NOx formation from ammonia impurities and its effects on fuel oxidation is not well understood for high-pressure supercritical CO2 oxy-combustion conditions. This effect is investigated computationally and experimentally in the present work. A chemical kinetic analysis revealed that the reaction between NH radical and CO2 plays an important role in determining the rate of NOx formation at these conditions. It was also found that a significant reduction in the NOx formation from ammonia oxidation occurs with sCO2 oxy-combustion at 300 atm when compared to traditional gas turbine conditions. A chemical reactor network simulation of realistic sCO2 cycle conditions confirmed the reduced NOx emissions. Monte Carlo simulations used to study the sensitivity of model input variables on the emissions showed that CO2 cooling impacts the CO emissions while most of the NOx are generated in the flame zone. Uncertainty analysis showed that the reaction between NH and CO2 to form HNO radical is an important contributor to the model uncertainty under sCO2 oxy-combustion conditions. The presence of combustion-generated NOx in the recycled-CO2 can impact the fuel oxidation in the primary zone of the combustor. Hence, to understand the effect of NOx on ignition, high-pressure shock tube ignition delay time experiments were performed at conditions relevant to sCO2 oxy-combustion. The ignition delay time measurements were made for syngas and CH4 fuels with and without NO addition under supercritical conditions using CO2 as the bulk diluent at nominal pressures around 100 atm. Experimental data showed that the presence of NO promotes ignition at these conditions. The effect is more pronounced for CH4 compared to syngas. Nitric oxide acts as a chemical catalyst to promote ignition by increasing the combustion radical pool. The catalytic cycle involves the conversion of NO to NO2 which also contributes to CH4 oxidation by H-atom abstraction to generate CH3 radical.

The impact of chain length on the time histories of key intermediate species that form upon first-stage ignition was studied experimentally in a shock tube using multi-color laser absorption diagnostics. CO, CH2O, OH, and composite time histories of heavier carbonyls were measured simultaneously during the low temperature oxidation of n-octane & n-decane in the temperature range of 720-860 K, and pres-sure range of 4.1-5.8 atm. The test mixtures were 0.64% & 0.6% fuel in oxygen, respectively. A two-color diagnostic was also developed and used in tandem with other diagnostics to quantify the evolution of temperature during oxidation. To our knowledge, this work reports the first measurement of CH2O, OH, and temperature during oxidation of C8 or heavier hydrocarbon fuels at low temperatures. The measured time histories were also compared against the predictions of two recent detailed kinetic models. Signifi-cant differences in the measured and predicted first-stage ignition delay times, absolute concentration of species post-ignition and the maximum rate of formation of species were observed. Our measurements will serve as targets for further refinement of these detailed kinetic models and in the development of rate rules for similar classes of fuels. The measurements also revealed significant differences in the rela-tive reactivity of n-octane and n-decane. Taken together with previous speciation studies conducted in n-heptane, we observe a strong correlation between the species time histories, reactivity, and chain length of straight chain alkanes. This observation lends further support to the selection of CO and CH2O as target species for development of Low-Temperature Hybrid Chemistry (HyChem) models for real transportation and aviation fuels in future.(c) 2023 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

We address the role of the linear mixing rule in the kinetics of the H 2 O 2 decomposition system by reporting the rate constant for H 2 O 2 + M = 2OH + M (M = Ar and CO 2 ) in the temperature range of 1087-1234 K at low pressures in a mixture of 20% CO 2 in Argon. The reaction rate constant was inferred from H 2 O concentrations monitored by using a laser-absorption spectroscopy-based water diagnostic. To the best of our knowledge, this is the first measurement of the rate constant of this reaction in a mixture to be reported in literature. A significant discrepancy was found between the rate constants derived using the traditional linear mixing rule and the reduced pressure linear mixing rule. This discrepancy can have serious implications on the predictive accuracy of these kinetic models, especially under conditions relevant to the operation of supercritical CO 2 (sCO 2 ) power cycles that rely on oxy-fuel combustion in a working fluid comprised almost entirely of CO 2 .& COPY; 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

High-temperature cyclopentadiene pyrolysis was examined behind reflected shock waves in a heated shock tube using several laser absorption diagnostic schemes. A two-color, online-offline sensor near 3335 cm-1 was used to measure time histories of acetylene, while a three-color scheme of diagnostics at 10.532, 10.675, and 11.345 μm yielded measurements of cyclopentadiene and ethylene. Species time histories of cyclopentadiene decomposition and acetylene formation as well as ethylene yields are reported from 1319 to 1678 K at 1.2-1.5 atm. In addition, the overall decomposition rate of cyclopentadiene is reported, and comparisons are made to a number of kinetic models.

This work presents a new diagnostic for quantitative measurements of formaldehyde (CH2O) in the tem-perature range of 70 0-150 0 K and pressures from 10 to 60 atm targeted for combustion applications. A two-color, online-offline strategy using laser absorption spectroscopy allows CH2O to be measured with-out interference from heavy carbonyls, hydrocarbons, water, and other combustion intermediates. The online and offline wavenumbers were chosen as 1745.2 cm -1 and 1738.38 cm -1 (near the Q-branch of formaldehyde's C = O stretch), respectively, after a careful line selection process to maximize sensitivity over the temperature (T), pressure (P) range of interest. Absorption cross sections of CH2O diluted in Ar and O2 at these wavelengths were determined by shock-heating 1,3,5 trioxane (C3H6O3) as a formalde-hyde precursor in the Stanford high-pressure shock tube between 13 and 52 atm. Analytical expressions for the cross sections at the two colors as a function of temperature and pressure are provided, with an uncertainty of +/- 5%. Applications of the diagnostic have been demonstrated in three different classes of experiments of interest in combustion studies: 1) Pyrolysis of dimethyl ether (DME)/Ar; 2) pyrolysis of ethanol/Ar; and 3) oxidation of heavy hydrocarbons, including a three-component gasoline surrogate (TPRF-60), two high-performance gasolines, and a jet fuel (Jet A). The measured CH2O time histories were compared with predictions of their respective chemical kinetic models, which provided insights into the deficiencies in these models. To the best of the authors' knowledge, this work presents the first quanti-tative measurement of formaldehyde during the oxidation of these fuels at engine-relevant conditions. (c) 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Two novel laser absorption diagnostics for allene and propyne are presented. Through examination of the high temperature spectra of the two C3H4 isomers via two fast-wavelength-scanning, broadband external cavity quantum cascade lasers, 1930.38 cm(-1) and 1233.3 cm(-1) were chosen as the wavelengths for a two-species, two-wavelength diagnostic pair that minimizes interference from other species. Absorption cross sections for allene and propyne were measured at each of these wavelengths over 1196-1502 K, at 1.3-1.6 atm. Once characterized, these diagnostics were employed to measure the mutual isomerization rates of allene and propyne over the same temperature range. The two rates are reported as k(A->P) (1202-1502 K) = 2.64 x 10(12)exp(-59,214/RT) +/- 14% and k(P->A) (1196-1494 K) = (4.19 x 10(12))exp(-61,912/RT) +/- 17%. These rates were found to be in excellent agreement with limited past experimental determinations and recently computed rates. Using these rates, the equilibrium constant is also reported as a function of temperature. (C) 2021 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

The direct-fired supercritical-CO2 (sCO2) cycle has demonstrated the ability to produce clean energy by burning hydrocarbon feed stocks under oxy-fuel conditions. High-pressure operation of the direct-fired cycle allows for more economic extraction of CO2 for carbon capture and storage. However, the presence of nitrogen impurities in the oxidizer (i.e., N2) and in fuel feed stocks (e.g., NH3) can generate NOx in the exhaust. The presence of NOx in the recycled-CO2 stream can impact the combustion process as well as the structural integrity of the system. Also, even trace amounts of nitrogen oxides (considered acid gases) can be detrimental for CO2 capture, transportation and storage at supercritical conditions. Therefore, it is critical to understand and accurately model the effects of nitrogen impurities on NOx formation and the impact of NOx in the recycled CO2 on combustion kinetics under oxy-fuel sCO2 conditions. It is also important to understand the effects of pressure with a sCO2 medium as the direct-fired sCO2 cycle operates up to 300 atm pressure. In this work, experimental and modeling work were performed to study the effect of nitrogen species on emissions as well as effect of NOx on ignition of CH4 and syngas fuels at sCO2 conditions. A chemical reactor network simulation was used to investigate the effects of nitrogen impurities in fuel and oxidizer stream on emissions from a direct-fired combustor condition. Monte Carlo simulations were also carried out to study the impact of model input variables on the emission profile. High-pressure shock tube ignition delay time experiments were performed to investigate the effect of NOx on ignition at conditions relevant to direct-fired oxy-fuel sCO2 combustion. The ignition delay time measurements were made for syngas and CH4 fuels with and without NO addition using CO2 as bulk diluent at nominal pressures around 100 atm. Experimental data showed that the presence of NO promotes the ignition at the oxyfuel sCO2 combustion conditions. Reaction sensitivity analyses and model uncertainty analyses were conducted to identify important reactions and their rate uncertainty on the model predictions, respectively.

The rate constants of two unimolecular decomposition channels of ethanol, C2H5OH(+Ar) = CH3 + CH2OH(+Ar), and C2H5OH(+Ar) = C2H4 + H2O(+Ar) were measured near 1 atm and 10 atm behind reflected shock waves between 1190 and 1550 K by tracking the evolution of CO and C2H4, respectively. Use of sensitive laser diagnostics in conjunction with carefully selected test mixtures enabled the suppression of sensitivity of CO and C2H4 time-histories to secondary reactions, resulting in a significant reduction of uncertainty in the measured rate constants. Additionally, CH3CHO time-histories measured during the pyrolysis of 2% C2H5OH/Argon mixtures were used to determine the rate constant of the most dominant H-abstraction channel, C2H5OH + H = sC(2)H(4)OH + H-2 between 1290 and 1530 K. The low temperature CH3CHO time-histories were also used to infer the rate constant of sC(2)H(4)OH = CH3CHO+H. The measured rate constants were used to generate an updated kinetic model, whose performance was evaluated against the species time-history data reported in recent shock tube pyrolysis studies. The performance of the updated model was also evaluated against ignition delay times (IDTs) of stochiometric C2H5OH/Oxygen/Nitrogen mixtures, measured between 100 0 and 1500 K near 20 atm in this study. The updated kinetic model was found to show excellent agreement with both shock tube pyrolysis and IDT studies. Additionally, the reduction in the uncertainty of the rate constants studied in this work resulted in a significant reduction in the uncertainty of the model predictions. The rate constant expressions listed below were derived from the measurements in this work, and are valid in the temperature range of 1200-1600 K. kC(2)H(5)OH(+Ar) <-> CH3 + CH2OH(+Ar),1 atm = 2.125 x10(13) e(-34070/T) s(-1) ( +/- 21%) kC(2)H(5)OH(+Ar) <-> CH3 + CH2OH(+Ar),10 atm = 3.079 x10(14) T(0.23)e(-38030/T) s(-1) ( +/- 28%) kC(2)H(5)OH(+Ar) <-> C2H4 + H2O(+Ar),10 atm = 6.062 x10(36) T-6.45 e(-42180/T) s(-1) (+/- 31%) ksC(2)H(4)OH <-> CH3CHO + H, 1 atm = 5.817 x10(9) e(-11372/T) s(-1) (+/- 55%) kC(2)H(5)OH + H <-> sC(2)H(4)OH + H-2 = 1.780 x10(7) T-1.94 e(-1835/T) s(-1) (+/- 28%) (C) 2021 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

The focus of the present work is to understand the effect of H2 and C2H4 on the ignition characteristics of other gaseous fuels, namely, CH4, C2H6 and C3H6. Atmospheric-pressure flow reactor experiments were performed to measure the ignition delay times of CH4, C2H6 and C3H6 binary mixtures with H2 or C2H4 between 800 and 950 K. Most of the experiments were conducted at stochiometric conditions with 21% O2. Select tests were performed to examine the effects of equivalence ratios and oxygen concentrations. Ignition delay time were also measured for ternary mixtures of C3H6-C2H4-H2 at select conditions. Based on the experimental data, the overall effectiveness of H2 or C2H4 in reducing the ignition delay time of the binary mixtures can be listed in the following order: CH4 > C3H6 > C2H6. The experimental data were used to refine and validate the chemical kinetic mechanism for allyl-HO2 system relevant to low-temperature ignition chemistry of C3H6. A detailed sensitivity analysis is presented to identify the important reaction pathways, and their implications for the experimental observations are discussed. Monte Carlo simulations were used to quantify the model uncertainty for ignition delay time predictions, and to identify reactions that have significant contribution to the model uncertainty. Among the binary mixtures studied, CH4-H2 mixture produced the largest model uncertainty, primarily due to the sensitive reactions involving HO2 radical.

Time-histories of the concentration of major products of ethanol pyrolysis at 1 atm and 10 atm in the temperature range of 120 0-160 0 K were measured behind reflected shock waves using fixed-wavelength laser absorption spectroscopy. Measurement of absorbance at nine wavelengths enabled the determination of the evolution of CH 4 , C 2 H 4 , C 2 H 5 OH, CO, CH 2 O and CH 3 CHO during the pyrolysis of 2% C 2 H 5 OH/Argon mixtures. These measurements enabled almost complete tracking of oxygen and carbon during pyrolysis. The performance of five recently developed kinetic models was evaluated against the measurements. It was found that none of these models could reconcile all the measured time-histories simultaneously. Sensitivity and Rate-of-production analysis revealed that the thermal decomposition and H-abstraction reactions of ethanol are the key reactions that have a controlling influence on the species time-histories. Additional experiments were designed and conducted to infer the rate constants of these reactions. The details of these experiments, the rate constant determination methodology, and the implications of the updated rate constants on the model performance are discussed in the companion paper [1] . To the best of our knowledge, this work represents the first detailed study of ethanol pyrolysis involving simultaneous measurement of multiple species in a shock tube using laser absorption spectroscopy. (c) 2021 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Collisional excitation kinetics for atomic oxygen is studied behind reflected shock waves in 1%O_{2}/Ar mixtures over 10 000-11 000K using laser absorption spectroscopy of the O(3s^{5}S^{o}) to O(3p^{5}P_{3}) transition at 777 nm and the O(3p^{5}P_{3}) to O(3d ^{5}D_{2,3,4}^{o}) transitions at 926 nm. Four time histories are inferred simultaneously from the absorbance of the two transitions: the population density of level 4 of atomic oxygen, i.e., the O(3s ^{5}S^{o}) state, n_{4}; the population density of level 6 of atomic oxygen, i.e., the O(3p^{5}P_{3}) state, n_{6}; the electron number density, n_{e}; and the heavy-particle translational temperature, T_{tr}. Atomic oxygen in the levels 4 and 6 are not in equilibrium with the ground-state atomic oxygen as the measurements of n_{4} and n_{6} are generally 3-20 times smaller than the corresponding values under Boltzmann equilibrium at T_{tr}. However, these two states are close to partial equilibrium with each other within the test time, indicating strong heavy-particle cross coupling between levels 4 and 6 of atomic oxygen. A simplified two-temperature collisional-radiative (CR) model is developed to study the thermal and chemical nonequilibrium of atomic oxygen following shock heating. The four measured time histories are used to optimize the 12 collisional rate constants in the CR model using a stochastic gradient descent (SGD) algorithm. The time-history results, diagnostic methods, and collisional-radiative model presented in the current study are potentially useful in studies of high-enthalpy air, plasma processing, or other applications involving weakly ionized oxygen.

A high-pressure shock tube was used to study ignition delay times (IDT) of CH4/O-2/Ar and natural gas/O-2/Ar mixtures behind reflected shock waves. Reaction progress was monitored using sidewall pressure and direct laser absorption diagnostics of CH4 near 3.175 mu m and ethylene near 10.532 mu m. Stoichiometric, fuel-rich and fuel-lean mixtures of CH4/O-2, highly dilute in argon, were studied over a temperature range from 1450 to 1850 K and pressures between 10 and 55 atm. Of note are the experiments conducted with fuel-rich mixtures, as there is a lack of literature data in this regime. In the current study, the methane absorption diagnostic provided a unique tool enabling both speciation of methane and a clear definition of ignition delay time. In addition to methane oxidation, we have measured ignition delay times of commercial natural gas blends over a temperature range of 1408-1541 K, at pressures near 12 atm, and at an equivalence ratio of 1. To understand the effects of minor constituents (such as ethane and propane) in commercial natural gas blends, ethylene concentration during pyrolysis experiments was monitored using a two-wavelength scheme (10.532 mu m and 10.674 mu m) using a CO2 gas laser. The deficiency of existing kinetic models towards predicting the high-temperature kinetics of natural gas blends was highlighted through our measurements. Therefore, this study also provides data critical for refining these models. Extensive sensitivity analysis emphasizes the importance of the reaction CH3+C2H6 -> CH4+C2H5 during natural gas pyrolysis, and the accuracy of the chemical kinetic models is significantly improved by using a revised reaction rate constant (Shao et al. 2019) for this reaction. These measurements extend the test conditions of earlier studies of methane and commercial natural gas. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

The speciation of jet-fuel pyrolysis plays an important role in the development of predictive models for jet-fuel combustion. In this work, a mull-wavelength speciation technique was applied to a large dataset of shock-tube laser-absorption measurements of jet-fuel pyrolysis. A novel absorbance model was developed to interpret the measurements at common conditions, and pyrolysis product time-histories and early-time yields are reported for three jet fuels at five high-temperature conditions. Specifically, methane, ethylene, and lumped vinyl-group time-histories, in addition to aromatics (benzene and toluene) yields, were quantified using eight wavelengths over conditions 1040-1480 K, 1-3 atm, for jet fuels JP8, Jet-A, and JP5. Results are presented at 1150 K, 1200 K, 1250 K, 1300 K, and 1350 K. Additional relationships between vinyl-group-containing species, namely propene, 1-butene, and 1,3-butadiene, are explored. The mole fraction time-histories exhibit good agreement with the literature and extend the speciation of these jet fuels to the limits of current spectroscopic techniques.

The overall reaction rate for JP10+OH→products was measured directly via laser absorption of OH in shock tube experiments from 931-1308 K and 0.94-1.44 atm. The JP10 concentration of test gas mixtures was measured in the shock tube for several experiments using a 3.39 µm laser fuel diagnostic. Measured JP10 concentrations indicated fuel losses due to adsorption of 11-31% compared to values calculated manometrically from mixture preparation. OH was generated via rapid thermal decomposition of tert-butyl hydroperoxide behind reflected shock waves, and post-shock OH profiles were measured via laser absorption at 308.6 nm. Measured OH profiles were fit with a chemical kinetic model for JP10 chemistry to determine the overall JP10+OH reaction rate. A recommendation is made for the JP10+OH overall reaction rate over the temperature range explored in this study as k1(931-1308 K) = 1.622x1014 exp(-1826/T[K]) ±12%. To the authors' knowledge, this provides the first direct measurement of the JP10+OH reaction rate.

The pyrolysis of propane plays an important role in determining the combustion properties of natural gas mixtures and offers insight into the cracking patterns of larger fuels. This work investigates propane pyrolysis behind reflected shock waves with a multiwavelength laser-absorption speciation technique. Nine laser wavelengths, sensitive to key pyrolysis species, were used to measure absorbance time histories during the decomposition of 2% propane in argon between 1022 and 1467 K, 3.7-4.3 atm. Absorbance models were developed at each diagnostic wavelength to interrogate common initial conditions, and time histories of all major species are reported at 1250, 1290, 1330, 1370, and 1410 K. Nearly complete carbon recovery observed at lower temperatures enabled the inference of hydrogen formation from atomic conservation, while decaying carbon recovery at high temperatures suggests the formation of allene and 1-butene. The results show systematically faster pyrolysis than predicted by kinetic modeling and motivate further study into the kinetics of propane pyrolysis.

Homogeneous and inhomogeneous ignition modes of n -heptane were studied using high-speed imaging in a high-pressure shock tube (HPST). n -Heptane, a fuel with strong negative temperature coefficient (NTC) behavior, was mixed with 4%-21% oxygen in argon or nitrogen and ignited over a wide temperature range (700-1250 K) and at elevated pressures ( > 10 atm). Ultraviolet (UV) images of OH * emission were captured through a sapphire shock-tube end wall using a high-speed camera and a UV intensifier. The current study demonstrates the capability to study auto-ignition modes using high-speed imaging in a high-pressure shock tube. Both homogeneous and inhomogeneous auto-ignition events were observed with the latter generally confined to intermediate temperatures and reactive n -heptane mixtures. We also observed that conventional sidewall diagnostic signals are, in many cases, sufficient to identify inhomogeneous ignitions that are not accurately modeled under the assumption of spatially uniform chemistry. (c) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

We report the first shock tube measurements of formaldehyde (CH 2O) during the first stage ignition of n-heptane, 2-methylhexane and 3,3-dimethylpentane, in highly diluted fuel/oxygen mixtures in the pressure range of 7-10 atm and temperature range of 700-880 K. Combined time histories of all carbonyl (-C = O) species, CO and fuel were also measured simultaneously in an effort to study the impact of fuel structure on the concentration and the rate of evolution of first stage ignition products. Of the three isomers studied in this work, n-heptane was found to be the fastest, while 3,3-dimethylpentane was found to be the slowest. The differences in the time scale of formation, and plateau concentration of the intermediates between the isomers across the entire range of test conditions suggests a strong dependency of the measured time histories to fuel structure. These species therefore act as markers of the Negative Temperature Coefficient (NTC) behavior of fuels and can be used as targets for developing semi-empirical, hybrid chemistry models of complex, multicomponent petroleum derived gasoline and jet fuels. The time histories reported in this work should prove very useful in the refinement of detailed kinetic models of n-heptane, and development of rate rules for branched alkane isomers. (c) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Ethane pyrolysis chemistry plays a critical role in the combustion behavior of natural gas and gives insight into the decomposition patterns of larger alkanes. In this work, ethane pyrolysis was studied behind reflected shock waves with a convex optimization-based laser absorption speciation technique. Species time-histories of ethane, ethylene, methane, and acetylene were quantified in the decomposition of 1% and 2% ethane in argon at conditions between 1178 and 1527 K and 3.1-4.2 atm. Six laser wavelengths, simultaneously probing sensitive regions of each species' spectra, enabled the sensitive detection of all major pyrolysis products and confirmed the negligible formation of other species. The time-history data presented in this work paint a coherent picture of ethane decomposition that motivates specific refinements to state-of-the-art detailed kinetic models.