Grantee Research Project Results

1997 Progress Report: Modeling Gas-Phase Chemistry and Heterogeneous Reaction of Polycyclic Aromatic Compounds

EPA Grant Number: R824970C007
Subproject: this is subproject number 007 , established and managed by the Center Director under grant R824970
(EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
Center: HSRC (1989) - Northeast HSRC
Center Director: Sidhu, Sukh S.
Title: Modeling Gas-Phase Chemistry and Heterogeneous Reaction of Polycyclic Aromatic Compounds
Investigators: Howard, Jack B.
Institution: Massachusetts Institute of Technology
EPA Project Officer: Hahn, Intaek
Project Period: January 1, 1992 through June 30, 1998
Project Period Covered by this Report: January 1, 1996 through June 30, 1997
Project Amount: Refer to main center abstract for funding details.
RFA: Center on Airborne Organics (1993) Recipients Lists
Research Category: Targeted Research

Objective:

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. 

Rationale:  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 to describe the basic flame chemistry and PAH growth up to a mass of 400 amu, and aerosol dynamics to describe all species (both PAH and soot) with masses above 400 amu.  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 using ABACUSS, a differential algebraic equation solver, has been developed to isolate sub-mechanisms of the model by incorporating  experimental data.  This technique is used to test the various inputs that make up the over all model.  The interface between the gas-phase kinetics and aerosol dynamics (the soot nucleation step) is a sub-mechanism of the current model to be further developed with the above technique.  The role of PAH as soot growth species is another sub-mechanism addressed by the ABACUSS technique and the application of data sets from one-dimensional flames.  The predictive capability of the model is tested using data obtained previously from the jet-stirred reactor/plug-flow reactor (JSR/PFR) experimental apparatus. 

Status:  Through the use of sensitivity analysis, the PAH/soot model has been drastically reduced in size.  The model now consists of 481 reactions and 107 species and gives essentially identical results to the previous model with 3674 reactions and 174 species.  The reduction in size, combined with the use of a dedicated workstation, have reduced computation times by over an order of magnitude, and the practical simulation of one-dimensional flames is now possible.  Extensive simulations of JSR/PFR system were performed.  The model was used to assess a hypothesized mechanism that rationalizes the data of Marr for the case of naphthalene injection into the PFR.  The data show that naphthalene injected into the PFR quickly drops to concentrations which are, surprisingly, below the baseline case of no injected naphthalene.  The model qualitatively predicts this counterintuitive behavior when soot is treated as a sink to which the PAH add directly, consistent with the PAH being a growth species for soot.  At the same time, the PAH concentrations are affected by the presence of soot. 

A software interface has been written to use CHEMKIN interpreter output to assemble the differential equations describing the model-simulated combustion in the PFR in a format compatible with ABACUSS.  This allows the versatility of using several features of ABACUSS to further develop and test the model.  Foremost is the ability to incorporate experimental concentration profiles into the calculation of the model equations, thereby eliminating unknowns and isolating the error that may be introduced by other portions of the model into a particular sub-mechanism.  ABACUSS also provides the ability to have time dependent rate constants and optimization of model parameters. 

DEMMUCOM, an error analysis software tool, has been used to demonstrate the ability to analyze the error of model concentration predictions based on uncertain rate constants in the PAH/soot model.  It has been found that the error in concentration predictions typically is attributable to the error of ~5 reaction rate constants, with inclusion of the error of more rate constants not changing the error of the output significantly. 

Future Plans:  Future plans include improving the PAH formation model as well as the ability to account for PAH-soot interactions and their effect on PAH concentrations.  In addition, a greater number of 2-10 ring PAH including species containing 5-membered ringes will be included in the model, and new types of PAH reactions will be added to the model. 

Key Personnel 

Graduate Student:  David Kronholm

Supplemental Keywords:

RFA, Scientific Discipline, Air, Waste, particulate matter, Environmental Chemistry, Incineration/Combustion, Atmospheric Sciences, aerosols, hydrocarbons, aerosol dynamics, ambient aerosol, emission control strategies, source attribution studies, atmospheric transport, emission controls, kinetc models, combustion, soot profiles, gas-phase transformation, air pollution, ambient pollution control, PAH

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Final

Main Center Abstract and Reports:

R824970    HSRC (1989) - Northeast HSRC

Subprojects under this Center: (EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
R824970C001 Chemical Kinetic Modeling of Formation of Products of Incomplete Combustion from Spark-ignition Engines
R824970C002 Combustion Chamber Deposit Effects on Engine Hydrocarbon Emissions
R824970C003 Atmospheric Transformation of Volatile Organic Compounds: Gas-Phase Photooxidation and Gas-to-Particle Conversion
R824970C004 Mathematical Models of the Transport and Fate of Airborne Organics
R824970C005 Elementary Reaction Mechanism and Pathways for Atmospheric Reactions of Aromatics - Benzene and Toluene
R824970C006 Simultaneous Removal of Soot and NOx from the Exhaust of Diesel Powered Vehicles
R824970C007 Modeling Gas-Phase Chemistry and Heterogeneous Reaction of Polycyclic Aromatic Compounds
R824970C008 Fundamental Study on High Temperature Chemistry of Oxygenated Hydrocarbons as Alternate Motor Fuels and Additives
R824970C009 Markers for Emissions from Combustion Sources
R824970C010 Experimental Investigation of the Evolution of the Size and Composition Distribution of Atmospheric Organic Aerosols
R824970C011 Microengineered Mass Spectrometer for in-situ Measurement of Airborne Contaminants