FINAL REPORT A NIST Kinetic Data Base for PAH Reaction and Soot Particle Inception during Combustion SERDP Project WP-1198 DECEMBER 2007 Wing Tsang, NIST Samuel Manzello, NIST Collen Stroud, NIST Michael Donovan, NIST Valeri Babushok, NIST Kermit Smyth, NIST NIOT Charles Frayne David Lehnert Sean McGivern AFRL Scott Stouffer, UDRI Vishwanath R. Katta, ISSI This document has been approved for public release. This report was prepared under contract to the Department of Defense Strategic Environmental Research and Development Program (SERDP). The publication of this report does not indicate endorsement by the Department of Defense, nor should the contents be construed as reflecting the official policy or position of the Department of Defense. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the Department of Defense. 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Ring Expansion Reactions in the Thermal Decomposition 100 of t-Butyl-1,3-Cyclopentadiene F. NIST Well Stirred Reactor/Plug Flow Reactor Facility 124 (WSR/PFR) G. Soot Particle Size Distributions in the PFR Section 134 H. Gas-Phase Analysis 147 I. Use of Laser Diagnostics to Determine Soot 157 Inception in WSR/PFR J. Publications and Presentations 167 Acknowledgments sdsdsdsds a fdfdfd l Acknowledgements 3 1. Summary and Conclusions This is the final report of Project (SERDP # 1198) (A NIST Kinetic Data Base for PAH Reaction and Soot Particle Inception during Combustion) It results from an experimental and analytical program designed to improve existing chemical kinetic databases used for the simulation of heptane combustion with special emphasis on the formation of PAH and soot. This is a joint work of Building and Fire Research and Chemical Sciences and Technology Laboratory at NIST. The original principal investigators were Linda Blevins, George Mulholland and Wing Tsang. Dr Blevins has since joined the DOE. Dr. Mulholland has retired. Dr. Samuel Manzello has taken their place. The experimental studies involve studies in a heated pulse shock tube and a stirred flow-plug flow reactor especially constructed for the present studies. The former was focussed on determining the mechanisms and rate constants for the unimolecular decomposition and isomerization of all the radicals formed as the initial heptyl radicals degrades to smaller unsaturated structures. Altogether there are 21 radicals undergoing 29 beta bond scissions and 11 reversible isomerizations or a total of 51 reactions. In addition, for completeness the decomposition of heptane and the 1-olefins from 1-hexene to 1-butene are also included. The mechanisms and rate constants for a majority of these reactions are directly determined through single pulse shock tube experiments. Others were generated from existing data. The unique features of the reactions are the relatively large size of the radicals, the low reaction thresholds and the presence of isomerization processes. These raise a number of unique problems not usually described for the simple high reaction threshold reactions found in the literature. These have been solved in the course of this work. A special feature of these high temperature reactions is the contribution of energy transfer effects leading to the presence of pressure dependences. Use have been made of a NIST program for the determination of such effects. They lead to reductions of rate constants by as much as an order of magnitude at temperatures in excess of 1000 K at pressures of 1 bar. These effects are lowered to a factor of 2 to 3 at 100 bars pressure. As a result it is not possible to express results in terms of a single rate expression for a single process. This is an extra complication and the correction factors are tabulated in extensive tables. Also included in the report are the thermodynamic properties of the 21 radicals in JANNAF format. When these processes are combined with the well established bimolecular processes involving radical attack on the stable compounds it will be possible to describe completely the pyrolytoc cracking of heptane The integration of the database for cracking into that for heptane combustion is intended to extend the range of the database to cover increasingly rich systems. The reader is referred to http://kinetics.nist.gov/CKMech/ for a listing reactions in the database. It also contains listing of other databases and is extremely useful for making comparisons. .This is of course of vital importance for the description of PAH and soot formation, since obviously such phenomenan can only be formed as the combustion mixture becomes richer. We have thus intergrated 51 unimolecular decomposition and 4 isomerization reactions and 24 species processes into a heptane combustion data base consisting of 347 species and 1745 reactions. This is a small fraction of the total database needed to fit the whole range of conditions for heptane combustion. Nevertheless they represent the only set of reactions invovling a fuel such as heptane where the rate constants are directly measured as opposed to the standard methodology of deriving them from estimates established under very different conditions or used to fit global results. Since existing heptane databases fit certain sets of experimental data on heptane combustion it is important that the inclusion of extra reactions do not significantly degrade the existing fits. Fortunately, the pyrolytic reactions do not appear to have large affects on the the original fits. This is partly due to the pyrolytic reactions being at conditions and temperatures that are different than the combustion processes. Nevertheless some changes in the rate constants in the existing databases have been made to marginally improve the fits. In additon we have also captured in the new database the reactions leading to PAH formation from small unsaturated compounds. Thus this database permits the simulation of combustion phenomenon across the entire range of stoichiomtries. However, in the course of work it became clear that there are severe problems in the established heptane combustion databases. Some have been put together many years ago and therefore do not reflect the latest understanding of the nature of the reactions in the database. In order to match global results adjustments have ben made in other rate constants, thus compounding the error . The large size of these databases is another factor in making it extremely difficult to systematically bring these databases up to modern standards. Nevertheless there is probably no alternative. We illustrate the situation with the reaction of a fuel radical with oxygen. The consequence is a fuel peroxyl radical that is vibrationally excited. This can then undergo a whole series of isomerizaion processes leading ultimately to the production of OH. All of these are expressed as thermal rate constants in current databases. This cannot be the case for high temperature systems. Thus the postulated rate constants are probably wrong and the pressure dependence is ignored. The above deal with known reactions. In the case of the formation of aromatics it is suspected that there are pathways that has not been enumerated. Thus in present study, in an attempt to study the reactions of cyclopentadienyl radical a pathway for formation of benzene has been discovered. Probably the main problem in studying the formation of larger PAHs and soot is the unavailability of samples of precursors to possible intermediates in the reaction chain. Thus it is not possible to possible to test mechanisms in the traditional fashion used by chemical kineticists. In order to fill in the gaps in mechanisms in the transformation of small unsaturates to PAH and soot a stirred-flow plug-flow apparatus has been constructed at NIST. The entire system has the advantage of well defined boundary conditions and removes some of the spacial ambiguities present in direct studies with flames. The facility is equiped with a nano differential mobility analyzer to determine particle size, gas samples can be extracted and analyzed with gc-ms.