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Molecular weight growth pathways in fuel-rich combustion

By 1967- David Franklin Kronholm

Abstract

Polycyclic aromatic hydrocarbons (P AH) and soot are formed when a hydrocarbon fuel is oxidized under fuel-rich conditions. The distinction between what constitutes the largest P AH molecule and the smallest soot particle is arbitrary; the formation processes of both can be placed under the heading of molecular weight growth. Evidence exists for the carcinogenicity of many P AH molecules. Soot is used as a component of dyes and as an additive to rubbers as well as being an undesirable atmospheric pollutant. Both are emitted from many typical combustion processes such as diesel engines, wood fires, furnaces, etc. Though the area has received much attention, the fundamental chemical mechanisms for formation of both P AH and soot are still uncertain. Much debate has centered on the identity of the soot surface growth reactant, in particular whether the dominant surface growth reactant is P AH or acetylene. Though several models of soot formation exist, none has demonstrated through comparison to experimental data a thorough knowledge of the fundamental chemical processes of soot formation. The goal of this research was to further the understanding of these fundamental chemical processes. Since the chemistry of P AH and soot are intertwined, PAH was a necessary subcomponent of the soot formation research. The research was accomplished by obtaining soot particle size distribution data for the jet-stirred reactor/ plug-flow reactor (JSR/PFR), development of kinetics modeling methods, and the development of a kinetics model of soot formation. The JSR/PFR has been used extensively in the past to investigate P AH and soot formation, providing much data for concentrations of light-gas species, P AH, and soot under various conditions of equivalence ratio, temperature, and PFR additives. No experimental data have been obtained for soot particle size distribution in the JSR/PFR, so a study was undertaken here to obtain the soot particle size distributions for two conditions previously studied by Marr, premixed atmospheric ethylene combustion at equivalence ratio 2.2 and temperatures of 1520 K and 1620 K. Thermophoretic sampling was used to obtain soot samples for transmission electron micrograph analysis. Software was written and used to obtain soot particle sizes from electron micrographs. The chemical environment in a fuel-rich flame consists of many hundreds of species and thousands of chemical reactions. To isolate particular portions of the chemistry, a calculational technique was developed, data incorporation, that replaces chosen portions of the chemistry in kinetics models with functions of data concentrations. This technique was then used to isolate the process of P AH molecular weight growth and soot nucleation through the use of a discrete sectional model, and rate coefficients for hydrogen-atom abstraction, acetylene-addition, and PAH radical addition to PAH were obtained by comparisons to data from Marr for the 1620 K condition described above and the same condition with naphthalene injection into the PFR. The data incorporation technique was then used to expand the discrete sectional model to include sections describing soot, and the experimental soot size distribution data described above was used with previously available PFR data to obtain values for rate coefficients of PAHaddition to soot and coagulation of soot particles. Five PFR conditions were used to develop the soot formation model in these calculations, and the dominant mechanisms of soot formation present under these conditions appear to be present in the model. Quantitative agreement is obtained to all of the available data, including simultaneous agreement of soot mass and particle size, without significant deviation in the rate coefficients required to obtain agreement. Calculations were performed using both PAH and acetylene as the dominant soot surface growth reactant. It was found that P AH had far more consistent rate coefficient values (constant to within a factor of 4) than acetylene ( constant to within a factor of 59) to describe the data for all of the conditions. An analysis of the above five sets of conditions in the PFR, an additional three for the PFR, and three for premixed one-dimensional flames of acetylene, ethylene, and benzene, for which concentrations of acetylene, P AH. and soot, and in the case of the one-dimensional flames, soot particle size data, were available, were analyzed with the aim of understanding the dominant characteristics of the soot surface growth reactant. Soot mass growth rates were calculated for all of the conditions, and deviate markedly between the PFR and one-dimensional flames. Soot growth rate increases and oscillates in the PFR and sharply declines in the one-dimensional flames in the region of soot growth after initial particle inception. Under all of these conditions, PAH show the characteristics required of the dominant surface growth reactant: increases and oscillations in the PFR and sharp declines in the one-dimensional flames. For acetylene to be the dominant surface growth reactant, anomalous behavior of acetylene-suot reactivity would be required that cannot be explained by soot aging or radical intermediates. This leads to the observation that the long-held notion of declining soot reactivity in premixed one-dimensional flames similar to the ones studied here is a result of variations in the PAH intermediates and not a real phenomenon in the region after soot particle inception. An approximate method of uncertainty analysis of kinetics models was used to place an uncertainty bound of a factor of 3 on the rate coefficient parameters calculated. The approximate method was compared to more precise techniques and used to show that the uncertainty of concentration predictions with PAH kinetics models is of very large magnitude. The approximate uncertainty analysis technique was also used to show that the data incorporation technique reduces the uncertainty in calculated rate parameters by over two orders of magnitude. A kinetics model reduction algorithm was developed and implemented to reduce a PAH kinetics model fro.n 722 reactions and 187 species to 93 reactions and 52 species, maintaining naphthalene conc1;;ntration to within 9% of the original model. This technique was also used by Dinaro to redm:e a benzene oxidation model from 545 to 41 reactions for use in super-critical water oxidation applications.by David Franklin Kronholm.Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2000.Includes bibliographical references

Topics: Chemical Engineering.
Publisher: Massachusetts Institute of Technology
Year: 2000
OAI identifier: oai:dspace.mit.edu:1721.1/8996
Provided by: DSpace@MIT
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