Species Concentration Measurements Research Articles

Calibration techniques for quantitative NO measurement using Laser-Induced Fluorescence

Laser-Induced Fluorescence (LIF) is an essential optical diagnostic technique for the high-resolution and low-uncertainty measurement of combustion species concentration in a variety of applications and conditions. Two different calibration techniques are explored in this study to obtain quantitative Nitric Oxide (NO) concentration measurements in flames. The first technique, the most employed in the literature, uses the extrapolation of the fluorescence signal from seeded to nascent NO and is only valid under negligible NO reburn conditions. The second technique uses the optical calibration of the experimental setup to relate it to a modelled LIF signal and can be applied regardless of NO reburn. Both of these techniques are explored under two different assumptions: constant and non-constant interfering LIF signal on the NO absorption spectrum. While the former is most often used in the literature, the latter is necessary when the LIF signal from interfering species cannot be distinguished from the NO-LIF signal, especially in high pressure conditions. Hence, a total of four techniques are presented in this work and are found to be in excellent agreement when performed in different flame conditions. The calibration techniques are applied to three lean, atmospheric, laminar, premixed, methane-air flames to explore their field of applicability. Specifically, the study explores the relevance of the techniques in reburn conditions, which occur mostly in high pressure, rich, highly-seeded, or NH3-containing flames. This study aims to offer the reader a portfolio of calibration techniques to use according to the conditions in which they need to be applied. While this study was carried out measuring NO concentration in a stagnation flame burner, the concepts and equations presented can be transposed to the measurement of other species and to other experimental configurations.

Quantitative Analysis of the Li-Ion Solvation Structure in Li-Ion Battery Electrolytes Using ATR-FTIR Spectroscopy.

Attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy is widely used to study condensed materials due to its convenient sample preparation and ability to avoid absorption saturation. Recently, it has been applied to in situ and in operando observations of chemical reactions within electrochemical devices, such as lithium-ion batteries. However, because ATR-FTIR spectroscopy relies on frequency-dependent attenuated reflectance, quantitative concentration measurements of chemical species using the Beer-Lambert law are challenging. Despite the availability of several correction methods, discrepancies remain in the solvation structures around Li+ ions when comparing transmission-type FTIR and ATR-FTIR spectroscopy results, which complicate the determination of solvation and desolvation energies. In this study, we investigate ATR-FTIR correction algorithms, elucidate the reasons for the discrepancies between ATR-FTIR and transmission FTIR spectroscopy results, and develop a method to correct the ATR-FTIR spectrum to accurately determine the solvation structures around Li+ ions.

Diagnosis of multiple gases using a multi-pass ring cavity to enhance Raman scattering

Spontaneous Raman scattering is widely used to enable simultaneous concentration measurements of all relevant species with overcomes whose signal is weak and susceptible to fluorescence interference. A multi-pass ring-cavity scheme is proposed to enhance Raman scattering signal. The Raman spectra of CO2, N2, O2 and H2O in the air are acquired under condition of low excitation power which is 200 mW laser of 532 nm. Compared with the results obtained by single collection without ring cavity, the Raman signal level is enhanced 40 times and the signal-to-noise ratio is improved by 43 times at maximum. Power of pump is taken into consideration to analyze its impact on signal. A limit of detection (3σ criterion standard) of 83 ppm for CO2 is calculated for the multi-pass ring-cavity. The limit of detection of 14 ppm for CH4 is also estimated by the theoretical ratio of cross section values of CO2 and CH4. When power of excitation is low, high level of signal of multi-species gases is acquired with fine detection sensitivity through ring cavity.

Experimental and numerical investigation of high-pressure methane catalytic synthesis from H2 and CO2

The catalytic synthesis of methane from H2 and CO2, which is an efficient way for the production of synthetic natural gas (SNG) by utilizing the H2 originating from carbon-free renewable power sources and the greenhouse gas CO2, was investigated in a non-isothermal channel-flow reactor coated with Ni/Al2O3 at pressures 1.5–10 bar, H2:CO2 volumetric ratios 4–6, and surface temperatures 450–760 K. In situ, spatially resolved Raman measurements of gas-phase species concentrations across the channel height were complemented by gas chromatography analysis of the exhaust gas. Two-dimensional simulations were carried out, using elementary catalytic and gaseous chemical reaction mechanisms and detailed transport. Comparisons of the Raman data and simulations revealed that the preferential diffusion of H2 drastically altered the local H2:CO2 ratios at the catalytic walls, which attained values up to 8 times higher than the corresponding inlet values. This behavior exemplified the caution needed when performing kinetic studies in the mixed kinetically/transport controlled regime. The production of methane increased with increasing pressure and decreasing H2:CO2 ratio. At the highest ratio H2:CO2 = 6, the simulations underpredicted (overpredicted) the measured reactant (product) mole fractions, however, as H2:CO2 decreased, the trend was reversed. The discrepancies reduced with increasing pressure, resulting in a model overprediction of methane by 7.4% at 10 bar. Methane was produced from C(s) via successive hydrogenation. C(s) was in turn produced from COOH(s), followed by dissociation of COOH(s) to CO(s), and finally H-assisted dissociation of CO(s) to C(s). Gas-phase chemistry was negligible over the investigated conditions. The in-channel Raman measurements in conjunction with the exhaust gas analysis facilitated the construction of a global step for the studied reaction. The methane synthesis was assessed to follow a ∼p0.41 pressure dependence over the temperature range 520–680 K.

Four-dimensional laser absorption cinematography of species and temperature dynamics at 2 kHz in reacting flows.

A four-dimensional (4D) mid-infrared laser absorption imaging technique has been developed and demonstrated for quantitative, time-resolved, volumetric measurements of temperature and species concentration in dynamic combustion flows. This technique employs a dual high-speed infrared camera setup to capture turnable radiation from a quantum cascade laser near 4.85 µm to resolve rovibrational absorption transitions of carbon monoxide at two orthogonal projection angles. The laser is modulated with a customized waveform to adaptively resolve two target transitions with an increased density of data samples in proximity to the transition peaks, therefore ensuring accurate and quantitative spectral interpretation while minimizing the required frame rate. A 3D masked Tikhonov regularized inversion was performed to reconstruct spectrally resolved absorbance at every grid point of each frame, which enables subsequent interpretation of local gas properties in time. These methods are applied to achieve quantitative 4D cinematography of temperature and carbon monoxide in a propagating C2H4/O2 flame with a spatial pixel resolution of ∼70 µm and a temporal resolution of 2 kHz.

Transient analysis of solar pyrolysis and hydrogen yield via interband cascade laser absorption spectroscopy of methane, acetylene, ethylene, and ethane

A mid-infrared laser absorption sensing method has been developed to measure species concentrations of four hydrocarbons and gas temperature over a range of temperatures in mixture compositions relevant to the pyrolytic decomposition of natural gas. The four measured species (methane, acetylene, ethylene, and ethane) are the most abundant hydrocarbons during the pyrolysis of natural gas, and provide a means to monitor decomposition progress and hydrogen yield through molar balance. In this work, time-division multiplexed signals of three distributed-feedback interband cascade lasers are used to make simultaneous measurements of select C-H stretch rovibrational transitions of the target hydrocarbons in the 3.00–3.34 μm range. The sensor was validated over a range of temperatures and pressures (300–1000 K, 0.03–1 atm) at relevant mixture compositions, with correction methods developed to mitigate cross-species interference. The sensor was demonstrated on a solar-thermal pyrolysis reactor, where time-resolved measurements of species mole fractions were performed across a range of insolation conditions to capture the transient response of the reactor.

Laser absorption tomography of complex combustion fields based on finite element node strategy and adaptive edge optimization algorithm

Laser absorption spectroscopy tomography is a technique for combustion diagnosis that provides two-dimensional temperature and species concentration measurements. However, its current implementation assumes a homogeneous distribution of discrete grids, introducing errors and uncertainties in the reconstruction results due to large gradients between adjacent grids and information loss within a grid. This work aims to develop a finite element node strategy (FES) to address these issues. To enhance the reconstruction performance, we propose an adaptive edge optimization algorithm (AEO) that can effectively reduce artifacts and preserve edges. We validate the effectiveness of the algorithm through comprehensive simulations and experiments. The simulation results show that the error of integrated projection value exhibits a trend of rise with increasing resolution, and it is also true that the more complex the reconstruction distribution is, the larger the error becomes. In the simulation, the maximum relative error by using the discrete grids strategy (DGS) reached 1.3 % for complex distribution, whilst it was only 0.1 % when the FES was used. Two representative flame phantoms were used in the simulation. The reconstruction error of 4.51 % was achieved in one phantom by using the FES over 5.11 % for using DGS, in the other phantom, the error was 3.14 % for the FES compared with 3.88 % for the DGS. With the addition of the adaptive edge optimization algorithm (FES-AEO), the temperature reconstruction errors were reduced to 1.49 % and 1.77 % respectively, and the characteristic peaks and edge values were closer to the true values. The experimental results show that when the resolution is 1.67 mm, the mean relative error of the DGS measurements is 1.26 %, while the FES-AEO is only 0.65 %.

Interference-free laser-based temperature and CO-concentration measurements for shock-heated isooctane and isooctane/ethanol blends

Species concentration (e.g. CO) and temperature measurements in the combustion field require fast-response technique without interfering species. In the last decade, tunable diode lasers have been established as strong technique to measure species such as CO, CO2, and H2O as well as temperature with high sensitivity. The drawback is the degree of interference that might hamper the robustness of the technique. In this work simultaneous measurements of temperature and CO concentration were carried out using an interference-free mid-infrared laser-based absorption technique behind reflected shock waves. Two transition lines of CO (P(v″ = 0, J″ = 21) and P(v″ = 1, J″ = 21)) in the fundamental vibrational band near 4.87 and 4.93 μm, respectively, were selected. Absorbance interferences from CO2 and H2O at room and high temperatures were evaluated. Spectroscopic parameters for the development of the system were measured: line strengths and collisional broadening coefficients (in Ar) of both lines were obtained at 1020–1950 K by using the scanned-wavelength direct-absorption method. The technique was demonstrated for non-reactive and reactive mixtures. For the non-reactive case, temperature and CO concentration were measured at 1030–1910 K and 1.0–3.7 bar. For the reactive case, oxidation of i-C8H18/O2/Ar and i-C8H18/C2H5OH/O2/Ar mixtures were investigated at three equivalence ratios of 2.0, 1.0, and 0.5. The two newly adopted lines exhibited good performance in the detection of CO concentration and are immune to interferences from CO2 and H2O. In addition, the simulated data from the state-of-the-art isooctane/ethanol mechanisms in literature were compared with the measured data, showing overall good agreement.

Gas mixture conditions conducive to backdraft phenomenon

This work examines the influence of gas mixture compositions on backdraft phenomenon. A series of experiments were conducted in a reduced-scale enclosure subjected to various fire configurations, including a 25.0 kW and a 37.5 kW methane fire and a 16.7 kW and a 25.0 kW propane fire. Three different metrics were used to evaluate backdraft phenomenon: (1) internal conditions within the enclosure before an anticipated backdraft; (2) ignition of a local gas mixture resulting in a backdraft; and (3) intensity of resulting backdraft. The gas mixture composition within the compartment before a backdraft was determined using measurements obtained from an enhanced phi meter and gas analyzer. Ignition resulting in a backdraft was evaluated from the equivalence ratio and gas species concentration measurements surrounding a triggered spark ignitor. Backdraft intensity was analyzed by quantifying the total heat release of the exiting fireball via carbon dioxide generation calorimetry. Real-time equivalence ratio measurements showed the progressive state of the gas mixture composition within the compartment before ignition. Backdraft was observed to occur when the gas mixture surrounding the ignition soure was greater than stoichiometric conditions. The total heat release of backdrafts was found to correspond to the initial mass fraction of fuel residing within the compartment for experiments with propane fires.

Balanced Fast-SpaciMS capillary configurations provide practically noninvasive channel-average measurements in catalytic monoliths

Spatially resolved capillary-inlet mass spectrometry (SpaciMS) provides a detailed picture of the spatiotemporal evolution of reaction network in catalytic monoliths. In the present work, we combine the SpaciMS experiments with a newly developed non-isothermal 3D CFD model for heterogeneously catalyzed reactive flows, including diffusion and permeation through the coated catalyst and channel wall. We explore how the capillary size and sampling rate can be balanced to minimize the impact on probed-channel conversions and provide species concentration measurements representative of the free channel average. In all studied configurations, the balanced sampling-rate fraction is noticeably higher than the corresponding capillary occlusion fraction. For a typical 350-micron capillary that occupies 11% of the channel cross- section, the balanced sampling rate represents 43% of the channel flow. The present work shows that together, a balanced SpaciMS configuration and numerical simulation provide accurate channel-averaged information of a monolithic catalyst under realistic operation conditions.

Enhancing radiological monitoring of 137Cs in coastal environments using taxonomic signals in brown seaweeds

With the rapidly expanding global nuclear industry, more efficient and direct radiological monitoring approaches are needed to ensure the associated environmental health impacts and risk remain fully assessed and undertaken as robustly as possible. Conventionally, radiological monitoring in the environment consists of measuring a wide range of anthropogenically enhanced radionuclides present in selected environmental matrices and using generic transfer values for modelling and prediction that are not necessarily suitable in some situations. Previous studies have found links between taxonomy and radionuclide uptake in terrestrial plants and freshwater fish, but the marine context remains relatively unexplored. This preliminary study was aimed at investigating a similar relationship between brown seaweed, an important indicator in radiological monitoring programmes in the marine environment, and Caesium-137, an important radionuclide discharged to the marine environment. A linear mixed model was fitted using REsidual Maximum Likelihood (REML) to activity concentration data collected from literature published worldwide and other databases. The output from REML modelling was adjusted to the International Atomic Energy Agency (IAEA) quoted transfer value for all seaweed taxa in order to produce mean estimate transfer value for each species, which were then analysed by hierarchical ANalysis Of VAriance (ANOVA) based on the taxonomy of brown seaweeds. Transfer value was found to vary between taxa with increasing significance up the taxonomic hierarchy, suggesting a link to evolutionary history. This novel approach enables contextualisation of activity concentration measurements of important marine indicator species in relation to the wider community, allows prediction of unknown transfer values without the need to sample specific species and could, therefore, enhance radiological monitoring by providing accurate, taxon specific transfer values for use in dose assessments and models of radionuclide transfer in the environment.

Tomographic measurements of temperature and species concentration via deep learning and wavelength modulated absorption spectroscopy

To achieve the 2-D nonintrusive measurements of temperature and species concentration in the combustion field, a new framework, combining calibration-free wavelength modulation absorption spectroscopy (CF-WMS) with a designed convolutional neural network (CNN), was developed. The principle of the CF-WMS, along with the architecture of the CNN net, the training, and the performance of the network, has been investigated. The region of interest was discretized into 24 × 24 pixels2, and 48 probing beams with six targeted frequencies were used to verify the feasibility of the designed CNN with WMS 2f/1f signal for temperature and species concentration reconstruction. 20 000 samples of temperature and water vapor concentration distributions are randomly fabricated, featuring three randomly positioned Gaussian distributions. Reconstructed images of the phantoms agreed well with the original distributions with the relative error of about 5.0%–9.2% and 8.0%–12.4% using 17 000 training datasets with different beam arrangements for temperature and H2O species, respectively. Several representative beam arrangements with a limited number have been examined and compared. The beam arrangement BA2 and BA3 exhibited better performance than BA1 with average errors of about 5% and 8% for temperature and H2O species, respectively. Such a method can provide an effective way to achieve spatially and temporally resolved, real-time, in situ monitoring in practical combustion environments.

Combustion Characteristics of Natural Gas/Air Flat Premixed Laminar Flames in a Developed Matrix Burner

The natural gas flat flame characteristics are experimentally and numerically investigated under different equivalence ratios. A matrix burner is designed so that the natural gas-air mixture from the matrix burner makes up the flat flame. The combustion characteristics, such as the flame lift, the flame thickness, the gas temperatures, and the species concentration, are measured. The measured species concentrations are compared to the computed concentrations using different chemical kinetic mechanisms. The work was carried out at atmospheric pressure and room temperature at different equivalence ratios of 0.6, 0.8, 1, 1.2, and 1.4. A close agreement occurs between the experimental and the simulation results at lean and stoichiometric conditions with a mean absolute error value ranging from 0.9:3. At stoichiometric conditions, the maximum measured gas temperature, the flame thickness, and the minimum flame lift-off all coincide with the maximum simulated mole fractions of CO2 and NO. In addition, the centreline axial temperature is low just above the burner top (i.e., the flame lift region). However, those temperatures increase as the distance above the burner top increases.

Spatially resolved broadband absorption spectroscopy measurements of temperature and multiple species (NH, OH, NO, and NH[formula omitted]) in atmospheric-pressure premixed ammonia/methane/air flames

The temperature and multi-species concentration distributions, including NH, OH, NO, and NH3, are of great importance to the research on ammonia/methane combustion mechanisms. However, there is scarce research simultaneously measuring these parameters in atmospheric-pressure ammonia/methane flames. In this work, we proposed a measurement method based on the ultraviolet broadband absorption spectroscopy (BAS) to realize the simultaneous measurement of temperature and multiple species concentrations for the first time (absolute concentrations for OH, NH, and NO, and relative concentrations for NH3). Due to the strong absorption of the electronic transitions in the ultraviolet range, the proposed method offers low detection limits for concentration measurements. Taking NH radicals as an example, the detection limit is as low as 31 ppb at 1700 K. High sensitivity for temperature measurements is also achieved due to the multi-line spectra of OH broadband absorption. Moreover, a fitting strategy was proposed to separate the NO and NH3 spectra near 225 nm, enabling a simultaneous determination of both species. We established an optical system based on the proposed BAS method with a spatial resolution of approximately 0.1 mm to perform spatially resolved measurements of temperature and multiple species on atmospheric ammonia/methane/air flames. The ammonia/methane flames were investigated at different equivalence ratios (0.9, 1.0, and 1.1) and different ammonia mole fractions in fuels (10%, 30%, and 50%). As investigated, NH and NH3 concentrations increased linearly with increasing ammonia mole fractions, while OH and NO concentrations decreased in similar trends with increasing equivalence ratios. The experimentally measured temperature and multi-species concentrations were compared with computational fluid dynamics (CFD) simulations using the mechanism of the CRECK modeling group. Good agreements were achieved in absolute values, spatial profiles, and variations with equivalence ratios and ammonia mole fractions.

Fuel-rich hetero-/homogeneous combustion of C3H8/O2/N2 mixtures over rhodium

The catalytic (heterogeneous) and gas-phase (homogeneous) combustion of C3H8/O2/N2 mixtures over rhodium was investigated experimentally and numerically at 5 bar and at fuel-rich equivalence ratios φ = 2.0-3.5 relevant to propane Catalytic Partial Oxidation (CPO). In situ spatially-resolved Raman measurements of major gas-phase species concentrations and Planar Laser Induced Fluorescence (PLIF) of formaldehyde were applied in an optically accessible channel-flow reactor to monitor the catalytic and gas-phase processes, respectively, while accompanying 2D simulations were carried out with detailed hetero-/homogeneous chemical reaction mechanisms. Due to the high gas-phase reactivity of propane, homogeneous chemistry could not be ignored over most of the reactor's oxidation zone length (upstream zone where the deficient reactant oxygen is not fully consumed). The presence of gas-phase chemistry deteriorated the otherwise high catalytic syngas (H2 and CO) selectivities over the oxidation zone. Raman measurements of major gas-phase species concentrations over the restricted oxidation zone length without appreciable gas-phase chemistry showed that the catalytic reaction mechanism slightly underpredicted (overpredicted) the H2 (CO) formation. The same behavior was also attested over the remaining length of the oxidation zone where combined catalytic and gas-phase chemistry was present. The production of considerable amounts of H2 at the highest investigated equivalence ratio of 3.5 accelerated the onset of homogeneous ignition and the formation of strong flames. The discrepancies between measured and predicted homogeneous ignition distances were less than 6.8% in all cases, illustrating the validity of the employed hetero-/homogeneous kinetic schemes. Contrary to past methane CPO studies, the contribution of gas-phase chemistry and the formation of strong flames in propane CPO was detrimental to syngas production.

Measurements of Temperature and Species Concentrations of n-Pentane Mixture during Autoignition Using the Corrected Filtered Natural Emission of Species

Measurements of Temperature and Species Concentrations of <i>n</i>-Pentane Mixture during Autoignition Using the Corrected Filtered Natural Emission of Species

Experimental and model investigation of the low temperature oxidation and pyrolysis of 2-methyl-2-butene in a jet-stirred reactor

Species concentrations for 2-methyl-2-butene oxidation and pyrolysis were measured in a jet-stirred reactor at atmospheric pressure, in the temperature range of 650 to 1200 K and the residence time of 1.5 s. The model presented in literature was modified based on the high-level quantum calculated rate coefficients published, including 1,2-dimethyl-allyl (AC5H9C) + HO2, 1,2-dimethyl-allyl + O2, 1,2-dimethyl-allyl + 1,2-dimethyl-allyl, isoprene + H reaction classes. The modified model was validated against the measured species concentrations, ignition delay times and laminar flame speeds. The modified model can accurately predict experimental data under various conditions. Moreover, flux analysis showed that the major pathways were abstraction reactions that consumed more than 40% 2-methyl-2-butene. At lower temperatures, reactions of HO2 addition, O2 addition and radical self-combination were the dominant consumption path for allylic pentenyl radicals. Sensitivity analysis showed that HO2 additions to AC5H9C radicals, forming AC5H9COOH, BC5H9-AOOH, and C5H9O oxy-radicals + OH played important roles in promoting the overall reactivity.

Experimental and chemical kinetic study on the low temperature oxidation of 1,3-butadiene in a jet-stirred reactor

Species concentrations for 1,3-butadiene oxidation were measured in a jet-stirred reactor (JSR) at ϕ = 0.5, 1.0 and 2.0, at 1 atm, in the temperature range of 650 to 1200 K and the residence time of 2 s. AramcoMech 3.0 was modified based on the high-level quantum calculated rate coefficients published, including OH/H/HO2 addition to 1,3-butadiene and subsequent reactions. The modified model was validated against the measured species concentrations in this work and ignition delay times, laminar flame speeds proposed in literatures. Good agreement with experimental data can be observed. In addition, kinetic investigation was carried out. Flux analysis and sensitivity analysis showed that OH/H addition reactions play important roles in determining the reactivity in 1,3-butadiene consumption.

Evaluation of pressure and species concentration measurement using uncertainty propagation

This paper presents a probabilistic uncertainity evaluation method as described in the Guide to the Expression of Uncertainty in Measurements (GUM) and its application to probe measurements on pressure and fuel concentration. All sources of unceratinties are expressed as probability distributions. Consequently, the overall standard uncertainty of the quantity can be calculated using the Gaussian error propagation formula. The result of the uncertainty evaluation yields the most probable value of the measurand and describes its distribution in terms of rectangular (standard uncertainty) or gaussian (“expanded” uncertainty) distribution. A pitot-static probe and a fuel-concentration stem probe are used in order to demonstrate the principle of the probabilistic uncertainty evaluation method. The uncertainty induced by the pressure and concentration data acquisition system as well as the calibration of the fuel-concentration probe are included in the analysis. The overall “expanded” uncertainties for the measured and calculated values are presented as a function of different inlet fuel flows. In addition to this, the individual sources of uncertainty to the overall standard uncertainty are presented and discussed. Moreover, the transformation of standard uncertainty to “expanded” uncertainty will provide the deviation of the measurement in a 95% or 99% normal distributed interval instead of a 67% rectangular distributed interval.

Chemical effects of carbon dioxide in ethylene, ethanol and DME counter-flow diffusion flames: An experimental reference for the fictitious CO2 flame

The chemical effects of carbon dioxide on flame chemistry were investigated numerically and experimentally in ethylene, ethanol, and dimethyl-ether (DME) counter-flow diffusion flames. In this work, a method that could add an experimental reference flame for the fictitious CO2 flame (FCO2) was proposed. The fuel/oxidant carrier gas (FCO2+argon+helium) in the FCO2 flames was replaced by an inert gas mixture (nitrogen+argon+helium) whose composition was optimized to maintain approximately the same thermal and dilution effects as FCO2 addition. The differences in the flame temperatures and the measured species concentration between CO2 flame and the inert gas mixture flame could then be attributed to the pure chemical effects. The new experimental strategy was applied to ethylene, ethanol, and dimethyl-ether (DME) counter-flow diffusion flames. Good agreement was found between the experimental and the computational results. The chemical effects of CO2 addition, indicated by both the numerical FCO2 flame and the experimental inert gas mixture flame, both decreased the peak flame temperatures and the concentration of CH4, H2, and C2H2 except CO. The effects of CO2 addition on the production pathways of CO and C2H2 were very similar in ethylene, ethanol, and DME flames. Decreased CO oxidation rate through CO+OH = CO2+H was a significant feature of the chemical effects identified by both the FCO2 flame and the inert gas mixture flame. Thermal effects of CO2 addition decreased the formation rate of C2H2 through C2H3(+M) = H+C2H2(+M) while the chemical effects on this pathway were weaker.