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Energy & Fuels 2006, 20, 1024-1032
A Wide-Range Kinetic Modeling Study of Oxidation and Combustion of Transportation Fuels and Surrogate Mixtures Eliseo Ranzi* CMIC Department. Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy ReceiVed January 19, 2006. ReVised Manuscript ReceiVed March 9, 2006
Liquid fuels, such as gasolines and jet and diesel fuels, are usually refined products from the processing of crude oil. Their composition is mainly based on major physical properties and combustion performance indexes. For these reasons, real transportation fuels contain thousands of compounds that greatly vary with the feedstock origins, the seasons, and the economic factors that are imposed by the refinery. Regardless of this complexity, the chemical species contained in the fuels belongs to only four hydrocarbon classes: linear or branched alkanes, alkenes, cycloalkanes, and aromatics. Moreover, the physical properties (such as vapor pressure and flash point) and the combustion properties (such as octane or cetane numbers and smoke point) are regularly variable with composition. On these bases, it is viable to define surrogate mixtures to reproduce the most important chemical and physical properties of real transportation fuels. These surrogate fuels are then very useful both for the design of more reproducible experimental tests and for the development of reliable kinetic models, which are always projected to a deeper understanding of combustion processes. This paper analyzes some critical features in the definition of surrogates and in the development of detailed kinetic schemes of the pyrolysis and combustion of liquid fuels and also discusses experiments and simulation results obtained under very different conditions. These examples not only relate to ideal reactors (such as plug flow, jet stirred, shock tube, or rapid compression devices), but also concern the knock propensity of hydrocarbon mixtures in internal combustion engines as well as the combustion behavior of liquid fuel droplets and the structure of premixed and diffusion flames.
1. Introduction The combustion of fossil fuels in the transportation sector accounts for a large portion of the carbon dioxide and total greenhouse gas emissions. These emissions from human activities are likely causing changes in the Earth’s atmosphere and have a relevant environmental, economic, and ecological impact. As these effects intensify, the improvements of engine design and the optimal fuel efficiency of vehicles, with respect to the potential to lower greenhouse gas emissions, become a target for the sustainable development and the “zero emission” combustion process. The significance of the kinetic modeling of pyrolysis and combustion of real fuels is emphasized in the regular workshops on the combustion of real transportation fuels.1 Moreover, the future trend on the use of different fuels confirms the continuous interest in conventional liquid fuels, including gasoline, diesel, and aviation fuels, as predicted by the United States Department of Energy (US DOE).2 Future engines will require stringent control of the combustion environment, which is determined by complex interactions of the combustion chamber with the chemical composition and physical and combustion properties of the fuel. The kinetic modeling of fuel combustion is crucial to the development or improvement of key emerging engine technologies (for instance, the homogeneous charge compression ignition (HCCI) engines). * Author to whom correspondence should be addressed. Tel.: 39 02 2399 3250. E-mail:
[email protected]. (1) Hudgens, J. W., Ed. Workshop on Combustion Simulation Databases for Real Transportation Fuels; National Institute for Standards and Technology (NIST): Gaithersburg, MD, September 4-5, 2003. (2) Annual Energy Outlook 2002 with Projections to 2020. Technical Report No. 0383/2002, United States Department of Energy (US DOE/ EIA), Washington, DC, 2002.
There is a general agreement on the need of reliable databases and chemical kinetic models for the coupled applications of chemical kinetics and computational fluid dynamics to simulate combustion processes realistically. Liquid hydrocarbon fuels are complex mixtures of large molecules, containing hundreds of constituents that meet general physical property specifications. These properties, such as volatility, heat of combustion, freeze point and so on, can be met by an infinite variety of hydrocarbon mixtures, even if the relative amount of different species is constrained by the property requirements. For reproducibility reasons and for wellcontrolled fundamental modeling and experimental studies, it is useful to carefully select defined mixtures of reference species with fixed chemical compositions. These mixtures are the surrogates of the different transportation fuels and they must reasonably describe the important characteristics of real fuels.3 The large reference species undergo a sequential reduction in size during combustion; therefore, simulations of real fuels must include and refer to existing chemical kinetic models that detail the chemistry of small species. Together with pyrolysis and oxidation reactions that convert large molecules to smaller molecules and radicals, it is also necessary to include condensation and dealkylation reactions that govern the progressive growth of polycyclic aromatic hydrocarbons (PAHs) and soot. In fact, detailed kinetic models of combustion processes become very useful tools only if they are reliable not only for the evaluation of knocking tendency of the different fuels, but also for the prediction of pollutant formation in terms of PAH, soot, and NOx. (3) Edwards, T.; Maurice, L. Q. J. Propul. Power 2001, 17 (2), 461466.
10.1021/ef060028h CCC: $33.50 © 2006 American Chemical Society Published on Web 04/04/2006
Oxidation and Combustion of Transportation Fuels Table 1. Carbon Number, Boiling Temperature, and Number of Alkane Isomers of Different Petroleum Fractionsa carbon number
boiling temp [°C]
number of paraffin isomers
petroleum fraction
8 10 12 15 20 25 30 35
126 174 216 271 344 402 449 489
1.8 × 101 7.5 × 101 3.55 × 102 4.347 × 103 3.66 × 105 3.67 × 107 4.11 × 109 4.93 × 1011
gasoline and naphthas kerosene jet fuels diesel fuels light gas oil gas oil heavy gas oil atmospheric residue
a
Adapted from ref 4.
This paper briefly reviews the major properties of real transportation fuels and then discusses some relevant features of the kinetic modeling of these systems. This paper finally shows a few validation examples derived from different applications of this detailed kinetic scheme. 2. Properties of Liquid Fuels Liquid fuels are constituted by complex hydrocarbon mixtures derived from the refinery. A clear example of this complexity is given in Table 1, where the boiling temperatures and the number of alkane isomers of different petroleum fractions are reported, versus the carbon number.4 2.1. Gasolines and Naphthas. Virgin or straight-run naphthas are complex mixtures of a large number of different isomers directly obtained from crude distillation in a refinery. These streams are generally characterized by specific gravity and distillation curves (ASTM D86 or TBP curves). Distillation curves provide a measure, in terms of volatility, of the relative proportions of all the different hydrocarbon components of the fuel. Naphthas can constitute 10-25 vol % of the crude oil. Because of its use in automotive engines for industrial and domestic purposes, gasoline is an indispensable energy source and is the primary product of most petroleum refineries. Gasolines are volatile mixtures of practically all the hydrocarbon isomers distilling between ∼30 °C and 220 °C and consist of compounds in the C4-C12 fraction. The ASTM distillation curve defines the temperatures at which 10%, 50%, and 90% of the gasoline is evaporated, as well as the maximum end-point temperature. The distillation curve defines and controls starting, warmup, acceleration, vapor lock, and crankcase dilution. The distillation profile of conventional gasolines changes in summer and winter conditions. In winter conditions, as in cold-weather starting, when only a small portion of the fuel is evaporated, the low-temperature end of the curve is the most important. However, for higher temperatures, such as those in the intake of a hot engine, the shape of the low-temperature end of the curve is much less important, because all of this part of the fuel is evaporated anyway. Oil companies vary the 10% distillation point seasonally to correspond to the weather conditions.5 Gasolines are produced by blending naphthas and different refinery streams to meet the required performance specifications. The composition of the hydrocarbon component of a gasoline can vary widely, depending on the type and nature of the crude processed, the process conditions, the overall balance of demand between gasoline and other refinery products, the season, and the product specifications. Many gasolines also contain blending (4) Altgelt, K. H.; Boduszynski, M. M. Composition and Analysis of HeaVy Petroleum Fractions; Marcel Dekker: New York, 1994. (5) http://www.chevron.com/products/prodserv/fuels/bulletin/motorgas/ 1_drivingperformance.
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components of a nonpetroleum origin, especially oxygenates such as ethers and alcohols, and additives may be used to boost particular performance features. 2.2. Kerosenes and Jet Propellant Fuels. Kerosenes are complex mixtures that contain C9-C16 hydrocarbons, and their composition is dependent on the crude source and the refinery process. The typical distillation range is 140-300 °C; therefore, the boiling points of the carcinogenic 3-7 fused-ring PAHs are well above this range.6 Kerosene-type jet fuels are relatively nonvolatile, with respect to wide-cut jet fuels. The major components of kerosenes are linear and branched alkanes (35%-45%) and cycloalkanes or naphthenes (30%-35%). Oneand two-ring aromatic hydrocarbons normally comprise
