Experimental Investigation of Flow Coking and Coke Deposition of

Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX

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Experimental Investigation of Flow Coking and Coke Deposition of Supercritical Hydrocarbon Fuels in Porous Media Yinhai Zhu, Shuai Yan, Ran Zhao, and Peixue Jiang* Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Energy and Power Engineering, Tsinghua University, Beijing 100084, China ABSTRACT: Transpiration cooling technology is a feasible thermal protection method for hypersonic vehicles. To ensure the safety of transpiration cooling structures, the coke deposition rule of hydrocarbon fuels in porous media is a key problem that has to be considered. In this paper, experimental studies are reported on coke deposition of n-decane and HF-II fuel in sintered bronze porous media at different pressures and a temperature ranging from 350 to 638 °C. Results showed that the temperature was a dominant factor for coke generation from hydrocarbon fuels. Two types of coking behaviors, i.e., thermal-oxidative coking and pyrolysis coking, were observed, which resulted in the coke surface deposition rate exhibiting a double-peak curve. At a certain temperature, the mass flow rate determined the fluid residence time in porous channels. Thermal-oxidative coking was affected by the residence time and the dissolved oxygen concentration, so the coke surface deposition rate increased at first and then decreased with the mass flow rate. However, pyrolysis coking was only affected by the residence time, and in this case, the coke surface deposition rate decreased monotonously with the mass flow rate.

1. INTRODUCTION The hypersonic vehicle with a Mach number of 5 or more is an important development direction for the future aerospace industry. The scramjet engine providing the propulsion for this type of aircraft is also known as the third revolution in aviation history.1 An aircraft receives strong aerodynamic heating during flight at high Mach numbers, which requires high performance thermal protection techniques. The regenerative cooling technology using the hydrocarbon fuel carried by the aircraft as a coolant is the most preferred method in active thermal protection. Active regenerative cooling has been widely used in liquid rocket engines and gas turbines and has been proved to be very effective. As shown in Figure 1, in the active regenerative cooling for hypersonic vehicles, the hydrocarbon fuel first flows through the combustion chamber wall and is then injected into the combustion chamber.2 The hydrocarbon fuel, which has a high physical and chemical heat sink, can remove heat from hot walls of the aircraft combustor.3 However, the leading edge, fuel injection strut, and other parts of the aircraft are in a severely high temperature and high heat flux environment, and transpiration cooling technology with a higher cooling efficiency is a feasible option.4−6 Transpiration cooling was first put forward for the thermal protection of rocket engine throats in the 1940s. The principle of transpiration cooling is schematically shown in Figure 2.7 The cooling fluid flows through the porous wall into the main stream and forms a continuous thin film over the protected wall. At the same time, it weakens the heat transfer from the high temperature main stream to the wall. The transpiration cooling method has a high cooling efficiency. According to numerical simulation studies, the maximum cooling capacity of transpiration cooling can reach up to 6 × 107−1.4 × 109 W/m2, so it can effectively protect components in a working environment with high heat flux density.8 During transpiration cooling, coke is formed due to increasing temperature of hydrocarbon fuels, and it may © XXXX American Chemical Society

block the cooling channel and reduce the cooling efficiency, so that the cooling fails. In an even worse situation, the fuel oil system can be blocked, seriously affecting the safety of aircraft operations. Therefore, it is necessary to study the coking and coke deposition of hydrocarbon fuels in the porous structure. Existing studies on hydrocarbon fuel coking and coke deposition have mostly focused on the coking phenomenon in the pipe. Edwards9,10 and Spadaccini11 carried out in-depth studies on the thermal oxidative coking of various hydrocarbon fuels, revealing the formation process of coke products in hydrocarbon fuels. Moreover, they analyzed the patterns in the effects of temperature, dissolved oxygen, mass flow rate, and other factors on the thermal oxidative coking. Their studies have shown that traditional fuel can undergo a self-oxidation reaction with dissolved oxygen at temperatures above 163 °C, and the macromolecular colloidal particles that are produced become a coke body after deposition on the pipe wall. The temperature determines the extent of the thermal oxidative coking reaction and is the most important influencing factor. The dissolved oxygen concentration and the mass flow rate affect the material balance of the chemical reaction and the residence time, respectively, thus affecting the characteristics of thermal oxidative coking. Tao et al.12 and Zhu et al.13 also drew similar conclusions from their studies on the thermal oxidative coking of aviation kerosene RP-3 in the smooth pipe. Gül et al.14 studied the pyrolysis of hydrocarbon fuels and found that pyrolysis was accompanied by coking and suggested that this type of coking was formed when the free radicals that were generated from the pyrolysis of the fuel itself underwent dehydrogenative condensation under metal catalysis. Furthermore, they showed that the amount of coke was proportional to Received: November 7, 2017 Revised: February 5, 2018 Published: February 6, 2018 A

DOI: 10.1021/acs.energyfuels.7b03436 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 1. Active regenerative cooling and passages in a scramjet engine.2

In this paper, the coking characteristics of n-decane and HFII fuel23 in the porous media structure under supercritical pressure were studied experimentally. The patterns of coke deposition of fuels in oxidative coking and pyrolysis coking were analyzed, and the pattern for the effects of temperature, mass flow rate, pressure, and fuel type on the coking and flow resistance of hydrocarbon fuels were revealed.

2. EXPERIMENTAL SYSTEM AND DATA MEASUREMENT 2.1. Experimental System. The experimental system is shown in Figure 3a, which was developed by Zhu et al.24 The hydrocarbon fuel in the fuel tank was pressurized by the high pressure plunger pump, preheated through the preheating section, and then heated through the heating section to the set temperature. Aerogel and insulation wool were used to insulate the preheating section and heating section. After the hydrocarbon fuel flowed out of the heat section, the coke in the fluid was first filtered by a filter and then the fuel entered the coking section. The high-temperature fuel was cooled in the cooler before it entered the sampling system. Figure 3b shows the schematic of the coking section. A thin porous medium was mounted in the groove of the flange, and the outer edge of the porous medium was wrapped in a layer of mica paper to ensure close contact between the porous medium and the flange. The graphite gasket was installed between the flanges to achieve complete seal under high pressure. The porous medium had a diameter of 5 mm and a thickness of 3 mm and was sintered from the particles of tin bronze with a particle diameter of 95 μm (as shown in Figure 4). The porous structure of the porous medium was analyzed by a mercury porosimeter. It was found that the pore diameters were mainly between 25 and 50 μm, the average pore diameter was 42 μm, and the average porosity of the porous medium was 31.4%. During the experiment, the bypass was first opened, and the system was preheated until the inlet temperature of the coking section was about 300 °C, and then we opened the main path and closed the bypass. The pressure difference in the experimental section was monitored. When the pressure difference became nearly unchanged, the coke deposition in the porous medium was considered to be stable. The test time for each case was about 1 to 1.5 h. 2.2. Parameter Measurement. A variety of physical parameters were measured in this experiment, including temperature, pressure, pressure difference, mass flow rate, experimental time, and amount of coke. 2.2.1. Temperature. The K-type thermocouple was used to measure the fluid temperature at the inlet of the coking section and the wall temperature. The armored thermocouple is 2 mm in diameter and placed at the inlet of the coking section as shown in Figure 3b. The errors for the temperature ranges of 0−400 and 400−1000 °C were 1.6 °C and ±0.4%, respectively. 2.2.2. Pressure and Pressure Difference. The pressure transmitter EAJ430A was used to measure the pressure of the experimental system; it had a measurement range of 0−4 MPa and a measurement accuracy of ±0.075%. The differential pressure transmitter EAJ130A with a measurement range of 0−120 kPa and a measurement accuracy of ±0.075% was used to measure the pressure difference of the coking section.

Figure 2. Schematic of transpiration cooling.7

the degree of pyrolysis of the fuel. Liu et al.15 and Xu et al.16 performed numerical simulation studies on the pyrolysis of RP3 in the pipe and the effect of coking on the fluid flow resistance and heat transfer and concluded that the dynamic process of coke deposition and detachment could cause a fluctuation in flow pressure difference; furthermore, the coke product deposition could lead to a worse heat transfer between the wall and fluid, and local high temperature could occur on the wall. In addition, the duration of coking reaction in the pipe and the metal elements in the pipe material could also have a certain effect on the coking reaction. Clark et al.17 found that the rate of coking deposition in the smooth pipe was proportional to the experimental time to the power of 1.7. Different metal elements in the pipe material have different catalytic or suppressive effects on the coking reaction. The metal elements Ni, Cr, Fe, and Cu in the pipe can catalyze the production of coke, while the elements Al, Ti, Nb, and Ta can inhibit coking by forming a passive film on the wall.18,19 Gascoin et al.20 and Fau et al.21 conducted the pyrolysis of ndodecane in a smooth pipe and porous media and preliminarily studied the pyrolysis and coking phenomenon of hydrocarbon fuels in porous media. The results showed that the pyrolysis products, the amounts of coke, and the coke deposition rates of the fuels in the two channels were quite different. In the porous media, the amount of coke had a good linear relationship with the pyrolysis gas production. Tabach et al.22 further conducted a numerical simulation study to obtain the quasi-linear relationship between the amount of coke and the permeability on the logarithmic graph. The coking phenomenon of hydrocarbon fuels under a supercritical pressure has been studied for decades. Most studies have focused on the coke phenomenon of fuels in the smooth pipe. However, the study of fuel coking in the porous medium under supercritical pressure is lacking. Further investigations of the effects of temperatures, pressures, mass flow rates, and fuel type on the coke deposition in the porous medium are required. B


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Figure 3. Experimental system and test section. where m2 is the weight of the porous medium after the coking experiment and m1 is the weight of the porous medium before the experiment. The minimum amount of coke measured in all cases was 0.091 mg, so the maximum uncertainty for the measurement of the amount of coke was δmC = mC

δm2 2 + δm12 min{mC}

= 0.031

(2)

The equation for the specific surface area coefficient of a spherical packed bed was adopted. That is, the porosity of the porous medium ε was used to calculate the specific surface area coefficient of the porous medium α, which was multiplied by the volume V of the porous medium to give the inner surface area of the porous medium S:

Figure 4. Scanning electron micrographs of the porous medium. 2.2.3. Mass Flow Rate. The mass flow rate was measured using a Siemens MASS 2100 Coriolis mass flowmeter with a measurement error of ±0.1%. 2.2.4. Amount of Coke. After the experiment, the porous medium was placed in an oven and dried for 2 h until the mass did not change. The measurement error of the deposit weight was ±0.001 mg. 2.3. Data Processing and Error Analysis. The amount of coke deposit was calculated by mC = m2 − m1 (1)

S = αV =

6 πd 2L 3 (1 − ε) = (1 − ε)πd 2L dp 4 2d p

(3)

where the uncertainty of the porosity ε was about 3% and the uncertainty of the particle diameter dp was about 5%. Therefore, the uncertainty of the inner surface area measurement was C


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Figure 5. Test of experimental repeatability. δS = S

2 ⎛ δε ⎞2 ⎛ δd p ⎞ ⎛ δL ⎞2 ⎛ δd ⎞2 ⎟ ⎜ ⎟ + ⎜ + ⎜ ⎟ + ⎜2 ⎟ = 0.060 ⎜ ⎟ ⎝ε⎠ ⎝ L⎠ ⎝ d ⎠ ⎝ dp ⎠

(4)

Finally, the average coke surface deposition rate ṁ c of the hydrocarbon fuel in the porous medium was obtained by dividing the amount of coke mc by the experimental time t and the inner surface area S of the porous medium: m ṁ C = C (5) tS where the measurement error of experimental time was ±3.5 s and the shortest time of coking experiment was 1 h. Therefore, the maximum uncertainty of the average coke surface deposition rate was

δṁ C = ṁ C

⎛ δmC ⎞2 ⎛ δt ⎞2 ⎛ δS ⎞2 ⎜ ⎟ +⎜ ⎟ + ⎜ ⎟ = 0.068 ⎝S ⎠ ⎝ min{t } ⎠ ⎝ mC ⎠

Figure 6. Coke surface deposition rate of n-decane at different temperatures.

(6)

rate of n-decane to produce coke precursor material increased, so that the amount of coke showed an upward trend with increasing temperature. After the peak value and before the initiation of pyrolysis coking, the limited dissolved oxygen in the fuel was rapidly consumed, and the rate of formation of the coke precursors was saturated, resulting in a decreasing tendency of the coke deposition rate curve with increasing temperature. When the temperature was above 600 °C, the coking rate increased rapidly with increasing temperature. The coke products at this moment were mainly generated by the pyrolysis of the fuel itself to produce a variety of alkenes. The degree of pyrolysis of n-decane and the reaction rate were mainly determined by the temperature; as the temperature increased the rate of producing free radicals by pyrolysis increased significantly. The rate of formation of the unsaturated hydrocarbons, aromatic hydrocarbons, and other coke precursors increased, and the amount of coke and the coke deposition rate also increased rapidly with increasing temperature. According to the above results, coking was the result of a series of complex chemical reactions and physical deposition. The temperature is the macroscopic characterization of the energy level of the system, directly determining the type and the degree of chemical reactions that can occur in the system and also determining the rate at which coke products are produced. Therefore, the temperature is the most important factor affecting the coking characteristics of hydrocarbon fuels. Under the pyrolysis coking condition, the formation of methane, the formation of alkenes, the ring forming reaction of conjugated dienes, the generation of high-molecular-weight

3. EXPERIMENTAL RESULTS AND ANALYSIS 3.1. Experimental Facility Reliability Verification. The coking process is very complex, particularly the coking and coke deposition process in the porous media. In order to verify experimental reliability, three sets of repeatability experiments were performed on each of the two state points of n-decane. The two states both had an inlet pressure of 3 MPa and a mass flow rate of 3 kg h−1; however, one had a coking temperature of 400 °C, while the other had a coking temperature of 600 °C. Figure 5 shows the results of the measurement results of the amounts of coke in the three experiments under these two temperatures. From the results of repeatability experiments, the maximum error of the difference in the amounts of coke in the three experiments with respect to the average amount of coke was 4.7%. The results indicated that the experimental system had good reproducibility. 3.2. Temperature Effect. Figure 6 shows the experimental results of the change in the coke surface deposition rate with temperature of n-decane at a pressure of 3 MPa and a mass flow rate of 3 kg h−1. It can be seen that the temperature had a significant effect on the coking process. The coke surface deposition rate of the n-decane under the supercritical pressure in the porous medium showed a double-peak feature with temperature. At 400 °C, the first peak occurred. At this time, the fuel temperature was relatively low; thus, the main chemical reaction was carried out between the fuel and dissolved oxygen to produce coke products. This peak was called the oxidative coking peak.9,10 Before the peak appeared, as the temperature increased, the energy in the system increased, and the reaction D


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Figure 8 shows the macroscopic images of the porous media after the coking reaction. Results showed that a large amount of

aromatic hydrocarbons, and the dehydration and condensation reactions associated with these reactions are the keys to determining the degree of coking. We further collected the pyrolysis gas and pyrolysis liquid in the coking experiment and analyzed the components using gas chromatography mass spectrometry (GC−MS). Figure 7a,b shows the components of pyrolysis gas and pyrolysis liquid in the coking characteristic curve, respectively.

Figure 8. Images of porous media after coking test.

coke deposits resulted in an apparently darker color of the porous media when the coke surface deposition rate was relatively high in the coking characteristic curve (such as the oxidative coking peak point at 400 °C and the points at 600 and 636 °C for the occurrence of the pyrolysis reaction). In order to more clearly observe the morphology of the coke products and the distribution of the coke products in the porous medium, the microstructure of the porous medium after the coking experiment was examined by scanning electron microscopy as shown in Figure 9. Coking deposits were mostly distributed in small pores and few in large pores, which were typical filamentous carbons.

Figure 7. Component analysis of the pyrolysis gas and pyrolysis liquid products of n-decane

The mole fractions of methane and ethylene, propylene, and other alkenes in the gas-phase products increased with increasing temperature as shown in Figure 7a. Results showed that the small-molecular-weight alkenes increased when the ndecane was heated to a higher temperature. In Figure 7b, liquid products of alkenes with less than 10 carbon atoms at 636 °C were more than double that at 600 °C, especially the cyclohexene and decahydronaphthalene. According to the above results, thermal oxidative coking could be affected by many factors, such as chemical reaction rate, reactant concentration, and diffusion rate. The effects of various factors on the deposition rate of thermal oxidative coking at different temperatures were different, thus leading to the occurrence of peak points of thermal oxidative coking at 400 °C. The pyrolysis coking was mainly controlled by the coking chemical reaction rate, so the temperature was the most critical factor determining the deposition rate, and the amount of coke increased monotonically with increasing temperature.

Figure 9. Microscopic images of porous media after coking.

3.3. Effect of Mass Flow Rate. At a certain temperature, the mass flow rate determines the residence time of n-decane in the porous media structure, which affects the process of the coking reaction. Most studies have shown that, with increasing mass flow rate, the residence time of hydrocarbon fuels in the test section is reduced, which will inevitably lead to a decrease in the amount of coke. However, in this experimental study, it was found that the effect of an increase in the mass flow rate on E


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Energy & Fuels coke deposition rate under oxidative coking condition was not monotonic. Figure 10 shows the change in the coke surface

Figure 11. Effect of pressure on the coking process of n-decane.

of pyrolysis decreased. For example, at 3 MPa in the figure, there was no apparent pyrolysis phenomenon at 550 °C, and there was also a lack of occurrence of coke products. However, at 5 MPa, a significant coke product deposition phenomenon of pyrolysis coking occurred at this temperature. From the point of view of flow, the n-decane density under supercritical pressure increased with increasing pressure, and the flow rate of the fluid was slowed in the case of constant mass flow rate, resulting in an increase in the residence time of n-decane in the porous medium, which is favorable for the production of more coking products. However, the deposition rate at 5 MPa is lower than that of 3 MPa as shown in Figure 11. Note that the deposition rate is affected by both the production rate and the detachment rate of the coke in the porous medium. The mechanism of the fluid pressure on the production rate and the detachment rate need to be further studied. 3.5. Effect of Fuel Types. The hydrocarbon fuel for the actual engineering application is a mixture of various components. Figure 12 shows the coking characteristic curve

Figure 10. Coke surface deposition rate at different mass flow rates.

deposition rate of n-decane at 3 MPa with various mass flow rates at thermal oxidative coking temperature (400 °C) and pyrolysis coking temperature (600 °C). As shown from the figure, under the oxidative coking condition, the coke deposition rate decreased after the initial increase with increasing mass flow rate. According to the mechanism of oxidative coking formation, the oxidative coking at a certain temperature was constrained by the concentration of dissolved oxygen and the diffusion of coke products to the flow channel surface. The effects of these two factors on thermal oxidative coking at different flow rates were different: in the experiment, n-decane was heated to 400 °C in the heating section, so there was a consumption of dissolved oxygen in the pipe. For a small flow rate working condition with a mass flow rate of less than 3 kg h−1, the residence time of n-decane in the pipe was longer and more dissolved oxygen was consumed, so that the dissolved oxygen concentration in the coking section of the porous medium was reduced accordingly. Under this situation, an increase in the mass flow rate could increase the dissolved oxygen concentration in the coking section, thereby promoting the thermal oxidative coking reaction. In the working condition with a high flow rate of above 3 kg h−1, an increase in the mass flow rate made the flow rate faster, which is not conducive to the deposition of coke products in porous media. The pyrolysis coking of n-decane at a certain temperature was determined by its residence time in porous media, and an increase in the mass flow rate reduced the residence time of ndecane, so that the coke surface deposition rate was decreased monotonically. It is important to note that, since some thermal oxidative coking probably occurs in the heating section, the pipe length of the heating section will affect the coke deposition in the porous medium. However, the totally test time for each case is more than 1 h, which is much longer than the residence time of n-decane in the heating section. Therefore, the results of average coking rate are still reliable. 3.4. Effect of Pressure. Studies have shown that inlet pressure affects the coking characteristics of hydrocarbon fuels, but its mechanism is more complex. Figure 11 shows the coking characteristic curves of n-decane with a mass flow rate of 3 kg h−1 at 3 and 5 MPa, respectively. As the pressure increased, the coke deposition rate of n-decane in the porous media was reduced to some extent. For the pyrolysis coking condition, on the one hand, the increase in pressure was favorable for the pyrolysis of n-decane25 so that the temperature at the beginning

Figure 12. Coking characteristic curves of two hydrocarbon fuels.

of n-decane and engineering fuel HF-II23 at an inlet pressure of 3 MPa and a mass flow rate of 3 kg h−1. As can be seen from the curve, similar to n-decane, the HF-II fuel also showed a double-peak structure in the coking characteristic curve. Due to a large number of components in the HF-II fuel, the thermal oxidative coking peak temperatures of different components were different. The characteristic curve shown in the experiment was the result of the combined effect of many components, so the coking deposition rate curve near the peak point was relatively flat. In the experiment, a small amount of produced pyrolysis gas at 550 °C could be detected, indicating that the engineering fuel had started undergoing F


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lead to a rapid deterioration in the permeability of the porous medium, and therefore, the flow resistance increased rapidly. On the other hand, the large pores in the porous medium were less likely to be blocked and the coke deposits in the large pores of the porous medium were also more easily washed away by the fluid. After a certain amount of time, the deposition rate and the detachment rate of the coke layer reached a dynamic balance, so that the pressure difference of the coking section was generally unchanged. The Darcy permeability of the porous medium, KD, can reflect the flow characteristics of the fluid in the porous medium. The Darcy−Forchheimer equation was used to calculate the mean Darcy permeability in the porous medium, which was calculated as26

pyrolysis at this temperature and the initial temperature of pyrolysis coking was certainly lower than 550 °C. Overall, the coking capacity of the HF-II fuel was obviously stronger than that of n-decane. This is due to the fact that the HF-II fuel itself contains toluene and other aromatic hydrocarbon coke precursors, which are more likely to produce the initial coke products deposited in the fine pores of the porous medium. In addition, some of the alkane components with high carbon numbers in the HF-II fuel are more susceptible to pyrolysis reactions at high temperatures, increasing the rate of coke production from pyrolysis coking. Based on the above two reasons, the coke surface deposition rate of the HF-II fuel in the same working conditions was generally greater than that of ndecane. 3.6. Effect of Coke Deposition on Flow Pressure Drop. The effect of coke deposition on flow resistance can be reflected by changes in the pressure difference of the coking section. It is generally believed that as the coking time increases, the coke products will continue to deposit and gradually block the channels in the porous medium, thereby increasing the fluid flow resistance. In this study, we found that the increase in pressure drop of hydrocarbon occurred mainly in the initial stage of coking reaction; after a certain time, the increase in the pressure drop was slow, or did not even change. Figure 13 shows the change in the pressure drop of the coking

μ ΔP = u+ L KD

F ρu 2 KD

(7)

where ΔP is the pressure difference of the experimental section, L is the thickness of the porous medium, u is the Darcy speed of the hydrocarbon fuel, μ and ρ are the dynamic viscosity and density of the hydrocarbon fuel, respectively, and F is the inertia constant. Figure 14 shows the calculation result of the Darcy permeability with the experimental time. It can be seen that,

Figure 14. Change in the Darcy permeability with experimental time. Figure 13. Change in the pressure drop with experimental time.

for the temperature corresponding to the high coke surface deposition rate, the final permeability of the porous medium after the coking was remarkably deteriorated. Using this model, the deposition situation of coke products in porous medium could be approximately described by measuring the pressure difference and mass flow rate.

section at 3 MPa and 3 kg h−1. The initial pressure drop of the porous medium was 24.1 KPa, which was measured based on the unheated fuel flowing through the porous medium at 3 kg h−1. The coke deposition process is fast at the beginning. In addition, during the experiment, the fuel first flowed through the bypass line as shown in Figure 3b and then switched to the porous medium until it reached the set temperature. In summary, the pressure drop changes at the beginning are fast and may be affected by the switch process. Therefore, the pressure drop changes at the beginning were not presented in Figure 13. In Figure 13, under several working conditions of oxidative coking, the increase in the pressure difference of the coking section mainly occurred within the first 30 min after the experiment started, and the time for the pressure difference to stabilize was shortened with increasing fluid temperature. The experimental results showed that coke deposition at the initial stage had a more significant effect on the fluid flow, mainly because the coke products preferentially deposited in the region with the smallest pore size in the porous media structure. At this moment, a very small amount of product deposition might

4. CONCLUSIONS In this paper, the coking characteristics of n-decane and an engineering fuel under a supercritical pressure in a porous medium were studied. The effects of fluid temperature, mass flow rate, and inlet pressure on the coking performance of hydrocarbon fuels were revealed. The microscopic morphology of coke deposits in the porous media structure was obtained, the gas and liquid phases distribution of the pyrolysis products was measured, and the coking characteristics under the pyrolysis condition were analyzed. The main conclusions were as follows: (1) The fluid temperature was the dominant factor in the formation of coke products for hydrocarbon fuels in the porous media. G


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(11) Spadaccini, L. J.; Sobel, D. R.; Huang, H. Deposit Formation and Mitigation in Aircraft Fuels. J. Eng. Gas Turbines Power 2001, 123 (4), 741−746. (12) Tao, Z.; Xu, G. Q.; Fu, Y. C.; et al. Experimental Study on Influences of Physical Factors to Supercritical RP-3 Surface and Liquid-Space Thermal oxidative coking. Energy Fuels 2014, 28 (9), 6098−6106. (13) Zhu, K.; Xu, G. Q.; Tao, Z.; Jia, Z. Surface Deposition Characteristics of Supercritical Kerosene RP-3 Fuel within Treated and Untreated Stainless-Steel Tubes. Part 1: Short Thermal Duration. Energy Fuels 2016, 30 (4), 2687−2693. (14) Gű l, Ő .; Rudnick, L. R.; Schobert, H. H. Effect of the reaction temperature and fuel treatment on the deposit formation of jet fuels. Energy Fuels 2008, 22, 433−439. (15) Liu, Z.; Bi, Q. Dynamic behaviors of coke deposition during pyrolysis of China RP-3 aviation fuel. 20th AIAA International Space Planes and Hypersonic Systems and Technologies Conference, 2015, 10.2514/6.2015−3686. (16) Xu, K.; Meng, H. Numerical study of fluid flows and heat transfer of aviation kerosene with consideration of fuel pyrolysis and surface coking at supercritical pressures. Int. J. Heat Mass Transfer 2016, 95, 806−814. (17) Clark, R. H.; Thomas, L. An Investigation of the Physical and Chemical Factors Affecting the Performance of Fuels in the JFTOT. SAE Technical Paper 881533, 1988, 10.4271/881533. (18) Albright, L. F.; Marek, J. C. Mechanistic Model for Formation of Coke in Pyrolysis Units Producing Ethylene. Ind. Eng. Chem. Res. 1988, 27 (5), 755−759. (19) Wickham, D. T.; Alptedin, G. T.; Engel, J. R. Additives to reduce coking in endothermic heat exchangers. AIAA-99−2215. (20) Gascoin, N.; Gillard, P.; Bernard, S.; Bouchez, M. Characterization of coking activity during supercritical hydrocarbon pyrolysis. Fuel Process. Technol. 2008, 89 (12), 1416−1428. (21) Fau, G.; Gascoin, N.; Gillard, P.; et al. Fuel Pyrolysis through Porous Media: Coke Formation and Coupled effect on Permeability. J. Anal. Appl. Pyrolysis 2012, 95, 180−188. (22) Tabach, E.; Chetehouna, K.; Gascoin, N.; Gaschet, F. Modeling the spatio-temporal evolution of permeability during coking of porous material. 20th AIAA International Space Planes and Hypersonic Systems and Technologies Conference 2015−3663. (23) Jiang, R.; Liu, G.; Zhang, X. Thermal cracking of hydrocarbon aviation fuels in regenerative cooling microchannels. Energy Fuels 2013, 27 (5), 2563−2577. (24) Zhu, Y. H.; Liu, B.; Jiang, P. X. Experimental and Numerical Investigations on n-Decane Thermal Cracking at Supercritical Pressures in a Vertical Tube. Energy Fuels 2014, 28 (1), 466−474. (25) Jiang, P. X.; Yan, J. J.; Yan, S.; Lu, Z. L.; Zhu, Y. H. Thermal cracking and heat transfer of hydrocarbon fuels at supercritical pressures in vertical tubes. Heat Transfer Eng. 2019, 40 (5−6) 010.1080/01457632.2018.1432026. (26) Jiang, P. X.; Lu, X. C. Numerical Simulation of Fluid Flow and Convection Heat Transfer in Sintered Porous Plate Channels. Int. J. Heat Mass Transfer 2006, 49 (9−10), 1685−1695.

(2) There were two coking behaviors of thermal oxidative coking and pyrolysis coking in hydrocarbon fuels, leading to a double-peak structure of the coke surface deposition rate. (3) At a certain temperature, the mass flow rate determined the residence time of the fuel in the fluid channel. As the thermal oxidative coking was affected by the residence time and the dissolved oxygen concentration, the coke surface deposition rate increased first and then decreased with increasing mass flow rate. The pyrolysis coking was mainly affected by the residence time, and the coke surface deposition rate decreased monotonically with increasing mass flow rate. (4) The pressure drop in the porous media increased rapidly at the beginning of the coke deposition process, and then the change became less after the deposition rate and the detachment rate of the coke layer reached a dynamic balance.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yinhai Zhu: 0000-0002-7700-4689 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported by the National Natural Science Foundation of China (No. 51536004) and the Science Fund for Creative Research Groups of NSFC (No. 51621062).

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REFERENCES

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