 were presented in the bottom ash, further aggravating ash sintering and bed agglomeration. However, with quartz sand being the bed material, fusion temperatures of bottom ash were still high. Likewise, the fly ash tended to generate Na2SO4 and CaCl2 of low meltingpoints and would decrease its fusion temperatures. However, as the furnace exit temperature was as low as 680 °C, these particles would retain their solid phases and were less sticky. The application of FGR would therefore mitigate the deposition and sintering propensity of fly ash, but would aggravate the agglomeration and sintering of the bottom ash. For ash deposition on P1 at combustion temperature 950 °C and

3.3. Chemical characterisation and fusion temperature Fig. 12(a) shows the Na contents in the fly ash, bottom ash, and the P2S deposits at FGR730 and FGR680. The Na contents in bottom ash at FGR730 and FGR680 were determined as 31.7 and 35.2 mg g−1, both of which were higher than that without FGR of 27.4 mg g−1 [33]. This suggests that more Na was indeed retained in the bottom ash, consistent with the XRD and SEM-EDX analysis as above. In comparison, Na contents in the fly ash at FGR730 and FGR680 were 16.4 and 15.7 mg g−1, which were less than that without FGR of 21.8 mg g−1. This indicates that less Na was released into gas phase, in response to a retention of Na in the bottom ash. Likewise, Na content in the P2S deposit when without FGR was 31.8 mg g−1, but decreased to 21.4 mg g−1 at FGR730 and 20.2 mg g−1 at FGR680, respectively. As Na was mainly retained in the bottom ash, Na content in fly ash and the deposits thus decreased. As a result, the deposition propensity of the fly lessened, whereas agglomeration and sintering of bed particle in the bottom ash aggravated due to Na retention. To illustrate the fusion characteristics of the bottom ash and fly ash, fusion temperatures of the collected ash were determined and the results are shown in Fig. 12(b). It is found that the deformation temperature (DT), softening temperature (ST), hemispherical temperature (HT), and flowing temperature (FT) of the bottom ash at FGR730 and FGR680 were all higher than 1460 °C. This suggests that the bottom ash 9

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Fig. 13. The effect of FGR on ash deposition and Na migration characteristics during CFB combustion burning ZD.

decreasing the fusion temperatures of the fly ash. The fly ash was however less sintered due to low furnace exit temperature and the absence of Na. (3) When FGR was applied, the P1 deposits at FGR730 and FGR680 comprised of sub-micron particles and agglomerates but with lessened sintering degree. Na content in the deposits decreased, mitigating its deposition propensity. (4) The P2 deposits at FGR730 and FGR680 consisted of discrete particles rich in Ca sulphate and aluminosilicates but depleted in Na. At the low flue gas temperature, the ash particles were less sticky and thus had less propensity to deposit on the probe.

without FGR [33], Na/Ca aerosols had deposited on the P1 surface of 550 °C by means of condensation or thermophoresis [3,6,9,10], forming sintered or melted agglomerates enriched in Ca/Na sulphates. Once FGR was applied, the deposit at FGR730 became sub-micron particles and agglomerates rich in Ca/Mg/Na sulphates and aluminosilicates with irregular shape. Ash sintering and melting were lessened due to the decrease in flue gas temperature around P1. At temperature as low as 680 °C, these deposited ash particles were less sticky, consequently mitigating its deposition propensity on the surfaces within the furnace. For ash deposition on surfaces at the cyclone outlet, the P2S deposit when without FGR [33] consisted of sintered particles with layered structures enriched in CaSO4 and Na2Si2O5. With the application of FGR, these deposited particles turned into irregular discrete particles and agglomerates where Na content decreased. Less ash particles were found deposited on P2S due to the decrease in flue gas temperature. In terms of the P2T deposit without application of FGR, sintering and melting between ash particles were identified among Ca/Na sulphates were abundant. With FGR is applied, these Ca/Na sulphates became granular and not sintered or fused. The decrease in furnace exit temperature would thus decrease the ash sintering and deposition propensity during Zhundong lignite combustion in circulating fluidized bed.

Declaration of competing interest The authors declare that they have no conflict of interest. Acknowledgement This work was supported by the Youth Fund of the National Natural Science Foundation of China (Grant Number: 51706028). References

4. Conclusions

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