Grantee Research Project Results
Final Report: Products of Incomplete Combustion in the Incineration of Brominated Hydrocarbons
EPA Grant Number: R828193Title: Products of Incomplete Combustion in the Incineration of Brominated Hydrocarbons
Investigators: Senkan, Selim M.
Institution: University of California - Los Angeles
EPA Project Officer: Hahn, Intaek
Project Period: July 1, 2000 through June 30, 2003
Project Amount: $350,000
RFA: Combustion Emissions (1999) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Air
Objective:
The overall objective of this research project was to develop fundamental and practical insights on the formation and control of potentially toxic products of incomplete combustion in the incineration processes of halogenated hydrocarbons. The specific objectives of this research project were to: (1) experimentally determine the identities and absolute concentrations of the major, minor, and trace products of incomplete combustion (PIC) species, as well as temperature and soot levels in the atmospheric-pressure laminar, premixed and diffusion flames of brominated hydrocarbons (BHCs) such as CH3Br, in mixtures with hydrocarbon fuels, including methane, acetylene, butadiene, and heptane, as well as in pure hydrocarbon flames; and (2) numerically establish both the fate and role bromine plays in the combustion and PIC formation mechanisms of hydrocarbons. Incineration is an effective treatment method for the disposal of organic hazardous wastes, including those that contain halogenated hydrocarbons. The combustion of halogenated hydrocarbons, however, is associated with the formation of trace toxic byproducts such as aromatics and polycyclic aromatic hydrocarbons (PAHs), halogenated dibenzodioxins, halogenated dibenzofurans, biphenyls, and pyrenes. Because some of the isomers of these PICs are potent carcinogens, the development of a better understanding of their origins and fate is important for the continued safe utilization of the incineration technology.
Summary/Accomplishments (Outputs/Outcomes):
The objectives were reached by undertaking a systematic and integrated program of experimental and theoretical/computational studies pertaining to flame combustion. To accomplish our first objective, species concentrations were determined by withdrawing samples from within flames using heated microprobes, followed by gas analysis by online high-resolution gas chromatography/quadrupole mass spectrometry (GC/QMS). Temperature measurements also were made with thermocouples using the rapid insertion technique to prevent excessive soot accumulation on thermocouple beads. An important aspect of the program was to assess how the addition of brominated compounds perturb the flame chemistry and reaction mechanisms of regular hydrocarbons. This comparative approach was undertaken because it builds on our comprehensive accumulated knowledge base of the hydrocarbon combustion kinetics and mechanisms. To exploit this comparative approach, however, the flame structures of regular hydrocarbons also must be determined first under conditions identical to the flames containing BHC. Detailed chemical kinetic mechanisms describing the formation and destruction of PIC in BHC-containing flames also were developed in view of the better established hydrocarbon and chlorinated hydrocarbon combustion mechanisms.
During the project, we conducted the first set of premixed and diffusion flame experiments with pure CH3Br in O2/Ar using the opposed jet diffusion flame system shown in Figure 1. Unfortunately, these flames produced excessive amounts of soot under stable operating conditions, rendering microprobe sampling impractical when CH3Br is the only fuel used. Even the large orifice diameter sampling probes (e.g., 500 microns [0.5 mm]) were plugged within 100 milliseconds, preventing the acquisition of representative flame samples.
Our initial experiments with CH3Br demonstrated the production of significant levels of acetylene and 1,3-butadiene together with soot. These results were consistent with the generally believed notion that acetylene and 1,3-butadiene are the major hydrocarbon intermediates in flames responsible for the production of aromatics, polyaromatics, dioxins, furans, PAHs, and ultimately soot. Because there are no prior detailed flame structure studies on both 1,3-butadiene or acetylene under ambient pressure conditions, we undertook systematic studies to map the chemical structures of their diffusion flames at different strain rates and carbon densities. These complementary studies were undertaken to better evaluate the effects of residence time and dilution on aromatic and PAH formation, respectively.
Figure 1. Experimental Setup of Counter-Flow Diffusion Flame
Our early studies also demonstrated the need to undertake experiments where CH3Br represents a small fraction of the fuel. Again, these studies were not useful because at low CH3Br concentrations, the flames exhibited chemical structures that were similar to the parent hydrocarbon fuels used. That is, no BHCs other than the CH3Br were observed. Consequently, we also focused our studies on the flames of hydrocarbons that were relevant to incineration. In this regard, we studied the flames n-heptane as a representative liquid hydrocarbon that is used as an auxiliary fuel in incineration.
Detailed kinetic modeling work also was undertaken regarding the combustion of CH3Br. The mechanism developed involved the participation of more than 40 species in more than 200 reversible elementary reactions. The mechanism has been built on H2-O2 and CH4-O2 combustion reaction mechanisms, and developed analogous to the CH3Cl-O2 mechanism by properly taking into account the C-Br, Br-Br, and H-Br bond dissociation energies. Reaction-rate parameters [k=Atnexp(-E/RT)] were obtained from published and evaluated experimental data when such information was available. In the absence of experimental data, kinetic parameters were estimated using analogies, empirical methods, structure-activity relationships, or calculated using computational quantum chemical methods. Comparisons of model predictions with the experimental data were not undertaken because of the lack of detailed flame-chemistry data on BHCs. We did, however, complete a comprehensive modeling study of 1,3-butadiene flames and explored the effects of mixing on PAH and soot formation.
Effects of Equivalence Ratio on Species and Soot Concentrations in Premixed n-Heptane Flames
The microstructure of laminar premixed, atmospheric-pressure, and fuel-rich flames of n-heptane/oxygen/argon has been studied at two equivalence ratios (C/O=0.63 and C/O=0.67). A heated quartz microprobe coupled to an online GC/MS (HP 5890 Series II/HP 5972) has been used to establish the identities and absolute concentrations of stable major, minor, and trace species by the direct analysis of samples withdrawn from the flames. Benzene was the most abundant aromatic compound identified. The largest PAHs detected were the family of C18H10 (molecular weight of 226) that includes cyclopenta[cd]pyrene and benzo[ghi]fluoranthene, with peak concentrations reaching 8 ppm and 6 ppm, respectively. Soot-particle diameters, number densities, and volume fractions were determined using classical light scattering and extinction measurements. The largest soot-particle diameter measured was about 18 nm, and the soot- volume fraction reached the amount of 4.9 x 107.
Effects of Oxygenate Additives on PAH and Soot Formation
Effects of three oxygenate additives (methanol, ethanol, and methyl tertiary butyl ether) on the formation of PAHs and soot in laminar, premixed, atmospheric-pressure, and fuel-rich flames of n-heptane have been studied at an equivalence ratio of 2.10. All of the oxygenate additives studied reduced the mole fractions of aromatic and PAH species, as well as soot formation. The reduction in soot formation for different oxygenates, however, was comparable under the experimental conditions investigated.
Experimental and Artificial Neural Network Modeling Study of Soot Formation in Premixed Hydrocarbon Flames
The formation of soot in premixed flames of methane, ethane, propane, and butane was studied at three different equivalence ratios. Soot-particle sizes, number densities, and volume fractions were determined using classical light scattering measurement techniques. The experimental data revealed that the soot properties were sensitive to the fuel type and the combustion parameter equivalence ratio. An increase in the equivalence ratio increased the amount of soot formed for each fuel. In addition, methane flames showed larger particle diameters at higher distances above the burner surface and propane, ethane, and butane flames came after the methane flames, respectively. Three-layer, feed-forward type artificial neural networks having seven input neurons, one output neuron, five hidden neurons for soot-particle diameter predictions, and seven hidden neurons for volume-fraction predictions were used to model the soot properties. The network could not be trained and tested with sufficient accuracy to predict the number density because of a large data range and greater uncertainty in determination of this parameter. The number of complete data sets used in the model was 156. There was good agreement between the experimental and predicted values, and neural networks performed better when predicting output parameters (i.e., soot-particle diameters and volume fractions) within the limits of the training data.
Effects of Oxygen Addition on PAH Formation
The effect of 3 percent O2 addition to the fuel on detailed chemical structure of a 1,3-butadiene counter-flow diffusion flame has been investigated using heated microprobe sampling and online GC/MS. Centerline gas temperature and species mole fraction profiles were measured both in the absence and presence of oxygen on the fuel side. The rapid thermocouple insertion method was used to obtain the flame temperature profiles. Although the addition of oxygen to the fuel side did not significantly change the shape and peak values of the temperature profiles, species mole fraction profiles were altered significantly. Flame reactions started earlier in the presence of oxygen in the fuel, resulting in the shift of the reaction zone towards the fuel burner port. The presence of oxygen in the fuel led to decreased peak mole fractions of the aromatic and two-ring PAH species. In contrast, the peak mole fractions of PAHs having three or more rings significantly increased in the presence of oxygen in the fuel stream. A preliminary analysis of the data suggests the need to invoke new reaction mechanisms describing the formation and destruction of PAHs.
Kinetic Modeling of Counter Flow Diffusion Flames of Butadiene and Mixing Effect
A comprehensive, semidetailed kinetic scheme was used to simulate the chemical structures of counter-flow diffusion and fuel-rich premixed 1,3-butadiene flames to better understand the formation of PAHs. The results showed that model predictions were in good agreement with the experiments for most of the species in both of the flames. In the counter-flow flames, higher molecular weight products are slightly over predicted. The pathways characterizing the pollutant formation are very different in the premixed and counter-flow flames, confirming or suggesting the need to verify and refine the detailed mechanisms tuned for premixed conditions when they are extrapolated and used in diffusion flames. Reaction path analysis for PAH formation in the counter-flow flame shows that both the hydrogen-abstraction/carbon-addition mechanism and the resonantly stabilized radicals are important for the growth of PAH. The kinetic model was unsuccessful in predicting the increased reactivity in O2-doped diffusion flames, indicating the need for improved models and the opportunity for new experiments of butadiene oxidation in the intermediate temperature region.
Acetylene is a ubiquitous combustion intermediate that also is believed to be the major precursor for aromatics, PAHs, and soot formation both in hydrocarbon and halogenated hydrocarbon flames. Despite its important role as a flame intermediate, however, the detailed chemical structures of pure acetylene diffusion flames have not been studied in the past. We compared the detailed chemical structures of counter-flow diffusion flames of acetylene at strain rates of 37.7 and 50.3 s-1. Both flames possessed the same carbon density of 0.37 g/L corresponding to acetylene mole fraction of 0.375 in argon at the fuel side, and an oxygen mole fraction of 0.22 in argon at the oxidizer side. Concentration profiles of a large number of major, minor, and trace species, including a wide spectrum of aromatics and PAHs, have been determined by direct sampling from flames using a heated quartz microprobe coupled to an online GS/mass selective detector. Temperature profiles were made using a thermocouple and the rapid insertion technique. Although the major species concentrations were nearly the same in both flames, the mole fraction profiles of trace combustion byproducts were significantly lower in the higher strain rate flame by nearly two orders of magnitude for PAH. These comparative results provide new information on the trace chemistries of acetylene flames and should be useful for the development and validation of detailed chemical kinetic mechanisms describing the formation of toxic byproducts in the combustion of hydrocarbons and halogenated hydrocarbons.
Effects of Carbon Density on PAH Formation in Acetylene Diffusion Flame
PAHs in counter-flow diffusion flames of acetylene have been studied as a function of carbon density. Rapid insertion technique has been applied to temperature measurements by using silica-coated Pt/Pt+13 percent Rh thermocouple (R type). Butadiyne was the most abundant pyrolysis product in the acetylene flames followed by vinylacetylene (1-buten-3-yne); benzene was the most abundant aromatic compound followed by phenylacetylene (ethynylbenzene). As particular characteristics of acetylene flames, acenaphthylene was more abundant than naphthalene, and paraffins such as methane, ethane propane, butane, and heavier compounds were not detected. The largest PAHs detected were the group of C18H10 (molecular weight of 226), such as cyclopenta[cd]pyrene and benzo[ghi] fluoranthene.
Journal Articles on this Report : 6 Displayed | Download in RIS Format
Other project views: | All 10 publications | 6 publications in selected types | All 6 journal articles |
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Granata S, Faravelli T, Ranzi E, Olten N, Senkan S. Kinetic modeling of counterflow diffusion flames of butadiene. Combustion and Flame 2002;131(3):273-284. |
R828193 (2002) R828193 (Final) |
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Inal F, Senkan SM. Effects of equivalence ratio on species and soot concentrations in premixed N-heptane flames. Combustion and Flame 2002;131(1-2):16-28. |
R828193 (2002) R828193 (Final) |
Exit Exit Exit |
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Inal F, Senkan S. Effects of oxygenate additives on polycyclic aromatic hydrocarbons (PAHs) and soot formation. Combustion Science and Technology 2002;174(9):1-19. |
R828193 (2002) R828193 (Final) |
Exit Exit |
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Inal F, Tayfur G, Senkan SM. Experimental and artificial neural network modeling study on soot formation in premixed hydrocarbon flames. Fuel 2003;82(12):1477-1490. |
R828193 (2002) R828193 (Final) |
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Olten N, Senkan S. Effect of oxygen addition on polycyclic aromatic hydrocarbon formation in 1,3 butadiene counter-flow diffusion flames. Combustion and Flame 2001;125(1-2):1032-1039. |
R828193 (2002) R828193 (Final) R826730 (2000) |
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Yamamoto M, Duan S, Senkan S. The effect of strain rate on polycyclic aromatic hydrocarbon (PAH) formation in acetylene diffusion flames. Combustion and Flame 2007;151(3):532-541. |
R828193 (Final) R830896 (Final) |
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Supplemental Keywords:
toxic combustion byproduct, polycyclic aromatic hydrocarbon, PAH, gas chromatography, GC, quadrapole mass spectrometry, QMS, halogenated hydrocarbon, dioxin, furan, aromatic, detailed kinetic modeling, elementary reaction, soot formation, premixed flame, diffusion flame, flame sampling, detailed flame structure, toxics, waste, analytical chemistry, atmospheric sciences, ecology, ecosystems, ecosystem protection, environmental exposure and risk, environmental chemistry, environmental monitoring, fate and transport, hazardous air pollutants, HAPs, combustion, chemical mixtures, pesticides, biphenyl, dibenzofurans, detailed chemical kinetics, hazardous waste incinerators, hydrocarbons, incineration, mass spectrometry, MS, products of incomplete combustion, PIC, soot profiles., RFA, Scientific Discipline, Toxics, Waste, Ecosystem Protection/Environmental Exposure & Risk, Environmental Chemistry, HAPS, Fate & Transport, Analytical Chemistry, chemical mixtures, Environmental Monitoring, Ecology and Ecosystems, Atmospheric Sciences, Incineration/Combustion, fate and transport, mass spectrometry, products of incomplete combustion (PIC), Dibenzofurans, hazardous waste incinerators, PAH, gas chromatography, furans, Biphenyl, hydrocarbons, dioxins, incineration, detailed chemical kinetics, toxic by-productsRelevant Websites:
Senkan Reseach Laboratory Exit
Progress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.