1998 Annual Report

Combustion Chemistry of Polycyclic Aromatic Compounds Jack B. Howard, Massachusetts Institute of Technology

Goal

The objective of this study is to enhance predictive capabilities of the model previously developed for polycyclic aromatic hydrocarbons (PAH) formation, for use in source attribution studies and in the development of emission control strategies.

Background

Polycyclic aromatic compounds are major contributors to air pollution from combustion sources. These compounds as well as oxy-PAH and soot particles formed from them are all of health concern. Basic understanding of the factors that govern the detailed chemical composition of the effluents from combustion systems is necessary for the identification of signatures for source attribution and the development of control strategies. The mechanistic and kinetic model developed in this project provides basic understanding of PAH generation in combustion. The model is currently being improved and extended to more complicated flow systems.

Approach

A predictive model of PAH formation in flames is being developed using elementary reactions describing basic flame chemistry, including representative PAH growth. Aerosol dynamics describe species at the molecular weight boundary of PAH and soot and larger. Sectional aerosol equations for soot formation, growth, and oxidation are expressed in a form suitable for concurrent soot aerosol modeling and detailed gas-phase kinetic modeling. The soot model predicts the effects of soot upon the concentrations of gas-phase species, including PAH of interest. A technique has been developed (data incorporation technique, DIT) that uses functions of data to take the place of submechanisms of a kinetic model, and thus the isolation of particular pathways. This is accomplished by transforming the ordinary differential equation (ODE) system describing the mass action kinetics into a differential algebraic equation (DAE) system. The DIT technique is used to test the various portions that make up the overall model. Newly postulated pathways and predictive capability are tested against data obtained previously from the jet-stirred reactor/plug-flow reactor (JSR/PFR) experimental apparatus.

Summary of Progress and Accomplishments

The DIT has been applied to analyze consumption pathways for PAH and used to develop a soot nucleation model that includes PAH-PAH coagulation reactions. Using the data of Marr for the JSR/PFR, the PAH-PAH coagulation pathway is found to be important, and an optimal solution was found for collision efficiencies for PAH radical/PAH stable and PAH radical/PAH radical reactions. The results of the newly formulated model were compared to a standard model that does not include the PAH-PAH coagulation pathway. This model corresponds to the portion of the overall PAH/soot model at the interface of the gas phase chemistry and soot aerosol dynamics, and will be used to improve the overall PAH/soot model.

The progress made with the new nucleation model is the result of a novel automated optimization technique that makes use of the results of thousands of model calculations spanning a three dimensional rate coefficient parameter sub-space. The intersection of two such mappings at extrema of the experimental data constrain the solutions to an optimal set. An analysis in the reduction in kinetics model error with the use of the DIT has been accomplished for a modified version of MarrŐs PAH model. With the incorporation of H2, C2H2, C2H4, and C10H8 experimental data functions for simulations of C2H4/air combustion in the JSR/PFR at an equivalence ratio of 2.2, the uncertainty of rate coefficients based on comparisons to the prediction of overall tar is reduced by over an order of magnitude, based on calculations using sensitivity coefficients. By comparing the curves labeled "Full Model with Data" and "Model with No Data" the effect of including data on the predictions is clearly seen.

Status

The proposal for this research had the goals of: (1) ability to account for PAH-soot interactions and their effects on PAH concentrations; (2) a greater number of 2-6 ring PAH including species containing 5-membered rings; and (3) new types of PAH reactions, including intermolecular rearrangements and aromatics oxidation. The soot nucleation model mentioned above addresses all three in that it (1) improves the ability to describe PAH-soot interactions because of the improvements to the soot model; (2) broadens the description of PAH to include the whole spectrum of molecular weights from 100 amu to 1600 amu (2 rings up to soot nuclei); and (3) allows for the new pathway of PAH coagulation.

Future Plans

This report represents the completion of the project.

Key Personnel

Graduate Student:

David Kronholm



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