The Relationship between Coking Behavior in Hydrocarbon Fuels

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The Relationship between Coking Behavior in Hydrocarbon Fuels Pyrolysis and Surface Roughness Shiyun Tang, Xiyue Luo, Cheng Cai, Jianli Wang, and Anjiang Tang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03140 • Publication Date (Web): 05 Jan 2018 Downloaded from http://pubs.acs.org on January 5, 2018

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The Relationship between Coking Behavior in Hydrocarbon Fuels Pyrolysis and Surface Roughness Shiyun Tang a, b, *, Xiyue Luo a, Cheng Cai a, Jianli Wang c, Anjiang Tang a

College of Chemical Engineering, Guizhou Institute of Technology, Guiyang, 550003, P. R.

China b

Guangdong Provincial Key Laboratory of New and Renewable Energy Research and

Development, Guangzhou, 510640, P. R. China c

Key Laboratory of Green Chemistry & Technology of the Ministry of Education, College of

Chemistry, Sichuan University, Chengdu, 610064, P. R. China

ABSTRACT: The pyrolysis of hydrocarbon fuels can give rise to the formation of coke on metal substrate surfaces. Until now, there are few research reports about how the nature of these surfaces affects the formation of coke, especially the effect of surface roughness. A series of samples of different surface roughness was obtained by mechanical polishing, and the coking property with changed surface roughness was evaluated by cyclohexane cracking under one atmosphere at 730-810 oC. The coke obtained was then analyzed by SEM, TEM, Raman spectrum and TPO. The results showed that reducing the surface roughness can effectively decrease the amount of coke in different ranges of cracking temperatures and cracking times, especially in high temperature ranges (above 770 oC). The polishing process reduced the metal

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catalytic activity of coking to inhibit coke formation without changing the mechanism of cyclohexane cracking. Surface roughness could significantly affect the morphology of coke. The filamentous coke exhibited a cylindrical cross section with small amounts of amorphous carbon on its outer surface. The coke on the surface of unpolished substrate had a higher degree of graphitization than that on polished substrate, and the degree of graphitization gradually reduced as the surface roughness decreased. Generally, the decreased surface roughness was not only unfavorable to coke adhesion but also changed the properties of coke in the coking process.

KEYWORDS: Coke; Surface roughness; Hydrocarbon fuels; Pyrolysis.

1. INTRODUCTION The pyrolysis of hydrocarbon fuels can give rise to the formation of carbonaceous solid deposits (also known as coke) on the surface of metal substrates. Both the process and the structure of the coke are very complicated. Studies have shown that the coke is mainly composed of polycyclic aromatic hydrocarbons (PAHs). Wornat et al.1, 2 studied the solid products of catechol pyrolysis at a temperature of 1000 oC in a laminar flow reactor and found that the main structure of the coke is PAHs. They also studied the solid products of JP-7 cracking under high pressure and temperature, and obtained similar results. To understand the behavior of coke growth during the pyrolysis of organic compounds, much effort has been devoted to the mechanism of coke formation in the thermal cracking process. The mechanisms of coke formation proposed by Albright and Marek in 1988 are generally accepted—that the formation of carbon deposition mainly assigns to three ways: thermal oxidation, condensation of aromatic compounds, and metal


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3, 4

catalytic coking.

Generally, metal catalytic coking is the main factor for the large amount of

coke and the PAH structure of the solid products among them.

5, 6

Many conditions can influence the carbon deposition process in the pyrolysis of the organic compounds, and these can be summed up in five aspects: the raw materials, the operating conditions, the extent of cracking, the flow area, and the surface effect. For different raw materials (i.e., organic compounds with different chemical structures), the amount and properties of the carbon deposition are very different. Depending on the operating conditions (including the reaction temperature, pressure, reaction time and fuel flow rate), temperature can affect the rate of coking, especially at high temperatures (often above 400 oC); the coking rate will increase dramatically as the temperature rises. The extent of cracking mainly affects the coking rate by changing the species and concentration of the coking precursor. The flow area is significant because as the organic compounds flow and pyrolysis occurs in the reactor, conversion will change at different locations and the flow state will change and affect the coking process. The surface effect, both the surface roughness and elementary composition, has a strong effect on the coking process, thus it can influence the physicochemical properties and amount of coke. There have been many studies of the first four factors, but little research has addressed the 7

surface effect, especially the effect of surface roughness. Crynes et al. studied if polishing Incoloy 800 metal coupons significantly reduced coke formation during the pyrolysis of light hydrocarbon feedstocks. They found that after the polishing process, the surface roughness of these coupons was reduced from the original 71 rms to 6 rms. They then compared the coking effect of polished and unpolished Incoloy 800 metal coupons using methane, ethane, ethylene,


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propane, propene, and isobutane at 700 oC, at one atmosphere for one hour. Their results showed that the coking rate was sharply reduced after polishing treatment. In particular, the ratio of carbon formed from the unpolished coupons to that from polished coupons varied from 5.6 for isobutane up to 28.1 for ethylene, and the polishing effect had little influence on the extent of gas conversion and major product yields. Crynes et al. believed that polishing leads to lower surface temperatures (radiation effects), fewer mechanical surface defects, and possible changes in surface chemistry. Similarly, Marek and Albright3, 4, Gregg and Leach8 also found less coke on polished metal surfaces after the pyrolysis of hydrocarbons. However, when Durbin and Castle9 polished a variety of materials to which they applied the acetone pyrolysis. They found that the amount of coke on polished specimens was not less than the coke on unpolished specimens. They explained that this may be due to the increasing temperature of the metal surface in the process of heat treatment. Thus, the existing research results are contradictory. There is no doubt that the relationship between coking behavior in hydrocarbon fuel pyrolysis and surface roughness needs further study. The quantitative assessment of the relationship between coke properties and surface roughness is critical to understanding the reactivity and coking behavior that occurs in the pyrolysis of organic compounds. This could be used to determine the final utilization of coke, such as serving as a type of solid fuel in the pulverized furnace or as a filler material of electrodes for aluminum manufacture. Following from our previous work on coking behavior on the surface of TiN, TiO2 and TiC coatings,10-13 in this paper we try to show the effect of surface roughness on coking behavior in the pyrolysis of hydrocarbon fuels.


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2. EXPERIMENTAL SECTION 2.1. Materials and Pretreatment. This experiment used 310S stainless steel (SS310) foils (10 mm × 10 mm × 0.9 mm) as the metal substrate, with the chemical composition as shown in Table 1. Mechanical polishing was used to obtain a series of samples of different surface roughness. We chose 400 to 2000 mesh SiC emery paper to polish the 310S foils inch by inch. Before the polishing process, the 310S foils were washed in a soap solution, rinsed with acetone and ethanol, and then dried in a vacuum drying oven at 80 °C for one hour. Table 1. The Chemical Composition of SS310 Foils Contents wt./% Name 310S

Ni

Cr

Si

Mn

C

S

P

Fe

19.00-22.00

24.00-26.00

≤1.50

≤2.00

≤0.08

≤0.03

≤0.045

rest

2.2. Coking Tests of the 310S Foils. In order to evaluate the coking property of the 310S foils, cyclohexane with a boiling point of 80.7 °C and density of 0.78 g/cm3 at 20 °C was cracked in a tubular reactor made of quartz (16-mm internal diameter and 350 mm in length) which was placed horizontally inside an electrical furnace. Before the tests, the polished and unpolished SS310 specimens were ultrasonically cleaned in acetone and dried in high purity nitrogen (HP-N2) for 30 min. A specimen was then placed in the center of the tube reactor and HP-N2 was passed through the reaction system for 30 min to purge the air inside. Subsequently, the reactor was heated to fixed temperatures (730-810 oC) at a rate of 15 oC/min under the HP-N2 atmosphere. After reaching the designated temperature, a


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cyclohexane steam oil bath with a mass flow rate of 0.15 g/min at 50 °C was carried by purified HP-N2. The preheating temperature of the furnace was fixed at 400 oC to eliminate the possible existence of trace oxygen in the hydrocarbon fuel.14 The flow rates of the carrier and dilution gas were 75 ml/min and 25 ml/min, respectively. The inlet and outlet pressures of the reactor tube were both at atmospheric pressure during anti-coking tests because of the low gas flow rate. Our previous tests found that 0.5-2.5 h is the desired time for a suitable amount of coke formation. The reactor was turned off after the test, and cooled to room temperature in an HP-N2 atmosphere. The amount of coke on the 310S foils was calculated by weighing the specimens before and after coking with a microbalance as follows: Amount of coke =

M −m *100% S

In this calculation, the M denotes the total mass of the coke and substrate, m denotes the mass of the substrate, and S denotes the area of the substrate. The total amount of coke on all specimens was reproducible to within 3% by weight of the deposit’s mass.

2.3. Analysis Method. The morphology of the coke on the 310S surface was characterized by scanning electron microscopy (SEM, Hitachi-S-4800, Japan). Transmission electron microscopy (TEM) was recorded on a Tecnai G2 F20 S-TWIN (FEI, USA) electron microscope at an accelerating voltage of 200 kV. Atomic force microscope (AFM) was performed using a MFP-3D-BIO AFM operated in


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tapping mode and with an ultra-sharp silicon tip. Measurements with 512 × 512 data points were taken at three positions on each sample with scan areas of 20 µm × 20 µm. Image Pro Plus Software was used to analyze the images. Average surface roughness was measured on five line scans per sample across the whole diameter. The Raman spectra were measured using LabRAM HR (HORIBA Co., Ltd., France) with around 0.65 cm–1 resolution and 532 nm radiation from an Nd: YAG laser as the excitation source. The system was operated with an output power of 5 mW and a focal spot in the order of a few micrometers (1–2 µm2). The back-scattered Raman signals were collected and recorded from 100 to 2000 cm–1. In order to reduce experimental error in the material balance and study the influence of surface roughness on the product distribution in the pyrolysis process, the gaseous products were collected and measured at an interval of 5 min using gas chromatography (GC-2000 III, Shanghai Institute of Computing Technology, China). The test conditions were a flame ionization detector (FID) and Al2O3/S capillary column (0.53 mm×50 m, 5 µm). All gas components were identified and quantified by comparing with standard gas samples. The coke properties of the carbon deposits were measured by temperature programmed oxidation (TPO) in a CO2 infrared analyzer (GXH-1050, Beijing Junfang Research Institute). In the TPO process, carbon on the 310S surface was oxidized to carbon dioxide by high pure O2 (600 ml/min) in a furnace and over a Pt/Al2O3 oxidation catalyst bed at 300 °C (CO was completely converted CO2).15, 16


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3. Results and Discussion 3.1. The Influence of Surface Roughness on the Amount of Coke. The surface roughness of 310S coupons under different polishing conditions was calculated by AFM. The amount of coke was used to evaluate the efficacy of the surface roughness under 10 min of cyclohexane pyrolysis at 800 oC and one atmosphere. Table 2 shows the results of surface roughness and the carbon amount for polished and unpolished coupons. It can be seen that the surface roughness reduced rapidly from the original 238 nm to 6.14 nm on coupons polished with 2000 mesh, and the amount of coke also decreased sharply from the 3.383 µg/cm2 on unpolished coupons to 0.689 µg/cm2 on these coupons, simultaneously. The polishing process reduced the surface roughness by 97.4%, and the amount of coke decreased by 79.6%. Reducing the surface roughness effectively decreased the amount of coke, but this effect was limited. The result was in accordance with our prediction because, as was found in our previous work, there were more significant effects of anti-coking coating, such as TiN and TiC coating. It is worth pointing out that polishing treatment was a kind of anti-coking pretreatment in most cases because of the limited ability in reducing coke. Table 2. The Surface Roughness and Amount of Carbon for Polished and Unpolished Coupons Value Number Mesh

Ra (nm)

Amount of coke (µg/cm2)

1#

unpolished

238

3.383

2#

400

123

1.767

3#

600

37.4

1.226

4#

800

28.9

1.002

5#

1000

20.8

0.970


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6#

1500

14.3

0.909

7#

2000

6.14

0.689

Figure 1(a) and 1(b) show that the amount of coke of 1# (unpolished) and 3# (polished with 600 mesh) varied as a function of the cracking temperature and cracking time. Figure 1(a) shows that the amount of coke on unpolished coupons at different cracking temperatures were all higher than the amount on polished coupons. In particular, when the cracking temperature was below 770 oC, the amount of coke increased slowly on unpolished coupons but increased sharply when the cracking temperature was above 770 oC. The carbon deposition rate on polished coupons remained at a low level for the entire temperature range. Figure 1(b) shows that with the increase of cracking time, the amount of coke increased rapidly in the first 60 min for unpolished coupons, then the rate of growth decelerated. In comparison, the amount of coke showed little change when the cracking time was increasing for polished coupons. These results suggest that reducing surface roughness effectively decreases the amount of coke at different cracking temperatures and times, especially in high temperature ranges (above 770 oC). 4

(a)

(b)

1# 3#

6

1# 3#

3

4 2

M( mg/cm )

M( mg/cm2)

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

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2

2

1

0 720

740

760

780

800

820

0 40

o

Test temperature / C

80

120

160

Reaction time /min

Figure 1. Amount of coke with the cracking temperature (a) and cracking time (b): 1# is the unpolished coupon; 3# is the 600-mesh polished coupon.


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3.2. The Influence of Surface Roughness on the Distribution of Gaseous Products. The distribution of cracking gaseous products is a key parameter in characterizing the cracking severity of hydrocarbons. Figure 2 shows the distribution of gaseous products of unpolished coupons and coupons polished with different meshes at a cracking temperature of 770 o

C. As can be seen from Figure 2(a), the components of gas products for both polished and

unpolished coupons after cyclohexane pyrolysis were the same, and thus were unrelated to the polishing process. Comparison of the content of each component for the unpolished and different mesh polished coupons showed all deviations to be within 3%, which is within the range of experimental uncertainty for the off-line gas chromatography (GC) method used. This indicates that surface roughness had no effect on the components of gaseous products during the cyclohexane pyrolysis. The result is in agreement with the findings of Crynes et al.7 The most abundant components in the gas products were ethylene (C2H4) and 1,3-butadiene (C4H6) for both unpolished and coupons polished with different meshes in more than 62%, as shown in Figure 2(b). The small molecule level was a key parameter representing the extent of hydrocarbon fuel pyrolysis. Here, the total methane, ethane and ethylene content were about 60%, which indicated severe cyclohexane cracking at high temperature. In conclusion, the polishing process did not change the mechanism of cyclohexane cracking.


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(a)

(b)

o

Figure 2. The distribution of cracking gaseous products of cyclohexane at 770 C (a); total of

abundant components and small molecules of methane, ethane and ethylene (b). 3.3. The Influence of Surface Roughness on the Morphology of Coke. Figure 3(a)–(f) shows SEM images of the polished and unpolished 310S coupons before and after coking. As shown in Figure 3(a), the surface of 310S substrate was rough with many cracks and gaps. A large number of studies have indicated that catalytic coking caused by nickel (Ni) and iron (Fe) particles on stainless steel surfaces is one of the main coking reasons in hydrocarbon cracking. 17-19 These cracks and gaps may be the best active sites for catalytic coking. Figure 3(b) and 3(c) demonstrate that polishing significantly changed the surface morphologies of 310S substrate and a new spindly polishing line was formed. The surface finish of polished coupon was generally much better than that of unpolished coupons. The morphological evolution of carbon deposits formed under severe catalytic coking from unpolished 1000-mesh polished and 2000-mesh polished coupons can be clearly seen in Figure 3(d)–(f): First, extensive interwoven filamentous coke covered the cracks and gaps of unpolished 310S substrate completely with the bright points being found at the filament tips or in the


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filaments. According to Kulnitskiy et al.20 and Xu et al.21, these bright points on the filamentous tips were most likely metallic atoms extracted from the tube that catalyzed the filamentous growth. The filamentous coke had a diameter of about 70 nm and a coarse skin. This might be due to the coke precursors in the gas phase reacting with the coke surface through radical reactions and initiating the lateral growth of the filaments. Second, the 1000-mesh polished coupons showed much filamentous coke intermixed with irregular granular coke. Note that both the diameter and length of the filamentous coke on the 1000-mesh polished coupons were clearly smaller than those of the unpolished coupons. The irregular granular coke may be attributed to carbon precipitation from metal carbides occurring in the process of forming filamentous coke. Third, only irregular granular coke appeared on the 2000-mesh polished coupons, and this was more abundant than that found on 1000-mesh polished coupons. As a conclusion, by comparing the SEM images of unpolished and polished coupons after coking, we can see that surface roughness could significantly affect the morphology of coke. This is because filamentous coke often corresponded to severe coking, and the extent of coking was gradually reduced with a decrease in surface roughness.


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Figure 3. SEM images of the 310S coupons before and after coking: (a) unpolished, (b) 1000-mesh polished, (c) 2000-mesh polished, (d) coke for unpolished, (e) coke for 1000-mesh polished, (f) coke for 2000-mesh polished.

3.4. The Influence of Surface Roughness on the Structure of Coke. Generally, the structure of coke contained the following three types: the solid carbon nanofibers (CNF), the hollow carbon nanotubes (CNT), and the amorphous carbon. Figures 4(a)–4(c) show TEM images of coke on the 310S surface of unpolished, 1000-mesh polished and 2000-mesh polished coupons. It is clear that the coke on all the different 310S surfaces comprised filamentous carbon deposits with branches. The average diameters of filamentous coke were about 62 nm (the branch diameter was about 45 nm) for unpolished coupons, 62 nm for 1000-mesh polished coupons and 55 nm for 2000-mesh polished coupons, respectively. These filamentous cokes showed a cylindrical cross section with small amounts of amorphous carbon on their outer surface. The multi-walled carbon nanotube with a hollow structure can be clearly seen in Figure 4c. Some metal particles covered with carbon at the tips or in the CNT


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could be found. This result is consistent with the previous SEM result. According to Snoeck et al., the appearance of the hollow structure is related to the relative rates of nucleation of carbon filaments, diffusion of carbon through the catalytic metal particle, and to differences in the diffusion path lengths on the metal/carbon interface. When the rate of nucleation was lower compared to that of diffusion, filamentous coke without a solid structure was formed; the opposite caused the CNT growth.22 The amorphous carbon on the outer surface of the filamentous coke could be attributed to coke precursors in the gas phase reacting with the coke surface through radical reactions and lateral growth of the filaments.23, 24 Therefore, the main feature of catalytic coking was the formation of filamentous coke with some metal particles inside the filaments or at their tips. These filaments were excellent collection sites for the precursors of pyrolytic coke and to promote its formation.

Amorphous carbon

Metal Metal

Figure 4. TEM images of coke on the 310S surface of unpolished (a), 1000-mesh polished (b) and 2000-mesh polished (c) coupons.

3.5. The Influence of Surface Roughness on the Other Properties of Coke. To further illustrate the influence of surface roughness on the other properties of coke, the coke on the surface of unpolished, 600-mesh polished, 1000-mesh polished and 2000-mesh polished 310S substrate were selected for Raman spectra and TPO analysis. Figure 5 shows a


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typical Raman spectrum of coke on unpolished, 600-mesh polished, 1000-mesh polished and 2000-mesh polished coupons. All the typically Raman spectra of coke showed several features in common, the so-called D and G peaks, which lay at around 1340 and 1580 cm–1 for visible excitation. According to Reshetenko et al.25, the relative intensity ratio of D-band to G-band (ID/IG) can be used as a function of the degree of graphitization of coke. Table 3 summarizes the Lorentz fitting of coke on the 310S surface with different polishing treatments. On one hand, neither the Raman shift (ω) nor the width (Г) of the D-band and G-band showed any obvious law. On the other hand, the value of ID/IG was 1.48 for unpolished substrate was significantly lower than that of polished substrate. It is clear that the value of ID/IG increased gradually as the surface roughness of the metal substrate decreased after polishing. This implies that the coke on the surface of unpolished substrate had a higher degree of graphitization than that on the polished substrate, and the degree of graphitization gradually reduced as the surface roughness decreased. 600 400

1344

(a)

D

1338

(b)

1583

1577

D

G

Intensity (a.u.)

Intensity (a.u.)

G 300

200 800

1200

1600

2000

ID/IG=1.48

400

800

1200

1600

2000

ID/IG=1.77

200

100

500

1000

1500

2000

500

1000

Raman shift/cm-1 1347

(c)

1500

1594

G

D

1343

(d)

600

Intensity (a.u.)

400

800

1200

1600

2000

Raman shift/cm-1

D

1592

G

600

Intensity (a.u.)

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

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2000

ID/IG=2.47

400

800

1200

1600

2000

ID/IG=2.77

200

200

0

0

500

1000

1500

2000

500

-1

Raman shift/cm


1000

Raman shift/cm-1

1500

2000

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Figure 5. Typical Raman spectrum of: (a) unpolished coupon (1#); (b) 600-mesh polished coupon (3#), (c) 1000-mesh polished coupon (5#) and (d) 2000-mesh polished coupon (7#).

Table 3. Lorentz Fitting of Coke on the 310S Surface under Different Polishing Treatment Value Couple

ωD(cm-1)

ωG(cm-1)

ГD(cm-1)

ГG(cm-1)

ID/IG

1#

1344

1583

79

59

1.48

3#

1338

1577

148

71

1.77

5#

1347

1594

209

82

2.47

7#

1343

1592

190

72

2.77

Note: ωD-Raman shift of D-band; ωG-Raman shift of G-band; ГD-width of D-band; ГG-width of G-band.

TPO is a widely used technique to study the properties of solid carbons, such as nanotubes and nanofibers, anthracites, and other carbon materials.26, 27 This technique is based on the fact that the reaction of solid carbons with molecular oxygen takes place at specific active sites, such as at structural defects or carbon atoms on the edges of graphene layers, thus we can relate the oxygen reactivity of solid carbons to their properties. The TPO profiles of coke on unpolished and polished coupons obtained by CO2 infrared analyzer are compiled in Figure 6. All TPO curves of samples presented a single and asymmetric peak corresponding to the maximum oxidation temperature (Tmax) of 660 oC for unpolished, 625 oC for 600-mesh polished, 555 oC for 1000-mesh polished, and 530 oC for 2000-mesh polished coupons. It is well known that the main structure of the coke is PAHs. Alonso-Morales et al.28, Mohan et al.29 and Altin et al.30 found that hydrogen rich, structurally disordered deposits were oxidized at lower temperatures, while hydrogen lean structurally ordered deposits were oxidized at higher temperatures. At the same time, a higher degree of graphitization corresponded to a higher oxidation temperature. Therefore,


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the hydrogen content of coke gradually increased as the surface roughness decreased, but the degree of graphitization of coke showed the opposite trend. This result was consistent with the previous Raman result.

Carbon deposition/ppm

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

Energy & Fuels

400 300 200 100 0 400 300 200 100 0 450

o

660 C

#

1

o

625 C

#

3

300 o

#

150

555 C

5

0 450 300

#

7

150

o

530 C

0 300

450

600

750

900

o

Temperature/ C

Figure 6. TPO profiles of coke on the 310S surface of 1#, 3#, 5# and 7# coupons.

In summary, surface roughness was reduced by polishing, and this process could reduce the sizes of the metal particles on the surface of the metal substrate. This reduced the metal catalytic activity of coking to inhibit coke formation without changing the mechanism of cyclohexane cracking. Hence, the decrease in surface roughness was not only unfavorable to coke adhesion but also changed a series of properties of coke in the coking process.

4. CONCLUSIONS The quantitative assessment of the relationship between the properties of coke and surface roughness is both necessary and important for understanding the reactive coking behavior in the pyrolysis of organic compounds. A series of samples of different surface roughness were obtained by a mechanical polishing process, and the coking property with the changed surface


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roughness was evaluated by cyclohexane cracking under one atmosphere and temperatures of 730-810 oC. The results suggest that surface roughness can effectively reduce the amount of coke at different cracking temperatures and cracking times, especially in the high temperature range (above 770 oC). The polishing process only reduced the metal catalytic activity of coking to inhibit coke formation without changing the mechanism of cyclohexane cracking. Surface roughness could significantly affect the morphology of coke, which presented a filamentous morphology intermixed with irregular granular coke. These filamentous cokes exhibited a cylindrical cross section with small amounts of amorphous carbon on their outer surface. Moreover, the coke on the surface of unpolished substrate had a higher degree of graphitization than that on polished substrate, and the degree of graphitization gradually reduced as the surface roughness decreased. As a result, the decrease in surface roughness was not only unfavorable to coke adhesion, but also changed a series of properties of coke in the coking process.

AUTHOR INFORMATION *Corresponding author. Phone/Fax: +86-0851-88210651. E-mail addresses: [email protected]

ACKNOWLEDGMENTS The research was supported by S&T Plan Project Approving in Guizhou (No. Guizhou branch in LH word [2016] 7104 and [2015] 7095) and Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development (No. Y607sb1001).


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The Relationship between Coking Behavior in Hydrocarbon Fuels

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