1
www.nasa.gov HOMO dimer LUMO dimer E s-t of 20 kcal/mol Estimated binding energy of 33 kcal/mol Towards a Deeper Understanding of Thermochemical Energy Conversion through Flames Enoch Dames [email protected] Department Chemical Engineering Massachusetts Institute of Technology Cambridge, MA, 02139-4307, USA Soot formation: an unresolved problem Quantum chemistry and rate theory for advanced fuels Los Angeles Hong Kong The number of transport vehicles (e.g., cars and planes) is expected to increase by 35% over the next 25 years. 1 And unfortunately, soot’s contribution to global climate forcing effects was recently shown to be significantly greater than previously thought and only second to CO 2 2 : CO 2 – 1.7 W/m 2 Soot - 1.1 W/m 2 Although millions of premature deaths per year are attributed to soot production, the scientific community still doesn’t have a clear picture of how soot is formed. Thus arises the age-old question – how can you change something you don’t understand? 1 http://www.eia.gov/ 2 Bond et al. 2013, J. Geophys. Res. A dual purpose vision: uncovering aspects of soot formation, which can lead to A – new insights for mitigation, B – novel uses and materials fuel + oxidizer CO, H 2 , CO 2 , H 2 O, C 2 H 2 fuel + oxidizer CO, H 2 , CO 2 , H 2 O, C 2 H 2 combustion soot formation Current applications for flame synthesis of nanoparticles: • carbon black for tires • TiO 2 and other metal oxide nanopowders for paint, solar and fuel cells • thin diamond films for semiconductors • optoelectronics applications & luminescent materials • more recently: carbon nanotubes and buckminsterfullerenes , quantum dots, composites, catalysts 0 10 20 30 40 50 60 0 2 4 6 8 M06-2X/6-31+G(d,p) Isodesmic Scwarc 1951 ONIOM Central BDE (kcal/mol) 1 Szwarc, M. Proc. Roy. Soc. (London) A Math. Phys. Sci. 1951, 207, 5. 2 Vreven, T.; Morokuma, K. J. Phys. Chem. A 2002, 106, 6167. Synthesizing chemicals, fuels, and particles The evidence: • H 2 and CO are major combustion products (see second picture to the right) • FTIR, SEM, AFM evidence for aliphatic functional groups on nacent soot (Wang, 2012 Proc. Combust. Inst.) • [Persistent] free radicals shown to exist in soot (radicals are necessary for molecular growth processes and can act as catalytic sites for polymerization of hydrocarbon chains) Goal: uncover thermodynamic driving force(s) for bulk phase carbon growth and surface growth through a combined multi-scale computational-experimental approach 2 2 1 1 2 n m m CO H CH HO n n unique structure-specific electronic properties may lead to better understanding of PAH stacking at high temperature Propulsion and the role of combustible energy sources Tesla Model S battery: 1 MJ/kg Ford Model T gasoline engine: 45 MJ/kg *pictures form Tesla and the Antique Automobile Club of America Like it or not, fossil fuels continue to be relied upon. 1) they’re accessible and cheap 2) they have high energy density 3) less developed nations can’t afford to embark on advanced alternative fuel projects My research plans in the area of propulsion concern the multi-scale modeling and chemistry necessary for advancing air and ground transportation technologies. This work involves identifying key rate limiting steps for a wide range of engine types and conditions, with focus on aeropropulsion and high speed/hypersonic [turbulent] combustion, as well as high-performance and efficient combustion (e.g. HCCI and RCCI engines) Target compounds and fuels are those that can be generated from petroleum/fossil fuel alternatives. (e.g., biofuels, solar fuels, Fischer-Tropsch fuels) Boeing R. Reitz, U Wisconsin The picture to the right serves as a model for a particular bond type possible in soot, across two PAH/graphene sheets. Very weak carbon-carbon bond dissociation energies (BDEs), illustrated below, can lead to persistent free-radicals in soot. This poster details my academic research proposal and interests, previous work, and path leading to the present day. I am a physical chemist and a mechanical engineer. Soot formation and nanoparticle synthesis, catalytic upconversion of syngas, propulsion, the chemistry of alternative fuels and natural gas, are all among my evolving academic foci. For a list of my publications, my teaching and service experience, please refer to my CV (which can be found at http://cheme.scripts.mit.edu/green-group/enoch-dames) B.S., Engineering Science and Mechanics B.A., Chemistry High Temperature Gas Dynamics Lab Biophysics (2002) Tissue Engineering (2004) Maui Space Surveillance Site GE Global Research, Thermal System Lab DOE/Princeton Fellowship 2012 2005 2006 2002 2004 Ph.D., Mechanical Engineering Hypothesis: Fischer Tropsch like kinetics are occurring on the surface of soot: Burner-stabilized premixed flame 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0 500 1000 1500 2000 0 0.5 1 1.5 2 2.5 3 3.5 4 T (K) Distance From Burner (cm) Mole Fraction O 2 CO fuel H 2 O T H 2 CO 2 90 Although soot has been studied for decades, it has only recently been observed that hydrocarbon chains can form on its surface under some flame conditions – can we exploit this process to upconvert syngas or natural gas to valuable chemical feedstocks, or even transportation fuels? Relative Radical Conc. Time Hours Days Months Indefinite adopted from Gehling and Dellinger, 2013, Environ. Sci. Tech. Vander Wal and Tomasek, 2003, Combust. Flame 2003 A dirty snow cone melts faster Flames offer scalable means of producing nanoparticles Looking back at the experiment (Abid et al. 2008), where does this surface growth process occur? liquid-like shells of soot particles Much of my previous and current work has involved developing detailed kinetic models for H 2 /CO, natural gas, alkanes, aromatics, and oxygenated fuels. Quantum chemistry and advanced rate theory are utilized to predict phenomenological rates as a function of temperature and pressure. An array of methods are used to develop and validate detailed kinetic models. Generalized rate rules for oxygenated fuel combustion are also currently being developed. 10 5 10 6 10 7 10 8 10 9 10 10 10 11 10 12 0.80 1.00 1.20 1.40 1.60 1.80 2.00 2.20 1000 K / T rate, s -1 6pp 6pp 6pt 6ps Dashed lines: H-shifts, nomenclature: 6ps = 6-membered ring TS between a primary and secondary radical site Solid lines: -scission to CH 2 O and an alkyl radical : crossover temperature See http://rmg.mit.edu/ for more information on automated mechanism development 0 50 100 150 200 250 300 0 500 1000 1500 2000 2500 3000 [OH], ppm Time, microseconds OH 1542 K 1.42 atm 1486 K 1.52 atm 1377 K 1.54 atm 1343 K 1.61 atm 0 20 40 60 80 0 50 100 150 200 250 0 50 100 150 200 250 300 0 500 1000 1500 2000 2500 3000 Time, microseconds OH 0 20 40 60 80 0 50 100 150 200 250 1542 K 1.42 atm 1486 K 1.52 atm 1377 K 1.54 atm 1343 K 1.61 atm Literature model with data Improved model with data Dames et al., 2013, Combust. Flame (data have uncertainty bars of 10 %) Utilizing the synergy between experiments and theory for high-fidelity model development: shock tube modeling of 3-pentanone oxidation Defining reaction classes and making rate rules applicable to a wide range of compounds improves the performance of computer-generated mechanisms. In the process, I’m also discovering new chemistry not currently considered in models, which will play an important role in understanding the combustion characteristics of future oxygenated alternative fuels. 400 ppm 3- pentanone with 2800 ppm O 2 in Ar ( = 1.0). High fidelity detailed kinetic models can be developed in concert with high-quality experimental data. In this case, 3-pentanone – a potential alternative fuel surrogate – was studied behind reflected shock waves. More post-shock concentration profiles were measured for this species than have ever been obtained for a single compound of this size. As a result, the 46 species profiles of OH, H2O, CO, C 2 H 4 , CH 3 , and 3-pentanone were used to greatly improve the ‘foundational fuel chemistry’ of a previous model, as illustrated by comparing the plots to the left. 10 5 10 6 10 7 10 8 10 9 10 10 10 11 10 12 0.80 1.00 1.20 1.40 1.60 1.80 2.00 2.20 1000 K / T rate, s -1 10 2 10 3 10 4 10 5 10 6 10 7 0.01 0.1 1 10 100 1000 k , s -1 P, atm Hippler et al. 2001 1 808 K 748 K 713 K 678 K uncertainty bands Oguchi et al. 2000 2 740 K 710 K 684 K simulation results From: Dames and Golden, JPCA, 2013 1 Hippler, Striebel, Viskolcz, PCCP 2001 2 Oguchi, Miyoshi, Koshi, Matsui, Bull. Chem. Soc. Jpn. 2000 i d AE dt rate of collisional production of A at energy level j collisional rate loss of A at energy level i - rate loss of A i due to reaction - † 0 exp tot B B tot Q kT E k T kT h Q † † , , r in r in W E Q kE Q h E Master Equation Analysis and RRKM theory I’d like to develop models that take into account enhanced binding of species due to electronic excited states accessible at high T, especially for those with low singlet – triplet gaps, like this one below: CH 3 O decomposition 1, 2 2, 2 3, 1 2, 3 3, 3 breaking bond k calculated with energies obtained at the M08SO/MG3S level of theory and benchmarked against high level coupled cluster theory 3 3 Energy density comparison With up to 60% thermal efficiencies, reactivity controlled compression ignition engines rely on two different fuels – understanding their autoignition characteristics is therefore critical this new technology. Methoxy/hydroxymethyl potential energy surface
