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
要
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
