Surrogate Fuels for Transportation Fuels

Transcription

1 Surrogate Fuels for Transportation Fuels Charles Westbrook Lawrence Livermore National Laboratory December 5, 2007 SEDP Meeting Washington, DC

2 The fuel situation in 1922 looks pretty familiar Thomas Midgley, Chief of Fuels Section for General Motors, 1922 US Geological Survey years left of petroleum reserves Production of 5 billion gallons of fuel in 1921 Potential new sources of petroleum il shale ils from coal Alcohol fuels from biomass igher efficiency a high priority for conservation reasons People will not buy a car lacking in acceleration and hill climbing Solution is higher compression ratio, then at about 4.25 : 1 bstacle is engine knock, whose origin is unknown esult was development of TEL as antiknock Phenomenological picture with no fundamental understanding 2

3 Explanation of engine knock, N, antiknocks, diesel ignition, and CCI ignition came in the 1990 s from DE/BES theoretical chemistry and supercomputing and EEE knock working group eactant Transition state C C Alkylperoxy radical isomerization rates are different in paraffin and cyclic paraffin hydrocarbons Low temperature chain branching paths Most work has been done for alkane fuels, and many questions remain for aromatics, cyclic paraffins, large olefins 3

4 Chemistry of alkylperoxy radical isomerization has reached street-level awareness eat release rates in CCI combustion of two fuels, iso-octane with no low T heat release, and PF-80 with two stage heat release We are seeing researchers debating which makes the better CCI fuel. Both debaters have completely accepted the existence and source of the low T reactivity Low temperature heat release PF80 iso- ctane This is serious, black-belt fuel chemistry and computational chemistry Crank Angle esults from experiments of Sjöberg and Dec, SNL

5 ecent revolution in understanding of diesel engine combustion processes As recently as 1990, the entire basis of diesel combustion was poorly understood Fuel evaporating from a Droplets being shed from a liquid jet liquid core and then burning 5

6 eavy-duty Diesel Engine esearch Approach: Investigate the processes in the cylinder of an operating diesel engine using advanced optical diagnostics Modified heavy-duty truck engine provides good optical access while maintaining the basic combustion characteristics of a production engine. Data from multiple advanced laser diagnostics have substantially improved our understanding of diesel combustion and emissions formation. 6

7 New conceptual picture developed Extended role of multiple advanced laser diagnostic techniques developed under BES program, used by EEE program Team led by John Dec, SNL Explains 2 stages in diesel burning ignition and cetane sooting logic This is serious, black-belt optical physics science Lots still unknown, soot chemistry, spray dynamics fuel effects, etc. 7

8 omogeneous Charge Compression Ignition (CCI) engine delivers high efficiency, and low particulate and Nx emissions: Advantages: Technical challenges: low N x low particulate matter high efficiency engine control multi-cylinder balancing startability low power output high C and C emissions 8

9 ave we have come a long way since 1922? We still are looking to oil shale, oil sands and biomass for the future owever, our understanding of knocking, antiknocks and low T chemistry has grown enormously e.g., Current engine designers debate how much low T heat release is best, and take its sources for granted Conceptual model for diesel combustion has led to breakthroughs e.g., Understanding of anti-sooting action of oxygenates Entirely new concept engine (CCI) is being developed Great majority of this progress is due to basic science understanding, e.g., optical diagnostics, quantum chemistry and electronic structure theory, high performance computing, etc. We have used basic science advances to make big jumps in understanding, but we are back to trial and error in many cases 9

10 ctane numbers of heptanes are due exclusively to their different molecular structures This was recognized in 1920 s but no explanation in fundamental terms had been provided prior to our work 10

11 Engine knock is an undesirable thermal ignition 11

12 Kinetic features of engine knock istory of octane numbers and empirical observations End gas self-ignition prior to flame arrival Actual ignition driven by 2 2 decomposition at ~ 900K Kinetic influence of molecular size and structure Effects of additives, both promoters and inhibitors educed models must retain 2 2 decomposition reaction to describe ignition Issue of real SI engine fuel being complex mixture of components Lots of kinetics research still needed (aromatics, cyclics, etc.) 12

13 US Auto/il program was an essential advance in emissions research Joint project between auto manufacturers and oil companies in 1980 s and 1990 s Engine emissions shown to be a combination of unburned fuel and products of incomplete combustion Both depend on the specific molecular structure of the fuel molecules Therefore, prediction of emissions requires a detailed knowledge of fuel composition Single-component representations of practical fuels are therefore completely inadequate, and a thorough picture of the fuel components is essential 13

14 Most transportation fuels consist of complex mixtures of many chemical species Natural gas Gasoline Diesel Jet fuel ocket fuel These fuels contain too many components for detailed mechanisms Gasoline, diesel and jet fuel have hundreds of components (even natural gas) 14

15 Classes of compounds in practical fuels 15

16 Gasoline has many branched alkanes Jet fuel has the highest n-alkane Gasoline is lower in cycloalkanes 16

17 Diesel fuel and a surrogate Diesel fuel is made up of straight-chain alkanes, branched-chain alkanes,cyclic alkanes, simple aromatics, alkylated aromatics, polycyclic aromatics and others Example test: Surrogate diesel: n-heptane: cetane no. of 56 branched chain component: iso-octane cyclic alkane component: cyclohexane or methyl cyclohexane aromatic component: toluene Most common surrogate is 100% n-heptane 17

18 Fuel Surrogate Palette for Diesel Surrogate Fuel Component Selection tetralin n-dodecane n-tridecane n-tetradecane n-pentadecane n-hexadecane n-decyl-benzene alpha-methyl-naphthalene hepta-methyl-nonane n-alkane branched alkane cycloalkanes aromatics others butylcyclohexane decalin 18

19 We have greatly extended the components in the palette that can be modeled in the high molecular weight range: Surrogate Fuel Component Selection n-octane (n-c818) n-nonane (n-c920) n-decane (n-c1022) n-undecane (n-c1124) n-dodecane (n-c1226) n-tridecane (n-c1328) n-tetradecane (n-c1430) n-pentadecane (n-c1532) n-hexadecane (n-c1634) 19

20 Fuel components that have higher molecular weights are needed Surrogate Fuel Component Selection Diesel fuel has mostly C14 to C24 components centered around C16 eal Diesel Amount (SAE Presentation) Molecular Weight C10 C16 C24 20

21 To span the cetane number scale, easily ignitable components (e.g. n-hexadecane) and less-ignitable components (aromatics, iso-alkanes) are needed Surrogate Fuel Formulation ecommended components from Diesel Surrogate Fuel Working Group (SAE ): n-hexadecane heptamethylnonane n-decylbenzene 1-methylnapthalene 21

22 Chemical kinetic mechanism for nc8-nc16 surrogate components: Detailed Chemical Kinetics for Components 2116 species 8130 reactions Low and high temperature chemistry => can use to investigate low temperature combustion strategies Same reaction rate rules as highly validated n- heptane mechanism Tailor the mechanism to fit specific fuels for computational efficiency 22

23 Detailed Chemical Kinetics for Components Includes high and low temperature ignition chemistry: Important for predicting low temperature combustion regimes 23

24 Family of ignition simulations a valuable analysis tool n-decane, φ = 1.0, 13 bar pressure Same approach used by Petersen et al. for propane ignition analyses 24

25 EP ignition 100 ignition dela EP Umich /T A new diagnostic technique for analysis of ignition kinetics 25

26 EP ignition 100 ignition dela EP Umich model /T We are familiar with model/experiment comparisons 26

27 EP ignition 100 ignition dela EP Umich model galway /T Combining multiple sets of experimental results can provide additional mechanism validation 27

28 Surrogate fuels past use of n-heptane surrogate for diesel many similarities between all large n-alkanes n-decane surrogate for kerosene (Dagaut) n-hexadecane surrogate for biodiesel n-decane and methyl decanoate similarities role of methyl ester group potential of n-cetane + methyl decanoate or smaller methyl ester for biodiesel surrogate 28

29 2,2,4,4,6,8,8-eptamethyl nonane mechanism Primary reference fuel for diesels ighly branched C 16 molecule Same reaction rate rules as iso-octane and n-heptane Low T kinetics only for MN and not for its immediate products Mechanism complete, in validation test phase 29

30 Cycloalkanes: methyl cyclohexane Cycloalkanes are interesting due to oil sands methylcyclohexane 30

31 il-sand derived fuels have focused attention on cyclo-alkanes Slide courtesy Phil Smith, University of Utah 31

32 trillion barrels of bitumen in place in the oil sands of Alberta, Canada More oil than the known reserves of the Middle East Composition of il Sands Graphic: pp. 194, Athabasca il Sands The Karl A. Clark Volume 32

33 Asphaltene molecule typical of oil sands 33

34 Cyclic ring structure changes the energy environment of important reactions and requires new kinetic descriptions of ignition eactant Transition state C C Alkylperoxy radical isomerization rates are different in paraffin and cyclic paraffin hydrocarbons Low temperature chain branching paths Most work has been done for alkane fuels, and many questions remain for aromatics, cyclic paraffins, large olefins 34

35 Biodiesel fuels Biodiesel fuels produced from various oleaginous plants US: soybean / Europe: rapeseed triglyceride + 3 C 3 3 C 3 + methanol methyl ester glycerol ( = hydrocarbon chain) 35

36 Composition of Biodiesels methyl palmitate % Soybean apeseed C16:0 C18:0 C18:1 C18:2 C18:3 methyl stearate methyl oleate methyl linoleate methyl linolenate 36

37 Comparison with n-decane Ignition Delay Times 1.E+02 P = 12 atm Equivalence ratio: 1, in air Ignition delay times very close P = 50 atm 1.E+01 Ignition Delay Time (ms) 1.E+00 1.E-01 n-alkanes: cannot reproduce the early formation of C 2 but Symbols: n-decane experiments (Pfahl et al.) Line: methyl decanoate mechanism 1.E /T (K -1 ) reproduce the reactivity of methyl esters very well 37

38 Summary There is a need for kinetic modeling capabilities for practical fuels Surrogates offer a way to include real fuel effects There is a continuing need to increase the level of detail in surrogate fuel mixtures There is a corresponding need to provide kinetic models of fuel components, with steadily growing molecular structure complexity 38