C2H5OH + (M) â C2H4+H2O+(M). Following the recent suggestions of Saxena and Williams [1], both these reactions are assumed as. Troe type fall off acts.
Kinetic Modelling of Ethanol Effect on Gasoline Combustion Properties A. Frassoldati, A. Cuoci, T. Faravelli, E. Ranzi Dipartimento di Chimica Materiali e Ingegneria Chimica, Politecnico di Milano, Italy 1. Introduction The recent dramatic increase of the crude oil barrel cost and the environmental impact of fossil fuels on the global warming is pushing toward alternative energy sources. Transportation gives a significant contribution to the greenhouse effect. New strategies address the attention to the renewable fuels, which can answer to both the demands: reduction of the oil dependency and control of CO2. In this context ethanol is nowadays playing an important role. Ethanol comes from the fermentation of sugars contained in many biomasses and shows quite good properties to be used in conventional internal combustion engines: it is an octane number enhancer and stands as a candidate for conventional transportation fuel replacement. Moreover, ethanol is an oxygen carrier, and oxygen content in the automotive fuels allow a reduction of the emission and in particular of carbon monoxide and soot. On the other side, car companies have been working on increasing the engine efficiencies and reducing emissions. Traditional Spark Ignition (SI) and Compression Ignition (CI) engines are continuously improved in order to reduce fuel consumption without lessening the performance. Considerable effort has also been invested to develop new operating paradigms such as HCCI (Homogeneous Charge Compression Ignition) engines that offer the prospect of combining the best features of both SI and CI. This combustion strategy is characterized by high dilutions to reduce the temperature peaks and to limit NOx formation and by premixed, gas phase combustion to avoid soot production. Combustion phasing is entirely controlled by chemical kinetics. Unfortunately HCCI is best suited to steady-state, mid-range conditions and hybrid systems which have the capability of switching between HCCI combustion and either SI or CI are needed for other conditions. Due to the kinetic control of HCCI, developing the complex control system necessary to maintain stable and flexible combustion requires understanding the in-cylinder chemistry. With the increasing importance of simulation to engine design, the need of detailed kinetic mechanisms of hydrocarbon oxidation has also increased. In this work the chemical behavior of ethanol and ethanol Primary Reference Fuels (PRF), i.e. nheptane and iso-octane, mixtures is analyzed. The mechanism is then included in a quasi 1D engine simulator to investigate the operability regions of a HCCI engine.
2. Kinetic mechanism Primary reactions of ethanol involve the radical initiation to form methyl and hydroxyl-methyl radicals C2H5OH + (M) → CH3+CH2OH+(M) And the molecular decomposition to ethylene and water C2H5OH + (M) → C2H4+H2O+(M) Following the recent suggestions of Saxena and Williams [1], both these reactions are assumed as Troe type fall off acts. This molecular reaction prevails in almost all the field of interest (below 2000K), whilst the overall radical reaction paths remain the dominant ones. The initiation reaction to form OH+C2H5 is neglected because of its higher activation energy.
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- H2 O
C2H4
C2H5OH
CH2OH + CH3
CH3CHOH
[CH3CH2O]
- OH CH2CH2OH + O2
+ O2 - HO2
HOC2H4OO
-H
-H
- CH3
CH3CHO
CH2O
Figure 1. Primary Reaction diagram of ethanol oxidation
0.01
0.005 O2
0.008
0.004
0.006
0.003
Mole Fraction
CO
H2 O
0.004 0.002
0.002 0.001
C2H5OH
0 0.0008
CO2
0 0.0006
0.0006
0.0004
0.0004
CH3CHO
C2H4
0.0002
0.0002 0
C2H6 0
CH4
CH2O
0
0 0.4 0.8 1.2 Time (s) Figure 2. Time evolution of major and minor species during VPFR oxidation of stoichiometric C2H5OH/O2/N2 mixture (initial molar fuel concentration 0.3%). Experimental data from [2]: -closed black symbols and black lines: P=12 atm; T = 800 K - open red symbols and red lines: P=3 atm; T = 950 K 0.4
0.8
1.2
H-abstractions produce three isomer radicals (CH3CHOH, CH2CH2OH and CH3CH2O). The pathway toward the first radical is the most favorable. In order to reduce the overall complexity of the kinetic scheme and to save computing time, in the present mechanism the ethoxy is considered to be instantaneously decomposed to both acetaldehyde and H and to formaldehyde and CH3. CH3CHOH forms acetaldehyde either depleting a hydrogen radical or interacting with oxygen. CH2CH2OH can β-decompose to C2H4 and OH or can react with oxygen forming the radical HOC2H4OO, which in turns forms the ethylene oxide and OH or formaldehyde and OH. Figure 1 briefly summarizes these pathways. The mechanism was validated in comparisons with different experimental data. Comparisons with measures in flow reactors, shock tubes and flames at different pressures and mixtures are presented. Figure 2 shows the prediction of the temperature and pressure effect on a stoichiometric C2H5OH/O2/N2 mixture, with an initial ethanol mole fraction of 0.003. The experiments [2] were
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carried out in the adiabatic Variable Pressure Flow Reactor (VPFR) of Princeton. The agreement is good for most of the species, even though some discrepancies are present, like the underestimation of ethylene. At lower temperature CO2 is significantly underpredicted. Anyway, the experimental deviation of the overall carbon balance is of the same order of magnitude. What is more important is the capability of the model to catch the differences between the different operating conditions. Increasing the temperature and simultaneously decreasing the pressure, the system becomes more reactive, with larger formation of CH4, C2H4 and C2H6, whilst the peaks of the oxygenated compound do not change significantly. The model was also tested in comparison with counterflow diffusion flames to verify the capability to reproduce the flame structure, together with the species profiles at higher temperatures (~2000 K). Saxena and Williams [1] recently presented experimental data performed on both partially premixed and pure diffusion systems. This is another excellent test of model predictions. 0.3
0.3
(a)
(c)
0.2
N2/3 O2 H2O C2H5OH CO2 CO H2
Mole fraction
0.1
0 0.02
(b)
C2H2+C2H4 CH4 C2H6×2
0.2
N2/3 O2 H2O C2H5OH CO2 CO H2
0.1
0 0.03
0.02
(d)
C2H2+C2H4 CH4 C2H6×2
0.01 0.01
0
0 12 0 4 8 12 Distance (mm) Figure 3. Species profiles of C2H5OH partially premixed (a and b) and diffusion (c and d) counterflow flames. Symbols: experimental data [1]; lines: predictions 0
4
8
The two flames were carried out with air as oxidizer and at a strain rate of 100 s-1. The non premixed flame was fed in the fuel side with a mixture of ethanol (30% mol) and N2 (70% mol), whilst the fuel inlet composition (mol fraction) of the premixed flame was C2H5OH/O2/N2 = 0.1385/0.1812/0.6803. Figure 3 shows the results of the flame measurements together with the theoretical estimations. The agreement is good not only for the major species, but also for minor products. The effect of premixing is quite evident in the H2O profile, which is wider in the partially premixed flame, because of the larger range of oxidation. CO moves toward the fuel side of the flame and the pyrolysis products (CH4, C2H4 and C2H2) increase, being the temperature in the rich side higher. The diffusion and the variation in the number of moles are responsible of the minimum of the nitrogen profile. The ignition of ethanol were studied by different authors [3,4]. Fig. 4 shows the comparison between measures and predictions of the delay times of C2H5OH/O2/Ar mixtures carried out at relatively low different pressures (1-5 bar). The agreement with the data of Natarajan and Bhaskaran [3] is quite good, with the pressure effect well predicted. The comparison with the measures of Dunphy and Simmie is less satisfactory. Even though the pressure dependence is IV-2, 3
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similar to the previous case, there is a general overestimation of the delay times, especially at the higher pressures. Moreover, the estimated apparent activation energy, i.e. the slope of the curve, is lower. Finally the chemical interaction between ethanol and PRF has been recently investigate by Fikri et al. [5]. The ignition delay time of a stoichiometric mixture nC7H16/iC8H18/C2H5OH 18/62/20 (% liquid volume) was measured in a heated high-pressure shock tube in a temperature range 690 < T < 1200 K at pressures of 10, 30 and 50 bars. This fuel composition mimics an oxygenated gasoline with RON 95.1 and MON 89.5. 10000
Ignition delay time (μs)
1000
P=1 P=1
P=3
1000
100
P = 4.5 P=2 100
10
(b)
(a) 1
10
0.7 0.8 0.9 0.8 1000/T (K-1) Figure 4. Ignition delays of C2H5OH/O2/Ar stoichiometric mixtures (C2H5OH = 2.5%.) at different pressures (atm) Experimental data from : a) [3]. Results for P=2 atm have been reduced by half for clarity b) [4] 0.6
0.7
Ignition delay time (μs)
100000
(a)
p = 30 bar
(b)
p = 30 bar
10000
p = 10 bar p = 50 bar
1000 100 10
0.8
1 1.2 1.4 0.8 1 1.2 1.4 -1 1000/T (K ) 1000/T (K-1) Figure 5. Ignition delays of nC7H16/iC8H18/C2H5OH air stoichiometric mixture at different pressures. (a) Comparison between experimental data [5] (points) and predictions (lines) (b) Computed delay times of PRF (dashed line) compared with the mixture with ethanol (plain line)
Figure 5 shows the comparison between the experimental data and the model predictions. Both the effect of temperature and pressure are correctly reproduced by the model. The octane improving capacity of ethanol is evident: the ON reduces to 77.5, removing the ethanol. The dashed line of figure 5b, which shows the estimated ignition delay times for the pure PRF at 30 bar, highlights this effect. C2H5OH is responsible of the disappearance of the NTC region, favoring in this way the ignition propensity in the intermediate T region (900
