Article pubs.acs.org/EF
Effect of Vanadium on the Petroleum Coke Ash Fusibility Jiazhou Li,†,‡ Jiantao Zhao,*,‡ Xin Dai,†,‡ Jin Bai,‡ and Yitian Fang‡ †
University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi 030001, People’s Republic of China
‡
ABSTRACT: Ash fusibility is closely related to ash slagging, which has significant impact on the clean and efficient utilization of petroleum coke (petcoke). Some mineral elements in petcoke, especially vanadium (V), are considered to be responsible for ashrelated slagging during thermal conversion of petcoke. This study investigates the effect of vanadium pentoxide (V2O5) on synthetic petcoke ash fusibility from different perspectives, including V2O5 content variation, temperature rising, and atmosphere change. X-ray diffraction (XRD) and scanning electronic microscopy (SEM) were used to determine the mineral transformation and surface morphology of ash and slag at a high temperature. The ash-melting process was simulated by thermodynamic equilibrium calculations via a multicomponent system. The results show that ash fusion temperatures (AFTs) of synthetic ash samples vary markedly with the V2O5 content increasing. Moreover, AFTs exhibit a significant difference between reducing and oxidizing atmospheres, which can be ascribed to the different transformation behaviors of minerals in ash under different atmospheres. With the V2O5 content increasing, the V-containing species formed under a reducing atmosphere are vanadium trioxide (V2O3) and coulsonite (FeV2O4). Both can contribute to the progressive increase of AFTs because of their high melting points. However, AFTs under an oxidizing atmosphere initially decrease and then increase slowly. When the addition of V2O5 is below 25%, low-melting calcium pyrovanadate (Ca2V2O7) formed under an oxidizing atmosphere decreases the AFTs. When the V2O5 content continues to increase, the formations of high-melting spinel NiAl2O4 and V-bearing amorphous phase lead to the increase of AFTs.
1. INTRODUCTION With the further processing of heavy crude oils, the production of petroleum coke (petcoke) is steadily increasing.1 There is also gradually more inferior petcoke in response to the quality deterioration of crude oil in refineries. Both combustion and gasification are well-proven technologies that use inferior petcoke more economically and reasonably. However, inferior petcoke contains high levels of heavy metals V, Fe, and Ni, especially V, whose concentration can be up to 1500 ppm in petroleum samples.2 V is one of the most concerned trace elements because it is responsible for ash-related slagging during thermal conversion of petcoke.3−5 Characteristics affecting ash slagging include fusibility and rheological property, of which, fusibility is considered to be the most significant parameter.6 Four characteristic temperatures can be applied for describing ash fusibility and ash fusion temperatures, namely, initial deformational temperature (DT), softening temperature (ST), hemispherical temperature (HT), and flow temperature (FT).7 Petcoke ash fusibility is closely associated with the ash composition. It should be mentioned that the mass percentage of V2O5 in ash accounts for about 20% and sometimes even higher.3 In the petcoke ash-melting process, V-containing minerals may be produced by the reactions of V with other associated minerals and then affect the ash fusibility to some extent. Therefore, it is of significance to explore the effect of V on ash fusibility and the mineral transformation mechanism. There have been a number of studies focused on the transformations and reactions of V. Wang et al.8 investigated the effect of V on the coal ash fusion temperatures (AFTs). They found that AFTs increased monotonically under a © XXXX American Chemical Society
reducing atmosphere. V2O5 could react with CaO to form calcium orthovanadate (Ca3V2O8). Simultaneously, V2O5 was reduced to V2O3 initially at 1100 °C. In contrast, AFTs significantly decreased under an oxidizing atmosphere. Ca2V2O7 was formed at 1100 °C, while it decomposed into CaO and V2O5(l) above 1100 °C because Ca2V2O7 was not stable above 1200 °C. Frandsen et al.9 studied the behavior of V by thermal conversion calculations. It was demonstrated that V2O3(s) was the stable form of V below 1447 °C in standard reducing conditions, but the predominant V-containing species formed under an oxidizing atmosphere was V2O5(s) below 1077 °C. Chen and Lu10 reported that almost all V remained in ash during petcoke combustion. C3CaOV2O and V2O5 were the main two kinds of V-containing compounds in bottom ash. Jia et al.11 investigated the V compounds in ash from a circulating fluidized bed combustion (CFBC) boiler, firing 100% petcoke. It was shown that Ca2V2O7 was the main V species in ash. Bunt et al.12 examined the behavior of V in a fixed bed gasifier. The simulation demonstrated that, above 325 °C, V2O5 was reduced to V2O3 without volatilization of V species occurring. V2O3 was kept stable within the process of gasification. Nakano et al.13 studied the phase equilibria in synthetic coal−petcoke slags. They pointed out that, under reducing (CO/CO2 = 1.8) conditions, crystallized V2O3 and spinel (VFe2O4) phase were found in high-temperature (1200 °C) slag. Although there are several works on the transformation behavior of V in ash, little has been reported about the Received: October 31, 2016 Revised: February 17, 2017 Published: February 20, 2017 A
DOI: 10.1021/acs.energyfuels.6b02858 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 1. Ash Composition of a Real Shanxi Petcoke (wt %) petcoke
V2O5
NiO
SiO2
Al2O3
Fe2O3
CaO
MgO
TiO2
SO3
K2O
Na2O
P2O5
Shanxi
33.89
7.55
20.20
10.54
7.40
13.92
1.18
0.83
0.75
0.76
1.74
0.54
stable. Six analytical reagents were mixed evenly by blending them in pure ethyl alcohol and then drying at 100 °C for more than 10 h.24 2.2. AFT Test. The high-temperature ash fusibility tester (KY Corporation, China) was used to determine the characteristic temperatures. The measurements were carried out following the Chinese standard GB/T219-2008 under reducing (6:4 CO/CO2) and oxidizing (air) atmospheres.28 An ash cone with specific geometry was heated to 900 °C at 15 °C/min and then changed to 5 °C/min. In the procedure, DT, ST, HT, and FT were determined and recorded on the basis of the variation of the specific shapes of ash cones. 2.3. Quenching Experiments. Quenching experiments were performed in a high-temperature tube furnace, which provided the conditions, such as temperature and atmosphere, in the ash-melting process. The schematic representation of the furnace apparatus is shown in Figure 1. The synthetic ash sample filled in the corundum
influence of V on ash fusibility. The emphasis of this study is mainly on exploring the effect of V on ash fusibility and mineral transformation behavior in the ash-melting process. V can exhibit triple valences (V3+, V4+, and V5+) under different atmospheres at high temperatures;14 i.e., the behavior of V is tightly associated with the atmosphere. Consequently, it is necessary to investigate the impact of the atmosphere on the AFTs of ash containing V. AFTs obtained in a reducing atmosphere (6:4 CO/CO2) and an oxidizing atmosphere (air) have been widely used as the guide for ash slagging of gasification and combustion, respectively.8 In this study, the reducing (6:4 CO/CO2) and oxidizing (air) atmospheres were selected as experimental atmospheres. The ash-melting process was also predicted by thermodynamic equilibrium calculations using FactSage. This software15,16 based on the Gibbs energy minimization theory has been successfully performed to study the thermal conversion of minerals in ash17−19 because many data, such as ash liquidus temperature and slag content, cannot be obtained through the experimental method alone.19−21 As a result of the impurities and complex phases in ash, many investigations6,22−27 have attempted to simplify the system using surrogate materials. Ash is a mixture of oxides; the properties of ash at a high temperature might be similar to the synthetic ash composed by primary oxides.22,23 Moreover, the chemical composition of synthetic ash can be easily controlled. Therefore, synthetic ash samples with the main components of V2O5, NiO, SiO2, Al2O3, CaO, and Fe2O3 were taken as the research subjects. The V2O5 content was set as a variable, ranging from 0 to 40 wt %. That allows for systematic investigation of the mechanisms of the effect of V2O5 on petcoke ash fusibility and mineral transformation.
Figure 1. Schematic representation of the furnace apparatus.
crucible was heated under reducing (6:4 CO/CO2) and oxidizing (air) atmospheres. The thermal profile is set according to the AFT test. After the scheduled temperature and residence time (15 min) were reached, the sample was taken out, quenched by water immediately, and then collected for later testing. 2.4. Characterization and Testing. The chemical composition of the real Shanxi petcoke was analyzed by X-ray fluorescence (XRF) spectrometry (XRF-1800, Shimadzu, Japan) with a Rh target X-ray tube (50 kV and 40 mA). A RIGAKU D/max-rB X-ray power diffractmeter using Cu Kα radiation (40 kW, 100 mA, and Kα1 = 0.154 08 mm) was used to detect the mineral composition of quenched ash samples. The samples were scanned in the range of 10− 90° at 4° 2θ/min scanning speed with a step size of 0.01°. In addition, JSM-7001F scanning electron microscopy (SEM) was employed to observe the microstructure of the quenched ash samples. 2.5. Thermodynamic Equilibrium Calculations. The thermodynamic database FactSage was applied to calculate the components of the solid phase and change of the liquid phase at high temperatures (1100−1500 °C, with a 100 °C increment) with multiple components of V2O5−NiO−SiO2−Al2O3−CaO−Fe2O3. FactPS and FToxid databases were chosen for phase formation data, and the solution species formed were possibly all chosen from the FToxid database. Thermodynamic equilibrium calculations were carried out under reducing (6:4 CO/CO2) and oxidizing (air) atmospheres at 0.1 MPa. All possible reactions (homogeneous and heterogeneous) will reach thermodynamic equilibrium, provided that the Gibbs free energy of the system is at its minimum value. The lower the Gibbs free energy, the higher the priority of the reaction.29
2. EXPERIMENTAL SECTION 2.1. Synthetic Ash Samples. The ash composition of a real Shanxi petcoke is shown in Table 1. On the basis of this composition, synthetic ash samples were prepared with six pure oxides (V2O5, NiO, SiO2, Al2O3, CaO, and Fe2O3) that constitute more than 93 wt % of the components in real petcoke ash. Low-content oxides, such as MgO, TiO2, and K2O, are not included in synthetic ash samples to simplify the ash components. Considering the high levels of V2O5 in petcoke ash, the V2O5 content of synthetic ash samples is set up to 40 wt %. Table 2 shows the chemical composition of synthetic ash samples. The ratio of all components to each other (except V2O5) was
Table 2. Chemical Composition of Synthetic Ash Samples composition (wt %) sample
SiO2
Al2O3
CaO
Fe2O3
NiO
V2O5
1 2 3 4 5 6 7 8 9
40.40 38.00 36.00 34.60 32.80 31.00 29.40 28.00 26.20
20.20 19.00 18.00 17.30 16.40 15.50 14.70 14.00 13.10
18.70 18.00 17.10 15.70 14.50 13.60 12.30 10.70 9.70
10.50 10.20 9.60 8.90 8.40 7.70 7.00 6.40 5.70
10.20 9.80 9.30 8.50 7.90 7.20 6.60 5.90 5.30
0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 B
DOI: 10.1021/acs.energyfuels.6b02858 Energy Fuels XXXX, XXX, XXX−XXX
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3. RESULTS AND DISCUSSION 3.1. Effect of V on Ash Fusibility under a Reducing Atmosphere. The experimental AFTs of the synthetic ash samples under a reducing atmosphere are shown in Figure 2. It
Figure 3. XRD patterns of six synthetic ash at 1300 °C in a reducing atmosphere.
may be that FeV2O4 reacts with other minerals, such as Si and Al, to generate V-bearing amorphous matter. The peaks of V2O3 in the ash with 40% V2O5 are much stronger than those in the ash with 30% V2O5, which can be explained by the formation of V2O3 in reaction 2. To verify the experimental results, mineral transformation behavior at 1300 °C under a reducing atmosphere was calculated by the multicomponent system SiO2−Al2O3− CaO−Fe2O3−NiO−V2O5 in the thermodynamic FactSage modeling, as shown in Figure 4. With the addition of V2O5,
Figure 2. Effect of the V2O5 content on AFTs under a reducing atmosphere.
can be seen that AFTs increase monotonically with the addition of V2O5. When the addition is higher than 30%, FT has been beyond 1500 °C. The trend indicates that the formation of high-melting minerals occurs after V2O5 is added to ash under a reducing atmosphere. According to the literature,8,12,30,31 V is mainly present as V2O3 under reducing conditions. The melting point of V2O3 is 1940 °C, indicating that AFTs increase significantly when the content of V2O3 increases progressively. Furthermore, the interactions of V2O5 with other minerals, such as Fe, to form a high-melting eutectic may also result in the higher AFTs. 3.2. Effect of V on Mineral Transformation Behavior under a Reducing Atmosphere. During the ash-melting process, the interactions between V2O5 and other minerals (Ca, Fe, and Al) can change the mineral components and their content and then further influence the AFTs. Hence, the ash fusibility can be probed effectively on the basis of mineral components of ash at a certain temperature.32 The mineral composition of six synthetic ash samples (V2O5 content is 0, 5, 10, 20, 30, and 40%, respectively) at 1300 °C in a reducing atmosphere was identified by XRD to investigate the AFT modification mechanism. The XRD patterns of six synthetic ash samples are present in Figure 3. At 1300 °C, the minerals in the ash without V2O5 are mainly anorthite (CaAl2Si2O8), quartz (SiO2), and elemental nickel (Ni). However, the formation of FeV2O4 occurs when the addition of V2O5 is 5%, which may result from the reaction: 2V2O5 + Fe2O3 + 5CO → 2FeV2O4 (s) + 5CO2
Figure 4. Effect of the V2O5 content on minerals calculated by FactSage at 1300 °C in a reducing atmosphere.
the CaAl2Si2O8 and CaSiO3 contents decrease continuously, of which CaAl2Si2O8 changes from 55.1% (V2O5 content is 0%) to 35.7% (V2O5 content is 40%). SiO2 is kept almost unchanged (about 8%). When the addition of V2O5 is lower than 10%, the V-containing species formed in the system is only FeV2O4, which is consistent with the results of XRD. When the V2O5 content continues to increase, V2O3 is formed and its mass fraction increases progressively up to 28.3% (V2O5 content is 40%). Similarly, the peaks of V2O3 in XRD patterns become stronger when the V2O5 content increases from 20 to 40%. However, there exists discrepancy for FeV2O4 between XRD analysis and FactSage calculations. In thermodynamic calcu-
(1)
The high melting point (above 1500 °C) of FeV2O4 can be responsible for the increasing AFTs. When the addition of V2O5 reaches 20%, diffraction peaks of V2O3 appear because excessive V2O5 is reduced to V2O3 following the reaction: 2V3O5 + CO → 3V2O3 + CO2
(2)
V2O3 further leads to the increase of AFTs. Diffraction peaks of FeV2O4 disappear when the V2O5 content is 30%. The reason C
DOI: 10.1021/acs.energyfuels.6b02858 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
micrographs. As shown in Figure 5, panels b, d, and f represent synthetic ash samples that are magnified 3700 times. The micrograph of synthetic ash with the addition of 5% V2O5 shows a smooth surface and has a dense layer structure (panel b). With the V2O5 proportion further increasing, the structure turns to be looser and rougher in the order of synthetic ash with 5% V2O5 (panel b), synthetic ash with 10% V2O5 (panel d), and synthetic ash with 40% V2O5 (panel f), which suggests that the molten extent of synthetic ash with 5% V2O5 is the highest. These are consistent with the variation of AFTs in Figure 2. 3.3. Effect of the Temperature on Mineral Transformation Behavior under a Reducing Atmosphere. Ash fusion is a high-temperature process. Along with increasing the temperature, minerals in ash will undergo complex chemical transformations, which then further influence the ash fusibility. The mineral evolution of a synthetic ash sample (V2O5 content is 30%) from 1100 to 1500 °C under a reducing atmosphere was characterized in this study. As shown in Figure 6, the
lations, there is a increase from 1.9% (V2O5 content is 5%) to 8.8% (V2O5 content is 20%), with a decline to 1.3% (V2O5 content is 40%). However, in XRD patterns, the formation of FeV2O4 only occurs in the V2O5 content range of 5−20%. Once the V2O5 content exceeds 20%, the peaks of FeV2O4 disappear. According to the preceding analysis, it is concluded that the V2O5 content has a great impact on the mineral transformation behavior under a reducing atmosphere in the ash-melting process. Along with an increasing V2O5 content, the Vcontaining species formed in the system are V2O3 and FeV2O4. Both V-containing species can contribute to the increase of AFTs as a result of their high melting points. Ash fusion characteristics can be identified on the basis of the change of ash surface morphology.33,34 Figure 5 shows the
Figure 6. XRD patterns of synthetic ash with the V2O5 content of 30% from 1100 to 1500 °C in a reducing atmosphere.
synthetic ash between 1100 and 1200 °C is mainly composed of CaAl2Si2O8, SiO2, Ni, FeV2O4, and V2O3. It should be noted that the variation of diffraction peak intensity of mineral in XRD patterns indicates the change of its content.35−37 With the temperature rising constantly, the V2O3 content increases because the diffraction peak height of V2O3 increases markedly. The peaks of FeV2O4 disappear above 1300 °C. There may be two reasons to explain the decomposition of FeV2O4. The first is that FeV2O4 decomposes to form V2O3 above 1300 °C following the reaction:
Figure 5. SEM photomicrographs of synthetic ash samples with different V2O5 contents of (a and b) 5%, (c and d) 10%, and (e and f) 40% at 1300 °C under a reducing atmosphere.
FeV2O4 (s) → V2O3(s) + FeO(s)
(3)
The second is FeV2O4 reacting with other minerals, such as Si and Al, to generate amorphous matter. V2O3 remains stable from 1100 to 1500 °C and is the only crystal V-containing species above 1300 °C. 3.4. Effect of V on Ash Fusibility under an Oxidizing Atmosphere. Figure 7 shows the experimental AFTs of synthetic ash samples under an oxidizing atmosphere against the addition of V2O5. AFTs drop monotonically as the V2O5 content increases until it reaches 25%. However, with the V2O5
microstructure of synthetic ash samples with the addition of 5, 10, and 40% V2O5, which are quenched in ice water from 1300 °C under a reducing atmosphere. The quenched ash is considered to keep the morphology of high-temperature ash.8 It is generally believed that the melted particles have a dense layer structure and smooth surface, while unmelted particles have a loose structure and rough surface.23 Therefore, we can judge the melting extent of synthetic ash on the basis of D
DOI: 10.1021/acs.energyfuels.6b02858 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
The melting point of Ca2V2O7 is 1288 °C. Thus, the formation of Ca2V2O7 under an appropriate V2O5 content (
