Article pubs.acs.org/EF
Experiment and Mechanism Study on the Effect of Kaolin on Melting Characteristics of Zhundong Coal Ash Mingqiang Li,† Zhongxiao Zhang,‡ Xiaojiang Wu,§ and Junjie Fan*,† †
School of Environment and Architecture, University of Shanghai for Science and Technology, Shanghai 20093,China School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200245, China § Shanghai Boiler Works Co. Ltd., Shanghai 200245, China ‡
ABSTRACT: Methods of experimental research and quantum chemical calculation were combined to study the effect of kaolin on ash melting characteristics of Zhundong high alkali coal and the mineral evolution law during ash melting processes. Furthermore, the behavior mechanism of kaolin capturing alkali metal was also studied from the perspective of mineral microstructure features. The results show that the melting temperature of Zhundong high alkali coal ash have rapid rise first and then gradually decreased with the amounts of kaolin increasing. When the adding proportion of kaolin is greater than 10%, the improving efficiency on the ash melting temperature become weakened. Once kaolin was added in Zhundong high alkali coal, anorthite, anhydrite, albite, and muscovite would be generated among the temperature range from 1000 to 1200 °C, Na+ or K+ that easy to volatile and form eutectics have been captured by the Si−Al system, some mullite generated among the temperature range from 1200 to 1300 °C. Investigating the capture mechanism, it indicates that O (26) and O (22) have electrophilic reaction with Na+ and K+ easily, which would promote the rupture of aluminum−oxygen bonds. The O2− of alkali metal or alkaline earth metal oxide would easily have nucleophilic reaction with Si (6) and Si (8) and prompt the rupture of bridging oxygen bonds (Si−O−Si). Kaolinite would be transformed into high melting point minerals that contain Na+ or K+ which have trend to form eutectics or evaporate into the flue gas easily, and the degree of fouling and slagging on the heating surface can be reduced based on the two easiest reaction paths. Fe) Si2O6], omphacite [NaCaMgAl (Si2O6)2] and sanidine stone [K(AlSi3O8)], and the melting temperature of them are all ranged from 900 to 1000 °C.3 Rich Na, K, Cl, S, etc. were found in the cohesive fine particles that belong to the bottom slagging layer of the cross-sectional slices sample by scanning electron microscopy and energy spectrum analyzer.3 Literature on K release shows that organic Cl are more easier to combine with K than inorganic Cl.5 K is easily volatilized during the ashing processes, so the slagging characteristics of coal ash could not be reflected by the test under the standard ashing temperature. It was also found in the study of alkali metal distribution with the flue gas temperature that the sediments contain large amounts of S and Cl under different flue gas temperature, but with the decreasing of flue gas temperature, the contents of K and Cl reduce. Meanwhile, Na and S contents have a high level in the interface; Si and Mg contents are high in the inner layer, and Ca and Cl contents are high in outer layer.6 Combined with the methods of CFD simulation, thermal equilibrium chemistry and fine particle dynamics, processes of alkali metal vapor condense into tiny particles in a preheating boiler were simulated, and it presents a good match with the operation data of a waste heat boiler, especially for ash particle composition.7 As a effective additive for removal of alkali metal, kaolin has been studied by many researchers on its removal mechanism. Partial kaolinite could be easily decomposed at a high
1. INTRODUCTION Zhundong, located in the east of Junggar Basin, Xinjiang, China, with the forecast reserves of 390 billion tons is the largest intact coalfield in China and even in the world. According to the current coal consumption in China, the Zhundong coal could meet the needs for coal consumption in China for the next 100 years.1,2 However, a large amount of alkali metal elements are contained within the primary minerals in this region, such as Na, K, etc.1 Compared with the conventional power coal, it has a very different ash fouling and slagging behaviors during the burning processes, which result in that it could not be stable long-term used in most coal-fired power plants and greatly increase running cost and energy consumption. Many scholars has focused on the mechanism study of alkali metal behavior during the burning processes of high alkali coal. Li Gengda expresses that a significant slagging appeared near the side wall of the burner at 1150 °C, while the Zhundong coal was fired in a 25KW down-fired furnace, it was also found that the ash collection efficiency is about 4% at the location of 800 °C, which shows a strong deposition trend than burning bituminous coal or biomass.3 The more alkali metal contents are in the coal, and the smoother surface is for the ash particle. In addition, with the increase of coal particle size, the deposition rate also increases.4 Studies have shown that NaCl is easy to form a slagging layer at the bottom place by the condensation on ash particles when the flue gas temperature is 550 °C.5 Due to the rich content of alkali metal, a large number of low-melting wollastonite and eutectics existed in the clinker from the main combustion zone, such as pyroxene [(Ca, Mg, © 2016 American Chemical Society
Received: June 22, 2016 Revised: August 17, 2016 Published: August 23, 2016 7763
DOI: 10.1021/acs.energyfuels.6b01525 Energy Fuels 2016, 30, 7763−7769
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Energy & Fuels temperature condition.8 And Linak’s research9 has proved that kaolin was excessive molten between 1373 and 1573 K during the burning processes, which lead to it inactivated and cannot continue to be used for further capture alkali metal. Gale also found that, when the temperature increased to 1573 K, the kaolin crystal structure would be collapsed10 and the sintering phenomenon would also be occurred by the excessively melten of eutectics, both of which would sharply decrease the specific surface area and porosity of kaolin and lead to the deactivation of kaolin, so the adsorption capacity of kaolin for metal would be obviously reduced.9 Gale also found that the melting phenomena only happened when the eutectics existed, and no melting phenomenon occurs when single kaolin was burned.11 The physical adhesion effect is much less than chemical adsorption during the adsorption process of kaolin on alkali metals, some stable silicon aluminate would be formed after solidifying alkali metal within its lattice.11 As the main ingredient of kaolin, the removal mechanism of silica on alkali metal was studied by many scholars. Some scholars mix lignite with silica additives in the 30 MW subcritical boiler to burning ten months and found that the main ingredient of sediment near burner is fragile calcium sulfate, which is a hierarchical structure with calcium silicate as the core and calcium sulfate in the surface layer. When silica addition amount is 4%, the effect of capture for Na is best, and the result of this capture is to make alkali metal transfer to the big slag, rather than by acidification, which reduced the fouling degree of sulfate in convection area.12 With the development of computer technology, in addition to the above conventional experiments to explore the mechanism of slagging for high alkali coal, the quantum chemistry theory provides a theoretical basis and methods to delve into the complex physical−chemical reactions and reaction paths in the ash melting process at high temperatures.13−15 Based on the density functional theory, Fukui function is an important indice for predicting molecular reactive sites, which demonstrated the rationality of electronic theory for chemical reaction.16−18 Therefore, methods of quantum chemistry theoretical and experimental study were combined to study the evolution of minerals during the coal ash melting processes while different proportions of kaolin were added in the Zhundong high alkali coal. Quantum chemistry theory was innovatively used for the study on the capture mechanism of alkali metal by kaolin, and the influence factors and evolution mechanism on the coal ash melting characteristic were revealed both by the macrocosm and microcosm level, which provide guidance and a theoretical basis for the optimization and selection of the additive.
Table 1. Properties of High Alkali Coal and Kaolin Zhundong coal
kaolin
proximate analysis Mar (wt %) 20.48 0.0 Mad (wt %) 8.86 0.0 Aar (wt %) 14.23 100 Vdaf (wt %) 35.98 0.0 Qnet,ar (MJ/kg) 18.3 0.0 ultimate analysis (wt %) Cdaf 78.77 Hdaf 4.59 Odaf 15.06 Ndaf 0.84 Sdaf 0.74 ash fusion temperature (°C) (Reducing atmosphere) DT 1070 >1500 ST 1080 >1500 HT 1090 >1500 FT 1110 >1500 ash compositions (wt %) SiO2 35.95 61.99 Al2O3 14.60 21.73 Fe2O3 16.32 1.39 CaO 11.72 0.61 MgO 5.69 0.48 K2O 0.78 0.61 Na2O 4.46 0.21 TiO2 0.96 0.59 SO3 9.10 0.83 a
DT, deformation temperature; ST, softening temperature; HT, hemispherical temperature; FT, flowing temperature.
method of the national standard GB212. The products were labeled as sample+0%, sample+5%, sample+10%, and sample+20%, respectively. According to the GB/T219−2008 method, the ash fusion temperature was tested under the condition of weak reducing atmosphere by the HR-4 ash melting point apparatus to study the effect of kaolin addition on fusing characteristics of Zhundong high alkali coal. Moreover, in order to further study the effect of kaolin on mineral evolution characteristics during the process of the coal ash melting, the above standard ash samples were heated to 1000, 1100, 1200, or 1300 °C and held at each temperature for 15 min. The heated samples were rapidly cooled within the dry ice container, which was used for XRD analysis after drying and grinding processes. 2.2. Computational Methods and Model. Kaolinite is the main mineral composition of kaolin, whose crystal structure was used as the calculation model.19 All calculations were performed using Materials Studio software20 within the framework of the density functional theory (DFT). The optimized structure of kaolinite is refined using the CASTEP21 program with a plane wave basis set and pseudopotentials within the DFT formalism.22−24 The valence electron wave functions were expanded in a plane wave basis set represented by a kinetic energy cutoff of 380 eV. The electron−ion interactions were described by Perdew, Burke, and Ernzerhof (PBE) ultrasoft pseudopotentials.22 These were consistent with the description of the exchange-correlation effects by the generalized gradient approximation (GGA) density functional, specifically (GGA) PBE,25 which describes molecular bonding to a greater accuracy than does the local density approximation (LDA). The geometry optimizer was BroydenFletcher-Goldfarb-Shanno (BFGS),26,27 and the electronic method was ensemble density functional theory (EDFT).28 The Brillouin zone integrations were performed on a Monkhorst−Pack29 grid of 3 × 2 × 2 k-points as this gave convergence to within the error bound just described. Further convergence details per BFGS iteration are as follows, energy change per atom: 5 × 10−6 eV; maximum force: 0.01
2. EXPERIMENTAL SECTION AND COMPUTATIONAL DETAILS 2.1. Experimental Coal Sample and Method. Zhundong high alkali coal and kaolin were adopted as the test samples in this paper, and their analysis data are shown in Table 1. It can be seen from Table 1 that Zhundong coal ash contains high Fe2O3(16.32%), Na2O(4.46%), and MgO(5.69%) and the content of Al2O3 (14.6%) is low. Kaolin has a very high refractory temperature (about 1735 °C) and sintering temperature, due to the higher ingredients of SiO2(61.99%) and Al2O3(21.73%). In addition, it also contains a small amount of montmorillonite, mica, quartz, and so on. Zhundong raw coal was first mixed with kaolin according to certain proportion (adding kaolin for 0%, 5%, 10%, and 20%, respectively) and then fully ground in an agate mortar. The standard ash samples were prepared at the temperature of 815 °C (±10 °C) with the 7764
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Energy & Fuels eV/Å; maximum stress: 0.02 GPa; maximum displacement: 0.0005 Å. The calculation was spin polarized. All lattice parameters and atomic coordinates were allowed to relax to equilibrium within the convergence criteria previously stated. Other calculations were performed by means of the DMol3 program30,31 with double numerical extra polarization (DNP) basis set.30,32 The exchange-correlation function is treated with the generalized gradient approximation (GGA) with PBE. 22 The convergence tolerance quality level of self-consistent field was fine with settings 1.0 × 10−5 Ha (1 Ha = 27.2 eV) of the energy, 0.002 Ha/ Å for the maximum force, and 0.005 Å for the maximum displacement. Following full relaxations of the lattice and ions to the convergence criteria described above, the chemical reactivity of different position in the structure of kaolinite was analyzed using the electrostatic potential (ESP),33 the frontier molecular orbital (LUMO-lowest unoccupied molecular orbital, HOMO-highest occupied molecular orbital)34,35 and Fukui function (nucleophilic attack f +, electrophilic attack f −).17 ESP is the interaction energy between a positive charge located in the point r and the current system, the bigger the absolute value of the negative (positive) electrostatic potential is in the area, the more possible the reaction would occur under the attack of electrophilic (nucleophilic) reagent.33 According to the frontier molecular orbital theory, LUMO of the reactants will be occupied by electrons during the chemical reaction processes. Once the ionization occurred, electron would transfer away from HOMO of reactant.36 To a certain extent, f − and f + in the Fukui function is related with HOMO and LUMO.17 However, the Fukui function not only includes the frontier molecular orbital theory but also covers orbit relaxation when the system increases or loses electrons.37,38
that kaolin has a good effect on improving melting characteristics of high alkali coal ash, but there exists a optimal adding proportion. 3.2. Effect of Kaolin on Mineral Evolution under Different Temperatures. Figure 1 shows that the melting point of high alkali coal ash increased significantly and the fouling characteristics were well improved by adding kaolin. When the proportion is greater than 10%, the rise trend of ash melting temperature become slower. So sample+0%, sample +5%, sample+10% were chosen to further study the form evolution of minerals vary from different temperature by XRD. Figure 2a−c shows the effect of temperature on evolution of the main mineral composition in sample+0%, sample+5%, and sample+10%, respectively. It can be seen from Figure 2a that quartz (SiO2, melting point: 1710 °C) content gradually decreased with increase of temperature. When the temperature increased to 1000∼1100 °C, some high melting point minerals would be formed, such as anorthite (CaO·Al2O3·2SiO2, melting point: 1553 °C), anhydrite (CaSO4, melting point: 1460 °C), and hematite (Fe2O3, melting point: 1457 °C). Due to the existence of kaolinite composition (SiO2, Al2O3) in ash, some part of Na and K would be fixed as muscovite and albite. As the temperature further increased to 1200 °C, most of the coal ash turned into liquid, and the amorphous molten state is the main existence for Zhundong coal ash. The ash melting temperature was still very low, although some high melting point minerals were generated during the heating process of the original coal ash samples. Some eutectics would be formed during the heating process due to the high content of Na, such as Na2SO4+NaCl (melting point: 721 °C), Ca2SO4+Na2SO4 (melting point: 918 °C), SiO2+Na2O+K2O (melting point: 540 °C), MgSO4+Na2SO4 (melting point: 680 °C), etc.39 In Figure 2b, when 5% of kaolin was added in high alkali coal, a certain amount of anhydrite, anorthite, and hematite were still existed among the temperature range from 1000 to 1100 °C.40 Albite content was significantly higher than sample+0% in the same temperature range. Moreover, K2O content is only 0.78% in ash, so no muscovite was found under this condition. But when the temperature rised to 1200 °C, quartz is the main ingredient, and a small amount of anorthite and hematite were also found in the ash. To some extent, kaolin would react with alkali metal, which can reduce the production of eutectics. The main reactions are listed as follows:
3. RESULTS AND DISCUSSIONS 3.1. Effect of Kaolin on Melting Characteristics of High Alkali Ash. Coal ash melting temperature is one of the important indicators for the evaluation of coal ash fouling and slagging. Figure 1 shows the effect of kaolin proportion on
Na 2O + Al 2O3 ·2SiO2 + 4SiO2 → Na 2O·Al 2O3 ·6SiO2 (albite) Figure 1. Effect of kaolin on ash fusion temperature of Zhundong high alkali coal ash.
In Figure 2c, when 10% of kaolin was added in high alkali coal, the content of refractory minerals quartz was reduced gradually with the increase of temperature. However, it still exists at 1200 °C. Compared with sample+5%, there are more parts of Na would be transformed into albite. That is to say, more of them would be fixed as the form of silicate among the temperature range from 1000 to 1100 °C, and katophorite and rankinite would also be generated in this the temperature range. When the temperature increased to the range from 1200 to 1300 °C, the amount of mullite (3Al2O3·2SiO2, melting point:1860 °C) is highter than sample+5% in the same temperature range, which leads to the rising of coal ash melting temperature. Through the change of mineral composition of three samples under different temperature we can know: On the one hand, kaolin addition increase the content of refractory minerals in
melting temperature of high alkali coal. It can be seen that the ash melting temperature of Zhundong high alkali coal has a rapid rising trend at first, and then the rise becomes slower gradually with the increase of kaolin proportion. When the kaolin proportion is less than 10%, the increased rate of ash melting temperature is bigger, but when the proportion is greater than 10%, the rise trend of the ash melting temperature becomes slower. When the rate of kaolin increased to 5%, the ash melting temperature increased by 72−159 °C. When it increased to 10%, the ash melting temperature is higher than that of the rate of 5% for 37−95 °C approximately. When it further increased to 20%, its corresponding melting temperature only increases about 9−45 °C than before. They indicate 7765
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proportion added to Zhundong coal, the catch reaction on alkali metal would become further strengthen. 3.3. Mechanism Analysis on Kaolinite Changing Melting Characteristics of Zhundong Coal Ash. In order to further explore the microscopic mechanism of the kaolinite catching alkali metal, MS was used to get the stabilized structure of kaolinite, on the basis of the optimized structure, further calculation and analysis for the reactivity of kaolinite was made. Figure 3 is the optimized stable structure of
Figure 3. Optimized structure of kaolinite.
kaolinite, and Figure 4 is the HOMO and the LUMO after calculation; the HOMO of kaolinite is mainly contributed by oxygen atoms belonging to chemical bonds of aluminum− oxygen (Al−O) and oxygen atoms connecting the aluminum and silicon (Al−O−Si), and the LUMO of kaolinite is provided by the silicon, oxygen, and hydrogen, that is scattered relatively. ESP isosurface is shown in Figure 5a. The blue areas occupy a larger range, which are mainly concentrated around the O atoms of kaolinite. The red areas are mainly distributed around the atoms of Si and Al, which is easy to attract the reaction of nucleophilic reagent. Figure 5b,c shows that the maximum value of Fukui function f − of kaolinite are concentrated around the aluminum−oxygen octahedron, it would be attacked by the electrophilic reagent, which indicates that the broken of Al−O bond in kaolinite are mainly caused by the reaction between electrophilic reagent and kaolinite. And the site with the maximum value of Fukui function f + stay around the silicon−oxygen tetrahedron is the preferential attachment site for nucleophilic reagent, which could joint into the kaolinite crystal by the transformation of electron with silicon−oxygen tetrahedron. In order to distinguish the different chemical reactivity of different position in the molecular structure of kaolinite, the condensed Fukui function ( f − and f +) obtained based on the Mulliken population analysis are shown in Table 2. According to the definition of Fukui function, the site where have the maximum of f − and f +would be the priority attack position by electrophilic reagents and nucleophilic reagent. Data in Table 2 indicate that the broken of Al−O bonds would be mainly caused by electrophilic reagent attacked on O (26) and O (22) preferentially. Correspondingly, Si(6) and Si(8) would be preferentially attacked by the nucleophilic reagent, which caused the broken and restructure of Si−O bond.
Figure 2. XRD patterns of samples under different temperatures [a: XRD patterns of sample+0% under different temperatures, b: XRD patterns of sample+5% under different temperatures, c: XRD patterns of sample+10% under different temperatures), Q: SiO2(quartz), A: CaSO4(anhydrite), Bl: Na2Mg(SO4)2·4H2O(bloedite), Bi: KFeMg 2 (AlSi 3 O 10 ) (OH) 2 (biotite), M: KAl 3 Si 3 O 11 (Muscovite), H: Fe2O3(hematite), An: CaAl2Si2O8(anorthite), Al: NaAlSi3O8(albite), K: Na 2 Ca(M g,Fe) 4 A l (Si 7 A l )O 2 2 (OH) 2 (kato phorite), R : Ca3Si2O7(rankinite), Mu-3Al2O3·2SiO2(mullite)].
coal, such as quartz, mullite etc to raise the coal ash melting temperature. On the other hand, kaolin is easy to have reaction with alkali metal such as Na in Zhundong high alkali coal to form albite which have a high melting temperature relatively.41 So alkali metal was fixed in the mineral phase with high melting point to prevent them from forming eutectics or evaporating into the flue gas.5 Moreover, with the increase of the kaolin 7766
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Figure 4. HOMO and LUMO schematic of kaolinite. The blue and yellow parts present the distribution of the wave function with positive and negative values, respectively.
Figure 5. ESP isosurface and the Fukui function of kaolinite. The color gradient from blue to red represents the value of electrostatic potential and Fukui function change from minimum to maximum.
Table 2. Condensed Fukui Functions (f −and f +) of Kaolinite atom
f−
f+
atom
f−
f+
Al(1) Al(2) Al(3) Al(4) Si(5) Si(6) Si(7) Si(8) O (9) O (10) O (11) O (12) O (13) O (14) O (15) O (16) O (17)
0.047 0.044 0.047 0.044 0.036 0.037 0.036 0.037 0.017 0.017 0.012 0.015 0.011 0.017 0.017 0.012 0.015
0.058 0.047 0.058 0.047 0.158 0.168 0.158 0.168 −0.015 −0.014 −0.023 −0.030 −0.023 −0.015 −0.014 −0.022 −0.030
O (18) O (19) O (20) O (21) O (22) O (23) O (24) O (25) O(26) H (27) H (28) H (29) H (30) H (31) H (32) H (33) H (34)
0.011 0.042 0.056 0.037 0.061 0.042 0.055 0.037 0.062 0.012 0.017 0.015 0.025 0.012 0.017 0.015 0.025
−0.023 −0.020 −0.010 −0.008 −0.013 −0.020 −0.010 −0.008 −0.013 0.076 0.050 0.032 0.064 0.077 0.050 0.032 0.065
muscovite. The first path is that Na+ and K+ in molecular structure of the alkaline minerals could react with O(26) and O(22) in kaolinite, which causes alkali matal transform into other minerals by the priority broken of aluminum−oxygen bond. The second path is that O2− in the alkali/alkaline earth metal could have nucleophilic reaction with Si(6) and Si(8) in kaolinite. At the same time, the bond length of Si(6)−O(10), Si(6)−O(11), Si(6)−O(13), Si(8)-(12), Si(8)−O(15), Si(8)− O(16) and Si(8)−O(18) are 1.622 Å, 1.641 Å, 1.654 Å, 1.635 Å, 1.622 Å, 1.654 and 1.641 Å respectively. Si(6)−O(13) and Si(8)−O(16) have the relative longest bond length, which should be much easier to be broken during the catching processes.14 Therefore, the structural distortions of kaolinite would be easily triggered from these sites by reacting with alkali metal in ash, and then transformed into the minerals with high melting temperature.41
4. CONCLUSIONS (1) Kaolin presents a good effect on improving melting characteristics of high alkali coal ash. With the increase of kaolin proportion, ash melting temperature of Zhundong high alkali coal has a rapid rising trend at first and then become slower gradually once the adding proportion is greater than 10%.
It is shown in Figure 6 that there are two easiest reaction paths for kaolinite catching alkali metal when it was added in Zhundong coal. The formation of eutectics would be inhibited due to products of high melting point, such as albite and 7767
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Figure 6. Two easiest reaction paths for kaolin catching alkali metal. by-products from two coal-fired power plants in Xinjiang Province, Northwest China. Fuel 2012, 95 (95), 446−456. (3) Li, G. D.; Li, S. Q.; Huang, Q.; Yao, Q. Fine particulate formation and ash deposition during pulverized coal combustion of high-sodium lignite in a down-fired furnace. Fuel 2015, 143, 430−437. (4) Namkung, H.; Xu, L. H.; Shin, W. C.; Kang, T. J.; Kim, H. T. Study on deposition tendency of coal ash under various gasification environments through DTF. Fuel 2014, 117, 1274−1280. (5) Thiel, C.; Pohl, M.; Grahl, S.; Beckmann, M. Characterization of mineral matter particles in gasification and combustion processes. Fuel 2015, 152, 88−95. (6) Chen, G. Y.; Zhang, N.; Ma, W. C.; Rotter, V. S.; Wang, Y. Investigation of chloride deposit formation in a 24 MWe waste to energy plant. Fuel 2015, 140, 317−327. (7) Leppanen, A.; Tran, H.; Taipale, R.; Valimaki, E.; Oksanen, A. Numerical modeling of fine particle and deposit formation in a recovery boiler. Fuel 2014, 129, 45−53. (8) Lin, X. C.; Wang, C. H.; Miyawaki, J.; Wang, Y. G.; Yoon, S.; Mochida, I. Analysis of the transformation behaviors of a Chinese coal ash using in-/ex-situ XRD and SEM-EXD. Asia-Pac. J. Chem. Eng. 2015, 10 (1), 105−111. (9) Linak, W. P.; Miller, C. A.; Wood, J. P.; Shinagawa, T.; Yoo, J.-I.; Santoianni, D. A.; King, C. J.; Wendt, J. O.; Seo, Y.-C. High temperature interactions between residual oil ash and dispersed kaolinite powders. Aerosol Sci. Technol. 2004, 38 (9), 900−913. (10) Gale, T. K.; Wendt, J. O. High-temperature interactions between multiple-metals and kaolinite. Combust. Flame 2002, 131 (3), 299−307. (11) Gale, T. K.; Wendt, J. O. Mechanisms and models describing sodium and lead scavenging by a kaolinite aerosol at high temperatures. Aerosol Sci. Technol. 2003, 37 (11), 865−876. (12) Dai, B. Q.; Wu, X. J.; De Girolamo, A.; Zhang, L. Inhibition of lignite ash slagging and fouling upon the use of a silica-based additive in an industrial pulverised coal-fired boiler. Part 1. Changes on the properties of ash deposits along the furnace. Fuel 2015, 139, 720−732. (13) Zhang, Z. X.; Wu, X. J.; Zhou, T.; Chen, Y. S.; Hou, N. P.; Piao, G. L.; Kobayashi, N.; Itaya, Y.; Mori, S. The effect of iron-bearing mineral melting behavior on ash deposition during coal combustion. Proc. Combust. Inst. 2011, 33, 2853−2861. (14) Wu, X. J.; Zhang, Z. X.; Chen, Y. S.; Zhou, T.; Fan, J. J.; Piao, G. L.; Kobayashi, N.; Mori, S.; Itaya, Y. Main mineral melting behavior
(2) Contents of quartz and mullite increase with the adding of kaolin. Additionally, alkali metal would be fixed into the aluminosilicate by reacting with kaolinite, which can not only prevent the formation of eutectics but also reduce the evaporation of alkali metal elements. (3) Electrophilic reagents of Na+ and K+ could react with O(26) and O(22) in kaolinite and lead to the broken of aluminum−oxygen bond. Correspondingly, bridging oxygen bonds (Si−O−Si) would be also broken by the nucleophilic reaction between O2− from alkali/alkaline earth metal oxide and Si(6) or S(i8) in kaolinite. Albite and muscovite could be generated through the two easiest reaction paths, which present the micromechanism on ash melting characteristic changing due to kaolin catching alkali metal.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +86-18939750581. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful to thank The National Natural Science Foundation of China (NFSC, 51276212), The National Key Technology R&D Program of China (2015BAA04B01), and The Key Project of Science and Technology Commission of Shanghai Municipality (15dz1206500).
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REFERENCES
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DOI: 10.1021/acs.energyfuels.6b01525 Energy Fuels 2016, 30, 7763−7769
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DOI: 10.1021/acs.energyfuels.6b01525 Energy Fuels 2016, 30, 7763−7769
