Experimental and Numerical Investigation into the Effect of Fuel Type

Feb 13, 2017 - State Key Laboratory of Automotive Simulation and Control, Jilin University, Changchun 130025, China. Energy Fuels , 2017, 31 (3), pp 2...
5 downloads 15 Views 3MB Size
Download PDF
Article pubs.acs.org/EF

Experimental and Numerical Investigation into the Effect of Fuel Type and Fuel/Air Molar Concentration on Autoignition Temperature of n‑Heptane, Methanol, Ethanol, and Butanol Runzhao Li, Zhongchang Liu, Yongqiang Han,* Manzhi Tan, Yun Xu, Jing Tian, Daoguang Chong, Jiahong Chai, Jiahui Liu, and Zhengnan Li State Key Laboratory of Automotive Simulation and Control, Jilin University, Changchun 130025, China ABSTRACT: The effect of fuel type, fuel molar concentration, and air molar concentration on the autoignition temperature (AIT) of n-heptane, methanol, ethanol, and butanol, under a wide range of conditions (φ = 0.4−2.0, Pinit = 80−500 psi), is investigated using a constant volume bomb. The results indicate that the AIT generally decreases as the chain length and molecular weight each increase. The AIT decreases as the fuel molar concentration, as well as air molar concentration, increase. The AIT of n-heptane remains approximately constant (280 °C) as the fuel/air molar concentration increase. While the AIT of methanol, ethanol, and butanol decrease from 548 °C to 479 °C, from 450 °C to ∼371 °C, and from 401 to 342 °C, respectively, with increasing fuel/air molar concentration. Numerical study is performed to identify the cause of AIT variation using validated comprehensive reaction mechanisms of n-heptane and methanol. The AIT of n-heptane remains almost constant under wide parameters, because of the low activation energy and multiple reaction pathways of keto-hydroperoxides formation and decomposition, which are the chain branching reactions of n-heptane low-temperature oxidation. Conversely, the AIT of methanol is susceptible to external factors such as fuel/air molar concentration may be attributed to the high energy barriers and relatively simple routes of H atom abstraction from methanol by hydroperoxyl radical and hydrogen peroxide decomposition, which are the chain branching reactions of methanol oxidation.

1. INTRODUCTION The ignition of a flammable mixture as the result of heat release due to an exothermic oxidation reaction in the absence of an external ignition source is known as autoignition. The autoignition temperature (AIT), which is also called the spontaneous ignition temperature, is the minimum temperature at which a flammable mixture will produce hot-flame ignition under specific test conditions.1−3 The major application of AIT can be divided into three categories, including: (1) Improve the combustion efficiency and reduce pollutant emission by optimizing the combustion process about the internal combustion engine. The engine knock of spark ignition engine and ignition quality of compression ignition engine are directly connected to fuel ignition properties. Furthermore, the relationship between the cetane and octane rating is also an important content of engine research.4 (2) Quantitatively evaluate the static fire hazards associated with flammable vapor or liquid leakage.5,6 (3) Classify the electrical equipment to define the maximum acceptable exposed surface temperature that operates at specific gases or vapors.7−9 The AIT data reported by the compilations and literatures are diverse, because there are many factors that influence the value. The influencing factors usually fall into three types: (i) mixture parameters such as fuel type, pressure, and equivalence ratio;4 (ii) specifications about the test apparatus, such as vessel shape, size, surface-to-volume ratio and surface material, etc; and (iii) the criterion application to determine autoignition (for example, visual criterion and temperature/pressure rise criterion).10 © XXXX American Chemical Society

This work mainly concentrates on the effect of mixture parameters on the AIT of n-heptane, methanol, ethanol, and butanol. From the perspective of fuel type, AIT of straight-chain hydrocarbons decreases as the chain length and molar weight each increase. Meanwhile, the branch-chain hydrocarbons usually have a higher AIT, compared to those of straight-chain hydrocarbons.11 From the perspective of pressure, AIT generally decreases as the pressure increases, which adheres to Semenov’s equation. The equation can be expressed as described in eq 1 with a limited temperature range.12−14 ⎛P ⎞ E ln⎜ c ⎟ = A + C ⎝ T0 ⎠ 2RT ̅ 0

(1)

Here, Pc is the mixture pressure, T0 the autoignition temperature, EA the activation energy, R̅ the universal gas constant, and C a constant term. The constant term C is a function of the surfaceto-volume ratio. From the perspective of equivalence ratio, the lowest AIT usually occurs at the margin between the stoichiometric condition and the upper flammability limits, except for methane. The methane/air mixture reaches minimum AIT (640 °C) within an equivalence ratio range of 0.295−0.830.15 This work principally investigates into the effect of mixture parameters on the AIT of n-heptane, methanol, ethanol, and butanol. n-Heptane is the most commonly used diesel fuel surrogate in computed fluid dynamics (CFD) simulations of the internal combustion engine. Also, it is a typical straight-chain Received: November 7, 2016 Revised: January 12, 2017 Published: February 13, 2017 A

DOI: 10.1021/acs.energyfuels.6b02940 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels alkane with a high cetane number, reaching 56.16−18 Methanol is the simplest alcohol with a high octane number (RON = 106, MON = 104) that has only one methyl group connected to the hydroxyl group.19,20 Methanol, as an oxygenated fuel, usually serves as a gasoline additive, to enhance fuel economy and reduce the emissions of unburned hydrocarbons. In recent years, the emergence of the Reactivity-Controlled CompressionIgnition (RCCI) technology adopts a dual fuel to construct a fuel reactivity gradient, an equivalence ratio gradient, and a temperature gradient to accurately control the combustion process. The experimental results show that RCCI has the potential to achieve high thermal efficiency and low NOx/soot emission.21,22 The requirement of the fuel properties should require a large reactivity difference between them. Specifically, it means that one should have a high cetane number (readily to undergo spontaneous ignition) and the other should have a high octane number (resistant to undergo spontaneous ignition). As a result, the n-heptane and alcohols (methanol, ethanol, and butanol) are selected in this work, which have a high cetane number and high octane numbers, respectively. The widespread standard test methods of chemical spontaneous ignition include DIN 51794, EN 14522, ASTM E659-15, and ASTM D2883-95.1−3,7,8,23 The summary of AIT-related standards is listed in Table 1. However, there are some limitations in practice use of these standards, including:

lead to lower AIT values, which may be hazardous in practice use.4,10,26 (3) The vessel size in the mentioned standards is almost always 10%, along with a temperature increase of >50 °C within 10 min (after heating initiation), is indicative that autoignition is occurring. This paper investigates the effect of mixture parameters on the AIT of n-heptane, methanol, ethanol, and butanol. The n-heptane and alcohols have high cetane numbers (readily to undergo spontaneous ignition) and high octane numbers

Table 1. Comparison between Different Standard Test Methods for Auto-Ignition Temperature Determination scope

test vessel

heating device ignition criterion time criterion

scope test vessel

heating device ignition criterion time criterion

European Standard EN 14522

German Standard DIN 51794

P = 1 atm, T ≤ 650 °C, flammable gas or vapor in mixture with air or air/inert gas narrow-necked Erlenmeyer borosilicate glass open vessel, V = 200 mL hot air oven uniformly heated

P = 1 atm, T ≤ 650 °C, flammable gas or vapor in mixture with air or air/inert gas narrow-necked Erlenmeyer borosilicate glass open vessel, V = 200 mL hot air oven uniformly heated

visual criterion

visual criterion

t ≤ 5 min

t ≤ 5 min

ASTM Standard E659-15

ASTM Standard D2883-95

P = 1 atm, liquid chemical in air borosilicate round-bottom, short necked boiling flask open vessel, V = 200 mL electrically crucible furnace or fluidized sand bath visual criterion and temperature rise criterion t ≤ 10 min

P = low vacuum (∼0.8 MPa), T ≤ 650 °C, liquids and solids round bottom, long-neck, AISI Type 316 stainless steel flask with 25.4 mm Corning ferrule closed vessel, V = 1L fan-assisted air-circulating oven visual criterion and temperature rise criterion t ≤ 10 min

(1) These standards mostly adopt an open vessel as the combustion chamber, in that the natural convection definitely plays an important role in ignition. The fuel evaporates and is transported outside the reservoir, while the cold air flows into the flask from the surroundings. As a result of this convection process, the AIT of specific fuel under isochoric conditions is relatively lower than that under isobaric conditions.24,25 (2) These standards generally apply to atmospheric pressure, except ASTM D2883-95, because of the open container. However, the process industry is usually carried out under nonatmospheric conditions. Increases in pressure generally B

DOI: 10.1021/acs.energyfuels.6b02940 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

Figure 1. Framework of this article. this work prefers to investigate the effects of air molar concentration and fuel molar concentration on the AIT, rather than pressure. The definitions of fuel molar concentration and air molar concentration are shown in eqs 2 and 3. The molar concentration directly reflects the amount of substance per unit volume. The pressure itself is a secondary variable that can be expressed by eq 4. nfuel ρfuel ̅ = (2) V

(resistant to undergo spontaneous ignition), respectively, which can be applied to RCCI combustion mode. Three groups of experimentscalled the air constant condition, the fuel constant condition, and the stoichiometric condition, respectivelyare designed to investigate the effect of fuel type, fuel molar concentration, and air molar concentration on the AIT. On the other hand, a numerical study of n-heptane and methanol oxidation is performed to identify the predominant reactions and interpret the cause of AIT variation, from the perspective of chemical kinetics. The framework of this paper is illustrated in Figure 1 and the key procedures of chemical kinetic analysis are summarized as described below: (1) To identify the ignition indicator by temperature and active radicals histories analysis; (2) To determine predominant reactions and ignition related species by performing the species sensitivity analysis of the ignition indicator; (3) To clarify the reaction pathway of fuel oxidation by conducting the rate of production (ROP) analysis. (4) To interpret the AIT variation of n-heptane and methanol, based on their reaction pathways and species sensitivity analysis.

nair V

(3)

p = ρ ̅ RT ̅

(4)

ρair̅ =

For convenience, the pressure is still adopted in this paper, since the vessel volume is constant that changing the air molar concentration is equal to alter the air pressure. However, the design of experimental programs concentrate on the influence of fuel molar concentration and air molar concentration on AIT. 2.2. Apparatus and Experimental Procedures. The experimental investigation is conducted using a constant volume combustion bomb system, as shown in Figure 2. The combustion bomb system consists of an intake system, a fuel supply system, a bomb vessel, an injection system, an exhaust system, a temperature control system, and a data acquisition system. The test vessel is a vertical cylinder 83 mm high and 132 mm in diameter, with a volume of 1.014 L. The required amount of air is supplied by adding appropriate pressure into the closed vessel. After reaching the desired air pressure, a specific amount of fuel is injected into cylinder to attain combustible mixture with a specific equivalence ratio. The injection pressure is always 50 bar higher than the pressure inside the combustion chamber, to facilitate the formation of a homogeneous mixture. After the fuel injection into the combustion chamber, 3 min would be set aside to

2. EXPERIMENTAL APPARATUS AND METHODS In this section, the devices of autoignition temperature measurement, ignition criterion, and experimental programs are illustrated in detail. 2.1. Terms and Definitions. The AIT generally decreases with increasing pressure;10 however, the pressure itself may not reflect the influence of vessel volume and reagent amounts directly. As a result, C

DOI: 10.1021/acs.energyfuels.6b02940 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

Figure 2. Experimental setup. allow uniform fuel-air mixing before heating began. The formation of homogeneous fuel-air mixture is a prerequisite of chemical kineticscontrolled spontaneous ignition; otherwise, the global equivalence ratio would be meaningless. The electric heating wire is mounted inside the cylinder and located within the range from 106 mm to 116 mm in the radial direction. In addition, an external electric heating wire is available to promote temperature uniformity. The mixture temperature can be adjusted readily by the continuously adjustable transformer. The dynamic pressure and temperature histories are measured by pressure transducer and temperature transducers. The pressure signal is amplified by a charge amplifier (Kistler, Model 5015A). There are three temperature sensors, which are located at radii of 0, 66, and 88 mm from the center and they all are maintained at the same elevation (42 mm above the bottom). The thermocouples used in this study are nickel chromium Type K thermocouples. The temperature measurement range and accuracy of the sheathed thermocouples are 0−1300 °C and ±2.5 °C, respectively. According to the temperature calibration result, the maximum temperature appears at the region that range from 50 mm to 88 mm in the radial direction and the temperature difference in this region is within 50 °C. A high-speed synchro data acquisition device synchronizes the pressure/temperature signals and then transmits to the LabVIEW system. Before each experiment, compressed air is used to purge the product gases over 3 min to minimize the residual gas concentration. The main apparatus used in the combustion bomb system are listed in Table 2. 2.3. Ignition Criterion. A combined temperature and pressure criterion is applied to determine autoignition. The pressure elevation more than 10% of the initial intake air pressure and the temperature increase more than 50 °C within 10 min (after heating initiation) is categorized as autoignition. While the pressure rise is air molar concentration

3.2. Primary Oxidation Mechanism Analysis of n-Heptane. Since the n-heptane and methanol reach the maximum and minimum AIT variation, they are selected for chemical kinetic analysis in order to determine the cause of reactivity difference. The numerical simulation is performed using Chemkin Pro software, by adopting a closed homogeneous batch reactor model, assuming a constant volume, temperature uniformity, and adiabatic conditions. The combustibles ignite instantly after reaching the critical temperature. The chemical time-scale is much shorter than the physical time-scale during the ignition process (the heat losses are negligible, because of the limited heat dissipation rate); therefore, the adiabatic assumption may not be very different from the practical application. The chemical kinetic mechanism and thermodynamic parameters of n-heptane and methanol are derived from Lawrence Livermore National Laboratory17,18,34 and Princeton University,35,36 respectively. The n-heptane comprehensive kinetics mechanism has been validated by the comparison with experimental data over a wide range of experimental regimes: a temperature range of 550−1700 K, an initial pressure range of 1−42 atm, and an equivalence ratio range of 0.3−1.5. The scope of methanol comprehensive kinetics mechanism covers the temperature range of 633−2050 K, an initial pressure range of 0.26−20 atm, and an equivalence ratio of 0.05−2.6. Both of these reaction mechanisms have been validated over wide parameter ranges and various physical conditions (including ignition delay time, static reactor, jet stirred reactor, flow reactor, laminar flame speed, etc.). The objective of this numerical study is not to further validate or improve those comprehensive kinetic mechanism but rather to apply them to elucidate the reactivity diversity between n-heptane and methanol. With the objective of representing the experimental data and reducing the computational cost, n-heptane and methanol oxidation behavior are simulated under the stoichiometric condition and an initial intake air pressure of 240 psi, which is identical to the experimental condition. The calculated ignition delay times of the stoichiometric n-heptane/ air mixture at 80, 160, and 240 psi are plotted in Figure 5. The n-heptane has distinct two-stage ignition behavior (cool flame ignition and hot flame ignition) and negative temperature coefficient (NTC) behavior (located between the two symbols),

Figure 6. Calculated temperature and species profiles for constant volume bomb experiment; stoichiometric n-heptane/air mixture, Tinit = 280 °C, Pinit = 240 psi.

Table 6. Inputs Used for Chemkin Simulation of n-Heptane Oxidation item problem type end time temperature pressure volume heat loss surface temperature (°C) equivalence ratio fuel mixture oxidizer mixture complete combustion products

value constrain volume and solve the energy equation 10 s 280 °C 240 psi 0.001014 m3 0 cal/s same as gas temperature 1 N−C7H16 O2/N2 = 21/79 (vol %) CO2/H2O/N2

history exhibits a typical two-stage ignition characteristic that the first stage occurs at 944.7 ms, then reaching 1020 K. The mixture temperature subsequently rises up to 1266 K until 944.82 ms. The second stage ignition then occurs and the mixture temperature increases dramatically as the fuel is completely consumed. Preceding the second stage ignition, the fuel is consumed gradually and the H2O2 accumulates gradually as the temperature increases to 1000 K. When the mixture temperature exceeds 1000 K, the H2O2 decomposes into two hydroxyl radicals via eq 7. These active radicals build up and accelerate the fuel consumption that elevates the mixture temperature rapidly. The key reactions of this chain-branching process are presented in eqs 5−7:37

Figure 5. Computed ignition delay times for stoichiometric n-heptane/ air mixture (Pinit = 80, 160, and 240 psi). F

H + O2 + M → HO2 + M

(5)

RH + HO2 → R + H 2O2

(6)

H 2O2 + M → OH + OH + M

(7) DOI: 10.1021/acs.energyfuels.6b02940 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

Figure 7. Sensitivity coefficients of OH species for constant volume bomb simulation; stoichiometric n-heptane/air mixture, Tinit = 280 °C, pinit = 240 psi.

Figure 8. Simplified kinetic scheme for primary mechanism of n-heptane oxidation.

to identify the dominant reactions of OH production and consumption, as illustrated in Figure 7. The sensitive coefficient of each reaction is normalized in order to make the comparison more convenient. A positive sensitive coefficient indicates the OH radical production and an overall reactivity increase, while a negative sensitive coefficient indicates OH radical consumption and a decreased overall reactivity. The results of OH species

According to ref 37, the autoignition of hydrocarbon is featured by H2O2 decomposition. However, the OH production is more closely related to the temperature inflection, as shown in Figure 6. Besides, OH production is mainly derived from H2O2 decomposition when the mixture temperature is >1000 K. Therefore, the OH radical can be regarded as the ignition indicator. A species sensitivity analysis is performed G

DOI: 10.1021/acs.energyfuels.6b02940 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

Table 7. Inputs Used for Chemkin Simulation of Methanol Oxidation item problem type end time temperature pressure volume heat loss surface temperature (°C) equivalence ratio fuel mixture oxidizer mixture complete combustion products

Figure 9. Computed ignition delay times for stoichiometric methanol/ air mixture (Pinit = 80, 160, and 240 psi).

These five reaction types listed above are part of the lowtemperature oxidation reaction pathway of the hydrocarbons. The primary oxidation mechanism is plotted in Figure 8; for a detailed description, please refer to refs 17, 18, 34, 38, and 39. Therefore, only a brief description, which concentrates on the low-temperature oxidation and autoignition, is provided in this paper. Hydrogen atom abstraction from fuel by hydroxyl to form alkyl radicals is the significant initiation step of n-heptane oxidation. The activation energy of R1981 (as shown in Figure 7) is −596 kJ/mol, which manifests that the reaction rate increases as the temperature decreases. In the lowtemperature regime (∼500−600 K), the additions involving the alkyl radical to O2 proceed rapidly to form an alkylperoxy radical (ROO•). The alkyl peroxy radical isomerizes to produce •QOOH. The second addition of •QOOH to an oxygen molecule produces a peroxyalkylhydroperoxide (•OOQOOH) radical. The peroxyalkylhydroperoxide radicals undergo isomerization to form dihydroperoxyalkyl (•U(OOH)2) by internal H atom transfer, and then it decomposes to form keto-hydroperoxides and hydroxyl. The formation of a carbonyl radical (XO•) and a hydroxyl radical is produced from the decomposition of keto-hydroperoxides. The formation and decomposition of keto-hydroperoxides is of critical importance in the low-temperature oxidation of high-carbon-number alkanes (Cn ≥ 4), because these processes produce two hydroxyl radicals and initiate the chain-branching reaction. The chain-branching reaction corresponds to the n-heptane autoignition occurring.

Figure 10. Calculated temperature and species profiles for constant volume bomb experiment (stoichiometric methanol/air mixture, Tinit = 460 °C, Pinit = 240 psi).

sensitive analysis suggest that five types of reactions mainly influence the n-heptane autoignition process, as shown in eqs 8−12: keto‐hydroperoxides → XO• + •OH

(8)

•U(OOH)2 → keto‐hydroperoxides + •OH

(9)

ROO• → •QOOH

(10)

•QOOH + O2 → •OOQOOH

(11)

RH + •OH → R• + H 2O

(12)

value constrain volume and solve the energy equation 10 s 460 °C 240 psi 0.001014 m3 0 cal/s same as gas temperature 1 CH3OH O2/N2 = 21/79 (vol %) CO2/H2O/N2

Figure 11. Sensitivity coefficients of OH species for constant volume bomb simulation (stoichiometric methanol/air mixture, Tinit = 460 °C, Pinit = 240 psi). H

DOI: 10.1021/acs.energyfuels.6b02940 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

Figure 12. Production rate for major species of methanol oxidation (Tinit = 460 °C, Pinit = 240 psi).

For n-heptane, the ability to undergo spontaneous ignition at almost the same temperature over a wide range of conditions may be described as shown below:

(1) The favor of secondary site and tertiary site hydrogen atom abstraction from n-heptane by hydroxyl radical is due to the reaction rate increasing along with the temperature decreasing. I

DOI: 10.1021/acs.energyfuels.6b02940 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels (2) There are 18 reactions (R2325−R2342) about peroxyalkylhydroperoxide decomposition to form keto-hydroperoxides and hydroxyl radicals. The relatively low energy barrier associated with the peroxyalkylhydroperoxide decomposition plays an important role in hydroxyl radical accumulation. In particular, the activation energy of reactions R2327 and R2326 is 18.95 and 21 kcal/mol. (3) There are 18 reactions (R2343−R2360) about ketohydroperoxides decomposition that involve branching reactions and are likely to occur with an activation energy of 39 kcal/mol. It is appreciably lower than that of hydrogen peroxide (H2O2) decomposition to form hydroxyl radicals (EA = 48.43 kcal/mol) from the low-temperature oxidation of methanol. (4) The multiple routes and very low energy barrier for ketohydroperoxides formation and decomposition facilitate the tremendous production of hydroxyl radicals and the branched chain-reaction that occur under a wide range of conditions. 3.3. Primary Oxidation Mechanism Analysis of Methanol. The process of methanol primary oxidation mechanism analysis is similar to that for n-heptane. The ignition delay time for the methanol/air stoichiometric mixture decreases as the mixture temperature increases, as shown in Figure 9. The temperature history of the stoichiometric methanol/air mixture exhibits distinct single stage ignition, as shown in Figure 10. The inputs used for the Chemkin simulation of methanol oxidation are listed in Table 7. The temperature almost remains constant, preceding single stage ignition. The single stage ignition occurs at ∼1000 K. The methanol is consumed slightly and the H2O2 builds up steadily before the temperature approaches 1000 K. When the mixture temperature exceeds 1000 K, the H2O2 decomposes rapidly to produce OH radicals via eq 7. Because of the relatively high activation energy of H2O2 decomposition (48.43 kcal/mol), the critical decomposition temperature of H2O2 under the present conditions is ∼1000 K. The increasing concentration of OH radical accelerates the methanol consumption at an exponential rate that results in a dramatic elevation in mixture temperature. Therefore, the OH radical also acts as the ignition indicator of methanol. The species sensitivity analysis of the OH radical is performed to identify the dominant reactions and ignition-related species, as shown in Figure 11. According to the results of species sensitivity analysis, the methanol autoignition behavior is intimately associated with the chain-branching reactions, as shown in Figure 11, and the sensitivity coefficients of other reactions are negligible. A rate of production (ROP) analysis about ignitionrelated species is carried out and the relation of these species are illustrated as shown in Figure 12. The simplified kinetic scheme of methanol primary oxidation mechanism is plotted in Figure 13, based on ROP analysis results and refs 35, 36, and 40−42. For a detailed description of the methanol oxidation mechanism, please refer to the references mentioned above. It is important to note that the dominant reactions listed in Figure 11 pertain to the initiation of low-temperature oxidation, hydrogen atom abstraction from alcohol, and branchedchain reactions. Reaction R′89 is the initial step of methanol oxidation that is important to produce free radicals. However, the methanol and oxygen can react only when the high energy barrier (EA = 44.9 kcal/mol) is overcame. The H atom abstraction from methanol by hydroperoxyl (HO2) accounts for 80%−90% of the total methanol destruction, which is the primary formation path for hydrogen peroxide (H2O2). Another important route for hydrogen peroxide formation is the H atom abstraction from formaldehyde by the hydroperoxyl radical. Both

Figure 13. Simplified kinetic scheme for primary mechanism of methanol oxidation.

of these abstraction reactions constitute the major pathway of hydrogen peroxide accumulation. As previously mentioned, the hydrogen peroxide decomposition to form reactive hydroperoxyl (OH) radicals (reaction R′20) is a branching reaction that signifies the emergency of spontaneous ignition. Nevertheless, only if the hydrogen peroxide is generated by the reaction form of eq 13, the sequential reaction of hydrogen peroxide decomposition stringently satisfies the conditions of the chainbranching reaction (more radicals are produced than consumed). In other words, one relatively unreactive radical HO2 produces two reactive OH radicals through reactions R′91 and R′20, as shown in Figure 11. On the other hand, if the decomposition reaction R′20 is preceded by reaction R′19, it does not result in a net gain of free radicals. This is the reason why the sensitivity coefficients of reactions R′19 and R′18 are negative. RH + HO2 → R + H 2O2

(13)

The spontaneous ignition temperature variation of methanol under a wide range of conditions may be attributed as described below: (1) The high energy barriers of the addition of methanol to the oxygen molecule (EA = 44.90 kcal/mol) and hydrogen peroxide decomposition to form hydroxyl radicals (EA = 48.43 kcal/mol) cause the methanol to be unable to undergo autoignition at relatively low temperature. J

DOI: 10.1021/acs.energyfuels.6b02940 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels Table A-1. Extraction of n-Heptane Reaction Mechanisma (Rate Constant: k = ATn exp[−EA/(RT)])

a

number

reaction

A (cm3 mol−1 s−1 K−n)

n

EA (cal/mol)

R2325 R2326 R2327 R2328 R2329 R2330 R2331 R2332 R2333 R2334 R2335 R2336 R2337 R2338 R2339 R2340 R2341 R2342 R2343 R2344 R2345 R2346 R2347 R2348 R2349 R2350 R2351 R2352 R2353 R2354 R2355 R2356 R2357 R2358 R2359 R2360

C7H14OOH1−2O2 = NC7KET12 + OH C7H14OOH1−3O2 = NC7KET13 + OH C7H14OOH1−4O2 = NC7KET14 + OH C7H14OOH1−5O2 = NC7KET15 + OH C7H14OOH2−1O2 = NC7KET21 + OH C7H14OOH2−3O2 = NC7KET23 + OH C7H14OOH2−4O2 = NC7KET24 + OH C7H14OOH2−5O2 = NC7KET25 + OH C7H14OOH2−6O2 = NC7KET26 + OH C7H14OOH3−1O2 = NC7KET31 + OH C7H14OOH3−2O2 = NC7KET32 + OH C7H14OOH3−4O2 = NC7KET34 + OH C7H14OOH3−5O2 = NC7KET35 + OH C7H14OOH3−6O2 = NC7KET36 + OH C7H14OOH3−7O2 = NC7KET37 + OH C7H14OOH4−1O2 = NC7KET41 + OH C7H14OOH4−2O2 = NC7KET42 + OH C7H14OOH4−3O2 = NC7KET43 + OH NC7KET12 → NC5H11CHO + HCO + OH NC7KET13 → NC4H9CHO + CH2CHO + OH NC7KET14 → NC3H7CHO + CH2CH2CHO + OH NC7KET15 → C2H5CHO + C3H6CHO-1 + OH NC7KET21 → CH2O + NC5H11CO + OH NC7KET23 → NC4H9CHO + CH3CO + OH NC7KET24 → NC3H7CHO + CH3COCH2 + OH NC7KET25 → C2H5CHO + CH2CH2COCH3 + OH NC7KET26 → CH3CHO + C3H6COCH3−1 + OH NC7KET31 → CH2O + NC4H9COCH2 + OH NC7KET32 → CH3CHO + NC4H9CO + OH NC7KET34 → NC3H7CHO + C2H5CO + OH NC7KET35 → C2H5CHO + C2H5COCH2 + OH NC7KET36 → CH3CHO + C2H5COC2H4P + OH NC7KET37 → CH2O + C3H6COC2H5−1 + OH NC7KET41 → CH2O + NC3H7COC2H4P + OH NC7KET42 → CH3CHO + NC3H7COCH2 + OH NC7KET43 → C2H5CHO + NC3H7CO + OH

2.0 × 1011 2.5 × 1010 3.125 × 109 3.906 × 108 1.0 × 1011 1.0 × 1011 1.25 × 1010 1.563 × 109 1.953 × 108 1.25 × 1010 1.0 × 1011 1.0 × 1011 1.25 × 1010 1.563 × 109 1.953 × 108 1.563 × 109 1.25 × 1010 1.0 × 1011 1.0 × 1016 1.0 × 1016 1.0 × 1016 1.0 × 1016 1.0 × 1016 1.0 × 1016 1.0 × 1016 1.0 × 1016 1.0 × 1016 1.0 × 1016 1.0 × 1016 1.0 × 1016 1.0 × 1016 1.0 × 1016 1.0 × 1016 1.0 × 1016 1.0 × 1016 1.0 × 1016

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

26000.0 21000.0 18950.0 22150.0 23450.0 23450.0 17450.0 15650.0 18650.0 17450.0 23450.0 23450.0 17450.0 15650.0 18650.0 15650.0 17450.0 23450.0 39000.0 39000.0 39000.0 39000.0 39000.0 39000.0 39000.0 39000.0 39000.0 39000.0 39000.0 39000.0 39000.0 39000.0 39000.0 39000.0 39000.0 39000.0

Data taken from ref 18.

(2) The contribution order of AIT variation is

(2) The chain-branching reaction mainly proceeds through H atom abstraction from methanol by the hydroperoxyl radical, followed successively by the hydrogen peroxide decomposition. The chain-branching reactions of n-heptane advance through the formation and decomposition of keto-hydroperoxides, which both have 18 reactions. The relatively simple oxidation reaction pathway of methanol make the autoignition be susceptible to external factors, such as temperature, fuel molar concentration, and air molar concentration.

fuel type > fuel molar concentration > air molar concentration

The variation in AIT is dependent not only on the external factors such as temperature, fuel molar concentration, and air molar concentration, but also on the dominant reactions of fuel oxidation. (3) The AIT of n-heptane remains almost constant (∼ 280 °C) under a wide range of conditions (φ = 0.4−2.0, Pinit = 80−500 psi) and its variation is