Migration and Transformation of Vanadium and Nickel in High Sulfur Petroleum Coke During Gasiﬁcation Processes

energies

Article Migration and Transformation of Vanadium and Nickel in High Sulfur Petroleum Coke during Gasiﬁcation Processes

Wei Li 1, Ben Wang 1,*, Jun Nie 1, Wu Yang 1, Linlin Xu 2 and Lushi Sun 1,* 1 State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, China; [email protected] (W.L.); [email protected] (J.N.); [email protected] (W.Y.) 2 Wuhan Huayu Energy-Burning Engineering Technology Co. Ltd., G3, New Energy Building, Future City, No. 999 Gaoxin Avenue, East Lake High-tech Zone, Wuhan 430206, China; [email protected] * Correspondence: [email protected] (B.W.); [email protected] (L.S.); Tel.: +86-27-87542417 (B.W.); +86-27-87542417 (L.S.) Received: 26 July 2018; Accepted: 14 August 2018; Published: 18 August 2018

Abstract: The volatilization characteristics and occurrence forms of V and Ni in petroleum coke (petcoke) were investigated during steam (H2O) and carbon dioxide (CO2) gasification on a fixed bed reactor at 800–1100 ◦C. The Tessier sequential chemical extraction procedure was employed to determine the different forms of V and Ni. The results showed their volatilities were not dependent on the gasification atmosphere, but rather relied mainly on the reaction temperature. The CO2 atmosphere accelerated the conversion of organic-bound nickel to residual form at low temperature and promoted Fe-Mn oxides formation at high temperature. However, the H2O atmosphere was conducive to form vanadium bound to Fe-Mn oxides and promoted the decomposition of residual forms. In addition, the thermodynamic equilibrium calculations showed the volatilization of Ni ◦ mainly released Ni3S2 between 800–1100 C. The H2O atmosphere was favorable to generate the more stable NixSy compound, thereby suppressing the volatilization of Ni, while the presence of CO2 led to an increase in residual V and decrease of Fe-Mn oxides. The V and Ni mainly caused erosion problems under the CO2 atmosphere while the fouling and slagging obviously increased under the H2O atmosphere with impacts gradually weakened with the increase of temperature.

Keywords: high sulfur petcoke; vanadium; nickel; gasiﬁcation; sequential extraction; migration

1. Introduction The shortage of energy resources and the improvement of petroleum processing capacity have led to a continuous increase in the production of petcoke [1–3]. Petcoke is mainly used in the electrolytic aluminum industry and boilers as a combustion fuel, however it releases massive amounts of polluting gases such as SO2 under combustion conditions, resulting in serious environmental problems [4]. Petcoke has high calorific value, low ash and low price qualities, which endow petcoke with the potential to be a gasification feedstock to produce synthetic gas and recycle sulfur through the Claus process. Therefore, it is an efficient and clean utilization way, which has broad prospects for making waste profitable [5,6]. Unfortunately, petcoke is enriched with Ni and V concentrations of more than 1000 ppm, resulting in serious fouling, slagging and erosion problems of gasification equipment and pipes [7]. V2O5 can react with refractory and metal oxides [8], exposing the surface of the metal to an oxidizing atmosphere and accelerating its oxidation rate. High concentrations of V, Ni and sulfur in petcoke can cause severe pollution, including erosion and corrosion to the furnace and equipment, during gasification [9]. Therefore, a thorough understanding of the chemical speciation migration of V and Ni in the process of gasification and the complex chemical reactions that may occur with

Energies 2018, 11, 2158; doi:10.3390/en11082158 www.mdpi.com/journal/energies Energies 2018, 11, 2158 2 of 14 gas reactants and mineral components is important to predict and control the fouling, slagging and erosion behavior during the process of gasification. There are three gasification technologies, including fixed bed, fluidized bed, and entrained flow bed, of which the entrained flow bed appears to be a promising gasification technology with the advantages of high output, high thermal conversion efficiency because of its higher operating temperatures. However, the high temperature also accelerates the fouling, slagging and erosion problems caused by vanadium and nickel in the petcoke. Park et al. [10] investigated slagging of petroleum coke ash. The main components of petcoke under gasification conditions were vanadium, nickel and iron. Among them, vanadium existed in the form of V2O3 with a melting point of up to ◦ 1940 C. V2O3 in petcoke ash remained solid at the typical gasification temperature and the solid V2O3 caused the slag viscosity to increase. For the petcoke with high ash content, the effect of V2O3 on slag viscosity must be taken into consideration when the minimum gasification temperature was estimated. Nakano et al. [11] studied a synthetic slag and found that V2O3 could form solid solutions with Al2O3 and Fe2O3 to reduce the slag viscosity. Duchesne et al. [12] examined the viscosity characteristics of coal and petcoke blended ash. Petcoke ash was mainly composed of V and Ni, and the spinel formed by vanadium could increase the ash viscosity. Mahapatra et al. [13] measured the release of vanadium and nickel during the pyrolysis process, reporting that vanadium and nickel tended to accumulate in the coke compared to fly ash. Wang et al. [7] investigated that the formation of ash and slag was closely related to the conversion of vanadium, nickel, and sulfur during the gasification process. Brüggemann et al. [14] also observed that large amounts of gaseous nickel-containing compounds were formed in the synthesis gas. Therefore, the above problems can be relieved by using a relatively low reaction temperature. The investigation of the occurrence and migration behavior of V and Ni has been studied by many researchers. Querol et al. and Dai, et al. [15,16] expressed that vanadium existed in the form of organic matter and aluminosilicate minerals in coal. Brüggemann et al. [14] indicated that nickel was transformed into NixSy and elemental Ni under high H2S and low H2S partial pressure, which was calculated from the thermodynamics of the conversion of Ni during heavy oil gasification. Wang et al. [17] found that V2O5 reacts with CaO to form Ca3V2O8 when blended with coal ash in a reducing atmosphere, and the decomposition of Ca3V2O8 could react with Fe2O3 to form FeV2O4 when the temperature was over 1300 ◦C. Li et al. [18] reported that the vanadium reacted with the mineral component to form a new thermal stable compound when the gasification temperature was above 1300 ◦C. However, a detailed systematic research on primary speciation of V and Ni in petcoke and its release and migration characteristics during gasification has not been accomplished. Therefore, in this study, the behavior of V and Ni in the CO2 and H2O gasification of petcoke was studied by fixed bed gasification experiments. The Tessier sequential chemical extraction procedure and ICP-MS were used to obtain the forms of V and Ni in the gasification product under different gasification conditions. Furthermore, the release and transformation mechanism of V and Ni were elaborated deeply. Moreover, thermodynamic equilibrium calculations, the use of the FACTSAGE software, were carried out to study the reaction mechanism of V and Ni with the main associated mineral elements during gasification. Finally, a detailed assessment on the pernicious effects on gasification equipment by the mineral elements V and Ni was gained.

2. Experiments

2.1. Petcoke Samples The petcoke samples were obtained from the Wuhan branch of Sinopec, ground and sieved through 100 mesh sieves. Before the gasiﬁcation experiment, the samples were dried at 80 ◦C in advance in a vacuum drying oven for 24 h. The petcoke ash was prepared at 805 ◦C for 2 h according to the Chinese standard procedures (GB/T 1574-2007). Table1 shows the ultimate and proximate analysis of the samples. Table2 lists the ash compositions (by XRF) of the petcoke. Energies 2018, 11, 2158 3 of 14

Energies 2018, 11 3 of 14 Table 1. Proximate and ultimate analysis of petcoke samples (wt %). Table 1. Proximate and ultimate analysis of petcoke samples (wt%). Mad Aad Vad FCad Cad Had Nad Sad Oad * Mad Aad Vad FCad Cad Had Nad Sad Oad * 1.46 0.50 8.56 89.48 85.43 3.63 1.81 2.01 7.12 1.46 0.50 8.56 89.48 85.43 3.63 1.81 2.01 7.12 ad: the proximate and ultimate analysis of the samples are on the air-dry basis; *: the ultimate analysis of O is adquantiﬁed: the proximate by the subtraction and ultimate method. analysis of the samples are on the air-dry basis, *: the ultimate analysis of O is quantified by the subtraction method. Table 2. Ash composition of petcoke samples (wt %). Table 2. Ash composition of petcoke samples (wt%).

NiO V2O5 Fe2O3 SiO2 SO3 Al2O3 TiO2 CaO Na2O MnO NiO V2O5 Fe2O3 SiO2 SO3 Al2O3 TiO2 CaO Na2O MnO 20.11 14.88 20.119.41 14.88 9.4118.18 18.183.873.87 13.5913.59 0.960.9614.54 14.544.29 0.174.29 0.17

2.2.2.2. Fixed Fixed B Beded G Gasiﬁcationasification A Apparatuspparatus and and E Experimentalxperimental P Proceduresrocedures OurOur laboratory laboratory-scale-scale reactor reactor system system (Figure (Figure 1)1) contains contains a steam steam generator generator and and a a fixed ﬁxed-bed-bed reactor reactor thatthat is is heated heated by by an an electrical electrical furnace. furnace. The The total total length length of of the the quartz quartz tube tube is is 500 500 mm mm and and the the inner inner diameterdiameter is is 40 40 mm. mm. T Thehe quartz quartz tube tube is is heated heated by by an an electrothermal electrothermal furnace, furnace, which which can can reach reach a a ◦ maximummaximum temperature temperature of of 1100 1100 °CC under under control control by by a a temperature temperature controller. controller. The The steam steam generator generator is is mainlymainly composed composed of of an an electrical electrical heating heating furnace furnace and and a ahigh high-precision-precision syringe syringe pump pump which which precisely precisely controlscontrols the the water water injection injection (0.736 (0.736 m mL/minL/min of of deionized deionized water water provides provides 1.00 1.00 L/min L/min steam steam at at ambient ambient temperattemperature).ure). The The deionized deionized water water is is evaporated evaporated in in the the preheating preheating furnace, furnace, and and then then carried carried by by the the carriercarrier gas gas of of N N22. The. The mixture mixture gas gas enters enters the the quartz quartz tube tube through through a a heat heat insulating insulating pipeline pipeline wrapped wrapped withwith a a heating heating tape tape..

FigureFigure 1. 1. DiagramDiagram of of the the fixed ﬁxed bed bed gasifi gasiﬁcationcation system. system.

In the petcoke gasification experiments, a N2 flow of 500 mL/min was maintained to remove air In the petcoke gasification experiments, a N2 flow of 500 mL/min was maintained to remove from the system. When the specified temperature was reached, the atmosphere was switched to air from the system. When the specified temperature was reached, the atmosphere was switched to carbon dioxide gas or steam carried by nitrogen. The flow rate of CO2 was kept at 500 mL/min for all carbon dioxide gas or steam carried by nitrogen. The flow rate of CO2 was kept at 500 mL/min for all CO2 gasification experiments, while that of N2 was kept at 150 mL/min and H2O at 350 mL/min for CO2 gasification experiments, while that of N2 was kept at 150 mL/min and H2O at 350 mL/min for all H2O gasification experiments, respectively. Meanwhile, basket with 1 g samples was rapidly all H2O gasification experiments, respectively. Meanwhile, basket with 1 g samples was rapidly placed placed in the quartz mesh in the reaction zone. The reaction time was 30 min. After gasification, the in the quartz mesh in the reaction zone. The reaction time was 30 min. After gasification, the basket basket was pulled to the up and cold position and rapidly cooled to room temperature by N 2. The was pulled to the up and cold position and rapidly cooled to room temperature by N2. The gasification gasification chars were collected, weighted, and subsequently analyzed. chars were collected, weighted, and subsequently analyzed.

2.3.2.3. Sequential Sequential C Chemicalhemical E Extractionxtraction SequentialSequential chemical chemical extraction extraction wa wass an an effective effective way way to to quantify quantify the the chemical chemical forms forms of of trace trace elementselements [19 [19––2121]].. InIn this this research, research, ﬁve five chemical chemical forms forms of V andof V Ni and in petcoke Ni in and petcoke char afterand gasiﬁcation char after gasificationwere extracted were by extracted the Tessier by sequential the Tessier extraction sequential procedure extraction [22 ].procedure In the extraction [22]. In procedures the extraction [23], procedures [23], the Ni and V were extracted and partitioned into five forms: (1) exchangeable, extracted by MgCl2 (1 mol/L); (2) bound to carbonates, extracted by CH3COONa (1 mol/L); (3) bound to Fe-Mn oxides, extracted by NH2OH·HCl (0.04 mol/L) which dissolved in CH3COOH (25%, v/v); 4)

Energies 2018, 11, 2158 4 of 14

the Ni and V were extracted and partitioned into ﬁve forms: (1) exchangeable, extracted by MgCl2 (1 mol/L); (2) bound to carbonates, extracted by CH3COONa (1 mol/L); (3) bound to Fe-Mn oxides, extracted by NH2OH·HCl (0.04 mol/L) which dissolved in CH3COOH (25%, v/v); 4) bound to organic matter, extracted by HNO3 (0.02 mol/L) and H2O2 (30%, v/v); After four extraction steps, Ni and V still remaining in the residue were labeled as residual forms, which might be present in the insoluble organic matrix or occur in minerals such as aluminosilicates, titanium dioxide, zircon, sulphides and so on [18,24].

2.4. Calculation of V and Ni Volatility The V and Ni contents in petcoke and its gasiﬁcation char and their extracted solutions were measured by inductively coupled plasma mass spectrometry (ICP-MS). V and Ni volatility is determined by the equation:

C m C Ry Rv(%) = 1 − 1 1 × 100% = 1 − 1 × 100% (1) C0m0 C0

Char yield after petcoke gasiﬁcation is calculated by Ry, which is assessed as follows:

m Ry(%) = 1 × 100% (2) m0 where Rv is the volatility of V or Ni, %; Ry is the yield of gasification char, %; C1, the concentration of the V or Ni in gasification char, ug/g; C0, the concertration of V or Ni in petcoke, ug/g ; m1, the weight of gasification char, g; m0, the weight of petcoke, g.

2.5. Thermodynamic Equilibrium Calculations The FACTSAGE program is used to facilitate our explanation of the transformation mechanisms of V and Ni during gasification and verify the experimental results. FACTSAGE [25] software is based on the Gibbs energy minimization method. The Equilib module utilized with the FactPS and FToxid databases can detect the concentration of chemical species when specific elements or compounds react to equilibrium. The precision of the software can meet the demands of simulation. In this research, modeling employed main trace elements on behalf of the main composition of petcoke. Simulation calculations were carried under the temperature range between 100 ◦C to 1500 ◦C at normal atmospheric pressure. The quantity of petcoke feedstock to be fed to the gasifier was 1 kg and the CO2 or H2O was 1.5 kg. It was noticeable that the quantity of gasification agent and mineral elements was in units of g/kg-petcoke in the gasification experiment. However, the unit (g/kg-petcoke) was modified to mol/kg-petcoke for the purpose of calculation. The input information is listed in Table3. There is an assumption that all the sulfur in petcoke is presented in the vapor phase as H2S (g) initially because all of gasifiers produce H2S (g) [18,26].

Table 3. Input information into the FACTSAGE.

Reactants mol/kg-Raw Petcoke Reactants mol/kg-Raw Petcoke CO 71.19 NiO 1.31 × 10−2 −2 H2 17.52 V2O5 0.40 × 10 −2 N2 0.65 Fe2O3 0.29 × 10 −2 H2S 0.63 SiO2 1.49 × 10 1 −2 CO2 * 34.00 Al2O3 0.65 × 10 2 −2 H2O* 83.33 TiO2 0.06 × 10 - - CaO 1.27 × 10−2 −2 - - Na2O 0.34 × 10 - - MnO 0.01 × 10−2 1 2 * only in the CO2 gasiﬁcation, * only in the H2O gasiﬁcation. Energies 2018, 11, 2158 5 of 14

3. ResultsEnergies and 2018,Discussion 11 5 of 14

Energies 2018, 11 5 of 14 3.1. The3. Results Char Yield and fromDiscussion Two Types of Gasification Atmospheres 3. Results and Discussion The3.1. The char Char yield Yield under from COTwo2 Tandypes H of2 OGasification gasification Atmospheres conditions are shown in Figure2. The yield tends ◦ to decrease3.1. with The C increasinghar Yield from gasification Two Types of G temperatures.asification Atmospheres The char reduces from 89.8% (800 C) to 64.9% ◦ The char yield under CO2 and H2O gasification conditions are shown in Figure◦ 2. The yield tends ◦ (1100 C) in CO2 gasification while the char yield declines from 80.8% (800 C) to 29.0% (1100 C) to decreaseThe with char increasing yield under gasification CO2 and H2O temperatures. gasification conditions The char are reduces shown in from Figure 89.8% 2. The (800 yield °C) tends to 64.9% in H2O gasification.to decrease with Apparently, increasing gasification the atmosphere temperatures. plays The anchar important reduces from role 89.8% for (800 the °C) yield to 64.9% of char and (1100 °C) in CO2 gasification while the char yield declines from 80.8% (800 °C) to 29.0% (1100 °C) in petcoke has(1100 high °C) gasification in CO2 gasification reactivity while underthe char H yield2O atmosphere.declines from 80.8% From (800 the °C) reduction to 29.0% (1100 trend °C) of in char yield, H2O gasification. Apparently, the atmosphere plays an important role for the yield of char and it can be seenH2O theregasification. are two Apparently, stages inthe the atmosphere CO and plays H Oan gasification.important role Infor stagethe yield 1 (before of char and 1000 ◦C), the petcoke has high gasification reactivity under2 H2O 2atmosphere. From the reduction trend of char petcoke has high gasification reactivity under H2O atmosphere. From the reduction trend of char reduction trend of char yield in H O atmosphere is dramatically higher than that in CO atmosphere. yield,yield, it can it becan seen be seen there there are are two 2two stages stages in in the the CO22 andand H H2O2O gasification gasification. In .stage In stage 1 (before 1 (before 10002 °C1000), °C), It is interestingthe reductionthe reduction that trend the trend of reduction of char char yield yield trend in in HH of22OO char atmosphere yield inis is thedramatically dramatically CO2 and higher Hhigher2 Othan gasification than that inthat CO in2 in CO stage2 2 ◦ (afteratmosphere. 1000atmosphere.C) is It opposite is Itinteresting is interesting to stage that that the1 thebecause reduction reduction CO trend2 molecules of of char char yield yield canin inthe the cross CO CO2 and the2 and H2 reactionO H gasification2O gasification energy in barrier in stage 2 (after 1000 °C) is opposite to stage 1 because CO2 molecules can cross the reaction energy leadingstage to 2 an (after increase 1000 of°C the) is opposite char consumption to stage 1 because yield while CO2 molecules the H2O can gasification cross the almostreaction complete energy at barrier leading to an increase of the char consumption yield while the H2O gasification almost barrier leading to an increase of the char consumption yield while the H2O gasification almost this temperature.complete at this temperature. complete at this temperature.

Figure 2. Char yield (Ry%) during petcoke gasification from 800 °C to 1100◦ °C . ◦ Figure 2. Char yield (Ry%) during petcoke gasification from 800 C to 1100 C. 3.2. Occurrence Modes of Nickel in Petcoke and Its Gasification Char Figure 2. Char yield (Ry%) during petcoke gasification from 800 °C to 1100 °C . 3.2. OccurrenceThe Modes migration of Nickel of occurrence in Petcoke forms and of nickel Its Gasification during CO2 Charand H2O gasification is shown in Figure 3.2. Occurrence3a,b. A few M Feodes-Mn ofoxides Nickel with in Pa etcokeproportion and I ofts 7.9%Gasification is in the C rawhar petcoke while the content of nickel The migrationexists in exchangeable of occurrence and carbonates forms are of tiny nickel amounts during of 0.7% CO and2 0.3%.and The H2 majorityO gasification of nickel is is in shown in Figure3a,b.Theorganic A migration few (62.7%) Fe-Mn andof occurrence oxides residual with form forms (28.4%). a proportion of nickel After gasification, during of 7.9% CO issignificant2 and in theH2Oraw change gasification petcoke of the different is while shown forms the in contentFigure of nickel3a,b exists. ofA nickelfew in exchangeable Fe occurs-Mn oxides in the gasification andwith carbonatesa proportion char. are of tiny7.9% amountsis in the raw of 0.7%petcoke and while 0.3%. the The content majority of nickel of nickel exists in exchangeable and carbonates are tiny amounts of 0.7% and 0.3%. The majority of nickel is in is in organic (62.7%) and residual form (28.4%). After gasification, significant change of the different organic (62.7%) and residual form (28.4%). After gasification, significant change of the different forms forms of nickel occurs in the gasification char. of nickel occurs in the gasification char.

(a) (b)

Figure 3. (a) Occurrence modes of Ni in raw petcoke and char in CO2 atmosphere; (b) Occurrence

modes of Ni in raw petcoke and char in H2O atmosphere.

(a) (b)

Figure 3. (a) Occurrence modes of Ni in raw petcoke and char in CO2 atmosphere; (b) Occurrence Figure 3. (a) Occurrence modes of Ni in raw petcoke and char in CO2 atmosphere; (b) Occurrence modes of Ni in raw petcoke and char in H2O atmosphere. modes of Ni in raw petcoke and char in H O atmosphere. 2 Energies 2018, 11, 2158 6 of 14

For the CO2 gasification, as shown in Figure3a, when the gasification temperature increases to 800 ◦C, most of the organic-bound nickel is converted to the residual form. The residual form of nickel increases from 28.4% to 79.7% while the content of organic-bound sharply reduces from 62.7% to 9.7%. The Ni bound to Fe-Mn oxides is basically constant. As the gasification temperature increases from 800 ◦C to 900 ◦C, the residual form of nickel begins to decompose and mainly transforms into organic and Fe-Mn oxides. At 1000 ◦C, the fraction of the organic-bound nickel also transforms into Fe-Mn oxides which reaches a maximum of 14.9%. When the temperature reaches 1100 ◦C, organic and Fe-Mn oxides are significantly reduced and converted into the residual form of nickel. The content of the residual form of nickel reaches 79.4%, which is basically consistent with the content in the residue of nickel form at 800 ◦C. We note that comparing the char samples at 800 ◦C and 1100 ◦C, it can be found that the content of nickel affiliated with the exchangeable, carbonates, Fe-Mn oxides and residual forms are basically unchanged. After gasification at 800 ◦C, Ni associated with organic matter sharply drops 53.0% while the residual form rises acutely (51.3%) and very low level of volatile Ni (1.4%) emerges. At 1100 ◦C, as residual form remains unchanged, Ni affiliated with organic matter further decreases 6.7% and converts into volatile Ni. In this sense, from room temperature to 1100 ◦C, there are 51.3% and 6.7% of organic matter Ni converted into residual form and volatile form, respectively. It can be deduced that the organic-bound nickel prefers to form residual fraction rather than volatilize. For the H2O gasification, as shown in Figure3b, a large amount of organic-bound nickel decomposes from 62.7% to 28.7% and is mainly converted into the residual form (from 28.7% to 57.3%), with a small part converted into Fe-Mn oxides at 800 ◦C. The residual form of nickel decreases by 13.1% at 900 ◦C and is mainly converted into organic and Fe-Mn oxides, while the transformation behavior from 900 ◦C to 1000 ◦C shows an opposite trend. When the temperature reaches 1100 ◦C, organic and Fe-Mn oxides completely decompose and convert into the residual form. In general, the gasification temperature mainly affects the transformation of the organic-bound nickel and the residual form nickel, accompanied by a small amount of other forms. Comparing the CO2 and H2O gasification processes, the trends of the different forms are roughly the same, but the ◦ contents differs greatly. At 800–900 C, the new generated residual form nickel in CO2 atmosphere is nearly three times higher than that of H2O atmosphere, indicating that CO2 atmosphere is conducive to the conversion of the organic-bound nickel to the residual form at low temperature. At 1000–1100 ◦C, the organic-bound and the residual form nickel in the CO2 atmosphere are lower than in the H2O atmosphere, indicating that the CO2 atmosphere promotes the formation of Fe-Mn oxides at high temperature. In both atmospheres, the organic-bound nickel gradually decreases with temperature, but the amount in the H2O one is significantly higher than that in the CO2 atmosphere, indicating that the CO2 gasification are favorable to the decomposition of organic-bound nickel. The trends of the residual form Fe-Mn oxides are also generally the same. The main difference is that the peak of ◦ the content of Fe-Mn oxides in H2O atmosphere is advanced to 900 C, which is probably due to the relatively low activity of CO2 and the decomposition of bound to Fe-Mn oxides requires higher energy to carry out, leading to the transformation of most of Fe-Mn oxides into the residual form of nickel at ◦ 1000 C under CO2 atmosphere. As the temperature increases, the volatilization characteristic of nickel gradually increases during H2O and CO2 gasification, illustrated in Figure4. It is remarkable that H 2O and CO2 gasification display similar volatilization behavior. The nickel volatility in H2O and CO2 gasification reaches the highest Rv value 9.0% and 9.5% at 1100 ◦C. It is apparent that the difference in the nickel volatility between H2O and CO2 gasification is ignorable, so it can be concluded that the volatility of nickel is not dependent on the gasification atmosphere, but rather mainly on the reaction temperature. Energies 2018,, 1111, 2158 77 of 14 Energies 2018, 11 7 of 14

FigureFigure 4. Volatility 4. Volatility (Rv (Rv%)%) of of nickel nickel during during petcokepetcoke gasification gasiﬁcation from from 800 800 °C ◦toC 1100 to 1100 °C . ◦C. Figure 4. Volatility (Rv%) of nickel during petcoke gasification from 800 °C to 1100 °C . 3.3. Occurrence Modes of Vanadium in Petcoke and Its Gasification Char 3.3. Occurrence ModesModes of VanadiumVanadium in PetcokePetcoke andand ItsIts GasiﬁcationGasification CharChar The transformation of different forms of vanadium in the process of CO2 and H2O gasification

Thecan be transformation seen from Figure of 5a different and Figure forms 5b. The of vanadium mass fraction inin of thethe vanadium processprocess bound ofof COCO to22 and andexchangeable HH22O gasiﬁcationgasification and can becarbonates seen from can Figure Figure be barely 55aa,b. and detected The Figure mass by 5b. fraction ICP The-AES. mass of The vanadium fraction occurrence of bound vanadium of vanadium to exchangeable bound in petcoke, to exchangeable and which carbonates is and cancarbonates besimilar barely can to detectedthat be of b nickel,arely by ICP-AES.detected is mainly inby The organic ICP occurrence-AES. matter The (65.78%) of occurrence vanadium and residual inof petcoke,vanadium form (28.29%), which in petcoke, is with similar a small which to that is amount of Fe-Mn oxides form, which is consistent with Li et al. [18] who reached the conclusion that ofsimilar nickel, to isthat mainly of nickel, in organic is mainly matter in organic (65.78%) matter and (65.78%) residual and form residual (28.29%), form with (28.29%), a small with amount a small of Fe-Mnthe oxides organic form, and residual which is form consistents are the with predominant Li et al. [ 18vanadium] who reached chemical the form conclusions while the that content the organic of amountvanadium of Fe-Mn occurring oxides in form, exchangeable, which is carbonates,consistent andwith Fe Li-Mn et oxidesal. [18] are who negligible reached for the raw conclusion petcoke. that andthe organic residual and forms residual are the form predominants are the predominant vanadium chemical vanadium forms chemical while form thes content while the of vanadiumcontent of occurringvanadium in occurring exchangeable, in exchangeable, carbonates, carbonates, and Fe-Mn and oxides Fe- areMn negligibleoxides are for negligible raw petcoke. for raw petcoke.

(a) (b)

Figure 5. (a) Occurrence modes of V in raw petcoke and char in CO2 gasification; (b) Occurrence modes of V in raw petcoke and char in H2O gasification. (a) (b) For the CO2 gasification, most of the organic-bound vanadium converts to the residual form of Figurevanadium 5. ( aat) 800Occurrence °C. The contentmodes of the V in organic raw petcoke-bound vanadiumand char in sharply CO2 gasification reduces from; (b 65.8%) Occurrence to 6.0% Figure 5. (a) Occurrence modes of V in raw petcoke and char in CO2 gasification; (b) Occurrence modesand the of vanadium V in raw petcokebound to and Fe- Mnchar oxides in H2O decreases gasification. from 5.9% to 2.2%, resulting in the residual form modes of V in raw petcoke and char in H2O gasification. vanadium reaching a maximum of 90.8%. As temperature increases to 1000 °C, the residual form of Forvanadium the CO begins2 gasification, to decompose, most mainly of the convertingorganic-bound into vanadium vanadium bound converts to Fe-Mn to theoxides, residual while theform of vanadiumForother the forms at CO800 remains2 °C.gasification, The basically content most unchanged. of the of organic the organic-boundWhen-bound the temperature vanadium vanadium reaches sharply converts 1100 reduces °C, tothe from the Fe-Mn residual 65.8% oxides to form6.0% ◦ ofand vanadium thereach vanadium the at maximum 800 boundC. The valueto Fe content -Mn of 11.0%, oxides of the while decreases organic-bound the organic from 5.9%- vanadiumbound to vanadium2.2%, sharply resultin is reducesalmostg in the completely from residual 65.8% form to 6.0%vanadiumdecomposed and the reaching vanadium. After a maximum gasification bound to of Fe-Mn at 90.8%. 800 oxides°C, As V temperature associated decreases with from increases organic 5.9% to tomatter 2.2%,1000 sharply resulting°C, the drops residual in the 59.8% residual form, of ◦ formvanadiumwhile vanadium thebegins residual reaching to decompose, form a acutely maximum mainly rises 62.5% of converting 90.8%. with Asno in content temperatureto vanadium of V volatility increases bound emerging to toFe 1000-Mn. At oxides,C, 1100 the °C,while residual V the associated with organic matter decreases 64.7% while the residual form acutely rises 53.3% with a formother offorms vanadium remains begins basically to decompose, unchanged. mainly When convertingthe temperature into vanadium reaches 1100 bound °C, the to Fe-Mn Fe-Mn oxides,oxides little V volatility (1.8%) emerging. It is obvious that the organic-bound vanadium prefer◦s to form whilereach the the other maximum forms remains value of basically 11.0%, unchanged. while the organicWhen the-bound temperature vanadium reaches is almost 1100 C, completely the Fe-Mn oxidesdecomposed reach. the After maximum gasification value at of 800 11.0%, °C, whileV associated the organic-bound with organic vanadium matter sharply is almost drops completely 59.8%, ◦ decomposed.while the residual After form gasification acutely rises at 800 62.5%C, Vwith associated no content with of organicV volatility matter emerging sharply. At drops 1100 59.8%, °C, V ◦ whileassociated the residual with organic form matter acutely decreases rises 62.5% 64.7% with while no content the residual of V volatility form acutely emerging. rises 53.3% At 1100 withC, a little V volatility (1.8%) emerging. It is obvious that the organic-bound vanadium prefers to form

Energies 2018, 11, 2158 8 of 14

V associated with organic matter decreases 64.7% while the residual form acutely rises 53.3% with a little V volatility (1.8%) emerging. It is obvious that the organic-bound vanadium prefers to form residual fraction rather than volatilize. No vanadium bound to exchangeable is found throughout the whole gasification process. Energies 2018, 11 8 of 14 For the H2O gasification, as shown in Figure5b the organic-bound vanadium decomposes to the residual form at 800 ◦C. The organic-bound vanadium sharply reduces from 65.8% to 20.8% and the residual fraction rather than volatilize. No vanadium ◦bound to exchangeable is found throughout the residualwhole form gasification increases process. from 28.3% to 71.8%. At 900 C, the residual form of vanadium decreases by 20.0% andFor changes the H2O to gasification, the Fe-Mn as oxides shown and in Figure carbonates 5b the organic while the-bound organic-bound vanadium decomposes vanadium to remainsthe ◦ consistent.residual As form the at temperature 800 °C. The organic increases-bound to vanadium 1000 C, bothsharply organic-bound reduces from 65.8% and the to 20.8% residual and the form of vanadiumresidual simultaneously form increases reduce from 28.3% and convertto 71.8%. into At 900 Fe-Mn °C, the oxides residual while form the of carbonates vanadium remainsdecreases constant. by At 110020.0%◦C, and the changes vanadium to the bound Fe-Mn oxides to carbonates and carbonates converts while to the the organic residual-bound form. vanadium The organic-bound remains vanadiumconsistent. shows Asa the downward temperature trend increases while to Fe-Mn 1000 °C, oxides both display organic- anbound upward and the tendency residual during form of steam gasification.vanadium No simultaneously vanadium bound reduce to exchangeableand convert into is detected. Fe-Mn oxides while the carbonates remains constant. At 1100 °C, the vanadium bound to carbonates converts to the residual form. The organic- Comparing the two kinds of atmosphere, the variation of the forms are obviously different. bound vanadium shows a downward trend while Fe-Mn oxides display an upward tendency during In the CO atmosphere, the dominant conversion is the organic-bound vanadium to the residual form. steam2 gasification. No vanadium bound to exchangeable is detected. However,Comparing the forms the in thetwo steam kinds atmosphereof atmosphere, are the more variation diverse, of the by forms changing are obviously from the different. organic-bound In vanadiumthe CO to2 atmosphere, other forms the of dominant residue, Fe-Mnconversion oxides is the and organic carbonates.-bound vanadium In both atmospheres, to the residual increasingform. temperatureHowever, promotes the forms thein the decomposition steam atmosphere of organic-bound are more diverse, vanadium, by changing but from the the total organic amount-bound in the H2Ovanadium is significantly to other higher forms of than residu thate, inFe-Mn the oxides CO2 atmosphere, and carbonates. indicating In both atmospheres, that the CO increasing2 gasification atmospheretemperature is favorable promotes for the the decomposition decomposition of organic of organic-bound-bound vanadium, vanadium. but the It istotal worth amount noting in the that the vanadiumH2O is bound significantly to Fe-Mn higher oxides than are that gradually in the CO increasing2 atmosphere, in both indicating atmospheres, that the but CO the2 gasification amount in the atmosphere is favorable for the decomposition of organic-bound vanadium. It is worth noting that steam atmosphere is three times higher than that in the CO2 atmosphere, indicating that the steam the vanadium bound to Fe-Mn oxides are gradually increasing in both atmospheres, but the amount atmosphere is conducive to the formation of Fe-Mn oxides. The presence of large amounts of Fe-Mn in the steam atmosphere is three times higher than that in the CO2 atmosphere, indicating that the oxides affects the transformation of various forms in the steam gasification process. At the same time, steam atmosphere is conducive to the formation of Fe-Mn oxides. The presence of large amounts of the totalFe-Mn amount oxides ofaffects the residual the transformation form vanadium of various in the forms steam in the atmosphere steam gasification is significantly process. lowerAt the than that ofsame the t COime,2 theatmosphere, total amount indicating of the residual that the form H2 vanadiumO atmosphere in the promotessteam atmosphere the conversion is significantly of residual formlower vanadium than that into of other the CO forms.2 atmosphere, indicating that the H2O atmosphere promotes the conversion Asof residual the temperature form vanadium increases, into other the volatilizationforms. characteristic of vanadium gradually increases during H2AsO andthe temperature CO2 gasification, increases, as described the volatilization in Figure characteristic6. It is apparent of vanadium that H 2 graduallyO and CO increases2 gasification during H2O and CO2 gasification, as described in Figure 6. It is apparent that H2O and CO2 display similar vanadium volatilization behavior. The vanadium volatility in H2O and CO2 gasification reachesgasification the highest displayRv value similar 1.5% vanadium and 1.8% volatilization at 1100 ◦C. behavior. It is obvious The vanadium that the difference volatility inin theH2O vanadium and CO2 gasification reaches the highest Rv value 1.5% and 1.8% at 1100 °C. It is obvious that the volatility between H2O and CO2 gasification is ignorable, so it can be concluded that the volatility difference in the vanadium volatility between H2O and CO2 gasification is ignorable, so it can be of vanadium during gasification is not dependent on the gasification atmosphere, but mainly on the concluded that the volatility of vanadium during gasification is not dependent on the gasification reactionatmosphere, temperature. but mainly Li et al. on [18 the] found reaction the temperature. volatilization Li of et vanadium al. [18] found is strongly the volatilization dependent of on the vanadiumvanadium in the is strongly organic dependent matter in petcoke.on the vanadium in the organic matter in petcoke.

FigureFigure 6. Volatility 6. Volatility (Rv (Rv%)%) of of vanadium vanadium duringduring petcoke gasification gasiﬁcation from from 800 800 °C ◦toC 1100 to 1100 °C. ◦C. 3.4. Ni Speciation in Different Gasification Atmospheres

Energies 2018, 11, 2158 9 of 14

3.4. NiEnergies Speciation 2018, 11 in Different Gasiﬁcation Atmospheres 9 of 14 Energies 2018, 11 9 of 14 As FactSage software platform is based on the Gibbs free energy minimization principle and As FactSage software platform is based on the Gibbs free energy minimization principle and the the systemsystemAs isFactSageis inin a a thermodynamicthermodynamic software platform equilibrium equilibrium is based on state, the state, Gitibbs is it ismainly free mainly energy used used minimization to analyze to analyze the principle conversion the conversion and the of of inorganicsysteminorganic substances. is substances.in a thermodynamic Calculation Calculation results equilibrium results of of CO CO state,2 and2 and it H H is22O Omainly gasiﬁcationgasification used to associated associated analyze withthe with conversion Ni Ni are are shown shown of in Figuresinorganicin Figure7 and8s substances.. 7 and 8. Calculation results of CO2 and H2O gasification associated with Ni are shown in Figures 7 and 8.

Figure 7. Calculation results of CO2 gasification associated with Ni. Figure 7. Calculation results of CO2 gasiﬁcation associated with Ni. Figure 7. Calculation results of CO2 gasification associated with Ni.

Figure 8. Calculation results of H2O gasification associated with Ni. Figure 8. Calculation results of H2O gasification associated with Ni. Figure 8. Calculation results of H2O gasification associated with Ni. For the CO2 gasification, the only product is NiS 2 (s) below 200 °C. For the 300–400 °C temperatureFor the range CO2 gasification,, the only product the only changes product to Ni 6 isS5 (s). NiS Above2 (s) below 400 °C, 200 the °C main. For product the 300 is Ni–4003S2 (s).°C ◦ ◦ temperatureForLi et the al. [18] CO2 predicted rangegasification,, the the only morphology the product only product changes of vanadium isto NiSNi6S 2in5 (s)(s). the below Above gasification 200 400 °C,C. of Forthepetcoke. main the 300–400 Asproduct the temperature isC Ni temperature3S2 (s). ◦ range,Liincreas theet al. only [18]ed, Ni productpredicted reacted changes thewith morphology S to to form Ni6 SNiS 5of(s). vanadium2, Ni Above9S8, Ni in 4007S 6the, and C,gasification Ni the3S main2. The of productmelting petcoke. point is As Ni thes3 Sof 2temperature NiS(s). Li(s) etand al. [18] predictedincreasNi3S2 (s) theed, are morphology Ni 645 reacted °C and with 797 of S vanadium °C.to form It can NiS be in 2found, theNi9S gasification 8that, Ni7 Sthe6, and S content ofNi3 petcoke.S2. Thedecreases melting As thewith point temperature thes increase of NiS (s) of increased, and Ni, Ni3S2 (s) are 645 °C and 797 °C. It can be found that the S content decreases with the increase of Ni, Ni reactedwhile the with melting S to form point NiS and2, stability Ni9S8, Ni of7 NiS6,xS andy compounds Ni3S2. The gradually melting increase. points of When NiS the (s) andtemperature Ni3S2 (s) are 645 ◦whileCreaches and the 797 1300 melting◦C. °C, It can point gaseous be and found Nistability that(g) and the of Ni S NiS contentxSy compounds(g) are decreases produced. gradually with The the increase. thermodynamicincrease When of Ni, the while temperatureequilibrium the melting reachessimulation 1300 in °C,[27 ] gaseousshowed Ni that (g) Ni andxSy was NiS the (g) stable are produced. form of nickel The thermodynamicuntil 1000 °C in equilibrium a reducing ◦ point and stability of NixSy compounds gradually increase. When the temperature reaches 1300 C, simulationgasification in atmosphere [27] showed (containing that NixS Hy 2wasS gas). the When stable the form temperature of nickel untilwas up 1000 to 1000°C in °C, a reducingthe main gaseous Ni (g) and NiS (g) are produced. The thermodynamic equilibrium simulation in [27] showed gasification atmosphere (containing H2S gas). When the temperature was up to 1000 °C, the main stable nickel form was elemental Ni. For the H2O◦ atmosphere, the distribution of Ni is basically the that NixSy was the stable form of nickel until 1000 C in a reducing gasification atmosphere (containing stablesame as nickel in the form CO 2was atmosphere, elemental but Ni. the For formation the H2O atmosphere, temperature the of Ni distribution6S5 (s) and ofNi Ni3S2 is(s) basically are lowered the H S gas). When the temperature was up to 1000 ◦C, the main stable nickel form was elemental Ni. 2 sbyame 100 as °C. in Thisthe CO indicates2 atmosphere, that steam but isthe favorable formation to generatetemperature the moreof Ni 6stableS5 (s) andNixS Niy compound,3S2 (s) are lowered thereby For thebysuppressing 100 H2O °C. atmosphere, This the indicates volatilization the that distribution steam of the is Ni favorable- ofcontaining Ni is to basically generate compound, the the samemore which stable as is in identical theNixS COy compound, with2 atmosphere, the volatility thereby but the ◦ formationsuppressingresult temperatureof Ni in the Figure volatilization of4. When Ni6S5 theof(s) the temperature and Ni Ni-containing3S2 (s) rises are comto lowered 1400pound, °C, there bywhich 100 is issome C.identical This solid indicates elementalwith the volatility thatNi apart steam is favorableresultfrom to gaseousof generateNi in Figure Ni the(g) 4. and moreWhen NiS stable the (g). temperature NiNi xelementSy compound, rises does to 1400not thereby react°C, there with suppressing is othersome solidmetal the elemental elements volatilization Ni at apart this of the Ni-containingfrom gaseous compound, Ni (g) and which NiS (g). isidentical Ni element with does the not volatility react with result other of metal Ni in elements Figure4. at When this the

Energies 2018, 11, 2158 10 of 14

Energies 2018, 11 10 of 14 temperature rises to 1400 ◦C, there is some solid elemental Ni apart from gaseous Ni (g) and NiS (g). temperature. There are more elemental Ni (s) at high temperature in the H2O atmosphere than that Ni element does not react with other metal elements at this temperature. There are more elemental Ni in CO2 atmosphere, which is also consistent with the residual content of Ni at 1100 °C in the (s) at high temperature in the H2O atmosphere than that in CO2 atmosphere, which is also consistent experiment (Figure 3). with the residual content of Ni at 1100 ◦C in the experiment (Figure3). When comparing the experimental data (Figure 4) with the simulation results, it is found that When comparing the experimental data (Figure4) with the simulation results, it is found that there there is reasonable agreement on the volatilization characteristic of Ni. In the gasification process, is reasonable agreement on the volatilization characteristic of Ni. In the gasiﬁcation process, nickel is nickel is mainly present in the form of Ni3S2 (with a low melting point◦ of 797 °C). When the mainly present in the form of Ni3S2 (with a low melting point of 797 C). When the temperature temperature reaches 800 °C, the volatilization of Ni species occurs, which can also be verified by the reaches 800 ◦C, the volatilization of Ni species occurs, which can also be veriﬁed by the Rv changes in Rv changes in Figure 4. Wang et al. [7] investigated the corrosion and fouling in syngas cooler tubes Figure4. Wang et al. [ 7] investigated the corrosion and fouling in syngas cooler tubes and reported the and reported the Ni3S2 would be vaporized instantly during the devolatilization process due to the Ni S would be vaporized instantly during the devolatilization process due to the high temperature. high3 2temperature. It can be deduced that the main volatilization component of Ni comes from the It can be deduced that the main volatilization component of Ni comes from the Ni3S2 in the high Ni3S2 in the high temperature range (800–1100 °C). temperature range (800–1100 ◦C).

3.5.3.5. V V S Speciationpeciation in in D Differentifferent G Gasiﬁcationasification A Atmospherestmospheres

Figures 9 and 10 show thermodynamic calculation results of vanadium under CO2 and H2O Figures9 and 10 show thermodynamic calculation results of vanadium under CO 2 and H2O gasificationgasification process. process. The The results, results, under under the the gasification gasification conditions of of 800 800–1100–1100 °C,◦C, indicate indicate that that the the gaseousgaseous vanadium compounds formedformed in in the the two two atmospheres atmospheres are are almost almost the the same, same, revealing revealing that that the theatmosphere atmosphere has has little little effect effect on the on distribution the distribution of V speciesof V species at different at different temperatures, temperatures, which proveswhich provethat thes that volatility the volatility of V during of V gasification during gasification does not does depend not on depend the gasification on the gasification atmosphere. atmosphere. Below 200 Below◦ 200 °C, the main product is V3O5, and it changes to V2O3 as the temperature rises from◦ 300 °C C, the main product is V3O5, and it changes to V2O3 as the temperature rises from 300 C to 1500 to◦C. 1500 Below °C. 1300Below◦C, 1300 there °C, is there no volatility is no volatility of V, which of V, is which consistent is consistent with the with experiment the experiment results ofresultsRv in of Rv in Figure 6. Above◦ 1300 °C, a little VO2 gas is generated (less than 5% of the total amount of V). Figure6. Above 1300 C, a little VO2 gas is generated (less than 5% of the total amount of V). However, However, V2O3 is still the main product in the gasification process of petcoke. V does not react with V2O3 is still the main product in the gasification process of petcoke. V does not react with other metal otherelements metal in elements this temperature in this temperature range. The range. migration The andmigration conversion and conversion mechanism mechanism of V in the of process V in the of prpetcokeocess of gasification petcoke gasification is similar to is thatsimilar of coal to that gasification of coal gasification and the conclusions and the conclusions agree well agree with somewell withliterature some [literature28,29] about [28, the29] about presence the presence of vanadium of vanadium in coal gasification in coal gasification reaction. reaction.Frandsen Frandsen et al. [28 et] al. [28] found that the stable V species was V 2O3 (s) and the volatilization of VO2 (g) emerges when found that the stable V species was V2O3 (s) and the volatilization of VO2 (g) emerges when the the temperature exceeded 1525◦ °C. Bunt et al. [29] found that V2O3 remained stable while VO2 gas temperature exceeded 1525 C. Bunt et al. [29] found that V2O3 remained stable while VO2 gas (0.1%) (0.1%)was observed was observed above 1225above◦C. 1225 °C.

Figure 9. Calculation results of CO2 gasification associated with V. Figure 9. Calculation results of CO2 gasiﬁcation associated with V.

Energies 2018, 11, 2158 11 of 14 Energies 2018, 11 11 of 14

Figure 10. Calculation results of H2O gasification associated with V. Figure 10. Calculation results of H2O gasiﬁcation associated with V.

When comparing the experimental data (Figure 4) with the simulation results, it is found that When comparing the experimental data (Figure4) with the simulation results, it is found that there is reasonable agreement on the volatilization characteristic of V. References [17,18,30] illustrate there is reasonable agreement on the volatilization characteristic of V. References [17,18,30] illustrate that the occurrence forms of V in gasification char were mainly V 2O3, FeV2O4 and other forms of that the occurrence forms of V in gasiﬁcation char were mainly V O , FeV O and other forms of minerals, of which there might be the following reaction during the2 gasification:3 2 4 minerals, of which there might be the following reaction during the gasiﬁcation:

2V2O3 + CO + Fe2O3 → 2FeV2O4(s)+ CO2 (3) 2V2O3 + CO + Fe2O3 → 2FeV2O4(s) + CO2 (3) Thermodynamic equilibrium calculation results show that the content of V 2O3 (s) in H2O and CO2

gasificationThermodynamic remain stable equilibrium at 0.004 mol calculation from 300 results °C to 1300 show °C that, but the it contentbegins to of decrease V2O3 (s) at in 1400 H2O °C and in ◦ ◦ COboth2 gasificationH2O and CO remain2 gasification. stable The at 0.004 modeled mol fromresults 300 of theC tocontent 1300 ofC, V but2O3 (s) it beginsin H2O togasification decrease at ◦ 1400 °CC and in both 1500 H °C2O is and 0.00396 CO2 molgasification. and 0.00371 The mol modeled while that results in CO of2 the gasification content of is V 0.003982O3 (s) mol in H and2O ◦ ◦ gasification0.00385 mol. atThere 1400 is anC andinflection 1500 pointC is at 0.00396 1400 °C mol which and sho 0.00371ws the mol trend while of the that change in CO of2 Vgasification compound. isTherefore, 0.00398 molthe andcontent 0.00385 of V mol.2O3 in There H2O isgasification an inflection is lower point than at 1400 that◦C in which CO2 gasification shows the trend at high of thetemperature, change of mainly V compound. because Therefore, high concentration the content of CO of V2 inhibits2O3 in H the2O positive gasification reaction is lower (3), resulting than that the in COincrease2 gasification of residual athigh V and temperature, decrease of mainlyFe-Mn oxides, because which high is concentration also consistent of COwith2 inhibits the residual the positive content reactionof V at 1100 (3), resulting°C in the theexperiment increase (Figure of residual 5). Combined V and decrease with the of Fe-Mn occurrence oxides, patterns which of is alsovanadium, consistent the withproportion the residual of the contentresidual of decreases V at 1100 and◦C inthe the volatile experiment fraction (Figure increases5). Combined as the temperature with the occurrencerises, which patternsis also consistent of vanadium, with the the simulation proportion results of the residualtrend. This decreases shows that and the the simulation volatile fraction results increases can predict as thewell temperature the conversion rises, trend which of V. is also consistent with the simulation results trend. This shows that the simulation results can predict well the conversion trend of V. 3.6. Fouling, Slagging and Erosion Tendency Assessment for Ni and V Occurrence Modes 3.6. Fouling, Slagging and Erosion Tendency Assessment for Ni and V Occurrence Modes Many researchers [31–33] have found that the behavior and effects of heavy metals could not be predictedMany and researchers explained [31 by–33 their] have total found amount that thein the behavior environment, and effects but ofrather heavy were metals related could to notthe bedistribution predicted of and forms explained and the by differences their total amountin migration in the and environment, transformation but rather behaviors were of related the var toious the distributionforms, which of determined forms and thethe differencesvolatility at in high migration temperatures. and transformation Zhang et al.behaviors [31] concluded of the that various the forms,exchangeable which, determined acid-soluble the and volatility organic matter at high as temperatures. soluble form could Zhang be et leachable al. [31] concluded under appropriate that the exchangeable,natural conditions, acid-soluble while insolubleand organic form matters, including as soluble the form Fe- couldMn oxides be leachable and residue under appropriateform, were naturaldefined conditions,as the form while that could insoluble not be forms, leached including. The analysis the Fe-Mn of morphological oxides and residue changes form, of heavy were defined metals asin the the process form that of fly could ash notheating be leached. indicated The that analysis the soluble of morphological form of minerals changes decreased of heavy with increasing metals in thetemperature, process of and fly ashthe heatingtotal of indicatedresidual and that Fe the-Mn soluble oxides form form of increased minerals decreasedaccordingly, with summarizing increasing temperature,that high temperature and the total benefits of residual the stability and Fe-Mnof minerals oxides [31 form]. Liu increased et al. [32] accordingly,found a relatively summarizing uniform thatdistribution high temperature of five forms benefits of Ni the in stabilitydry sludge of minerals which were [31]. Liumainly et al. distributed [32] found in a relativelythe Fe-Mn uniform oxides distributionform and residue of five form forms after of thermal Ni in dry treatment. sludge which With the were increase mainly of distributed temperature, in the the ratio Fe-Mn of state oxides to formunstable and state residue increased form after from thermal 0.4:1 in dry treatment. sludge to With a maximum the increase of 10.4:1 of temperature, at 700 °C, indicating the ratio ofthat state Ni towas unstable present state in a increasedmore stable from form 0.4:1 after in dry thermal sludge treatmen to a maximumt. Zorpas of et 10.4:1 al. [33 at] 700 found◦C, that indicating significant that conversions of minerals converted to a less mobile form or released by vaporization during heating process. Wang et al. [7] researched the corrosion and fouling in syngas tubes, and found that the

Energies 2018, 11, 2158 12 of 14

Ni was present in a more stable form after thermal treatment. Zorpas et al. [33] found that significant conversions of minerals converted to a less mobile form or released by vaporization during heating process. Wang et al. [7] researched the corrosion and fouling in syngas tubes, and found that the insoluble form was relatively stable and it could not be easily volatilized during the heating process, suggesting the problem of being more inclined to the erosion on the metal tube wall. Correspondingly, the soluble form was more likely to cause fouling and slagging. Therefore, it can be seen from Figures3 and5 that during the gasification reaction, V and Ni dominate the erosion problem under the CO2 atmosphere. The V and Ni insoluble mass fractions are not affected by the temperature change, remaining basically at 93% and 85%, respectively. Compared with the CO2 atmosphere, the contribution rate of V and Ni to the fouling and slagging is obviously increased under the H2O atmosphere, but this impact gradually weakens with the increase of temperature. The influence of atmosphere to the fouling and slagging problems is about the same at the temperature of 1100 ◦C. In addition, it can be noticed that the erosion problem caused by V is more serious than that by Ni under the two atmospheres, especially in the CO2 atmosphere, so attention should be paid to the ratio of CO2 concentration during actual operation.

4. Conclusions

The migration and transformation of V and Ni in high sulfur petcoke during CO2 and H2O gasiﬁcation were investigated in the range from 800 ◦C to 1100 ◦C in a laboratory-scale ﬁxed-bed reactor. The V and Ni volatilization behavior, migration and transformation behavior, and distribution of occurrence modes were studied. The main conclusions were drawn as follows:

(1) The organic matter and residual forms were the dominant chemical forms of both V and Ni in petcoke and the Fe-Mn oxides only accounted for a small proportion. (2) The volatilization of V and Ni was not dependent on the gasiﬁcation atmosphere, but mainly on the reaction temperature.

(3) The CO2 atmosphere was conducive to the conversion of the organic-bound nickel to the residual form Fe-Mn oxides. The H2O atmosphere favors the formation of diverse forms. (4) Factsage results conﬁrmed the H2O atmosphere was favorable to suppress the volatilization of Ni-containing compounds. More residual content of V was formed in the CO2 gasiﬁcation. (5) The V and Ni mainly caused erosion problems under the CO2 atmosphere while the fouling and slagging obviously increased under the H2O atmosphere, with this impact being gradually weakened with the increase of temperature. The erosion problem caused by V was more serious than that caused by Ni, especially in the CO2 atmosphere.

Author Contributions: W.L., B.W. and L.S. conceived, designed and performed the experiments; J.N. and W.Y. performed the sequential chemical extraction; W.L. and L.X carried simulation calculation; W.L. and B.W. analyzed and wrote the paper. Funding: This research was funded by the Financial support of the National Natural Science Foundation of China (NSFC) (NO. 51506068, NO. 51661145010). Acknowledgments: This work was supported by the Financial support of the National Natural Science Foundation of China (NSFC) (NO. 51506068, NO. 51661145010). The work was also supported by the China Scholarship Council. The authors also thank the help from the Analytical and Testing Center of Huazhong University of Science and Technology. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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