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Catalysis and Kinetics

Experimental investigation on pyrolysis of n-decane initiated by nitropropane under supercritical pressure in a miniature tube Zhenjian Jia, Weixing Zhou, Wenli Yu, and Zhixiong Han Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00593 • Publication Date (Web): 24 Apr 2019 Downloaded from http://pubs.acs.org on April 24, 2019

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Experimental investigation on pyrolysis of n-decane initiated by nitropropane under supercritical pressure in a miniature tube Zhenjian Jiaa, Weixing Zhoub, Wenli Yub, Zhixiong Hanb a

College of Petroleum and Chemical Engineering, Beibu Gulf University, Qinzhou, Guangxi 535011, PR China

b

School of Energy Science and Engineering, Harbin Institute of Technology, Harbin, Heilongjiang 150001, PR China

Abstract: Adding an initiator is an effective method of promoting hydrocarbon pyrolysis and improving the heat sink of fuels. Nitropropane was proposed as an initiator with a good performance, owing to its lower reaction activation energy for C–N bond cleavage. To study the effects of this initiator on hydrocarbon pyrolysis, a miniature tube reactor that can simulate a real heating procedure in an aeroengine was used to investigate n-decane pyrolysis with and without nitropropane under experimental supercritical conditions. The results demonstrate that the nitropropane initiator promotes the pyrolysis of fuel as it flows through a tube with a large length-diameter ratio within a certain temperature range. The initial decomposition temperature of n-decane is reduced by approximately 100 K, and the increase of the conversion leads to a higher heat sink for n-decane, which can result in decreases in the fuel and reactor temperatures under the same heating condition and within the effective temperature range. A stronger promoting effect can be achieved by increasing the concentration of the nitropropane initiator. The variation laws for the n-decane pyrolysis reaction rate along the flow reactor are changed by the initiator, the presence of nitropropane greatly accelerates the pyrolysis reaction of fuel at a lower temperature, and the opposite tendency appears as the fuel temperature increases, which is caused by the consumption of the initiator. In addition, the selectivity of the methane, propane and alkenes, especially ethylene, increases because of the propyl radical generated by the C–N dissociation of nitropropane before the initiator is consumed.

Key words: n-decane, initiated pyrolysis, nitropropane, endothermic hydrocarbon fuel

1 Introduction With the development of aeroengines for advanced aircrafts with hypersonic or supersonic speed[1,2], endothermic hydrocarbon fuel has been proposed as a good solution for thermal management issues resulting from high intensity combustion in the combustion chamber and high friction with air at high flight speeds[3-6]. Endothermic hydrocarbon fuels with a chemical heat sink due to the occurrence of endothermic pyrolysis reactions can take the place of conventional fuels in which the heat-absorbing capacity is limited by a physical heat sink[6,7]. Improving the chemical endothermic capacity is essential to increasing the heat absorption of endothermic hydrocarbon fuel. However, endothermic hydrocarbon fuels reach useful pyrolysis rates at a high temperature, generally during the pyrolysis process, which will cause large amounts of coke deposition and reduce the stress intensity of cooling channel ACS Paragon Plus Environment

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materials[8,9]. The slowest step could dominate the overall pyrolysis reaction rate of hydrocarbon fuels according to the free radical mechanism. In addition, the C–C dissociation produces free radicals through an initial decomposition reaction and is a typical rate-limited step in the pyrolysis of hydrocarbon fuels[10]. Thus, initiators can easily generate more active free radicals at a lower temperature, which could change the initial decomposition path and accelerate the pyrolysis of fuel[11]. In recent years, many studies have been performed in this particular area. The improvement effect of initiators on the heat sink capacity[12-16] and conversion rate[11,14-26] of fuel were studied experimentally. These previous experimental results demonstrated that initiators could improve the heat sink capacity and greatly accelerate the reaction rate of jet fuel. Recent experimental conditions for the initiated pyrolysis of hydrocarbon fuels are summarized in Tab. 1. It can be observed that a nonflow reactor[17-19,24,25] and a low flow rate with a large-diameter flow reactor[11-13,20,21,23,26] were adopted during studies on homogeneous initiators. However, this setup is quite different from the operational conditions of the practical cooling channels in advanced aircraft. Using the scramjet as an example, which has nearly the most severe thermal environment among the air breathing aeroengines[27], the typical hydraulic diameter of a cooling channel with a large length-diameter ratio has a range of 0.5-2 mm [28-31], and the flow rate of endothermic hydrocarbon fuels is in the range of 0.125-4 g/s. Therefore, it is necessary to investigate the characteristics of initiated pyrolysis in endothermic hydrocarbon fuel under the actual operational conditions in the cooling channels. Tab. 1 Previous experimental conditions for the initiated pyrolysis of hydrocarbon fuels. Parameters of reactor Reactor

i.d.

length

(mm)

(mm)

--

--

--

--

Flow tube

3.18

305

Flow tube

7

600

Flow tube

6

670

Flow tube

3

800

Flow tube

1

1000

Autoclave Tubing bomb

Initiator DTBP, NM,

Flow rate

Temperatur

(ml/s)

e

Pressure

(K)

(MPa)

Ref.

--

683

5.0

[17-19]

NP, TEA, TEMPO

--

693-728

3.11-4.3

[24,25]

AIBN, Acetone

0.09

723-894

3.7

[11-13]

0.005

823-923

0.1

[20,23]

TEA

0.033

833-923

3.5

[21]

DTBP, DIPA, TEA

0.017

773-813

2.93

[26]

80

873-993

3.5

[14-16,32]

acetone

TEA TBA

hyperbranched polymers

DTBP: di-tert-butyl peroxide; NP: nitropropane; TEA: triethylamine; TBA: tributylamine; NM: nitromethane TEMPO: 3,6,9-triethyl-3,6,9-trimethyl-1,4,7-triperoxonane; AIBN: 2,2-azobisisobutyronitrile; DIPA: diisopropylamino

The subject of this paper was to investigate the initiated pyrolysis processes from when n-decane passes through an electric heated miniature flow tube under supercritical pressure. A detailed investigation on the pyrolysis of practical hydrocarbon fuels is difficult to perform owing to their complicated composition. Using a surrogate is also a viable option. N-decane was used in this study because it is one of the primary components of transportation fuel and is usually employed as a representative of long straight alkane ingredients in surrogates for diesel and jet fuel, especially endothermic fuel (such ACS Paragon Plus Environment

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as China RP-3), and n-decane is often employed as a single component surrogate fuel or a primary component in surrogate fuel to study the flow, convective heat transfer and combustion characteristics of endothermic hydrocarbon fuel[33-36]. In addition, nitropropane high-performance additive or fuel initiator[25,37] was used as the initiator in the experiments. In this study, supercritical pyrolysis and the initiated pyrolysis of n-decane was investigated over a range from 323 K to 960 K. The total and chemical heat sinks of n-decane were calculated by using an energy analysis based on the direct measurement of the outer surface temperature distribution along the reactor. The liquid and gas pyrolysis species were separated for the specific analysis using a gas chromatography-mass spectrometer (GC-MS). Furthermore, the conversion of n-decane was determined and compared under different heating power and initiator concentration conditions. The effects of nitropropane initiator on the pyrolysis and heat sink of fuel were studied and discussed in detail. The following sections will present the experimental apparatus, the analysis method for the heat sink, the analysis method of the pyrolysis species, and the results in detail.

2 Experimental methods 2.1 Experimental apparatus Fig. 1 shows a one-stage heating system that was established and used to investigate the pyrolysis of hydrocarbons under supercritical pressures. A system with direct current heating was applied in this study because it can be used to simulate the actual heating process of the cooling channel in aeroengines. The geometrical dimensions and heat flux of the flow tube reactor are consistent with those of the cooling channel in an actual aeroengine. The facilities consist primarily of fuel and initiator tanks, two high-pressure constant-flux pumps, a direct current heating power, a flow tube reactor and a fuel cooler. The mass flow rate of the fuel and initiator were measured by two mass flow meters (Micro Motion Elite CMF010). There are two advantages to adding the initiator to the feed stream through a separate line rather than mixing the initiator with fuel. First, it was possible to establish steady-state conditions with fuel only and then measure the change in the reaction rate caused by adding the initiator. The second advantage is that it was easy to vary the initiator concentration during a run by changing the flow rates of both pumps[11]. The reactor is a GH3128 superalloy tube (1000 mm in length, 3 mm in outer diameter, and 1 mm in inner diameter) horizontally placed on an experimental platform, and fuel can easily be heated by the reactor from ambient temperature up to 1000 K. Two copper electrodes are used to wrap both ends of the tube. The tube is heated with a direct current supply, which has a power ranging from 0 to 5 kW, and the heating power is controlled by adjusting the voltage. Downstream from the reactor, the pyrolytic fuel was cooled to ambient temperature by flowing through a water-fed condenser and then filtered before passing through a needle valve. The function of the needle valve is to regulate the experimental pressure and ensure that the outlet pressure of the reactor can be kept constant during the experiments. Fig. 2a shows that the outer surface temperature distribution along the reactor is measured by thirty-three K-type thermocouples (0.1 mm in diameter) welded onto the surface of the tube. The fuel temperatures at the inlet and outlet of the tube are measured by K-type thermocouples ACS Paragon Plus Environment

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(0.2 mm in diameter) inserted into the central flow of the fuel. The experimental pressure is measured with two pressure gauges installed at the outlet of the pump and the upstream back pressure valve. The uncertainty associated with the measurement of the surface and fuel temperatures is estimated to be less than 3 K, whereas the uncertainties of the pressure and mass flow measurement are less than 1%. All the experimental data were recorded for analysis by a data acquisition system.

Fig. 1 One-stage fuel heating and pyrolysis system.

2.2 Experimental procedure Before the experiment, the dissolved oxygen was removed by pumping argon into the n-decane and nitropropane, and the reactor was oxidized at 1000 K to form a passive film, which will prevent the pyrolysis of hydrocarbon from the catalysis of metal[38]. The system was purged with argon for 10 minutes to exclude air from the entire reactor, and then n-decane and the initiator were loaded and regulated by high-pressure pumps that transport the reactants into the miniature tube. As the pyrolytic fuel flowed out from the tube and was cooled to ambient temperature, the gaseous species were analyzed by gas chromatograph (GC) and the liquid species were analyzed by gas chromatograph/mass spectrometry (GC-MS). In the present study, the mass flow rate was 0.8 g/s, the experimental pressure was 3.2 MPa, and the concentrations of nitropropane were 0, 1, 2, and 4 wt%. To maintain the same heating conditions for n-decane with different concentrations of initiator, during each run, the heating power was kept constant and the flow rate of the fuel and initiator were regulated by pumps.

2.3 Analysis of pyrolysis products The pyrolysis products were sampled from a gas-liquid separator as the pyrolytic fuel exited the heat exchanger. Before the analysis of the pyrolysis species, the fluid species was collected for 1 minute and weighed, and the volume flow rate of the gaseous species was measured by a gas flowmeter. The liquid and gas hydrocarbon species were analyzed quantitatively with an Agilent 7890A gas chromatograph (GC) equipped with a flame ionization detector (FID), and the hydrogen was analyzed quantitatively with a Techcomp GC7900 equipped with a thermal conductivity detector (TCD). The gaseous species were identified and quantified by using standard gas mixtures, and the liquid species were identified by using an Agilent 5975C electron impact mass spectrophotometer (EI-MS). The mole fractions of the pyrolysis products can be calculated by mass conservation based on the data obtained through the above measurements. The conversion of n-decane and the mole ACS Paragon Plus Environment

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fraction of the pyrolysis species were calculated by using the methods proposed in a previous study[39]. The uncertainty is estimated to be 10% for all the species. Detailed analysis methods for the pyrolysis products are shown in Tab. 2. Tab. 2 Analysis methods for pyrolysis products Products

Detector

Gas

FID

Liquid

FID MS

H2

TCD

Column HP-PLOT Al2O3/S (30 m×0.25 mm×15 μm) HP-5 MS (50 m×0.53 mm×15 μm) OV-101(1 m×4 mm) TDX-01(2 m×4 mm)

Temperature

Carrier gas

constant at 100 °C

He (7 ml/min)

from 20 to 200 °C at 15 °C/min with a final isothermal period of 10 min at 200 °C from 50 to 200 °C at 20 °C/min with an initial isothermal period of 18 min at 20 °C

He (1 ml/min) N2 (25 ml/min)

2.4 Calculation of the heat sink The total heat sink of the fuel was calculated by subtracting the heat lost to the surroundings from the total heat input based on the method proposed in previous studies[39,40]. The method for measuring the heat flux is shown in Fig. 2. The tube is first heated without fuel to find how the heat loss varies relative to the outer surface temperature. The total heat sink of the fuel includes physical and chemical heat sinks. The physical heat sink of fuel equals the enthalpy change due to the increase in temperature, and the chemical heat sink is calculated by finding the simple energy balance relationship. The enthalpy of fuel can be determined by using the NIST Reference Fluid Thermodynamic and Transport Properties (NIST REFPROP)[41] as follows: H sen  h Th   h Tl  (1)

The chemical heat sink of fuel is calculated using the following: H endo  Qsin k  H sen (2)

where H sen and H endo are the physical and chemical heat sinks, respectively, in J/kg.

Fig. 2 Schematic diagram of the reactor and the heat flux measurement.

3 One-dimensional steady-state flow simulation To investigate the effects of the initiator on the pyrolysis process of fuel, a one-dimensional steady-state method[42,43] is used to simulate the flow and reaction process of n-decane, and there is an assumption that the variations in fuel properties and flow velocities in the radial direction can be ignored. In addition, the decomposition of n-decane is represented by a global ACS Paragon Plus Environment

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chemical mechanism that can be expressed as n-decane → products. Thus, the governing equations are given by the following:   u   0 (3) t x   4   h     uh   q f t x d

(4)

p      u     u u   0 (5) x t x

   Y     uY    (1  Y )k (6) t x

Eq. (3), Eq. (4), Eq. (5) and Eq. (6) are the equations for mass conservation, energy conservation and momentum conservation, respectively, where, ρ is density, u is velocity, h is specific enthalpy, Y is the mass conversion of n-decane, and k is the reaction rate constant. The physical properties of the pyrolysis species mixture were determined by using the NIST Reference Fluid Thermodynamic and Transport Properties (NIST REFPROP)[41]. In the fuel flow simulation, the rate of pyrolysis and initiated pyrolysis of hydrocarbon fuel in the volume element can be approximated as first-order[8,25], and the reaction rate constant is determined by the following: k  Ae



Ea RT

(7)

where A and Ea are the pre-exponential factor and the apparent activation energy. The known parameters are the flow rate and the inlet temperature of n-decane and the heat flux distributions along the reactor. As shown in Fig. 2, in every volume element (L = dx), the heat input causes the fuel temperature to increase and the fuel decompose. In addition, the conversion of n-decane and mole fractions of pyrolysis species versus the fuel temperature were measured. Thus, the global mechanism of n-decane decomposition can be obtained using the experimental results, and the reaction rate constant k is an unknown. In the simulation process, the reaction rate constants were optimized within the parameter range given in previous studies[25,39,44]. The objective of the optimization was to ensure good consistency between the simulation results of the n-decane conversion with the experimental results. It should be noted that lnk is linear with -1/T, and the linear relationships between lnk and -1/T were obtained through the optimization.

Fig. 3 Schematic diagram of simulation for fuel pyrolysis; the black squares are the experimental results and the solid line is the simulated result Fig. 3 shows the comparison method for the simulated and experimental results. Because the pyrolysis species was ACS Paragon Plus Environment

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sampled at the outlet of the reactor, the simulated conversion of n-decane at 100 cm (outlet of the reactor) is plotted as a function with the outlet temperature of fuel, and they are validated against the experimental results. In addition, the global mechanisms and the reaction rate constants for the decomposition of n-decane are shown in section 4.2.

4 Results and discussion 4.1 Effect of nitropropane on heat sinks Fig. 4 and Fig. 5 show comparisons of the total and chemical heat sinks among pure n-decane and n-decane with different concentrations of nitropropane at different temperatures. The total and chemical heat sinks of n-decane increase as the fuel temperature increases, and the growth trends in the fuel heat sink with different concentrations of nitropropane (including pure n-decane) versus the temperature are different. The differences are obvious over temperatures ranging from 630 K to 950 K, and the reason is that the chemical heat sink of n-decane-initiated pyrolysis is higher than that of n-decane pyrolysis within this temperature range. As shown in Fig. 5, the experimental results demonstrate that adding nitropropane to n-decane caused the initial decomposition temperature to decrease by 100 K, from 730 K to 630 K, approximately. The experimental results also show that the improved effect of nitropropane on the chemical heat sink of n-decane is over a temperature range of 630 – 950 K, and additions of 1 wt%, 2 wt% and 4 wt% nitropropane increased the chemical heat sink, for improvements of 0.14 MJ/kg to 0.33 MJ/kg, 0.38 MJ/kg, and 0.41 MJ/kg at 887 K. Additionally, using nitropropane as an initiator that flows through a miniature tube with fuel is effective within a specific temperature range. This phenomenon can be explained by the fact that the nitroalkane is consumed faster than n-decane due to its lower activation energy and higher reaction rate[45]. Fig. 4 and Fig. 5 show that the changes in the nitropropane concentrations influence the total and chemical heat sinks of n-decane, and a higher concentration results in a higher heat sink at the same fuel temperature, but the effective temperature range of the initiator exhibits almost no change as the concentration changes.

Fig. 4 Comparison of total heat sinks in fuel at different outlet fuel temperatures.

Fig. 5 Comparison of chemical heat sinks of fuel at different outlet fuel temperatures.

4.2 Effect of nitropropane on n-decane pyrolysis 4.2.1 Effects on distribution of pyrolysis products Although the concentration of the added initiator in the fuel is low, the experimental results show that the distributions ACS Paragon Plus Environment

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of pyrolysis products are influenced by the addition of nitropropane. Fig. 6 shows the selectivity of the primary products, including n-alkanes such as methane and C2-C9 n-alkanes, and alkenes such as C2–C9 1-alkenes. Here, the selectivity of the products is defined as the C atom mole number of products over the C atom mole number of n-decane consumed[45] and then calculated using the experimental measurement. Compared with the n-decane pyrolysis without the initiator, the pyrolysis initiated by nitropropane promotes the formation of methane, ethylene and propane when the conversion of n-decane is lower than 20%. During the pyrolysis of pure n-decane, methane and propane are generated via H-abstractions of n-decane by methyl and propyl, respectively. The primary decomposition of nitroalkanes is dominated by the reaction of the C-N bond rupture, generating alkyl radicals and NO2 [37,45,46]. The addition of nitropropane to the initiated pyrolysis greatly increases the formation of propyl (R1), which further promotes the production of propane by H-abstraction reaction (R2). Simultaneously, the radical propyl also easily decomposes via C–C β-scission reaction to produce ethylene and methyl (R3), and methyl is primarily consumed through an H-abstraction reaction to produce methane (R4). Thus, the addition of nitropropane promotes the generation of methane and ethylene. C3H7NO2 = C3H7 + NO2

(R1)

n-C10H22 + C3H7 = C3H8 + C10H21

(R2)

C3H7 = C2H4 + CH3

(R3)

n-C10H22 + CH3 = CH4 + C10H21

(R4)

However, the hydrocarbon fuel decomposes primarily via H-abstraction reactions, generating alkyl radicals. In n-decane pyrolysis, the generated decyl radicals are primarily consumed by β-scission reactions of C-C or C-H bonds to generate C2– C9 1-alkenes and smaller alkyl radicals (≤ C9); a small amount of the C3–C10 alkyl radicals isomerizes to other alkyl radicals through intramolecular H-migration reactions, which are also important for the generation of 1-alkenes. To summarize, these 1-alkenes are primarily produced by the β-scission reactions of alkyl radicals, and they are the major products during n-decane pyrolysis. During the initiated pyrolysis, the addition of nitropropane can promote the generation of alkyl radicals from ndecane, which indirectly promotes the formation of 1-alkenes, and hence, the presence of nitropropane increases the formation of alkenes, especially ethylene, before the initiator is consumed. The addition of nitropropane can lead the selectivity of other alkenes (C3-C9) to show a small increase (less than 10%), and the difference in selectivity for the pyrolysis of n-decane with and without the initiator decreases gradually while the conversion of n-decane increases and the initiator is consumed. The addition of nitropropane has similar effects on the selectivity of alkanes. The above results also show that the initiator can cause effects at a certain temperature or conversion range.

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Fig. 6 Selectivity of major pyrolysis products versus the conversion of fuel. The symbols are experimental results, and the lines are the fitting of the experimental results.

4.2.2 Effect of nitropropane on n-decane conversion According to the distributions of the primary pyrolysis species, the global chemical mechanism is n-C10H22→c1H2+c2CH4+c3C2H4+c4C3H6+c5C4H8+c6C5H10+c7C6H12+c8C7H14+c9C8H16+c10C9H18+c11C2H6+c12C3H8+ c13C4H10+c14C5H12+c15C6H14+c16C7H16+c17C8H18+c18C9H20 where ci (i = 1 to 18) is the stoichiometric number, and the values of the stoichiometric numbers are shown in the supporting information. A comparison of the n-decane pyrolysis conversion with different nitropropane concentrations is shown in Fig. 7. According to the experimental temperature, reaction rate constant k was optimized at different temperature stages. The prediction of the fuel conversion is qualitatively consistent with the experimental results, and the results illustrate that an accurate reaction rate constant k was obtained through optimization during the simulation process. The addition of an initiator accelerates the conversion of fuel, which could explain the results in Fig. 4 and Fig. 5 perfectly. The improvement effect on the conversion is first increased and then decreased, and the addition of 1 wt%, 2 wt% and 4 wt% nitropropane increased the conversion of n-decane from 9.5% to 21.6%, 24.9% and 26.8% at 887 K. In the presence of the initiator, the C–C bond cleavage reaction is no longer the initial decomposition reaction. By contrast, the C–N bond cleavage reaction of nitropropane, which produces propyl and nitro groups, changes the initial decomposition pathways and promotes the pyrolysis of the fuel[37,47]. The promoting effect of the initiator can be reflected in the reaction rate constant, which was calculated and is shown in Fig. 8. Compared to n-decane pyrolysis without an initiator, nitropropane as a good performance initiator additive ACS Paragon Plus Environment

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to fuel increases the reaction rate constant when the fuel temperature is lower than 830 K, and the reaction rate constant of initiated pyrolysis decreases gradually to the same value of n-decane pyrolysis without an initiator.

Fig. 7 Comparing the conversion rate of fuel with different nitropropane concentrations; the symbols are the experimental results and the solid line is the simulated result.

Fig. 8 Arrhenius plots for the rate constants of pyrolysis and initiated pyrolysis of n-decane.

The difference between hydrocarbon fuel pyrolysis with and without initiator is embodied in the process of the radical initiation reaction of the chain, the initiated pyrolysis process is listed in Tab. 3. In the initiated pyrolysis process, the activation energy of initiator initial decomposition R (5) and H-abstraction R (6) is much lower than that of the initial hydrocarbon decomposition ( RH  R -x  R -y ), and therefore, it can be concluded that the radicals generated by the initiator can reduce the activation energy of fuel pyrolysis. In addition, the concentration of the initiator is very low, and it is assumed that the addition of nitropropane influences the reaction rate of n-decane pyrolysis by changing the activation energy in this study. Tab. 3 Initiated pyrolysis process Reaction Initiation

Chain transfer

Termination

Formula

No.

A  B  A -  B-

R (5)

A -  R  AH  R -

R (6)

A -  H  R1  AH  R1-

R (7)

R1-  R -2  Alkene

R (8)

R -  H -  Alkene

R (9)

R1-  R -2  Alkane

R (10)

For the pyrolysis of pure n-decane, the pre-exponential factor and apparent activation energy are 1.8×1015 and 251151 J/mol (60 kcal/mol), and the values of these two parameters are the same as they are in the previous studies[39,44]. Under the assumption that the pre-exponential value is constant, the distribution of activation energy versus the fuel temperature is shown in Fig. 9. In the initiated pyrolysis process, the activation energy increases from 238593 J/mol to 251151 J/mol, with the fuel temperature increasing from 630 K to 890 K, 910 K and 940 K, corresponding to the concentration of nitropropane 1 wt%, 2 wt% and 4 wt%, respectively. The changes in activation energy can be divided into three stages, including two stages of remaining unchanged and one stage of increasing sharply. The effects of the initiator concentration on the activation energy ACS Paragon Plus Environment

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primarily lies in the increasing stage, and a higher concentration makes the initiator enhance the pyrolysis of fuel under higher fuel temperatures.

Fig. 9 Distribution of activation energy of n-decane pyrolysis, with different concentrations of nitropropane versus the fuel temperature. The results in Fig. 8 and Fig. 9 illustrate that the higher initiator concentration could accelerate the pyrolysis of fuel under a wider range of temperature. This result different from the chemical heat sink results and the conversion rate of n-decane shown in Fig. 5 and Fig. 7, which only reflects the overall effects of nitropropane. In fact, the heat absorption process is significantly influenced by the variation in the conversion rate as fuels flow through the practical flow reactor and cooling channel [48]. The conversion rate of n-decane per unit length is calculated by taking the dc/dL (c and L are the conversion of n-decane and the length of the tube, respectively) to analyze the effect of nitropropane on the conversion rate of n-decane[45]. Fig. 10 shows the calculation results along the flow tube reactor corresponding to the heating condition of 3661 W, which is the highest heating power in this study. For the pyrolysis of pure n-decane, the result shows that the value of dc/dL is first increased and then decreased from 85 cm to the end of the reactor. The pyrolysis reaction rate increases because of the gradual increase in the temperature, which consequently promotes n-decane pyrolysis. However, the concentration of n-decane continually decreases with the consumption of n-decane as it flows through the reactor. Although the reaction temperature along the flow direction is still increasing, the concentration of fuel dominates the conversion rate by degrees. There is a peak value in the conversion rate along the reactor that results from the two aspects discussed above. For n-decane pyrolysis with nitropropane, the dc/dL is much different from that of pure n-decane pyrolysis. Using the fuel with 4 wt% initiator as an example, there is a maximum and a minimal value at the 59 cm and 75 cm positions, respectively. Before the 59 cm position, the presence of the initiator greatly promotes the conversion of fuel, and the decrease in the dc/dL between 59 cm and 75 cm is due to the consumption of the initiator. After the 75 cm position, the pyrolysis reaction rate increases due to the gradual increase in the fuel temperature. As shown in Fig. 10, the dc/dL of n-decane with different concentrations of nitropropane has a similar variation law. Adding nitropropane to fuel dramatically promotes the conversion of n-decane, which results in a higher heat sink at a lower temperature. This result indicates that endothermic hydrocarbon fuel with an initiator will achieve a lower temperature by ACS Paragon Plus Environment

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absorbing the same amount of heat when the initiator plays its role in comparison with the pyrolysis of pure fuel. Fig. 11 shows the fuel and wall temperature along with the reactor at the heating power of 3661 W. The addition of nitropropane decreases both the fuel and wall temperature between the 30 cm and 82 cm positions by increasing the conversion rate of ndecane (shown in Fig. 10) under the same heating condition. Additionally, the maximum reductions in the wall and fuel temperatures are 30 K and 35 K, respectively.

Fig. 10 Conversion rate of n-decane per unit length along with the reactor corresponding to a heating power of 3661 W. Solid lines indicate the conversion rate, and dashed lines show the dc/dL.

Fig. 11 The fuel temperature (solid lines) and wall temperature (dashed lines) along with the reactor at a heating power of 3661 W.

5 Conclusions N-decane pyrolysis with and without nitropropane initiator was investigated in a miniature tube, which can be used to simulate the actual heating process of the cooling channel in the aeroengine to study the effects of the initiator on the pyrolysis of hydrocarbon fuel. The endothermicity and pyrolysis characteristics were discussed using the experimental results obtained by analyzing the heat sink and the products. The result shows that the initial decomposition temperature of n-decane decreases from 730 K to 630 K, and the heat sink increases at a lower temperature due to the addition of nitropropane; a stronger improvement effect can be achieved by increasing the concentration of nitropropane initiator. The flow of nitropropane initiator through a miniature tube with fuel is effective over a temperature range from 630 K to 950 K. Adding 1 wt%, 2 wt% and 4 wt% nitropropane increased the conversion of n-decane from 9.5% to 21.6% and 24.9% and 26.8%, and the corresponding chemical heat sink was improved from 0.14 MJ/kg to 0.33 MJ/kg, 0.38 MJ/kg, and 0.41 MJ/kg at 887 K. There is peak value in the n-decane consumption rate along the tube, and different variation laws for the pyrolysis rate are obtained in n-decane pyrolysis with different concentrations of initiator. The N-decane pyrolysis reaction rate greatly increases because of the addition of nitropropane at a lower temperature, but the rate declines when the fuel temperature is higher, which is caused by the consumption of initiator and n-decane. The addition of nitropropane decreases both the fuel and wall temperatures by increasing the fuel conversion under the same heating power within the effective temperature range, and the maximum reductions in the wall and fuel temperatures are 30 K and 35 K, respectively. In n-decane pyrolysis initiated by nitropropane, radicals produced during the decomposition of nitropropane enhance the chain reaction of fuel. The initial ACS Paragon Plus Environment

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decomposition reaction for the C–N dissociation in nitropropane produces propyl, which promotes the generation of methane, propane and alkenes, especially ethylene, before the initiator is consumed.

Acknowledgments The authors are grateful for financial support from the Natural Science Foundation of China (91741204) and Guangxi Natural Science Foundation (2017GXNSFBA198106).

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Fig. 1 One-stage fuel heating and pyrolysis system. 790x340mm (96 x 96 DPI)

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Fig. 2 Schematic diagram of the reactor and the heat flux measurement. 627x462mm (96 x 96 DPI)

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Fig. 3 Schematic diagram of simulation for fuel pyrolysis; the black squares are the experimental results and the solid line is the simulated result 740x287mm (96 x 96 DPI)

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Fig. 4 Comparison of total heat sinks in fuel at different outlet fuel temperatures. 729x575mm (96 x 96 DPI)

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Fig. 5 Comparison of chemical heat sinks of fuel at different outlet fuel temperatures. 731x568mm (96 x 96 DPI)

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Fig. 6 Selectivity of major pyrolysis products versus the conversion of fuel. The symbols are experimental results, and the lines are the fitting of the experimental results. 723x474mm (96 x 96 DPI)

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Fig. 7 Comparing the conversion rate of fuel with different nitropropane concentrations; the symbols are the experimental results and the solid line is the simulated result. 761x573mm (96 x 96 DPI)

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Fig. 8 Arrhenius plots for the rate constants of pyrolysis and initiated pyrolysis of n-decane. 736x572mm (96 x 96 DPI)

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Fig. 9 Distribution of activation energy of n-decane pyrolysis, with different concentrations of nitropropane versus the fuel temperature. 762x591mm (96 x 96 DPI)

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Fig. 10 Conversion rate of n-decane per unit length along with the reactor corresponding to a heating power of 3661 W. Solid lines indicate the conversion rate, and dashed lines show the dc/dL. 787x565mm (96 x 96 DPI)

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Fig. 11 The fuel temperature (solid lines) and wall temperature (dashed lines) along with the reactor at a heating power of 3661 W. 823x574mm (96 x 96 DPI)

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