Nick M. Marinov

A DETAILED CHEMICAL KINETIC MODEL FOR HIGH TEMPERATURE ETHANOL OXIDATION

A detailed chemical kinetic model for ethanol oxidation has been developed and validated against a variety of experimental data sets. Laminar flame speed data (obtained from a constant volume bomb and counterflow twin-flame), ignition delay data behind a reflected shock wave, and ethanol oxidation product profiles from a jet-stirred and turbulent flow reactor were used in this computational study. Good agreement was found in modeling of the data sets obtained from the five different experimental systems. The computational results show that high temperature ethanol oxidation exhibits strong sensitivity to the fall-off kinetics of ethanol decomposition, branching ratio selection for C2H5OH + OH Products, and reactions involving the hydroperoxyl (HO2) radical. The multichanneled ethanol decomposition process is analyzed by RRKM/Master Equation theory, and the results are compared with those obtained from earlier studies. The ten-parameter Troe form is used to define the C2H5OH(+M) CH3 + CH2OH(+M) rate expression as k∞ = 5.94E23 T−1.68 exp(−45880 K/T) (s−1) ko = 2.88E85 T−18.9 exp(−55317 K/T) (cm3/mol/sec) Fcent = 0.5 exp(−T/200 K) + 0.5 exp(−T/890 K) + exp(−4600 K/T) and the C2H5OH(+M) C2H4 + H2O(+M) rate expression as k∞ = 2.79E13 T0.09 exp(−33284 K/T) (s−1) ko = 2.57E83 T−18.85 exp(−43509 K/T) (cm3/mol/sec) F cent = 0.3 exp(−T/350 K) + 0.7 exp(−T/800 K) + exp(−3800 K/T) with an applied energy transfer per collision value of = 500 cm−1. An empirical branching ratio estimation procedure is presented which determines the temperature dependent branching ratios of the three distinct sites of hydrogen abstraction from ethanol. The calculated branching ratios for C2H5OH + OH, C2H5OH + O, C2H5OH + H, and C2H5OH + CH3 are compared to experimental data.

Aromatic and polycyclic aromatic hydrocarbon formation in a laminar premixed n-butane flame

Experimental and detailed chemical kinetic modeling work has been performed to investigate aromatic and polycyclic aromatic hydrocarbon (PAH) formation pathways in a premixed, rich, sooting, n-butane–oxygen–argon burner stabilized flame. An atmospheric pressure, laminar flat flame operated at an equivalence ratio of 2.6 was used to acquire experimental data for model validation. Gas composition analysis was conducted by an on-line gas chromatograph/mass spectrometer technique. Measurements were made in the main reaction and post-reaction zones for a number of low molecular weight species, aliphatics, aromatics, and polycyclic aromatic hydrocarbons (PAHs) ranging from two to five-fused aromatic rings. Reaction flux and sensitivity analysis were used to help identify the important reaction sequences leading to aromatic and PAH growth and destruction in the n-butane flame. Reaction flux analysis showed the propargyl recombination reaction was the dominant pathway to benzene formation. The consumption of propargyl by H atoms was shown to limit propargyl, benzene, and naphthalene formation in flames as exhibited by the large negative sensitivity coefficients. Naphthalene and phenanthrene production was shown to be plausibly formed through reactions involving resonantly stabilized cyclopentadienyl and indenyl radicals. Many of the low molecular weight aliphatics, combustion by-products, aromatics, branched aromatics, and PAHs were fairly well simulated by the model. Additional work is required to understand the formation mechanisms of phenyl acetylene, pyrene, and fluoranthene in the n-butane flame.

Modeling of Aromatic and Polycyclic Aromatic Hydrocarbon Formation in Premixed Methane and Ethane Flames

Detailed chemical kinetic modeling has been performed to investigate aromatic and polyaromatic hydrocarbon formation pathways in rich, sooting, methane and ethane premixed flames. An atmospheric pressure, laminar flat flame operated at an equivalence ratio of 2.5 was used to acquire experimental data for model validation. Gas composition analysis was conducted by an on-line gas chromatograph / mass spectrometer technique. Measurements were made in the flame and post-flame zone for a number of low molecular weight species, aliphatics, aromatics, and polycyclic aromatic hydrocarbons (PAHs) ranging from two to five-aromatic fused rings. The modeling results show the key reaction sequences leading to aromatic and polycyclic aromatic hydrocarbon formation primarily involve the combination of resonantly stabilized radicals. In particular, propargyl and I-methylallenyl combination reactions lead to benzene and methyl substituted benzene formation, while polycyclic aromatics are formed from cyclopentadienyl and f...

Reaction mechanisms in aromatic hydrocarbon formation involving the C5H5 cyclopentadienyl moiety

The quantum chemical BAC-MP4 and BAC-MP2 methods have been used to investigate the reaction mechanisms leading to polycyclic aromatic hydrocarbon (PAH) ring formation. In particular we have determined the elementary reaction steps in the conversion of two cyclopentadienyl radicals to naphthalene. This reaction mechanism is shown to be an extension of the mechanism occurring in the H atomassisted conversion of fulvene to benzene. The net reaction involves the formation of dihydrofulvalene, which eliminates a hydrogen atom and then rearranges to form naphthalene through a series of ring closures and openings. The importance of forming the -CR(·)-CHR-CR′=CR″- moiety, which can undergo rearrangement to form three-carbon atom ring structures, is illustrated with the C4H7 system. The ability of hydrogen atoms to migrate around the cyclopentadienyl moiety is illustrated both for methyl-cyclopentadiene, C5H5CH3, and dihydrofulvalene, C5H5C5H5, as well as for their radical species, C6H7 and C5H5C5H4. The mobility of hydrogen in the cyclopentadienyl moiety plays an important role both in providing resonance-stabilized radical products and in creating the -CR(·) CHR-CR′=CR″- moiety for ring formation. The results illustrate the radical pathway for converting five-membered rings to aromatic six-membered rings. Furthermore, the results indicate the important catalytic role of H atoms in the aromatic ring formation process.

Towards the development of a chemical kinetic model for the homogeneous oxidation of mercury by chlorine species

The potential for regulation of mercury emissions from coal-fired boilers is a concern for the electric utility industry. Field data show a wide variation in the fraction of mercury that is emitted as a vapor vs. that retained in the solid products. The reason for this variation is not well-understood. Near the end of the flue gas path, mercury exists as a combination of elemental vapor and HgCl2 vapor. The data show that HgCl2 is more likely to be removed from the flue gas. Thus, the degree of oxidation is considered to be a critical factor that tends to reduce emission. Mercury is certain to exist as elemental vapor in the flame, with the oxidation occurring at some point in the post-flame environment. At present, the mechanism promoting this oxidation is not quantitatively known, particularly under the low chlorine concentrations afforded by many coals. In the present work, we measure mercury oxidation from a furnace operating between 860°C and 1171°C. These data are compared with similar results from the literature. The possible elementary reactions that may lead to oxidation are reviewed and a chemical kinetic model is proposed. This model yields good qualitative agreement with the data and indicates that mercury oxidation occurs during the thermal quench of the combustion gases. The model also suggests that atomic chlorine is the key oxidizing species. The oxidation is limited to a temperature window between 700°C and 400°C that is defined by the overlap of (1) a region of significant superequilibrium Cl concentration, and (2) a region where oxidized mercury is favored by equilibrium. Above 700°C, reverse reactions effectively limit oxidized mercury concentrations. Below 400°C, atomic chlorine concentrations are too low to support further oxidation. The implication of these results are that homogeneous oxidation is governed primarily by (1) HCl concentration, (2) quench rate, and (3) background gas composition.

Aromatic and polycyclic aromatic hydrocarbon formation in a premixed propane flame

Experimental and detailed chemical kinetic modeling has been performed to investigate aromatic and polycyclic aromatic hydrocarbon (PAH) formation pathways in a premixed, rich, sooting, propane-oxygen-argon burner stabilized flame. An atmospheric pressure, laminar flat flame operated at an equivalence ratio of 2.6 was used to acquire experimental data for model validation. Gas composition analysis was conducted by an on-line gas chromatograph/mass spectrometer (GC/MS) technique. Measurements were made in the main reaction and post-reaction zones for a number of low molecular weight species, aliphatics, aromatics, and polycyclic aromatic hydrocarbons (PAHs) ranging from two to five-fused aromatic rings Reaction flux and sensitivity analysis were used to help identify the important reaction sequences leading to aromatic and PAH growth and destruction in the propane flame. Benzene formation was shown to be dominated by the propargyl recombination reaction. A secondary benzene formation pathway occurred from ...

An experimental and kinetic calculation of the promotion effect of hydrocarbons on the NO-NO2 conversion in a flow reactor

Experimental and detailed chemical kinetic modeling work has been performed to investigate the role of hydrocarbon oxidation in NO-NO 2 conversion. An atmospheric pressure., quartz flow reactor was used to examine the dependence of NO oxidation to NO 2 by hydrocarbon type, reaction temperature, and residence time. The five hydrocarbons examined were methane, ethylene, ethane, propene, and propane. In the experiment, probe measurement of the species concentrations was performed in the flow reactor using a mixture of NO(20 ppm)/air/hydrocarbon(50 ppm) at residence times from 0.16 to 1.46 s and temperatures from 600 to 1100 K. In the chemical kinetic calculation, the time evolution of NO, NO 2 , hydrocarbons, and reaction intermediates were evaluated for a series of the hydrocarbons and the temperatures. The chemical mechanism consisted of 639 reversible reactions and 126 species. Experimental results indicate that, in general, ethylene and propane effectively oxidize NO to NO 2 while methane is less effective. The calculation indicates the important chemical kinetic features that control NO-NO 2 conversion for each hydrocarbon type. The dependence of NO-NO 2 conversion with hydrocarbon type and temperature is qualitatively reproduced by the calculation. The calculation indicates that all five hydrocarbons oxidize NO to NO 2 predominantly through NO+HO 2 ahNO 2 +OH and that the contribution of oxidation by RO 2 and HORO 2 is minor. Highest effectiveness comes from hydrocarbons that produce reactive radicals (i.e., OH, O atom) that promote hydrocarbon oxidation and lead to additional HO 2 production. On the other hand, if hydrocarbons produce radicals, such as methyl and allyl, which resist oxidation by O 2 , then these radicals tend to reduce NO 2 to NO. Experimental results show that the effectiveness of hydrocarbons varies appreciably with temperature and only within the low-temperature range. Propane shows the greatest NO-NO 2 conversion for the lowest temperatures. This ability is primarily due to the hydroperoxy-propyl plus O 2 reactions as indicated by the sensitivity analysis results.

Experimental and modeling investigation of aromatic and polycyclic aromatic hydrocarbon formation in a premixed ethylene flame

Experimental and detailed chemical kinetic modeling has been performed to investigate aromatic and polyaromatic hydrocarbon formation pathways in a rich, sooting, ethylene-oxygen-argon premixed flame. An atmospheric pressure, laminar flat flame operated at an equivalence ratio of 2.5 was used to acquire experimental data for model validation. Gas composition analysis was conducted by an on-line gas chromatograph/mass spectrometer (GC/MS) technique. Measurements were made in the flame and post-flame zone for a number of low molecular weight species, aliphatics, aromatics and polycyclic aromatic hydrocarbons (PAHs) ranging from two to five-aromatic fused rings. The modeling results show the key reaction sequences leading to aromatic and polycyclic aromatic hydrocarbon growth involve the combination of resonantly stabilized radicals. In particular, propargyl and 1-methylallenyl combination reactions lead to benzene and methyl substituted benzene formation, while polycyclic aromatics are formed from cyclopentadienyl radicals and fused rings that have a shared C{sub 5} side structure. Naphthalene production through the reaction step of cyclopentadienyl self-combination and phenanthrene formation from indenyl and cyclopentadienyl combination were shown to be important in the flame modeling study. The removal of phenyl by O{sub 2} leading to cyclopentadienyl formation is expected to play a pivotal role in the PAH or soot precursor growth process under fuel-rich oxidation conditions.

Detailed chemical kinetic modeling of diesel combustion with oxygenated fuels

The influence of oxygenated hydrocarbons as additives to diesel fuels on ignition, NOx emissions and soot production has been examined using a detailed chemical kinetic reaction mechanism. N-heptane was used as a representative diesel fuel, and methanol, ethanol, dimethyl ether and dimethoxymethane were used as oxygenated fuel additives. It was found that addition of oxygenated hydrocarbons reduced NOx levels and reduced the production of soot precursors. When the overall oxygen content in the fuel reached approximately 25% by mass, production of soot precursors fell effectively to zero, in agreement with experimental studies. The kinetic factors responsible for these observations are discussed.

The Importance of the Nitrous Oxide Pathway to NOx in Lean-Premixed Combustion

This study addresses the importance of the different chemical pathways responible for NO x formation in lean-premixed combustion, and especially the role of the nitrous oxide pathway relative to the traditional Zeldovich pathway. NO x formation is modeled and computed over a range of operating conditions for the lean-premixed primary zone of gas turbine engine combustors. The primary zone, of uniform fuel-air ratio, is modeled as a micromixed well-stirred reactor, representing the flame zone, followed by a series of plug flow reactors, representing the postflame zone. The fuel is methane. The fuel-air equivalence ratio is varied from 0.5 to 0.7. The chemical reactor model permits study of the three pathways by which NO x forms, which are the Zeldovich, nitrous oxide, and prompt pathways. Modeling is also performed for the well-stirred reactor alone. Three recently published, complete chemical kinetic mechanisms for the C1−C2 hydrocarbon oxidation and the NO x formation are applied and compared. Verification of the model is based on the comparison of its NO x output to experimental results published for atmospheric pressure jet-stirred reactors and for a 10 atm. porous-plate burner. Good agreement between the modeled results and the measurements is obtained for most of the jet-stirred reactor operating range. For the porous-plate burner, the model shows agreement to the NO x measurements within a factor of two, with close agreement occurring at the leanest and coolest cases examined. For lean-premixed combustion at gas turbine engine conditions, the nitrous oxide pathway is found to be important, though the Zeldovich pathway cannot be neglected. The prompt pathway, however, contributes small-to-negligible NO x . Whenever the NO x emission is in the 15 to 30 ppmv (15 percent O 2 , dry) range, the nitrous oxide pathway is predicted to contribute 40 to 45 percent of the NO x for high-pressure engines (30 atm), and 20 to 35 percent of the NO x for intermediate pressure engines (10 atm). For conditions producing NO x of less than 10 ppmv (15 percent O 2 , dry), the nitrous oxide contribution increases steeply and approaches 100 percent. For lean-premixed combustion in the atmospheric pressure jet-stirred reactors, different behavior is found. All three pathways contribute; none can be dismissed. No universal behavior is found for the pressure dependence of the NO x . It does appear, however, that lean-premixed combustors operated in the vicinity of 10 atm have a relatively weak pressure dependence, whereas combustors operated in the vicinity of 30 atm have an approximately square root pressure dependence of the NO x