Experimental Study on Co-gasification of Coal Liquefaction Residue

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Experimental Study on Co-gasification of Coal Liquefaction Residue and Petroleum Coke Xin Liu, Zhi-jie Zhou, Qi-jing Hu, Zheng-hua Dai, 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: An experimental study on co-gasification of coal liquefaction residue and petroleum coke in carbon dioxide was investigated by thermogravimetric analysis. The temperature of the experiment was 1173
1323 K, and the isothermal (1273 K) kinetics were compared to evaluate the effect of loading of coal liquefaction residue on the gasification reactivity of petroleum coke. The gasification reactivity, X-ray diffraction, scanning electron microscopy images, and Brunauer
Emmett
Teller specific surface area were investigated. The results confirm that the gasification reactivity of petroleum coke was improved greatly by the catalytic components in coal liquefaction residue. The catalytic effect of the catalytic components in coal liquefaction residue was influenced by the temperature and loading. Under the condition of reaction kinetic control, higher temperature promoted the catalytic effect of coal liquefaction residue, and the catalytic effect also increased with the loading.

1. INTRODUCTION Petroleum coke is the final byproduct during the refining process in delay-coke equipment. With a continuous increase in the worldwide supply of heavy crude oil and the installation of more petroleum deep conversion process units, the output of petroleum coke is steadily increasing.1,2 Some reports about the gasification of petroleum coke have already been published.3
7 However, the dull reactivity of petroleum coke is still a problem for industry application mainly because of the high carbon/hydrogen ratio, high sulfur, and the low content of combustible volatiles and ash. It is critical to improve the gasification reactivity of petroleum coke. On the other hand, coal liquefaction technology is the leading technology for high coal conversion technology and the development of cleaner energy in the future, and it has strategic significance in the development of energy technology in China.8,9 On the basis of liquefaction conditions of coal, the coal liquefaction residue contains heavy liquid hydrocarbons and solids, which include carbonaceous matters, almost all minerals in coal, and remaining liquefaction catalysts.10 The coal liquefaction residue accounts for 30% of the total amount of raw coal sometimes, among which the contents of carbon, sulfur, and inorganic matter are higher.11
14 Therefore, taking into account the economy and environment, it is important to make effective use of coal liquefaction residue. To improve the gasification reactivity of petroleum coke, adding coal liquefaction residue as a catalyst for co-gasification reaction of coal liquefaction residue and petroleum coke has a comparatively large advantage because of its high content of alkali metals, alkaline-earth metals, and iron
sulfur catalysts.15 Therefore, the aims of the present work are to study the co-gasification reactivity of petroleum coke and coal liquefaction residue at different temperatures and loadings of coal liquefaction residue. 2. EXPERIMENTAL SECTION 2.1. Samples. Petroleum coke (petcoke) was supplied by the Jinling Refinery Plant in Nanjing, China, and coal liquefaction residue (CLR) r 2011 American Chemical Society

was supplied by the Shenhua Group coal liquefaction pilot plant in Beijing, China. The samples were sieved to within a size range of 83
165 μm. The proximate and ultimate analyses and the high heating values of the samples are presented in Table 1. The component content analysis of CLR is shown in Table 2. It can be seen that the solid content of CLR is more than 40%, including the unreacted carbonaceous matters, almost all minerals in coal, and remaining liquefaction catalysts. The composition analysis of ash in CLR is shown in Table 3. It shows that there are plenty of alkali- and alkaline-earth-metallic (AAEM) species and iron oxygen in the CLR. They are effective catalysts for combustion and gasification of carbonaceous materials. The samples were dissolved in 50% HCl and 50% HNO3 for 24 h, separately, and the solution was diluted and measured by atomic absorption spectrometry (AAS). The content of metal in CLR before and after acid wash is shown in Table 4. 2.2. Analysis Methods. The scanning electron microscopy (SEM) analysis was performed on a JSM-6360LV electron microscope. The specific surface area and pore volume analysis was conducted on an ASAP 2020 physical adsorption instrument using N2 adsorption. The crystal structure of the sample was determined with a JSM-6360 LV X-ray diffraction (XRD) device using Cu KR radiation. The characteristic parameters (d002 and Lc) of the crystalline structure of coke samples are calculated according to the following equations:16 d002 ¼

Lc ¼

λ 2 sin θ002

ð1Þ

0:94λ β002 cosðθ002 Þ

ð2Þ

where d002 and Lc are the interplanar spacing and the stacking height of the carbon crystal, respectively. λ is the wavelength of the X-ray radiation, θ002 is the position of the (002) peak, and β002 is the angular width at halfmaximum intensity of the (002) peak. Received: March 15, 2011 Revised: June 8, 2011 Published: June 20, 2011 3377

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Table 1. Proximate and Ultimate Analyses and the High Heating Values of the Samples proximate analysis (wt %, ad) sample

Madc

petroleum coke

1.80

CLR

0.05

ultimate analysis (wt %, dry ash free)

Vadc

FCadc

C

H

N

S

Oa

HHVb (MJ/kg)

0.26

9.34

88.60

89.15

3.72

0.75

2.08

4.04

36.08

14.46

30.69

54.81

76.97

2.12

1.71

1.98

2.76

31.98

Aadc

Calculated by difference. b HHV refers to high heating value. cad: “M”: moisture, “A”: ash; “V”: volatile; “FC”: fixed carbon, “ad”: air dry, which are all air dry basis.

a

Table 2. Component Content Analysis of CLR

content (wt %)

Table 4. Content of Metal in CLR before and after Acid Wash

oil

asphaltene

solid

26.38

33.59

40.03

metal element content (wt %, dry basis) sample CLR

Table 3. Composition Analysis of Ash in CLR

acid-washed CLR

Fe

Ca

Na

Mg

K

10.86

4.34

15.52

1.58

6.82

2.98

1.22

0.95

0.09

0.57

Al2O3 CaO Fe2O3 TiO2 SiO2 Na2O K2O MgO ash content (wt %) 3.36 12.71 25.04 12.02 45.86 0.55 0.46 0.45

2.3. Measurement of Sample Co-gasification Reactivity. The measurement of the gasification reactivity of coke was carried out on a Thermo-Cahn Thermax 500 thermogravimetric analyzer. In each experiment, a 7
8 mg sample of coke was used. 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 1273K. The gasification started by switching nitrogen to carbon dioxide at the desired temperature and proceeded isothermally until no mass loss occurred. The mass conversion (x, %) is calculated according to the following equation: x¼

m0
m  100 m0
mash

ð3Þ

where m0 is the sample mass (g) on a dry basis at the initial time, m is the sample mass (g) on a dry basis at time t, and mash is the mass (g) of the ash in the sample and the loading of CLR is defined as the ratio of the CLR to the total mass of the mixture.

3. RESULTS AND DISCUSSION 3.1. Morphology of the Samples. The SEM pictures of CLR, petroleum coke, and co-gasification char with 20% CLR loading are presented in Figure 1. It can be seen from Figure 1a that the particle of CLR kept its irregular shape and had a relatively compact structure. From Figure 1b it can be seen that petroleum coke is a layered structure, is dense in texture, and is somewhat regular in arrangement. Figure 1c
f shows the SEM images of the co-gasification char gasified at 1273 K, and the gasification conversions (x) of the mixture are 36%, 46%, 61%, and 72%, respectively. As shown in Figure 1c
f the pore structure of the co-gasification char has emerged during the gasification process. Asphaltene, heavy oil, medium oil, and ash dissolve out to form the co-gasification char, and the pore structure appears when conversion of the co-gasification samples reaches 36%. With gasification reactions in progress, asphaltene volatize and the inorganic mineral substance in ash catalyzes the co-gasification reaction, and the pores of char become larger. When the conversion (x) of the mixture reaches 72% during the co-gasification process, the micropores merge and the macropores collapse. Therefore, the pores become larger and porosity becomes smaller.

3.2. Effect of the Loading of CLR. The experiments were carried out at 0%, 5%, 20%, 30%, 50%, and 100% CLR loading. The results of co-gasification reactivity at 1273 K are presented in Figure 2. It shows that the gasification rate depends on the CLR loading. The high loading of CLR could increase the catalytic content in CLR. Therefore, the gasification rate of the mixture was obviously higher than that of pure petroleum coke but lower than that of CLR. Table 1 shows there is 14.46% ash in the CLR. The composition of CLR and the ash composition of CLR are shown in Tables 2 and 3. It can be seen that, during the direct liquefaction technological process, there are carbon, hydrogen, iron-based catalyst, alkali and alkaline-earth metals (Ca, Na, K, Mg, etc.), and some asphaltene and emollient oils that are still unreacted in CLR. These AAEM species17
20 and iron
sulfur species21 both have a catalytic effect on gasification of carbonaceous matter. With increasing CLR loading, the ratio of the catalytic content of AAEM species and iron
sulfur species increased. Therefore, the gasification reactivity of the petroleum coke increases substantially with increasing CLR loading because high CLR loading leads to more catalytic components. 3.3. Effect of Asphaltene and Emollient Oils. Li22found that pyrolysis at 1173 K can remove the asphaltene and emollient oils. Therefore, some CLR was pyrolyzed in N2 (99.99%) for 30 min at 1173 K to find out the effect of asphaltene, heavy oil, and medium oil in CLR during the co-gasification process. The treated sample was named “CLR char”. Figure 3 shows the FTIR spectra of CLR and CLR char. The aromatic structures of CLR before and after pyrolysis are different. The C
H stretching vibration absorption peak of the aromatic ring around 3400 cm
1, the CdC skeleton vibration absorption peak between 1400 and 1600 cm
1, and the C
H plane deformation vibration absorption peak between 650 and 910 cm
1 of CLR char are all weaker than those of CLR, which indicates that the condensation degree of the aromatic ring increased and its structure tended to graphitization after pyrolysis. Figure 4 shows the XRD spectrum of CLR and CLR char, and the microcrystallite parameters of CLR and CLR char are shown in Table 5. The results demonstrate that the (002) crystal plane diffraction maximum corresponding to the angle shifts to large angle after pyrolysis and is closer to 26.6° of native graphite. In addition, after pyrolysis, the interplanar spacing d002 decreases and the crystallite length Lc increases. Therefore, the spatial 3378

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Figure 1. SEM images of the samples: (a) CLR; (b) petroleum coke; (c) co-gasification char of x = 36%; (d) co-gasification char of x = 46%; (e) co-gasification char of x = 61%; (f) co-gasification char of x = 72%.

Figure 4. XRD spectrum of CLR and CLR char. Figure 2. Carbon conversion (x) vs t at different CLR loadings.

Table 5. Microcrystallite Parameters of CLR and CLR Char 2θ/deg

d002/nm

Lc/nm

CLR

25.44

0.348

8.510

CLR char

25.96

0.343

8.519

sample

Figure 3. FTIR spectra of CLR and CLR char.

arrangement of the aromatic layer of CLR char becomes more regular, and the aromatic structure of CLR char tends to be that of graphite. Figure 5 shows the result of co-gasification of petroleum coke with CLR and CLR char. From the results, it can be seen that the catalytic effect of CLR char is almost 3 times higher than that of

CLR. It can also be seen that the degree of graphitization of CLR char is larger than that of CLR. However, its catalytic effect is better than that of CLR. It can be concluded that the asphaltene and emollient oils have no catalytic effect. During the pyrolysis process, asphaltene and emollient oils volatilized and formed a porous structure, resulting in the catalytic effect of CLR char being better than that of CLR. 3.4. Effect of Alkali, Alkaline-Earth, and Transition Metals. To study the influence of the alkali, alkaline-earth, and transition metals in CLR on petroleum coke during the co-gasification process, some CLR was washed by acid to remove these elements. The metal elemental content of CLR before and after acid wash can be seen in Table 4. The carbon conversions of different samples were compared, including pure petroleum coke and mixtures of acid-washed CLR and petroleum coke, coal and petroleum coke, CLR and 3379

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Energy & Fuels petroleum coke, and CLR ash and petroleum coke. The additive loading is 5% in the first three samples, while the CLR ash loading is 1%. The order of gasification reactivity is petroleum coke with CLR ash > petroleum coke with CLR > petroleum coke with coal > petroleum coke with acid CLR > pure petroleum coke. On one hand, with acid wash, AAEM species, which are some of the catalytic components in coal, CLR, and CLR ash, are mostly removed. On the other hand, the 1% highly dispersed iron-based catalyst was added in the coal direct liquefaction process. Meanwhile, the co-gasification can be strengthened due to the existence of such a catalyst. In this study, the concentrations of AAEM and iron-based catalyst are higher in the CLR ash.

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Therefore, the catalysis of CLR ash is better than that of other samples in Figure 6. 3.5. Effect of the Co-gasification Temperature. Figure 7 shows the catalytic activity of CLR on co-gasification of petroleum coke at different temperatures between 1173 and 1223 K. The loading of CLR is 20%. It can be seen that, during the control stage of the chemical reaction, the catalytic effect is better with increasing temperature. The catalytic gasification reactivity of the mixture is almost 3 times higher than that of pure petroleum coke when the temperature reaches 1323 K, while it is nearly 1.5 times higher when the temperature is 1173 K. Due to the high ratio of C to H, the petroleum coke is composed of polycyclic aromatic hydrocarbons and is rich in aromatics with lots of aromatic rings.23 The compounds in petroleum coke are various due to the different amounts of rings in the aromatics. The polycyclic aromatics of dull reactivity would not be gasified at low temperatures. Temperature is one of the most important factors that influence the gasification of carbonaceous matter with a catalyst and without a catalyst.24
26 Zou found that the specific surface area and pore volume of the coal increase with increasing temperature.5 The reason is that a blind pore is easier to break open at higher temperature in the condition in which the gasifying agent is sufficient. The increasing BET surface area is favorable to the contact of petroleum coke with the mineral composition in CLR and the increase of the pore volume. Therefore, they lead to a further increase in the effective contact area of the gasification reaction and finally to an increase in the gasification reaction rate.

Figure 5. Carbon conversion (x) vs t for co-gasification of petcoke with CLR and CLR char.

4. CONCLUSIONS AAEM species and iron
sulfur species in CLR are effective catalysts for co-gasification of petroleum coke and CLR. The temperature and CLR loading are both great factors that influence the catalytic capacity. Under the condition of reaction kinetic control, higher temperature has a higher catalytic effect on co-gasification of the mixture of CLR and petroleum coke. The catalytic effect of CLR on the gasification of petroleum coke increased with an increase of the CLR loading. The alkali metals, alkaline-earth metals, and transition metals in CLR promote the co-gasification reaction, while asphaltene and oils have no effect. ’ AUTHOR INFORMATION Corresponding Author Figure 6. Carbon conversion (x) vs t for different samples.

*Phone: 86-021-64250784. Fax: 86-021-64251312. E-mail: wfch@ ecust.edu.cn.

Figure 7. Carbon conversion (x) vs t at different temperatures: (a) petroleum coke; (b) petroleum coke + CLR. 3380

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’ ACKNOWLEDGMENT This research is supported by the National Key State Basic Research Development Program of China (973 Program, Grant 2010 CB 227000), Program for Changjiang Scholars and Innovative Research Team in University (Grant PCSIRT-IRT 0620), and Shanghai Outstanding Academic Leaders Subsidy Scheme (Grant 08 XD 1401300). ’ REFERENCES (1) Gary, J. H.; Handwerk, G. E. Petroleum Refining: Technology and Economics; CRC Press: Boca Raton, FL, 2001. (2) Wang, J.; Anthony, E. J.; Abanades, J. C. Clean and Efficient Use of Petroleum Coke for Combustion and Power Generation. Fuel 2004, 83 (10), 1341–1348. (3) Ginter, D.; Somorjai, G.; Heinemann, H. Factors Affecting the Reactivity of Chars and Cokes during Low-Temperature Steam Gasification. Energy Fuels 1993, 7 (3), 393–398. (4) Revankar, V.; Gokarn, A.; Doraiswamy, L. Studies in Catalytic Steam Gasification of Petroleum Coke with Special Reference to the Effect of Particle Size. Ind. Eng. Chem. Res. 1987, 26 (5), 1018–1025. (5) Zou, J. H.; Zhou, Z. J.; Wang, F. C.; Zhang, W.; Dai, Z. H.; Liu, H. F.; Yu, Z. H. Modeling Reaction Kinetics of Petroleum Coke Gasification with CO2. Chem. Eng. Process. 2007, 46 (7), 630–636. (6) Zou, J. H.; Yang, B. L.; Gong, K. F.; Wu, S. Y.; Zhou, Z. J.; Wang, F. C.; Yu, Z. H. Effect of Mechanochemical Treatment on Petroleum Coke-CO2 Gasification. Fuel 2008, 87 (6), 622–627. (7) Trommer, D.; Steinfeld, A. Kinetic Modeling for the Combined Pyrolysis and Steam Gasification of Petroleum Coke and Experimental Determination of the Rate Constants by Dynamic Thermogravimetry in the 500-1520 K Range. Energy Fuels 2006, 20 (3), 1250–1258. (8) Liu, Z.; Shi, S.; Li, Y. Coal Liquefaction Technologies—Development in China and Challenges in Chemical Reaction Engineering. Chem. Eng. Sci. 2010, 65 (1), 12–17. (9) Hengfu, S.; Zhenyi, C.; Chunbao, X. Recent Advances in Direct Coal Liquefaction. Energies 2010, 3, 155–170. (10) Cui, H.; Yang, J.; Liu, Z.; Bi, Z. Effects of Remained Catalysts and Enriched Coal Minerals on Devolatilization of Residual Chars from Coal Liquefaction. Fuel 2002, 81 (11
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