Investigation on regulation mechanism of red mud on the ash fusion characteristics of high ash-fusion-temperature coal

Fuel 257 (2019) 116036

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Full Length Article

Investigation on regulation mechanism of red mud on the ash fusion characteristics of high ash-fusion-temperature coal

T

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Fenghai Lia,b,c, , Bing Yub, Hongli Fana, Mingxi Guoa, Tao Wanga, Jiejie Huangc, Yitian Fangc a

School of Chemistry and Chemical Engineering, Heze University, Heze, Shandong 274015, China School of Chemistry and Chemical Engineering, Henan Polytechnic University, Jiaozuo 454003, China c State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi 030001, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Ash fusion behaviors High AFT coal Red mud Regulation mechanism

Red mud (RM) used as a flux to decrease coal ash fusion temperature (AFT) has great economic and environmental attractions. The effects of Bayer RM (BRM) and Sintering RM (SRM) on the AFTs of high silicon-aluminum coal (Jiaozuo coal (ZJ)) or high aluminum coal (Pingsuo coal (PS)) were investigated by X-ray fluorescence analysis, X-ray diffraction, and FactSage calculation. With increasing BRM/SRM mass ratio, the AFTs of JZ or PS mixture firstly decreased quickly and then slowly. When ash flow temperatures were decreased to < 1400 °C, for ZJ mixtures, the mass ratio of BRM or SRM should not be < 1.38% or 0.65%; while for PS, BRM or SRM should not be < 2.76% or 1.81%, respectively. SRM lower AFT’s of two coals compared to the addition of BRM resulted from that SRM higher CaO content (20.12 wt%) than that of BRM (8.87 wt%), and its lower A/B value (1.15) than BRM (1.53). The increasing contents of Na2O, Fe2O3, and CaO inhibited high melting-point (MP) mullite formation and prompted the formations of low MP albite, hercynite, and anorthite with increasing RM mass ratio. This caused the respective decrease in the AFTs. The mineral evolution based on FactSage calculation could explain the AFT changing trend with increasing BRM or SRM mass ratio.

1. Introduction To keep the balance between economic development and environmental protection, gasification technology has become one of the most promising alternatives for using these carbonaceous materials (e.g., coal, biomass, and petroleum coke) to produce power through Integrated Gasification Combined Cycle (IGCC), clean fuels (e.g., synthesized natural gas, hydrogen, and methanol), and chemical products [1,2]. Among existing gasification technologies (fixed-bed, fluidizedbed, and entrained-flow bed (EFB)), EFB gasification has gained great interests worldwide due to its advantages of high carbon conversion, feedstock diversification, and near-zero pollutant emissions [3]. In the EFB gasifier, the feedstock particles are entrained in a strong turbulent flow state [4], under the operating conditions of high temperature and pressure, the reaction activity difference is small. And the ash fusion and flow characteristics, which gives the indicators of its slagging and fouling [5], is becoming the key parameter for the stable and long-term operation of the EFB gasification [6]. For the smooth running of a typical EFB gasifier, its flow temperature (FT) should be < 1400 °C [7], and the ash/slag viscosity ranges over 10–25 Pa·s under operating temperature. When high ash fusion temperature (AFT, FT > 1400 °C)

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coals directly gasified in the EFB, a decrease in gasification efficiency and even passive shutdown of the gasifier may occur because of the ash/slag poor fluidity of these coals [8,9]. Moreover, the high-AFT coals account for > 50% Chinese reserves [10]. Thus, it is of importance to modify these coal AFTs. A decrease in the AFT is usually achieved by adding flux agent [11,12], coal blending or biomass [9,13,14]. The essence of these methods is to change the mineral compositions of ash/slag at high temperature. The formations of low melting-point (MP) mineral and its low-melting-point eutectics (LME) are the main reason to make AFT decrease [15]. Under a given condition, these LMEs may convert into amorphous matter, and lead to the AFTs decrease further [16]. Comparatively speaking, biomass can make coal AFT decrease to some extent, however it rarely used in practice because of its low energy density, high drying and collection cost, seasonal shortage, and low ash content [17]. Although coal blending is more efficient and economic due to raw material complementary and product structure optimization [18], it is obviously restricted by high transport cost because the coal with large difference in ash components was likely to occur in different areas; single flux is relatively expensive. Thus, it is required to find cheap complex fluxes to decrease the AFTs of high-AFT coals, and the

Corresponding author at: School of Chemistry and Chemical Engineering, Heze University, Heze, Shandong 274015, China. E-mail address: [email protected] (F. Li).

https://doi.org/10.1016/j.fuel.2019.116036 Received 11 May 2019; Received in revised form 20 July 2019; Accepted 17 August 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.

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waste materials from factory with high content of flux composition are economically attractive. Red mud (RM) is the powdery solid waste from the process of producing alumina [19], and generally contains large amounts of alkaline and small amount of radioactive elements [20,21]. In industrial process, 1–2 ton RM are generally discharged when one ton alumina are produced. The worldwide reserve of RM is over 2.7 billion tons, and the amount of RM produced in 2015 was near 160 million tons worldwide and China produced about 88 million tons in 2016 [19,22]. Stacking and burial are the most traditional method to the treatment of RM. However, RM stacking brings the degradation to environment, such as atmospheric pollution, soil alkalization, and groundwater contaminate [23]. Thus, the utilization and recycling of RM is currently a crucial issue and has become a global issue. Up to date, RM has been widely used in the fields of adsorbent [24], building or road materials [22], magnetic zeolite synthesis [25], rare earth metal extraction, wastewater treatment, soil remediation and so on [19,26,27]. In the field of coal conversion, RM was used as an oxygen-carrier for coal chemical-looping combustion [28], a catalyst for coal liquefaction, a desulfurization agent for sulfur removal [22], and a hydrogenation agent for anthracene oil [29]. RM has a high content of basic elements (e.g., Ca, Fe, and Na), which are generally used as flux agent to decrease coal AFT; and a previously published paper has been manifested RM can decrease coal AFT effectively [30]. However, the published one only described the effects of Sintering-RM (SRM) on the high-silicaaluminum coal (HSAC, Al2O3 + SiO2 > 80 wt%) AFTs, and explored the AFT variation from the mineral transformation based on the XRD patterns, the regulation mechanism of RM on AFT has not been clarified yet and is needed to investigate further. According to alumina production technology, RM can be divided into Bayer RM (BRM), SRM, and Combined bayer-sintering (CRM), and the composition of SRM and CRM is similar. Thus, the objective of this paper were to investigate the effects of BRM or SRM on two typical high AFT-coals [HSAC and highaluminum coals (HAC, Al2O3 in ash content in the range of 38–51 wt%] AFTs, and to explore their variation mechanism from the perspective of thermodynamics. This might provide the references to regulate the ash fusion characteristics during high-AFT coal gasification.

Table 1 Proximate and ultimate analyses of coal samples. Sample

JZ

PS

Proximate analysis on an air-dried basis (wt. %) Moisture 3.27 Volatile matter 8.45 Ash 11.16 Fixed carbon 77.12

3.16 19.96 12.38 64.50

Ultimate analysis on dry ash free basis (wt. %) Carbon 89.13 Hydrogen 4.83 a 3.12 Oxygen Nitrogen 2.07 Sulfurb 0.85

82.24 5.69 9.58 1.12 1.37

a b

Calculated by difference. Total sulfur.

the particle size < 0.198 mm were added into ZJ or PS laboratory ashes at a certain mass ratios (0, 5, 10, 15, 20, and 25%), and then were manually mixed until reached uniformity. The mixed ash samples were put into the AFA and heated to temperature, then kept at that temperature for 5 min, respectively. After that, the samples were put into ice water as soon as possible to prevent crystal segregation and phase change. Finally, the quenched samples were transferred into a vacuum oven at 105 °C for 24 h, and then stored in a vacuum drying oven before analyses. 2.3. The measurements of AFTs and ash compositions The AFTs of samples and their mixtures under reducing atmosphere (1:1, H2/CO2, volume ratio) were conducted on the AFA strictly following the Chinese standard test method (GB/219-2008). To ensure the AFT accuracy, the average value of the three testing values is adopted as experimental result and shown in Table 2. An X-ray fluorescence spectrometer (XRF-1800, Shimadzu, Japan) with an Rh target X-ray tube (50 kV, 40 mA) was used to analyze their ash compositions, and the results are also presented in Table 2. HSAC and HAC (reserves are abundant, and mostly distributed in Northern China [32].) are two typical of high-AFT coal. From the perspective of ash compositions (Table 2), JZ is HSAC (Al2O3 + SiO2 = 80.19 wt%), and PS belongs to HAC (Al2O3: 42.18 wt%). Thus, JZ and PS were selected for the experiment. The AFTs of BRM and SRM are generally low (FT both lower than 1150 °C) because of their high basic oxide contents. The high contents of Al2O3 (32.58 wt%) and Na2O (13.36 wt%) in BRM resulted from the refined process of alumina (Bauxite was dissolved in sodium hydroxide (NaOH) solution, and aluminum hydroxide was precipitated from sodium aluminates solution under the conditions of adding seed crystals and constant stirring)[19]; the comparatively high contents of CaO (20.12 wt%) and Na2O (7.26 wt%) in SRM derived from the usage of sodium carbonate and lime during the sintering of alumina.

2. Experimental 2.1. Raw materials and their characteristics The two air-dried coals, i.e., Jiaozuo coal (from Henan Province, China) and Pingsuo coal (from Shanxi Province, China), were provided by Institute of Coal Chemistry (ICC), Chinese Academy of Sciences (CAS), and referred to as JZ and PS, respectively. The BRM and SRM samples came from Shanxi Aluminum Plant (Hejin city, Shanxi Province, China). The proximate analyses and ultimate analyses conducted following Chinese standards (GB/T212-2008 and GB/T313912015) are presented in Table 1, respectively. The contents of moisture (JZ: 3.27 wt%; PS: 3.16 wt%) and total sulfur (JZ: 0.85 wt%; PS: 1.37%) were relatively low, while the fixed carbon contents (JZ:64.50 wt%; PS:77.12 wt%) were relatively high. The volatile matter of PS was higher than that of JZ. The two samples were crushed to < 0.198 mm, dried at 105 °C for 18 h in nitrogen atmosphere, and kept in a drying oven before usage.

2.4. Ash analytical methods The mineralogy was determined by X-ray diffraction (XRD) using a D/max 2400X power diffractmeter (Rigaku co., Tokyo, Japan), the operating conditions were 40 kV, 0.15408 nm CuKa and 100 mA. The samples were scanned from 10° to 70° at 5° 2-Theta/min at the step size of 0.01° 2-Theta. The software package MDI Jade 6.5 was used to identify the mineralogy and quantification was determined by a semiquantitative normalized reference intensity ratio (RIR) method [33–36]. Its precision varies with phase diffraction intensity (generally is ± 25% for weakly diffracting phases and ± 10% for strongly diffracting phases), the amorphous matter content was obtained by subtracting total crystalline phase content from the ash bulk chemical

2.2. Ash preparation The laboratory ashes of the JZ or PS samples were prepared following Chinese standard procedures (GB/T1574-2001), and it has been described in detail in the previous paper [30]. The mixed samples at different temperatures were prepared in a modified ALHR-2 AFT analyzer (AFA, Fig. 1, its design has been described in detail [31]) with the highest temperature of 1500 °C (Ao-lian Co. Ltd., China) (to keep the same condition as the AFT measurements). The two RM samples with 2

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Fig. 1. Modified schematic diagram of ash fusion point determination meter. Legend: 1. oxygen gas cylinder; 2. carbon dioxide cylinder; 3. gas valve; 4. mass flow meter; 5. mixed chamber; 6. computer; 7. round bearing, 8. corundum tube; 9. corundum tube; 10. aluminum boat; 11. ash cone; 12. thermoelectric couple.

3. Results and discussions

Table 2 AFTs and compositions of coals ash and RMs. Samples

JZ

PS

BRM

SRM

AFTa(oC) DT ST HT FT

> 1500 > 1500 > 1500 > 1500

> 1500 > 1500 > 1500 > 1500

1044 1192 1122 1142

1019 1062 1084 1125

47.69 42.18 3.04 4.36 0.31 0.24 0.85 0.21 0.95 0.17 10.00

22.66 32.58 15.35 8.87 0.64 0.31 0.64 13.36 5.23 0.36 1.53

26.24 23.83 10.58 20.12 2.13 2.93 3.42 7.26 2.25 1.24 1.15

Ash composition (wt./%) SiO2 46.83 Al2O3 34.36 Fe2O3 7.92 CaO 3.59 MgO 1.99 K2O 0.83 SO3 1.12 Na2O 1.21 TiO2 1.17 P2O5 0.98 A/Bb 4.62

3.1. The AFT variation of two coals with increasing RM mass ratio The coal AFT variation with different RM addition is presented in Fig. 2. Addition of BRM and SRM reduces the AFTs of JZ and PS. For AFTs of JZ mixture, with increasing BRM/SRM mass ratio, the AFTs of the JZ mixture firstly decrease quickly and then more slowly. When the AFTs reach the requirements of EFB (FT < 1400 °C), BRM mass ratio is required to be > 11.05% (accounts for about 1.38 wt% for JZ mixtures); for SRM, it is only required > 5.50 wt% (about 0.65%) (Fig. 2a, Table 2). This difference results from that SRM contain high content of basic oxide with low ionic potential (e.g., Fe2+, Ca2+, and Na+) (Table 2), it tends to prevent polymer formation, and make the AFT decrease. The decreasing effects of two RMs on PS mixture AFT aren’t obvious than that of ZJ mixtures, because the aluminum in HAC may in the form of boehmite (AlOOH), which transform into the high melting point (MP) alumina (MP:2054 °C) [39,40]. To meet the requirement of EFB, > 18.25 wt% BRM mass ratio are needed (2.76% BRM for JZ mixtures) from the perspective of AFT; for SRM, > 12.75 wt% mass ratio was required (1.81% SRM) (Fig. 2 a, Table 2). Thus, it might reasonable to select RM as flux during EFB gasificaion of high-AFT coal. Generally speaking, the acidic oxides (A, the total content of SiO2, Al2O3 and TiO2) with high ionic potential tend to form the networks of silicate and alumino-silicate, and lead to the AFT increase; the basic oxides (B, the total content of Na2O, K2O, CaO, MgO, and Fe2O3) promote the transformation of the stable network from tectosilicates, inosilicates, cyclosilicates, and sorosilicates to nesosilicates, which cause a decrease in the AFT [31,41]. The variations in the A/B value (A/ B = (SiO2 + Al2O3 + TiO2)/(Fe2O3 + CaO + MgO + Na2O + K2O), which account for most ash compositions, can reflect the AFT changing trend to some extent. The A/B values of four raw material ashes are also listed in Table 2, and the mixed JZ and PS ash A/B values with different mass ratio RMs (based on the Table 2) are presented in Fig. 3. For ZJ and PS, the addition of SRM made their mixture A/B value decrease significantly more than that of BRM. This might explain the decreasing AFT of SRM was more effective than that of BRM. And for the addition of BRM or SRM at the same ratio, the AFT of ZJ mixture decreased significantly more than that of PS mixture resulted from the difference in their A/B values (JZ: 4.62; PS: 10.00).

AFT: ash fusion temperature; DT:; DT – deformation temperature, ST – softening temperature, HT – hemispheric temperature, FT – flow temperature. b A/B = (SiO2 + Al2O3 + TiO2)/(Fe2O3 + CaO + MgO + Na2O + K2O). a

composition, following the procedures described by Ward et al. [37].

2.5. Thermodynamic calculation The Equilib module of FactSage software (version 7.1) are used to assess the theoretical mineralogy of ash samples in a reducing atmosphere (H2/CO2, 1:1, volume ratio) at atmospheric pressure (0.02 MPa) with increasing temperature. The Equilib module can simulate the composition of chemical species when given compounds react under idea chemical equilibrium condition according to the theory of Gibbs free energy minimization [38]. The oxides SiO2, Al2O3, CaO, Fe2O3, Na2O, and K2O (the total content of SiO2, Al2O3, CaO, and Fe2O3 accounts for > 85 wt% of ash composition, although the content of Na2O and K2O is low, it may transform into feldspar through the reaction with high contents of Al2O3 and SiO2) were selected as input data in the equilibrium module with the database of FToxid and FACTPS. The calculations were carried out from 900 °C to 1500 °C with the interval of 20 °C.

3.2. The variation in mineral composition. 3.2.1. Mineral evolution in original coal ashes with increasing temperature. The ash fusion characteristics are mainly dependent on the formation of mineral kinds and their contents with the increasing 3

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Fig. 2. The coal AFT variation with the RM addition. (a) JZ; (b) PS. Note: 1500 °C is referred to the temperature is more than 1500 °C

900 °C, the formation of cordierite (ferroan) ((Mg, Fe)2Al4Si5O18) and a decrease in hematite content resulted from the following reactions: hematite(Fe2O3) → wustite (FeO), wustite (FeO) + magnesia (MgO) + metakaolin (Al2Si4O7) → cordierite (ferroan) ((Mg, Fe)2Al4Si5O18) + quartz(SiO2) [42]. Mullite (Al6Si2O13) and anorthite (CaAl2Si2O8) appeared at 1000 °C due to the following reactions: metakaolin (Al2Si4O7) → mullite (Al6Si2O13); anhydrite (CaSO4) → calcium oxide (CaO) + sulfur dioxide (SO3); calcium oxide (CaO) + mullite (Al6Si2O13) → anorthite (CaAl2Si2O8). As for PS ashes, the appearance of alumina at 815 °C resulted from the dehydration of

temperature, which can be measured by XRD. To investigate the AFT decreasing mechanism of high-AFT coal with RM addition, it is needed to explore the mineral evolution in the process of the ash being heated. The ash samples of ZJ and PS at different temperatures (815, 900, 1000, 1100, 1200, and 1300 °C) were prepared in the AFA under the reducing atmosphere (CO2/H2 1:1, volume ratio). Fig. 4 shows the XRD patterns of ZJ and PS ash samples at different temperatures. As shown in Fig. 4a, the minerals of JZ ashes at 815 °C and 900 °C are mostly in the forms of quartz (SiO2), anhydrite (CaSO4), hematite (Fe2O3) and metakaolin (Al2Si4O7, derived from kaolinite); At

Fig. 3. A/B variations with the RM addition. (a) PS; (b) JZ. A/B = (SiO2 + Al2O3 + TiO2)/(Fe2O3 + CaO + MgO + Na2O +K2O). 4

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Fig. 4. XRD patterns of coal ashes at different temperatures. (a) JZ; (b) PS. 1. quartz (SiO2); 2. anhydrite (CaSO4); 3. hematite (Fe2O3); 4. metakaolin (Al2Si4O7); 5. cordierite (ferroan) ((Mg, Fe)2Al4Si5O18); 6. anorthite (CaAl2Si2O8); 7. mullite (Al6Si2O13); 8. alumina (Al2O3).

mixture decrease with increasing RM (BRM or SRM) content. With increasing RM mass ratio, the contents of Na2O, Fe2O3, and CaO in ZJ mixed ashes gradually increased (Table 3). The main reactions and their Gibbs free energy related to the three oxides are presented in Table 5. The priority order of the oxides being with alumina and silica-oxygen is Na2O > CaO > FeO. The low MP albite (NaAlSi3O8) formed firstly due to the reaction of Na2O + 6SiO2 + Al2O3 → 2NaAlSi3O8 (ΔG: –397.51 kJ/mol, 1100 °C). Because Na2O content was low (< 5.0% in the samples), and then anorthite generated through the reaction of CaO + Al2O3 + 2SiO2 → CaAl2Si2O8 (ΔG: –133.60 kJ/mol, 1100 °C) [2,45]. After that cordierite (ferroan)((Mg, Fe)2Al4Si5O18) emerged. The ΔG of the formation of mullite (–18.06 kJ/mol, 1100 °C), almost the same as the hercynite formation (–16.32 kJ/mol, 1100 °C), was higher than that of albite, anorthite, and cordierite (ferroan), respectively (Table 5). Thus, the increasing contents of Na2O, Fe2O3, and CaO inhibited the formation of high MP mullite, and resulted in a corresponding decrease in the AFT [46]. Because the ionic potential of Ca2+ (20 nm−1) is lower than that of Fe2+ (reduction of Fe2O3 under gasification condition) (26.32 nm−1), Ca2+ has higher potential to change bridging Si-O-Si

boehmite (400–500 °C), which were present in HAC [25,43]. As the temperature increased, the mullite formed due to the reaction of alumina and quartz. At 1300 °C, the dominant phase in JZ and PS ashes was high MP mullite (1850 °C), which resulted in their high AFTs. 3.2.2. Variations in mineral evolution with increasing RM content. During the process of ash fusion, the mineral interact and fuse into liquid. Thus, it is reasonable to assess ash fusion characteristics according to the mineral composition of ash sample at a certain temperature [31,44]. The ash compositions of ZJ or PS mixed with RM (BRM and SRM) calculated according to Table 2 are shown in Table 3. To investigate the AFT variation mechanism of mixed ZJ or PS with the additions of different RMs, the temperature of 1100 °C was selected to prepare the mixed ash samples because at this temperature mineral contents in the samples were moderate and could better reflect AFT variation [16]. The XRD patterns of ZJ mixed ash samples at 1100 °C with different mass ratio BRM or SRM are shown in Fig. 5 and the mineral content calculated according to RIR method are presented in Table 4. A decrease in high MP mullite content and the increases in low MP albite, hercynite and amorphous matter resulted in the AFTs of ZJ Table 3 Ash compositions of ZJ or PS mixed with RMs. Ash samples

ZJ + 5%BRM ZJ + 10%BRM ZJ + 15%BRM ZJ + 20%BRM ZJ + 25%BRM ZJ + 5%SRM ZJ + 10%SRM ZJ + 15%SRM ZJ + 20%SRM ZJ + 25%SRM PS + 5%BRM PS + 10%BRM PS + 15%BRM PS + 20%BRM PS + 25%BRM PS + 5%SRM PS + 10%SRM PS + 15%SRM PS + 20%SRM PS + 25%SRM

Ash composition (wt./%) SiO2

Al2O3

Fe2O3

CaO

MgO

K2O

SO3

Na2O

TiO2

P2O5

45.61 44.41 43.21 42.00 40.79 45.80 44.77 43.74 42.71 41.68 46.44 45.19 43.93 42.68 41.43 46.62 45.55 44.47 43.40 42.33

34.26 34.18 34.09 34.00 33.92 33.83 33.31 32.78 32.25 31.73 41.70 41.22 40.74 40.26 39.78 41.26 40.34 39.43 38.51 37.59

8.29 8.66 9.03 9.40 9.78 8.05 8.19 8.32 8.45 8.59 3.66 4.27 4.89 5.50 6.12 3.42 3.80 4.17 4.55 4.93

3.85 4.12 4.38 4.65 4.91 4.42 5.24 6.07 6.90 7.72 4.58 4.81 5.04 5.26 5.49 5.15 5.94 6.72 7.51 8.30

1.91 1.85 1.79 1.72 1.65 2.00 2.01 2.01 2.02 2.03 0.33 0.34 0.36 0.38 0.39 0.40 0.49 0.58 0.68 0.77

0.80 0.78 0.75 0.73 0.70 0.93 1.04 1.14 1.25 1.36 0.24 0.25 0.25 0.25 0.26 0.37 0.51 0.64 0.78 0.91

1.10 1.07 1.05 1.02 1.00 1.23 1.35 1.47 1.58 1.69 0.84 0.83 0.82 0.81 0.80 0.98 1.10 1.24 1.36 1.49

1.82 2.43 3.03 3.64 4.25 1.51 1.81 2.12 2.42 2.72 0.87 1.52 2.18 2.84 3.49 0.56 0.91 1.27 1.62 1.98

1.37 1.58 1.78 1.98 2.18 1.22 1.28 1.33 1.39 1.44 1.16 1.38 1.59 1.80 2.02 1.02 1.08 1.15 1.21 1.27

0.99 0.92 0.89 0.86 0.82 0.99 1.00 1.02 1.03 1.04 0.18 0.19 0.20 0.21 0.22 0.22 0.28 0.33 0.38 0.43

5

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Fig. 5. XRD patterns of JZ mixed ashes with RM addition. (a) BRM, (b) SRM. 1. quartz (SiO2); 2. cordierite (ferroan) ((Mg, Fe)2Al4Si5O18); 3. anorthite (CaAl2Si2O8); 4. mullite (Al6Si2O13); 5. albite (NaAlSi3O8); 6. hercynite (FeAl2O4).

proportion ranged from 15% to 25%, the anorthite content increases and mullite content decreases resulted in a slow decrease in the AFTs of the PS mixture.

bonds into non-bridging terminal oxides through Lewis acid-base reactions with the anionic SiO4 network than that of Fe2+, and thus has more inhibitory effects on the formation of the polymers [38,47]. Thus, the decreasing effect of CaO on the AFT was more obvious than that of FeO [48]. The increasing Fe2O3 content in the mixed ashes (0.54%: 8.59%–8.05%) by the addition of SRM is higher than that of BRM (0.49%: 9.78%–8.29%), and the increasing CaO content in the mixed ashes (3.3%:7.72%–4.42%) by SRM addition is higher than that of BRM (1.06%, 4.91%–3.85%). Thus, the decreasing effects of SRM on the AFT were greater than that of BRM. Moreover, the MP of anorthite derived from the reaction of calcium oxide and mullite was relatively high. This resulted in the AFTs decrease firstly quickly and then slowly with the additions of the two RMs. Fig. 6 shows the XRD patterns of PS mixed ash samples with different mass ratio BRM or SRM, and their mineral contents based on RIR method are shown in Table 6. With increasing BRM or SRM mass ratio, the formations of low MP albite, hercynite and anorthite and increases in their contents led to the AFT decrease. With the increasing BRM mass ratio, the content of Fe2O3 and Na2O in the mixed PS ashes increased gradually (Table 3). Fe2O3 changed into ferrous oxide (FeO) under the atmosphere of 50% H2 and 50% CO2, and then converted into low MP hercynite through the reaction of ferrous oxide (FeO) + alumina (Al2O3) → hercynite (FeAl2O4). The sodium oxide reacted with alumina and quartz and resulted in low MP albite formation [49]. These caused the AFT of PS mixture to decrease gradually with BRM addition. As for the increasing SRM mass ratio (0–15%), the decreasing contents of high MP mullite and alumina caused their AFT decrease. And when the SRM

3.2.3. Simulation calculation of mineral evolution in mixed ash using FactSage software To further investigate the mineral evolution mechanism during the heating of the ash, the variations of mineral and relative liquid phase compositions of ashes at high temperature were simulated by the FactSage software. Although some deviations exist, the thermodynamic calculation is still used to explain the AFT change under a given condition [4,50]. Fig. 7 shows the variations in mineral and relative liquid phase compositions of ZJ and mixed ashes with different RM mass ratios. As shown in Fig. 7a, quartz, mullite, feldspar (e.g., anorthite), cordierite and a small amount of ferrous sulfide (FeS) appeared in the JZ ashes. With increasing temperature increased, the quartz disappeared first, and then ferrous sulfide, feldspar disappeared at about 1250 °C, and cordierite at about 1300 °C. The FeS diffraction peak wasn’t detected in the XRD patterns (Figs. 4 and 5) might be attributed to the FactSage calculation based on the ideal equilibrium state and the formation of FeS-FeO low MP eutectic (MP: 940 °C) in the real experimental process (which melted before 1100 °C). As can be seen clearly that when BRM and SRM was at the same mass ratio, the high MP mullite contents in the mixed ZJ-BRM ash were higher than that in the ZJ –SRM ash; while the contents of cordierite and feldspar with relative low MP in the mixed ZJ-BRM ash were lower than that in the mixed ZJSRM ash. This resulted in the SRM addition decreased the ZJ mixed AFT

Table 4 Mineralogical composition of ZJ mixture ash samples by RIR at 1100 °C. Ash samples

ZJ ZJ + 5%BRM ZJ + 10%BRM ZJ + 15%BRM ZJ + 20%BRM ZJ + 25%BRM ZJ + 5%SRM ZJ + 10%SRM ZJ + 15%SRM ZJ + 20%SRM ZJ + 25%SRM

Mineral (wt.%) quarz

cordierite, (ferroan)

anorthite

mullite

albite

hercynite

Amorphous matter

10.32 9.56 7.92 6.53 5.23 3.43 9.24 6.71 5.95 4.86 3.25

30.12 28.27 25.89 21.15 15.28 13.30 27.83 24.85 20.32 14.56 12.92

23.42 25.16 26.93 27.12 30.12 31.29 26.34 28.36 29.84 33.92 35.08

16.87 13.29 10.74 8.18 7.12 6.97 11.93 7.21 5.02 3.57 3.34

– 2.73 3.26 4.29 5.36 6.12 2.06 2.86 3.24 4.56 5.95

– – 1.96 3.07 4.08 4.86 – 1.83 2.29 3.85 4.27

19.23 20.91 23.30 29.64 32.91 34.03 22.60 28.18 33.34 34.68 35.19

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Table 5 Main reactions during ash fusion process and their Gibbs free energy variation. ΔG (kJ/mol)

Reaction

(1) (2) (3) (4) (5) (6)

CaO + Al2O3 + 2SiO2 → CaAl2Si2O8 (anorthite) Al2O3 + Na2O + 6SiO2 → 2NaAlSi3O8 (albite) 3Al2O3 + 2SiO2 → Al6Si2O13 (mullite) 2Al2O3 + 2FeO + 5SiO2 → Fe2Al4Si5O18 (magnesium-cordierite) FeO + Al2O3 → FeAl2O4 (hercynite) 2Al2O3 + 2MgO + 5SiO2 → Mg2Al4Si5O18 (ferro-cordierite)

1000 °C

1100 °C

1200 °C

−131.64 −395.71 −15.40 −43.76 −16.27 −115.06

−133.60 −397.51 −18.06 −46.07 −16.32 −117.44

−135.62 −396.89 −20.70 −48.37 −16.37 −119.87

Fig. 6. XRD patterns of PS mixed ashes with RM addition. (a) BRM, (b) SRM. 1. quartz (SiO2); 2. anorthite (CaAl2Si2O8); 3. mullite (Al6Si2O13); 4. alumina (Al2O3); 5. cordierite, (ferroan) ((Mg, Fe)2Al4Si5O18); 6. albite (NaAlSi3O8); 7. hercynite (FeAl2O4). Table 6 Mineralogical composition of PS mixture ash samples by RIR at 1100 °C. Ash samples

PS PS + 5%BRM PS + 10%BRM PS + 15%BRM PS + 20%BRM PS + 25%BRM PS + 5%SRM PS + 10%SRM PS + 15%SRM PS + 20%SRM PS + 25%SRM

Mineral (wt.%) quarzt

anorthite

mullite

alumina

cordierite (ferroan)

albite

hercynite

Amorphous matter

12.35 10.65 9.02 4.25 3.43 2.23 10.92 9.23 3.72 3.12 2.35

22.37 24.31 25.29 27.31 28.83 30.38 25.07 26.29 30.32 34.08 36.42

38.47 37.19 34.85 30.12 28.39 23.56 34.12 28.85 25.12 22.27 18.16

18.27 15.92 12.37 7.56 – – 13.19 9.42 4.32 – –

– 4.85 4.23 3.69 2.93 2.47 4.14 3.99 3.08 2.75 2.16

– – 3.17 4.36 5.17 6.72 – 2.07 3.92 4.68 5.41

– – – 2.72 4.12 5.13 – – 2.68 3.96 4.07

8.54 11.93 17.47 19.99 27.43 29.51 12.56 20.15 26.84 29.14 32.43

mullite in the PS-SRM ash decreased more than that in PS-BRM ash, and the solid phase content at 1500 °C decreased correspondingly (Fig. 8b and c). Moreover, the temperature that feldspar disappeared increased gradually with increasing feldspar content, which led to an increase in solid phase content at a certain temperature (Fig. 8). The higher feldspar (anorthite) content generated in the PS-SRM ash than that in the PS-BRM ash was due to the relative high CaO content in SRM compared with that in BRM (SRM:20.12%; BRM: 8.87%). This led to the FT of PSSRM mixture being higher than that of PS-BRM when RM at high mass ratio (25%). With the increasing mass ratio of two RMs, the solid phase content at 1500 °C decreased correspondingly (Fig. 8 b and c). All these might explain AFT variation in PS mixture with the additions of two RMs.

more obviously than that of BRM. Moreover, for the addition of two RMs, the content of solid phase at 1500 °C (95%ZJ + 5%BRM, 85% JZ + 15%BRM) 95%ZJ + 5%SRM, 85%JZ + 15%SRM) decreased, respectively. These might explain the AFT variation trend of ZJ mixture with the additions of different RMs. The mineral and relative liquid phase composition variation of PS ash and their mixture with increasing temperature are shown in Fig. 8. The main minerals in the PS ash were quartz, mullite, feldspar (e.g., anorthite), cordierite (Fig. 8a). Compared with Fig. 7a, ferrous sulfide didn’t form because of its low sulfur trioxide (SO3) content (0.31 wt%). Ferrous sulfide and hercynite appeared in the PS mixed ash samples with BRM or SRM addition because the two RM contained relatively high SO3 and Fe2O3 content, which led to decreases in their mixture AFTs. When the two RMs were at the same mass ratio, the content of 7

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Fig. 7. Mineral compositions of ZJ and its mixed ashes with RM addition. (a) ZJ; (b) ZJ + BRM; (c) PS + SRM.

8

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Fig. 8. Mineral compositions of PS and its mixed ashes with RM addition. (a) PS; (b) PS + BRM; (c) PS + SRM.

9

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4. Conclusions [18]

With increasing BRM/SRM mass ratio, the AFTs of JZ or PS mixture firstly decrease quickly and then decrease more slowly. To meet requirement of EFB, for ZJ mixture, the mass ratio of BRM or SRM should not be < 1.38% or 0.65%; while for PS mixture, BRM or SRM should not be < 2.76% or 1.81%, respectively. The AFTs of JZ (HSAC) mixture decrease more obviously than that of and PS (HAC) when RM at the same ratio. Both the CaO content in SRM (20.12%) was higher than that in BRM (8.87%) and its A/B value (1.15) was lower than that of BRM (1.53) resulted in the AFT decreasing effects of SRM on coals were more obvious than that of BRM. The increasing contents of Na2O, Fe2O3, and CaO inhibited high melting-point (MP) mullite formation and prompted the formations of low MP albite, hercynite and anorthite (relative high MP) with increasing RM mass ratio. This led to the AFTs of mixtures decreasing. Simulation calculation of mineral evolution according to FactSage software could explain the coal AFTs variations with increasing BRM or SRM mass ratio.

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Acknowledgements [29]

This research was financially supported by the Natural Science Foundations of China (21875059), Natural Science Foundations of Shandong Province, China (ZR2018MB037, ZR2017BB063), and the Strategic Priority Research Program of the Chinese Academy of Sciences, China (XDA07050103).

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