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Application of biomass leachate in regulating the fusibility of coal ash
Fuel 268 (2020) 117338
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Full Length Article
Application of biomass leachate in regulating the fusibility of coal ash a
a,⁎
a
a
a
Jiajian Wang , Xia Liu , Qinghua Guo , Juntao Wei , Xueli Chen , Guangsuo Yu a b
a,b,⁎
T
Institute of Clean Coal Technology, East China University of Science and Technology, Shanghai 200237, PR China State Key Laboratory of High-efficiency Coal Utilization and Green Chemical Engineering, Ningxia University, Yinchuan 750021, PR China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Leaching Biomass Additive Fusibility Gasification
Biomass leachate was rich in water-soluble alkali and alkali earth metals (AAEM) species, which can effectively reduce the ash fusion temperatures (AFTs) of coal ash. In this study, a high silicon-aluminium coal (SA) with high AFTs and typical agricultural biomasses were selected to investigate the application of biomass leachate in regulating the fusibility of coal ash. The results showed that with the addition of biomass leachate, the AFTs of high silicon-aluminium coal with high-melting temperatures could be effectively reduced by greater than 70 °C, and the effect of biomass leachate on reducing the AFTs of coal ash is stronger than that of biomass. The ash chemical composition of the biomass alone could not fully reflect the ability of biomass leachate in regulating the fusibility of coal ash, the content of water-soluble AAEM and the evaporation of alkali metal elements in ashing process also had a significant influence. By adding biomass or biomass leachate into the SA, low-melting leucite and gehlenite, which would react with anorthite to generate low-melting eutectic, formed, resulting in the remarkable decrease of AFTs of SA. Exothermic peak temperature (EPT) and flow temperature (FT) showed an approximate linear relationship as FT ≈ EPT + 65 °C. The prediction results of FactSage were basically correspond to the experimental results.
⁎
Corresponding authors at: Institute of Clean Coal Technology, East China University of Science and Technology, Shanghai 200237, PR China. E-mail addresses: [email protected] (X. Liu), [email protected] (G. Yu).
https://doi.org/10.1016/j.fuel.2020.117338 Received 30 October 2019; Received in revised form 22 January 2020; Accepted 5 February 2020 0016-2361/ © 2020 Elsevier Ltd. All rights reserved.
Fuel 268 (2020) 117338
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1. Introduction
consumed in the preparation of coal water slurry, therefore, for a long time, utilizing industrial effluent in preparing coal water slurry to save water resources is an important study realm. It is a good method not only to deal with industrial effluent, but also to improve the fusibility of coal ash to some extent due to the water-soluble AAEM species contained in biomass leachate [24,25]. Hence, it is of great significance in environmental protection to evaluate application of biomass leachate in regulating the fusibility of coal ash. In this study, a typical high silicon-aluminium coal with high ash fusion temperatures (AFTs) and two typical biomasses (rice straw and cotton stalk) were selected as raw materials to explore the application of biomass leachate in regulating the fusibility of coal ash. Experimental apparatuses such as ash fusion temperature analyzer, the high-temperature stage microscope system, ICP-OES, X-ray diffractometer (XRD), and thermogravimetric analyzer and differential thermal analyzer (TG-DTA) were applied to explore the ash fusibility of mixed samples and the influence mechanism of biomass leachate, providing theoretical support for applying biomass leachate in preparing coal water slurry of gasification.
Biomass is expected to be applied in combustion and power generation due to the energy crisis and environmental pollution problems. Nowadays, researchers have carried out a series of development and utilization of traditional agricultural waste such as straw and bagasse, and new biomass fuel such as sludge and algae [1–3]. However, high contents of Cl and K in biomass lead to a low ash fusion temperature of biomass [4]. Therefore, it is easy for biomass ash to deposit in the combustion process, which will affect the long-term stable operation of biomass boiler and affect economic benefits [5]. Meanwhile, many coals hardly meet the operating needs of gasifiers and boilers with the depletion of high-quality coals. Hence, methods such as additive [6–8], co-combustion [9,10], co-gasifying [11–13] and leaching [14–16] are widely used in the combustion and gasification of biomass and coal. According to previous studies, the AFTs of coal ash with high contents of Al and Si were usually high [17]. Meanwhile, the ratio of silicate to aluminium (SiO2/Al2O3) in coal ash also had a significant effect on AFTs of coal. Yan et al. [18] proposed that the AFTs of coal decreased with the increase of SiO2/Al2O3 ratio when the SiO2/Al2O3 ratio was lower than 2, while it increased with the increase of SiO2/ Al2O3 ratio when the SiO2/Al2O3 ratio was higher than 2. It was generally believed that alkali and alkali earth metals (AAEM) could increase the fusibility of coal ash. Adding a certain proportion of AAEM into coal ash could effectively reduce the AFTs of coal ash. Biomass was rich in AAEM, and blending coal with a certain proportion of biomass could not only increase the content of AAEM but also dilute the content of Si and Al in coal ash, resulting in a decrease of AFTs of coal. Ma et al. [19] blended biomass with high silica-alumina coal, studied the fusibility of the blended coal and pointed out that the peanut shell, bean stalk and corn cob could effectively increase the fusibility of high silicaalumina coal. Biomass rich in Ca and K was a good fluxing agent for high silica-alumina coal. Xiong et al. [20] pointed out when mixed with a certain proportion of biomass (pine sawdust or corn stalk), vanadium trioxide disappeared and low melting-temperature leucite formed, and the ash fusion temperatures of the mixture of biomass and petroleum coke decreased. Certainly, biomass did not simply increase or decrease the ash fusion temperatures of coal. Li et al. [21] proposed that biomass mass ratio affected the types and quantities of minerals generated. When appropriate chestnut shell was added into Shenmu coal, generation of anorthite and gehlenite at high temperature could reduce the AFTs of coal to some extent, while the generation of high meltingtemperature kalsilite, lime and magnesia increased the AFTs when about 30–50% chestnut shell was added into Shenmu coal. Although many researchers have studied the influence of biomass additive on the fusibility of coal, the utilization and the role as a fluxing agent of biomass leachate, which was derived from leaching treatment of biomass and ample in water-soluble (including alkali chlorides, sulfates, carbonates and alkali earth chlorides) inorganic species, have not ever been explored [22]. Biomass leaching treatment removes soluble AAEM in biomass so that improves the fusibility of biomass ash, and avoids the ash deposition problem during combustion [23]. As a kind of industrial effluent, the discharge of biomass leachate is a problem for biomass-fired industry. It is well known that plenty of water is
2. Materials and methods 2.1. Characteristics of raw material A typical high silicon-aluminium coal (SA), rice straw (RS) and cotton stalk (CS) were used as raw materials, which were ground and sieved to the particle size of < 200 μm. The AFTs of high silicon-aluminium coal is generally high, so the entrained-flow gasifier utilizing this kind of coal needs to be operated at high temperature to meet the requirements of slag discharge [26]. Usually, high silicon-aluminium coal is pre-treated before gasification. Meanwhile, rice straw and cotton stalk are common agricultural waste and are often used for co-gasification and co-combustion with coal [6,27]. Therefore, the selected materials are available and representative. The results of proximate analyses (Chinese Standard GB/T212-2008 for coal and Chinese Standard GB/T28731-2012 for biomass) and ultimate analyses (Chinese Standard GB/T476-2001) of three samples were shown in Table 1. As shown in Table 1, the fixed carbon yield in SA was much higher than that of RS and CS, while volatile yield of SA was relatively low. The ash yield of CS was low, while the ash content of SA was close to that of RS. Ash chemical compositions of raw materials were determined by Xray fluorescence (XRF, Thermo Fisher Scientific Company, Waltham, MA) spectrometer and the XRF analysis data of the samples was given in Table 2. As shown in Table 2, SA was a typical high silicon-aluminium coal. The content of SiO2 in SA ash was up to 66.98 wt% and the content of Al2O3 was around 17.85 wt%. In addition, SA has a relatively high iron content (Fe2O3: 6.69 wt%) while contents of other elements were low. RS (K2O: 9.58 wt%; Al2O3: 0.31 wt%) and CS (K2O: 23.12 wt %; Al2O3: 0.92 wt%) were rich in potassium and poor in aluminum. The ash composition of RS had the characteristics of high silicon (SiO2: 68.80 wt%), which was similar to that of SA. On the contrary, CS has a lower silicon content (SiO2: 6.16 wt%) but a higher calcium content than RS.
Table 1 Proximate and ultimate analyses of raw samples. Sample
SA RS CS
Proximate analysis (wt/%, ad)
Ultimate analysis (wt/%, ad)
M
A
V
FC
C
H
Oa
N
S
4.09 5.15 5.98
9.04 9.81 3.00
32.60 70.45 71.20
54.27 14.59 19.82
72.25 35.23 42.01
4.58 6.32 6.49
6.56 42.31 39.65
1.11 1.18 0.85
2.37 0.00 2.02
ad: air dry base. a By difference. 2
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Afterwards, the sample was heated up to 1400 °C at a heating rate of 10 °C/min. The fusion process of the ash sample during the heating process was observed and recorded.
Table 2 Ash chemical composition of raw materials. Samples
SA RS CS
Ash composition/% SiO2
Al2O3
K2O
Na2O
CaO
Fe2O3
MgO
Others
66.98 68.80 6.16
17.85 0.31 0.92
1.21 9.58 23.12
1.15 3.51 11.73
2.42 5.14 19.40
6.69 1.39 4.48
1.40 4.45 10.74
2.3 6.82 23.45
2.3.3. XRD analysis of quenched ash samples Quenching is an effective method to preserve the mineral composition and slag structure of coal ash at high temperature [30]. The major procedures were shown below. The ash sample was placed in a ceramic crucible and heated up to over 800 °C at a heating rate of 50 °C/min. Then the ceramic crucible was heated up to the specified temperatures at a heating rate of 10 °C/min. After that, the ceramic crucible was kept at a constant temperature for 30 min. Afterwards, the ceramic crucible was taken out and cooled quickly in ice water to prevent the crystal phase transformation and maintain high-temperature mineral composition. The obtained quenched slags were ground to less than 200 μm and used for XRD analysis. XRD patterns were recorded by a PANalytical X'pert Powder X-ray powder diffractometer with Cu Kα radiation. The operating conditions were 40 kV and 40 mA. The samples were scanned from 2θ = 10-80° with 0.01° step size.
2.2. Sample preparation 2.2.1. Biomass leaching treatment Before leaching, SA, RS and CS were dried in the oven at 105 °C for 10 h. Then pulverized RS and CS with a diameter of less than 150 μm were mixed with ultrapure water in a beaker at a mass ratio of 1:40, respectively. Next, the biomass-water mixtures were stirred at 70 °C for 24 h [14]. Then, the mixtures were filtered to obtain two types of leachates. 2.2.2. Coal blending In order to explore the effect of biomass leachate on the fusibility of coal ash, in addition to blending coal with biomass leachate, coal blending with biomass directly was used as control groups in the study. Coal blending with biomass leachate and coal blending with biomass directly were prepared based on the mass ratio of coal and biomass of 1:1 according to previous studies by Fermoso et al. [28] and Li et al. [21]. The main element composition of two biomass leachates were measured via inductively coupled plasma with an optical emission spectroscopy as detector (Agilent 725, ICP-OES). Then two types of leachates were mixed with SA, which had the same mass as biomass before leaching, respectively. After being fully stirred at 90 °C until the formation of a thickened mass, the new mixtures were dried at 105 °C for 10 h and grounded into the particle size of < 200 μm. The dried mixtures of RS leachate and SA was named as RSL, while the dried mixtures of CS leachate and SA was named as CSL. RS and CS were mixed with SA under equal mass respectively, and the obtained mixtures were named as RSB and CSB. The elemental composition of the five ash samples (SA, RSL, CSL, RSB and CSB) was measured via ICP-OES after digestion. The digestion steps have been described in previous studies [29]. Aqua regia and hydrofluoric acid were used as the digestion solutions.
2.3.4. TG-DTA analysis Thermal gravimetric analyzer (NETZSCH STA 2500 Regulus, Germany) included differential thermal analyzer (DTA) and thermal gravimetric analyzer (TGA). It could study the thermal and mass changes of samples exactly under the same test conditions so that it was used to investigate the mineral decomposition and transformation during ash fusion process [31]. About 10 mg of sample was placed in a ceramic crucible. The operation procedures were the same as that on the high-temperature stage to ensure that the reactions and products were unchanged. Thermal and mass changes of ash samples during heating process were recorded and analyzed. Repeated TGA experiments were conducted at least three times for each sample so as to ensure the accuracy of TGA data. 2.3.5. Thermodynamic equilibrium calculations Thermodynamic calculation software FactSage was widely used in high temperature phase equilibrium calculation of coal ash. FactSage 7.1 with database of FToxid and FactPS was used to predict mineral compositions and proportions of the liquid and solid phases under inert atmosphere. The main compositions SiO2, Al2O3, K2O, CaO, Na2O, MgO and Fe2O3 were selected for calculation in the equilib module. The calculations were carried out from 900 to 1500 °C with an interval of 20 °C.
2.3. Experimental methods
3. Results and discussion
2.3.1. Measurement of AFTs According to Chinese Standard GB/T212-2008, five kinds of ash samples were prepared in a muffle furnace at 815 °C. Ash cones were prepared according to Chinese Standard GB/T219-2008 and the AFTs were determined by the 5E-AF4000 ash fusion temperature determination meter (Kaiyuan Company, Changsha, China) in a weak reductive atmosphere.
3.1. Fusibility of ash samples Elemental composition of five ash samples and main element composition of biomass leachate measured by ICP-OES were shown in Table 3 and Table 4, respectively. As shown in Table 3, compared with SA, the content of Si, Al and Fe in the other ash samples was decreased, while the content of K, Na, Ca and Mg was increased. The content of K,
2.3.2. High-temperature morphology observation The high-temperature stage microscope system was usually used to in-situ observe the transformation in the apparent morphology and structure of samples during ash fusion process. The system was mainly composed of high temperature heater (Linkam, UK) and microscope (DM4500P Leica, Germany). The operation procedures were as follows. Firstly, the sample was dispersed on a sapphire substrate (99.9% purity of Al2O3, 7.0 mm diameter and 0.2 mm thickness), which was placed in the reaction zone. Under the continuous flow of N2 (100 mL/min, 99.999% purity), the slag sample was heated up to 800 °C at a heating rate of 50 °C/min, then it was heated up to 800–1000 °C at a heating rate of 20 °C/min.
Table 3 Elemental composition of five ash samples measured by ICP-OES. Samples
SA RSL RSB CSL CSB
3
Elemental composition/% Si
Al
Ca
Fe
Na
K
Mg
25.00 20.00 23.00 20.00 19.00
5.40 3.80 2.30 4.60 3.70
2.30 2.30 2.30 2.70 4.10
4.70 3.70 4.00 4.00 4.30
0.82 3.00 1.50 1.20 1.20
1.30 5.20 4.90 6.50 7.10
0.48 1.10 0.85 0.83 1.20
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Containing less Al and more water-soluble AAEM especially Na may be the reason why AFTs of RSL was lower than CSL. AFT was a numerical representation for morphology transformation of coal ash [33], while it could not picture the continuous fusion process well. In order to verify the improvement effect of biomass leachate on the AFTs of SA, the morphology change of ash samples during the heating process was in-situ observed through the high-temperature stage microscope system. The high-temperature melting process of SA, RSL and CSL was shown in Fig. 1. Three ash samples exhibited different fusibility. The SA sample showed obvious shrinkage at around 1250 °C, and reached the apparent molten state at around 1350 °C. RSL exhibited obvious shrinkage at 1100 °C and reached the apparent molten state when the temperature reached 1250 °C. The RSB sample showed obvious shrinkage at around 1150 °C, and reached the apparent molten state when the temperature reached 1300 °C. Compared the shrinkage temperature and the apparent molten state temperature of the three samples with the AFTs listed in Table 5, it was obviously found that the shrinkage temperature and the apparent molten state temperature corresponded to DT and FT, respectively. The addition of biomass leachate or biomass changed the chemical composition of coal ash, resulted in the formation of minerals with a low melting temperature or low melting-temperature eutectic in the melting process, and consequently reduced the AFTs of coal ash. The fusion process of ash samples was validated and verified through observation of in-situ morphology in high-temperature stage. However, our understanding of the impact of biomass leachate on the fusibility of coal ash was still apparent. XRD analysis of quenched slags at different temperatures and TG-DTA analysis of five ash samples were needed in studying the influence mechanism of biomass and its leachate on reducing the AFTs of coal ash.
Table 4 Main element composition of biomass leachate measured by ICP-OES. Samples
RS CS
Elemental composition/mg·g−1 Si
Al
Ca
Fe
Na
K
Mg
0.06 0.02
– –
0.04 0.08
– –
0.80 0.45
1.80 1.40
0.14 0.07
Na, Mg in RSL was higher than that in RSB, while the content of Si was lower than that in RSB, indicating that RS contained lots of water-soluble AAEM such as K, Na, Mg, which was demonstrated in Table 4. Compared with blending RS with SA directly, the addition of RS leachate in SA could change the ash chemical composition of SA to a greater extent. According to Table 1, Tables 2 and 3, the content of K and Na in CS was higher than in RS, the content of K2O in CS was 23.12%, and the content of Na2O was 11.73%, while the content of K in CSL and CSB was not high, and the content of Na was similar to that in RSL. It was speculated that the low content of ash in CS had a lesser impact on the chemical composition of SA coal ash under the same raw material ratio. On the other side, as shown in Table 4, lower content of water-soluble AAEM and the evaporation of alkali metal elements in ashing process also may contributed to the relatively less content of K and Na in CSL and CSB. The AFTs of five samples were shown in Table 5, including four characteristics temperatures, i.e., deformation temperature (DT), softening temperature (ST), hemispherical temperature (HT) and flow temperature (FT). As shown in Table 5, the range of AFTs of raw SA was 1230 ± 80 °C. Combined with the ash composition of raw materials shown in Table 2, SA was rich in Si and Al which resulted in high AFTs, and RS and CS had a high content of AAEM, which could effectively reduce AFTs of coal [32]. Therefore, adding biomass directly into SA could dilute the content of Si and Al and thus reduce AFTs of SA. Compared with raw coal ash, the AFTs of SA could be reduced by blending with biomass or biomass leachate. Consequently, it was obvious that the AFTs of SA was significantly reduced by the addition of biomass as shown in Table 5. Nevertheless, RS leachate had the most obvious influence on the ash fusibility of SA among four additives. Compared with SA, AFTs of RSL decreased by more than 70 °C, especially the FT decreased by nearly 140 °C. The effect of CS leachate on the fusibility of SA was relatively weaker than RS leachate but stronger than raw biomass. Biomass leachate contained only water-soluble substances and there were almost no Si, Al or Fe existed in the biomass leachate. Therefore, adding biomass leachate into coal could reduce Si and Al in SA to a lower content, which was verified in Table 3. Since RS and CS almost contained no Al, the total amount of Al in blending coal was nearly not affected. However, adding biomass leachate and directly adding biomass brought different amounts of ash into SA, so the dilution in the content of Al was different. Therefore, the content of Al in RSL and CSL was higher than that in RSB and CSB, respectively. This also applied to Si in CS. Hence it could be inferred that adding biomass leachate into coal had a more obvious effect on reducing AFTs than directly adding biomass to coal, which was consistent with the results shown in Table 5.
3.2. Mineral behaviors during ash fusion process The XRD patterns of the five ash samples at different temperatures (815 °C, 1000 °C, 1200 °C, 1400 °C) under inert atmosphere were illustrated in Fig. 2. As shown in the Fig. 2, the diffraction intensity of minerals in samples decreased as temperature rose, which reflected the mineral content in samples decreased gradually as temperature rose. As shown in Table 5, DT (Deformation Temperature) of five sample ashes all ranged from 1000 °C to 1200 °C. As a consequence, all samples were partly melted at the temperature of 1200 °C. DCL and DCB had lower deformation temperature. Therefore, the typical hill between 20° and 30° in the XRDs of DCL and DCB caused by amorphous silicates was more obvious. As shown in Fig. 2(a), the SA ash was mainly composed of quartz (SiO2), calcite (CaCO3), anhydrite (CaSO4) and hematite (Fe2O3). Hematite was transformed from ferrous disulfide (FeS2) in the ashing process. As the temperature raised up to 1000 °C, calcite and anhydrite decomposed into CaO and combined with SiO2, Al2O3 to form anorthite (CaAl2Si2O8) [34]. The formation of anorthite was accompanied by the decrease of quartz, which proved that quartz was involved in the reaction of anorthite formation. At 1200 °C, the diffraction peak of anhydrite disappeared because anhydrite decomposed between 1000 °C and 1200 °C [19]. At 1400 °C, there was still a certain amount of quartz and anorthite in the sample, which played an important role in high AFTs of SA. Compared with pure SA coal ash, RSL (as shown in Fig. 2(b)) mainly contained minerals such as quartz, calcite, anhydrite, hematite and gehlenite (Ca2Al2SiO7). As the temperature raised up to 1000 °C, anorthite was formed, and the diffraction intensity of gehlenite increased slightly, while the diffraction intensity of hematite and anhydrite changed slightly. At 1200 °C, there were no mineral diffraction peaks in the samples. It was inferred that anhydrite reacted with gehlenite and eutectic with a low melting-temperature generated between 1000 °C and 1200 °C [35]. By comparing Fig. 2(b) and (c), it was found that the types and
Table 5 Ash fusion temperatures of samples. Sample
SA RSL RSB CSL CSB
Ash fusion temperature (Weak reducing atmosphere)/℃ DT
ST
HT
FT
1149 1079 1052 1155 1154
1218 1118 1191 1194 1210
1236 1148 1211 1213 1232
1311 1170 1237 1230 1259
4
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Fig. 1. Micrograph of ash samples at different temperatures: (a) SA; (b) RSL; (c) CSL.
generate eutectic with a low melting temperature. Therefore, only a small amount of quartz, gehlenite and leucite were left at 1200 °C. There were no diffraction peak of minerals in CSB at 1400 °C. CSL and CSB had similar mineral types at the same temperatures, and there was only a little distinction in diffraction peaks of each mineral, so the differences between AFTs of CSL and CSB were small.
contents of minerals in RSL and RSB were basically the same at the same temperature. However, diffraction peaks of cristobalite only existed in RSB between 815 °C and 1000 °C, which decomposed between 1000 and 1200 °C, and resulted in that quart, leucite and gehlenite existed at 1200 °C in RSB while there were no mineral presenting in RSL. Therefore, the AFTs of RSL was lower than RSB. As shown in Fig. 2(d), the CSL ash at 815 °C was mainly composed of quartz, calcite, hematite, gehlenite and muscovite. As the temperature raised up to 1000 °C, muscovite decomposed into leucite (KAlSi2O6) while anhydrite reacted with gehlenite to generate eutectic with a low melting temperature. At 1200 °C, there were only a little quartz gehlenite and leucite in the sample. There were no mineral diffraction peaks in the sample at 1400 °C. As shown in Fig. 2(e), the sample contained quartz, calcite, hematite, gehlenite and muscovite at 815 °C. At 1000 °C, muscovite decomposed into leucite and Al2O3, while calcite decomposed into CaO. Al2O3 and CaO reacted with a part of quartz to form anorthite. As the temperature rose to 1200 °C, anhydrite combined with gehlenite to
3.3. TG-DTA analysis of ash samples The mass and heat analysis of five ash samples was performed and the results were shown in Fig. 3. As shown in Fig. 3, it could be found that there are continuous exothermic and endothermic peaks in the five ash samples above 1000 °C. Between 1000 °C and 1200 °C, the DTA curve of SA exhibited an exothermic peak. Combined with the mineral transformation in Fig. 2(a), the weight loss between 800 °C and 1100 °C in Fig. 3(a) are assigned to the decomposition of calcite and part of anhydrite, and the products combined with quartz and alumina to form anorthite and 5
1
1200 1000
1
40
50
60
70
80
815 10
20
30
40
2ș/°
2400 1600 800 0 1400
1
40
50
80
1000
1600
1
70
80
800
1 6 1
6
0 1400
6 7
6 5 74
1
1 2 7 1 4 1
8
1
1
1 1
1200
1 1000
1
m pe
ra
1
60
2400
re
1
3200
1
815
Te
30
70
4000
1200
1
1
815 20
60
(d)
tu
1
Diffraction intensity(CPS)
4000 3200
10
50
2ș/°
(c)
19 1 3 5 74 1 1 9 1 8 32 74 1 1 1 1
1000
1
10
20
30
40
2ș/°
50
60
70
Diffraction intensity(CPS)
30
1
re
20
1
1200
1
1
tu
10
1
1
73 2 7 4 1 1 1
Te
815
1 73 5 7 4 1
1
Te
1
1 1
1
ra
41 1 1
1
1
80
m pe
3 2
1
41 1
0 1400
1
Te
35
800
1
2ș/°
(e)
4000 3200 2400 1600 800
1
1
1000
1 815
10
20
30
40
50
60
70
re
1200
1
1
tu
1
ra
6 5 7 41 1 1
2 7 41 1 1
1400
1
80
pe
8
1
0
16 7
m
6 1
6 1
Te
1
Diffraction intensity(CPS)
1
41
0 1400
re
5
5
1600
tu
1
1000
ra
1
m pe
1
2400
re
2000
3200
tu
3000
1
4000
(b)
ra
4000
m pe
Diffraction intensity(CPS)
5000
(a)
Diffraction intensity(CPS)
Fuel 268 (2020) 117338
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2ș/°
Fig. 2. XRD patterns of five ash samples at different temperatures: (a) SA; (b) RSL; (c) RSB; (d) CSL; (e) CSB. [Legend: (1) quartz (SiO2), (2) calcite (CaCO3), (3) anhydrite (CaSO4), (4) hematite (Fe2O3), (5) anorthite (CaAl2Si2O8), (6) leucite (KAlSi2O6), (7) gehlenite (Ca2Al2SiO7), (8) muscovite (KAl2Si3AlO10(OH)2), (9) cristobalite (SiO2).]
the decomposition of muscovite, the formation of leucite and the combination of gehlenite and anorthite. Above 1200 °C, the mass of SA coal ash showed little change, while the other four kinds of ash samples still had mass loss. Cl and alkali metals such as K were easily volatilized at high temperature [36]. Hence it was inferred that the mass loss was due to the evaporation of alkali metals contained in RS, CS and their corresponding leachate at high temperatures. The mass loss of coal ash blended with biomass leachate was similar to that of coal ash blended with biomass, which proved that there were a lot of alkali metals in biomass leachate close to those in biomass. There were exothermic peaks in the DTA curves of RSL, RS and CSL between 1300 °C and 1400 °C. Operating temperature was below 1400 °C due to the limited specifications of experimental instrument. But it was easy to infer from the trend of DTA curves that there would be exothermic peaks above 1400 °C in Fig. 3(a) and (e). Comparing
release heat. For RSL and RSB, there were continuous exothermic and endothermic peaks on the DTA curves. Combined with the mineral transformation in Fig. 2(b), it was inferred that the two exothermic peaks were assigned to the combination of gehlenite and anorthite generating low melting-temperature eutectic, and the decomposition of anhydrite. The former was exothermic and the latter was endothermic. According to the mineral transformation results shown in Fig. 2(c), the decomposition of calcite and part of anhydrite, and the formation of anorthite and low melting-temperature eutectic all occurred in the heating process of RSB. The reactions and mineral transformation during heating process of RSB were similar to those during heating process of RSL. Therefore, the DTA curve of RSB was similar to RSL. The decomposition of cristobalite occurred only in RSB may prolong continuous exothermic process, which explained the increase of the width of exothermic peak. For CSL and CSB, each of them had a wide exothermic peak between 900 °C and 1200 °C, which may be the results of 6
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TG(%) DTA(uV/mg)
100
(b)
1
TG(%) DTA(uV/mg)
100 0
99
2
99
1
98
0
-1 -2
97
-3 -4
96
-1
96
TG(%)
98
DTA(uV/mg)
TG(%)
97
-2
95 94
-3
93
DTA(uV/mg)
(a)
-4
92 -5 91
EPT=1238 0
200
400
600
800
1000
1200
1400
-6 1600
90 0
200
400
600
T
(c)
(d)
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TG(%)
-2
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DTA(uV/mg)
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-3
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-4
93
-5
92
-5
92 -6
EPT=1168
91 90 0
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-6
EPT=1173
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(e)
TG(%) DTA(uV/mg)
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TG(%)
96 -3
95 94
-4
93
DTA(uV/mg)
TG(%)
800
T
TG(%) DTA(uV/mg)
100
-5
EPT=1104
DTA(uV/mg)
95
-5
92 -6
EPT=1203
91 90 0
200
400
600
800
1000
1200
1400
-7 1600
T Fig. 3. TG-DTA analysis of five ash samples: (a) SA; (b) RSL; (c) RSB; (d) CSL; (e) CSB.
and FT of five samples showed an approximate linear relationship as FT ≈ EPT + 65 °C. Therefore, it was assumed that most of minerals in coal ash melted at EPT. Around this temperature, most of substances in the ash sample were converted to liquid phase, which was conductive to the flow of slag. Combined with XRD analysis of quenched slags at different temperatures and TG-DTA analysis of five ash samples, the influence mechanism of biomass and its leachate on reducing the AFTs of coal ash were basically clear. However, the composition of low melting-temperature minerals eutectic materials generated in the fusion process
AFTs of these five ash samples in Table 2, it could be found that the AFTs of RSL, RSB and CSL were relatively low. It was speculated that AAEM in biomass and biomass leachate promoted the formation and heat release of low-melting eutectic between 1300 °C and 1400 °C, which led to the reduction of the melting temperatures of these three ash samples. By comparison with Table 2 and Fig. 3, it was obvious that the exothermic peak temperature (EPT) of the five ash samples was in the order of SA > CSB > CSL > RS > RSL, and the FT of the five ash samples was also in the order of SA > CSB > CSL > RS > RSL. EPT 7
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(b)
100
80
Slag
High-Albite
60
Propotion
100
80
Slag
Ortho-enstatite
60
Proto-enstatite
Propotion
(a)
Cordierite 40
Anorthite
20
Andradite Diopside
20
SiO2
Leucite
Hematite
0 900
40
1000
1100
Hematite
0 1200
1300
1400
1500
900
1000
1100
Temperature
(c)
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1300
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(d)
100
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80
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Slag Diopside
60
Propotion
Propotion
60
40
20
Sanidine
1000
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20
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0 900
Sanidine 40
Andradite
Hematite 1100
1200
1300
Hematite
0 1400
1500
900
1000
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Temperature
1200
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Temperature
(e) 100 80
Slag
Propotion
60
Diopside 40
Leucite 20
Andradite
Hematite
0 900
1000
1100
1200
1300
1400
1500
Temperature Fig. 4. Factsage prediction of solid fraction of ash samples: (a) SA; (b) RSL; (c) RSB; (d) CSL; (e) CSB.
results were shown in Fig. 4. The main solids of SA at 1000 °C partly agreed with the XRD analysis results were shown in Fig. 2(a), SiO2, anorthite and hematite were the main minerals at 1000 °C. Minerals such as proto-enstatite and cordierite were not found in XRD analysis, which may be the main result for their small proportion. The existence of anhydrite was not predicted by FactSage, but its diffraction peak existed at 1000 °C in Fig. 2(a). Thus, it was inferred that anhydrite was an intermediate of
were still unclear. Therefore, thermodynamic equilibrium calculations were carried out.
3.4. FactSage predictions of equilibrium condition The solid proportions at high temperature (greater than900 °C) based on the element composition in Table 3 in pure inert atmosphere were predicted by FactSage 7.1 using FToxid and FactPS databases. The 8
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the reaction and the isothermal time was too short in quenching procedure. On the other hand, the short reaction time also could account for the situation why SiO2 existed until 1400 °C and Fe2O3 existed until 1200 °C in Fig. 2(a) while the calculating results shown in Fig. 4(a) was the opposite. In general, the thermodynamic equilibrium calculation results of SA at high temperature were basically consistent with the experimental results. The calculation results from FactSage showed that after adding biomass or biomass leachate, cordierite in SA disappeared, the content of quartz was greatly reduced, and low-melting andradite, diopside and luecite were formed, which led to the increase of the liquid phase content and the decrease of the AFTs of SA. However, simulation results of the other four ash samples did not fit the experimental results well. FactSage calculation was based on the oxides contents in original ash and used the ideal mixing of species to form mineral phases. But during actual experiments, the ideal equilibrium state predicted by FactSage could not be achieved due to the limitation of reaction time and reactivity of reactants. In addition, FactSage was more widely used in subjects such as coal chemistry. Therefore, predictions of ash rich in alkali metals at high temperatures may be not much accurate. The simulation results could reflect the liquid–solid ratio and the melting characteristic of the ash samples to some extent. As shown in Fig. 4, simulation results calculated by FactSage reflected that the AFTs of SA were reduced effectively with the addition of biomass and biomass leachate.
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
4. Conclusions
[12]
The biomass leachate could effectively reduce the AFTs of high silicon-aluminium coal with high-melting temperature. The positive effect of biomass leachate on reducing the AFTs of coal ash was stronger than that of biomass and RS leachate showed the more obvious positive effect. SA ash was mainly composed of quartz, calcite, anhydrite and hematite. By adding biomass or biomass leachate into the SA, the contents of Si and Al in SA were relatively reduced and the contents of AAEM species such as K and Na in SA were relatively increased. Therefore, low-melting leucite and gehlenite which would react with anorthite to generate low-melting eutectic, formed, thus resulting the remarkable decrease of AFTs of SA. The prediction results of FactSage reflected that the AFTs of SA were reduced effectively with the addition of biomass and biomass leachate, and were basically correspond to the experimental results.
[13] [14]
[15]
[16]
[17]
[18] [19]
[20]
CRediT authorship contribution statement
[21]
Jiajian Wang: Data curation, Formal analysis, Writing - original draft. Xia Liu: Project administration, Writing - review & editing. Qinghua Guo: Conceptualization, Resources. Juntao Wei: Investigation, Visualization. Xueli Chen: Software, Validation. Guangsuo Yu: Funding acquisition, Methodology, Supervision.
[22] [23]
[24]
Declaration of Competing Interest
[25]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[26]
[27]
Acknowledgement This work has been partially supported by National Natural Science Foundation of China (21878093).
[28]
[29]
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Fuel journal homepage: www.elsevier.com/locate/fuel
Full Length Article
Application of biomass leachate in regulating the fusibility of coal ash a
a,⁎
a
a
a
Jiajian Wang , Xia Liu , Qinghua Guo , Juntao Wei , Xueli Chen , Guangsuo Yu a b
a,b,⁎
T
Institute of Clean Coal Technology, East China University of Science and Technology, Shanghai 200237, PR China State Key Laboratory of High-efficiency Coal Utilization and Green Chemical Engineering, Ningxia University, Yinchuan 750021, PR China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Leaching Biomass Additive Fusibility Gasification
Biomass leachate was rich in water-soluble alkali and alkali earth metals (AAEM) species, which can effectively reduce the ash fusion temperatures (AFTs) of coal ash. In this study, a high silicon-aluminium coal (SA) with high AFTs and typical agricultural biomasses were selected to investigate the application of biomass leachate in regulating the fusibility of coal ash. The results showed that with the addition of biomass leachate, the AFTs of high silicon-aluminium coal with high-melting temperatures could be effectively reduced by greater than 70 °C, and the effect of biomass leachate on reducing the AFTs of coal ash is stronger than that of biomass. The ash chemical composition of the biomass alone could not fully reflect the ability of biomass leachate in regulating the fusibility of coal ash, the content of water-soluble AAEM and the evaporation of alkali metal elements in ashing process also had a significant influence. By adding biomass or biomass leachate into the SA, low-melting leucite and gehlenite, which would react with anorthite to generate low-melting eutectic, formed, resulting in the remarkable decrease of AFTs of SA. Exothermic peak temperature (EPT) and flow temperature (FT) showed an approximate linear relationship as FT ≈ EPT + 65 °C. The prediction results of FactSage were basically correspond to the experimental results.
⁎
Corresponding authors at: Institute of Clean Coal Technology, East China University of Science and Technology, Shanghai 200237, PR China. E-mail addresses: [email protected] (X. Liu), [email protected] (G. Yu).
https://doi.org/10.1016/j.fuel.2020.117338 Received 30 October 2019; Received in revised form 22 January 2020; Accepted 5 February 2020 0016-2361/ © 2020 Elsevier Ltd. All rights reserved.
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1. Introduction
consumed in the preparation of coal water slurry, therefore, for a long time, utilizing industrial effluent in preparing coal water slurry to save water resources is an important study realm. It is a good method not only to deal with industrial effluent, but also to improve the fusibility of coal ash to some extent due to the water-soluble AAEM species contained in biomass leachate [24,25]. Hence, it is of great significance in environmental protection to evaluate application of biomass leachate in regulating the fusibility of coal ash. In this study, a typical high silicon-aluminium coal with high ash fusion temperatures (AFTs) and two typical biomasses (rice straw and cotton stalk) were selected as raw materials to explore the application of biomass leachate in regulating the fusibility of coal ash. Experimental apparatuses such as ash fusion temperature analyzer, the high-temperature stage microscope system, ICP-OES, X-ray diffractometer (XRD), and thermogravimetric analyzer and differential thermal analyzer (TG-DTA) were applied to explore the ash fusibility of mixed samples and the influence mechanism of biomass leachate, providing theoretical support for applying biomass leachate in preparing coal water slurry of gasification.
Biomass is expected to be applied in combustion and power generation due to the energy crisis and environmental pollution problems. Nowadays, researchers have carried out a series of development and utilization of traditional agricultural waste such as straw and bagasse, and new biomass fuel such as sludge and algae [1–3]. However, high contents of Cl and K in biomass lead to a low ash fusion temperature of biomass [4]. Therefore, it is easy for biomass ash to deposit in the combustion process, which will affect the long-term stable operation of biomass boiler and affect economic benefits [5]. Meanwhile, many coals hardly meet the operating needs of gasifiers and boilers with the depletion of high-quality coals. Hence, methods such as additive [6–8], co-combustion [9,10], co-gasifying [11–13] and leaching [14–16] are widely used in the combustion and gasification of biomass and coal. According to previous studies, the AFTs of coal ash with high contents of Al and Si were usually high [17]. Meanwhile, the ratio of silicate to aluminium (SiO2/Al2O3) in coal ash also had a significant effect on AFTs of coal. Yan et al. [18] proposed that the AFTs of coal decreased with the increase of SiO2/Al2O3 ratio when the SiO2/Al2O3 ratio was lower than 2, while it increased with the increase of SiO2/ Al2O3 ratio when the SiO2/Al2O3 ratio was higher than 2. It was generally believed that alkali and alkali earth metals (AAEM) could increase the fusibility of coal ash. Adding a certain proportion of AAEM into coal ash could effectively reduce the AFTs of coal ash. Biomass was rich in AAEM, and blending coal with a certain proportion of biomass could not only increase the content of AAEM but also dilute the content of Si and Al in coal ash, resulting in a decrease of AFTs of coal. Ma et al. [19] blended biomass with high silica-alumina coal, studied the fusibility of the blended coal and pointed out that the peanut shell, bean stalk and corn cob could effectively increase the fusibility of high silicaalumina coal. Biomass rich in Ca and K was a good fluxing agent for high silica-alumina coal. Xiong et al. [20] pointed out when mixed with a certain proportion of biomass (pine sawdust or corn stalk), vanadium trioxide disappeared and low melting-temperature leucite formed, and the ash fusion temperatures of the mixture of biomass and petroleum coke decreased. Certainly, biomass did not simply increase or decrease the ash fusion temperatures of coal. Li et al. [21] proposed that biomass mass ratio affected the types and quantities of minerals generated. When appropriate chestnut shell was added into Shenmu coal, generation of anorthite and gehlenite at high temperature could reduce the AFTs of coal to some extent, while the generation of high meltingtemperature kalsilite, lime and magnesia increased the AFTs when about 30–50% chestnut shell was added into Shenmu coal. Although many researchers have studied the influence of biomass additive on the fusibility of coal, the utilization and the role as a fluxing agent of biomass leachate, which was derived from leaching treatment of biomass and ample in water-soluble (including alkali chlorides, sulfates, carbonates and alkali earth chlorides) inorganic species, have not ever been explored [22]. Biomass leaching treatment removes soluble AAEM in biomass so that improves the fusibility of biomass ash, and avoids the ash deposition problem during combustion [23]. As a kind of industrial effluent, the discharge of biomass leachate is a problem for biomass-fired industry. It is well known that plenty of water is
2. Materials and methods 2.1. Characteristics of raw material A typical high silicon-aluminium coal (SA), rice straw (RS) and cotton stalk (CS) were used as raw materials, which were ground and sieved to the particle size of < 200 μm. The AFTs of high silicon-aluminium coal is generally high, so the entrained-flow gasifier utilizing this kind of coal needs to be operated at high temperature to meet the requirements of slag discharge [26]. Usually, high silicon-aluminium coal is pre-treated before gasification. Meanwhile, rice straw and cotton stalk are common agricultural waste and are often used for co-gasification and co-combustion with coal [6,27]. Therefore, the selected materials are available and representative. The results of proximate analyses (Chinese Standard GB/T212-2008 for coal and Chinese Standard GB/T28731-2012 for biomass) and ultimate analyses (Chinese Standard GB/T476-2001) of three samples were shown in Table 1. As shown in Table 1, the fixed carbon yield in SA was much higher than that of RS and CS, while volatile yield of SA was relatively low. The ash yield of CS was low, while the ash content of SA was close to that of RS. Ash chemical compositions of raw materials were determined by Xray fluorescence (XRF, Thermo Fisher Scientific Company, Waltham, MA) spectrometer and the XRF analysis data of the samples was given in Table 2. As shown in Table 2, SA was a typical high silicon-aluminium coal. The content of SiO2 in SA ash was up to 66.98 wt% and the content of Al2O3 was around 17.85 wt%. In addition, SA has a relatively high iron content (Fe2O3: 6.69 wt%) while contents of other elements were low. RS (K2O: 9.58 wt%; Al2O3: 0.31 wt%) and CS (K2O: 23.12 wt %; Al2O3: 0.92 wt%) were rich in potassium and poor in aluminum. The ash composition of RS had the characteristics of high silicon (SiO2: 68.80 wt%), which was similar to that of SA. On the contrary, CS has a lower silicon content (SiO2: 6.16 wt%) but a higher calcium content than RS.
Table 1 Proximate and ultimate analyses of raw samples. Sample
SA RS CS
Proximate analysis (wt/%, ad)
Ultimate analysis (wt/%, ad)
M
A
V
FC
C
H
Oa
N
S
4.09 5.15 5.98
9.04 9.81 3.00
32.60 70.45 71.20
54.27 14.59 19.82
72.25 35.23 42.01
4.58 6.32 6.49
6.56 42.31 39.65
1.11 1.18 0.85
2.37 0.00 2.02
ad: air dry base. a By difference. 2
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Afterwards, the sample was heated up to 1400 °C at a heating rate of 10 °C/min. The fusion process of the ash sample during the heating process was observed and recorded.
Table 2 Ash chemical composition of raw materials. Samples
SA RS CS
Ash composition/% SiO2
Al2O3
K2O
Na2O
CaO
Fe2O3
MgO
Others
66.98 68.80 6.16
17.85 0.31 0.92
1.21 9.58 23.12
1.15 3.51 11.73
2.42 5.14 19.40
6.69 1.39 4.48
1.40 4.45 10.74
2.3 6.82 23.45
2.3.3. XRD analysis of quenched ash samples Quenching is an effective method to preserve the mineral composition and slag structure of coal ash at high temperature [30]. The major procedures were shown below. The ash sample was placed in a ceramic crucible and heated up to over 800 °C at a heating rate of 50 °C/min. Then the ceramic crucible was heated up to the specified temperatures at a heating rate of 10 °C/min. After that, the ceramic crucible was kept at a constant temperature for 30 min. Afterwards, the ceramic crucible was taken out and cooled quickly in ice water to prevent the crystal phase transformation and maintain high-temperature mineral composition. The obtained quenched slags were ground to less than 200 μm and used for XRD analysis. XRD patterns were recorded by a PANalytical X'pert Powder X-ray powder diffractometer with Cu Kα radiation. The operating conditions were 40 kV and 40 mA. The samples were scanned from 2θ = 10-80° with 0.01° step size.
2.2. Sample preparation 2.2.1. Biomass leaching treatment Before leaching, SA, RS and CS were dried in the oven at 105 °C for 10 h. Then pulverized RS and CS with a diameter of less than 150 μm were mixed with ultrapure water in a beaker at a mass ratio of 1:40, respectively. Next, the biomass-water mixtures were stirred at 70 °C for 24 h [14]. Then, the mixtures were filtered to obtain two types of leachates. 2.2.2. Coal blending In order to explore the effect of biomass leachate on the fusibility of coal ash, in addition to blending coal with biomass leachate, coal blending with biomass directly was used as control groups in the study. Coal blending with biomass leachate and coal blending with biomass directly were prepared based on the mass ratio of coal and biomass of 1:1 according to previous studies by Fermoso et al. [28] and Li et al. [21]. The main element composition of two biomass leachates were measured via inductively coupled plasma with an optical emission spectroscopy as detector (Agilent 725, ICP-OES). Then two types of leachates were mixed with SA, which had the same mass as biomass before leaching, respectively. After being fully stirred at 90 °C until the formation of a thickened mass, the new mixtures were dried at 105 °C for 10 h and grounded into the particle size of < 200 μm. The dried mixtures of RS leachate and SA was named as RSL, while the dried mixtures of CS leachate and SA was named as CSL. RS and CS were mixed with SA under equal mass respectively, and the obtained mixtures were named as RSB and CSB. The elemental composition of the five ash samples (SA, RSL, CSL, RSB and CSB) was measured via ICP-OES after digestion. The digestion steps have been described in previous studies [29]. Aqua regia and hydrofluoric acid were used as the digestion solutions.
2.3.4. TG-DTA analysis Thermal gravimetric analyzer (NETZSCH STA 2500 Regulus, Germany) included differential thermal analyzer (DTA) and thermal gravimetric analyzer (TGA). It could study the thermal and mass changes of samples exactly under the same test conditions so that it was used to investigate the mineral decomposition and transformation during ash fusion process [31]. About 10 mg of sample was placed in a ceramic crucible. The operation procedures were the same as that on the high-temperature stage to ensure that the reactions and products were unchanged. Thermal and mass changes of ash samples during heating process were recorded and analyzed. Repeated TGA experiments were conducted at least three times for each sample so as to ensure the accuracy of TGA data. 2.3.5. Thermodynamic equilibrium calculations Thermodynamic calculation software FactSage was widely used in high temperature phase equilibrium calculation of coal ash. FactSage 7.1 with database of FToxid and FactPS was used to predict mineral compositions and proportions of the liquid and solid phases under inert atmosphere. The main compositions SiO2, Al2O3, K2O, CaO, Na2O, MgO and Fe2O3 were selected for calculation in the equilib module. The calculations were carried out from 900 to 1500 °C with an interval of 20 °C.
2.3. Experimental methods
3. Results and discussion
2.3.1. Measurement of AFTs According to Chinese Standard GB/T212-2008, five kinds of ash samples were prepared in a muffle furnace at 815 °C. Ash cones were prepared according to Chinese Standard GB/T219-2008 and the AFTs were determined by the 5E-AF4000 ash fusion temperature determination meter (Kaiyuan Company, Changsha, China) in a weak reductive atmosphere.
3.1. Fusibility of ash samples Elemental composition of five ash samples and main element composition of biomass leachate measured by ICP-OES were shown in Table 3 and Table 4, respectively. As shown in Table 3, compared with SA, the content of Si, Al and Fe in the other ash samples was decreased, while the content of K, Na, Ca and Mg was increased. The content of K,
2.3.2. High-temperature morphology observation The high-temperature stage microscope system was usually used to in-situ observe the transformation in the apparent morphology and structure of samples during ash fusion process. The system was mainly composed of high temperature heater (Linkam, UK) and microscope (DM4500P Leica, Germany). The operation procedures were as follows. Firstly, the sample was dispersed on a sapphire substrate (99.9% purity of Al2O3, 7.0 mm diameter and 0.2 mm thickness), which was placed in the reaction zone. Under the continuous flow of N2 (100 mL/min, 99.999% purity), the slag sample was heated up to 800 °C at a heating rate of 50 °C/min, then it was heated up to 800–1000 °C at a heating rate of 20 °C/min.
Table 3 Elemental composition of five ash samples measured by ICP-OES. Samples
SA RSL RSB CSL CSB
3
Elemental composition/% Si
Al
Ca
Fe
Na
K
Mg
25.00 20.00 23.00 20.00 19.00
5.40 3.80 2.30 4.60 3.70
2.30 2.30 2.30 2.70 4.10
4.70 3.70 4.00 4.00 4.30
0.82 3.00 1.50 1.20 1.20
1.30 5.20 4.90 6.50 7.10
0.48 1.10 0.85 0.83 1.20
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Containing less Al and more water-soluble AAEM especially Na may be the reason why AFTs of RSL was lower than CSL. AFT was a numerical representation for morphology transformation of coal ash [33], while it could not picture the continuous fusion process well. In order to verify the improvement effect of biomass leachate on the AFTs of SA, the morphology change of ash samples during the heating process was in-situ observed through the high-temperature stage microscope system. The high-temperature melting process of SA, RSL and CSL was shown in Fig. 1. Three ash samples exhibited different fusibility. The SA sample showed obvious shrinkage at around 1250 °C, and reached the apparent molten state at around 1350 °C. RSL exhibited obvious shrinkage at 1100 °C and reached the apparent molten state when the temperature reached 1250 °C. The RSB sample showed obvious shrinkage at around 1150 °C, and reached the apparent molten state when the temperature reached 1300 °C. Compared the shrinkage temperature and the apparent molten state temperature of the three samples with the AFTs listed in Table 5, it was obviously found that the shrinkage temperature and the apparent molten state temperature corresponded to DT and FT, respectively. The addition of biomass leachate or biomass changed the chemical composition of coal ash, resulted in the formation of minerals with a low melting temperature or low melting-temperature eutectic in the melting process, and consequently reduced the AFTs of coal ash. The fusion process of ash samples was validated and verified through observation of in-situ morphology in high-temperature stage. However, our understanding of the impact of biomass leachate on the fusibility of coal ash was still apparent. XRD analysis of quenched slags at different temperatures and TG-DTA analysis of five ash samples were needed in studying the influence mechanism of biomass and its leachate on reducing the AFTs of coal ash.
Table 4 Main element composition of biomass leachate measured by ICP-OES. Samples
RS CS
Elemental composition/mg·g−1 Si
Al
Ca
Fe
Na
K
Mg
0.06 0.02
– –
0.04 0.08
– –
0.80 0.45
1.80 1.40
0.14 0.07
Na, Mg in RSL was higher than that in RSB, while the content of Si was lower than that in RSB, indicating that RS contained lots of water-soluble AAEM such as K, Na, Mg, which was demonstrated in Table 4. Compared with blending RS with SA directly, the addition of RS leachate in SA could change the ash chemical composition of SA to a greater extent. According to Table 1, Tables 2 and 3, the content of K and Na in CS was higher than in RS, the content of K2O in CS was 23.12%, and the content of Na2O was 11.73%, while the content of K in CSL and CSB was not high, and the content of Na was similar to that in RSL. It was speculated that the low content of ash in CS had a lesser impact on the chemical composition of SA coal ash under the same raw material ratio. On the other side, as shown in Table 4, lower content of water-soluble AAEM and the evaporation of alkali metal elements in ashing process also may contributed to the relatively less content of K and Na in CSL and CSB. The AFTs of five samples were shown in Table 5, including four characteristics temperatures, i.e., deformation temperature (DT), softening temperature (ST), hemispherical temperature (HT) and flow temperature (FT). As shown in Table 5, the range of AFTs of raw SA was 1230 ± 80 °C. Combined with the ash composition of raw materials shown in Table 2, SA was rich in Si and Al which resulted in high AFTs, and RS and CS had a high content of AAEM, which could effectively reduce AFTs of coal [32]. Therefore, adding biomass directly into SA could dilute the content of Si and Al and thus reduce AFTs of SA. Compared with raw coal ash, the AFTs of SA could be reduced by blending with biomass or biomass leachate. Consequently, it was obvious that the AFTs of SA was significantly reduced by the addition of biomass as shown in Table 5. Nevertheless, RS leachate had the most obvious influence on the ash fusibility of SA among four additives. Compared with SA, AFTs of RSL decreased by more than 70 °C, especially the FT decreased by nearly 140 °C. The effect of CS leachate on the fusibility of SA was relatively weaker than RS leachate but stronger than raw biomass. Biomass leachate contained only water-soluble substances and there were almost no Si, Al or Fe existed in the biomass leachate. Therefore, adding biomass leachate into coal could reduce Si and Al in SA to a lower content, which was verified in Table 3. Since RS and CS almost contained no Al, the total amount of Al in blending coal was nearly not affected. However, adding biomass leachate and directly adding biomass brought different amounts of ash into SA, so the dilution in the content of Al was different. Therefore, the content of Al in RSL and CSL was higher than that in RSB and CSB, respectively. This also applied to Si in CS. Hence it could be inferred that adding biomass leachate into coal had a more obvious effect on reducing AFTs than directly adding biomass to coal, which was consistent with the results shown in Table 5.
3.2. Mineral behaviors during ash fusion process The XRD patterns of the five ash samples at different temperatures (815 °C, 1000 °C, 1200 °C, 1400 °C) under inert atmosphere were illustrated in Fig. 2. As shown in the Fig. 2, the diffraction intensity of minerals in samples decreased as temperature rose, which reflected the mineral content in samples decreased gradually as temperature rose. As shown in Table 5, DT (Deformation Temperature) of five sample ashes all ranged from 1000 °C to 1200 °C. As a consequence, all samples were partly melted at the temperature of 1200 °C. DCL and DCB had lower deformation temperature. Therefore, the typical hill between 20° and 30° in the XRDs of DCL and DCB caused by amorphous silicates was more obvious. As shown in Fig. 2(a), the SA ash was mainly composed of quartz (SiO2), calcite (CaCO3), anhydrite (CaSO4) and hematite (Fe2O3). Hematite was transformed from ferrous disulfide (FeS2) in the ashing process. As the temperature raised up to 1000 °C, calcite and anhydrite decomposed into CaO and combined with SiO2, Al2O3 to form anorthite (CaAl2Si2O8) [34]. The formation of anorthite was accompanied by the decrease of quartz, which proved that quartz was involved in the reaction of anorthite formation. At 1200 °C, the diffraction peak of anhydrite disappeared because anhydrite decomposed between 1000 °C and 1200 °C [19]. At 1400 °C, there was still a certain amount of quartz and anorthite in the sample, which played an important role in high AFTs of SA. Compared with pure SA coal ash, RSL (as shown in Fig. 2(b)) mainly contained minerals such as quartz, calcite, anhydrite, hematite and gehlenite (Ca2Al2SiO7). As the temperature raised up to 1000 °C, anorthite was formed, and the diffraction intensity of gehlenite increased slightly, while the diffraction intensity of hematite and anhydrite changed slightly. At 1200 °C, there were no mineral diffraction peaks in the samples. It was inferred that anhydrite reacted with gehlenite and eutectic with a low melting-temperature generated between 1000 °C and 1200 °C [35]. By comparing Fig. 2(b) and (c), it was found that the types and
Table 5 Ash fusion temperatures of samples. Sample
SA RSL RSB CSL CSB
Ash fusion temperature (Weak reducing atmosphere)/℃ DT
ST
HT
FT
1149 1079 1052 1155 1154
1218 1118 1191 1194 1210
1236 1148 1211 1213 1232
1311 1170 1237 1230 1259
4
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Fig. 1. Micrograph of ash samples at different temperatures: (a) SA; (b) RSL; (c) CSL.
generate eutectic with a low melting temperature. Therefore, only a small amount of quartz, gehlenite and leucite were left at 1200 °C. There were no diffraction peak of minerals in CSB at 1400 °C. CSL and CSB had similar mineral types at the same temperatures, and there was only a little distinction in diffraction peaks of each mineral, so the differences between AFTs of CSL and CSB were small.
contents of minerals in RSL and RSB were basically the same at the same temperature. However, diffraction peaks of cristobalite only existed in RSB between 815 °C and 1000 °C, which decomposed between 1000 and 1200 °C, and resulted in that quart, leucite and gehlenite existed at 1200 °C in RSB while there were no mineral presenting in RSL. Therefore, the AFTs of RSL was lower than RSB. As shown in Fig. 2(d), the CSL ash at 815 °C was mainly composed of quartz, calcite, hematite, gehlenite and muscovite. As the temperature raised up to 1000 °C, muscovite decomposed into leucite (KAlSi2O6) while anhydrite reacted with gehlenite to generate eutectic with a low melting temperature. At 1200 °C, there were only a little quartz gehlenite and leucite in the sample. There were no mineral diffraction peaks in the sample at 1400 °C. As shown in Fig. 2(e), the sample contained quartz, calcite, hematite, gehlenite and muscovite at 815 °C. At 1000 °C, muscovite decomposed into leucite and Al2O3, while calcite decomposed into CaO. Al2O3 and CaO reacted with a part of quartz to form anorthite. As the temperature rose to 1200 °C, anhydrite combined with gehlenite to
3.3. TG-DTA analysis of ash samples The mass and heat analysis of five ash samples was performed and the results were shown in Fig. 3. As shown in Fig. 3, it could be found that there are continuous exothermic and endothermic peaks in the five ash samples above 1000 °C. Between 1000 °C and 1200 °C, the DTA curve of SA exhibited an exothermic peak. Combined with the mineral transformation in Fig. 2(a), the weight loss between 800 °C and 1100 °C in Fig. 3(a) are assigned to the decomposition of calcite and part of anhydrite, and the products combined with quartz and alumina to form anorthite and 5
1
1200 1000
1
40
50
60
70
80
815 10
20
30
40
2ș/°
2400 1600 800 0 1400
1
40
50
80
1000
1600
1
70
80
800
1 6 1
6
0 1400
6 7
6 5 74
1
1 2 7 1 4 1
8
1
1
1 1
1200
1 1000
1
m pe
ra
1
60
2400
re
1
3200
1
815
Te
30
70
4000
1200
1
1
815 20
60
(d)
tu
1
Diffraction intensity(CPS)
4000 3200
10
50
2ș/°
(c)
19 1 3 5 74 1 1 9 1 8 32 74 1 1 1 1
1000
1
10
20
30
40
2ș/°
50
60
70
Diffraction intensity(CPS)
30
1
re
20
1
1200
1
1
tu
10
1
1
73 2 7 4 1 1 1
Te
815
1 73 5 7 4 1
1
Te
1
1 1
1
ra
41 1 1
1
1
80
m pe
3 2
1
41 1
0 1400
1
Te
35
800
1
2ș/°
(e)
4000 3200 2400 1600 800
1
1
1000
1 815
10
20
30
40
50
60
70
re
1200
1
1
tu
1
ra
6 5 7 41 1 1
2 7 41 1 1
1400
1
80
pe
8
1
0
16 7
m
6 1
6 1
Te
1
Diffraction intensity(CPS)
1
41
0 1400
re
5
5
1600
tu
1
1000
ra
1
m pe
1
2400
re
2000
3200
tu
3000
1
4000
(b)
ra
4000
m pe
Diffraction intensity(CPS)
5000
(a)
Diffraction intensity(CPS)
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2ș/°
Fig. 2. XRD patterns of five ash samples at different temperatures: (a) SA; (b) RSL; (c) RSB; (d) CSL; (e) CSB. [Legend: (1) quartz (SiO2), (2) calcite (CaCO3), (3) anhydrite (CaSO4), (4) hematite (Fe2O3), (5) anorthite (CaAl2Si2O8), (6) leucite (KAlSi2O6), (7) gehlenite (Ca2Al2SiO7), (8) muscovite (KAl2Si3AlO10(OH)2), (9) cristobalite (SiO2).]
the decomposition of muscovite, the formation of leucite and the combination of gehlenite and anorthite. Above 1200 °C, the mass of SA coal ash showed little change, while the other four kinds of ash samples still had mass loss. Cl and alkali metals such as K were easily volatilized at high temperature [36]. Hence it was inferred that the mass loss was due to the evaporation of alkali metals contained in RS, CS and their corresponding leachate at high temperatures. The mass loss of coal ash blended with biomass leachate was similar to that of coal ash blended with biomass, which proved that there were a lot of alkali metals in biomass leachate close to those in biomass. There were exothermic peaks in the DTA curves of RSL, RS and CSL between 1300 °C and 1400 °C. Operating temperature was below 1400 °C due to the limited specifications of experimental instrument. But it was easy to infer from the trend of DTA curves that there would be exothermic peaks above 1400 °C in Fig. 3(a) and (e). Comparing
release heat. For RSL and RSB, there were continuous exothermic and endothermic peaks on the DTA curves. Combined with the mineral transformation in Fig. 2(b), it was inferred that the two exothermic peaks were assigned to the combination of gehlenite and anorthite generating low melting-temperature eutectic, and the decomposition of anhydrite. The former was exothermic and the latter was endothermic. According to the mineral transformation results shown in Fig. 2(c), the decomposition of calcite and part of anhydrite, and the formation of anorthite and low melting-temperature eutectic all occurred in the heating process of RSB. The reactions and mineral transformation during heating process of RSB were similar to those during heating process of RSL. Therefore, the DTA curve of RSB was similar to RSL. The decomposition of cristobalite occurred only in RSB may prolong continuous exothermic process, which explained the increase of the width of exothermic peak. For CSL and CSB, each of them had a wide exothermic peak between 900 °C and 1200 °C, which may be the results of 6
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TG(%) DTA(uV/mg)
100
(b)
1
TG(%) DTA(uV/mg)
100 0
99
2
99
1
98
0
-1 -2
97
-3 -4
96
-1
96
TG(%)
98
DTA(uV/mg)
TG(%)
97
-2
95 94
-3
93
DTA(uV/mg)
(a)
-4
92 -5 91
EPT=1238 0
200
400
600
800
1000
1200
1400
-6 1600
90 0
200
400
600
T
(c)
(d)
1
1000
1200
1400
TG(%) DTA(uV/mg)
100
-6 1600
1
99
0
99
0
98
-1
98
-1
-3
95 94
-4
93
-2
96
TG(%)
-2
96
97
DTA(uV/mg)
97
-3
95 94
-4
93
-5
92
-5
92 -6
EPT=1168
91 90 0
200
400
600
800
1000
1200
1400
-6
EPT=1173
91 90
-7 1600
0
200
400
T
600
800
1000
1200
1400
-7 1600
T
(e)
TG(%) DTA(uV/mg)
100
1
99
0
98
-1
97
-2
TG(%)
96 -3
95 94
-4
93
DTA(uV/mg)
TG(%)
800
T
TG(%) DTA(uV/mg)
100
-5
EPT=1104
DTA(uV/mg)
95
-5
92 -6
EPT=1203
91 90 0
200
400
600
800
1000
1200
1400
-7 1600
T Fig. 3. TG-DTA analysis of five ash samples: (a) SA; (b) RSL; (c) RSB; (d) CSL; (e) CSB.
and FT of five samples showed an approximate linear relationship as FT ≈ EPT + 65 °C. Therefore, it was assumed that most of minerals in coal ash melted at EPT. Around this temperature, most of substances in the ash sample were converted to liquid phase, which was conductive to the flow of slag. Combined with XRD analysis of quenched slags at different temperatures and TG-DTA analysis of five ash samples, the influence mechanism of biomass and its leachate on reducing the AFTs of coal ash were basically clear. However, the composition of low melting-temperature minerals eutectic materials generated in the fusion process
AFTs of these five ash samples in Table 2, it could be found that the AFTs of RSL, RSB and CSL were relatively low. It was speculated that AAEM in biomass and biomass leachate promoted the formation and heat release of low-melting eutectic between 1300 °C and 1400 °C, which led to the reduction of the melting temperatures of these three ash samples. By comparison with Table 2 and Fig. 3, it was obvious that the exothermic peak temperature (EPT) of the five ash samples was in the order of SA > CSB > CSL > RS > RSL, and the FT of the five ash samples was also in the order of SA > CSB > CSL > RS > RSL. EPT 7
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(b)
100
80
Slag
High-Albite
60
Propotion
100
80
Slag
Ortho-enstatite
60
Proto-enstatite
Propotion
(a)
Cordierite 40
Anorthite
20
Andradite Diopside
20
SiO2
Leucite
Hematite
0 900
40
1000
1100
Hematite
0 1200
1300
1400
1500
900
1000
1100
Temperature
(c)
1200
1300
1400
1500
1400
1500
Temperature
(d)
100
100
Aegirine 80
80
Slag
Slag Diopside
60
Propotion
Propotion
60
40
20
Sanidine
1000
Leucite
20
Andradite Diopside
0 900
Sanidine 40
Andradite
Hematite 1100
1200
1300
Hematite
0 1400
1500
900
1000
1100
Temperature
1200
1300
Temperature
(e) 100 80
Slag
Propotion
60
Diopside 40
Leucite 20
Andradite
Hematite
0 900
1000
1100
1200
1300
1400
1500
Temperature Fig. 4. Factsage prediction of solid fraction of ash samples: (a) SA; (b) RSL; (c) RSB; (d) CSL; (e) CSB.
results were shown in Fig. 4. The main solids of SA at 1000 °C partly agreed with the XRD analysis results were shown in Fig. 2(a), SiO2, anorthite and hematite were the main minerals at 1000 °C. Minerals such as proto-enstatite and cordierite were not found in XRD analysis, which may be the main result for their small proportion. The existence of anhydrite was not predicted by FactSage, but its diffraction peak existed at 1000 °C in Fig. 2(a). Thus, it was inferred that anhydrite was an intermediate of
were still unclear. Therefore, thermodynamic equilibrium calculations were carried out.
3.4. FactSage predictions of equilibrium condition The solid proportions at high temperature (greater than900 °C) based on the element composition in Table 3 in pure inert atmosphere were predicted by FactSage 7.1 using FToxid and FactPS databases. The 8
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the reaction and the isothermal time was too short in quenching procedure. On the other hand, the short reaction time also could account for the situation why SiO2 existed until 1400 °C and Fe2O3 existed until 1200 °C in Fig. 2(a) while the calculating results shown in Fig. 4(a) was the opposite. In general, the thermodynamic equilibrium calculation results of SA at high temperature were basically consistent with the experimental results. The calculation results from FactSage showed that after adding biomass or biomass leachate, cordierite in SA disappeared, the content of quartz was greatly reduced, and low-melting andradite, diopside and luecite were formed, which led to the increase of the liquid phase content and the decrease of the AFTs of SA. However, simulation results of the other four ash samples did not fit the experimental results well. FactSage calculation was based on the oxides contents in original ash and used the ideal mixing of species to form mineral phases. But during actual experiments, the ideal equilibrium state predicted by FactSage could not be achieved due to the limitation of reaction time and reactivity of reactants. In addition, FactSage was more widely used in subjects such as coal chemistry. Therefore, predictions of ash rich in alkali metals at high temperatures may be not much accurate. The simulation results could reflect the liquid–solid ratio and the melting characteristic of the ash samples to some extent. As shown in Fig. 4, simulation results calculated by FactSage reflected that the AFTs of SA were reduced effectively with the addition of biomass and biomass leachate.
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
4. Conclusions
[12]
The biomass leachate could effectively reduce the AFTs of high silicon-aluminium coal with high-melting temperature. The positive effect of biomass leachate on reducing the AFTs of coal ash was stronger than that of biomass and RS leachate showed the more obvious positive effect. SA ash was mainly composed of quartz, calcite, anhydrite and hematite. By adding biomass or biomass leachate into the SA, the contents of Si and Al in SA were relatively reduced and the contents of AAEM species such as K and Na in SA were relatively increased. Therefore, low-melting leucite and gehlenite which would react with anorthite to generate low-melting eutectic, formed, thus resulting the remarkable decrease of AFTs of SA. The prediction results of FactSage reflected that the AFTs of SA were reduced effectively with the addition of biomass and biomass leachate, and were basically correspond to the experimental results.
[13] [14]
[15]
[16]
[17]
[18] [19]
[20]
CRediT authorship contribution statement
[21]
Jiajian Wang: Data curation, Formal analysis, Writing - original draft. Xia Liu: Project administration, Writing - review & editing. Qinghua Guo: Conceptualization, Resources. Juntao Wei: Investigation, Visualization. Xueli Chen: Software, Validation. Guangsuo Yu: Funding acquisition, Methodology, Supervision.
[22] [23]
[24]
Declaration of Competing Interest
[25]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[26]
[27]
Acknowledgement This work has been partially supported by National Natural Science Foundation of China (21878093).
[28]
[29]
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