Indian Journal of Chemical Technology
Vol. 11, May 2004, pp 410-422
Low temperature oxidation of ethanol
S S Verma
Department of Physics, Sant Longowal Institute of Engineering and Technology, Longowal, 148 106, India
Received 21 September 2002; revised received 28 October 2003; accepted 29 January 2004
Oxidation characteristics of ethanol in the temperature range 200 to 1000 oC suitable to its use in automobiles are
studied to highlight the role and optimum values of different combustion controlling parameters for its clean combustion.
Sensitivity evaluation of different reaction paths has concluded some important reactions responsible for the ethanol
oxidation. It is observed that clean combustion of ethanol can be obtained with a proper combination of combustion
parameters viz., temperature, C2H5OH/O2 mole ratio and residence time.
IPC Code: C07B 33/00
Keywords: Ethanol, oxidation, high-octane fuel
Ethanol is a high-octane fuel with high oxygen
content (35% oxygen by weight) and when blended
properly in gasoline produces a cleaner and more
complete combustion. Recently the interest1-7 in
ethanol as a fuel extender, octane enhancer,
oxygenate, and a neat fuel has increased dramatically
because of the environmental concerns associated
with conventional transportation fuels. Currently,
ethanol and methyl tetra-butyl ether (MTBE) are the
two oxygenated fuels most widely used in a number
of developed countries. However, ethanol appears to
be an attractive oxygenate over MTBE as it is
produced from biomass; which is a renewable fuel
and has roughly double the oxygen content than
MTBE on an oxygen to carbon basis. Ethanol
advocates1-7 base their arguments for promoting the
use of this fuel on three main issues: air quality,
energy security, and farm income. Ethanol is a quality
fuel alternative (anti-knock) rating when blended with
gasoline. It improves combustion and keeps fuel
systems clean. Ethanol combustion can also
contribute towards the control of global warming.
As for ethanol and NOx, while there is some
difference of opinion, it appears that combustion of
ethanol does produce more tailpipe emissions of NOx
than gasoline, because the ethanol blend can be
burned as a leaner (i.e., with low fuel/air ratio)
__________
E-mail: [email protected]; Fax: 01673-84657
mixture, resulting in higher combustion temperatures
than with gasoline. With regulations on pollutant
emissions becoming strict, the amount of oxygenated
fuel like ethanol in gasoline needs to be increased.
Therefore, a full understanding of the reaction
pathways of oxidation of ethanol and of the pollutant
species that may be produced during its combustion is
needed. This understanding will allow industry and
regulatory agencies to better evaluate the feasibility
and relationship between the combustion process and
pollutant emissions when using ethanol. Reaction
pathway and sensitivity analysis are used to help
identify those reactions and accompanying rate
constants that exhibit a strong influence on the
ethanol oxidation process.
The presently considered temperature range (2001000 oC) is drawn from the literature1-4 representing
the homogeneous gaseous phase as well as catalytic
oxidation of ethanol at low temperatures. The
oxidation kinetics of ethanol in this temperature range
are investigated in order to highlight the product
composition characteristics under different operating
conditions such as temperature, fuel/oxygen mole
ratio and residence time. Sensitivity evaluation of the
chemical reactions involved in the oxidation of
ethanol and the reaction involved in the formation of
other important combustion species is also carried out
to highlight the possible reaction paths for ethanol
oxidation.
VERMA: LOW TEMPERATURE OXIDATION OF ETHANOL
Computer modeling of reaction kinetics
8-12
Computational methods
based on numerical
integration of kinetic schemes have played a
significant role in advances of reaction kinetics.
Chemical systems are always complex, involving
many reaction paths and equilibrium. Understanding
of these complex processes is needed for application
in larger or real systems. Many overall processes
comprise complex reaction systems that can be
represented by a number of elementary reactions,
where numerical integration of the elementary
reaction kinetics is used for the analysis. Chemical
mechanisms, often involving hundreds of elementary
reactions, are routinely used to predict species profiles
in relation to time, or to engineer and optimize a given
process. Although, it is now considered completely
realistic to compute transient combustion schemes
that allow combustion events to be studied involving
full blown three dimensional thermo-fluid solutions
coupled to large scale detailed kinetics schemes. But
these said computation study schemes take very large
computer times and the results are also not found in
much disagreement with the presently adopted simple
chemical kinetics scheme.
Present software
Presently used integration software, REACT for
windows11,12 gives the convenience of simulating and
modeling chemical mechanisms. It can be used to
simulate complex mechanisms to predict the time
dependence of reaction species concentrations to
optimize synthetic processes or other chemical
processes, e.g. atmospheric and combustion
chemistry. The REACT code illustrates and applies
numerical integration of elementary kinetic reaction
equations for analysis of complex (real) chemical
systems. REACT is a numerical integrator for
variable-sized, simple or complex chemical kinetic
systems. Elementary reactions representing a
chemical system (series of chemical reactions) and
their rate constants can be entered numerically (nonArrhenius form). Initial reaction conditions (times,
species concentrations) are specified and REACT
then numerically integrates species profiles versus
time. The program illustrates the ease with which
complex kinetic systems with many elementary
reactions can be evaluated, or how parameters (initial
concentrations, added reactions, etc.) can be changed
for evaluation of sensitivity or optimization.
411
Regarding data
Twenty six main chemical species namely H, O,
CO, H2, O2, OH, CH3, CH4, CO2, H2O, HCO, HO2,
C2H5, CH2O, H2O2, CH2OH, CH3CO, CH3OH,
C2H5OH, CH3CHO, CH3CHOH, N, N2, NO, NO2 and
N2O were considered in present study. Forward and
backward reaction rates for considered reaction
mechanism representing the interaction between these
chemical species in 71 chemical reactions are given in
Table 1. The forward rate coefficients for the
considered 71 chemical reactions are drawn from the
literature13-18. Efforts are made to include the optimum
number of chemical reactions involved in the ethanol
oxidation in order to give representation to different
possible reaction pathways. Few termolecular
reactions given earlier13-18 have been re-written as
bimolecular reactions in the present reaction set
considering their importance in this form. This is done
due to the limitation of presently used chemical
kinetic software; which deals only with unimolecular
and bimolecular reactions. However, this change is
expected to affect the output to a very small extent, as
termolecular reactions are slow in nature.
The backward rate coefficients are calculated
from the forward rate coefficients and the equilibrium
constants (Kp). The equilibrium constant (at P=1 atm)
for each reaction was calculated from Gibbs free
energy polynomials18 for each species using Kp= exp
(-ΔG0T /RT). Here, it is emphasized that the rate data
and the literature citation considered presently are not
necessarily the "best" available data for a given
reaction. In many cases, there are other citations
giving the same or nearly same rate and other
publications, which give a different rate for the same
reaction. But, the effect of the variations in data will
not change the trend of the interactions. However, it
may affect the accuracy of the results. In the present
investigations, the aim is just to study the interaction
possibility and its dependence on operating
parameters e.g., temperature, input values of
C2H5OH/O2 mole ratio and residence time etc.
Results and Discussion
The kinetics of the considered reaction scheme
given in Table 1 for ethanol oxidation under the
isothermal combustion condition was studied in the
lower temperature range from 200-1000 oC in order to
highlight the effect of different combustion
controlling parameters along with the sensitivity
analysis of various reactions involved in the
412
INDIAN J CHEM TECHNOL, VOL 11, MAY 2004
Table 1 ⎯ Reaction rate coefficients (K=ATn e-E/RT) for reversible homogeneous gas-phase chemical reactions. Units of K are (cm3/mols),
T is temperature and R is the ideal gas constant
S.No.
Chemical Reaction
Forward
Backward
A
n
E
A
n
E
1
CH3OH=CH3+OH
3.020E+18
0.00
80000.0
1.778E+11
0.00
-10985.0
2
CH3OH+O2=CH2OH+HO2
3.981E+10
0.00
50910.0
1.459E+04
0.00
37972.0
3
CH3OH+H=CH3+H2O
5.248E+12
0.00
5340.0
2.071E+12
0.00
36950.0
4
CH2OH=CH2O+H
2.511E+13
0.00
29000.0
4.144E+13
0.00
-35030.0
5
CH2OH+O2=CH2O+HO2
1.000E+12
0.00
6000.0
8.736E+17
0.00
-8298.0
6
CH2OH+H=CH2O+H2
3.012E+12
0.00
0.0
7.693E+18
0.00
43410.0
7
CH4=CH3+H
1.995E+17
0.00
88000.0
4.935E+10
0.00
-19920.0
8
CH4+O2=CH3+HO2
7.943E+13
0.00
56000.0
1.040E+13
0.00
-2188.0
9
CH4+H=CH3+H2
2.239E+04
3.00
8750.0
8.571E+03
3.00
8270.0
10
CH4+OH=CH3+H2O
3.467E+03
3.08
2000.0
5.746E+03
3.08
16675.0
11
CH4+O=CH3+OH
1.175E+07
2.08
7630.0
1.975E+06
2.08
505.0
12
CH4+HO2=CH3+H2O2
1.995E+13
0.00
18000.0
1.042E+13
0.00
1448.0
13
CH2O+OH=HCO+H2O
7.586E+12
0.00
170.0
2.623E+12
0.00
29997.0
14
CH2O+H=HCO+H2
3.311E+14
0.00
10500.0
2.644E+13
0.00
25172.0
15
CH2O+O=HCO+OH
5.012E+13
0.00
4600.0
1.758E+12
0.00
17177.0
16
CH2O+HO2=HCO+H2O2
1.000E+12
0.00
8000.0
1.090E+11
0.00
6600.0
17
HCO=H+CO
1.445E+14
0.00
19000.0
6.123E+08
0.00
1558.0
18
HCO+O2=CO+HO2
3.311E+12
0.00
7000.0
7.427E+12
0.00
39290.0
19
HCO+OH=CO+H2O
1.000E+14
0.00
0.0
2.839E+15
0.00
105133.0
20
HCO+H=CO+H2
1.995E+14
0.00
0.0
1.311E+15
0.00
89998.0
21
HCO+O=CO+OH
1.000E+14
0.00
0.0
2.880E+14
0.00
87903.0
22
CO+O=CO2
5.888E+15
0.00
4100.0
4.482E+23
0.00
131780.0
23
CO+O2=CO2+O
2.512E+12
0.00
47690.0
2.221E+13
0.00
53910.0
24
CO+OH=CO2+H
1.513E+08
1.30
-770.0
1.695E+10
1.30
21565.0
25
CO+HO2=CO2+OH
5.754E+13
0.00
22930.0
6.533E+14
0.00
84763.0
26
H2+O2=OH+OH
1.700E+13
0.00
47780.0
5.895E+11
0.00
29570.0
27
OH+H2=H2O+H
1.170E+09
1.30
3626.0
5.066E+09
1.30
18781.0
28
O+OH=O2+H
4.000E+14
-0.50
0.0
5.066E+15
-0.50
16115.0
29
O+H2=OH+H
5.060E+04
2.67
6290.0
2.222E+04
2.67
4195.0
…Contd.
413
VERMA: LOW TEMPERATURE OXIDATION OF ETHANOL
Table 1 ⎯ Reaction rate coefficients (K=ATn e-E/RT) for reversible homogeneous gas-phase chemical reactions. Units of K are (cm3/mols),
T is temperature and R is the ideal gas constant ⎯ Continued
S.No.
Chemical Reaction
Forward
Backward
A
n
E
A
n
E
30
H+O2=HO2
3.610E+17
-0.72
0.0
1.911E+23
-0.72
49732.0
31
OH+HO2=H2O+O2
7.500E+12
0.00
0.0
9.493E+13
0.00
72863.0
32
H+HO2=OH+OH
1.400E+13
0.00
1073.0
1.798E+13
0.00
56685.0
34
OH+OH=O+H2O
6.000E+08
1.30
0.0
5.915E+09
1.30
17250.0
35
H+H=H2
1.000E+18
-1.00
0.0
1.548E+24
-1.00
107440.0
36
H+OH=H2O
1.600E+22
-2.00
0.0
1.072E+29
-2.00
122593.0
37
O+O=O2
1.890E+13
0.00
-1788.0
1.627E+21
0.00
119672.0
38
H+HO2=H2+O2
1.250E+13
0.00
0.0
3.654E+13
0.00
57708.0
39
HO2+HO2=H2O2+O2
2.000E+12
0.00
0.0
7.980E+12
0.00
41636.0
40
H2O2+H=HO2+H2
1.600E+12
0.00
3800.0
1.172E+12
0.00
19872.0
41
H2O2+OH=H2O+H
1.000E+13
0.00
1800.0
5.992E+07
0.00
-16705.0
42
C2H5OH=CH3+CH2OH
3.020E+18
0.00
75470.0
2.787E+04
0.00
22860.0
43
C2H5OH+O2=CH3CHOH+H2
3.981E+10
0.00
50000.0
3.137E+06
0.00
27702.0
44
C2H5OH+OH=CH3CHOH+H2O
3.020E+13
0.00
5960.0
3.013E+10
0.00
56525.0
45
C2H5OH+H=CH3CHOH+H2
4.365E+12
0.00
4570.0
1.006E+09
0.00
39980.0
46
C2H5OH+O=CH3CHOH+OH
6.760E+12
0.00
1510.0
6.840E+08
0.00
34825.0
47
C2H5OH+HO2=CH3CHOH+H2O2
6.309E+12
0.00
15000.0
1.984E+09
0.00
34338.0
48
C2H5OH+CH3=CH3CHOH+CH4
3.980E+12
0.00
9690.0
2.396E+09
0.00
45580.0
49
CH3CHOH=CH3CHO+H
5.012E+13
0.00
21850.0
4.029E+10
0.00
-31870.0
50
CH3CHOH+O2=CH3CHO+HO2
1.000E+13
0.00
5560.0
4.255E+15
0.00
1572.0
51
C2H5OH+H=C2H5+H2O
5.248E+12
0.00
5000.0
1.565E+12
0.00
34510.0
52
CH3CHO=CH3+HCO
7.079E+15
0.00
81760.0
4.647E+07
0.00
-1898.0
53
CH3CHO=CH3CO+H
5.012E+14
0.00
87860.0
1.428E+08
0.00
-115.0
54
CH3CHO+O2=CH3CO+HO2
1.995E+13
0.50
42200.0
3.008E+12
0.50
3957.0
55
CH3CHO+H=CH3CO+H2
3.981E+13
0.00
4200.0
1.755E+13
0.00
23665.0
56
CH3CHO+OH=CH3CO+H2O
1.000E+13
0.00
0.0
1.908E+13
0.00
34620.0
57
CH3CHO+O=CH3CO+OH
5.012E+12
0.00
1790.0
9.702E+11
0.00
19160.0
58
CH3CHO+CH3=CH3CO+CH4
1.698E+12
0.00
8430.0
1.945E+12
0.00
28375.0
59
CH3CHO+HO2=CH3CO+H2O2
1.698E+12
0.00
10700.0
1.022E+12
0.00
14093.0
⎯ Contd.
414
INDIAN J CHEM TECHNOL, VOL 11, MAY 2004
Table 1 ⎯ Reaction rate coefficients (K=ATn e-E/RT) for reversible homogeneous gas-phase chemical reactions. Units of K are (cm3/mols),
T is temperature and R is the ideal gas constant ⎯ Continued
S.No.
Chemical Reaction
Forward
Backward
A
n
E
A
n
E
60
CH3CO=CH3+CO
3.020E+13
0.00
17240.0
2.950E+06
0.00
4115.0
61
CO2+N=NO+CO
1.900E+11
0.00
3400.0
4.593E+09
0.00
29130.0
62
HO2+NO=NO2+OH
2.110E+12
0.00
-479.0
8.453E+12
0.00
8238.0
63
NO2+H=NO+OH
3.500E+14
0.00
1500.0
8.860E+12
0.00
32281.0
64
NO2+O=NO+O2
1.000E+13
0.00
600.0
3.206E+12
0.00
47496.0
65
N2O+H=N2+OH
7.600E+13
0.00
15200.0
3.697E+12
0.00
80375.0
66
N2O=N2+O
1.600E+14
0.00
51600.0
1.145E+07
0.00
11430.0
67
N2O+O=N2+O2
1.000E+14
0.00
28200.0
6.161E+13
0.00
109490.0
68
N2O+O=NO+NO
1.000E+14
0.00
28200.0
2.908E+12
0.00
66250.0
69
N+NO=N2+O
3.270E+12
0.30
0.0
1.481E+13
0.30
75190.0
70
N+O2=NO+O
6.400E+09
1.00
6280.0
1.368E+09
1.00
38230.0
71
N+OH=NO+H
3.800E+13
0.00
0.0
1.029E+14
0.00
48065.0
formation/destruction of different chemical species. In
the present studies, isothermal calculation at constant
pressure condition is considered. Though, with the
difficulty in quick removal of heat to keep the
temperature constant, particularly, for very small
residence times, an adiabatic calculation would be
further valuable. A low temperature oxidation study
of ethanol reported in literature2,3 is generally in
presence of catalysts that involves both homogeneous
and heterogeneous reactions with large residence
times. Present studies consider only gaseous phase
homogeneous reactions and results under these
conditions are well in comparison with the quoted
results4. Moreover, the present results will help in
understanding of concentration characteristics of
ethanol combustion species at lower temperatures,
which will be useful in optimizing its (ethanol)
catalytic oxidation.
Effect of temperature
Figure 1 shows the combustion characteristics of
ethanol left after combustion vs. residence time for
different temperatures. It is found from Fig. 1, that
significant ethanol combustion does not take place up
to a temperature of 300 oC. But for a temperature of
400oC, the combustion of ethanol moves towards
complete combustion. Similar trend is observed for
higher temperatures also with a drastic reduction in
residence time value. The formation and destruction
of various combustion species in general and
undesired chemical species such as NOx and CO in
particular from ethanol combustion was studied in the
lower temperature range (i.e., 200-1000oC) as a
function of input C2H5OH/O2 mole ratio and residence
time. C2H5OH/O2 mole ratio of 0.001/0.20 was found
to be optimum in terms of controlled emission of CO
and NO from C2H5OH combustion. Table 2 shows the
detailed formation characteristics as well as order of
magnitude for all 26 chemical species under
consideration with combustion temperatures and
residence time from ethanol combustion. From the
Table 2 it is clear that the orders of magnitude of
equilibrium formation for significant chemical species
for a temperature rise from 500 to 1000oC change
very slowly. Which means that a lower value in the
range of 500-1000oC can be set for optimum
combustion temperature with minimum/optimized
emission of undesired gaseous products. However, the
strong influence of combustion temperature on the
formation of nitric oxides in the absence of fuel
VERMA: LOW TEMPERATURE OXIDATION OF ETHANOL
415
Fig. 1 ⎯ Oxidation profile of ethanol left after combustion with residence time for different combustion temperatures
nitrogen as well as the prominence of transit chemical
species at higher temperatures have been the basis for
the selection of temperature value to 1000oC in the
present investigations and C2H5OH/O2 mole ratio of
0.01/02.0 (an optimum value) was taken as input.
clear that even with a combustion temperature of
1000oC (highest value selected in temperature range
for present investigations), the formation of chemical
species viz., NO, N2O, NO2, N and HCO is to a
minimum level.
Figures 2-5 show the formation/destruction
behavior of various considered chemical species with
residence time for a temperature of 1000oC and
C2H5OH/O2 input value of 0.01/0.20. Figure 2 shows
the formation of major chemical species CH4, CO2
and H2O along with CO, CH2OH and CH3CHO.
Production of species like CH4 and CH2OH is also
predicted4 to be dominant in the considered
temperature range. Figure 3 shows formation of semi
major chemical species such as C2H5, H2, CH3CHOH
and H2O2. Further species viz., CH3OH, CH3 and HO2
formed in minor quantities are shown in Fig. 4. The
chemical species formed in small quantities but
mainly responsible in chemical kinetics are shown in
Fig. 5. Other few left species viz., NO, N2O, NO2, N
and HCO are all formed in very small concentrations
(i.e., less than 10-10 mole/L) as seen from Table 2. The
formation and destruction of these various chemical
species shown in Figs 2 to 5, are a strong function of
residence time and are shown for larger temperature
only in the considered lower temperature range. It is
Effect of C2H5OH/O2 ratio
The C2H5OH/O2 mole ratio was varied with its
values of 0.005/0.205, 0.01/0.20, 0.05/0.16, 0.10/0.11
and 0.15/0.06 keeping nitrogen content constant and
these values are represented in the figures with scripts
'a', 'b', 'c', 'd' and 'e' respectively. Though the thermodynamic properties of the combustion products may
change as the composition changes, while the
reactions proceed but are considered to be invariant in
the present investigations. The C2H5OH combustion
characteristics were studied for a combustion
temperature of 1000oC. Fig. 6 shows the C2H5OH
combustion characteristics for different input values
of C2H5OH/O2 mole ratio for a temperature of
1000oC. Figure 6 indicates the complete combustion
of ethanol irrespective to its input value. This trend
was found similar for any temperature higher than the
minimum required temperature for ethanol
combustion. Only there is a reduction in the residence
time with the increase in temperature. Fig. 7 and 8
show the dependence of CO and NO formation with
416
INDIAN J CHEM TECHNOL, VOL 11, MAY 2004
Fig. 2 ⎯ Concentration of major chemical species with residence time for ethanol oxidation
Residence time (μs)
Fig. 3 ⎯ Concentration of semi major chemical species with residence time for ethanol oxidation
residence time for different input values of
C2H5OH/O2 mole ratio. From these figures it is clear
that CO formation increases whereas NO
concentration decreases with the increase in ethanol
concentration in the reactants. This indicates the
preference for higher input values of ethanol for low
NO output. However, the order of magnitude of
nitrogen oxides is small enough even with a
VERMA: LOW TEMPERATURE OXIDATION OF ETHANOL
417
Fig. 4 ⎯ Concentration of minor chemical species with residence time for ethanol oxidation
Fig. 5 ⎯ Concentration of other minor chemical species with residence time for ethanol oxidation
temperature of 1000oC and with minimum input value
of ethanol. Whereas the CO and CO2 concentration
rise considerably for higher input values of ethanol.
From the present studies, therefore, ethanol/oxygen
mole ratio between 0.01/0.20 (b) and 0.05/0.16 (c)
comes to be an optimum input value in order to
balance the output concentrations of CO, CO2 and NO
from ethanol combustion in the lower temperature
range.
Effect of residence time
Residence time required for the equilibrium
formation/destruction of various chemical species
depends largely on temperature of combustion.
Table 2 shows that residence time becomes smaller
for higher temperatures. Keeping same residence time
for different temperature or for same temperature
with different residence times will not affect the
418
INDIAN J CHEM TECHNOL, VOL 11, MAY 2004
Table 2 ⎯ Variation of concentration (≈) of different chemical species with temperature from ethanol combustion in the lower temperature
range with input conditions of C2H5OH=1%, N2=79% and O2=20%
Order of concentration (Moles/liter)(≈10-x), x is given in the table below
S.No.
Temperature (oC) (Residence time)
Chemical
1000
Species
200
300
400
500
600
800
(0-10s)
(0-10s)
(0-0.5s)
(0-0.1s)
(0-0.003s)
(0-0.0001s)
(0-2x10-6s)
1
H
30
22
18
15
12
09
07
2
N
30
30
30
30
29
17
14
3
O
30
21
19
17
14
10
07
4
CO
15
04
03
03
03
03
03
5
H2
22
12
09
06
05
04
04
6
N2
*
*
*
*
*
*
*
7
NO
30
30
30
29
25
16
13
8
O2
*
01
01
01
01
01
01
9
OH
30
18
14
13
11
09
07
10
CH3
22
14
12
11
09
08
05
11
CH4
15
05
03
02
02
02
02
12
CO2
25
07
04
03
03
03
03
13
H2O
22
07
04
03
03
03
03
14
HCO
30
24
20
17
16
13
12
15
HO2
14
09
08
07
06
05
05
16
N2O
30
18
16
14
13
11
09
17
NO2
30
30
29
25
22
21
17
18
C2H5
22
12
09
07
06
04
04
19
CH2O
18
15
13
11
10
08
07
20
H2O2
10
04
03
02
02
02
04
21
CH2OH
19
12
09
07
06
04
03
22
CH3CO
21
11
10
09
09
08
07
23
CH3OH
30
15
12
11
10
08
07
24
C2H5OH
*
03
03
**
**
**
**
25
CH3CHO
10
05
03
03
03
03
03
26
CH3CHOH
18
08
06
05
05
04
04
*-Indicates species concentration changed by less than 0.001% during the reaction
**-Shows overall consumption of species concentration
VERMA: LOW TEMPERATURE OXIDATION OF ETHANOL
Fig. 6 ⎯ Profiles of ethanol combustion as a function of C2H5OH/O2 input ratio
Fig. 7 ⎯ Formation characteristics of CO with residence time as a function of C2H5OH/O2 input ratio
419
420
INDIAN J CHEM TECHNOL, VOL 11, MAY 2004
Fig. 8 ⎯ Formation characteristics of NO with residence time as a function of C2H5OH/O2 input ratio
equilibrium values of species but only expedite or
slow their (species) formation/destruction for higher
or lower temperatures respectively. Further studies
indicate that residence time for the equilibrium values
of species does not depend on the input value of
C2H5OH/O2.
Sensitivity evaluation
Sensitivity analysis of the various chemical
reactions related to the ethanol dissociation and
formation of end products viz., CO, CO2 and NO was
carried out to highlight the reaction paths responsible
for oxidation. The input conditions were taken with
C2H5OH/O2 mole ratio equal to 0.15/0.06 (that means
the fuel-rich condition) and combustion temperature
of 1000oC. With these input conditions, ethanol
combustion can act as a low source of nitric oxides.
Ethanol combustion
Sensitivity evaluation of chemical reactions R42,
R43, R44, R45, R46, R47, R48 and R51 involved in
the ethanol consumption was carried out and it is
concluded that the presence of all these reactions
contribute towards 100% combustion of ethanol at
temperature ≥ 400oC. However, the main chemical
reactions, which contribute towards 90% combustion
of ethanol are R42 and R48. Chemical reaction (R42)
individually contribute towards 50% consumption of
ethanol.
C2H5OH=CH3 + CH2OH
…
(R42)
C2H5OH + CH3 = CH3CHOH + CH4
…
(R48)
CH3CO formation
Sensitivity evaluation of chemical reactions R53,
R54, R55, R56, R57, R58 and R59 involved in the
dissociation of CH3CHO into CH3CO show that the
combination of R54 and R55 is mainly responsible for
the formation (about 90%) of CH3CO.
CH3CHO + O2 = CH3CO + HO2
…
(R54)
CH3CHO + H = CH3CO + H2
…
(R55)
CO formation
Almost all the CO formation during ethanol
combustion in the lower temperature range takes
place through chemical reaction CH3CO= CH3 + CO
(R60)
CO2 formation
Sensitivity evaluation of chemical reactions R22,
R23, R24 and R25 involved in the CO2 formation
from CO shows that a combination of R22 and R24 is
VERMA: LOW TEMPERATURE OXIDATION OF ETHANOL
responsible for about 74% conversion of CO into CO2
and rest 25% is being converted with help of R23 and
R25.
421
CO and CH2OH; however, for large residence
times and with the rise in temperature chemical
species CH4 and CH2OH further oxidize into final
equilibrium species like CO, CO2 and H2O
CO + O = CO2
…
(R22)
CO + OH = CO2 + H
…
(R24)
•
Sensitivity evaluation of main chemical reactions
R69 and R71 involved in production of NO pointed
out that the main reaction responsible in NO
formation in the lower temperature range is the
reverse reaction of R69 and contributes alone towards
99.9% formation of NO at T = 1000oC
Sensitivity analysis shows that the ethanol
combustion proceeds in the following manner
with reaction paths mainly responsible given in
the brackets C2H5OH [R42 + R48] → CH3CHO
[R54 + R55] → CO [R60] → CO2 [R22 + R24]
•
In the lower temperature range clean combustion
(i.e., less formation of CO and CO2) of ethanol
can be achieved with leaner-fuel conditions
because the order of different nitrogen oxides
(NO, NO2 and N2O) formation up to a temperature
of 1000oC is less than 10-10 moles/liter.
NO formation
N+NO=N2+O
…
(R69)
From this sensitivity evaluation of various
chemical reactions responsible for the formation and
destruction of different chemical species, the overall
reaction chain responsible for the ethanol combustion
can be laid down as:
Acknowledgement
C2H5OH [C2H5OH=CH3 + CH2OH (R42); C2H5OH +
CH3 = CH3CHOH + CH4 (R48)] → CH3CHO
[CH3CHO + O2 = CH3CO + HO2 (R54); CH3CHO +
H = CH3CO + H2 (R55)] → CO [CH3CO= CH3 + CO
(R60)] → CO2 [CO + O=CO2 (R22); CO + OH=CO2
+ H (R24)]
References
The financial support by the Council of Scientific
and Industrial Research (CSIR), Govt. of India as a
research project no. 22(0317)/EMR-II is highly
acknowledged.
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•
Combustion of ethanol takes
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•
Further rise in temperature leads to a complete
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place
at
a
•
Combustion with higher input values of ethanol in
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•
•
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422
INDIAN J CHEM TECHNOL, VOL 11, MAY 2004
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