Article pubs.acs.org/EF
Effect of V and Ni on Ash Fusion Temperatures Zhigang Wang,†,‡ Jin Bai,*,† Lingxue Kong,† Zongqing Bai,† and Wen Li† †
State Key Lab of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China University of Chinese Academy of Sciences, Beijing 100049, China
‡
ABSTRACT: Ash fusion temperatures (AFTs) of ashes with different ratio of V2O5 and NiO were investigated under mild reducing (CO/CO2 = 6:4) and oxidizing (air) atmosphere. FactSage, X-ray diffraction (XRD), and scanning electron microscopy/electron dispersive X-ray (SEM-EDX) were applied to determine liquidus temperatures, minerals transformation, and morphology of ash and slag at high temperature. AFTs vary with the content of V2O5, NiO, and VNiO (V2O5 and NiO) and a significant difference was exhibited between reducing and oxidizing atmosphere. Redox reactions of V2O5 and NiO with atmospheres are the major reason for the difference. V2O3 and spinel decrease the melting rate of minerals and increase the AFTs. V2O5 of low liquidus temperature may form eutectic matter with anorthite; so, AFTs significantly decreased under oxidizing atmosphere. Ni aggregation formed regular ball increases AFTs slightly under reducing atmosphere. AFTs with VNiO approach to AFTs with V2O5 because V2O3 is the most refractory minerals under reducing atmosphere and V2O5 forms eutectic matter with spinel under oxidizing atmosphere. The association of refractory minerals and liquid phase influenced the distribution of solid minerals during melting in ash and slag, which was also an important factor to illustrate the influence on AFTs. The correlation between liquidus temperature and AFTs of oxidizing atmosphere for ash containing Ni was established, but the similar correlation for ash containing V was proved not to be feasible. The linear regression relationships of AFTs with the content of V2O5, NiO, and VNiO were established for predicting the fusion temperatures of ash with V and VNi.
1. INTRODUCTION Coal gasification is a well-proven technology that started with the production of coal gas for urban areas, progressed to the production of fuels, such as oil and synthetic natural gas (SNG), chemicals, and also to large-scale Integrated Gasification Combined Cycle (IGCC) power generation. The entrained-flow gasifier operated at very high temperatures of high efficiency and low emission and is now becoming the tendency in gasification technology.1 Meanwhile, the multiple feeding for gasifier is necessary for reducing coal consumption and other environmental issues. Petroleum coke, a byproduct of the oil refining industry, has been and is expected to continue to be produced increasingly as a result of the growing consumption of petroleum oil. Petroleum coke has high heating value and low ash content (below 1.2%) and is regarded as an important fuel for gasifier,2 especially for gasifiers in oil refining factory, but the amount of petroleum coke is not enough to run the gasifier. Co-feeding of petroleum coke and coal is the excellent choice for petroleum coke utilization and also will decrease the ash content in the blending.3,4 However, ash composition of coal is generally Ca, Fe, Si, Al, K, Na, and Mg, which exist as clay and oxides at low temperature ash and as silicates and aluminosilicates at high temperature. Additionally, petroleum coke ash is mainly composed of vanadium and nickel. Although ash compositions of different petroleum coke exhibit large distinction, the content of vanadium and nickel remains relatively high. The vanadium mass percentage is generally about 20% and sometimes even higher. The nickel mass percentage is usually higher than 10%.5 The large difference exists between ash composition of petroleum coke and coal may cause problems for slagging in the gasifier, because the transformation of V and Ni are very © XXXX American Chemical Society
different from the general ash composition at high temperatures. V exhibits triple valences V3+, V4+, and V5+ at different atmospheres at high temperatures.6 V2O3 is stable under reducing atmosphere, and transforms into V2O5 in oxidizing atmosphere. Meanwhile, Ni exists in a wide range of oxidation states as Ni, Ni2+, and Ni3+ in high temperature range. The fate of V and Ni in the thermal conversion calculation of coal was investigated by Frandsen et al.7 When the air excess number (λ) was set to equal 1.2, solid V2O5 was the major stable form of vanadium below 1077 °C, and solid NiO was stable up to 1327 °C. In standard reducing conditions (λ = 0.6), solid V2O3 was present at lower than 1477 °C and Ni was formed at 772 °C and present below 1427 °C. However, the influences of coal minerals on the transformation of V and Ni were not taken into account. In the petroleum coke combustion process, deposits formed by coke ash was primarily a solid solution of nickel vanadate [Ni3(VO4)2] and vanadium pentoxide as V2O5.5 V and Ni occur as high valence in oxidizing condition of combustion. In the reducing condition of gasification, Bunt et al.8 demonstrated that when the temperature was above 325 °C, reduction of the V species to the vanadium trioxide (V2O3) remained stable throughout the rest of the gasification in fixed bed gasifier. Park et al.4 indicated V2O3 formed under reducing atmosphere remained solid at typical gasification temperatures and 3.4% V2O3 increased the slag critical viscosity temperature from 1288 to 1321 °C. In the study of Nakano et al.,9 crystallized karelianite (V2O3) and spinel (VFe2O4) phase were found in high temperature slag under the reducing atmosphere (CO/CO2 = 1.8). Received: August 19, 2013 Revised: November 18, 2013
A
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Table 1. Ash Compositions of Samples samples
SiO2
Al2O3
Fe2O3
CaO
MgO
TiO2
SO3
K2O
Na2O
P2O5
raw ash ACa
50.36 45.18
33.42 29.99
3.70 3.32
5.49 15.64
1.29 1.16
1.25 1.12
1.83 1.64
0.87 0.78
0.98 0.88
0.32 0.29
2.2. Measurement of AFTs. The ash fusion temperatures auto detecting system developed by KY Corporation, China, was used to determine AFTs with different additions of V and Ni (wt %, coal basis). The measurements were performed following the Chinese standard procedures (GB/T219-2008) under reducing (CO/CO2 = 6:4) and oxidizing (air) atmospheres. In the procedure, an ash cone with specific geometry was heated at a rate of 15 °C/min up to 900 °C, and then, the heating rate was changed to 5 °C/min. The shape change of ash cone was recorded with video camera in the process. Five samples could be measured at once. The initial deformational temperature (DT), softening temperature (ST), hemispherical temperature (HT), and flow temperature (FT) were recognized and recorded by the auto detecting system with the accuracy of 1 °C. Repeated AFT experiments were performed. The average value of the AFTs was used, and the error range is within 10 °C. 2.3. Quenching Experiment. Quenching experiment was performed to obtain the slag sample at certain temperature for composition and morphology. The sample was heated in the horizontal electricity tube furnace under reducing (CO/CO2 = 6:4) or oxidizing (air) atmosphere. The thermal profile is set based on the ATFs test. Then, the sample was heated to the set temperature, taken out, and quenched in the ice water. 2.4. Instrumental Analysis. Coal ash or slag samples were ground to less than 0.074 mm. A RIGAKU D/max-rB X-ray powder diffract meter was applied for XRD patterns using Cu Kα radiation (40 kV, 100 mA, Kα1 = 0.15408 nm). The samples were scanned with a step size of 0.02° at 4°/min over the 2θ range 5−80°. A JSM-7001F scanning electron microscope was employed to assess the microstructure of the slag. Energy dispersive X-ray spectroscopy (EDX) was carried out to identify the composition of solid and liquid in the samples. 2.5. Thermodynamic Equilibrium Calculations. FactSage 6.2 was used to calculate component of solid phase and change of liquid phase at different temperatures with these oxides from ash composition (SiO2−Al2O3−Fe2O3−CaO−MgO−TiO2−SO3−K2O− Na2O−V2O5/NiO). FactPS and FToxid database were selected for phase formation data, and FToxid is first to avoid selecting duplicate compounds. The solution species formed possibly were all selected from FToxid database in equilibrium calculation. FToxid calculations were carried out between the solid temperature and liquidus temperature in a mild reducing (CO/CO2 = 6:4) or oxidizing (O2/ N2 = 2:8) atmosphere under 0.1 MPa. At a given temperature and composition range, the calculation method of FactSage is based on Gibbs’ energy minimization for each of the samples. Phases formed at concentrations below 0.01 wt % were ignored because the convergence of the algorithms is slow and sensitive.
Although the role of ash composition in ash fusion characteristics and slag viscosity is widely studied, little research has been carried out to investigate the effects of V and Ni on ash flow temperature. However, other metals of multiple valences also exist in coal ash. Valence of iron varied with atmosphere in coal ash or slag, which influences the melting behaviors. Nowok et al.10 determined redox ratios of iron and structural position of Fe3+ and Fe2+ ion in the quenched slag in mild reducing atmosphere and air. The Fe3+/∑Fe ratio at 1370 °C was 0.12 in mild reducing atmosphere and increased to 0.84 in air. Vargas et al.11 also reviewed other factors influenced the redox ratios of iron including concentration of other species and acidity, but the atmosphere brought stronger influences than other factors. The effect of V and Ni are also tightly related with atmosphere. V and Ni existed as different oxides and silicates under different atmosphere. It is necessary to investigate the influence of atmosphere on AFTs of ash containing V and Ni. The gas products of different gasification technology exhibit large difference. Although the entrained flow gasifier usually operates in a reducing atmosphere (H2 + CO), the minerals particles also first move through the oxidation zone and then reduction zone of gasifier and form slag to tap. AFTs obtained in the reducing atmosphere (CO/CO2 = 6:4) of the Chinese standard procedures (GB/T219-2008) are still widely used as a guide for slagging of gasification. AFTs obtained in the oxidizing atmosphere (air) are widely used as a guide for slagging of combustion. In this work, the reducing atmosphere (CO/CO2 = 6:4) and oxidizing atmosphere (air) were selected as experimental atmospheres and the influence of the two atmospheres on AFTs was investigated. Considering the match of reactivity of cofeeding materials, petroleum coke should be blended with coal with low reactivity for entrained-flow gasification. The typical anthracite from Shanxi, China, was selected for blending with petroleum coke for reactivity matching. Also, petroleum coke in the blending will decrease the ash content of the mixture, since the ash content of Shanxi anthracite is usually over 30% before washing. In the study, AFTs adding different ratios of V2O5 and NiO at reducing and oxidizing atmospheres were investigated. FactSage, X-ray diffraction (XRD) and scanning electron microscopy/electron dispersive X-ray (SEM-EDX) were applied for accounting for the transformation of V and Ni containing minerals and the mechanism of influences on AFTs by V and Ni. The prediction equation is established between V and Ni content and AFTs for guiding the blend of petroleum coke and coal.
3. RESULTS DISCUSSION 3.1. Effect of V on Ash Fusion Behavior in Reducing Atmosphere. As shown in Figure 1, AFTs increased with the ratio of V2O5 monotonically in reducing atmosphere. It was obviously that the increasing extend of AFTs was different with increasing vanadium in ash. There ranges were divided based on the increasing extent caused by addition of V2O5. When the addition of V2O5 was lower than 6%, flow temperature (FT) was almost constant. AFTs increased slightly while the addition of V2O5 exceeded 6%, and the significant increase appeared when the addition was above 21%. FT was beyond 1550 °C, as the content of V2O5 was higher than 27%. AFTs were also related with mineral contents. The trend indicated that refractory minerals were formed with V2O5 under reducing
2. EXPERIMENTAL SECTION 2.1. Samples. The typical anthracite from Shanxi, China, was selected for this study. Coal ash was prepared at 815 °C according to GB/T1574-2007. Inductively coupled plasma−atomic emission spectrometry (ICP-AES) was used to characterize ash composition based on ASTM D6349. CaO (12%) is added on dry ash basis for satisfying slag tapping in entrained flow gasifier. Raw ash and CaO were blended completely in agate mortar, and the mixture is denoted as ACa. The chemical compositions of raw ash and ACa are given in Table 1. ACa was blended with different ratios of V2O5 and NiO. The blends were stored in a desiccator after blending sufficiently. B
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when the addition of V2O5 was lower than 6%. AFTs increased significantly when V2O3 content continues to increase. FT was generally defined as the temperature at which the formed liquid phase reaches 75% of the total material.12 Therefore, the significant increase of FT appeared when the content of solid was above around 25%. This contributes to the higher increase rate of FT when the addition is above 21%. The addition of V2O5 increases the surface stickiness of ash slightly because DT is broadly identified with an index of surface stickiness. ST shows an association with HT, which is broadly identified plastic distortion or sluggish flow.13 HT increased with increasing ST, and the temperature intervals between them also gradually increased because V2O3 leads to higher viscosity and lower solution activity. Tliq increased sharply to 1940 °C after V2O5 was added into coal ash and kept constant with increasing content of V2O5. Tliq is determined by melting of all minerals, and AFTs are related to the subliquidus phase formed during heating. Thermodynamic results by FactSage indicated no other V-minerals existed and no reactions between V2O3 and ash composition happened even at high concentration of vanadium. Above 1475 °C, V2O3 did not form eutectic matter with other minerals. Vanadium was only included as solid or liquid compounds and not as slag solution species. It suggests that V2O3 is stable in reducing conditions.4,8 FactSage did not have the databases about the interaction between VOx and slag;6,9 so, results by FactSage are not sufficient to explain interaction between VOx and slag, and Tliq is not accurate. Jak14 indicated the linear relationship between AFTs and Tliq, and also proved by others, was expressed as AFTs = a + b × Tliq.15 However, AFTs of ash containing vanadium did not fit the rule established between AFTs and liquidus temperature. The linear relation between AFTs and vanadium content existed instead. In Figure 1, the linear relationship between FT and V2O5 content could be established in the three ranges of V2O5 content, and R2 values are all above 0.94. The relationship can be used for predicting AFTs of ash with V under reducing atmosphere. During the process of ash fusion, minerals interact and fuse into liquid, which leads to the variations in the mineral components and their contents. Therefore, the fusibility of ash may be determined by the components and the quantities of crystals, which can be easily determined by XRD. Thus, the mineral evolution during the process of ash fusion was investigated in this work. The transformations of minerals in raw ash were characterized for comparison. In Figure 3a, raw ash was mainly composed of quartz, illite (KAl2AlSi3O10(OH)2), and lime. Anorthite and gehlenite formed at 1100 °C, and the contents of lime and quartz decreased. Lime disappeared at 1200 °C. Gehlenite decomposed and transformed into anorthite at 1300 °C. Anorthite is a major crystalline mineral from 1300 to 1500 °C. With 20% V2O5 added under reducing temperature, a large portion of lime transformed with V 2 O 5 into calcium orthovanadate (Ca3V2O8), but lime was still found at 1200 °C, which may be from the decomposition of calcium orthovanadate (Ca3V2O8) initially at 1200 °C. V2O5 was reduced to karelianite (V2O3) from 1100 °C and stayed stable at 1500 °C. Karelianite is only crystal containing V above 1400 °C. According to the melting points in Table 2, karelianite caused the increase of AFTs under reducing atmosphere. Karelianite exhibits the solubility in slag solution,9 and low content of karelianite could be into slag solution; so, AFTs
Figure 1. Influences of V2O5 on ash fusion temperatures under reducing atmosphere.
atmosphere. In a mild reducing atmosphere, vanadium exists as V2O3 according to the thermodynamic calculation by FactSage in Figure 2. It is consistent with the literature that vanadium
Figure 2. Phase assemblage−temperature curves calculated from FactSage software with SiO2−Al2O3−Fe2O3−CaO−MgO−TiO2− SO3−K2O−Na2O−V2O5, including V2O5 contents under reducing atmosphere: (a) ACa; (b) ACa including 20% V2O5).
mainly exists as V2O3 in the reducing atmosphere.4,8 All vanadium remained as V2O3, and its melting point is 1940 °C. In coal ash, FT is related to the rate of solution of the most refractory minerals. FT of ash containing V is mostly influenced by the rate of solution of the V2O3. A small amount of V2O3 may be dissolved in the liquid slag for V2O3 forms an equilibrium solid solution with Fe2O3 or Al2O3, 9 so FT is stable C
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Figure 4. Ash fusion temperatures vs content of V2O5.
1077 and 1372 °C.7 In this study, VO2(g) may not be formed because the interaction between V2O5 and other minerals blocks the formation of gas VO2. Phase assemblage of slag containing V from FactSage software was shown in Figure 5. CaO(V2O5) formed with V2O5 decomposes at 1100 °C and generates V2O5(l), which transforms to V2O4(l). The interaction between liquid V2O5 and slag is not able to be predicted for lack of solution data in FactSage database. The formation of CaO(V2O5) suggests that the reaction between CaO and V2O5 may happen in oxidizing
Figure 3. XRD patterns of different temperature slags added 20% V2O5: (a) ACa in reducing atmosphere, (b) ACa added 20% V2O5 in reducing atmosphere. L: lime (CaO). V: V2O5. M: mullite (3Al2O3· 2SiO2). Q: quartz (SiO2). G: gehlenite (Ca2Al2SiO7). An: anorthite (Ca(Al2Si2O8). Vo: V2O3. C1: (Ca3V2O8). C2: (Ca2V2O7). P: portlandite (Ca(OH)2).
Table 2. Melting Temperatures of Pure Compounds Containing V and Ni in Slag compound
symbol
melting temp. (°C)
vanadium trioxide vanadium dioxide vanadium pentoxide calcium metavanadate calcium pyrovanadate calcium orthovanadate nickel nickel oxide nickel disulfide spinel
V2O3 VO2 V2O5 CaV2O6 Ca2V2O7 Ca3V2O8 Ni NiO Ni3S2 NiAlO4
1940 1377 675−690 1051 1288 1653 1455 1984 788 1727
showed no obvious variation when content of V2O5 is lower than 6%. 3.2. Effect of V on Ash Fusion Behavior in Oxidizing Atmosphere. The variation of ash fusion temperatures under oxidizing atmosphere against addition of V2O5 was presented in Figure 4. AFTs decreased linearly with increasing V2O5 content. It indicates that minerals formed with V2O5 in the oxidizing atmosphere are fluxing minerals. V2O5 is a major stable formation of vanadium oxides below 1077 °C. V2O5 melts at 690 °C and forms eutectic with other minerals, which decreased AFTs. V2O5 also transforms into VO2(g) between
Figure 5. Phase assemblage-temperature curves calculated from FactSage software with SiO2−Al2O3−Fe2O3−CaO−MgO−TiO2− SO3−K2O−Na2O−V2O5, including different V2O5 contents under oxidizing atmosphere: (a) 10% V2O5, (b) 20% V2O5. D
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atmosphere. This is consistent with the literature in which CaV2O6, Ca3V2O8 and CaV3O7 are observed at 600−900 °C in vanadium-containing slag calcium roasting process.16 Tliq is consistent with melting temperature for subliquidus phase and is not changed by addition V2O5. Tliq also kept as a constant in the experimental range with increasing V2O 5 content. Obviously, the relationship between AFTs and Tliq could not to be established, but the linear fitting between AFTs and V2O5 existed and R2 was 0.99. The relationship is feasible to predict the AFTs of ash with V under oxidizing atmosphere. XRD patterns of different temperature slags with addition of 20% V2O5 under oxidizing temperature are presented in the Figure 6. Comparing with quenched slags under reducing Figure 7. Ash fusion temperatures vs content of NiO in reducing atmosphere.
Figure 6. XRD patterns of different temperature slags added 15% V2O5 in oxidizing atmosphere air. L: lime (CaO). V: V2O5. M: mullite (3Al2O3·2SiO2). Q: quartz (SiO2). G: gehlenite (Ca2Al2SiO7). An: anorthite (CaAl2Si2O8). Vo: V2O3. C1: (Ca3V2O8). C2: (Ca2V2O7). P: portlandite (Ca(OH)2).
Figure 8. XRD patterns of different temperature slags containing 20% NiO in reducing atmosphere. L: lime (CaO). N: NiO. Q: quartz (SiO2). An: anorthite (CaAl2Si2O8). Ni: nickel (Ni). G: gehlenite (Ca2Al2SiO7).
atmosphere, lime diffraction peaks in the quenched slags under oxidizing temperature disappeared and peaks of Ca2V2O7 were observed at 1100 °C, which suggested that the reaction between CaO and V2O5 was faster in oxidizing atmosphere. While the temperature was over 1100 °C, calcium vanadium oxide decomposed into CaO and V2O5(l) because Ca2V2O7 was not stable over 1200 °C. Comparing with ACa slag, it is found that no peaks of crystal anorthite appeared at 1500 °C. This indicates that V2O5(l) formed eutectic matter with anorthite, promoted dissolution of anorthite, and decreased AFTs. 3.3. Effect of Ni on Ash Fusion Behavior in Reducing Atmosphere. AFTs increased slowly with NiO content under reducing atmosphere, but FT only rises to 1432 °C as NiO content increases to 35%. NiO was reduced to solid Ni in the reducing atmosphere (Figure 7).7 More NiO was added, and a higher content of solid Ni was attained, which also explained the slowly increasing trend. To verify the transformation of Ni in reducing atmosphere, XRD was also applied to analyze the quenched sample at different temperatures. In Figure 8, the reducing reaction of NiO began at 1100 °C, and it was fully reduced until the temperature was 1400 °C under reducing atmosphere. Ni is still observed as crystalline even at 1500 °C, as the melting point was around 1455 °C. Based on the XRD results, nickel may not participate in the reactions with other silicates or aluminosilicates under reducing atmosphere, and formed refractory mineral, because no other crystal minerals containing nickel are found from 1100 to 1500 °C. Hence, the
solid Ni is the major reason for increasing AFTs. AFTs only increase slowly with content of nickel. It is suggested that the effect of solid phase on AFTs also depends on the properties of solids and its interaction with other minerals. Phase composition attained by FactSage also proved that no Ni-containing silicates or aluminosilicates were found under the reducing atmosphere, as shown in Figure 9. However, Tliq was not changed as NiO addition increase, and it was consistent with the dissolution temperature of anorthite. The linear relation between AFTs and NiO content was also established as FT = 0.98[NiO] +1400.37. It was clear that the influence by NiO on AFTs in reducing atmosphere was weak. 3.4. Effect of Ni on Ash Fusion Behavior in Oxidizing Atmosphere. In Figure10, AFTs under oxidizing atmosphere first decreased slightly as NiO was added in ash. No significant variation of FT was observed in the range of NiO from 3% to 19%. FT increased significantly when NiO content was above 19%. NiO is stable in combustion condition without considering effect of coal minerals.7 The melting point of NiO is 1984 °C, but AFTs decrease slightly at first and increased significantly with addition of NiO. It indicates that refractory and fluxing minerals are generated by NiO and other minerals. In Figure 11, refractory and fluxing minerals were Ni-spinel and Ni-olivine respectively. The olivine is melted around 1300 E
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Figure 12. XRD patterns of different temperature slags containing NiO under oxidizing atmosphere. L: lime (CaO). N: NiO. Q: quartz (SiO2). An: anorthite (CaAl2Si2O8). S: spinel (NiAl2O4). O: olivine (Ni2SiO4). G: gehlenite (Ca2Al2SiO7).
Figure 9. Phase assemblage-temperature curves calculated from FactSage software with SiO2−Al2O3−Fe2O3−CaO−MgO−TiO2− SO3−K2O−Na2O−NiO including 10% NiO contents under reducing atmosphere.
AFTs. Ni-spinel is the major refractory mineral above 1400 °C, and the content increased at high temperatures over 1400 °C; thus, AFTs increased with Ni content. Meanwhile, the intensity of anorthite diffraction peaks decreased with temperature increasing and disappeared at 1500 °C. It indicated that Al2O3 and SiO2 from anorthite decomposition transformed into Nispinel. Ni-spinel is more stable than anorthite, which is consistent with thermodynamic calculation by FactSage. In the pseudoternary phase diagram SiO2−Al2O3−CaO− NiO (Figure 13), the composition of coal ash initially lay in the
Figure 10. Ash fusion temperatures vs content of NiO in oxidizing atmosphere.
Figure 13. Calculated liquidus temperatures in the SiO2−AlO3− CaO−NiO system on the pseudoternary section in oxidizing atmosphere. Figure 11. Phase assemblage-temperature curves calculated from FactSage software with SiO2−Al2O3−Fe2O3−CaO−MgO−TiO2− SO3−K2O−Na2O−NiO including 20% NiO contents under oxidizing atmosphere.
anorthite region and transformed to the spinel (NiAl2O4) region with NiO content increasing; so, AFTs increased greatly with high concentration of NiO. So, Tliq first decreased slightly and then increased with the addition of NiO in Figure 8. The variation with NiO content showed similar trend to AFTs. Tliq is feasible to predicate FT by FT = 427.50 + 0.63Tliq (R = 0.69). 3.5. Co-effects of V and Ni on Ash Fusion Behavior. The petroleum coke simultaneously contains V and Ni, but the V and Ni content of different petroleum cokes varied in a large
°C. Obviously, the formation of olivine decreased the AFTs, but the spinel increased AFTs, as the melting point of spinel was around 1727 °C. As shown in Figure 12, Ni-spinel and Niolivine are formed. Ni-olivine is only observed at 1200 °C, and this is consistent with thermodynamic calculation by FactSage. Ni-olivine turns to solution at 1300 °C, and decreased the F
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range. For investigating additivity of effects of V2O5 and NiO on ash fusibility, V2O5 and NiO with weight ratio of 1:1 were added in ACa. The coeffects of V2O5 and NiO on ash fusibility were investigated. The coexisting of V2O5 and NiO was labeled as VNiO in this study. Figure 14 shows the plots of fusion
Figure 14. Flow temperature vs content of V2O5 and NiO curves in reducing atmosphere.
temperatures against the content sum of V2O5 and NiO. FT of coal ash samples did not change significantly with increasing addition of VNiO until the VNiO content reached about 6%. At higher concentration of VNiO, AFTs significantly increased. When VNiO content was above 22%, AFTs increased at a higher rate than AFTs of low VNi ash. FT was above 1550 °C when VNiO content was more than 35%. Generally, this trend is very similar with the variation of AFTs with V2O5, and the variation curve of AFTs with VNiO lies between V2O5 and NiO. V2O3 is more refractory than Ni. FT is mostly influenced by the rate of solution of V2O3. This could explain the AFTs with VNiO approached to AFTs with V2O3. The effects of V2O5 and NiO on AFTs do not show good additivity. Figure 15a showed XRD patterns of different temperature slags under reducing atmosphere containing VNiO. NiO transformed into Ni at 1100 °C, and was fully reduced to Ni at 1400 °C. This trend was consistent with the slags containing NiO and without V2O5. V2O3 peaks occurred at 1400 °C, which was the same with the slags with V2O5 and without NiO. XRD show no reaction between V2O3 and Ni; so, V2O3 is still the last melting mineral in the slag with VNiO, and this is consistent with the slag with vanadium. When the content of V2O3 reaches to a certain amount, the effect of V2O3 on AFTs tends to be significant, and AFTs with VNiO deviates from AFTs with NiO evidently. XRD also indicated that the influences on AFTs by VNiO were caused by V2O3 when the addition content was higher than 21%. In Figure 16, AFTs in oxidizing atmosphere decreased with increasing addition of VNiO until the VNiO content reached about 22%. At higher addition of VNiO, AFTs of coal ash samples increased, but more slowly than the decreasing rate. The variation curve of AFTs with VNiO also lies between V2O5 and NiO. The significant interaction effect of V2O5 and NiO on AFTs was also not observed under oxidizing atmosphere. Liquid matter formed with V2O5 promote dissolution of spinel, and V2O5 and spinel may form eutectic matter. When the addition ratio is higher than 22%, the effect of spinel formation increased FT.
Figure 15. XRD patterns of different temperature slags containing VNiO: (a) reducing atmosphere CO/CO2 = 6:4, (b) oxidizing atmosphere air. C: lime (CaO). V: V2O5. N: NiO. Q: quartz (SiO2). An: anorthite (CaAl 2 Si 2 O 8 ). Ni: nickel (Ni). G: gehlenite (Ca2Al2SiO7). Vo: V2O3. S: spinel (NiAl2O4).
Figure 16. Flow temperatures vs content of V2O5 and NiO in oxidizing atmosphere.
XRD patterns of different temperature slags containing VNiO under oxidizing atmosphere were presented in Figure 15b. It was observed that NiO was fully changed into spinel at 1300 °C, but NiO still existed in slag only containing NiO at 1300 °C. The difference indicated that V promoted the formation of spinel at high temperature.4 Ca2VO7 peaks were not observed in XRD for the low concentration of Ca2VO7, and it was difficult to be detected by routine XRD but approved by FactSage.17 Formation of Ca2VO7 consumed a certain amount of CaO and reduced anorthite generation at low temperature, and more Al2O3 was left and participated in the reaction with G
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Figure 17. SEM-EDX of slags at 1500 °C: (a) ash slag under reducing condition; (b) slag containing vanadium under reducing condition; (c) slag containing vanadium under oxidizing condition, (d) vanadium dispersion in the slag under oxidizing condition.
Table 3. EDX Analysis Results of Slags Containing Ni at 1500 °C EDX analysis results (at. %) samples
Ni
Al
Si
Ca
Fe
S
K
Na
O
1 2 3 4 5 6
59.90 86.87 0.00 100 1.02 13.19
0.00 0.00 26.07 0.00 19.62 50.31
0.00 0.00 35.97 0.00 34.81 4.00
0.00 0.00 11.28 0.00 12.82 0.00
0.00 9.23 1.25 0.00 0.00 3.61
40.10 0.00 0.00 0.00 0.00 0.00
0.00 0.00 1.02 0.00 0.00 0.00
0.00 0.00 1.39 0.00 0.00 0.00
0.00 3.90 23.02 0.00 31.73 28.89
The effects of solid matter and liquid matter on AFTs exhibit no additivity. It is usually considered that the eutectic matter was formed between solid matter and liquid matter. Hence, AFTs tend to be lower than excepted value when refractory mineral and liquid mineral are added simultaneously in the ash. AFTs showed different sensitivity to the various types of solid matter. In addition, the effects of different types of solid matter on AFTs exhibited no additivty. It also indicated that the
NiO; so, V played the positive effect on the formation of spinel. Liquid phase V2O5 should be regenerated by the decomposition of Ca2VO7 at 1300 °C and lower the AFTs. The concentration of spinel, increasing with addition of NiO, elevated AFTs obviously. Comparing with slag containing NiO at 1500 °C, crystal spinel in slag containing VNiO at 1500 °C also proved that liquid matter composed of V2O5 forms eutectic matter and promote dissolution of spinel. H
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interaction of solid matter with liquid matter is different, and was also an important factor influencing AFTs. 3.6. Morphology of V and Ni during Melting. While V and Ni was added into coal ash, phase separation appeared even at higher temperatures; so, SEM-EDX was applied for the quenched slags to further clarify the effect of behaviors of refractory minerals on the fusibility of ash. The quenched slag is regarded to keep the morphology of high temperature. Under SEM observation, the melted liquid presented dense layer structure and had a smooth surface, and unmelted particles presented a loose structure and had irregular shape.18 Figure 17a indicated 1500 °C ash slag formed homogeneous liquid siliceous slag. After vanadium was added, both uniformed phase by liquid and unmelted irregular particles were found. EDX analysis indicated that the irregular particles were richer in vanadium compared with the composition of liquid region (Table 3). Liquid phase only contained 7.93% V (atom percent); the unmelted particles contain 81.84% V. The certain content V in liquid matter suggests that little V2O3 dissolves in liquid slag, which may be the reason that AFTs is not variable as V content is below 6%. The formation of refractory particles enriched V increased AFTs when the amount of particles increased with V content. The 100% V2O3 particles were not found in the slag and vanadium always combined with minor proportion Al and several other elements. It suggested that Al2O3 and Fe2O3 had some extent solubility in V2O3. This was consistent with the literature.19 Figure 17c showed that under oxidizing condition homogeneous liquid slag was formed at 1500 °C, and the vanadium was evenly dispersed in the slag (Figure 17d). It also proved that the crystal anorthite formed eutectic matter with the liquid solution. Liquid V2O5 fully dissolved in the liquid region. The strengthened association exists between V2O5 and slag under either atmosphere. The spherical particles around 900 μm were found at 1500 °C in Figure 18a, and the cross section and surface of the regular particle were analyzed by EDX. The outside surface of the particle was golden by visual observation. Ni3S2 was observed to cover on the Ni particles because of the reaction between nickel and SOx. The size of the particle indicated that Ni particle was prone to agglomerate with other Ni particles at high temperature.20 Agglomerated Ni particles absorbed Ni3S2, which also contains nickel. The melting point of Ni3S2 is as low as 600 °C; so, the melted layer may promote the particle to move easily. Other regular Ni particles of smaller sizes were found embedded in aluminosilicates (Figure 18b). The agglomeration behavior of Ni indicated the interaction between nickel and normal slag composition was rare under reducing atmosphere. The uniform phase was liquid phase at 1500 °C, but only a trace amount of Ni was found in the liquid phase, which was also significantly lower than Ni average content in ash. Results by FactSage also indicated liquid Ni was not soluble with slag. Ni was less involved in the chemical reactions and physical melting process of slag, which explained slight increase of AFTs with NiO under reducing atmosphere. In Figure 18c, it was noticed that the slag containing NiO under oxidizing condition was also heterogeneous. Infusible mineral presented as short dash line forms existed in the liquid slag. The infusible matter was mainly composed of nickel aluminate by EDX (Table 4). In addition, in the liquid phase the content of Ni was 1.02% (2.19 wt %). It indicated that major proportion of NiO participated in the formation of Nispinel, contributes to the increase of AFTs.
Figure 18. SEM photomicrographs of slags containing 20% NiO at 1500 °C: (a) SEM-EDX of a spherical particle under reducing atmosphere; (b) SEM-EDX of slag surface under reducing atmosphere; (c) SEM-EDX of slag surface under oxidizing atmosphere.
SEM-EDX of slag with VNiO under reducing atmosphere was shown in Figure19a. It was clear that some unmelted particles dispersed uniformly in the liquid phase with a dense structure. The unmelted minerals were richer in V than in the liquid area and did not contain Ni. In addition, a regular spherical particle wrapped with mainly nickel sulphide was also observed in Figure 19a. This was same with the particle of Figure 18a. The particle was aggregation Ni covered by nickel sulphide. The particle was formed mainly with aggregation of Ni and without V. The V containing phase is usually associated I
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Table 4. EDX Analysis Results of Slags Containing V and Ni at 1500 °C EDX analysis results (at. %) samples
V
Ni
Al
Si
Ca
Fe
S
Mg
O
1 2 3 4 5
59.90 0.00 0.60 3.72 5.56
0.00 63.67 0.00 13.71 1.39
14.97 0.00 23.51 28.13 34.81
11.89 0.00 34.99 30.08 37.16
3.84 0.00 15.36 4.89 14.71
0.00 0.00 0.00 2.90 1.65
0.00 36.33 0.00 0.00 0.00
0.00 0.00 0.00 1.41 0.66
9.79 0.00 25.55 15.15 18.24
melting temperatures; so, V was the major factor decreasing AFTs. The fusibility of ash was determined by relative concentrations of the fluxing minerals and the refractory minerals.13 The formations of karelianite, liquid V2O5, and spinel changed the fusibility significantly. However, Ni melted and agglomerated to form regular spherical particles at high temperature only influenced the fusibility slightly. It seems that the association of refractory and fluxing minerals is an important factor for fusibility, as indicated in Figure 19. Karelianite exhibited as refractory minerals associated with Al, Ca, and Si, and also spinel that occurred with Si and Fe. Karelianite and spinel associated with the liquid formed with fluxing minerals played a role of framework to block the fusion of ash; so, karelianite and spinel changed the fusibility significantly. The strong association exists between liquid V2O5 and other minerals for V2O5 evenly dispersed in the slag. It was clear that liquid V2O5 promotes ash fusion. On the contrary, Ni melted and agglomerated to form regular particles, and no association was found between Ni and other minerals; as a result, Ni only caused a slightly change on AFTs. The effect of association of refractory minerals and fusible minerals on ash cone fusion is schematically, and two different association patterns between solid mineral and liquid phase are classified in Figure 20. The coal ash cone in the AFT test is a
Figure 19. SEM-EDX of slags containing 20% VNiO at 1500 °C: (a) slags under reducing atmosphere; (b) slags under oxidizing atmosphere).
with Al and several other elements, which coincided with slag only with V added at 1500 °C. Ni and V were not found simultaneously in any infusible particles, and also, no association between Ni and V appears in the uniformed phase. V containing minerals associated with Al and several other elements formed refractory matter, which caused the increase of AFTs under reducing atmosphere, which also explained the influence of VNi under reducing atmosphere being almost identical with the influence of V. In Figure 19b, slag under oxidizing atmosphere was a heterogeneous slag and was composed of the smooth liquid and the unmelted matter. The infusible matter exhibited similar form and dispersion in the liquid phase with the slag of Figure 18c. Higher content of nickel in unmelted matter was detected than that in the liquid region, and the infusible matter was Ni−spinel. Spinel formed with NiO and Al2O3 contributes to the increase of AFTs. The content of V in the liquid region was far more than that in infusible matter, and V containing minerals exhibited low
Figure 20. Association between mineral and liquid phase during melting.
heterogeneous and consists of various particles with different components. As the temperature increases, liquid phase formed by minerals of low melting temperature. The association of unmelt particles and liquid phase is initially established. When the strong association is formed, as shown in Figure 20A, the solids are uniformly distributed in the liquid phase, because the chemical association is larger than the surface tension driving the agglomerate of solid particles. The uniformed solid particles are regarded as the skeleton in the ash cone; so, the dissolution rate of solids decreases and AFTs increases obviously. In Figure J
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Notes
20B, due to the weak association between solid matter and liquid phase, the solid matter tends to pile up under or on the liquid phase determined by the different density of solid and liquid phase. The influence of unmelted solids is weakened on AFTs. In this study, the 100% V2O3 particles were not found in the slag, and vanadium was always combined with minor proportion of Al and several other elements, because V atoms in karelianite could be substituted with other elements,19 such as Al and Fe. The reaction between with karelianite and aluminosilicate formed the intermediate phase which correlated the refractory minerals with liquid aluminosilicates. The association promoted distribution of refractory minerals and then increased AFTs. Ni was prone to agglomerate at high temperature for surface tension between nickel and liquid phase was high; so, the association of nickel particles and liquid aluminosilicates was not established, which explained AFTs did not change significantly with NiO increasing under reducing atmosphere.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from the National Basic Research Program of China (2010CB227005-02), the National Natural Science Funds (21006121), the Youth Foundation of Shanxi Province (2010021008-2), and Joint Foundation of Natural Science Foundation of China and Shenhua Group Corporation Ltd. (U1261209) was gratefully appreciated.
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4. CONCLUSIONS The transformation of minerals including V2O5 and NiO at high temperatures was affected by redox property of atmosphere. In the reducing atmosphere, V minerals are present as stable V2O3 and NiO was reduced to Ni. In the oxidizing atmosphere, V2O5 is a major stable form and NiO forms olivine and spinel with SiO2 and Al2O3, respectively. A small amount of V2O3 can dissolve in slag solution; so, AFTs keep stable when V content is below 6%. V2O3 associated with Al, Ca, and Fe forms refractory matter, which causes the increase of AFTs significantly when V content is above 6%. V2O5 with low melting point forms eutectic matter with anorthite and decreased AFTs significantly. NiO was reduced to Ni, which aggregates to regular small balls and increases AFTs slightly. NiO forms olivine and spinel with SiO2 and Al2O3, respectively, and decreases anorthite production; so, AFTs first decreased slightly and then increased significantly. The effects of V2O5 and NiO on AFTs did not show great additivity. Although the variation curve of AFTs with VNiO almost lies between V2O5 and NiO, AFTs with VNiO approached to AFTs with V2O5 because V2O3 is the most refractory minerals under reducing atmosphere. V2O5 forms eutectic matter with spinel under oxidizing atmosphere. The liquidus temperature calculated by FactSage was not able to predict the fusion temperatures of ash containing V2O5 under both atmospheres and those of NiO in reducing atmosphere. However, the linear relationship between FT and V2O5 or NiO content can be established. AFTs show different sensitivity to various solid minerals. Except for the melting point and content of solid matter, the association between refractory minerals and liquid phase is also an important factor influencing effect of solid matter on AFTs. Tight relation between mineral and liquid phase causes the uniformed distribution of unmelted solid particles and increases AFTs significantly. Conversely, weak association leads to the agglomerate of solid particle in liquid phase and influences AFTs weakly. The strength of association may be relate with the surface tension and chemical interaction between solid and liquid.
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