A comparative study of combustion between biofuels and fossil fuels

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1 A comparative study of combustion between biofuels and fossil fuels Antonella Ingenito 1 University of Rome La Sapienza, Rome, Italy, 184 Roberto Andriani 2, Milan Politechnic, Milan, Italy, Antonio Agresta 3 and Fausto Gamma 4 University of Rome La Sapienza, Rome, Italy, 184 Current global energy supplies are dominated by fossil fuels, with much smaller contributions from nuclear power and hydropower. Bioenergy provides about 1% of the total energy supplies, making it by far the most important renewable energy source used; solar, wind and other renewable energy sources have nowadays a very small contribution. On average, in the industrialized countries biomass contributes less than 1% to the total energy supplies, but in developing countries the proportion is as high as 2-3%. Bioenergy could play a bigger role, especially in the industrial countries that consume a lot of fossil energy and are therefore the main contributors to atmospheric pollution and global warming. Therefore, biofuels can contribute to reducing the dependency on fossil fuels and to lowering greenhouse gas emissions due to transport and other activities and can be considered as both renewable and sustainable energy sources. Indeed, these are recognized as net zero CO 2 emission fuels due to the very short-term carbon cycle. Thus the goal of this paper is to investigate the possibility to reduce NOx and CO emissions still maintaining the same conventional fuel performance. In particular, in the following, a comparative study of the bio-fuels ignition delay time, flame temperature ad emissions at different operative conditions with those of conventional fuels, has been performed. Results have actually shown that renewable fuels could play a critical role in pollutants abatement and for the next generation energy sources. I. Introduction IOFUELS are generally composed by a mixture of multiple monoalkyl esters of long-chain fatty acids. Most B biofuels used in the world are made from soy oil and rapeseed oil by transesterification with an alcohol. The soy- and rapeseed-derived biofuels are complex mixtures composed saturated and unsaturated methyl esters (when methanol is used for the transesterification process) and alcohols. Development of detailed chemical kinetic models for biofuels oxidation is still at the beginning due to the very large size of the branched or unsaturated carbon chains that make up biofuels together with the chemical variability of biofuels. However, several recent studies describe kinetic mechanisms for those fuels, assuming a conspicuous number of species and reactions. The oxidation chemistry of Methyl Decanoate is modeled using a reduced mechanism developed by Z.Luo [1, 2, 3], T.F. Lu et al. which contains 837 reactions among 118 species specifically suitable for high temperature oxidation (T > 1K) and it's based on the related detailed Lawrence Livermore Nation Lab (LLNL) mechanism. Also based on the LLNL mechanism, the Bio- detailed chemical kinetic mechanism model [4], contains 431 species and 2336-step process. The chemical kinetic mechanism for JP1 is a reduced 39 species,189-reaction mechanism [5, 6] while the n- Dodecane is modeled through 289 reactions among 56 species [7]. 1 Dr, Dep. of Mechanical and Aerospace Engineering, Via Eudossiana 18, 184, Rome, Italy, AIAA Member. 2 Prof., Department of Energy, Via Lambruschini, 4, 2156, Milan, Italy. 3 Eng., Dep. of Mechanical and Aerospace Engineering, Via Eudossiana 18, 184, Rome, Italy. 4 Prof., Dep. of Mechanical and Aerospace Engineering, Via Eudossiana 18, 184, Rome, Italy. 1

2 Furthermore, to predict NOx formation, the NO kinetics based on the UCSanDiego mechanism, which models 52 reactions among 23 species has been added to any mechanism above mentioned [8]. So far, no consensus is found in literature regarding their performance. For example, many authors report that biofuels increases NOx emissions compared to hydrocarbon fuel [9, 1], but some other authors report a decrease [11, 12]. In this work, a comparison between the different biofuels, i.e. methyl decanoate (C11H22O2) and bio-butanol (C4H9OH), and fossil fuels, i.e., n-dodecane (C12H26) and the JP1(exo-tetrahydrodicyclopentadiene, C1H16) has been done in terms of ignition delay, flame temperature and emissions (carbon monoxide and nitrogen oxide). II. Fuels Characterization In the following, a summary of the selected fuels has been reported. Among the fossil fuels, the n-dodecane aviation fuel and the JP1 aircraft launcher fuel have been analyzed; as for the biofuels, the biobutanol and the methyl decanoate have been selected. The n-dodecane is a liquid alkane hydrocarbon with the chemical formula CH3(CH2)1CH3 (or C12H26), also known as duodecane. It is an oily liquid of the paraffin series. In recent years, n-dodecane has garnered attention as a possible surrogate for kerosene-based fuels such as Jet-A, S-8, and other conventional aviation fuels. It is considered a second-generation fuel surrogate designed to emulate the laminar flame speed, largely supplanting n-decane, primarily due to its higher molecular mass and hydrogen to carbon ratio which better reflect the n-alkane content of jet fuels. The simplifed University of San Diego n-dodecane oxidation model kinetic is composed 56 species, 289 reactions [13]. JP-1 is essentially a single hydrocarbon exo-tetrahydrodicyclopentadiene. It has a minimum volumetric heat content of 39,434 MJ/m3 (141,5 Btu/gal). For comparison, Jet A or JP-8 has a volumetric energy content of about 35, MJ/m3 (125,8 Btu/gal), about 11 percent lower. JP-1 is a fuel developed for demanding applications, such as aircraft-launched missiles. The required properties are: maximum volumetric energy content, clean burning, and good low-temperature performance. To achieve these properties, this fuel is formulated with high-density naphthenes in nearly pure form. These fuels only are used in limited volumes and in situations where price is a minor consideration. The [14] also called biobutanol when produced biologically, i.e., by fermentation of biomass by bacteria (e.g. Clostridium acetobutylicum) or other microorganisms. production from biomass and agricultural byproducts could be more efficient (i.e. unit engine motive power delivered per unit solar energy consumed) than ethanol or methanol production [7]. The biobutanol is a primary alcohol with a 4 carbon structure and the molecular formula of CH3(CH2)3OH (or C4H1O). It belongs to the higher alcohols and branched-chain alcohols. [1] may be used as a fuel in an internal combustion engine and as aviation fuel. has been demonstrated to work in vehicles designed for use with gasoline without modification. The oxidation model kinetic is composed by 431 species and 2336 reactions [15]. The methyl decanoate is a saturated fatty acid. Its formula is CH3(CH2)8COOCH3 (or C11H22O2). The chemical kinetics mechanism is composed of 118 species and 837 reactions [16]. In Tab.1 a summary of the fuels proprierties is reported. Fuel Properties at T = K and P = 1 atm Density ρ [g/cm 3 ] Viscosity μ [mpa s] JP Bio Tab. 1 Densities and Viscosities of the fuels under investigation 2

3 III. Results In this Section a comparison in terms of ignition delay, flame temperature and emissions between JP1, N- Dodecano, Methyl-Decanoate and Bio- has been done at different pressures, temperatures and equivalence ratios. Ignition delays for the fuels in pure air have been reported in Refs. [17, 18, 19]. t_id [s].9.8 JP Fig. 2 t id [s] vs φ: P=1 atm, Tin=12 K JP Fig. 2 t id [s] vs φ: P=5 atm, Tin=12 K t_id [s].9.8 JP Fig. 3 t id [s] vs φ: P=1 atm, Tin=12 K t_id [s] JP /T [1/K] Fig. 4 t id [s] vs 1/T: P=1 atm, φ=.4 tid [s] JP /T_[1/K] Fig. 5 t id [s] vs 1/T: P=1 atm, φ=1 3 Ignition delay time, t id, for the purposes of this discussion is defined as the time for the gas mixture to reach a temperature 4 K higher than initial in a perfect stirred reactor. Lean to reach mixtures of fuel and air are considered for a range of initial temperatures from 1 K to 14 K at three different pressures: 1atm, 5 and 1 atm (note that, at those pressures, no autoignition is foreseen at temperatures below 1 K). Methyl decanoate and n-dodecane show similar autoignition characteristics as function of equivalence ratio at a given temperature of 12K, though there are quantitative differences (see Fig. 1-Fig.3). JP1 shows a very different ignition delay characteristics with respect to all other fuels, showing a minimum t_id of.18 s at very lean mixtures. For lean mixtures (φ<.6 at p=1tm and φ<.8 at p=5 and 1 atm), the longer ignition delay is for the bio-butanol. Increasing the equivalence ratio, the longer ignition delay is shown for JP1 for all three pressures. At the higher pressures and equivalence ratios (i.e. P= 1 atm and φ>1), the biobutanol shows a lower ignition delay than fossil fuels.

4 As for the methyl-decanoate has the shortest delay of all fuels: i.e., at P=1 atm and φ=.4, it is ~ 68% shorter than JP1 and and 122% than ; at φ=2, it is ~ 122% shorter than, 19% than n- Dodecane and 39% than JP1. Therefore, shows very challenging characteristics in terms of ignition delay time, and accordingly in terms of the flame anchoring. Fig. 4 and Fig. 5 show that the methyl decanoate has the lower ignition delay through a range of initial temperatures from 1 to 14 K, at the two different equivalence ratios. It is mainly due to its chemical formula that makes its decomposition faster than that of all others fuels examined in this work. As for the biobutanol, its autoignition characteristic, instead, depends on the equivalence ratio: in fact, at the stoichiometric equivalence ratio, its ignition delay is shorter than that of the others two fossil fuels, instead at φ=.4 it is longer than that of JP1. The n-dodecane has a longer ignition delay with respect to all other fuels, and it does not ignite below 11 K at p=1 atm. In Tab. 2, a summary of ignition delay variation percentage is shown. Fuel =.4 = 1. 1 K 14 K 1 K 14 K -1% -43% -64% -48% Bio- +14% +5% -49% -18% Tab. 2- Percent variation in t_id [s] with respect of JP1 As for the adiabatic flame temperature (see Figg. 6-8), the higher flame temperatures are reached by the fossil fuels, whereas the lowest temperature is for the biobutanol JP Fig. 6 T [K] vs φ: P=1 atm, Tin=12 K JP Fig. 7 T [K] vs φ: P=5 atm, Tin=12 K JP Fig. 8 T [K] vs φ: P=1 atm, Tin=12 K 4 Actually, the difference is not significant, showing a maximum variation of 5 K. This means that in terms of efficiency, bio fuels show very good performance. As for the NOx and CO emissions calculated after a reactor residence time of.1s, Fig. 9- Fig.14 show that all fuels have similar emissions characteristics as a function of equivalence ratio at a given temperature and pressure (T=12 K and p=1 atm). Higher NOx emissions in terms of ppmvd (part per million volume dry) are found for fossil fuels, in particular for n-dodecane, whereas the lowest NOx ppmvd emissions are found for the butanol. A ppmvd variation between n-decane and biobutanol reaches its highest value of ~15% at φ=.8.

5 JP Fig. 9 NOx ppmvd: P=1 atm, Tin=12 K ppm@15%o2_co JP Fig. 1 CO ppmvd: P=1 atm, Tin=12 K ppm@15%o2_nox JP1 ppm@ 15%O2_CO JP Fig. 11 NOx ppmvd: P=5 atm, Tin=12 K Fig. 12 CO ppmvd: P=5 atm, Tin=12 K 6 7 ppm@15%o2_nox JP1 ppm@15%o2_co JP Fig. 13 NOx ppmvd: P=1 atm, Tin=12 K Fig. 14CO ppmvd: P=1 atm, Tin=12 K Due to the very short-term carbon cycle, the biobutanol has also the lower CO emissions: therefore in terms of fuel emissions, butanol seems to be the best choice. Depending on the initial conditions (p, T and φ), the methyl decanoate has a different behavior in terms of CO and NOx emissions. In fact, it has higher CO emissions than n-dodecane at p=1 atm and p=5 atm, the opposite is at p=1 atm and φ<1.6. Note that, because NOx emissions increase even after the flame temperature has been reached, depending on the residence time different ppmvd are predicted (see Fig. 15 and Fig. 16). In fact NOx emissions decrease substantially by decreasing the reactor residence time. Therefore, depending on the combustor geometry and turbulence times, i.e. depending on the Damkäohler non dimensional number (it is the ratio between the convective and chemical flow 5

6 time), a different amount of NOx emissions can be found. In Fig. 17, the NOx emissions history of JP1 and is shown NOx ppmvd 15%O E.R.=.4 E.R.=.72 E.R.=1 E.R.=1.36 E.R.=1.68 E.R.=2 NOx ppmvd 15%O E.R.=.4 E.R.=.72 E.R.=1 E.R.=1.36 E.R.=1.68 E.R.= t [s] t [s] Fig. 15 NOx ppmvd vs t[s] for JP1: P=1 atm, Tin=12 K and its zoom (right) 3 3 NO x ppm vd 15% O E.R.=.4 E.R.=.72 E.R.=1 E.R.=1.36 E.R.=1.68 E.R.=2 NOx ppmvd 15%O E.R.=.4 E.R.=.72 E.R.=1 E.R.=1.36 E.R.=1.68 E.R.= t [s] t [s] Fig. 16NOx ppmvd vs t[s] for : P=1 atm, Tin=12 K and its zoom (right) ppm@15%o2_nox JP ppm@15%o2_co JP Fig. 17 NOx and CO ppmvd vs t[s].4: P=1 atm, Tin=12 K 6

7 Fig. 17 shows that, for the biobutanol fuel, depending on the equivalence ratio and on the residence time, it is possible to decrease the NOx emissions at values lower than 2 ppmvd: actually this is a interesting strategy to be proposed in terms of combustor geometry for the emission reduction to tolerable values. Furthermore, Fig. 16 shows that decreasing the initial temperature, the NOx ppmvd decreases for all fuels: In fact, it is well known that the most relevant NOx sources are due to the thermal NOx process that is highly temperature dependent. Fuel =.4 = 1. 1 K 14 K 1 K 14 K n-dodecane +.45% -1.5% -57.5% -.45% +16.3% -2.6% Bio % -3.13% -7.4% -5.4% Tab.3- Percent variation of NOx emission at 15%O2, with respect of JP1 values A summary of the NOx variation percentage with respect to the JP1 fuel is shown in Tab. 3. It is clearly shown that depending on the initial conditions, biofuels can give good responses both in term of performance than in terms of emissions requirement. IV. Conclusions Ignition delay, flame temperature and NOx and CO formation rates of JP1,, compared with biofuels such as methyl decanoate and bio-butanol are investigated employing the CHEMKIN constant-pressure perfectly stirred reactor. The methyl decanoate is a saturated fatty acid, while the also called biobutanol when produced biologically, is a primary alcohol with a 4 carbon structure. The ignition delay analysis shows that for pressure of 1 atm and 5 atm, methyl decanoate and have similar behaviour though the latter has constantly longer autoignition times. JP1 and bio-butanol show a counter-trend. For lean mixtures ( <.8) butanol always has longer ignition delays while for richer mixture its delay starts decreasing, up until, for P = 1 atm and = 2, it becomes shorter than that of the other two fossil fuels. On the other hand, JP1 has an upward trend where, for >.8 it increases its ignition delay. At all the pressure values investigated, methyl decanoate showed the shortest autoignition delays, reaching its minimum at P = 1 atm and = 2, when it autoignites 39% earlier than JP1. Methyl decanoate, therefore, due to its chemical structure, shows the best characteristics in terms of ignition delay and, hence, flame anchoring. NOx and CO emissions study is conducted considering a reactor time residence of.1s. In this case, the best behaviour is to be assigned to biobutanol, as it shows the lowest emissions in terms of ppmvd for both NOx and CO. This behaviour is consistent with the fact that, concerning with the NOx emissions, butanol also shows the lowest value of adiabatic flame temperature, and, as for the CO emissions, it has a very shortterm carbon cycle. With respect to present day standards, biobutanol showed a trend such that, depending on the equivalence ratio and on the residence time, it is possible to decrease the NOx emissions at values lower than 2 ppmvd. The methyl decanoate has a different behavior in terms of CO and NOx emissions and as a matter of fact, it has higher CO emissions than n-dodecane at p=1 atm and p=5 atm, while the opposite at p=1 atm and φ<1.6. The present work partially shows that the idea of a complete replacement of fossil fuels is a challenging but possible development for future combustor designs. References 1. Z. Luo, T.F. Lu, M.J. Maciaszek, S. Som, and D.E. Longman, A Reduced Mechanism for High Temperature Oxidation of Biodiesel Surrogates, Energy & Fuels, Vol. 24 No. 12 pp , Sarathy, S.M., Pitz W.J., Thomson M.J., and Lu T.F., An Experimental and Kinetic Modeling Study of Methyl Decanoate Combustion, Proc. Combust. Inst., doi:1.116/j.proci , 21 7

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