exothermic oxidative degradation that followed injec- tion ... - J-Stage

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Oct 6, 1997 - during the preignition period2). Inomata et al. studied the effects of isopropyl nitrate and di-tert-butyl perox- ide on the spontaneous ignition of n- ...
石 油 学 会 誌

Sekiyu

Gakkaishi,

41,

(5),

341-347

(1998)

341

[Note]

Chemical

Reaction

Kohtaro

Dept.

of Chemical

Mechanism

HASHIMOTO*†1),

System

of Cetane

Yoshiaki

Engineering,

School

AKUTSU,

To

clarify

have ence

of

lar

to

reaction to

cetane those

Next,

the

oxides

mechanism

calculate

number used

of

the

The

nitromethane,

wich

Alkyl

produced

role

in the

reduction

tion

of the

ignition

were

agents no

cetane

from of

the

delay

reduced

delay

of

effect, of

period.

n-butane

The

were

main

Introduction Critically

resulted

increasing

in the use

lates.

Diesel

may

induce

ing in cold ignition fuels

weather, be

The

of

poor

ignition and

need

ignition

in terms

cetane

fuels

properties

engine-start-

to have

improved

properties

of diesel number.

improving

to improve

has

distil-

of their cetane

number

diesel fuels is one method

diesel

part of cracked

as knock

thus they

rated

addition

for

having

problems

properties.

can

The

fuels such

demand

of a greater

agents

to

the ignition prop-

erties of diesel fuels. In the

1940s

peroxides improving ment

and

1950s,

alkyl nitrates and

were found to be effective agents1). Li et al. examined

in cetane

number

due to some

organic

cetane number the improve-

nitrates and organic

peroxides and suggested that the improvement in cetane number correlated with the number of free radicals produced during

by thermal

the preignition

the effects of isopropyl ide on rapid

important

tion

of

the

Al-Rubaie organic

factor was

additive

period

whom

†1) (Present Niizominami,

and

and

was

Toda,

of n-butane

the

Saitama

that by

preignition

should

be

oxida-

of some

the ignition

that the primary

Technical

a the

period3).

the effectiveness

generation

JOMO

using

concluded

nitrates in reducing

correspondence address)

et al. studied

di-tert-butyl perox-

the heat released

concluded

heat

and

during

et al. examined peroxides

additives *To

ignition

machine

of the additive

Inomata

nitrate and

the spontaneous compression

most

delay

decomposition

period2).

through

role

rapid

of and

addressed. Research

Center,

3-17-35

Sekiyu

Gakkaishi,

Bunkyo-ku,

were

as

calculated to

alkyl

Tokyo

did

of

show

alkyl

113-8656

effect

improving

reactions

of

using

simple

the

alkyl

the

on

On the

radicals

sensitivity

in

of and

the

the

play

involved

we pressimi-

model.

organic

As

ignition may

the

conditions

validity

nitrites,

agents

agents,

n-butane

volume.

n-butane.

any

number

the

the

at constant

period

not

improving of under

confirm

nitrates,

conditions

delay

number ignition

per-

a result, other delay

all

hand, period.

an

important

in

the

reduc-

analysis.

exothermic oxidative degradation that followed injectionintothe cylinder4). Clothieret al.made engine measurement with several additives at temperatures below those normally found in operating diesel engines5). Clothier et al. also reviewed how cetane number improving agents worked6). Our recent study suggests thatfree radicals in the preignitionperiod should have an important role in improving the ignition properties7) and that azo compounds, which are known to be radical generating agents,improve the cetane number8). However, the reactionmechanisms of cetane number improving agents are not well known. Also, there are relatively few studies on the effects of cetane number improving agents on the ignition delay periods of hydrocarbons. Westbrook et al. studied the effectsof pro-knock additives an the ignitiondelay periods of hydrocarbon9),10) To clarifythe effectsof cetane number improving agents from the standpointof reactionmechanisms, itis necessary to complement the model reactionfor spontaneous ignition of hydrocarbons with reactions imvolving cetane number improving agents,and to calculate the effects of the cetane number improving agents on theirignitiondelay periods by using computer simulation. Recently, some detailed models for spontaneous ignition of hydrocarbons were proposed11)-13) by assuming the rateconstantsof elementary reactions14),15). In this study, effectson spontaneous ignitionof nbutane in the presence of cetane number improving agents have been studied. First, ignition delay periods

335-8585 石 油 学 会 誌

TAMURA

Hongo,

cetane

periods

such

cetane

determined

of

spontaneous

machine

ignition

improving

Masamitsu

7-3-1

the

adiabatic

decomposition

ignition

period

agents

the

number

delay

under

of Tokyo,

effects

for

compression

improving examined

thermal

the

profile ignition

and

Agents

6, 1997)

elucidating

a rapid

number

periods

has

for

First,

using

cetane

delay

order

agents.

improving

radicals

in

Improving

ARAI,

University

October

pressure-temperature

experiment

known

ignition

number

the

improving

into

effects

on

cetane

1.

the

attempted

Mitsuru

of Engineering,

(Received

Number

Vol.

41,

No.

5,

1998

342

Table 1

M

Reactions Added to the n-Butane Spontaneous IgnitionModel Reaction rate constants in cm3 mol J unit,k=ATBexp

is the third body.

a: estimated

by

the reaction

n-C4H10+CH3O→n-C4H9+CH3OH

b: estimated

by

the reaction

CH4+NO2→CH3+HNO2

c: estimated

by

the reaction

CH3+NO2→CH3O+NO

and

n-C4H10+CH3O→s-C4H9+CH3OH

(ref. 13)).

(ref. 23)).

peroxides, which are known as cetane number improving agents, on ignition delay periods were examined under adiabatic conditions at constant volume and their reaction mechanisms were discussed by simple sensitivityanalysis. Calculation

the reaction

(ref. 23)).

were calculated under conditions similar to those of the experiment with a rapid compression machine and validity of the reaction model was confirmed. Next, the effects of alkyl nitrates,alkyl nitritesand organic

2.

(-E/RT)

Calculations One

involved

similar

were

performed

adiabatic

to those

under

compression,

in the case

two

conditions.

conditions

of the rapid

which

compression

machine experiments performed by Inomata et al.3) to confirm validity of the model. The compression ratio was 14.6:1 and compression was completed at 22ms. The initial pressure was 0.33atm and CO2 was mixed with N2 as an inert gas in order to adjust the temperature when compression was completed. Ignition delay periods were determined as the periods from the points at which the compressions were completed to the points

Method

when temperatures reached 400K higher than those

The pressure-temperature profile of n-butane was calculated using the program SENKIN16) from the Sandia National Laboratories. Kozima's model13) partly revised by adding thermal decomposition reactions of cetane number improving agents, and nitromethane, and by adding the reactions of nitric dioxides, was used as the reaction model for spontaneous ignition model of n-butane. Unknown thermochemical data of chemical species in the model were estimated using the program THERM 17).

石 油 学 会 誌

Sekiyu

Gakkaishi,

when the compressions were comleted. Di-tert-butyl peroxide was used as the model of cetane number improving agent. Two involved adiabatic conditions at constant

vol-

ume to investigate the reaction mechanisms of cetane number improving agents such as n-amyl nitrate, npropyl nitrite, n-amyl nitrite, t-amyl nitrite and di-tertbutyl peroxide. The initial pressure was 20atm, the initial temperature was 800K, and the concentration the additive was 0.05-0.5 vol%. The effects of initial

Vol.

41,

No.

5,

1998

of

343

pressure and initialtemperature were also investigated in the presence of 0.1 volalon-amyl nitrite. Ignition delay periods were determined as the periods from the startof calculationto the point at which the temperatures were higher than the initialtemperatures by 400 K. Pritchard's data18) for thermal decomposition of alkyl nitrate, Batt et al.s' data19)-21) for thermal decomposition of alkyl nitrites, and Shaw et al. s' data22) for organis peroxide were used in this study. Slack et al.s' data23) were used for reaction of NO2 and Dewer et al.s' data24) were used for thermal decomposition reaction of nitromethane. Table 1 shows the reactions added. The reactionrateconstantisshown as k=ATBexp(-E/RT). Additive:di-tert-butyl

3. Results and Discussion

Initial ●

Calculated Values in Comparison with That of Observed Values Inomata et al. measured the ignitiondelay period of n-butane in the presence of di-tert-butyl peroxide using the rapid compression machine3). They measured the

peroxide,

pressure:

Calculated

7.7 △

vol%,

Observed by

Fig.1

gas

temperature:

19.5

vol%,

O2:

et

al.3),

77.4

Compression

vol%,

Comparison

gas temperature after completing compression. However, the temperature decreased rapidlyjust after compression because of the reflectionof the piston. Therefore, the calculated results were not in agreement with the experimental results when the compressed gas temperatures were the same. It was not possible to simulate the temperature profileof the experiment, so the calculated compressed gas temperature was decreased such thatignitiondelay period of calculation was the same as thatof experiment when di-tert-butyl

700K,

n-C4H10:

N2:

3.1

69.6

vol%,

CO2:

vol%.

value

Inomata

N2+Ar:

temperature:297K,

value

Compression

3.1.

Initial

0.33atm.

O2:

of

195

gas vol%,

Calculated

temperature:

n-C4H10:

3.1vol%.

Values

with

860K,

Observed

Values

peroxide was absent. In Fig. 1, calculation results were compared with experimental resultswhen di-tertbutyl peroxide was present. The calculated compressed gas temperature was 700K and that of the experiment was 860K. The reducing effectof ignition delay period using di-tert-butyl peroxide could be well simulated. Thus, this reaction model may be valid for the simulation of the spontaneous ignitionof Initial temperature: 800K, Initial pressure: 20atm, N2: 76.5 n-butane in the presence of a cetane number improving vol%, O2: 20.4 vol%, n-C4H10: 3.1%. agent. Further comparison was not performed ■: n-amyl nitrate, ○: n-amyl nitrite, △: t-amyl nitrite, □: nbecause the validityof the n-butane ignitionmodel was propyl nitrite, ●: nitromethane, ▲: di-tert-butyl peroxide. not confirmed below 700K. Fig.2 Effects of Cetane Number Improving Agents on 3.2. Effects of Cetane Number Improving Agents Ignition Delay Period of n-Butane by Calculation on the IgnitionDelay Periods of n-Butane Figure 2 shows the effects of cetane number improving agents used in this study on the ignition delay period of n-butane. Figure 2 shows also that position of cetane number improving agents. Alkyl nitrate,alkyl nitriteand organic peroxide decomposed the additionof cetane number improving agents to nbutane reduce itsignitiondelay period. This reaction very quickly in comparison with the ignition delay perimodel indicates the effectiveness of the cetane numberod under the these conditions. The n-amyloxy radical and NO2 were produced from the decomposition of one improving agents by reducing the ignitiondelay period n-amyl nitrate molecule. The n-amyloxy radical, one of the hydrocarbon fuelinvolved. Figure 3 shows the mechanisms of thermal decomt-amyloxy radical, n-propyloxy radical, and NO were 石 油 学 会 誌

Sekiyu

Gakkaishi,

Vol.

41,

No.

5,

1998

344

Initial

temperature:

●:

additive,

no

800K. ▲:

vol%,

O2:

Thermal

Decomposition

Number

Improving

Mechanisms

Agents

Used

of

20.4

nitrite

0.1 vol%.

vol%,

N2:

3.1

Fig. 4

The Effect of Initial Pressure Period by Calculation

decomposition Fig. 3

n-amyl

n-C4H9:

of cetane

76.5

number

vol%.

on

the Ignition Delay

improving

agents

played an important role in the reduction of the ignition delay period of n-butane. The effects of n-butyl radical and n-amyl nitriteon the ignition delay period of n-

Cetane

in This Study

butane were same. These results also suggested the importance of alkyl radicals for the reduction of igniproduced one

from

t-amyl

Two

the

t-butoxy

position of alkoxy

and

were

alkyl

ethyl

loxy and

radical

and

n-butyl

had

no

effect

because

on

culation.

These

experimental authors7)

delay

results

suggested

cetane

number

that

of n-amyl

n-butane

with

nitrate and

indicated

that

tant.

Also,

slight

periods

the the

on

without

that

delay

gested

NO2

acetone

of

n-butane by

cal-

with

the

those

of

the

little effect

on

number. that

methane23), was

improvement7).

effects

ignition

of

rad-

and

decreased

has

et al. reported

period

methyl

et al.3) and

cetane

pro-

n-propy-

agreement

ketone

or on

Dewer delay

or

nand

formaldehyde,

period not

in

of Inomata

period

and

had

were

aldehyde

of

produced

delay

period

aldeone

radical

formaldehyde

results

that

radical

ignition

results

Although ignition

was

the delay

an of

radical

t-amyloxy

radical

Adding

ignition

ignition

ethyl

of t-butoxy

acetone.

and

acetone, β-scission

produced

β-scission

ical

and

decom-

β-scission

β-scission

of

radical

nitrite,

the

The

radicals

produced

formaldehyde, β-scission duced

from

peroxide.

immediately.

radical

n-amyl

nitrite, respectively.

produced

produced

a ketone

amyloxy

of one

n-propyl

di-tert-butyl

radicals

or

one

radicals

of one

hyde

decomposition

nitrite

NO2

reduced

our not By

the

ignition

the

reaction

the

experimental

involved

in

the

comparing delay of

the

period

NO2,

of

and

it

differences

between

these

observed.

Thus,

it sug-

were

reactions

of

alkyl

radicals

NO2

were produced

not

very from

石 油 学 会 誌

importhermal

Sekiyu

Gakkaishi,

tion delay of n-butane. Figure 2 shows that the effect of reduction of namyl nitriteon n-butane ignition delay period was the highest among the three alkyl nitrites,and the effects of t-amyl nitriteand n-propyl nitritewere about the same although differences may not be so obvious. Our previous experimental results showed that the improving effect of n-amyl nitriteon cetane number was highest while such effects of t-amyl nitriteand n-propyl nitrite were almost the same7). The result of calculation shows a similar tendency to that of experiment. Figure 2 shows that ignition delay period was not significantlyaffected by adding nitromethane. During the preignition period of n-butane, nitromethane was not drastically decomposed. The half life of nitromethane at 800K is more than 1s, which is much longer than the ignition delay period of n-butane. Thus the reason that nitromethane had no marked cetane number improving effect might have been due to itsslow decomposition reaction. Figures 4 and 5 show the effects of initialpressure and temperature on the ignition delay period of nbutane, respectively. These figures also indicate the effects of cetane number improving agents on the ignition delay period under the conditions given. 3.3. Reaction Mechanisms of Alkyl Radicals Reactions of alkyl radicals involving the reduction of the ignition delay period of n-butane were determined

Vol.

41,

No.

5,

1998

345

Initial

temperature:

●:

additive,

▲: n-amyl

n-C4H9:

3.1

O2:

Fig. 5

The

no

vol%,

20atm.

20.4

nitrite

0.1vol%.

vol%,

N2:

76.5

Effect of InitialTemperature

vol%.

on the Ignition Delay

Period by Calculation

Fig. 6

Reaction

Mechanisms

of n-Butyl

Radical

by simple sensitivity analysis, that is, the influence of the variation of individual rate coefficients by a factor of 10on the calculated ignition delay period was determined. Reactions that reduced the ignition delay period to less than half when those rate coefficients were changed were picked up. Sensitivity analysis of the program SENKIN could not be performed because there were too many reactions involved. The results were clear because reaction mechanism shown in Figs. 6-8 played the important roles in the reduction of the ignition delay period of n-butane, since the reactions reduced the ignition delay period to less than half when the rate coefficients were changed. Figure 6 shows reaction mechanisms of n-butyl radical during the preignition phase of n-butane. The n-

Fig. 7

Reaction

Mechanisms

of Ethyl Radical

butyl peroxy radical is produced by the oxygen molecule-addition reaction. The intramolecular hydrogen abstraction reaction follows it to produce hydroperoxy butyl radical. The hydroperoxy butyl peroxy radical is produced by the oxygen molecule-addition reaction. The decomposition reaction of the hydroperoxy butyl peroxy radical follows it to produce hydroxy radical and complex hydroperoxide. These reactions play important roles in the reduction of the ignition delay period of n-butane. Figure 7 shows reaction mechanisms

of ethyl radi-

cal during the preignition phase of n-butane. The ethyl peroxy radical is produced by oxygen moleculeaddition reaction. The hydrogen-abstraction reaction from one n-butane molecule produces s-butyl radical and ethyl hydroperoxide. It is difficult to expect intramolecular hydrogen-abstraction reaction to produce hydroperoxy ethyl radical because of its large strain energy. The difference in the effects of alkyl

石 油 学 会 誌

Sekiyu

Gakkaishi,

Fig. 8

Reaction

Mechanisms

of Methyl

Radical

nitriteson ignition delay periods may be due to the difference in the reactivity of n-butyl radical and ethyl radical. From the result of sensitivity analysis, decomposition reaction of ethyl hydroperoxide is not important. Figure 8 shows reaction mechanisms of methyl radical during the preignition phase of n-butane. The methyl peroxy radical is produced by oxygen moleculeaddition reaction. The hydrogen abstraction reaction from n-butane molecule produces s-butyl radical and methyl hydroperoxide. From the result of sensitivity analysis, decomposition reaction of methyl hydroperoxide is not important.

Vol.

41,

No.

5,

1998

346 4.

Conclusion

In this study, the spontaneous ignition of n-butane in the presence of cetane number improving agents has been examined. First, the ignition delay periods were calculated under the conditions similar to those of the experiment using a rapid compression machine to investigate the validity of the reaction model. As the effect of reducing the ignition delay period using ditert-butyl peroxide could be well simulated, this reaction model might be valid for the evaluation of the spontaneous ignition of n-butane in the presence of cetane number improving agents. Second, the effects of such known cetane number improving agents as alkyl nitrates,alkyl nitrites,and organic peroxides on ignition delay periods were calculated under adiabatic conditions at constant volume. As a result, all cetane number improving agents showed reduction effects on the ignition delay periods of n-butane. The alkyl radicals produced from the thermal decomposition of cetane number improving agents were involved in the reduction of the ignition delay period. Reactions of alkyl radicals, which play important roles in the reduction of ignition delay period of n-butane were determined by use of simple sensitivityanalysis.

4) Al-Rubaie, M. A. R., Griffiths, J. F., Sheppard, C. G. W., SAE Paper, 91233 (1991). 5) Clothier, P. Q. E., Moise, A., Pritchard, H. O., Combust. Flame, 82, 242 (1990). 6) Clothier, P. Q. E., Aguda, B. D., Moise, A., Pritchard, H. O., Chem. Soc. Reviews, 22, 101 (1993). 7) Hashimoto, K., Kawakatsu, Y., Arai, M., Tamura, M., Nippon Enerugi Gakkaishi, 74, 200 (1995). 8) Hashimoto, K., Akutsu, Y., Arai, M., Tamura, M., Sekiyu Gakkaishi, 39, (2),166 (1996). 9) Westbrook, C. K., Pitz, W. J., Leppard, W. R., SAE Paper, 912314 (1991). 10) Chevalier, C., Pitz, W. J., Warantz, J., Westbrook, C. K., Melenk, H., Twenty-fourth Symposium (International) on Combustion, The Combustion Institute, Pittsburg, 1992, p. 93. 11) Westbrook, C. K., Warnatz, J., Pitz, W. J., Twenty-second Symposium (International)on Combustion, The Combustion Institute, Pittsburg, 1988, p. 893. 12) Westbrook, C. K., Pitz,W. J.,SAE Paper, 890990 (1989). 13) Kozima, S.,Combust. Flame, 99, 87 (1994). 14) Benson, S. W., Prog. Energy Combust. Science, 7, 125 (1981). 15) Benson, S.W., "Thermochemical Kinetics," John Wiley and Sons, New York (1968). 16) Lutz, A. E., Kee, R. J., Miller, J. A., SENKIN: A FORTRAN PROGRAM FOR PREDICTING HOMOGENEOUS GAS PHASE CHEMICAL KINETICS WITH SENSITIVITY ANALYSIS, SAND87-8248, 1988. 17) Ritter, E. D., Bozzelli, J. W., Int. J. Chem. Kinet., 23, 767

(1991). Pritchard,H. O., Combust. Flame, 75, 415 (1989). Batt,L., Milne, R. T., Int.J. Chem. Kinet.,8, 59 (1976). Batt,L., Milne, R. T., Int.J. Chem. Kinet.,9, 549 (1977). References Batt, L., Islam, T. S. A., Rattray,G. N., Int.J. Chem. Kinet.,10, 931 (1978). 1) Robbins,W. E.,Audette,R. R., Reynolds,N. E.,SAE Quart. 22) Shaw, D. H., Pritchard, D. H., Can. J. Chem., 46, 2721 (1968). Trans., 5, 404 (1951). 23) Slack, M. W., Grillo,A. R., Combust. Flame, 40, 155 (1981). 2) Li, T., Simmons, R. F., Twenty-First Symposium (International) 24) Dewer, M. J. S., Ritchie, J. P., Alster,J., J. Org. Chem., 50, on Combustion, The Combustion Institute, Pittsburg, 1986, p. 1031 (1985). 455. 25) Baldwin, A. C., Barker, J. R., Golden, D. M., Hendry, D. G., J. 3) Inomata, T., Griffiths, J. F., Pappin, A. J., Twenty-third Phys. Chem., 81, 248 (1977). 18) 19) 20) 21)

Symposium (International) on Combustion, The Combustion Institute, Pittsburg, 1990, p. 1759.

石 油 学 会誌

Sekiyu

Gakkaishi,

Vol.

41,

No. 5,

1998

347



旨 セ タ ン価 向 上 剤 の化 学 反 応 メ カ ニ ズ ム

橋本

公 太 郎 †1),阿 久 津

好 明,

新井

充,

東 京 大 学 大 学 院工 学 系 研 究 科化 学 シス テ ム 工 学 専 攻, 113-8656東 †1) (現住所)(株)JOMOテ

田村

昌三

京都 文 京 区 本郷7-3-1

ク ニ カ ル リサ ー チ セ ン ター 研 究 グル ー プ , 335-8585埼

玉 県 戸 田市 新 曽南3-17-35

セ タ ン価 向 上 剤 の 向 上 に 関 す る 化 学 反 応 メ カニ ズ ム を解 明

ブ タ ンの 着 火 遅 れ 時 間 が 減 少 す る こ とが わ か っ た 。 一 方, セ

す る た め, n-ブ タ ンの 自然発 火 反 応 にセ タ ン価 向上 剤 の 熱分 解

タ ン価 向 上 効 果 の 認 め ら れ な い ニ トロ メ タ ン に は 着 火 遅 れ 時

反 応 お よ び 熱 分 解 生 成 物 の 化 学 反 応 を 加 え た 反 応 モ デ ル を作

間 減 少 効 果 は 認 め られ な か っ た 。 セ タ ン価 向 上 剤 は, 熱 分 解

成 し, 反 応 シ ミュ レ ー シ ョン を 行 っ た 。 ま ず, 計 算 条 件 を 急

で 生 成 す る ア ル キ ル ラ ジ カ ル が 着 火 遅 れ 時 間 減 少 に 寄 与 して

速 圧 縮 機 に よ る実 験 と類 似 した 条 件 で 行 い, 計 算 の 妥 当 性 を

い る と考 え られ る 。 セ タ ン価 向 上 剤 の 熱 分 解 で 生 成 す る ア ル

確 認 した 。 次 に, セ タ ン価 向 上 剤 と して 知 られ て い る 硝 酸 エ

キ ル ラ ジ カ ル のn-ブ タ ン着 火 遅 れ 時 間減 少 に寄 与 す る 反 応 を

ス テ ル, 亜 硝 酸 エ ス テ ル お よ び 有 機 過 酸 化 物 の 着 火 遅 れ 時 間

簡 単 な感 度 計 算 に よ り抜 き出 した。

に 及 ぼ す 影 響 を計 算 した結 果, セ タ ン価 向 上 剤 添 加 に よ りn-

Keywords Additive,

Cetane

index, Gas oil, Reaction

mechanism,

石 油 学 会 誌

Sekiyu

Computer

Gakkaishi,

simulation,

Vol.

41,

Spontaneous

No.

5,

1998

ignition