The research is designed to provide experimental time-resolved data on soot particle size and structure during soot growth and burnout under well defined conditions pertinent to practical flames. The work builds on previous time-resolved measurements of soot mass concentration and gas composition, including PAH, and currently available modeling capability for handling the formation and interaction of PAH and soot. The specific objectives are: (1) to extend previous jet-stirred/plug-flow reactor data to include measurements of particle size and structure at different residence times corresponding to different particle formation and burnout stages including any oxidative fragmentation; (2) to extend the present PAH-soot model to include oxidative enlargement of pores and fragmentation of soot particles; (3) to critically test the model against data so as to evaluate and improve the mechanistic understanding and accuracy of model predictions of soot particle and PAH emissions from combustion; and (4) to use the model to identify potential combustor design features and operation conditions for improved control of soot and PAH emissions.
Soot formation in flames is a major source of the fine airborne particles of current health concern. The mechanism of soot formation involves particle nucleation or inception, mass addition to the particles by reaction with gas-phase molecules, coagulation of particles through particle-particle sticking collisions, mass removal from particles by pyrolytic elimination of functional groups accompanied by dehydrogenation and structural rearrangement or carbonization of the condensed material, and oxidation. Although the details of soot formation and oxidation are not completely understood, there is reason to believe that the fine soot particles of health concern may originate in combustors in different ways. The soot formation and oxidation processes may be terminated at different stages through dilution, thermal quenching or other mechanisms occurring in combustion equipment. The resulting particles found in the exhaust may have characteristics of the formation stage, the destruction state, or both, depending on the combustor design and operating conditions. For example, larger numbers of smaller particles could be produced by quenching the formation process at an early stage of formation before particle number is extensively reduced by coagulation, or by incomplete oxidation of particles under conditions where pore development leads to particle fragmentation. Recent observations from diesel engine combustion indicate that new design features which reduce the mass of soot emitted can actually increase the number of particles and the associate toxicity. These and other complexities of environmental soot sources are poorly understood, and basic guidance is needed for the development and optimization of control strategies. To help provide the understanding and guidance that is needed, the present research is designed to provide improved understanding of the origin and characteristics of fine soot particles produced in combustion.
This work focuses on improving the basic information on the origin and characteristics of ultrafine particles in combustion. Time-resolved measurements of soot particle size and concentration are being performed for different flame configurations and conditions, and the results are used to study the mechanisms of soot particle generation and growth in flames. A Scanning Mobility Particle Size Analyzer (SMPS) and transmission electron microscopy (TEM) are used to determine soot particle size, morphology, and structure in the ultrafine region. A primary dilution sampling probe was designed for in-flame extraction of soot. A program was written in MATLAB to assist in particle sizing of the TEM images, and the results are compared with those obtained with the SMPS. TEM images also provide valuable information on soot structure and agglomerate formation, which is needed for complete characterization of the soot.
The SMPS has been coupled with a new sampling probe to analyze ultrafine soot particles (1000 nm down to 5 nm) in methane/oxygen and ethylene/air pre-mixed flames at different equivalence ratios. The sampling probe was specially designed to withstand flame temperatures and quickly quench the extracted soot and combustion products. Rapid quenching prevents chemical reactions and suppresses aerosol dynamics by cooling and dilution. The sampling probe also prevents thermophoretic losses of soot by radial injection of the dilution gas thus confining the sample to the center of the probe and away from the cool walls. Sampling is performed in the luminous region of the flames at different heights above the burner to obtain time-resolved measurements.
A high dilution ratio of 60 to 100 is used to prevent soot particles from agglomerating and also to reduce the concentration of soot to within the limits of the SMPS. The dilution ratio is an important factor when comparing in-flame soot concentrations. The dilution ratio is accurately measured by adding a tracer to the combustion gases and measuring its concentration in the diluted sample using gas chromatography/mass spectrometry. In this way, the in-flame concentrations of different samples can be compared even when the dilution ratio is varied.
Data for the soot size distribution and number density are being obtained for a jet-stirred reactor (JSR)/plug-flow reactor (PFR) system used previously in this laboratory. Measurements are being performed by means of thermophoretic sampling and laser scattering for the conditions of 2.2 equivalence ratio in pre-mixed C2H4/Air/O2 combustion. Profiles of total soot mass and concentrations of gas phase species including many PAH were measured in the previous work for the same conditions. The aim is to combine the detailed soot aerosol characterization with the previous data to provide a more comprehensive set of fuel-rich combustion data for use in the development and testing of PAH and soot formation models.
The thermophoretic sampling technique consists of rapid insertion (exposure time of 25 ms - 500 ms) of an ambient temperature TEM grid into the hot (1620 K) gases of the PFR to collect soot particles. Subsequently, images of samples are obtained using the TEM and analyzed with in-house software to determine the particle size distribution. Measurements of laser scattering at different distances or residence times in the PFR and performed using a 512 nm Nd:YAG laser to obtain information on soot particle number density through the use of the Rayleigh approximation and the aforementioned size distribution.
Work to date consists of determination of the optimal TEM substrate material and exposure times for conditions of interest, completion of the size analysis software, and calibration of the laser system. The remaining work consists of TEM analysis and size characterization of more samples and obtaining laser scattering data for the conditions of interest.
The work to date has focused on the experimental task of extending previous data to include measurements of particle size and structure at different combustion residence times corresponding to different particle formation and burnout stages. Both the mobility analyzer and electron microscope techniques have been implemented and the collection of the data needed for the modeling work is underway.
Stephen W. Lasher