Effect of Ash Composition (Ca, Fe, and Ni) on Petroleum Coke Ash

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Effect of ash composition (Ca, Fe and Ni) on petroleum coke ash fusibility Jiazhou Li, Xiaodong Chen, Yubo Liu, Qingan Xiong, Jiantao Zhao, and Yitian Fang Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 12 Jun 2017 Downloaded from http://pubs.acs.org on June 12, 2017

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Effect of ash composition (Ca, Fe and Ni) on petroleum coke ash fusibility

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Jiazhou Li,†, ‡ Xiaodong Chen,†, ‡ Yubo Liu,†, ‡ Qingan Xiong,†, ‡ Jiantao Zhao,*, †

3

Yitian Fang†

4

†

5

Academy of Sciences, Taiyuan, Shanxi 030001, China

6

‡

7

ABSTRACT: Ash fusion behavior is closely associated with ash-related problems

8

including fouling, sintering and slagging, which result in negative effect on the

9

utilization of petroleum coke (petcoke). Petcoke ash contains high levels of

10

vanadium (V), nickel (Ni), iron (Fe) and calcium (Ca). The chemical composition of

11

ash plays an intrinsic role in determining ash fusibility. To better understand the

12

modification mechanism of the ash fusion temperatures (AFTs), this study

13

investigates the influences of ash composition (CaO, Fe2O3 and NiO) on the

14

synthetic petcoke ash fusibility from the perspectives of ash composition change and

15

temperature rising. The AFTs of synthetic ash samples were identified by ash

16

fusibility tester. X-ray diffraction (XRD) and scanning electronic microscopy (SEM)

17

were applied to explore the relationships between the experimental AFTs and the

18

variation of mineral composition and microstructure of high-temperature ash slag.

19

Moreover,

20

SiO2-Al2O3-V2O5-CaO-Fe2O3-NiO system based on the FactSage modeling. The

21

results show that the AFTs of petcoke are closely related to the ash chemical

22

composition. As the CaO and Fe2O3 content increases, AFTs exhibit continuous

23

decline, while first decrease slightly and then increase with the increasing NiO

24

content, which are ascribed to the different mineral transformation behaviors of

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese

University of Chinese Academy of Sciences, Beijing 100049, China

the

ash

melting

process

was

1

ACS Paragon Plus Environment

predicted

by

the

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high-temperature ash slag. The dominant crystalline minerals formed in

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high-temperature ash slag with different CaO, Fe2O3 and NiO content are anorthite

27

(CaAl2Si2O8), nickel orthosilicate (Ni2SiO4), calcium pyrovanadate (Ca2V2O7) and

28

quartz (SiO2). Fe may form Fe-bearing amorphous matter with other minerals. The

29

synergistic effect between high-melting Ni2SiO4 and low-melting Ca2V2O7 may

30

contribute to the variation of AFTs, which was well validated through

31

thermodynamic equilibrium calculations.

32

1. INTRODUCTION

33

The production of petcoke is steadily expanding with the continuous

34

consumption of heavy crudes in refineries [1-4]. Petcoke is an attractive fuel because

35

of its particular advantages including low price, high fixed carbon, and low ash

36

content [5, 6]. It can be burned in boilers for heat and power production around the

37

world. However, the main disadvantages of this process are fireside slagging, fouling

38

and corrosion problems, which cause economic losses and reduce the lifetime of

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equipment [7, 8]. Thus, how to avoid slagging and fouling has been becoming a

40

significant research topic both in science and engineering.

41

The ash fusibility is considered to be an important factor for understanding the

42

slagging and fouling processes inside boilers [9]. Many experimental techniques

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[10-15] have been successfully applied to investigate the ash fusibility, such as ash

44

fusion temperatures (AFTs) test, thermomechanical analysis (TMA) and high

45

temperature processing microscope (HTPM), of which AFTs test is most widely used

46

to explore the ash fusibility. AFTs are able to give detail information on the ash

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softening and melting behavior. Moreover, AFTs also provide the indication on the

48

progressive melting of ash to slag [9]. Four characteristic temperatures, deformational

49

temperature (DT), softening temperature (ST), hemispherical temperature (HT) and 2


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flow temperature (FT) are identified by the variation of the specific shape of an ash

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cone. The changed shape indicates the melting extent of ash slag.

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There have been many studies performed to predict the AFTs. Jak [16] related

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AFTs to equilibrium phase diagram to put forward a new AFTs simulation approach.

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They found that the new developed F*A*C*T thermodynamic database for the

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Al-Ca-Fe-O-Si system could be successfully applied to predict AFTs. Rizvi et al. [17]

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predicted the ash fusion behavior by simultaneous thermal analysis (STA). It was

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demonstrated that STA could assess the physical and chemical changes occurred in

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the process of ash melting, as well as the possible reactions. Yin et al. [18] established

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a back-propagation (BP) neural network model to predict the AFTs. In comparison to

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traditional techniques, such as regression relationships and ternary equilibrium phase

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diagrams, BP neural network was more convenient and could achieve a better

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prediction effect of AFTs. AFTs have also been correlated with the minerals and

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chemical composition of coal ash. Both experiments and thermodynamic calculations

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are used to investigate the influence of coal ash composition on the ash fusibility.

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Song et al. [19, 20] combined experiments with FactSage to study the effects of CaO,

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Fe2O3 and MgO on the AFTs. The results showed that the AFTs had an initial decline

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and then followed an increase with CaO and Fe2O3 content increasing, which was

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well validated through FactSage. Van Dyk JC [21] investigated the effects of acidic

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components (Si, Al and Ti) on the ash flow temperature. It was shown that all acidic

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components SiO2, Al2O3 and TiO2 had positive effects on the ash flow temperature.

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Liu et al. [22] examined the relationship between ash composition (CaO, Fe2O3, K2O)

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and the AFTs. They pointed out that the AFTs decreased with the increasing Fe2O3

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content while changed little as the K2O content varied. For the effect of CaO, the

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AFTs initially decreased and then increased. Wang et al. [23] studied the influence of 3


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sodium oxides on the ash fusibility. It was found that Na2O decreased the AFTs

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effectively. Although the role of coal ash composition in ash fusibility is widely

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studied, little research has been performed to relate petcoke ash composition to ash

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fusion characteristics.

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The ash composition of coal is generally composed of Si, Al, Ca, Fe, K, Na and

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Mg. These inorganic elements will undergo complex mineral transformations in ash

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melting process, influencing the ash fusibility. However, the petcoke ash mainly

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consists of V, Ni, Ca and Fe [24]. The existing compositional difference between two

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ashes may cause different and unpredictable slagging and fouling problems due to the

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presence of V and Ni. The V transformation behavior and the effect of V on petcoke

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ash fusibility had been investigated in our previous works [25-27]. Thus, in this study,

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the fundamental experiments are carried out to explore the influence of petcoke ash

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composition (CaO, Fe2O3, NiO) on the ash fusibility, which may provide a solid

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foundation for the efficient and environmental utilization of petcoke.

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Due to the presence of the impurities and complex phases in ash, many

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researchers [19, 22, 23, 28-32] have been attempting to simplify the system using

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surrogate materials. The properties of ash at high temperatures might be similar to the

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synthetic ash composed by primary oxides [22, 28]. Furthermore, synthetic ash

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composition can be controlled accurately [23]. Thus, synthetic ash samples with the

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main components of SiO2, Al2O3, V2O5, CaO, Fe2O3 and NiO were selected as the

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research subjects. Note that, the content of CaO, Fe2O3 and NiO is set as variables,

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which allows to systematically investigate the effects of CaO, Fe2O3 and NiO on the

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AFTs and the mechanism of mineral transformations respectively. The ash fusibility

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tester is applied to measure the four characteristic temperatures, i.e., DT, ST, HT and

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FT. Moreover, two characterization methods of XRD and SEM are also combined to 4


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identify the mineral composition and microstructure of ash samples. Simultaneously,

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thermodynamic equilibrium calculations by FactSage are employed to establish

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multiple composition system of SiO2-Al2O3-V2O3-CaO-Fe2O3-NiO for predicting the

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ash melting process, which is to compare and then validate the experimental results.

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2. EXPERIMENTAL SECTION

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2.1. Synthetic Ash Samples.

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Five petcoke samples from Shanxi local oil refinery, Wuhan petrochemical Co.,

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Zhenhai petrochemical Co., Zibo Qilu Petrochemical Co., Ltd and Daqing Oilfield

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respectively, were collected for this study. Petcoke ashes were prepared at 700 oC for

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24 h in a muffle furnace according to the American Society of the International

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Association for Testing and Materials (ASTM-International) standard designation

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D4422-13, Standard Test Method for Ash in Analysis of Petroleum Coke [33]. Table

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1 shows the chemical composition of five petcoke ashes. On the basis of these ash

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composition, synthetic ash samples were prepared. In order to simplify the ash

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components and make the theoretical calculations and experiments clearer,

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low-content oxides in petcoke ash, such as TiO2, K2O and P2O5, were not taken into

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account. Synthetic ash samples consisted of six major oxides (analytical reagents),

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namely, SiO2, Al2O3, V2O5, CaO, Fe2O3 and NiO, of which the concentrations of CaO,

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Fe2O3 and NiO were variables. Six analytical reagents were blended well in pure ethyl

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alcohol and then dried at 100 °C for more than 10 h [29]. The chemical composition

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of six-components synthetic ash samples is shown in Table 2.

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2.2. AFTs Test.

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The high-temperature ash fusibility tester (KY company, China) was used to

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measure the AFTs of synthetic ash samples. According to the Chinese standard

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procedure GB/T219-2008 [34], the tests were carried out under oxidizing (air) 5


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atmosphere, which provided guides for the slagging of combustion [35]. In the

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procedure, an ash cone with specific geometry was heated to 900 °C at 15 °C/min and

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then changed to 5 °C/min. The variation of specific shape of ash cone was recorded

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automatically during ash melting process. Table 3 lists the experimental AFTs of five

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real petcoke samples.

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2.3. Quenching Experiments.

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Quenching experiments were carried out in a tube furnace with the upper

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temperature limit of 1600 °C to obtain the ash slag samples at certain temperature.

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The schematic diagram of the furnace apparatus is shown in Figure 1. The ash sample

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(1.0 g) filled in corundum crucible was heated to target temperature under oxidizing

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(air) atmosphere. The thermal profile is set on the basis of AFTs test. After reaching

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the preset temperature, the sample was immediately taken out, quenched by ice water

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and collected for later testing.

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2.4. Characterization and Testing.

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X-ray fluorescence (XRF) spectrometry (XRF-1800, Shimadzu, Japan) with a Rh

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target X-ray tube (50 kV, 40 mA) was used to analyze the chemical composition of

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five real petcoke ash. The mineral composition of quenched ash samples was detected

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by a RIGAKU D/max-rB X-ray power diffractometer using Cu Ka radiation (40 kW,

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100 mA, Ka1=0.15408 mm). The samples were scanned between 2θ values of 10° and

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80° at 4° 2θ/min scanning speed and a 0.01° step size. Moreover, the surface

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morphologies of the quenched ash samples were examined by a JSM-7001F scanning

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electron microscope. The samples were magnified 270 and 3700 times respectively to

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identify different levels of microstructure.

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2.5. Thermodynamic Equilibrium Calculations.

6


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The thermodynamic database FactSage based on the Gibbs free energy

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minimization was performed to establish multiple components system of

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V2O5-NiO-SiO2-Al2O3-CaO-Fe2O3 to analyze the chemical reactions and mineral

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transformations during ash melting process. In terms of the components, FToxid and

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FactPS databases were chosen for phase formation data, and the solution species

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formed were chosen from FToxid database. Thermodynamic equilibrium calculations

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were carried out in the oxidizing (air) atmosphere under 0.1 MPa. When the Gibbs

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free energy of the system is at its minimum value, the system and all possible

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reactions (homogeneous and heterogeneous) will reach thermodynamic equilibrium.

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Those reactions with negative Gibbs energy may take place. The lower the Gibbs

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energy, the higher the priority of the reaction [36].

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Thermodynamic database FactSage is a powerful method that has proven to yield

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insight into the ash melting process. However, it has some shortcomings when applied

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to ash melting process. Chemical kinetics are not taken into account in

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thermodynamic calculations. Moreover, temperature and composition gradients are

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also ignored. Despite these drawbacks, thermodynamic calculations do provide

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valuable guides for the species equilibrium distributions and the reaction mechanisms

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[37].

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3. RESULTS AND DISCUSSION

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3.1. Effect of CaO Content on AFTs.

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CaO is commonly used as a fluxing agent for decreasing the AFTs [38, 39].

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Figure 2 shows the plots of the experimental AFTs against the addition of CaO for

171

synthetic ash samples. Considering the CaO content in ash is highly variable, the CaO

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content of synthetic ash samples in this work varies from 5% to 35%. The AFTs

173

exhibit a continuous and impressive decline with increasing CaO content. When the 7


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addition reaches 30%, the FT has been below 1100 °C. The reasonable explanation

175

might be as follows: CaO reacts with other minerals under an appropriate content to

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generate low-melting eutectics, which may further cause the decomposition of

177

high-melting minerals. Thus, the formation of low-melting eutectics and the

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decomposition of high-melting minerals may decrease the AFTs.

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3.2. Effect of CaO on Mineral Transformation Behavior At High Temperatures.

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Ash melting is a high temperature process. Interactions among minerals could

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cause the variation in mineral components and their concentrations, which may

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markedly influence the AFTs. Therefore, the ash fusion characteristics can be

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analyzed on the basis of the mineral phases in ash slag at a certain temperature [39].

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The mineral composition of three synthetic ashes (CaO content is 5, 20 and 30%,

185

respectively) at 1200 °C was identified by XRD to explore the variation mechanism

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of AFTs. In XRD patterns, the diffraction peak intensity of mineral is approximately

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proportional to the mineral content [40-42]. As shown in Figure 3, the minerals in the

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ash with 5% CaO are mainly Ni2SiO4, CaAl2Si2O8 and SiO2. It is deduced that V is

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present as V2O5(l) because V2O5 melts at 690 °C. High melting point (1700 °C)

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Ni2SiO4 may be responsible for the relatively high AFTs. When the addition of CaO

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reaches 20%, Ca2V2O7 emerges following the reaction [27]:

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2CaO + V2 O5 → Ca 2 V2 O7 (s).

(1)

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The formation of low melting point (1288 °C) Ca2V2O7 contributes to the decline of

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AFTs to some extent. Moreover, in comparison to the ash with 5% CaO, the Ni2SiO4

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content decreases because of the reduced diffraction peak height of Ni2SiO4, which

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further reduces the AFTs. When the CaO content increases to 30%, diffraction peaks

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of Ni2SiO4 disappear, suggesting that the AFTs is further decreased. The 8


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decomposition of Ni2SiO4 can be explained by the fact that Ni2SiO4 reacts with other

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minerals, such as Ca and Al, to form amorphous silicate and aluminosilicate.

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To validate the crystalline minerals in ash slag, the phase assemblage at 1200 °C

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was calculated by the multicomponent system SiO2-Al2O3-V2O5-CaO-Fe2O3-NiO in

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thermodynamic equilibrium calculations FactSage. As shown in Figure 4, with

203

increasing CaO content, there is little variation in CaAl2Si2O8. Ca2V2O7 initially

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increases (CaO content is less than 25%) and then follows an impressive decline. It is

205

also noted that calcium orthovanadate (Ca3V2O8) is generated (CaO content is over

206

30%) because of the reaction of Ca2V2O7 with CaO.

Ca 2 V2O7 (s) + CaO → Ca 3V2O8 (s).

207

(2)

208

Another V-bearing solid species formed in the system is Fe(VO3)2 according to

209

thermodynamic calculations. However, diffraction peaks of Ca3V2O8 and Fe(VO3)2 in

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XRD patterns do not appear. The reason may be that both V-bearing species are

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thermodynamically unstable. High-melting Ni2SiO4 shows a slowly decline until it

212

disappears (CaO content is 30%), which is consistent with the results of XRD.

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It should be mentioned that, along with the temperature rising, high temperature

214

mineral transformations occurred in ash melting process will affect the AFTs. Figure

215

5 presents the XRD patterns of synthetic ash with 20% CaO heated from 1000 to

216

1400 °C. It can be seen that CaAl2Si2O8, SiO2, Ca2V2O7 and Ni2SiO4 are the major

217

crystalline

218

thermodynamically stable within the temperature range. When the temperature is

219

higher than 1300 °C, diffraction peaks of Ni2SiO4 disappear. This suggests that

220

Ni2SiO4 has either decomposed, or formed amorphous matter with other minerals at

221

high temperatures (>1300 °C).

minerals,

of

which

CaAl2Si2O8,

SiO2

9


and

Ca2V2O7

keep

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Ash fusibility can be reflected by the variation of ash surface morphology, i.e.,

223

the micromorphology can indicate the ash melting extent [43, 44]. Furthermore, the

224

melted particles present the dense layer structure and smooth surface, while the

225

unmelted particles present the loose structure and rough surface [22]. Figure 6 shows

226

the micromorphology of synthetic ash samples with 5, 20 and 30% CaO, which are

227

quenched in ice water from 1200 °C. It is generally believed that the quenched ash

228

keeps the morphology characteristics of high-temperature ash well [35]. As shown in

229

panels b, d and f, synthetic ash samples are magnified 3700 times. When the addition

230

of CaO is 5%, the ash is composed of many fine irregular particles (panel b). With the

231

addition proportion further increasing, small particles have been agglomerating

232

(panels d and f). Meanwhile, the structures of synthetic ashes turn to be denser and

233

more smooth (panels d and f), which indicates that the synthetic ash with 30% CaO

234

has the highest molten extent. This is consistent with the AFTs’ trend in Figure 2.

235

3.3. Effect of Fe2O3 Content on AFTs.

236

Fe is predominantly present as Fe3+ in oxidizing atmosphere while forms Fe2+ or

237

even Fe in reducing atmosphere. Thus, the valence state of Fe could influence the ash

238

fusibility. The experimental AFTs of synthetic ash samples with different Fe2O3

239

content are shown in Figure 7. The variation of AFTs is similar to that from synthetic

240

ashes with different CaO content (Figure 2), i.e., the AFTs drop monotonically within

241

the entire Fe2O3 content range. However, the existing discrepancy is that the AFTs

242

show a slight decline in Figure 7. The FT changes only from 1274 °C (Fe2O3 content

243

is 2%) to 1240 °C (Fe2O3 content is 20%).

244

3.4. Effect of Fe2O3 on Mineral Transformation Behavior At High Temperatures.

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XRD patterns of three synthetic ash samples (Fe2O3 content is 2, 11 and 17%,

246

respectively) heated at 1200 °C are present in Figure 8. It can be seen that the mineral 10


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phases in ashes closely resemble those from synthetic ashes with different CaO

248

content, i.e., CaAl2Si2O8, SiO2, Ca2V2O7 and Ni2SiO4 are the main components.

249

Moreover, Ni2SiO4 remains stable within the whole ash melting process in

250

comparison to synthetic ashes with different CaO content. No diffraction peaks of

251

Fe-bearing species appear with increasing Fe2O3 content in XRD patterns. Thus, it

252

could be deduced that Fe may be present as Fe-bearing amorphous matters. With

253

regard to the slight decline in AFTs, it may be ascribed to the synergistic effect

254

between low-melting Ca2V2O7 and high-melting Ni2SiO4.

255

Phase assemblage of synthetic ash samples with different Fe2O3 content

256

calculated by thermodynamic FactSage modeling at 1200 °C is present in Figure 9.

257

With the Fe2O3 content increasing, SiO2 keeps almost unchanged (about 5.6%).

258

CaAl2Si2O8, Ca2V2O7 and Ni2SiO4 vary in a similar way: all of them decrease linearly

259

and slowly, of which Ca2V2O7 declines from 22.7% (Fe2O3 content is 2%) to 15.8%

260

(Fe2O3 content is 20%), and Ni2SiO4 decreases from 13.6% (Fe2O3 content is 2%) to

261

8.4% (Fe2O3 content is 20%). The formation of Fe-bearing species Fe(VO3)2 occurs in

262

the system and its content progressively increases up to 45.1% (Fe2O3 content is 20%).

263

However, diffraction peaks of Fe(VO3)2 have not appeared in XRD patterns,

264

suggesting that Fe(VO3)2 is not formed. On the basis of thermodynamic calculations,

265

the reduced Ni2SiO4 content may contribute to the decrease of AFTs.

266

The mineral composition of the synthetic ash with the addition of 11% Fe2O3

267

heated from 1000 to 1400 °C is shown in Figure 10. The temperature has an

268

insignificant effect on the mineral speciation transformations because the mineral

269

phases in ash are mainly CaAl2Si2O8, SiO2, Ca2V2O7 and Ni2SiO4 within the entire

270

temperature range. Diffraction peaks of Fe2O3 only appear at 1100 °C. The behavior

271

of Ni2SiO4 is similar to that from the synthetic ash with 20% CaO from 1000 to 11


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1400 °C. With the temperature rising constantly, Ni2SiO4 gradually increases until the

273

temperature reaches 1200 °C and then follows a rapid decline according to the

274

variation of diffraction peaks height of Ni2SiO4. Ni2SiO4 disappears completely at

275

1400 °C. With the combination of Figures 9 and 10, we further conclude that the

276

presence of Ni2SiO4 plays a key role in the variation of AFTs.

277

Figure 11 shows the surface morphologies of the synthetic ashes with different

278

Fe2O3 (Fe2O3 content is 2, 11 and 17%, respectively) heated at 1200 °C. Synthetic ash

279

samples in panels b, d and f have magnified 3700 times. When the addition of Fe2O3

280

is 2%, the micromorphology of synthetic ash shows a rough surface and has a loose

281

layer structure (panel b) corresponding to the high AFT. As the Fe2O3 content further

282

increases, some fine particles have agglomerated into large particles (panels d and f).

283

Furthermore, there exists an evident trend that the structures of synthetic ashes turn to

284

be denser and more smooth in the order of the synthetic ash with 2% Fe2O3 (panel b),

285

the synthetic ash with 11% Fe2O3 (panel d) and the synthetic ash with 17% Fe2O3

286

(panel f). Hence, it is deduced that the molten extent becomes higher from the

287

synthetic ash with 2% Fe2O3 to the synthetic ash with 11% Fe2O3 to the synthetic ash

288

with 17% Fe2O3. This corresponds to the variation of AFTs in Figure 7.

289

3.5. Effect of NiO Content on AFTs.

290

Ni is considered to be one of the most concerned trace elements in petcoke. It

291

can exist in the oxidation states as Ni, Ni2+ and Ni3+ at high temperatures [35]. Thus,

292

the presence of Ni in ash may change the ash fusibility to some extent. Figure 12

293

presents the experimental AFTs of synthetic ash samples as a function of the NiO

294

content. Obviously, the AFTs are strongly dependent on the mass fraction of NiO.

295

The AFTs show a small decline before the NiO content reaches 8% and then follow a

12


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impressive rise as the NiO content increases to 14%. The AFTs further have a slight

297

increase until the NiO content reaches 20%.

298

3.6. Effect of NiO on Mineral Transformation Behavior At High Temperatures.

299

Synthetic ash samples with the addition of 2, 11 and 17% NiO were heated at

300

1200 °C and then identified by XRD to investigate the transformation behaviors of

301

minerals in ash slag. As shown in Figure 13, the minerals in the synthetic ash with the

302

addition of 2% NiO are present in the form of CaAl2Si2O8, SiO2 and Ca2V2O7. As the

303

NiO content increases to 11%, diffraction peaks of Ni2SiO4 appear in XRD patterns.

304

With regard to the slight decline of AFTs in the NiO content range of 2%-8%, this

305

may be ascribed to the synergistic effect between Ca2V2O7 and Ni2SiO4. Low-melting

306

Ca2V2O7 takes the lead in the AFTs rather than high-melting Ni2SiO4 because the

307

mass fraction of Ni2SiO4 is lower. The reason why the AFTs rapidly increase in the

308

NiO content range of 8%-14% may be that Ni2SiO4 increases continuously and takes

309

the initiative. With the addition of NiO further increasing to 17%, CaAl2Si2O8, SiO2,

310

Ca2V2O7 and Ni2SiO4 keep thermodynamically stable.

311

Figure 14 shows the phase assemblage of synthetic ash samples with the addition

312

of NiO calculated by FactSage modeling at 1200 °C. It is clear that CaAl2Si2O8,

313

Fe(VO3)2, Ca2V2O7 and SiO2 are the dominant minerals in the system when the NiO

314

content is 2%. The content of CaAl2Si2O8, Fe(VO3)2 and Ca2V2O7 is 32.2, 24.1 and

315

21.8%, respectively. However, it should be mentioned that Fe(VO3)2 is not formed on

316

the basis of experimental data. With the addition of NiO further increasing, the

317

Ni2SiO4 content linearly increases up to 26.3% (NiO content is 20%). Conversely, the

318

content of CaAl2Si2O8, Ca2V2O7 and SiO2 linearly decreases until the NiO content

319

reaches 20%. For Fe(VO3)2, there is a rapid increase from 24.1% (NiO content is 2%)

320

to 29.3% (NiO content is 5%), with a slow decline to 23.1% (NiO content is 20%). 13


Energy & Fuels

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321

According to the preceding analyses, the variation of minerals in ash slag and their

322

content in Figures 13 and 14 can validate the AFTs’ trend in Figure 12.

323

The mineral evolution of a synthetic ash sample (NiO content is 11%) heated

324

from 1000 to 1400 °C was characterized by XRD. As shown in Figure 15, the

325

crystalline minerals in high-temperature ash slag closely resemble those from the

326

synthetic ash with 11% Fe2O3 (Figure 10). The mineral components are composed of

327

CaAl2Si2O8, Ni2SiO4, Ca2V2O7 and SiO2. But there exists a difference in Ni2SiO4. The

328

temperature that Ni2SiO4 disappears completely is 1300 °C rather than 1400 °C.

329

Moreover, the formation of Fe2O3 does not occur at 1100 °C. There are two reasons to

330

explain the disappearance of Ni2SiO4 at 1300 °C. The first is that Ni2SiO4 forms

331

Ni-bearing amorphous matters with other minerals. The second is that Ni2SiO4

332

decomposes to generate gaseous Ni-containing species at high temperatures, such as

333

NiO.

334

SEM analysis is carried out to obtain the microstructure of synthetic ash samples

335

with NiO content of 2, 11 and 17%, as shown in Figure 16. Panels b, d and f represent

336

synthetic ash samples that are magnified 3700 times. The synthetic ash consists of

337

many fine particles when the addition of NiO is 17% (panel f) while the synthetic ash

338

with 11% NiO has agglomerated and some large particles are formed (panel d). When

339

the NiO content is lower, larger particles have partial molten surfaces (panel b), and

340

show denser and more smooth structure, which means the higher melting extent. This

341

is consistent with the AFTs’ trend in Figure 12.

342

4. CONCLUSIONS

343

The effect of petcoke ash composition (CaO, Fe2O3 and NiO) on ash fusion

344

characteristics was investigated in this study from the perspectives of content change

345

and temperature rising. The ash fusibility tester, XRD, SEM and FactSage were used 14


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346

to

determine

the

AFTs,

mineral

transformations

and

microstructure

347

high-temperature ash slag. The main conclusions of our research are as follows:

of

348

(1) The AFTs closely relate to petcoke ash chemical composition (CaO, Fe2O3

349

and NiO). With the content of CaO and Fe2O3 increasing, the AFTs show a

350

continuous decline. For the effect of NiO content, the AFTs first decrease slightly

351

while then increase.

352

(2) The variation of AFTs are mainly dependent on the mineral transformations

353

of high-temperature ash slag. With the addition of CaO and NiO, low-melting

354

Ca2V2O7 and high-melting Ni2SiO4 are formed in ashes, respectively. Their

355

synergistic effect contributes to the AFTs’ tendency to some extent. However, Fe may

356

forms Fe-bearing amorphous matter with other minerals.

357

(3) The crystalline minerals of the ashes with different CaO, Fe2O3 and NiO

358

content mainly consist of CaAl2Si2O8, SiO2, Ca2V2O7 and Ni2SiO4, of which

359

CaAl2Si2O8, SiO2 and Ca2V2O7 keep thermodynamically stable within the temperature

360

range (1000-1400 °C). The decomposition of Ni2SiO4 occurs at temperatures higher

361

than 1300 °C.

362

(4) The higher melting extent of high-temperature ash slag leads to the

363

agglomerate of fine particles and has the more smooth and denser structure.

364

AUTHOR INFORMATION

365

Corresponding Author

366

E-mail address: [email protected]

367

ACKNOWLEDGEMENTS

368

The authors gratefully acknowledge the financial support of the National Natural

369

Science Foundation of China (No. 21576276).

370

REFERENCES 15


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Prediction of biomass ash fusion behavior by the use of detailed characterization

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methods coupled with thermodynamic analysis. Fuel 2015, 141, 275-284.

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Petroleum Coke Ash Fusibility. Energy Fuels 2017, 31(3), 2530-2537.

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Energy & Fuels

Table 1. Ash Composition (Bottom Ash) of Five Real Petcoke (wt %) Petcoke

V2O5

NiO

SiO2

Al2O3

Fe2O3

CaO

MgO

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

Wuhan

28.58 10.52 21.35 11.36 4.36

17.33 1.85

0.74

0.89

0.81

1.15

0.60

Zhenhai

29.82 12.37 25.44 7.60

10.12 8.79

1.21

0.35

1.35

0.96

0.98

0.32

Zibo

31.04 6.87

18.35 6.11

11.79 19.41 0.99

0.55

1.64

0.84

0.74

0.78

Daqing

32.35 9.75

17.74 7.04

5.35

0.64

0.97

1.35

1.05

0.61

21.35 0.74

TiO2

21


SO3

K2O Na2O

P2O5

Energy & Fuels

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Page 22 of 39

Table 2. Chemical Composition of Synthetic Ash Samples Composition (wt %) Number

SiO2

Al2O3

V2 O5

CaO

Fe2O3

NiO

Ashes of different CaO content 1 2 3 4 5 6 7

24.20 23.10 21.90 20.80 19.40 18.10 16.90

12.10 37.90 5.00 11.55 36.00 10.00 10.95 34.20 15.00 10.40 32.30 20.00 9.70 30.50 25.00 9.05 28.60 30.00 8.45 26.75 35.00 Ashes of different Fe2O3 content

9.20 8.75 8.40 7.95 7.60 7.15 6.80

11.60 10.60 9.55 8.55 7.80 7.10 6.10

1 2 3 4 5 6 7

23.40 22.60 22.00 21.40 20.60 20.00 19.20

11.70 36.80 15.10 11.30 36.30 14.50 11.00 35.40 13.80 10.70 34.60 13.20 10.30 34.00 12.50 10.00 33.20 11.90 9.60 32.60 11.30 Ashes of different NiO content

2.00 5.00 8.00 11.00 14.00 17.00 20.00

11.00 10.30 9.80 9.10 8.60 7.90 7.30

1 2 3 4 5 6 7

23.60 23.00 22.20 21.60 20.60 20.00 19.40

10.80 10.20 9.60 8.70 8.20 7.60 7.00

2.00 5.00 8.00 11.00 14.00 17.00 20.00

11.80 11.50 11.10 10.80 10.30 10.00 9.70

37.00 36.20 35.60 34.90 34.40 33.60 32.70

14.80 14.10 13.50 13.00 12.50 11.80 11.20

22


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Energy & Fuels

Table 3. AFTs of Five Petcoke Ash Samples (oC) Ash samples

DT

ST

HT

FT

Shanxi

1215

1236

1268

1274

Wuhan

1164

1198

1225

1240

Zhenhai

1125

1153

1175

1190

Zibo

1266

1302

1333

1349

Daqing

1301

1325

1357

1369

DT deformation temperature; ST softening temperature; HT hemispherical temperature; FT flow temperature.

23


Energy & Fuels

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Figure 1. The schematic representation of the furnace apparatus

24


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Energy & Fuels

Figure 2. Effect of CaO content on the AFTs.

25


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. XRD patterns of three synthetic ash with 5, 20, 30% CaO at 1200 °C.

26


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Energy & Fuels

Figure 4. Effect of CaO content on the mineral transformation calculated by FactSage at 1200 °C.

27


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5. XRD patterns of synthetic ash with 20% CaO from 1000 to 1400 °C.

28


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Energy & Fuels

(a)

(b)

(c)

(d)

(e)

(f)

Figure 6. SEM photomicrographs of synthetic ash samples with different CaO content of (a and b) 5%, (c and d) 20% and (e and f) 30% at 1200 °C. 29


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 7. Effect of Fe2O3 content on the AFTs.

30


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Energy & Fuels

Figure 8. XRD patterns of three synthetic ash with 2, 11, 17% Fe2O3 at 1200 °C.

31


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 9. Effect of Fe2O3 content on the mineral transformation calculated by FactSage at 1200 °C.

32


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Energy & Fuels

Figure 10. XRD patterns of synthetic ash with 11% Fe2O3 from 1000 to 1400 °C.

33


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Page 34 of 39

(a)

(b)

(c)

(d)

(e)

(f)

Figure 11. SEM photomicrographs of synthetic ash samples with different Fe2O3 content of (a and b) 2%, (c and d) 11% and (e and f) 17% at 1200 °C. 34


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Energy & Fuels

Figure 12. Effect of NiO content on the AFTs.

35


Energy & Fuels

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Figure 13. XRD patterns of three synthetic ash with 2, 11, 17% NiO at 1200 °C.

36


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Energy & Fuels

Figure 14. Effect of NiO content on the mineral transformation calculated by FactSage at 1200 °C.

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Energy & Fuels

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Figure 15. XRD patterns of synthetic ash with 11% NiO from 1000 to 1400 °C.

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Energy & Fuels

(a)

(b)

(c)

(d)

(e)

(f)

Figure 16. SEM photomicrographs of synthetic ash samples with different NiO content of (a and b) 2%, (c and d) 11% and (e and f) 17% at 1200 °C. 39


Effect of Ash Composition (Ca, Fe, and Ni) on Petroleum Coke Ash

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