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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2023 , Vol. 35 >Issue 12: 1895 - 1910

DOI: https://doi.org/10.7536/PC230417

Review

Endothermic Reaction of High Heat Sink Hydrocarbon Jet Fuel

  • Zhenquan Fang 1 ,
  • Shugen Jiang 2 ,
  • Xinghua Zhang , 1, * ,
  • Qi Zhang 1 ,
  • Lungang Chen 1 ,
  • Jianguo Liu 1 ,
  • Longlong Ma 1
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  • 1 Southeast University, School of Energy and Environment, Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education (Southeast University),Nanjing 210009, China
  • 2 Nanjing Wuzhou Refrigeration Group Co., Ltd,Nanjing 211100, China
*Corresponding author e-mail:

Received date: 2023-04-14

  Revised date: 2023-08-30

  Online published: 2023-11-30

Supported by

National Key R&D Program of China(2022YFB4201803)

Key Program of the National Natural Science Foundation of China(52236010)

Key Program of the National Natural Science Foundation of China(52376173)

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Abstract

Hypersonic vehicle is not only the significant development direction in the field of air and space, but also the important symbol of the overall scientific and technological strength of a country. In order to combine cooling and propulsion functions, endothermic hydrocarbon fuel need to possess the basic characteristics of high heat sink, high density, high calorific value, high thermal stability, low freezing point, low coking and low cost. In this paper, the research progress of endothermic reactions of endothermic hydrocarbon fuels was summarized. This paper focusing on the effects of thermal cracking, catalytic cracking and catalytic steam reforming on heat sink. Firstly, the effects of pyrolysis conditions such as temperature, pressure and residence time on heat sink were analyzed. And then, the correlation between fuel composition, molecular structure and thermal cracking, and the effects of molecular sieves, nanoparticles and initiators on the catalytic cracking behavior and heat sink of endothermic hydrocarbon fuels were summarized. Furthermore, the influence of molecular sieve, nanoparticles and initiator on the catalytic cracking behavior and heat sink of endothermic hydrocarbon fuels, and the coking and inhibition technology in the process of endothermic reaction is summarized. Finally, the future research directions of endothermic hydrocarbon fuels are proposed in the light of the current development.

Contents

1 Introduction

2 Effect of thermal cracking on heat sink

2.1 Pyrolysis conditions

2.2 Fuel composition

3 Effect of catalytic cracking on heat sink

3.1 Molecular sieve

3.2 Nanoparticles

3.3 Initiator

4 Effect of catalytic steam reforming on heat sink

5 Comprehensive comparison of heat absorption technology

6 Coking and inhibition technology

7 Conclusion and outlook

Key words： endothermic hydrocarbon fuel; thermal cracking; catalytic cracking; catalytic steam reforming; heat sink

Cite this article

Zhenquan Fang , Shugen Jiang , Xinghua Zhang , Qi Zhang , Lungang Chen , Jianguo Liu , Longlong Ma . Endothermic Reaction of High Heat Sink Hydrocarbon Jet Fuel[J]. Progress in Chemistry, 2023 , 35(12) : 1895 -1910 . DOI: 10.7536/PC230417

1 Introduction

The research and development of hypersonic vehicles (> Mach 5) is a hot topic and focus in the field of aerospace. However, when the flight speed reaches Mach 7, the temperature of the combustor will exceed 2000 K. When the flight speed reaches Mach 12, the temperature will reach an astonishing 4950 K[1]. The traditional method is to carry coolant to cool down, but it will increase the weight of the aircraft and shorten the range. In this context, researchers proposed a new strategy to use the fuel itself as a coolant, and first proposed the concept of "Endothermic hydrocarbon fuel" (EHF) in 1971[2,3][4].
EHF is a class of hydrocarbons with high endothermic capacity under limited coking. As shown in Fig. 1, before entering the combustor, EHF first undergoes temperature rise, phase change (physical heat sink), and then pyrolysis at high temperature (chemical heat sink), becoming the cooling source for hypersonic vehicles[6]. Methylcyclohexane (MCH) is a first-generation endothermic fuel, which undergoes selective dehydrogenation under the catalysis of Pt/Al2O3 to produce toluene (C7H8) and H2. This process can provide a total heat sink of nearly 4549 kJ/kg (chemical heat sink 2186 kJ/kg, physical heat sink 2363 kJ/kg), which can absorb the heat released by aircraft with Mach numbers of 4 – 6[7,8]. However, the platinum catalyst and MCH for this process are expensive. Catalytic cracking and catalytic steam reforming of hydrocarbon fuels have been proposed and used for hypersonic vehicle cooling to achieve engine cooling objectives while improving cost status. Sobel and Spadaccini first found that Norpar 12 cracked into low molecular weight olefins, alkanes, and H2 at 1000 K under operating conditions simulating high-speed flight, with a total heat sink of 3950 kJ/kg[7].
图1 高超声速飞行器及其发动机冷却的总体示意图[5]

Fig. 1 Overall schematic representation of the hypersonic vehicle with its cooled engine[5]. Copyright 2021, American Chemical Society

Generally, EHF with a density around 0.78~0.80 g/cm3 (e.g.: JP-7, RP-1, RP-2, etc.) can provide a heat sink of 3500 kJ/kg[9]. However, due to the limitation of kinetics and the formation of a large number of alkane products, the reaction enthalpy of actual fuel cracking is much lower than the theoretical value. In addition, the formation of polycyclic aromatic hydrocarbons in the cracking process will stimulate the formation of coke, reduce the selectivity of cracking and reduce the heat sink. At present, there are two ways to improve the heat sink of fuels: (1) to develop fuels with high thermal stability and increase the use temperature of endothermic fuels, thereby increasing the physical heat sink of fuels; (2) Strengthen the endothermic reaction process and improve the chemical heat sink of the fuel.
The physical heat sink is mainly related to the composition of the fuel itself, and the physical heat sink at a given temperature depends on the enthalpy change of EHF. Most EHFs can produce a physical heat sink of 1600 ∼ 1800 kJ/kg at 810 K, which simply cannot meet the requirements at high Mach numbers (≥ 5)[10,11]. When the Mach number reaches 8, the required heat sink is as high as 4600 kJ/kg (Fig. 2). In this case, it is imperative to improve the chemical heat sink. Chemical heat sink is usually closely related to endothermic reactions such as cracking and catalytic steam reforming[12]. Cracking includes thermal cracking and catalytic cracking, in which thermal cracking is a free radical mechanism, which generally occurs at high temperatures and is usually accompanied by coking, and the reaction orientation is poor. Catalytic cracking follows the carbocation mechanism and can be achieved at relatively low temperatures. The desired endothermic reaction can be selectively achieved by regulating the reaction path through the appropriate use of catalysts[13]. Catalytic steam reforming is another strong endothermic reaction besides thermal cracking and catalytic cracking. EHF molecules contact and react with water molecules on the surface of the catalyst to produce relatively small molecular weight molecules such as H2, CH4 and CO, while absorbing a large amount of heat.
图2 所需热沉与马赫数的关系[4]

Fig. 2 Heat sink required as a function of Mach number[4]. Copyright 2019, Elsevier.

In this paper, the effects of thermal cracking, catalytic cracking and catalytic steam reforming on the endothermic reaction of EHF were mainly discussed, and the advantages of catalytic cracking in the endothermic reaction of EHF were emphatically analyzed, the coking and inhibition technologies in the endothermicreaction process were summarized, and the future development of endothermic reactions of EHF was prospected.

2 Effect of pyrolysis on heat sink

Pyrolysis is to adjust the pyrolysis degree, reaction path and product distribution of EHF by regulating the pyrolysis conditions, so as to enhance the endothermic capacity of EHF and improve the heat sink. The cracking of EHF can be measured by the conversion rate and gas production rate. Increasing the conversion rate and gas production rate of EHF cracking can strengthen its endothermic reaction and improve the heat sink. Generally, the process of EHF cracking to produce saturated hydrocarbons such as CH4, C2H6, and C3H8 is an exothermic reaction, while the process to produce unsaturated hydrocarbons such as C2H4 and C3H6 is an endothermic reaction. In order to effectively improve the heat sink of EHF, it is necessary to increase the olefin/paraffin ratio while ensuring the overall conversion. At present, the main factors affecting the heat sink are temperature, residence time and pressure.

2.1 Pyrolysis condition

2.1.1 Temperature and residence time

The effect of temperature on the heat sink is mainly realized by changing the cracking degree of the fuel and the component distribution of the pyrolysis products. First of all, the increase of temperature will further deepen the cracking degree of fuel. Secondly, the increase of temperature will increase the content of unsaturated hydrocarbons, thus increasing the heat sink of the fuel[14]. Generally, in a certain temperature range, the higher the temperature, the deeper the cracking degree and the higher the heat sink. Hou et al. Studied the heat transfer and pyrolysis behavior of kerosene. When the temperature is > 500 ℃, pyrolysis plays an important role in the endothermic process, and the total heat sink increases with the increase of temperature[15]. In contrast, Zhong et al. Studied the cracking behavior and heat sink change of aviation kerosene in a heating tube under the conditions of temperature of 780 ~ 1050 K, pressure of 3 ~ 4.5 MPa, and residence time of 0.6 ~ 3 s[16]. It is found that the chemical heat sink does not always increase with the increase of pyrolysis temperature, and the maximum endotherm occurs at 900 ~ 960 K. When the fuel conversion rate is 45%, the chemical heat sink is the highest, about 500 kJ/kg. Therefore, the influence of temperature on the heat sink is not uniform. In addition to temperature, residence time is another important factor. Edwards demonstrated that residence time is critical to the cracking of hydrocarbons and the improvement of heat sink[17]. Other studies have pointed out that, within a certain range, longer residence time will increase the conversion rate of fuel and make the reaction closer to chemical equilibrium, when the overall reaction is exothermic[18]. Shorter residence time increases the olefin/paraffin ratio in the product, which leads to more endothermic cracking process. Therefore, within a certain range, the heat sink is inversely proportional to the residence time.

2.1.2 Pressure

EHF needs to be used in supercritical conditions due to the operational requirements of hypersonic vehicles or spacecraft. Therefore, it is of great significance to study the endothermic reaction of EHF under supercritical conditions. The change of pressure will affect the physical properties of EHF (density, viscosity, specific heat, etc.), and the change of these basic properties will make EHF have different heat transfer characteristics, pyrolysis mechanism and coking behavior, thus affecting the heat sink[19][20]. Compared with atmospheric pressure, the increase of pressure will increase the residence time at a fixed mass flow rate, thus promoting fuel conversion and increasing the chemical heat sink[21]. For example, the increase of pressure (3.45 – 11.38 MPa) can improve the conversion of n-decane, enhance the conversion process of n-alkane (C5~C9) products, and improve the gas yield[22]. The conversion of heptane under supercritical conditions was three times higher than that under subcritical conditions[23]. The conversion of kerosene based hydrocarbon fuel increases linearly with pressure, and the EHF conversion at 6. 0 MPa is 3. 3-5. 7 times higher than that at 0. 7 MPa in the temperature range of 650 ~ 720 ℃, and the heat sink is increased by about 50%[20].
The change of pressure also changes the olefin/paraffin ratio in the pyrolysis product, and the pyrolysis gas with a higher olefin/paraffin ratio has a better heat absorption capacity, that is, a higher heat sink[24,25]. The main cracking products of n-decane at supercritical pressure (3,4,5,6 MPa) are CH4 and C2~C9 alkanes and alkenes. As shown in Fig. 3, with the increase of pressure, the selectivities of H2, CH4 and C2H6 increase with the increase of conversion, and the selectivities of ethylene and propylene decrease with pressure, which makes the olefin/alkane decrease gradually, and the heat sink shows a certain degree of decline[26,27]. However, the effect of pressure on ZH-100 conversion and gas yield is not purely promotion or inhibition. When the pressure is 0.1 MPa, the conversion and gas yield of ZH-100 are the lowest. When the pressure is 1. 5 MPa, the conversion and gas yield of ZH-100 are the highest. When the pressure is 2. 5 and 3 MPa, the conversion and gas yield of ZH-100 are intermediate. The main reason for the different variation law from the previous one is that the pressure range at this time crosses the critical pressure, and the ZH-100 undergoes a severe phase transformation at the critical pressure, which affects the heat and mass transfer in the reaction process, thus showing a different variation law from that at the critical pressure[24]. It can be seen that the influence of pressure on EHF heat sink is not uniform, which needs further exploration and analysis.
图3 热解产物中烯烃/烷烃的浓度比[26]

Fig. 3 Concentration ratio of alkenes/alkanes in pyrolysis products[26]. Copyright 2014, American Chemical Society.

2.2 Fuel composition

Chemical structure essentially determines behavior[2]. Billingsley et al. Pointed out that the variability of fuel composition in space propulsion system not only affects physical properties such as density, viscosity and calorific value, but also affects chemical properties such as thermal decomposition performance[28]. Lovestead et al. Provided a basis for comparing the basic thermophysical properties of JP-7, RP-1, and RP-2 fuels, and reached similar conclusions[29]. DeBlase et al. Used quadrupole mass spectrometry to characterize four hydrocarbons (n-hexane, cyclohexane, n-dodecane and decalin) in situ, and found that these hydrocarbons had unique pyrolysis products[30]. For example, the main products of n-hexane pyrolysis are C3 and C4 species, while the main products of cyclohexane pyrolysis are C4 and C5 intermediates. McDonald et al. Compared the pyrolysis of n-dodecane, MCH, and 2,2,4,4,6,8,8-heptamethylnonane (isohexadecane) in a shock tube from 990 to 1520 K[31]. While the yield of C2H4 was in the following order: n-dodecane > MCH > iso-hexadecane.
According to the functional group structure, high-performance EHF is mainly composed of n-alkane, iso-alkane and cycloalkane mixed in appropriate proportion[32⇓⇓~35]. The pyrolysis behavior of EHF is quite different with different mixing ratios. This is because the cracking process of hydrocarbons is complex, and the components not only undergo chain scission, isomerization, dehydrocyclization, dealkylation, homolysis, dehydrocrosslinking, macromolecular free radical β bond cleavage and coking reactions, but also undergo corresponding reactions with each other[36,37]. In addition, the bond energies of C — C and C — H bonds of iso-alkanes are smaller than those of the corresponding C — C and C — H bonds of n-alkanes, and the bond energy of C — C bond is smaller than that of the C — H bond. Therefore, according to the bond energy, the order of cracking or dehydrogenation is: iso-alkane C — C bond > n-alkane C — C bond > naphthenic C — H bond, that is, the stability of iso-alkane is lower than that of n-alkane with the same number of carbon atoms, while that of n-alkane is lower than that of naphthenic[38,39]. In addition to bond energy, the reactivity and the amount of free radicals generated are also important factors affecting the pyrolysis behavior. Isoparaffins are more reactive because their molecular structure is more prone to the formation of reaction intermediates and thus to cracking. The amount and type of free radicals produced in the pyrolysis of iso-alkanes and n-alkanes are different, which affects the rate and selectivity of the reaction. Jiang et al. Explored the pyrolysis behavior of mixtures with different n-dodecane: isododecane ratios under supercritical conditions (550 ~ 680 ℃, 4.0 MPa)[40]. It is found that isododecane has a promoting effect on the pyrolysis of n-dodecane, and the promoting effect is related to the ratio of n-dodecane to isododecane. When the ratio is 3 ∶ 1, the promoting factor is close to 4. The reasons for the above promotion may be: (1) Activation effect: Due to the different molecular structures, isoalkanes have unstable bonds or steric hindrance effect inside their molecules, which makes these bonds easier to break at high temperatures, thus triggering the cracking reaction. (2) Number of free radicals produced: Isoparaffins tend to produce more free radicals during pyrolysis, which can participate in the transfer process of the reaction chain, thus accelerating the reaction. (3) Carbon-carbon bond cleavage: Carbon-carbon bond cleavage in isoparaffins requires less energy and is thus more likely to occur. Sun et al. Found that for n-alkanes, the gas yield and heat sink increase with the increase of carbon chain length, and the anti-coking performance of alkanes with even carbon numbers is better[41]. For the same carbon number of alkanes, the gas yield of iso-alkanes is higher. However, for iso-alkanes containing more than two methyl groups, the heat sink value, the yield of H2 and the yield of gas with carbon number ≤ 4 are much lower. For example, as the number of methyl groups increases, the heat sink values of octane isomers gradually decrease. From n-octane to n-decane, the gas molar yield increases from 9. 11 mmol/G to 16. 55 mmol/G, which also shows that the length of the carbon chain has a greater impact on the heat sink of n-alkanes[42].
In addition to the carbon chain length, the hydrogen/carbon ratio (H/C) of EHF is also an important factor. Relevant studies have found that the larger the H/C, the faster the pyrolysis rate and the stronger the endothermic capacity[44]. It should be noted that the relationship between the cracking performance of single alkane and H/C is not consistent with the actual fuel used. However, the fuel in practical application is composed of a variety of alkanes, and the cracking performance of the fuel can still be analyzed and predicted from a macro perspective by H/C. Yue et al. Explored the effect of H/C on the pyrolysis behavior of five kerosene-based EHF and seven model fuels (C9H20~C13H28, ethylcyclohexane and decalin) under supercritical conditions (600 – 750 ° C, 3.5 MPa)[44]. The results show that for supercritical cracking, the gas production and heat sink of hydrocarbon fuel with higher H/C are greater than those of fuel with lower H/C, so the order of total heat sink is C9>C10>C11>C12>C13. Therefore, the cracking performance and heat sink value of hydrocarbon fuel can be reasonably predicted to a certain extent according to the H/C value. The total heat sink of C13 pyrolysis at 750 ° C is 3760 kJ/kg, and that of decalin is 3140 kJ/kg. However, many hydrocarbons with specific structures, especially isoparaffins with high carbon numbers, have been commercialized in small quantities, which limits the further understanding of the pyrolysis behavior of isoparaffins[20]. In order to solve this problem, Liu et al. Studied the pyrolysis of C8~C16 alkanes (9 n-alkanes, 14 iso-alkanes and 8 cycloalkanes) in a high-pressure micro-pyrolysis reactor equipped with GC-MS/FID at 600 ~ 700 ℃ and 3.0 MPa, and analyzed their heat sink.The heat dissipation index HSI of EHF (HSI (X) = (HS (X)/HS (nC10), HS = HSp + HSc, X is the hydrocarbon to be tested, HS is the total heat sink, HSp is the physical heat sink, HSc is the chemical heat sink, and the total heat sink of nC10 at 700 ℃ is selected as the reference value) is defined to characterize its heat absorption capacity (Fig. 4)[43]. For alkanes with the same carbon number, the HSI of isoparaffin with a single substituent on the central carbon chain is the highest, followed by n-alkane and dimethylalkane, indicating that the position and number of substituents have a significant effect. While for cycloalkanes, n-alkylcyclohexane has a relatively high HSI. For the same hydrocarbon group (n-alkane, 3-methylalkane, n-alkylcycloalkane), the HSI of EHF increases significantly with the number of carbon atoms. In addition, the heat sink of alkane is related to the position, number and type of substituents, and the HSI increases gradually when the monomethyl substituent moves to the center of the main chain. Obviously, the HSI of cycloalkanes is generally lower than that of normal or isoalkanes with the same carbon number.
图4 烃类在700℃、3.0 MPa下热解的HSI-分子结构关系[43]

Fig. 4 HSI-molecular structure relationship of hydrocarbons pyrolysis at 700℃ and 3.0 MPa[43]. Copyright 2020, Elsevier.

The addition of another fuel can also significantly affect the chemical heat sink. Li et al. Found that due to the instability of JP-10 structure, the conversion of JP-10 and the yield of olefins increased after the addition of isooctane, while the cracking of isooctane was inhibited[45]. Other researchers compared the pyrolysis behavior of several common hydrocarbons under the same conditions, and explored the effects of fuel mixing on pyrolysis behavior (conversion, product distribution, heat dissipation, etc.). For example, Sun et al. Found that there were mutual inhibition and competition effects in the co-pyrolysis of n-dodecane (C12) and decalin (DHN)[46]. The increase in the proportion of C12/DHN has a positive effect on the gas production rate of EHF, but has little effect on the selectivity (≤C4) of H2 and olefins. The heat sink of EHF depends strongly on the gas yield. The C12/DHN ratio has a great influence on the amount and morphology of EHFs coking. Zhang et al. Carried out a study on the controllable decomposition of kerosene fuel and methanol co-feeding[47]. The results show that the addition of methanol helps to improve the overall heat sink, while increasing the ethylene yield and significantly reducing the carbon deposit by producing H2 as a diluent. The gas yield and heat sink increase with increasing carbon chain length of n-alkanes. Isoparaffins can produce higher gas yields than n-alkanes. However, for isoparaffins containing more than two methyl groups, the H2 and olefin yield are low, and the resistance to carbon deposition is poor[41]. Secondly, the even-carbon n-alkanes (NPEC) exhibit better anti-coking properties than the odd-carbon n-alkanes (NPOC). When designing high performance EHF, there is an optimum value of NPEC/NPOC ratio to balance heat sink and anti-coking performance. Compared with naphthenes, alkanes are easier to crack and have higher olefin selectivity and heat sink[48].
At present, most of the research focuses on the pyrolysis of model compounds, and there are few studies on the pyrolysis of real fuels. Therefore, there is still much room for the study of the mechanism, the construction of detailed reaction kinetic models, the product distribution, and the reaction pathway of the pyrolysis of real fuels with complex composition. The pyrolysis process conditions and real working conditions were simulated, and the effects of reaction conditions (temperature, reaction time, pressure, fuel composition, etc.) On the heat sink of EHF were systematically investigated, especially the pyrolysis behavior of EHF under real supercritical/subcritical fluid working conditions.

3 Effect of catalytic cracking on heat sink

Catalytic pyrolysis refers to the use of catalysts to promote EHF to produce carbocations and free radicals to drive pyrolysis, regulate pyrolysis pathways and control product distribution, thereby improving EHF conversion and heat sink. During the use of EHF, the catalyst must have high catalytic activity, high selectivity and certain stability, and the superposition of these properties can maximize the heat sink. At present, the main catalysts used are molecular sieves, nanoparticles and initiators.

3.1 Molecular sieve

Molecular sieves are the most widely used catalysts because of their good acidity, shape selectivity, hydrothermal stability, high activity, unsaturated hydrocarbon selectivity, coking resistance and economy[12,49,50]. Sobel et al. Investigated the cracking performance of Norpar12 jet fuel over SAPO-34 and SAPO-5 molecular sieves, which produced more unsaturated hydrocarbons and effectively improved the heat sink[7]. Among the molecular sieves used, H-ZSM-5 has better product selectivity, less isomerization and more endothermic products, and is widely used in the catalytic cracking of EHF[51][49,51,52]. In the catalytic cracking reaction of n-dodecane, the conversion of n-dodecane reached 20% at 550 ℃ after contacting with H-ZSM-5 for 30 min, which was twice as much as that of thermal cracking, and the catalytic activity of H-ZSM-5 zeolite increased with the increase of Si/Al ratio[53][54]. Compared with the unmodified H-ZSM-5 zeolite, the Ag-modified H-ZSM-5 zeolite prepared by impregnation method showed higher n-dodecane cracking conversion efficiency[55]. However, the application of H-ZSM-5 is limited due to its single microporous structure (about 0. 5 nm in diameter), long diffusion path, many strong acid sites, and easy carbon deposition leading to catalyst deactivation[56]. Simple H-ZSM-5 is difficult to meet the needs of practical applications. In order to overcome the above limitations, researchers have introduced many new ideas to improve the structural and electronic properties of H-ZSM-5, such as the modification of different elements, the preparation of hierarchical pore zeolites and the preparation of ultra-thin nano-heulandite[57][58⇓⇓⇓~62][63⇓~65][66,67].
Designing molecular sieves with good morphology to make them have excellent diffusivity and acidity is an important research direction of molecular sieve modification. Transition metal ions have unique redox States and can absorb or release electrons from carbocations or hydrocarbons during the reaction, thereby promoting the formation of carbocations and radicals. Using appropriate transition metals to modify molecular sieves can effectively improve the distribution of acid sites of molecular sieves, so that molecular sieves have higher coking resistance. In addition, the introduction of transition metal ions can also adjust the overall acidity of H-ZSM-5, thus achieving effective catalytic cracking of hydrocarbon fuels under supercritical conditions. At the same time, the carbon deposition on the outer surface of H-ZSM-5 can also be effectively inhibited, which is conducive to maintaining its catalytic performance at high temperature, thus maximizing the gas production and heat sink. In addition to transition metals, F, P and other non-metallic modification can also effectively improve the porosity, acid strength and acid site distribution of molecular sieves, thereby modulating the catalytic performance of molecular sieves. For example, phosphorus modification can stabilize the framework aluminum structure of H-ZSM-5 to resist severe hydrothermal environment, which is an effective method to improve the catalytic activity and lifetime of H-ZSM-5[68]. In addition, the strong acid sites of H-ZSM-5 decreased and the propylene selectivity increased after phosphorus modification[69]. The catalytic activity of H-ZSM-5 in n-dodecane cracking was greatly improved by phosphorus modification. The gas production rate is 64. 2%, and the heat sink is 3 094 kJ/kg, which is 20. 2% higher than that of the unmodified catalyst (2 576 kJ/kg). In addition, H-ZSM-5-0.5P and H-ZSM-5-1P could still maintain stable (gas production rate > 50%) activity and high heat sink (3000 kJ/kg) after 60 min of reaction[70]. In addition to phosphorus modification, fluorine modification is also very effective in improving the catalytic activity of H-ZSM-5 in various reactions[71,72]. However, the mechanism of fluoride modification of H-ZSM-5 is not clear, and its application in EHF cleavage has rarely been reported.
Hierarchical pore molecular sieves contain both micropores and mesopores, and have the advantages of both, so they have excellent catalytic performance in the endothermic reaction of EHF. For example, a series of micro-mesoporous H-ZSM-5 synthesized by hydrothermal method have obtained better selectivity of light olefins and longer service life in the catalytic cracking of n-decane under supercritical conditions[73]. However, the activity of the hierarchically porous H-ZSM-5 synthesized by alkali treatment decreased rapidly after 30 min in the supercritical catalytic cracking of n-dodecane[74]. This is attributed to the carbon deposition inside the microreactor covering the acid sites, which leads to the rapid deactivation of the catalyst. In this case, the amount of carbon deposition will increase sharply, blocking the reactor, and the pressure will rise rapidly at high temperature, which will lead to the risk of explosion in practical application. For this reason, ordered mesoporous H-ZSM-5 with Al-MCM-41 shell came into being.
The molecular sieve with "core-shell" structure is a new type of molecular sieve with compound structure, which is formed by using the pretreated mesoporous molecular sieve as the core and the external molecular sieve as the shell. In practical applications, EHF molecules first enter the outermost shell and undergo a pre-cleavage reaction to produce molecules with relatively small molecular weight. Then, due to diffusion, these molecules with relatively small molecular weight will further enter the nucleus and react. Because the core is a mesoporous molecular sieve with large pore size, the reaction product is easy to diffuse, and the generated product is easy to separate in time. Therefore, the "core-shell" molecular sieve has more exposed acid sites and better carbon deposition resistance. Compared with conventional H-ZSM-5, the ordered mesoporous H-ZSM-5 with Al-MCM-41 shell (4.5 – 10 nm) exhibited 28% higher catalytic activity and 25% lower deactivation rate. With the increase of pore size in the shell of Al-MCM-41, the accessibility of acid sites gradually increases, which leads to the increase of catalytic activity and the decrease of secondary reaction ability, thus contributing to the improvement of heat sink[75].
In addition to the above methods, the preparation of ultrathin nanoflake zeolites can also effectively improve the catalytic activity of H-ZSM-5. Due to the short diffusion length of its micropores, the diffusion rate of reactants in the intracrystalline mesopores is higher, which makes the catalytic cracking activity of H-ZSM-5 nanosheets for n-dodecane higher than that of conventional H-ZSM-5[76]. When the temperature is 600 ℃, the isomorphous MFI (Mobil-type Five) nanoflake zeolites (MFI-Al, MFI-Ga and MFI-Fe) synthesized by hydrothermal method perform well in the catalytic cracking of n-dodecane, and the heat sink reaches 3120, 3380 and 3220 kJ/kg, respectively[59]. Due to its synergistic catalytic effect, the heat sink of n-decane can be as high as 4640 kJ/kg with the addition of 0.1 wt% Co3O4 nanosheet-encapsulated H-ZSM-5 composite at 758 ° C, which is much higher than the thermal cracking of pure conventional H-ZSM-5 (2990 kJ/kg at 687 ° C) and n-decane (3770 kJ/kg at 728 ° C)[77].

3.2 Nanoparticles

After the 1990s, the improvement of EHF heat sink by designing nanocatalysts with different acidities has been extensively studied. Generally speaking, Lewis (L) acid on the catalyst can initiate the carbocation of alkane, and Brnsted (B) acid can initiate the carbocation of alkene[78]. The carbocation is the reactive intermediate and is readily cleaved at the β site with subsequent generation of a new carbocation. Related studies have shown that nanocatalysts play a key role in the formation of carbocations[79]. Therefore, the design and synthesis of new nanocatalysts with controlled acidity has become a research hotspot in the past decades. At present, the applied nanocatalysts can be divided into zeolite nanoparticles, metal nanoparticles and composite oxides. In addition, precious metal catalysts (Pd), non-precious metal catalysts (Ni) and transition metals are also widely used in the catalytic cracking of EHF.

3.2.1 Zeolite nanoparticles

As a new type of catalytic material, zeolite nanoparticles have a larger specific surface area, which can expose more active sites, effectively eliminate the diffusion effect, and give full play to the efficiency of the catalyst. In addition, it has more external orifices, which are not easily blocked by deposits, and is conducive to prolonging the reaction cycle. Therefore, it shows superior performance in improving catalyst utilization, increasing macromolecular conversion capacity, reducing deep reaction, improving selectivity and reducing coking deactivation. Catalytic cracking of nano-HZSM-5 under supercritical conditions is an effective strategy to improve the heat sink of EHF[80,81]. For example, the highly dispersed nano-HZSM-5 obtained by modifying HZSM-5 with trimethylchlorosilane can realize low homogeneous catalytic cracking of EHF and improve the total heat sink[82]. The HZSM-5 catalyst coated with NiO nanoparticles/nanosheets contributes to the cracking of n-decane under supercritical conditions, which can increase the total heat sink from 3770 kJ/kg (728 ° C) to 4590 kJ/kg (780 ° C)[83]. The synergistic effect of NiO and HZSM-5 can change the cracking path of n-decane. Through monomolecular beta cracking, the fuel produces more unsaturated hydrocarbons such as ethylene and propylene, thereby increasing the endothermic capacity and effectively inhibiting the formation of carbon deposits. Other studies have shown that nano-HZSM-5 modified by fluoride has better catalytic activity and inhibits the formation of coke to a certain extent[84]. With the increase of fluoride concentration, the conversion of n-decane increased gradually at first, and then decreased slightly[85]. In addition to HZSM-5, the conversion of JP-10 can reach 63. 3% at 700 ℃ and the heat sink can reach 2800 kJ/kg by hydrothermal synthesis of β-zeolite with high hydrocarbon dispersion[86]. Although molecular sieves can improve the total heat sink of EHF, the structure is single, the selectivity of cracking products is difficult to adjust, and the coking is serious.

3.2.2 Metal nanoparticle

In order to further improve the heat sink of EHF and overcome the above problems, researchers have further designed metal nanoparticles[87]. It was found that the conversion of JP-10 catalyzed by Ni nanoparticles was significantly enhanced compared with thermal decomposition[88]. Ni and Ni-B nanoparticles prepared by Li et al. And Au nanoparticles protected by Gemini surfactant showed good stability and cracking catalytic activity[88,89][90]. In addition to Ni and Au, Pt-based materials are also a class of classical and efficient catalysts, which are widely used in many catalytic reactions[91,92]. They show superior performance in catalytic cracking and dehydrogenation of hydrocarbons, which can effectively improve the heat sink. Pt nanoparticles (Pt NPs) have excellent catalytic dehydrogenation/cracking performance, and Kim et al. Found that the loading of Pt further promoted the dehydrogenation of MCH and JP-10, the generation of olefins and the improvement of heat sink[93]. In addition, loading Pt NPs onto Al2O3 enables MCH to provide a heat sink of about 4652 kJ/kg, which can theoretically meet the cooling demand below Mach 10. Compared with the addition of Al2O3,CaO, it is beneficial to the formation of oxygen vacancy defects, thus increasing the electron density of Pt NPs (electronic effect), promoting the encapsulation of Pt NPs (geometric effect), and further improving its catalytic performance[94].
In general, the key factors controlling the performance of Pt-based catalysts are the dispersion and sintering resistance of Pt species on the support[95,96]. However, under harsh supercritical conditions, the catalyst is prone to deactivation due to carbon deposition and sintering at the Pt metal sites upon EHF cracking. In order to solve this problem, hyperbranched polymer-coated metal nanoparticles have emerged. Because the catalyst has good hydrocarbon solubility, it can be used to form nanofluid with EHF. Yue et al. Prepared a nanofluid composed of decalin and Pd nanoparticles modified by octadecanethiol and octadecylamine[87]. The conversion rate of the nanofluid is 80. 5% at 750 ℃, and the heat sink is 3500 kJ/kg. Wu et al. Prepared Pt NPs coated with hydrophobic hyperbranched polyglycerol and formed a nanofluid with MCH, and the pyrolysis performance and endothermic capacity of the nanofluid were significantly improved[97]. At 650 ℃, the heat sink of the nanofluid is 2390 kJ/kg, which is increased by 20. 7% compared with the thermal cracking. At the same time, Wu et al. Synthesized modified hyperbranched polyethyleneimine-coated hydrocarbon dispersed Pt NPs with an average particle size of 2.4 nm by phase transfer method, and formed nanofluids with MCH[13]. The results show that the conversion of methylcyclohexane increases from 12. 4 wt% to 33. 0 wt% at 650 ℃, and the heat sink value increases from 1 990 kJ/kg to 2 240 kJ/kg. Similarly, Ye et al. Synthesized Pt NPs with NaBH4 as a reducing agent, and then encapsulated with hyperbranched polyamidoamine as a quasi-homogeneous catalyst to catalyze decalin cracking (Fig. 5)[98]. At 675 ℃, the conversion of decalin increased from 22. 3% to 50. 7% and the heat sink increased from 2180 kJ/kg to 2620 kJ/kg with the addition of 50 ppm.
图5 超支化聚酰胺-胺包裹Pt NPs对十氢萘裂解的影响[98]

Fig. 5 Effect of Pt NPs encapsulated by hyperbranched polyamide-amine on the cracking of decalin[98]. Copyright 2019, American Chemical Society

It can be seen that due to its special physical and chemical properties such as thermal conductivity and stability, the conversion rate, gas production rate and heat sink of EHF-based nanofluids are better than those of EHF under the same conditions. Therefore, nanofluids have great potential in the endothermic reaction of EHF. However, the relationship between the size and concentration of nanoparticles and the stability, viscosity and fluidity of nanofluids should be fully considered in the application process to prevent the aggregation and deposition of nanoparticles. In addition, the preparation of EHF-based nanofluids should be reasonably regulated according to their specific application objectives, so as to meet the requirements of use.

3.2.3 Complex oxide

Composite oxides, that is, oxides with multiple components, at least one of which is a transition metal oxide. Components may interact with each other, and the situation of interaction varies according to conditions. As far as catalysis and function are concerned, some components are main catalysts, while others are cocatalysts or carriers. Complex oxides, which can be acidic or basic, undertake different tasks in the endothermic reaction of EHF. Compared with zeolite nanoparticles, composite oxides have higher stability. For example, it has better catalytic cracking activity and high temperature stability than Cu/ZSM-5,Pt/CeO2-Al2O3 in the catalytic cracking reaction of domestic No.3 jet fuel (RP-3)[99]. Compared with metal nanoparticles, composite oxides have better resistance to carbon deposition. At present, complex oxides have been widely used in the cracking of EHF[100⇓~102]. Among them, amorphous SiO2-Al2O3, as a typical composite oxide, has been reported earlier in the field of hydrocarbon conversion because of its large specific surface area, weaker acidity than zeolite, and higher stability[103]. For example, the heat sink of n-decane for Pt NPs supported SiO2-Al2O3 catalyst was as high as 3900 kJ/kg at 750 ° C under supercritical conditions, and the activity did not decrease significantly after standing for 30 min[104]. Compared with pure ZrO2 or/and TiO2, the ZrO2-TiO2 composite oxide has larger specific surface area, better acid property and stronger mechanical strength[105]. However, there are some defects in the application of ZrO2-TiO2 composite oxides as supports in kerosene cracking, and the stability at higher temperatures (> 750 ℃) needs to be improved. For this purpose, a series of Pt/ZrO2-TiO2-Al2O3 composite oxides with different Al2O3 mass ratios (ZrO2∶TiO2∶Al2O3=1∶1∶x,x=1,2,3,4,5) were synthesized[106]. Compared with thermal cracking, the gas yield and heat sink of RP-3 were significantly increased under the catalysis of the above composite oxides. In addition, the catalytic cracking activity and high temperature stability of the catalyst largely depend on the mass ratio of Al2O3. Subsequently, a series of ZrxT i 0.9 - x Al0.1O2 composite oxides with different ZrO2∶TiO2(ZrO2∶TiO2∶Al2O3=x∶(0.9-x)∶0.1,x=0,0.225,0.45,0.675,0.9) ratios were successfully prepared and applied to the catalytic cracking of RP-3 at high temperature and high pressure[107]. The gas yield and heat sink were significantly increased, and in addition, the catalytic cracking activity of RP-3 aviation kerosene was greatly affected by different ZrO2:TiO2 molar ratios. The catalyst at ZrO2∶TiO2=0.45∶0.45 has excellent cracking activity and high temperature stability. When the temperature is 700 ℃, the gas yield, the olefin content in the gas phase and the heat sink are 40. 7%, 51.5% and 3 410 kJ/kg, respectively.
Typically, metal doping creates new active metal sites and significantly changes the hydrocarbon cracking reaction path[108]. Metal sites can greatly affect the dissociation of C — H bonds, promote the generation of hydrogen and olefins, and enhance the heat sink of hydrocarbon fuels. It has been reported that Fe, Mn, Mo, W are effective acidic promoters to adjust the surface acidity of the catalyst[109,110]. In general, Fe and W modified catalysts are dominated by L acid sites, while Mn/Mo modified catalysts are dominated by B acid sites[111]. Compared with Mn, Fe and W, the Mo-modified ZrO2-TiO2-Al2O3(ZTA) composite oxide prepared by impregnation method has appropriate acidity and shows more ideal anti-coking ability than other catalysts. The heat sink of n-decane under Mo-ZTA catalysis is the highest, with an increase of 220 kJ/kg, 340 kJ/kg and 300 kJ/kg at temperatures of 650 ° C, 700 ° C and 750 ° C, respectively. When the temperature is 750 ℃, the conversion of n-decane can reach 88. 4% and the heat sink can reach 3790 kJ/kg by the Mo-promoted composite oxide. For all catalysts (except W-ZTA), there is a good positive correlation between the heat sink value and the corresponding conversion, following the order: Mo-ZTA > Fe-ZTA > Mn-ZTA > ZTA without catalyst. However, the heat sink under W-ZTA catalysis was close to that under Fe-ZTA catalysis despite the low conversion[112]. In addition to the introduction of Mo element, the addition of MoO3 can also significantly change the nature and concentration of acid sites. Both B and L acid strength were enhanced after modification with MoO3 species[110]. Among the studied ZrO2-MoO3 catalysts, the catalyst containing 3.2% Mo exhibited the best MCH conversion due to having the highest concentration of acid sites[113]. At the same time, the addition of MoO3 is also helpful to improve the selectivity of light hydrocarbons and enhance the stability of the catalyst to some extent[114]. Therefore, supported MoO3 catalysts are widely used in petroleum refining, chemical production, and pollution control[115]. Compared with thermal cracking, the n-decane conversion and heat sink over Pt/MoO3/ZrO2-TiO2-Al2O3 composite oxides with different MoO3 contents were significantly improved, and the n-decane conversion was as high as 89. 7% and the heat sink was up to 3790 kJ/kg at 750 ℃. The catalyst with a MoO3 content of 7.0 wt% exhibited the best catalytic cracking activity. This is consistent with it having the highest total acid amount and strong acid sites[116]. Compared with MoO3, there are few studies on WOx modified catalysts, which can only show that a small amount of WOx(1 wt%) can lead to a significant increase in the acid strength of catalysts[117].

3.3 Initiator

Radical chain reaction is a widely accepted mechanism for the thermal cracking of any hydrocarbon in which radical formation is the rate-determining step[118]. The initiator has a radical coating effect, which can produce a large number of active radicals at a relatively low temperature, thus leading to an increase in the overall cracking rate[119]. Therefore, researchers have developed soluble initiators to solve the deposition problem of heterogeneous or quasi-homogeneous catalysts. Among them, triethylamine, tributylamine, azobisisobutyronitrile or peroxide are the most widely used[120]. According to the research of Wang et al., triethylamine and tributylamine have obvious promotion effect on the cracking of n-heptane, and the addition of 2 ~ 10 wt% can significantly improve the cracking rate and conversion rate[121]. According to Wickham et al., although 2wt% azo/peroxide can improve the cracking rate of n-heptane, the product distribution is not significantly affected, and the improvement of heat sink is limited[122]. Liu et al. Compared the effects of three initiators (1-nitropropane (NP), triethylamine (TEA) and 3,6,9-triethyl-3,6,9-trimethyl-1,4,7-triperoxycyclohexane (TEMPO)) on the cracking of n-dodecane under supercritical conditions[123]. When the concentration of initiator increased to 2%, the conversion of n-dodecane increased by 20% ~ 150%, and then remained unchanged. The catalytic activity was NP > TEMPO > TEA. Although the above soluble initiators can improve the cracking and heat sink of EHF to a certain extent, their decomposition temperatures are low (for azo compounds, 50 ~ 65 ℃; For peroxide compounds (50 ~ 100 ℃), only a large amount of peroxide compounds (> 2 wt%) can produce a significant acceleration effect, so it can not promote the cleavage of EHF well[124].
In contrast, macroinitiators with highly branched structure and degree of polymerization can solve the problems faced by small molecule initiators. Hyperbranched polymers are mainly derived from the polymerization of ABm(m≥2) monomers, which have highly branched structure and unique properties[126]. Hyperbranched macromolecules can be viewed as "soft" nanoparticles in contrast to "hard" metal nanoparticles. These macroinitiators with hyperbranched structure can not only dissolve well in EHF, but also act as a "free radical package" to supply a large number of free radicals with a small amount of addition (usually less than 0. 1 wt%), thus achieving a good promotion effect at a relatively low amount of addition. In addition, it has a suitable starting temperature, good solubility and excellent long-term storage stability, which can meet the technical requirements of operation[127]. Ye et al. Explored the pyrolysis behavior of JP-10 under supercritical conditions using hexadecyl hyperbranched polyethylenimine (CHPEI) and monomeric triethylamine (TEA) as initiators, respectively (Fig. 6)[125]. It was found that both initiators with C — N bond could promote the thermal cracking of JP-10, while CHPEI with high molecular weight had a similar promotion effect as TEA when the addition amount of CHPEI was only one tenth of that of TEA. The addition of 0. 20 wt% CHPEI increased the conversion of JP-10 from 16. 4% to 33. 9%, and the corresponding heat sink value increased from 1960 kJ/kg to 2180 kJ/kg at 675 ℃.
图6 十六烷基超支化聚乙烯亚胺对JP-10裂解的影响[125]

Fig. 6 Effect of hexadecyl hyperbranched polyethyleneimine on the pyrolysis of JP-10[125]. Copyright 2019, Elsevier

He et al. Used palmitoyl hyperbranched polyamidoamine as a macroinitiator to explore its effect on MCH cleavage[128]. When the temperature is 690 ℃, the conversion of MCH increases from 39.5% to 56.3%, and the corresponding heat sink increases from 2480 kJ/kg to 2910 kJ/kg. He et al. Further designed palmitoyl hyperbranched polyglycerol (PHPG), which can effectively promote the cleavage of n-tridecane (Fig. 7)[129]. When the temperature is 690 ℃, the conversion of n-tridecane is increased by 17.6%, and the corresponding heat sink is increased from 3000 kJ/kg to 3500 kJ/kg. At the same time, the addition of alcohols can significantly improve the solubility of PHPG in EHF, thus further improving the heat sink of EHF[130]. Similarly, Mi et al. Modified hyperbranched polyethylenimine (HPEI) with phenols and synthesized a series of multifunctional polymer additives BHPEI and CBHPEI[131]. The conversion of JP-10 at 675 ° C increased from 16.4% to 33.4% and the corresponding heat sink value increased from 1980 kJ/kg to 2150 kJ/kg at the dosage of 0.1 wt%.
图7 棕榈酰基超支化聚甘油的合成示意图[129]

Fig. 7 Synthesis scheme of palmitoyl hyperbranched polyglycerol[129]. Copyright 2015, Elsevier

It can be seen that the addition of initiator can effectively improve the cracking rate and conversion rate of EHF. However, whether the heat sink of EHF can be greatly improved is closely related to the amount of initiator added. Therefore, the design and synthesis of new initiators should not only focus on their effects on the cracking rate and conversion of EHF, but also study the optimal amount of initiators to ensure that the heat sink of EHF can be maximized.

4 Effect of catalytic steam reforming on heat sink

Catalysis steam reforming (CSR) is another strong endothermic reaction besides thermal cracking and catalytic cracking. CSR technology originated from the chemical industry to produce H2 and NH3, and then it has been widely developed in the fields of fuel cells, internal combustion engines, gas generators and so on[132][133]. In recent years, this technique has been used to improve the heat sink of hypersonic vehicles. In the CSR process, EHF molecules will contact and react with water molecules on the surface of the catalyst to produce relatively small molecular weight molecules such as H2, CH4 and CO, which can absorb a lot of heat at the same time. As a strong endothermic catalytic reaction, CSR has the characteristics of large endothermic heat, high hydrogen production and low carbon deposition[134]. Compared with pyrolysis, CSR can increase the chemical heat sink by nearly 50%, and the hydrogen production rate can reach 70%, which can effectively inhibit the generation of coke. In addition, the increase of H2 content can also improve the combustion efficiency and reduce the outlet temperature[135]. Therefore, steam reforming is a method to improve the heat sink of fuel, which has received much attention in recent years. In fact, CSR often coexists with thermal cracking, especially when the reaction temperature is high. Under supercritical conditions, CSR can effectively improve the heat sink of EHF, and the higher the temperature, the greater the difference between the improvement of EHF heat sink by CSR and that by pyrolysis. In addition, the H2 produced by CSR will further promote the polymerization and Diels-Alder reaction to proceed in the opposite direction, thus reducing the rate of coke formation. For example, under supercritical conditions, CSR can not only improve the total heat sink and conversion of RP-3, but also greatly reduce the amount of coking[136,137].
The chemical heat sink of CSR reaction is higher than that of pyrolysis, especially in the high temperature range. In this process, the CSR reaction and the pyrolysis reaction compete with each other. When the temperature is less than 500 ℃, the CSR is dominant, which can convert the fuel into small molecular gas products H2 and CO. CSR and thermal cracking reactions occur simultaneously at temperatures above 500 ℃. CSR reaction can reduce the degree of thermal cracking and inhibit the polymerization of small molecules produced by cracking into aromatics, thereby reducing carbon deposition. Zheng et al. Explored the effect of CSR technology on the heat sink of several high energy density fuels (Exo-tetrahydrodicyclopentadiene (Exo-THD), Exo-tetrahydrotricyclopentadiene (Exo-THT), 1,3-dimethyladamantane (DMA), and tetracycloheptane (QC))[138]. The results show that CSR technology can significantly improve the overall heat sink value of endothermic fuels, especially those with higher H/C and (H/C)*Ma(M: molecular weight, a: fitting coefficient), such as 1,3-dimethyladamantane (DMA) and Exo-tetrahydrodicyclopentadiene (Exo-THD). High temperature and high S/C (S is vapor) ratio contribute to the CSR reaction. Feng et al. Found that under the combined influence of CSR endothermic and exothermic reactions, the axial distribution of chemical heat sink has a maximum value, showing a trend of first increasing and then decreasing[139]. The key parameters affecting the CSR reaction were compared and analyzed, and the results showed that the maximum value of the chemical heat sink decreased by 15% when the pressure increased from 3 MPa to 5 MPa. The chemical heat sink increases with the increase of moisture content, and the maximum value of the chemical heat sink can increase by about 50% when the moisture content increases from 5% to 10%. Some research teams have built a two-stage sequential catalytic cracking unit consisting of CSR and catalytic steam gasification, and found that the catalytic steam gasification process can effectively inhibit the formation of coke, which is a strong endothermic process and can improve the chemical heat sink of fuel[140]. The results show that the two-stage catalytic structure can reduce coke by 50% and increase chemical heat sink by 50% compared with the single CSR process combined with catalytic steam gasification.
Although CSR can effectively improve the heat sink of EHF, water content is an important parameter in the reaction process. Therefore, the key of CSR is how to reduce the water content as much as possible while ensuring the conversion rate and gas production rate of EHF cracking. At present, the coupling relationship among moisture content, CSR and heat sink has not been well explained, and further exploration is still needed.

5 Comprehensive comparison of endothermic technologies

Thermal cracking, catalytic cracking and catalytic steam reforming are important means to improve the heat sink of EHF, which are comprehensively compared and analyzed in Table 1. Compared with catalytic cracking and catalytic steam reforming, thermal cracking does not require the intervention of catalyst, so it does not involve complex catalyst synthesis, selection and regeneration[141]. Generally speaking, pyrolysis can increase the heat sink of EHF by 20% ~ 30%. In order to obtain the same heat sink value as catalytic cracking and catalytic steam reforming, it is necessary to maintain the thermal cracking at a higher temperature. This will bring a series of side reactions, which not only leads to poor product selectivity of thermal cracking and 10% ~ 30% coking, but also makes the overall energy efficiency only 10% ~ 30%.
表1 吸热技术综合对比

Table 1 Comprehensive comparison of heat absorption technology

Reaction rate Endothermic value Temperature(℃) Coking value Product selectivity Catalyst Energy efficiency*
Thermal cracking[143⇓~145] Slow Increase by20%~30% 300~1000 10%~30% Worse Not require 10%~30%
Catalytic cracking[146⇓⇓~149] Fast Increase by 30%~50%,or even higher 500~1000 5%~20% Good Require 50%~70%
Catalytic steam
reforming[136; 138; 139; 150]		 Fast Increase by 20%~50%,or even higher 400~1000 0.1%~10% Good Require 60%~80%

* Energy efficiency = (cracked gas energy/energy input) ×100%

Compared with thermal cracking, catalytic cracking can produce as many small molecular olefins as possible, which can increase the heat sink of EHF by 30% -50%, or even higher. The introduction of catalyst can not only effectively reduce the activation energy required for the reaction, but also reduce the initial temperature required for the reaction and enhance the reaction rate[142]. Catalytic cracking also has the advantages of good product selectivity and high energy efficiency (50% ~ 70%). However, catalytic cracking also faces some disadvantages, such as large amount of coking (5% ~ 20%), the need to select the appropriate catalyst, and the complex operation of catalyst regeneration.
CSR is another strong endothermic reaction besides thermal cracking and catalytic cracking. In the CSR process, EHF molecules will contact and react with water molecules on the surface of the catalyst to produce molecules with relatively small molecular weights, such as H2, CH4 and CO, which can absorb a large amount of heat and can increase the heat sink of EHF by 20% – 50%, or even higher. The hydrogen production rate is as high as 70%, which can effectively inhibit the generation of coke, and the amount of coke is only 0.1% ~ 10%[134]. CSR also has the advantages of fast reaction rate, good product selectivity, and high energy efficiency (60% – 80%)[135]. Although CSR has the above advantages, it requires the intervention of catalyst and water, so it faces the problems of complex operation and high water quality requirements for catalyst regeneration.

6 Coking and inhibition technology

In the cracking process of EHF, due to the influence of operating conditions, reaction materials, reactor materials and other conditions, coking is usually accompanied. Coking mechanisms mainly include metal catalytic coking mechanism, aromatic condensation coking mechanism and free radical growth coking mechanism. In practice, the coking reaction in EHF cracking process is usually caused by several coking mechanisms[151⇓~153].
Temperature has a great influence on the coking reaction. It is generally believed that thermal oxidation deposition coking mainly occurs at temperatures < 260 ℃, thermal cracking coking mainly occurs at temperatures > 400 ℃, and 260 ~ 400 ℃ is a transitional state, and the physical and chemical properties of the fuel itself directly determine its coking characteristics[20,46,154]. Thermal oxidation deposition and coking refers to the formation of alkyl peroxides through a series of chemical reactions between EHF molecules and dissolved oxygen in the cracking process, which has poor stability and will further generate free radicals. Subsequently, these free radicals will form aldehydes/ketones through hydrogen abstraction and branching reactions, and finally deposit on the surface of the reaction tube to form colloid[155]. Dissolved oxygen is the decisive factor for thermal oxidation deposition and coking. At present, the addition of antioxidant is mainly used to weaken the thermal oxidation deposition and coking[156].
The service temperature of EHF is generally greater than 400 ℃, and pyrolysis coking is more harmful than thermal oxidation deposition coking at this temperature. Cracking coking can be divided into catalytic/non-catalytic coking. Catalytic coking refers to the chemical adsorption between the precursor and the active surface, and the dehydrogenation coking under the catalytic action. Non-catalytic coking refers to the formation of non-volatile carbon deposits of polycyclic polyenes by polymerization, hydrogen transfer and other reactions of coking precursors on the surface of the reactor at high temperature[157]. These carbon deposits can be divided into two types, one is fibrous and the other is amorphous[158]. Non-catalytic coking usually results in the formation of blocky coke, which is mainly formed by the overlapping of granular carbon, and its surface is rough[159]. In contrast, the catalytic coke is rod-like or fibrous. Therefore, the occurrence of catalytic coking can be judged according to the morphology of coke.
Coking in EHF cracking is usually affected by many factors, among which temperature, cracking depth and surface effect are the most important factors. The cracking reaction of EHF is closely related to temperature. It is generally believed that the increase of temperature will help the cracking reaction, but at the same time, the content of coking parent (small molecular olefins) will increase with the increase of temperature, thus aggravating the coking reaction[160]. Studies have shown that the coking rate increases with the increase of temperature, and the relationship between coking rate and temperature conforms to the Arrhenius equation[161,162]. Cracking depth is the product of reaction residence time and EHF cracking reaction rate constant, which mainly affects the coking rate[163]. Due to the composition and performance differences of EHF itself, the influence of pyrolysis depth is very different. The surface effect refers to the physical and chemical properties of the surface of the reaction tube that affect the coking process. The severity of coking is positively related to the roughness of the reactor surface, that is, the greater the roughness of the reactor surface, the deeper the severity of coking. This is mainly because the thickness of the stagnant layer increases with the increase of roughness, which increases the residence time of the coking precursor in the boundary layer and intensifies the severity of the coking reaction[142].
At present, there are two main ways to inhibit coking, one is to passivate the metal active substances on the surface of the reaction tube by physical or chemical treatment to inhibit the catalytic effect of the tube wall. It is found that the inert coating can cover the active metal sites on the reaction tube wall, which can not only suppress the filamentary coke, but also reduce the amorphous coke[164]. The other is to add coking inhibitor and hydrogen donor to the fuel to inhibit homogeneous/heterogeneous reaction coking by changing the free radical reaction mechanism. At present, the commonly used coking inhibitors are mainly phosphorus/sulfur compounds, metal salts/oxides, aliphatic polysiloxane, alkali metal compounds, borides and so on[165]. Thiophene has a good inhibition effect on the coking of n-heptane cracking, and the addition of thiophene with a mass fraction of 1×10-3 can inhibit 92% of the coking[166].

7 Conclusion and prospect

EHF can absorb the heat generated by high Mach number aircraft through physical and chemical heat sinks, and has both propulsion and cooling functions. At present, the development of EHF mainly focuses on further improving the fuel heat sink by means of thermal cracking, catalytic cracking and catalytic steam reforming, so as to meet the requirements of high Mach number flight of aircraft. However, EHF is a highly coupled and challenging subject, and many basic scientific problems have not been well explained, so a lot of basic research work needs to be carried out urgently.
(1) At present, the pyrolysis behavior of fuels is mainly studied by using model compounds such as n-decane, and there is a lack of in-depth excavation of the actual pyrolysis behavior of EHF. The relationship and law of the actual composition-structure-heat sink of fuels need to be further strengthened in the future.
(2) The core of catalytic cracking lies in the research and development of high-performance catalysts. Although the current catalysts have achieved good results in improving the heat sink of fuels, the selectivity and stability of catalysts are still difficult, and the relationship between the structure, morphology and mechanism of action of catalysts has not yet been well explained, which is also the focus of future research.
(3) The use of nanofluids and the introduction of macroinitiators can greatly improve the EHF heat sink. The development of nanofluids and macroinitiators with higher activity will be an important means to improve the heat sink of EHF in the future.
(4) Catalytic steam reforming, as an effective way to improve the heat sink of EHF, has the advantages of large heat absorption, high hydrogen production and low carbon deposition. However, the role of catalytic reforming reaction conditions, catalysts and other factors in the reforming reaction process is not yet clear, and the basic theoretical research on the coupling relationship among water content, CSR and heat sink needs to be strengthened.
In the future, it is necessary to combine the adjustment of EHF structure with the optimization of pyrolysis/catalytic cracking and catalytic steam reforming reaction conditions, and to deepen the understanding of basic theory (reaction path, catalytic mechanism, etc.).Only in this way can we develop EHF with the basic characteristics of "four high and three low", such as high heat sink, high density, high calorific value, high thermal stability, low freezing point, low coking and low cost, to meet the national aerospace strategic needs and help the development of the national aerospace industry.

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