Effect of Iron Species and Calcium Hydroxide on High-Sulfur

Jan 24, 2012 - ABSTRACT: The effect of iron species on petroleum coke CO2 ... The catalytic gasification residue of petroleum coke was identified by X...
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Effect of Iron Species and Calcium Hydroxide on High-Sulfur Petroleum Coke CO2 Gasification Zhi-jie Zhou,* Qi-jing Hu, Xin Liu, Guang-suo Yu, and Fu-chen Wang Key Laboratory of Coal Gasification, Ministry of Education, East China University of Science and Technology, Shanghai 200237, People’s Republic of China ABSTRACT: The effect of iron species on petroleum coke CO2 gasification was studied in the present work. The effects of the temperature (1173−1673 K), the catalyst types, catalyst loading (ranging from 0 to 5 wt %), and composition during the gasification of petroleum coke CO2 were discussed. The catalytic gasification residue of petroleum coke was identified by X-ray diffraction (XRD). The XRD result shows that the iron species exist as Fe3C at the initial stage of gasification. Then, most Fe3C is transformed into reduced iron Fe0; subsequently, some of these species are oxidized to FeO, Fe2O3, and Fe3O4, while some of which quickly reacted with S in petroleum coke to form stable FeS, leading to catalyst deactivation. The changes during gasification are discussed in terms of the oxygen-transfer mechanism. It is clearly demonstrated that there exists a significant synergistic effect between calcium hydroxide and iron species during petroleum coke gasification. The random pore model is suitable to describe catalytic gasification of petroleum coke. The activation energy of petroleum coke CO2 gasification with 5 wt % FeCl3 is 168.06 kJ mol−1, with the temperature range of 1223−1373 K, which is 29.9 kJ mol−1 lower than pure petroleum coke CO2 gasification.

1. INTRODUCTION Petroleum coke is the residue of petroleum refining. With the continuous increasing of heavy crude oil in worldwide supply and the development of crude oil refining technology, the yield of petroleum coke is steadily increased. It is an urgent problem to solve the disposal of petroleum coke effectively, especially the high-sulfur petroleum coke.1 The entrained-bed gasification, with its high reaction temperature and pressure, has been proven to be an efficient and economic process for coal, biomass, petroleum coke, and waste to produce useful syngas (CO + H2) with near-zero pollution emissions.2−4 Because of the high heating value, high carbon content, and good compatibility between petroleum coke and oil, great efforts5−7 have been put into study of petroleum coke gasification in recent years. However, previous studies3,5 found that the gasification reactivity of petroleum coke was low because of its low combustible, low ash content and compact carbon structure, which greatly restricted its applicability for feedstocks of gasifiers. Therefore, it is important and valuable to improve the gasification reactivity of petroleum coke. It is well-known that the gasification activity of carbon in carbonaceous materials (such as coals and coal chars) can be greatly enhanced by various alkali and alkaline earth metal compounds,6−10 which provides some guidance to study the catalytic gasification of petroleum coke. Alkali metal salts act as effective catalysts for coal combustion and gasification11−13 at an appropriate range of temperatures. However, taking into account the economic benefits and environmental effect, it is economically unfeasible to use pure alkali metal salts as catalysts during gasification and difficult to recover after gasification. Many efforts have been put into searching for a cheaper and more effective catalyst. The catalytic effect of black liquor on petroleum coke gasification has been investigated. The result © 2012 American Chemical Society

showed that the gasification reactivity of petroleum coke was improved greatly by black liquor.1 From an economical point of view, iron species seem to be much more promising catalysts with nearly unlimited availability. Waste acids from titanium dioxide production and metal pickling plants are preferred sources of iron salts, mainly of iron sulfate and iron chloride.14,15 However, the application of iron species as catalysts of carbon or coal gasification involves numerous problems, for example, the primary problem being the dispersion of iron species on the carbon or coal surface. Agglomeration and sulfur poisoning during gasification have already been detected to occur,16,17 which lead to a loss of active sites. Therefore, some methods have been reported to enhance the catalytic activity of iron species.16,18 FeCl3 can be converted to a highly active, highly dispersed, Cl−-free Fe catalyst proposed by Ohtsuka and Asami in NH3/NH4Cl solution.18 However, there remains several problems in this method. One problem is that NH3/NH4Cl is more expensive than FeCl3. Another problem is that this method may be just restricted to low-rank coals. Asami et al. studied that the iron catalyst precipitated from FeCl3 solution using inexpensive Ca(OH)2 promoted the gasification of brown coal and char with CO2.16 However, it may also just be restricted to coals with higher oxygen contents and lower mineral contents than brown coal markedly. In this paper, we use a pre-oxidation method to enhance the oxygen content of the coke, which is proven to effectively break the restriction in the work by Asami et al.16 The addition of other catalysts leading to the synergistic effect of catalysts is also a good choice to enhance the catalytic Received: August 17, 2011 Revised: January 23, 2012 Published: January 24, 2012 1489

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Table 1. Proximate and Ultimate Analyses of the Samplesa proximate analysis (wd, %)

a

ultimate analysis (wd, %)

samples

ash

volatile matter

fixed carbon

C

H

N

S

O

coke

0.29

10.33

89.38

92.24

3.20

1.48

2.77

0.31

wd refers to the mass fraction on a dried basis.

Figure 1. Process of catalyst preparation. Ca(OH)2/FeCl3 of 1.5 was added to the mixture of petroleum coke after nitric acid oxidation pretreatment and FeCl3 solution, and the resulting mixture was stirred for 2 h at room temperature and filtered to remove solution. The remained sample was washed with deionized water repeatedly to remove the Cl ions and Ca ions and then dried at 378 K for 2 h, which was denoted as petcoke + FeCl3(C). (3) FeCl3− Ca(OH)2 binary catalyst: we loaded FeCl3−Ca(OH)2 binary catalyst with the method of ion exchange. Ca(OH)2 powder at an experimental designed mass ratio of Ca(OH)2/FeCl3 was added to the mixture of petroleum coke after nitric acid oxidation pretreatment and FeCl3 solution. The resulting mixture was stirred for 24 h at room temperature and then dried at 378 K for 2 h. The process of catalyst preparation is given in Figure 1. 2.3. Atmospheric Gasification Procedure. The measurement of the gasification reactivity of petcoke was carried out on a ThermoCahn Thermax 500 thermogravimetric analyzer (TGA). In each experiment, a 7−8 mg sample of petcoke was used. A nitrogen gas of high purity (99.99%) was purged at a flow rate of 1000 mL/min when the sample was heated at a heating rate of 25 K/min until the temperature reached the set temperature (1223, 1273, 1323, 1373, and 1653 K). The gasification started when nitrogen was switched by carbon dioxide at the desired temperature and proceeded isothermally until no mass loss occurred. In preliminary tests,7 it was found that reaction rates do not increase any more when the sample weight is less than 10 mg, the partical size is smaller than 74 μm, and the gas flow is more than 60 mL min−1. It means that the effects of external and intragranular diffusion have been eliminated in the experimental condition. The kinetic measurements are under reaction control. 2.4. Calculation of Conversion. The gasification conversion (X) was calculated according to eq 1. The gasification rate (r) at time t is generally determined by eq 2

activity of iron species. In the case of the synergistic effect of catalysts, several works have been published.19−22 Liu et al.21 reported that inorganic matter (CaO, Al2O3, and K2CO3) had synergistic effects on coal gasification, but these effects were related to the temperature and coal rank. Woon et al.22 tested the catalytic activity of alkali and transition-metal salt mixtures for steam char gasification. An equimolar mixture of K2SO4 and Ni(NO3)2 exhibits the highest catalytic activity and synergistic effect. However, the synergistic effect of FeCl3 and Ca(OH)2 during petroleum coke gasification has not yet been reported. This paper is based on fundamental studies of the above problems. In this study, iron species catalysis of petroleum coke CO2 gasification was performed to investigate the catalytic effects of iron species, temperature, catalyst loading, composition of catalysts, and synergistic effect of calcium hydroxide and iron species during the gasification process.

2. EXPERIMENTAL SECTION 2.1. Samples. Petroleum coke (petcoke) from Nanjing Jinling Refinery Plant in China was used in this study. Petroleum coke was denoted as coke. The sample were dried at 378 K until no weight loss, ground, and sieved to particles with a size fraction Fe(NO3)3 > FeSO4. Iron chloride is readily available as acid wastes from steel pickling and titanium oxide production plants. Also, it shows the highest catalytic activity of the three types of iron species chosen. Therefore, it is the most desirable raw material for iron species catalysts.14,15 However, Cl−containing compounds inevitably evolve during gasification, and they may cause some serious problems, such as corrosion on various parts of materials and increasing capacity needed for gas cleaning. Therefore, it would be necessary to develop a new method to convert iron chloride to active catalysts, avoiding such pollutants. 3.2. Effect of the FeCl3 Loading Method on Petroleum Coke Gasification. Figure 3 shows the gasification reactivity

Table 2. Contents of Fe, Ca, and Cl in Petroleum Coke sample

Fe (wt %)

Ca (wt %)

Cl (wt %)

coke coke + 5% FeCl3(C)

0.002 1.800

0.005 0.0070

0.007 0.008

Cl and Ca inherently present in the original petroleum coke. Therefore, Cl− in FeCl3 was completely removed by water washing, which implied no Cl contamination, and the Cl−-free Fe catalyst was prepared. These results show that the precipitation method using Ca(OH)2 is suitable for preparing the chlorine-free, active iron catalyst from FeCl3 solution. Therefore, the Fe(C) catalyst alone will be used in the following sections. 3.3. Effect of the Temperature on Petroleum Coke Catalytic Gasification. Figure 5 shows the effect of the gasification temperature on petroleum coke with 5 wt % FeCl3 loading. With the high ratio of C/H, the petroleum coke is composed by polycyclic aromatic hydrocarbons and is rich in the aromatics with lots of rings.23 The polycyclic aromatics with low reactivity are gasified slowly at low temperatures. It is concluded from Figure 5d that the big aromatic ring systems start gasification rapidly from 1373 K. Figure 5f shows that the gasification conversion at 1223 K is higher at any time than that at 1323 K without catalyst. This means that the use of an iron catalyst can lower the gasification temperature by more than 100 K in this experimental condition. The temperature is one of the most important factors influencing the catalytic activity of Fe. The difference between the reactivity of petroleum coke

Figure 3. Variation of X with t at 1273 K for petroleum coke with 5 wt % FeCl3 by different loading methods. 1491

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Figure 5. Variation of X with t at different temperatures for petroleum coke with 5 wt % FeCl3.

increasing. Agglomeration of the iron catalyst on brown coal char proceeds more rapidly at a higher charring temperature, which leads to a lower degree of catalyst dispersion.16 The rate at 1273 and 1373 K in the latter part of the gasification decreased rapidly with the increase of conversion. Such a rate drop has been reported by many researchers12,14,16,18 and is considered as the main drawback in the catalysis of coal gasification by iron. X-ray diffraction (XRD) measurements reveal catalyst aggregation of iron species from the Fe(C) catalyst.24 Catalyst agglomeration results from the consumption of most of the petroleum coke, which results in rapid catalyst deactivation. Table 3 shows the porosity state of petroleum coke with FeCl3 at different gasification conversions. The specific surface area and pore volume increase with gasification conversion before 50%, and after reaching their maximum, they decrease with gasification conversion after 50%. The specific surface area and pore volume are particularly low when gasification conversion reaches 90%, which further leads to the reactivity decreasing markedly.

with Fe and without Fe becomes less with the temperature increasing. The gasification rate of the iron-catalyzed gasification is plotted as a function of the petroleum coke gasification conversion in Figure 6. In comparison to the uncatalyzed

Figure 6. Gasification rate with X at different temperatures for petroleum coke with 5 wt % FeCl3.

gasification, the decrease of the gasification rate with the increase of conversion is more marked with the temperature 1492

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Table 3. Porosity State of Petroleum Coke with FeCl3(C) at Different Conversions samples

BET surface area (m2/g)

pore volume (cm3/g)

pure coke 0% 30% 50% 70% 90%

1.63 5.71 6.07 7.37 1.24 0.93

0.0058 0.050 0.073 0.074 0.050 0.025

3.4. Effect of the Catalyst Loading on Petroleum Coke Catalytic Gasification. The gasification reactivity is usually quantified by a reactivity index, Rs, which is defined as 0.5 Rs = τ0.5 (3)

Figure 8. XRD spectra of the 5 wt % ferric chloride catalytic gasification residue of petroleum coke at different gasification conversions (X): A, Fe3C; B, FeS; C, Fe2O3; D, Fe3O4; and E, Fe0.

iron during gasification with CO2 and H2O has usually been explained in terms of the oxygen-transfer mechanism.25−27 This involves the following solid−gas and solid−solid reactions:

where τ0.5 is the time (in minutes) needed for the carbon conversion of 50%. This definition is commonly used in the literature for the comparison of the gasification reactivity of different coals. Figure 7 presents Rs values of different catalyst

FexOy + CO2 → FexOy (O) + CO

(4)

FexOy (O) + C → FexOy + CO

(5)

At the initial stage of gasification, Fe3C and CO2 react to form CO and metallic iron, which react with CO2 to form CO and iron oxides. Then, iron oxides are subsequently reduced by carbon. Fe0 formed when gasification conversion reached 20%, which suggests rapid reduction of iron oxides to metallic iron on the solid phase. It has been well-established in the literature16,26 that, after iron loading, iron may be associated with certain functional groups in Figure 4, including carboxylic groups. At the beginning of gasification, smaller iron particles with a high degree of dispersion and high reactivity formed more reactive sites in the petroleum coke,16 which lead to rapid solid-phase reactions. A schematic picture of iron-catalyzed carbon dioxide gasification of petroleum coke is given in Figure 9.

Figure 7. Relation between the gasification reactivity and catalyst loadings at 1273 K.

loadings for the CO2 gasification reaction at 1273 K. It can be seen that FeCl3 could effectively increase the petroleum coke gasification rate. The higher the Fe loading, the better the increasing gasification reactivity. However, when FeCl3 loading is increased to 5 wt % petroleum coke, the gasification reactivity is slightly increased. Taking that into account, 2 wt % loading is a saturation loading. This result is consistents with the work by Sams et al.25 In this study, to prevent the possible deviation of catalyst loading affecting the gasification reaction rate, we chose 5 wt % as the catalyst loading rather than the saturation loading of 2 wt %. 3.5. XRD Analysis of Catalyst Composition during Catalytic Gasification. XRD is used in the present paper to analyze the transformation of iron species in the petroleum coke. Figure 8 shows XRD analysis of the 5 wt % ferric chloride catalytic gasification residue of petroleum coke at different conversions. As shown in Figure 8, at an initial stage of gasification, Fe3C was the main species. When the ferricchloride-bearing petroleum coke was subsequently gasified with CO 2, the XRD changed drastically. When gasification conversion reached 20%, most Fe3C disappeared, and instead, reduced Fe0, Fe2O3, FeS, and Fe3O4 became predominant. Some of these reduced iron Fe0 are oxidized to FeO, Fe2O3, and Fe3O4 subsequently when gasification conversion reached 40%, while others quickly reacted with S in petroleum coke to form stable FeS, leading to catalyst loss. The catalytic action of

Figure 9. Reaction scheme of iron-catalyzed carbon dioxide gasification of petroleum coke.

3.6. Synergistic Effect between Calcium Hydroxide and Iron Species. It is evident in Figure 10 that there exists a significant synergistic effect between calcium hydroxide and iron species during petroleum coke gasification. As a strong iron species catalyst poison, sulfur in coals and high-sulfur petroleum coke exist in both organic-, especially 1493

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Figure 11. Arrhenius curve.

Figure 10. Gasification rate with X at 1273 K for petroleum coke gasification with pure and mixed catalysts by the ion-exchange method.

coke noncatalytic gasification with CO2 was 198.05 kJ mol−1 in the same conditions, which is in accordance with the reported data.33 By comparison, the activation energy of petroleum coke CO2 gasification with 5 wt % FeCl3 is lower (29.9 kJ mol−1) than petroleum coke noncatalytic gasification.

28

heterocyclic, and inorganic-bound sulfur forms. The main sources of inorganic-bound sulfur are the iron disulfides pyrite and marcasite. Inorganic-bound sulfur could be removed by chemical desulfurization.29 Calcium carbonate, hydroxide, and oxide are most effective in sulfur-capture processes.30−32 The formation of CaS reported by Jaffri and Zhang32 suggests that calcium hydroxide can work as not only a synergistic catalyst but also an in situ desulfurization agent during coal gasification. 3.7. Kinetics Analysis of Petroleum Coke Catalytic Gasification. The shrinking core model, integrated model, random pore model, and normal distribution function model18−21 are used to fit the kinetic data of petroleum coke gasification with CO2. Their formulation are as follows:

dX = K (1 − X )2/3 dt

(6)

dX = K (1 − X )n dt

(7)

dX = r0(1 − X ) 1 − ϕ ln(1 − X ) dt

(8)

4. CONCLUSION The order of catalytic activity of iron species catalysts is FeCl3 > Fe(NO3)3 > FeSO4. The catalytic effect of FeCl3 can be enhanced by the addition of Ca(OH)2 to obtain a Cl−-free Fe catalyst. The gasification reactivity of petroleum coke increases with an increasing iron species catalyst loading and gasification temperature. The changes during gasification are discussed in terms of the oxygen-transfer mechanism. There exists a significant synergistic effect between calcium hydroxide and iron species during petroleum coke gasification. The random pore model well describes the kinetic curve for catalytic gasification of petroleum coke among the four kinetic models, whose correlation coefficient exceeds 0.96.

■

AUTHOR INFORMATION

Corresponding Author

⎛ (X − X )2 ⎞ dX max ⎟ = rmax exp⎜⎜ − ⎟ dt 2ω2 ⎝ ⎠

*Telephone: 86-021-64252974. Fax: 86-021-64251312. E-mail: [email protected].

(9)

Notes

The fitting results are shown in Table 4. It is clear that the random pore volume among all other models can best describe

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The research is financially supported by the National Key State Basic Research Development Program of China (973 Program, 2010 CB 227000), the National High Technology Research and Development Program (863 Program, 2011AA050106), the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT-IRT 0620), and the Shanghai Outstanding Academic Leaders Subsidy Scheme (08 XD 1401300).

Table 4. Correlation Coefficient of Kinetic Models R2 model

1223 K

1273 K

1323 K

1373 K

random pore volume shrinking core model integrated model normal distribution function model

0.96657 0.90177 0.68493 0.89569

0.99878 0.98746 0.91515 0.94686

0.99704 0.97109 0.86403 0.96077

0.99485 0.97651 0.85296 0.98632

■

the variation of the gasification rate with conversion. The correlation factor R2 values of the normal distribution function model at different temperatures are all more than 0.96. The maximal gasification rate rm increases with an increasing temperature. We found ln rm has a linear relationship with 1/T, as shown in Figure 11. It indicates that the relationship between the parameter rm and the temperature obeys the Arrhenius law rm = A0 exp(−Ea/RT). Therefore, the activation energy Ea and frequency factor A0 can be calculated: Ea = 168.06 kJ mol−1, and A0 = 1.97 × 107 min−1. The activation energy of petroleum

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